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Volcanostratigraphic framework and magmatic evolution of the Oyu Tolgoi porphyry Cu-Au district, South… Wainwright, Alan John 2008

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Volcanostratigraphic framework and magmatic evolution of the Oyu Tolgoi porphyry Cu-Au district, South Mongolia by  ALAN JOHN WAINWRIGHT B.Sc., McGill University, 2000 M.Sc., University of Toronto, 2003  A DISSERTATION SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF  DOCTOR OF PHILOSOPHY  in  THE FACULTY OF GRADUATE STUDIES (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver)  November 2008  © Alan John Wainwright, 2008  ABSTRACT The super-giant Oyu Tolgoi porphyry copper-gold deposits in the South Gobi desert, Mongolia, consist of multiple discrete porphyry centers aligned within a northnortheast trending, >6.5 km long, arc-transverse mineralized corridor. The porphyries are linked to a tectono-magmatic event at ~372 Ma within a Devonian to Carboniferous volcanic arc, and U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma. The Oyu Tolgoi district underwent at least three discrete periods of syn- to post-mineral shortening and there is evidence for at least three unconformities within the Paleozoic sequence.  Although the deposits were formed in an active orogenic  environment characterized by rapid uplift, their preservation is a reflection of climactic effects as well insulation from erosion by rapid burial under mass-wasted and pyroclastic material in the volcaniclastic apron of late-mineral dacitic volcanoes. The porphyry copper-gold deposits are spatially and temporally related to medium- to high-K calc-alkaline quartz monzodiorite (~372 Ma) and granodiorite (~366 Ma) intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC).  Adakite-like wholerock compositions as well as zircon grains with high  CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and late-mineral porphyry intrusions are different from younger intrusions that are not related to porphyry Cu-Au deposit formation. Moreover, mixed zircon populations within OTIC intrusions indicate that efficient assimilation of material from different host rocks by a convecting magma chamber occurred. Mafic to intermediate volcanic units evolved from tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of marine arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline, regardless of age. Nd-isotopic geochemistry from all suites is consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc and lead isotopic compositions indicate that the sulfides in the porphyry Cu-Au deposits are genetically linked to the Late Devonian magmas. Magma mixing, adakite-like magmatism and rapid uplift and erosion in a juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from other settings in active and fossil volcanic arcs. ii  TABLE OF CONTENTS ABSTRACT....................................................................................................................... ii TABLE OF CONTENTS ................................................................................................ iii LIST OF TABLES ......................................................................................................... viii LIST OF FIGURES ......................................................................................................... ix ACKNOWLEDGEMENTS ........................................................................................... xii CO-AUTHORSHIP STATEMENT ............................................................................. xiii Chapter 1 - Introduction .................................................................................................. 1 Objectives of the dissertation.......................................................................................... 1 Porphyry copper deposits................................................................................................ 3 Volcanic arcs that host porphyry deposits ...................................................................... 4 Magmas related to porphyry copper deposits ................................................................. 5 Overview of the dissertation ........................................................................................... 6 References....................................................................................................................... 9 Chapter 2 - Volcanostratigraphic framework of the Oyu Tolgoi porphyry Cu-Au district; South Gobi region, Mongolia .......................................................................... 12 Introduction................................................................................................................... 12 Tectonic setting............................................................................................................. 13 Geology of the Oyu Tolgoi porphyry Cu-Au district ................................................... 16 District stratgraphic sequence ....................................................................................... 19 Devonian supracrustal rocks – Alagbayan Group ........................................................ 23 Carboniferous supracrustal rocks – Gurvankharaat Group........................................... 30 Devonian subvolcanic intrusions .................................................................................. 33 Carboniferous subvolcanic intrusions........................................................................... 38 Regional granitoid plutons............................................................................................ 39 U-Pb geochronology ..................................................................................................... 40 Analytical method..................................................................................................... 41 Results........................................................................................................................... 58 Older recycled zircons .............................................................................................. 58 Devonian magmatic activity ..................................................................................... 58 Carboniferous magmatic activity............................................................................. 60  iii  Discussion ..................................................................................................................... 67 Volcano-sedimentary sequences and depositional environments............................. 67 Geochronology of the stratigraphic sequence........................................................... 67 Late Devonian volcanic environment ....................................................................... 68 Carboniferous volcanic environment ........................................................................ 73 Pre-Devonian activity and onset of oceanic arc magmatism .................................... 76 Conclusions................................................................................................................... 77 References..................................................................................................................... 78 Chapter 3 - Upper Devonian event sequence and paleogeography; Oyu Tolgoi porphyry Cu-Au district, South Mongolia ................................................................... 82 Introduction................................................................................................................... 82 Geologic setting ............................................................................................................ 83 Devonian volcanic arc sequences ................................................................................. 84 Field relations - Late Devonian intrusive rocks............................................................ 89 U-Pb geochronology ..................................................................................................... 92 Analytical method..................................................................................................... 92 Results....................................................................................................................... 93 Pb isotopic compositions .............................................................................................. 97 Analytical method..................................................................................................... 97 Results....................................................................................................................... 97 Discussion ..................................................................................................................... 99 Upper Devonian stratigraphic sequence ................................................................... 99 Isotopic constaints on the event sequence .............................................................. 101 Timing of mineralization ........................................................................................ 103 Geologic and chronologic implications for volcanic arcs....................................... 105 Paleogeography of the Oyu Tolgoi district............................................................. 106 Conclusions................................................................................................................. 106 References................................................................................................................... 108 Chapter 4 - Petrology, petrochemistry and timing of Late Devonian intrusions associated with the super-giant Oyu Tolgoi Cu-Au deposits; South Gobi region, Mongolia ........................................................................................................................ 111  iv  Introduction................................................................................................................. 111 Geologic framework ................................................................................................... 112 Petrology of Late Devonian intrusions ....................................................................... 114 Intrusive sequence....................................................................................................... 116 Lithogeochemistry ...................................................................................................... 119 Geochemical characteristics........................................................................................ 122 Major elements........................................................................................................ 122 Alkalinity ................................................................................................................ 122 Minor elements, trace elements and REE ............................................................... 124 U-Pb geochronology ................................................................................................... 127 Analytical method................................................................................................... 127 Results..................................................................................................................... 129 Discussion ................................................................................................................... 143 Petrologic and petrochemical evolution of OTIC intrusions .................................. 143 U-Pb ages of ore-stage to late-mineral OTIC intrusions ........................................ 143 Petrochemical characteristics of fertile island arcs ................................................. 146 Conclusions................................................................................................................. 150 References................................................................................................................... 151 Chapter 5 - Cooling, fractionation and mixing of magmas in the super-giant Oyu Tolgoi Cu-Au porphyries, South Mongolia; SHRIMP-RG trace element geochemistry of zircon.................................................................................................. 156 Introduction................................................................................................................. 156 Geologic setting .......................................................................................................... 157 Analytical method....................................................................................................... 159 Results......................................................................................................................... 160 Zircon morphology ..................................................................................................... 170 REE patterns ............................................................................................................... 170 Hf versus TiO2-in-zircon thermometer ....................................................................... 170 Trace element variations ............................................................................................. 176 Multi-valent elements Eu and Ce................................................................................ 177 Discussion ................................................................................................................... 182  v  Evidence and implications for magma mixing ....................................................... 182 Oxidized porphyry magmas .................................................................................... 183 Spatial-temporal and wholerock geochemical considerations ................................ 185 Conclusions................................................................................................................. 185 References................................................................................................................... 187 Chapter 6 – Geochemical evolution of Devonian to Carboniferous volcanic arc rocks in the South Gobi region; a reconnaissance petrochemical and isotopic study from the Oyu Tolgoi porphyry Cu-Au district, Mongolia.................................................. 190 Introduction................................................................................................................. 190 Geologic setting of the Oyu Tolgoi porphyry Cu-Au district..................................... 191 Devonian to Carboniferous igneous suites ................................................................. 193 Wholerock petrochemistry.......................................................................................... 195 Results......................................................................................................................... 199 Mafic to intermediate rocks .................................................................................... 199 Intermediate to felsic rocks..................................................................................... 203 Nd isotopic geochemistry ........................................................................................... 207 Analytical method................................................................................................... 207 Results..................................................................................................................... 207 Pb isotopic geochemistry ............................................................................................ 210 Analytical method................................................................................................... 210 Results..................................................................................................................... 210 Discussion ................................................................................................................... 213 Origin and wholerock petrochemical evolution of Devonian to Carboniferous igneous rocks at Oyu Tolgoi ................................................................................... 213 Isotopic compositions and petrogenesis of Devonian to Carboniferous igneous rocks at Oyu Tolgoi .......................................................................................................... 214 Geodynamic framework of the Paleozoic volcanic arc at Oyu Tolgoi ................... 215 Conclusions................................................................................................................. 215 References................................................................................................................... 217 Chapter 7 – Summary and Conclusions ..................................................................... 220 Future research directions ........................................................................................... 222  vi  Structure and stratigraphy ....................................................................................... 222 Geodynamic framework.......................................................................................... 223 Geochemistry .......................................................................................................... 223 References................................................................................................................... 224 Appendix 1 – CL images and SHRIMP-RG spot locations (zircon) ........................ 225 Appendix 2 - Wholerock geochemistry for OTIC intrusions.................................... 237 Appendix 3 – Zircon spot locations for trace element study..................................... 247 Appendix 4 - Wholerock geochemistry for Devonian-Carboniferous rocks ........... 253 Appendix 5 – Detection limits for wholerock geochemical analyses (ALS CHEMEX*) ................................................................................................................... 263  vii  LIST OF TABLES Table 2.1 Groups, Formations and names of informal lithologic package designations used in this study............................................................................................................... 22 Table 2.2 Summary table of volcanostratigraphic lithofacies. ........................................ 36 Table 2.3 SHRIMP-RG analytical data for Oyu Tolgoi district geochronology samples. ........................................................................................................................................... 46 Table 2.4 Summary of SHRIMP-RG U-Pb (zircon) ages................................................ 57 Table 2.5 TIMS U-P data for Augite Basalt (chemically-abraded grains) and UDS (airabraded grains) units. ........................................................................................................ 59 Table 3.1 Summary of Upper Devonian isotopic ages for layered rocks and intrusions within the Oyu Tolgoi porphyry Cu-Au district. .............................................................. 94 Table 3.2 TIMS U-Pb data for granodiorite and QMD samples...................................... 96 Table 3.3 Pb isotopic compositions for QMD-P1 clast samples AJW-06-488 and AJW06-425. .............................................................................................................................. 98 Table 4.1 Petrologic characteristics of Late Devonian intrusions. ................................ 118 Table 4.2 Representative geochemical analyses for Late Devonian intrusions............. 120 Table 4.3 SHRIMP-RG analytical data for Late Devonian intrusions. ......................... 133 Table 4.4 TIMS U-Pb Analytical data for Late Devonian intrusions. ........................... 139 Table 4.5 Summarized U-Pb (zircon) ages for Late Devonian intrusions..................... 142 Table 4.6 Nickel and chromium concentrations from ore-stage intrusive rocks in Cu-Au porphyry deposits. The SiO2 (wt. %) range for the data sets is approximately 55 wt. % to 70 wt. %. ......................................................................................................................... 149 Table 5.1 Summary of U-Pb (zircon) ages for the intrusions in the study. ................... 161 Table 5.2 Trace element concentrations (ppm) for zircons. .......................................... 162 Table 6.1 Igneous rock suites from the Oyu Tolgoi district .......................................... 194 Table 6.2 Representative geochemical analyses for igneous rock suites from the Oyu Tolgoi district.................................................................................................................. 196 Table 6.3 Nd isotopic geochemistry of rocks from the Oyu Tolgoi porphyry Cu-Au district. ............................................................................................................................ 208 Table 6.4 Pb isotopic geochemistry from Oyu Tolgoi sulfide and feldspar separates. . 211  viii  LIST OF FIGURES Figure 1.1 Simplified tectonic map of Mongolia............................................................... 2 Figure 2.1 Tectonic evolution of the Central Asian Orogenic Belt. ................................ 14 Figure 2.2 Simplified regional geology map. .................................................................. 18 Figure 2.3 Property-scale geology and sample locations................................................. 20 Figure 2.4 Contoured bedding data from oriented drill core ........................................... 21 Figure 2.5 Facies distribution in Devonian or older to Carboniferous rocks................... 24 Figure 2.6 Polished slab photographs of textural variation in Devonian or older rocks . 28 Figure 2.7 Facies distribution for Carboniferous rocks at Oyu Tolgoi............................ 31 Figure 2.8 Polished slab photographs of textural variations in Carboniferous rocks. ..... 34 Figure 2.9 Representative cathodoluminescence images of zircon grains from samples dated by SHRIMP-RG. ..................................................................................................... 44 Figure 2.10 Weighted mean 207Pb-corrected 238U/206Pb age diagrams ............................ 63 Figure 2.11 Concordia plots of two chemically-abraded single-grain TIMS U-Pb analyses for the sample of Bulagbayan Formation basalt (AJW-03-204) and six airabraded single-grain fractions for the unmineralized welded ignimbrite (UDS unit; AJW03-148).............................................................................................................................. 65 Figure 2.12 Zircon spot age histogram for tuffaceous sandstone samples from the Sedimentary Sequence (Gurvankharaat Group). .............................................................. 66 Figure 2.13 Composite summary section of Paleozoic rocks in the Oyu Tolgoi porphyry Cu-Au district.................................................................................................................... 70 Figure 2.14 Schematic sequence of volcanic and sub-volcanic events in the Devonian (or prior) ................................................................................................................................. 72 Figure 2.15 Schematic sequence of volcanic and sub-volcanic events in the Carboniferous.................................................................................................................... 75 Figure 3.1 Bedrock geology map of the Oyu Tolgoi district ........................................... 86 Figure 3.2 Generalized stratigraphic sequence of Upper Devonian rocks....................... 87 Figure 3.3 Detailed stratigraphic sections for the contact between mineralized and unmineralized rocks .......................................................................................................... 88 Figure 3.4 Polished slab photographs showing the textural variations in Upper Devonian volcanic, volcaniclastic and intrusive rocks...................................................................... 90 ix  Figure 3.5 Concordia diagrams for TIMS U-Pb results for Upper Devonian rocks........ 95 Figure 3.6 Pb isotopic geochemistry of QMD-P1 clasts................................................ 100 Figure 3.7 Generalized interpretation of the Upper Devonian event sequence. ............ 104 Figure 4.1 Bedrock geology map of the Oyu Tolgoi district ......................................... 113 Figure 4.2 Representative polished slab images of Late Devonian intrusive rocks from the Oyu Tolgoi district .................................................................................................... 115 Figure 4.3 Photographs of breccias showing intrusive relationships documented in the OTIC. .............................................................................................................................. 117 Figure 4.4 Major element and petrochemical discrimination diagrams. ....................... 123 Figure 4.5 Chondrite-normalized rare-earth element plots for Late Devonian intrusions ......................................................................................................................................... 125 Figure 4.6 Silica versus REE and adakite discrimination plots. .................................... 126 Figure 4.7 Representative cathodoluminescence images from Late Devonian intrusion samples dated by SHRIMP-RG. ..................................................................................... 132 Figure 4.8 SHRIMP-RG U-Pb age diagrams for Late Devonian OTIC phases. ........... 138 Figure 4.9 Concordia diagrams showing TIMS U-Pb data for Late Devonian intrusions. ......................................................................................................................................... 141 Figure 4.10 U-Pb (zircon) age histograms for OTIC intrusions. ................................... 145 Figure 4.11 Geochemistry of intrusions from worldwide porphyries............................ 148 Figure 5.1 Bedrock geology map of the Oyu Tolgoi district ......................................... 158 Figure 5.2a Representative cathodoluminescence images of zircons from QMD intrusions......................................................................................................................... 172 Figure 5.2b Representative cathodoluminescence images of granodiorite zircons ....... 173 Figure 5.2c Representative cathodoluminescence zircon images for Carboniferous samples............................................................................................................................ 174 Figure 5.3 Chondrite-normalized REE patterns for the zircons in the study................. 175 Figure 5.4 Hf variation diagrams for zircon. ................................................................. 178 Figure 5.5 Y vs. Yb/Gd and Th/U vs. Yb/Gd for zircon. .............................................. 180 Figure 5.6 Hf vs. multi-valent element concentrations.................................................. 181 Figure 6.1 Bedrock geology map of the Oyu Tolgoi district ......................................... 192  x  Figure 6.2 Geochemical and tectonic discrimination diagrams: mafic to intermediate rocks................................................................................................................................ 200 Figure 6.3 Primitive mantle-normalized extended trace element diagrams: mafic to intermediate rocks........................................................................................................... 201 Figure 6.4 Chondrite-normalized rare earth element diagrams for mafic to intermediate volcanic and subvolcanic rocks. ..................................................................................... 202 Figure 6.5 Geochemical discrimination diagams for intermediate to felsic rocks from the Oyu Tolgoi district.......................................................................................................... 204 Figure 6.6 Chondrite-normalized rare-earth element (REE) plots for intermediate to felsic subvolcanic rocks. ................................................................................................. 205 Figure 6.7 Variation of adakite-like characteristics and REE ratios.............................. 206 Figure 6.8 Nd isotopic geochemistry ............................................................................. 209 Figure 6.9 Pb isotopic geochemistry of feldspar and sulfide seperates from Oyu Tolgoi rocks................................................................................................................................ 212  xi  ACKNOWLEDGEMENTS Thanks to Ivanhoe Mines for financial and logistical support, in particular Doug Kirwin and Charlie Forster. SHRIMP-RG work was facilitated by Joe Wooden and Frank Mazdab and their input regarding U-Pb (zircon) dating and trace element geochemistry of zircons was invaluable. Rich Friedman performed the TIMS U-Pb analyses and is thanked for his input and advice. Thanks to Janet Gabites and Dominique Weis for providing the radiogenic isotopic compositions and their constructive comments. Hai Lin is thanked for contributing to the heavy mineral separation work. Committee members James Scoates and Jim Mortensen are thanked for their input and constructive criticism. Kelly Russell is thanked for discussions on the volcanic sequences and Karie Smith and Arne Toma are thanked for their help with technical and managerial matters at MDRU.  The Hugh E. McKinstry grant from the Society of Economic  Geologists provided funding for the zircon chemistry chapter. Special thanks to Imants Kavalieris, David Crane, Alastair Findlay, Cyrill Orssich, Oyunchimeg Rinchin, Dale Sketchley, Elisabeth Ronacher, Odnaa Ayush, Peter Lewis and all employees of Ivanhoe Mines at Oyu Tolgoi for providing logistical help and input on geologic issues in the field. Special thanks to Dick Tosdal for supervising the project and support on all aspects of the Ph.D. degree.  Finally, thanks go to friends, colleagues (especially Claire  Chamberlain and Adam Simmons) and family for support and encouragement during the study.  xii  CO-AUTHORSHIP STATEMENT The five chapters in this dissertation are written as manuscripts for publication in refereed journals. Each represent primarily my work and initiative, except where noted below. I plan to submit all of the manuscripts with my supervisor, Richard Tosdal, as a co-author. He participated in all stages of the research including development of the project, field work to establish the project and as a field check, establishing laboratory protocols, and editing of the manuscripts. Chapter 2 Volcanostratigraphic framework of the Oyu Tolgoi porphyry Cu-Au district; South Gobi region, Mongolia. Authors: Alan J. Wainwright, Charles N. Forster, Douglas J. Kirwin, Peter D. Lewis, Richard M. Tosdal, Joseph L. Wooden and Frank K. Mazdab. Charles Forster and Douglas Kirwin (Ivanhoe Mines) were involved in the research guidance and financial support for the project. Much time was spent in the field with Peter Lewis discussing the various geologic problems and he also contributed to the editing of Chapter 2. Joseph Wooden and Frank Mazdab contributed to the work at the SHRIMP-RG lab, including interpretation of the data for 15 samples. They will all provide revisions of the manuscript before publication. Chapter 3 Upper Devonian event sequence and paleogeography; Oyu Tolgoi porphyry Cu-Au district, South Mongolia. Authors: Alan J. Wainwright, Peter D. Lewis, Richard M. Tosdal and Richard M. Friedman. Much time was spent in the field with Peter Lewis discussing the various geologic problems and he also contributed to the editing of Chapter 3. Richard Friedman provided TIMS U-Pb (zircon) geochronological results for 2 samples, including chemistry, analysis and data reduction and assisted with the interpretation of the data. They will all provide revisions of the manuscript before publication.  xiii  Chapter 4 Petrology, petrochemistry and timing of Late Devonian intrusions associated with the super-giant Oyu Tolgoi Cu-Au deposits; South Gobi region, Mongolia. Authors: Alan J. Wainwright, Richard M. Tosdal, Joseph L. Wooden, Frank K. Mazdab and Richard M. Friedman. Joseph Wooden and Frank Mazdab contributed to the work at the SHRIMP-RG lab, including sample preparation and interpretation of the data for 8 samples. Richard Friedman provided TIMS U-Pb (zircon) geochronological results for 4 samples, including chemistry, analysis and data reduction and assisted with the interpretation of the data. They will all provide revisions of the manuscript before publication. Chapter 5 Cooling, fractionation and mixing of magmas in the super-giant Oyu Tolgoi Cu-Au porphyries; SHRIMP-RG trace element geochemistry of zircons. Authors: Alan J. Wainwright, Richard M. Tosdal, Joseph L. Wooden and Frank K. Mazdab. Joseph Wooden and Frank Mazdab contributed to the analytical work, data interpretation and editing of the chapter. They will provide revisions of the manuscript before publication. Chapter 6 Geochemical evolution of Devonian to Carboniferous volcanic arc rocks in the South Gobi region; a reconnaissance petrochemical and isotopic study from the Oyu Tolgoi porphyry Cu-Au district, Mongolia. Authors: Alan J. Wainwright, Richard M. Tosdal, James S. Scoates, Janet E. Gabites and Dominique Weis. Janet Gabites provided the lead isotopic geochemistry for 18 samples and Dominique Weis provided the Nd isotopic geochemistry for 17 samples. James Scoates assisted with the selection of appropriate samples for the isotope study and contributed to editing of the chapter.  They will all provide revisions of the manuscript before  publication.  xiv  Chapter 1 - Introduction Objectives of the dissertation This dissertation constrains the history of volcanism, magma evolution and mineral deposit formation in the Oyu Tolgoi porphyry Cu-Au district in the southern Gobi Desert of Mongolia (Figure 1.1). The Paleozoic Cu-Au deposits are important as they represent a new mineral exploration discovery in a remote area underlain by poorly understood geology. The South Gobi desert is comprised of accreted volcanic arcs, and the chronometric and tectono-magmatic character of these arcs is not well understood. As such, the project has allowed the opportunity to contribute to the knowledge of the geologic and metallogenic evolution of Central Asia.  In addition to the geologic  problems presented by the complex stratigraphic and structural architecture of the region, this research also addresses fundamental questions that have implications for our understanding of the development of giant porphyry copper-gold deposits within the framework of volcanic arc evolution, in addition to the characteristics of magmas associated with the Cu-Au porphyry deposits. The Oyu Tolgoi district hosts one of the largest accumulations of copper, gold and sulfur on Earth with measured and indicated resources that include 1.15 Gt grading 1.27% copper and 0.48 g/t gold and an additional inferred resource of 1.44 Gt grading 1.11% copper and 0.28 g/t gold (0.60% copper equivalent cut-off grade) (Ivanhoe Mines press release, 2006).  More recently, additional porphyry mineralization has been  discovered approximately five kilometers south-southwest of the Oyu Tolgoi porphyries, within the same mineralized corridor. The Heruga Deposit is estimated to contain an inferred resource of 760 Mt grading 0.48% copper, 0.55 g/t gold and 142 ppm molybdenum (0.60% copper equivalent cut-off grade) (Ivanhoe Mines press release, 2008). The volcanic and plutonic rocks within the Oyu Tolgoi district span at least 70 m.y. in the Paleozoic and record the geologic history of the region. Early studies on the district (Perello et al., 2001; Kirwin et al., 2005; Lewis, 2005; Ayush, 2006; Khashgerel et al., 2006) focused on the geologic characteristics of the deposits proper, alteration and  1  Figure 1.1 Simplified tectonic map of Mongolia (modified after Badarch et al., 2002). The Oyu Tolgoi porphyry Cu-Au deposits are located in the Gurvansayhan Terrane of the southern Gobi desert.  2  sulfide mineralization as well as stable isotopes. Other larger-scale studies have made important contributions to the understanding of the geologic evolution of the southern Gobi Desert (e.g. Lamb and Badarch, 1997; Lamb and Badarch, 2001; Badarch et al., 2002; Helo et al., 2006). In this study, field mapping and drill core logging have been integrated with petrochemical, mineral-chemical and radiogenic isotopic geochemistry to provide first-order constraints on the regional and local stratigraphic framework as well as the tectono-magmatic history of the arc. Uranium-lead (zircon) isotopic ages from Oyu Tolgoi district rocks have allowed the establishment of an absolute temporal framework within the porphyry copper-gold complex in addition to defining its position within the evolution of the volcanic arc. Furthermore, the distinction between igneous rocks associated with giant porphyry deposits and volcanic arc rocks without associated porphyry copper mineralization is not well understood and this study contributes to the petrochemical and mineral-chemical criterion that improves the understanding of the genesis of giant ore deposits in the South Gobi Desert of Mongolia and in porphyry belts worldwide. Porphyry copper deposits Intermediate to felsic intrusions are spatially and temporally related to some of the world’s largest deposits of base and precious metals. Porphyry Cu-Au-Mo deposits, related to these intrusions, are typically large (hundreds of megatonnes) and low-grade (0.3-1% Cu), and are the world’s primary source of Cu and Mo, and an important source of Au (e.g. Seedorf et al., 2005). Examples of these ore deposits include the Grasberg porphyry copper-gold deposit in Indonesia (e.g. McDowell et al., 1996; Pollard and Taylor, 2002), the Far Southeast–Lepanto porphyry and epithermal Cu-Au deposits in the Philippines (Hedenquist et al., 1998) and the El Salvador porphyry Cu-Mo deposit in Chile (Gustafson and Hunt, 1975; Cornejo et al., 1997). The porphyry deposits usually form at convergent plate margins, above zones of active subduction, in settings that range from primitive island arcs to continental arc margins.  Cenozoic porphyry deposits  related to subduction zones are widespread in South America and the Southwest Pacific whereas older examples such as the Paleozoic Cadia-Ridgeway deposits in the Lachlan fold belt of eastern Australia are relatively uncommon. Mineralization events of this type  3  may be a fairly common feature in subduction zones today and may have been throughout much of Earth history.  However, their likelihood of preservation and  discovery has lead to a relatively sparse distribution of porphyry deposits in magmatic arcs of all ages. Porphyry deposits are associated with unusually oxidized, water-rich calc-alkaline to alkaline arc magmas. Water saturation occurs during cooling of a batholithic root intrusion in the upper crust at about 50% crystallinity. This leads to the separation of an aqueous fluid phase rich in sulfur, potassium and metals (Cu, Mo and Au) that accumulates in the cupola of the subvolcanic magma chamber and is ultimately released due to fluid overpressuring or tectonic fracturing along with the intrusion of shallow porphyry dikes (Dilles, 1987).  The fluids and metals are introduced into both the  porphyry intrusions and host wall rocks.  Sulfide minerals typically linked to the  porphyry event include pyrite, chalcopyrite and bornite associated with biotite, orthoclase, sericite and quartz (Rose, 1970; Gustafson and Hunt, 1975). Subsequent establishment and inward collapse of a convective groundwater system which reacts with the cooling mineralized rocks will generate an advanced argillic overprint assemblage characterized by pyrophillite, alunite and other clay minerals. A variety of ore minerals are associated with the later hydrothermal stage including pyrite, covellite and enargite (Gustafson and Hunt, 1975; Hedenquist et al., 1998; Garza et al., 2001). Volcanic arcs that host porphyry deposits The geodynamic framework of convergent plate margins is crucial to understanding the emplacement mechanisms, location and timing of porphyry-forming events. Porphyry deposits of similar ages tend to occur in linear groups and are variably associated  with  compressional,  extensional  and  transtensional  environments.  Furthermore, certain tectonic events within the framework of volcanic arc evolution are thought to be associated with copper-gold porphyry deposits. Arc reversals (Solomon, 1990), subduction of very young lithosphere, very slow or oblique convergence (Mungall, 2002), flat subduction (Kay and Mpodozis, 2001) or the cessation of subduction (Sillitoe, 1997) provide suitable geotectonic environments that can generate fertile magmas. Known porphyry deposits are not present in all modern island arc  4  settings. For example, the Sunda-Banda/Luzon arc is known to host large porphyry CuAu deposits (e.g. Tampakan, Batu Hijau, Santo Tomas, Lepanto) whereas the Lesser Antilles/New Britain/Aleutian arcs do not host known porphyry districts, which could be a function of the geodynamic architecture of the individual arcs or arc segments. Magmas related to porphyry copper deposits Typically, porphyry deposits are associated with the shallow emplacement of a complex series of porphyritic stocks and/or dikes in and above the cupola zone of a calcalkaline batholith (Gustafson and Hunt, 1975; Dilles, 1987).  However, copper  introduction tends to be related to only one or two of the early intrusive phases, whereas the younger phases tend to truncate or dilute grade (Lickfold et al., 2003). Within the porphyry intrusive sequence, textural and compositional variations through time have been documented in a small number of complexes, and there appears to be a broadly similar sequence related to a fractionating root intrusion, and progressively shallow crystallization depths (e.g. Lang and Titley, 1998; Lickfold et al., 2003). A variety of petrologic and petrochemical characteristics are common in many of the world’s porphyry belts, yet relatively uncommon in the geologic record of volcanic arcs. In general, the intrusions associated with porphyry deposits are hydrous, volatilerich and strongly oxidized (e.g. Dilles, 1987; Mueller et al., 2001), but otherwise not dissimilar to granitoid magmas of similar composition (Cline and Bodnar, 1991). Furthermore, copper-bearing porphyries are disproportionately associated with adakitelike rocks (Thieblemont et al., 1997; Sajona and Maury, 1998; Oyarzun et al., 2001), and gold-rich porphyries with alkaline rocks, in particular of shoshonitic compositon (Mueller and Groves, 1993; Sillitoe, 1997, Mueller et al., 2001). There is an association between unusually oxidized arc magmas and igneous complexes associated with the formation of porphyry deposits (Candela, 1992; Blevin and Chappell, 1992; Hedenquist and Lowenstern, 1994; Mungall, 2002, Sun et al., 2004). Furthermore, the addition of primitive melt into an intermediate magma chamber may be an important factor in the genesis of giant porphyry deposits (Cornejo et al., 1997; Waite et al., 1998; Hattori and Keith, 2001; Maughan et al., 2002; Pollard and Taylor, 2002).  5  Overview of the dissertation This dissertation is presented as five chapters (Chapters 2-6), each of which represents a manuscript to be submitted to a refereed journal for publication. Field work and sample collection was conducted during three separate field seasons: five months in Summer/Fall 2003, five months in Summer/Fall 2004 and one month in Spring 2006. Field observations were made during detailed drill core logging of several kilometers of diamond drill core on-site (outdoors) in addition to outcrop mapping in Mongolia in temperatures that ranged from -15oC to +45oC. SHRIMP-RG work (geochronology and trace element-in-zircon work) was conducted at the USGS-Stanford facility during six separate 5-7 day periods. Each involved sample analysis 24-hours per day in conjunction with 1-2 other UBC researchers, supervised by SHRIMP-RG staff. Chapter 2 is focused on the overall regional geologic framework of the district, is largely field-based and the majority of the U-Pb (zircon) geochronologic data is presented. Some of the first precise U-Pb (zircon) ages (15 SHRIMP-RG and 2 TIMS) are presented for the Oyu Tolgoi porphyry district in this chapter.  Moreover, the  stratigraphic nomenclature for the area is defined and the first detailed facies descriptions for volcanic and volcaniclastic units in the area are presented.  All heavy mineral  separates were produced by A.J. Wainwright using UBC equipment, assisted by Hai Lin on some occasions. Joseph Wooden and Frank Mazdab contributed to the work at the SHRIMP-RG lab, including sample preparation and interpretation of the data. Richard Friedman provided both TIMS/CA-TIMS U-Pb geochronological results, including chemistry, analysis and data reduction and he also assisted with the interpretation of the data. Peter Lewis and Richard Tosdal contributed by way of constructive geological discussions in the field as well as to the editing of the chapter. Chapter 3 presents additional field data that is concerned specifically with the Upper Devonian event sequence and Cu-Au porphyry deposit formation within this context, based on detailed drill core logging of several 10-50 m intervals by A.J. Wainwright in the South Gobi desert. Additional U-Pb (zircon) geochronolgy (TIMS and CA-TIMS) as well as Pb isotopic geochemistry is presented in this chapter as well that constrains the event sequence within this critical time-stratigraphic interval in addition to the age of mineralization in the Oyu Tolgoi district. The heavy mineral 6  fractions were separated by A.J. Wainwright.  Richard Friedman provided two  TIMS/CA-TIMS U-Pb (zircon) geochronological results, including chemistry, analysis and data reduction and assisted with the interpretation of the data. A.J. Wainwright separated the sulfide fractions for the Pb isotopic study and Janet Gabites produced the Pb isotopic results for two samples. Peter Lewis and Richard Tosdal contributed by way of constructive geological discussions in the field as well as to the editing of the chapter. In Chapter 4 the geochemistry and U-Pb geochronologic data that is relevant to the intrusions that are spatially and temporally related to the Cu-Au deposits is presented. The chapter is the first comprehensive study that dates the intrusions and links wholerock petrochemistry to the U-Pb (zircon) ages in the Oyu Tolgoi district. All heavy mineral separates were produced by A.J. Wainwright using UBC equipment, assisted by Hai Lin on some occasions. Joseph Wooden and Frank Mazdab contributed to the work at the SHRIMP-RG lab, including sample preparation and interpretation of the data. Richard Friedman provided all TIMS/CA-TIMS U-Pb (zircon) geochronological results, including chemistry, analysis and data reduction and assisted with the interpretation of the data.  Wholerock geochemistry was produced by ALS CHEMEX of North  Vancouver. Chapter 5 discusses the trace element-in-zircon work and is one of the first studies to apply the interpretation of such extended trace element data from zircons to rocks from a porphyry copper district. All heavy mineral separates were produced by A.J. Wainwright using UBC equipment, assisted by Hai Lin on some occasions. Joseph Wooden and Frank Mazdab contributed to the analytical work, data interpretation and editing of the chapter. Chapter 6 discusses the geochemical evolution (whole-rock geochemical and isotopic) of the Paleozoic volcanic arc that hosts the deposits. This chapter is the first study that links absolute geochronologic ages to the petrochemical data for the South Gobi region.  James Scoates and Dominique Weis assisted A.J. Wainwright in the  selection of appropriate samples for the Nd isotope study and the Nd isotope geochemistry was produced by Dominique Weis at PCIGR. A.J. Wainwright separated the feldspar and sulfide fractions for the Pb-isotopic study and Janet Gabites produced  7  the Pb-isotopic results. Wholerock geochemistry was produced by ALS CHEMEX of North Vancouver. Although all contributions focus on different aspects of the geology of the Oyu Tolgoi district, each has been prepared as a stand-alone publication, which has resulted in some overlap and repetition between the chapters.  8  References Ayush, O., 2006, Stratigraphy, geochemical characteristics and tectonic interpretation of Middle to Late Paleozoic arc sequences from the Oyu Tolgoi porphyry Cu-Au deposit, unpublished M.Sc. thesis. Badarch, G., Dickson C.W. and Windley, B.F., 2002, A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia, Journal of Asian Earth Sciences, v. 21, p. 87-110. Blevin, P.L. and Chappell, B.W., 1995, Chemistry, origin, and evolution of mineralized granites in the Lachlan fold belt, Australia; the metallogeny of I- and S-type granites, Economic Geology, v. 90, no.6, p.1604-1619. Candela, P.A., 1992, Controls on ore metal ratios in granite-related ore systems; an experimental and computational approach, Special Paper – Geological Society of America v. 272, p. 317-326. Cline, J.S. and Bodnar, R.J., 1991, Can economic porphyry copper mineralization be generated by a typical calc-alkaline melt?, Journal of Geophysical Research, v. 96, no. B5, p. 8113-8126. Cornejo, P. C., Tosdal, R.M., Mpodozis, C., Tomlinson, A.J., Rivera, O. and Fanning, C. M., 1997, El Salvador, Chile porphyry copper deposit revisited; geologic and geochronologic framework, International Geology Review, v. 39, no. 1, p. 22-54. Dilles, J.H., 1987, Petrology of the Yerington batholith, Nevada: Evidence for evolution of porphyry copper fluids, Economic Geology, v. 82, p. 1750-1789. Garza, R.A.P., Titley, S.R. and Pimentel, F.B., 2001, Geology of the Escondida porphyry copper deposit, Antofagasta Region, Chile, Economic Geology, v. 96, no. 2, p. 307324. Gustafson, L.B. and Hunt, J.P., 1975, The porphyry copper deposit at El Salvador, Chile, Economic Geology, v. 70, no. 5, p. 857-912. Hattori, K.H. and Keith, J.D., 2001, Contribution of mafic melt to porphyry copper mineralization: evidence from Mount Pinatubo, Philippines, and Bingham Canyon, Utah, USA, Mineralium Deposita, v. 36, p. 799-806. Hedenquist, J.W., Arribas, A. and Reynolds, T.J., 1998, Evolution of an intrusioncentered hydrothermal system: Far Southeast–Lepanto porphyry and epithermal CuAu deposits: Phillipines, Economic Geology, v. 93, no. 4, p. 373-403. Hedenquist, J.W. and Lowenstern, J.B., 1994, The role of magmas in the formation of hydrothermal ore deposits, Nature, v. 370, p. 519-527. Helo, C., Hegner, E., Kroener, A., Badarch, G., Tomurtogoo, O., Windley, B.F. and Dulski, P., 2006, Geochemical signature of Paleozoic accretionary complexes of the Central Asian Orogenic Belt in South Mongolia: Constraints on arc environments and crustal growth, Chemical Geology, v. 227, p. 236-257. Kay, S.M. and Mpodozis, C., 2001, Central Andean ore deposits linked to evolving shallow subduction systems and thickening crust, GSA Today, v. 11, no. 3, p. 4-9. Khashgerel, B-E, Rye, R.O., Hedenquist, J.W. and Kavalieris, I., 2006, Geology and reconnaissance stable isotope study of the Oyu Tolgoi porphyry Cu-Au system, South Gobi, Mongolia, Economic Geology, v. 101, no. 3, p. 503-522.  9  Kirwin, D.J., Forster, C.N., Kavalieris, I., Crane, D., Orssich, C., Panther, C., Garamjav, D., Munkhbat, T.O. and Niislelkhuu, G., 2005, The Oyu Tolgoi copper-gold porphyry deposits, South Gobi, Mongolia, in IAGOD Guidebook series 11, Seltmann R., Gerel, O. and Kirwin, D.J. (eds.), p. 156-168. Lamb, M.A. and Badarch, G., 1997, Paleozoic sedimentary basins and volcanic arc systems of Southern Mongolia: New stratigraphic and sedimentilogic constraints, International Geology Review, v. 39, p. 342-576. Lamb, M.A. and Badarch, G., 2001, Paleozoic sedimentary basins and volcanic arc systems of southern Mongolia: New geochemical and petrographic constraints: in Paleozoic and Mesozoic tectonic evolution of central Asia, From continental assembly to intracontinental deformation,Geological Society of America Memoir 194, p. 117149. Lang, J.R. and Titley, S.R., 1998, Isotopic and geochemical characteristics of Laramide magmatic systems in Arizona and implications for the genesis of porphyry copper deposits, Economic Geology, v. 93, no. 2, p.138-170. Lewis, P.D., 2005, Thrust-controlled formation of the giant Hugo Dummett Cu-Au porphyry deposit, Oyu Tolgoi, Mongolia, Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 97. Lickfold, V., Cooke, D.R., Smith, S. G., and Ullrich, T.D., 2003, Endeavor copper-gold porphyry deposits, Northparkes, New South Wales: Intrusive history and fluid evolution, Economic Geology, v. 98, p.1607-1636. Kay, S.M. and Mpodozis, C., 2001, Central Andean ore deposits linked to evolving shallow subduction systems and thickening crust, GSA Today, v. 11, no. 3, p. 4-9. Maughan, D.T., Keith, J.D., Christiansen, E.H., Pulsipher, T., Hattori, K. and Evans, N. J., 2002, Contributions from mafic alkaline magmas to the Bingham porphyry Cu– Au–Mo deposit, Utah, USA, Mineralium Deposita, v. 37, p. 14-37. McDowell, F.W., McMahon, T. P., Warren, P. Q., and Cloos, M., 1996, Pliocene Cu-Aubearing igneous intrusions of the Gunung Bijih (Ertsberg) District, Irian Jaya, Indonesia: K-Ar geochronology, Journal of Geology, v. 104, p. 327-340. Mueller, D., Franz, L., Herzig, P.M. and Hunt, S., 2001, Potassic igneous rocks from the vicinity of epithermal gold mineralization, Lihir Island, Papua New Guinea, Lithos, v.57, no.2-3, p.163-186. Mueller, D. and Groves, D.I., 1993, Direct and indirect associations between potassic igneous rocks, shoshonites and gold-copper deposits, Ore Geology Reviews, v. 8, no. 5, p. 383-406. Mungall, J.E., 2002, Roasting the mantle: Slab melting and the genesis of major Au and Au-rich Cu deposits, Geology, v. 30, n. 10, p. 915-918. Oyarzun, R., Marquez, A., Lillo, J., Lopez, I. and Rivera, S., 2001, Giant versus small porphyry copper deposits of Cenozoic age in northern Chile: adakitic versus normal calc-alkaline magmatism, Mineralium Deposita, v. 36, p. 794-798. Perello, J., Cox, D., Garamjav, D., Sanjdorj, S., Diakov, S., Schissel, D., Munkhbat, D. and Oyun, G., 2001, Oyu Tolgoi, Mongolia; Siluro-Devonian porphyry Cu-Au-(Mo) and high-sulfidation Cu mineralization with a Cretaceous chalcocite blanket, Economic Geology, v. 96, no. 6, p. 1407-1428.  10  Pollard, P.J. and Taylor, R.G., 2002, Paragenesis of the Grasberg Cu-Au deposit, Irian Jaya, Indonesia; results from logging section 13, Mineralium Deposita, v. 37, no.1, p.117-136. Rose, A.W., 1970, Zonal relations of wallrock alteration and sulfide distribution at porphyry copper deposits, Economic Geology, v. 65, p. 920-936. Sajona, F.G. and Maury, R.C., 1998, Association of adakites with gold and copper mineralization in the Philippines, C.R. Acad. Sci. Paris Sci. Terre Planetes v. 326, p. 27-34. Seedorf, E., Dilles, J.H., Proffett, J.M., Einaudi, M.T., Zurcher, L., Stavast, W.J.A., Johnson, D.A. and Barton, M.D., 2005, Porphyry deposits: Characteristics and origin of hypogene features, Economic Geology 100th anniversary volume, p. 251-298. Sillitoe, R.H., 1997, Characteristics and controls of the largest copper-gold and epithermal gold deposits in the circum-Pacific region, Australian Journal of Earth Sciences, v. 44, p. 374-388. Solomon, M., 1990, Subduction, arc reversal, and the origin of porphyry copper-gold deposits in island arcs, Geology, v. 18, p. 630-633. Sun, W.D., Arculus, R.J., Kamenetsky, V.S. and Binns, R.A., 2004, Release of goldbearing fluids in convergent margin magmas prompted by magnetite crystallization, Nature, v. 431, p. 975–978. Thieblemont, D., Stein, G. and Lescuyer, J.L., 1997, Gisements epithermaux et porphyriques; la connexion adakite. Epithermal and porphyry deposits; the adakite connection, Comptes Rendus de l'Academie des Sciences, Serie II. Sciences de la Terre et des Planetes, v. 325, no. 2, p. 103-109. Waite, K.A., Keith, J.D., Christiansen, E.H., Whitney, J.A., Hattori, K., Tingey, D.G. and Hook, C.J., 1998, Petrogenesis of the volcanic and intrusive rocks associated with the Bingham Canyon porphyry Cu-Au-Mo deposit, Utah, in Geology & Ore Deposits of the Oquirrh & Wasatch Mountains, Utah: Soc. Econ. Geol. Guidebook, John, D.A. and Ballantyne, G. (eds.), v.29, p.69-90.  11  Chapter 2 - Volcanostratigraphic framework of the Oyu Tolgoi porphyry Cu-Au district; South Gobi region, Mongolia1 Introduction The super-giant Oyu Tolgoi porphyry copper-gold deposits are located in the South Gobi region of Mongolia, 650 km south of the capital, Ulaanbaatar, and 80 km north of the Chinese border (Figure 2.1a). The discovery identifies the South Gobi volcanic arc terrane as a belt that is important for geologic models of Central Asia as well as for mineral exploration.  To that end, a critical step toward developing an  understanding of the metallogenic and tectonic evolution of a mineral belt is to place precise chronostratigraphic constraints on ore deposit districts.  Such studies are  important as mineral deposits within a convergent margin belt are frequently contemporaneous and located in specific rock units. Furthermore, volcanic arc sequences evolve in response to tectonic events such as arc rifting or collision with aseismic ridges and continental margins (e.g. Collot and Fisher 1991; Gill et al. 1994; Clift 1995; Lee et al. 1995; Draut and Clift, 2006). These changes in geodynamic conditions can also relate to environments that are conducive to porphyry copper-gold deposit formation such as arc reversals (Solomon, 1990), subduction of very young lithosphere, very slow or oblique convergence (Mungall, 2002), flat subduction (Kay and Mpodozis, 2001) or the cessation of subduction (Sillitoe, 1997). The goal of this chapter is to reconstruct the Paleozoic stratigraphic framework of the Oyu Tolgoi district with particular focus on the volcanic and volcaniclastic rocks. Field observations are presented in addition to uranium-lead (zircon) ages from igneous and sedimentary rocks that allow the definition of the district stratigraphic sequence and link the formation of the super-giant Oyu Tolgoi porphyry Cu-Au deposits to the evolution of the Paleozoic volcanic arc. The sequences are generally not fossiliferous and many do not contain material suitable for U-Pb dating.  Hence, cross-cutting  1  A version of this chapter will be submitted to a refereed journal for publication: Wainwright, A.J., Forster, C.N., Kirwin, D.J., Lewis, P.D., Tosdal, R.M., Wooden, J.L. and Mazdab, F.K., in prep., Volcanostratigraphic framework of the Oyu Tolgoi porphyry Cu-Au district; South Gobi region, Mongolia.  12  intrusions and contact relationships are used where necessary in order to define the temporal framework. Tectonic setting The Oyu Tolgoi Cu-Au district is located in the Central Asian Orogenic Belt (CAOB) (e.g. Mossakovsky et al., 1994; Badarch et al., 2002; Buchan et al., 2002; Helo et al., 2006), also referred to as the Altaids (Sengor et al., 1993; Sengor and Natal’in, 1996). The CAOB extends across Asia for 5000 km, is 1000 to 2000 km wide and is situated between Precambrian cratons; the Siberian craton to north and the Tarim and North China cratons to the south (e.g. Badarch et al., 2002; Dobretsov et al., 1995; Figure 2.1b). The tectonic collage is characterized by an assemblage of microcontinental blocks and mobile belts of different ages including relicts of island arcs, Precambrian continental crust and Neoproterozoic to Paleozoic ophiolites (e.g. Badarch et al., 2002; Buchan et al., 2002). In general, the ages of the orogenic complexes within the belt decrease from north to south away from the Siberian craton margin (present coordinates) (Kovalenko et al., 2004). To the west, other orogenic belts, the Baikalides and the PreUralides, separate the CAOB from the Baltica craton (Yakubchuk, 2004). Yet another mobile belt south of the CAOB, termed the Manchurides, makes up the northern part of the North China craton. The Manchurides are separated from the CAOB by the Solonker Suture, which is thought to form the southern boundary of the Central Asian Orogenic Belt (Sengor and Natal’in, 1996). There is considerable debate regarding the Paleozoic tectonic evolution of Central Asia. Contradictory explanations for the current geometry of the CAOB have arisen due to differing approaches to explaining the origin of slivers of Precambrian crust and Neoproterozoic to Paleozoic ophiolites found within the tectonic collage. Sengor et al. (1993) proposed that a single, giant subduction-accretion complex was contorted by oroclinal bending (Figure 2.1c). This structure was subsequently dismembered by strikeslip faulting in order to generate the current geometry. In this model, Precambrian crust was rifted from a combined Siberia-Baltica craton west of the Tuva-Mongol and Kipchak  13  14  Figure 2.1 Tectonic evolution of the Central Asian Orogenic Belt. A) Location map of the Oyu Tolgoi porphyry Cu-Au district and accreted arc terranes in Mongolia (modified after Badarch et al., 2002); B) Tectonic setting of the Central Asian Orogenic Belt (CAOB) modified after Dobretsov et al. (1995); Paleotectonic reconstructions for the Late Devonian: C) Sengor et al. (1993) and D) Mossakovsky et al. (1994).  volcanic arc systems.  Conversely, it has been proposed that the CAOB grew by  subduction and accretion of multiple oceanic basins and by development of individual magmatic arcs (Figure 2.1d) (Coleman, 1989; Mossakovsky et al., 1994; Buchan et al., 2002).  In this scenario, Precambrian material would have been derived from the  Gondwana supercontinent on the east side of a Paleoasian ocean, in addition to the Siberia and Baltica cratons.  The two models differ mainly in that one suggests a  prolonged and steady period of subduction-accretion, followed by large-scale deformation of a single giant magmatic arc complex, whereas the latter suggests punctuated accretion of multiple individual arc terranes and microcontinents. During the Paleozoic, rocks that now comprise the South Gobi region of Mongolia underwent accretionary episodes that assembled a number of island and continental margin magmatic arcs, rifted basins, accretionary wedges and continental margins (Zonenshain et al., 1990; Sengor et al., 1993; Sengor and Natal’in, 1996; Lamb and Badarch, 1997). The Oyu Tolgoi porphyry deposits are located within a sequence of Devonian rocks in the Gurvansayhan Terrane (Badarch et al. 2002; Helo et al., 2006), which is part of the larger assemblage of volcanic arc complexes that trend through western China and the Altai Mountains in western Mongolia across to northeastern Mongolia (Figure 2.1a).  The Gurvansayhan Terrane consists of highly deformed  accretionary complexes and volcanic arc assemblages. The structure of the terrane is complex and dominated by imbricate thrust sheets, dismembered blocks, mélanges and high strain zones (Badarch et al., 2002). The Devonian rocks are interpreted to represent a juvenile island arc assemblage (Helo et al., 2006; Chapter 6). Arc volcanism ceased by the Permian in the South Gobi area (present coordinates), but tectonic activity continued.  In the Permian, accreted material in  southern Mongolia underwent basin and range style extension accompanied by bimodal basalt-peralkaline granite magmatism (Kovalenko and Yarmolyuk, 1995) in a mature continental setting (Lamb and Badarch, 1997). The fact that all of the amalgamated terranes in the South Gobi are intruded by Carboniferous to Permian granites suggests that amalgamation occurred prior to the Carboniferous (Sengor and Natal’in, 1996).  15  Geology of the Oyu Tolgoi porphyry Cu-Au district The Oyu Tolgoi district consists predominantly of Paleozoic mafic to intermediate volcanic and volcaniclastic rocks that have been intruded by Devonian to Permian felsic plutons (Figure 2.2). These rocks are unconformably overlain by poorly consolidated Cretaceous sedimentary rocks and younger unconsolidated sedimentary deposits. The Oyu Tolgoi copper-gold porphyries are situated in a poorly-exposed inlier of Devonian mafic to intermediate volcanic, volcaniclastic and sedimentary rocks and Late Devonian intrusions surrounded by Carboniferous volcanogenic and sedimentary rocks.  The western margin of the prominent Permian Hanbogd peralkaline granite  complex is located approximately 4 km east of the Oyu Tolgoi porphyry deposits. This geologic feature is 60 km across, consists of two circular, overlapping lobes, and is clearly visible on satellite images of the area. A potassium-argon age of 287 ± 2 Ma (riebeckite) has been obtained for the complex (Kovalenko and Yarmolyuk, 1995; Perello et al., 2001). High Yb, Y and Ta values discriminate the Hanbogd intrusions from older volcanic arc magmas and suggest that the complex is part of a younger within-plate magmatic suite (Kovalenko and Yarmolyuk, 1995). More generally, granitoids from the district represent volcanic arc magmatic suites that range in age from Devonian to Carboniferous (Gerel et al., 2005). These bodies include the Javhalant Mountain pluton (324 ± 3 Ma; see below), 12 km southwest of Oyu Tolgoi, and the OT18 pluton (321 ± 4 Ma; see below), located 18 km north of the porphyry deposits. The Oyu Tolgoi Cu-Au deposits are divided into two zones: the Hugo Dummett zone to the north and the more deeply eroded Southern Oyu deposits (South Oyu, Southwest Oyu and Central Oyu). The surface traces and surface projection of the distinct porphyry centers define a north-northeast trending mineralized corridor underlain by east-dipping panels of Upper Devonian or older layered sequences intruded by quartz monzodiorite and granodiorite stocks and dikes (Figure 2.3). In the northern part of the corridor, these rocks are cut by a north-northeast-trending Late Devonian granodiorite porphyry dike complex, and are in fault contact to the west and east with Carboniferous volcaniclastic rocks.  A large (~10 km2) equigranular quartz monzodiorite intrusion  flanks these layered rocks to the west (364 ± 4 Ma; Chapter 4), and a post-mineral equigranular granodiorite intrusion (350 ± 4 Ma; see below) is in fault contact with the 16  Devonian rocks at the northern end of the corridor. Younger rocks in the north half of the corridor are separated from older rocks in the south half by a north-dipping normal fault that separates the Hugo Dummett deposits from the Southern Oyu deposits. South of the mineralized corridor, the north-northeast-striking Devonian rocks are in reversefault contact with a sequence of west to northwest-striking Upper Devonian to Carboniferous layered sequences. Thrust faults with related numerous steeply-dipping transverse or dip-slip faults, overturned folds and polyphase fold patterns characterize the structural geology of the Oyu Tolgoi porphyry Cu-Au district, and developed over multiple discrete deformation events (Lewis, 2005; Ayush, 2006). The Hugo Dummett deposits are bound to the east and west by north-northeast trending dip-slip faults that separate mineralized Devonian or older rocks from Carboniferous strata. Moreover, the Hugo Dummett deposits are separated from the Southern Oyu deposits by an arcuate, east-west trending north-dipping normal fault and are overlain by a thrust fault-emplaced, overturned sequence with a bedding-parallel axial surface. Fold interference patterns in sinuous outcrops of andesites, clastic sedimentary rocks and a bedding-parallel rhyolite sill (330 ± 2 Ma; see below) ~4 km southeast of the South Oyu deposit provide the best outcrop example of the polyphase shortening. Polyphase fold interference structures are also observed at meter-scale in small fold structures with non-linear axial surfaces in addition to large-scale dome and basin patterns visible in layered rock sequences on satellite images of the area.  17  18  Figure 2.2 Simplified regional geology map. Map generated by AJW and Ivanhoe Mines geology staff based on satellite images and limited field visits. UTM coordinates are WGS84 zone 48, northern hemisphere.  There is a north-northeast-striking shear zone approximately 3 km west of the porphyry deposits that deforms andesitic rocks, granite as well as quartz-feldspar porphyry (QFP) dikes (Figure 2.3a). The orientation of the QFP dikes changes from northwest to sub-parallel to the plane of shearing as the rocks become increasingly mylonitic within the zone of intense ductile deformation. The shear zone consists of mylonitic to ultramylonitic rocks in the centre, grading outward over about 200 meters to rocks without deformation fabrics. As well, mesoscopic shear-sense indicators such as shear bands and S-C fabrics in deformed granite are consistent with dextral shearing. A U-Pb (zircon) age of 335 ± 1 Ma (see below) on a sample from the QFP dike swarm places a maximum age on the shear zone. Structural measurements from oriented drill core indicate that aside from local variations, the bedded rocks within the deposit area are largely concordant and dip moderately to steeply eastward (Figure 2.4).  However, despite the appearance of  concordant bedding, the rocks need not be conformable. Slight angular discordances are possible within the sequence, and moderate to large angular discordances are documented between Devonian and Carboniferous sequences adjacent to the deposit area. District stratgraphic sequence A number of authors have defined groups, formations and members in the South Gobi region (e.g. Durante, 1976 cited in Minjin et al., 2004; Goldenberg et al., 1978 cited in Minjin et al., 2004), and strata within the Oyu Tolgoi district have traditionally been assigned to the established nomenclature based on broad lithologic similarities as well as biostratigraphic data (Minjiin et al., 2004). None of the type localities for the formally defined units are proximal to Oyu Tolgoi; rather, they are tens to hundreds of kilometers away. Nonetheless, the lithologic character of units at Oyu Tolgoi supports correlation with the defined units at the group level, though not in most instances at the formation level.  Accordingly, informal lithologic nomenclature is assigned to the various  supracrustal rock packages as opposed to formal Formational and Member designations (summarized in Table 2.1).  19  20  Figure 2.3 Property-scale geology and sample locations. A) Geologic map generated by Ivanhoe Mines geology staff and AJW based on outcrop mapping, drill hole data and trenches; B) Location map for U-Pb geochronology samples (denoted by prefix AJW) and measured pseudo-section sites (denoted by prefixes OTD, OT and EGD). UTM locations of the samples are given in Table 2.3. A and B represent the same area. See inset in figure 2.2 for map location.  Figure 2.4 Contoured bedding data from oriented drill core collected by Ivanhoe Mines geology staff (lower hemisphere stereographic projection). A) 1296 contoured poles to sedimentary bedding from all bedded Carboniferous and Devonian packages; B) 115 contoured poles to fiamme foliation from the UDS sequence; C) 23 poles to bedding from the OTHS sequence; D) 46 contoured poles to bedding from the Sedimentary Sequence. See text for unit descriptions.  21  Table 2.1 Groups, Formations and names of informal lithologic package designations used in this study.  Age  Group  Formation  Lithologic package  Carboniferous  Gurvankharaat  Sainshandhudag  Upper Volcaniclastic Sequence (UVS)  Unconformity Carboniferous  Gurvankharaat  Sainshandhudag  Lower Volcaniclastic Sequence (LVS)  Carboniferous  Gurvankharaat  Sainshandhudag  Sedimentary Sequence  Carboniferous  Gurvankharaat  Sainshandhudag  Polylithic Breccia Sequence (PBS)  Unconformity Devonian or older  Unassigned  Unassigned  Oyu Tolgoi Hanging Wall Sequence (OTHS)  Thrust fault Upper Devonian  Alagbayan  Unassigned  Unmineralized Dacite Sequence (UDS)  Unconformity Devonian or older  Alagbayan  Unassigned  Mineralized Fragmental Sequence (MFS)  Devonian or older  Alagbayan  Bulagbayan  Augite Basalt unit  22  Based on detailed drill core logging and field mapping, two Paleozoic stratigraphic sequences are defined in the Oyu Tolgoi district. These include a Devonian or older sequence of basaltic rocks, pyroclastic sequences and sedimentary strata, and an overlying Carboniferous volcanic arc sequence containing basaltic to rhyolitic flows, tuffs and volcaniclastic rocks as well as conglomerate, siltstone and sandstone. Based on age and lithologic similarities, these sequences are correlated with the Alagbayan Group and Gurvankharaat Group respectively.  The Middle to Upper Devonian Alagbayan  Group was first defined by Goldenberg et al. (1978) (cited in Minjin et al., 2004) as a formation in the Alag Bayan hills of Khan Bogd Sum (county), ~80 km northeast of the Oyu Tolgoi deposits. The Alagbayan Group lies in fault contact above the Tsavchir Formation turbidite, thought to be Devonian, which in turn overlies Lower Paleozoic metasedimentary rocks of the Undaan Formation (Minjin et al., 2004). The type locality of the Gurvankharaat Group was described by Goldenberg et al. (1978) (cited in Minjin et al., 2004) near the near the boundary of Manlai and Khan Bogd sums, 100 km northnortheast of the Oyu Tolgoi deposits. At Oyu Tolgoi, these two sequences consist of eight principal lithofacies, which are summarized in Table 2.2 and described below. The currently accepted International Commission on Stratigraphy age of 359 ± 2.5 Ma is used as the boundary between the Devonian and Carboniferous Periods (Gradstein et al., 2004). Devonian supracrustal rocks – Alagbayan Group Outcrops of Devonian volcanic rocks at Oyu Tolgoi are extremely limited and lithostratigraphy is based on detailed logging of representative drill holes (Figure 2.5; drill hole locations in Fig. 2.3b). Devonian lithologic units are divided into a basal Augite Basalt sequence succeeded by a Mineralized Fragmental Sequence (MFS), an Unmineralized Dacite Sequence (UDS), and a structurally overlying volcano-sedimentary package referred to as the allochthonous Oyu Tolgoi Hanging Wall Sequence (OTHS). The basal unit is assigned to the Bulagbayan Formation, while the latter three differ significantly from formally defined units, and are therefore assigned informal unit names.  23  24  Figure 2.5 Facies distribution in Devonian or older to Carboniferous rocks. Measured drillcore sections are adjusted to the base of the Unmineralized Dacite Sequence (UDS) and show the heterogeneous distribution of the facies. The stratigraphic level at A corresponds to the level at point A’ in Figure 2.7. See Figure 2.3 for drill hole locations. All drill holes dip steeply to moderately to the west and are approximately perpendicular to the regional bedding.  Bulagbayan Formation (Augite Basalt): The lowermost rock unit known in the Oyu Tolgoi district is at least 250 m thick and consists of fine-grained to porphyritic basaltic lavas, fragmental basalts, and fine-grained laminated volcaniclastic and sedimentary rocks. The sequence is correlated with the Bulagbayan Formation based on lithologic similarities (Minjin et al., 2004). Coherent facies are characterized by coarse augite phenocrysts up to 8 mm in diameter in a fine-grained groundmass dominated by feldspar and clinopyroxene (Figure 2.6a). The porphyritic facies, as well as coherent aphyric units, are interbedded with monomictic breccias with 0.5 to 3 cm angular, fine-grained basalt clasts (Figure 2.6b) in addition to subordinate zones of dark, laminated, finegrained sedimentary rocks (Figure 2.6c). The latter consist of grey to green, finelylaminated siltstone, tuff, and fine-grained sandstone, and occur primarily in the deepest drillhole intercepts, particularly in the southwestern periphery of the Southwest Oyu deposit area. The coherent facies, which could be mafic lavas or shallow intrusions, comprise >90% of logged sections for this unit. These rocks are usually altered by the superposed hydrothermal system, obscuring protolith textures and composition. Mineralized Fragmental Sequence (MFS): Overlying the Bulagbayan Formation is a unit consisting of feldspar-bearing fragmental rocks within which intense clay alteration, abundant quartz veins, and sulfide mineralization have masked primary textural features and composition, rendering the protolith uncertain. Despite intense alteration, relict fragmental features such as pseudo(?)-eutaxitic texture (Figure 2.6d), angular, lapillisized clasts (Figure 2.6e) as well as amoeboid-shaped clasts are recognized in some drill holes. These may represent bonafide juvenile volcanic rock-derived or re-sedimented clasts that are primary, pre-alteration features. Otherwise, it is also possible that in some cases the textures are a result of irregular alteration in a homogeneous medium. A conservative interpretation is favoured whereby a protolith is not assigned, although it is likely that the fragmental textures are indeed related to primary features. The unit is zircon-undersaturated (based on heavy mineral separate yields) which suggests that there is a petrologic difference between it and the overlying zircon-saturated UDS package (Chapter 3).  25  Unmineralized Dacite Sequence (UDS): Interbedded coherent facies dacite, welded and unwelded lapilli tuffs (Figures 2.6f and 2.6g), ash tuffs, monomictic volcaniclastic breccias with block-sized lithic fragments, and polymictic breccias and conglomerates form the UDS unit. The stratigraphic sequence of these facies varies through the deposit area and not all are universally present. Various unmineralized rock types at the base of the UDS immediately overlie the mineralized unit, including fresh dacite tuffs, polymictic conglomerates, and breccias. Mineralized clasts are common in the lower part of the UDS and decrease in abundance upwards. Typically, these clasts are composed of quartz vein fragments with disseminated pyrite, and less commonly of altered, mineralized feldspar porphyritic intrusive (Chapter 3).  Fragmental dacite facies represent  approximately 70% of measured sections and there are frequently multiple welded zones separated by unwelded intervals of lapilli tuff within a single drill hole. Volumetricallydominant dacitic lithic clasts are typically brown, feldspar-phyric and contain subordinate quartz phenocrysts in a fine-grained feldspathic groundmass. Clast-supported facies with juvenile, amoeboid-shaped clasts are locally observed within restricted intervals. The top of the UDS sequence usually consists of sedimentary rocks such as polylithic conglomerates, carbonaceous siltstones and sandstones. This sedimentary package is not always present and overlies the dacitic volcanogenic rocks, but is below the fault defining the base of the OTHS. The combined thickness of the MFS and UDS package ranges from 130 to 700m. The UDS typically ranges from 100 to 300 m, whereas the MFS is more diffcult to confidently measure due to alteration, which obscures the contact between the MFS unit and the Bulagbayan Formation. Oyu Tolgoi Hanging Wall Sequence: An unmineralized sequence of up to 600 m of sedimentary rocks interbedded with mafic to intermediate coherent and fragmental rocks forms the Oyu Tolgoi Hanging Wall Sequence (OTHS). The OTHS structurally overlies lower units along a fault zone that is approximately parallel to layering in both underlying and overlying units. Nowhere is a stratigraphic contact between the OTHS and the structurally lower rocks observed.  Thus the relative stratigraphic levels of the two  packages are consequently unknown.  Primary volcanic rocks in the OTHS are  26  dominantly basaltic in composition, and are fine-grained to clinopyroxene-porphyritic with phenocrysts up to 6 mm (Figure 2.6h). Groundmass is dominated by fine-grained clinopyroxene  and  microphenocrysts.  feldspar  as  well  as  intergrown  magnetite  and  apatite  Minor andesite porphyry and volcaniclastic andesite breccia are  intercalated within the sequences of sedimentary rocks and augite basalts (Figure 2.6i), and are characterized by variably-aligned 2-4mm feldspar and hornblende phenocrysts in a fine-grained groundmass. Clast-supported to matrix-supported breccias are common in these rocks as are abundant calcite veins, calcite-filled vesicles and strong chlorite alteration. Lobate-clast breccias and clasts with irregular curviplanar margins, interpreted as peperite, are commonly observed at the contacts between basalts and adjacent sedimentary rocks (Figure 2.6j).  Intercalated sedimentary rocks consist of laterally  discontinuous horizons of fine- to medium-grained sandstone, mudstone, siltstone and subordinate conglomerate. Primary sedimentary structures indicate that much of the OTHS is overturned, and facing direction reversals and bedding markers imply that major overturned folds are present. The overturned character of the sequence, together with the fault at its base, indicate that this unit is allochthonous and was thrust-superposed onto the underlying UDS unit (Lewis, 2005; Ayush, 2006).  27  Figure 2.6 Polished slab photographs of textural variation in Devonian or older rocks. A) Mineralized augite-porphyritic basalt (Bulagbayan Fm.); B) Mineralized volcaniclastic basalt (Bulagbayan Fm.); C) Mineralized fine-grained sedimentary facies (Bulagbayan Fm.); D) Mineralized sample with relict welding textures (MFS); E) Strongly altered fragmental rock with lapilli-sized clasts (MFS).  28  Figure 2.6 continued. F) Dacite lapilli tuff with silicified clast (UDS); G) Welded tuff (UDS); H) Mafic to intermediate polymictic breccia (OTHS); I) Andesite breccia (OTHS); J) Basalt clasts with cuspate margins (OTHS).  29  Carboniferous supracrustal rocks – Gurvankharaat Group Carboniferous strata at Oyu Tolgoi and in surrounding areas overlie the Devonian strata along a regional disconformity or low-angle unconformity. Lithofacies within the Carboniferous sequence include mafic to silicic coherent and fragmental lavas, tuffs and subvolcanic intrusions interstratified with volcaniclastic breccia and conglomerate, and sedimentary facies including conglomerate, sandstone, siltstone, and mudstone (Figures 2.7 and 2.8).  These rocks are divided into four informal units: Polylithic Breccia  Sequence (PBS), Sedimentary Sequence, Lower Volcaniclastic Sequence (LVS) and Upper Volcaniclastic Sequence (UVS). The units are separated by depositional contacts and assigned to the Sainshandhudag Formation of the Gurvankharaat Group. These four sequences occur in a consistent order although not all are present in all areas. Polylithic Breccia Sequence: Monotonous, structureless polylithic breccia up to 200 m thick forms the lowermost part of the Carboniferous sequence in most of the Oyu Tolgoi area. The breccias are matrix-supported and composed of heterolithic lapilli-sized clasts including volcanic clasts, and metasedimentary clasts (Figure 2.8a).  Matrix material is  characterized by euhdral to broken feldspar crystals, as well as fine-grained chloritealtered material.  Subordinate welding textures and armoured lapilli occur within  restricted intervals (Figure 2.8b).  30  31  Figure 2.7 Facies distribution for Carboniferous rocks at Oyu Tolgoi. The stratigraphic level at A’ corresponds to the level at point A in Figure 2.5. See Figure 2.3 for pseudo-section locations. All drill holes dip steeply to moderately to the west and are approximately perpendicular to the regional bedding.  Sedimentary Sequence: A clastic sedimentary package up to 200 m thick overlies the PBS unit. The Sedimentary Sequence consists of interbedded polymictic conglomerate, fine- to coarse-grained sandstone, and variably carbonaceous mudstone and siltstone. In some drill holes, a green cross-bedded tuffaceous sandstone unit occurs within the upper part of this unit. Euhedral to fragmental feldspar phenocrysts and pumice fragments characterize the tuffaceous samples. The beds range in thickness from mm-scale to cmscale. Convolute folds with wavelengths of less than 5 cm are commonly present in the finest-grained material (Figure 2.8c) as are small mm- to cm-scale fault offsets. Lower Volcaniclastic Sequence (LVS): A feldspar- and hornblende–phyric andesitic lava flow 2-70 m thick overlies the Sedimentary Sequence. Variably-aligned 2-4 mm feldspar and hornblende phenocrysts occur within an aphanitic groundmass and the lower contact is commonly characterized by andesite that intrudes and brecciates the subjacent sedimentary rocks, while flow-top breccias occur along the upper contact. The andesite flow is overlain by a thick, structureless matrix-supported sequence of green to maroon volcaniclastic conglomerate and breccia containing abundant clasts similar in composition to the underlying andesite lavas, as well as fine-grained mafic to intermediate clasts. Clasts are lapilli- to block-sized, rounded to sub-angular (Figure 2.8d) and are contained in a plagioclase-rich or locally aphanitic hematitic matrix. Coherent fine-grained basalt intervals within the unit locally display peperitic textures along contacts with surrounding sedimentary material (Figure 2.8e). The intermediate to mafic volcaniclastic rocks are intruded by, overlain by, and intercalated with feldsparphyric to aphanitic coherent and fragmental dacites. The dacite typically consists of 5 mm feldspar phenocrysts (35%) and 1-3 mm ferromagnesian phenocrysts (5%) set in a red-colored aphanitic groundmass. In one drill hole along the northeast side of the Oyu Tolgoi deposits, porphyritic dacite occurs in the LVS unit as angular to irregular, locally amoeboid to spindle-shaped clasts mixed with rounded mafic clasts (Figure 2.8d). Moreover, thin (<5 m) dacite sills with chilled upper and lower margins cut the mafic volcaniclastic sequences in outcrop. Thick sequences (>20 m) of coherent facies dacite are restricted to the northwest side of the Hugo Dummett deposits. The total thickness of  32  the LVS package is at least 800 m and 90% of logged sections comprise the mafic to intermediate volcaniclastic facies with subangular to rounded clasts. Upper Volcaniclastic Sequence (UVS):  The youngest Carboniferous rocks in the  district comprise an upper volcaniclastic sequence that crops out approximately 3 km east of the Oyu Tolgoi deposits. Southeast of the deposits, the UVS unit is 2-5 m thick and contains fine-grained, coherent-facies andesite overlain by variably-welded rhyolitic ignimbrite. These rocks are overlain by fine-grained grey and beige mudstones and siltstones with characteristic conchoidal fracture and abundant plant stem fossils. Northeast of the deposits, the UVS comprises a concordant sequence of welded and unwelded volcaniclastic andesite with lapilli- to block-sized angular to sub-angular fragments (Figure 2.8f and 2.8g), overlain by unwelded dacitic fragmental tuff (Figure 2.8h). These rocks are overlain by coherent-facies feldspar-porphyritic and fine-grained coherent facies andesite (Figure 2.8i). Devonian subvolcanic intrusions Quartz monzodiorite:  The oldest intrusions at Oyu Tolgoi are medium-grained  hornblende-quartz monzodiorites with equigranular to crowded porphyritic textures (Chapter 4). Typically they contain 1-2 mm euhedral feldspar phenocrysts (60%), 1-3 mm hornblende phenocrysts (20%) and <1 mm quartz phenocrysts (<10%) in a finegrained groundmass dominated by K-feldspar and accessory zircon, apatite and magnetite. There are multiple phases; however the textures of these intrusions as well as contacts between phases or with other units are largely obscured by intense alteration, quartz veins, and sulfide mineralization. In one location, however, an altered feldsparphyric intrusion displays a chilled margin against strongly altered and mineralized rocks of the MFS unit. As well, a fresh equigranular quartz monzodiorite with a chilled margin intrudes fine-grained Augite Basalt of the Bulagbayan Formation. Quartz monzodiorite intrusions, altered or fresh, have never been observed to cut rocks younger than the mineralized MFS unit (Chapter 3).  33  Figure 2.8 Polished slab photographs of textural variations in Carboniferous rocks. A) Polylithic andesite tuff (PBS); B) Armoured lapilli in andesite tuff (PBS); C) Convolute folds in laminated siltstone (Sedimentary Sequence); D) Juvenile clast of dacite in volcaniclastic basalt sequence (LVS); E) Lobate basalt clasts in siltstone interpreted as peperite (LVS).  34  Figure 2.8 continued. F) Volcaniclastic dacite (UVS); G) Dacitic crystal-ash tuff (UVS); H) Welded tuff UVS); I) Andesite tuff (UVS).  35  36  Composition  AndesiteDaciteRhyolite  Basalt-Dacite  Clastic sedimentary  Andesite  BasaltAndesite  Dacite  Lithofacies  UVS  LVS  Sedimentary Sequence  PBS  OTHS  UDS  Porphyritic clasts (0.2–20 cm; up to 30%), feldspar and quartz phenocrysts (0.1-0.5 cm, <10%), aphanitic feldspathic groundmass.  Fine-grained to porphyritic, clinopyroxene (<8 mm; 30-40%), plagioclase (0.2-1mm; 3540%), groundmass. Andesites are characterized by trachytic, feldspar-porphyritic textures.  Feldspathic matrix–supported (<4 mm; 5070%), fine-grained angular sedimentary, metamorphic and volcanic clasts.  Dominated by mudstones, siltstones, sandstones with local conglomerate. Tuffaceous sandstones are characterized by pumice fragments and broken feldspar phenocrysts.  Aphanitic to feldspar-porphyritic basaltandesite, red feldspar-porphyritic to aphanitic dacite with groundmass-hornblende.  Fine-grained feldspar-hornblende andesite, broken feldspar-quartz ash tuffs with local welding and lithic fragments.  Petrology  Table 2.2 Summary table of volcanostratigraphic lithofacies.  Includes ash and lapilli tuffs, breccias with block-sized clasts and intercalated polymictic, clast-supported to matrix-supported conglomerates and breccias. Includes subordinate amoeboidclast breccias. Characterized by multiple cooling units and abundant mineralized clasts.  Peperitic basalts intercalated with andesitic volcanic and volcaniclastic rocks and sedimentary sequences. Characterized by strong chlorite alteration and abundant calcite veining.  Structureless, immature sediment and local pyroclastic fall deposit with lapilli-sized angular fragments. Local welding and armoured lapilli textures are present.  Interbedded sandstone, silstone, mudstone and conglomerate. Soft-sediment deformation structures are present. A volcanogenic tuff occurs high in the sequence.  Debris flow facies with rounded to subangular clasts of mafic to intermediate material intercalated with juvenile fragments of dacite. Dacites also occur as sills, dikes and domes that intrude the volcaniclastic facies.  Fine-grained coherent to fragmental facies andesite overlies and underlies thin sequences of locally welded intermediate to felsic ash tuffs and clastic sedimentary rocks with plant stem fossils.  Characteristics  37  Composition  unknown  Basalt  Lithofacies  MFS  Augite Basalt  Fine-grained to porphyritic, clinopyroxene (<8 mm; 40-50%), plagioclase (0.2-1mm; 2530%), groundmass includes cpx, plag, apatite, magnetite.  Porphyritic clasts locally observed (0.2–3 cm; up to 30%), feldspar phenocrysts in aphanitic groundmass.  Petrology  Dominated by fine-grained to porphyritic coherent facies with subordinate intervals of matrix-supported volcaniclastic basalt. Lowest intervals include fine grained laminated tuff and laminated siltstone facies. Weakly to strongly altered and mineralized.  Relict welded lapilli tuffs, unwelded lapilli tuffs and amoeboidclast breccias. Strongly altered and mineralized.  Characteristics  Granodiorite: Granodiorite porphyries intrude all of the Devonian or older supracrustal packages and are not observed to intrude the Gurvankharaat Group rocks. Granodiorite porphyries typically contain 1-6 mm euhdral to subhedral feldspar phenocrysts (40%) and subordinate 1-2 mm quartz (<10%) in a fine-grained groundmass dominated by plagioclase feldspar and biotite with accessory apatite and zircon. Carboniferous subvolcanic intrusions Carboniferous intrusions in the Oyu Tolgoi deposit area are limited to narrow (typically <20 m) fine-grained or porphyritic dikes, and range in composition from basalt to rhyolite.  All phases are unaltered and unmineralized with well-defined intrusive  contacts that allow the establishment of a relative chronologic sequence. Hornblende-biotite andesite: Andesite porphyry dikes are typically meters to tens of meters wide and sparsely porphyritic with 2-5 mm skeletal feldspar phenocrysts (50%), 2 mm hornblende phenocrysts (20%) and 1-2mm biotite phenocrysts (5-10%) in a finegrained brick-red groundmass with rare calcite amygdules. Accessory phases include zircon and magnetite.  The hornblende-biotite andesite dykes intrude the andesite  volcaniclastic rocks of the PBS and they are cut by rhyolite dikes. Rhyolite: Rhyolite dikes are typically beige to orange-pink to green, fine–grained to aphanitic, and contain sparse 1-3 mm quartz and feldspar phenocrysts (<20%) and accessory apatite and zircon. They are laminated on a mm-scale and commonly display folds and flow fabrics. Fragments of rhyolite are locally enclosed within basalt dikes (see below) at the contact between the two units, indicating that the basalt dikes are younger. Similarly, chilled margins with sharp planar contacts indicate that the rhyolites post-date hornblende-biotite andesite porphyry intrusions. Dikes are abundant in the mineralized corridor and rhyolite also forms a folded sill exposed several kilometers southeast of the deposits. They also crop out in a variably mylonitized northwest- to north-northeast trending arcuate dike swarm on the west side of the porphyry deposits.  38  Basalt: Two distinct types of basalts intrude all Devonian to Carboniferous rocks in the district. Microgabbro is conspicuously magnetic and consists of fine- to medium-grained hornblende and plagioclase with accessory magnetite.  A north-northwest-striking  microgabbro dike cuts across the Oyu Tolgoi mineralized corridor, and forms a prominent feature on ground magnetic surveys. Thin, fine–grained, non-magnetic grey basalt dikes with abundant amygdules form the second type, and are more widely distributed through the deposit area. Amygdules are spherical, elongate, or irregularly shaped, 1-5 mm, and invariably filled by carbonate. Cross-cutting relationships between fine-grained basalt and the microgabbro are ambiguous as there are examples of anastamosing chilled margins of microgabbro in contact with basalt as well as an example of a clast of microgabbro within fine-grained basalt. The fine-grained basalt dikes are resistant to weathering relative to granite plutons that they intrude in the district including the Permian Hanbogd Mountain complex, and crop out extensively as laterallycontinuous, 1-5 m wide sinuous bodies. Regional granitoid plutons Carboniferous to Permian felsic intrusive bodies are present throughout the southern Gobi region, and form preferentially uneroded red-pink colour anomalies on satellite images that are easily distinguished from surrounding dark green volcanic and volcaniclastic rocks. For the purposes of this study, the following plutons are defined: OT North pluton:  The OT North pluton, located immediately north of the Hugo  Dummett deposit is at least 3 km across and intrudes mafic to intermediate volcanic and volcaniclastic host rocks. The pluton is cut by the ductile shear zone 3 km west of the porphyry Cu-Au deposits and is in fault contact with the Sedimentary Sequence (Gurvankharaat Group) at the north end of the Hugo Dummett deposit. The equigranular hornblende granodiorite is coarse-grained, pink with 3-4 mm potassium-feldspar (30%), 3-4 mm plagioclase (30%), 3-5 mm hornblende (30%) and 1-2 mm quartz (5%). Accessory phases include apatite and zircon.  39  Javhalant Mountain pluton:  The 2.5 km wide Javhalant pluton crops out  approximately 12 km southwest of the Oyu Tolgoi deposits and intrudes mafic to intermediate volcaniclastic rocks and sedimentary rocks.  The top of the Javhalant  Mountain consists of hornfelsed andesitic roof pendants and the southern contact with sedimentary rocks is faulted.  The granite intrusion consists of 2-4 mm potassium  feldspar crystals (30-40%), 1-3 mm plagioclase (30%) and 1-2 mm quartz phenocrysts (10-20%). Accessory phases include apatite, zircon and titanite. OT18 pluton: This granite pluton is 4 km across and located approximately 18 km north of the Oyu Tolgoi porphyry deposits, defined by a conspicuous pink outcrop feature on satellite images of the area. Host rocks for this intrusion do not crop out. The pluton consists of coarse-grained, equigranular hornblende-biotite granite with 2-5 mm plagioclase feldspar (30-40%), 2-3 mm K-feldspar (30-40%), 3-4 mm hornblende (510%) and 1-3 mm quartz (5-10%). Accessory phases include zircon and apatite. U-Pb geochronology Prior to this study, limited isotopic ages were available for the Oyu Tolgoi porphyry copper-gold district and surrounding area. A Late Devonian age of 411 ± 3 Ma (K-Ar) determined on biotite from a potassic alteration assemblage in the South Oyu deposit was interpreted as the age of mineralization (Perello et al., 2001). In addition, the Tavan Tahil syenite (25 km southeast of Oyu Tolgoi) was dated at 307 ± 4 Ma (K-Ar; biotite; Perello et al., 2001). The Hanbogd complex has been radiometrically dated (KAr; riebeckite) with an interpreted crystallization age of 287 ± 3 Ma (Kovalenko and Yarmolyuk, 1995). Lamb and Cox (1998) reported an Ar-Ar age of 364.9 ± 3.5 Ma on sericite and a K-Ar age on mica from a monzonite dike of 313 ± 2.9 Ma from the Tsagaan Suvarga porphyry copper deposit, 150 km northeast of Oyu Tolgoi. Kirwin et al. (2005) and Khashgerel et al. (2006) reported that Re-Os ages on molybdenite samples from Southwest Oyu, Central Oyu and South Hugo range from 373 to 370 ± 1.2 Ma. A similar Re-Os age of 370 ± 1.2 Ma was determined on molybdenite from the Tsagaan Suvarga copper-molybdenum deposit (Watanabe and Stein, 2000).  40  Analytical method Fifteen samples were selected for the SHRIMP-RG (Sensitive High-Resolution Ion Microprobe; Reverse-Geometry) U-Pb geochronologic study and additional TIMS (Thermal Ionization Mass Spectrometer) ages were obtained from two samples to better constrain the sequence and to provide an internal check on the interpreted SHRIMP-RG ages. Five to fifteen kilogram samples for U-Pb dating were processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator. Individual zircon grains were then handpicked for U-Pb isotopic analysis. SHRIMP-RG The U-Pb dating was performed at the USGS – Stanford University SHRIMP-RG lab and reviews of the applied techniques are in Williams (1996). Zircon grains were mounted in epoxy, polished, photographed, and coated with ~10 nm of gold. Polished mounts were cleaned with soap, HCl, and distilled water and dried prior to coating with gold. During U-Pb analysis by SHRIMP-RG, a primary beam of 16O2- ions (about 10–20 nA) was used to raster an area about 50 × 50 µm for about 90 seconds to remove the gold coat and surface contamination (common Pb). The beam was focused to create flatfloored (about 2 µm-deep) elliptical pits for analysis (about 30 × 35 µm). Data were collected for four to five scans per spot. In advance of uranium-lead dating, zircon grains were examined under cathodoluminescence (CL) and reflected light to reveal if the single grain candidates for SHRIMP-RG analysis contained imperfections such as cracks, holes or obvious inherited zircon cores (Figure 2.9). This information guided selection of the best grains and locations for the spot analyses.  The zircon grains show a wide variey of sizes,  morphologies and colours that range from clear to pale yellow or orange. Many grains are sub-angular, however they did not consistently yield older U-Pb ages. Cathodoluminescence imaging shows oscillatory banding in most grains, sector zoning in some grains, and does show some evidence for inherited cores which was confirmed in 41  several analyses. Most spot analyses, however, were performed on grain rims or tips in order to identify the youngest, magmatic ages (Figure 2.9). The measured U-Pb isotopic data were referenced to the USGS-Stanford University zircon age standards RG6 (1440 Ma) and R33 (419 Ma). Isotopic ratios were reduced using the programs ISOPLOT and SQUID (Ludwig, 1999). Analysis of the results used weighted mean  207  Pb-corrected  206  Pb/238U age plots (Figure 2.10). As the  zircon grains from the Oyu Tolgoi district are relatively young, only the 206  207  Pb-corrected  Pb/238U ages can be reliably used to determine the age of these rocks for two reasons.  One reason is the generally poor measuring statistics on  204  Pb, the normal isotope used  for assessing common Pb during the age calculation, and the other is the low U contents of the zircons and young age, which limits the amount of radiogenic  207  Pb produced  during in situ decay. Hence, the 207Pb-corrected 206Pb/238U ages are used throughout, and plots of weighted mean age (2σ error bars) used to determine U-Pb ages are shown in Figure 2.10. Individual zircon spot ages are stated with 1σ error. Sufficient individual SHRIMP-RG zircon spot analyses were made for each sample such that at least eight analyses showing similar ages were obtained. These similar ages were then used in the calculation of the overall rock age. In the case of some of the fragmental volcanic samples, fewer zircons were used (n > 5) in this calculation as xenocrysts are common in these rocks. The xenocrysts were identified based on the known ages of older igneous material. Only zircons with  207  Pb-corrected  206  Pb/238U ages that fall between 300 and 400  Ma are displayed in the diagrams (Figure 2.10). Zircons that are younger than 300 Ma are interpreted to have undergone significant lead-loss since crystallization and zircons with U-Pb ages that exceed 400 Ma are clearly either xenocrysts or have inherited cores as they are significantly older. These xenocrystic zircons nonetheless have important implications for tectonic models of the CAOB. Analytical data for fifteen SHRIMP-RG samples are presented in Table 2.3 and the results are summarized in Table 2.4. Representative cathodoluminescence images of the zircon grains dated by SHRIMP-RG are presented in Figure 2.9 and all CL images are located in Appendix 1. As well, 40 and 24 zircons, respectively, were analyzed by SHRIMP-RG from two tuffaceous sandstone  42  samples (AJW-04-245 and AJW-04-354) and histograms are used to display the age results for these samples in Figure 2.12. TIMS Analytical work was perfomed at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at The University of British Columbia. All grains were hand-picked under magnification in alcohol and pre-treated using air abrasion or chemical abrasion techniques. After air abrasion (Krogh, 1982), grains are washed and ultrasonicated in 3N HNO3 to remove fine dust from pyrite abrasion media, rinsed with acetone followed by ethanol, and transferred to 10 mL pyrex beakers; they are ready for final washing prior to dissolution. The chemical abrasion technique (CA-TIMS) employed is modified from procedures outlined in Mundil et al. (2004) and Mattinson (2005), and the PCIGR CATIMS technique is described in detail in Scoates and Friedman (2008). Isotopic ratios are measured with a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Uranium fractionation was determined directly on individual runs using a  233-235  U tracer, and Pb  isotopic ratios were corrected for fractionation (0.23-0.35%/amu, throughout the course of this study) based on replicate analyses of the NBS 982 Pb reference material and the values recommended by Thirlwall (2000). Reported precisions for Pb/U and Pb/Pb dates were determined by numerically propagating all analytical uncertainties through the entire age calculation using the technique of Roddick (1987).  Standard concordia  diagrams were constructed and concordia/weighted mean ages were calculated with Isoplot 3.00 (Ludwig, 2003). Unless otherwise noted, all errors are quoted at the 2σ level. A TIMS U-Pb age for the sample of welded ignimbrite (AJW-03-148) as well as CA-TIMS U-Pb ages on two xenocrystic grains from the Bulagbayan Formation Augite Basalt (AJW-03-204) are reported in Table 2.5. Concordia diagrams for these samples are presented in Figure 2.11.  43  Figure 2.9 Representative cathodoluminescence images of zircon grains from samples dated by SHRIMPRG. Circles represent the spot locations of the analyses and numbers correspond to results in Table 2.3.  44  Figure 2.9 continued.  45  46  U (ppm)  Th (ppm)  53 78 111 63 109 76 409 85 63 115 199 146 110  24 58 59 28 29 29 251 64 25 87 84 77 70  AJW-04-246-1 AJW-04-246-2 AJW-04-246-3 AJW-04-246-4 AJW-04-246-5 AJW-04-246-6 AJW-04-246-7 AJW-04-246-8 AJW-04-246-9 AJW-04-246-10 AJW-04-246-11  102 137 80 39 101 181 96 58 65 104 53  80 122 40 18 65 86 48 27 39 74 37  AJW-04-246 PBS unit polylithic breccia  AJW-03-148-1 AJW-03-148-2 AJW-03-148-3 AJW-03-148-4 AJW-03-148-5 AJW-03-148-6 AJW-03-148-7 AJW-03-148-8 AJW-03-148-9 AJW-03-148-10 AJW-03-148-11 AJW-03-148-12 AJW-03-148-13  AJW-03-148 UDS unit welded ignimbrite  Spot  232  0.8067 0.9233 0.5188 0.4738 0.6713 0.4902 0.5176 0.4853 0.6180 0.7316 0.7249  0.4682 0.7714 0.5465 0.4632 0.2788 0.3878 0.6349 0.7762 0.4102 0.7861 0.4363 0.5486 0.6604  Th/ U  238  5 7 4 2 5 9 5 3 3 5 3  3 4 6 3 6 4 21 4 3 6 10 7 5  Pb* (ppm)  206  17.55 17.43 18.03 17.06 18.37 17.79 17.57 17.35 17.98 17.94 17.59  16.85 17.13 16.04 17.07 14.91 17.1 16.86 16.96 16.61 17.56 16.61 17.12 17.25  Total 238 U/206Pb  Table 2.3 SHRIMP-RG analytical data for Oyu Tolgoi district geochronology samples.  2.2 1.9 2.0 2.3 2.0 1.8 2.0 2.2 2.6 2.0 2.2  2.1 1.9 1.8 2.0 2.2 2.1 1.9 2.2 2.2 2.0 1.9 2.0 2.0  ± (%)  0.0575 0.0532 0.0544 0.0556 0.0542 0.0542 0.0534 0.0528 0.0506 0.0519 0.0574  0.0550 0.0538 0.0536 0.0561 0.0572 0.0562 0.0556 0.0549 0.0541 0.0549 0.0534 0.0554 0.0561  Total 207 Pb/206Pb  4.2 2.6 3.5 5.1 3.3 2.4 3.3 4.3 4.2 3.3 4.4  4.7 4.0 3.2 4.3 3.9 3.9 1.8 3.7 4.6 3.1 2.4 2.8 3.4  ± (%)  356 360 348 366 341 352 357 362 350 350 355  371 366 390 366 418 365 371 369 377 356 377 365 362  Pb/238U Age  206  1σ  8 7 7 9 7 6 7 8 9 7 8  8 7 7 7 9 8 7 8 8 7 7 7 7  47  61 75 471 73 226 64  U (ppm) 44 46 572 42 119 43  Th (ppm) 0.7455 0.6315 1.2562 0.5915 0.5442 0.6949  Th/ U  238  AJW-04-354-1 AJW-04-354-2 AJW-04-354-3 AJW-04-354-4 AJW-04-354-5 AJW-04-354-6 AJW-04-354-7 AJW-04-354-8 AJW-04-354-9 AJW-04-354-10 AJW-04-354-11 AJW-04-354-12 AJW-04-354-13 AJW-04-354-14 AJW-04-354-15 AJW-04-354-16 AJW-04-354-17 AJW-04-354-18 AJW-04-354-19 AJW-04-354-20 AJW-04-354-21  106 160 145 224 452 120 305 210 97 421 352 320 218 452 315 209 314 242 276 223 330  65 132 112 164 495 91 186 165 39 350 227 191 109 237 266 169 215 118 152 90 175  0.6374 0.8519 0.7999 0.7592 1.1311 0.7804 0.6291 0.8124 0.4194 0.8591 0.6663 0.6166 0.5168 0.5406 0.8724 0.8367 0.7084 0.5039 0.5693 0.4174 0.5481  AJW-04-354 Sedimentary Sequence tuffaceous sandstone  AJW-04-246-12 AJW-04-246-13 AJW-04-246-14 AJW-04-246-15 AJW-04-246-16 AJW-04-246-17  Spot  232  5 8 7 11 23 6 13 11 5 21 17 15 11 22 16 10 17 12 14 12 17  3 4 23 4 11 3  Pb* (ppm)  206  16.99 17.23 17.34 17.66 16.66 16.8 19.83 16.63 16.8 17.61 17.43 17.81 17.01 17.4 16.95 17.71 16.25 17.39 17.51 16.08 17.05  17.73 18.05 17.69 17.21 17.44 18.42  Total 238 U/206Pb  1.9 1.8 1.9 1.8 2.0 2.0 1.8 1.8 2.1 2.0 1.8 1.8 1.9 1.8 1.9 2.0 1.8 1.8 1.8 1.8 1.8  2.6 2.1 1.7 2.1 1.8 2.1  ± (%)  0.0546 0.0535 0.0562 0.0551 0.0542 0.0588 0.0583 0.0541 0.0558 0.0544 0.0548 0.0531 0.0532 0.0544 0.0521 0.0533 0.0544 0.0555 0.0550 0.0533 0.0528  0.0519 0.0548 0.0550 0.0586 0.0551 0.0502  Total 207 Pb/206Pb  2.9 2.4 2.6 2.1 1.5 3.1 2.2 2.2 3.7 1.9 2.0 2.3 2.8 2.0 2.5 3.3 1.8 2.2 2.2 2.4 1.8  4.6 3.8 1.5 3.8 2.9 4.5  ± (%)  368 364 360 355 376 371 315 377 372 356 359 352 369 360 370 354 385 360 358 390 368  355 347 354 362 359 342  Pb/238U Age  206  1σ  7 7 7 6 7 7 6 7 8 7 6 6 7 6 7 7 7 6 6 7 6  9 7 6 8 6 7  48  271 766 311  U (ppm) 181 776 164  Th (ppm) 0.6915 1.0471 0.5448  Th/ U  238  AJW-04-245-1 AJW-04-245-2 AJW-04-245-3 AJW-04-245-4 AJW-04-245-5 AJW-04-245-6 AJW-04-245-7 AJW-04-245-8 AJW-04-245-9 AJW-04-245-10 AJW-04-245-11 AJW-04-245-12 AJW-04-245-13 AJW-04-245-14 AJW-04-245-15 AJW-04-245-16 AJW-04-245-17 AJW-04-245-18 AJW-04-245-19 AJW-04-245-20 AJW-04-245-21 AJW-04-245-22 AJW-04-245-23 AJW-04-245-24  205 114 165 122 263 203 218 138 216 209 148 216 159 165 136 197 440 204 132 140 383 140 167 170  177 104 111 68 259 122 276 121 120 163 100 128 51 105 77 114 278 169 81 94 199 84 135 119  0.8933 0.9477 0.6942 0.5743 1.0183 0.6193 1.3066 0.9052 0.5747 0.8060 0.6993 0.6101 0.3295 0.6541 0.5840 0.5985 0.6535 0.8558 0.6358 0.6975 0.5359 0.6193 0.8382 0.7225  AJW-04-245 Sedimentary Sequence tuffaceous sandstone  AJW-04-354-22 AJW-04-354-23 AJW-04-354-24  Spot  232  10 5 9 6 13 10 11 7 11 11 7 11 8 8 7 10 22 10 7 7 19 7 8 8  14 33 15  Pb* (ppm)  206  18.42 17.93 15.23 17.37 17.54 17.66 17.47 17.56 16.71 17.13 17.73 17.44 17.62 17.84 17.25 17.41 17.13 17.34 17.28 17.38 17.32 17.51 17.87 17.33  16.9 20.14 17.98  Total 238 U/206Pb  1.8 2.0 1.8 2.2 1.7 1.7 1.7 1.8 1.7 1.7 1.8 1.7 1.8 1.8 1.8 1.7 1.6 1.7 1.8 1.8 1.7 1.8 1.8 1.8  1.8 1.7 1.9  ± (%)  0.0536 0.0559 0.1432 0.0529 0.0538 0.0552 0.0572 0.0554 0.0545 0.0561 0.0580 0.0523 0.0509 0.0573 0.0533 0.0534 0.0546 0.0564 0.0556 0.0561 0.0533 0.0535 0.0570 0.0530  0.0525 0.0557 0.0529  Total 207 Pb/206Pb  2.7 3.5 1.9 3.4 2.3 2.7 2.5 3.1 2.6 2.6 3.0 2.6 3.0 2.9 3.2 2.6 1.8 2.6 3.2 3.1 1.9 3.2 2.8 2.9  2.0 1.2 2.6  ± (%)  341 349 366 361 357 354 357 356 375 365 352 360 357 350 364 360 365 360 362 360 362 358 350 362  371 311 349  Pb/238U Age  206  1σ  6 7 9 8 6 6 6 6 6 6 6 6 7 6 7 6 6 6 7 7 6 6 6 6  7 5 7  49  129 194 144 150 155 614 196 219 160 126 146 156 158 219 156 316  U (ppm)  AJW-04-270-1 AJW-04-270-2 AJW-04-270-3 AJW-04-270-4 AJW-04-270-5 AJW-04-270-6 AJW-04-270-7 AJW-04-270-8.1 AJW-04-270-8.2 AJW-04-270-9  121 349 1100 130 390 109 114 84 49 108  114 220 974 76 192 96 58 66 30 96  76 179 100 87 114 907 187 161 87 63 133 135 123 112 86 158  Th (ppm)  AJW-04-270 LVS unit andesite flow  AJW-04-245-25 AJW-04-245-26 AJW-04-245-27 AJW-04-245-28 AJW-04-245-29 AJW-04-245-30 AJW-04-245-31 AJW-04-245-32 AJW-04-245-33 AJW-04-245-34 AJW-04-245-35 AJW-04-245-36 AJW-04-245-37 AJW-04-245-38 AJW-04-245-39 AJW-04-245-40  Spot  232  0.9730 0.6527 0.9148 0.6034 0.5081 0.9139 0.5301 0.8116 0.6369 0.9183  0.6100 0.9536 0.7224 0.5983 0.7620 1.5261 0.9868 0.7587 0.5574 0.5193 0.9383 0.8923 0.8061 0.5295 0.5732 0.5173  Th/ U  238  6 16 40 6 13 5 5 4 2 5  6 9 7 7 8 32 10 11 8 6 7 8 8 11 8 16  Pb* (ppm)  206  18.66 18.29 23.91 19.75 25.83 17.54 18.4 18.13 18.15 17.4  17.4 17.71 17.4 17.41 17.09 16.62 17.47 17.18 17.14 16.81 17.41 17.49 17.19 17.09 16.98 17.32  Total 238 U/206Pb  1.4 1.0 0.9 1.4 1.1 1.4 1.4 1.6 1.9 1.4  1.8 1.7 1.8 1.8 1.8 1.6 1.7 1.7 1.8 1.8 1.9 1.8 1.8 1.7 1.8 1.7  ± (%)  0.0504 0.0550 0.0508 0.0534 0.0517 0.0523 0.0583 0.0550 0.0534 0.0561  0.0561 0.0535 0.0551 0.0548 0.0556 0.0555 0.0520 0.0523 0.0528 0.0580 0.0562 0.0603 0.0532 0.0539 0.0545 0.0535  Total 207 Pb/206Pb  4.1 2.3 2.0 3.9 2.7 4.2 3.9 4.5 6.2 3.8  3.2 2.7 3.0 3.0 3.0 1.5 2.7 2.6 3.0 3.2 3.4 2.8 2.9 2.5 2.9 2.1  ± (%)  338 343 264 318 245 358 339 346 346 359  359 354 360 360 366 376 360 365 366 371 359 356 365 367 369 362  Pb/238U Age  206  1σ  5 4 2 5 3 5 5 6 7 5  7 6 6 6 7 6 6 6 7 7 7 6 6 6 7 6  50  U (ppm)  93 212 210 81 135 215 73 144 76 155  373 336 75 121 255 330 148 138 284 113 122 665 606  AJW-03-192 UVS unit dacite tuff  AJW-03-221-1 AJW-03-221-2 AJW-03-221-3 AJW-03-221-4 AJW-03-221-5 AJW-03-221-6 AJW-03-221-7 AJW-03-221-8 AJW-03-221-9 AJW-03-221-10 AJW-03-221-11 AJW-03-221-12 AJW-03-221-13  AJW-03-221 UVS unit rhyolite tuff  AJW-03-205-1 AJW-03-205-2 AJW-03-205-3 AJW-03-205-4 AJW-03-205-5 AJW-03-205-6 AJW-03-205-7 AJW-03-205-8 AJW-03-205-9 AJW-03-205-10  221 162 58 55 220 317 125 71 117 56 53 353 383  62 70 156 49 94 53 34 118 48 117  Th (ppm)  AJW-03-205 LVS unit dacite flow  Spot  232  0.6120 0.4969 0.7942 0.4675 0.8937 0.9934 0.8746 0.5270 0.4256 0.5103 0.4496 0.5476 0.6530  0.6864 0.3422 0.7673 0.6336 0.7167 0.2547 0.4856 0.8431 0.6604 0.7795  Th/ U  238  17 16 4 6 12 15 7 6 14 6 5 30 27  4 10 10 4 6 10 4 7 4 7  Pb* (ppm)  206  18.7 18.56 17.31 18.44 19.09 18.45 17.79 18.43 17.82 17.32 19.26 18.823 19.253  18.12 17.94 17.84 18.01 18.01 18.28 17.83 18.19 18.01 18.39  Total 238 U/206Pb  0.6 0.6 1.2 1.0 0.6 0.6 0.9 0.9 0.8 1.1 0.9 0.5 0.5  1.7 1.5 1.5 1.8 1.6 1.5 1.8 1.6 1.8 1.6  ± (%)  0.0525 0.0544 0.0521 0.0541 0.0543 0.0532 0.0510 0.0537 0.0536 0.0635 0.0533 0.1240 0.0549  0.0555 0.0528 0.0543 0.0566 0.0529 0.0523 0.0523 0.0545 0.0510 0.0527  Total 207 Pb/206Pb  1.8 1.9 3.9 3.4 2.1 2.0 3.1 2.8 2.0 3.6 3.2 17.0 1.5  2.7 1.9 1.8 3.0 2.4 1.9 3.3 2.3 3.4 2.4  ± (%)  336 338 363 340 329 340 354 341 352 358 326 305 326  345 350 351 347 349 344 352 345 349 342  Pb/238U Age  206  1σ  2 2 4 4 2 2 3 3 3 4 3 10 2  6 5 5 6 5 5 6 5 6 5  51  151 315 148 301 224 309 613 188 473 283 163 181  U (ppm)  AJW-03-183-1 AJW-03-183-2 AJW-03-183-3 AJW-03-183-4 AJW-03-183-5 AJW-03-183-6 AJW-03-183-7 AJW-03-183-8 AJW-03-183-9 AJW-03-183-10 AJW-03-183-11 AJW-03-183-12 AJW-03-183-13 AJW-03-183-14 AJW-03-183-15 AJW-03-183-16  320 119 150 171 124 210 155 181 293 231 228 85 160 71 578 165  AJW-03-183 Andesite dike  AJW-03-192-1 AJW-03-192-2 AJW-03-192-3 AJW-03-192-4 AJW-03-192-5 AJW-03-192-6 AJW-03-192-7 AJW-03-192-8 AJW-03-192-9 AJW-03-192-10 AJW-03-192-11 AJW-03-192-12  Spot  190 116 56 61 35 81 68 67 148 120 77 38 76 47 373 197  117 214 72 185 125 202 387 168 360 315 84 79  Th (ppm)  232  0.6151 1.0050 0.3814 0.3690 0.2872 0.3992 0.4513 0.3844 0.5241 0.5355 0.3496 0.4630 0.4894 0.6879 0.6658 1.2369  0.7991 0.7000 0.5033 0.6352 0.5790 0.6734 0.6516 0.9214 0.7859 1.1488 0.5294 0.4536  Th/ U  238  15 8 7 8 6 10 7 9 14 11 12 4 7 3 19 6  38 15 7 14 10 11 29 8 16 11 8 8  Pb* (ppm)  206  18.3 12.34 17.9 17.62 18.04 17.87 18.27 17.85 17.68 18.55 16.55 17.32 18.33 22.27 26.25 24.99  3.446 18.36 18.83 18.21 18.66 24.77 18.434 19.04 25.18 21.9 18.59 18.62  Total 238 U/206Pb  1.4 1.6 1.6 1.5 1.7 1.5 1.6 1.5 1.4 1.5 1.5 1.7 1.8 1.9 1.4 1.6  0.7 0.6 0.9 0.7 0.8 0.7 0.5 0.8 0.5 0.8 1.3 0.9  ± (%)  0.0537 0.0600 0.0524 0.0736 0.0544 0.0521 0.0563 0.0538 0.0530 0.0561 0.0546 0.0535 0.0550 0.0512 0.0512 0.0529  0.1054 0.0544 0.0544 0.0536 0.0528 0.0549 0.0545 0.0535 0.0517 0.0549 0.0519 0.0564  Total 207 Pb/206Pb  1.6 2.1 2.3 1.9 2.5 2.0 2.2 2.1 1.7 1.8 1.8 2.9 2.3 3.8 1.4 2.6  0.9 2.1 3.0 2.2 2.6 2.6 1.6 2.7 2.0 2.4 3.4 2.9  ± (%)  343 501 351 347 347 352 342 351 355 337 378 362 342 283 241 252  1634 341 333 345 337 254 340 330 251 287 338 336  Pb/238U Age  206  1σ  5 8 5 5 6 5 5 5 5 5 5 6 6 5 3 4  10 2 3 2 3 2 2 3 1 2 5 3  52  87 274 218 438 456 288  U (ppm)  946 229 175 151 262 144 400 326 117 188 212 458 322  AJW-03-203-1 AJW-03-203-2 AJW-03-203-3 AJW-03-203-4 AJW-03-203-5 AJW-03-203-6  372 124 233 785 238 224  AJW-03-203 Rhyolite dike  AJW-03-180-1 AJW-03-180-2 AJW-03-180-3 AJW-03-180-4 AJW-03-180-5 AJW-03-180-6 AJW-03-180-7 AJW-03-180-8 AJW-03-180-9 AJW-03-180-10 AJW-03-180-11 AJW-03-180-12 AJW-03-180-13  AJW-03-180 Rhyolite dike  AJW-03-183-17 AJW-03-183-18 AJW-03-183-19 AJW-03-183-20 AJW-03-183-21 AJW-03-183-22  Spot  314 76 178 1249 192 151  1020 174 80 227 153 79 197 253 54 158 68 409 229  98 102 81 244 237 156  Th (ppm)  232  0.8714 0.6367 0.7912 1.6439 0.8312 0.6972  1.1141 0.7833 0.4743 1.5521 0.6020 0.5674 0.5079 0.8010 0.4774 0.8675 0.3306 0.9226 0.7350  1.1658 0.3847 0.3847 0.5769 0.5365 0.5587  Th/ U  238  17 6 11 36 11 10  28 10 8 7 12 7 21 15 5 8 10 21 15  3 18 10 15 21 14  Pb* (ppm)  206  18.83 18.84 18.8 18.71 18.67 18.62  28.99 18.92 18.22 18.71 18.12 18.41 16.69 18.52 19.11 19.08 17.67 18.39 18.42  25.36 13.42 18.36 25.06 18.57 18.15  Total 238 U/206Pb  1.5 1.6 1.5 1.5 1.5 1.6  0.5 0.8 0.9 0.9 0.8 1.0 0.7 0.7 1.0 0.8 0.7 0.6 0.6  1.8 1.4 1.5 1.4 1.4 1.4  ± (%)  0.0537 0.0533 0.0533 0.0528 0.0532 0.0536  0.0856 0.0531 0.0569 0.0552 0.0549 0.0545 0.0549 0.0528 0.0543 0.0555 0.0529 0.0533 0.0537  0.0515 0.0567 0.0546 0.0516 0.0535 0.0549  Total 207 Pb/206Pb  1.7 3.0 2.2 1.2 2.2 2.2  4.5 2.6 3.8 3.0 2.3 3.2 1.9 2.3 3.4 2.7 2.4 1.9 2.1  3.6 1.4 1.9 1.6 1.4 1.7  ± (%)  333 333 334 336 336 337  209 332 343 335 346 340 375 339 328 328 355 341 341  249 463 341 252 338 345  Pb/238U Age  206  1σ  5 5 5 5 5 5  2 3 3 3 3 3 3 2 3 3 3 2 2  5 6 5 4 5 5  53  1076 907 549 378 178 52  U (ppm)  AJW-03-193-1 AJW-03-193-2 AJW-03-193-3 AJW-03-193-4 AJW-03-193-5 AJW-03-193-6 AJW-03-193-7 AJW-03-193-8 AJW-03-193-9 AJW-03-193-10 AJW-03-193-11 AJW-03-193-12 AJW-03-193-13 AJW-03-193-14 AJW-03-193-15 AJW-03-193-16 AJW-03-193-17 AJW-03-193-18 AJW-03-193-19 AJW-03-193-20 AJW-03-193-21 AJW-03-193-22  274 1294 849 244 273 176 573 134 283 250 374 85 422 288 191 273 257 363 381 386 266 294  AJW-03-193 Rhyolite sill  AJW-03-203-7 AJW-03-203-8 AJW-03-203-9 AJW-03-203-10 AJW-03-203-11 AJW-03-203-12  Spot  195 285 706 116 140 90 307 73 176 172 239 36 366 178 126 134 172 306 285 315 113 131  956 564 572 618 174 20  Th (ppm)  232  0.7358 0.2277 0.8591 0.4912 0.5304 0.5277 0.5540 0.5579 0.6420 0.7081 0.6615 0.4320 0.8957 0.6390 0.6831 0.5089 0.6915 0.8719 0.7718 0.8435 0.4379 0.4618  0.9175 0.6423 1.0762 1.6902 1.0065 0.3974  Th/ U  238  12 42 28 10 12 8 36 6 13 11 17 9 22 14 9 13 11 17 18 17 12 14  48 42 25 17 6 2  Pb* (ppm)  206  18.94 26.33 26.35 20.58 19.16 19 13.57 18.44 18.8 19.15 19.07 7.73 16.71 18.33 19.26 18.67 19.53 18.8 18.68 19.12 18.48 18.64  19.1 18.7 18.83 18.9 25.23 18.58  Total 238 U/206Pb  1.8 1.6 1.6 1.7 1.7 1.7 1.6 2.0 1.7 1.7 1.7 1.8 1.7 1.7 1.7 1.7 1.7 1.7 1.6 1.9 1.8 1.8  1.4 2.4 1.5 1.5 1.6 1.9  ± (%)  0.0538 0.0504 0.0518 0.0558 0.0560 0.0536 0.0557 0.0540 0.0633 0.0530 0.0538 0.0668 0.0723 0.0713 0.0516 0.0513 0.0544 0.0530 0.0520 0.0534 0.0532 0.0521  0.0540 0.0543 0.0533 0.0542 0.0564 0.0536  Total 207 Pb/206Pb  2.2 1.2 1.5 2.4 2.1 2.7 1.3 3.1 4.1 2.3 1.9 2.2 1.6 1.9 2.7 2.2 2.2 2.3 1.9 1.9 2.7 2.6  1.0 1.1 1.4 1.7 2.7 4.5  ± (%)  331 241 240 305 327 331 459 340 330 328 329 783 366 335 327 337 321 334 337 328 340 337  329 335 333 332 249 338  Pb/238U Age  206  1σ  6 4 4 5 6 6 7 7 6 6 5 14 6 6 6 6 5 6 6 6 6 6  5 8 5 5 4 7  54  259 384 346 199 392 298 372  U (ppm)  442 111 145 142 101 89 84 166 149 142 172 275  AJW-03-132-1 AJW-03-132-2 AJW-03-132-3 AJW-03-132-4 AJW-03-132-5 AJW-03-132-6  402 539 347 426 624 422  AJW-03-132 Javhalant pluton  AJW-03-074-1 AJW-03-074-2 AJW-03-074-3 AJW-03-074-4 AJW-03-074-5 AJW-03-074-6 AJW-03-074-7 AJW-03-074-8 AJW-03-074-9 AJW-03-074-10 AJW-03-074-11 AJW-03-074-12  AJW-03-074 OT North pluton  AJW-03-193-23 AJW-03-193-24 AJW-03-193-25 AJW-03-193-26 AJW-03-193-27 AJW-03-193-28 AJW-03-193-29  Spot  230 346 151 172 411 262  487 64 85 107 69 61 49 115 101 78 128 311  192 260 239 95 276 203 234  Th (ppm)  232  0.5925 0.6639 0.4481 0.4165 0.6802 0.6420  1.1377 0.5990 0.6020 0.7756 0.7055 0.7050 0.6060 0.7179 0.6976 0.5662 0.7682 1.1673  0.7645 0.7014 0.7134 0.4917 0.7280 0.7051 0.6492  Th/ U  238  16 23 14 18 25 18  21 5 7 7 5 4 4 8 7 7 9 13  9 18 16 9 18 14 17  Pb* (ppm)  206  21.54 20.13 20.81 20.27 21.24 19.7  18.07 17.84 17.11 17.45 17.43 17.46 18.56 17.52 17.6 17.94 17.05 18.08  23.63 18.7 18.55 18.72 19.23 18.91 19.07  Total 238 U/206Pb  0.6 3.1 0.6 0.6 0.5 0.9  1.6 1.8 2.2 2.1 1.9 2.3 1.9 1.7 1.8 1.8 1.9 1.7  1.7 1.7 1.7 1.8 1.7 1.8 1.6  ± (%)  0.0524 0.0533 0.0540 0.0654 0.0521 0.0566  0.0536 0.0551 0.0543 0.0513 0.0557 0.0579 0.0510 0.0526 0.0551 0.0543 0.0538 0.0545  0.0537 0.0518 0.0533 0.0507 0.0519 0.0532 0.0518  Total 207 Pb/206Pb  2.1 1.8 2.2 1.7 1.7 2.0  1.8 3.3 2.9 3.0 3.4 3.4 3.9 2.7 2.7 2.8 2.6 2.2  3.0 2.1 2.3 3.1 2.2 2.5 1.9  ± (%)  292 312 302 306 297 318  347 351 366 360 359 357 339 358 356 349 368 347  267 336 339 336 327 332 330  Pb/238U Age  206  1σ  2 9 2 2 2 3  6 6 8 7 7 8 6 6 6 6 7 6  5 6 6 6 5 6 5  55  520 288 201 489 482 576 449 159 253 340 269 1118 229 309 1029 162 363 113 274 133 193  U (ppm)  AJW-03-116-1 AJW-03-116-2 AJW-03-116-3 AJW-03-116-4 AJW-03-116-5 AJW-03-116-6 AJW-03-116-7  138 135 146 130 188 193 176  AJW-03-116 OT18 pluton  AJW-03-132-7 AJW-03-132-8 AJW-03-132-9 AJW-03-132-10 AJW-03-132-11 AJW-03-132-12 AJW-03-132-13 AJW-03-132-14 AJW-03-132-15 AJW-03-132-16 AJW-03-132-17 AJW-03-132-18 AJW-03-132-19 AJW-03-132-20 AJW-03-132-21 AJW-03-132-22 AJW-03-132-23 AJW-03-132-24 AJW-03-132-25 AJW-03-132-26 AJW-03-132-27  Spot  48 71 42 64 116 104 99  208 106 75 241 356 356 262 74 133 192 182 1203 129 181 869 90 193 44 146 76 101  Th (ppm)  232  0.3634 0.5426 0.2968 0.5093 0.6390 0.5564 0.5802  0.4124 0.3821 0.3839 0.5097 0.7646 0.6381 0.6033 0.4817 0.5440 0.5842 0.6967 1.1118 0.5849 0.6049 0.8734 0.5763 0.5500 0.4062 0.5488 0.5893 0.5398  Th/ U  238  6 6 5 6 8 9 8  23 13 9 20 22 26 20 7 11 15 12 52 10 15 41 7 17 5 12 6 9  Pb* (ppm)  206  19.43 19.99 25.7 19.53 19.4 19.48 19.1  19.398 19.48 19.42 20.76 18.751 19.018 19.11 18.44 19.35 19.81 19.56 18.474 20.07 18.05 21.362 19.55 18.81 18.29 18.99 18.96 19.25  Total 238 U/206Pb  1.9 1.9 2.0 1.9 1.8 1.9 1.9  0.5 0.8 1.0 0.6 0.5 0.5 0.6 1.1 0.7 0.6 0.7 0.4 0.8 0.7 0.4 0.9 0.6 1.1 0.7 1.0 0.8  ± (%)  0.0508 0.0526 0.0597 0.0535 0.0550 0.0574 0.0540  0.0541 0.0537 0.0522 0.0526 0.0536 0.0540 0.0525 0.0568 0.0538 0.0542 0.0529 0.0530 0.0528 0.0530 0.0611 0.0531 0.0518 0.0541 0.0543 0.0557 0.0538  Total 207 Pb/206Pb  3.3 3.1 3.2 3.1 2.6 2.9 2.6  1.7 2.4 3.1 2.0 1.8 1.8 2.0 3.9 2.5 2.2 2.5 1.2 2.7 3.0 1.3 3.2 2.1 3.7 2.3 3.7 2.9  ± (%)  324 315 243 322 323 321 329  324 322 324 303 335 330 329 339 325 317 321 340 313 348 292 321 335 343 330 330 326  Pb/238U Age  206  1σ  6 6 5 6 6 6 6  2 3 3 2 2 2 2 4 2 2 2 1 2 2 1 3 2 4 2 3 3  56  AJW-03-116-8 AJW-03-116-9 AJW-03-116-10  Spot 152 126 116  U (ppm) 78 59 50  Th (ppm)  232  0.5261 0.4838 0.4457  Th/ U  238  Pb* (ppm)  206  7 5 5  19.89 19.99 18.68  Total 238 U/206Pb 1.9 2.0 1.9  ± (%) 0.0542 0.0547 0.0503  Total 207 Pb/206Pb 2.8 3.1 3.2  ± (%) 316 314 337  Pb/238U Age  206  1σ 6 6 7  57  Lithofacies  UDS  PBS Sedimentary Sequence Sedimentary Sequence LVS LVS UVS UVS  OT North pluton Javhalant pluton OT18 pluton  Regional granitoid plutons  Andesiteg Rhyolite dikeh Rhyolite dike Rhyolite sill  n/a n/a n/a  n/a n/s n/a n/a  Carboniferous intermediate to silicic subvolcanic units  Polylithic brecciab Tuffaceous sandstonec Tuffaceous sandstoned Andesite flowe Dacite domef Rhyolite tuff Dacite tuff  Carboniferous intermediate to silicic volcanic units  Welded ignimbritea  Upper Devonian intermediate to silicic volcanic units  Rock type  AJW-03-074 AJW-03-132 AJW-03-116  AJW-03-183 AJW-03-180 AJW-03-203 AJW-03-193  AJW-04-246 AJW-04-354 AJW-04-245 AJW-04-270 AJW-03-205 AJW-03-221 AJW-03-192  AJW-03-148  sample  649467 637204 654143  650667 651025 646882 655310  651907 652070 651907 651907 651270 652828 655611  652017  easting  4768096 4752491 4782912  4763367 4764201 4766519 4761560  4765317 4765775 4765317 4765317 4767399 4759666 4767603  4766476  northing  12 27 10  22 13 12 29  17 24 40 10 10 13 12  13  No. of analyses  350 324 321  345 340 335 330  354 <350 <350 346 347 339 339  367  Age* (Ma)  ±7 ±3 ±4  ±2 ±3 ±1 ±2  ±2 n/a n/a ±8 ±3 ±2 ±2  ±3  2σ  3 10 2  9 5 2 8  3 n/a n/a 3 0 8 6  4  Rejected zircons  Table 2.4 Summary of SHRIMP-RG U-Pb (zircon) ages for volcanic, sub-volcanic and plutonic rocks from the Oyu Tolgoi porphyry Cu-Au district. All location references are given in UTM WGS zone 48 coordinates.  Table 2.4 footnotes *  Ages are reported as 207Pb-corrected 206Pb/238U weighted mean Sample collected from Diamond Drill Hole OTD 463A at 746m (Hugo Dummett zone) b Sample collected from Diamond Drill Hole OTD462 at 446m (Hugo Dummett zone) c Sample collected from Diamond Drill Hole OTD467 at 253m (Hugo Dummett zone) d Sample collected from Diamond Drill Hole OTD462 at 230m (Hugo Dummett zone) e Sample collected from Diamond Drill Hole OTD462 at 156m (Hugo Dummett zone) f Sample collected from Diamond Drill Hole OTD402 at 412m (Hugo Dummett zone) g Sample collected from Diamond Drill Hole OTD307 at 170m (Southwest Oyu) h Sample collected from Diamond Drill Hole OTD217 at 392m (Central Oyu) a  Results Older recycled zircons A number of xenocrytic zircons that pre-date the dominant Late Devonian to Carboniferous magmatic activity are present in the Oyu Tolgoi district samples. The xenocrystic SHRIMP-RG individual zircon ages (1σ error) include Proterozoic (1634 ± 10 Ma and 783 ± 14 Ma), Cambrian (501 ± 8 Ma), Ordovician (463 ± 6 Ma and 459 ± 7 Ma) and Silurian grains (418 ± 9 Ma) (Table 2.3). Ancient xenocrysts detected by SHRIMP-RG in Late Devonian intrusions include a Proterozoic zircon (1104 ± 15 Ma) and two Silurian zircons (441 ± 1 Ma and 416 ± 6 Ma) (Chapter 4). All of these grains are much older than the remainder of the zircons dated during the SHRIMP-RG study, which were all younger than 390 ± 7 Ma (this chapter) and 393 ± 5 Ma (Chapter 4). Moreover, chemically-abraded single-grain TIMS (zircon) analyses from two rounded 75-125 micron zircons from a sample of Bulagbayan Formation Augite Basalt (AJW-03204) also yielded concordant old ages of 516 ± 1 Ma and 441 ± 1 Ma (Figure 2.11; Table 2.5). Devonian magmatic activity SHRIMP-RG U-Pb results for the unmineralized sample of welded ignimbrite (UDS package; sample AJW-03-148) suggests that xenocrystic zircons are abundant (~15%) and impossible to distinguish from younger magmatic age zircons under cathodoluminescence. The grains are relatively small (<100 µm), colourless to orange  58  59  U2  (ppm)  Wt  (µg)  (ppm)  Pb*3  Pb4 Pb  206 204  (pg)  Pb5  Th/U6 Pbc  Pb*7  19  18  15  9  9  B  C  D  E  G  43  58  118  98  83  131  2.6  3.6  7.2  6.0  5.3  8.0  812  846  617  613  575  507  0.6  CA5  428  374  33.1  32.2 1221  4509 0.9  0.4  1.8  2.3  10.8  10.9  10.7  10.7  0.67  0.46  0.54  0.57  0.46  0.45  0.51  0.48  21.2  75.3  13.5  14.2  10.0  9.9  9.4  8.3  Pb/238U  0.07086 ± 0.12  0.08338 ± 0.12  0.05902 ± 0.53  0.05882 ± 0.18  0.05905 ± 0.19  0.06006 ± 0.13  0.06101 ± 0.15  0.05854 ± 0.17  206  Pb/235U  0.5456 ± 0.41  0.6655 ± 0.22  0.4407 ± 0.98  0.4373 ± 0.55  0.4401 ± 1.41  0.4500 ± 1.35  0.4559 ± 1.51  0.4371 ± 1.69  207  207  Pb/206Pb  0.05585 ± 0.37  0.05789 ± 0.18  0.05416 ± 0.80  0.05392 ± 0.49  0.05405 ± 1.33  0.05434 ± 1.28  0.05420 ± 1.43  0.05415 ± 1.61  Isotopic ratios ±1σ,% 8  0.45108  0.60302  0.57422  0.46855  0.46687  0.54878  0.55051  0.53445  ρ9  1.2  1.8  2.1  -0.2  0.9  2.4  -0.6  2.9  discordant  % 10 Pb/238U  441.3 ± 1.0  516.3 ± 1.2  369.6 ± 3.8  368.4 ± 1.3  369.9 ± 1.4  376.0 ± 1.0  381.7 ± 1.1  366.7 ± 1.2  206  Pb/235U  442.1 ± 3.0  518.0 ± 1.8  370.7 ± 6.1  368.3 ± 3.4  370.3 ± 8.7  377.3 ± 8.5  381.4 ± 9.6  368.2 ± 10.5  207  2  Pb/206Pb  446.3 ± 16.5/16.7  525.5 ± 7.8/7.8  377.5 ± 35.7/36.6  367.7 ± 21.9/22.2  373.2 ± 58.8/61  385.0 ± 56.4/58.5  379.5 ± 63.2/65.8  377.4 ± 70.8/74.1  207  Apparent ages ±2σ,Ma 8  All analyzed zircon grains were air abraded or chemically abraded; fraction names: A, B, etc., air abraded; CA1, CA2, etc., chemically abraded. U blank correction of 0.2-1.0 pg ± 20%; U fractionation corrections were measured for each analysis with a double 233-235U spike. 3 Radiogenic Pb 4 Measured ratio corrected for spike and Pb fractionation of 0.23-0.35/amu ± 20% (Daly collector), which was determined by repeated analysis of NBS Pb 982 reference material throughout the course of this study. 5 Total common Pb in analysis based on blank isotopic composition. 6 Model Th/U derived from radiogenic 208Pb and the 207Pb/206Pb age of fraction. 7 Ratio of radiogenic to common Pb 8 Blank and common Pb corrected; blank Pb based on procedural blanks measured throughout the course of the study: amount, 0.4-10 pg; composition, 206Pb/204Pb = 18.5 ± 3%, 207 Pb/204Pb = 15.5 - 15.0 ± 3%, 208Pb/204Pb = 36.4 ± 3%. Common Pb compositions are based on Stacey-Kramers model Pb at the 207Pb/206Pb age of the fraction or the interpreted age of the rock (Stacey and Kramers, 1975). 9 Correlation coefficient. 10 Discordance in % to origin.  1  1.0  CA2  AJW-03-204 Bulagbayan Fm. Augite Basalt  11  A  AJW-03-148 Unmineralized Dacite Sequence welded ignimbrite  Fraction1  Table 2.5 TIMS U-P data for Augite Basalt (chemically-abraded grains) and UDS (air-abraded grains) units. Sample AJW-03-204 is from diamond drill hole OTD318 at 216m (Southwest Oyu; WGS84 UTM 649526 4762742; Figure 2.3). See Table 2.4 for sample AJW-03-148 details.  and euhedral. Nine of 13 grains are interpreted to yield a crystallization age of 367 ± 3 Ma for the ignimbrite. Two rejected grains were interpreted as xenocrystic (418 ± 9 Ma and 390 ± 7 Ma) and two as having undergone lead-loss (356 ± 7 Ma and 362 ± 7 Ma) (Figure 2.10a). Similar results were achieved from air-abraded single-grain TIMS U-Pb analyses (Table 2.5). The data for three out of six grains cluster at 369 ± 1 Ma (concordia age) or 369 ± 2 Ma (weighted mean  207  Pb-corrected  206  Pb/238U age) whereas two grains  that are older than 375 Ma are interpreted as xenocrysts (382 ± 1 Ma and 376 ± 1 Ma), and one grain age was interpreted to have resulted from slight lead-loss (367 ± 1 Ma) (Figure 2.11). Attempts to extract zircons or other mineral chronometers from two 10-15 kg samples of strongly altered fragmental rock from the MFS unit were unsuccessful. Syn-mineral quartz monzodiorite dikes that intrude the basement Augite Basalt sequence have been dated as well and the interpreted crystallization age for the unit is ~372 Ma, although some of the dikes may be as old as 374 ± 3 Ma (Chapter 4). Late mineral granodiorite porphyry dikes cut the Augite Basalt unit, the MDS and UDS sequences as well as the OTHS unit and are interpreted to have a crystallization age of ~366 Ma (Chapter 3; Chapter 4). Carboniferous magmatic activity Fourteen clear, euhedral zircons from a sample of polylithic breccia (PBS unit) yielded a SHRIMP-RG crystallization age of 354 ± 2 Ma. Three spot ages were not included in the calculation; two have younger ages due to lead-loss (341 ± 7 Ma and 342 ± 7 Ma) and one older grain is interpreted as xenocrystic (366 ± 9 Ma; Figure 2.10b). Dating of two samples of green tuffaceous sandstone (AJW-04-354 and AJW-04-245) from the overlying Sedimentary Sequence yielded a bimodal population of zircon spot ages: a dominant group of ~360 Ma spot ages and a subordinate group of ~350 Ma spot ages (Figure 2.12). Most of the zircons for these two age groups are euhedral and it was not possible to differentiate age populations based on morphology or CL images. If the dominant age population of 360 Ma is correct, it suggests the possibility that an older unit (Sedimentary Sequence) overlies a younger unit (PBS; 354 ± 2 Ma), which would imply that there is an unrecognized thrust fault in the district. Alternatively, the younger age is  60  more consistent with the stratigraphic position (Sedimentary Sequence overlying the PBS unit) of units observed in outcrop and drillcore. Seven out of ten small (<50 µm) clear, euhedral zircons from andesite porphyry lava at the base of the LVS mafic volcanic sequence yielded a SHRIMP-RG crystallization age of 346 ± 8 Ma (AJW-04-270; Figure 2.10c). Three analyses were interpreted as lead-loss (318 ± 5 Ma, 264 ± 2 Ma and 245 ± 3 Ma). This age is within the uncertainty of the youngest population of zircons from the tuffaceous sandstone in the underlying Sedimentary Sequence.  A slightly younger age than the Sedimentary  Sequence is consistent with contact relations between the two units. As well, a sample of feldspar porphyritic dacite that occurs in drill core above mafic volcaniclastic rocks from the LVS unit yielded a crystallization age of 347 ± 3 Ma (10 analyses, no rejects; Figure 2.10d), consistent with the contact relationships and also within the uncertainty of the SHRIMP-RG ages. The UVS stratigraphic sequence documented at two locations east of the Oyu Tolgoi porphyry deposits (sections OT-NE and OT-SE) are petrologically similar and both contain pyroclastic rocks with the same 207Pb-corrected 206Pb/238U SHRIMP-RG age (339 ± 2 Ma). Sample AJW-03-221 yielded four xenocrystic zircons (363 ± 4 Ma, 358 ± 4 Ma, 354 ± 3 Ma and 352 ± 3 Ma) that are much older than the weighted mean age of five similar grains. Four younger analyses (329 ± 2 Ma, 326 ± 3 Ma, 326 ± 2 Ma and 305 ± 10 Ma) are interpreted as lead-loss (Figure 2.10e). A second sample, AJW-03-192, yielded a similar age and four spot analyses were rejected; one due to discordance, three interpreted as lead-loss (287 ± 2 Ma, 254 ± 2 Ma and 251 ± 1 Ma) and one as a xenocryst (1634 ± 10; Figure 2.10f). Four Carboniferous dikes from this study yielded U-Pb (zircon) ages. Thirteen spot analyses from the hornblende-biotite andesite porphyry (AJW-03-183) indicate a SHRIMP-RG crystallization age of 345 ± 2 Ma. Four older grains were interpreted as xenocrysts (501 ± 8 Ma, 463 ± 6 Ma, 378 ± 5 Ma and 362 ± 6 Ma) and five younger grains were attributed to lead-loss (283 ± 5 Ma, 252 ± 4 Ma, 252 ± 4 Ma, 249 ± 5 and 241 ± 3 Ma; Figure 2.10g). Eight spot analyses from the rhyolite dike sample (AJW-03-180) yield a SHRIMP-RG U-Pb crystallization age of 340 ± 3 Ma where two grains were interpreted as xenocrysts (375 ± 3 Ma and 355 ± 3 Ma) and three younger spot ages  61  attributed to lead-loss (328 ± 3 Ma, 328 ± 3 Ma and 209 ± 2 Ma; Figure 2.10h). These UPb ages are consistent with cross-cutting relationships of intrusions as well as their host layered rocks. A quartz-feldspar porphyry dike (AJW-03-203) from the dike swarm that is deformed by the ductile shear zone 3 km west of the porphyry deposits yielded a SHRIMP-RG U-Pb crystallization age of 335 ± 1 Ma indicating a maximum age for the shear zone. Two out of 12 grains are interpreted as lead-loss (329 ± 5 Ma and 249 ± 4 Ma; Figure 2.10i). A polydeformed rhyolite sill (AJW-03-193) was sampled from the southeast corner of the district and 21 grains yielded a SHRIMP-RG crystallization age of 330 ± 2 Ma indicating that multiple episodes of shortening post-date this age. Five grains were excluded from the calculation as they are significantly younger than 330 Ma, and interpreted to have resulted from lead-loss (321 ± 5 Ma, 305 ± 5 Ma, 267 ± 5 Ma, 241 ± 4 Ma and 240 ± 4 Ma). Moreover, three grains that were older than 350 Ma were rejected, interpreted to be xenocrystic (783 ± 14 Ma, 459 ± 7 Ma and 366 ± 6 Ma; Figure 2.10j). Nine out of 12 zircons from a sample from the OT North pluton (AJW-03-074) define a weighted mean crystallization age of 350 ± 7 Ma where two zircons were interpreted as xenocrysts (368 ± 7 Ma and 366 ± 8 Ma) and one younger grain interpreted as lead-loss (339 ± 6 Ma; Figure 2.10k). Seventeen of 27 zircons from the Javhalant Mountain pluton (AJW-03-132) yielded a weighted mean age of 324 ± 3 Ma. There is considerable scatter in the data as lead-loss grains (302 ± 2 Ma, 297 ± 2 Ma, 292 ± 2 Ma and 292 ± 1 Ma) and xenocrystic grains (348 ± 2 Ma, 343 ± 4 Ma, 340 ± 1 Ma, 339 ± 4 Ma, 335 ± 2 Ma and 335 ± 2 Ma) are abundant in this zircon population (Figure 2.10l). Finally, 8 out of 10 zircons from the OT18 pluton (AJW-03-116) define a weighted mean crystallization age of 321 ± 4 Ma. One older zircon was interpreted as a xenocryst (337 ± 7 Ma), whereas one younger zircon was interpreted to be the result of lead-loss (243 ± 5 Ma; Figure 2.10m).  62  Figure 2.10 Weighted mean 207Pb-corrected 238U/206Pb age diagrams. Filled black boxes were used in the weighted mean age calculations and grey-filled boxes were excluded.  63  Figure 2.10 continued.  64  Figure 2.11 Concordia plots of two chemically-abraded single-grain TIMS U-Pb analyses for the sample of Bulagbayan Formation basalt (AJW-03-204) and six air-abraded single-grain fractions for the unmineralized welded ignimbrite (UDS unit; AJW-03-148).  65  Figure 2.12 Zircon spot age histogram for tuffaceous sandstone samples from the Sedimentary Sequence (Gurvankharaat Group).  66  Discussion Volcano-sedimentary sequences and depositional environments Devonian strata in the Oyu Tolgoi district record the eruption and intrusion of mafic marine arc volcanic rocks (Chapter 6) followed by the eruption of more silicic subaerial or shallow subaqeous pyroclastic rocks as well as the intrusion and eruption of basalts into sedimentary basins. These lithofacies are overlain by Carboniferous volcanic deposits, fluvial to shallow marine sedimentary facies, and voluminous mafic to intermediate volcaniclastic material intercalated with volumetrically minor but widespread erupted rocks that range from dacite to rhyolite in composition. All of the volcanic activity occurred prior to late shortening that post-dates ~330 Ma dikes and sills. Within this stratigraphic framework, there are time periods of up to ~10 m.y. of nondeposition or erosion as well as the structural juxtaposition of the OTHS unit above the Upper Devonian UDS unit (Figure 2.13). The presence of large Cu-Au porphyry deposits within the sequence suggests that some change in tectonic plate movement such as a slabstall event or an arc-reversal occurred in the Upper Devonian. This event was linked to the transition from subaqueous basalt magmatism to subaerial dacite caldera eruption, thrust-faulting, uplift and erosion; all of which occurred in less than ~5 m.y. Geochronology of the stratigraphic sequence A number of intrusions were dated that constrain the age of the Devonian or older strata (Chapter 3; Chapter 4). The oldest quartz monzodiorite intrusion age (374 ± 3 Ma; Chapter 4) suggests a minimum age for the Bulagbayan Formation basalts and the MFS unit. Moreover, the Bulagbayan Formation Augite Basalt must be younger than 441 ± 1 Ma, the age of the youngest xenocryst detected in the unit; however the vast majority of the zircons (SHRIMP-RG), including grains that are interpreted as xenocrysts are <390 Ma (Upper Devonian or younger) and the onset of marine arc magmatism likely occurred at this time. Lewis (2005) and Ayush (2006) interpret the graded bedding sequences within sedimentary facies, repetition of strata, as well as fault gouge located at the contact between the OTHS and UDS packages as consistent with a thrust-fault-emplaced, 67  overturned fold. If so, shortening occurred soon after the formation of the porphyry deposits due to the similarity in age between the mineralized quartz monzodiorite porphyries (~372 Ma; Chapter 4), and the granodiorite porphyry units (~366 Ma; see Chapter 3 for discussion), which cut the porphyry deposits and the structurally overlying OTHS unit. The PBS unit (354 ± 2 Ma) is ca. 10 m.y. younger than the granodiorite dike that cuts the underlying OTHS (~366 Ma; Chapter 3; Chapter 4). Therefore a significant unconformity is present at the base of the Gurvankharaat Group.  The age of the  Sedimentary Sequence remains uncertain and depends on the interpretation of the tuffaceous sandstone that was sampled for U-Pb dating. The Sedimentary Sequence is likely no younger than 350 Ma, the youngest zircons in the rock, and the unit was deposited immediately following the deposition of the PBS unit. If the rock is closer to 360 Ma, the dominant age of zircons in the tuff, then an intervening thrust fault is required to superpose an older unit on the ~350 Ma PBS sequence. The age of the LVS sequence has not been directly constrained, however the feldspar–porphyritic dacite dikes (347 ± 3 Ma) cut the volcaniclastic unit and juvenile fragments of dacite are present in volcaniclastic facies, suggesting an age for that unit. This age is consistent with both the ~350 Ma age for underlying tuffaceous sandstone from the Sedimentary Sequence as well as that of the andesite flow (346 ± 8 Ma) at the base of the LVS. All sequences are cut by rhyolite dikes with a U-Pb crystallization age of 340 ± 3 Ma. The 347 ± 3 Ma age of the uppermost stratigraphic units documented within the mine sequence suggests that UVS strata (339 ± 2 Ma) that crops out west of the Cu-Au deposits unconformably or disconformably overlies the LVS. The crystallization age of the OT North pluton at 350 ± 4 Ma, the suite of rhyolitic dikes and pyroclastic deposits at ca. 340 Ma to ca. 330 Ma, and that of the Javhalant Mountain and OT18 plutons at ca. 320 Ma indicate that felsic magmatic activity spanned much of the Carboniferous in the South Gobi volcanic arc terrane. Late Devonian volcanic environment The  occurrence  of  coherent-facies  porphyritic  rocks  and  monomictic  volcaniclastic breccias intercalated with laminated rocks suggests that the basement  68  Devonian volcanic environment (Augite Basalt unit; Bulagbayan Formation) was at least partially or intermittently subaqueous.  Greater than 90% of the material logged in  detailed sections is either coherent facies basalt or volcaniclastic basalt with angular fragments, and the remaining material is localized tuffaceous or sedimentary facies, which indicates a vent-proximal environment such as the roots or upper flanks of a marine arc volcano. The lack of distinctive textures such as pillows precludes a definitive subaqueous volcanic facies interpretation; however, oceanic arc environments are characterized by long-term deep water depths and only locally does volcanic construction form islands and shallow water depositional environments. Typically, debris aprons composed of turbidites and debris flow deposits occur in close proximity to arc volcanoes and form a relatively continuous record of mass wasted volcaniclastic rocks (Draut and Clift, 2006). The presence of interbedded laminated sedimentary facies suggests the possibility that some of the material is related to turbidites as these sedimentary rocks are also present in Devonian rocks elsewhere in the South Gobi (e.g. Lamb and Badarch, 1997; Lamb and Badarch, 2001). Wholerock geochemical and Nd isotopic compositions from the basement Augite Basalt sequence indicate that the mafic volcanic unit is tholeiitic, depleted in high field-strength elements and derived from depleted mantle in a juvenile marine arc setting (Chapter 6), consistent with the interpretation of volcanic and volcaniclastic facies logged in drill core. Conversely, the OTHS allochthonous basalt suite has high Nb concentrations, more similar to a within-plate geochemical signature and its origin and position is more poorly constrained (Chapter 6) due to the structural juxtaposition of the unit by a thrust fault. Welding textures in volcanic deposits from the Devonian MFS and UDS units suggest a subaerial or possibly shallow subaqueous depositional environment for both lithofacies. Welding textures, associated with hot emplacement temperatures above the  69  70  Figure 2.13 Composite summary section of Paleozoic rocks in the Oyu Tolgoi porphyry Cu-Au district. MFS- Mineralized Fragmental Sequence; UDSUnmineralized Dacite Sequence; OTHS- Oyu Tolgoi Hanging Wall Sequence; LVS- Lower Volcaniclastic Sequence; UVS – Upper Volcaniclastic Sequence.  glass transition phase boundary, are typically associated with calderas in modern volcanic environments (e.g. Cas and Wright, 1987). There is no evidence for caldera centers in the region due to inherent problems associated with a lack of outcrop and working with drill core; however, vent-proximal volcaniclastic facies have been detected in drill core within the MFS and UDS units as suggested by the presence of amoeboid-shaped juvenile clasts and by the presence of extremely large porphyritc dacite blocks (>2 m) or coherent dacite porphyry that may be lava flows or shallow intrusions. The generally small (lapilli- to block-sized) maximum size of clasts, limited development of megabreccia facies and relative thinness (<20 m) of individual welded ignimbrite units are features of resembling outflow (extracaldera) ignimbrite sheets. Rapid vertical and lateral facies changes and presence of multiple cooling units suggest that there was a dynamic volcanic environment with multiple eruptions. The Upper Devonian fragmental sequences are structurally overlain by mafic sequences with intercalated sedimentary facies. Lobate-clast breccias and irregular curviplanar margins at the contacts between basalt and sedimentary rocks are consistent with interaction between unlithified, water-saturated sediment and shallow intrusions or flows (peperite). This suggests that the activity was subaqueous, possibly in a shallow marine or lacustrine basin environment. The succession of stratigraphic sequences suggests that the Devonian environment in the South Gobi terrane consisted of a submarine arc that became emergent during the Upper Devonian.  Initial basement architecture consisted of subaqueous basaltic  stratovolcanoes with localized development of sedimentary and volcaniclastic deposits (Figure 2.14a). Pyroclastic eruptions of relatively silicic volcanoes (possibly dacitic calderas) followed once sufficient basement had been constructed in the oceanic island arc to develop a shallow subaqueous or subaerial depositional setting (Figure 2.14b). A complex volcanic terrane was built upon the eroded porphyry district involving multiple dacite eruptions, mass wasting of volcanoes, and intercalated fluvial environments (Figure 2.14c). These rocks are overlain by a thrust-fault emplaced (Lewis, 2005; Ayush, 2006) allochthonous volcano-sedimentary sequence that consisted of mafic volcanic edifices that erupted into a shallow marine basin or a series of lakes (Figure 2.14d) of uncertain relative age and original position. A possible modern analog for the Devonian  71  Figure 2.14 Schematic sequence of volcanic and sub-volcanic events in the Devonian (or prior). A) Island arc mafic volcanic sequence in the Devonian (or older); B) Deposition of fragmental sequence in the Devonian or earlier, intrusion of monzonite porphyries and formation of porphyry Cu-Au deposits in the Upper Devonian; C) Eruption of dacite calderas in the Upper Devonian following an erosion period; D) Emplacement of an allochthonous sequence of peperitic basalts and intercalated sedimentary rocks prior to the end of the Devonian.  72  volcanic environment at Oyu Tolgoi may be the Philippine archipelago, which contains mafic/felsic igneous complexes, large calderas, adakite-like rocks and porphyry coppergold deposits, as well as being proximal to relatively ancient continental basement. In the Philippines, Miocene to modern igneous activity is associated with porphyry Cu-Au and epithermal Au mineralization throughout the Central Luzon Cordillera (Sillitoe and Gappe, 1984). Moreover, Sajona and Maury (1998) have highlighted the temporal and spatial association of Philippine adakites (see Chapter 4 for discussion of adakite-like rocks at Oyu Tolgoi) with porphyry and epithermal deposits in Luzon, Mindanao and Negros. Carboniferous volcanic environment The PBS unit (354 ± 2 Ma) is ca. 10 m.y. younger than the granodiorite dike that cuts the underlying OTHS (~366 Ma; Chapter 4). Therefore a significant unconformity is present at the base of the Gurvankharaat Group. The PBS unit is not always present and is of variable thickness, suggesting that the basement morphology at the time of eruption could have been uneven, consistent with an erosional surface at the unconformity. Alternatively, unrecognized erosion of parts of the tuff may have resulted in an intraformational disconformity.  The massive polylithic breccia is heterolithic with  angular clasts and generally unwelded. The angular clasts suggest that the rock is an immature sedimentary or volcaniclastic deposit, however, some textures are present such as the armoured lapilli and apparent welding textures, thus locally, the breccia appears to be volcanic in origin. Following deposition of the PBS unit, the sequence was drowned by a series of shallow marine or lacustrine deposits and intercalated fluvial sediments. Overall fine grain-size suggest low-energy, suspension-dominated sedimentation conditions and the absence of abundant arkosic deposits suggests that the sedimentary sequences are relatively mature and not derived from proximal feldspar-bearing basement material. The lack of penetrative cleavage and chaotic nature of the folds suggests that fold structures may be soft-sediment deformation features, not related to regional shortening. Moreover, the presence of pyroclastic features such as angular pumice fragments support a primary,  73  syn-volcanic origin for the sample of tuffaceous sandstone located toward the top of the sedimentary sequence. Peperitic contacts between basalt and adjacent fine-grained matrix material in the LVS unit suggests magma injection into recently deposited wet sediment.  The  voluminous volcaniclastic deposits are characterized by rounded to sub-angular clasts, lack of stratification, aphanitic to porphyritic basalt/andesite clast textures, and matrix material that is muddy to feldspar-phyric. These characteristics are consistent with debris flow facies volcaniclastic conglomerates that are usually strongly linked to the immediate environs of arc volcanoes. Deposition in these environments is interpreted to have occurred by primary volcanic processes and gravity-driven mass-wasting.  Highly  irregular-shaped clasts and spindles of dacite amongst relatively mature lapilli to blocksized mafic clasts suggest that dacite eruptive activity was active in epiclastic depositional environments in the volcaniclastic aprons of mafic to intermediate eruptive centers. Thick intersections of dacite on the west side of the Hugo Dummett deposit suggest the possibility of a dome complex that might be the source of juvenile clasts. A fine-grained dacite flow with a carapace breccia seen in drill core in the northeast corner of the concession suggests the possible location of another vent. Other than these known locations, dacite occurs as dikes and sills that cut mafic volcaniclastic debris flow deposits. The Upper Volcaniclastic Sequence in the northeast and southeast corners of the Oyu Tolgoi area consists of volcaniclastic or coherent facies andesite overlain by ignimbrite, fine-grained clastic sedimentary rocks, and coherent facies andesite. These likely represent aerially extensive subaerial to shallow subaqueous felsic ignimbrites, volcaniclastic deposits and flows intercalated with lacustrine basins. The Carboniferous paleogeography in the South Gobi terrane consisted of an irregular erosional surface, onto which a fall deposit and immature sediments were erupted and deposited (Figure 2.15a), and subsequently drowned by shallow marine or lacustrine sedimentary deposits (Figure 2.15b). Mafic to intermediate volcanoes were constructed on this substrate, the subvolcanic roots of which intruded into subaqueous muddy environments. The volcaniclastic aprons of these edifices are preserved as debris flow deposits which were intruded by localized dacitic eruptive centers at 347 ± 3 Ma (Figure 2.15c). Following an erosion period, a similar sequence of mafic to intermediate  74  Figure 2.15 Schematic sequence of volcanic and sub-volcanic events in the Carboniferous. A) and B) Immature sediment deposition and pyroclastic eruptions followed by fluvial to shallow marine sedimentary sequences in the Lower Carboniferous; C) Mafic to intermediate volcanic and volcaniclastic sequence dominated by mass wasting products in volcaniclastic apron; D) Mafic to intermediate volcanic rocks with minor intercalated ignimbrite and clastic sedimentary rocks are intruded by basalt and rhyolite dikes.  75  volcanic and volcaniclastic deposits intercalated with felsic subvolcanic and volcanic rocks as well as clastic sedimentary facies was deposited (Figure 2.14d). All of these units underwent at least two periods of shortening and were intruded by basalt dikes. The Gurvankharat Group mafic to intermediate volcanic rocks are of dominantly calc-alkaline composition which is interpreted to reflect arc maturation and thickening upon the Devonian or older tholeitiic Augite Basalt unit substrate (Chapter 6). Pre-Devonian activity and onset of oceanic arc magmatism The presence of Proterozoic to Silurian zircon xenocrysts in the Devonian (or older) to Carboniferous rock samples requires that relatively ancient continental material and/or ancient arcs that mantled the cratons were proximal to the outboard oceanic arc. The pre-Devonian zircons may have been sourced from arcs that framed a combined Baltica-Siberia craton or possibly Gondwana fragments as in the models of Sengor et al. (1993) and Mossakovsky et al. (1994), respectively. In the Oyu Tolgoi district, certain horizons within the Bulagbayan Formation may have been the locus of sedimentation for which some material (including zircons) was derived from the ancient crust, possibly incorporated into turbidite sequences. The Sengor et al. (1993) (see also Senor and Natal’in, 1996) model proposes that in the Middle to Late Devonian (390 Ma) a marine arc lay outboard and east of a Middle Cambrian-Silurian (510-420 Ma) accretionary complex, in the vicinity of a VendianCambrian (610-530 Ma) accretionary complex. All of these units were butted against an elongate belt of rifted continental crust derived from the Angara (Baltica) Craton (summarized in Figure 2.1c).  Conversely, the fragments of cratons proposed by  Mossakovsky et al. (1994) are from East Gondwana in addition to Siberia and/or Baltica (summarized in Figure 2.1d). Cambrian to Silurian xenocrysts present in the Oyu Tolgoi dataset could be consistent with both the Sengor et al. (1993) and Mossakovsky et al. (1994) models (i.e. derived from the accreted arcs that mantled the Siberian craton or from Gondwana, respectively).  However, there are no xenocrystic zircons that are  Paleoproterozoic-Archean as might be expected in the environment proposed by Sengor. Conversely the xenocrysts that we have detected may be consistent with some magmatic episodes in the younger Gondwanan supercontinent. A prominent peak in Gondwanan  76  detrital zircons at 450-500 Ma was determined by Chew et al. (2007), which may be consistent with many of the Oyu Tolgoi xenocystic zircon ages. The lack of significant evidence (Archean zircon xenocrysts) does not preclude the Sengor et al., (1993) model; however a model is tentatively supported whereby the xenocrysts were derived from rifted younger fragments of Gondwana. Thus, the modern Southwest Pacific archipelago may be a better analog for the South Gobi region, rather than the single giant arc complex that should have incorporated ancient xenocrysts from a combined Baltica-Siberia craton. The Philippine island arc system is a composite terrane that is made up of two major blocks: the Palawan microcontinental block and the Philippine Mobile Belt (e.g. Gervasio 1971; McCabe et al. 1985).  The Palawan microcontinental block was rifted from  Mainland Asia (e.g. Holloway 1982) and this scenario may be similar to that in the the CAOB whereby Paleozoic mobile belts incorporated the rifted craton fragments of Gondwana into the orogenic collage. Conclusions The variation in stratigraphic sequences suggest that the Oyu Tolgoi district in Mongolia is underlain by a submarine arc that became emergent during the Upper Devonian. These sequences were subsequently drowned by shallow marine sediments and buried by the eruption and mass wasting products of subsequent volcanoes throughout much of the Carboniferous. U-Pb (zircon) geochronology records magmatic activity from ~390 Ma to ~320 Ma, and provides evidence for at least three unconformities within the Paleozoic sequence. The onset of oceanic arc magmatism occurred at ~390 Ma and xenocrystic zircon populations suggest that craton fragments from East Gondwana were present in the vicinity of the arc. 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Stacey, J.S. and Kramer, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two-stage model, Earth and Planetary Science Letters, v. 26, p. 207-221. Thirlwall, M F., 2000, Inter-laboratory and other errors in Pb isotope analyses investigated using a 207Pb-204Pb double spike, Chemical Geology, v. 163, no. 1-4, p. 299-322. Watanabe, Y. and Stein, H.J., 2000, Re-Os ages for the Erdenet and Tsagaan Suvarga porphyry Cu-Mo deposits, Mongolia, and tectonic implications, Economic Geology, v. 95, no. 7, p. 1537-1542. Williams, I.S., 1996, U-Th-Pb geochronology by ion microprobe, Reviews in Economic Geology, v. 7, p. 1–36. Yakubchuk, A., 2004, Architecture and mineral deposit settings of the Altaid orogenic collage: a revised model, Journal of Asian Earth Sciences, v. 23, p. 761-779. Zonenshain, L.P., Kuzmin, M.I. and Natapov, L.M., 1990, Geology of the USSR; a platetectonic synthesis, Geodynamics Series, v. 21, 242 pages.  81  Chapter 3 - Upper Devonian event sequence and paleogeography; Oyu Tolgoi porphyry Cu-Au district, South Mongolia2 Introduction Fossil magmatic-hydrothermal centers in the Oyu Tolgoi district (Mongolia) host one of the largest repositories of sulfur, copper and gold found to date on Earth. The district measured and indicated resources include 1.15 Gt grading 1.27% copper and 0.48 g/t gold and there is an additional inferred resource of 1.44 Gt grading 1.11% copper and 0.28 g/t gold (0.60% copper equivalent cut-off grade) (Ivanhoe Mines press release, 2006).  More recently, additional porphyry mineralization has been discovered  approximately 5 kilometers south-southwest of the Oyu Tolgoi porphyries, within the same mineralized corridor. The Heruga Deposit is estimated to contain an inferred resource of 760 Mt grading 0.48% copper, 0.55 g/t gold and 142 ppm molybdenum (0.60% copper equivalent cut-off grade) (Ivanhoe Mines press release, 2008). There are two other significant porphyry deposits in Mongolia that have recently been in production; Erdenet, located in northern Mongolia (1700 Mt at 0.54% Cu, 016% Mo and 0.01 ppm Au) and Tsagaan Suvarga, located 150 km northeast of Oyu Tolgoi (240 Mt @ 0.53% Cu, 0.018% Mo) (Sillitoe et al., 1996). Prior to the discovery of Oyu Tolgoi, Tsagaan Suvaarga was the only porphyry deposit known in the South Gobi region of Mongolia (Perello et al., 2001). The tectonic and paleogeographic setting of a porphyry deposit is difficult to constrain in ancient terranes as much of the information is lost, if the porphyry is preserved at all, due to erosion and deformation in the orogenic belts that host the deposits.  Uplift rates in modern island arc settings can be relatively rapid (0.5-3  mm/year; Garwin et al., 2005) which suggests that many porphyry deposits would likely be eroded shortly following their formation. Therefore it is unusual that Cu-Au deposits are preserved in a relatively ancient (Paleozoic) orogenic belt at Oyu Tolgoi. In addition to contributing to exploration by defining the complex volcanic and structural 2  A version of this chapter will be submitted to a refereed journal for publication: Wainwright, A.J., Lewis, P.D., Tosdal, R.M. and Friedman, R.M., in prep., Upper Devonian event sequence and paleogeography; Oyu Tolgoi porphyry Cu-Au district, South Mongolia.  82  architecture of the region, detailed study of the Devonian stratigraphy provides insight into the tectonomagmatic and paleogeographic environment of this ancient volcanic arc at the time of porphyry deposit formation.  In Chapters 2 and 4, U-Pb (zircon)  geochronology is used to constrain the Paleozoic geology and magmatic evolution of the district. In this chapter, additional U-Pb (zircon) as well as Pb isotopic constraints are provided which focus on the stratigraphic interval that hosts and immediately overlies the porphyry Cu-Au deposits. The detailed volcano-sedimentary succession and intrusions are examined in order to determine the sequence of geologic events in the Upper Devonian, constrain the age of porphyry Cu-Au mineralization and by inference, the paleogeographic environment in the Oyu Tolgoi district. Geologic setting The Oyu Tolgoi porphyry district is located in the Central Asian Orogenic Belt (CAOB); a tectonic supercollage that contains craton fragments, Proterozoic to Paleozoic ophiolites, and accreted volcanic arcs (Badarch et al., 2002; Buchan et al., 2002). The Cu-Au porphyry district is underlain by a sequence of Carboniferous and Devonian rocks of the Gurvansayhan terrane (Badarch et al., 2002; Helo et al., 2006), interpreted to represent a juvenile oceanic island arc (Helo et al., 2006; Chapter 6). The copper-gold deposits are hosted in an inlier of Late Devonian intrusions and Devonian or older volcanic, volcaniclastic and sedimentary rocks of the Alagbayan Group surrounded by disconformably overlying Carboniferous volcanic, volcaniclastic and sedimentary rocks of the Gurvankharaat Group (Chapter 2; Figure 3.1). The porphyry deposits are divided into two zones: Southern Oyu (South Oyu, Southwest Oyu and Central Oyu) and the Hugo Dummett zone. In the Hugo Dummett zone, Cu-Au mineralization is hosted by Bulagbayan Formation Augite Basalt sequences, Mineralized Fragmental Sequence (MFS) rocks, and Late Devonian intrusions. These packages are unconformably overlain by rocks from the Unmineralized Dacite Sequence (UDS) and have been overthrust by the allochthonous Oyu Tolgoi Hanging Wall Sequence (OTHS) (Chapter 2; Figure 3.2). The more deeply eroded Southern Oyu deposits are hosted entirely in the Augite Basalt unit (Bulagbayan Formation) and Late Devonian porphyry intrusive phases (Chapter 2). Wholerock petrochemical as well as Nd isotopic data indicate that the Augite Basalt unit 83  is a primitive marine arc tholeiitic sequence, whereas the overlying Carboniferous sequences are calc-alkaline to alkaline and consistent with a thickened arc (Chapter 6). The copper-gold deposits are spatially, temporally and genetically related to a sequence of Late Devonian intrusive phases that define the Oyu Tolgoi Igneous Complex (OTIC). These intrusive rocks include high-K calc-alkaline quartz monzodiorites (QMDE and QMD-P1 phases) and medium- to high-K calc-alkaline granodiorites (GDi-P2 and GDi-P3 phases) with U-Pb (zircon) crystallization ages of ~372 Ma and ~366 Ma, repectively (Chapter 4). The quartz monzodiorite phases intrude Augite Basalt unit rocks as well as MFS the unit, however, they have never been observed to cut rocks from the UDS unit, nor the OTHS sequence. Conversely, granodiorite intrusions cut all four of these sequences (Chapter 2; see below). Devonian volcanic arc sequences The Augite Basalt unit (Bulagbayan Formation) consists of coherent facies finegrained to augite porphyritic basalts, fragmental facies fine-grained to porphyritic basalts, and fine-grained laminated volcaniclastic rocks and sedimentary rocks.  The basalts  typically contain unbroken augite phenocrysts up to 8 mm. Fragmental facies contain fine-grained brown to dark green angular lapilli-sized clasts, augite-porphyritic fragments, as well as vesiculated fine-grained fragments. In the Hugo Dummett zone, the northernmost and largest of the porphyry deposits (Figure 3.1), a vertically- and laterally-variable sequence of Devonian volcaniclastic rocks overlies the Bulagbayan Formation basalts. This sequence is divided into a lower strongly altered Mineralized Fragmental Sequence (MFS) and an upper mostly unaltered Umineralized Dacite Sequence (UDS). The UDS unit contains locally strong pyrite as well as rare chalcopyrite veins. The UDS volcanic succession is structurally overlain by a laterally discontinuous sequence of largely vesiculated, fragmental and coherent, basalts and andesites interstratified with sedimentary rocks named the Oyu Tolgoi Hanging Wall Sequence (OTHS). Contacts between the volcanic and sedimentary components locally exhibit peperitic textures (Chapter 2). Primary sedimentary structures indicate that much of the OTHS is overturned, and facing direction reversals and bedding markers imply that major overturned folds are present. The overturned position of the sequence, together 84  with the bedding-parallel fault at its base, indicate that this unit is allochthonous and was thrust-superposed onto the underlying UDS unit (Lewis, 2005; Ayush, 2006). Within the MFS sequence, intense alteration as well as abundant quartz veins and sulfides have largely destroyed primary textural features.  This alteration mineral  assemblage includes quartz, pyrophyllite, kaolinite, dickite veins, with lesser K-alunite, Al-phosphate-sulfate minerals, zunyite, diaspore, topaz, corundum, and andalusite (Khashgerel et al., 2006).  Despite the alteration, fragmental textures that resemble  welding fabrics, angular, lapilli-sized clasts and amoeboid-shaped clasts are observed in some drill holes. These likely represent bonafide primary volcanic clasts, but in some instances the textures may be pseudo-fragmental resulting from irregular alteration of a non-fragmental protolith. Limited immobile trace element data in wholerock samples from the altered rocks suggests that the protolith may be dacitic (Kavalieris, I., pers. comm., 2007). Primary textures and lithologic variation in the overlying UDS stratigraphic sequence are readily identifiable, as the rocks are not strongly altered. These relatively well-preserved primary textures define a complex volcanic sequence comprising ignimbrites with multiple cooling units, ash to block and ash tuffs, and volumetrically minor intervals of polymictic conglomerates, breccias and fine-grained sedimentary intervals (Chapter 2). The base of the UDS is probably an unconformity (see discussion below) as it represents a shift in volcanic style and composition, and also is commonly marked by a typical, unconformity-base fragmental unit. Typically, strongly altered rocks with either relict fragmental textures or with no recognizable protolith are located directly beneath an abrupt contact with unmineralized rock types that comprise the base of the UDS sequence. The base of the unmineralized succession varies, and can include fresh dacite tuffs as well as polymictic conglomerates and breccias with mineralized clasts (Figure 3.3). The polymictic conglomerates and breccias in the UDS package are poorly- to moderately-sorted, clast- to matrix-supported, and contain abundant aphanitic to porphyritic dacite fragments. The mineralized clasts occur as lapilli-sized angular fragments within dacite tuff, larger rounded clasts within conglomerate, or as angular lapilli- to block-sized clasts within polymictic breccias. Mineralized clasts are most common near the base of the UDS sequence and decrease in  85  Figure 3.1 Bedrock geology map of the Oyu Tolgoi district, Mongolia, drawn by Ivanhoe Mines geology staff and AJW from mapping of drill holes and trenches through younger cover sequences, as well as outcrop mapping. Inset map shows the location of Oyu Tolgoi in the southern Gobi desert. Detailed core logs for the drill holes are presented in Fig. 3.3. Coordinates are UTM zone 48, northern hemisphere (WGS84). Grade shell outlines correspond to a cut-off grade of 0.3% copper equivalent. In the Hugo Dummett zone, the grade shell is a projection to surface.  86  87  Figure 3.2 Generalized stratigraphic sequence of Upper Devonian rocks from the Oyu Tolgoi district. MFS: Mineralized Fragmental Sequence. UDS: Unmineralized Dacite Sequence. Diagram not to scale.  88  Figure 3.3 Detailed stratigraphic sections for the contact between mineralized and unmineralized rocks. See Figure 3.1 for diamond drill hole locations. UDS: Unmineralized Dacite Sequence. MFS: Mineralized Fragmental Sequence.  abundance upwards (Figures 3.4a and 3.4b). Typically, the clasts are composed of quartz with disseminated pyrite, however, less commonly, clasts of altered, mineralized feldspar porphyry texturally similar to the QMD-P1 phase (Chapter 4) occur. In one drillhole, a clast of silicified, chlorite-sericite-altered QMD-P1 porphyry contains a quartz vein terminating against enclosing fresh dacite pyroclastic tuff (Figure 3.4a). Quartz veins that cut both matrix and clasts have never been observed in the UDS unit. Contacts observed in drill core between mineralized fragmental or intrusive rocks and overlying unaltered and unmineralized rocks are abrupt and can occur within centimeters (Figure 3.4c). The contacts are locally obscured by several centimeters of black, fine-grained gouge, calcite vein material or clay. Sulfides below the contact with unaltered rocks are unoxidized and alteration assemblages as determined by PIMA spectrometry within 10 meters below the contact are characterized by pyrophyllite, kaolinite and dickite. Within 10 meters above the contact, the alteration mineral assemblage is typically characterized by chlorite, illite and montmorillonite. Locally, advanced argillic mineral assemblages occur in the UDS package, stratigraphically above the strongly mineralized rocks (Kavalieris, I., pers. comm., 2006). Field relations - Late Devonian intrusive rocks Cross-cutting relationships and U-Pb (zircon) geochronology (SHRIMP-RG and TIMS) indicate that the OTIC intrusions in the district are separated into two chronologic groups:  Quartz monzodiorites (QMD) ~372 Ma and granodiorites (GDi) ~366 Ma  (Chapter 4). Late Devonian quartz monzodiorite intrusions intrude Augite Basalt from the Bulagbayan Formation and in one case, strongly altered feldspar porphyry was observed with a chilled lower margin adjacent to strongly altered volcaniclastic breccia with lapilli-sized fragments (MFS unit). The QMD intrusions, altered or fresh, have never been observed to cut rocks from the UDS or OTHS units. Late Devonian GDi-P3 intrusions intrude layered rocks from both the MFS and UDS units as well as the OTHS (Figure 3.4d), in addition to truncating strongly altered and mineralized QMD porphyries. Granodiorite dikes (GDi- P2 and GDi-P3) are locally sericite-altered with disseminated pyrite and more rarely, they are cut by chalcopyrite veinlets.  89  Figure 3.4 Polished slab photographs showing the textural variations in Upper Devonian volcanic, volcaniclastic and intrusive rocks. A) Clast of quartz monzodiorite porphyry (QMD-P1; sample AJW-06488) in dacite tuff; B) Typical matrix-supported polymictic breccia with abundant altered porphyry clasts; C) Abrupt contact between strongly altered/mineralized feldspar porphyry and unmineralized dacite tuff; D) Intrusive contact between a granodiorite dike (GDi-P3; sample AJW-04-250) and basalt breccia from the OTHS unit.  90  Figure 3.4 (continued)  91  U-Pb geochronology Isotopic ages from a number of Devonian rocks in the Oyu Tolgoi district place constraints on the sequence of stratigraphic, structural, and magmatic events (Table 3.1). Two new ages are presented in this chapter that add further geologic constraints and the relationships between the Devonian (or older) rock units are summarized in Figure 3.7. Uranium-lead (zircon) geochronology indicates that the age of unmineralized dacite tuff (UDS unit) in the Hugo Dummett zone is 369 ± 1 Ma (Chapter 2). Khashgerel et al. (2006) reported that mineralized and unmineralized QMD porphyry intrusions from Southwest Oyu yielded U-Pb ages (SHRIMP; zircon) of 378 ± 3 and 371 ± 3 Ma, respectively. SHRIMP-RG U-Pb (zircon) ages from Chapter 4 include 374 ± 3 Ma for a mineralized QMD-E intrusion in South Oyu, 369 ± 2 Ma for QMD-P1 from the Hugo Dummett zone, and TIMS U-Pb ages of 372 ± 1 Ma and 373 ± 1 Ma for mineralized QMD-P1 porphyries for Central and Southwest Oyu, respectively. GDi-P3 granodiorite porphyry intrusions yielded SHRIMP-RG U-Pb ages of 367 ± 2 Ma, 366 ± 4 Ma and 363 ± 4 Ma (Chapter 4). Re-Os ages for molybdenite samples of 372 ± 1.2 Ma and 373 ± 1.2 Ma were reported in Khashgerel et al., 2006 from Southwest Oyu and Central Oyu, respectively (Table 3.1). Analytical method Analytical work was perfomed at the Pacific Centre for Isotopic and Geochemical Research at The University of British Columbia. All grains were hand picked under magnification in alcohol and pre-treated using air abrasion or chemical abrasion techniques. After air abrasion (Krogh, 1982) grains are washed and ultrasonicated in 3N HNO3 to remove fine dust from pyrite abrasion media, rinsed with acetone followed by ethanol, and transferred to 10 mL pyrex beakers, ready for final washing prior to dissolution. The chemical abrasion technique (CA-TIMS) employed is modified from procedures outlined in Mundil et al. (2004) and Mattinson (2005), and the PCIGR CATIMS technique is described in detail in Scoates and Friedman (2008). Isotopic ratios are measured with a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Uranium 92  fractionation is determined directly on individual runs using a  233-235  U tracer, and Pb  isotopic ratios are corrected for fractionation (0.23-0.35%/amu, throughout the course of this study) based on replicate analyses of the NBS 982 Pb reference material and the values recommended by Thirlwall (2000). Reported precisions for Pb/U and Pb/Pb dates were determined by numerically propagating all analytical uncertainties through the entire age calculation using the technique of Roddick (1987).  Standard concordia  diagrams were constructed and concordia/weighted mean ages were calculated with Isoplot 3.00 (Ludwig, 2003). Unless otherwise noted, all errors are quoted at the 2σ level. Results Two rocks were sampled from drill core to further constrain the event sequence in the Upper Devonian.  Sample AJW-04-250 is a sericite-altered, weakly pyritic  granodiorite porphyry (GDi-P3) that cuts the OTHS unit. The contact between the intrusion and basalt breccia in the OTHS is characterized by a fine-grained, irregular chilled margin (Figure 3.4d).  Four single-grain air-abraded fractions yielded U-Pb  (TIMS) crystallization ages of 373 ± 1 Ma, 368 ± 1 Ma, 365 ± 1 Ma and 363 ± 1 Ma and seven chemically-abraded single-grain fractions yielded U-Pb (TIMS) ages between 370 ± 1 Ma and 373 ± 1 Ma (Table 3.2; Figure 3.5). A second sample, AJW-06-488 is a clast of sericite-altered, silicified, pyritic quartz monzodiorite (Figure 3.4a), texturally-similar to the QMD-P1 phase (Chapter 4). The clast was sampled from a fresh unaltered dacite volcaniclastic interval in the UDS unit and two chemically-abraded single grain fractions yielded concordant TIMS U-Pb crystallization ages of 374 ± 2 Ma and 371 ± 2 Ma, respectively (Table 3.2; Figure 3.5).  93  94  rock  QMD – Southwest Oyu (mineralized) QMD – Southwest Oyu (unmineralized) QMD-E – South Oyu (mineralized) QMD-P1 – Central Oyu (mineralized) QMD-P1 – Southwest Oyu (mineralized) Molybdenite – Southwest Oyu Molybdenite – Central Oyu QMD-P1 – Hugo Dummett Unmineralized welded tuff GDi-P3 – Hugo Dummett GDi-P3 – Hugo Dummett GDi-P3 – Southwest Oyu GDi-P3 – Hugo Dummett QMD-P1 – Hugo Dummett  Sample  n/a n/a AJW-03-182 AJW-03-181 AJW-03-178 n/a n/a AJW-04-356 AJW-03-148 AJW-03-184 AJW-03-185 AJW-03-179 AJW-04-250 AJW-06-488  378 ± 3 Ma 371 ± 3 Ma 374 ± 3 Ma 372 ± 1 Ma 373 ± 1 Ma 372 ± 1.2 Ma 373 ± 1.2 Ma 369 ± 2 Ma 369 ± 1 Ma 367 ± 2 Ma 366 ± 4 Ma 363 ± 4 Ma See text See text  age SHRIMP-RG SHRIMP-RG SHRIMP-RG TIMS TIMS Re-Os Re-Os SHRIMP-RG TIMS SHRIMP-RG SHRIMP-RG SHRIMP-RG TIMS TIMS  method Khashgerel et al., 2006 Khashgerel et al., 2006 Chapter 4 Chapter 4 Chapter 4 Khashgerel et al., 2006 Khashgerel et al., 2006 Chapter 4 Chapter 2 Chapter 4 Chapter 4 Chapter 4 This chapter This chapter  reference  Table 3.1 Summary of Upper Devonian isotopic ages for layered rocks and intrusions within the Oyu Tolgoi porphyry Cu-Au district.  Figure 3.5 Concordia diagrams for TIMS U-Pb results for Upper Devonian rocks. Sample AJW-04-250 includes chemically-abraded single-grain fractions (unfilled error ellipses) as well as air-abraded singlegrain fractions (grey ellipses). Sample AJW-06-488 includes chemically-abraded fractions only. See text for interpretation of the data.  95  96  U2  (ppm)  Wt  (µg)  14  59  26  9.6  10.8  5.8  12.5  CA1  CA2  CA4  CA5  CA6  CA7  CA8  85  88  118  122  69  48  175  130  123  289  129  5.1  5.5  7.1  7.3  4.2  2.9  10.5  7.9  7.2  19.0  Pb  204  2179  1112  5387  4230  2628  4030  3933  0.9  CA4  168  402 10.2  25.8 776  1045 0.7  0.9  1.8  1.7  0.9  1.0  2.5  2.6  2.3  6.1  6.8  3.3  1.9  (pg)  Pb5  0.41  0.62  0.38  0.55  0.37  0.38  0.42  0.36  0.36  0.39  0.33  0.81  0.41  Th/U6  12.6  17.8  35.2  18.6  87.5  69.4  43.2  64.8  63.5  6.4  5.3  22.6  24.1  Pbc  Pb*7 Pb/ U  238  0.05931 ± 0.22  0.05973 ± 0.26  0.05927 ± 0.14  0.05948 ± 0.11  0.05958 ± 0.12  0.05913 ± 0.10  0.05921 ± 0.15  0.05950 ± 0.12  0.05932 ± 0.12  0.05954 ± 0.13  0.05878 ± 0.20  0.05830 ± 0.13  Pb/ U  235  0.4425 ± 2.49  0.4471 ± 1.09  0.4414 ± 0.37  0.4402 ± 0.53  0.4421 ± 0.27  0.4415 ± 0.25  0.4395 ± 0.34  0.4432 ± 0.37  0.4436 ± 0.24  0.4444 ± 0.69  0.4359 ± 0.96  0.4346 ± 0.43  0.4310 ± 0.51  207  207  Pb/ Pb  206  0.05411 ± 2.34  0.05429 ± 1.00  0.05401 ± 0.35  0.05367 ± 0.50  0.05383 ± 0.23  0.05416 ± 0.22  0.05384 ± 0.28  0.05403 ± 0.33  0.05423 ± 0.20  0.05414 ± 0.63  0.05378 ± 0.87  0.05406 ± 0.38  0.05391 ± 0.47  Isotopic ratios ±1σ,% 8  0.05799 ± 0.11  206  0.71857  0.46280  0.31544  0.42892  0.51535  0.48067  0.56910  0.46228  0.57191  0.54982  0.53379  0.50491  0.48237  ρ9  1.2  2.4  0.1  -4.4  -2.6  2.0  -1.9  0.0  2.5  1.0  -1.8  2.3  1.1  discordant  % 10 Pb/ U  238  371.4 ± 1.6  374.0 ± 1.9  371.2 ± 1.0  372.5 ± 0.8  373.0 ± 0.9  370.3 ± 0.7  370.8 ± 1.1  372.6 ± 0.9  371.5 ± 0.9  372.8 ± 0.9  368.2 ± 1.4  365.3 ± 0.9  363.4 ± 0.8  206  Pb/235U  372.0 ± 15.5  375.3 ± 6.8  371.2 ± 2.3  370.4 ± 3.3  371.8 ± 1.7  371.3 ± 1.5  369.9 ± 2.1  372.5 ± 2.3  372.8 ± 1.5  373.4 ± 4.3  367.3 ± 5.9  366.4 ± 2.6  363.9 ± 3.1  207  Pb/206Pb  375.7 ± 102/108.9  383.1 ± 44.2/45.5  371.6 ± 15.8/16  357.3 ± 22.2/22.5  363.7 ± 10.4/10.4  377.7 ± 9.7/9.7  364.2 ± 12.7/12.8  372.4 ± 14.7/14.8  380.8 ± 8.8/8.9  376.7 ± 28/28.5  362.0 ± 38.7/39.7  373.7 ± 16.9/17.1  367.4 ± 20.9/21.1  207  Apparent ages ±2σ, Ma 8  6  Total common Pb in analysis based on blank isotopic composition. Model Th/U derived from radiogenic 208Pb and the 207Pb/206Pb age of fraction. 7 Ratio of radiogenic to common Pb 8 Blank and common Pb corrected; blank Pb based on procedural blanks measured throughout the course of the study: amount, 0.4-10 pg; composition, 206Pb/204Pb = 18.5 ± 3%, 207Pb/204Pb = 15.5 15.0 ± 3%, 208Pb/204Pb = 36.4 ± 3%. Common Pb compositions are based on Stacey-Kramers model Pb at the 207Pb/206Pb age of the fraction or the interpreted age of the rock (Stacey and Kramers, 1975). 9 Correlation coefficient. 10 Discordance in % to origin.  5  2  All analyzed zircon grains were air abraded or chemically abraded; fraction names: A, B, etc., air abraded; CA1, CA2, etc., chemically abraded. U blank correction of 0.2-1.0 pg ± 20%; U fractionation corrections were measured for each analysis with a double 233-235U spike. 3 Radiogenic Pb 4 Measured ratio corrected for spike and Pb fractionation of 0.23-0.35/amu ± 20% (Daly collector), which was determined by repeated analysis of NBS Pb 982 reference material throughout the course of this study.  1  0.6  CA2  413  346  1264  1480  Pb4  206  AJW-06-488 Quartz monzodiorite clast  5  5  D  4  B  C  6  A  7.6  (ppm)  Pb*3  AJW-04-250 Granodiorite dike  Fraction1  Table 3.2 TIMS U-Pb data for granodiorite and QMD samples. Fractions with prefix CA are chemically-abraded. Fractions A,B,C and D are air-abraded.  Pb isotopic compositions Pyrite separates were collected from QMD-P1 clast samples AJW-06-488 and AJW-06-425 to compare the isotopic signature to that from the stratigraphically underlying Oyu Tolgoi porphyry deposits (Chapter 6). Both samples are clasts that were cut out of otherwise unmineralized dacite tuff from the UDS unit. Sulfides contain almost no uranium, therefore the measured Pb isotope composition should be close to the Pb isotope composition at the time of crystallization (Mukasa, 1986; DeWolf and Mezger, 1994; Nebel et al., 2007) and therefore only the measured data is plotted and interpreted. Analytical method Analytical work was perfomed at the Pacific Centre for Isotopic and Geochemical Research at The University of British Columbia. Pyrite grains were picked from the sample under a binocular microscope. Leaching procedures followed those of Housh and Bowring (1991): grains were leached using 7N HNO3 for 30 minutes on a hotplate (125oC); the residue was rinsed with Milli-Q H2O, leached by 6N HCl on a hotplate for 30 minutes and rinsed with Milli-Q H2O; this residue was leached with 5% HF + 0.5N HBr (8:1) for 10 min on a hotplate stirring every 2 minutes followed by rinsing twice with Milli-Q H2O. This last step was repeated until the sample was white with no visible black inclusions. The final residue was dissolved by concentrated HF and 7N HNO3. An aliquot was taken for ICP-MS analyses to determine the parent/daughter abundance ratios. Isotopic ratios were measured with a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Results The 17.77, the  206  207  Pb/204Pb ratios for samples AJW-06-488 and AJW-06-425 are 17.76 and  Pb/204Pb ratios are 15.54 and 15.40 and the  208  Pb/204Pb ratios are 37.43 and  37.75, respectively (Table 3.3). Most of the data plots within the fields outlined by the pyrite, chalcopyrite and bornite separates from the mineralized porphyry and mineralized Augite Basalt samples from Chapter 6 (Figure 3.6). Sample AJW-06-425 lies  97  Table 3.3 Pb isotopic compositions for QMD-P1 clast samples AJW-06-488 and AJW-06-425. Sample  Mineral  AJW-06-488 AJW-06-425  pyrite pyrite  206  Pb/204Pb 17.76 17.77  2σ % 0.10 0.07  207  Pb/204Pb 15.45 15.40  2σ % 0.08 0.08  208  Pb/204Pb 37.43 37.75  2σ % 0.12 0.10  98  immediately adjacent to the field outlined by the Oyu Tolgoi samples on the  206  Pb/204Pb  versus 208Pb/204Pb plot. Discussion Upper Devonian stratigraphic sequence The presence of mineralized clasts in polymictic conglomerate and breccia at the base of the UDS package immediately above the contact with the MFS unit, in addition to the abrupt nature of the contact, suggest that the Hugo Dummett deposits are truncated by an unconformity, associated with an Upper Devonian erosional event that exhumed the deposits. This interpretation is consistent with the limitation of the porphyry-stage quartz monzodiorite intrusions to the sub-unconformity sequence. To better constrain the timing of the inferred erosional event, attempts were made to extract zircons or other mineral chronometers from two large 5-10 kg samples of strongly altered fragmental material with lapilli-sized clasts from the MFS unit; these failed on both occasions. The lack of zircon recovery in heavy mineral separates from the MFS package also lends support to a break in the volcanic sequence at this interval, as zircon undersaturation in MFS rocks suggests that there is a petrologic difference between it and overlying unmineralized, zircon-saturated rocks from the UDS unit. Unfortunately, the strong alteration and lithic content of the MDS unit limits the applicability of major and trace element geochemistry to test the potential difference between the two units. Markedly different alteration mineral assemblages separated by an abrupt contact also suggest a break in the volcanic sequence. The upper limit of kaolinite stability in geothermal systems is about 200°C (Reyes, 1990, 1991), whereas pyrophyllite has a typical temperature range of 250° to 350°C (Hemley et al., 1980; Reyes, 1990, 1991). The simplest explanation for the separation of kaolinite-bearing assemblages (UDS unit) from pyrophyllite-bearing assemblage (MFS unit) by as little as 10 cm is an unconformity, rather than an extremely steep temperature/fluid composition gradient. Some geological features in the Oyu Tolgoi district are at odds with the inferred unconformity. Firstly, although the contact between strongly altered and mineralized  99  Figure 3.6 Pb isotopic geochemistry of QMD-P1 clasts. Lead growth curves from Zartman and Doe (1981): MN: Mantle; LC: Lower Crust. The average crustal growth curve (S-K) is taken from Stacey and Kramers (1975). The grey fields correspond to the Oyu Tolgoi sulfide samples from mineralized intrusions and host rocks (Chapter 6).  100  rocks and unaltered rocks is generally abrupt, some alteration/mineralization occurs locally within UDS rocks. This localized alteration may be related to the granodiorite rocks (GDi-P3 and/or GDi-P2) which cut both the UDS and OTHS sequences and are locally sericite-altered, pyritic and cut by chalcopyrite veinlets, although much less so than the quartz monzodiorite intrusions. Secondly, there is no oxidation of sulfides in the uppermost parts of the mineralized MFS package. There is no evidence for long term surface exposure of any of the volcanic deposits from the UDS unit such as oxidation rinds on volcanic clasts or quartz-pyrite clasts, goethite coating on fractures or regolith development. This requires that burial of both primary and mass wasting products related to UDS volcanism was rapid and is consistent with the presence of unoxidized sulfides below the unconformity that cannot have been exposed to weathering periods at surface for prolonged time. Isotopic constaints on the event sequence The single-grain CA-TIMS results for the mineralized QMD clast (AJW-06-488; 372 ± 1 Ma and 374 ± 1 Ma) are consistent with the stratigraphic relations and are permissive of a model whereby the UDS sequence unconformably overlies the mineralized rocks. That is, the CA-TIMS U-Pb (zircon) ages from the clast are the same as the crystallization age of mineralized intrusions (~372 Ma; Chapter 4), and older than the age of the unmineralized dacite tuff (369 ± 1 Ma; Chapter 2). As there are only two dated grains, these provide a maximum age constraint for the rock. These U-Pb results are consistent with the sedimentological data that suggests the presence of this unconformity. The interpretation of TIMS results for the sample of granodiorite porphyry (AJW03-250) is more complex. Three out of four air-abraded fractions (368 ± 1 Ma, 365 ± 1 Ma and 363 ± 1 Ma) are consistent with stratigraphic relations as well as the SHRIMPRG data (~366 Ma; Chapter 4) and one older air-abraded grain is interpreted to be xenocrystic (373 ± 1 Ma). The chemically-abraded grains from this sample (~371 Ma; n = 7) are more problematic. One possible scenario suggests that the ~369 Ma age of the 101  UDS unit is correct and that the ~372 Ma porphyries were eroded prior to the eruption of the dacite volcanoes within less than ~3 Ma. This is geologically possible in island arcs (e.g. Batu Hijau, Tombulilato) due to rapid uplift rates documented in these districts (Garwin, 2002; Perello, 1994; Garwin et al., 2005), and requires that the ~371 Ma chemically-abraded grains in sample AJW-04-250 are xenocrysts. Conversely, another possible scenario is that the ~369 Ma grains in the UDS unit are the result of lead-loss and the magmatic age of the unit is in fact 376 Ma or 382 Ma, or perhaps slightly younger: the TIMS age of grains that have been interpreted as xenocrysts (Chapter 2). In this case, the UDS unit would have been pre-mineral and the scenario allows for the GDiP3 sample (AJW-04-250) to be ~371 Ma, as suggested by the seven chemically-abraded single-grain zircon fractions. This latter scenario requires that 1) the UDS unit is largely unmineralized for some reason; 2) the mineralized clasts in the UDS unit were derived from a different, older hydothermal system; 3) the ~372 Ma and ~374 Ma single-grain fractions from the mineralized QMD clast (AJW-06-488) are the result of lead-loss; and 4) the vast majority of SHRIMP-RG spot analyses from petrologically and petrochemically similar GDi-P3 samples (~366 Ma; Chapter 4) are the result of lead-loss. The ~371 Ma chemically-abraded grains from GDi-P3 sample AJW-04-250 most likely reflect a maximum age for the sample and the the air-abraded grains may be either magmatic age or possibly xenocrysts that underwent lead-loss, or a combination of both. The field constraints as well as the interpreted ages of the other GDi-P3 granodiorites (~366 Ma) that have been dated that are petrologically and petrochemically similar to sample AJW-04-250 (Chapter 4) are permissive of this more conservative interpretation of the U-Pb TIMS data. Xenocrystic zircon grains were likely abundant in the magma chamber at the time of granodiorite magmatic activity as zircon saturation was achieved by the slightly older quartz monzodiorite suite based on the large zircon grain yields in heavy mineral separates.  Moreover, xenocrystic grains were detected in some  granodiorite samples (GDi-P2 and GDi-P3) during SHRIMP-RG and TIMS work (Chapter 4) and these intrusions likely cannibalized material including zircons from the older units. Recent studies suggest that this phenomenon is not unusual as assignment of unique pluton crystallization ages based on zircon dating is complicated by zircons that  102  survive the multiple intrusive events that culminate in a large pluton (e.g. Miller et al., 2007). After the deposition of the UDS rocks, strata from the OTHS sequence were thrust over the UDS package, prior to the intrusion of late-mineral GDi-P3 dikes at ~366 Ma, the best age based on interpretation of all U-Pb data related to these rocks (this chapter; Chapter 4). The granodiorite dikes that cut all Devonian supracrustal rocks are weakly altered and mineralized and may be responsible for localized hydrothermal alteration of dacite tuff that overlies the unconformity. Shortening would have occurred almost immediately after (or possibly during) the formation of the porphyry deposits due to the similarity in age between the youngest mineralized quartz monzodiorite, unmineralized welded ignimbrite and the granodiorite porphyry units (Figure 3.7). Finally, the Pb isotopic results from pyrite extracted from two clasts that lie in UDS tuff, stratigraphically above the proposed unconformity in the Hugo Dummett zone, are permissive of their being derived from the underlying ore deposits and thus supportive of the model that includes an erosion surface. Timing of mineralization In the Hugo Dummett zone, mineralized QMD-P1 porphyry with a SHRIMP-RG age of 369 ± 2 Ma (Chapter 4) suggests a maximum age for mineralization. The TIMS age for overlying unmineralized UDS package dacite of 369 ± 1 Ma (Chapter 2) provides a minimum age for the mineralization event there. Mineralization in the Southern Oyu deposits must post-date the ages of mineralized porphyry intrusions in those deposits (374 ± 3 Ma; South Oyu, 372 ± 1 Ma; Central Oyu and 373 ± 1 Ma; Southwest Oyu; Chapter 4). A minimum U-Pb age is not directly constrained in the Southern Oyu deposits by overlying unmineralized volcanic sequences, but rather by the age of granodiorite dikes, which is likely ~366 Ma. As the intrusions from the different deposits are of similar age (within uncertainty) and composition, it is inferred that the main porphyry deposit formation event at Oyu Tolgoi (all deposits) was between 372 ± 1 Ma (youngest TIMS U-Pb age of mineralized QMD; Central Oyu) and 369 ± 1 Ma (TIMS U-Pb age of unmineralized dacite tuff in the Hugo Dummett zone which is presumably eroded in the Southern Oyu  103  Figure 3.7 Generalized interpretation of the Upper Devonian event sequence. Strongly mineralized fragmental rocks are separated from fresh dacite sequences (369 ± 1 Ma) by an unconformity. Late-mineral GDi-P3 dikes are weakly altered/mineralized and likely responsible for localized hydrothermal alteration of dacite tuff that overlies the unconformity. Not drawn to scale.  104  deposits). These results are consistent (within uncertainty) with Re-Os ages of 372 ± 1.2 Ma and 373 ± 1.2 Ma from molybdenite in the Southwest and Central Oyu deposits (Kirwin et al., 2005; Khashgerel et al., 2006). The U-Pb ages for QMD intrusions that may be older than ~372 Ma (374 ± 3; South Oyu and 378 ± 3 Ma; Southwest Oyu; Table 3.1) suggests the possibility that there may have been pre-mineral QMD phases that intruded into basement and served as passive host rocks during the magmatichydrothermal Cu-Au event (Figure 3.7). Geologic and chronologic implications for volcanic arcs The Upper Devonian volcanic environment in the Oyu Tolgoi district is similar to that inferred from stratigraphic sequences documented in other mineralized island arc districts. Galore Creek (Panteleyev, 1975), and Batu Hijau (Garwin, 2002; Garwin et al., 2005) are examples of mineralized volcanic sequences that consist of a mafic volcanic basement succeeded by relatively evolved rocks, all of which became subsequently mineralized during the intrusion of porphyry-age dikes. Mueller et al. (2001) suggested that an erosional period followed the porphyry mineralization episode at Lihir whereby the transition from porphyry to epithermal style mineralization and alteration was triggered by rapid decompression during the partial slope failure of the host stratovolcano and accompanied ingress of seawater. Similarly, continuous syn-mineralization uplift and erosion were interpreted to have removed some 2 km of rock in the past 3 m.y. at Tombulilato (Perello, 1994), in order to superpose epithermal systems over porphyry CuAu deposits. Uplift rates in modern southwest Pacific subduction environments are rapid, on the order of 0.5-3 mm/year (Garwin et al., 2005) and as high as 24 mm/year in certain specific environments such as oblique arc-continent collisional zones characterized by high-angle thrust faults (Yu and Kuo, 2001). This suggests that uplift rates in volcanic arcs are of sufficient magnitude to erode porphyry deposits during the lifetime of the hydrothermal system. At Oyu Tolgoi, the small difference between the U-Pb (zircon) ages of Late Devonian mineralized quartz monzodiorites and those of the unconformably overlying unmineralized Upper Devonian volcanic rocks requires that the erosion of hanging wall sequence was rapid (<3 m.y.). With an uplift rate of 3 mm/year, as much as 9 km of rock could have been uplifted and eroded, sufficient to expose any porphyry 105  systems. An important exploration implication for the rapid transition from an uplifting porphyry system to burial is that late- to post-mineral volcanic piles may conceal blind, unconformity-bound porphyry deposits elsewhere in the Oyu Tolgoi district as well as in other marine arc terranes that are characterized by rapid shortening, uplift and continued volcanism. Paleogeography of the Oyu Tolgoi district Late Devonian paleotectonic reconstructions place Oyu Tolgoi and the South Gobi volcanic arcs at ~60oN latitude (Sengor et al., 1993; Mossakovsky et al., 1994; Sengor and Natal’in, 1996; Perello et al., 2001; Yakubchuk, 2005). The end-Devonian coincided with a period of glaciation (Raymond and Metz, 2004, Brand et al., 2004) and extinction related to global cooling (Caplan and Bustin, 1999). This cold environment may have slowed the surface oxidation of sulphides (Elberling, 2005; Meldrum et al., 2001), contributing to the lack of oxidation below the unconformity at Oyu Tolgoi. This occurrence of unoxidized surfaces below an unconformity is not unique to Oyu Tolgoi, as the Pebble Copper porphyry deposit in Alaska is truncated by a Tertiary unconformity with no oxidation below that surface (J. Lang, pers. comm., 2007). Arctic conditions in the Upper Devonian at Oyu Tolgoi, as well as rapid mass wasting of steep-sided dacite volcanoes that outpaced supergene weathering processes likely contributed to the preservation of hypogene sulfide facies immediately below the unconformity that truncates the Hugo Dummett Cu-Au ore body. Conclusions U-Pb (zircon) geochronology indicates that copper-gold mineralization at Oyu Tolgoi occurred between 372 ± 1 Ma and 369 ± 1 Ma. Furthermore, the combined stratigraphic and U-Pb (zircon) results indicate that uplift and erosion of the porphyries occurred prior to the eruption of late-mineral volcanoes at 369 ± 1 Ma and that shortening and superposition of the thrust-emplaced allochthonous OTHS sequence occurred before the intrusion of late-mineral granodiorite porphyries at approximately 366 Ma. Moreover, arctic conditions during the Upper Devonian at 60oN latitude as well as rapid mass wasting that outpaced supergene processes contributed to the preservation of 106  hypogene sulfide facies immediately below the unconformity that truncates the Hugo Dummett zone ore body. 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Research pertaining to the magmas associated with the ore deposits is important to explorationists as well as to those that are interested in tectonomagmatic processes in volcanic arcs. Investigations in recent decades have suggested that typical, copper-bearing porphyry deposits are associated with upper-crustal (1-3 km) emplacement of a complex series of porphyritic stocks and dikes in and above the cupola zone of a calc-alkaline batholith (Gustafson and Hunt, 1975; Dilles, 1987). Furthermore, a variety of petrologic and petrochemical characteristics are common in many of the world’s porphyry belts, yet relatively uncommon in the geologic record of volcanic arcs. In general, the intrusions associated with porphyry deposits are hydrous, volatile-rich and strongly oxidized (e.g. Dilles, 1987; Mueller et al., 2001), but otherwise not dissimilar to granitoid magmas of similar composition (Cline and Bodnar, 1991).  Furthermore,  copper-bearing porphyries are disproportionately associated with adakite-like rocks (Thieblemont et al., 1997; Sajona and Maury, 1998; Oyarzun et al., 2001), and gold-rich porphyries with alkaline rocks, in particular of shoshonitic compositon (Mueller and Groves, 1993; Sillitoe, 1997, Mueller et al., 2001). Moreover, input from mafic magmas into the intermediate to felsic porphyry complex has been suggested for various deposits (Cornejo et al., 1997; Hattori and Keith, 2001; Maughan et al., 2002; Pollard and Talylor, 2002). The goal of this chapter is to present petrologic observations in addition to lithogeochemistry and U-Pb geochronology that allows the paragenetic sequence of Late  3  A version of this chapter will be submitted to a refereed journal for publication: Wainwright, A.J., Tosdal, R.M., Wooden, J.L., Mazdab, F.K. and Friedman, R.M., in prep., Petrology, petrochemistry and timing of Late Devonian intrusions associated with the super-giant Oyu Tolgoi Cu-Au deposits; South Gobi region, Mongolia.  111  Devonian intrusive rocks to be defined in the Cu-Au porphyry deposits at Oyu Tolgoi. These results are compared to porphyry intrusions from other Cu-Au districts as well as to rocks that are common in island arc environments in order to identify the characteristics of the Oyu Tolgoi Igneous Complex (OTIC) porphyries that are different from most arc rocks and place the OTIC intrusions into a global context. Geologic framework The Oyu Tolgoi Cu-Au district is located in the Central Asian Orogenic Belt; a tectonic supercollage that contains craton fragments, Proterozoic to Paleozoic ophiolites and accreted volcanic arcs (Badarch et al., 2002; Buchan et al. 2002). The Oyu Tolgoi porphyry deposits occur within a sequence of Devonian rocks in the Gurvansayhan Terrane (Badarch et al., 2002; Helo et al., 2006), interpreted to represent a juvenile oceanic island arc (Helo et al., 2006; Chapter 6). The copper-gold deposits are hosted in an inlier of Late Devonian intrusions and Devonian volcanic and volcaniclastic rocks that are surrounded by disconformably-overlying Carboniferous volcanogenic packages (Chapter 2; Figure 4.1). The five Cu-Au deposits that constitute the Oyu Tolgoi district are aligned in a >6.5 km, north-northeast-trending corridor, divided into two zones: the Hugo Dummett zone and the more deeply eroded Southern Oyu deposits (South Oyu, Southwest Oyu and Central Oyu). Recent exploration has led to the discovery of another porphyry center, the Heruga deposit, a further 5 km south-southwest of the Southern Oyu deposits, within the same mineralized corridor (Ivanhoe Mines press release, 2008). As a district, the Oyu Tolgoi porphyry deposits display a significant breadth of alteration and mineralization characteristics. South and Southwest Oyu are characterized by quartz vein stockworks, chalcopyrite and bornite mineralization hosted in biotite-altered basaltic rocks as well as quartz monzodiorite intrusions.  Conversely, Central Oyu and the Hugo Dummett  deposits contain quartz vein-hosted chalcopyrite-bornite overprinted by hypogene chalcocite, covellite and minor enargite associated with pyrophyllite, kaolinite and dickite-bearing alteration assemblages. Furthermore, a Cretaceous supergene chalcocite blanket is developed exclusively in the Central Oyu deposit, but is lacking in the other deposits (Kirwin et al., 2005). Khashgerel et al. (2006) suggest that the Oyu Tolgoi 112  Figure 4.1 Bedrock geology map of the Oyu Tolgoi district, Mongolia, drawn by Ivanhoe Mines geology staff and AJW from mapping of drill holes and trenches through younger cover sequences in addition to outcrop mapping. Inset map shows the location of Oyu Tolgoi in the southern Gobi desert of Mongolia. Coordinates are WGS 84 zone 48. The copper grade shell for the Hugo Dummett zone is projected to surface.  113  deposits are typical of porphyries formed in island arc environments; however, somewhat unusual alteration and sulfide mineral assemblages in the high-grade Hugo Dummett deposits, hosted in fragmental rocks that overlie the basalt basement, resulted from a cooling magmatic hydrothermal system that transgressed outward over enclosing advanced argillic alteration. Petrology of Late Devonian intrusions Late Devonian intrusive rocks in the Oyu Tolgoi porphyry Cu-Au district consist of a sequence of pre- to late-mineral, equigranular to porphyritic quartz monzodiorites and granodiorites.  The sequence of rocks is divided into equigranular to crowded  porphyritic (E-type) and porphyritic with aphanitic to holocrystalline groundmass (Ptype) petrologic varieties (Table 4.1).  Typically, alteration is present and locally  completely texturally destructive and consists of chlorite-sericite alteration with variable amounts of silica and carbonate introduction. Quartz monzodiorite rocks range from porphyritic with a holocrystalline groundmass (QMD-P) to relatively equigranular (QMD-E). The QMD-E intrusions are characterized by 2-4 mm plagioclase (20-30%), 2-4 mm hornblende (20-30%), 1-3 mm K-feldspar (10-20%) with minor quartz and biotite and accessory zircon, apatite and magnetite (Figure 4.2a). Phenocryst componentry in the QMD-P1 phase (30-50%) is dominated by 3-5 mm plagioclase, 3-5 mm hornblende with K-feldspar and minor quartz present in the finer-grained holocrystalline groundmass. Minor biotite is present as are accessory zircon, magnetite and apatite. Porphyritic quartz monzodiorite phases are typically hypidiomorphic and long axes of plagioclase and hornblende phenocrysts are commonly aligned (Figure 4.2b). Where unaltered, these rocks are characterized by a distinct overall red colouration, but more typically, the quartz monzodiorites are greygreen to buff white, cut by quartz veins, sericite-altered with abundant disseminated and vein-hosted pyrite ± chalcopyrite ± bornite. The granodiorite porphyries are subdivided into a coarse, sparsely-porphyritic phase (GDi-P2) and a fine-grained crowded porphyritic (GDi-P3) phase (Table 4.1). The GDi-P2 porphyries are characterized by 30-50% 5-10 mm locally zoned plagioclase phenocrysts set in a fine-grained, granular to aphanitic feldspathic groundmass with 114  Figure 4.2 Representative polished slab images of Late Devonian intrusive rocks from the Oyu Tolgoi district. Quartz monzodiorite samples include A) equigranular (QMD-E) and B) porphyritic (QMD-P1) varieties. Granodiorite samples include C) coarse-grained porphyries (GDi-P2) and D) fine-grained porphyries (GDi-P3).  115  abundant apatite microphenocrysts (Figure 4.1c). The GDi-P3 phase contains >80% phenocrysts of 1-3 mm plagioclase and minor quartz (<10%) set in a fine-grained greybrown to beige-yellow feldspathic groundmass (Figure 4.1d). The granodiorites are poor in ferromagnesian minerals and opaque phases, and are biotite-dominant compared to the QMD phases which are hornblende-dominant. Zircon is a common accessory mineral in the GDi phases. The granodiorite intrusions are relatively fresh, but can have sericitealtered plagioclase phenocrysts as well as disseminated pyrite.  In rare cases, the  granodiorite intrusions are cut by chalcopyrite veinlets, but more generally they truncate strongly mineralized rocks with fine-grained chilled margins adjacent to quartz-sulfiderich zones. These rocks thus post-date most of the porphyry system. The QMD intrusions in the deposit areas occur as dikes and stocks that are tens of meters wide. As well, a large QMD-E quartz monzodiorite pluton is located immediately west of the Central Oyu deposit. Airborne gravity interpretation, limited outcrop and drilling suggest that the intrusion is elliptical, with the long axis oriented north-northeast over a 5 x 8 km area (Khashgerel et al., 2006). The GDi-P3 phases form a NNE-trending dike swarm that ranges from several meters to over 600 m wide, and widens upward where they cut through the Late Devonian hanging wall in the Hugo Dummett zone (Kirwin et al., 2005). Intrusive sequence In general, the quartz monzodiorite phases are always more strongly altered and mineralized than nearby granodiorite phases which typically cross-cut the relatively intensely altered rocks.  Irrespective of the superposed hydrothermal events, chilled  margins and intrusion-cemented breccias permit establishment of a relative chronologic sequence of intrusive rock phases. A clast of sericite-altered QMD-P1 porphyry is part of an intrusion-cemented breccia, which is characterized by a groundmass composed of GDi-P2 porphyry (Figure 4.3a). In addition, GDi-P3 phases commonly have chilled margins adjacent to strongly altered and mineralized QMD phases. Furthermore, chilled margins in GDi-P3 dikes adjacent to GDi-P2 phases are present as well as a clear relation in an intrusion-cemented breccia. In the latter case, an angular clast of GDi-P2 phase intrusive is set in a groundmass of GDi-P3 material (Figure 4.3b). 116  Figure 4.3 Photographs of breccias showing intrusive relationships documented in the OTIC. A) Intrusion-cemented breccia with clast of QMD-P1 set in GDi-P2 groundmass. B) Intrusion-cemented breccia with clast of GDi-P2 set in GDi-P3 groundmass.  117  118  Rock type  Quartz monzodiorite  Quartz monzodiorite porphyry  Biotite granodiorite porphyry  Biotite granodiorite porphyry  Phase  QMD-E  QMD-P1  GDi-P2  GDi-P3 Porphyritic with aphanitic groundmass  Porphyritic with aphanitic groundmass  Porphyritic with holocrystalline groundmass. Aligned phenocrysts are common.  Coarse-grained, equigranular  Texture  Table 4.1 Petrologic characteristics of Late Devonian intrusions.  0.5-3 mm euhdral to subhedral plagioclase phenocrysts (40%), 0.5-1 mm quartz phenocrysts (10%), 5-10% finegrained biotite, and a brown aphanitic to microcrystalline groundmass with abundant apatite and zircon.  4-6 mm euhdral to subhedral plagioclase phenocrysts (40%), 0.5-1 mm quartz phenocrysts (10%), 1-2 mm biotite phenocrysts (10%) set in in a fine-grained brown to beige-yellow groundmass with abundant apatite microphenocrysts.  3-4 mm plagioclase phenocrysts (50%), 2-3 mm hornblende phenocrysts (20%) and 0.5-1 mm biotite phenocrysts (10%), 1-2 mm quartz phenocrysts (5%) and holocrystalline groundmass K-feldspar. Accessory minerals include zircon, apatite and magnetite.  3-4 mm plagioclase (40%), 1-3 mm K-feldspar (30%) and 2 mm quartz crystals (5%). Ferromagnesian phases include hornblende and biotite and accessory minerals include zircon, apatite and magnetite.  Mineral componentry  Lithogeochemistry Thirty-seven samples reflecting the petrologic range of Late Devonian intrusive rocks in the district were analyzed for major and trace elements. All samples were analyzed at ALS Chemex Laboratories Limited in North Vancouver, Canada. Major element analyses and selected trace element concentrations were determined by X-ray fluorescence spectrometry (XRF), rare earth element (REE) and remaining trace element concentrations were determined by inductively coupled plasma mass spectrometry (ICPMS). The analytical detection limits from ALS Chemex for wholerock geochemical samples are given in Appendix 5. Although great care was taken to collect least altered samples through field screening and thin section evaluation, some alteration was inevitable. As these rocks are associated with giant porphyry Cu-Au deposits, invariably some of the analyzed samples are partially altered. Alteration usually takes place in the form of sericite replacing feldspar and chlorite replacing hornblende. Wherever possible, a diamond saw was used to cut off visibly altered material during preparation of material for analysis. Representative analyses are presented in Table 4.2 and all analyses for the intrusive suites are presented in Appendix 2.  119  Table 4.2 Representative geochemical analyses for Late Devonian intrusions. Sample phase SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th  AJW-06-452 QMD-E 59.04 0.6 17.98 <0.01 5.39 2.75 0.12 1.53 3.89 4.92 3.13 0.24 0.12 0.1 2.88 99.94 <1 956 32.8 9.5 <10 1.74 187 3.35 1.93 1.13 20.6 3.54 3 0.63 15.9 0.32 13 6.7 15.5 5 7 3.78 64.3 3.49 1 1055 0.4 0.58 2.63  AJW-03-098 QMD-P1 57.42 0.58 17.77 0.01 5.19 2.32 0.19 1.48 4.52 5.48 3.13 0.27 0.12 0.09 3.37 99.62 <1 770 30.9 9 70 1.4 114 3.3 2.1 1.2 19 3.8 3 0.7 15.4 0.3 5 6 16.1 <5 8 3.7 58.6 3.5 <1 1035 <0.5 0.6 3  AJW-06-406 GDi-P2 67.76 0.49 15.22 <0.01 3.09 2.12 0.06 1.16 1.7 2.86 2.79 0.19 0.04 0.27 3.51 99.15 <1 2910 40.3 7.8 20 4.27 92 2.02 1.09 0.89 22.6 3.19 3.9 0.36 18.4 0.14 <2 8 18.3 11 65 4.54 74.2 3.4 1 265 0.6 0.44 3.9  AJW-06-415 GDi-P3 65.5 0.41 14.84 <0.01 2.84 1.34 0.07 1.55 2.85 3.56 2.77 0.19 0.04 0.03 5.05 99.7 <1 447 41.6 8 20 2.78 336 2.57 1.26 1.12 22.3 3.38 4.3 0.47 19.1 0.16 <2 8.2 18.6 14 10 4.59 76.5 3.63 1 375 0.6 0.5 3.82  120  Sample phase Tl Tm U V W Y Yb Zn Zr  AJW-06-452 QMD-E <0.5 0.27 1.54 142 3 16.5 2 67 111  AJW-03-098 QMD-P1 <0.5 0.3 1.3 150 1 19.2 2.1 59 91.9  AJW-06-406 GDi-P2 <0.5 0.14 2 67 8 8.8 0.85 89 139  AJW-06-415 GDi-P3 <0.5 0.17 2.18 71 11 11.7 1.15 70 150  121  Geochemical characteristics Major elements The Devonian intrusive rocks at Oyu Tolgoi range in composition from 55.2 to 68.2 wt % SiO2 and the QMD-P1 and QMD-E rocks plot as monzonites, whereas the GDi-P2 and GDi-P3 rocks as granodiorites on the Le Bas et al. (1986) SiO2 versus total alkalies petrochemical discrimination plot (Figure 4.4a). MgO ranges from 0.4 to 2.4 wt % and Al2O3 ranges from 14.8 to 18.4 wt %. Two linear trends are evident on SiO2 versus MgO and SiO2 versus Al2O3 plots; each are characterized by decreasing MgO and Al2O3 with increasing SiO2 (Figures 4.4c and 4.4d) and at a given SiO2 content of 60 wt %, the QMD phases have lower MgO contents than the GDi phases. Some outliers are present in the dataset, which may reflect alteration in the form of alkali or silica introduction. Alkalinity The OTIC rocks are relatively alkaline with K2O contents that range from 2.1 to 4.5 wt % (average 3.2 wt %) with K2O/Na2O ratios ranging from 0.5 to 1.6 (average 0.8). Based on the Peccerillo and Taylor (1976) classification of SiO2 versus K2O, the OTIC rocks are divided into two fields; dominantly high-K calc-alkaline (QMD-E, QMD-P1, GDi-P2 phases) and dominantly medium-K calc-alkaline (GDi-P3) (Figure 4.4b). Alteration effects can increase K2O contents; however uniformity within the rock suites suggests that this effect is minor as the least-altered samples would not likely be consistently affected by alteration to an identical extent.  Moreover, the lack of  correlation between loss-on-ignition (a proxy for hydrothermal alteration) and K2O (Figure 4.4e) suggests that there is no direct link between alteration and the alkalinity, supporting the interpretation.  122  Figure 4.4 Major element and petrochemical discrimination diagrams. A) Total alkalies vs. silica diagram from Le Bas et al. (1986); B) Oyu Tolgoi wholerock geochemistry plotted on silica vs. K2O diagram based on Peccerillo and Taylor (1976). Island arc reference data field (n = 232) is based on a GEOROC (http://georoc.mpch mainz.gwdg.de/georoc/) query for samples with appropriate elements from the Aleutian, Bismarck-New Britain and Lesser Antilles arcs. Fertile arc reference data field (n = 55) is based on a query for samples from the Sunda-Banda and Luzon arcs. Fertile arc is defined as one that hosts known porphyry deposits (see text for explanation); C) SiO2 versus MgO; D) SiO2 versus Al2O3; E) Losson-ignition (LOI) vesus K2O.  123  Minor elements, trace elements and REE The Oyu Tolgoi samples are characterized by low high field-strength element concentrations such as Nb which ranges from 3.5 ppm to 9 ppm (average of 6.8 ppm) and TiO2, ranging from 0.3 wt % to 0.8 wt % (average of 0.5 wt %), characteristic of volcanic arcs (e.g. Wilson 1989; Foley and Wheller 1990; Brenan et al., 1994). The Ba/La ratios (9 to 158) and U/Th ratios (0.3 to 0.6) in the OTIC samples are also relatively low, which precludes the involvement of sedimentary input in these rocks (Patino et. al., 2000). All of the rocks from the suites are heavy-rare earth element-depleted and display weak to absent negative europium anomalies (Figure 4.5). However, markedly different groups are apparent when the light-to-heavy (La versus Yb) and medium-to-heavy (Sm versus Yb) REE ratios are plotted. The GDi-P2 and GDi-P3 groups display distinctive, high La/Yb (7.9 to 21.6) and Sm/Yb (1.5 to 4) ratios that distinguish them from the QMD phases (6.9 to 19 and 1.5 to 3.7, respectively) (Figures 4.6a and 4.6b). The QMD rocks have Ni contents below, at or slightly greater than the detection limit of 5 ppm. Conversely, the GDi-P2 and GDi-P3 rock groups have significantly higher average Ni contents; 17 ppm (GDi-P2) and 21 ppm (GDi-P3). Similarly, the QMD samples have Cr contents that are at or below the detection limit of 10 ppm, whereas the granodiorite suites have average Cr contents of 64 ppm (Gdi-P3) and 38 ppm (GDi-P3) (Table 4.3). GDi-P3 monzonite intrusions are characterized by low Yb (<1.8 ppm) and low Sr/Y ratios (<40). Most of the QMD-P1 and QMD-E have high Sr/Y ratios (>40) and relatively high Yb (>1.8 ppm) contents. The GDi-P2 phases are both high Sr/Y (>40) and low Yb (<1.8 ppm). The samples from the Oyu Tolgoi district generally fit the major element definition for adakites, however the Sr/Y ratios and Yb contents of these rocks, with the exception of the GDi-P2 granodiorite porphyries excludes most of the rocks from the Defant and Drummond (1993) adakite classification (Figure 4.6c).  124  Figure 4.5 Chondrite-normalized rare-earth element plots for Late Devonian intrusions (after Sun and McDonough, 1989).  125  Figure 4.6 Silica versus REE and adakite discrimination plots. SiO2 versus La/Yb (A) and Sm/Yb plots (B). Yb vs. Sr/Y with adakite field. Adakite field is defined by Defant and Drummond (1993). Island arc reference data field (n = 232) is based on a GEOROC (http://georoc.mpch mainz.gwdg.de/georoc/) query for samples with appropriate elements from the Aleutian, Bismarck-New Britain and Lesser Antilles arcs. Fertile arc reference data field (n = 55) is based on a query for samples from the Sunda-Banda and Luzon arcs.  126  U-Pb geochronology Sensitive High-Resolution Ion Microprobe – Reverse Geometry (SHRIMP-RG) ages for porphyry intrusions in the district are reported in this chapter. Two other SHRIMP U-Pb zircon ages for mineralized and unmineralized quartz monzodiorite intrusions from Southwest Oyu deposit, undertaken at the Australian National University by George Brimhall in 2003, are 378 ± 3 and 371 ± 3 Ma, respectively (cited in Khashagerel et al., 2006).  Eight samples were selected for this SHRIMP-RG  geochronologic study and additional Thermal Ionization Mass Spectrometer (TIMS) UPb dates were obtained as well to produce isotopic ages with smaller uncertainties and to provide an internal check on the interpreted SHRIMP-RG ages.  Results for the  SHRIMP-RG and TIMS samples are summarized in Table 4.5. Analytical method SHRIMP-RG Five to fifteen kilogram samples for U-Pb dating were processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator. Individual zircon grains were then handpicked for U-Pb isotopic analysis. The dating was performed at the USGS – Stanford University SHRIMP-RG lab (Sensitive High mass-Resolution Ion MicroProbe-Reverse Geometry). Reviews of the applied techniques are in Williams (1996). Zircons were mounted in epoxy, polished, photographed, and coated with ~10 nm of gold. Polished mounts were cleaned with soap, HCl, and distilled water and dried prior to coating with gold. During U-Pb analysis on the SHRIMP-RG, a primary beam of 16  O2- ions (about 10–20 nA) was used to raster an area about 50 × 50 µm for about 90  seconds to remove the gold coat and surface contamination (common Pb). The beam was focused to create flat-floored (about 2-µm-deep) elliptical pits for analysis (about 30 × 35 µm). Data were collected for four to five scans per spot and were referenced to the zircon standards R33 (419 Ma) and RG6 (1440 Ma).  127  In advance of uranium-lead dating, zircon grains were examined under cathodoluminescence (CL) and reflected light to reveal if the single grain candidates for SHRIMP RG analysis contained imperfections such as cracks, holes or obvious inherited zircon cores. This information guided selection of the best grains and locations for the spot analyses. The zircon grains show a wide variey of sizes, morphologies and colours that range from clear to pale yellow or orange. Many grains are sub-rounded which could mean that they are relatively old xenocrysts, however they did not consistently yield older U-Pb ages. CL imaging shows oscillatory banding in most grains shows some evidence for inherited cores which was confirmed in several analyses; however, most spot analyses were performed on grain rims or tips in order to identify youngest, magmatic ages. Cathodoluminescence zircon images for the SHRIMP-RG samples are presented in Appendix 1, with representative images in Figure 4.7. Zircon data was reduced using the programs ISOPLOT and SQUID (Ludwig, 1999) and analysis of the data used weighted mean  207  Pb-corrected  206  Pb/238U age  diagrams. Sufficient individual SHRIMP-RG zircon spot analyses were made for each sample such that at least eight analyses showing similar ages were obtained. These similar ages were then used in the calculation of the overall rock age. As the zircons from the Oyu Tolgoi district are relatively young, only the 206Pb/238U results can be used to determine the age of these rocks because of poor measuring statistics on  204  Pb, the  normal isotope used for removing common Pb during the age calculation as well as the generally low U contents of the zircons. This data is used throughout and weighted mean age diagrams (2σ error bars) of U-Pb ages for individual grains are shown in Figure 4.8. Individual zircon spot ages discussed in the text use 1σ errors. Only zircons with corrected  207  Pb-  206  Pb/238U ages that fall between 320 and 400 Ma are displayed. Zircons that  are younger than 320 Ma are interpreted to have undergone significant lead-loss and zircons that are older than 400 Ma are clearly either xenocrysts or have inherited older cores.  The SHRIMP-RG U-Pb data is presented in Table 4.3 and weighted  207  Pb-  corrected 206Pb/238U age diagrams are shown in Figure 4.8.  128  TIMS Analytical work was perfomed at the Pacific Centre for Isotopic and Geochemical Research at The University of British Columbia. All grains were hand-picked under magnification in alcohol and are pre-treated using air abrasion or chemical abrasion techniques. After air abrasion (Krogh, 1982), grains were washed and ultrasonicated in 3N HNO3 to remove fine dust from pyrite abrasion media, rinsed with acetone followed by ethanol, and transferred to 10 mL pyrex beakers; they are ready for final washing prior to dissolution. The chemical abrasion technique (CA-TIMS) employed is modified from procedures outlined in Mundil et al. (2004) and Mattinson (2005) and the PCIGR CATIMS technique is described in detail in Scoates and Friedman (2008). Isotopic ratios were measured with a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Uranium fractionation is determined directly on individual runs using a  233-235  U tracer, and Pb  isotopic ratios were corrected for fractionation (0.23-0.35%/amu, throughout the course of this study) based on replicate analyses of the NBS 982 Pb reference material and the values recommended by Thirlwall (2000). Reported precisions for Pb/U and Pb/Pb dates were determined by numerically propagating all analytical uncertainties through the entire age calculation using the technique of Roddick (1987).  Standard concordia  diagrams were constructed and concordia/weighted mean ages were calculated with Isoplot 3.00 (Ludwig, 2003). Unless otherwise noted, all errors are quoted at the 2σ level. The TIMS U-Pb data is presented in Table 4.4 and concordia diagrams are shown in Figure 4.9. Results Quartz monzodiorite samples The oldest dated rock currently identified in the district is a strongly altered and mineralized, coarse-grained equigranular quartz monzodiorite from the South Oyu deposit (QMD-E; AJW-03-182). Zircons are large (>200 µm), euhedral, colourless and display both concentric and sector zonation patterns under cathodoluminescence. Twelve  129  of the grain analyses define a weighted mean  207  Pb-corrected  206  Pb/238U age of 374 ± 3  Ma which is interpreted to be the crystallization age of the sample (Figure 4.8a). A red, medium-grained, equigranular to crowded porphyritic mineralized porphyry from the Hugo Dummett zone (QMD-P1; AJW-04-356) yielded moderately large (150-200 µm), euhedral, colourless and concordantly zoned zircons. Eight of 12 zircons define a weighted mean  207  Pb-corrected  206  Pb/238U age of 369 ± 2 Ma which is  interpreted to be the crystallization age of the sample. Two zircons with ages of ~380 Ma are interpreted as xenocrysts and two zircons with ages of ~350 Ma were interpreted to be the result of lead-loss (Figure 4.8b). An altered and mineralized QMD sample from Southwest Oyu was dated by SHRIMP-RG and TIMS (QMD-P1; AJW-03-178). The zircons are moderate-sized to large (100-200 µm), euhedral, colourless, concordantly zoned and generally elongate. Twelve of 13 zircons define a weighted mean SHRIMP-RG  207  Pb-corrected  206  Pb/238U  age of 369 ± 5 Ma and one zircon with an age of ~385 Ma was interpreted as a xenocryst (Figure 4.8c). Five air-abraded single-grain TIMS fractions were analyzed from AJW03-178. Two grains yielded ages greater than 375 Ma and are interpreted as xenocrysts. One fraction with a TIMS U-Pb age of 371 ± 1 Ma is interpreted to have undergone some lead-loss. The inferred crystallization age is based on two grains (372 ± 1 Ma and 373 ± 1 Ma) that yield a U-Pb concordia age of 373 ± 1 Ma or a 2-point weighted mean 206  Pb/238U age of 373 ± 1 Ma (Figure 4.9). Zircons from a typical QMD-P1 mineralized quartz monzodiorite porphyry  collected from Central Oyu (AJW-03-181) are large (150-200 µm), euhedral, colourless to yellowish and typically concordantly zoned. Twelve of 15 zircons define a weighted mean 207Pb-corrected 206Pb/238U age of 368 ± 3 Ma. Two zircon grains with ages of >400 Ma and ~390 Ma were interpreted as xenocrysts, whereas one zircon with an age of ~335 Ma was interpreted to be the result of lead-loss (Figure 4.8d). The U-Pb (TIMS) age for this sample confirms the SHRIMP-RG age. Six air-abraded and two chemically-abraded single-grain fractions were analyzed in addition to one leachate from the chemical abrasion samples. The two oldest air-abraded grains as well as the two analyses from the chemically-abraded fractions define a weighted  206  Pb/238U age of 372 ± 1 Ma, the  interpreted crystallization age. The two remaining air-abraded fractions were slightly  130  younger at ca. 370 Ma, interpreted to have undergone slight lead-loss and the leachate was much younger at ca. 313 Ma (Figure 4.9b). The QMD pluton west of Central Oyu (AJW-04-385) yielded very large grains that are commonly greater than 300 µm and were generally well-faceted, equant and zoned with oscillatory growth bands. The weighted mean 207Pb-corrected 206Pb/238U age of 364 ± 4 Ma and intrerpreted crystallization age is defined by 12 of 12 zircons (Figure 4.8h). One final QMD sample from Central Oyu (AJW-06-467) yielded four air-abraded single-grain fractions that ranged in age from 371 ± 1 Ma to 365 ± 1 Ma. Granodiorite samples Two samples of GDi-P3 intrusions from the Hugo Dummett deposit were analysed by SHRIMP-RG. Sixteen out of 17 zircons yielded a 206Pb/238U age of 367 ± 2 Ma for sample AJW-03-184 (sericite-altered) with one grain (>400 Ma) interpreted as a xenocryst (Figure 4.8e). Twelve out of 16 zircons for sample AJW-03-185 (relatively unaltered) define a weighted mean  207  Pb-corrected  206  Pb/238U age of 365 ± 4 Ma. Four  older zircons with ages of ~370 Ma to ~380 Ma were interpreted as xenocrysts (Figure 4.8f). An additional unmineralized sample of GDi-P3 porphyry from Southwest Oyu was dated as well (AJW-03-179), where twelve of sixteen zircons define a weighted mean 207  Pb-corrected 206Pb/238U age of 363 ± 3 Ma and four remaining zircons were ~370 Ma  and interpreted as xenocrysts (Figure 4.7g). Dating of a sample of GDi-P2 by U-Pb TIMS (AJW-06-476) yielded five grains spread between 388 ± 2 Ma and 356 ± 2 Ma (air abraded), two of which were ~365 Ma, and two chemically-abraded grains with ages of 379 ± 2 Ma and 371 ± 2 Ma (Figure 4.9). One chemically-abraded grain was clearly xenocrystic with an age of 441 ± 1 Ma.  131  Figure 4.7 Representative cathodoluminescence images from Late Devonian intrusion samples dated by SHRIMP-RG. Circles refer to the locations of the spot analyses and the numbers correspond to analytical results in Table 4.3.  132  133  U (ppm)  84 132 70 168 111 101 72 56 101 98 95 68  AJW-04-356-1 AJW-04-356-2 AJW-04-356-3 AJW-04-356-4 AJW-04-356-5 AJW-04-356-6 AJW-04-356-7 AJW-04-356-8 AJW-04-356-9 AJW-04-356-10 AJW-04-356-11  75 127 65 237 154 72 105 78 135 90 102  38 81 29 110 67 31 41 32 70 42 56  AJW-04-356 Hugo Dummett QMD-P1  AJW-03-182-1 AJW-03-182-2 AJW-03-182-3 AJW-03-182-4 AJW-03-182-5 AJW-03-182-6 AJW-03-182-7 AJW-03-182-8 AJW-03-182-9 AJW-03-182-10 AJW-03-182-11 AJW-03-182-12  46 54 42 109 70 39 19 25 66 58 60 29  Th (ppm)  AJW-03-182 South Oyu QMD-E  Spot  232  0.5272 0.6642 0.4553 0.4793 0.4490 0.4436 0.4022 0.4227 0.5330 0.4843 0.5724  0.5622 0.4244 0.6250 0.6715 0.6544 0.3999 0.2769 0.4738 0.6762 0.6140 0.6545 0.4453  Th/ U  238  4 7 4 8 6 5 4 3 5 5 5 4  4 6 3 12 8 4 5 4 7 4 5  Pb* (ppm)  206  Table 4.3 SHRIMP-RG analytical data for Late Devonian intrusions.  16.90 17.00 16.62 16.98 16.69 16.25 17.71 16.49 17.09 17.23 17.68  17.01 16.68 16.66 17.12 16.79 16.45 16.68 16.31 16.60 16.63 16.87 16.52  Total 238 U/206Pb ± (%)  1.4 1.2 1.5 1.1 1.2 1.6 1.3 1.5 1.2 1.4 1.4  1.4 1.2 1.5 1.4 1.3 1.4 1.5 1.6 1.9 1.3 1.3 1.5  0.0551 0.0579 0.0522 0.0544 0.0561 0.0589 0.0580 0.0544 0.0523 0.0568 0.0548  0.0531 0.0548 0.0533 0.0526 0.0530 0.0541 0.0539 0.0576 0.0523 0.0541 0.0532 0.0543  Total 207 Pb/206Pb ± (%)  3.8 2.8 4.2 2.2 2.7 3.7 3.3 3.8 3 3.5 3.3  3.6 2.9 3.9 2.6 3.1 3.3 3.8 4.2 3.3 3.3 3.3 3.9  370 367 378 369 374 383 352 379 367 362 354  369 375 376 367 374 381 376 382 378 377 372 379  Pb/238U Age  206  1σ  5 5 6 4 4 6 5 6 5 5 5  5 5 6 5 5 5 5 6 7 5 5 6  134  199  U (ppm)  49 67 47 88 50 53 53 37 67 124 105 46 106  AJW-03-181-1 AJW-03-181-2 AJW-03-181-3 AJW-03-181-4 AJW-03-181-5 AJW-03-181-6 AJW-03-181-7 AJW-03-181-8 AJW-03-181-9 AJW-03-181-10 AJW-03-181-11  76 63 123 107 62 113 103 114 151 138 52  44 29 78 60 22 51 60 88 60 84 21  16 21 16 34 18 16 20 12 20 52 35 14 39  86  Th (ppm)  AJW-03-181 Central Oyu QMD-P1  AJW-03-178-1 AJW-03-178-2 AJW-03-178-3 AJW-03-178-4 AJW-03-178-5 AJW-03-178-6 AJW-03-178-7 AJW-03-178-8 AJW-03-178-9 AJW-03-178-10 AJW-03-178-11 AJW-03-178-12 AJW-03-178-13  AJW-03-178 SW Oyu QMD-P1  AJW-04-356-12  Spot  232  0.5971 0.4694 0.6506 0.5749 0.3754 0.4645 0.5948 0.7996 0.4115 0.6263 0.4146  0.3415 0.3188 0.3540 0.4001 0.3784 0.3116 0.3893 0.3395 0.3103 0.4357 0.3404 0.3131 0.3833  0.4430  Th/ U  238  4 3 6 5 3 7 5 6 8 7 3  2 3 2 5 2 3 3 2 3 6 5 2 6  10  Pb* (ppm)  206  17.11 16.55 16.81 17.30 17.04 14.41 17.10 15.55 16.84 16.90 17.22  17.31 16.66 17.34 16.75 17.21 17.04 16.91 17.07 17.37 16.63 16.53 17.33 16.18  17.00  Total 238 U/206Pb ± (%)  1.5 1.7 1.4 1.3 1.7 1.4 1.3 1.4 1.2 1.2 1.7  1.7 1.5 1.7 1.4 1.7 1.7 1.6 1.9 1.5 1.2 1.3 1.6 1.6  1.1  0.0563 0.0550 0.0553 0.0547 0.0586 0.0872 0.0522 0.1910 0.0602 0.0541 0.0514  0.0557 0.0552 0.0566 0.0513 0.0537 0.0543 0.0530 0.0582 0.0547 0.0554 0.0531 0.0568 0.0557  0.0547  Total 207 Pb/206Pb ± (%)  3.9 4.2 3.3 3.3 4.5 3 3.5 2.7 2.7 2.9 4.8  4.8 3.8 4.7 3.5 4.7 4.6 4.4 5.4 4.2 3 3.3 4.5 3.2  2.6  365 378 372 362 366 416 367 335 369 371 365  361 375 360 375 364 367 371 365 361 376 379 360 386  368  Pb/238U Age  206  1σ  5 6 5 5 6 6 5 10 4 5 6  6 6 6 5 6 6 6 7 6 5 5 6 6  4  135  51 128 54 82  U (ppm)  119 97 133 124 106 137 285 253 162 83 55 74 103 133 73 65 89  AJW-03-185-1 AJW-03-185-2 AJW-03-185-3 AJW-03-185-4  105 63 128 88  AJW-03-185 Hugo Dummett GDi-P3  AJW-03-184-1 AJW-03-184-2 AJW-03-184-3 AJW-03-184-4 AJW-03-184-5 AJW-03-184-6 AJW-03-184-7 AJW-03-184-8 AJW-03-184-9 AJW-03-184-10 AJW-03-184-11 AJW-03-184-12 AJW-03-184-13 AJW-03-184-14 AJW-03-184-15 AJW-03-184-16 AJW-03-184-17  32 18 57 40  45 33 35 79 64 96 127 186 67 52 16 24 58 72 26 18 44  21 46 20 32  Th (ppm)  AJW-03-184 Hugo Dummett GDi-P3  AJW-03-181-12 AJW-03-181-13 AJW-03-181-14 AJW-03-181-15  Spot  232  0.3150 0.2898 0.4583 0.4707  0.3913 0.3492 0.2737 0.6595 0.6290 0.7275 0.4611 0.7599 0.4239 0.6530 0.2964 0.3267 0.5854 0.5577 0.3636 0.2805 0.5113  0.4371 0.3743 0.3746 0.3976  Th/ U  238  3 7 3 4  5 3 6 5  6 5 7 6 5 7 46 13 8 4 3 4 5 7 4 3 4  Pb* (ppm)  206  17.51 16.57 16.95 16.37  17.45 17.22 17.18 17.41 17.40 17.16 5.35 17.35 16.87 17.17 17.27 17.29 17.08 16.93 17.22 17.22 17.41  16.76 15.25 16.88 17.27  Total 238 U/206Pb ± (%)  1.6 1.7 1.5 1.7  1.5 1.6 1.5 1.7 1.5 1.5 1.4 1.4 1.5 2 1.8 1.6 1.6 1.5 1.7 1.7 1.6  1.7 1.2 1.7 1.5  0.0566 0.0552 0.0558 0.0514  0.0548 0.0521 0.0544 0.0505 0.0550 0.0518 0.0773 0.0548 0.0548 0.0586 0.0524 0.0556 0.0546 0.0533 0.0548 0.0541 0.0527  0.0482 0.0871 0.0573 0.0517  Total 207 Pb/206Pb 5 2.6 4.6 4  3 3.9 2.7 3.4  2.9 3.2 2.7 2.9 3 2.7 0.86 1.9 2.4 3.3 4.3 3.7 3 2.7 3.7 3.9 3.3  ± (%)  357 377 369 384  359 365 365 362 360 366 1104 361 371 363 364 362 367 370 364 364 360  376 393 370 364  Pb/238U Age  206  1σ  6 6 6 6  5 6 5 6 6 5 15 5 5 7 6 6 6 6 6 6 6  6 5 6 5  136  51 60 68 78 107 55 38 70 105 218 100 83  U (ppm)  AJW-03-179-1 AJW-03-179-2 AJW-03-179-3 AJW-03-179-4 AJW-03-179-5 AJW-03-179-6 AJW-03-179-7 AJW-03-179-8 AJW-03-179-9 AJW-03-179-10 AJW-03-179-11 AJW-03-179-12 AJW-03-179-13 AJW-03-179-14 AJW-03-179-15 AJW-03-179-16  68 99 88 317 52 69 100 61 64 102 127 124 65 86 130 121  AJW-03-179 SW Oyu GDi-P3  AJW-03-185-5 AJW-03-185-6 AJW-03-185-7 AJW-03-185-8 AJW-03-185-9 AJW-03-185-10 AJW-03-185-11 AJW-03-185-12 AJW-03-185-13 AJW-03-185-14 AJW-03-185-15 AJW-03-185-16  Spot  28 58 47 204 23 27 59 27 25 58 54 69 29 26 90 64  24 17 28 42 62 22 14 20 32 93 40 32  Th (ppm)  232  0.4275 0.6034 0.5525 0.6656 0.4590 0.4080 0.6096 0.4574 0.4042 0.5833 0.4375 0.5748 0.4574 0.3078 0.7150 0.5458  0.4936 0.3011 0.4295 0.5527 0.5993 0.4130 0.3825 0.2919 0.3157 0.4432 0.4171 0.3998  Th/ U  238  3 5 5 16 3 3 5 3 3 5 6 6 3 4 6 6  3 3 3 4 6 3 2 3 5 11 5 4  Pb* (ppm)  206  17.25 17.03 16.74 16.78 17.29 17.23 17.57 17.08 17.39 17.38 17.51 17.28 16.70 17.26 17.27 16.76  17.02 17.59 16.93 17.04 16.51 16.85 16.80 17.17 17.16 17.65 17.57 17.00  Total 238 U/206Pb ± (%)  1.8 1.7 1.7 1.5 2.1 1.8 2.3 2 1.9 1.7 1.6 1.6 1.9 1.7 1.6 1.6  1.8 1.8 1.7 2 1.6 1.8 2.3 1.7 1.6 1.5 1.7 1.7  0.0541 0.0536 0.0529 0.0541 0.0543 0.0559 0.0537 0.0568 0.0531 0.0549 0.0554 0.0546 0.0520 0.0536 0.0559 0.0520  0.0555 0.0571 0.0581 0.0600 0.0571 0.0521 0.0650 0.0542 0.0535 0.0548 0.0543 0.0549  Total 207 Pb/206Pb ± (%)  3.3 2.8 2.9 1.6 3.8 3.3 2.8 3.5 3.5 2.7 2.4 2.5 3.5 3 2.5 2.5  4.3 3.9 3.7 5 2.9 4.3 16 3.7 2.6 1.8 2.6 2.8  363 368 375 373 362 363 357 366 361 360 357 362 376 363 362 375  367 355 368 365 378 373 368 365 365 355 357 368  Pb/238U Age  206  1σ  7 6 6 5 8 7 8 7 7 6 6 6 7 6 6 6  7 6 6 7 6 7 10 6 6 5 6 6  137  U (ppm)  AJW-04-385-1 AJW-04-385-2 AJW-04-385-3 AJW-04-385-4 AJW-04-385-5 AJW-04-385-6 AJW-04-385-7 AJW-04-385-8 AJW-04-385-9 AJW-04-385-10 AJW-04-385-11 AJW-04-385-12  106 96 141 89 144 58 57 147 109 96 87 81  AJW-04-385 OT West QMD-E  Spot  38 37 73 50 45 25 25 50 59 35 29 50  Th (ppm)  232  0.3710 0.3998 0.5330 0.5873 0.3265 0.4537 0.4498 0.3557 0.5620 0.3776 0.3420 0.6289  Th/ U  238  Pb* (ppm)  206  5 5 7 5 7 3 3 8 5 5 4 4  17.08 17.20 17.42 16.69 17.17 17.64 17.64 16.79 17.44 16.90 17.27 17.36  Total 238 U/206Pb ± (%)  1.7 1.4 1.2 1.4 1.2 1.6 1.6 1.2 1.3 1.3 1.4 1.6  0.0545 0.0532 0.0548 0.0526 0.0537 0.0533 0.0579 0.0548 0.0534 0.0555 0.0559 0.0526  Total 207 Pb/206Pb ± (%)  3.3 3.5 2.9 3.6 2.9 4.6 4.4 2.7 3.3 4.3 3.6 3.8  367 365 359 376 365 356 354 373 360 370 362 362  Pb/238U Age  206  1σ  6 5 4 5 4 6 6 4 5 5 5 6  Figure 4.8 SHRIMP-RG U-Pb age diagrams for Late Devonian OTIC phases. Ages corresponding to black-filled symbols were used in the 207Pb-corrected 206Pb/238U weighted mean age calculation. Ages corresponding to grey-filled symbols were not used in the weighted mean age calculation.  138  139  U2  (ppm)  Wt  (µg)  (ppm)  Pb*3  Pb4 Pb  206 204  7  6  4  3  B  C  D  E  87  293  108  43  120  18.7  6.8  2.7  7.6  5.3  29  24  21  15  32  11  ~30  A4  A5  A7  CA1  CA2  CAL  29  A2  A3  30  A1  135  75  48  48  62  43  63  56  59  6.9  4.7  3.0  2.9  3.8  2.7  3.9  3.4  3.6  944  529  142  420  266  125  1293  598  630  489  717  1156  1268  1295  AJW-03-181 QMD-P1 Central Oyu  9  A  AJW-03-178 QMD P-1 Southwest Oyu  Fraction1  116.0  2.4  9.8  4.3  10.1  5.4  5.9  4.8  5.0  3.5  3.2  7.7  7.6  11.5  (pg)  Pb5  0.46  0.56  0.46  0.48  0.42  0.50  0.46  0.45  0.44  0.60  0.49  0.47  0.60  0.41  Th/U6  1.8  22.0  9.7  10.2  7.7  11.7  18.9  20.7  21.1  16.1  8.6  2.1  6.9  4.1  Pbc  Pb*7 Pb/238U  0.04976 ± 0.42  0.05936 ± 0.17  0.05928 ± 0.19  0.05917 ± 0.22  0.05934 ± 0.14  0.05896 ± 0.16  0.05913 ± 0.13  0.05932 ± 0.14  0.05894 ± 0.11  0.05948 ± 0.11  0.06040 ± 0.19  0.06091 ± 0.34  0.05921 ± 0.17  0.05953 ± 0.19  206  Pb/235U  0.3710 ± 2.02  0.4398 ± 0.78  0.4406 ± 1.79  0.4406 ± 0.86  0.4395 ± 0.65  0.4392 ± 0.57  0.4392 ± 0.36  0.4408 ± 0.38  0.4388 ± 0.34  0.4421 ± 0.83  0.4461 ± 1.64  0.4572 ± 2.94  0.4360 ± 0.94  0.4392 ± 1.11  207  Isotopic ratios ±1σ,% 8 Pb/206Pb  0.05407 ± 1.80  0.05374 ± 0.72  0.05390 ± 1.69  0.05401 ± 0.79  0.05372 ± 0.58  0.05402 ± 0.52  0.05387 ± 0.31  0.05389 ± 0.33  0.05400 ± 0.30  0.05390 ± 0.78  0.05356 ± 1.57  0.05444 ± 2.78  0.05341 ± 0.87  0.05352 ± 1.02  207  0.60038  0.43249  0.59003  0.42845  0.55875  0.44216  0.52370  0.52023  0.48494  0.45953  0.45424  0.52147  0.46851  0.51883  ρ9  16.7  -3.3  -1.2  0.2  -3.6  0.7  -1.3  -1.4  0.5  -1.5  -7.4  2.2  -7.3  -6.5  discordant  % 10 Pb/238U  313.1 ± 2.6  371.8 ± 1.2  371.2 ± 1.3  370.6 ± 1.6  371.6 ± 1.0  369.3 ± 1.2  370.3 ± 0.9  371.5 ± 1.0  369.2 ± 0.8  372.5 ± 0.8  378.1 ± 1.4  381.1 ± 2.5  370.8 ± 1.2  372.8 ± 1.4  206  Pb/235U  320.4 ± 11.1  370.1 ± 4.8  370.7 ± 11.1  370.7 ± 5.3  369.9 ± 4.0  369.7 ± 3.6  369.7 ± 2.2  370.8 ± 2.4  369.4 ± 2.1  371.7 ± 5.2  374.5 ± 10.3  382.3 ± 18.8  367.4 ± 5.8  369.7 ± 6.9  207  Pb/206Pb  374.0 ± 79.2/83.3  360.1 ± 32.2/32.9  367.1 ± 74.3/77.9  371.4 ± 35.2/36  359.1 ± 26.1/26.5  371.9 ± 23.4/23.7  365.6 ± 14.1/14.2  366.6 ± 15/15.1  371.1 ± 13.6/13.7  367.1 ± 35/35.7  352.7 ± 69.3/72.4  389.3 ± 120.2/130  346.2 ± 39/40  350.7 ± 45.5/46.8  207  Apparent ages ±2σ,Ma 8  Table 4.4 TIMS U-Pb Analytical data for Late Devonian intrusions. Fractions with prefix ”CA” are chemically-abraded. CAL is the leachate fraction. All other fractions are air-abraded.  140  U2  (ppm)  Wt  (µg)  (ppm)  Pb*3 Pb  204  7  7  7  C  D  E  140  119  110  176  8.6  7.3  6.8  10.9 216  608  771  4  8  7  5  E  CA1  CA2  CA4  145  157  68  167  278  159  344  155  8.6  10.1  9.6  4.3  9.4  15.9  9.1  21.6  2657  1921  612  538  244  264  81  1619  1.2  2.1  3.4  4.5  13.3  6.9  41.8  2.4  15.1  4.7  6.0  27.1  (pg)  Pb5  0.28  0.47  0.49  0.30  0.28  0.27  0.39  0.25  0.53  0.56  0.50  0.50  Th/U6  42.1  32.0  10.0  8.3  3.5  3.9  1.0  25.3  3.3  10.0  12.6  3.1  Pbc  Pb*7 Pb/238U  0.07088 ± 0.10  0.05917 ± 0.28  0.06052 ± 0.22  0.05674 ± 0.24  0.05830 ± 0.21  0.05821 ± 0.20  0.06197 ± 0.82  0.05721 ± 0.16  0.05824 ± 0.20  0.05836 ± 0.19  0.05920 ± 0.16  0.05884 ± 0.29  206  Pb/235U  0.5421 ± 0.34  0.4442 ± 0.64  0.4683 ± 1.88  0.4227 ± 2.26  0.4303 ± 0.95  0.4290 ± 2.65  0.4704 ± 2.68  0.4209 ± 0.51  0.4299 ± 2.46  0.4344 ± 0.5  0.4404 ± 0.35  0.4442 ± 1.38  207  Isotopic ratios ±1σ,% 8 Pb/206Pb  0.05547 ± 0.30  0.05444 ± 0.60  0.05613 ± 1.76  0.05403 ± 2.12  0.05353 ± 0.83  0.05346 ± 2.54  0.05505 ± 2.21  0.05336 ± 0.47  0.05354 ± 2.34  0.05399 ± 0.42  0.05396 ± 0.27  0.05476 ± 1.23  207  0.46844  0.35672  0.61343  0.64604  0.60222  0.58110  0.67107  0.39928  0.57019  0.57593  0.66727  0.61538  ρ9  -2.5  5.0  17.7  4.6  -4.0  -4.9  6.6  -4.3  -3.9  1.4  -0.4  8.6  discordant  % 10 Pb/238U  441.4 ± 0.8  370.6 ± 2.0  378.8 ± 1.6  355.8 ± 1.7  365.3 ± 1.5  364.7 ± 1.4  387.6 ± 6.2  358.6 ± 1.1  364.9 ± 1.5  365.6 ± 1.4  370.7 ± 1.2  368.6 ± 2.1  206  Pb/235U  439.8 ± 2.4  373.2 ± 4.0  390.0 ± 12.2  358.0 ± 13.7  363.4 ± 5.8  362.5 ± 16.2  391.4 ± 17.4  356.7 ± 3.1  363.1 ± 15.0  366.3 ± 3.1  370.6 ± 2.2  373.2 ± 8.6  207  Pb/206Pb  431.2 ± 13.4/13.5  389.4 ± 26.6/27  457.5 ± 76.1/79.9  372.4 ± 92.5/98.1  351.4 ± 37.3/38.1  348.1 ± 111/119.2  414.2 ± 95.8/101.9  344.3 ± 21.3/21.5  351.6 ± 102.6/109.6  370.6 ± 19/19.2  369.4 ± 12.2/12.3  402.3 ± 54/55.8  207  Apparent ages ±2σ,Ma 8  2  All analyzed zircon grains were air abraded or chemically abraded; fraction names: A, B, etc., air abraded; CA1, CA2, etc., chemically abraded. U blank correction of 0.2-1.0 pg ± 20%; U fractionation corrections were measured for each analysis with a double 233-235U spike. 3 Radiogenic Pb 4 Measured ratio corrected for spike and Pb fractionation of 0.23-0.35/amu ± 20% (Daly collector), which was determined by repeated analysis of NBS Pb 982 reference material throughout the course of this study. 5 Total common Pb in analysis based on blank isotopic composition. 6 Model Th/U derived from radiogenic 208Pb and the 207Pb/206Pb age of fraction. 7 Ratio of radiogenic to common Pb 8 Blank and common Pb corrected; blank Pb based on procedural blanks measured throughout the course of the study: amount, 0.4-10 pg; composition, 206Pb/204Pb = 18.5 ± 3%, 207Pb/204Pb = 15.5 15.0 ± 3%, 208Pb/204Pb = 36.4 ± 3%. Common Pb compositions are based on Stacey-Kramers model Pb at the 207Pb/206Pb age of the fraction or the interpreted age of the rock (Stacey and Kramers, 1975). 9 Correlation coefficient. 10 Discordance in % to origin.  1  3  3  D  2  B  C  7  A  AJW-06-476 GDi-P2 Hugo Dummett  10  B  206  Pb4  206  AJW-06-467 QMD-P1 Central Oyu  Fraction1  Figure 4.9 Concordia diagrams showing TIMS U-Pb data for Late Devonian intrusions. Data point errorellipes are 2σ. Filled ellipses correspond to air-abraded single-grain fractions and unfilled ellipses correspond to single-grain chemically-abraded fractions. The one analysis not included in the Concordia plot for AJW-06-476 was interpreted as a xenocryst (441 Ma). The one analysis not included for sample AJW-03-181 was the leachate (313 Ma). See text for interpretation of the TIMS U-Pb data for these samples.  141  Table 4.5 Summarized U-Pb (zircon) ages for Late Devonian intrusions. Intrusive phase  Sample  Location  U-Pb Age (Ma)  2σ  Method  QMD-E QMD-P1 QMD-P1 QMD-P1 QMD-P1 QMD-P1 GDi-P2 GDi-P3 GDi-P3 GDi-P3 GDi-P3 QMD-P1 QMD-E  AJW-03-182 AJW-04-356 AJW-03-181 AJW-03-181 AJW-03-178 AJW-03-178 AJW-06-476 AJW-03-185 AJW-03-184 AJW-04-250 AJW-03-179 AJW-06-467 AJW-04-385  South Oyu Hugo Dummett Central Oyu Central Oyu Southwest Oyu Southwest Oyu Hugo Dummett Hugo Dummett Hugo Dummett Hugo Dummett Southwest Oyu Central Oyu Central Oyu West  374 369 371 368 369 372 see text 366 367 see text 363 see text 364  ±3 ±2 ±1 ±3 ±5 ±1 n/a ±4 ±2 n/a ±4 n/a ±4  SHRIMP-RG SHRIMP-RG TIMS SHRIMP-RG SHRIMP-RG TIMS TIMS SHRIMP-RG SHRIMP-RG TIMS SHRIMP-RG TIMS SHRIMP-RG  Grains used in age calculation 12 8 4 12 12 3 n/a 12 16 n/a 12 n/a 12  Reject grains 0 4 5 3 1 2 n/a 4 1 n/a 4 n/a 0  142  Discussion Petrologic and petrochemical evolution of OTIC intrusions The QMD phases are petrologically distinct from the younger, weakly altered and mineralized GDi phases that contain less ferromagnesian and opaque phenocrysts, are Kfeldspar-poor and strongly apatite-saturated relative to the QMD intrusions. The OTIC quartz monzodiorite intrusions are hornblende- and magnetite-bearing and lack Euanomalies suggesting that they are derived from an oxidized melt with high H2O content, likely a prerequisite for an igneous system that generates a porphyry deposit (Dilles, 1987; Mueller et al., 2001). The abundance of hydrous minerals in general, such as hornblende and biotite phenocrysts as well as apatite microphenocysts, is consistent with the volatile-rich nature of the magmas (Mueller and Forrestal, 1998). The association of mineralization with multi-phase intrusions is common in porphyry systems (e.g. Gustafson and Hunt, 1975; Sillitoe and Gappe, 1984; Carten et al., 1988). Similarly, evolution from mafic to felsic compositions and toward increasingly porphyritic textures has been documented in porphyry districts (Lang and Titley, 1998; Lickfold et al., 2003). The OTIC intrusions fall into two discrete petrochemical groups, one that is siliceous and rich in nickel and chromium with high Sm/Yb and La/Yb (GDi), and one that is poor in nickel and chromium with less fractionated REE (QMD). These two groups are also apparent as discrete linear arrays in major-element oxide plots, indicating that the two series are representative of distinct magmatic lineages. Distinctively high Ni and Cr contents in the granodiorite porphyries may have been the result of the injection of mafic magma into an evolved magma chamber or possibly by assimilation of xenolithic mafic material.  Magma mixing evidence may also be manifested as mixed zircon  populations within single rock samples based on mineral-chemical data (Chapter 5). U-Pb ages of ore-stage to late-mineral OTIC intrusions Individual SHRIMP-RG ages for QMD rock samples range from 374 ± 3 Ma to 364 ± 4 Ma and GDi samples range in age from 367 ± 2 Ma to 363 ± 2 Ma, however there are likely two main intrusion events in the Late Devonian; one at ~372 Ma, and one at ~366 Ma (Figure 4.10). The petrologic and petrochemical similarities between rocks  143  from each suite as well as the overlapping uncertainties between dated rocks within the suites are compelling and permit this interpretation. Most of the SHRIMP-RG spot ages are slightly younger than these interpreted ages (i.e. ~372 Ma and ~366 Ma), and also younger than the air-abraded and chemically-abraded grains from TIMS U-Pb analysis and this probably reflects some lead-loss in these grains. The weighted mean age for all SHRIMP-RG spot analyses from QMD samples samples is 370 ± 2 Ma (n = 64; 5 samples) and the weighted mean age for all spot analyses from the granodiorites is 365 ± 2 Ma (n = 48; 3 samples). These ages are interpreted to be slightly younger than those interpreted as magmatic age grains from the TIMS U-Pb work as a result of lead-loss. Moreover, a peak at approximately 374 Ma on the age versus number of grains histogram for QMD samples (Figure 4.10) suggests the possibility that there are pre-mineral zircons and possibly pre-mineral intrusions such as sample AJW-03-182 (QMD-E; 374 ± 3 Ma). U-Pb (zircon) results for samples AJW-06-476 (GDi-P2) and AJW-06-467 (QMD-P1) are more ambiguous as there is a range of ages among the single-grain fraction analyses, likely the result of mixed populations of xenocrystic, magmatic-age and lead-loss grains, although it is significant that the GDi-P2 sample has two grains at ~365 Ma, the preferred age for the GDi suite.  144  Figure 4.10 U-Pb (zircon) age histograms for OTIC intrusions. A) Granodiorite samples and B) quartz monzodiorite samples.  145  Petrochemical characteristics of fertile island arcs Two additional compositional fields have been plotted along with OTIC geochemistry to compare rocks from this study to that from modern island arc settings in order to place the OTIC dataset within a global context.  Inasmuch, the Sunda-  Banda/Luzon arc (fertile arc reference suite), is known to host large porphyry Cu-Au deposits (e.g. Tampakan, Batu Hijau, Santo Tomas, Lepanto) whereas the Lesser Antilles/New Britain/Aleutian arcs (infertile arc reference suite) do not host known porphyry districts. To varying degrees, the QMD and GDi suites are more similar to the fertile arc suite in that the rocks are generally more alkaline (particularly the QMD suite) (Figure 4.4b), have higher La/Yb and Sm/Yb ratios (Figure 4.6a and 4.6b), are more similar to adakites (Figure 4.6c), and some of the intrusive phases have higher nickel and chromium contents compared to both the fertile and unfertile arc suites (Table 4.6). It should be noted, however, that in general, intrusive rocks from large porphyry Cu-Au deposits in oceanic island and continental arcs are not necessarily potassic, nor adakitelike (Figure 4.11), nor do they necessarily have high nickel and chromium concentrations (Table 4.6). Linkages between high-K magmas and gold-rich magmatic-hydrothermal ore deposits have been recognized by previous workers (e.g. Mueller and Groves, 1993; Sillitoe, 1997; Mueller and Forrestal, 1998; Mueller et al., 2001).  Sillitoe (1997)  furthermore suggests that although less that 3% of rocks in the geologic record of the Pacific Rim are potassic, greater than 70% of the porphyry to epithermal deposits are associated with these rocks. Mueller and Groves (1993) suggest that alkaline rocks associated with porphyry copper–gold and epithermal gold mineralization are characterized by high halogen concentrations, particularly Cl, and by high oxygen fugacities. The presence of sufficient halogens, such as Cl, is thought to control the abundances of Au and Cu in saline aqueous fluids that exsolve from magma (Webster, 1992). Potassium enrichment in volcanic arc rocks is common (Peccerillo and Taylor 1976; Foley and Peccerillo 1992) and a number of models have been proposed to explain the origin of potassic magmas. Whitford et al. (1979) suggests that primary high-K calc-  146  alkaline magmas are formed by 5–15% mantle melting at 40–60 km depth in convergent margins, whereas tholeiitic magmas are produced by larger degrees of partial melting of mantle peridotite at shallower depths. Price et al. (1999) proposed that high-K island arc magmas may be attributed to rising geothermal gradients at the base of the crust and mixing of pristine arc melts with partially-melted amphibolitic underplated lower crust. Alkaline magmas may also be generated from mantle that had been enriched during previous episodes of subduction (Kennedy et al., 1990). Other processes that may cause K-enrichment are thought to be related to the role of wedge metasomatism by fluids and/or melts derived from the down-going slab during subduction (e.g. Rogers and Setterfield, 1994; Feldstein and Lange, 1999). A spatial and temporal association between adakite-like magmas and porphyries has been previously recognized (Thieblemont et al., 1997, Sajona and Maury, 1998; Oyarzun et al., 2001; Mungall, 2002), although the origin of the adakite geochemical signature is uncertain as is their role in the formation of ore deposits (e.g. Castillo et al., 1999; Richards, 2002; Richards and Kerrich, 2007). Moreover, some authors suggest that porphyry Cu-Au deposits require the injection of mafic magma into a sulfur-saturated intermediate magma chamber in order to supply required sulfur, volatiles and chalcophile elements to the system (e.g. Waite et al., 1998; Hattori and Keith, 2001; Maughan et al., 2002).  This event may be manifested as an increased concentration of nickel and  chromium in relatively evolved rocks (Maughan et al., 2002). Both fertile and unfertile arc magma suites have relatively low nickel and chromium contents compared to some porphyry districts such as Bingham Canyon, Batu Hijau and Bajo de la Alumbrera. (Table 4.6), therefore the enrichment of these elements is probably not generally arcwide, rather restricted to specific segments or possibly the porphyry districts. Regardless of the origin of the geochemical signature, the OTIC intrusions have high Sr/Y and and low Yb compared to most arc-related rocks and the GDi-P2 phase is distinctly adakitelike (Figure 4.6c). The localized enrichment in nickel and chromium is present in the Oyu Tolgoi district as well, although the Ni- and Cr- anomalous samples are related to late-mineral granodiorite intrusions that largely truncate strongly mineralized rocks.  147  Figure 4.11 Geochemistry of intrusions from worldwide porphyries. SiO2 vs. K2O after Peccerillo and Taylor (1976) (A) and SiO2 vs. Sr/Y (B). Data is from Oyu Tolgoi (this study) and a selection of other large porphyry Cu-Au deposits in the world. Data for Bingham Canyon is from Waite et al. (1998); data for Grasberg is from McMahon (1994); data for Batu Hijau is from Garwin (2001); data for Cadia Hill is from Holliday et al. (2002); data for Goonumbla is from Heithersay and Walshe (1995); data for Bajo de la Alumbrera is from Ulrich and Heinrich (2001).  148  Table 4.6 Nickel and chromium concentrations from ore-stage intrusive rocks in Cu-Au porphyry deposits. The SiO2 (wt. %) range for the data sets is approximately 55 wt. % to 70 wt. %.  Locality Bingham, Utah Grasberg, Irian Jaya  Batu Hijau, Indonesia  Cadia Hill, NSW, Australia Bajo de la Alumbrera, Argentina  OTIC average QMD-E OTIC average QMD-P1 OTIC average GDi-P2 OTIC average GDi-P3 Infertile island arc Fertile island arc  Ni (ppm)  Cr (ppm)  52 6 5 3 6 4 3 7 5 4 4 6 6 6 56 181 174 65 58 <5 6 17 21 18 11  166 6 6 9 8 9 4 270 249 161 123 269 9 10 208 298 281 168 207 <10 <10 64 38 33 26  Reference Waite et al. (1998) McMahon (1994)  Garwin (2001)  Holliday et al. (2002) Ulrich and Heinrich (2001)  This study  http://georoc.mpchmainz.gwdg.de/georoc/  149  Conclusions Intrusive rocks derived from at least two separate magmatic lineages comprise the OTIC of southern Mongolia; quartz monzodiorites and granodiorites. The intrusions were emplaced at ~372 Ma (QMD) and ~366 Ma (GDi) and rocks from the OTIC have petrologic characteristics that are similar to igneous complexes in other porphyry districts. These include the presence of hornblende, apatite and magnetite phenocrysts, small negative Eu-anomalies, an evolution from mafic to felsic compositions and equigranular to porphyritic textures through time. Discrete high La/Yb intrusive phases, adakite-like phases and Ni- and Cr-rich phases suggests a complex, possibly replenished magma chamber. Finally, Oyu Tolgoi porphyry deposits are conspicuously affiliated with relatively alkaline rocks, adakite-like rocks as well as somewhat elevated Ni and Cr contents; characteristics that discriminate these rocks from others that are common in modern island arcs.  150  References Badarch, G., Dickson C.W. and Windley, B.F., 2002, A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia, Journal of Asian Earth Sciences, v. 21, p. 87-110. 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As a common accessory mineral found in many rock types in diverse environments and able to survive multiple geologic episodes, trace element compositions from zircons have been used in provenence studies (e.g. Hoskin and Ireland, 2000; Belousova et al., 2002; Grimes et al., 2007), mineral deposit studies (Ballard et al., 2002; Liang et al., 2006; Harris et al., 2007), as well as in petrogenetic studies for rocks as old as Hadean (Watson and Harrison, 2005). Application of trace element geochemistry of zircon is important in porphyry copper systems as specific igneous geochemical characteristics and magma chamber events may be related to the genesis of large deposits.  For example, there is an  association between unusually oxidized arc magmas and igneous complexes associated with the formation of porphyry deposits given the linkage between oxidized magmas and magmatic-hydrothermal Cu-Au mineralization (e.g. Blevin and Chappell, 1992; Candela, 1992; Hedenquist and Lowenstern, 1994; Mungall, 2002, Sun et al., 2004). This likely involves redox control on the speciation/solubility of magmatic sulfur and its influence on the fractionation of chalcophile elements (e.g. Cline and Bodnar, 1991; Pasteris, 1996; Jugo et al., 1999; Ballard et al., 2002). Furthermore, the addition of primitive melt into an intermediate magma chamber may be an important factor in the genesis of giant porphyry deposits. Felsic intrusions may contain enough copper to account for large porphyry deposits; however, due to low solubility of sulfur in felsic melts, an unreasonable volume of material would be required to account for the large sulfur anomaly observed in the deposits. Sulfur is relatively soluble in basaltic melts and a 4  A version of this chapter will be submitted to a refereed journal for publication: Wainwright, A.J., Tosdal, R.M., Wooden, J.L. and Mazdab, F.K., in prep., Cooling, fractionation and mixing of magmas in the super-giant Oyu Tolgoi Cu-Au porphyries; SHRIMP-RG trace element geochemistry of zircons.  156  small addition of mafic material into a felsic porphyry root pluton could supply the required sulfur to the hydrothermal system (Dilles and Proffett, 1995). Injection of mafic magma into porphyry systems has been suggested as a contributor to giant porphyry copper deposit formation at Bingham Canyon (Waite et al., 1998; Hattori and Keith, 2001; Maughan et al., 2002), Grasberg (Pollard and Taylor, 2002) and El Salvador (Cornejo et al., 1997). An understanding of physico-chemical factors is critical to the development of models that explain ore deposit genesis, deposit location in space and time, as well as providing geologic criterion that improve exploration programs for metal deposits associated with magmas. In this chapter, SHRIMP-RG trace element geochemistry of zircon is used to investigate the evolution of the Oyu Tolgoi Intrusive Complex (OTIC) and possible linkages to the development of super-giant copper-gold deposits in the district. Geologic setting The Oyu Tolgoi Cu-Au district is located in the Central Asian Orogenic Belt; a tectonic supercollage that contains craton fragments, Proterozoic to Paleozoic ophiolites and accreted volcanic arcs (Badarch et al., 2002; Buchan et al., 2002). The Oyu Tolgoi porphyry district is underlain by a sequence of Devonian or older rocks in the Gurvansayhan terrane (Badarch et al., 2002; Helo et al., 2006), interpreted to represent a juvenile oceanic island arc (Helo et al., 2006, Chapter 6). The copper-gold deposits are hosted in an inlier of Late Devonian intrusions and Devonian or older volcanic and volcaniclastic rocks (Alagbayan Group) surrounded by disconformably-overlying Carboniferous supracrustal packages (Gurvankharaat Group) (Chapter 2; Figure 6.1). The copper-gold deposits are spatially and temporally related to a sequence of Late Devonian intrusive phases that comprise the Oyu Tolgoi Igneous Complex (OTIC). Quartz monzodiorite phases (QMD) are porphyritic (QMD-P1) to coarsely equigranular (QMD-E), characteristically red when fresh and high-K calc-alkaline with moderate La/Yb ratios. Granodiorites (GDi) are divided into two groups: 1) fine-grained, mediumto high-K calc-alkaline porphyries with low Sr/Y (GDi-P3) and 2) coarse-grained, high-K calc-alkaline porphyries with high Sr/Y (GDi-P2). All Late Devonian intrusive phases 157  Figure 5.1 Bedrock geology map of the Oyu Tolgoi district, Mongolia, drawn by Ivanhoe Mines geology staff and AJW from mapping of drill holes and trenches through younger cover sequences as well as outcrop mapping. Inset map shows the location of Oyu Tolgoi in the southern Gobi desert of Mongolia. Coordinates are WGS 84 zone 48. Copper grade shell for the Hugo Dummett zone is projected to surface.  158  are HREE-depleted with small negative Eu-anomalies and both granodiorite phases display extremely high La/Yb ratios and high Ni and Cr concentrations compared to the QMD series. Distinct linear arrays on SiO2 vs. MgO and SiO2 vs. Al2O3 plots as well as different La/Yb ratios and nickel and chromium concentrations that characterize the rock suites suggest that magmas with separate petrogenetic lineages are present, as well as the possibility that a magma recharge event occurred (Chapter 4). Cross-cutting relationships and U-Pb (zircon) geochronology (SHRIMP-RG and TIMS) indicate that the OTIC intrusions are separated into two age groups: Quartz monzodiorites (~372 Ma) and granodiorites (~366 Ma) (Chapter 4).  U-Pb (zircon)  crystallization ages for post-mineral granitoid plutons and dikes in the Oyu Tolgoi district range from 350 ± 4 Ma to 321 ± 4 Ma. These rocks include andesite to rhyolite dikes as well as coarse-grained granitoids (Chapter 2). Analytical method Five to fifteen kilogram rock samples were processed using a Rhino jaw crusher, a Bico disk grinder equipped with ceramic grinding plates, and a Wilfley wet shaking table equipped with a machined Plexiglass top, followed by conventional heavy liquids and magnetic separation using a Frantz magnetic separator. Individual zircon grains were then handpicked for analysis. The analytical work was performed at the Stanford-USGS Sensitive High mass-Resolution Ion MicroProbe-Reverse Geometry (SHRIMP-RG) lab. Zircons were mounted in epoxy, polished, photographed, and coated with ~10 nm of gold. Polished mounts were cleaned with soap, HCl, and distilled water and dried prior to coating with gold. A summary of the applied techniques are in Mazdab and Wooden (2006).  Prior  to  SHRIMP-RG  analysis,  zircons  were  examined  under  cathodoluminescence (CL) and reflected light to reveal zonation patterns, discordant cores and if the single grains contained imperfections such as cracks or holes. This information guided selection of the grains and grain locations for the spot analyses. A variety of different growth zones were analyzed that include oscillatory-zoned core-torim pairs, sector-zoned areas and discordant cores. Furthermore, on some of the larger grains, core-to-rim traverses were performed with three or more analytical spots. Representative CL images of the zircons are presented in Figure 5.2 and CL images for 159  all of the analytical spot locations are located in Appendix 3. The sample characteristics and U-Pb ages are listed in Table 5.1 and the trace element data is presented in Table 5.2. Results Trace element concentrations (SHRIMP-RG) for zircons are from twelve Oyu Tolgoi samples: five quartz monzodiorite samples, four granodiorite samples as well as three Carboniferous intermediate to felsic intrusions (Table 5.1).  All of the quartz  monzodiorite samples are equigranular to porphyritic and the granodiorite samples are sparsely to crowded feldspar-porphyritic (Chapter 4). The Carboniferous rocks include hornblende-biotite andesite porphyry (345 ± 2 Ma), fine-grained rhyolite (340 ± 3 Ma) and coarse, equigranular hornblende-biotite granite (324 ± 3 Ma) (Table 5.1; Chapter 2). Due to petrologic and petrochemical similarities of rocks within each suite as well as the uncertainty in SHRIMP-RG ages, one main quartz monzodiorite intrusion event at ~372 Ma is interpreted and the granodiorite intrusion event is interpreted to post-date the quartz monzodiorite event at ~366 Ma (Chapter 4). Hence, data is compared from three main magmatic-geochronologic groups: syn-mineral quartz monzodiorite intrusions (~372 Ma) and late- mineral granodiorite intrusions (~366 Ma) that comprise the OTIC, as well as a group of younger Carboniferous intrusions (~345 to ~324 Ma) that are not related to known porphyry Cu-Au mineralization. Although some zircon spot analytical locations have both a U-Pb age in addition to trace element data, it is inappropriate to link the two datasets due to the large uncertainty on the individual spot ages (typically ± 5 to 10 Ma; 1σ). Therefore all zircon analytical spots from an individual rock sample are assigned the interpreted age of that rock.  160  161  Intrusive phase  QMD-E QMD-P1 QMD-P1 QMD-P1 QMD-P1 QMD-P1 QMD-E GDi-P3 GDi-P3 GDi-P3 GDi-P2 Andesite dike Rhyolite dike Granite  Sample  AJW-03-182 AJW-04-356 AJW-03-181 AJW-03-181 AJW-03-178 AJW-03-178 AJW-04-385 AJW-03-179 AJW-03-185 AJW-04-250 AJW-06-476 AJW-03-183 AJW-03-180 AJW-03-132  South Oyu Hugo Dummett Central Oyu Central Oyu Southwest Oyu Southwest Oyu Central Oyu West Southwest Oyu Hugo Dummett Hugo Dummett Hugo Dummett Southwest Oyu Central Oyu 12km SW of OT  Location 374 369 372 368 369 373 364 363 366 ~366 ~366 345 340 324  U-Pb Age (Ma) ±3 ±2 ±1 ±3 ±5 ±1 ±4 ±4 ±4 n/a n/a ±2 ±3 ±3  2σ (Ma) SHRIMP-RG SHRIMP-RG TIMS SHRIMP-RG SHRIMP-RG TIMS SHRIMP-RG SHRIMP-RG SHRIMP-RG TIMS TIMS SHRIMP-RG SHRIMP-RG SHRIMP-RG  Method Chapter 4 Chapter 4 Chapter 4 Chapter 4 Chapter 4 Chapter 4 Chapter 2 Chapter 4 Chapter 4 Chapter 3 Chapter 4 Chapter 2 Chapter 2 Chapter 2  Reference  Table 5.1 Summary of U-Pb (zircon) ages for the intrusions in the study. QMD refers to quartz monzodiorite; GDi refers to granodiorite.  162  Ti  Y  La  Ce  14 12 16 14 10 13 8 11 9 13 10 18 18 15  1063 536 1293 546 603 554 1223 584 759 1487 600 1019 631 532  0.01 0.01 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.01 0.01 0.01  13 9 13 9 12 10 22 12 22 16 11 10 10 10  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.1  Pr  1.1 1.2 1.3 1.4 2.1 2.2 3.1 4.1 4.2 5.1  11 13 12 8 14 7 8 15 4 17  1033 453 617 621 557 558 507 1086 109 1117  0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.01  12 11 16 18 9 14 13 13 4 12  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1  AJW-04-385 QMD-E West of Central Oyu  1.1 1.2 13.1 13.2 13.3 2.1 2.2 3.1 3.2 6.1 6.2 9.1 9.2 9.3  AJW-03-181 QMD-P1 Central Oyu  spot  0.9 0.2 0.3 0.3 0.2 0.2 0.2 1.2 0.0 1.2  0.8 0.2 1.4 0.3 0.2 0.2 0.4 0.2 0.3 1.3 0.2 1.0 0.3 0.2  Nd  2.7 0.8 1.0 0.8 1.0 0.6 0.8 3.2 0.1 3.3  2.6 0.9 3.6 0.9 0.9 0.9 1.9 0.8 1.1 3.4 0.7 3.0 1.1 1.7  Sm  1.1 0.3 0.4 0.3 0.4 0.2 0.3 1.3 0.0 1.1  1.1 0.4 1.4 0.4 0.3 0.4 0.7 0.3 0.3 1.3 0.4 1.2 0.4 0.6  Eu  44 18 25 24 22 22 22 46 4 45  44 22 54 23 24 22 51 23 30 59 24 42 25 40  Ho  25 8 10 9 11 7 9 27 1 27  25 9 29 10 10 9 23 10 12 30 9 25 11 17  Gd  9 3 4 4 4 3 4 9 0 9  9 4 10 4 4 4 9 4 5 11 4 9 4 7  Tb  103 40 55 51 50 45 49 106 7 106  102 47 120 50 53 48 114 48 64 135 48 100 56 88  Dy  202 90 125 128 110 115 112 207 24 214  196 104 243 109 122 112 250 118 154 294 117 194 126 208  Er  46 23 31 32 27 29 28 48 7 48  45 26 55 27 30 28 59 30 38 70 28 45 31 52  Tm  399 212 282 300 239 282 251 410 88 423  382 230 476 239 270 254 518 277 345 520 248 394 278 476  Yb  84 45 60 64 51 62 53 85 27 87  81 48 100 52 58 57 107 58 74 95 56 80 63 104  Lu  10743 10233 11489 11304 10106 13991 11040 10805 19123 9998  10623 10760 10413 10795 11165 11091 13244 12090 12731 9322 11328 10775 10222 20751  Hf  52 26 43 50 26 39 32 54 77 52  48 24 67 27 31 25 88 37 69 52 26 52 31 27  Th  90 65 110 132 55 119 81 91 760 85  78 55 110 59 72 65 165 88 173 83 66 85 74 72  U  780 796 789 751 804 733 749 810 699 818  799 785 815 803 768 793 744 780 754 792 768 827 827 810  Temp  Table 5.2 Trace element concentrations (ppm) for zircons. (Temp) is the minimum temperature in oC based on the TiO2-in-zircon thermometer of Watson and Harrison (2005) (with aTiO2 = 0.7). Ti values are 49Ti, also used in the calculation of temperature.  163  11 14 10 12 12 9  5.2 6.1 6.2 8.1 8.2 8.3  573 689 1077 1147 539 660  Y  0.01 0.01 0.01 0.00 0.01 0.01  La 10 13 23 13 10 13  Ce  8 6 8 7 6 9 10 8 7 8 8 7  462 432 500 505 627 533 660 418 228 389 520 186  0.13 0.00 0.00 0.00 0.03 0.01 0.01 0.00 0.03 0.00 0.00 0.01  1.1 10.1 10.2 11.1 11.2 2.1 2.2 3.1  15 20 19 24 25 18 13 22  614 1143 605 891 488 1325 893 1067  0.01 0.02 0.00 0.01 0.00 0.04 0.00 0.01  AJW-03-182 QMD-E South Oyu  1.1 1.2 1.3 1.4 11.1 12.1 3.1 3.2 5.1 6.1 7.1 8.1  7 10 8 8 10 24 14 10  7 10 9 10 8 17 11 9 8 8 14 5  AJW-03-178 QMD-P1 Southwest Oyu  Ti  spot  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1  0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1  0.1 0.1 0.1 0.1 0.1 0.1  Pr  0.5 1.4 0.3 1.0 0.3 0.7 0.3 1.3  1.3 0.1 0.2 0.1 0.5 0.2 0.7 0.1 0.1 0.1 0.2 0.0  0.2 0.3 0.3 1.0 0.2 0.2  Nd  1.2 3.4 1.1 2.6 0.9 1.8 1.2 3.1  2.5 0.4 0.6 0.6 1.0 0.5 1.8 0.5 0.3 0.3 0.7 0.2  0.9 1.0 1.2 3.1 0.8 0.8  Sm  0.4 1.0 0.3 1.0 0.4 0.9 0.5 1.2  2.0 0.3 0.3 0.3 0.7 0.4 0.9 0.3 0.2 0.3 0.3 0.1  0.3 0.4 0.5 1.1 0.3 0.3  Eu  23 44 24 31 19 45 30 43  16 14 19 19 22 17 26 16 8 11 20 6  22 29 42 45 21 25  Ho  11 27 11 20 10 18 13 25  14 4 7 7 9 6 14 6 2 3 8 2  10 11 14 26 9 9  Gd  4 9 4 7 4 7 5 9  4 2 3 3 4 2 5 3 1 1 3 1  4 5 6 9 3 4  Tb  50 104 53 73 42 95 67 101  39 26 40 40 44 31 59 32 14 19 42 11  49 59 83 107 45 55  Dy  111 210 110 140 91 233 155 196  76 84 94 98 116 98 120 79 45 74 102 37  111 133 219 218 105 133  Er  27 49 26 32 21 57 37 44  17 24 23 24 31 28 28 20 13 22 25 11  27 33 57 49 26 33  Tm  240 405 235 270 191 509 330 374  161 257 214 217 318 293 251 180 150 257 231 124  259 284 525 425 236 295  Yb  52 83 49 55 41 105 70 78  42 69 47 47 80 75 51 38 43 74 49 33  55 62 120 86 51 63  Lu  9678 9869 8963 7153 9821 6583 6219 9357  9820 9469 9163 9861 8245 8272 8936 9152 10991 7929 10439 10358  10536 14079 11800 9802 9756 10709  Hf  19 47 23 28 28 32 23 43  30 33 13 14 36 130 27 11 30 18 19 10  29 38 65 53 22 30  Th  48 77 46 42 55 73 51 64  68 88 39 42 67 178 43 34 72 56 49 32  65 80 215 93 55 79  U  808 838 830 856 863 826 792 849  744 728 750 738 730 759 773 751 738 751 748 735  780 798 768 788 783 763  Temp  164  20 12 7 12 7 20  4.1 6.1 6.2 6.3 7.1 8.1  1356 657 355 951 439 1060  Y  0.01 0.00 0.01 0.01 0.01 0.01  La 13 21 13 16 8 11  Ce  9 21 17 16 14 12 11 15 12 19  1177 1431 462 855 562 583 722 1981 954 1454  0.00 0.02 0.01 0.01 0.01 0.00 0.00 0.08 0.01 0.02  16 13 8 11 9 11 19 41 13 14  7.1 7.2 8.1 8.2 8.3 9.1 9.2 1.1 1.2 1.3  14 4 5 6 2 9 9 4 14 24  1253 198 887 872 100 914 3590 127 549 1307  0.01 0.17 0.04 0.05 0.01 0.01 0.09 0.01 0.01 0.05  35 6 11 17 4 19 31 6 18 35  AJW-04-250 GDi-P3 Hugo Dummett  1.1 2.1 3.1 4.1 4.2 4.3 4.4 5.1 6.1 8.1  AJW-04-356 QMD-P1 Hugo Dummett  Ti  spot  0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1  0.1 0.1 0.1 0.1 0.1 0.1  Pr  0.5 0.3 0.7 0.6 0.0 0.4 2.4 0.0 0.3 1.1  0.6 1.4 0.2 0.8 0.2 0.2 0.3 1.6 0.5 1.6  1.3 0.3 0.1 1.1 0.1 1.0  Nd  1.9 0.3 1.4 1.4 0.1 1.2 5.8 0.1 0.9 2.6  2.4 3.3 0.8 1.9 0.8 0.9 1.2 3.0 2.0 3.7  3.4 1.0 0.3 2.8 0.4 2.9  Sm  0.8 0.2 0.8 0.6 0.1 0.5 2.1 0.1 0.3 1.1  0.8 1.2 0.3 0.7 0.3 0.3 0.4 1.3 0.7 1.3  1.3 0.5 0.2 1.3 0.3 1.0  Eu  48 6 26 26 3 32 101 4 20 51  47 55 17 32 21 22 28 80 37 58  54 24 12 36 14 41  Ho  19 2 13 13 1 12 52 1 9 23  24 30 8 17 9 9 11 28 20 33  31 11 3 23 4 25  Gd  8 1 4 5 0 5 19 0 4 9  9 10 3 6 4 4 4 12 7 12  11 4 1 8 2 9  Tb  101 9 53 58 6 71 217 7 44 113  108 125 38 71 46 49 59 160 87 136  127 56 21 88 23 102  Dy  255 35 150 138 19 163 448 25 104 262  218 251 85 152 105 105 140 391 176 261  248 126 75 175 87 195  Er  56 9 38 31 6 36 92 7 24 57  51 58 20 36 26 25 34 94 41 60  57 31 23 40 27 45  Tm  550 112 439 306 72 325 815 98 223 554  435 491 180 328 234 231 319 823 356 503  481 269 255 345 303 387  Yb  109 31 105 66 21 64 151 30 43 112  89 102 40 70 50 50 69 176 73 103  98 56 71 71 86 80  Lu  8267 9613 7829 7575 9381 8560 5222 11982 8093 8452  11316 9098 9859 9648 9803 10077 11737 9464 10715 9441  9807 9149 11804 8516 8274 9883  Hf  78 20 67 38 13 56 101 23 26 168  68 72 21 37 23 26 68 115 49 73  71 37 77 33 20 42  Th  124 59 123 70 52 118 141 86 62 218  132 108 53 71 51 58 154 229 88 105  107 76 182 49 74 66  U  800 699 703 729 640 758 758 683 804 856  758 843 821 817 804 789 776 809 788 833  835 788 732 786 737 838  Temp  165  13 11 9 5 18 16 15 3 19 10 2  2.1 3.1 3.2 3.3 4.1 4.2 5.1 5.2 6.1 6.2 6.3  808 373 473 224 1742 1399 603 86 3466 429 87  Y  0.02 0.01 0.02 0.01 0.03 0.04 0.03 0.02 0.06 0.01 0.00  La 13 11 12 6 6 6 12 4 43 10 3  Ce  1.1 10.1 10.2 10.3 11.1 11.2 12.1 12.2 2.1 3.1 4.1 5.1 5.2 6.2 7.1 7.2 8.1 9.1  12 7 7 8 9 9 6 7 4 6 14 5 11 14 8 5 29 9  574 922 446 830 696 783 2138 780 273 884 1204 696 1771 373 932 1675 5509 385  0.01 0.06 0.81 0.02 0.02 0.01 0.01 0.24 0.01 0.02 0.59 0.01 1.24 0.01 0.00 0.01 0.38 0.01  13 17 18 19 20 23 57 26 7 24 8 20 31 10 27 44 120 9  AJW-06-476 GDi-P2 Hugo Dummett  Ti  spot  0.0 0.0 0.1 0.1 0.1 0.0 0.1 0.2 0.0 0.1 0.1 0.0 1.0 0.1 0.0 0.1 0.4 0.0  0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1  Pr  0.3 0.4 0.6 0.3 0.3 0.4 0.3 1.4 0.1 0.7 1.2 0.2 13 0.2 0.4 0.1 7.1 0.1  0.4 0.1 0.2 0.1 1.8 1.7 0.4 0.0 3.1 0.2 0.0  Nd  0.9 1.1 0.6 1.1 0.7 1.7 1.0 2.8 0.2 1.5 2.4 0.7 20.4 0.7 1.8 0.6 15.3 0.3  1.3 0.5 0.5 0.2 3.4 3.3 0.9 0.1 7.1 0.5 0.1  Sm  0.4 0.8 0.3 0.4 0.4 1.2 0.7 1.6 0.2 0.8 0.4 0.3 10.2 0.2 1.2 0.5 9.9 0.2  0.5 0.2 0.3 0.2 0.6 0.6 0.4 0.1 3.0 0.2 0.0  Eu  23 34 14 30 22 28 59 26 9 34 52 27 76 14 33 46 216 11  29 12 17 8 61 50 21 2 125 16 2  Ho  9 14 6 12 8 16 16 15 3 14 25 9 90 6 18 10 130 3  14 4 5 2 33 26 9 0 69 5 1  Gd  4 6 2 5 3 6 7 5 1 6 10 4 24 2 6 5 46 1  5 2 2 1 12 9 4 0 25 2 0  Tb  48 73 30 63 46 71 102 56 15 70 121 52 207 30 75 77 506 19  67 24 31 14 146 115 47 4 286 31 4  Dy  115 177 82 164 134 133 370 126 62 188 220 157 310 72 155 299 927 72  145 70 100 45 297 239 114 16 568 84 16  Er  25 41 21 38 33 27 100 28 19 45 43 39 64 16 32 81 175 20  31 17 25 13 61 49 25 5 116 21 5  Tm  252 406 214 381 353 242 1142 254 254 491 382 421 593 154 305 940 1503 243  283 178 292 164 528 428 233 60 1008 161 73  Yb  52 86 46 78 79 46 275 52 77 111 74 95 114 31 60 236 273 61  55 38 69 48 99 82 47 18 186 37 25  Lu  10540 10865 7907 8111 7787 8205 8333 8942 11510 10934 13827 11201 12647 8405 9611 7967 6402 7658  8322 7869 10421 11929 7746 7922 7131 9890 6547 8581 9752  Hf  26 66 59 51 55 161 65 132 28 163 117 107 238 15 211 48 702 20  25 22 33 27 82 64 19 11 154 17 20  Th  56 299 132 133 134 373 161 419 100 295 365 292 731 32 567 135 661 51  42 51 91 85 138 114 34 47 158 54 86  U  783 740 739 745 755 758 722 739 682 722 798 705 779 801 747 713 880 754  795 780 756 708 824 812 807 661 832 772 652  Temp  166  Ti  Y  La  Ce  18 14 15 15 13 12 12 11 9 7 12 6 14 14 10 16 18 14 19  1398 407 1131 1210 752 1279 1173 678 1078 318 735 175 1064 525 233 997 423 507 706  0.03 0.02 0.02 0.01 0.01 0.08 0.11 0.01 0.00 0.00 0.01 0.01 0.02 0.02 0.01 0.02 0.02 0.02 0.02  16 11 15 16 20 18 17 16 19 11 14 6 16 10 6 13 11 15 17  1.1 1.2 10.1 10.2 10.3 11.1 12.1  7 6 19 11 5 9 10  162 169 1662 447 384 397 706  0.01 0.00 0.03 0.02 0.02 0.39 0.01  7 6 19 10 8 11 12  AJW-03-185 GDi-P3 Hugo Dummett  1.1 1.2 10.1 10.2 10.3 12.1 12.2 12.3 12.4 12.5 14.1 14.2 2.1 2.2 3.1 3.2 5.1 5.2 5.3  AJW-03-179 GDi-P3 Southwest Oyu  spot  0.1 0.0 0.2 0.1 0.1 0.1 0.1  0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1  Pr  0.1 0.1 2.2 0.3 0.3 0.4 0.3  1.8 0.2 1.2 1.3 0.5 1.4 1.2 0.3 0.8 0.1 0.3 0.1 1.6 0.3 0.1 1.1 0.2 0.3 0.6  Nd  0.2 0.2 4.0 0.7 0.5 0.6 1.0  3.5 0.6 2.8 3.2 1.2 3.1 2.6 1.0 2.1 0.3 0.9 0.2 3.4 0.9 0.3 2.7 0.7 0.9 1.3  Sm  0.2 0.2 1.8 0.4 0.4 0.3 0.6  1.3 0.2 1.1 1.2 0.5 1.1 1.0 0.4 0.7 0.1 0.4 0.2 1.4 0.4 0.1 1.1 0.2 0.3 0.6  Eu  5 5 69 16 11 13 28  53 14 42 51 30 47 40 25 42 11 27 6 45 19 8 41 14 21 28  Ho  2 2 37 6 4 4 12  28 6 25 28 13 27 20 10 19 3 11 2 26 9 3 22 6 8 13  Gd  1 1 13 2 2 1 4  10 2 8 10 5 9 7 4 7 1 4 1 9 3 1 8 2 3 5  Tb  9 9 155 30 21 23 57  114 28 98 114 64 104 84 51 84 20 55 10 100 38 16 87 28 41 59  Dy  30 32 306 85 63 83 143  242 66 193 226 149 213 171 120 193 62 129 36 201 88 40 182 69 99 137  Er  9 10 68 22 18 26 37  55 15 42 51 36 47 36 29 45 17 33 10 45 21 10 42 16 23 32  Tm  111 120 583 232 208 293 357  490 154 380 442 335 430 350 268 396 172 308 123 398 195 97 367 154 216 297  Yb  32 36 115 55 60 81 81  101 31 77 91 72 87 72 59 82 39 64 38 83 42 21 75 33 46 62  Lu  10392 9554 9047 10032 11387 10477 9954  9444 8953 9851 11232 12568 9814 8138 10153 11177 12569 9511 12157 11028 8756 9713 10288 9156 11773 10362  Hf  17 14 74 21 49 52 20  62 17 45 62 73 60 51 35 57 19 26 32 54 18 6 41 19 31 45  Th  56 45 99 52 109 87 67  91 39 71 99 170 94 78 83 106 60 66 98 88 39 22 74 42 75 100  U  731 727 832 782 705 756 772  827 800 806 806 792 785 790 782 757 742 783 718 803 798 769 816 825 800 832  Temp  167  4 8 11 11 10 9 22 14 4 5 7 12 21 10 6 6 6 5 2  13.1 15.1 3.1 4.1 4.2 4.3 5.1 5.2 6.1 6.2 6.3 7.1 8.1 9.1 A.1 A.2 A.3 B.1 B.2  192 198 1171 478 606 646 1212 990 390 367 616 673 640 914 162 176 222 232 242  Y  0.01 0.00 0.02 0.02 0.02 0.01 0.02 0.00 0.03 0.01 0.05 0.00 0.01 0.04 0.01 0.15 0.01 0.05 0.03  La 9 6 21 13 13 15 33 19 15 17 23 16 13 16 8 5 6 9 9  Ce  1.1 2.1 2.2 3.1 4.1 4.2 5.1 5.2 6.1 6.2  18 6 4 8 10 6 8 6 6 6  781 919 915 580 3569 1342 831 884 664 569  0.72 0.60 0.13 0.02 2.12 0.64 1.72 0.23 0.09 0.30  15 22 61 12 145 76 19 47 14 22  AJW-03-132 Carboniferous granite  Ti  spot  0.1 0.3 0.1 0.1 0.6 0.1 0.1 0.1 0.1 0.1  0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.0 0.1 0.1  Pr  1.0 1.0 0.6 0.2 8.1 1.2 1.1 0.7 0.3 0.3  0.1 0.1 0.5 0.3 0.4 0.3 0.8 0.9 0.1 0.1 0.3 0.4 0.4 1.0 0.0 0.1 0.1 0.1 0.1  Nd  1.6 2.0 1.3 0.8 12.2 2.3 1.4 1.4 1.0 0.9  0.2 0.3 1.5 0.8 1.0 0.9 2.0 2.5 0.3 0.2 0.8 1.2 1.1 2.3 0.2 0.2 0.2 0.2 0.3  Sm  0.4 0.5 0.5 0.2 5.8 0.9 0.4 0.6 0.2 0.4  0.2 0.2 0.7 0.4 0.4 0.4 0.9 1.0 0.1 0.1 0.4 0.5 0.5 1.0 0.1 0.1 0.1 0.2 0.2  Eu  29 32 36 20 135 47 28 31 23 18  6 7 45 19 26 25 48 39 11 11 23 26 24 36 5 6 7 7 7  Ho  14 15 15 8 86 24 13 13 9 9  2 2 17 8 11 10 20 21 2 2 9 11 11 19 2 2 2 2 3  Gd  5 6 6 3 29 9 5 5 4 3  1 1 7 3 4 4 8 7 1 1 4 4 4 7 1 1 1 1 1  Tb  63 73 79 44 329 112 61 64 47 40  10 11 93 41 54 53 97 87 18 17 48 55 53 80 10 11 14 11 14  Dy  141 164 188 107 638 241 146 145 108 93  40 38 217 89 124 121 230 178 78 73 118 126 120 172 32 37 42 45 44  Er  29 38 42 24 132 53 33 31 23 21  12 12 51 22 30 29 55 41 26 24 29 31 30 41 9 11 11 15 14  Tm  261 370 404 246 1221 489 318 293 225 203  156 129 463 201 280 267 498 349 315 306 280 280 265 376 111 141 133 187 177  Yb  51 76 83 50 232 95 65 59 47 42  47 39 94 43 60 58 101 70 85 86 59 58 58 81 29 45 33 56 59  Lu  8891 9974 13306 9755 8234 9222 8934 9024 10179 8394  12295 10061 9097 11734 12889 12064 9143 10223 12591 13038 11507 10043 9381 10940 12295 11213 10110 11394 13501  Hf  55 143 408 47 1125 422 70 224 51 122  33 18 76 35 39 37 68 39 35 41 33 25 18 53 11 21 9 111 62  Th  112 276 675 124 1016 591 152 353 119 223  100 51 148 78 89 87 112 63 103 114 83 59 42 83 37 59 33 237 228  U  829 723 691 752 767 728 748 717 719 730  695 752 778 776 772 759 848 799 699 700 739 788 842 772 730 719 725 709 646  Temp  168  18 8 10 9 8 108  7.1 7.2 8.1 8.2 9.1 9.2  1207 1139 1484 1068 791 2250  Y  0.04 0.40 3.96 11.87 0.60 38.10  La 15 34 97 81 19 281  Ce  9 9 4 6 14 6 10 6 18 10 24  1078 1712 2109 2385 3893 1188 3790 3200 3059 1933 885  0.04 0.28 0.00 0.05 0.04 0.01 0.02 1.19 0.28 0.01 4.39  14 42 11 13 61 16 33 47 112 29 20  10.1 12.1 13.1 18.1 2.1 3.1 3.2 4.1 4.2  13 20 13 23 21 11 9 13 13  1515 1024 1502 467 912 600 858 1638 624  0.11 0.04 0.13 0.02 0.15 0.01 0.01 0.17 0.02  7 12 8 5 26 4 7 9 7  AJW-03-183 Carboniferous andesite  1.1 10.1 2.1 3.1 4.1 5.1 6.1 6.2 7.1 8.1 9.1  AJW-03-180 Carboniferous rhyolite  Ti  spot  0.3 0.1 0.2 0.1 0.2 0.0 0.1 0.1 0.1  0.1 0.2 0.0 0.1 0.1 0.0 0.1 0.4 0.6 0.1 0.2  0.0 0.1 1.0 1.3 0.2 5.1  Pr  2.4 0.9 1.7 0.5 3.9 0.4 0.6 2.1 0.5  0.7 2.7 0.3 0.6 2.7 0.5 1.1 4.3 12.8 1.1 1.7  0.9 0.8 5.3 7.2 1.2 29.1  Nd  4.3 1.9 3.2 1.3 5.1 1.2 1.5 4.3 1.2  1.9 4.6 1.3 2.1 7.4 1.4 3.7 5.9 18.4 2.9 1.8  2.2 1.9 5.2 3.2 1.6 14.3  Sm  0.3 0.8 0.4 0.0 1.7 0.1 0.2 0.2 0.1  0.8 1.2 0.4 0.7 3.6 0.6 1.6 1.6 7.5 1.5 0.9  0.8 0.5 1.4 0.5 0.4 1.8  Eu  66 34 57 18 34 21 34 60 26  37 57 82 77 136 39 126 123 131 64 30  42 45 57 33 31 73  Ho  38 17 30 11 28 10 15 34 11  18 31 18 24 68 14 44 46 112 30 15  19 19 34 16 13 56  Gd  13 6 10 4 8 4 5 12 4  7 12 11 12 25 6 20 21 32 11 6  7 8 12 6 5 18  Tb  156 73 124 44 80 45 71 134 53  87 134 162 158 303 83 267 267 325 142 65  92 96 140 71 67 192  Dy  288 168 253 76 128 95 165 254 115  190 252 427 369 597 193 621 577 556 313 146  206 224 278 154 152 327  Er  62 43 56 15 25 21 40 54 27  40 52 96 79 123 43 131 125 108 66 31  45 48 60 34 34 71  Tm  515 382 494 121 209 192 356 461 234  373 462 929 715 1080 404 1210 1141 998 617 291  417 465 556 320 332 627  Yb  101 85 99 23 41 39 76 88 48  75 87 181 135 202 81 228 212 193 121 61  84 93 109 66 67 117  Lu  9764 8623 8791 10100 7418 7795 10143 9235 11404  6595 7632 13525 8980 7105 7753 7975 11013 7992 6355 4749  6882 10614 10668 8904 11305 8515  Hf  93 31 84 65 77 24 53 94 69  57 232 50 43 157 40 104 255 290 82 48  32 135 370 314 88 1289  Th  178 67 193 165 77 68 165 169 162  117 284 191 101 181 98 203 487 136 141 59  63 254 464 431 190 864  U  794 839 793 851 845 780 761 791 790  755 762 684 722 798 717 770 729 825 770 855  826 744 770 764 750 1049  Temp  169  Ti  13 10 15 14 14 11 13 9 16  spot  5.1 5.2 7.1 7.2 8.1 9.1 A.1 A.2 B.1  2090 588 1245 484 602 639 746 707 552  Y  0.07 0.01 0.06 0.03 0.04 0.02 0.55 0.01 0.02  La 14 5 6 6 6 6 8 8 9  Ce 0.1 0.1 0.2 0.1 0.2 0.1 0.2 0.0 0.1  Pr 2.3 0.4 1.7 0.3 0.4 0.4 2.4 0.5 0.5  Nd 5.3 1.0 3.3 0.8 1.5 1.2 2.4 1.3 1.2  Sm 0.4 0.1 0.2 0.1 0.1 0.1 0.5 0.2 0.2  Eu 95 22 49 21 25 24 31 29 25  Ho 52 10 28 9 11 10 16 13 11  Gd 19 4 10 4 4 4 5 5 4  Tb 223 49 113 46 55 52 67 63 50  Dy 436 99 214 97 121 111 150 136 106  Er 95 23 46 22 28 25 36 31 23  Tm 784 207 398 201 244 226 320 270 206  Yb 153 41 79 41 50 46 67 56 44  Lu 10645 9039 8679 11827 11098 9299 10342 10724 9521  Hf 281 41 61 36 45 49 43 77 14  Th 453 118 130 105 129 122 127 186 38  U  791 767 807 801 801 777 793 758 814  Temp  Zircon morphology Zircon size and morphology varies widely throughout all samples with long axes that typically range from 100 to 300 µm (Figure 5.2). The zircon shapes vary from elongate, well-terminated grains to relatively equant, faceted grains and some subrounded grains. Cathodoluminescence images reveal significant internal complexity. Concentric bands are most common, although sector-zoned patterns and discordantconcentric banding are also locally present. REE patterns The chondrite-normalized rare-earth element patterns of all samples are characterized by HREE enrichments relative to LREE, negative Eu-anomalies and positive Ce-anomalies (Figure 5.3). The Carboniferous samples have relatively flat REE patterns compared to the Devonian samples and one sample, AJW-03-132, the Carboniferous Javhalant pluton has average Nd<Pr, which is distinctive within the dataset as the remaining average samples have Nd>Pr. Hf versus TiO2-in-zircon thermometer Zirconium has a much higher zircon/melt partition coefficient compared to Hf and as magmas undergo cooling and fractional crystallization, Hf will increase in the melt relative to Zr. Hence, Hf can be used as an index of fractionation (Claiborne et al., 2006; Wooden et al., 2006). Furthermore, the TiO2-in-zircon thermometer can be used (Watson and Harrison, 2005; using aTiO2 = 0.7; Claiborne et al., 2006) to determine minimum crystallization temperatures for the zircons. The thermometer was calibrated under rutilesaturated conditions (aTiO2 = 1); rutile saturation in magmas is rare, but aTiO2 is approximately fixed at values >0.5 in most felsic magmas by saturation in other titaniumbearing phases (Watson et al., 2006; Claiborne et al., 2006) and aTiO2 = 0.7 is used, appropriate for titanite and titanomagnetite saturation. The isotope 49Ti was selected over the more abundant  48  Ti for calculation of temperature to avoid interference from  96  Zr  (Claiborne et al., 2006).  170  In general, zircon cores tend to be high-temperature and low Hf compared to their corresponding rim analyses (Figure 5.2). However, there are some exceptions where low-temperature, high Hf cores are surrounded by high-temperature, low Hf growth zones as well as grain traverses where T and Hf behave discordantly. Hafnium concentrations in the quartz monzodiorite samples ranges from 6219 ppm to 20751 ppm and the TiO2-in-zircon thermometer suggests that minimum temperatures in the quartz monzodiorite zircons range from 699oC to 863oC. The Hf versus temperature plot for quartz monzodiorite samples indicates that there is a correlation between increasing Hf and decreasing temperature (Figure 5.4a). The zircon spots from sample AJW-03-178 (Southwest Oyu) do not fit this trend as they have both low minimum temperature as well as low Hf. Hafnium concentrations in the Late Devonian granodiorite suite ranges from 5222 ppm to 13827 ppm and minimum temperatures range from 640 to 880oC. The zircon analyses from granodiorite sample AJW-03-179 tend to be relatively high temperature, whereas the samples from AJW-05-476 are relatively low compared to the remainder of the granodiorite zircon suite. Granodiorite samples AJW-04-250 and AJW-03-185 have analyses that span the temperature range for the entire sample suite. Hafnium concentrations in the Carboniferous intrusions ranges from 4749 ppm to 13525 ppm and minimum temperatures range from 684oC to 855oC with one outlier at 1049oC.  Zircons from andesite sample AJW-03-183 have relatively high minimum  temperatures whereas analyses from granite sample AJW-03-132 are relatively low temperature. Zircons from a rhyolite sample (AJW-03-180) span the temperature range of the Carboniferous sample suite.  171  Figure 5.2a Representative cathodoluminescence images of zircons from QMD intrusions with spot analytical locations, minimum temperatures (oC) and hafnium concentrations (ppm) indicated.  172  Figure 5.2b Representative cathodoluminescence images of granodiorite zircons with spot analytical locations, minimum temperatures (oC) and hafnium concentrations (ppm) indicated.  173  Figure 5.2c Representative cathodoluminescence zircon images for Carboniferous samples (andesite porphyry, AJW-03-183; rhyolite, AJW-03-180; granite, AJW-03-132) with spot analytical locations, minimum temperatures (oC) and hafnium concentrations (ppm) indicated.  174  175  Figure 5.3 Chondrite-normalized REE patterns for the zircons in the study. Black solid lines represent average zircon analysis from each rock; shaded areas enclose all analyses. Chondrite values are from McDonough and Sun (1995).  Trace element variations Th/U in zircon values for QMD ranges from 0.1 to 0.7 and hafnium versus Th/U plots suggests that Th/U decreases with increasing Hf concentration (Figure 5.4b). Both the QMD and GDi suites have two trends of decreasing Th/U; one with higher Hf and one with lower Hf. The ratio plots follow the observation that smaller ions are in relatively higher abundance in zircon vesus a larger ion at increased fractionation. The Yb/Gd ratio measures the steepness of the REE pattern (heavy REE to light REE ratio). Yb/Gd ratios vary from 12 to 88 in the quartz monzodiorite samples, from 7 to 142 for the granodiorite samples and from 7 to 51 for the Carboniferous samples. Many of the Devonian samples plot with higher Yb/Gd compared to the Carboniferous zircons (Figure 5.4c). Y concentration in the QMD porphyries ranges from 109 to 1981 ppm (Figure 5.4d) and the Y versus Yb/Gd plot (Figure 5.5a) separates the data into three groups: 1.  High Y, low Yb/Gd with zircon spots from all samples except AJW-03-178.  2.  Moderate Y, moderate Yb/Gd with analytical spots from all rocks.  3.  Low Y and high Yb/Gd dominated by zircon spots from sample AJW-03-178  with some analyses from AJW-03-182. The GDi samples are distributed along an array where Y ranges from 86 to 5509. There is some clustering of data which suggests that there may be two discrete groups; 1. High Yb/Gd and low Y with zircons from samples AJW-06-476, AJW-04-250 and AJW-03-185. 2. Low Yb/Gd and high Y with zircons from all GDi represented. Two zircons from sample AJW-06-476 (GDi-P2) have both high Y and high Yb/Gd. Zircons from AJW-03-185 (GDi-P3) as well as AJW-03-250 (GDi-P3) have some low Y grains with higher Yb/GD than any other QMD or GDi analyses. In the Carboniferous intrusions, Y ranges from 467 to 3893 and. The Yb/Gd data is uniformly low for all values of Y. In general, all of the district rock suites have a large number of samples that have Y that ranges from ~500-1800 with low Yb/Gd (~20). The difference between the Devonian suites and the Carboniferous samples is the presence of a high Yb/Gd group of  176  zircons that are not confined to single rock samples. Rather, the high Yb/Gd-low Y zircons are from rocks that also have zircon representatives in the low Yb/Gd-moderate Y group. On the Th/U vs. Yb/Gd plot (Figure 5.5b), most analyses from QMD define an array that increases in Th/U with decreasing Yb/Gd. Some spot analyses from AJW-03178, AJW-03-182 and AJW-04-385 are distinct from the rest of the spot analyses with higher Yb/Gd. Zircon Th/U values for GDi samples range from 0.2 to 1.1. Most values plot on an array of decreasing Th/U with increasing Yb/Gd. Zircons from sample AJW06-476 are unusual in that there are analyses with the same Th/U value, however they plot above and below the field that contains the majority of the other analyses. The three Carboniferous intrusions have Th/U values that range from 0.3 to 2.1. The majority of the samples all plot with approximately the same range of Th/U values. The Devonian and Carboniferous samples are similar in terms of Th/U values; however the Devonian zircons are conspicuous in that there are high Yb/Gd values present as well in the data set. Multi-valent elements Eu and Ce The EuN/EuN* ratio is used to quantify the magnitude of the negative Eu anomalies in the REE patterns (where the subscript indicates chondrite normalization and EuN* = ((SmN*GdN)0.5). The EuN/EuN* values range from 0.3 to 1.1 in the QMD samples where most samples plot between 0.3 and 0.5 and there is a broad trend where EuN/EuN* decreases with increasing Hf concentrations (Figure 5.6a). There is considerable scatter in the GDi data for EuN/EuN*, which ranges from 0.2 to 1.0. Most samples plot at about 0.4, however many of the analyses are higher, between 0.5 and 1, considerably different from the QMD suite. The EuN/EuN* values in younger Carboniferous intrusions ranges from 0 to 0.5 (lower than the Devonian grains) and and there is a broad decrease in EuN/EuN* with increasing Hf. Zircons from sample AJW-03-183 (andesite) have lower EuN/EuN* than all samples analyzed (<0.2). Similarly, an analogous equation is used to quantify the Ce-anomaly.  The  CeN/CeN* values for quartz monzodiorite zircons range from 12 to 1172, from 5 to 767  177  178  Figure 5.4 Hf variation diagrams for zircon. A) Hf (ppm) versus minimum temperature; B) Hf (ppm) versus Th/U. The arrows indicate the fractionation direction.  179  Figure 5.4 continued. C) Hf (ppm) versus Yb/Gd; D) Hf (ppm) versus Y (ppm).  180  Figure 5.5 Y vs. Yb/Gd and Th/U vs. Yb/Gd for zircon. A) Y versus Yb/Gd. Yb/Gd is a measure of the steepness of the HREE pattern and varies with changes in Y; B) Th/U versus Yb/Gd for zircon.  181  Figure 5.6 Hf vs. multi-valent element concentrations. A) Hf (ppm) versus EuN/EuN*; B) Hf (ppm) versus CeN/CeN* for zircon. The arrows indicate the fractionation direction.  for granodiorite zircons and from 3 to 299 for zircons from the three Carboniferous rocks (Figure 5.6b). There is a large amount of scatter on all three plots and no clear trends. All three groups have low CeN/CeN* zircons, however, the Devonian suites have a number of high CeN/CeN* zircons that differentiate them from the Carboniferous samples. Individual rock samples from the QMD and GDi suites contain zircons with high CeN/CeN* as well as grains with low CeN/CeN*. Furthermore, the variable CeN/CeN* does not appear to be a function of hafnium concentrations (i.e. high and low CeN/CeN* values occur in zircons with low and high Hf concentrations). Discussion Evidence and implications for magma mixing The presence of zircons with different Yb/Gd ratios and Y concentrations that were derived from a single rock sample provides evidence for source heterogeneity and/or mixing of material from multiple sources or mixing of discrete melts that experienced distinctive fractionation histories. Furthermore, core-to-rim pairs indicate that early, high-T, unfractionated melt environments evolved toward low-temperature, fractionated environments.  In general, discordant cores as well as the interiors of  concentrically-, oscillatory-zoned grains tend to be higher temperature and low-Hf compared to their corresponding rim analyses (Figure 5.2). There are a number of exceptions, however, where cooler, high-Hf cores are surrounded by hotter, low Hf rims as well as grain traverses where temperature and Hf concentrations behave discordantly. Hotter, less fractionated rim analyses compared to corresponding cores may represent mafic melt injection events or possibly incorporation of a zircon core that crystallized in a different part of the magma chamber. A variety of mixing scenarios are possible that could account for the presence of discrete zircon populations within a single rock sample. For example, it is possible that the magma mixing is related to chamber dynamics such that material from early chilled walls is subsequently mixed into new magma batches that intrude older solid intrusions. The new magma batches would have to scour the chamber walls, potentially breaking off fragments and remelting all but refractory minerals. Conversely, it is also possible that a sill-like chamber could be compositionally zoned. Possible scenarios include magmatic  182  layering due to fractional crystallization and settling processes or perhaps the roof of the intrusion incorporated fallen material from host rocks. A contaminated roof would be compositionally distinct from other sectors in the intrusion and convection or flow of material would be required to mix the zircons and melt into the heterogenous populations that are seen in the datasets. Recent field and geochronologic evidence supports complex histories and abundance of recycled material.  For example, complex sequences of  intrusion, reintrusion, partial mixing, disruption and remobilization of slightly older pulses have been documented in plutonic rocks elsewhere such as the Sierra Nevada Batholith, California (Miller et al., 2001). Similarly, in the Spirit Mountain Batholith, Nevada, zircons recycled into new pulses of magma document remobilization of previously emplaced crystal mush (Walker et al., 2007). The lack of discrete, mixed populations from rock samples that are not related to porphyry Cu-Au deposits at Oyu Tolgoi suggests that the physical processes that were actively mixing discrete aliquots of magma and zircons in the OTIC melts may be somehow fundamental to the production of Cu-Au fertile magmas. A turbulent, lowviscosity magma chamber would be capable of generating melts with mixed populations of zircons in the individual aliquots via scouring material from wallrock and moving refractory minerals to the various sectors of the subvolcanic chamber. Turbulent melts with high volatile content would be conducive to brecciating wallrocks and injecting melt and fluids into open space, as documented in porphyry copper environments. Oxidized porphyry magmas Many of the Late Devonian zircon analyses have higher EuN/EuN* and CeN/CeN* than the Carboniferous zircons. Ballard et al. (2002) found that intrusive complexes associated with porphyry deposits in northern Chile were oxidized, having zircons with unusually high Ce(IV)/Ce(III) (>300) as well as EuN/EuN* (>0.4). Similar results were obtained by Liang et al. (2006) in the Yulong porphyry district (Tibet) and Harris et al. (2007) in the Habo South porphyry district (South China). The trace element data from Oyu Tolgoi zircons cannot be used to infer Ce(IV)/Ce(III) and compared to that reported by Ballard et al. (2002), Liang et al., 2006 and Harris et al., 2007 for the following reasons:  183  1. Although a wholerock geochemical database for OTIC rocks now exists (Chapter 4), those analyses (specifically Ce analyses) cannot be applied to all of the rock samples from which zircons were extracted for this study, necessary for the calculation. Specifically, these include samples of mineralized QMD intrusions. 2. The zircon populations in OTIC rocks are complex as there are discordant growth bands, inherited cores and xenocrysts. In this study, a wide variety of spot locations in different growth bands were used instead of predominantly grain tips/rims in angular grains. The latter spot locations are preferred for use in a strategy where one wishes to determine a “magmatic age” rather than track changes during growth history.  This suggests that some of the zircon  geochemistry in this system may be unrelated to wholerock geochemistry of enclosing igneous material. 3. Cerium and europium contents in zircon may depend on factors other than oxidation state such as temperature (Liang et al., 2006) and plagioclase feldspar fractionation (Ballard et al., 2002). Regardless, similar to the findings of Ballard et al. (2002), average EuN/EuN* ratios from the OTIC exceed 0.4, a minimum value that is linked to fertile intrusions in Northern Chile. The zircon spots with EuN/EuN* ratios that are greater that 0.4 are dominantly from samples AJW-03-182 and AJW-03-178. This suggests that oxidation state (if that is the dominant factor governing EuN/EuN* ratios) was not consistent throughout the intrusion events and that contributions of oxidized magma may have occurred repeatedly during the evolution of the system. More generally, the EuN/EuN* values in the entire OTIC dataset decrease with increasing fractionation (i.e. increasing Hf; Figure 5.6a). There is no correlation between temperature and EuN/EuN*, however it is notable that the many zircons from AJW-03-178 and AJW-03-182 have low minimum temperatures and very high EuN/EuN*. This is unusual as the first order influence on the magnitude of the Eu-anomaly is feldspar crystallization, which dominates removal of Eu2+ from melt. It should therefore be expected that as fractionation proceeds, Hf content will increase in zircons and Eu3+ contents will increase in melt from which plagioclase was removed (J. Wooden, pers. comm., 2007). It is possible that the oxidation state of the OTIC magmas is governed by intrinsic properties of the source and addition of  184  increasingly reduced melt rather than fractional crystallization or that the Eu and Hf behavior in the system correlate poorly due to late zircon saturation relative to early influence of plagioclase. Spatial-temporal and wholerock geochemical considerations Trace element geochemical data from three samples (AJW-03-181, AJW-04-385 and AJW-04-356) lie on similar fractionation trends (Fig. 5.4a) suggesting that they may be derived from the same parent magma.  Conversely, zircons from AJW-03-178  (Southwest Oyu) and some zircons from AJW-03-182 (South Oyu) yield different trace element geochemistry suggesting that the grains were derived from a fundamentally unique source or that the melts were mixed with material that did not influence the other group.  Based on zircon geochemistry, sample AJW-03-182 (South Oyu) may be  transitional between that of AJW-03-178 (SW Oyu) and the other set which suggests the possibility that mixing occurred between two end-members. These QMD samples are the southernmost samples, so there appears to be a geographic correlation with the zircon chemistry as well. The granodiorite samples are divided into two suites based on texture and wholerock geochemistry: coarse, sparsely porphyritic adakite-like rocks (GDi-P2) and medium-grained crowded porphyritic non-adakite-like rocks (GDi-P3) (Chapter 4). In the zircon geochemical dataset, sample AJW-06-476 (GDi-P2) is different from the remainder of the GDi samples (GDi-P3) in that it has outlier zircons (high Y, high Yb/Gd) on the Y versus Yb/Gd plot as well as zircons with very high CeN/CeN*. The source of the high Y, high Yb/Gd material is unknown, however the unusual CeN/CeN* values could be sensitive to subtle variations in La and Pr values or possibly related to oxidation state effects. Conclusions The trace element geochemistry of zircon suggests that the OTIC intrusions are derived from a multi-component, fractionating, cooling magmatic system; however, the fractionation and cooling trends in the OTIC has been complicated by mixing of magmas from distinct reservoirs. These reservoirs have distinct Th/U, Yb/Gd, EuN/EuN* and  185  CeN/CeN* ratios as well as Hf and Y contents. Temperature and Hf concentrations (differentiation) characteristics are not significantly different between the syn-mineral, late-mineral and younger intusions.  The presence of zircons with high CeN/CeN*,  EuN/EuN* and Yb/Gd in the sample populations for syn- and late-mineral intrusions differentiates them from younger intrusions that are not related to porphyry Cu-Au formation. The study highlights the complexity of magma chambers related to the super-giant porphyry Cu-Au deposits at Oyu Tolgoi and possibly the importance of oxidation state effects and mixing above other parameters such as composition and temperature in melts that are capable of generating these deposits. A turbulent, low-viscosity magma chamber would be capable of generating melts with mixed populations of zircons in the individual aliquots via scouring material from wallrock and moving refractory minerals to the various sectors of the subvolcanic chamber.  186  References Badarch, G., Dickson C.W. and Windley, B.F., 2002, A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia, Journal of Asian Earth Sciences, v. 21, p. 87-110. Ballard, J.R., Palin, J.M. and Campbell, I.H., 2002, Relative oxidation states of magmas inferred from Ce(IV)/Ce(III) in zircon: application to porphyry copper deposits of northern Chile, Contributions to Mineralogy and Petrology, v. 144, p. 347-364. Belousova, E.A., Griffin, W.L., O’Reilly, S.Y. and Fisher, N.J., 2002, Igneous zircon: Trace element composition as an indicator of source rock type, Contributions to Mineraloy and Petrology, v. 143, p. 602-622. 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Pasteris, J.D., 1996, Mount Pinatubo volcano and ‘‘negative’’ porphyry copper deposits, Geology, v. 24, no. 12, p. 1075–1078. Pollard, P.J. and Taylor, R.G., 2002, Paragenesis of the Grasberg Cu-Au deposit, Irian Jaya, Indonesia; results from logging section 13, Mineralium Deposita, v. 37, no.1, p.117-136. Sun, W.D., Arculus, R.J., Kamenetsky, V.S. and Binns, R.A., 2004, Release of goldbearing fluids in convergent margin magmas prompted by magnetite crystallization, Nature, v. 431, p. 975–978. Waite, K.A., Keith, J.D., Christiansen, E.H., Whitney, J.A., Hattori, K., Tingey, D.G. and Hook, C.J., 1998, Petrogenesis of the volcanic and intrusive rocks associated with the Bingham Canyon porphyry Cu-Au-Mo deposit, Utah, in Geology & Ore Deposits of the Oquirrh & Wasatch Mountains, Utah, Soc. Econ. Geol. Guidebook, John, D.A. and Ballantyne, G. (eds.), v.29, p.69-90. 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Wooden J.L., Mazdab F.K., Barth A.P., Miller C.F. and Lowery L.E, 2006, Temperature (Ti) and compositional characteristics of zircon: Early observations using High Mass Resolution on the USGS Stanford SHRIMP-RG, Geochimica et Cosmochimica Acta Supplement, v. 70, no. 18, p. 707.  189  Chapter 6 – Geochemical evolution of Devonian to Carboniferous volcanic arc rocks in the South Gobi region; a reconnaissance petrochemical and isotopic study from the Oyu Tolgoi porphyry Cu-Au district, Mongolia5 Introduction Paleozoic tectono-magmatic activity in the Central Asian Orogenic Belt (CAOB) is poorly understood, yet critical to models that describe the evolution of the region. The volcanic and plutonic rocks within the Oyu Tolgoi porphyry copper-gold district span the Upper Devonian and much of the Carboniferous and record the tectonic history of the region within their isotopic, major and trace element geochemical compositions. This study provides constraints on the tectonomagmatic history of the region and thus on the evolution of the CAOB.  Moreover, the genesis of and distinction between giant,  marginal and barren porphyry complexes are not well understood both in global copper districts as well as at Oyu Tolgoi. Constraining the geochemistry of ore-stage intrusions is critical to the development of possible models that explain why some igneous complexes such as that found at Oyu Tolgoi are responsible for generating large coppergold deposits. Geodynamic settings for ancient rocks can be inferred using geochemical criterion (e.g., Pearce and Cann, 1973; Floyd and Winchester; 1975, Rollinson, 1993). In this chapter the tectono-magmatic history of the region is constrained by wholerock geochemistry as well as wholerock and mineral isotopic geochemistry from rocks that have already been dated by U-Pb (zircon) methods (Chapter 2; Chapter 3; Chapter 4). The sample suite provides a comprehensive stratigraphic and temporal distribution of mafic to felsic rocks that can be used to assess the variations in wholerock and isotope geochemistry for the arc. Based on this, the geodynamic evolution of the arc is inferred.  5  A version of this chapter will be submitted to a refereed journal for publication: Wainwright, A.J., Tosdal, R.M., Scoates, J.S., Gabites, J.E. and Weis, D., in prep., Geochemical evolution of Devonian to Carboniferous volcanic arc rocks in the South Gobi region; a reconnaissance petrochemical and isotopic study from the Oyu Tolgoi porphyry Cu-Au district, Mongolia.  190  Geologic setting of the Oyu Tolgoi porphyry Cu-Au district The Oyu Tolgoi porphyry copper-gold district is located in the Central Asian Orogenic Belt (CAOB), a tectonic supercollage that contains craton fragments, Proterozoic to Paleozoic ophiolites and accreted volcanic arcs (Badarch et al., 2002; Buchan et al., 2002). The Oyu Tolgoi porphyry district is underlain by a sequence of Devonian or older rocks in the Gurvansayhan terrane (Badarch et al., 2002; Helo et al., 2006), interpreted to represent a juvenile oceanic island arc (Helo et al., 2006; this chapter). The copper-gold deposits are hosted in an inlier of Late Devonian intrusions and Devonian or older volcanic and volcaniclastic rocks (Alagbayan Group) surrounded by disconformably overlying Carboniferous supracrustal packages (Gurvankharaat Group) (Chapter 2; Figure 6.1). Eight main lithofacies have been defined, which are dominated by mixed effusive, pyroclastic, subvolcanic and volcaniclastic rocks as well as sedimentary units which are typical of volcanic arc terranes. Devonian lithologic units are divided into a basal Augite Basalt sequence succeeded by a Mineralized Fragmental Sequence (MFS), an Unmineralized Dacite Sequence (UDS), and a structurally overlying volcano-sedimentary package referred to as the allochthonous Oyu Tolgoi Hanging Wall Sequence (OTHS). The Devonian volcanic succession is disconformably overlain by Carboniferous mafic to silicic volcanic and volcaniclastic rocks intercalated with clastic sedimentary sequences. These rocks are separated into four informal units: Polylithic Breccia Sequence (PBS), Sedimentary Sequence, Lower Volcaniclastic Sequence (LVS) and Upper Volcaniclastic Sequence (UVS) (Chapter 2). The Cu-Au deposits are related to Late Devonian pre- to late-mineral intrusions from the Oyu Tolgoi Intrusive Complex (OTIC; Chapter 4). All of the Paleozoic layered rock units within the district are intruded by post-mineral Carboniferous to Permian or younger dikes that range in composition from basalt to rhyolite (Chapter 2). Uraniumlead (zircon) geochronology indicates that the crystallization age of igneous rocks in the area ranges from at least 374 ± 3 Ma (Chapter 4) to 321 ± 3 Ma (Chapter 2). The majority of zircon xenocrysts are no older than approximately 390 Ma, which is inferred to be the age of the onset of arc magmatism and possibly the age of the Augite Basalt sequence. The age of the allochthonous OTHS unit is more poorly constrained. The unit  191  Figure 6.1 Bedrock geology map of the Oyu Tolgoi district, Mongolia, drawn by Ivanhoe Mines geology staff and AJW from mapping of drill holes and trenches through younger cover sequences as well as outcrop mapping. Inset map shows the location of Oyu Tolgoi in the southern Gobi desert of Mongolia. Coordinates are UTM zone 48, northern hemisphere (WGS84). Copper grade shell for the Hugo Dummett zone is projected to surface.  192  must be older than the ~366 Ma granodiorite dikes that intrude it (Chapter 3); however a magmatic or maximum age for the sequence has not been defined. Devonian to Carboniferous igneous suites Rocks from the Oyu Tolgoi district are subdivided into several suites based on UPb geochronology, petrology and stratigraphic position. The rocks are divided into two major groups: 1) mafic to intermediate rocks include all basaltic to andesitic lavas, breccias and subvolcanic intrusions, and 2) intermediate to felsic rocks include dacitic to rhyolitic composition dikes, stocks and plutons as well as alkaline series dikes and stocks (Table 6.1). The hornblende-biotite andesite dike suite is included in the latter group due to the similar textures to the quartz monzodioite intrusions from the alkaline suite (Table 6.1). Wholerock geochemical characteristics of the OTIC intrusions are discussed in the context of global porphyry-related magmas in Chapter 4, and wholerock geochemistry for the Oyu Tolgoi Hanging Wall Sequence (OTHS) basalts and andesites are discussed in Ayush (2006), referenced herein. OTHS unit igneous rocks have also been sampled for this study and the results are discussed in this chapter. U-Pb (zircon) ages have been assigned to the rocks from the various suites including an age of 390 Ma to the Augite Basalt; the interpreted age for the onset of oceanic arc magmatism, based on xenocryst populations.  No specific age has been assigned to the OTHS unit aside from the  minimum age of ~366 Ma as defined by cross-cutting dikes (Chapter 2). Mafic to intermediate rocks in the district consist of coherent flows and/or shallow intrusions as well as fragmental volcaniclastic rocks. In the case of fragmental facies rocks, clasts were sampled that are large enough for analysis (>60 g). From lowest stratigraphic position to highest, the sequences are divided into a basement Augite Basalt sequence which is part of the Bulagbayan Formation, the allochthonous Oyu Tolgoi Hanging Wall Sequence (OTHS), mafic to intermediate rocks from the Lower Volcaniclastic Sequence (LVS) and andesitic rocks of the Upper Volcaniclastic Sequence (UVS). These sequences range in age from Upper Devonian or older to Carboniferous (Chapter 2; Table 6.1). Intermediate to felsic rocks in the district consist of aphanitic to porphyritic dikes as well as coarse-grained equigranular rocks. These are divided into equigranular and 193  Table 6.1 Igneous rock suites from the Oyu Tolgoi district. Detailed unit descriptions are in Chapters 2 and 4. Magmatic suite  age  Rock types  comment  Basalt  Mineralized basement  Basalt to andesite Basalt to Andesite  Allochthonous sequence overlies basement, possibly older than Augite Basalt. Dominantly volcaniclastic sequence  Andesite  Coherent and fragmental rocks associated with felsic ignimbrites and clastic sedimentary rocks.  Equigranular quartz monzodiorite Porphyritic quartz monzodiorite Porphyritic granodiorite, coarse phenocrysts Porphyritic granodiorite, fine-grained phenocrysts Equigranular granodiorite Dacite  Syn-mineral intrusions  Mafic to intermediate rocks Augite basalt OTHS basalt LVS mafic to intermediate rocks UVS andesite  >374 ± 3 Ma, < ~390 Ma > ~366 Ma 346 ± 8 Ma 339 ± 2 Ma  Intermediate to felsic rocks QMD-E intrusions  374 ± 3 Ma  QMD-P1  372 ± 1 Ma  GDi-P2  ~366 Ma  GDi-P3  ~366 Ma  350- intrusion  350 ± 4 Ma  LVS dacite  347 ± 2 Ma  Hb-Bt andesite  345 ± 2 Ma  Rhyolite  340 ± 3 Ma  320-intrusions  ~320 Ma  Porphyritic andesite Rhyolite dikes and sills Granite  Syn-mineral intrusions Late-mineral intrusions, high Sr/Y  Late-mineral intrusions, low Sr/Y  Pluton north of Hugo Dummett zone Flows, sills and juvenile clasts in volcaniclastic deposits Petrologically-similar to QMD, cuts postmineral strata Cuts post-mineral strata Regional granitoid plutons  194  porphyritic quartz monzodiorites (QMD-E and QMD-P1), granodiorite porphyries (GDiP3 and GDi-P2), an equigranular granodiorite referred to as the 350-intrusion, LVS unit dacite flows, hornblende-biotite andesite porphyry rocks, rhyolite dikes and regional granitoid plutons.  These rocks range in age from Late Devonian to Carboniferous  (Chapter 2; Chapter 4; Table 6.1). Wholerock petrochemistry Seventy-three samples reflecting the petrologic range of Devonian to Carboniferous igneous rocks in the district were analyzed for major and trace elements. The OTIC sample geochemistry (n = 37), discussed in this chapter and included in the overall sample suit (n = 73), is presented in the context of global porphyry copper deposit-related intrusions in Chapter 4. All samples were analyzed at ALS Chemex Laboratories Limited in North Vancouver, Canada. Major element oxides and selected trace element concentrations were determined by X-ray fluorescence spectrometry (XRF), rare earth elements (REE) and remaining trace element concentrations were determined by inductively coupled plasma mass spectrometry (ICP-MS). In this chapter, all 73 analyzed samples are plotted in the figures. Representative analyses for the suites are presented in Table 4.2 (OTIC samples) and Table 6.2 (all other suites).  All  geochemical analyses for the igneous suites are presented in Appendix 2 (OTIC intrusions) and Appendix 4 (all other samples) and the analytical limits from ALS CHEMEX are given in Appendix 5. Although great care was taken to collect least altered samples through field screening and thin section evaluation, some samples are weakly altered as these rocks are associated giant porphyry Cu-Au deposits. Some of the analyzed samples are partially altered and the alteration usually takes place in the form of chloritization of mafic minerals, sericite replacing feldspars as well as carbonate and silica introduction. Wherever possible, a diamond saw was used to cut off visibly altered material during preparation of material for submission to ALS Chemex. Relatively soluble elements such as Si, Fe, Mg, Ca, Na and K are known to be mobile during hydrothermal events, therefore more emphasis is placed on lithogeochemical characterization based on rare-  195  196  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  suite  Sample OTHS 45.98 0.87 12.06 0.11 9.35 6.52 0.25 7.18 7.19 1.2 1.55 0.2 0.03 0.01 12.65 98.63 <1 69.9 35.4 46.4 840 4.61 72 3.92 2.37  44.35 1.42 13.11 0.02 14.3 6.11 0.28 8.5 11.84 2.04 0.64 0.18 0.05 0.03 3.08 99.81 <1 152.5 13.1 42.4 160 0.2 127 3.6 2.1  AJW-04263  AJW-03204 Augite Basalt 46.79 1.79 16.44 <0.01 9.38 5.33 0.17 2.25 7.55 3.38 3.33 0.56 0.15 0.09 7.15 99.03 <1 827 106 25.8 20 4.63 61 5.55 2.75  OTHS  AJW-06485  55.84 0.93 16.67 0.01 7.32 3.81 0.13 3.54 5.98 2.95 2.59 0.31 0.07 0.08 3.51 99.92 <1 674 51.9 21.5 140 1.4 57 4.1 2.4  AJW-03055 LVS andesite 56.44 1.01 17.56 <0.01 6.64 1.41 0.06 1.3 2.38 5.73 2.06 0.3 0.02 0.08 4.58 98.16 <1 782 36.7 13.5 40 3.17 35 3.81 2.31  UVS  AJW-04371  64.07 0.5 16.59 <0.01 4.22 1.85 0.03 1.58 2.86 3.83 3.82 0.17 0.05 0.1 2.1 99.93 <1 1040 36.2 8.9 80 0.5 10 2.6 1.7  AJW-03074 350 intrusions  Table 6.2 Representative geochemical analyses for igneous rock suites from the Oyu Tolgoi district.  63.99 0.51 16.75 <0.01 3.65 0.32 0.06 0.74 2.19 3.13 3.14 0.12 0.04 0.01 4.31 98.63 <1 148.5 52.8 6.1 <10 6.27 89 4.43 2.91  AJW-03205 LVS dacite 54.85 0.76 14.06 0.05 6.38 3.8 0.12 5.46 4.42 3.49 1.14 0.23 0.08 0.04 8.68 99.75 <1 289 42.3 25.2 360 4.4 58 3 1.7  AJW-03082 HbBt Andesite 77.05 0.06 13.34 0.01 1.31 0.77 0.02 0.21 0.49 2.22 2.84 0.06 0.02 0.07 2.27 99.96 <1 685 22.5 0.6 70 3.9 5 2.2 1.3  rhyolite  AJW-03062  70 0.4 14.72 0.03 2.84 1.29 0.06 0.81 1.95 3.66 4.27 0.11 0.03 0.09 0.93 99.9 <1 720 52.1 5.2 200 2.6 43 4.5 2.9  AJW-03116 320 intrusions  197  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb  suite  Sample OTHS 1.19 15.1 4 3.2 0.73 16.5 0.33 <2 6.4 18.1 232 11 4.5 43 4.1 1 345 0.4 0.64 3.73 <0.5 0.32 0.97 196 3 19.7 2.16  1 16 3.2 1 0.7 5.3 0.3 2 2 9.1 51 8 1.8 7.8 3 <1 478 <0.5 0.6 <1 <0.5 0.3 <0.5 499 2 20.3 1.8  AJW-04263  AJW-03204 Augite Basalt 2.66 20.2 7.84 6.1 0.94 55.8 0.31 <2 47.4 44.9 9 15 12.2 80.1 8.34 2 1540 2.7 0.99 5.12 <0.5 0.36 1.2 216 1 24.8 2.15  OTHS  AJW-06485  1.5 19 5 5 0.8 26.6 0.4 2 10 25.5 36 12 6.4 60.5 5.4 1 571 0.7 0.7 6 <0.5 0.3 1.9 179 <1 22.5 2.2  AJW-03055 LVS andesite 1.28 25.4 4.27 3.6 0.79 17.3 0.31 2 5.7 20.8 16 9 4.95 39.2 4.85 1 219 0.4 0.71 3.8 <0.5 0.33 2.04 188 6 20.6 2.17  UVS  AJW-04371  0.9 17 3 4 0.5 17.7 0.3 2 6 15.9 <5 6 4.2 79.9 3.1 1 471 0.5 0.4 7 <0.5 0.3 1.9 96 <1 15.6 1.8  AJW-03074 350 intrusions 0.95 20.3 4.56 8.6 0.92 24.1 0.47 <2 12.9 24.7 <5 21 6.47 78.5 5.49 2 313 0.8 0.74 7.99 <0.5 0.45 3.24 31 2 23.7 3.18  AJW-03205 LVS dacite 1 16 3.8 4 0.6 21.1 0.2 <2 8 18.6 96 13 4.6 29 3.8 1 719 0.5 0.5 6 <0.5 0.2 1.7 141 1 16.4 1.6  AJW-03082 HbBt Andesite 0.3 14 2 3 0.4 10.9 0.2 <2 7 9 <5 9 2.7 65.5 2.1 1 148 0.7 0.4 6 <0.5 0.2 1.3 <5 1 13.4 1.5  rhyolite  AJW-03062  0.8 16 4.6 7 0.9 23.6 0.5 3 9 23.9 6 12 6.2 135.5 4.7 2 282 1 0.7 9 <0.5 0.5 2.9 44 1 27.6 3.1  AJW-03116 320 intrusions  198  Zn Zr  suite  Sample OTHS 104 107  104 38.9  AJW-04263  AJW-03204 Augite Basalt 115 265  OTHS  AJW-06485  79 167  AJW-03055 LVS andesite 88 134  UVS  AJW-04371  27 131  AJW-03074 350 intrusions 82 315  AJW-03205 LVS dacite 65 129  AJW-03082 HbBt Andesite 36 60.7  rhyolite  AJW-03062  40 199  AJW-03116 320 intrusions  earth elements (REE) and high field-strength elements (HFSE), which are relatively immobile under low-grade metamorphic conditions and hydrothermal events. Results Mafic to intermediate rocks The mafic to intermediate flows, breccia clasts and shallow intrusions range in composition from picro-basalts to trachyandesites based on silica versus total alkalies, with the Devonian or older units being more primitive (lower SiO2) than the Carboniferous rocks (43.0 wt.% to 58.8 wt.% SiO2; Figure 6.2a). The Ti/1000 versus V tectonic discrimination plot (Shervais, 1982) suggests that the Augite Basalt samples are dominantly island arc tholeites, whereas the other mafic volcanic units are rocks plot in the field of back-arc basalts or MORB (Figure 6.2b). Two of four samples from the OTHS basalt suite have distinctly high Nb concentrations, 31 ppm and 47 ppm, whereas all remaining samples from the suites range from 2 ppm to 13 ppm. This is probably not due to contamination as our results are similar to those of Ayush (2006), who examined OTHS geochemistry with similar results. All samples plot in the volcanic arc field of the Th-Hf-Nb ternary diagram, except for the two high-Nb samples that plot in the oceanic island basalt (rift) field and one OTHS basalt sample that plots just outside of the arc basalt field (Figure 6.2c). All units have continental arc affinities with the exception of the basement Augite Basalts, which have oceanic arc characteristics in the Zr versus Zr/Y plot (Figure 6.2d). Most of the rock suites plot as transitional to calc-alkaline on the Zr versus Y plot (Figure 6.2e), whereas the Augite Basalt samples are distinctly tholeiitic. The Nb/Yb versus Zr/Yb plot suggests that the OTHS basalts have an enriched mantle signature, that the Augite Basalt have the least enriched mantle signature and the remaining suites are intermediate between the two (Figure 6.2f). Primitive mantle-normalized diagrams suggest that all of the suites are depleted in Nb and Ta, with the noteable exception of two of the four samples from the OTHS suite (Figure 6.3). This is consistent with results from Ayush (2006), who determined that the OTHS basalts are divided into two groups based on high field-strength element (HFSE) content. The large-ion lithophile element contents of the samples are erratic, which likely  199  Figure 6.2 Geochemical and tectonic discrimination diagrams: mafic to intermediate rocks. A) Silica versus total alkalies (Le Bas et al., 1986); B) Ti/1000 versus V (Shervais, 1982); C) Th-Hf-Nb ternary diagram (Wood, 1980); D) Zr versus Zr/Y (Pearce, 1983); E) Zr versus Y plot (Barrett et al., 2005); F) Nb/Yb versus Zr/Yb plot (Pearce and Peate 1995).  200  Figure 6.3 Primitive mantle-normalized extended trace element diagrams: mafic to intermediate rocks. Primitive mantle values after Sun and McDonough (1989).  201  Figure 6.4 Chondrite-normalized rare earth element diagrams for mafic to intermediate volcanic and subvolcanic rocks. Chondrite values after Sun and McDonough (1989).  202  reflects alteration effects that preferentially influence the mobile elements. Rare earth element patterns are characterized by small negative europium anomalies and variable enrichment in light rare-earth elements (Figure 6.4). Rare-earth element fractionation is greatest in the OTHS basalt suite.  Intermediate to felsic rocks Intermediate to felsic rocks in the Oyu Tolgoi district range in composition from basaltic andesite to rhyolite (52.6 wt% to 78.1 wt.% SiO2; Figure 6.5a) and plot as calcalkaline to high-K calc-alkaline on the SiO2 versus K2O diagram (0.8 wt.% to 5.0 wt.% SiO2; Figure 6.5b). The majority of these samples plot as high-K calc-alkaline, except for the ~340 Ma rhyolites, which have dominantly medium-K calc-alkaline composition. All of the samples plot as volcanic arc granitoids on the Yb + Nb versus Rb plot (Figure 6.5c). Most samples have Ba/La that exceeds 20, consistent with an arc, as opposed to back-arc, magma with exceptions for the hornblende-biotite andesite and 320 Ma granitoid suites, which have some rocks with low Ba/La (Figure 6.5d).  Rare-earth  element patterns are concave-upward and characterized by LREE enrichment and variable negative europium anomalies (Figure 6.6). There is a gradual trend toward decreasing Sr/Y ratios through time (Figure 6.7a) and no such pattern is apparent with Yb through time which is locally above or below 1.8 ppm (Figure 6.7b). The granodiorite suites (~366 Ma) have unusually low Yb compared to the remaining suites. La/Yb ratios vary through time with no overall trend (Figure 6.7c). The quartz monzodiorite suite at ~372 Ma has lower Sm/Yb than the granodiorite suite at ~366 Ma, however following this, the igneous suites progressively decrease in Sm/Yb with decreasing age (Figure 6.7d).  203  Figure 6.5 Geochemical discrimination diagams for intermediate to felsic rocks from the Oyu Tolgoi district. A) Silica versus total alkalies after Le Bas et al. 1986; B) Silica versus K2O after Peccerillo and Taylor (1976); C) Y + Nb versus Rb plot (after Pearce et al., 1984); D) Silica versus Ba/La. The boundary between the arc and back-arc fields in the Ba/La vs. silica plot is from Kay et al. (1994).  204  Figure 6.6 Chondrite-normalized rare-earth element (REE) plots for intermediate to felsic subvolcanic rocks. Chondrite values from Sun and McDonough (1989). QMD-E, QMD-P1, GDi-P2 and GDi-P3 data is from Chapter 4.  205  Figure 6.7 Variation of adakite-like characteristics and REE ratios with time for intermediate to felsic rocks. Ages are from U-Pb (zircon) data in chapter 2. The field definitions for adakites in the Yb vs. age and Sr/Y vs. age diagrams are from Defant and Drummond (1993).  206  Nd isotopic geochemistry The suite chosen for Nd isotopic geochemistry includes two samples of mafic to intermediate rocks: one Augite Basalt from the Bulagbayan Formation and one sample of andesite from the Lower Volcanic Sequence (LVS) unit. Intermediate to felsic samples include one QMD sample, three granodiorites (two GDi-P2 and one GDi-P3), one dacite from the LVS unit, one hornblende-biotite andesite, two regional granitoid plutons and two rhyolites (Table 6.3). Analytical method Seventeen wholerock samples representing the compositional and age range were analyzed for Nd isotopic ratios at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) lab at The University of British Columbia. The samples were selected based on field-screening, loss-on-ignition (LOI) characteristics from previously obtained wholerock geochemical analyses at ALS Chemex as well as thin section evaluation. Relatively unaltered samples with low LOI were selected and the span of ages and compositions were largely covered within the igneous rocks suites. Prior to isotopic analysis, the samples were leached repeatedly in an ultrasonic bath with 6N HCl following the procedure described by Weis et al. (2005, 2006). At PCIGR, Sr and Nd isotopic ratios were determined using a thermal ionization mass spectrometer (Triton). See Weis et al. (2006) for details of the analytical protocols. Results The measured 143Nd/144Nd for the samples ranges from 0.512629 to 0.512949 and 147  Sm/144Nd values range from 0.09200 to 0.19932 (Table 6.3; Figure 6.7a). The age-  corrected εNd(t) values for the suites range from +3.1 to +7.5 (Table 6.3; Figure 6.8b). There is a general trend toward increasing εNd(t) with time, although there are exceptions in the Augite Basalt, LVS and GDi suites that plot at lower εNd(t) than the trend. Depleted mantle model ages (TDM) for the samples range from 511 to 2081 Ma.  207  208  Granite Granite Rhyolite Rhyolite Hb-Bt andesite Hb-Bt andesite LVS-andesite LVS-Dacite LVS-Dacite Granodiorite GDi-P2 GDi-P2 GDi-P2 GDi-P3 GDi-P3 QMD-E QMD-E QMD-E QMD-P1 Augite basalt Augite basalt Augite basalt  AJW-03-116 AJW-03-132 AJW-03-107 AJW-03-120 AJW-03-183 AJW-03-183 dup AJW-03-055 AJW-03-091 AJW-03-091 dup AJW-03-074 AJW-03-093 AJW-06-476 AJW-06-476 dup AJW-06-415 AJW-03-179 AJW-03-068 AJW-03-068 dup AJW-04-385 AJW-06-451 AJW-03-204 AJW-03-204 dup AJW-04-272  321 324 335 335 345 345 346 347 347 350 366 366 366 366 366 372 372 372 372 390 390 390  age (Ma) 23.9 9.2 11.7 12 20.5 20.5 25.5 24.5 24.5 15.9 17.3 15.8 15.8 18.6 11.2 23.7 23.7 17.4 14.9 9.1 9.1 13.8  Nd (ppm)* 4.7 1.4 2.1 2.2 4.4 4.4 5.4 5 5 3.1 3.5 3.16 3.16 3.63 2.4 5.3 5.3 3.75 3.27 3 3 4.25  Sm (ppm)*  Nd/144Nd  0.512811 0.512729 0.512766 0.512805 0.512817 0.512805 0.512662 0.512802 0.512815 0.512755 0.512643 0.512629 0.512635 0.512659 0.512795 0.512818 0.512821 0.512844 0.512799 0.512949 0.512948 0.512841  143  *Nd and Sm concentrations determined by ICP-MS at ALS CHEMEX, North Vancouver.  rock  sample  Table 6.3 Nd isotopic geochemistry of rocks from the Oyu Tolgoi porphyry Cu-Au district.  0.000006 0.000008 0.000006 0.000007 0.000007 0.000007 0.000007 0.000006 0.000008 0.000007 0.000007 0.000005 0.000007 0.000007 0.000007 0.000007 0.000010 0.000010 0.000006 0.000007 0.000006 0.000010  2σ  Sm/143Nd  0.11889 0.09200 0.10851 0.11084 0.12976 0.12976 0.12803 0.12338 0.12338 0.11787 0.12231 0.12091 0.12091 0.11799 0.12955 0.13520 0.13520 0.13030 0.13268 0.19932 0.19932 0.18620  147  7.5 6.9 6.8 7.5 6.7 6.5 3.8 6.7 6.9 6.0 3.3 3.1 3.2 3.8 5.9 6.0 6.1 6.8 5.8 5.1 5.1 3.6  εNdi  544 527 557 511 604 624 866 586 565 627 844 853 843 781 642 642 637 558 659 2064 2081 1686  TDM (Ma)  Figure 6.8 Nd isotopic geochemistry of A) measured 147Sm/144Nd versus 143Nd/144Nd plot for Devonian to Carboniferous rocks at Oyu Tolgoi; B) Distribution of zircon age-corrected εNd(t) values. Averages of duplicate analyses for some samples are indicated on the diagram.  209  Pb isotopic geochemistry Twelve separates of sulfides and six separates of K-feldspar from the Oyu Tolgoi district were analyzed for lead isotopic compositions. The sulfides, which included samples of pyrite, chalcopyrite or bornite, were separated from quartz monzodiorite and granodiorite intrusions in addition to Augite Basalt basement from each of the discrete Oyu Tolgoi porphyry centers. The potassium feldspar fractions were separated from Late Devonian QMD intrusions as well as from the Carboniferous regional granitoid plutons (Table 6.4). As K-feldspar and sulfides contain almost no uranium, the measured Pb isotope composition of feldspars should be near to the Pb isotope composition at the time of crystallization (Mukasa, 1986; DeWolf and Mezger, 1994; Nebel et al., 2007) and therefore only the measured data has been plotted and interpreted. Analytical method Analytical work was performed at the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at The University of British Columbia. Feldspar and sulfide grains were picked using a binocular microscope. Leaching procedures followed those of Housh and Bowring (1991): grains were leached using 7N HNO3 for 30 min on a hotplate (125oC); the residue was rinsed with Milli-Q H2O, leached by 6N HCl on a hotplate for 30 min and rinsed with Milli-Q H2O; this residue was leached with 5% HF + 0.5N HBr (8:1) for 10 min on a hotplate stirring every 2 minutes followed by rinsing twice with Milli-Q H2O. This last step was repeated until the sample was white with no visible black inclusions. The final residue was dissolved by concentrated HF and 7N HNO3. An aliquot was taken for ICP-MS analyses to determine the parent/daughter abundance ratios. Isotopic ratios are measured with a modified single collector VG-54R thermal ionization mass spectrometer equipped with an analogue Daly photomultiplier. Results The 207  206  Pb/204Pb ratios for feldspar seperates range from 17.97 to 18.72,  Pb/204Pb ratios range from 15.44 to 15.64 and the 208Pb/204Pb ratios range from 37.61 to  38.59. The 206Pb/204Pb ratios for sulfide seperates range from 17.64 to 18.24, the  210  211  Location  North of OT 18km north of OT Javhalant Mountain Javhalant Mountain Central Oyu Central Oyu SW Oyu SW Oyu SW Oyu SW Oyu Central Oyu South Oyu Hugo North SW Oyu Central Oyu Hugo North SW Oyu South Oyu Central Oyu Hugo North  Sample  AJW03-074 AJW03-116 AJW03-132 AJW03-132 AJW04-385 AJW04-385 AJW06-448 AJW06-451 AJW03-178 AJW03-179 AJW03-181 AJW03-182 AJW03-184 AJW03-204 AJW06-467 AJW06-476 AJW03-178 AJW03-182 AJW04-385 AJW04-356  Granite pluton Granite pluton Granite pluton Granite pluton Wk mineralized QMD-E Wk mineralized QMD-E fresh QMD-P1 fresh QMD-P1 Str mineralized QMD-P1 Wk mineralized GDi-P3 Str mineralized QMD-P1 Str mineralized QMD-E Wk mineralized GDi-P3 Wk mineralized basalt Str mineralized QMD-P1 Wk mineralized GDi-P2 Str mineralized QMD-P1 Str mineralized QMD-E Wk mineralized QMD-E Str mineralized QMD-P1  Description feldspar feldspar feldspar feldspar feldspar feldspar feldspar feldspar pyrite pyrite pyrite pyrite pyrite pyrite pyrite pyrite chalcopyrite chalcopyrite chalcopyrite bornite  Mineral  Table 6.4 Pb isotopic geochemistry from Oyu Tolgoi sulfide and feldspar separates. 206  18.72 18.04 18.21 18.35 18.07 18.17 17.97 18.12 17.75 17.77 17.73 17.99 17.90 17.64 17.69 17.82 17.75 17.80 18.24 17.91  Pb/204Pb 0.06 0.10 0.05 0.04 0.08 0.05 0.04 0.04 0.06 0.09 0.04 0.39 0.05 0.17 0.08 0.33 0.04 0.19 0.07 0.05  2σ (%) 15.47 15.46 15.48 15.50 15.44 15.52 15.49 15.50 15.44 15.43 15.41 15.59 15.46 15.37 15.39 15.44 15.44 15.42 15.44 15.51  Pb/204Pb  207  0.07 0.11 0.06 0.06 0.08 0.06 0.06 0.06 0.07 0.07 0.06 0.26 0.06 0.10 0.07 0.18 0.06 0.17 0.07 0.06  2σ (%)  208  38.26 37.65 37.89 37.85 37.61 37.85 37.65 37.76 37.41 37.46 37.42 37.95 37.57 37.13 37.27 37.31 37.43 37.57 37.90 37.74  Pb/204Pb  0.09 0.12 0.09 0.08 0.11 0.08 0.08 0.08 0.10 0.12 0.13 0.42 0.09 0.20 0.12 0.45 0.08 0.23 0.01 0.08  2σ (%)  Figure 6.9 Pb isotopic geochemistry of feldspar and sulfide seperates from Oyu Tolgoi rocks. Lead growth curves from Zartman and Doe (1981): MN: Mantle; LC: Lower Crust. The average crustal growth curve (S-K) is taken from Stacey and Kramers (1975).  212  207  Pb/204Pb ratios range from 15.37 to 15.51 and the 208Pb/204Pb ratios range from 37.13 to  37.90 (Table 6.4). All samples plot near the mantle growth curve from Zartman and Doe (1981) and below the average crustal growth curve of Stacey and Kramers (1975) (Figure 6.9). Moreover, the data show that the Pb isotope ratios are relatively uniform between the samples the sulfide and the feldspar data both overlap in 207  206  Pb/204Pb versus  Pb/204Pb space, although the feldspars have higher 206Pb/204Pb in general. Two outliers  are present; one sample of QMD feldspar and one of the regional granitoid plutons. These are characterized by high 208Pb/204Pb and high 206Pb/204Pb ratios. Discussion Origin and wholerock petrochemical evolution of Devonian to Carboniferous igneous rocks at Oyu Tolgoi Mafic to intermediate volcanic rocks in the Oyu Tolgoi district are dominated by tholeiitic, transitional and calc-alkaline island arc rocks with local occurrences of intermediate alkaline rocks. The sequences exhibit weakly to moderately fractionated REE patterns likely due to amphibole fractionation. Middle to heavy REE are compatible in hornblende, which can be an early fractionating phase in calc-alkaline arc rocks (Dostal et al., 1983). The rocks are characterized by depleted HFSE consistent with volcanic arc environments (Wilson 1989; Foley and Wheller 1990; Brenan et al., 1994); however one allochthonous basalt suite (OTHS unit) has some samples that are characterized by high Nb, more similar to a within-plate geochemical signature. Delong et al. (1975) suggested that the subduction of linear features such as a fracture zone, or possibly the lateral edge of the subduction zone, can lead to unusual magmatism such as the generation of alkaline basalts in otherwise normal subduction zones. At Oyu Tolgoi, the presence of the high Nb basalts in a sequence that contains normal low Nb rocks as well suggests that there was mixing between rocks erupted rocks from entirely different magma sources. One possible model is a discrete plumbing system of non-subductionrelated rocks, such as those from a within-plate (enriched mantle) source that erupted in the same area as the subduction-source rocks. Conversely, Nb-enriched basalts are also reported in island arc rocks from the Philippines, associated with adakites. Sajona et al. (1996) suggest that trace element characteristics of the Nb-enriched basalts imply that  213  amphibole and ilmenite (and therefore Nb and Ti) were added metasomatically to the mantle by slab melts. At Oyu Tolgoi, the oldest rocks (Augite Basalt) are tholeiitic compared to the younger Carboniferous sequences that succeed this unit, which are calc-alkaline to alkaline. The OTHS basalts are exceptional (with high Nb) and poorly constrained as the original position of the unit is uncertain as well as its age and volcanostratigraphic relation to the remainder of the sequence. Calc-alkaline to alkaline volcanic rocks are thought to represent thick and mature island arcs, whereas tholeiitic basalts are derived from immature arcs or possibly a backarc basins (e.g., Kuno, 1966; Miyashiro, 1974; Gill, 1976).  The Carbonifeous calc-alkaline LVS andesites and the alkaline UVS  sequence andesites are probably the result of magmatism in a thickening arc compared to the tholeiitic Augite Basalt sequence in a juvenile subduction zone setting. Intermediate to felsic rock suites are dominantly high-K calc-alkaline and a decrease in Sr/Y ratios through time suggesting that magmatism was fundamentally different during and shortly after the mineralization event at ~372 Ma. A primitive tholeiitic basaltic arc was constructed into which intruded adakite-like rocks (QMD and GDi units). Melting of subducted hot oceanic crust in addition to portions of the mantle wedge can occur during the subduction of unusually hot, young, buoyant crust (Defant and Drummond, 1993). The presence of adakite-like samples soon after the onset of arc magmatism at no older than ~390 Ma may be consistent with this model. Isotopic compositions and petrogenesis of Devonian to Carboniferous igneous rocks at Oyu Tolgoi The Nd isotopic compositions are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc with εNd(t) that ranges from +3 to +7, and that the source of melt (depleted mantle) became increasingly depleted between ~390 Ma and ~320 Ma. The two GDi-P2 samples and one LVS andesite sample with lower εNd(t) of +3 may be the result of derivation from a somewhat enriched mantle source or potentially contamination with relatively radiogenic material not observed within the other samples. All of the lead isotopic compositions lie on or near the mantle growth curve and the source for the lead in much younger Carboniferous intrusions is the same as those from  214  the fertile Late Devonian intrusions.  The higher  206  Pb/204Pb ratios in the feldspar  seperates probably reflects the fact that uranium is slightly more compatible in feldspar than it is in sulfides, therefore more radiogenic lead would be expected today in the feldspars. Furthermore, these results are consistent with a juvenile arc origin, whereby the melts did not assimilate old radiogenic crust, consistent with low yield of ancient xenocrystic zircons in heavy mineral seperates (Chapter 2; Chapter 4). Geodynamic framework of the Paleozoic volcanic arc at Oyu Tolgoi Helo et al. (2006) reported Nd-isotopic compositions from eight samples from the Gurvansayhan Terrane with εNd(t) that ranged from +2.7 to +9.3 and they suggest that the terrane comprises intra-oceanic island arc-forearc–backarc assemblages.  Lamb and  Badarch (2001) presented geochemistry for five samples and interpreted high concentrations of alkali metals in Carboniferous volcanic rocks as evidence for an alkaline composition and possibly a within-plate origin. The results from this study support the interpretation of Helo et al. (2006), that the Devonian Guvansayhan Terrane rocks are largely related to island arc magmatism and unlike Lamb and Badarch (2001), who detected Carboniferous within-plate basalts, the within-plate geochemical signatures (similar to Ayush, 2006) were detected in an allochthonous Devonian or older sequence. Moreover, unlike Lamb and Badarch (2001), the Carboniferous rocks from this study are ubiquitously of volcanic arc affinity, albeit likely a relatively thickened arc with some alkaline samples. It is possible that there are examples in the Carboniferous sequences in the South Gobi where there were eruptions of within-plate melts among otherwise normal subduction zone rocks, similar to findings in the Devonian or older OTHS sequence at Oyu Tolgoi. Conclusions The igneous rocks at Oyu Tolgoi, South Mongolia are largely characterized by depleted HFSE consistent with volcanic arc environments; however one Devonian or older allochthonous basalt suite has high Nb, more similar to a within-plate geochemical signature. Mafic to intermediate volcanic units evolved fom tholeiitic to calc-alkaline and alkaline compositions, which is interpreted to be a reflection of arc maturation and 215  thickening. Felsic rock suites are dominantly high-K calc-alkaline and a decrease in Sr/Y ratios through time suggests that magmatism was fundamentally different during and shortly after the mineralization event at ~372 Ma, possibly related to a unique slab-melt event due to subduction of hot, young crust. Nd isotopic geochemistry from all suites are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc with εNd(t) that ranges from +3 to +7. Lead isotopic geochemistry indicates that the sulfides in the porphyry Cu-Au deposits are genetically linked to the magmas, and that there is little difference between ore-stage magmas and magmas not associated with mineralization. Exceptional characteristics for the volcanic arc include the presence of unusual high-Nb basalts as well as the adakite-like wholerock characteristics of syn- to late-mineral intrusions in the Devonian.  216  References Ayush, O., 2006, Stratigraphy, geochemical characteristics and tectonic interpretation of Middle to Late Paleozoic arc sequences from the Oyu Tolgoi porphyry Cu-Au deposit, unpublished M.Sc. thesis. Badarch, G., Dickson C.W. and Windley, B.F., 2002, A new terrane subdivision for Mongolia: implications for the Phanerozoic crustal growth of Central Asia, Journal of Asian Earth Sciences, v. 21, p. 87-110. Barrett, T.J., MacLean, W.H. and Areback, H., 2005, The Palaeoproterozoic Kristineberg VMS deposit, Skellefte district, northern Sweden. Part II: chemostratigraphy and alteration, Mineralium Deposita, v. 40, p. 368–395. Brenan, J.M., Shaw, H.F., Phinney, D.L. and Ryerson, F.J., 1994, Rutile-aqueous fluid partitioning of Nb, Ta, Hf, Zr, U and Th; implications for high field strength element depletions in island-arc basalts, Earth and Planetary Science Letters, v.128, no.3-4, p.327-339. Buchan, C., Pfaender, J., Kroener, A., Brewer, T.S., Tomurtogoo, O., Tomurhuu, D., Cunningham, D. and Windley, B.F., 2002, Timing of accretion and collisional deformation in the Central Asian Orogenic Belt: implications of granite geochronology in the Bayankhongor Ophiolite Zone, Chemical Geology, v. 192, p. 23-45. Defant, M.J. and Drummond, M.S., 1993, Mount St. Helens: potential example of the partial melting of the subducted lithosphere in a volcanic arc, Geology, v. 21, p. 547– 550. DeLong, S. E. Hodges, F. N. and Arculus, R. J., 1975, Ultramafic and mafic inclusions, Kanaga Island, Alaska, and the occurrence of alkaline rocks in island arcs, Journal of Geology, v. 83, no. 6, p. 721-736. DeWolf, C.P. and Mezger, K., 1994, Lead-isotope analyses of leached feldspars – constraints on the early crustal history of the Grenville-orogen, Geochimica Cosmochimica Acta, v. 58 p. 5537–5550. Dostal, J., Dupuy, C., Carron, J.P., Le Guen de Kerneizon, M. and Maury, R.C., 1983, Partition coefficients of trace elements: application to volcanic rocks of St. Vincent, West Indies, Geochimica Cosmochimica Acta, v. 47, p. 525-533. Floyd, P.A. and Winchester, J.A., 1975, Magma type and tectonic setting discrimination using immobile elements, Earth and Planetary Science Letters, v. 27, no. 2, p.211218. Foley, S.F. and Wheller, G.E., 1990, Parallels in the origin of the geochemical signatures of island arc volcanics and continental potassic igneous rocks: the role of residual titanites, Chemical Geology, v. 85, p. 1–18. Gill, J.B., 1976, Composition and age of Lau Basin and ridge volcanic rocks: implications for evolution of an interarc basin and remnant arc, Geological Society of America, Bulletin v. 87, p.1384–1395. Helo, C., Hegner, E., Kroener, A., Badarch, G., Tomurtogoo, O., Windley, B.F. and Dulski, P., 2006, Geochemical signature of Paleozoic accretionary complexes of the Central Asian Orogenic Belt in South Mongolia: Constraints on arc environments and crustal growth, Chemical Geology, v. 227, p. 236-257.  217  Housh, T. and Bowring, S.A., 1991, Lead isotopic heterogeneities within alkali feldspars: Implications for the determination of initial lead isotopic compositions, Geochim. Cosmochim. Acta, v. 55, p. 2309–2316. Kay S.M., Mpodozis, C., Tittler, A. and Cornejo, P., 1994, Tertiary magmatic evolution of the Maricunga mineral belt in Chile, International Geology Review, v. 36, p. 1079–1112. Kuno, H., 1966. Lateral variation of basalt magma types across continental margins and island arcs, Bulletin of Volcanology, v. 29, p.195–222. Lamb, M.A. and Badarch, G., 2001, Paleozoic sedimentary basins and volcanic arc systems of southern Mongolia: New geochemical and petrographic constraints: in Paleozoic and Mesozoic tectonic evolution of central Asia: From continental assembly to intracontinental deformation, Geological Society of America Memoir 194, p. 117149. Le Bas, M. J.., Le Maitre, R.W., Streckeisen, A. and Zanettin, B., 1986, Chemical classification of volcanic rocks based on the total alkali-silica diagram, Journal of Petrology, v. 27, no. 3, p. 745-750. Lewis, P.D., 2005, Thrust-controlled formation of the giant Hugo Dummett Cu-Au porphyry deposit, Oyu Tolgoi, Mongolia, Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 97. Miyashiro, A., 1974, Volcanic rock series in island arcs and active continental margins, American Journal of Science, v. 274, p. 321– 355. Mukasa, S.B., 1986, Common Pb isotopic compositions of the Lima, Arequipa, and Topqepala segments of the coastal batholith, Peru implications for magmagenesis, Geochim. Cosmochim. Acta, v. 50, p. 771–782. Nebel, O., Münker C., Nebel-Jacobsen Y.J., Kleine, T., Mezger K. and Mortimer N., 2007, Hf–Nd–Pb isotope evidence from Permian arc rocks for the long-term presence of the Indian–Pacific mantle boundary in the SW Pacific, Earth and Planetary Science Letters, v. 254, p. 377–392. Pearce, J.A., 1983, Role of the sub-continental lithosphere in magma genesis at active continental margins, in Continental Basalts and Mantle Xenoliths, C.J.Hawkesworth, M.J. Norry (eds.), Nantwich, England: Shiva, p. 230-249. Pearce, J.A., and Cann, J.R., 1973, Tectonic setting of basic volcanic rocks determined using trace element analyses, Earth and Planetary Science Letters, v. 19, p. 290–300. Pearce, J. A., Harris, N. B. W. and Tindle, A. G., 1984, Trace element discrimination diagrams for the tectonic interpretation of granitic rocks, Journal of Petrology, v. 25, no. 4, p. 956-983. Pearce, J.A., and Peate, D.W., 1995, Tectonic implications of the composition of volcanic arc magmas, Annual Review of Earth and Planetary Sciences, v. 23, p. 251–285. Peccerillo, A. and Taylor, S.R., 1976, Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, northern Turkey, Contributions to Mineralogy and Petrology, v. 58, no.1, p. 63-81. Rollinson, H., 1993, Using geochemical data: evaluation, presentation, interpretation, Prentice Hall, 352 pages. Sajona, F.G., Maury, R.C., Herve, B., Cotten, J. and Defant, M., 1996, High Field Strength Element Enrichment of Pliocene-Pleistocene Island Arc Basalts, Zamboanga Peninsula, Western Mindanao (Philippines), Journal of Petrology, v. 37, no. 3, p. 693-  218  726. Shervais, J.W., 1982, Ti-V plots and the petrogenesis of modern and ophiolitic lavas, Earth and Planetary Science Letters, v. 59, no. 1, p.101-118. Stacey, J.S. and Kramers, J.D., 1975, Approximation of terrestrial lead isotope evolution by a two stage model, Earth Planet Science Letters, v. 26, p. 207–221. Sun, S.S. and McDonough, W.F., 1989, Chemical and isotopic systematics of oceanic basalts: implications for mantel composition and processes. In: Saunders, A.D. and Norry, M.J. (eds.), Magmatism in Ocean Basins, Geol. Soc. London. Spec. Pub. 42, p. 313-342. Weis, D., B. Kieffer, C. Maerschalk, W. Pretorius, and J. Barling, 2005, High-precision Pb-Sr-Nd-Hf isotopic characterization of USGS BHVO-1 and BHVO-2 reference materials, Geochem. Geophys. Geosyst., v. 6, no. 2, p. 1-10. Weis, D., Kieffer, B., Maerschalk, C., Barling, J., de Jong, J., Williams, G.A., Hanano, D., Pretorius, W., Mattielli, N., Scoates, J.S., Goolaerts, A., Friedman, R.M. and Mahoney, J.B., 2006, High-precision isotopic characterization of USGS reference materials by TIMS and MC-ICP-MS, Geochem. Geophys. Geosyst., v. 7, no. 8, p. 130. Wilson, M., 1989, Igneous petrogenesis, Unwin Hyman, London, 466 pages. Wood, D.A., 1980, The application of a Th-Hf-Ta diagram to problems of tectonomagmatic classification and to establishing the nature of crustal contamination of basaltic lavas of the British Tertiary volcanic province, Earth and Planetary Science Letters, v. 50, no. 1, p. 11-30. Zartman, R. E. and Doe, B. R., 1981, Plumbotectonics; the model, Tectonophysics, v.75, no.1-2, p.135-162.  219  Chapter 7 – Summary and Conclusions The main goals of this study were to establish geologic constraints on the volcanic stratigraphy as well as to characterize the magmas in a poorly-studied, poorly-exposed volcanic arc sequence in the South Gobi desert of Mongolia in order to better understand the genesis of Cu-Au mineralization within the framework of arc evolution. The volcanic stratigraphy has been established that constrains the relationship between rock units and mineralization and defines a descriptive nomenclature for the volcano-sedimenrtary successions. Furthermore, wholerock and mineral geochemistry indicates differences between fertile and unfertile magma suites and also characterizes the geodynamic framework of the Paleozoic arc. The variation in stratigraphic sequences suggests that the Oyu Tolgoi district is underlain by a submarine arc that became emergent during the Upper Devonian. These sequences were subsequently drowned by shallow marine sediments and buried by the eruption and mass wasting products of subsequent volcanoes throughout much of the Carboniferous. The mineralization age is constrained between 372 ± 1 Ma and 369 ± 1 in the Oyu Tolgoi porphyry deposits. Furthermore, uplift and erosion of the porphyry deposits occurred prior to the intrusion of late-mineral granodiorite porphyries at ~366 Ma. Arctic, arid conditions are inferred during the Upper Devonian at 60oN latitude as well as rapid mass wasting that outpaced supergene processes contributed to the preservation of hypogene facies immediately below the unconformity. The Oyu Tolgoi porphyry-copper gold deposits are spatially and temporally related to intrusive phases that comprise the Late Devonian Oyu Tolgoi Igneous Complex (OTIC). Quartz monzodiorite phases (QMD) are porphyritic to coarsely equigranular, characteristically red when fresh and high-K calc-alkaline with moderate La/Yb ratios. Granodiorites (GDi) are divided into two groups: 1) fine-grained, medium- to high-K calc-alkaline porphyries with low Sr/Y and 2) coarse-grained, high-K calc-alkaline porphyries with high Sr/Y. All Late Devonian intrusive phases are HREE-depleted with small negative Eu-anomalies and both GDi phases display extremely high La/Yb ratios and high Ni and Cr compared to the QMD series. Distinct linear arrays on SiO2 vs. MgO and SiO2 vs. Al2O3 plots as well as different La/Yb ratios and nickel/chromium  220  concentrations that characterize the rock suites suggest that magmas with separate petrogenetic lineages are present, as well as the possibility that a magma recharge event occurred. Cross-cutting relationships and U-Pb (zircon) geochronology (SHRIMP-RG and TIMS) indicate that the QMD intrusions were emplaced at ~372 Ma and GDi inrusions at ~366 Ma. The U-Pb results suggest the possibility as well that there were pre-mineral QMD intrusions at approximately 374 Ma or earlier. Trace element geochemistry of zircons suggests that fractionation and cooling trends in the Late Devonian Oyu Tolgoi Igneous Complex (OTIC) have been complicated by mixing of magmas from distinct reservoirs. These reservoirs have distinct Th/U, Yb/Gd, EuN/EuN* and CeN/CeN* ratios as well as Hf and Y concentrations. The TiO2-inzircon thermometer estimates that minimum temperatures for the spot analyses range from 640oC to 880oC and zircons typically exhibit a transition from high-temperature cores with low Hf toward low-temperature rims with high Hf concentrations. Temperature and differentiation characteristics are not significantly different between the syn-mineral, late-mineral and younger intusions; however, the presence of zircons with high CeN/CeN*, EuN/EuN* and Yb/Gd in the sample populations from syn- and latemineral porphyry intrusions distinguishes them from younger intrusions that are not related to porphyry Cu-Au formation. A turbulent, low-viscosity magma chamber would be capable of generating melts with mixed populations of zircons in the individual aliquots via scouring material from wallrock and moving refractory minerals to the various sectors of the subvolcanic chamber. The igneous rocks at Oyu Tolgoi are largely characterized by depleted HFSE consistent with volcanic arc environments; however one Devonian or older allochthonous basalt suite has high Nb, more similar to a within-plate geochemical signature. Mafic to intermediate volcanic units evolved fom tholeiitic to calc-alkaline compositions, which is interpreted to be a reflection of arc maturation and thickening. Felsic rock suites are dominantly high-K calc-alkaline and a decrease in Sr/Y ratios through time suggests that magmatism was fundamentally different during and shortly after the mineralization event at ~372 Ma, possibly related to a unique slab-melt event due to subduction of hot, young crust. Nd isotopic geochemistry from all suites are consistent with magma derivation from depleted mantle in an intra-oceanic volcanic arc with εNd(t) that ranges from +3 to  221  +7. Lead isotopic geochemistry suggests that the sulfides in the porphyry Cu-Au deposits are genetically linked to the magmas, and that there is little difference between ore-stage magmas and magmas not associated with mineralization. Magma mixing, adakite-like magmatism and rapid shortening and uplift in a Devonian juvenile marine arc setting differentiate the ore-stage geologic environment at Oyu Tolgoi from those settings that are more commonplace in volcanic arcs of all ages. Future research directions Although significant work has been done (Perello et al., 2001; Kirwin et al., 2005; Lewis, 2005; Ayush, 2006, Khashgerel et al., 2006; this study), the Oyu Tolgoi porphyry Cu-Au district is complex and much remains to be learned. Exploration in the district is ongoing and the newly discovered Heruga deposits 5 km south-southwest of the Southern Oyu deposits in the same mineralized corridor require study and the framework of mineralization in the district will be expanded. Structure and stratigraphy A maximum age has been constrained for the shortening events, however the minimum age is not known (one dike sampled that cross-cut the folding did not yield zircons or other mineral chronometers). Now that an informal nomenclature for the stratigraphic units in the district has been established, future stratigraphers can properly define Groups, Members and Formations for these based on fully concordant volcanic arc successions measured in outcrop in the South Gobi desert. As well, future work can better constrain the architecture of the deposit area and in particular how deformation has affected the geometry of the deposits. Although work has been done during this study and by Ayush (2006), the nature or the OTHS sequence is poorly constrained and more work is needed to understand its origin and geochemistry. Specifically, very little is known about its original position within the stratigraphic sequence, nor the unusual association between the high- and low-Nb basalts. Another enigmatic unit is the MFS sequence due to intense alteration in the study area, as little is known except for the fact that it is likely fragmental. More stratigraphic work outside of the study area should add  222  to the knowledge of its petrology as well as its relationship with underlying Augite Basalt rocks. Geodynamic framework The question of the tectonic history of the Central Asian Orogenic Belt requires more work. Specifically the discrepancies between the oroclinal bend (e.g. Sengor et al., 1993; Sengor and Natal’in, 1996) and punctuated accretion models (e.g. Mossakovsky et al., 1994) remains to be resolved. A larger-scale detrital zircon study would be able to detect zircon age populations that could be assigned to Siberia-Baltica or Gondwana origins. Geochemistry There is a lack of samples of OTIC intrusions from the Hugo Dummett zone due to intense alteration of intrusions in this area that rendered the rocks unsuitable for wholerock geochemical analysis. As drilling continues and mining will also expose new rocks, appropriate samples may become available. As the trace-element-in-zircon study has highlighted differences between the Southern Oyu deposits compared to the Hugo Dummett zone, the petrochemistry of the intrusions is important. The sample database for trace element in zircons needs to be enlarged and compared with a large database of ore-stage intrusions from other systems as well as to rocks from volcanic arcs that are not related to Cu-Au mineralization.  223  References Ayush, O., 2006, Stratigraphy, geochemical characteristics and tectonic interpretation of Middle to Late Paleozoic arc sequences from the Oyu Tolgoi porphyry Cu-Au deposit, unpublished M.Sc. thesis. Khashgerel, B-E, Rye, R.O., Hedenquist, J.W. and Kavalieris, I., 2006, Geology and reconnaissance stable isotope study of the Oyu Tolgoi porphyry Cu-Au system, South Gobi, Mongolia, Economic Geology, v. 101, no. 3, p. 503-522. Kirwin, D.J., Forster, C.N., Kavalieris, I., Crane, D., Orssich, C., Panther, C., Garamjav, D., Munkhbat, T.O. and Niislelkhuu, G., 2005, The Oyu Tolgoi copper-gold porphyry deposits, South Gobi, Mongolia, in IAGOD Guidebook series 11, Seltmann R., Gerel, O. and Kirwin, D.J. (eds.), p. 156-168. Lewis, P.D., 2005, Thrust-controlled formation of the giant Hugo Dummett Cu-Au porphyry deposit, Oyu Tolgoi, Mongolia, Geological Society of America Abstracts with Programs, v. 37, no. 7, p. 97. Mossakovsky, A.A., Ruzhentsev, S.V., Samygin, S.G., Kheraskova, T.N., 1994, Central Asian foldbelt: geodynamic evolution and formation history, Geotectonics, v. 27, p. 445– 474. Perello, J., Cox, D., Garamjav, D., Sanjdorj, S., Diakov, S., Schissel, D., Munkhbat, D. and Oyun, G., 2001, Oyu Tolgoi, Mongolia; Siluro-Devonian porphyry Cu-Au-(Mo) and high-sulfidation Cu mineralization with a Cretaceous chalcocite blanket, Economic Geology, v. 96, no. 6, p. 1407-1428. Sengor, A.M.C., Natal'in, B.A. and Burtman, V.S., 1993, Evolution of the Altaid tectonic collage and Paleozoic crustal growth in Eurasia, Nature, v. 364, p.299-307. Sengor, A.M.C. and Natal’in, B.A., 1996, Turkic-type orogeny and its role in the making of the continental crust, Annual Review of Earth and Planetary Sciences, v. 24, p. 263-337.  224  Appendix 1 – CL images and SHRIMP-RG spot locations (zircon) Cathodoluminescence images from SHRIMP-RG zircon mounts. Numbers refer to spot analyses for each sample from Tables 2.3 and 4.3.  225  226  227  228  229  230  231  232  233  234  235  236  237  AJW-03-101 QMD-E 57.83 0.57 17.26 <0.01 5.14 0.93 0.16 1.57 3.59 5.03 2.73 0.23 0.1 0.16 5.56 99.95 <1 1240 28.4 12.2 10 2.85 22 2.72 1.71  AJW-06-452 QMD-E  59.04 0.6 17.98 <0.01 5.39 2.75 0.12 1.53 3.89 4.92 3.13 0.24 0.12 0.1 2.88 99.94 <1 956 32.8 9.5 <10 1.74 187 3.35 1.93  Sample phase  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  60 0.53 17.26 <0.01 4.47 2.24 0.14 1.33 4.16 5.02 3.09 0.25 0.13 0.09 3.01 99.47 <1 943 34.4 7.9 <10 1.16 35 3.43 2.15  AJW-06-456 QMD-E 58.75 0.57 18.46 <0.01 5.37 3.2 0.12 1.48 2.69 5.34 3.38 0.24 0.08 0.09 3.23 99.8 <1 837 33.9 8.7 <10 1.8 214 3.03 1.9  AJW-06-445 QMD-E  Appendix 2 - Wholerock geochemistry for OTIC intrusions  60 0.57 17.49 <0.01 4.56 2.63 0.14 1.37 3.33 5.04 3.77 0.24 0.1 0.09 3.34 100 <1 910 31.5 7.6 <10 1.22 21 3.28 1.97  AJW-06-450 QMD-E 65.15 0.46 16.91 <0.01 2.28 1.48 0.04 0.6 2.23 4.34 4.13 0.11 0.07 0.08 3.29 99.69 1 738 26.6 1.9 <10 1.74 1930 2.3 1.34  AJW-06-463 QMD-E 60.45 0.53 17.21 <0.01 4.83 2.17 0.09 1.06 3.28 4.75 3.96 0.22 0.08 0.1 3.38 99.93 <1 765 33.7 8.1 10 2.52 354 3.45 2.26  AJW-04-385 QMD-E  58.92 0.68 18 <0.01 5.84 3.39 0.18 1.85 3.92 3.95 4.03 0.27 0.07 0.12 2.11 99.93 <1 1105 45.7 10.9 50 1.6 59 4.3 2.6  AJW-03-068 QMD-E  238  AJW-03-101 QMD-E 0.98 18.2 3.24 3.2 0.57 14.6 0.26 3 5.8 14.2 9 9 3.47 68.5 3.05 1 879 0.4 0.47 2.94 <0.5 0.25 1.52 172 3 16.4 1.86 63 101  AJW-06-452 QMD-E  1.13 20.6 3.54 3 0.63 15.9 0.32 13 6.7 15.5 5 7 3.78 64.3 3.49 1 1055 0.4 0.58 2.63 <0.5 0.27 1.54 142 3 16.5 2 67 111  Sample phase  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  1.17 20.4 3.77 3.9 0.68 16.5 0.36 5 7.1 15.9 5 7 3.91 68.1 3.6 1 1155 0.5 0.61 3.38 <0.5 0.3 1.68 132 3 17.1 2.16 60 141  AJW-06-456 QMD-E 1.13 20.1 3.37 3.8 0.61 16.3 0.31 45 6.7 16 5 8 3.79 74.1 3.45 1 683 0.5 0.52 2.94 <0.5 0.25 1.82 126 6 14.1 2.01 81 141  AJW-06-445 QMD-E 1.07 19.3 3.7 2.9 0.67 15.1 0.28 17 6.5 15.1 <5 27 3.65 83.7 3.52 1 826 0.4 0.57 2.92 <0.5 0.27 1.61 134 1 16.7 2.09 97 102  AJW-06-450 QMD-E 0.8 19.7 2.54 2.7 0.43 13.5 0.22 27 5.4 11.1 <5 5 2.81 93.9 2.27 1 492 0.3 0.38 2.41 <0.5 0.21 1.38 94 5 11.9 1.44 35 104  AJW-06-463 QMD-E 1.11 18.2 4 4.6 0.77 17.2 0.36 21 7.1 17.4 8 7 4.32 89.1 3.75 1 759 0.5 0.6 4.42 <0.5 0.35 2.14 149 2 20.9 2.49 35 156.5  AJW-04-385 QMD-E 1.2 19 4.6 6 0.9 20.9 0.4 5 9 23.7 <5 13 5.8 96.1 5.3 2 658 0.6 0.7 6 <0.5 0.4 2.4 113 1 24.8 2.8 205 183.5  AJW-03-068 QMD-E  239  AJW-06-407 QMD-E 64.68 0.44 15.24 <0.01 3.51 2.25 0.13 1.66 2.89 3.83 2.32 0.18 0.04 0.04 4.04 99.01 <1 450 37 9.8 30 2.32 251 2.24 1.12  AJW-03-087 QMD-E  65.47 0.44 16.25 <0.01 2.93 1.62 0.05 1.16 2.89 3.67 2.62 0.16 0.04 0.1 3.83 99.6 <1 836 35.4 5.9 70 2.1 6 2 1  Sample phase  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  62.99 0.48 15.77 <0.01 3.52 2.38 0.15 1.82 2.78 3.9 2.57 0.18 0.05 0.08 5.22 99.48 <1 691 36.2 8.3 30 2.22 28 2.04 1.04  AJW-04-402 QMD-E 57.42 0.58 17.77 0.01 5.19 2.32 0.19 1.48 4.52 5.48 3.13 0.27 0.12 0.09 3.37 99.62 <1 770 30.9 9 70 1.4 114 3.3 2.1  AJW-03-098 QMD-P1 59.46 0.59 17.76 <0.01 4.46 1.67 0.09 1.17 3.19 5.27 3.44 0.26 0.09 0.09 4.05 99.91 <1 848 34.8 20.2 <10 1.62 191 2.8 1.56  AJW-06-446 QMD-P1 58.73 0.56 17.89 <0.01 5.3 2.37 0.12 1.37 4.06 5.52 3.18 0.27 0.12 0.09 2.76 99.97 <1 926 32.1 11.7 <10 0.96 191 3.26 2.01  AJW-06-448 QMD-P1 58.65 0.57 17.91 <0.01 5.09 2.43 0.13 1.31 3.94 5.43 3.31 0.25 0.1 0.11 3.16 99.96 <1 917 33.6 11.9 <10 1.05 173 3.26 2.07  AJW-06-449 QMD-P1 61.42 0.49 17.57 <0.01 4.57 2.05 0.12 1.31 3.54 5.03 3.51 0.23 0.12 0.09 2.08 100.05 <1 896 32.5 7.4 <10 0.99 42 3.23 1.99  AJW-06-451 QMD-P1  240  AJW-06-407 QMD-E 0.96 20.5 2.96 3.4 0.41 16.6 0.13 <2 6.8 16.9 20 9 4.12 61.2 3.2 1 300 0.5 0.44 3.31 <0.5 0.14 1.44 66 6 9.9 1.04 79 125  AJW-03-087 QMD-E  0.9 20 2.8 4 0.4 17.4 0.1 <2 7 16.7 6 12 4.2 72.7 3.3 1 307 0.5 0.4 4 <0.5 0.1 1.8 62 3 10.4 0.9 73 110  Sample phase  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  0.94 20 3.16 4.3 0.38 18.7 0.15 <2 7.2 17.5 19 12 4.43 78.2 3.4 1 416 0.5 0.41 4.14 <0.5 0.15 1.64 86 4 11.2 1 93 136.5  AJW-04-402 QMD-E 1.2 19 3.8 3 0.7 15.4 0.3 5 6 16.1 <5 8 3.7 58.6 3.5 <1 1035 <0.5 0.6 3 <0.5 0.3 1.3 150 1 19.2 2.1 59 91.9  AJW-03-098 QMD-P1 1.16 18.4 3.39 3 0.55 17 0.26 30 5.8 15.9 5 10 3.85 77.7 3.46 1 765 0.4 0.53 2.67 <0.5 0.22 1.68 122 3 12.8 1.52 60 104  AJW-06-446 QMD-P1 1.09 20.1 3.62 2.7 0.64 15.4 0.3 21 6.3 15.4 5 8 3.7 68 3.42 1 1045 0.4 0.57 2.69 <0.5 0.3 1.71 139 3 17 1.92 69 89  AJW-06-448 QMD-P1 1.12 19.7 3.61 2.9 0.65 15.8 0.29 29 6.6 15.9 5 16 3.78 69.9 3.38 1 852 0.5 0.6 2.86 <0.5 0.3 1.67 138 10 17.2 1.98 81 108  AJW-06-449 QMD-P1 1.04 19.6 3.3 3.2 0.63 15.5 0.31 <2 6.9 14.9 <5 6 3.66 74 3.27 1 1010 0.5 0.55 3.79 <0.5 0.27 1.95 124 3 16.4 2.12 57 110  AJW-06-451 QMD-P1  241  AJW-06-457 QMD-P1 58.92 0.56 17.95 <0.01 4.72 2.49 0.14 1.33 3.84 5.55 3.26 0.24 0.12 0.1 3.13 99.85 <1 894 33 7.7 10 1.29 70 3.43 2.06  AJW-06-454 QMD-P1  58.45 0.55 18 <0.01 5.61 3.45 0.13 1.77 3.26 5.18 3.16 0.29 0.08 0.1 3.11 99.69 <1 869 36.9 12.8 10 1.32 288 3.68 2.06  Sample phase  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  57.92 0.57 18.04 <0.01 4.91 2.62 0.14 1.54 3.84 5.34 3.26 0.26 0.09 0.08 3.52 99.5 <1 830 32.9 9.8 <10 1.44 132 3.55 2.06  AJW-06-458 QMD-P1 55.16 0.76 17.02 0.02 6.4 4.18 0.21 2.41 4.75 4.97 2.87 0.33 0.07 0.1 4.5 99.57 <1 835 38.7 14.8 60 1.8 63 3.9 2.3  AJW-03-100 QMD-P1 60.97 0.43 17.31 <0.01 4.36 1.99 0.09 1.08 2.96 5.12 3.67 0.18 0.07 0.09 3.25 99.55 <1 786 27.4 7.3 10 2.26 314 2.55 1.68  AJW-04-336 QMD-P1 59.71 0.5 17.62 0.01 4.43 2.19 0.13 1.55 3.37 5.13 2.92 0.22 0.1 0.1 3.28 99.06 <1 827 32.3 8.2 100 1.9 118 2.9 2  AJW-03-109 QMD-P1 61.54 0.66 16.61 0.01 5.58 2.17 0.07 1.27 2.03 2.2 2.8 0.18 0.03 0.05 6.45 99.48 <1 388 38.3 9.9 140 4.51 210 3.2 2  AJW-06-405 GDi-P2 62.64 0.55 15.24 0.01 4.37 3.27 0.13 1.64 2.27 2.24 3.14 0.19 0.09 0.12 7.03 99.67 <1 1210 42.4 11.3 40 3.5 424 2.82 1.51  AJW-06-412 GDi-P2  242  AJW-06-457 QMD-P1 1.12 19.7 3.55 3.2 0.67 15.7 0.32 14 6.2 15.8 9 8 3.84 69.7 3.74 1 1045 0.4 0.61 2.88 <0.5 0.29 1.49 133 3 17.1 2.1 64 117  AJW-06-454 QMD-P1  1.23 20.8 3.8 3.2 0.7 17.4 0.32 2 7.5 17.4 6 8 4.17 64.4 3.85 1 748 0.4 0.62 3.1 <0.5 0.33 1.61 140 <1 18 2.16 71 127  Sample phase  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  1.09 19.6 3.55 4.1 0.67 15.6 0.3 17 6.1 15.9 5 9 3.8 73.1 3.54 1 816 0.4 0.58 2.61 <0.5 0.3 1.52 126 1 17 2.12 64 161  AJW-06-458 QMD-P1 1.2 18 4.5 4 0.8 19.2 0.4 2 6 19.8 8 10 4.8 65.1 4.5 2 609 <0.5 0.7 4 <0.5 0.4 1.9 146 1 22.8 2.4 78 133  AJW-03-100 QMD-P1 0.92 18.4 3.11 3.1 0.55 15.2 0.27 11 5.4 13.6 6 6 3.3 80 2.87 1 718 0.4 0.44 3.13 <0.5 0.25 2.02 134 2 16.4 1.84 36 101.5  AJW-04-336 QMD-P1 1 19 3.3 4 0.6 17 0.3 6 6 15.2 5 7 3.7 58.6 3.2 1 828 <0.5 0.5 3 <0.5 0.3 1.6 128 2 17.4 1.9 45 100  AJW-03-109 QMD-P1 0.87 21.4 3.15 4 0.65 18 0.27 <2 8.5 15.2 46 43 3.94 70.8 3.2 2 222 0.6 0.52 4.57 <0.5 0.24 1.67 99 12 16.3 1.77 152 154  AJW-06-405 GDi-P2 1.07 21.2 3.63 3.5 0.51 19.7 0.18 <2 7.5 18.4 21 14 4.66 103 3.79 1 747 0.5 0.56 3.85 0.5 0.18 1.47 77 3 12.9 1.17 126 134  AJW-06-412 GDi-P2  243  AJW-03-131 GDi-P2 65.16 0.38 16.39 0.02 3.62 1.87 0.07 1.06 1.73 4.44 3.88 0.15 0.09 0.11 1.97 99.06 1 889 27.9 5.7 110 1.8 2360 2.3 1.5  AJW-06-476 GDi-P2  64.04 0.6 15.82 <0.01 4.21 3.14 0.1 1.26 1.68 2.31 3.46 0.19 0.07 0.15 5.76 99.63 <1 1485 36.8 11.8 40 3.48 446 2.43 1.23  Sample phase  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  68.2 0.37 15.27 <0.01 4.25 0.64 0.07 0.4 1.36 4.52 2.07 0.14 0.04 0.12 2.46 99.24 <1 937 17.2 7.2 20 2.3 25 1.61 1.04  AJW-04-360 GDi-P2 67.76 0.49 15.22 <0.01 3.09 2.12 0.06 1.16 1.7 2.86 2.79 0.19 0.04 0.27 3.51 99.15 <1 2910 40.3 7.8 20 4.27 92 2.02 1.09  AJW-06-406 GDi-P2 66.85 0.51 14.95 0.01 2.78 1.6 0.08 1.25 2.03 3.63 2.79 0.17 0.07 0.04 4.37 99.54 <1 397 41.1 5.9 40 2.91 2060 2.53 1.48  AJW-06-413 GDi-P2 65.72 0.49 15.93 <0.01 3.34 2.55 0.06 1.3 2.52 4.07 2.97 0.2 0.05 0.1 2.99 99.75 <1 925 37.3 6.7 100 1.3 10 2 1.1  AJW-03-093 GDi-P2 65.41 0.53 15.89 <0.01 3.6 2.12 0.09 1.11 1.35 0.07 4.51 0.19 0.02 0.03 5.52 98.33 <1 292 37 8.3 20 4.06 4370 2.78 1.4  AJW-06-474 GDi-P3 65.01 0.42 15.36 <0.01 3.06 1.67 0.04 1.44 3.06 2 3.03 0.17 0.02 0.01 5.5 99.13 <1 126.5 32.4 7.7 40 4.22 18 1.86 1.06  AJW-04-250 GDi-P3  244  AJW-03-131 GDi-P2 0.7 17 2.5 4 0.5 14.6 0.3 30 7 12.3 5 8 3.1 77.2 2.6 1 739 <0.5 0.4 4 <0.5 0.2 2.5 85 3 14.2 1.6 37 115.5  AJW-06-476 GDi-P2  0.91 21.2 3.3 3.7 0.41 17.1 0.16 <2 7.9 15.8 19 14 3.98 106.5 3.16 1 510 0.6 0.47 3.96 0.5 0.17 1.73 74 12 11 1.1 128 143  Sample phase  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  0.61 17.6 1.8 2.5 0.34 9.2 0.18 7 3.5 8.6 11 10 2.07 51.7 1.75 1 349 0.2 0.27 1.3 <0.5 0.16 0.6 64 1 10 1.14 59 81.1  AJW-04-360 GDi-P2 0.89 22.6 3.19 3.9 0.36 18.4 0.14 <2 8 18.3 11 65 4.54 74.2 3.4 1 265 0.6 0.44 3.9 <0.5 0.14 2 67 8 8.8 0.85 89 139  AJW-06-406 GDi-P2 0.92 17.8 3.23 3.5 0.45 19 0.18 3 7.6 17.7 15 6 4.57 71.2 3.54 1 621 0.5 0.49 3.65 <0.5 0.16 1.72 68 5 12.2 1.31 54 131  AJW-06-413 GDi-P2 0.9 20 2.8 4 0.4 17.8 0.2 2 7 17.3 6 19 4.5 59.4 3.5 1 417 0.5 0.4 4 <0.5 0.1 2.1 69 1 10.5 1 78 118  AJW-03-093 GDi-P2 1.13 22.9 3.58 4 0.46 16.8 0.17 <2 7.6 17 15 6 4.16 127.5 3.54 1 97.5 0.5 0.54 3.71 0.6 0.17 2.4 75 7 12.1 1.23 135 149  AJW-06-474 GDi-P3 0.67 19.7 2.45 3.4 0.34 14.5 0.14 <2 6.3 14 18 8 3.56 59.3 2.74 1 142.5 0.5 0.35 3.5 <0.5 0.13 1.36 62 7 8.7 0.95 58 125  AJW-04-250 GDi-P3  245  AJW-06-415 GDi-P3 65.5 0.41 14.84 <0.01 2.84 1.34 0.07 1.55 2.85 3.56 2.77 0.19 0.04 0.03 5.05 99.7 <1 447 41.6 8 20 2.78 336 2.57 1.26  AJW-06-414 GDi-P3  61.14 0.58 16.09 <0.01 4.01 2.94 0.16 2.22 2.33 2.15 3.44 0.22 0.04 0.06 7.09 99.56 <1 787 41.5 14.3 60 4.2 685 2.62 1.35  Sample phase  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er  62.19 0.49 15.05 <0.01 2.88 1.98 0.05 1.67 4.06 3.27 2.46 0.18 0.03 0.1 6.36 98.79 <1 767 42.2 6.6 50 5.95 32 2.49 1.26  AJW-03-185 GDi-P3 60.07 0.55 17.88 0.01 5.12 2.44 0.1 1.44 2.46 4.64 3.85 0.22 0.07 0.09 3.02 99.52 <1 755 32.3 8 60 2.8 663 3.3 2.1  AJW-03-112 GDi-P3 66.46 0.32 15.59 0.02 3.07 1.35 0.08 1.02 2.67 4.18 2.36 0.12 0.04 0.05 3.16 99.13 <1 298 26.6 4.8 180 2.9 108 2.1 1.4  AJW-03-179 GDi-P3  246  AJW-06-415 GDi-P3 1.12 22.3 3.38 4.3 0.47 19.1 0.16 <2 8.2 18.6 14 10 4.59 76.5 3.63 1 375 0.6 0.5 3.82 <0.5 0.17 2.18 71 11 11.7 1.15 70 150  AJW-06-414 GDi-P3  0.93 20.7 3.48 3.8 0.45 19.4 0.13 3 7 18.3 38 26 4.62 103.5 3.63 1 343 0.5 0.48 4.14 <0.5 0.15 1.64 84 6 11.6 1.12 202 141  Sample phase  Eu Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  1.08 19.2 3.71 3.8 0.46 20.8 0.18 2 6.8 19.6 19 7 4.95 67.3 3.67 1 326 0.5 0.48 4.14 <0.5 0.17 1.26 98 5 13.2 1.17 44 127  AJW-03-185 GDi-P3 1.1 19 3.8 3 0.6 15.9 0.3 4 6 16.4 <5 10 3.8 86.3 3.5 1 564 <0.5 0.5 2 <0.5 0.3 1 140 1 18.9 2 76 88.8  AJW-03-112 GDi-P3 0.6 16 2.4 4 0.4 14.2 0.3 6 7 11.2 7 13 2.9 72.6 2.4 1 336 <0.5 0.3 4 <0.5 0.2 2.1 72 2 13.3 1.6 45 111.5  AJW-03-179 GDi-P3  Appendix 3 – Zircon spot locations for trace element study Cathodoluminescence images from zircon mounts. Numbers refer to spot analyses for each sample from Table 5.2.  247  248  249  250  251  252  253  AJW-03-130 Aug Bas 45.42 1.02 18.21 <0.01 11.14 5.86 0.09 5.68 8.67 2.88 1.94 0.27 0.07 0.02 3.7 99.12 1 144.5 18.8 26.9 30 1.2 959 4.1 2.6 1.3  AJW-03-070 Aug Bas  51.41 0.76 19.64 <0.01 9.99 3.34 0.21 1.1 2.28 0.68 4.96 0.55 0.04 0.27 7.1 98.99 <1 2720 41.3 17.8 30 8.9 431 4.4 2.6 1.7  Sample suite  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu  44.35 1.42 13.11 0.02 14.3 6.11 0.28 8.5 11.84 2.04 0.64 0.18 0.05 0.03 3.08 99.81 <1 152.5 13.1 42.4 160 0.2 127 3.6 2.1 1  AJW-03-204 Aug Bas 45.8 1.36 15.66 0.01 12.37 6.33 0.22 6.28 10.13 2.41 1.56 0.25 0.06 0.04 1.99 98.14 <1 414 20.2 47.5 90 2.17 296 5.03 3.27 1.4  AJW-04-272 Aug Bas 45.39 1.15 14.85 0.02 11.19 7.39 0.14 5.47 6.5 2.26 1.92 0.45 0.05 0.03 8.83 98.26 <1 252 45.3 42.5 190 4.22 171 3.74 2.09 1.62  AJW-04-251 OTHS  Appendix 4 - Wholerock geochemistry for Devonian-Carboniferous rocks  45.98 0.87 12.06 0.11 9.35 6.52 0.25 7.18 7.19 1.2 1.55 0.2 0.03 0.01 12.65 98.63 <1 69.9 35.4 46.4 840 4.61 72 3.92 2.37 1.19  AJW-04-263 OTHS 43.25 1.64 14.15 0.01 11.8 7.67 0.19 4.63 8.6 1.72 2.61 0.49 0.08 0.09 9.4 98.66 <1 785 78.6 40.1 30 5.36 122 4.73 2.55 2.25  AJW-06-483 OTHS  46.79 1.79 16.44 <0.01 9.38 5.33 0.17 2.25 7.55 3.38 3.33 0.56 0.15 0.09 7.15 99.03 <1 827 106 25.8 20 4.63 61 5.55 2.75 2.66  AJW-06-485 OTHS  254  AJW-03-130 Aug Bas 20 3.6 2 0.8 8.3 0.4 7 3 12.4 21 <5 2.7 61.9 3.5 1 733 <0.5 0.6 1 <0.5 0.4 0.5 382 2 22.9 2.3 42 51.5  AJW-03-070 Aug Bas  25 4.9 2 0.9 20.3 0.4 <2 4 24.2 <5 20 5.5 125 5.4 1 331 <0.5 0.8 2 0.6 0.4 1.3 184 4 24.2 2.5 97 65.4  Sample suite  Ga Gd Hf Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  16 3.2 1 0.7 5.3 0.3 2 2 9.1 51 8 1.8 7.8 3 <1 478 <0.5 0.6 <1 <0.5 0.3 <0.5 499 2 20.3 1.8 104 38.9  AJW-03-204 Aug Bas 20.4 4.53 1.8 1.1 8.4 0.45 <2 2.8 13.8 34 12 2.88 47.5 4.25 1 699 0.2 0.81 0.66 <0.5 0.45 0.26 439 1 28.3 3 116 63  AJW-04-272 Aug Bas 18.1 5.01 2.4 0.69 22.2 0.29 <2 13 23.2 79 13 5.78 52 5.13 1 552 0.8 0.69 2.99 <0.5 0.3 0.8 364 3 18.2 1.84 108 81  AJW-04-251 OTHS 15.1 4 3.2 0.73 16.5 0.33 <2 6.4 18.1 232 11 4.5 43 4.1 1 345 0.4 0.64 3.73 <0.5 0.32 0.97 196 3 19.7 2.16 104 107  AJW-04-263 OTHS 18.8 6.57 4.6 0.87 40 0.29 <2 31.5 35.2 17 9 9.31 55.5 6.79 1 800 1.8 0.88 4.1 <0.5 0.31 1.03 282 1 21.7 1.92 118 177  AJW-06-483 OTHS 20.2 7.84 6.1 0.94 55.8 0.31 <2 47.4 44.9 9 15 12.2 80.1 8.34 2 1540 2.7 0.99 5.12 <0.5 0.36 1.2 216 1 24.8 2.15 115 265  AJW-06-485 OTHS  255  AJW-03-075 LVS 67.83 0.52 15.12 0.02 2.67 0.9 0.06 0.91 1.67 5.08 3.1 0.16 0.03 0.09 2.64 99.9 <1 685 41.8 3.9 100 0.9 5 3.8 2.4 1 13 4.2 6  AJW-03-055 LVS  55.84 0.93 16.67 0.01 7.32 3.81 0.13 3.54 5.98 2.95 2.59 0.31 0.07 0.08 3.51 99.92 <1 674 51.9 21.5 140 1.4 57 4.1 2.4 1.5 19 5 5  Sample suite  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf  65.85 0.48 16.69 <0.01 3.08 0.39 0.05 0.55 2.06 3.06 4.74 0.12 0.04 0.07 3.3 100.1 <1 646 49.9 3.2 20 5.6 30 4.1 2.6 0.9 19 4.5 9  AJW-03-091 LVS 53.67 0.84 17.72 <0.01 7.54 2.75 0.18 2.31 3.71 6.39 0.82 0.31 0.04 0.01 5.1 98.64 <1 91.3 33.8 31.2 20 2.12 24 4.03 2.41 1.18 19.2 4.95 3.2  AJW-03-092 LVS 63.99 0.51 16.75 <0.01 3.65 0.32 0.06 0.74 2.19 3.13 3.14 0.12 0.04 0.01 4.31 98.63 <1 148.5 52.8 6.1 <10 6.27 89 4.43 2.91 0.95 20.3 4.56 8.6  AJW-03-205 LVS 58.76 0.91 15.12 0.01 4.99 3.39 0.09 2.85 3.75 3.61 3.22 0.27 0.07 0.11 4.9 98.65 <1 944 47.3 19 120 1.53 68 3.92 2.28 1.47 16.8 4.84 4.8  AJW-04-270 LVS 43.01 1.01 14.31 0.17 8.31 3.45 0.17 4.15 10.69 0.26 1.59 0.31 0.02 0.01 14.55 98.56 <1 88.4 40.8 43.9 1230 3.13 73 4.4 2.44 1.55 17 5.11 4.3  AJW-04-285 LVS 53.59 0.97 16.44 <0.01 7.06 3.4 0.12 2.74 6.88 3.35 1.08 0.27 0.07 0.06 5.8 98.42 <1 519 39.5 22.4 30 1.29 46 3.72 2.34 1.32 19.7 4.23 3.3  AJW-04-228 UVS  256  AJW-03-075 LVS 0.8 20.1 0.4 2 9 20.5 12 7 5 50.6 4.4 1 260 0.5 0.7 6 <0.5 0.4 2.2 31 1 23.3 2.5 41 179.5  AJW-03-055 LVS  0.8 26.6 0.4 2 10 25.5 36 12 6.4 60.5 5.4 1 571 0.7 0.7 6 <0.5 0.3 1.9 179 <1 22.5 2.2 79 167  Sample suite  Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  0.9 22.8 0.5 <2 12 24.5 <5 8 6.3 111 5 2 345 0.8 0.7 7 <0.5 0.4 1.3 27 1 24.4 2.8 46 269  AJW-03-091 LVS 0.77 14.1 0.35 <2 4.4 20.6 18 11 4.69 21.6 5.01 1 403 0.3 0.69 2.38 <0.5 0.35 0.42 247 2 21.2 2.27 116 102  AJW-03-092 LVS 0.92 24.1 0.47 <2 12.9 24.7 <5 21 6.47 78.5 5.49 2 313 0.8 0.74 7.99 <0.5 0.45 3.24 31 2 23.7 3.18 82 315  AJW-03-205 LVS 0.77 22.1 0.31 <2 8.9 23 40 17 5.93 60.7 5.06 1 602 0.6 0.67 6.06 <0.5 0.3 1.95 145 2 19.7 2.11 63 165  AJW-04-270 LVS 0.84 18.1 0.35 <2 7.2 22.1 261 8 5.41 35.9 5.23 1 208 0.5 0.72 4.1 <0.5 0.35 1.19 198 3 21.1 2.31 87 147  AJW-04-285 LVS 0.76 17.9 0.28 <2 5.2 20.7 14 17 4.94 23.3 4.85 1 668 0.3 0.66 3.33 <0.5 0.31 1.14 183 2 19.7 2.09 79 121  AJW-04-228 UVS  257  UVS 56.44 1.01 17.56 <0.01 6.64 1.41 0.06 1.3 2.38 5.73 2.06 0.3 0.02 0.08 4.58 98.16 <1 782 36.7 13.5 40 3.17 35 3.81 2.31 1.28 25.4 4.27 3.6  UVS  58.13 0.79 17.59 <0.01 6.22 2.69 0.15 2.18 5.48 3.66 2.13 0.23 0.07 0.09 2.16 98.87 <1 766 35.6 20.2 60 0.74 46 3.26 2.03 1.19 19.2 3.74 4.2  suite  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf  AJW-04-371  AJW-04-343  Sample  64.07 0.5 16.59 <0.01 4.22 1.85 0.03 1.58 2.86 3.83 3.82 0.17 0.05 0.1 2.1 99.93 <1 1040 36.2 8.9 80 0.5 10 2.6 1.7 0.9 17 3 4  350 intrusions  AJW-03-074  54.33 0.99 15.65 0.15 7.11 4.25 0.14 3.36 5.22 3.2 2.95 0.3 0.06 0.07 6.4 99.94 <1 554 46 17.9 100 1.6 16 3.8 2.2 1.3 19 4.6 5  AJW-03-019 HbBt Andesite 56.33 0.74 14.17 0.04 6.17 4.7 0.14 5.36 4.69 2.38 3.07 0.23 0.06 0.1 6.03 99.5 <1 776 38.7 22.7 300 1.1 37 2.7 1.6 0.9 16 3.5 4  AJW-03-021 HbBt Andesite 55.47 0.7 14.48 0.02 4.33 3.15 0.15 3.26 6.37 2.94 2.02 0.25 0.05 0.03 9.78 99.85 <1 119 42.7 16.7 180 5 21 2.7 1.6 1.1 17 3.6 4  HbBt Andesite  AJW-03-032  52.58 0.73 13.74 0.04 6.34 4.18 0.13 4.95 5.88 2.26 1.85 0.22 0.06 0.05 10.95 99.78 <1 352 41.7 25.5 340 4.7 47 3.1 1.8 1.1 16 3.7 4  AJW-03-081 HbBt Andesite 54.85 0.76 14.06 0.05 6.38 3.8 0.12 5.46 4.42 3.49 1.14 0.23 0.08 0.04 8.68 99.75 <1 289 42.3 25.2 360 4.4 58 3 1.7 1 16 3.8 4  AJW-03-082 HbBt Andesite  258  UVS 0.79 17.3 0.31 2 5.7 20.8 16 9 4.95 39.2 4.85 1 219 0.4 0.71 3.8 <0.5 0.33 2.04 188 6 20.6 2.17 88 134  UVS  0.64 17.4 0.31 <2 5.8 17.8 30 10 4.5 44.2 3.82 1 620 0.4 0.56 3.65 <0.5 0.3 1.45 151 2 17.4 1.99 84 151  suite  Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  AJW-04-371  AJW-04-343  Sample  0.5 17.7 0.3 2 6 15.9 <5 6 4.2 79.9 3.1 1 471 0.5 0.4 7 <0.5 0.3 1.9 96 <1 15.6 1.8 27 131  350 intrusions  AJW-03-074  0.8 21.5 0.3 <2 7 21.7 14 27 5.5 72 4.7 1 508 <0.5 0.7 5 <0.5 0.3 1.5 164 1 21.8 2.2 147 156  AJW-03-019 HbBt Andesite 0.5 18.8 0.2 <2 8 17.6 80 49 4.4 61.3 3.5 1 548 0.5 0.5 5 <0.5 0.2 1.7 138 2 16 1.5 161 117.5  AJW-03-021 HbBt Andesite 0.5 21 0.2 <2 9 19.4 43 9 4.9 55.9 3.7 1 418 0.6 0.5 4 <0.5 0.2 1.6 125 1 15.3 1.5 89 132  HbBt Andesite  AJW-03-032  0.6 20.7 0.3 <2 7 18.7 95 9 4.6 40.5 3.8 1 544 0.5 0.5 6 <0.5 0.2 1.7 135 1 16.5 1.7 68 138  AJW-03-081 HbBt Andesite 0.6 21.1 0.2 <2 8 18.6 96 13 4.6 29 3.8 1 719 0.5 0.5 6 <0.5 0.2 1.7 141 1 16.4 1.6 65 129  AJW-03-082 HbBt Andesite  259  HbBt Andesite 56.21 0.76 15.07 0.03 6.49 4.47 0.15 5.73 4.31 2.44 3.21 0.24 0.06 0.1 4.92 99.71 <1 942 47.4 24.3 310 1.5 66 3.4 2 1.2 18 4.1 4  53.01 0.7 17.87 0.01 7 3.6 0.23 2.48 5.24 5.24 2.39 0.42 0.15 0.1 4.48 99.31 <1 860 28.3 14.6 50 1.8 162 3.4 2.1 1.3 19 4.1 3  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf  suite  AJW-03-121  AJW-03-110 HbBt Andesite  Sample  57.71 0.71 17.19 0.01 5.77 3.67 0.14 2 3.36 4.7 4.43 0.32 0.05 0.1 3.04 99.55 <1 850 40.6 11.6 100 1 48 4 2.4 1.2 17 4.4 4  AJW-03-183 HbBt Andesite 78.14 0.12 12.4 0.01 0.74 0.28 0.01 0.14 0.25 3 3.7 0.03 0.01 0.09 1.14 99.78 <1 820 21.1 0.6 150 1.1 34 2.2 1.5 0.3 13 2 2  rhyolite  AJW-03-001  68.6 0.12 12.93 0.01 1.51 1.22 0.2 0.99 3.49 5.39 0.96 0.06 0.03 0.19 5.15 99.62 <1 1675 27.4 0.6 110 0.6 6 2.3 1.5 0.6 14 2.4 3  rhyolite  AJW-03-029  77.05 0.06 13.34 0.01 1.31 0.77 0.02 0.21 0.49 2.22 2.84 0.06 0.02 0.07 2.27 99.96 <1 685 22.5 0.6 70 3.9 5 2.2 1.3 0.3 14 2 3  rhyolite  AJW-03-062  76.63 0.06 12.86 0.02 1.09 0.84 0.06 0.16 0.79 5.84 0.76 0.02 0.03 0.04 1.49 99.85 <1 278 31.2 0.9 190 0.5 8 2 1.4 0.3 12 2.2 3  rhyolite  AJW-03-107  75.31 0.08 13.5 0.02 1.19 0.9 0.12 0.14 0.52 4.49 3.37 0.03 0.04 0.09 1 99.9 <1 731 32 0.9 150 0.7 6 2.2 1.5 0.4 14 2.3 3  rhyolite  AJW-03-120  260  HbBt Andesite 0.7 23.6 0.3 3 9 21.8 69 16 5.6 68.3 4.6 1 641 0.6 0.6 6 <0.5 0.3 2.1 149 3 18 1.8 74 131.5  0.7 14.4 0.3 2 4 15.7 <5 7 3.5 43.2 3.9 1 1330 <0.5 0.6 2 <0.5 0.3 0.9 201 1 19 1.9 72 71.1  Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  suite  AJW-03-121  AJW-03-110 HbBt Andesite  Sample  0.8 19.2 0.4 2 7 20.5 9 10 4.8 100 4.4 1 412 <0.5 0.7 5 <0.5 0.4 1.9 123 2 23.4 2.4 65 140.5  AJW-03-183 HbBt Andesite 0.5 10.5 0.3 <2 9 8.5 <5 <5 2.5 74.6 2.1 1 70 0.8 0.3 6 <0.5 0.2 2.3 8 2 14.5 1.7 21 49.8  rhyolite  AJW-03-001  0.5 12.8 0.3 2 9 11.4 <5 9 3.2 20 2.6 <1 260 0.6 0.4 5 <0.5 0.3 1.7 <5 2 15.4 1.7 39 77.1  rhyolite  AJW-03-029  0.4 10.9 0.2 <2 7 9 <5 9 2.7 65.5 2.1 1 148 0.7 0.4 6 <0.5 0.2 1.3 <5 1 13.4 1.5 36 60.7  rhyolite  AJW-03-062  0.4 16.8 0.3 2 7 11.7 5 9 3.2 23 2.1 1 217 0.6 0.3 6 <0.5 0.2 2.4 <5 2 13.5 1.7 31 68.4  rhyolite  AJW-03-107  0.4 16.7 0.3 2 7 12 <5 12 3.5 73.7 2.2 2 310 0.6 0.3 6 <0.5 0.2 2.1 <5 1 13.9 1.6 35 86  rhyolite  AJW-03-120  261  AJW-03-116 320 intrusions 70 0.4 14.72 0.03 2.84 1.29 0.06 0.81 1.95 3.66 4.27 0.11 0.03 0.09 0.93 99.9 <1 720 52.1 5.2 200 2.6 43 4.5 2.9 0.8 16 4.6 7  AJW-03-180 rhyolite  75.64 0.11 12.88 <0.01 0.99 0.58 0.11 0.34 0.87 1.07 3.39 0.04 0.01 0.04 2.91 98.4 <1 385 27.2 0.9 10 4.45 <5 2.41 1.54 0.44 14.4 2.28 2.5  Sample suite  SiO2 (wt%) TiO2 Al2O3 Cr2O3 Fe2O3 FeO MnO MgO CaO Na2O K2O P2O5 SrO BaO LOI Total Ag (ppm) Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf  77.01 0.1 12.47 0.04 0.98 0.51 0.03 0.15 0.59 3.17 4.8 0.01 0.01 0.04 0.53 99.93 <1 195 37.4 1.4 300 2.4 9 1 0.7 0.2 13 1.5 3  AJW-03-132 320 intrusions 77 0.08 12.42 0.03 0.83 0.45 0.05 0.15 0.44 3.34 5.03 0.02 <0.01 0.02 0.47 99.88 <1 20 36.3 0.7 250 7.8 15 0.7 0.8 0.1 16 0.9 5  AJW-03-133 320 intrusions  262  AJW-03-116 320 intrusions 0.9 23.6 0.5 3 9 23.9 6 12 6.2 135.5 4.7 2 282 1 0.7 9 <0.5 0.5 2.9 44 1 27.6 3.1 40 199  AJW-03-180 rhyolite  0.47 13.3 0.28 <2 9.2 10.9 <5 16 3.18 84.1 2.27 1 70.4 0.8 0.39 6.17 <0.5 0.25 1 <5 2 14 1.74 50 58  Sample suite  Ho La Lu Mo Nb Nd Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  0.2 21.5 0.2 4 4 9.2 8 12 3.1 138.5 1.4 1 57 0.5 0.2 13 <0.5 0.1 1.6 11 2 6.6 0.8 10 67.4  AJW-03-132 320 intrusions 0.2 23.9 0.3 2 12 6.1 8 17 2.6 155 0.7 1 7.9 1.3 0.1 33 <0.5 0.2 5.1 <5 1 7.5 1.5 14 83.9  AJW-03-133 320 intrusions  263  0.01-100  Fe-VOL05 FeO  Method code Analytes & Ranges (%)  Method code Analytes & Ranges (ppm)  ME-MS81 Ag Ba Ce Co Cr Cs Cu Dy Er Eu Ga Gd Hf Ho La Lu Mo Nb Nd 1-1000 0.5-10000 0.05-10000 0.5-10000 10-10000 0.01-10000 0.2-10000 0.05-1000 0.03-1000 0.03-1000 0.1-1000 0.05-1000 0.2-10000 0.01-1000 0.01-1000 0.01-1000 2-10000 0.2-10000 0.1-10000  *For more information on analytical techniques at ALS Chemex, see http://www.alsglobal.com/Mineral/ALSContent.aspx?key=24.  0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100 0.01-100  ME-XRF06 SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O Cr2O3 TiO2 MnO P 2O 5 SrO BaO Loss on Ignition  Method code Analytes & Ranges (%)  Appendix 5 – Detection limits for wholerock geochemical analyses (ALS CHEMEX*)  Ni Pb Pr Rb Sm Sn Sr Ta Tb Th Tl Tm U V W Y Yb Zn Zr  5-10000 5-10000 0.03-1000 0.2-10000 0.03-1000 0.1-10000 0.1-10000 0.1-10000 0.01-1000 0.05-1000 0.5-1000 0.01-1000 0.05-1000 5-10000 1-10000 0.5-10000 5-10000 5-10000 0.5-10000  

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