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Geology, geochronology and structural reconstruction of the Cerro Bayo epithermal district, Chilean Patagonia Poblete, Jaime Andrés 2011

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GEOLOGY, GEOCHRONOLOGY AND STRUCTURAL RECONSTRUCTION OF THE CERRO BAYO EPITHERMAL DISTRICT, CHILEAN PATAGONIA by Jaime Poblete B.Sc., Universidad de Chile, 2008  A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE  in The Faculty of Graduate Studies (Geological Sciences)  THE UNIVERSITY OF BRITISH COLUMBIA (Vancouver) July 2011  © Jaime Poblete, 2011  Abstract  The Cerro Bayo low-sulfidation epithermal district is located in the Aysén region, Chilean Patagonia. The oldest rocks that crop out in the district correspond to mainly rhyolitic fragmental successions of the Jurassic Ibáñez Formation which erupted based on biotite 40Ar/39Ar and zircon U-Pb ages, between ca. 154 and 144 Ma. The veins at Cerro Bayo are spatially associated with N-S aligned rhyolitic domes dated at 146.50±0.21 and 146.3±0.2 Ma (zircon U-Pb), intruding the volcanic rocks. Dacitic domes dated at 83.0±0.21 and 82.6±0.2 Ma (zircon U-Pb) are present to the west of the district at Laguna Verde. Silver and gold mineralization is hosted by steeply dipping N-S to NW trending quartz veins, that were emplaced in three main episodes based on adularia 40Ar/39Ar ages: (1) Mallines: ca. 144-142; (2) Bahía Jara and Brillantes: ca .137-124 Ma; and (3) Laguna Verde: 114-111 Ma. Clay alteration mineralogy around the veins consists of illite±smectite±kaolinite–chlorite. At Mallines the illite+kaolinite–smectite mineral assemblage combined with the high illite crystallinity index indicates that these veins are not deeply eroded and the surface outcrops correspond to high levels within the low-sulfidation epithermal deposit model. At Bahia Jara the illite ± smectite – kaolinite and at Brillantes the illite–chlorite–kaolinite assemblages, and overall lower illite crystallinity indices indicate that the veins in these areas are more deeply eroded than at Mallines. Illite crystallinity index is lower for Brillantes than at Bahía Jara indicating that the level of exposure at Brillantes is deeper than at Bahía Jara. At Laguna Verde the mainly illite – smectite clay alteration assemblage plus the lowest illite crystallinity indices of the entire district indicate higher temperature of formation and, by inference, the most deeply eroded veins of the entire district. The level of exposure of the veins has no relation with age of mineralization and the veins suffered differential degree of uplift and erosion after the Late Cretaceous. Moreover, glacial erosion has not uniformly affected the area. The observed textures and clay alteration assemblages at Mallines indicate important potential at 150–250 m depth if classic published deposit models are considered.     ii   Table of contents Abstract .......................................................................................................................................... ii Table of contents............................................................................................................................ ii List of tables .................................................................................................................................. vi List of figures ............................................................................................................................... vii Acknowledgements ..................................................................................................................... xiii Dedication.................................................................................................................................... xiv 1 Introduction ................................................................................................................................ 1 1.1 Low-sulfidation epithermal deposits ..................................................................................... 1 1.2 Overview of the Cerro Bayo Ag-Au low-sulfidation epithermal deposit ............................. 4 1.3 Rationale and thesis objectives.............................................................................................. 8 1.4 Mining history ....................................................................................................................... 9 1.5 Methods ............................................................................................................................... 12 1.5.1 Volcanostratigraphy...................................................................................................... 12 1.5.2 Whole-rock geochemistry............................................................................................. 12 1.5.3 Geochronology ............................................................................................................. 13 1.5.4 Data compilation: ore geochemistry and fluid inclusions ............................................ 14 1.5.5 Alteration: clay mineralogy .......................................................................................... 14 2 Volcanic stratigraphy, geochronology and whole-rock geochemistry................................. 15 2.1 Regional stratigraphic context of the Cerro Bayo district ................................................... 15 2.2 Jurassic-Cretaceous volcanic stratigraphy and intrusive rocks at Cerro Bayo.................... 17 2.2.1 Unit 1: andesitic to dacitic coherent lavas and volcanoclastic basal succesions .......... 17 2.2.2 Unit 2: lower variably welded rhyolitic to rhyodacitic pyroclastic fragmental unit .... 21 2.2.3 Unit 3: volcanosedimentary unit................................................................................... 23 2.2.4 Unit 4: upper variably welded rhyolitic to rhyodacitic pyroclastic fragmental unit .... 24 2.2.5 Intrusive rocks .............................................................................................................. 26 2.3 Geochronology .................................................................................................................... 29 2.3.1 Zircon U-Pb geochronology ......................................................................................... 30 2.3.1.1 Methodology........................................................................................................... 30 2.3.2 Biotite 40Ar/39Ar geochronology .................................................................................. 38 2.3.2.1 Analytical procedures and results........................................................................... 38 2.4 Whole-rock geochemistry.................................................................................................... 44 2.4.1 Major elements characteristics ..................................................................................... 44 2.4.2 Immobile trace element characteristics ........................................................................ 45 2.5 Stratigraphic correlations and eruptive history at the Cerro Bayo epithermal district ........ 51 2.5.1 Correlations with the Chon Aike felsic large igneous province................................... 52 3 Epithermal veins: characteristics and chronology of emplacement .................................... 57 3.1 Structures in the Cerro Bayo district ................................................................................... 60 3.1.1 N-S striking fault south of Cerro Bayo dome............................................................... 61 3.1.2 Kinematic indicators at Mallines.................................................................................. 61 3.1.3 Kinematic indicators at Bahía Jara ............................................................................... 65 3.1.4 NE striking major faults ............................................................................................... 65 3.2 Mineralization in the Laguna Verde area ............................................................................ 67 3.3 Mineralization in the Bahía Jara area .................................................................................. 70    iii   3.3.1 Vein textures................................................................................................................. 71 3.4 Mineralization in the Brillantes area ................................................................................... 71 3.4.1 Vein textures................................................................................................................. 73 3.5 Mineralization in the Mallines area ..................................................................................... 73 3.5.1 Vein textures................................................................................................................. 73 3.6 Compiled geochemical data................................................................................................. 75 3.6.1 Element concentrations and element correlation analyses ........................................... 75 3.6.2 Element ratio analyses .................................................................................................. 82 3.6.3 Discussion of geochemistry.......................................................................................... 87 3.7 Fluid inclusion data compilation ......................................................................................... 88 3.7.1 Discussion of fluid inclusion data ................................................................................ 92 3.8 40Ar/39Ar adularia geochronology ....................................................................................... 93 3.8.1 Methodology................................................................................................................. 94 3.8.2 Results .......................................................................................................................... 94 3.9 Concluding remarks........................................................................................................... 103 4 Interlayered-clay alteration minerals ................................................................................... 106 4.1 Published work on XRD analyses in alteration clay mineralogy ...................................... 107 4.2 Clay studies by XRD methods........................................................................................... 107 4.2.1 Material and methods ................................................................................................. 108 4.2.2 Identification of clay and mixed-layered clay minerals ............................................. 109 4.3 Results on XRD methods: clay mineralogy and spatial distribution................................. 110 4.3.1 Mallines area............................................................................................................... 113 4.3.2 Bahía Jara and Brillantes areas................................................................................... 117 4.3.3 Laguna Verde area...................................................................................................... 120 4.4 Discussion of XRD results ................................................................................................ 122 4.5 Terraspec® analyses from host-rock adjacent to veins ..................................................... 124 4.5.1 Terraspec® analyses of the Delia vein host-rock (Laguna Verde area)..................... 127 4.5.2 Terraspec® analyses of host-rock from the Brillantes area........................................ 133 4.5.3 Terraspec® analyses of host-rock from the Bahía Jara area ...................................... 134 4.5.4 Terraspec® analyses of host-rock from the Mallines area ......................................... 136 4.6 Discussion on Terraspec® analysis and comparison to XRD results ............................... 139 5 Discussion ................................................................................................................................ 141 5.1 Volcanostratigraphy, clay alteration assemblage and erosional levels.............................. 141 5.2 Geochronology: volcanic stratigraphy, subvolcanic domes and mineralization ............... 143 5.3 Environment of vein emplacement.................................................................................... 144 6 Conclusions.............................................................................................................................. 148 6.1 Geologic framework of the Cerro Bayo district ................................................................ 148 6.2 Epithermal mineralization at the Cerro Bayo district........................................................ 148 6.3 Regional metallogenetic significances of ages.................................................................. 149 6.4 Clay mineralogy, vein geochemistry, fluid inclusions and geochronology relationships . 152 6.5 Exploration implications.................................................................................................... 153 References................................................................................................................................... 155 Appendices ................................................................................................................................. 163 Appendix A1. Outcrop map of the Cerro Bayo district.......................................................... 164 Appendix A2. Whole-rock geochemical sample locations..................................................... 165 Appendix A3. ALS Chemex geochemical methods and detection limits. Taken from http://www.alsglobal.com/....................................................................................................... 166    iv   Appendix A4. Whole-rock geochemistry analyses from host rock from the Ibáñez Formation at the Cerro Bayo Ag-Au epithermal district........................................................................... 168 Appendix A5. Geochemical field duplicates .......................................................................... 173 Appendix A6. Geochemical standard analysis ....................................................................... 174 Appendix A7. ID-TIMS U-Th-Pb isotopic data ..................................................................... 175 Appendix A8. Laser ablation U-Pb isotopic data ................................................................... 176 Appendix A9. 40Ar/39Ar biotite analytical data ...................................................................... 177 Appendix A10. 40Ar/39Ar adularia analytical data.................................................................. 179 Appendix A11. Comparative results for Ag geochemical analyses between ALS Chemex laboratories and Cerro Bayo laboratories. A. Ag vs Ag from vein drill-core samples. B. Ag vs Ag for surface vein samples .................................................................................................... 183 Appendix A12. Compiled geochemical data from Coeur d’Alene Mines and C. Hermosilla (2009, written commun.), showing the main, minimum and maximum values for the chosen elements in the different veins and areas................................................................................. 184     v   List of tables   Table 1.1. Cerro Bayo district historic production (from Sims, 2010) ......................................... 11  Table 1.2. Mineral resources and probable mineral reserves of Cerro Bayo as if March 1, 2010 (from Sims, 2010). Considered veins are: Dagny, Delia, Fabiola, Coyita, Dalila and Yasna from Laguna Verde; and Marcela Sur from Bahía Jara.................................... 11  Table 2.1. Biotite K-Ar available for the Ibañez Formation near Lake General Carrera, Chilean Patagonia. (Suárez and De La Cruz, 1997a). ............................................................... 29  Table 2.2. K-Ar available for the Cerro Bayo dome (De la Cruz and Suárez, 2008)................... 30  Table 3.1. Mean, minimum and maximum concentration values of ore forming elements from the Cerro bayo district, compiled from C. Hermosilla (2009, written commun.) and from Coeur d’Alenes mines internal databases. ................................................................... 76  Table 3.2. Pearson correlation matrix between the analyzed elements for vein drill-core samples. ...................................................................................................................................... 77  Table 3.3. Pearson correlation matrix between the analyzed elements for vein surface samples. 78  Table 3.4. Pearson correlation matrix between the analyzed elements for all the analyzed samples......................................................................................................................... 79  Table 3.5. Mean, minimum and maximum values of the different element ratios analyzed from different veins around the Cerro Bayo district. compiled from C. Hermosilla (2009, written commun.) and from Coeur d’Alenes mines internal databases ....................... 84  Table 3.6. Compiled fluid Inclusion data...................................................................................... 91  Table 3.7. K-Ar and 40Ar/39Ar adularia ages. ............................................................................... 93  Table 3.8. New adularia 40Ar/39Ar ages from the Cerro Bayo district, Chile (given in Ma). Plateau criteria include number of steps and % 39Ar. Ages in bold represent plateau according to the Isoplot default criteria. ...................................................................... 95  Table 4.1. XRD results for the samples taken in the Cerro Bayo epithermal district. The percentage of each clay mineral is based on the area under the curve method (Ouhadi and Young, 2003)....................................................................................................... 111  Table 4.2. Major absorption features (Hauff, 2005) ................................................................... 125  Table 4.3. Major absorption features for clay minerals (Hauff, 2005) ....................................... 127        vi   List of figures Figure 1.1. Schematic sketches showing the relation between fluid types and alteration zonation in low- and high-sulfidation epithermal deposits. Modified from White and Hedenquist (1995) and Tosdal et al. (2009). .............................................................. 4  Figure 1.2. Location map of the Cerro Bayo Ag – Au epithermal district. The map on the left is showing the political Aysén region which is enlarged in the map on the right hand side. ............................................................................................................................ 6  Figure 1.3. Simplified geologic map of the Aysen region, southern Chile, showing the main mines and prospects (modified from Townley et al., 2000)....................................... 7  Figure 1.4. Simplified geologic map of the Cerro Bayo district, showing the four main studied areas (modified from Coeur d’Alene Mines). ............................................................ 8  Figure 2.1. Chon Aike large felsic igneous province and the Patagonian Batholith outcrops modified from: Pankhurst et al., 1998-1999; Féraud et al., 1999; Pankhurst et al., 1999; Calderón et al., 2007; Hervé et al., 2007; Wallier, 2009................................ 16  Figure 2.2. Simplified geologic map of the Cerro Bayo district. The black dots are showing where the stratigraphic columns were documented. The main studied areas are indicated by dashed lines (Bahía Jara, Brillantes, Laguna Verde and Mallines). UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. For detail outcrop map of the Ibáñez Formation, see Appendix A1.............................. 18  Figure 2.3. Stratigraphic columns of the Cerro Bayo district. Appendix A2 provides geochemical and geochronological samples location. ............................................. 19  Figure 2.4. Generalized stratigraphic column of the Ibáñez Formation in the Cerro Bayo Ag-Au low sulfidation epithermal district. The approximate thickness of the entire section is ~700 meters. ......................................................................................................... 20  Figure 2.5. Unit 1 at the Laguna Verde Sur area. A: (270179E/4838961N) Purple andesitic volcanoclastic level with high hetero-lithic fragment content (300°/18°). B: Microphtograph of sample JP-199 (270149E/4839045N) that corresponds to a porphyritic (7% plagioclase phenocrysts) andesite (phenocrysts are partly replaced by clays and minor hematite), immersed in microlithic groundmass. ..................... 22  Figure 2.6. General view of the Pampa la Perra are where the stratigraphic column was performed. Units 2,3 and 4 can be observed. ........................................................... 22  Figure 2.7. Pampa la Perra volcanosedimentary unit (Unit 3; 276616E/4839618N) ................. 23  Figure 2.8. Cerro Torta W (284991E/4837072N). A: Upper oxidized levels of the Ibáñez Formation; ~30 meters below the basal contact of the Toqui Formation . B: JP-196 where fresh pumice surrounded by hematized ash matrix can be observed. ........... 25  Figure 2.9. Photograph taken from Bahía Jara looking S. N-S aligned rhyolitic subvolcanic domes intrude the Ibáñez Formation. ....................................................................... 27  Figure 2.10. Microphotograph of the Cerro Bayo dome (yellow line is 2.5 mm long). Devitrified fluidal texture showing rotated quartz phenocrysts.................................................. 27  Figure 2.11. Laguna Verde dacitic dome (270353E/4841771N). A: Flow banding. B: JP-141B where sericite veinlets cutting the flow banding can be noted................................. 28  Figure 2.12. Pictures showing the separated zircons for the U-Pb dating, where clear needleshaped and bigger cloudy zircons can be observed. A. Cerro Bayo dome - JP-65A. B. Laguna Verde dome - JP-141A. For analytical data see Appendix A7.............. 33  Figure 2.13. Concordia diagram for zircon separated from sample A. JP-65A (Cerro Bayo dome) and B. JP-78A (Cerro Lápiz dome) with calculated concordia ages (uncertainty    vii   reported as 2σ). Red ellipses reflect the errors of the individual grains associated with the concordia line. For analytical data see Appendix A7................................ 35  Figure 2.14. Concordia diagram for zircon separated from sample A. JP-141A (Laguna Verde dome) and B. JP-112A (Cañadón Verde dome) with calculated concordia ages (uncertainty reported as 2σ). Red ellipses reflect the errors of the individual grains associated with the concordia line. For analytical data see Appendix A7. ............. 36  Figure 2.15. Plots of 206Pb/238U zircon ages for individual LA-ICP-MS analysis from the dated samples (error bars are at ±2σ) rej: rejected. For analytical data see Appendix A8. .................................................................................................................................. 37  40 Figure 2.16. Ar/39Ar biotite samples. A: JP-168 from Cerro Torta E (285732E/4836275N). B: JP-197 from Cerro Torta W (285026E/4837063N). ................................................ 38  Figure 2.17. 40Ar/39Ar age spectra of biotite samples from Cerro Torta W. For analytical data see Appendix A9. .......................................................................................................... 40  Figure 2.18. 40Ar/39Ar age spectra of biotite samples from Cerro Torta E. For analytical data see Appendix A8. .......................................................................................................... 41  Figure 2.19. Field photograph of Cerro Torta showing where the samples for 40Ar/39Ar were taken. Note the NW dipping strata and that the sample JP-197 correspond to the higher levels of the Ibáñez Formation in this area. For samples location see Appendix A1. .......................................................................................................... 42  Figure 2.20. Alteration box plot for samples from the Cerro Bayo district (Large et al., 2001). AI = 100 (K2O + MgO)/(K2O + MgO + Na2O +CaO). CCPI = 100 (MgO +FeO)/(MgO + FeO + Na2O + K2O). Alteration minerals are indicated within the diagram as follows: ab = albite, ad = adularia, ank = ankerite, cc = calcite, chl = chlorite, dol = dolomite, ep = epidote, il = illite, py = pyrite................................... 46  Figure 2.21. Compositional Zr/Ti vs. Nb/Y discrimination diagram (Winchester and Floyd, 1977) for rhyolites, rhyodacites and andesites of the Cerro Bayo district, showing almost all samples plotting near the rhyolite-rhyodacite boundary, the rhyolitic domes and the more alkalic domes from the Laguna Verde area. For whole-rock geochemistry see Appendix A4; for whole-rock geochemistry field duplicates see Appendix A5; and for geochemical standard analysis see Appendix A6. ............. 47  Figure 2.22. Tectonic Nb vs. Y discrimination diagram (Pearce et al., 1984), for the samples taken in the Cerro Bayo district. Samples from the Ibáñez and the El Quemado (Argentinian equivalent for the Ibáñez Formation) formations are plotted for reference. .................................................................................................................. 47  Figure 2.23. Chondrite normalized rare earth elements (REE) patterns for A. Mallines B. Bahía Jara C. Brillantes and D. Laguna Verde areas.......................................................... 49  Figure 2.24. Chondrite normalized incompatible wlements patterns for, A. Mallines B. Cerro Bayo C. Brillantes and D. Laguna Verde areas........................................................ 50  Figure 2.25. Formations that comprise the Chon Aike large igneous province, plotted against the eruption age. Range of ages taken from Pankhurst et al. (1999 and 2000), Féraud et al. (1999), Calderón et al. (2007), Hervé et al. (2007) and this work. ..................... 54  Figure 2.26. Nb/Y vs. Zr/Ti diagram (Winchester and Floyd, 1977) for the studied area and compiled data from different formations of the Chon Aike large felsic province. 1: this study; 2: Pankhurst and Rapela, 1995; 3: Parada et al., 1997; 4: Pankhurst et al., 1999; 5: Parada et al., 2001; 6: Hervé et al., 2007. .................................................. 55  Figure 2.27. Y versus Nb diagram from Pearce et al. (1984). The dark grey shaded area corresponds to samples from the Ibáñez Formation at 46°S Lat. (Cerro Bayo district). It is possible to observe that the most andesitic formations (Bajo Pobre and Lonco Trapial) present lower Nb contents than the more differentiated rhyolitic    viii   Marifil, Chon Aike and Ibáñez formations. Samples from the Marifil Formation plot in a large area straddling the volcanic arc, the within plate and the anomalous ridge fields. 1: this study; 2: Pankhurst and Rapela, 1995; 3: Pankhurst et al., 1998; 4: Hervé et al., 2007. .................................................................................................... 55  Figure 2.28. Chondritic normalized spider diagram for incompatible elements. The orange shaded area corresponds to data from the Patagonian Batholith (Hervé, 2007), the grey shaded area corresponds to data from the Ibáñez Formation at ~46°S Lat. (this thesis); comparing with data from Parada et al. (1997, 2001). ................................ 56  Figure 2.29. Rare Earth Elements (REE) spider diagram showing the Patagonian Batholith (Hervé et al., 2007) and the basal levels of the Ibáñez Formation data near the Chacabuco River (Parada et al., 1997 and 2001). The grey shaded area represents the Ibáñez Formation at ~46°S Lat. (this thesis) and the purple shaded represents data from the Marifil and the Chon Aike formations (Pankhurst and Rapela, 1995) .................................................................................................................................. 56  Figure 3.1. Simplified geologic map of the Aysen region, southern Chile, showing the main mines and prospects (modified from Townley et al., 2000). The red square in the map indicates the study area..................................................................................... 59  Figure 3.2. Simplified geological map of the Cerro Bayo district, showing the different subareas studied in this thesis. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. ............................................................................................ 60  Figure 3.3. A. Flow banding that can be noted along the NS rhyolitic dikes, in the area south of Cerro Bayo dome. B. N355° grey quartz with fine pyrite dissemination cross-cutting the NS rhyolitic dikes. .............................................................................................. 61  Figure 3.4. Dip/Dip Direction equal area stereographic and rose plots for veins from A) System 1, B) System 2, C) System 3 and D) Sytem 4 for the Mallines area. ....................... 63  Figure 3.5. Extensional sinistral jog for System 2 in the Mallines area (281800E/4838215N). A. Field photograph figure. B. Schematic sketch of the structural relationship. .......... 64  Figure 3.6. Extensional sinistral jog for System 2 in the Malines area (2818858E/4837525N) (Plan view). Sample JP-221 (see Chapter 4) was taken from this vein. ................. 64  Figure 3.7. Map that shows where the structural work was focused (i.e., around the Raúl, SW of Cerro Bayo dome and the Laguna Salmonosa areas). Modified from Williams (2003) and from Coeur d’Alene Mines. See Chapter 2 for description of the volcanic units. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. .................................................................................................................... 66  Figure 3.8. A. Cerro Bayo distrital map that shows the veins characterized by Pizarro (2000), Townley (1996) and this study (same legend as Fig. 3.2). B. Inset black square corresponds to the characterized veins at Laguna Verde. Veins with asterisk correspond to those mentioned in Table 1.2............................................................ 69  Figure 3.9. Microphotographs from the Fabiola Vein at the Laguna Verde area. White boxes width is 0.125 mm. 3A. Massive calcedonic quartz with disseminated opaque crystals where reddish internal reflections of proustite-pyrargyrite are observed. 3B. Disseminated anhedral to subhedral pyrite with proustite-pyrargirite intergrwon are observed. .................................................................................................................. 70  Figure 3.10. Microphotograph of of a colloform band of the Roberta Vein. White bar length is 0.25 mm. A. Massive chalcedonic quartz associated with adularia. B. Pyrite with iron oxide rims associated to adularia. ..................................................................... 72  Figure 3.11. Roberta vein where crustiform texture is observed, suggesting fluid boiling during mineral depositation (Dong et al., 1995). The first stage of crustiform quartz    ix   deposition, is followed by a second stage of drusy quartz and post-mineralization dextral fault displacement (in blue dashed line)....................................................... 72  Figure 3.12. Photograph that shows a N110° (System 2, in red) striking vein that is cut by a N165° (System 3, in blue) vein at Mallines. It is possible to observe the dextral offset along the structure in which vein System 3 vein system was emplaced. Vein System 2 has been offset by ~60 cms. Inset at upper right shows sample JP-233 taken from System 3 before and after potassium staining. The presence of adularia is indicated by the yellow stain. 40Ar/39Ar geochronology results for this sample are presented below........................................................................................................ 74  Figure 3.13. Bar graphs showing the ore elements concentrations analyzed from different areas. Only surface samples were taken from the Brillantes area. ..................................... 80  Figure 3.14. Ag vs. Zn bivariant diagram showing the negative corelation in the Brillantes III vein for these elements at the Brillantes area ........................................................... 82  Figure 3.15. As / Ag vs Au / Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples ..................................................................................................................... 85  Figure 3.16. As/Ag vs As/Cu bivariant plot. A. Drill core vein samples. B. Surface vein samples .................................................................................................................................. 85  Figure 3.17. As/Ag vs Mo/Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples .................................................................................................................................. 86  Figure 3.18. As/Ag vs Hg/Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples .................................................................................................................................. 86  Figure 3.19. Map showing the veins that were analyzed by fluid inclusions thermometry in the Cerro Bayo district. Inset figure is an enlargement of the black rectangle for samples analyzed at the Laguna Verde area. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.................................................................................. 90  Figure 3.20. Average salinity vs. Th diagram for samples from different areas of the Cerro Bayo district. Data obtained from Townley (1996), Pizarro (2000), and Hermosilla (2009, written commun.). .................................................................................................... 92  Figure 3.21. 40Ar/39Ar age spectra of adularia samples from Mallines. A. System 4. B. System 2. For analytical data see Appendix A10. ................................................................... 97  Figure 3.22. 40Ar/39Ar age spectra of adularia samples from Mallines: System 3. For analytical data see Appendix A10............................................................................................ 98  Figure 3.23. 40Ar/39Ar age spectra of adularia samples from Roberta Vein, Brillantes. For analytical data see Appendix A10. .......................................................................... 99  Figure 3.24. 40Ar/39Ar age spectra of adularia samples from Guanaco block. A. Guanaco I vein. B. Guanaco III vein. For analytical data see Appendix A10................................. 100  Figure 3.25. 40Ar/39Ar age spectra of adularia samples from Taitao system, Laguna Verde area. For analytical data see Appendix A10. ................................................................. 101  Figure 3.26. Cerro Bayo district map showing all adularia 40Ar/39Ar and K-Ar geochronology results for this study and literature sources (see text and Tables 3.4 and 3.5 for details). Bold: this study; italic: (1) De la Cruz and Suárez, 2008; (2) Townley, 1996; (3) Tippet et al., 1991. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. .......................................................................................... 102  Figure 4.1. Simplified geological map of the Cerro Bayo Ag-Au epithermal district, showing the main areas of the district: Laguna Verde, Cerro Bayo and Mallines (modified from Coeur d’Alene Mines web page and Suárez and De la Cruz, 2008). A-A’-B and C-D are the cross-sections from Mallines to Brillantes and from Delia vein at Laguna Verde, respectively. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. .......................................................................................... 106     x   Figure 4.2. N-S long section showing the XRD results for the samples taken from Mallines, Cerro Bayo, and Brillantes areas. I.C: illite crystallinity;ill: illite; smc: smectite; kaol: kaolonite. See Fig. 4.1 for location of section line. ...................................... 112  Figure 4.3. Illite-Smectite-Kaolinite ternary diagram for XRD analyzed samples from fragmental rocks, showing the mineral distribution between the different study areas in the Cerro Bayo district. Each mineral percentage was calculated by the area under the curve method (Ouhadi and Young, 2003). ....................................................... 113  Figure 4.4. XRD pattern for samples adjacent to the Cascada and Guadalupe veins in the Mallines area. PCI: poorly-crystallized illite; WCI: well-crystallized illite; I/S: illite/smectite interlayered; Ill: illite; Kaol: kaolinite; Qtz: quartz. ........................ 114  Figure 4.5. XRD spectra for samples from System 1 and 2 veins at the Mallines area. I/S: illite/smectite interlayered; Ill: illite; Kaol: kaolinite; Qtz: quatz. Note the poor interlayered-clay mineral ordering (R=0) for two samples belonging to System 2. ................................................................................................................................ 115  Figure 4.6. XRD spectra for samples from System 2 veins at the Mallines area. Note the poor interlayered-clay mineral ordering (R=0) for sample JP-249, the northernmost sample of the four. PCI: poorly-crystallized illite; WCI: well-crystallized illite; Ill: illite; kaol: kaolinite; Qtz: quartz. .......................................................................... 116  Figure 4.7. XRD spectra for sample JP-235 from System 4 veins at the Mallines area. Ill: illite; Kaol: kaolinite. ....................................................................................................... 117  Figure 4.8. XRD spectra for samples from the Bahía Jara area veins. Note the well interlayeredclay mineral ordering (R=1) for samples from Marcela Sur, Lucero and Guanaco VII veins. Ill: illite; Kaol: kaolinite; Qtz: quartz. ................................................... 118  Figure 4.9. XRD spectra for one sample from the Bahía Jara area and three samples taken from the Brillantes area veins. Note from samples JP-255 the presence of chlorite and calcite; and calcite in sample JP-256 as the mineral alteration assemblage. Ill: illite; Kaol: kaolinite; Qtz: quartz. ................................................................................... 119  Figure 4.10. XRD spectra for samples taken from host-rock from different drill-cores at different depths from the Delia vein at the Laguna Verde area. Note the well ordered (R=1) illite/smectite interlayered for the samples that present smectite. Ill: illite; Qtz: quartz ...................................................................................................................... 121  Figure 4.11. Delia vein C-D cross section showing the clay mineralogy for the samples analyzed, and the available Ag, Au and Aueq concentrations for selected points from the vein. I.C: illite crystallinity; ill: illite; smc: smectite; kaol: kaolinite. ............................ 122  Figure 4.12. Delia vein cross-section showing the distribution of samples analyzed by Terraspec® immediately adjacent to the vein. ....................................................... 128  Figure 4.13. Terraspec® spectra from samples taken immediately adjacent to Delia vein at Laguna Verde. They present mainly illite as the major alteration mineral, with only minor or trace smectite interlayered. Typical spectra features are indicated. See text for explanation. I: illite; I+S: illite/smectite; J: jarosite. ........................................ 130  Figure 4.14. Terraspec® spectra from samples taken away from Delia vein at Laguna Verde. They present mainly illite/smectite as the major alteration mineral. Typical spectra features are indicated. See text for explanation. I: illite; I+S: illite/smectite; J: jarosite. ................................................................................................................... 131  Figure 4.15. Terraspec® spectra from samples taken from adjacent to the andesitic dike that cross cuts Delia vein at Laguna Verde. They present mainly illite as the unique alteration mineral. Typical spectra features are indicated. Red arrows indicate Fe content in the mineral lattices. I: illite; J: jarosite. ................................................. 132     xi   Figure 4.16. Terraspec® spectra for samples taken from adjacent to veins at Brillantes. They present mainly illite as the unique alteration mineral. Typical spectra features are indicated. See text for explanation. I: illite; S: smectite; K: kaolinite; Mg-Chl: magnesium-rich chlorite; J: jarosite. ...................................................................... 133  Figure 4.17. Terraspec® spectra of samples taken immediately adjacent to veins at the Bahía Jara area. They present mainly illite, minor illite/smectite interlayered and lesser kaolinite as the clay alteration mineral assemblage. Features indicated correspond to I: illite; S: smectite; K: kaolinite. ........................................................................... 135  Figure 4.18. Terraspec® spectra of samples taken immediately adjacent to veins from System 1 at the Mallines area. They present mainly illite and kaolinite, and trace smectite as the clay alteration mineral assemblage. I: illite; S: smectite; K: kaolinite. ............ 136  Figure 4.19. Terraspec® spectra for samples taken immediately adjacent to veins from System 2 at the Mallines area. They present illite and kaolinite as the clay alteration mineral assemblage. I: illite; K: kaolinite............................................................................ 137  Figure 4.20. Terraspec® spectra for samples taken immediately adjacent to veins from System 3 at the Mallines area. They present illite, kaolinite and trace smectite as the clay alteration mineral assemblage. I: illite; K: kaolinite; S: smectite. ......................... 138  Figure 5.1. General map of relative erosion levels of different structural blocks. Based on vein textures. Illite crystallinity, geochemistry and fluid inclusion data. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. ................................ 142  Figure 5.2. Temperature vs. depth graph showing the fluid inclusion data for the different studied areas. Boiling-point curves for H2O liquid (0 wt percent) and for brine of constant composition given in wt percent NaC1. The temperature at 0 meters of each curve is the boiling point for the liquid at 1.013 bars (1.0 atm) load pressure which is equivalent to the atmospheric pressure at sea level. The uncertainty is contained within the width of the lines. (Modified from Haas, 1971) ................... 146  Figure 5.3. Schematic environment reconstruction for veins from the different studied areas: A. Mallines. B. Bahía Jara/Brillantes. C. Laguna Verde. ........................................... 147  Figure 6.1. Main structures, deposits and prospects of the Aysén region, Chilean Patagonia. The three defined metallogenic epochs are indicated. The red line define the deposits of the first, Late Jurassic metallogenetic epoch related to the Deseado Massif mineralization events, the orange circles define the second metallogenetic epoch (Lower Cretaceous) and the green lines define the third, also lower Cretaceous metallogenetic epoch. Modified from Townley and Palacios (1999). UJ: Upper Jurassic; LK: Lower Cretaceous. ........................................................................... 151      xii   Acknowledgements The writer is extremely grateful to Coeur d’Alene Mines and its vice-presidentexploration, Alfredo Cruzat, who together made this study possible by allowing complete access to the Cerro Bayo district property, and to all drill core and geological records. Generous financial aid and support of field and laboratory work was also made available. The mine staff at the Cerro Bayo district was exceedingly helpful and particular thanks are due Ricardo Parra, Juan Moya, Nelson Ardiles, Cristian Alarcon for their thoughtful cooperation. Special thanks are extended to Manuel Rodríguez for many helpful comments and discussions relating to the geology of the orebodies and for indispensable assistance in the field. Professor Thomas Bissig provided invaluable guidance, geological insight, and inspiration in the field and in the laboratory. His, Jim Mortensen and Craig Hart critical reviews of the manuscript were enlightening and are sincerely appreciated. I acknowledge the diverse services from the Pacific Centre for Isotopic and Geochemical Research (PCIGR) at the University of British Columbia, namely Richard Friedman, Janet Gabites and Hai Lin. I also acknowledge the diverse contributions from MDRU, and especially Karie Smith and Arne Toma are thanked for their help with technical and managerial matters. I want to thank to numerous EOS-UBC friends, including Abdul Razique, Alfonso Rodríguez, Ayesha Ahmed, Betsy Friedlander, Bram Van Straaten, Brendan Scorrar, Esther Bordet, Jack Milton, Jenny Haywood, Jess Norris, Leanne Smar, Leif Bailey, Lindsay McClenaghan, Lizzie Stock, Moira Cruickshanks, Rosie Howard, Santiago Vaca, Shawn Hood, Tatiana Alva and Will Lepore for making UBC and Vancouver in general one of the best experiences in my life. Generous financial support provided by the Society of Economic Geologists, through their research grant programs, helped defray laboratory expenses.     xiii   Dedication  To my family     xiv   1 Introduction  Due to its chemical and physical properties such as excellent electrical and heat conductivity, ductility and resistance to extreme temperatures, silver cannot readily be substituted in most of its applications. Silver is widely used in medicine (e.g. silver sulfadiazine is the most effective known treatment for burns), polyester fabrics, silver-zinc batteries, technology devices, solar panels, as well as in jewelry and other items of decorative adornment. The number of applications that require silver is enormous and consumption is constantly growing; thus exploring for new sources of this metal is of great importance. Epithermal type deposits are the primary source for Ag (Graybeal and Vikre, 2010) and many Ag-rich epithermal deposits occur in Patagonia. This thesis describes the geology of an important Ag-rich low-sulfidation epithermal district in southern Chile.  1.1 Low-sulfidation epithermal deposits Epithermal ore deposits typically form in a shallow crustal environment, at less than 1.5 km depth and at temperatures below 300°C, mainly in subareal hydrothermal systems (Simmons et al., 2005). These hydrothermal systems are commonly associated with calc-alkaline to alkaline magmatism, in volcanic arcs at convergent plate margins, as well as in intra-arc, back-arc and post-collisional rift settings (White and Hedenquist, 1995). Several classifications have been proposed to describe epithermal ore deposits. The most common is to classify them as: (1) low-sulfidation and (2) high-sulfidation, which form from fluids of contrasting chemistry (Fig. 1.1). Low-sulfidation epithermal deposits form from nearneutral pH and reduced fluids in geothermal systems, while high-sufidation epithermal deposits are associated with acidic and relatively oxidized fluids in the magmatic-hydrothermal environment adjacent to young volcanoes (White and Hedenquist, 1995). They can also be differentiated by the characteristic hypogene alteration mineral assemblages: (1) quartz ± calcite ± adularia ± illite for low-sulfidation type and (2) quartz + alunite ± pyrophillite ± dickite ± kaolinite for high-sulfidation type (Simmons et al., 2005). Epithermal deposits have also been    1   classified into adularia – sericite and acid – sulfate types (Heald et al., 1987). This thesis is concentrated on the study of an example belonging to the low-sulfidation type. Low-sulfidation epithermal deposits are hosted by coeval and older volcanic rocks including andesite, dacite and rhyolite; and/or by underlying basement rocks, and rarely by subvolcanic intrusions. Most commonly, orebodies occur as veins with steep dips that formed through dilation and extension. Metals originated from the magma may have been transported to the site of ore entrapment by hydrothermal fluids or may have been extracted from adjacent wall rocks by exsolving magmatic fluids (Graybeal and Vikre, 2010). The ore deposition typically occurs within a vertical range of a few hundred meters (Dreier, 2005) and it is strongly influenced by fluid mixing and boiling. Silver-rich low-sulfidation epithermal deposits are commonly associated with Au, and Au occurs as microscopic to submicroscopic grains of electrum and rare tellurides, whereas Ag usually occurs as electrum, acanthite, sulfosalts (e.g., pyrargyrite-proustite, Ag-rich tetrahedrite) and/or Ag selenide minerals. These two precious metals are present with variable amounts of base metal sulfides like sphalerite, galena, and lesser chalcopyrite (Simmons et al., 2005). The hypogene alteration assemblage presents a lateral and a vertical zonation towards a mineralized vein. For instance, in the Manantial Espejo silver-rich epithermal deposit the Maria vein presents a zonation with adularia alteration proximal to the vein and at deeper level grading into sericite-illite alteration, and into distal, commonly pervasive propylitic alteration (Wallier, 2009). In some cases shallow-levels of intense silicification are present locally in that district; and supergene argillic alteration, commonly kaolinite, overprints the hypogene assemblages especially along fractures (Wallier, 2009). Low-sulfidation epithermal deposits are typically considered as extinct geothermal fields (Simmons and Browne, 2000b) and in active geothermal systems temperature can be measured in situ and related to the clay alteration minerals (Simmons and Browne, 2000b). In the Wairikei geothermal field located in New Zealand Harvey and Browne (1991) measured an increasing temperature from 100°C to 200°C coincident with an increase in illite layers in the illite/smectite interlayered minerals from 60% illite at 100°C to pure illite at 200°C over a 150 m elevation difference in lacustrine sediments. In New Zealand geothermal fields surface kaolinite, formed at    2   low pH, does not persist above about 60°C, although dickite is known in one well where measured temperatures are from 140°C to 150°C (Browne, 1978). In the Broadlands-Ohaaki geothermal system Simmons and Browne (2000b) found pattern of formation of clay minerals strongly influenced by the permeability of rocks and structures. The maximum temperature gradient occurs in the centre of the upflow where waters starts to boil at 300°C at >500 m depth forming a quartz-illite-adularia-albite-chlorite-calcite-pyrite assemblage derived from deep chloride waters, whereas in the periphery of the system illite-smectite-calcitesiderite assemblages are found. In in the centre of the upflow but higher in the system quartzadularia are formed, grading to illite (>200°C), illite-smectite interlayered and smectite (<150°C) to the periphery. Near surface in the CO2-rich steam-heated environment, hydrolytic alteration can be found which chalcedony quartz, K-mica, calcite and kaolinite assemblages are present and represent temperatures below 200°C. Well crystallized illite occurs to the deeper parts of the system, forming at >250°C complex crystal structures. Low-sulfidation epithermal deposits are also distinguished by the gangue mineral textures. Crustiform banded quartz is common, typically with interbanded, discontinous layers of sulfide minerals (mainly pyrite) and/or selenide minerals, adularia, and/or illite. At relatively shallow depths, the bands are colloform in texture and millimeter-scale, whereas at greater depths, the quartz is more coarsely crystalline. Lattice texture (tabular calcite or its pseudomorph quartz) occur as open-space filling in veins, and along with vein adularia indicate deposition from boiling fluids of near-neutral to alkaline pH. These textures are related to precious metal deposition (Simmons and Christenson, 1994; Simmons and Browne, 2000b). The majority of epithermal deposits are Tertiary or younger in age (Simmons et al., 2005), because generally they formed in high-relief volcanic-arc settings that are typically affected by rapid uplift and subsequent erosion (Simmons et al., 2005). However there are older deposits still preserved, such as the Mesozoic deposits located in the Chilean and Argentinean Patagonia (e.g., Manantial Espejo and Cerro Vanguardia, Argentina; Cerro Bayo district, Chile).     3   Figure 1.1. Schematic sketches showing the relation between fluid types and alteration zonation in low- and highsulfidation epithermal deposits. Modified from White and Hedenquist (1995) and Tosdal et al. (2009).  Silver-rich low-sulfidation epithermal deposits belong to the magmatic-hydrothermal class of deposits (Hedenquist and Lowenstern, 1994; Graybeal and Vikre, 2010) and most of the larger and high-grade silver-rich magmatic-hydrothemal deposits are of low-sulfidation type. The other genetic group of Ag-rich deposits correspond to: (1) volcanogenic massive sulfide (VMS); (2) sedimentary exhalative (SEDEX); and (3) lithogene (Graybeal and Vikre, 2010). The magmatichydrothemal deposits are commonly telescoped with high (>300°C) and low temperature (<200°C) mineral and metal associations in single orebodies, thus the distinction between epithermal and mesothermal is not always clear (Graybeal and Vikre, 2010).  1.2 Overview of the Cerro Bayo Ag-Au low-sulfidation epithermal deposit The geology of the Aysén region, located in southern Chile (between 44º and 47º30´ S; Fig. 1.2), consists of a Paleozoic metamorphic basement, Upper Jurassic-Cretaceous calc-alkaline andesitic to rhyolitic volcanic and related sedimentary rocks, the Patagonian Batholith (60-160 Ma and 3   4   20 Ma) and Upper Cretaceous to Miocene basalts and marine and continental sedimentary rocks. The Aysén region hosts mainly epithermal precious metal, Zn-Pb skarns and associated polymetalic vein deposits (Fig. 1.3). The Cerro Bayo epithermal Ag-Au district (46º32' S, 71º53' W; Fig. 1.2) comprises an area of ~50 km2 and it is located 110 km south of the city of Coyhaique, near the border towns Chile Chico (Chile) and Los Antiguos (Argentina), on the southeastern shoreline of the Lake General Carrera. The rocks that crop out in this district consist of Middle Jurassic felsic volcanic rocks of the Ibañez Formation, unconformably overlain by Lower Cretaceous fine to medium grained sandstones of the Toqui Formation, which in turn are overlain by Tertiary basalts. The Ibañez Formation is represented by rhyolitic to rhyodacitic pyroclastic flows and hosts the Ag-Au veins. The Cerro Bayo district consists of Ag-Au rich low-sulfidation deposits. It can be divided into four main subdistricts: (1) Laguna Verde; (2) Bahía Jara; (3) Brillantes; and (4) Mallines (Fig. 1.4). In all these sub-areas the mineralization is associated with quartz ± adularia veins, and different epithermal low-sulfidation textures can be observed. Of these sub-areas, Laguna Verde and Bahía Jara have been mined since the 1990’s and early 2000’s, respectively. Brillantes has not been studied in depth, and Mallines is apparently poorly mineralized, which has led to the interpretation of it being too deeply eroded (Pers. Comm., M. Rodríguez, 2010).     5   Figure 1.2. Location map of the Cerro Bayo Ag – Au epithermal district. The map on the left is showing the political Aysén region which is enlarged in the map on the right hand side.     6   Figure 1.3. Simplified geologic map of the Aysen region, southern Chile, showing the main mines and prospects (modified from Townley et al., 2000).     7   Figure 1.4. Simplified geologic map of the Cerro Bayo district, showing the four main studied areas (modified from Coeur d’Alene Mines).  1.3 Rationale and thesis objectives In the Cerro Bayo district although regional and district scale geological mapping has been carried out (Cornejo, 1989; Williams, 2006), little is known about the relationship of the volcanic stratigraphy with high grade ore shoots in the veins, because of the complex structural controls, and post mineral deformation which exposes different levels of the veins at the surface in different structural blocks. The district scale alteration zonation of mineralizing systems and fluid pathways are still poorly understood. Moreover the temporal relationship between the    8   mineralization and subvolcanic intrusive bodies has not been confirmed by geochronological data. All the above exposed geologic problems allow generating the principal research questions: what are the levels of exposure of the veins in the different structural blocks? and what are the principal controls in mineralization? To address these questions, this thesis contains two main parts: (1) the volcanostratigraphy and its related volcanic eruption history and (2) mineralization and alteration. The specific goals for topic 1 are: Define the volcanic stratigraphy to understand the geological framework. The aim of this  •  goal is to make a precise correlation between the stratigraphy of different structural blocks, to know the degree of preservation of the volcanic sequence in each studied subareas. •  Constrain the volcanic geologic events undertaking high-precision geochronology in the sub-volcanic domes and the volcanic stratigraphy. The specific goals for topic 2 are: Investigate on the basis of hydrothermal clay mineralogy, vein texture, and compiled vein  •  geochemistry and fluid inclusions, the exposure level of the veins at the different studied areas. •  Constrain the mineralizing geologic events undertaking high-precision geochronology of the veins. The integration of the specific goals will allow the major objective of this thesis of  building a district scale geological, ore genetic and exploration model for the Cerro Bayo district.  1.4 Mining history The exploration and mining history of the Cerro Bayo district is summarized from Sims (2010). Ag-Au mineralization at Cerro Bayo (termed Fachinal at that time) was discovered by Freeport     9   Chilean Exploration Company (FCEC) in the summer of 1984. Drilling of veins and potential bulk-minable stockworks commenced in 1986, and continued until mid 1989. Early 1990, Coeur d’Alene Mines Corporation acquired FCEC and extensive exploration concessions at Fachinal. Coeur d’Alene Mines Corporation resumed evaluation of the area in the second quarter of 1990, with infill and step-out drilling and tunneling, which led to a feasibility study and a production decision in mid 1994. Production started late May 1995 after the construction of a standard flotation mill. Laguna Verde is the area where mining started in the Cerro Bayo district in 1995. Mining was concentrated on several breccia bodies, large veins, and stockworks and three open pits were developed, whereas some veins were mined using underground methods. Due to declining metal prices and depletion of reserves the mine operations were suspended in November 2000. A drilling program in 2000, prior to mine suspension, outlined a high-grade vein system at Bahía Jara, near the Cerro Bayo dome, located 14 km west of the town of Chile Chico and 12 km east from Laguna Verde. Infill drilling and development of two underground ramps toward the main Lucero vein began in November 2001. Mine development followed shortly thereafter and the Laguna Verde processing plant was re-started in April 2002. Exploration and development drilling continued during 2003, as well as production from the underground areas and from two open pits. During 2005, production continued from both open pit and underground sources, but during 2006 through 2008 it was exclusively from underground sources. Mining and processing operations were suspended in late 2008 due to diminishing reserves and the need to evaluate the viability of mining several new veins discovered within the Coihues Este area (Laguan Verde: Dagny, Fabiola and Delia veins). Exploration continued during 2009 in the District, culminating with the delineation of Mineral Resources and Reserves in those veins. Cerro Bayo district was on care and maintenance from October 2008 to September 2010. Mandalay Resources Corp. purchased 100% of Compañía Minera Cerro Bayo in August 2010.     10   Table 1.1. Cerro Bayo district historic production (from Sims, 2010)  Table 1.2 Mineral resources and probable mineral reserves of Cerro Bayo as if March 1, 2010 (from Sims, 2010). Considered veins are: Dagny, Delia, Fabiola, Coyita, Dalila and Yasna from Laguna Verde; and Marcela Sur from Bahía Jara (see veins with asterisk in Figure 3.8 for location).     11   1.5 Methods  1.5.1 Volcanostratigraphy To define the volcanic stratigraphy ten stratigraphic columns were established in different areas where the volcanic successions are well exposed. Samples were taken in every volcanic level of the different stratigraphic columns and, the chosen samples were analyzed by petrography, for whole-rock geochemistry and for geochronology in order to correlate and reconstruct the entire volcanic eruptive history in the area. Sixty thin sections were prepared from representative rock samples of the stratigraphic columns. The thin sections have been described at the MDRU (Mineral Deposit Research Unit) petrographic laboratory at The University of British Columbia. The objective of this was distinguishing the units that were defined by Williams (2006) during district scale mapping; characterize the stratigraphic units petrographically; and make a more precise correlation between the stratigraphic columns from different areas in the Cerro Bayo district. The potentially diagnostic features that were documented in thin sections are crystal and crystal fragment content; degree of hydrothermal alteration; devitrification textures; degree of welding; cross-cutting relationships and flow banding. All these are present in variable amount and degree between the different units. Petrography also aids in defining the variability in a particular unit.  1.5.2 Whole-rock geochemistry Thirty-six samples have been analyzed at ALS Chemex for major and trace elements using lithium borate fusion and inductively coupled plasma atomic emission spectrometry (ICP-AES) for major elements and inductively coupled plasma mass spectrometry (ICP-MS) for trace elements (see Appendix A2 for sample coordinates). The analytical package corresponds to MEMS81D. For ALS Chemex analytical methodology see Appendix A3.     12   The analyzed samples were chosen from different stratigraphic columns and from levels with low content of lithic fragments. Almost all samples are affected by detectable hydrothermal alteration and immobile element diagrams have therefore been used for tectonic discrimination purposes. Major elements were used only to characterize the alteration by means of alteration indices.  1.5.3 Geochronology The age of prominent subvolcanic intrusions and domes in the district remains unconstrained and given the spatial proximity to mineralized veins the dating of these rocks has been prioritized in this thesis. Zircons from four samples have been dated by the U-Pb method using the chemical abrasion isotope dilution thermal ionization mass spectrometry (ID-TIMS) at the Pacific Centre for Isotope and Geochemical Research (PCIGR) at The University of British Columbia. One sample from the pyroclastic succession from the Pampa la Perra area and another sample from an andesitic dike that cross-cuts Delia vein at the Laguna Verde area were dated by the zircon laser ablation mass spectrometry (LA-MS) U-Pb method. These samples were analyzed with the aim to constrain the age of a pyroclastic flow outcropping at the base levels of Pampa la Perra, and to define a minimum mineralization age at Laguna Verde. Four new 40Ar/39Ar data on biotite from the acid pyroclastic sequence were obtained in the upper levels of the Ibáñez Formation, where the strata have variable bedding orientations which, on the basis of the available outcrop relationships may be interpreted as a thrust fault; or alternatively as an uncorfomity. The ages would potentially bracket an event of deformation. Veins from different areas present similar characteristics, but there are some key textural features observed in the field that distinguish veins between Mallines, Bahía Jara, Brillantes and Laguna Verde. Seven adularia samples from different areas taken from within the veins were analyzed by the 40Ar/39Ar method. The objective of this is to find age constraints for the mineralization events, and potentially to help understanding different ore-grades and ore shoot locations from different areas.     13   1.5.4 Data compilation: ore geochemistry and fluid inclusions Geochemical data from Coeur d’Alene Mines and from C. Hermosilla (2009, written commun.) were used to establish the geochemical variability among veins from different areas in the Cerro Bayo district. The samples were analyzed at the ALS Chemex Laboratories by the ME-ICP41 analytical package, which consists of Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) following Aqua Regia Digestion. Homogenization temperatures and salinity data from fluid inclusions trapped in epithermal quartz from Townley (1996), Pizarro (2000), and C. Hermosilla (2009, written commun.) were compiled for different veins around the district. These data were used only for broad scale comparisons due to the variable origin and documentation of fluid inclusion data.  1.5.5 Alteration: clay mineralogy XRD and ASD Terraspec® analyses were used to characterize the alteration clay mineralogy from samples adjacent to veins from the different mineralized areas and structural blocks. These two methods are compared to verify consistency and thus the applicability of the Terraspec® analysis as a low cost tool in low-sulfidation deposits exploration.     14   2 Volcanic stratigraphy, geochronology and whole-rock geochemistry  2.1 Regional stratigraphic context of the Cerro Bayo district The Mesozoic Chon Aike felsic magmatic province of Patagonia (Fig. 2.1) covers an area of about 1,000,000 km2 (Pankhurst and Rapela, 1995; Pankhurst et al., 1998). In the Patagonian Cordillera these Jurassic felsic rocks correspond to the Ibáñez Formation (Fig. 2.2) and were locally faulted, tilted and thrust during of Cretaceous-Tertiary Andean deformation. Along the Patagonian Cordillera (between ~42° and 50° Lat. S; Fig. 2.1) part of the felsic magmatic province is represented by a Jurassic belt of well-exposed volcanic sequences assigned to the Ibáñez (Chile), the Tobífera (Chile), and the El Quemado (Argentina) formations. North of 47° Lat. S, and thus north of Cerro Bayo, Haller & Lapido (1982) recognized that the base of the Ibáñez Formation in the Upper Jurassic consists of an andesitic pyroclastic sequence with dacitic intercalations. South of Cerro Bayo, at the El Faldeo pollymetallic Au – Zn district (47°25’S Lat; near the Chacabuco River; Fig. 2.1) Parada et al. (1997) described the lower ~200 m of the Ibañez Formation that unconformably overlies polymetamorphic and multiply-deformed schists, phyllites, quartzites, shales, marbles of the Paleozoic metamorphic basement. In this area, the Ibáñez Formation consists of a homoclinal sequence of sedimentary and volcanic beds. Comprising, from bottom to top: (1) a 50-m-thick sedimentary polymictic breccia, where the base is composed of fragments between 2 and 10 cm of schist, quartzite and phyllite, derived from the basement, in a clay matrix; overlain by breccias containing reworked tuff clasts, hydrothermal breccias and silicified igneous clasts. Then, (2) a 150 m thick succession of dacitic and rhyolitic tuffs, with a variation from lapilli to ash grainsize, containing clasts of felsic pumice, volcanic rocks fragments and crystal fragments of quartz and feldspar, felsic lavas, hydrothermal eruption breccias and their reworked sedimentary equivalents complete the succession.     15   A U-Pb zircon date from a tonalitic pluton that cross-cuts the basal levels of the Ibáñez Formation at El Faldeo gave an age of 155 ± 10 Ma (Parada et al., 1997), biotite  40  Ar/39Ar  analysis for the same pluton gave plateau ages of 158.86 ± 1.52 and 157.69 ± 1.51 Ma; and one biotite K-Ar analysis by Palacios et al. (1997) gave an age of 151 ± 4 Ma. Whole-rock K-Ar age determinations of pervasively sericitized felsic rocks of the El Faldeo deposit indicate alteration ages between 142 and 140 Ma (Palacios et al., 1997).  Figure 2.1. Chon Aike large felsic igneous province and the Patagonian Batholith outcrops modified from: Pankhurst et al., 1998; Féraud et al., 1999; Pankhurst et al., 1999; Calderón et al., 2007; Hervé et al., 2007; Wallier, 2009.     16   2.2 Jurassic-Cretaceous volcanic stratigraphy and intrusive rocks at Cerro Bayo Previous geological maps of the area include Williams (2006) who mapped the entire district and defined numerous units around the study area, as well as principal and secondary faults and lineaments. Regional mapping was carried out by Suárez and De la Cruz (1997a) and De la Cruz and Suárez (2008) who provided a basic volcanic stratigraphy, but also reported several geochronological constraints which will be discussed later in this chapter. The volcanic stratigraphy of the Ibáñez Formation in the Cerro Bayo area consists of a basal member of dacitic to andesitic coherent lavas and volcaniclastic rocks (Unit 1). These are overlain by a lower (Unit 2) and upper (Unit 4) pyroclastic succession consisting of variablywelded rhyolitic to rhyodacitic fragmental rocks. These two pyroclastic members are separated by a horizontally extensive volcanosedimentary unit (Unit 3; Figs. 2.3 and 2.4). In general, all the volcanostratigraphic units present in the district have a gentle inclination to the east (~ 5° to 25°). The felsic succession is intruded by rhyolitic domes and dykes at Cerro Bayo as well as dacitic domes near Laguna Verde (Fig. 2.4). The upper pyroclastic unit is overlain in angular unconformity by the El Toqui Formation of probable Lower Cretaceous age (De la Cruz and Suárez, 2008). Eocene to Pliocene basaltic plugs and lavas intrude and overlie the Mesozoic sequence. The stratigraphic subdivisions of the Ibáñez Formation as defined in this study are described below.  2.2.1 Unit 1: andesitic to dacitic coherent lavas and volcanoclastic basal succesions This unit crops out at the basal levels of the Laguna Verde and Laguna Verde Sur stratigraphic columns, and is well developed between 195 and 427 m depth in drill core FCH-312 from the Laguna Verde area (270972E/4840381N). At Laguna Verde Sur (Figs. 2.2 and 2.5), Unit 1 corresponds to a porphyric andesite (7 – 10% of the rock are phenocrysts), with plagioclase phenocrysts (~2.5 mm that are partly replaced by clays, lesser opaques and calcite) set in a fluidal microlitic groundmass. This coherent sequence has a middle level of andesitic autobreccia with a poorly-defined subhorizontal bedding.     17   Figure 2.2. Simplified geologic map of the Cerro Bayo district. The black dots are showing where the stratigraphic columns were documented. The main studied areas are indicated by dashed lines (Bahía Jara, Brillantes, Laguna Verde and Mallines). UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. For detail outcrop map of the Ibáñez Formation, see Appendix A1.     18   [m a.s.l.] (True thickness)  Figure 2.3. Stratigraphic columns of the Cerro Bayo district. Geochemical samples numbers and new geochronological dates are indicated. Appendix A2 provides geochemical and geochronological samples location.  GUANACO  Symbology  (252,7) 433  WEST  EAST - High iron oxide content - High qz eye content - Milimetric iron oxide veins - Silicified lithic fragments - Argillic alteration (Ill + Smc?) - Fine disseminated bowxork - JP-128 (277444E/4841078N) - Upward increasing qz eyes content - Carbonate + iron oxide < 8 cm wide veins -Moderately welded  LAGUNA VERDE [m a.s.l.]  - Carbonate + lithic fragments - Py dissemination - Moderately silicified - Moderately argillizied - Moderately welded  [m a.s.l.]  625  - Andesitic lava lithic fragments - Moderately to weakly welded  - Moderately to strongly welded - High quartz crystal content - Plag replaced by clays - Argillized matrix - Argillized fiammae locally with preserved glass - JP-110 (271034E/4842963N)  450  585  CAŇADON VERDE [m a.s.l.] 436  395  554  386 373  533  - Weakly porphyric texture - Chlorite patches - JP-200 (JP-1000) (270149E/4839045N)  365  - Andesitic volcanoclastic rocks with crude bedding 333 486  1/16  2  - Plag partly replaced by clay and limonite 64 mm.  322  294  1/16  2  - Strongly welded - Iron oxide + calcite veins - JP-91 (270614E/4842850N) - Moderately pinkish K-fld crystals; low qz eye content - Moderately welded at high levels - Coarse sandstones levels with plag fragments - Argillic alteration - Fine sandstones levels with cleavage - Completely replaced plag crytals by clays and calcite - Low qz fragment content - Moderately welded at high levels - Ill alteration in phenocrysts and shards - Argillic alteration in matrix - JP-88 (270568E/4842907N) - Plag replaced by clays - Moderately silicified - Calcite veins - Qz + K-fld + plag crystals - Tuffaceous sandstone 64 mm.  - Moderately argilizied - Boxwork after disseminated pyrite - Weakly to moderately welded  [m a.s.l.] (True thickness) - Weakly welded - Weakly welded - Incipiently argilizied - Calcite cemented - Qz replacing plag - Plag completely replaced by clays - Moderately to strongly welded - Fine py & boxwork filled with hmt - Fiammae replaced by clays - JP-04 (276557E/4839702N)  (180) 532 400  - Volcanosedimentary levels - Block-sized reddish dacitic tuff, and greenish andesitic lava lithic fragments. - Weakly to moderately welded - Carbonate veins - Moderately argillized  390 387 378 371  1/16  2  (164) 525 (153) 516 (142) 500  [m a.s.l.] 612 - Moderately argillic alteration - Andesitic lava & metamorphic lithic fragments - Moderately welded  - Barite veins  64 mm.  2  64 mm.  480 475  - Slightly welded - High plag and low qz contents  - Slighlty welded - Moderately plag and low qz contents - Moderatelly to strongly welded  425  1/16  2  398  1/16  2  - Moderately argillic alteration in matrix - Slight to moderatelly welded - JP-115 (270001E/4844765N) 64 mm.  pumice or relict pumice 1220  - Moderate qz eyes and plag content, and low bt content - JP-197 (285026E/4837063N) - 40Ar/39Ar biotite age: 144.90 ± 0.46 Ma - High hmt content in matrix - Weakly welded  1190  1/16  2  377  - Argillic alteration in clasts  366  - Argillic alteration - Iron oxide - Moderately qz + plag  355  CERRO TORTA E [m a.s.l.] (Real height)  - High content of qz eyes - Weakly to moderately welded - Iron oxide veins < 5 mm - Ill alteration in K-fld - Argillic alteration in matrix - JP-135B (280504E/4846090N) - High iron oxide content - Weakly welded - High K-fld content - Moderate qz content - JP-134 (280604E/4846053N)  290  64 mm.  - Argillic alteration - Carbonates + iron oxide veins - Weakly welded  253 238  1/16  2  64 mm.  spherulites / devitrified fiammae rounded, polymict lithic fragments  - Quartzite - Presence of oyster fossils  (155.9) 1290  - Weakly welded - Moderate qz eyes and plag content, and low bt content - Fiammae replaced by clays - JP-194 (284938E/4837145N) - 40Ar/39Ar biotite age: 153.27 ± 0.98 Ma  1133  - Strongly silicified - Fiammae replaced by clays - Abundant fine grained disseminated pyrite - High hmt content  angular, polymict lithic fragments - High iron oxide content - Moderately argillized - JP-169 (285630E/4836297N) - Plag replaced by calcite + clays - Moderately fresh bt - JP-168 (285732E/4836275N) - 40Ar/39Ar biotite age: 149.74 ± 0.72 Ma.  (128.9) 1277 (118.9) 1270  perlitic fracture carbonate / carbonate associated with iron oxides shard banded pumice  - Moderately welded - Ash matrix - Green fiammae - Moderately argillized - Moderate bt content - JP-167 (JP-1001) (285888E/4836270N) - 40Ar/39Ar biotite age: 154.0 ± 1.5 Ma.  (74.2) 1266  - Weakly welded - High plag and low qz contents  sand-size paticles, granular texture mud-size particles distinct planar bedding diffuse planar bedding  - Moderately welded - High qz eyes content - Highly silicified - JP-173 (JP-1002) (284526E/4837216 )  994 978  1/16  2  64 mm.  1238  jig-saw texture of fine-grained, moderately porphyritic rhyolite  - Moderately to intensely altered bt - Moderatel to strongly welded - Moderate argillizied  (13.3) 1241 (0)  316  fiammae / vitriclast or relict vitriclast  - Strongly welded  1170  - Slight to moderately welded - Moderately K-fld and low qz contents - Weakly to moderately welded - JP-103 (281950E/4842467N) 64 mm.  poorly to moderately porphyritic andesite  [m a.s.l.]  - Argillic alteration in clasts - Moderately welded  400  445 439  CERRO TORTA W  1181 1179  298 292 287  - Locally calcite cemented - Autobrecciated - Rhyolitic lithic fragments  - Moderately argillic alteration - High qz content  (0) 521  - Flow banded - Plag microliths in groundmass - Plag + pyroxene phenocrysts - Chl + ep alteration in phenocrysts - Fine disseminated oxided pyrite - JP-138B (280322E/4846302N)  - High hmt content - High plag content recrystallized to qz + K-fld - Moderate hmt content - Slightly welded  1/16  - Moderately welded - Plag phenocrysts and fragments replaced by clays - carbonates  BRILLANTES  - Massive - Metamorphic, volcanic lithic fragments - Calcite cemented  463  - Qz + carbonates & iron oxide + carbonates veins - K-Fld altered to clays (ill+smc?) - High qz eye and K-fld contents - JP-72 (JP-1003) (275367E/4839277N) - LA-ICP-MS zircon U/Pb date: 146.4 ± 1.1 Ma  [m a.s.l.]  (69.7) 513  (0) 488  - Upward increasing welding (weakly to moderate)  383  498  - Iron oxide + carbonates <0.2 mm wide veins - High qz eye content - Moderately argillizied plag content - Moderately to strongly welded - JP-129 (276537E/4841499N)  - Strongly welded - Argillic alteration particularly in fiammae - JP-106 (282490E/4842882N) - Weakly welded - Calcite veins - Weak argillic alteration in plag and matrix - Strongly welded  417 409 404  JUANITA  553  (10) 491  coherent dacitic lava  [m a.s.l.]  565  (200) 550  phenocryst-poor, variably welded rhyodacitic tuff  PORFIA  - Plag replaced by carbonates - clays - Argillized matrix - Moderately to strongly welded - JP-126 (277061E/4840898N)  PAMPA LA PERRA  phenocryst-poor, variably welded rhyolite tuff phenocryst-rich, variably welded rhyodacitic tuff  1/16  2  Mineral abbreviations  64 mm.  Qz: quartz Plag: plagioclase  Volcanostratographic units Unit 4 Unit 3 Unit 2 Unit 1  }  K-fld: potassium feldspar Bt: biotite Ill: illite  Ibáñez Formation  LAGUNA VERDE SUR  477 473  phenocryst-rich, variably welded rhyolitic tuff  Smc: smectite  Toqui Formation  Chl: chlorite Ep: epidote Py: pyrite Hmt: hematite  19    Figure 2.4. Generalized stratigraphic column of the Ibáñez Formation in the Cerro Bayo Ag-Au low sulfidation epithermal district. The approximate thickness of the entire section is ~700 meters.  20   2.2.2 Unit 2: lower variably welded rhyolitic to rhyodacitic pyroclastic fragmental unit This unit crops out at the basal levels of the Cañadón Verde, the Guanaco, the Juanita, the Laguna Verde, the Pampa la Perra (Fig. 2.6), and the Porfía stratigraphic columns (Fig. 2.2). Although, the base and the top of this unit is not exposed in the same stratigraphic column, up to ~150 m stratigraphic thickness is exposed in the Pampa la Perra and Juanita areas. This variably welded fragmental unit with ash matrix has similar features in the different studied areas. This unit has 5 to 10% crystal content considering the entire rock volume. In the western part of the district, quartz (30 - 50% of the crystals, <0.5 mm) and plagioclase (50 - 70%, <2 mm; partly replaced by clays ± carbonates) crystal fragments and hematite, probably replacing mafics, are present. Argillized fiammae, in some cases containing quartz crystals, were observed. Pumice fragments were also detected, along with cuspate argillized shards. In the eastern part of the district, near Guanaco and Porfía, Unit 2 comprises quartz (20 - 75% of the crystals, <3 mm, some fractured and enveloped by sericite), K-feldspar (20 – 40%, <1 mm, partly or totally replaced by clays ± sericite), plagioclase (30%; <0.3 mm; partly replaced by clays ± sericite) and muscovite (5 - 10%, <0.2 mm) crystal fragments. Bow tie and rounded spherulites inside argillized fiammae as well as devitrified glass pumice surrounded by clays are evident. Calcite crystals <1 cm in diameter replacing K-feldspars was observed in Unit 2 in the Guanaco and Porfía areas.     21   A  B  Plg Plg  2.5 mm  Figure 2.5. Unit 1 at the Laguna Verde Sur area. A: (270179E/4838961N) Purple andesitic volcanoclastic level with high hetero-lithic fragment content (300°/18°). B: Microphtograph of sample JP-199 (270149E/4839045N) that corresponds to a porphyritic (7% plagioclase phenocrysts) andesite (phenocrysts are partly replaced by clays and minor hematite), immersed in microlithic groundmass.  Figure 2.6. General view of the Pampa la Perra are where the stratigraphic column was performed. Units 2,3 and 4 can be observed.     22   2.2.3 Unit 3: volcanosedimentary unit This unit crops out in the middle stratigraphic levels, mainly in the Cerro Bayo and Laguna Verde areas. Due to its wide distribution in the entire district; this volcanosedimentary dominated unit can, despite some internal variability, be used as a stratigraphic marker that permits correlation between the different areas. The thickness varies from 20 to 80 meters, with the greatest thickness exposed in the Juanita area, south of Brillantes (Fig. 2.2).  Figure 2.7. Pampa la Perra volcanosedimentary unit (Unit 3; 276616E/4839618N)  In the middle levels of both the Pampa la Perra (Fig. 2.7) and the Guanaco stratigraphic columns, Unit 3 occurs as a crystal-rich volcanoclastic unit of ~15 m and ~60 m thickness respectively. Quartz (5% of the crystals, <0.4 mm), K-feldspar (20%, <0.8 mm, partly replaced by clays) and plagioclase (75%, completely replaced by quartz – clays – calcite) are observed. Abundant finegrained hematite and <0.2 mm wide hematite + calcite veinlets were observed.     23   2.2.4 Unit 4: upper variably welded rhyolitic to rhyodacitic pyroclastic fragmental unit This unit crops out in the same areas as Unit 2, in addition to the Mallines area (Fig. 2.2). It has a minimum exposed thickness of 20 m in Pampa la Perra area (top not exposed) to 300 meters in the Cerro Torta area (Mallines). In all areas Unit 4 displays a fragmental texture with crystal fragment contents that vary from 5 to 10% of the total volume of the rock, except for the basal levels of the Cerro Torta E stratigraphic column that contains up to 40% crystal fragments. At Cerro Bayo (including the Pampa la Perra, the Guanaco and the Porfía stratigraphic columns), Unit 4 is characterized by slightly lower quartz (<15 – 50% of the total crystal content; <1.5 mm) and higher plagioclase (50 - >85%; <2.5 mm; partly or totally replaced by clays + carbonates ± sericite) crystal fragments content, compared to Unit 2. Welding is represented by local greenish fiammae where they are completely replaced by clays; argillized shards are also observed. Hydratation and devitrification textures such as perlitic fractures in devitrified pumice and minor spherulites are present. Granitic and rounded volcanic lithic fragments constitute the lithic component of this unit. Abundant <0.1 mm wide calcite veins and calcite replacing plagioclase are characteristic for this unit. The upper portion of Unit 4 crops out at Mallines, specifically in the Cerro Torta stratigraphic columns (Figs. 2.2 and 2.3). In this area it is represented by fragmental rocks with porphyritic texture and variable amounts of crystal fragments in an ash matrix. It is unconformably overlain by the Toqui Formation (De la Cruz and Suárez, 2008). In the Cerro Torta W stratigraphic column, quartz (90% of the crystals, <3.5 mm) and plagioclase (10%, <1.5 mm; partly replaced by clays) crystal fragments are present. Abundant devitrified juvenile pumice clasts and lesser fiammae are present at these levels and considerable silicification has affected the rock in the proximity of the Cascada vein. It is important to remark that the upper ~40 m of the stratigraphic column in this area is composed by an incipiently welded, reddish succession (Fig. 2.8), containing ~3% crystal fragments including quartz (40% of crystals, <0.5 mm), plagioclase (30%, <1 mm; completely    24   replaced by clays) and biotite (30%, <0.3 mm; fresh). Fiammae, replaced by clays, and shards can be observed as well. At the top of the column a quartz (60% of crystals, <2.5 mm), plagioclase (20%, <2.5 mm) and biotite (20%, <1 mm; fresh) crystal fragment rich tuff was observed. This level is not welded and contains minor rounded pumice clasts. In thin section, a strong hematization of the ash matrix it is evident (Fig. 2.8B).  Figure 2.8. Cerro Torta W (284991E/4837072N). A: Upper oxidized levels of the Ibáñez Formation; ~30 meters below the basal contact of the Toqui Formation . B: JP-196 where fresh pumice surrounded by hematized ash matrix can be observed.  The basal levels of the Cerro Torta E stratigraphic column correspond to a greenish fragmental rock sequence that dips at ~ 21°S). It consists of quartz (70% of crystals, <1.5 mm; almost all surrounded by calcite), plagioclase (15%, <1.8 mm; partly replaced by clays) and biotite (15%, <1.8 mm, subhedral; minor clay replacement) crystal fragments. Minor spherulites were observed as well. This column continues with a fragmental rock succession with scarce crystal fragments (some isolated quartz and plagioclase <1 mm), moderate welding, fiammae (some replaced by fine-grained white micas and others by pale green clays). These levels present similar features to those present in Unit 4 in the Cerro Bayo area (mainly in the Pampa la Perra and the Guanaco areas).    25   The upper portion of this stratigraphic column is a subhorizontal fragmental sequence composed of quartz (10% of the crystals) and muscovite (90%, < 0.3 mm) crystal fragments with high hematite content. Both the Cerro Torta W and E columns are overlain by a ~30 meter thick quartzite units with oyster shells belonging to the Cretaceous Toqui Formation (De la Cruz and Suárez, 2008).  2.2.5 Intrusive rocks The main intrusive rocks present in the district include a series of N-S aligned subvolcanic domes (Figs. 2.2 and 2.9) that extend from Mallines to the Cerro Bayo and Bahía Jara areas. A volumetrically less important series of roughly NE-SW aligned domes also crop out from the Laguna Verde to the Cañadón Verde area.  2.2.5.1 N-S trending aligned domes The N-S aligned domes have an aphyric to fluidal porphyritic texture with phenocryst content generally increasing from south to north. Phenocrysts make up to 1-10% of the rock and include quartz (60 - 85% of crystals, ≤2 mm) and K-feldspar (15 - 40%, ≤4 mm; partly replaced by clay and quartz). The groundmass is mainly represented by devitrified (bow-tie texture) bands with central rotated strongly embayed quartz grains (Fig. 2.10), as well as spherulites, which have been replaced by K-feldspar. South of Cerro Bayo dome, N-S trending rhyolite dikes that are texturally similar to the Cerro Bayo dome, crop out. Flow-banding striking from N140° to N220° and dipping from 55°W to 64°E are locally observed, however the dominant orientation is N-S/~70°W, parallel to the principal structure along which the domes are aligned. Up to five cm wide N355° striking quartz veins with fine-grained disseminated euhedral pyrite cut the dikes, indicating that dome and dike emplacement occurred prior to the hydrothermal activity at Cerro Bayo.    26   Figure 2.9. Photograph taken from Bahía Jara looking S. N-S aligned rhyolitic subvolcanic domes intrude the Ibáñez Formation.  Figure 2.10. Microphotograph of the Cerro Bayo dome (yellow line is 2.5 mm long). Devitrified fluidal texture showing rotated quartz phenocrysts.     27   2.2.5.2 NE aligned domes A series of domes crop out between the Laguna Verde to the Cañadón Verde areas along a poorly defined NE-SW trend (Fig. 2.2). A porphyric fluidal texture with quartz (45% of the crystals), Kfeldspar (45%; some of them replaced by clays ± sericite) and biotite (10%) phenocrysts are present. In the Laguna Verde area light and dark bands can be observed in thin section, where the dark color is due to the presence of opaque minerals (Fig. 2.11B). A porphyritic texture, defined by 1%, relatively unaltered, phenocrysts, containing 90% K-feldspar, 9% quartz, <1% biotite phenocrysts, immersed in a fluidal fine-grained K-rich matrix (intensely yellow after K-feldspar hydrofluoric acid + sodium cobaltinitrite staining), can be observed in this dome  Figure 2.11. Laguna Verde dacitic dome (270353E/4841771N). A: Flow banding. B: JP-141B where sericite veinlets cutting the flow banding can be noted.     28   2.3 Geochronology Limited geochronological data are available in the literature for the volcanic succession in the Cerro Bayo area. Suárez and De La Cruz (1997a) present seven K-Ar biotite ages for the upper part of the Ibañez Formation ranging between 150 ± 4 Ma and 144 ± 3 Ma (Table 2.1) and De la Cruz and Suárez (2008) reported two SHRIMP U-Pb ages of 149.1 ± 3.2 Ma (Mallines area) and 141.9 ± 3.2 Ma (~5 km east of the Cerro Bayo dome). These ages all correspond to the Oxfordian – Kimmeridgian interval (Upper Jurassic). Suárez and De La Cruz (1997a) also report a K-Ar biotite age of 132 ± 3 Ma (Berriasian - Lower Cretaceous) for a tuffaceous sandstone that is interpreted as the basal level of the Toqui Formation.  Table 2.1. Biotite K-Ar available for the Ibañez Formation near Lake General Carrera, Chilean Patagonia. (Suárez and De La Cruz, 1997a).  Apart from the age constraints for the Ibáñez Formation, three K-Ar ages are available for the Cerro Bayo dome (De la Cruz and Suárez, 2008). One analysis on feldspar gave an age of 111 ± 4 Ma, and the other two on whole-rock gave ages of 92 ± 2 and 97 ± 2 Ma (Table 2.2). These are considered minimum ages because the feldspars that were analyzed are sericitized.     29   Table 2.2. K-Ar available for the Cerro Bayo dome (De la Cruz and Suárez, 2008).  The previously published age constraints on the volcanic stratigraphy and intrusive units are still ambiguous and for some units such as the domes at Laguna Verde no age constraints are available. Thus  40  Ar/39Ar and the U-Pb methods have been applied to clarify the timing of the  different events and the results are presented below.  2.3.1 Zircon U-Pb geochronology Zircons from six samples have been dated by the U-Pb method. Four samples were analyzed using the chemical abrasion isotope dilution thermal ionization mass spectrometry (ID-TIMS) at the Pacific Centre for Isotope and Geochemical Research (PCIGR) at The University of British Columbia. Those samples correspond to the Cerro Bayo (JP-65A), Cerro Lápiz (JP-78A), Laguna Verde (JP-141A) and Cañadón Verde (JP-112A) domes. Two samples were analyzed by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS); these samples were from Unit 2 (JP-72) of the Ibáñez Formation collected at the basal levels of the Pampa la Perra stratigraphic column, and an andesitic dyke (JP-158C) that cross-cuts the Delia vein in the Laguna Verde area.  2.3.1.1 Methodology The CA-ID-TIMS technique utilized in this study has been modified from CA-TIMS procedures outlined in Mundil et al. (2004), Mattinson (2005) and Scoates and Friedman (2008). After rock samples have undergone standard mineral separation procedures zircons are handpicked in alcohol. The clearest, crack- and inclusion-free grains are selected, photographed and then annealed in quartz glass crucibles at 900º C for 60 hours. Annealed grains are transferred into 3.5 mL PFA screwtop beakers, ultrapure HF (up to 50% strength, 500 mL) and HNO3 (up to 14 N,    30   50 mL) are added and caps are closed finger tight. The beakers are placed in 125 mL PTFE liners (up to four per liner) and about 2 mL HF and 0.2 mL HNO3 of the same strength as acid within beakers containing samples are added to the liners. The liners are then slid into stainless steel Parr™ high pressure dissolution devices, which are sealed and brought up to a maximum of 200º C for 8-16 hours (typically 175º C for 12 hours). Beakers are removed from liners and zircon is separated from leachate. Zircons are rinsed with >18 MΩ.cm water and sub-boiled acetone. Then 2 mL of subboiled 6N HCl is added and beakers are set on a hotplate at 80-130 C for 30 minutes and again rinsed with water and acetone. dimensions (volumes) of grains.  Masses are estimated from the  Single grains are transferred into clean 300 uL PFA  microcapsules (crucibles), and 50 mL 50% HF and 5 mL 14 N HNO3 are added. Each is spiked with a  233-235  U-205Pb tracer solution (typically 2 mL), capped and again placed in a Parr liner (8-  13 microcapsules per liner). HF and nitric acids in a 10:1 ratio, respectively, are added to the liner, which is then placed in Parr high-pressure device and dissolution is achieved at 240º C for 40 hours. The resulting solutions are dried on a hotplate at 130º C, 50 mL 6N HCl is added to microcapsules and fluorides are dissolved in high pressure Parr devices for 12 hours at 210° C. HCl solutions are transferred into clean 7 mL PFA beakers and dried with 2 mL of 0.5 N H3PO4. Samples are loaded onto degassed, zone-refined Re filaments in 2 mL of silicic acid emitter (Gerstenberger and Haase, 1997). Isotopic ratios are measured using a modified single collector VG-54R or VG-354S (the latter with Sector 54 electronics) thermal ionization mass spectorometer equipped with an analogue Daly detector. Analytical blanks are 0.2 pg for U and for Pb in the range of 1-10 pg. U fractionation was determined directly on individual runs using the 233-235U tracer, and Pb isotopic ratios were corrected for fractionation of 0.23%/amu, based on replicate analyses for the NBS982 Pb reference material and the values recommended by Thirwhall (2000). Data reduction employed the Microsoft Excel based program of Schmitz and Schoene (2007). Standard concordia diagrams were constructed and regression intercepts weighted averages calculated with Isoplot (Ludwig, 2003). Unless otherwise noted, all errors are quoted at the 2σ or 95% level of confidence. For samples JP-72 and JP-158C, U-Pb analyses were carried out using laser ablation (LA) ICPMS methods. Analyses were done using a New Wave UP-213 laser ablation system and a    31   Thermo Finnigan Element2 single collector, double-focusing, magnetic sector ICP-MS. The data acquisition and reduction protocol employed at the PCIGR has been described by Tafti et al. (2009), and is briefly summarized below. The best quality zircons were handpicked from the heavy mineral concentrate and mounted in an epoxy puck along with several grains of the 337 Ma Plešovice zircon standard (Sláma et al., 2007) and a 197 Ma in-house zircon monitor, and brought to a very high polish. The surface of the mount was washed for 10 minutes with dilute nitric acid and rinsed in ultraclean water prior to analysis. High quality portions of each grain, free of alteration, inclusions, or cores, were selected for analysis. Line scans rather than spot analyses were employed in order to minimize elemental fractionation during the analyses (Košler et al., 2008). Backgrounds were measured with the laser shutter closed for ten seconds, followed by data collection with the laser firing for approximately 29 seconds. The time-resolved signals were analyzed using GLITTER software (Van Achterbergh et al., 2001; Griffin et al., 2008), which automatically subtracts background measurements, propagates all analytical errors, and calculates isotopic ratios and ages. Corrections for mass and elemental fractionation were made by bracketing analyses of unknown grains with replicate analyses of the Plešovice zircon standard. A typical analytical session at the PCIGR consists of four analyses of the standard zircon, followed by two analyses of the 197 Ma in-house zircon monitor, four analyses of unknown zircons, two standard analyses, four unkown analyses, etc; and finally two analyses of the inhouse monitor and four standard analyses. Final interpretation and plotting of the analytical results employs ISOPLOT software (Ludwig, 2003). The amount of radiogenic  207  Pb in young  zircons is extremely low; hence, counting errors are correspondingly high, and calculated errors for  207  Pb/235U and  207  Pb/206Pb ages are also high. Interpreted ages for the samples dated in this  study are based on a weighted average of the individual calculated of measured  204  206  Pb/238U ages. The amount  Pb in all but a very small number of the analyses generated during the study is  negligible, so no correction was made for contained common Pb. Clear needle-shaped acicular ({101} prismatic) zircon crystals are common in rapidly crystallized, porphyritic, sub-volcanic intrusions, high-level granites, and gabbro (Corfu et al. 2003) indicating juvenile growth, where the age represents the igneous event. On the other hand, larger pink to brown, or opaque, zircons commonly represent inherited xenocrysts (Corfu et al.    32   2003), thus the age represents older igneous events. Most zircons analyzed in this study are clear and needle shaped although some older inherited zircons are probably present as well (Fig. 2.12).  Figure 2.12. Pictures showing the separated zircons for the U-Pb dating, where clear needle-shaped and bigger cloudy zircons can be observed. A. Cerro Bayo dome - JP-65A. B. Laguna Verde dome - JP-141A. For analytical data see Appendix A7.  Analytical data for ID-TIMS analyses are shown on conventional U-Pb Concordia plots in Figs. 2.13 and 2.14. The data fall on the concordia curve or they are slightly ‘normally’ discordant. The zircons yield a concordant age of 146.50 ± 0.21 Ma for the Cerro Bayo dome (Fig. 2.13A) and 146.3 ± 0.2 Ma for the Cerro Lápiz dome (Fig. 2.13B). The ages are based on four zircon fractions for each dome and ages are based on weighted average. The zircons from the Laguna Verde (Figs. 2.2 and 2.14A) and the Cañadón Verde (Figs. 2.2 and 2.14B) domes gave ages of 82.6 ± 0.2 Ma and 83.0 ± 0.2 respectively, and provide the first evidence for Late Cretaceous igneous activity in the study area. Minor inheritance is evident in one zircon grain of the Laguna Verde sample. The ages are based on three zircon fractions for each dome and ages are based on weighted average. To constrain Unit 4 and the minimum mineralization age at Laguna Verde samples were taken from the base levels at Pampa la Perra, adjacent to the plane of a normal fault where south of this fault plane Unit 4 crops out, and from an andesitic dyke that crosscuts the Delia vein immediately south of Laguna Verde. An additional sample (JP-138A) corresponding to the upper dacitic level of Brillantes was processed for zircon separation but the sample did not yield zircons for LA-MS U-Pb dating. Results for the two LA-MS dating results are shown in Figure 2.15 and for detailed    33   analytical data see Appendix A8. Sample JP-72 (Fig. 2.15A) that corresponds to Unit 4 in the Pampa la Perra area show an age of 146.4 ± 1.1 Ma; within error of the ages of the N-S trending aligned domes. Sample JP-158C (Fig. 2.15B), which corresponds to the andesitic dyke crosscutting the Delia vein at the Laguna Verde area, gave an age of 146.1 ± 1.2 Ma.     34   Figure 2.13. Concordia diagram for zircon separated from sample A. JP-65A (Cerro Bayo dome) and B. JP-78A (Cerro Lápiz dome) with calculated concordia ages (uncertainty reported as 2σ). Red ellipses reflect the errors of the individual grains associated with the concordia line. For analytical data see Appendix A7.     35   Figure 2.14. Concordia diagram for zircon separated from sample A. JP-141A (Laguna Verde dome) and B. JP-112A (Cañadón Verde dome) with calculated concordia ages (uncertainty reported as 2σ). Red ellipses reflect the errors of the individual grains associated with the concordia line. For analytical data see Appendix A7.     36   Figure 2.15. Plots of 206Pb/238U zircon ages for individual LA-ICP-MS analysis from the dated samples (error bars are at ±2σ) rej: rejected. For analytical data see Appendix A8.     37   2.3.2 Biotite 40Ar/39Ar geochronology 40  Ar/39Ar data for four biotite samples separated from fragmental rhyolite samples collected  around Cerro Torta to the SE of Veta Cascada at the Mallines are presented in this section (Figs. 2.2 and 2.19). Two samples were taken from ~500 m SE (set 1, Cerro Torta W, Fig. 2.16A) and a further two specimens were taken from ~1500 m SE of Veta Cascada (set 2, Cerro Torta E, Fig. 16B).  Figure 2.16. 40Ar/39Ar biotite samples. A: JP-168 from Cerro Torta E (285732E/4836275N). B: JP-197 from Cerro Torta W (285026E/4837063N).  2.3.2.1 Analytical procedures and results All  40  Ar/39Ar biotite dating was performed at the Pacific Centre for Isotope and Geochemical  Research (PCGIR), University of British Columbia, Canada. The analytical procedures were as follows. Firstly, the rocks were crushed with a steel mortar; then the crushed rocks were washed with deionized water to remove the finest and lighter grains. After this, the samples were dried under infrared light. Approximately twenty grains of biotite with grain sizes between ~0.25 and ~0.5 mm were finally handpicked. The samples were wrapped in aluminum foil and irradiated with similar-aged samples and interspersed with neutron flux monitors (Fish Canyon Tuff sanidine, 28.02 Ma; Renne et al., 1998). The samples were irradiated at the McMaster Nuclear Reactor in Hamilton, Ont., for 44     38   MWH, with neutron flux approximately 3 x 1016 neutrons/cm2. Analyses (n = 54) of 18 neutron flux monitor positions produced uncertainties of <0.5% in the J value. The samples were analyzed at the Noble Gas Laboratory, Pacific Centre for Isotopic and Geochemical Research (PCIGR), University of British Columbia, Vancouver, BC, Canada. The separate samples were step-heated at increasing laser powers in the defocused beam of a 10-W CO2 laser (New Wave Research MIR10) until fused. The gas evolved from each step was analyzed using a VG5400 mass spectrometer equipped with an ion-counting electron multiplier. All measurements were corrected for total system blank, mass spectrometer sensitivity, mass discrimination, radioactive decay during and subsequent to irradiation, as well as interfering Ar from atmospheric contamination and the irradiation of Ca, Cl and K (isotope production ratios: (40Ar/39Ar)K = 0.0302 ± 0.00006, (37Ar/39Ar)Ca = 1416.4 ± 0.5, (36Ar/39Ar)Ca = 0.3952 ± 0.0004, Ca/K = 1.83 (37ArCa/39ArK)). The plateau and correlation ages were calculated using Isoplot ver.3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. Samples JP-168 (Fig. 2.18A) and JP-197 (Fig. 2.17A) yielded statistically reliable plateau spectra, indicating minimal disturbance of the Ar systematics of the samples. They give ages of 149.74 ± 0.72 Ma and 144.90 ± 0.46 Ma, respectively. Sample JP-197 from Cerro Torta W underlies the Toqui Formation and represents the top of the Ibáñez Formation. Samples JP-194 and JP-167 gave ages of 153.27 ± 0.98 and 154.0 ± 1.5 Ma, respectively. The argon loss in samples JP-194 (Fig. 2.17B) and JP-167 (Fig. 2.18B) provides evidence for a later hydrothermal overprint affecting the argon systematic of the biotite. JP-194 is the sample closest to the Cascada Vein (Fig. 2.19). The fact that not all samples have been affected to the same degree by thermal resetting is evidence for a relatively localized hydrothermal effect rather than a regional thermal/metamorphic overprint. The age of the hydrothermal perturbation is not well constrained but on the basis of the apparent ages of the lower temperature steps, it is probably safe to assume that it was younger than ~130 Ma.     39   Figure 2.17. 40Ar/39Ar age spectra of biotite samples from Cerro Torta W. For analytical data see Appendix A9.     40   Figure 2.18. 40Ar/39Ar age spectra of biotite samples from Cerro Torta E. For analytical data see Appendix A8.     41       Figure 2.19. Field photograph of Cerro Torta showing where the samples for 40Ar/39Ar were taken. Note the NW dipping strata and that the sample JP-197 correspond to the higher levels of the Ibáñez Formation in this area. For samples location see Appendix A1.  42   2.3.3 Discussion of the geochronologic results The new biotite  40  Ar/39Ar ages overlap with the biotite K-Ar age range given for the Ibañez  Formation in Suárez and De La Cruz (1997a; Table 2.1) from samples also taken from Cerro Torta. All these samples correspond to the higher levels of the Ibáñez Formation in this area, (i.e, Unit 4), hence the youngest volcanism prior to the deposition of the Toqui Formation (De la Cruz and Suárez, 2008), and the N-S trending aligned rhyolitic domes are interpreted to be part of the same magmatic episode. The zircon U-Pb age of 146.4 ± 1.1 Ma from Pampa la Perra is probably reflecting the age of zircons from rocks from Unit 4 that remained attached to this rock wall after the slip of this unit along the plane fault. The younger ages for the Cerro Bayo rhyolitic dome presented by De la Cruz and Suárez (2008) probably indicate thermal resetting of the K-Ar systematics for the K-feldspar and the whole-rock system in general due to younger hydrothermal activity observed in the study area. The hematitized upper levels of the Ibáñez Formation stratigraphic sequence at Mallines are strongly oxidized, and overlain by the shallow marine Toqui Formation. The latter corresponds to early stages of transgressive successions that occurred during the Early Cretaceous Austral Basin (Suárez et al., 2009, 2010), and it is well exposed at Mallines, whereas in areas north of the Cerro Bayo district it is diachronous with the late stages of the Ibáñez Formation (Suárez et al., 2009). This suggests that the pyroclastic fragmental successions of the Ibáñez Formation were deposited in a continental to shallow marine environment. The dacitic domes from the Laguna Verde area were emplaced at ca. 83 – 82 Ma along a roughly NE aligned trend. This NE orientation may suggest that these domes are related to the postmineralization NE trending faults (Fig. 2.2). Zircons from the andesitic dike cutting the Delia vein yielded an age of 146.1 ± 1.2 Ma, and is interpreted to reflect the age of inherited xenocrysts incorporated into the dike due to the similar characteristics of these zircons compared to those extracted from the Ibáñez Formation samples.     43   This interpretation is also supported by the  40  Ar/39Ar on sericite related to the Laguna Verde  veins (Tippet et al., 1991; Townley, 1996; see also Chapter 3). The short distance (~10 km) between the Upper Jurassic Cerro Bayo and Cerro Lápiz domes with the Upper Cretaceous Laguna Verde and Cañadón Verde domes is consistent with the stationary nature of the arc which is reflected in the Meso-Cenozoic intrusive history of the Patagonian Batholith (Parada et al., 2001).  2.4 Whole-rock geochemistry  2.4.1 Major elements characteristics All samples analyzed have been plotted in the alteration box plot originally applied to volcanogenic massive sulfide (VMS) deposits by Large et al. (2001; Fig. 2.20). The alteration boxplot has also been applied to epithermal deposits (Gemmel, 2007). Most samples, particularly the rhyolite samples taken near veins and the dacitic dome from Laguna Verde, plot on an adularia + illite alteration trend. This means that the majority of the samples have gained K2O, and possibly lost Na2O and CaO during hydrothermal alteration. The Ishikawa alteration index (Ishikawa et al., 1976; AI= 100 (K2O + MgO)/(K2O + MgO + Na2O +CaO); Fig. 2.20) corresponds to the quantification of the intensity of sericite and chlorite alteration that occurs proximal to Kuroko type deposits. The reactions consist of the breakdown of sodic plagioclase and volcanic glass and their replacement by sericite and chlorite. There is a loss of Na2O (and CaO) and a gain of K2O involved to form sericite, as well as, a loss of K2O and gains in FeO and MgO, for chlorite formation. The latter two reactions are on the basis of constant Al2O3. The index varies from values of 20 to ~60 for unaltered rocks and between 50 and 100 for illite, adularia and chlorite altered rocks; and an AI=100 indicating a complete replacement of feldspar and glass by sericite and/or chlorite (Large et al., 2001). The chlorite-carbonate-pyrite index (CCPI= 100 (MgO +FeO)/(MgO + FeO + Na2O + K2O); FeO= FeO + Fe2O3; Fig. 2.20), has been defined to measure the increase in MgO and FeO    44   associated with hydrothermal Mg-Fe chlorite, that commonly replaces albite, K-feldspar, or sericite in the volcanic rock, leading to a loss of Na2O and K2O. This index is also affected by Mg-Fe carbonate alteration (dolomite, ankerite, or siderite) as well as pyrite, magnetite or hematite enrichments. The most altered samples, with respect to the Ishikawa alteration index (Ishikawa et al., 1976) (A.I. > 80; Fig. 2.20) were collected at Laguna Verde, Pampa la Perra, Guanaco and near the Cascada vein. The sample from the rhyolitic Bahía Jara dome yields an A.I. > 95, that indicates considerable hydrothermal alteration. No veins are observed in the immediate vicinity of this dome but they may be present under Holocene alluvial overburden (De la Cruz and Suárez, 2008). The Laguna Verde dacitic dome possibly plots in this field due to a weak hydrothermal enrichment in K2O content (8.92 wt.%) and a depletion in Na2O (0.11%) and CaO (1.01%), represented by fine-grained illite veinlets; which is consistent with the mineralogy documented in thin section (Fig. 2.11B). The rhyolitic sample JP-169 (285630E/4836297N) from the upper part of the Cerro Torta E stratigraphic column plots in the least altered andesitic/basalt field which can be readily be explained by the carbonate replacement of plagioclase observed in thin section (Fig. 2.20)  2.4.2 Immobile trace element characteristics Most of the rocks analyzed have a rhyodacitic to rhyolitic composition when using the Nb/Y versus Zr/TiO2 diagram of Winchester and Floyd (1977; Fig. 2.21), except for a few samples. Exceptions includ three andesitic lavas taken from the basal levels of the Laguna Verde Sur stratigraphic column (JP-200 and its field duplicate JP-1000; 270149E/4839045N) and Laguna Verde (FCH-312-284); and the domes taken from Laguna Verde (JP-141B; 270352E/4841771N) and Cañadón Verde (JP-112 and JP-112B; 274647E/4842839N) which plot in the trachytic and a trachyandesitic fields, respectively; with Nb/Y > 1.     45   The geochemical analyses show that the Ibáñez Formation in this area has bimodal-like characteristics (Fig. 2.21), with volcanic arc affinities based on the tectonic discrimination diagram Pearce et al. (1984; Fig. 2.22). The domes also show volcanic arc affinities, but those taken from the Laguna Verde area are geochemically different from those taken from Bahía Jara. The Cañadón Verde dome (ca. 83 Ma) has low Y (7.7 and 7.2 ppm) and HREE (Yb = 0.82 and 0.79 ppm) contents (Figs. 2.23 and 2.24) which explains why it plots in the trachyandesite field. The similar aged Laguna Verde dome has a Y (12 ppm) and Yb (1.38 ppm) content which is slightly higher than the Cañadón Verde dome, but still lower than the Y content of the Ibáñez Formation volcanic rocks. The Laguna Verde dome also presents the highest Nb values (28 ppm) whereas the Cañadón Verde dome present the highest Sr concentrations (360 and 363 ppm) and consequently the highest Sr/Y values (46.75 and 50.52) of the entire suite of samples. Chondrite normalized REE pattern are similar for rhyolitic and rhyodacitic fragmental rocks from the Cerro Bayo, the Mallines, the Brillantes and the Laguna Verde areas (Fig. 2.23). These samples have LREE enrichments (La/SmN between 2.80 – 6.30), where the highest La/SmN values correspond the Cerro Torta W area. They also have negative Eu anomalies (Eu/Eu* between 0.45-0.60; except for JP-88 (270568E/4842907N) in Unit 2 at Laguna Verde which has a Eu/Eu* value of 0.75) and only minor HREE fractionation ((Sm/Yb)N between 1.80 – 2.60).  Figure 2.20. Alteration box plot for samples from the Cerro Bayo district (Large et al., 2001). AI = 100 (K2O + MgO)/(K2O + MgO + Na2O +CaO). CCPI = 100 (MgO +FeO)/(MgO + FeO + Na2O + K2O). Alteration minerals are indicated within the diagram as follows: ab = albite, ad = adularia, ank = ankerite, cc = calcite, chl = chlorite, dol = dolomite, ep = epidote, il = illite, py = pyrite     46   Figure 2.21. Compositional Zr/Ti vs. Nb/Y discrimination diagram (Winchester and Floyd, 1977) for rhyolites, rhyodacites and andesites of the Cerro Bayo district, showing almost all samples plotting near the rhyolite-rhyodacite boundary, the rhyolitic domes and the more alkalic domes from the Laguna Verde area. For whole-rock geochemistry see Appendix A4; for whole-rock geochemistry field duplicates see Appendix A5; and for geochemical standard analysis see Appendix A6.  Figure 2.22. Tectonic Nb vs. Y discrimination diagram (Pearce et al., 1984), for the samples taken in the Cerro Bayo district. Samples from the Ibáñez and the El Quemado (Argentinian equivalent for the Ibáñez Formation) formations are plotted for reference.  At Mallines the REE patterns for rhyolitic and rhyodacitic tuffs (Fig. 2.23A) have a wider range of La/YbN ratios (6.14 – 12.61) compared to the other areas. The Cerro Torta W stratigraphic    47   column presents higher values for La/YbN (9.19-12.61) and La/SmN (5.33 – 6.30) ratios. On the other hand, dacitic samples from the highest stratigraphic level at Brillantes (JP- 138B; 280322E/4846302N) and from the basal rhyodacitic levels in Laguna Verde present the lowest values for La/YbN (4.76), probably representing the earliest events of the Ibañez Formation in the Cerro Bayo district. This can be correlated with the dacitic to rhyolitic basal levels of the Ibañez Formation observed south of the district by Parada et al. (1997) and, possibly also with andesitic lavas and tuffs from the Jurassic La Plata Formation (Ramos, 1976) outcropping north of the Cerro Bayo district at 45°S Lat. Both dacitic domes analyzed near Laguna Verde (Fig. 2.23D) present different REE pattern in comparison with the Ibáñez Formation. They exhibit high Eu/Eu* values, especially for the Cañadón Verde dome (0.91). They also present high La/YbN (11.17 for JP-112B and 11.39 for JP-141B) and La/SmN (4.29 for JP-112B and 5.27 for JP-141B) values compared with the other samples, probably reflecting some upper crustal contaminaton and indicating that garnet may have remained in the residuum during partial melting (Murphy, 2007). Figure 2.24 shows the chondrite normalized incompatible trace element patterns with elements plotted from left to right in order of increasing compatibility. The normalizing values are those of Thompson (1982). More mobile LIL elements correspond to Cs, Rb, K, Ba, Sr, Eu; and less mobile HFS elements to Y, Hf, Zr, Ti, Nb, Ta. On the one hand the LIL element concentrations may be a function of the behavior of a fluid phase, while the HFS element concentrations are controlled by the chemistry of the source and the crystal/melt processes which have taken place during the evolution of the rock (Rollinson, 1993). All rhyolitic to rhyodacitic rocks from Cerro Bayo district (Fig. 2.24) have very similar chondrite normalized incompatible trace element patterns, with high LIL concentrations, positive K and Rb anomalies, marked Nb, Sr, P and Ti troughs. The marked K and Rb enrichment may reflect hydrothermal alteration, and the replacement of K-Feldspar, shards and fiammae by K-rich clays. The Rb and Th peaks may indicate that the ascending magma has an important upper crustal contribution; and Nb and Ta troughs may be showing the inclusion of these two elements in residuum clinopyroxene in subduction-zone magma sources. The marked P and Ti depletion is probably controlled by the fractionation of minerals such as apatite and Fe-Ti oxides (ilmenite, magnetite, rutile or sphene) during melt generation (Rollinson, 1993).    48     Figure 2.23. Chondrite normalized rare earth elements (REE) patterns for A. Mallines B. Bahía Jara C. Brillantes and D. Laguna Verde areas.  49     50   Figure 2.24. Chondrite normalized incompatible wlements patterns for, A. Mallines B. Cerro Bayo C. Brillantes and D. Laguna Verde areas.  2.5 Stratigraphic correlations and eruptive history at the Cerro Bayo epithermal district The new geochronological data for the volcanic stratigraphy in the Cerro Bayo district contrains the volcanic activity between ca. 154 to 144 Ma. These ages overlap with published ages for this area (Suárez and De la Cruz, 1997a; and De la Cruz and Suárez, 2008), and are somewhat younger than ages reported from further south in the Tobífera Formation (Figs. 2.1 and 2.25; Pankhurst et al., 2000). All the biotite 40Ar/39Ar geochronology correspond to the higher levels of the Ibáñez Formation in this area (i.e. Unit 4); hence represent the youngest volcanic events prior to the deposition of the Toqui Formation. The Cerro Bayo and other domes were emplaced at ~146 Ma during the deposition of the volcanic fragmental successions of the Ibañez Formation in the Cerro Bayo district along a pronounced N-S trend, and are likely proximal to the source of the Ibáñez Formation in this area. These domes correspond to an Upper Jurassic-Lower Cretaceous magmatic event defined elsewhere in the region (160 – 130 Ma; Parada et al., 2001), and represent part of the siliceous LIP of Patagonia (Pankhurst et al., 2000). Parada et al. (2001) defined two magmatic domains, the northern and southern magmatic domains (NMD and SMD, respectively), in the back-arc region in the Aysén region (between ~45° and ~47°30’S Lat.) of the Patagonian Andes. One of the distinctive geological features of the SMD is the presence of a large metamorphic massif, which is exposed ~50 km to the SW of the study area. The predominantly rhyolitic to rhyodacitic composition of the rocks with slightly higher La/Yb and Sm/Yb ratios than for the NMD, together with the presence of metamorphic lithic fragments and the only scarce andesitic intercalations at the basal levels of the Ibañez Formation, allows correlation of the Cerro Bayo district rocks and the SMD. The origin of the felsic volcanism it is thought to have included a larger crustal contribution with the involvement of the Paleozoic metamorphic basement (Townley, 1996; Parada et al., 1997, 2001). The Laguna Verde and the Cañadón Verde dacitic domes emplacement ages (ca. 83 - 82 Ma) and their incompatible trace element pattern, suggests correlation with the Cretaceous (114 – 75 Ma.) volcanic event defined by Parada et al. (2001) in the Patagonian Cordillera. With these new U-Pb data it is evident that the magmatic history in the district lasted longer than previously thought.    51   The geochemical characteristics of the Cañadón Verde dome are consistent with a garnet and/or hornblende residuum. There are two possible origins for this situation: (1) slab melt; or (2) melt that evolved in the melting, assimilation, storage and homogenization (MASH) zone at the base of a thickened crust (Hildreth and Moorbath, 1988). Thickened crust is unlikely given the low elevation and limited erosion of the Jurassic – Cretaceous veins (Chapter 3) present in this area. Due to the low Y and heavy rare earth elements (HREE) contents, and the high Sr/Y ratios suggestive of an adakite-like composition, the dacites may have originated from partial melting of a subducting young hydrous slab (Drummond and Defant, 1990). This is consistent with the several episodes of seismic ridge subduction that occurred during the last 80 million years in the Patagonian Andes (Ramos, 2005). The Laguna Verde dome shows slightly different geochemical characteristics from the Cañadón Verde, with higher K2O and lower Na2O, CaO and Sr contents; this is probably due to hydrothermal gain and loss of mobile elements. Future detailed study will be necessary to corroborate the origin and context in which these post-mineralization domes occurred.  2.5.1 Correlations with the Chon Aike felsic large igneous province The main formational names of the Chon Aike Large Igneous Province around Patagonia (Fig. 2.1) are the Marifil (felsic) and the LoncoTrapial (andesitic) in northern Patagonian Massif, and the Bajo Pobre (andesitic) and Chon Aike (felsic) formations in the Deseado Massif. In the two massifs the volcanic rocks are predominantly flat-lying and undeformed, and overlie crystalline basement rocks of Precambrian to earliest Jurassic age. In the Andean Cordillera the Ibáñez, the El Quemado and the Tobífera formations are also recognized. These units form a narrow belt parallel to the trend of the Patagonian Andes, extending for > 3,000 km along the proto-Pacific margin, and cropping out in Patagonian South America and also in the Antartic Peninsula (Riley et al., 2001). In contrast to the units of the Northern Patagonian and Deseado Massif, the rocks distributed along the proto-Pacific margin are locally deformed, faulted, tilted and locally strongly affected by hydrothermal alteration. Together, these mostly felsic formations constitute one of the largest felsic magmatic provinces in the world (~ 235,000 km3, Pankhurst et al., 1998) and contain numerous and varied metallic deposit types (Chapter 3).     52   The basic rocks in the Bajo Pobre, Lonco Trapial and El Quemado formations are significantly modified from mantle-derived liquids, and have been suggested to be remelts of mafic lower crust (Storey and Alabaster, 1991; Pankhurst and Rapela, 1995). The Bajo Pobre and Lonco Trapial formations are somewhat more basic in composition (basaltic andesite to andesite) than the mainly rhyolitic formations of the Chon Aike LIP, and these andesitic to basaltic formations may be related to the mantle plume that is thought to have given rise to the flood basalt that were deposited during the early stages of Gondwana break-up (e.g., Karoo mantle plume; Rapela et al., 2005). From Figure 2.25 it is evident that the distribution of Jurassic felsic rocks in Patagonia reflects a migration of the locus of volcanic activity from NE to SW between 187 and 142 Ma, which has been interpreted to be related to the initiation of the Gondwana break-up (Pankhurst et al., 2000). These authors also defined three rhyolitic volcanic episodes: Early Jurassic (V1, 188-178 Ma) with within-plate affinities in northeast Patagonia (Marifil Formation) and the southern Antarctic Peninsula (Mount and Brennecke formations); Middle Jurassic (V2, 172-162 Ma) representing a movement of the focus of eruption to southern Patagonia (Chon Aike Formation) and the northern Antarctic Peninsula (Mapple Formation); and Late Jurassic (V3, 157-153 Ma) representing the Andean outcrops of rhyolitic ignimbrite (El Quemado and Ibáñez formations, and Tobífera Formation) and overlaps with the earliest known intrusions of the Patagonian Batholith, which ranges from 157-145 Ma, and Antarctic Peninsula batholiths. The V1 event is coincident with the peak of the extensive Karoo-Ferrar basaltic volcanism (Storey, 1995), which is thought to be related to one of the precursor mantle plumes for the Gondwana break-up (Rapela et al., 1995). The geochronology presented in Figure 2.25 suggests that the volcanic episodes defined by Pankhurst et al. (2000) are in fact more continuous rather than discrete episodes, because the ages overlap between the different rhyolitic volcanic successions. A separation by tectonic setting is more appropriate than by ages. The Nb vs. Y tectonic discrimination diagram (Fig. 2.27; Pearce et al., 1984) shows compositional fields for the different Jurassic formations at Patagonia. The oldest Marifil and Chon Aike formations have the widest range and highest Nb and Y compositions, plotting in the volcanic arc and within plate tectonic fields. The Ibáñez, El Quemado and Tobífera formations in    53   contrast, present a narrower range and lower Nb and Y compositions, and plot preferentially in the volcanic arc field but straddling the boundary to the ocean ridge field. This latter feature is consistent with the fact that these formations are strongly related to the formation of extensional Jurassic back-arc environments (Parada et al., 2001), like the Rocas Verdes basin (~52°S Lat; Calderón et al., 2007). The geochemical data from the Ibáñez Formation in the Cerro Bayo district plot in the volcanic arc field, as do data for the Patagonian Batholith (Fig. 2.27). The incompatible element spider diagram (Fig. 2.28) shows a slight enrichment in LIL for the Ibáñez Formation compared to the Patagonian Batholith, and both present a negative Nb anomaly, probably due to Nb included in clinopyroxene in subduction-zone magma sources residual magma. These features may suggest that the Patagonian Batholith is genetically related to the Ibáñez Formation (and to the equivalent El Quemado and Tobífera formations, Fig. 2.1), and that both units have a stronger subduction signature compared with age equivalent units further east.  [Ma]  SW  NE  140 145  145.5  Upper  150 155 160  170  Marifil Fm.  Chon Aike Fm.  Bajo Pobre Fm.  El Quemado Fm.  Ibáñez Fm.  190  Tobífera Fm.  185  Lower  180  Patagonian Batolith  175  JURASSIC  Middle  165  161  176  201.6  Figure 2.25. Formations that comprise the Chon Aike large igneous province, plotted against the eruption age. Range of ages taken from Pankhurst et al. (1999 and 2000), Féraud et al. (1999), Calderón et al. (2007), Hervé et al. (2007) and this work.     54   Figure 2.26. Nb/Y vs. Zr/Ti diagram (Winchester and Floyd, 1977) for the studied area and compiled data from different formations of the Chon Aike large felsic province. 1: this study; 2: Pankhurst and Rapela, 1995; 3: Parada et al., 1997; 4: Pankhurst et al., 1999; 5: Parada et al., 2001; 6: Hervé et al., 2007.  Figure 2.27. Y versus Nb diagram from Pearce et al. (1984). The dark grey shaded area corresponds to samples from the Ibáñez Formation at 46°S Lat. (Cerro Bayo district). It is possible to observe that the most andesitic formations (Bajo Pobre and Lonco Trapial) present lower Nb contents than the more differentiated rhyolitic Marifil, Chon Aike and Ibáñez formations. Samples from the Marifil Formation plot in a large area straddling the volcanic arc, the within plate and the anomalous ridge fields. 1: this study; 2: Pankhurst and Rapela, 1995; 3: Pankhurst et al., 1998; 4: Hervé et al., 2007.     55   Figure 2.28. Chondritic normalized spider diagram for incompatible elements. The orange shaded area corresponds to data from the Patagonian Batholith (Hervé, 2007), the grey shaded area corresponds to data from the Ibáñez Formation at ~46°S Lat. (this thesis); comparing with data from Parada et al. (1997, 2001).  Figure 2.29. Rare Earth Elements (REE) spider diagram showing the Patagonian Batholith (Hervé et al., 2007) and the basal levels of the Ibáñez Formation data near the Chacabuco River (Parada et al., 1997 and 2001). The grey shaded area represents the Ibáñez Formation at ~46°S Lat. (this thesis) and the purple shaded represents data from the Marifil and the Chon Aike formations (Pankhurst and Rapela, 1995)     56   3 Epithermal veins: characteristics and chronology of emplacement  The metallogeny of the Aysén region of Chilean Patagonia (between 44°15’ and 47°30’S Lat; Fig. 3.1) is dominated by important Zn-Pb and Ag-Au deposits, which contrasts the Cu dominated central and northern Chile (Townley and Palacios, 1999; Maksaev et al., 2007). The southern Andean segment is different from the central Andes because the Mesozoic volcanic arc in this region is not superimposed on the Paleozoic volcanic arc, and no west to east volcanic arc migration through time is observed (Hervé et al., 2007). Three groups of deposits have been defined by Townley and Palacios (1999) on the basis of geologic and tectonic setting, geographic location, and ore deposit characteristics. From north to south they are called: (1) The El Toqui (44°30’-45°50’S Lat.); (2) The Fachinal (46°05’ 47°05’S Lat.); and (3) The El Faldeo groups (47°05’S Lat. to the south) (Fig. 3.1). These deposits correspond to: epithermal low sulfidation precious metal and pollymetallic veins, massive sulphide veins, and Zn-Pb skarn; depending in which type of host rock these deposits are emplaced. Zn-Pb skarns are typically hosted in coquinoid limestones; marbles and black schists and phyllites; epithermal low sulfidation deposits are hosted by granodioritic and andesitic porphyries of the Patagonian Batholith and dacitic to rhyolitic tuff of the Ibáñez Formation (Fig. 3.1). Fluid inclusion data from the Santa Teresa and Katerfield low-sulfidation epithermal deposits from El Toqui Group, the Quebrada Chica, Río Amarillo and Halcones-Leones deposits from the Fachinal Group, and the El Faldeo deposit from the El Faldeo Group for low-sulfidation epithermal deposits indicate ranges from 1.3 to 5.3 wt.% NaCl eq. and Th of 123° - 321°C, assuming boiling conditions. A depth of formation from 50 to 400 m below paleowater table can therefore be assumed. For the El Toqui Zn – (Au) from El Toqui Group, Mina Silva and Rosillo Zn – Pb from the Fachinal Group, and El Faldeo Zn – Pb – (Au – Ag) from El Faldeo Group skarns deposits the fluid composition ranges from 1 to 20 wt.% NaCl eq. with Th of 160° to 456°C, which indicates an estimated depth of formation from 850 to 1500 m. With these data    57   Townley and Palacios (1999) deduced a mainly meteoric fluid with boiling evidence and magmatic-meteoric fluid origin for the epithermal low sulfidation and Pb-Zn skarn deposits, respectively. The Cerro Bayo low-sulfidation epithermal district is part of the Fachinal Group, which includes four precious metal deposits and prospects (Cerro Bayo, Quebrada Chica, Río Amarillo and Halcones-Leones) of the low-sulfidation type, as well as Mina Silva and Manto Rosillo, which are interpreted to be Zn-Pb and Zn skarns. Most of these deposits are hosted by Upper Jurassic to Lower Cretaceous andesitic to rhyolitic rocks, except for the skarns, which are hosted by rocks of the metamorphic basement. In this chapter, field mapping and compiled fluid inclusion and vein geochemical data, plus new adularia  40  Ar/39Ar ages are discussed. Field observations include surface vein textures,  mineralization, and structural relationships focused on the Mallines area, as well as the Bahía Jara and the Brillantes areas (Fig. 3.2). Ore and gangue parageneses are taken from Townley (1996) and Pizarro (2000), complemented by observations made during this study. Geochemistry of veins and host-rocks were taken from C. Hermosilla (2009, written commun.) and from a Coeur d’Alene Mines internal database; whereas fluid inclusion data were compiled from Townley (1996), Pizarro (2000) and C. Hermosilla (2009, written commun.). Adularia from different subdistricts was dated using the 40Ar/39Ar method to constrain the timing of mineralization in the district.     58   Figure 3.1. Simplified geologic map of the Aysen region, southern Chile, showing the main mines and prospects (modified from Townley et al., 2000). The red square in the map indicates the study area.     59   Figure 3.2. Simplified geological map of the Cerro Bayo district, showing the different sub-areas studied in this thesis. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.  3.1 Structures in the Cerro Bayo district Detailed structural observations were made around major NE striking faults that offset mineralized veins around the Raúl vein, south of Cerro Bayo dome, and north of Laguna Salmonosa (Fig. 3.2). Furthermore, some structural relationships were observed along the N-S striking Cerro Bayo fault where the N-S trending rhyolitic domes were emplaced. Kinematic indicators like slickensides were observed mostly around veins at Bahía Jara. In addition, crosscutting vein relationships were analyzed at Mallines.    60   3.1.1 N-S striking fault south of Cerro Bayo dome All strikes and dips given are on the basis of right hand rule. South of Cerro Bayo dome N-S trending rhyolite dikes similar to the Cerro Bayo dome occur. Here, flow banding with orientation from N140° to N220° and dips from 55°W to 64°E can be observed, but the main flow banding orientation is at 180/69°, parallel to the principal structure; this feature is consistent with the N-S trend of the rhyolitic domes (Fig. 3.3A). Vertical, up to five cm wide, 175° striking quartz veins with fine-grained disseminated euhedral pyrite cut the dikes, indicating that dome and dike emplacement preceded mineralization events at Cerro Bayo area (Fig. 3.3B).  Figure 3.3. A. Flow banding that can be noted along the NS rhyolitic dikes, in the area south of Cerro Bayo dome. B. N355° grey quartz with fine pyrite dissemination cross-cutting the NS rhyolitic dikes.  3.1.2 Kinematic indicators at Mallines Detailed mapping (attached map) reveals that four preferred vein orientations exist in the Mallines area (Fig. 3.4): 150° to 160° with subvertical dips (System 1); 110° to 135° with predominant 110° strike orientation and dipping from subvertical to 60° (System 2); 165° to 180° with vein dips from vertical to 60° (System 3); and W-E to NE (System 4). The cross-cutting relationships observed in the field show that the first mineralization event corresponds to System 1 (with no conclusive kinematic indicators), then a second event occurred with the emplacement of System 2 and the last one is the emplacement of System 3; the latter emplaced in dextral or    61   sinistral strike slip faults with offsets of less than 60 cm. System 3 crosscuts System 4, but no conclusive temporal relationship between System 4 and Systems 1 and 2 has been observed. One point at Mallines is of particular interest (coord: 281800E/4838215N; see attached map). Here a quartz vein, from System 2, has a strike orientation change (Fig. 3.5), from south to north: 140°  100°  130°. In the portion with a 100° strike a dilatational jog is observed, associated with a breccia. This matrix-supported breccia corresponds to a hematized matrix containing subangular silicified quartz vein fragments. The shape of this jog indicates a sinistral movement at the time of emplacement. The other point where kinematic indicators have been observed is at coord. 281858E/4837752N, where a 30 cm wide 110° striking vein (System 2) with calcedonic quartz and locally with striae can be observed. To the N, this vein is hydrothermally brecciated, attaining a width of 1.5 m, where the clasts correspond to calcedonic quartz in a milky quartz cement. In the vein, dextral post-mineralization faults sub-parallel to the vein can be observed. The brecciated area has a jog shape, suggesting a sinistral strike-slip structural regime (Fig. 3.6) at the time of emplacement.     62   Figure 3.4. Dip/Dip Direction equal area stereographic and rose plots for veins from A) System 1, B) System 2, C) System 3 and D) Sytem 4 for the Mallines area.     63   Figure 3.5. Extensional sinistral jog for System 2 in the Mallines area (281800E/4838215N). A. Field photograph figure. B. Schematic sketch of the structural relationship.  Figure 3.6. Extensional sinistral jog for System 2 in the Malines area (2818858E/4837525N) (Plan view). Sample JP221 (see Chapter 4) was taken from this vein.     64   3.1.3 Kinematic indicators at Bahía Jara Striae on vein walls of the Guanaco System, especially for Guanaco II and Guanaco III veins; and around the Cerro Bayo dome, at Lucero, Celia and Rosario veins were oberved. These veins are characterized by strikes of 115° to 170° with vertical to 65° dips, for veins near the Cerro Bayo dome and from the Guanaco block, respectively (Fig. 3.2). Mainly normal (Guanaco II vein) to normal-sinistral (all other veins mentioned above) movements were noted, possibly these veins are related to the N-S striking Cerro Bayo fault (Fig. 3.3).  3.1.4 NE striking major faults At Raúl and one km SSW of the Cerro Bayo dome a major subvertical 030° to 080° striking fault, which can be followed over a strike length of ~2 km, is exposed. Fault breccias and striae can be observed. In the southern block striae with a plunge/trend of 018°/230° were measured, and millimetric quartz veins (320°/60°) dextrally offset by the main fault are present. The striae indicate a post-mineralization sinistral movement, with a normal component (Fig. 3.7). At Laguna Salmonosa, a major 70° to 90° striking fault is present. Striae with plunge/trend of 020°/087° indicate a largely sinistral movement with a component of normal offset of the southern block. At this location an ash tuff with K-feldspar fragments associated with hematite and carbonates crops out, as well as centimeter scale 115° to 125° striking milky quartz veins. Volcanosedimentary intercalations with a locally developed cleavage, possibly related to fault movement, can be observed as well (Fig. 3.7).     65   Figure 3.7. Map that shows where the structural work was focused (i.e., around the Raúl, SW of Cerro Bayo dome and the Laguna Salmonosa areas). Modified from Williams (2003) and from Coeur d’Alene Mines. See Chapter 2 for description of the volcanic units. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.     66   3.2 Mineralization in the Laguna Verde area Pizarro (2000) characterized the ore, gangue and alteration mineralogy around Temer Sur, Cóndor I, Taitao I and Cristal III veins, that are located in the western part of the Laguna Verde area (Fig. 3.8). These veins mostly have a NS to NNW-SSE strike and crop out in different structural blocks. In the Temer Sur vein samples from three different levels were taken (330, 305 and 280 m a.s.l.) where variable amounts of pyrite (1 - 10% of total rock) at all levels were observed. Silver sulfosalts (proustite/pyrargirite), were documented on in level 330. The gangue mineralogy is composed of white sacaroidal quartz, with subordinate opaline grey quartz and scarce calcite. The Temer I vein (Fig. 3.8) was sampled on three levels (410, 370 and 270 m a.s.l.) by Townley (1996). At the 410 level metallic mineralization consists of fine disseminated native silver and gold (<20 µm), with presence of limonite as coatings and fillings. At the 370 level, euhedral to subhedral disseminated pyrite as well as in veinlets is present (partially replaced by hematite). Anhedral sphalerite overgrowing pyrite and showing chalcopyrite and minor tetrahedrite inclusions are observed. Anhedral to subhedral native silver and gold is present in quartz grains (<5 µm). At the 270 level mineralization consists of disseminated anhedral to subhedral pyrite, partially altered to hematite and /or limonite. Native silver and gold is rare, occurring as fine anhedral to subhedral disseminated grains in quartz (<5 µm). For the Cóndor I vein, four different levels (440, 354, 305 and 265 m a.s.l.) were investigated (Townley, 1996; Pizarro, 2000) and on all levels disseminated pyrite (3 – 5% of total rock), silver sulfosalts and electrum were observed. At level 440, hematite pseudomorphous after pyrite, as well as, fine disseminated native silver and gold in quartz (<10 µm) are observed. At level 305 pyrite intergrown with and overgrown by sphalerite, chalcopyrite and tetrahedrite are observed. Tetrahedrite overgrowing quartz, as well as chalcopyrite as inclusions in sphalerite along fractures or as disseminated anhedral grains in quartz are reported. At level 265 the sulfur content is high (up to 30%), and galena, sphalerite and silver sulfosalts are abundant. Gangue mineralogy consists of fine-grained white sacaroidal quartz.     67   From the Taitao I Sur vein, Pizarro (2000) sampled two levels (354 and 305 m a.s.l.), pyrite (3 – 5%) was recognized in both levels, and in the 305 level silver sulfosalts, electrum, argentite, chalcopyrite, arsenopyrite and tetrahedrite forms the sulfide assemblage. Gangue corresponds to white sacaroidal and opaline quartz, as well as calcite and fluorite. Townley (1996) documented the mineralization of the Taitao II vein (Fig. 3.8) at level 415 m a.s.l., where he recognized subhedral to anhedral pyrite (in veinlets, clusters and disseminated); anhedral disseminated chalcopyrite (<30 µm); and anhedral disseminated grains of sphalerite with chalcopyrite inclusions. Fine disseminated native gold and silver in quartz (<20 µm) was also reported (Townley, 1996). Samples at levels 430 and 390 m a.s.l. of the Cristal III vein were taken by Pizarro (2000), and pyrite was observed in both. In the upper level, hematite occurs in pyrite fractures and grain boundaries, as well as gold and silver in fractures. Sphalerite occurs in the deeper level. Gangue mineralogy in both levels consists of white sacaroidal quartz and calcite. Townley (1996) investigated a sample from the Cristal vein at 480 m a.s.l. and abundant limonite and/or hematite as coatings, stainings, fillings, and pseudomorphic replacement of pyrite was observed. At this level native silver, gold and electrum occur as finely disseminated anhedral and subhedral grains (<40 µm) in quartz. Gold was also observed as microfracture fillings possibly associated to oxidixed pyrite (boxwork). Townley (1996) investigated a sample from the Caiquenes vein at 530 m a.s.l. This vein shows limonite and hematite as fillings, coatings, stainings and pseudomorphic replacement of pyrite. Rare fine anhedral to subhedral native silver, gold and electrum grains (<5 µm) are found disseminated in quartz. At the Coihues este area (Fig. 3.3) N300° to N325° striking veins present grey, crustiform, calcedonic, drusy and sacaroidal textures. Locally hydrothermal breccia, with angular calcedonic quartz fragments (< 1 cm), in an iron oxide rich matrix with specular hematite and disseminated fine pyrite and silver sulfosalts is present.     68   Figure 3.8. A. Cerro Bayo distrital map that shows the veins characterized by Pizarro (2000), Townley (1996) and this study (same legend as Fig. 3.2). B. Inset black square corresponds to the characterized veins at Laguna Verde. Veins with asterisk correspond to those mentioned in Table 1.2.     69   In the Caiquenes system (Fig. 3.8) a vein with a N290° strike displays drusy, milky, crustiform, brecciated and locally lattice textures. The clast-suported breccia domain contains sub-rounded milky quartz clasts, with hematitized matrix. K-feldspar staining indicates that adularia is part of the gangue mineralogy.  Figure 3.9. Microphotographs from the Fabiola Vein at the Laguna Verde area. White boxes width is 0.125 mm. 3A. Massive calcedonic quartz with disseminated opaque crystals where reddish internal reflections of proustitepyrargyrite are observed. 3B. Disseminated anhedral to subhedral pyrite with proustite-pyrargirite intergrwon are observed.  3.3 Mineralization in the Bahía Jara area The Bahía Jara area corresponds to the east-central part of the district. It includes the area between the Raúl vein to the south and the Laguna Salmonosa to the north, as well as between from the Cerro Bayo dome to the Guanaco block (Fig. 3.2). Townley (1996) sampled the Guanaco I vein (Fig. 3.8) on three different vertical levels (410, 360 and 320 m a.s.l.). On level 410 mineralization consists of disseminated pyrite, pyrite boxwork, hematite and jarosite; with lesser disseminated fine-grained anhedral native silver (<15 µm) and gold (<20 µm). Tetrahedrite as pyrite overgrowth was also observed on this level. On level 360 disseminated pyrite, overgrown and intergrown with sphalerite is oberserved. Sphalerite is common as anhedral disseminated fine grains, with chalcopyrite intergrowths. Disseminated anhedral bornite, as well as bornite replacing sphalerite occurs in fractures; bornite is partly replaced by covellite along cleavage planes. Arsenopyrite is rare, only present as subhedral     70   disseminated grains, while native silver and gold are present as anhedral and subhedral grains, respectively (<20 µm). On level 320, mineralization consists of abundant disseminated subhedral pyrite, also present along crystal interstices and fractures in quartz, and in some cases overgrown by anhedral sphalerite. Sphalerite is also present as anhedral disseminated grains, in some cases having chalcopyrite inclusions or being partially replaced by chalcopyrite along fractures. Disseminated anhedral to subhedral bornite grains partially replaced, along fractures, by covellite is also observed. Fine disseminated anhedral and subhedral native silver and gold grains are documented as well.  3.3.1 Vein textures Immediately W from the Cerro Bayo dome the quartz veins present strike orientations from 315° to 330° and steep NE dips. The veins contain calcedonic, drusy and minor grey quartz. Locally, quartz intergrown with barite and quartz replacing tabular calcite (lattice texture) can be observed. In the Guanaco block (Fig. 3.8) vein orientations range from 135° to 150° with steep SW to W dips, and they present grey, sacaroidal and crustiform textures. In ore shoots sacaroidal quartz and locally hydrothermal breccia, where the clasts correspond to calcedonic quartz and the matrix to grey quartz, pyrite and silver sulfosalts, are the dominant textures.  3.4 Mineralization in the Brillantes area The Brillantes area is located in the northern part of the district (Fig. 3.2) and contains, from east to west, the Constanza, the Roberta, the Francisca and the Brillantes veins. The Constanza vein (Fig. 3.3) at surface consists of quartz, galena and copper carbonates (malachite and azurite).     71   The Roberta vein (Fig. 3.8) strikes at 190° and dips 75°, with an average width of 20 cm. At surface (500 m a.s.l.) the mineralization consists mainly of fine-grained disseminated subhedral pyrite crystals associated to chalcedonic quartz and adularia (Fig. 3.10)  Figure 3.10. Microphotograph of of a colloform band of the Roberta Vein. White bar length is 0.25 mm. A. Massive chalcedonic quartz associated with adularia. B. Pyrite with iron oxide rims associated to adularia.  Figure 3.11. Roberta vein where crustiform texture is observed, suggesting fluid boiling during mineral depositation (Dong et al., 1995). The first stage of crustiform quartz deposition, is followed by a second stage of drusy quartz and post-mineralization dextral fault displacement (in blue dashed line).     72   3.4.1 Vein textures Crustiform growth textures with quartz and minor adularia at the Roberta vein (Fig. 3.11) suggest periodic fluid boiling and multiple mineralizing events in the Brillantes area. Drusy white quartz cross-cuts the crustiform textures, suggesting a younger hydrothermal event after typical epithermal mineralization in this vein.  3.5 Mineralization in the Mallines area Townley (1996) sampled the Madre and Segunda veins (Fig. 3.8), at the 920 and 860 m a.s.l. levels. The Madre vein has highly corroded euhedral and subhedral disseminated pyrite and lesser arsenopyrite, overgrown by proustite-tetrahedrite. The sulfides are partially to completely replaced by hematite and/or limonite. Fine (<15 µm) disseminated native gold in quartz, spatially associated with proustite-tetrahedrite is observed. The Segunda vein has subhedral disseminated pyrite, partially to completely oxidized to hematite and/or limonite. Fine disseminated anhedral grains of native silver (<50 µm) and lesser freibergite, intergrown with silver, are also observed.  3.5.1 Vein textures The veins emplaced in System 1 have widths from 0.5 to 1 m. These veins present mainly sacaroidal, drusy, and lesser grey quartz with minor lattice texture of quartz replacing calcite. The hydrothermal alteration adjacent to the veins corresponds to silicification with illite, smectite and minor kaolinite. At surface it is possible to observe hematite, jarosite and minor goethite, associated with drusy quartz. Locally, chlorite accompanies drusy quartz and regular boxwork (some filled with hematite) and unidentified fine-grained sulfides. System 2 has vein widths up to ~1 m. Chalcedonic, sacaroidal and minor drusy and grey quartz, and locally crustiform textures are present in these veins. The alteration minerals are mainly illite, and minor kaolinite and silicification. Iron oxides and limonites are present and they occur as hematite (associated with calcedonic, and drusy quartz) jarosite (with grey and sacaroidal quartz) and minor goethite.    73   System 3 (Fig. 3.12) veins are from 0.2 to 10 m wide, with the Veta Madre being the widest and containing a variety of quartz textures: sacaroidal, calcedonic, drusy, bladed, crustiform, grey quartz and minor brecciation. The hydrothermal alteration around the veins consists mostly of silicification accompanied by illite, smectite and minor kaolinite. Weak mineralization within the veins corresponds to fine-grained pyrite dissemination associated with grey and bladed quartz. Fine-grained regular boxwork, hematite, goethite (the latter associated with drusy and sacaroidal quartz, and minor bladed quartz) and jarosite (associated with chalcedonic, drusy and grey quartz, and filling boxworks) are present.  Figure 3.12. Photograph that shows a N110° (System 2, in red) striking vein that is cut by a N165° (System 3, in blue) vein at Mallines. It is possible to observe the dextral offset along the structure in which vein System 3 vein system was emplaced. Vein System 2 has been offset by ~60 cms. Inset at upper right shows sample JP-233 taken from System 3 before and after potassium staining. The presence of adularia is indicated by the yellow stain. 40 Ar/39Ar geochronology results for this sample are presented below.     74   3.6 Compiled geochemical data Data from Coeur d’Alene Mines and from C. Hermosilla (2009, written commun.) were used to established the geochemical variability among veins from different areas at the Cerro Bayo district. The elements analyzed were Ag, Au, As, Cu, Hg, Mo, Pb, Sb, and Zn; from the Bahía Jara, Laguna Verde, Mallines and Brillantes areas (Fig. 3.2). The samples were analyzed at the ALS Chemex Laboratories in La Serena, Chile, by the MEICP41 analytical package (Inductively Coupled Plasma-Atomic Emission Spectrometry (ICPAES) following Aqua Regia Digestion). This geochemical package does not include Au, and Ag was not analyzed for drill-core samples from the Bahía Jara area by the ME-ICP41 method. Au and Ag analyses performed at the Cerro Bayo internal laboratories were therefore considered (Appendix A5). The geochemistry of individual areas are summarized in Table 3.1. Data for individual veins are presented in Appendix A6.  3.6.1 Element concentrations and element correlation analyses The geochemistry from vein drill-core samples indicates that the highest concentrations for almost all elements analyzed except As and Mo occur in the Bahía Jara area (Fig. 3.13). The Pearson correlation factor from vein drill-core samples in the Bahía Jara area between Ag-Au is 0.94; Au-Cu is 0.95; and Au-Mo is 0.89 (Table 3.2). Considering vein drill-core and vein surface samples the Pearson correlation factor between Ag-Cu, Ag-Sb and Cu-Sb is 0.95, 0.97 and 0.96, respectively (Table 3.4), consistent with the presence of freibergite ((Ag,Cu,Fe)12(Sb,As)4S13) and pyrargyrite (Ag3SbS3).     75   Table 3.1. Mean, minimum and maximum concentration values of ore forming elements from the Cerro bayo district, compiled from C. Hermosilla (2009, written commun.) and from Coeur d’Alenes mines internal databases.     76   Table 3.2. Pearson correlation matrix between the analyzed elements for vein drill-core samples.     77   Table 3.3. Pearson correlation matrix between the analyzed elements for vein surface samples.     78   Table 3.4. Pearson correlation matrix between the analyzed elements for all the analyzed samples.     79   Figure 3.13. Bar graphs showing the ore elements concentrations analyzed from different areas. Only surface samples were taken from the Brillantes area.     80   Figure 3.13. Cont.  Average Mo concentrations for vein samples are similar between the different studied areas (Fig. 3.13), and lie between 4.43 and 6.26 ppm. However, drill-core vein samples from the Laguna Verde area have elevated Mo concentrations with an average of 22.36 ppm. The Pearson correlation factor between Pb-Zn is 0.86, showing affinities for base metal mineralization in this area (Table 3.2). The Pearson correlation factor for surface vein samples between Cu-Pb, Cu-Sb and Cu-Zn are 0.72, 0.78 and 0.74 respectively, confirming important base metal affinities for this area (Table 3.3) The Mallines area shows the highest As values of the entire district (Table 3.1). Vein samples have As concentration values from 4 to 7910 ppm, and mean values of 743.56 and 158.54 ppm,    81   for vein drill-core and vein surface samples, respectively. The Pearson correlation factor between As-Sb is 0.92 considering vein drill-core samples (Table 3.2), which may be showing the solid solution proustite-pyrargirite (Ag3AsS3 – Ag3SbS3) in the silver sulfosalts mineralization in this area (Townley, 1996). Surface vein samples in the Brillantes area (Brillantes III vein), present the highest mean concentrations for Mo (6.17 ppm), and a poor correlation between Ag-Pb (-0.3) and Ag-Zn (-0.1) (Fig. 3.14; Table 3.3). The Pearson correlation factor for the Brillantes area between Cu-Zn is 0.76.  Figure 3.14. Ag vs. Zn bivariant diagram showing the negative corelation in the Brillantes III vein for these elements at the Brillantes area  3.6.2 Element ratio analyses Variability in element concentrations between different areas are evident especially for Ag, As, Cu, Hg, Mo and Zn (Table 3.1). This makes it possible to geochemically distinguish between different veins using elements ratios. A summary of the analyzed ratios is presented in Table 3.5.     82   The vein drill-core samples for the Bahía Jara area exhibit the lowest Au/Ag ratios (0.002 to 0.1), but the highest Ag concentrations (3 to 11824 ppm); whereas the highest Au/Ag ratios characterize the samples collected from surface at Mallines (up to 1.155; Fig. 3.15), where 75% of the samples have Au/Ag ratios over 0.03. Arsenic is one of the elements with highest variability between the different areas (Table 3.1), with higher concentrations at Mallines (4 to 7910 ppm) than in the other areas (1 to 1158 ppm at Bahía Jara; 8 to 234 ppm at Laguna Verde; and 2 to 84 ppm at Brillantes). The As concentration is inversely proportional to Ag and Cu (Fig. 3.13). Thus, the As/Ag vs As/Cu bivariant plot differentiate well between different areas (Fig. 3.16). The veins at Bahía Jara occupy the bottom left part of the plot, and the veins from Mallines plot in the upper right corner of the diagram. Figure 3.16 also shows that Laguna Verde and Brillantes vein samples plot in similar fields, with samples from Brillantes having a slightly lower As/Cu ratios than those from Laguna Verde, but similar Cu concentrations. This difference, hence, is attributed to the overall low As content of the Brillantes area veins. The Mo/Ag ratio it is also a good discriminator between the different areas. The highest ratios are present at Mallines (0.012 to 55, with a mean of 5.7), and the lowest ones at the Bahía Jara area (0.001 to 41.94, with mean 0.87). Brillantes and Laguna Verde present a similar range of Mo/Ag ratios values but with Brillantes having a higher mean value (3.9) than Laguna Verde (0.8) for surface vein samples. Thus, a relatively good correlation between Mo/Ag and As/Ag is also evident (Fig. 3.17). Veins at Brillantes have similar Mo/Ag values compared to surface vein samples from the Bahía Jara area. Bahía Jara vein drill-core samples present the highest Hg/Ag ratios, compared to all other samples (Fig 3.18). Laguna Verde, Mallines vein drill-core and surface samples, and Bahía Jara vein surface samples plot in similar areas in the As/Ag vs Hg/Ag diagram (Fig 3.18), compared to the veins from the Bahía Jara area.     83   Table 3.5. Mean, minimum and maximum values of the different element ratios analyzed from different veins around the Cerro Bayo district. compiled from C. Hermosilla (2009, written commun.) and from Coeur d’Alenes mines internal databases     84   Figure 3.15. As / Ag vs Au / Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples  Figure 3.16. As/Ag vs As/Cu bivariant plot. A. Drill core vein samples. B. Surface vein samples     85   Figure 3.17. As/Ag vs Mo/Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples  Figure 3.18. As/Ag vs Hg/Ag bivariant plot. A. Drill core vein samples. B. Surface vein samples     86   3.6.3 Discussion of geochemistry Ratios between certain elements allows to discriminate between the different study areas. The Bahía Jara area has the lowest As/Cu, As/Ag, Mo/Ag, Pb/Ag, Zn/Ag, and the highest Hg/Ag ratios, particularly if only the vein drill-core samples are considered. This contrasts Mallines which has the highest As/Ag, As/Cu, Mo/Ag ratios and the lowest Hg/Ag ratio. The Mallines veins show the highest As concentrations of the entire district and with the relative highest Au/Ag, As/Ag, As/Cu and Mo/Ag. They also present the lowest Cu, Pb, Zn concentrations of the analyzed samples, with the best Pearson correlation factor between Au and Hg (0.96) for vein drill-core samples. This is consistent with the surface outcrops of the veins representing relative shallow levels of exposure within the epithermal low-sulfidation deposit model (Buchanan, 1981), thus higher ore metal content may be present at depth. In Bahía Jara area the highest mean concentrations for Ag, Au, Cu, Hg and Mo, and the lowest As concentrations occur in vein drill-core samples; and Ag has good correlation (> 0.84) with Au, Cu, Hg, Mo and Sb. Further the highest Hg/Ag ratio and the lowest Au/Ag, As/Ag, Cu/Ag, Mo/Ag, Pb/Ag and Zn/Ag ratios are present for these samples, basically because of the highest Ag concentrations of the entire district. On the other hand, vein surface samples present the highest Au, Hg and Sb and the lowest Mo concentrations of all the analyzed samples, with the lowest Pb/Ag ratio. The above is showing that Ag mineralization in Bahía Jara is associated to Au, Cu and Mo and that high Hg concentrations may be a good a pathfinder towards mineralized veins. The Brillantes III vein in the Brillantes area yields the highest mean Mo and the lowest Ag and Au concentrations at surface, and negative correlations between Ag and Cu, Ag and Pb, and Ag and Zn, with the highest correlation between Cu and Zn. The Cu/Ag, Pb/Ag and Zn/Ag ratios are the highest of the surface vein analyzed samples. These features are possibly indicating that in depth Brillantes is more enriched in base metal, in comparison to the Bahía Jara and Mallines areas. Furthermore, at Brillantes, the high Pearson correlation factor (>0.87) between Au and As and As and S show that Au, if it is present, occur as microinclusions or as part of the crystal lattice in arsenian pyrite (Reich et al., 2005). Based on the general chemical signature, a relatively deep level of exposure is inferred for the Brillantes area.    87   The Laguna Verde area contain the highest mean concentration for Ag, Cu, Pb and Zn for surface vein samples, the lowest As/Ag, Cu/Ag, Mo/Ag and Zn/Ag ratios and a relative good (>0.72) correlation coefficient between Cu and Pb, Cu and Sb, and Cu and Zn. At depth, these veins have the highest mean Mo concentrations and increasing Cu, Pb and Zn; and decreasing Ag concentrations, the lowest mean Au/Ag ratio and the highest Pb/Ag considering vein drill-core samples; with relative good correlation factor (>0.81) between Pb and Hg, Zn and Hg and Pb and Zn. These features may be showing a potential for polymetallic veins mineralization in depth for this area.  3.7 Fluid inclusion data compilation Fluid inclusion data from Townley (1996), Pizarro (2000), and C. Hermosilla (2009, written commun.) from different veins around the district were used to characterize the hydrothermal fluid in terms of homogenization temperature and salinity; with the aim of finding key indicators to constrain the environment, depth of formation and ore fluids involved in the mineralization processes for the different areas and veins. Townley (1996) reported fluid inclusion measurements for thirty samples from the Cerro Bayo district from different subareas (Fig. 3.19). From the Mallines area, the Madre and Segunda veins yielded homogenization temperatures (Th) between 200° and 300°C (average of 236°C); and 160° and 300°C (average of 223°C), respectively. Salinity ranges from 0 to 4 wt.% NaCl eq., with an average of 1.43 wt.% NaCl eq. for the Madre vein and from 1 to 3 wt.% NaCl eq., with an average of 2.15 wt.% NaCl for the Segunda vein (Table 3.6). North of Mallines, in the Bahia Jara area the Guanaco I vein shows a Th range between 130° and 300°C, with an average of 209°C. Salinity ranges from 0 to 6 wt.% NaCl eq., with an average of 1.74 wt.% NaCl eq. Further west, in the Laguna Verde area four vein system and a breccia zone were analyzed for fluid inclusions. These are: (1) Cristal vein: with Th range between 180° and 300°C, with an average of 235°C; and salinity range between 0 and 6 wt.% NaCl eq., with an average of 2.26 wt.% NaCl eq; (2) Taitao 2 vein: with Th range between 290° and 360°C, with an average of 321°C; and salinities ranges between 2 and 3 wt.% NaCl eq., with an average of    88   2.25 wt.% NaCl eq; (3) Condor I vein: where in the upper level temperatures range between 100° and 150°C (average of 123°C). In the lower level temperatures ranges between 150° and 350°C, with two data clusters, one between 160° and 170°C and another between 210° and 220°C. The average Th value is 221°C. Salinities are similar for both levels, the upper having a range between 0 and 4 wt.% NaCl eq., and the lower between 0 and 5 wt.% NaCl eq; (4) Temer 1 vein: shows temperature ranges between 120° and 280°C. An upper level has an average Th of 239°C, while the lower level has an average temperature of 187°C. The salinity ranges between 0 and 10 wt.% NaCl eq., with 3.98 wt.% NaCl eq. on average (with most data between 1 and 3 wt.% NaCl eq.); (5) Breccia zone: shows Th ranges between 200° and 270°C, with an average of 230°C; and salinity range between 1 and 7 wt.% NaCl eq., with an average of 3.26 wt.% NaCl eq. (Townley, 1996). Townley (1996) did not find petrographic evidence for boiling, but temperature and salinity variations suggest that boiling was possible. Pizarro (2000) reported fluid inclusion measurements for twenty samples from the Cerro Bayo district in the Laguna Verde subdistrict (Fig 3.19, inset figure). For the Temer sur vein, two samples taken at different depth (305 and 280 m a.s.l.) were analyzed. For the lower level the temperature ranges from 118° to 318°C (average of 232°C), with two temperature groups: one between 160° and 220°C (average of 187°C), and another between 280° and 330°C (average of 297°C). Salinity ranges between 0.30 and 8.76 wt.% NaCl eq., with an average of 2.7 wt.% NaCl eq. A group between 0.30 and 1.32 wt.% NaCl eq. (with an average of 0.95 wt.% NaCl eq.) was documented. The upper level sample shows a Th range between 105° and 162°C (with an average of 129°C), and a marked group between 120° and 170°C (with an average of 162°C), and salinity range between 0.13 and 5.12 wt.% NaCl eq (average of 3.16 wt.% NaCl eq.). Pizarro (2000) also reported fluid inclusion data for the Cóndor I and Taitao 1 veins (also from the Laguna Verde subdistrict). For the Cóndor I vein Th ranges between 290° and 362°C, with two main groups: one between 290° and 320°C and the other one between 330° and 360°C. Both with an average of 327°C. Salinities ranges between 0.13 and 3.31 wt.% NaCl eq., with an average of 1.22 wt.% NaCl eq. For Taitao I vein the temperature ranges between 176° and 361°C, with a main cluster between 280° and 350°C, and an average of 306°C for the latter range. Salinities vary from 0.13 and 6.74 wt.% NaCl eq, with an average of 2.49 wt.% NaCl eq. The    89   salinities for the main cluster are between 0.13 and 0.81 wt.% NaCl eq, with an average of 0.62 wt.% NaCl eq. From comparing the range of fluid inclusion data from throughout the district it is evident that Laguna Verde area veins present the widest range of Th (123° to 327°C) and the highest average values for salinities (1.22 to 4.29 wt.% NaCl eq) (Fig. 3.20). The Bahía Jara veins present a narrow range of Th (196.6°C to 234°C), with medium salinity (0.79 to 3.6 wt.% NaCl eq.). Veins from the Brillantes and the Mallines areas present low salinity (0.1 to 0.35 wt.% NaCl eq. for Brillantes and 0 to 2.31 wt.% NaCl eq. for Mallines), but Mallines present varible Th (165.6° to 296.8°C) compared to Brillantes (205.7° to 212.9°C).  Figure 3.19. Map showing the veins that were analyzed by fluid inclusions thermometry in the Cerro Bayo district. Inset figure is an enlargement of the black rectangle for samples analyzed at the Laguna Verde area. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.     90   Table 3.6. Compiled fluid Inclusion data.     91   Figure 3.20. Average salinity vs. Th diagram for samples from different areas of the Cerro Bayo district. Data obtained from Townley (1996), Pizarro (2000), and Hermosilla (2009, written commun.).  3.7.1 Discussion of fluid inclusion data Veins at Mallines yield Th (165.6°C to 237°C) and salinities (0 to 2.31 wt.% NaCl eq.) compatible to a mix between low salinity boiling and pure boiling trend evolution (Wilkinson, 2001). Veins from the Bahía Jara area (including Brillantes III vein from the Brillantes area) present boiling fluid evolution with a narrow range of Th (196.6°C to 234°C) and the widest range of salinities (0.1 to 3.6 wt.% NaCl eq.); whereas the veins from the Laguna Verde area present a wide range of Th (123°C to 327°C), and overall higher salinities (1.22 to 4.29 wt.% NaCl eq.) compared to the veins from other areas. This may be indicating a transition from an epithermal to mesothermal environment for the Laguna Verde area. The results for Brillantes III show Th slightly over 200°C with low salinity (less than 0.5 wt.% NaCl eq.), consistent with an epithermal environment. The presence of drusy quartz textures along with the higher Mo and low Ag and Au contents in this area, allows to think that more fluid inclusions analysis can give a better constraint of the Th and salinity at this area.     92   The fluid inclusion data also show that for veins sampled at different depths (e.g. Temer Sur, Cóndor I, Guanaco I and Madre veins), Th and salinity is variable with depth, indicating that hydrothermal fluids mixing with different quantities of meteoric waters may have been an important ore deposition mechanism. Another, important deposition mechanism that occurred in the district is boiling, evidenced by the presence of crustiform textures (e.g. Roberta vein, Guanaco I veins), local hydrothermal breccia (e.g. Taitao II and Mallines System 2 veins), bladed quartz (Madre vein) and adularia (e.g. Guanaco I and Roberta veins).  3.8 40Ar/39Ar adularia geochronology Age constraints available previous to this study are generally ambiguous. Most previous results are based on the K-Ar method and analytical data are generally not available. De la Cruz and Suárez (2008) reported adularia  40  Ar/39Ar and K-Ar ages for different areas in the Cerro Bayo  district (Table 3.7). For the Mallines area they obtained a K-Ar age of 156 ± 5 Ma and a 40  Ar/39Ar age of 144.2 ± 1.0 Ma which is slightly younger than the Cerro Lápiz zircon U-Pb age  (146.3 ± 0.2 Ma), presented in Chapter 2. Four veins from the Bahía Jara area have been dated by these authors. For the Guanaco block, one K-Ar age of 145 ± 5 Ma and one 40Ar/39Ar age of 137.0 ± 1.4 Ma were reported. Approximately 1 km NW from the Cerro Bayo dome, one K-Ar age of 128 ± 9 Ma and one 40Ar/39Ar age of 128.4 ± 2.6 Ma were reported for the same adularia sample.  Table 3.7. K-Ar and 40Ar/39Ar adularia ages.     93   For the Laguna Verde area, two K-Ar ages are available from the literature. Tippet et al. (1991) reported a sericite age of 114 ± 3 Ma; and Townley (1996) gave a sericitized whole-rock age of 113 ± 2 Ma. These ages are interpreted as the timing of alteration. There are no age spectra available for any of the 40Ar/39Ar analyses, and no analytical data for the K-Ar ages mentioned above are published; it is not known if the  40  Ar/39Ar ages correspond to  plateau, correlation or total gas ages. Furthermore, especially for the K-Ar analyses, the errors are large which makes comparing the mineralization ages between different areas challenging. To corroborate the ages and to try to understand whether the large range of ages reported by Tippet et al. (1991), Townley (1996) and De la Cruz and Suárez (2008) is real, seven adularia samples collected from different veins in the Cerro Bayo district were dated by the 40Ar/39Ar method. All samples represent adularia grown within the vein and are not a product of wall rock alteration. Thus, the ages below represent vein emplacement rather than alteration or mixed ages between host-rock feldspar and hydrothermal adularia. 3.8.1 Methodology The samples were crushed in a ring mill, washed in distilled water and ethanol, and sieved when dry to -40+60 mesh. Appropriate mineral grains were picked out of the bulk fraction. Analytical procedures are the same as described in Chapter 2 for the biotite 40Ar/39Ar geochronology.  3.8.2 Results Initial data entry and calculations were carried out using the software ArArCalc (Koppers, 2002). The plateau and correlation ages were calculated using Isoplot ver.3.09 (Ludwig, 2003). Errors are quoted at the 2-sigma (95% confidence) level and are propagated from all sources except mass spectrometer sensitivity and age of the flux monitor. The best statistically-justified plateau and plateau age were picked based on the following default criteria used by Isoplot software (Ludwig, 2003): (1) three or more consecutive steps comprising more than 60% of the 39Ar; (2) probability of fit of the weighted mean age greater than 5%; (3) slope of the error-weighted line through the plateau ages equals zero at 5% confidence; (4) ages of the two outermost steps on a plateau are not significantly different from the weighted-mean plateau age (at 1.8σ, six or more    94   steps only); and (5) outermost two steps on either side of a plateau must not have nonzero slopes with the same sign (at 1.8σ, nine or more steps only). Table 3.8. New adularia 40Ar/39Ar ages from the Cerro Bayo district, Chile (given in Ma). Plateau criteria include number of steps and % 39Ar. Ages in bold represent plateau according to the Isoplot default criteria.  Out of the seven samples, three yielded reliable age plateaus (which are usually the preferred age) according to the rigorous criteria the isoplot software software uses. The age spectra for all samples present a plateau which can be visually defined, even if it does not strictly fit the Isoplot criteria. Thus, user defined plateau ages containing less than 60% of 39Ar have been defined for those samples for which Isoplot did not calculate a plateau age by default. All ages are presented in Table 3.8 and Figs. 3.21 to 3.25 and for analytical data see Appendix A10.  3.8.2.1 Mallines area Sample JP80, was taken in the Veta Segunda area and corresponds to a paragenetically early vein with a 015° strike orientation (System 4; mentioned above). The age spectrum for this sample exhibits 8 individual fractions with ages between ~140 and ~145 Ma containing more than 80% 39  Ar. The >140 Ma steps are bordered by younger aged steps at low and high temperature. The  spectrum overall has a slight bell shape, but contains a small plateau containing 3 steps with equivalent ages jointly defining a plateau age of 144.6 ± 1.5 Ma (user defined plateau only containing 32.7 % of  39  Ar), which is within error of the isochron age of 144.4 ± 1.1 Ma. The  inverse isochron age is less precise but at 141.1 ± 2.7 Ma also falls within error of the former. The preferred age for this sample is 144.4 ± 1.1 Ma.     95   Sample JP79B corresponds to a 113° striking (System 2; mentioned above) vein cross-cutting NS striking veins. The age spectrum shows evidence for minor excess 40Ar but contains a reliable plateau (on the basis of Isoplot default criteria) of 144.4 ± 1.6 Ma. The isochron correlation ages are with 142.5 ± 3.0 Ma and 142.4 ± 3.0 Ma, which are within error of the plateau age which is taken as the preferred age for this sample. Sample JP233 was taken from a paragenetically late N-S striking vein (System 3; mentioned above; Fig. 3.7). A reliable plateau age of 142.3 ± 1.6 Ma, with matching but less precise isochron ages of 140.1 ± 3.6 Ma (normal isochron) 141.7 ± 5.4 Ma (inverse isochron) was obtained for this sample. The age spectrum exhibits minor argon loss in the low temperature steps as well as minor excess  40  Ar in the highest temperature steps. The plateau age is taken as the  preferred age for this sample. Taking all ages together it is evident that hydrothermal activity at Mallines was concentrated around 144 Ma with the paragenetically younger veins possibly post-dating the older veins by 1-2 m.y., although the  40  Ar/39Ar data do not permit to distinguish the two events on a strictly  statistical basis. The adularia ages also indicate that the veins were emplaced slightly after the 146 Ma rhyolite domes at Cerro Bayo and Cerro Lapiz.     96   Figure 3.21. 40Ar/39Ar age spectra of adularia samples from Mallines. A. System 4. B. System 2. For analytical data see Appendix A10.     97   Figure 3.22.  40  Ar/39Ar age spectra of adularia samples from Mallines: System 3. For analytical data see Appendix A10.  3.8.2.2 Roberta vein, Brillantes Sample JP190 corresponds to the Roberta Vein. The sample yielded a user defined plateau age of 133.0 ± 1.5 Ma with similar but less precise isochron ages of 132.0 ± 6.3 Ma (normal) and 129.6 ± 5.8 Ma (inverse). The plateau contains 4 steps and 62.5% of 39Ar. The low temperature steps exhibit evidence for minor argon loss and two consecutive low T steps have apparent ages around 115 Ma. The heating steps at higher temperature than those of the plateau are only marginally different from the plateau age with ages around 130 Ma. The plateau age is taken as the preferred age for this sample.     98   Figure 3.23. 40Ar/39Ar age spectra of adularia samples from Roberta Vein, Brillantes. For analytical data see Appendix A10.  3.8.2.3 Guanaco area Sample JP247, corresponds to the Guanaco I vein. The age spectrum contains 8 steps (76% of 39  Ar) with ages between 122.5 and 128 Ma albeit not strictly attaining a plateau according to  Isoplot criteria. In the low temperature steps minor argon loss is evident. A user defined plateau of 124.9 ± 1.1 Ma based on 6 steps and 53.4% 39Ar was defined. An isochron age of 123.0 ± 4.5 Ma and an inverse isochron age of 122.6 ± 4.3 Ma are within error of the user defined plateau age which is considered the most reliable age estimate for this sample. Sample JP259 was taken from the Guanaco III vein. Ages of the individual heating increments are stepwise increasing at low and middle temperatures but attain a three step plateau of 111.7±1.2 Ma containing 42.5% of the 39Ar. The isochron ages are somewhat younger (107.2 ± 4.6 Ma: normal; 107.6 ± 4.9 Ma: inverse). The age spectrum is somewhat disturbed and exhibits moderate argon loss in the low temperature steps. The last heating step yielded an age of 124.9 ± 3.1 Ma. This age spectrum is less reliable than all others and may reflect thermal resetting of a 125 Ma age (i.e. similar to Guanaco I) during a hydrothermal pulse at ~111 Ma.    99   Figure 3.24. 40Ar/39Ar age spectra of adularia samples from Guanaco block. A. Guanaco I vein. B. Guanaco III vein. For analytical data see Appendix A10.     100   3.8.2.4 Laguna Verde area Sample JP265 corresponds to the Taitao vein system. The age spectrum exhibits evidence for excess 40Ar in the low and high temperature steps. A plateau age of 111.9 ± 2 Ma based on the Isoplot default criteria was defined and contains the steps with the lowest ages. The isochron ages of 110.9 ± 1.7 Ma (normal) and 111.5 ± 1.8 Ma (inverse) are equivalent. The plateau age is taken as the preferred age but due to the evidence for excess argon must be considered a maximum age. The true age may be slightly lower.  Figure 3.25. 40Ar/39Ar age spectra of adularia samples from Taitao system, Laguna Verde area. For analytical data see Appendix A10.     101   Figure 3.26. Cerro Bayo district map showing all adularia 40Ar/39Ar and K-Ar geochronology results for this study and literature sources (see text and Tables 3.4 and 3.5 for details). Bold: this study; italic: (1) De la Cruz and Suárez, 2008; (2) Townley, 1996; (3) Tippet et al., 1991. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.     102   3.9 Concluding remarks From the fluid inclusion data available (Townley, 1996; Pizarro, 2000; C. Hermosilla, 2009, written commun.) for the Cerro Bayo district, the Th and salinity ranges (Wilkinson, 2001) allow to define the Cerro Bayo district as an epithermal deposit. This is consistent with field observations showing typical epithermal textures such as crustiform textures and bladed quartz (quartz replacing calcite), plus high-grade Ag-Au quartz vein with locally high As and Hg concentrations that corroborate the epithermal environment for this district. The field observations of the vein textures at the Mallines area show variable amounts of bladed quartz (quartz replacing calcite) that is indicative of boiling and also that this boiling occurred high in the epithermal system (Buchanan, 1981). This type of texture suggests that the veins at Mallines are not deeply eroded. This is also supported by the geochemical data, which shows relatively high Hg and As but low Cu, Mo and Zn contents for this area. The fact that Bahía Jara veins yield the narrowest Th interval can be well correlated with the highest mean concentrations for Ag, Au, Cu, Pb and Zn for samples from vein drill-core (Fig. 3.15 and Table 3.1) in this area. Limited fluid inclusion data for the Brillantes area (only for Brillantes III vein) carried in finegrained quartz (C. Hermosilla, 2009, written commun.), suggest a narrow Th range possibly recording the Th and salinity from the crustiform epithermal low temperature events (e.g Roberta vein; Fig. 3.11) and may not be representative for the Th and salinity for the drusy quartz textures associated to Cu and Pb mineralization observed at Brillantes. The low Ag and Au concentrations and the elevated mean concentrations for Cu and Zn for surface vein samples for the Brillantes area, and despite the crustiform textures associated to drusy quartz texture at Roberta vein (Fig. 3.6), allows conclude that Brillantes has an additional potential for polymetallic base metal in addition to epithermal precious metal mineralization. The new 40Ar/39Ar adularia geochronology for the Cerro Bayo district is overall consistent with the previously published ages (Tippet et al., 1991; Townley, 1996; De la Cruz and Suárez, 2008). On the basis of all ages, three main mineralization episodes and their spatial extent can be defined: (1) Mallines, between ca. 145 and 142 Ma; (2) Bahía Jara, between ca. 137 and 124 Ma;    103   and (3) Laguna Verde, between ca. 114 and 111 Ma, the latter with overall higher salinity fluids and slightly elevated Mo. The structural setting has been evolving during the long history of vein emplacement. One of the oldest volcanic events recorded in the study area is the emplacement of the N-S trending rhyolitic domes at ca. 146 Ma (see Chapter 2). Based on field observation (to the south of the Cerro Bayo dome; Fig. 3.3), the rhyolitic domes and dykes emplacement occurred in a N-S striking, steeply W dipping normal fault. Associated to this main fault, in the Mallines area, between ca. 145-142 Ma the mainly N-S to WNW-ESE striking Mallines vein systems were emplaced. After the mineralization at Mallines, ore deposition occurred at Bahía Jara between ca. 137 and 125 Ma, possibly as late as ca. 111 Ma. Here, high-grade Ag veins were emplaced (e.g. Lucero vein; Appendix A12). These veins were emplaced in predominantly a normal-sinistral to pure normal system, where the veins near the Cerro Bayo dome present easterly dips and the veins from Guanaco block and the Brillantes area are W dipping. The latter, probably dip westward due to post-mineralization tilting to the E of the host fragmental volcanic sequence, and a predominantly subvertical dip is inferred for these veins at the time of emplacement. Based on the igneous and hydrothermal geochronology the main N-S fault (Cerro Bayo fault) is interpreted as the main conduit for the hydrothermal fluids for the Mallines, the Bahía Jara and the Brillantes areas. The mineralization ages from the Guanaco area are tentatively interpreted to record two hydrothermal events. The first event took place around ~125 Ma and was overprinted by hydrothermal activity around ~111 Ma and possibly an additional even younger event. Guanaco III largely records the late hydrothermal activity or, alternatively, thermal resetting of adularia during the ca. 114 – 111 Ma hydrothermal events documented for the Laguna Verde area; whereas at Guanaco I the first event is better preserved. The new adularia  40  Ar/39Ar age of 133.0 ± 1.5 Ma for the Roberta vein at Brillantes, allows to  temporally correlate the hydrothermal activity of this area to the Bahía Jara area. But, unlike the veins of the Bahía Jara area, geochemical analyses show that the Brillantes III vein presents the highest mean Mo concentrations and highest mean Pb/Ag and Zn/Ag ratios at surface and negative correlations between Ag-Pb and Ag-Zn. Also, along the Constanza quartz vein (in the    104   Brillantes area) azurite and galena can be observed. Moreover, all the Brillantes veins contain drusy quartz (Fig. 3.6), along with crustiform textures. All these observations suggest a deeper level of exposure compared to the Bahía Jara and Mallines areas. The structural block where these veins were emplaced was probably affected by tectonic uplift along the Laguna Salmonosa fault and subsequent erosion. This last event occurred possibly after 82-83 Ma, when the NE trend of dacitic domes were emplaced along the Laguna Salmonosa reverse fault (see Chapter 2). The last mineralization event in the district corresponds to the Laguna Verde area (ca. 114 – 111 Ma.) in the western part of the district. These veins contain the highest concentrations for Mo, Pb and Zn (see Appendix A12) which also coincide with the overall highest salinities. The oldest vein at Mallines are similar in age to mineralization in the Deseado Massif (ca. 150 in Huevos Verdes, mentioned in Wallier, 2009; ca. 154 Ma in Manantial Espejo; Wallier, 2009) and can be considered as the westernmost expression of volcanic activity and related mineralization of the Deseado Massif. In contrast veins from Bahía Jara, Brillantes and Laguna Verde may represent hydrothermal activity related to the long-lived magmatism of the active southern Andean continental margin represented in the Patagonian Batholith (Hervé et al., 2007). The geochronology demonstrates that the hydrothermal activity and epithermal mineralization occurred periodically over ca. 34 Ma.     105   4 Interlayered-clay alteration minerals  To help estimating erosion levels of the Ag-Au veins in different structural blocks (Fig. 4.1) and the post-mineralization fault movements, clay alteration mineral assemblages from the pyroclastic host rock around the veins were analyzed by XRD. Emphasis was given to the Mallines area. Due to the presence of illite and probably interlayered illite-smectite as the major alteration minerals in this type of ore deposits this technique may be used to estimate paleotemperature and by inference paleodepth (Ahmed, 2010) and post-mineralization block faulting.  Figure 4.1. Simplified geological map of the Cerro Bayo Ag-Au epithermal district, showing the main areas of the district: Laguna Verde, Cerro Bayo and Mallines (modified from Coeur d’Alene Mines web page and Suárez and De la Cruz, 2008). A-A’-B and C-D are the cross-sections from Mallines to Brillantes and from Delia vein at Laguna Verde, respectively. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.     106   4.1 Published work on XRD analyses in alteration clay mineralogy Some primary minerals in hosting rocks become unstable as a result of wall-rock/hydrothermal fluid interactions and are replaced by hydrothermal phases. There are large number of variables that influences and controls the mineralization and alteration mineral assemblage in hydrothermal systems; and these vary in relative importance from case to case. These factors are: (a) temperature (b) pressure, (c) rock type, (d) permeability, (e) fluid composition, and (f) duration of hydrothermal activity (Browne, 1978). Most work on mixed-layer clay alteration minerals in the epithermal environment was carried out in active geothermal systems (Browne and Ellis, 1970; Browne, 1978, 1984; Whitney, 1990; Harvey and Browne, 1991, 2000; Inoue and Kitagawa, 1994; Pandarinath et al. 2006). However, some authors have applied XRD techniques, specifically illite crystallinity, to sediment hosted epigenetic Cu exploration (Duba and Williams-Jones, 1983); for “disseminated Au” deposits (Kruse and Hauff, 1991); Carlin-type Au (Ahmed, 2010) and for porphyry Cu deposits (Zhandong et al., 2001; Franchini et al., 2007).  4.2 Clay studies by XRD methods Samples were taken immediately adjacent to the veins at Mallines and complement samples were collected from other veins along a N-S long section passing through the Raúl, Bahía Jara (Cerro Bayo and Guanaco) and Brillantes area (Fig. 4.1). In areas where veins are under cover (e.g. Marcela Sur), samples from host rock adjacent to veins were obtained from drill cores. These analyses were done with the aim of finding different alteration mineral assemblage between the different studied areas and their potential use as paleotemperature, and by inference, paleodepth indicators.     107   4.2.1 Material and methods Thirty-two samples of hydrothermally altered Ibáñez Formation pyroclastic rocks from the Cerro Bayo epithermal district were collected from surface in the Mallines, the Bahía Jara and the Brillantes areas; and from three drill cores, one from the Delia Vein in the Laguna Verde area, one from the Marcela Sur vein and one from the Lucero vein, the two latter from the Bahía Jara area. Clay mineral identification was done using XRD on the <2 µm fraction of the samples. To separate clay minerals from the volcanic rocks, between 10 and 20 g of crushed whole-rock sample with 200 mL distilled water were ground in an industrial blender for ~2-3 min at full power. The finest decanted material was disaggregated ultrasonically using a horn-type device for 2 min. For strongly silicified samples, up to four ultrasonic (~8 min) baths were performed with multiple decanting cycles. Subsequent to a short period of settling, the material was put in plastic test tubes, and then in the centrifuge for ~2-3 min, after which the fine fraction was decanted. After this, the settled material was separated from the water plus suspended material. Then, the last sample fraction was placed again in the ultrasonic bath for 2 min, and later in the centrifuge for ~5-6 min. Finally, the suspended material was centrifuged for ~1-2 h, enough to have the < 2 µm sized material settled. After the clay separation, the <2 µm fraction was identified for each samples using a standard Siemens (Bruker) D5000 Bragg-Brentano diffractometer at the Faculty of Earth and Ocean Science, The University of British Columbia, Vancouver. Air-dried and ethylene-glycol clay mineral separate (<2 µm) were analyzed from 2 to 80 °2θ and 2° to ~30 °2θ, respectively, at 0.5° 2θ per second with a step width of 0.037 °θ, with Cu-Kα radiation. Once the spectra were collected, the spectral peak interpreting software Diffrac Plus XRD Commander was used to aid in clay mineral analysis.     108   4.2.2 Identification of clay and mixed-layered clay minerals Illite was distinguished in air-dried and ethylene-glycol saturated samples by the sharp reflection of the 001 peak at ~10Å. Smectite was distinguished by the 001 peak shift from 12.4 to 14.5Å in air-dried samples to ~1Å higher after ethylene-glycol saturation. The illite crystallinity index was determined by measuring the width of the 001 illite peak in °2θ at half-peak height, using the method of Kübler (1964, 1967). Thus, the lower the illite crystallinity index the higher the crystallinity of the illite. For mixed-layered clay mineral identification, important changes after ethylene-glycol in the XRD diffraction pattern allow recognizing the presence of a smectite component. In this case a “x”/smectite is present, and “x” may be illite, chlorite, kaolinite, talc, serpentine, vermiculite, etc. The interstratification is random and probably rich in smectite if after ethylene-glycol saturation the sample produces a peak near 5.2 °2θ (Moore and Reynolds, 1997). The importance of the XRD pattern in the low angle region cannot be overemphasized. A reflection at 5 °2θ indicates random interstratification, and one near 6.5°2θ indicates R1 ordering, well-ordered. The latter reflection becomes broad and weak and does not move very much as the composition becomes more illitic. In general, also a wide peak slightly greater than 10Å will represent an illitic mineral (i.e. high proportion of illite) with either small diffracting domains or the presence of a small number of smectite layers in an illite/smectite mixed layer mineral. If the peak shifts to less than 10Å upon glycol saturation, the mineral contains smectite layers which in some cases define asymmetric peaks. The latter peaks are defined to be composed of two major components (Lanson, 1997), one wide and displaced to a position greater than 10Å, and another more narrow and centered at 10Å. The wide peak does not change position after glycol saturation and is called poorly crystallized illite (PCI), and is characterized by a shoulder. These wide peaks may be attributed to small grain size or a small proportion (< 5%) of interstratified smectite in the structure which does not fully expand after glycol saturation. The second peak is termed well crystallized illite (WCI) and it will be a mica layer structure with a narrow peak width, < 0.4°2θ (Meunier and Velde, 2004).    109   The intensities of the XRD pattern of an individual mineral are known to be proportional to the concentrations of the different minerals present. Therefore, by measuring the intensities of patterns, some idea of the relative amounts of each phase can be achieved (Ouhadi and Yong, 2003). Thus, for a quantitave identification of clay mineralogy the analysis based on peak areas was used (Ouhadi and Young, 2003; see section 4.3). This method consists of measuring the area under the curve of the 001 reflection from the XRD pattern of each clay mineral. Here it is assumed that the clay minerals detected correspond to the overall alteration assemblage present in the samples. It is important to remark that the samples studied herein present intense to moderate silicification. Other quantitave methods of analysis regarding the identification of clay minerals exist in the literature and they consist of: (1) using the identical mass absorption coefficient method; (2) mineral diagnostics based on an internal standard; and (3) quantitative mineral evaluation using an external standard (Ouhadi and Young, 2003).  4.3 Results on XRD methods: clay mineralogy and spatial distribution The A-A’-B cross section (see Fig. 4.1 for location) indicates the distribution of the hydrothermal clay mineralogy from the Mallines area to the Brillantes area (Fig. 4.2); and Fig. 4.11 shows the distribution of clay minerals in the Delia vein cross section (C-D cross section in Fig. 4.1), at the Laguna Verde area. The XRD results with their clay minerals proportion, illite peaks °2Θ values and illite crystallinity are shown in Table 4.1. X-ray diffraction analyses detected the presence of illite/smectite mixed layered, illite and kaolinite clay minerals (Table 4.1). Based on the clay distribution and amounts, three main zones can be distinguished: (1) Mallines: illite, minor kaolinite (mean: 26.26%) and trace smectite (mean: 13.1%); (2) Bahía Jara (including Brillantes): illite, minor smectite (mean: 17.63%), trace kaolinite (mean: 10.46%) and trace chlorite (sample JP-255 from Brillantes); and (3) Laguna Verde: illite, trace smectite (mean: 12.64%) and no kaolinite (Fig. 4.3). Illite is the most abundant clay mineral overall in the district. Kaolinite only occurs in the Mallines area and in lesser amounts in the Bahía Jara and the Brillantes areas (Fig. 4.2).    110   Table 4.1. XRD results for the samples taken in the Cerro Bayo epithermal district. The percentage of each clay mineral is based on the area under the curve method (Ouhadi and Young, 2003)     111     Figure 4.2. N-S long section showing the XRD results for the samples taken from Mallines, Cerro Bayo, and Brillantes areas. I.C: illite crystallinity;ill: illite; smc: smectite; kaol: kaolonite. See Fig. 4.1 for location of section line.  112   Figure 4.3. Illite-Smectite-Kaolinite ternary diagram for XRD analyzed samples from fragmental rocks, showing the mineral distribution between the different study areas in the Cerro Bayo district. Each mineral percentage was calculated by the area under the curve method (Ouhadi and Young, 2003).  4.3.1 Mallines area In this area, thirteen samples from the different vein systems, and from Cascada and Guadalupe veins were analyzed (Table 4.1). At Mallines six samples contain more than 90% illite, but it is also the area where the highest percentage of kaolinite is present. This is exemplified by samples JP-232 and JP-237, both from System 3, containing 82.79% and 100% kaolinite, respectively. Sample JP-173 (Fig. 4.4; Table 4.1) from Cascada vein outcroping at Cerro Torta W (Fig. 2.3) contains the largest amount of illite of all Mallines samples. A shoulder at 10.45Å is evident in the illite peak after glycol saturation. However, this peak has not shifted significantly compared    113   to the illite peak under air-dried conditions, indicating that the clay mineralogy of this sample is composed of a mix of well crystallized and poorly crystallized illite (WCI and PCI, respectively) with an illite crystallinity index of 0.74. Trace amounts of interlayered smectite (< 5%) may be present but are not evident from the XRD spectra.  Figure 4.4. XRD pattern for samples adjacent to the Cascada and Guadalupe veins in the Mallines area. PCI: poorlycrystallized illite; WCI: well-crystallized illite; I/S: illite/smectite interlayered; Ill: illite; Kaol: kaolinite; Qtz: quartz.  For the samples analyzed from the System 1 veins in the Mallines area the clay mineralogy corresponds to variable amounts of illite and kaolinite (Table 4.1; Fig. 4.3). The illite crystalinity index varies from 0.49 to 1.3, sample JP-234 having the lowest value and best crystallinity of all Mallines samples (Fig. 4.5). Sample JP-252 is the northernmost sample analyzed for System 1, and presents the highest illite (with an illite crystallinity index of 0.64) and lowest kaolinite percentage, and the highest °2θ for its illite 001 peak (10.13), meaning that is the most pure illite of the three analyzed samples for this system.     114   Figure 4.5. XRD spectra for samples from System 1 and 2 veins at the Mallines area. I/S: illite/smectite interlayered; Ill: illite; Kaol: kaolinite; Qtz: quatz. Note the poor interlayered-clay mineral ordering (R=0) for two samples belonging to System 2.  XRD spectra from veins from System 2 at the Mallines area indicate that, apart from illite and kaolinite, two samples contain smectite as an alteration phase for this mineralization event (Table 4.1; Fig 4.3). These two samples were taken in the northern part of the mapped area, around the Madre Norte vein, and correspond to JP-250 (~32% smectite) and to JP-174B (~4% smectite; Fig. 4.5). For all the samples analyzed, the illite crystallinity index range is variable from 0.49 to 1.33, with the lowest crystallinity for the samples containing smectite.     115   Four samples were analyzed from System 3 veins (Table 4.1; Fig. 4.6), they contain illite and kaolinite, with the highest percentage of kaolinite of all Mallines samples. Illite crystallinity index are similar for three of the samples, with values from 0.75 to 0.87. A fourth sample contains only kaolinite. The sample with the highest percentage of illite was taken in the northern part of the Mallines area.  Figure 4.6. XRD spectra for samples from System 2 veins at the Mallines area. Note the poor interlayered-clay mineral ordering (R=0) for sample JP-249, the northernmost sample of the four. PCI: poorly-crystallized illite; WCI: well-crystallized illite; Ill: illite; kaol: kaolinite; Qtz: quartz.  Only one sample was analyzed by XRD from System 4 in the Mallines area (Table 4.1; Fig. 4.7). This sample corresponds to JP-235 and presents illite (95.83%) and kaolinite (4.17%), and has the highest percentage of illite of all samples taken around the Madre vein area. The illite crystalinity index is 0.54, and °2θ for the illite peak is at 10.18° which is the highest of all samples collected at this area.     116   Figure 4.7. XRD spectra for sample JP-235 from System 4 veins at the Mallines area. Ill: illite; Kaol: kaolinite.  4.3.2 Bahía Jara and Brillantes areas Nine samples from the Bahía Jara area, and three samples from the Brillantes area were analyzed by XRD. These samples were chosen from different structural blocks, separated by NE trending post-mineralization faults (Fig. 4.1). The analyzed samples from Bahía Jara present abundant illite, trace percentage of smectite (only one sample with up to ~52%) and a small percentage of kaolinite (7 of 9 samples with kaolinite with 0.26 to 27.15%; Table 4.1). The illite crystallinity index is very similar among the samples and varies from 0.62 to 0.90, the most crystalline sample being from the Guanaco III vein (sample JP-259). Two out of three samples from around the Cerro Bayo dome (Fig. 4.1) present only illite in the clay alteration assemblage (Lucero and Luz Eliana veins; Fig. 4.8); and the third one from the Celia vein presents, apart from illite, smectite (~6%) and important amounts of kaolinite (~27%). This sample also has, with 0.9, the highest illite crystallinity index of the three samples (in contrast to 0.69 and 0.74).     117   Figure 4.8. XRD spectra for samples from the Bahía Jara area veins. Note the well interlayered-clay mineral ordering (R=1) for samples from Marcela Sur, Lucero and Guanaco VII veins. Ill: illite; Kaol: kaolinite; Qtz: quartz.     118   Four samples were taken from veins hosted in the Guanaco block (JP-132B, JP-247, JP-259 and JP-261; Table 4.1). Three samples show illite and minor kaolinite as the clay alteration assemblage (Figs. 4.8 and 4.9), and were taken from the volcanic Unit 4 (Chapter 2). In contrast, sample JP-132B has illite, minor smectite (with random interlayered illite-smectite order; R=0) and trace amounts of kaolinite as the clay alteration assemblage (Fig. 4.8) and was taken from the volcanic Unit 2 (Chapter 2). The other two samples were taken from the southern blocks of the Bahía Jara area. One correspond to the Raul vein (JP-241) at the other one to the Marcela Sur vein (Fig. 4.1; Table 4.1). The latter was taken from drill-core DMS-44 because of quaternary alluvial cover of the vein. These samples have illite, smectite and minor kaolinite but contain the largest amount of smectite of all the samples taken from the Bahía Jara area (between ~7 and ~52%), and they have well-ordered interlayered illite-smectite (R=1; Fig. 4.8)  Figure 4.9. XRD spectra for one sample from the Bahía Jara area and three samples taken from the Brillantes area veins. Note from samples JP-255 the presence of chlorite and calcite; and calcite in sample JP-256 as the mineral alteration assemblage. Ill: illite; Kaol: kaolinite; Qtz: quartz.     119   Three samples were taken from the Brillantes area, and their XRD analyses gave illite, minor kaolinite and lesser chlorite as the clay alteration assemblage (Table 4.1). They have illite crystallinity indices from 0.57 to 1.02. The Constanza vein (samples JP-254 and JP-255; described in Chapter 3) is partly hosted by a rhyolitic fragmental rock (illite crystallinity index of 0.59; volcanic Unit 2) and by an andesitic stock. The clay alteration mineralogy in the latter lithology presents illite and chlorite as the mineral alteration assemblage (Fig. 4.9), with an illite crystallinity index of 1.02. The sample taken from the Roberta vein hosted in a rhyolitic tuff has an illite crystallinity index of 0.57 (lowest value of the three samples in this area), and it has illite and kaolinite as the main clay alteration assemblage.  4.3.3 Laguna Verde area All samples analyzed from the Delia vein (Table 4.1; Figs. 4.10 and 4.11) have illite with minor smectite as the clay alteration phases. An illite crystallinity index between 0.49 and 0.69 was determined for the overall suite of samples. Samples FCH-366-46; FCH-369-196; and FCH-373302, taken adjacent to the vein, have the highest cristallinity (illite cristallinity index values from 0.49 to 0.55) of all the thirty-one samples analyzed (Table 4.1), and they present illite as the only alteration phase (Fig. 4.10). The other three samples at Laguna Verde were taken meters away from the vein and contain a significant smectite component (~2 to ~23%) compared to those taken immediately adjacent to the vein. Sample FCH-373-328 (Fig. 4.10) has the highest smectite content, and it is located 2 m away from a post-mineralization andesitic dyke that cross-cuts Delia vein.     120   Figure 4.10. XRD spectra for samples taken from host-rock from different drill-cores at different depths from the Delia vein at the Laguna Verde area. Note the well ordered (R=1) illite/smectite interlayered for the samples that present smectite. Ill: illite; Qtz: quartz     121   Figure 4.11. Delia vein C-D cross section showing the clay mineralogy for the samples analyzed, and the available Ag, Au and Aueq concentrations for selected points from the vein. I.C: illite crystallinity; ill: illite; smc: smectite; kaol: kaolinite.  4.4 Discussion of XRD results All illite analyses were performed on silicified rhyolitic to rhyodacitic pyroclastic fragmental rocks, except for sample JP-255 from Brillantes, that was taken from an andesitic subvolcanic rock. The illite cystallinity index can be considered as an indicator for paleotemperature of the hydrothermal fluids that circulated during the mineralization events in the different areas. The fluid temperature is directly related to the paleodepth within the low-sulfidation epithermal hydrothermal systems (Buchanan, 1981). In average, Mallines contains the samples with the highest illite crystallinity index (0.82; Table 4.1), followed by the Bahía Jara area which has an average of 0.74. The Laguna Verde area samples as well as the felsic samples from Brillantes have the highest illite cristallinity with a mean illite crystallinity index of 0.55 and 058, respectively. These values suggests that the veins at Mallines were emplaced at the shallowest     122   depth of all the areas, in contrast to Laguna Verde and Brillantes, where the illite crystallinity values suggests the deepest and highest temperature environment of vein formation. For the Mallines area a generally decreasing illite content from System 1 to 3 can be observed, where the lowest mean illite crystallinity index is calculated for System 1. From rock samples adjacent to System 1 veins, illite and kaolinite are the principal clay alteration minerals. In samples from System 2 kaolinite and illite are less abundant but smectite is present at a higher proportion, and finally in samples from System 3 veins the lowest percentage of illite and the highest percentage of kaolinite is observed. This is consistent with progressively lower temperature and shallower level of exposure from System 1 to System 3. Vein host-rocks from Bahía Jara and Brillantes present not only consistently increasing illite content (86.44% and 91.46 %, respectively) and purity (higher °2θ), but also decreasing kaolinite percentage compared to the Mallines area, confirming a deeper and hotter environment for these two areas. The Guanaco III vein from the Bahía Jara area exhibits the most crystalline illite indicating that this vein may have been eroded to a deeper level compared to other veins in this area. Alternatively, the Guanaco III vein may be related to the hydrothermal activity centered at Laguna Verde. This is indicated by the similar clay alteration mineralogy as well as geochronological data (Chapter 3). The Marcela Sur vein, buried under alluvial overburden, exhibits the lowest amount of illite and the highest amount of smectite, of all the analyzed veins from the Bahía Jara area. This indicates that this vein has probably not been affected by deep erosion and relatively shallow parts have been preserved due to post-mineralization normal faulting along NE trending faults (Fig. 3.7) Samples from Brillantes are from different lithologies. Sample JP-254 adjacent to the Constanza vein and JP-256 from Roberta vein are from rhyolitic tuff, and sample JP-255, also adjacent to the Constanza vein from an andesitic subvolcanic body. The latter is the only one that presents illite and chlorite as the alteration mineral assemblage, and a relatively high illite crystallinity index (1.02; i.e. lower crystallinity), compared to the low illite cristallinity index for the other two samples analyzed for this area (0.59 and 0.57). While the low illite crystallinity index indicates a relatively deep and high temperature setting of the samples taken from the rhyolitic host-rock, the    123   poorer crystallinity of illite in the andesitic host suggests a significant influence of host-rock composition on illite crystallinity at given temperature conditions. The samples from the Laguna Verde area present the lowest overall mean illite crystallinity index (0.55) of the entire district. They also present the highest illite percentage and relatively high °2θ peak positions. Where smectite is present it occurs as a well ordered illite/smectite interlayered phase. Thus, it is concluded that the exposed parts of the Laguna Verde and Brillantes veins were emplaced at the deepest level when compared to the other areas of the Cerro Bayo district. Around the Delia vein, the only vein where sufficient data are available, an alteration zonation can be defined on the basis of clay mineralogy. Samples immediately adjacent to the Delia vein have the lowest illite crystallinity index and almost only illite in the alteration assemblage. The other three samples were taken meters away from the vein and present a significant smectite percentage (2.21 to 23.37%; with illite crystallinity index from 0.53 to 0.69) compared to other samples mentioned above. Sample FCH-373-328 (Fig. 4.10) that contains the highest smectite percentage, was taken near an andesitic dyke that cross-cuts Delia vein.  4.5 Terraspec® analyses from host-rock adjacent to veins Reflectance spectroscopy is a technique that uses the energy within the visible (VIS; 400 to 700 nm), near infrared (NIR; 700 to 1300 nm) and short wave infrared (SWIR; 1300 to 2500 nm) regions of the electromagnetic spectrum; and can be applied to mineral analyzes. This technique is based on the spectral properties of materials, where certain atoms and molecules absorb energy as a function of their atomic structures; and the manifestation of this takes the form of a reflectance spectrum, with absorption features (Hunt, 1977 and Goetz et al., 1982). The absorption features are characteristic for different OH, CO2, SO4 or H2O bearing minerals and can be used to identify fine-grained clay minerals. Reflectance spectroscopy is a technique widely applied in exploration for epithermal mineralization due to the rapid data adquisition and low cost. However, this technique is qualitative only in contrast to XRD.     124   For the three major regions of spectral reflectance, different mechanisms cause the absorption features to appear in each one. In the VIS/NIR regions the dominant mechanisms are the charge transfer, crystal field effects and ligand identity, where minerals containing transition metals such as iron, nickel, chrome, manganese and REE can be recognized. In the SWIR region the dominant mechanism corresponds to vibrational transitions where cations coordinated with OH, SO4, CO3 generate a distinctive SWIR absorption response (Table 4.2). This is a function of the cation and the bond length. For instance hydroxyl absorption features, like Al-OH, are always in the 2200 nm region; FeOH bonds generate an absorption feature around 2280 – 2295 nm for clays; and CaCO3 bonds have a response around 2334 nm (Hunt, 1977 and Goetz et al., 1982).  Table 4.2. Major absorption features (Hauff, 2005)  Illite (K1-1.5Al4·(Si7-6.5Al1-1.5O20·(OH)4) features correspond to single absorptions in the ~1400 nm and the ~2200 nm regions that vary in sharpness and width as a function of crystallinity and temperature of formation, as well as for smectite content, the latter characterized by a shoulder at ~1456 nm in the spectra. The absorption feature from 1404 to 1415 nm is present for illite, illite/smectite, kaolinite, dickite, muscovite and smectite (Table 4.3). The wavelength shift in this feature is diagnostic and can be used to estimate Al content (Hauff, 2005). The absorption feature from 2188 to 2192 nm is also shown by dickite and smectite (Table 4.3); the wavelength shift in this feature is diagnostic and can be used to estimate Al content. The presence of two small features at ~2350 and ~2450 nm can differentiate between smectite and illite; in muscovite and well-crystallized illites, these features are larger and sharper, and the    125   ~2450 nm feature is minimized with poor crystallinity. Illite dominates in illite/smectite interlayered-clays when these features are present (Hauff, 2005). For kaolinite (Al2Si2O5·(OH)4) and dickite a double absorption feature at ~1392-1402 nm is diagnostic and is a function of vibrations within the OH molecule. Minima of the higher wavelength doublet occurs at ~2162-2168 nm (it may be confused with pyrophyllite, and some alunites) and ~2205-2214 nm (potentially mistaken for illite, alunite, or dickite). The very small doublet at ~1810-1848 nm is very diagnostic and a measuring tool of kaolinite crystallinity (Table 4.3). The lengths of the doublet, for kaolinite, are also a function of site occupancy in the octahedral layers of the crystal structure. Multiple site occupancy creates multiple and/or broad spectral absorption minima. Degree of structural ordering is manifested in the profile and symmetry of the kaolinite spectral features; and the presence of bound water indicates disorder in the structure, usually the presence of smectite layers. The Mg-Fe two end members for chlorite ((Mg,Fe)3·(Si,Al)4O10·(Mg,Fe)3·(OH)6) have some different features; the 2252 and 2340 features shift with Mg and Fe substitution, and the first one changes with Al content. In the 2350 nm region, Mg-chlorites have an absorption feature at lower wavelength (2310-2330 nm) than Fe-chlorites (2340-2370 nm; Table 4.3). Fe-bearing chlorite samples have broad features, and a positive slope from ~1500 to ~1900 nm. The Mg-bearing chlorites have three features in the lower wavelength regions around ~1400, where the ~1390 nm features is very sharp as Mg tends to order a crystal structure. The presence of water indicates another phase or the alteration of the chlorite to contain water-bearing layers. There are four distinctive diagnostic absorption features for jarosite (KFe3·(SO4)2·(OH)6). They occur at 1466-1478 nm, 1847-1857 nm, 2267-2271 nm and 2298-2304 nm. The combination between the ~1460 nm feature and the ~1510 nm shoulder is distinctive. The ~1847 and the ~2260 nm features are not always obvious in mixtures. Supergene jarosite can carry water in the structure and/or with associated clays; and hypogene jarosite are less likely to contain very much water (Hauff, 2005).     126   Water is a major problem relative to changing the appearance of a spectrum, and is usually associated with interlayered smectite. Surface water gives a rounded appearance to the 1400 nm and 1900 nm features, and the larger the water feature, the more capacity the sample has to take on or retain water. This can also be and indication of temperature and environmental conditions including ground water movements (Hauff, 2005).  Table 4.3. Major absorption features for clay minerals (Hauff, 2005)  4.5.1 Terraspec® analyses of the Delia vein host-rock (Laguna Verde area) ASD Terraspec® analyses was performed for host-rock drill-core samples from the Delia vein; with the aim of identifying different alteration mineral assemblages towards the vein. The Terraspec® spectra for samples taken from immediately adjacent to the Delia vein (Fig. 4.12), present illite as the principal alteration mineral assemblage. This is recognized by the single absorption feature at 1400 and 2200 nm, and also by the two small features at 2350 and 2450 nm (Fig. 4.13). In samples taken adjacent to the Delia vein, the 1400 and the 2200 nm absorption features present a decrease in sharpness and an increasing breadth towards the surface (Fig. 4.13), showing an    127   increase of illite crystallinity with depth. This suggests that between 74 and 124 m a.s.l. the fluids temperatures were highest in the Delia vein. From 208 m a.s.l and higher, the ~1400 nm illite/smectite absorption features can be observed. This is consistent with a change from Al-rich illite at depth to an Al depleted illite towards the surface, characterized by the shift from slightly less than 2200 nm minimum absorption at depth to slightly higher than 2200 nm above 182 m a.s.l. Minor jarosite absorption features at ~1840 nm, in almost all adjacent rocks analyzed can be recognized.  Figure 4.12. Delia vein cross-section showing the distribution of samples analyzed by Terraspec® immediately adjacent to the vein.     128   The 1900 nm absorption feature is sharp for all samples from the Delia vein, indicating that molecular water is not present between illite layering, and that water is mostly present as OH molecules as part of the crystal lattices (Fig. 4.13). Samples from some distance away from the Delia vein (Fig. 4.14) present absorption features attributable to illite and illite/smectite interlayered mineral assemblage. These are, absorption features at 1404 to 1415 nm, and a marked shoulder at ~1455 nm. The absorption feature at 2350 and 2450 is less pronounced for near surface samples, suggesting diminishing illite proportion in the illite/smectite interlayering. The larger absorption for 1900 nm, compared to 2200 also is indicative of smectite. From Figure 4.14, no conclusive pattern from samples taken from outside Delia vein can be observed. Some samples show the ~1840 nm jarosite absorption feature, and it is most pronounced at ~208 m a.s.l. and at elevations above ~230 m a.s.l. Nine samples from immediately adjacent to the andesitic dike that cross cuts Delia vein were analyzed by Terraspec®. They gave only illite as the alteration assemblage, and also five spectra indicate that they contain iron in their crystal lattice as shown by the positive slope between ~1300 to ~1900 nm (see red arrows in Figure 4.15); the latter feature is most notorious from ~257 to ~266 m a.s.l; and probably shows presence of Fe-rich Chlorite.     129   Figure 4.13. Terraspec® spectra from samples taken immediately adjacent to Delia vein at Laguna Verde. They present mainly illite as the major alteration mineral, with only minor or trace smectite interlayered. Typical spectra features are indicated. See text for explanation. I: illite; I+S: illite/smectite; J: jarosite.     130   Figure 4.14. Terraspec® spectra from samples taken away from Delia vein at Laguna Verde. They present mainly illite/smectite as the major alteration mineral. Typical spectra features are indicated. See text for explanation. I: illite; I+S: illite/smectite; J: jarosite.     131   Figure 4.15. Terraspec® spectra from samples taken from adjacent to the andesitic dike that cross cuts Delia vein at Laguna Verde. They present mainly illite as the unique alteration mineral. Typical spectra features are indicated. Red arrows indicate Fe content in the mineral lattices. I: illite; J: jarosite.     132   4.5.2 Terraspec® analyses of host-rock from the Brillantes area Four samples were analyzed in the Brillantes area: two from the Constanza vein, one from the Roberta vein and one from the Brillantes vein. Samples from the Constanza vein were taken from two different lithologies; JP-254 corresponds to a rhyolitic tuff and JP-255 to an andesitic subvolcanic intrusive. In this area the clay alteration mineral assemblage is variable, with illite/smectite, lesser Mg-rich chlorite and trace kaolinite (Fig. 4.16). The latter is only present in trace amounts adjacent to Roberta vein (JP-256) with illite/smectite interlayered, where drusy quartz and crustiform textures are the main vein textures (Fig 3.6).  Figure 4.16. Terraspec® spectra for samples taken from adjacent to veins at Brillantes. They present mainly illite as the unique alteration mineral. Typical spectra features are indicated. See text for explanation. I: illite; S: smectite; K: kaolinite; Mg-Chl: magnesium-rich chlorite; J: jarosite.     133   The sample from the Brillantes vein (JP-257) contains illite/smectite interlayered (with illite predominance) and jarosite with a noteworthly Fe-enrichment in the clay mineralogy (red arrow in Fig 4.16). Aside from interlayered illite-smectite, the sample from the Constanza vein presents Mg-rich chlorite as part of the alteration mineral assemblage.  4.5.3 Terraspec® analyses of host-rock from the Bahía Jara area For this area, the same samples analyzed for XRD were analyzed for Terraspec®. The main clay alteration assemblage found was illite, lesser smectite and minor kaolinite (Fig. 4.17). Specifically, samples analyzed from Bahía Jara show illite (Guanaco VII, JP-132B); illite/smectite (Lucero, BJH-523-101; Luz Eliana, JP-180B; Guanaco III, JP-259); illite/smectite and lesser kaolinite (Raúl, JP-241; Marcela Sur, DMS-44-249; Guanaco I, JP-247; Vizcacha system, JP-261); and illite with lesser kaolinite (Celia, JP-263) (Fig. 4.17). Around the Cerro Bayo dome, samples gave illite/smectite and minor kaolinite as the clay assemblage, whereas towards the west (Guanaco block) and southwest (Marcela Sur and Raúl veins) samples have illite/smectite with lesser kaolinite. Sample JP-132B from Guanaco VII presents only illite (Fig. 4.17). This is the westernmost sample taken from the Bahía Jara area and from the volcanic Unit 2 (see Chapter 2), and the closest from the Laguna Verde area. Samples from Guanaco III, Guanaco VII veins and Vizcacha system (Fig. 4.17) show the highest Al content, with the lowest position of the ~2200 nm absorption feature, similar to the samples analyzed from the Delia vein in Laguna Verde area. The presence of kaolinite is more marked by the doublet at the 1400 nm absorption for the Celia vein, which is consistent with the XRD results (Table 4.1). For the buried Marcela Sur vein the presence of kaolinite may be showing that this vein is well preserved and has not undergone significant erosion.     134   Figure 4.17. Terraspec® spectra of samples taken immediately adjacent to veins at the Bahía Jara area. They present mainly illite, minor illite/smectite interlayered and lesser kaolinite as the clay alteration mineral assemblage. Features indicated correspond to I: illite; S: smectite; K: kaolinite.     135   4.5.4 Terraspec® analyses of host-rock from the Mallines area For this area, samples from all vein systems (see Chapter 3) were analyzed by Terraspec®. The predominant clay alteration assemblage corresponds to illite and kaolinite, and trace illite/smectite. A large amount of molecular water in all Mallines samples (Figs. 4.18 to 4.20); is evident from the rounded absorption feature at ~1900 nm. In System 1 veins, the clay alteration mineralogy corresponds to illite and kaolinite for samples located mainly in the southern part of this area (JP-234; JP-238; Fig. 4.18). Samples taken further north, around Madre Norte vein, contain illite, kaolinite and trace interlayered illite/smectite (JP251; JP-252; Fig 4.18). Sample JP-252 is the only one containing larger amounts of illite, and the kaolinite 2160 nm absorption feature is not present.  Figure 4.18. Terraspec® spectra of samples taken immediately adjacent to veins from System 1 at the Mallines area. They present mainly illite and kaolinite, and trace smectite as the clay alteration mineral assemblage. I: illite; S: smectite; K: kaolinite.     136   In System 2, the clay alteration assemblage corresponds to illite and kaolinite for all the analyzed samples (Fig. 4.19). No significant differences in spectra between the different samples are evident, except that for sample JP-250 (northern part of the mapped area) the ~2100 nm kaolinite feature is not marked, indicating that in this sample illite is predominant.  Figure 4.19. Terraspec® spectra for samples taken immediately adjacent to veins from System 2 at the Mallines area. They present illite and kaolinite as the clay alteration mineral assemblage. I: illite; K: kaolinite.  In System 3 clay alteration assemblage corresponds to illite, kaolinite and trace amounts of smectite (Fig. 4.20). Samples (JP-220 and JP-226) with trace amounts of illite/smectite interlayered are located near Madre vein, the widest vein of the Mallines area (~8 m width). Sample JP-223, taken from the Madre vein host-rock it is the only one that present kaolinite and trace amounts of illite clay.     137   Figure 4.20. Terraspec® spectra for samples taken immediately adjacent to veins from System 3 at the Mallines area. They present illite, kaolinite and trace smectite as the clay alteration mineral assemblage. I: illite; K: kaolinite; S: smectite.     138   4.6 Discussion on Terraspec® analysis and comparison to XRD results In general Terraspec® analysis show consistent results when compared to the XRD results. For the host-rock of the Delia vein samples illite and minor illite/smectite interlayered and minor jarosite were detected, with the highest Al content in clays of the entire district. Particularly for samples immediately adjacent to the vein, progressively increasing in illite crystallinity towards greater depths, but increasing proportion of illite/smectite interlayered clays towards surface is evident in both XRD and Terraspec®. Using Terraspec® as a pathfinder towards mineralized veins has to be used with caution. Samples from meters away from Delia vein show illite/smectite interlayered clay alteration mineralogy, and with the 1900 nm molecular water absorption feature broader and less marked compared to samples adjacent to vein. Although, no consistent spectra meters away from this vein can be observed, a lower fluid/rock ratio can be deduced compared to samples immediately adjacent to veins. From samples adjacent to the andesitic dike, that cross cuts the Delia vein, some similarities to the samples adjacent to the Delia vein, presenting illite and minor jarosite as the clay mineralogy assemblage are evident. The difference is that these samples adjacent to the dike present higher Fe content, probably related to the relatively high Fe content (10.1% Fe2O3) of this dike, as well as high illite crystallinity. (see Appendix A4) Veins from the Bahía Jara area show consistent spectras compared with the XRD analyses. The presence of illite, minor smectite and trace kaolinite for the Marcela Sur and the Raúl veins, confirms the similar level of erosion for these veins, but with the Marcela Sur vein being under Quaternary cover, because of post-mineralization normal faulting. The presence of illite as the only clay mineral next to the Guanaco VII vein, compared to the other veins from Bahía Jara may be related the proximity of this vein to the Laguna Verde area (where higher temperature hydrothermal activity is documented), or because this sample was taken from Unit 2, in contrast for the other veins where samples from Unit 4 were taken. The Mallines Terraspec® analyses gave illite, kaolinite and trace smectite, showing consistency with the XRD results. The broad and rounded 1900 nm absorption feature may be showing the    139   high molecular H2O content of the clay, compared to the veins from the other areas, potentially representing large amount of fluid circulation. The broad 1400 nm absorption features for the System 3 veins shows a disorder in the kaolinite structure, consistent with a shallower level of exposure compared to the other two vein systems.     140   5 Discussion  5.1 Volcanostratigraphy, clay alteration assemblage and erosional levels The clay alteration mineralogy in general agrees well with the volcanic stratigraphy, the microthermometry and geochemistry of the veins. All data permit classifying structural blocks according to exposure levels. The deepest and oldest levels of the volcanic stratigraphy correspond to Unit 1 and 2, which mainly crop out at Brillantes, north of Laguna Verde and at Laguna Verde Sur areas. These areas are also those with the highest illite crystallinity and overall highest fluid inclusion salinity and homogeneization temperature. Unit 4 crops out mainly at Mallines and Bahia Jara, where illite-smectite-kaolinite clay alteration assemblages are present, and illite crystallinity is the lowest. The kaolinite in these two areas has two possible origins: (1) steam-heated or (2) supergene. Steam-heated kaolinite, forms near the surface at low temperatures (~100°C) according to the low-sulfidation model (Buchanan, 1981; Simmons and Browne, 2000b) together with K-rich mica, calcite and chalcedonic quartz (Simmons and Browne, 2000b). Supergene kaolinite is the result of weathering and hydration of K-feldspar (Best, 2003). Based on the observation that almost all the kaolinite at Mallines is accompanied by jarosite and boxwork most kaolinite at Mallines and Bahía Jara is interpreted as supergene with a lesser component of steam-heated kaolinite since some kaolinite is associated to chalcedonic quartz and K-rich mica (Simmons et al., 2000a). Furthermore the steam-heated kaolinite interpretation at Mallines is supported by the fact that blocks cropping out to the north do not contain much kaolinite compared to to the blocks to the south containing higher kaolinite amounts at the same elevation. These may be indicating relatively shallow levels of erosion at Mallines within the low-sulfidation epithermal model (Buchanan, 1981). Isotopic characterization of kaolinite combined with a systematic kaolinite cristallinity would potentially be helpful to elucidate the amount of kaolinite related to steam-heated or alternatively supergene environment. Bahía Jara vein samples taken from surface in Unit 4 contain up to ~27% of kaolinite in the clay size fraction associated with high amounts of jarosite and boxwork. However, a sample from    141   Marcela Sur host-rock taken from a drill-core from Unit 4 and one sample from Guanaco VII vein host-rock from Unit 2 present up to ~4% of kaolinite. Thus, the latter probably represents steam-heated kaolinite. At Bahía Jara trace amounts of up to 5% steam-heated kaolinite are inferred whereas samples from the surface are dominated by supergene kaolinite.  Figure 5.1. General map of relative erosion levels of different structural blocks. Based on vein textures. Illite crystallinity, geochemistry and fluid inclusion data. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19.  Comparing the clay mineralogy to the compiled geochemical data, blocks from Mallines contain the highest amounts of kaolinite and the largest concentration of As. Laguna Verde and Brillantes vein have affinities related to deeply eroded veins (e.g. high Mo concentrations). Laguna Verde and Brillantes, where well crystallized illite and no kaolinite are present and less products of supergene alteration such as jarosite and boxwork, are considered to be the deepest eroded veins.    142   The sample from Brillantes that contains kaolinite was collected from surface and the kaolinite is likely of supergene oroigin. Samples from Laguna Verde do not contain kaolinite or any other supergene products because all of them correspond to drill core, and no supergene alteration occurred at the sampling depth. The oldest veins from Mallines, emplaced in Unit 4, seem to be the best preserved of the entire district; whereas the veins from Brillantes, emplaced in Units 1 and 2 and the youngest veins from Laguna Verde are the most deeply eroded ones (Fig. 5.1). These features are probably related to differential erosion after the Late Cretaceous, likely during glaciation. Glacial erosion is supported by erratic metamorphic and granitic blocks observed at Laguna Verde; and striae in horizontal rock surface observed at Cerro Porfía in the Bahía Jara area.  5.2 Geochronology: volcanic stratigraphy, subvolcanic domes and mineralization The samples for geochronology were chosen to reconstruct the volcanic and mineralization history of the Cerro Bayo district and also to find the temporal relationship between mineralization and subvolcanic dome emplacement. The biotite  40  Ar-39Ar ages (ca. 154 – 144  Ma) and the zircon U/Pb age (ca. 146 Ma) from Unit 4 correlate well with the defined stratigraphy, being coeval with the zircon U/Pb ages (ca. 146 Ma) of the N-S aligned rhyolitic dikes and domes which in turn are consistent with the field observations that show quartz-pyrite veinlets cutting rhyolitic dikes to the south of the Cerro Bayo dome. The adularia for the 40Ar/39Ar dates was chosen from crustiform texture of veins, were the main material were milky quartz bands. In the Mallines area three veins were dated from System 2 (144.4 ± 1.6 Ma), System 3 (142.3 ± 1.6 Ma) and System 4 (144.6 ± 1.5 Ma), and the ages are consistet with cross cutting relationships. Although the age of System 4 is apparently older than System 2, 40Ar/39Ar analytical data are inconclusive as to what the relative age between the two is. However, System 1 can be considered as the oldest mineralization event of the entire district, but is likely not older than ca. 146 Ma because the rhyolitic domes are cut by quartz veins. The adularia 40Ar/39Ar ages from the Bahía Jara and Brillantes veins represent a movement of the focus of the hydrothermal activity to the north of Mallines. In these areas, hydrothermal activity    143   occurred from ca. 133 Ma to ca. 124 Ma. Although, the ca. 111 Ma Guanaco III vein in the western part of the Bahía Jara area may be considered as a younger mineralization event at Bahía Jara or reflects resetting of the Ar system due to the Laguna Verde hydrothermal activity (from ca. 114 Ma to 111 Ma). A resetting of the system is probably the less likely scenario because of the distance (~4 km) between the Guanaco III vein and the veins from Laguna Verde. The long time span of epithermal mineralization in the Cerro Bayo District is unusual when compared to other low-sulfidation epithermal districts where mineralization is usually constrained within a few m.y. at the most (e.g., El Peñon, Chile: Arancibia et al. 2006, Warren et al., 2008; Fresnillo, Mexico: Velador et al. 2010). Focussed and detailed petrography and fluid inclusion work comparing veins between Bahía Jara and Laguna Verde, potentially backed up by stable and radiogenic isotope studied on vein minerals, can elucidate the association of the Guanaco III vein.  5.3 Environment of vein emplacement The compiled fluid inclusion data show that the highest homogenization temperatures belong to the Laguna Verde area, whereas the Bahia Jara and Brillantes veins have restricted homogenization temperatures, and Mallines has low salinities for the hydrothermal fluids. Assuming that boiling occurred in all veins and plotting this data in the temperature vs. depth diagram from Haas (1971; Fig 5.2) shows that most data indicate a depth of emplacement between 400 and 100 m below the paleo-water table, but it does not differentiate between different areas well. Ouliers of higher temperatures and greater estimated depths of emplacement are geologically unreasonable and may be an artifact of fluid inclusion homogenization temperatures obtained from inclusions that have trapped liquid and vapor. On the other side, the Lower Cretaceous Toqui Formation represents the basal levels of the marine transgression of the Austral Basin that started in the Late Jurassic and is synchronous to the last events of the Ibáñez Formation during the Tithonian, Berriasian, Valanginian and early Hauterivian (Suárez et al., 2009). In the Cerro Bayo area and to the south of it the Toqui Formation has up to 20 m thickness and experienced erosion prior the deposition of the overlying strata (De la Cruz and Suárez, 2008). The Aptian-Albian sub-aereal volcanic rocks of the Divisadero Group croppig out ~20 km south of the Cerro Bayo district cover the Toqui Formation and is 50 to 100m thick. The ca. 132 Ma (K-Ar on biotite; Suárez and De la Cruz,    144   1997a) age for the basal levels of the Toqui Formation at Cerro Torta indicates that the transgression affected the Cerro Bayo district after the Mallines vein emplacement and synchronous to the Bahía Jara and Brillantes mineralization events (ca. 133 – 124 Ma; Fig. 5.3). During the Laguna Verde hydrothermal events and vein emplacement (ca. 114 – 111 Ma), the Ibáñez Formation was possibly covered by the Toqui Formation and volcanism related to the Divisadero Formation was occurring at that time (De la Cruz and Suárez, 2008; Fig. 5.3). Likely the Laguna Verde area hydothermal activity is related to the magmatism related to the Divisadero Formation and/or the Patagonian Batholith. Similar aged subvolcanic intrusions may be present under the Ibáñez Formation with no manifestation on surface. The formation of the veins 100 to 400 m below the paleo-water table puts a maximum thickness constraint on the Toqui Formation in the area. However, given the relatively deep level of erosion and the fact that their currently exposed part is hosted in Unit 2 of the Ibáñez Formation, the Toqui and Divisadero formations were likely only present as thin layers.     145   Figure 5.2. Temperature vs. depth graph showing the fluid inclusion data for the different studied areas. Boilingpoint curves for H2O liquid (0 wt percent) and for brine of constant composition given in wt percent NaC1. The temperature at 0 meters of each curve is the boiling point for the liquid at 1.013 bars (1.0 atm) load pressure which is equivalent to the atmospheric pressure at sea level. The uncertainty is contained within the width of the lines. (Modified from Haas, 1971)     146   Figure 5.3. Schematic environment reconstruction for veins from the different studied areas: A. Mallines. B. Bahía Jara/Brillantes. C. Laguna Verde.     147   6 Conclusions  6.1 Geologic framework of the Cerro Bayo district The Ibáñez Formation in the Cerro Bayo district is, based on the stratigraphic columns documented in this thesis, ~700 m thick and overlies a pre-Jurassic (Paleozoic) metamorphic basement. It can be subdivided into 4 members: (1) a basal andesitic to dacitic coherent lava and volcanoclastic succession; (2) a lower, variably welded rhyolitic to rhyodacitic pyroclastic fragmental succession; (3) a volcanosedimentary unit; and (4) an upper variably welded rhyolitic to rhyodacitic pyroclastic fragmental succession. The age of the Ibáñez Formation is constrained between ca. 154 and 144 Ma and is older and coeval with the ca. 146 Ma N-S aligned rhyolitic domes and dikes that crop out from the Mallines to the Bahía Jara area. The domes were emplaced in the N-S trending Cerro Bayo fault, which probably acted as the conduit for the pyroclastic successions, the domes representing the late stages of the volcanic activity for the Ibáñez Formation in this area. The ca. 83-82 Ma dacitic domes aligned in a NE-SW trend are younger magmatic events recorded for this area compared to the Ibáñez Formation, and they correspond to a previously undocumented Late Cretaceous igneous event.  6.2 Epithermal mineralization at the Cerro Bayo district Mineralization at Cerro Bayo is vein hosted in steeply N-S to NW striking faults, and less importantly NE striking faults. The new  40  Ar/39Ar adularia ages confirm periodic hydrothermal  activity over a time interval of 34 m.y for the Cerro Bayo district. Based on the new and published ages, three main mineralization episodes are defined: (1) Mallines: 144 ± 1.6 to 142.3 ± 1.6 Ma; (2) Bahía Jara and Brillantes: 137 ± 1 to 124.9 ± 1.1 Ma; and (3) Laguna Verde: 114 ± 3 to 111.9 ± 2 Ma. Mineralization at Mallines immediately post-dates the Ibáñez Formation,    148   whereas there are no age equivalent volcanic units related to the younger two episodes of hydrothermal activity outcropping within the study area but the Aptian-Albian Divisadero Formation has been documented ~20 km south of Cerro Bayo (De la Cruz and Suárez, 2008). Based on: (1) the gentle inclination to the east (~5° to 25°) of the volcanic pyroclastic succession; (2) the ~70°W dip of the Cerro Bayo fault; (3) the vertical dips for veins immediately west of the Cerro Bayo dome; and (4) the dips from suvertical to 65°W in the Guanaco block; and assuming a horizontal bedding for the Ibáñez Formation at the moment of the veins emplacement, it is deduced that the Cerro Bayo fault had a mainly subvertical dip at the moment of mineralization, whereas the veins around the Cerro Bayo dome and those from the Guanaco block where emplaced in ~70°E dipping fractures and vertical fractures, respectively. The proximity to the veins indicates that the Cerro Bayo fault was part of the plumbing system for the hydrothermal fluids.  6.3 Regional metallogenetic significances of ages Ore deposits along the Patagonian cordillera including the different mineralized areas at Cerro Bayo and in the Argentinian Deseado Massif can be classified into several metallogenetic episodes in Patagonia. The 40Ar/39Ar adularia ages for the Mallines area (ca. 145 – 142 Ma) are somewhat younger than the ages reported for Manantial Espejo (ca. 154 Ma; Wallier, 2009) in the southwestern part of the Deseado Massif, but similat to the alteration illite K-Ar ages (ca. 142 Ma; mentioned in Schalamuk et al., 1997) associated with mineralization at Cerro Vanguardia, to the south of this massif (Fig 2.1). The youngest mineralization events in Mallines correlate with the ages of the alteration events in the epithermal – mesothermal El Faldeo district (ca. 142 – 140 Ma; Fig. 5.1) further south from the Cerro Bayo district where steeply dipping N-S and NW structures similar to the Mallines vein orientations can be found. The adularia ages from the Bahía Jara and the Brillantes (ca. 137 – 124 Ma) areas can be correlated with an age of ca.130 Ma for a mineralized quartz-diorite porphyry (Townley, 1996) ~55 km south from the Cerro Bayo district, at Halcones-Leones (Fig. 6.1). The range of    149   mineralization ages for the Laguna Verde area (ca. 114 – 111 Ma) are similar to ages reported further north in the El Toqui Pb – Zn – Au skarn, where a range between ca. 116 and 106 Ma., based on biostratigraphy and an altered actinolite 40Ar/39Ar age, respectively (Townley, 1996) are reported. Further west, radiometric data from porphyries near the Mina Silva Pb – Zn and Manto Rosillo Zn skarns with uncertain relation to mineralization gave ages of 100 and 70 Ma (Townley, 1996). With these new 40Ar/39Ar ages it is possible to define three metallogenetic epochs in the Aysén region in the Chilean Patagonia, rather than the upper Jurassic and lower Cretaceous metallogenic epochs defined by Townley and Palacios (1999). The Upper Jurassic mineralization at the Faldeo district, the Lago Azul prospect and at Mallines in the Cerro Bayo district are the westernmost expressions of mineralization related to the Deseado Massif (Fig. 6.1), and they define the first metallogenetic epoch in the Late Jurassic from ca. 144 Ma to 141 Ma. Mineralization at Bahía Jara and Brillantes in the Cerro Bayo district, at the Halcones-Leones prospect, south of Cerro Bayo, and possibly Quebrada Chica and Río Amarillo epithermal prospects define a second, Lower Cretaceous metallogenetic epoch from ca. 137 Ma to 124 Ma (Fig. 6.1). On the other hand, mineralization of lower Cretaceous age from ca. 116 Ma to 106 Ma define the third epoch, a 275 km long NNE trend containing Zn – Pb – (Au) skarns, base metal, polymetallic and epithermal precious metal veins deposits and prospects (Fig. 6.1). In the case of the Laguna Verde area at Cerro Bayo and the Lago Azul prospect this younger epoch is overprinting the Upper Jurassic one. The metamorphic basement, the Ibáñez Formation, the sedimentary Coyhaique Group and younger volcanic sequences host mineralization assigned to this epoch.     150   Figure 6.1. Main structures, deposits and prospects of the Aysén region, Chilean Patagonia. The three defined metallogenic epochs are indicated. The red line define the deposits of the first, Late Jurassic metallogenetic epoch related to the Deseado Massif mineralization events, the orange circles define the second metallogenetic epoch (Lower Cretaceous) and the green lines define the third, also lower Cretaceous metallogenetic epoch. Modified from Townley and Palacios (1999). UJ: Upper Jurassic; LK: Lower Cretaceous.     151   6.4 Clay mineralogy, vein geochemistry, fluid inclusions and geochronology relationships Ore geochemistry and fluid inclusion data are consistent with the hydrothermal clay alteration mineralogy among the different areas. The hydrothermal clay alteration mineralogy around the veins corresponds to illite ± smectite ± kaolinite - chlorite. This mineral assemblage presents characteristic variations between the different areas. In the Mallines area, where the oldest veins were emplaced, the illite + kaolinite - smectite mineral assemblage and the low crystallinity (high crystallinity index) of the illite crystals show that the veins in this area are well-preserved and the surface vein outcrops correspond to high levels within the low-sulfidation epithermal deposit model. This is consistent with the highest As concentrations and the lowest Ag/Au ratios, as well as low Cu, Mo, Pb and Zn concentrations. Fluid inclusions have overall low homogenization temperatures and salinity which is consistent with the textural, alteration and geochemical observations. To the north of Mallines in the Bahía Jara area, the alteration mineral assemblage corresponds to illite ± smectite ± kaolinite, and at Brillantes it corresponds to illite – chlorite - kaolinite. These two synchronous mineralization events are, at ~137 – 124 Ma, are younger than Mallines and show alteration mineral assemblages slightly different between them. In similar rocks, the Brillantes veins show lower illite crystallinity index than those of the Bahía Jara area, and smectite is absent. Thus, Brillantes is representing a deeper environment of the epithermal system compared to the Bahía Jara area (Fig. 5.1). These findings are corroborated by the narrowest fluid inclusion homogenization temperature range for veins from Bahía Jara (196° to 234°C), which in turn have the highest Ag, Au and Cu concentrations of all areas. In contrast, the highest Mo concentrations for surface vein outcrops for the entire district are observed at Brillantes. This is in agreement with the clay mineralogy and a deeper level of exposure compared to Bahía Jara. Host-rocks of the Laguna Verde area show illite – smectite and the highest illite crystallinity (lowest crystallinity index) of all the analyzed samples. Thus, the youngest mineralizing event shows the clearest affinity with base metal mineralization, and in addition it presents the highest     152   homogenization temperatures and salinities of the entire district. This is consistent with the deepest and highest temperature ore fluid circulation within the district. There is no relation between age of mineralization and level of exposure of the veins from Cerro Bayo. It is concluded that the veins suffered differential degree of uplift and erosion after the Late Cretaceous, and degree of glacial erosion was an important factor of preservation or destruction of the veins in the different areas of the district. This is supported by the fact that veins exhibiting deep levels of exposure present little or no supergene minerals compared to the well-preserved veins at Mallines where abundant boxwork and limonites can be observed (Fig. 5.1).  6.5 Exploration implications Clay alteration mineralogy together with geochemistry and fluid inclusion data provide valuable information in the exploration phase of low-sulfidation precious metal deposits. The clay alteration assemblage for the Mallines area indicates mainly illite ± kaolinite with high illite crystallinity index (low crystallinity). These features as well as vein textures, and geochemical data may indicate that Mallines is highly prospective at depth. Possibly, high-grade Ag-Au ore-shoots are located 150 to 250 m below the present erosion surface based on the Buchanan (1981) low-sulfidation epithermal deposit model. Clay alteration assemblages containing illite ± smectite - kaolinite or pure illite, as Bahía Jara and Laguna Verde veins do, with low illite crystallinity values coincide with near ore-shoot levels and can be corroborated by characteristics of veins such as crustiform textures or the presence of adularia as well as geochemistry. At less than 50 to 100 m below the present erosion surface high-grade ore-shoots levels may be present. North of the Cerro Bayo district in the Aysén region, the lower Cretaceous Zn – Pb – (Au) El Toqui skarn deposit is hosted in mainly synchronous marine sediments of the Coyhaique Group (Fig. 3.1), which contains the El Toqui Formation; a unit immediately overlying the Ibáñez Formation in the study area. Thus, a NNE trending ~150 km long area, where volcanic rocks of the Ibáñez Formation are recorded and overlain by the Coyhaique Group and the lower    153   Cretaceous volcanic Divisadero Formation (Fig. 3.1), is potentially prospective for Au - Ag epithermal and Zn – Pb skarn mineralization. To the south of the district there is an area where the first, second and third metallogenetic epochs overlap (Fig. 6.1). This area is more deeply eroded than the northern part of the Aysén region, where the most common prospective outcrops correspond to the pre-Jurassic metamorphic basement and the Jurassic Ibáñez Formation, where hydrothermal alteration zones are observed especially along N-S structures (De la Cruz and Suárez, 2008) possibly following the same trend of the Cerro Bayo fault.     154   References  Ahmed, A.D. 2010. Beyond the Confines of the Ore Deposits: Mapping Low Temperature Hydrothermal Alteration Above, Within, and Beneath Carlin-type Gold Deposits. Unpublished M.Sc. 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Science in China (Series D), Vol. 44, No. 2, pp. 177 – 184.     162   Appendices     163   290000°E  289000°E  288000°E  287000°E  286000°E  285000°E  284000°E  283000°E  282000°E  281000°E  280000°E  279000°E  278000°E  277000°E  276000°E  275000°E  274000°E  273000°E  272000°E  271000°E  270000°E  269000°E  268000°E  267000°E  266000°E  265000°E  4847000°N  SYMBOLOGY Subvolcanic Dacitic Domes  Unit 4 Unit 3  Subvolcanic Rhyolitic Domes  Unit 2  Veins  Unit 1  4846000°N  4845000°N  Fault r r a  LAKE GENERAL CARRERA L  o g a  G  e  r e n  al  C  a r e  4844000°N  4843000°N  4842000°N  Laguna los Cisnes  LAGUNA VERDE  - Chico a Chile  CERRO BAYO Laguna Verde  4841000°N Es te r oE  lB añ  o  Oficinas Laguna Verde  Tranque de Relaves  78º  V POL  77º 76º  74º  IN OR  78º 88º  82º  78º 80º  73º  85º  80º 72º 80º  4840000°N  80º  80º 85º  80º  80º  75º  80º  85º  82º 70º  85º  78º  65º  85º  Pampa la Perra  70º  80º  80º  85º  74º  a  i ch Fa  na  l  80º  74º  85º 71º  Ti na  82º  76º  74º  Loguera Granja Temer  80º 78º  75º  Laguna los Patos  Es te ro  la  85º  4839000°N  76º  65º  Estero el Baño  Este  l ro e  eo Rod  Laguna Caiquenes  4838000°N  4837000°N  Appendix A1. Outcrop Cerro Bayo map showing the different units that crop out in the district. UTM projection, Provisional South American Datum (PSAD) 1969, zone 19. Compiled by Coeur, stratigraphy interpreted according to this thesis  4836000°N  CERRO BAYO DISTRICT 1:20,000 Scale  4835000°N 4834000°N  4833000°N  Appendix A2. Whole-rock geochemical samples location     165   Appendix A3. ALS Chemex geochemical analytical methods and detection limits. Taken from http://www.alsglobal.com/ The ME-MS81 package using Inductively Coupled Plasma - Mass Spectroscopy (ICP - MS) with Lithium Metaborate Fusion (FUS-LI01) was used for trace elements geochemical analysis. A prepared sample (0.200 g) is added to lithium metaborate flux (0.90 g), mixed well and fused in a furnace at 1000°C. The resulting melt is then cooled and dissolved in 100 mL of 4% HNO3 / 2% HCl solution. This solution is then analyzed by inductively coupled plasma - mass spectrometry. The ME-ICP06 package using Inductively Coupled Plasma – Atomic Emission Spectroscopy (ICP – AES) with Lithium Metaborate/Lithium Tetraborate (LiBO2/Li2B4O7) Fusion (FUS-LI01) was used for major elements geochemical analyses. A prepared sample (0.200 g) is added to lithium metaborate/lithium tetraborate flux (0.90 g), mixed well and fused in a furnace at 1000°C. The resulting melt is then cooled and dissolved in 100 mL of 4% nitric acid/2% hydrochloric acid. This solution is then analyzed by ICP-AES and the results are corrected for spectral interelement interferences. Oxide concentration is calculated from the determined elemental concentration and the result is reported in that format Sample preparation quality is monitored at ALS Minerals through the insertion of sample preparation duplicates. For every 50 samples prepared, an additional split is taken from the coarse crushed material to create a pulverizing duplicate. The additional split is processed and analyzed in a similar manner to the other samples in the submission. It should be noted that the precision of the preparation duplicate results is highly dependent on the individual sample mineralogy, analytes of interest and procedures selected for sample preparation. Therefore the data are most relevant at the client project level. All preparation duplicate data is automatically captured, sorted and retained in the QC Database and available on Webtrieve™ for client review. The data are also available on the QC Data Certificates. Quality control samples including certified reference materials, blanks, and duplicates are inserted within each analytical run. The blank is inserted at the beginning, standards are inserted at random intervals, and duplicates are analyzed at the end of the batch. The minimum number of quality control samples required to be inserted are based on the rack size specific to the method.    166   Appendix A3. Cont.     167   Appendix A4. Whole-rock geochemistry analyses from host rock from the Ibáñez Formation at the Cerro Bayo Ag-Au epithermal district     168   Appendix A4. Cont.     169   Appendix A4. Cont.     170   Appendix A4. Cont.     171   Appendix A4. Cont.     172   Appendix A5 Geochemical field duplicates     173   Appendix A6. Geochemical standard analysis     174     Appendix A7. ID-TIMS U-Th-Pb isotopic data.    175                                            Appendix A8. Laser ablation U-Pb isotopic data         176        Appendix A9. 40Ar/39Ar biotite analytical data   177     Appendix A9. Cont.    178   Appendix A10. 40Ar/39Ar adularia analytical data    179     Appendix A10. Cont.  180      Appendix A10. Cont.  181     Appendix A10. Cont.  182   Appendix A11. Comparative results for Ag geochemical analyses between ALS Chemex laboratories and Cerro Bayo laboratories. A. Ag vs Ag from vein drill-core samples. B. Ag vs Ag for surface vein samples     183   Appendix A12. Compiled geochemistry data from Coeur d’Alene Mines and C. Hermosilla (2009, written commun.), showing the main, minimum and maximum values for the chosen elements in the different veins and areas     184   Appendix A12. Cont.     185   Appendix A12. Cont.     186   Appendix A12. Cont.     187   Appendix A12. Cont.        188   

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