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Geological, structural and geochronological framework of the Veladero North area, Cordillera Frontal,… Charchaflie, Diego 2003

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GEOLOGICAL, STRUCTURAL AND GEOCHRONOLOGICAL FRAMEWORK OF THE VELADERO NORTH AREA, CORDILLERA FRONTAL, ARGENTINA By DIEGO CHARCHAFLIE Licenciado en Ciencias Geologicas, Universidad de Buenos Aires, 1994 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE In THE FACULTY OF GRADUATE STUDIES (Department of Earth and Ocean Sciences) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA January 2003 © Diego Charchaflie, 2003 In presenting this thesis in partial fulfilment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. Department The University of British Columbia Vancouver, Canada DE-6 (2/88) Abstract The Veladero North area, located in the northern part of the El Indio-Pascua Belt within the Cordillera Frontal of Argentina and Chile, contains gold reserves and resources that combined exceed 370 tons (12 Moz). Throughout the Belt, mineralization is related to Miocene magmatic activity and both high- and low-sulfidation systems have been mined. The Miocene volcanic rocks form several formations separated by unconformities from underlying Carboniferous to Triassic volcanic and plutonic rocks. Whereas age and structural relationship of these rocks are well known in Chile, the geology of the Argentinan flank is poorly documented. This study defines the stratigraphy, geology and tectonic evolution of the Veladero North area and clarifies the regional framework of the Cordillera Frontal in Argentina. Thrust faults that dip steeply (70° to 90°) to the west and back-thrusts that dip steeply (70° to 90°) to the east define the overall geometry of the Miocene fold and thrust belt around the Veladero North area. The thrust panels comprise Permian (259 ± 0.7 Ma to 254 ± 4.2 Ma, U-Pb) rhyolitic volcanic and volcaniclastic rocks assigned to the Guanaco Sonso Formation and late Oligocene to early Miocene (24.5 ± 0.2 Ma to 22.8 ± 1.7 Ma, U-Pb) andesitic to dacitic pyroclastic and sedimentary rocks equivalent to the Tilito Formation. Middle Miocene (15.8 ± 1 Ma, U-Pb and 12.7 ± 0.9 Ma to 11.0 ± 0.2 Ma, 4 0 Ar - 3 9 Ar ) pyroclastic rocks and shallow intrusives of the Cerro de las Tortolas Formation and Infiernillo Unit, and Vacas Heladas Formation in the study area unconformably overlie the fold and thrust belt. These strata are sub-horizontal or dip shallowly (<20°) to the east, and are separated from the thrusted rocks by two regional, low-relief unconformities. A Pliocene (2.1 ± 0.5 Ma, 4 0 Ar - 3 9 Ar ) rhyolitic dome in the eastern part of the district forms the Cerro de Vidrio Formation, the youngest recognized unit of the region. Previously published and new geochronological data suggest that late Paleozoic to Triassic magmatism in the Cordillera Frontal occurred as a series of plutonic and volcanic episodes isolated by periods of magmatic quiescence. The Choiyoi Group apparently comprises two independent volcanic units emplaced before and after a 25 m.y. volcanic lull, of which, the older and widespread Permian Guanaco Sonso Formation is exposed in the Veladero North area. Structural relations and geochronology indicate that the largest part of Tertiary deformation in the study area occurred throughout the Miocene, and is best explained by multiple phases of shortening. The youngest host rock of the Veladero North epithermal deposit is a volcanic succession dominated by heterolitic bedded breccias, volcaniclastic sandstones and dome-related rocks equivalent to the (16 Ma to 14.9 Ma) Cerro de las Tortolas Formation and its intrusive counterpart, the Infiernillo Unit. If the mineralization in Veladero North is the same age as that in the rest of El Indio-Pascua Belt (9.5 Ma to 6 Ma), the volcaniclastic package is then several million years older than the mineralization. Thus, the genesis of the package bears little relationship to gold mineralization except for forming a porous host rock for it. Table of contents Abstract ii Table of Contents iv List of Tables vii List of Figures viii Acknowledgements x Chapter 1 General Introduction General Introduction 1 Methodology 3 Presentation 4 Alteration 5 References 6 Chapter 2 Late Paleozoic to early Mesozoic rocks of the Cordillera Frontal of Argentina and Chile with emphasis on the Veladero North Area, San Juan Province, Argentina Abstract 8 Introduction 9 Regional late Paleozoic to early Mesozoic geologic framework 11 Morphostructural elements 11 Basement 13 Gondwana Magmatism 14 Volcanic rocks 17 Choiyoi Group 17 Plutonic rocks 20 Elqui Superunit 20 Ingaguas Superunit 21 Colanguil Batholith 23 Veladero North area geologic setting 26 Volcanic and volcaniclastic rocks 29 Rfo Taguas Package 29 Guanaco Zonzo Package 30 Potrerillos-Canito Package 33 Intrusive rocks 33 iv Geochemistry of late Paleozoic rocks of the Veladero North area 36 Major and trace element geochemistry 40 Rare earth element geochemistry 40 Discussion 41 Summary of Veladero North Sequence 41 Regional correlation 43 Conclusions 46 References 47 Chapter 3 Geological framework of the Veladero North area, Cordillera Frontal, Argentina Abstract 52 Introduction 53 Geologic setting of the El Indio Belt 54 Late Paleozoic to Jurassic basement rocks 58 Tertiary stratigraphy of the El Indio Belt 58 Eocene to early Oligocene rocks 58 Late Oligocene to late Pliocene rocks 61 Structure 63 Remanent late Miocene landforms 64 Miocene volcanic framework of the Veladero North area 64 Tilito Formation 65 Rio Taguas structural package 65 East Veladero structural package 67 West Veladero structural package 68 Andesitic intrusive rocks equivalent to the Tilito Formation 71 Cerro de las Tortolas Formation 78 Veladero Section ; 78 Turbio River Section 81 Fabiana Section 82 Infiernillo Intrusive Unit 83 Vacas Heladas Formation 86 Geochemistry 87 Structural framework of the Veladero North area 91 Lithologic association of the epithermal deposit 94 Timing of deformation 95 Conclusions 99 References 100 Chapter 4 Conclusions and recommendations for future research Conclusions 105 Geology 105 Late Paleozoic volcanic-plutonic arc 106 Miocene volcanism and deformation 106 Lithologic association of the epithermal deposit 107 Recommendations 108 References 109 Appendices Appendix I: Geochronology I l l Conventional U-Pb I l l Guanaco Sonso Formation 112 Tertiary rocks 114 SHRIMP 116 References 118 Appendix II Sample description and location 119 Appendix III XRF and ICP-MS Geochemistry 123 Whole-rock geochemistry 123 Detection limits 123 Appendix IV Alteration 125 List of Tables Chapter 2 Table 2-1: Lithology and geochemistry of Gondwanan magmatism 18 Table 2-2: Geochronologic studies in the Cordillera Frontal 28 Table 2-3: U-Pb geochronology in the Veladero North area 31 Table 2-4: Geochemistry of Permian rocks in the Veladero North area 37 Chapter 3 Table 3-1: Veladero North area SHRIMP data 72 Table 3-2: Veladero North area U-Pb data 74 Table 3-3: Veladero North whole-rock geochemistry 89 List of Figures Chapter 1 Figure 1-1: Location of the Veladero North area 2 Chapter 2 Figure 2-1: Distribution of late Paleozoic - early Mesozoic magmatism of western South America 10 Figure 2-2: Morphostructural elements of the Chilean-Argentinan Andes 12 Figure 2-3: Cordillera Frontal plutonic and volcanic units, major structures and location of geochronological studies 15 Figure 2-4: Magmatic stratigraphy of the Cordillera Frontal 16 Figure 2-5: Geology of the Pascua-Lama-Veladero North area 27 Figure 2-6: Rio Taguas east-west cross-section 29 Figure 2-7: Concordia Plots of Volcanic rocks in the Veladero North area 32 Figure 2-8: Guanaco Zonzo creek exposures 34 Figure 2-9: Concordia Plots of Intrusive rocks in the Veladero North area 35 Figure 2-10: Major and trace element concentration diagrams 38 Figure 2-11: REE diagrams 39 Figure 2-12: Geochronologic studies in the Cordillera Frontal 42 Figure 2-13: Summary of late Paleozoic to Mesozoic magmatic episodes in the Cordillera Frontal of Argentina and Chile 46 Chapter 3 Figure 3-1: Location of the El Indio-Pascua Belt and major Tertiary mineralization districts 54 Figure 3-2: Regional Geology of the El Indio-Pascua Belt and location of major deposits and mines 55 Figure 3-3: Convergence and obliquity between the Nazca and South America plates during the last 40 m.y 57 Figure 3-4: El Indio-Pascua Belt stratigraphy 59 Figure 3-5: Veladero-Pascua-Lama district geology 60 Figure 3-6: Tilito Formation, Rio Taguas structural package 66 Figure 3-7: Tilito Formation, east of Veladero North area 67 Figure 3-8: Tilito Formation, West Veladero structural package 70 Figure 3-9: Tilito Formation intrusives 71 Figure 3-10: SHRIMP data in Veladero North area 73 Figure 3-11: Concordia diagrams, Veladero North data 76 Figure 3-12: Cerro de las Tortolas Formation, Veladero Section 79 Figure 3-13: Cerro de las Tortolas Formation bedding 80 Figure 3-14: Cerro de las Tortolas Formation, south of Turbio river 82 Figure 3-15: Cerro de las Tortolas Formation, Fabiana Prospect area 83 Figure 3-16: Infiernillo Intrusive Unit 84 Figure 3-17: Vacas Heladas Formation 87 Figure 3-18: Whole-rock geochemistry of the Veladero North lithostratigraphic units 88 Figure 3-19: East-west cross section of the northern part of the study area. 92 Figure 3-20: East-west cross section of the southern part of the study area. 92 Figure 3-21: Schematic east-west cross section of the Veladero North area. 97 ix Acknowledgements I arrived to Vancouver in September 2000 with several vague ideas. At that time, the more clearly delineated ones were to complete a research program related to the geology of Veladero North deposit and to spend more than two weeks in a row with my girlfriend. Time went by very fast, many and diverse things happened and, a little more than two years later, I defended my M.Sc. thesis. There are many people that I would like to thank for their contributions to this thesis. First, I would like to thank Dick Tosdal for the countless revisions and suggestions to the manuscript. I am indebted to Dick for his support in academical as well as non-academical issues during all this time. I also wish to thank the members of the committee, Dick Chase, Jim Mortensen, James Scoates and Kelly Russell for their additional suggestions to the thesis. Particularly, I would like to thank Kelly who survived a visit the study area and fuelled interesting geologic discussions. This project was funded by MDRU and Homestake, with additional support from the Thomas and Marguerite MacKay Memorial Scholarship. This research would not have been possible without the collaboration and incentive of Homestake geologists: Don Lewis, Bill Wright and Ricardo Martinez. David Heberlain and Jay Hodgson from Barrick provided assistance when visiting Lama as well as a thoughtful discussion on the geochronologic results. I would like to thank Thomas Bissig for providing some of his figures, maps and data before its publication and Alfredo Vitaller for interesting discussion during the first part of this research. I am indebted to Veladero project geologists and staff for the logistical support in the field, largely provided by Ismael Chavez, Mary Carrizo, Jose Luis Gomez and Chris Jones. I will always remember Gabriel Campillay for managing to cook out -of - the-menu-then-tasty dishes that made life in the Cordillera more enjoyable. I would also like to thank my friends and fellows graduate students from UBC: Lawrence Winter, Geoffrey Bradshaw, Simon Haynes, Scott Heffernan and Pat Hayman who contributed to this thesis and to the insertion into a different culture in Canada. Although over the past years they were unsuccessful in attempting to introduce me to hockey, I am grateful for revealing to me other Canadian fundamentals. There are many people that I would like to thank for their support. My parents Chacha and Marie; my family Ivan, Machy, Marina, Ricardo, Alicia and Angel. Also my / spanish-speaking friends of Vancouver: Sergio and Paola Jaramillo, Carlos and Mar Ramirez Medina, Sergi Molins and Ursula Cass; my friends from Argentina: Benoit and Vanesa Remy, Juan Manuel and Alexandra Molinari, Fernando Ruarte, Laura Net and many others that I am probably forgetting. Finally, I would like to thank my wife Diana for her patience, companionship and encouragement during the past two years. Of all the individuals that supported this research, it is with her that I am most indebted. xi Chapter 1 General Introduction The Veladero North deposit is located in the northern part of the 150-km long El Indio-Pascua Belt that straddles the international border between Argentina and Chile (Figure 1-1). The belt is named after the El Indio mine, a multi-million ounce gold deposit discovered ca. 1978, and Pascua, one of the major ongoing exploration projects of the region. Veladero North, together with the nearby Pascua and Lama prospects represent one of the largest undeveloped gold and silver mining properties in the world with gold reserves of over 30 million ounces and 71.3 million ounces of silver reserves (Barrick data, 2002). To date (December 2002), reserves and resource estimation for the Pascua-Lama-Veladero are in aggregate nearly 2.5 times the total metal production from the El Indio-Pascua Belt. Mineralization and alteration throughout the El Indio-Pascua Belt was related to epithermal processes intimately associated to Tertiary magmatism, largely Miocene, and are hosted by late Paleozoic as well as Cenozoic rocks (Deyell, 2001; Bissig et al , 2001). Most deposits and exploration targets in the Belt are classified as high-sulfidation, and include venous (El Indio) and breccia-hosted (Tambo) mineralization. The Rio del Medio deposit is the only known occurrence of low-sulfidation veins of economic mineralization in the region. The El Indio-Pascua Belt occupies the centre of the present-day amagmatic segment of the Central Andes between the volcanically active Central and Southern Volcanic Zones (e.g. Jordan et a l . , 1983). The amagmatic nature of the segment has been related to the current shallow subduction angle (<10°) of the Nazca oceanic plate beneath the South American plate (Barazangi and Isacks, 1976). However, subduction-related magmatism and deformation in the general area of the El Indio-Pascua Belt were active throughout the Tertiary, as well as during the late Paleozoic to early } + « + + + + + + + + j + + + + ^  + +f / • + • • 4 •  ^  - • + +/+ * * * + \ * I J IN + +1* * * * * + fcSm S?r&+*I*. A V + + + + J + 4 + > + + , 4. \.Pofrerillosl * * \»+ + + 4 R i o W ^ ,--4 ^V.eladero, ' North K C Plate 1J + + + + +(+ + + + + + +\-+ /+ + + + + •• A + + + + + -Fabiana* 4. 4. . V ^^Veladero South' »1 V * 6 ^ / < / f V°° J + + + + jRenaca / . + + ^ + + 0 + / \ + + + + + J / + + RiofdehMedio^J* n + + J + «, + + + • + •£+ + + + + +-+ + + + . + 2 + + + + + + + + 6°. Dona Ana + C°. Escabrosp /O Deidad Rio Frio wacas'Heladas « + + + c + 5+ + + + o £ + + + • +TJ+ + + + ^ + ' f + + 30° S + + + + + + + + + + + + + + + + + + + + + + + + + _ Simplified geology Late Pliocene Cerro de Vidrio Formation Late Miocene Vallecito Formation Middle to late Miocene Vacas Heladas Formation Late Eocene to Middle Miocene Cerro de las Tortolas Formation Escabroso Formation Tilito Formation Bocatoma intrusive unit (- -t| Late Paleozoic to Jurassic Choiyoi Group, Elqui-Limari Batholith and Colanguil Batholith Faults >*4 Reverse faults » » * Lineaments X Mine O XX Exploration prospects «„ . . International border Argentina-Chile 10 km Figure 1 -1: Regional Geology of the El Indio-Pascua Belt and location of major alteration zones and mines. Modified from Bissig et al. (2001). Box represents the study area and the location of the 1:20,000 scale map in Plate 1. Abbreviations: BdTF=Bahosdel Toro Fault, P-V L- Pascua-Veladero Lineament, Co F=Colangiiil Fault. 2 Mesozoic (e.g. Martin et a l . , 1995). Porphyry and epithermal systems as well as Cenozoic and Paleozoic igneous units record the evolution of the convergent margin and define the tectonic setting in which they were emplaced. The absence of thick Mesozoic sedimentary units characterizes the Cordillera Frontal, and differentiates the morphostructural province where the El Indio-Pascua Belt is entirely comprised from the Cordillera Principal to the south (e.g. Herve et a l . , 1987). The purpose of this study is to clarify the geology of the Veladero North epithermal deposit at the northern end of the El Indio-Pascua Belt. The deposit is the southernmost of three epithermal deposits, Pascua, Lama and Veladero that are aligned along a WNW trending zone. Specifically, the aim is to map the distribution and structure of lithostratigraphic units and to determine their age. This work provides the foundation upon which to build a mineralization model of the Veladero North deposit, and is a prelude to further regional investigation in the Argentinan side of the El Indio-Pascua Belt. Methodology Geological mapping of the over 120 k m 2 study area was done at 1:10,000 scale using aerial photos and GPS to locate the scattered outcrops among the colluvial covered slopes. In total, approximately four months were spent in the field. The collected field data included lithology, alteration and structural fabrics as well as rock samples (see appendix II for data collection stations). Field mapping also recorded the location of planar landforms recently interpreted as Miocene erosion surfaces (Bissig et a l . , 2001). Eight volcanic units and four intrusive phases were defined based on stratigraphy and lithology. Ninety-two samples were prepared for petrographic analysis to describe unit lithologies and mineralogical assemblages. An initial suite of 68 samples representing the least altered rocks was submitted to Bondar-Clegg in Vancouver for major and minor element analysis via X-Ray Fluorescence (XRF). Based on the petrology and the initial geochemistry, 34 samples were selected and sent to Memorial University of Newfoundland for Inductively Coupled Plasma Mass Spectrometry (ICP-MS) analysis (See appendix III for detection limits). The high-quality of the ICP-MS data provided a basis to compare the igneous chemistry from the Veladero North area with previous studies in the El Indio-Pascua Belt (Bissig, 2001; Kay et a l . , 1987; Malizia et al , 1997; Mpodozis and Kay, 1992; Llambias and Sato, 1990). Twelve rock samples from intrusive and volcanic units were dated using conventional U-Pb techniques in the Geochronology Laboratory of the University of British Columbia. Three additional Sensitive High-Resolution Ion Microprobe (SHRIMP) U-Pb age determinations were completed in the facilities of Stanford University, California by Richard Tosdal (written communication, 2002). The U-Pb method was required in this study because the pervasive and locally intense hydrothermal alteration of the rocks around the Veladero North epithermal deposit preclude the use of other techniques (e.g. K-Ar system). Presentation This research is presented as two separate scientific communications that will be submitted to peer-reviewed journals for publication. The first communication, entitled "Late Paleozoic to early Mesozoic rocks of the Cordillera Frontal of Argentina and Chile with emphasis on the Veladero North Area, San Juan Province, Argentina" (Chapter 2), describes the Permian volcanic, volcaniclastic and intrusive rocks that account for as much as half of the outcrops of the study area, and although previously unrecognized, may be a significant host to gold, as they are at Pascua. The late Paleozoic rocks are unmetamorphosed and have the same overall petrologic character when altered as do the Cenozoic rocks, which are largely late Oligocene to Miocene in age (Martin et a l . , 1995). In many cases, major and trace-element geochemistry or geochronological studies can distinguish the rocks of such d i s p a r a t e a g e . T h e c h a p t e r a l s o r e v i e w s a v a i l a b l e . g e d c h r o n o l o g i c a n d g e o c h e m i c a l i n f o r m a t i o n o f t h e C o r d i l l e r a F r o n t a l o f A r g e n t i n a a n d C h i l e , a n d p r o p o s e s a r e v i s i o n o f t h e m a g m a t i c s t r a t i g r a p h y w i t h s p e c i a l a t t e n t i o n t o t h e v o l c a n i c r o c k s t h a t f o r m t h e C h o i y o i G r o u p . T h e l a t t e r v o l c a n i c r o c k s a r e a m a j o r r o c k u n i t i n t h e s t u d y a r e a . T h e s e c o n d c o m m u n i c a t i o n ( C h a p t e r 3) f o c u s e s o n t h e M i o c e n e v o l c a n i c s e q u e n c e s t h a t h o s t s t h e V e l a d e r o N o r t h e p i t h e r m a l d e p o s i t . T h e p r i m a r y o b j e c t i v e i s t o d o c u m e n t t h e c h a r a c t e r a n d a g e o f t h e v o l c a n i c r o c k s , s t r u c t u r a l g e o m e t r y a n d t i m i n g o f d e f o r m a t i o n i n t h e V e l a d e r o N o r t h a r e a . T h e l o c a l i n f o r m a t i o n i s t h e n c o m b i n e d w i t h r e g i o n a l d a t a t o p l a c e t h e s t u d y a r e a i n a g e o l o g i c c o n t e x t . T h i s c h a p t e r a l s o e v a l u a t e s w h e t h e r s p e c i f i c v o l c a n i c r o c k s m i g h t b e r e l a t e d t o t h e m i n e r a l i z a t i o n i n t h e a r e a . C o m p l e m e n t a r y i n f o r m a t i o n is p r e s e n t e d i n t h e a p p e n d i x e s o f t h e t h e s i s . T h e A p p e n d i x I is a d e t a i l e d d e s c r i p t i o n o f t h e m e t h o d o l o g y a n d r e s u l t s o b t a i n e d b y U - P b g e o c h r o n o l o g y a n d r e f e r r e d t o i n t h e t e x t . T h e A p p e n d i x e s I I t o I V g r o u p a d d i t i o n a l d a t a r e g a r d i n g s a m p l e l o c a t i o n , g e o c h e m i s t r y a n d a l t e r a t i o n . Alteration H y d r o t h e r m a l a l t e r a t i o n is s u p e r i m p o s e d o n p r a c t i c a l l y a l l r o c k s a r o u n d t h e V e l a d e r o N o r t h e p i t h e r m a l d e p o s i t . D e t e r m i n a t i o n o f t h e p r e c i s e n a t u r e a n d t i m i n g o f t h e h y d r o t h e r m a l a l t e r a t i o n , a n d h e n c e o f t h e m i n e r a l i z a t i o n i n t h e s t u d y a r e a , a r e b e y o n d t h e s c o p e o f t h i s r e s e a r c h . A l t h o u g h s u c h i n v e s t i g a t i o n s a r e c u r r e n t l y i n p r o g r e s s ( e . g . L a M o t t e , i n p r e p . ) , t h e p r e s e n t c o n t r i b u t i o n d o e s n o t b e n e f i t f r o m p r e l i m i n a r y o r u n p u b l i s h e d r e s u l t s . D i s c u s s i o n o f m i n e r a l i z a t i o n a n d a l t e r a t i o n o f t h e V e l a d e r o N o r t h d e p o s i t ( C h a p t e r 3) i s b a s e d o n d a t a c o l l e c t e d i m m e d i a t e l y n o r t h o f t h e s t u d y a r e a a s p a r t o f a r e g i o n - s c a l e i n v e s t i g a t i o n ( B i s s i g e t a l . , 2001). F o u r m a j o r h y d r o t h e r m a l a l t e r a t i o n e v e n t s h a v e b e e n d o c u m e n t e d i n t h e C e n o z o i c r o c k s o f El I n d i o - P a s c u a B e l t . G e o c h r o n o l o g i c s t u d i e s s u g g e s t t h a t , a l t h o u g h d i s c r e t e h y d r o t h e r m a l a c t i v i t y h a s b e e n a s s o c i a t e d w i t h e v e r y m a g m a t i c e p i s o d e between ~36 Ma and ~ 1 1 Ma, auriferous epithermal mineralization is limited to the 9.5 Ma to 6 Ma interval (Bissig et a l . , 2001). In addition, late Paleozoic to Jurassic subvolcanic porphyries also have associated alteration halos, although these appear to be barren and of more limited extent. In the Veladero North area, recognized alteration assemblages are propyllitic, advanced argillic and silicic (Deyell, 2001; Bissig, 2001; Jones et a l . , 1999). Typical assemblages in propylitic alteration zones consist of chlorite, clays (kaolinite-illite) with rare epidote. Fine-grained masses of clays partly replace feldspars and ferromagnesian minerals. Pyrite is an accessory component but, if present, is finely disseminated. The rock texture is normally preserved in this type of alteration. Advanced argillic alteration is characterized by alunite, quartz and clays (kaolinite, illite, dickite or pyrophyllite). The altered rock is pervasively replaced and the original texture is commonly obliterated. Silicified rocks are widespread in the study area. Commonly, if the clays have been leached out, the resulting alteration assemblage is formed exclusively by quartz, and the texture and the original mineralogy of the rock are completely obliterated. The spatial association of silicification and gold mineralization is of particular interest on the Veladero North area. Mineralogical and geochemical characteristics of the alteration assemblages were extensively described by Deyell (2001) and are listed in the Appendix IV. References Barazangi, M., and Isacks, B.L., 1976, Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America: Geology (Boulder), v. 4, p. 686-692. Barrick 2002, (http://www.barrick.com/6_About_Barrick/6_02_History.asp). Bissig, T., 2001, Metallogenesis of the Miocene El Indio-Pascua gold-silver-copper Belt, Chile/Argentina: geodynamic, geomorphological and petrochemical controls on epithermal mineralization [Ph.D. thesis]: Kingston, Queen's University. Bissig, T., Lee, J . , W, Clark, A., H, and Heather, K., B, 2001, The Cenozoic history of volcanism and hydrothermal alteration in the Central Andean Flat-Slab Region: New 4 0 A r - 3 9 A r constraints from the El Indio-Pascua Au (-Ag, Cu) Belt, 29°20'-30°30' S : International Geology Review, v. 43, p. 312-340. Bissig, T., Clark, A., H, Lee, J . , W, and Hodgson Jay, C , 2002, Miocene Landscape Evolution and Geomorphologic Controls on Epithermal Processes in the El Indio-Pascua Au-Ag-Cu Belt, Chile and Argentina: Economic Geology, v. 97, p. 971 -996. Deyell, C.L., 2001, Alunite and high sulfidation gold-silver-copper mineralization in the El Indio-Pascua belt, Chile-Argentina [Ph. D. thesis]: Vancouver, Mineral Deposit Research Unit. University of British Columbia. Canada. Herve, F., Godoy, E., Parada, M.A., Ramos, V., Rapela, C.W., Mpodozis, C , and Davidson, J . , 1987, A general view on the Chilean-Argentine Andes, with emphasis on their early history, in Monger, J .W.H. , and Francheteau, J . , eds., Circum-Pacific orogenic belts and evolution of the Pacific Ocean basin., Volume 18: Geodynamics Series: Washington, DC, United States, American Geophysical Union, p. 97 -113. Jones, P.J., Martinez, R.D., Vitaller, A.O., Chavez, I., Carrizo, M.M., La Motte, M.G., and Riveros, S.E. , 1999, El Deposito Epitermal Aurifero Veladero, San Juan, in Zappettini, E.O., ed. , Recusos Minerales de la Republica Argentina, Volume Anales 35: Buenos Aires, Instituto de Geologia y Recursos Minerales SEGEMAR, p. 1673-1648. Jordan, T.E., Isacks, B.L., Allmendinger, R.W., Brewer, J.A., Ramos, V.A., and Ando, C.J., 1983, Andean tectonics related to geometry of subducted Nazca Plate: Geological Society of America Bulletin, v. 94, p. 3 4 1 - 3 6 1 . Kay, S . M . , Maksaev, V., Moscoso, R., Mpodozis, C , and Nasi, C , 1987, Probing the evolving Andean lithosphere; mid-late Tertiary magmatism in Chile (29 degrees -30 degrees 30') over the modern zone of subhorizontal subduction: Journal of Geophysical Research, B, Solid Earth and Planets, v. 92, p. 6173-6189. Llambias, E.J., Sato, A . M . , and Castro, C.E., 1990, Relaciones entre el grupo Choiyoi y el Batolito de Colanguil, Actas del Decimo primer Congreso Geologico Argentino, Volume 1: San Juan, Asociacion Geologica Argentina, p. 7 9 - 8 2 . Malizia, D., Limarino, C O . , Sosa Gomez, J . , Kokot, R., Nullo, F.E., and Gutierrez, P.R., 1997, Hoja geologica Cordillera del Zancarron (Provincia de San Juan) N° 3169-26 y 25: Buenos Aires, Servicio de Geologia y Mineria de Argentina (SEGEMAR), 197 p. Martin, M.W., Clavero, J . , Mpodozis, C , and Cuitino, L., 1995, Estudio geologico regional de la franja El Indio Cordillera de Coquimbo, Servicio Nacional de Geologia y Mineria, Compania Minera San Jose, 238 p. Mpodozis, C , and Kay, S . M . , 1992, Late Paleozoic to Triassic evolution of the Gondwana margin; evidence from Chilean Frontal Cordilleran batholiths (28° S to 31° S) ; with Suppl. Data 9 2 - 2 2 : Geological Society of America Bulletin, v. 104, p. 999-1014. 7 Chapter 2 Late Paleozoic to early Mesozoic rocks of the Cordillera Frontal of Argentina and Chile with emphasis on the Veladero North Area, San Juan Province, Argentina Abstract In the Veladero North area, located in the Cordillera Frontal of Argentina and Chile, late Paleozoic rocks form homoclinal or smoothly folded volcanic sequences that define three north-south trending structural packages; each comprising similar volcanic, volcaniclastic and intrusive rocks. The sequence is included within the Choiyoi Group and is separated from Tertiary units by an angular unconformity or by Tertiary thrust or normal faults. Rhyolitic to dacitic ignimbrite with varying degrees of welding is the dominant extrusive facies, followed by interbedded volcaniclastic sandstone and conglomerate beds. Relatively small (less than 1 km 2 ) , intensely altered, plagioclase-and quartz-bearing dacitic to rhyolitic stocks intrude the volcanic rocks. Uranium-lead geochronology and distinct geochemical characteristics indicate that volcanic and intrusive rocks in the study area are equivalent, and were emplaced between 259 ± 0.7 Ma and 254 ± 4.2 Ma. Revision to published and new geochronologic data hint that late Paleozoic to Mesozoic magmatic activity in the Cordillera Frontal occurred during distinct episodes. Plutonism is represented by the (320 Ma to 280 Ma) Guanta and Cochiguas units, the (270 Ma to 325 Ma) Chollay-EI Leon Unit and the (220 Ma to 190 Ma) Los Colorados and Carricitos units. The (275 Ma to 250 Ma) Guanaco Sonso Formation and the (225 Ma to 210 Ma) Los Tilos Formation constitute two independent volcanic episodes that overlap partially with plutonic events, suggesting diachronism between plutonic and volcanic pulses. Introduction Paleozoic to early Mesozoic magmatism along the Pacific margin of southern South America (Gondwana) extended from northern Peru to southern Argentina over a distance of almost 5000 km (Mpodozis and Kay, 1990, 1992; Kay eta/ . , 1989; Ramos et al., 1988; Bell, 1987; Mpodozis and Ramos, 1989; Petersen, 1999; MacFarlane et al., 1999; Sato and Llambfas, 1993). Magmatism was accompanied by complex transpressional and extensional tectonic regimes that are poorly constrained (Mpodozis and Ramos, 1989; Kay etal., 1989; Gonzalez Bonorino, 1991). In particular, the recent proposals linking Paleozoic margin activity to early Paleozoic interactions between Gondwana and Laurentia (North America) remain a topic of current debate (Dalziel, 1997; Ramos, 1988; Herve, 1988). Establishing whether such a linkage existed is important because Paleozoic and early Mesozoic geology strongly influenced the younger and superposed middle Mesozoic to Recent Andean arc (e.g. Ramos eta/ . , 1996). The 2500-km long segment of late Paleozoic to early Mesozoic rocks between northern Chile-western Bolivia and southern Argentina is the best known part of the Gondwanan arc (Figure 2-1) . Through reconnaissance geological studies, plutonic complexes and volcanic successions with local formation names have been broadly grouped into Paleozoic arc rocks or into a Gondwana granite-rhyolite province, or simply referred to as pre-Andean magmatic rocks (Mpodozis and Kay, 1990, 1992; Ramos, 1988; Parada, 1990; Heredia etal., 2002). These rocks in central Chile have been well documented by mapping at different scales (Mpodozis and Cornejo, 1988; Nasi et al., 1990; Martin et al., 1995; 1999). Reconnaissance geochemical and geochronological studies support the established stratigraphic relationships (Pankhurst et al., 1996; Parada e ta/ . , 1991; Parada, 1990; Mpodozis and Kay, 1990; Ribba etal., 1988; Nasi et al., 1985). 9 072"°W P e r H ' , Bolivia -20°S Chanaral Granites -28°S Elqui-Limari Batholith (Elqui and Ingaguas Superunits) De la Costa Granites Argentina Colangiiil Plutons h-36°S r-44''S Granite-Rhyolite Province 1 Choiyoi Group Inferred limit Dased on Present day outcrops and drillhole intercepts. Rhyolitic lavas [—~\ Granitoids and lavas Figure 2-1: Distribution of late Paleozoic - early Mesozoic magmatism of western South America. Modified from Kay et al. (1989) and Petersen (1999). Southeastern boundary of the granite-rhyolite province (dashed line) is represented by the approximate contact with the Chon-Aike volcanic province (not shown). In stark contrast to Chile, the late Paleozoic and early Mesozoic Gondwanan arc in Argentina is poorly understood (Heredia etal., 2002; Malizia et al., 1997; Llambias and Sato, 1995). Most studies in the Argentinan Andes have focused on Tertiary rocks (e.g. Bissig, 2001). The volumetrically more important Paleozoic to early Mesozoic rocks have been largely ignored except for several local investigations of Paleozoic volcanic sequences and plutonic complexes (Sato and Llambias, 1993; Rapalini and Vilas, 1991; Llambias et al., 1990; Llambias and Sato, 1990, 1995; Llambias et al., 1987). As a 10 general rule, all Paleozoic volcanic rocks in Argentina are assigned to the Choiyoi Group, which is part of the granite-rhyolite province. Locally, the rocks have been subdivided into formations (Mirre, 1966; Coira and Koukharsky, 1976; Rodriguez Fernandez et al., 1997; Heredia et al., 2002), but the distribution of the formations throughout the granite-rhyolite province is not known, nor are their temporal relationships. Both features limit the regional utility of formation names and they are not used herein except for those in the region adjoining the study area. Miocene shortening in the Veladero North (Argentina) and Pascua-Lama (Chile-Argentina) (Figure 2-5) area has superposed contrasting late Paleozoic volcanic and subvolcanic rocks as well as granitic rocks (Nasi eta/ . , 1985; Nasi eta/ . , 1990; Martin et al., 1995, 1999; Bissig, 2001; Charchaflie et al., 2002). This chapter examines the Paleozoic rocks found in several of the Miocene thrust sheets in the Veladero North area. The 1:10,000 scale mapping, together with U-Pb geochronologic and geochemical data provide the basis for establishing a late Paleozoic geologic framework for the Veladero North and Pascua-Lama areas (Martin etal. 1995, 1999; Bissig, 2001). Collectively, the late Paleozoic igneous framework established in this portion of the Andean Cordillera provides a starting point for addressing the definition of a late Paleozoic framework for the eastern flank of the Andes. Regional late Paleozoic to early Mesozoic geologic framework Morphost ructura l e lements The Chile-Argentina Andes between 28°S and 33°S (Figure 2-2) comprise four morphostructural elements including, from west to east, Cordillera de la Costa, Cordillera Principal, Cordillera Frontal and Precordillera. Late Paleozoic sedimentary and volcanic rocks unconformably overlain by a thin i i Bolivia I—20°S Pacific Ocean Location of Figure 2 -r-28°S Calingasta-Uspallata Depression Central Valley -36°S -44°S 450 km Figure 2-2: Morphostructural elements of the Argentina-Chile Andes modified from Herve et al. (1987). Outline of location of figure 2-3. 12 veneer of Tertiary rocks underlie the Cordillera Frontal of Argentina and Chile. Towards the southwest, deformed Mesozoic rocks underlie the Cordillera Principal. A Tertiary (?) tectonic depression, the Central Valley, separates the Cordillera Frontal from the paired metamorphic belt that outcrops along the present coastline of Chile in the Cordillera de la Costa. To the east, the Calingasta-Uspallata depression defines the boundary between the Cordillera Frontal and the Precordillera. Allochthonous Ordovician to Devonian sedimentary rocks that form most of the Precordillera are not found in the adjacent Cordillera Frontal. Ramos and others (1984, 1986) proposed that Chilenia, the basement of the Cordillera Frontal, was accreted to South America during Devonian times. Since the Devonian, both morphostructural elements share a common geologic evolution. Carboniferous sedimentary rocks that cover the early to middle Paleozoic rocks in the Precordillera are also present in the Cordillera Frontal and constitute the youngest common lithological unit between these morphostructural elements. Basement Two early Paleozoic metamorphic complexes and sedimentary sequences form the basement of the Cordillera Frontal. The oldest known rocks are early Paleozoic (?) micaceous orthogneisses known as La Pampa gneisses, which were metamorphosed in the Silurian (415 ± 4 Ma, Ribba et al., 1988). The gneisses have a restricted distribution and are the metamorphosed remanent of the Chilenia terrane sialic crust (Ribba eta/ . , 1988). Middle Paleozoic metabasites, quartz-muscovite schists, quartzites and marbles form the El Transito Metamorphic Complex in northern Chile (Ribba etal., 1988; Herve, 1982). Whereas the age of the protolith is not known, Ribba and others (1988) argue that they were metamorphosed around 335 ± 20 Ma or 304 + 40 Ma, based on poorly defined Rb-Sr isochrons, referred to as "error-chrons". The El Transito Metamorphic Complex represents the Carboniferous forearc accretionary prism formed during eastward subduction beneath Chilenia terrane (Ribba etal., 1988; Ramos etal., 1984). Late Carboniferous and Early Permian quartz-feldspar sandstone and siltstone, as thick as 7000 metres, form the eastern margin of the Cordillera Frontal and are known in the western flank of the Andes (Figure 2-3). In Chile, these rocks, known as Las Placetas Formation, contain marine fossils and flora of Late Carboniferous age. Las Placetas Formation represents shallow marine and lacustrine deposits formed in intra-arc basins (Bell, 1985, 1987). In Argentina at the latitude of the El Indio Belt, equivalent rocks of the Cerro Agua Negra Formation consist of sandstones, greywackes, siltstones and minor conglomerates locally metamorphosed by late Paleozoic and Tertiary intrusives. Malizia et al. (1997) proposed that the sedimentary rocks evolved from marine nearshore and deltas in the lower section to continental facies in the middle and upper sections. This sedimentary sequence was deposited in a back arc basin located east of the Gondwana plutons (Ramos, 1988). G o n d w a n a M a g m a t i s m Late Paleozoic volcanic rocks in Argentina are included within the Choiyoi Group, one of the major rock packages of the granite-rhyolite province (Kay et al., 1989). The Choiyoi volcanic province and four batholiths, Montosa-EI Potro, Chollay and Elqui-Limari in Chile and the Colangiiil in Argentina constitute the Gondwana magmatic products in the northern part of the Cordillera Frontal. Batholiths in the western margin of South America have been divided into Units and Superunits following the petrologic division in the Coastal Batholith of Peru (Cobbing and Pitcher, 1972). Units that are common throughout the batholiths are defined on the basis of similarities in texture, colour, composition and contact relationships. An assemblage of units that occurs in close association and that has similar relative and absolute ages form superunits. In Chile, Nasi and others (1985) grouped the Montosa-EI Potro, Chollay, and Elqui-Limari batholiths into two Superunits: Elqui and Ingaguas. The Colanguil Batholith, although formed by independent intrusive units, is not referred to as a Superunit in the literature, but could be considered to constitute a similar set of related igneous rocks (Figure 2 -4) . Figure 2-3: Geology of the Cordillera Frontal of Argentina and Chile and geochronological data summary. Compiled from: 1: Nasi et al., 1985; 2: Mpodozis and Cornejo, 1990; 3: Martin et al., 1995; 4: Rodriguez Fernandez et al., 1997; 5: Linares and Llambias, 1974; 6: Llambias and Sato, 1995; 7: Sato and Kawashita, 1988; 8: Pankhurstetal., 1996; 9: Bissig, 2001; 10: Herve in Ribba, 1985; 11: Malizia et al., 1997; 12: Cardo et al., 2000. Uncertainties reported as published. 15 9 T Intrusive Sedimentary Metamorphic Volcanic o a < o 3 a 3 o Bl -1 CT o 3 O C Ul •a a Elqui Superunit Ingaguas Superunit 8 < _ _ io n c Qi-OJ- => =1 F+-r r n n Qi o =r o r- " © o tu n sr -i IQ oi 3! oi c QJ n O m si Pastos Blancos Group Z CD s I I 8 ° o •0 g — ro S i ? S o 10 8 n 3 a x II n n' Intrusive Sedimentary Metamorphic Volcanic Colangiiil Colanguil Granodiorite Granite o CD 3 o Q. o' < I -DT ID QJ 3 ui c re cr -i 01 CL 01 r- > oi i_ O •o ro ro S <"> ^ 0 1 Choiyoi Group _ c O 3 <"> £ 2? oi ro QJ l/l IQ 5 " oT Q1 c 3 i Q- ro o ro - „ 5 o 3 3 w • > CD m Z > II c? V o l c a n i c r o c k s Choiyoi Group The Choiyoi Group in Argentina is the extensive volcanic sequence outcropping throughout the Cordillera Frontal and to the north and southeast in the Cordillera Principal and Precordillera (Figures 2-1 and 2-3). Initially referred as "Choiyoilitense" after an Araucano name (Groeber, 1946), it was first defined as a formation in west-central Argentina (Rolleri and Criado Roque, 1969) and elevated to group status once regional studies defined an internal stratigraphy and demonstrated the lateral equivalence of locally delimited formations (Stipanicic e ta/ . , 1968). Equivalent rocks in Chile are known as far north as the Bolivian border over a distance close to 2500 km long (Figure 2-1) . The Choiyoi Group is divided into a lower, andesitic section and an upper rhyolitic section. In the western flank of the Cordillera Frontal (Chile), most rocks are part of the upper section; the lower member is not well known. In the eastern part of the Cordillera Frontal (Argentina), a relatively thin (~250 m) sequence of andesite, minor dacite, clastic rocks and basalt forms the lower andesitic section of the Choiyoi Group (Cortes, 1985; Llambias and Sato, 1993; Caminos, 1979). Volcanic rocks and lesser conglomerates from the lower section unconformably overlie Late Carboniferous rocks. K-Ar and Rb-Sr geochronology (297 Ma to 289 Ma) from volcanic and intrusive rocks indicate a Late Carboniferous age for the lower section (Vilas and Valencio, 1982; Mpodozis and Cornejo, 1988; Llambias and Sato, 1993). The basalts have arc affinities and reflect deposition in a normal-subduction setting (Poma and Ramos, 1994). Sato and Llambias (1993) postulated that the andesites are broadly equivalent to the calc-alkaline granodiorites of the Colangiiil Batholith and to the early phases of the Elqui Superunit (see below and Figure 2-4) . 17 Table 2-1: Lithology and geochemistry of Gondwana magmatic products in the Cordillera Frontal (Argentina-Chile) Unit Lithology Minera 1221. Geochemistrv "Sr l 1 ' 2> Guanta Tonalite Granodiorite ( 1 , 2 ) Montosa Granodiorite Coarse Grained 55-69% Si02 0.7058- 0.7063 Hornblende and Biotite Metaluminous ASI= 0.88 to 0.94 medium- high K Ta-Ti depleted /La Th-U-alkalis enriched LREE depleted Low Sr Negative Eu Concave-up REE End -2.9-3.7 Biotite and Hornblende 65-73% Si02 Peraluminous ASI= flat HREE 1-1.08 U. 2) cochiguas Granodiorites Monzodiorites Muscovite or Biotite (i, 2) E ! volcan Granite Granodiorite (l< 2) Los Carricitos Granodiorite (l- 2 ) Chollay (x- 2> El Leon Monzogranite Syenogranite Granodiorite 73-74% Si02 Peraluminous ASI = 1.14-1.18 Steep HREE Variable Eu anomaly 0.707 0.708- 0.724 Coarse to very coarse 71-77% Si02 Biotite Peraluminous ASI = 1.06-1.18 High K20 Low K20/ Na20 Large Negative Eu anomaly LREE flattest in Si02 rich rocks Medium grained 68-70% Si02 0.7046- 0.7052 Biotite and Hornblende Low K20 Alkalis enriched Ti-Ta depleted Meta to Peraluminous ASI = 0.95-1.14 High Sr Steep LREE Moderate to steep HREE Small Eu anomaly Coarse Grained Monzogranite Biotite ( 1 , 2 ) El Colorado Monzogranite Porphyritic rhyolites ( 3 ) Tabaquito Granodiorite Granodiorite 69-78% Si02 0.704 High Na20 (>3%) K20 variable Metaluminous to peraluminous ASI = 0.96-1.14 0.705 - 0.706 Flat REE 0.705 - 0.706 Moderate to large negative Eu anomaly Biotite and amphibole 65-70% Si02 Metaluminous high-K 0.7052-0.7065 (3,5,6) Colangiiil Granodiorites Granodiorites Quartz diorite Monzodiorite Biotite or amphibole High Sr/Rb High LIL/ HSF 55-67% Si02 0.7041-0.7064 Metaluminous to peraluminous A/CNK 0.82-1.17 more sodic than potassic High LIL/ HSF (3.s,6) colangiiil Granites (5,6) vizcachas Plutons (3,5,6) C h 0 i y 0 i Group Cordierite Granites Leucogranites Las Openas Tonalite Granites (Subvolcanic Andesites) Andesites Rhyolites 72-77% Si02 Peraluminous A/CKN 0.99-1.23 K20/Na20 1.24-1.99 High HSF (Nb, Y, Zr, Th) High-K metaluminous 0.93-1 0.7072-0.7045 0.7099-0.7130 0.7057 54-67% Si02 Metaluminous to peraluminous A/CNK 0.9-1.06 70-76% Si02 Peraluminous A/CNK 0.99-1.31 Depleted Sr and Ba 0.7081 (l):Mpodozis and Kay (1990, 1992); (2):Nasi et al. (1985); (3): Llambias and Sato (1995); (4): Sato and Llambias (1993); (5): Heredia et al. (2002); (6): Rodriguez Fernandez et al. (1999). Rhyolitic and dacitic flows with subordinate continental clastic rocks form the upper and better-known section of the Choiyoi Group. The thickness of the upper section is typically obscured by Tertiary faults, but may be as much 2000 m or more in thickness (Groeber, 1946; Caminos, 1976, Thiele, 1954; Mpodozis and Cornejo, 1988; Heredia e t a / . , 2002). Permian fossiliferous rocks, such as the Matahuaico Formation in Chile, are interfingered within the volcanic sequence. In contrast, Liassic marine sedimentary rocks or Late Triassic continental sequences cover the Choiyoi Group in the Cordillera Principal. In the El Indio Belt to the west of Veladero North, silicic rocks equivalent to the Choiyoi Group are currently known as the Pastos Blancos Group (Thiele, 1964; Nasi eta/. , 1990; Mpodozis and Cornejo, 1988; Martin etal., 1999). The volcanic Guanaco Sonso Sequence forms the base, and the bimodal Los Tilos Sequence forms the top of the Pastos Blancos Group. Rhyolitic welded ashflow tuffs and minor volcaniclastic rocks form the Guanaco Sonso Sequence. K-Ar biotite ages of 281.0 ± 6 Ma, 262.0 ± 6 Ma and 260.0 ± 6 Ma as well as U-Pb zircon age of 265.8 ± 5.6 Ma (Martin e ta/ . , 1999) indicate a Permian age for the rocks. Bimodal basaltic-andesitic to dacitic volcanic rocks and sedimentary rocks form the Los Tilos Sequence. A dacitic tuff from this unit yielded a Middle Triassic K-Ar biotite age of 235.0 ± 5.0 Ma (Martin eta/ . , 1999). Geochronological studies are not conclusive on the minimum age of Los Tilos Sequence. In Argentina, Rodriguez Fernandez et al. (1997) and Malizia et al. (1997) report Late Triassic K-Ar whole-rock ages (Table 2-2). Based on the gradational nature of the upper contact of Los Tilos Sequence with Early Jurassic Lautaro Formation, Martin et al. (1999) proposed that the transition is Early Jurassic in age, the estimated maximum age of the overlying formation. As defined, the Pastos Blancos Group lacks an andesitic basal section and tentatively extends into the Jurassic, but otherwise is comparable to the Choiyoi Group. The geochemistry of the upper, rhyolitic section of the Choiyoi Group has not been studied in detail. Sato and Llambias (1993) point out that the rhyolites and the granites of the Colangiiil Batholith have similar major and trace element geochemistry and 8 7 S r - 8 6 S r initial ratios. They propose a common magmatic origin because the granites and rhyolites are spatially related. Likewise, Martin and others (1999) correlate the Los Tilos Sequence to El Colorado Unit (see Ingaguas Superunit below) in the El Indio Belt. Late Permian and Early Triassic units form post-orogenic volcanic successions produced by crustal melting inferred to correspond to an extensional tectonic environment (Llambias and Sato, 1995; Sato and Llambias, 1993; Mpodozis and Kay, 1992; Heredia etal., 2002). P l u t o n i c r o c k s Elqui Superunit In west-central Chile, the Elqui Superunit as defined by Nasi and others (1985) comprises a series of tabular plutons that range from coarse grained and foliated gabbros to granite. These rocks constitute a belt in the western side of the Elqui-Limari Batholith (Figure 2-3). The Guanta, Cochiguas and El Volcan Units form the Elqui Superunit. Tonalite, hornblende-biotite granodiorite and minor diorite and gabbro of the Guanta Unit are the oldest known intrusive phase of the Gondwana magmatism. The intrusive rocks are Carboniferous based on K-Ar ages from amphiboles of 303 i 9 Ma and 297 ± 9 Ma (Nasi et al., 1985) and conventional U-Pb ages of 298 i 2 Ma (Tosdal, unpublished data) and 285.7 ± 1.5 Ma (Pankhurst etal., 1996). Granitoids of the Guanta Unit are metaluminous, calc-alkaline and have I-type affinities. Mpodozis and Kay (1992) associated the Guanta granitoids with a Late Carboniferous subduction zone. Granodiorite and monzogranite with biotite and muscovite are the main lithologies of the Cochiguas Unit. Granitoids from the El Volcan Unit in Chile also range in composition from monzogranites to granodiorites, but muscovite is not a primary igneous mineral. They form two different units, although transitional contacts are very 20 common (Nasi et al., 1985). El Volcan granitoids also crosscut Cochiguas plutons. Geochronologic data (K-Ar in muscovite of 301 ± 4 Ma from Mpodozis and Cornejo (1988) and a preliminary conventional U-Pb determination of 296 ± 3 Ma from Tosdal, unpublished data) indicate that the Cochiguas Unit is of similar age as the Guanta Unit. Geochronological study of the El Volcan Unit, however, is restricted to one K-Ar biotite age of 247 i 4 Ma (Mpodozis and Cornejo, 1988). Although that age is consistent with the intrusive relationship between El Volcan and Cochiguas Units, analysis of the age patterns of the volcanic and plutonic rocks (see below) argues that the Permian age is a minimum. Based on transitional intrusive relationships only, the El Volcan plutons are inferred to be broadly time-equivalent to the Guanta and Cochiguas Units rocks. Cochiguas and El Volcan are peraluminous granitoids with low N a 2 0 - K 2 0 ratios and high FeO / MgO. High K 2 0 content of El Volcan Unit is a typical S-type granitoid feature that is not present in the Cochiguas plutons. Mpodozis and Kay (1992) inferred that the source of the bulk of both units was relatively shallow and that the older Cochiguas granitoids were formed in a feldspar-poor garnet-bearing thickened continental crust. Mpodozis and Kay (1992) (see also Rapalini, 1989; Rapalini and Vilas, 1991) suggest an oblique subduction setting at the time of formation of El Volcan and Cochiguas Units, an environment that should also apply to the broadly contemporaneous Guanta Unit. Ingaguas Superuni t The Ingaguas Superunit forms the easternmost part of the Elqui-Limari Batholith (Figure 2-3). The Superunit is predominately granitic and granodioritic in composition but includes subordinate gabbro. The main differences with the Elqui Superunit and the most common characteristics of Ingaguas rocks are the fine grain size, absence of deformation fabrics, common presence of miarolic cavities and very restricted occurrence of xenoliths and mafic dikes (Nasi etal., 1985). Ingaguas plutons intrude the Elqui Superunit and rhyolitic units of the Pastos Blancos Group, which are equivalent to the upper section of the Choiyoi Group. Nasi and others (1985), based on mineralogical and textural similarities, distinguished five plutonic units: La Laguna, Los Carricitos, El Leon, Chollay and El Colorado. Previous geochronological studies (Mpodozis and Cornejo, 1988; Nasi e ta/ . , 1985) are not conclusive on the absolute age of the units and indicate a range of ages between 250 Ma and 207 Ma (Table 2-2). The oldest plutons of the Ingaguas Superunit are 250 Ma to 240 Ma granitoids. Pink monzogranite, granodiorite and syenogranite with biotite and rare amphibole form the Chollay and El Leon Units (Nasi et al., 1985). Martin and others (1999) prefer the composite name Chollay-EI Leon to refer to those plutons and they include dacitic and quartz porphyries into this Unit. Limited major element geochemistry of the granitoids shows that the Chollay Unit is less siliceous than the El Leon Unit and that both units range from metaluminous to peraluminous compositions. The granitoids are inferred to have been emplaced in a relatively thin crust, under an extensional regime that followed the collision of an unrecognized terrane (Mpodozis and Kay, 1992). The youngest plutons of the Ingaguas Superunit form a group that ranges in age from 212 Ma to 200 Ma, or as young as 190 Ma if 4 0 A r - 3 9 A r dating (Bissig, 2001) is taken into account. Based on geochemical and petrological characteristics, the younger group should be further subdivided into Los Carricitos and El Colorado Units. Biotite and hornblende granodiorite exclusively form the Los Carricitos Unit. The plutons of this unit represent a small volume of the Ingaguas Superunit, and they are intruded by the El Colorado Unit. Conventional U-Pb geochronology studies yields an age of 212 + 2 Ma (Tosdal, unpublished data) in agreement with a hornblende K-Ar age of 207 + 9 Ma (Nasi et al., 1985). Los Carricitos Unit displays a distinctive geochemistry (high Sr, steep REE pattern and very small Eu anomaly) typically associated with a relatively high-pressure, plagioclase-poor, garnet-bearing source. Los Carricitos plutons have been interpreted as post-collisional granitoids that were formed at the base of a thick crust and emplaced during the initial stages of extension and relaxation (Mpodozis and Kay, 1990, 1992). Distinctive red granites intrude Elqui Superunit granitoids, the older parts of the Ingaguas Superunit and the Pastos Blancos Group and constitute the El Colorado Unit (Nasi et al., 1985). Martin and others (1999), in their revision of the geology of the region, included within this unit quartz-feldspar rhyolitic porphyries and dikes, mafic intrusives that form dikes or hypabyssal bodies and medium to coarse-grained gabbro and diorite assigned to El Leon Unit and La Laguna Unit in previous 1:250000 scale maps (for example Nasi et al., 1990). The bimodal nature of the El Colorado Unit has been correlated to the succession of andesite and dacite flows of the Los Tilos sequence (Martin etal., 1999). From granitic rocks previously included in older plutonic suites, U-Pb geochronology yields an age of 201 i 5 Ma (Tosdal, unpublished data) for the granitoids whereas an 4 0 A r - 3 9 A r method on biotite from granite produced an age of 190.1 ± 3.2 Ma (Bissig, 2001). No intermediate rocks from the El Colorado Unit have been analyzed, but in general, the granites range from metaluminous to peraluminous compositions and none is peralkaline. REE patterns are consistent with a low-pressure, plagioclase-rich, garnet-free source (Mpodozis and Kay, 1992). El Colorado Unit, similar to Chollay-EI Leon plutons, was emplaced in an extensional, post-collisional regime (Mpodozis and Kay, 1990, 1992). Proposed models for both Chollay-EI Leon and El Colorado Units assume melting of large volumes of crust driven by the underplating of hot mantle material. Co langui l Bathol i th The Colanguil Batholith is located in west-central Argentina and represents the eastern extension of the Gondwana plutons. The north-striking belt formed by the Colanguil plutons was initially considered to represent an inner arc parallel to the outer Elqui-Limari Batholith (Llambias et al., 1987). Recent mapping in the Argentinan side of the Andes, supported by geochronological studies, extends the occurrence of late 23 Paleozoic to early Mesozoic plutons as far west as the international border between Argentina and Chile (Heredia e ta/ . , 2002; Rodriguez Fernandez etal., 1997). Thus, the two batholiths may have formed a single entity that was further dismembered by Mesozoic and Cenozoic tectonics. In west-central Argentina, the Colanguil Batholith contains a series of plutons that intrude Late Carboniferous sedimentary rocks and late Paleozoic volcanic rocks (Figure 2-3). The major units of the Colanguil Batholith are the Tabaquito Granodiorite, Las Piedritas Granodiorite, Colanguil Granite and the Vizcachas Plutons. They are monolithological and have similar known ages. Minor units include rhyolitic porphyries and dikes associated with the granitoids. The Colanguil plutons, unlike the Elqui Superunit, lack deformation fabrics and rocks with S i 0 2 comprised between 68 and 72 wt%. The silica gap and difference in some major and trace element geochemistry underline the genetic independence between granodiorites and granites (Llambias and Sato, 1995, 1990; Sato and Llambias, 1993; Rodriguez Fernandez eta/ . , 1997). Biotite and hornblende granodiorite forms the Tabaquito Granodiorite that is the oldest unit with a calculated Rb-Sr isochron age of 329 Ma to 326 Ma (Llambias and Sato, 1995). Geochemical analyses indicate that the Tabaquito Granodiorite is a calc-alkaline, high Sr intrusion formed in a volcanic arc related to a destructive margin (Llambias and Sato, 1995). The Las Piedritas Granodiorite includes five plutons (Las Piedritas, Romo, Tocota, Agua Negra and Los Leones) formed by amphibole and biotite granodiorite with reabsorbed xenoliths and fresh metamorphic inclusions. Isochron ages of the Las Piedritas plutons, calculated by the Rb-Sr method, span from 272 Ma to 260 Ma (Llambias and Sato, 1995, 1990). The calc-alkaline rocks formed by mixing of lower and upper crustal melts, emplaced during the waning stages of a subduction-related magmatic arc (Heredia etal., 2002; Llambias and Sato, 1995, 1990). The granodiorites and probably some granites (e.g. the Los Patos Granite of Rodriguez Fernandez et al. 24 ( 1 9 9 7 ) l o c a t e d a l o n g t h e A r g e n t i n a - C h i l e b o r d e r i n t h e c e n t r a l p a r t o f F i g u r e 2 - 3 ) o f t h e C o l a n g i i i l B a t h o l i t h a r e e q u i v a l e n t t o t h e G u a n t a U n i t i n C h i l e . F i v e b i o t i t e g r a n i t e p l u t o n s f o r m t h e C o l a n g i i i l G r a n i t e s U n i t ( F i g u r e 2 - 4 ) . T h e p l u t o n s c o m p o s i t i o n i s h o m o g e n e o u s t h r o u g h o u t t h e b a t h o l i t h . T h e L a s O p e n a s g r a n i t i c p l u t o n is t h e o n l y e x c e p t i o n , a n d i t i s a c o r d i e r i t e - b e a r i n g i n t r u s i o n ( L l a m b i a s a n d S a t o 1 9 9 5 , 1 9 9 0 ) . A R b - S r w h o l e - r o c k i s o c h r o n s h o w s t h a t t h e C o l a n g i i i l G r a n i t e s w e r e e m p l a c e d i n a v e r y s h o r t p e r i o d b e t w e e n 2 5 9 M a a n d 2 5 7 M a ( L l a m b i a s a n d S a t o , 1 9 9 5 ; S a t o a n d L l a m b i a s , 1 9 9 3 ) . E x c e p t f o r t h e L a s O p e n a s p l u t o n , t h e s l i g h t l y p o t a s s i c a n d p e r a l u m i n o u s C o l a n g i i i l G r a n i t e s s h a r e l i t h o l o g i c a l a n d g e o c h e m i c a l c h a r a c t e r i s t i c s w i t h t h e I n g a g u a s C o m p l e x i n C h i l e ( L l a m b i a s a n d S a t o 1 9 9 5 , 1 9 9 0 ) . A p o s t - c o l l i s i o n a l , e x t e n s i o n a l t e c t o n i c s e t t i n g h a s b e e n p r o p o s e d a t t h e t i m e o f e m p l a c e m e n t o f t h e g r a n i t i c C o l a n g i i i l p l u t o n s ( L l a m b i a s a n d S a t o 1 9 9 5 , 1 9 9 0 ; S a t o a n d L l a m b i a s , 1 9 9 3 ) . H e r e d i a e t al. ( 2 0 0 2 ) a n d R o d r i g u e z F e r n a n d e z e t al. ( 1 9 9 7 ) e x c l u d e f r o m t h e C o l a n g i i i l B a t h o l i t h a s e r i e s o f s m a l l p l u t o n s l o c a t e d i n t h e s o u t h e r n p a r t o f p r e - T e r t i a r y i n t r u s i v e b e l t ( F i g u r e 2 - 3 ) . T h e V i z c a c h a s g r a n o d i o r i t e is t h e l a r g e r i n t r u s i v e b u t g r a n i t e a n d t o n a l i t e a l s o f o r m t h e V i z c a c h a s P l u t o n . A s i n g l e K - A r w h o l e - r o c k a n a l y s i s o f t h e g r a n o d i o r i t e y i e l d s a n E a r l y J u r a s s i c a g e o f 2 0 0 ± 7 M a ( R o d r i g u e z F e r n a n d e z et al., 1 9 9 7 ) , s i m i l a r t o t h e a g e o f t h e y o u n g e s t p l u t o n s o f t h e I n g a g u a s S u p e r u n i t . P r e l i m i n a r y g e o c h e m i s t r y ( R o d r i g u e z F e r n a n d e z e t al., 1 9 9 7 ) r e v e a l s t h a t t h e V i z c a c h a s p l u t o n is f o r m e d b y h i g h - K c a l c - a l k a l i n e r o c k s w i t h A / C N K l e s s t h a n o n e . R o d r i g u e z F e r n a n d e z a n d o t h e r s ( 1 9 9 7 ) j u s t i f y t h e e x c l u s i o n f r o m t h e G o n d w a n a m a g m a t i c c y c l e b e c a u s e t h e V i z c a c h a s p l u t o n h a s g e o c h e m i c a l c h a r a c t e r i s t i c s t h a t c o n f l i c t w i t h t h e a n o r o g e n i c n a t u r e p r e d i c t e d b y L l a m b i a s a n d S a t o ( 1 9 9 5 ) . 25 Veladero North area geologic setting T h e El I n d i o B e l t , w h e r e P a s c u a - L a m a - V e l a d e r o N o r t h a r e a is l o c a t e d , is e n t i r e l y s i t u a t e d w i t h i n t h e C o r d i l l e r a F r o n t a l m o r p h o s t r u c t u r a l p r o v i n c e ( F i g u r e 2 - 2 ) . I n t h e s t u d y a r e a , l a t e P a l e o z o i c r o c k s f o r m h o m o c l i n a l o r g e n t l y f o l d e d s e q u e n c e s t h a t d e f i n e t h r e e n o r t h - s o u t h s t r u c t u r a l p a c k a g e s a n d c o n s t i t u t e t h e b a s e m e n t t o t h e T e r t i a r y v o l c a n i c s e q u e n c e ( F i g u r e 2 - 5 a n d P l a t e 1 ) . N o c o n t a c t w i t h y o u n g e r M e s o z o i c r o c k s i s d e f i n e d i n t h e V e l a d e r o N o r t h a r e a ; t h i s i s i n c o n t r a s t t o t h e g r a d a t i o n a l c o n t a c t r e p o r t e d i n C h i l e f o r t h e L a u t a r o F o r m a t i o n ( M a r t i n et al., 1 9 9 5 , 1 9 9 9 ) . T h e t o p o f t h e s e q u e n c e a n d t h e b a s e o f t h e T e r t i a r y s t r a t a i s a n a n g u l a r u n c o n f o r m i t y , b u t m o r e c o m m o n l y , P a l e o z o i c a n d C e n o z o i c r o c k s a r e f o u n d i n s e p a r a t e t h r u s t s h e e t s . I n t h e e a s t e r n p a r t o f t h e s t u d y a r e a , l a t e P a l e o z o i c r o c k s a r e t h r u s t o v e r O l i g o c e n e v o l c a n i c r o c k s . C o n v e r s e l y , i n t h e w e s t o f t h e m a p p e d a r e a , T e r t i a r y v o l c a n i c l a s t i c r o c k s u n c o n f o r m a b l y c o v e r P a l e o z o i c r o c k s o r t h e y w e r e b r o u g h t i n t o c o n t a c t b y n o r m a l f a u l t s . T h e a g e o f n o r m a l f a u l t i n g i s n o t p r e c i s e l y c o n s t r a i n e d b y g e o c h r o n o l o g i c a l d a t a o r f i e l d r e l a t i o n s , h o w e v e r , n o r m a l f a u l t s a r e m o r e l i k e l y t o b e T e r t i a r y s i n c e t h e y a l s o o f f s e t O l i g o c e n e ( ? ) v o l c a n i c l a s t i c r o c k s . T h r u s t s f a u l t s a r e T e r t i a r y a s s u p p o r t e d b y f i e l d e v i d e n c e a n d t h e a g e o f a s e r i e s o f d a c i t i c a n d a n d e s i t i c i n t r u s i v e s e m p l a c e d a l o n g t h e f a u l t s ( s e e C h a p t e r 3 f o r d i s c u s s i o n a n d t i m i n g o f d e f o r m a t i o n ) . F r o m e a s t t o w e s t , t h e s t r u c t u r a l p a c k a g e s a r e : R i o T a g u a s , G u a n a c o Z o n z o a n d P o t r e r i l l o s - C a n i t o . E a c h s t r u c t u r a l p a c k a g e c o n t a i n s s t r a t i g r a p h i c a l l y e q u i v a l e n t v o l c a n i c , v o l c a n i c l a s t i c a n d i n t r u s i v e r o c k s . T h e f o l l o w i n g d e s c r i p t i o n f o c u s o n t h e l i t h o l o g i c a l a n d g e o c h r o n o l o g i c a l a s p e c t s o f t h e e f f u s i v e a n d i n t r u s i v e r o c k s a n d t h e n s u m m a r i z e s t h e i r g e o c h e m i c a l c h a r a c t e r i s t i c s . 26 Table 2-2: Compilation of previous geochronological studies in the Cordillera Frontal (Argentina and Chile) Unit names, absolute ages and associated uncertainties are reported as published. | Sub-Unit Method Material Age Uncertainty Mallzia et al., 1997 a-b Choiyoi K-Ar WR 203 30 Malizia et al., 1997 a-b Choiyoi K-Ar WR 214 11 Martin et al., 1995 Los Tilos K-Ar Bl 235 5 Martin et al., 1995 Los Tilos U-Pb u-Pb 217 7.5 Martin et al., 1995 Guanaco Sonso K-Ar Bl 260 6 Martin et al., 1995 Guanaco Sonso K-Ar Bl 262 6 Martin et al., 1995 Guanaco Sonso K-Ar Bl 281 6 Rodriguez Fernandez et al., 1997 Choiyoi K-Ar WR? 214 5 Rodriguez Fernandez et al., 1997 Choiyoi K-Ar WR? 288 5 Malizia et al., 1997 a-b Choiyoi K-Ar WR 287 15 Malizia et al., 1997 a-b Choiyoi K-Ar WR 315 15 Malizia et al., 1997 a-b Choiyoi K-Ar WR 333 70 Sato and Llambias, 1993 Rhyolite Dyke Rb-Sr Isochrone 247.6 3 Sato and Llambias, 1993 Andesltes Rb-Sr isochrone 289.2 19.3 Bissig, 2001 Basement GS? Ar-Ar Bl 261 5.4 Martin et al., 1985 Guanaco Sonso U-Pb U-Pb 265.B 5.6 This study DC-119 Guanaco Sonso U-Pb U-Pb 263.7 0.7 This study DC-239 Guanaco Sonso U-Pb U-Pb 262.6 0.7 This study DC-181 GS intruslves U-Pb u-Pb 254 4.2 This study DC-142 GS intrusives U-Pb u-Pb 259 0.7 This study DC-162 GS intrusives U-Pb U-Pb 258 1.25 Martin et al., 1995 Colorado K-Ar Muse 221 5 Bissig, 2001 easement, Colorado? Ar-Ar Bl 190 3.2 Nasi et al., 1985 Chollay K-Ar Bl 238 6 Mpodozis and Cornejo, 1990 El Leon K-Ar Bl 238 4 Mpodozis and Cornejo, 1990 El Leon K-Ar Bl 276 4 Tosdal, unpublished El Leon U-Pb U-Pb 201 5 Martin et a!., 1995 Chollay-EI Leon U-Pb U-Pb 242 1.5 Martin et al., 1995 Chollay-EI Leon U-Pb U-Pb 242.5 1.5 Martin et al., 1995 Chollay-EI Leon U-Pb U-Pb 249.7 3.2 Nasi et al., 1985 Los Carricitos K-Ar Bl 221 3 Nasi et al., 1985 Los Carricitos K-Ar Bl 238 4 Nasi et al., 1985 Los Carricitos K-Ar Hbd 207 9 Tosdal, unpublished Los Carricitos U-Pb U-Pb 212 2 Parada et al., 1981 Ingaguas Rb-Sr Rb-Sr 197 5 Mpodozis and Cornejo, 1990 El Volcan K-Ar Bl 247 4 Mpodozis and Cornejo, 1990 Cochlguas K-Ar Muse 259 6 Mpodozis and Cornejo, 1990 Cochlguas K-Ar Muse 301 4 Martin et al., 1995 Cochlguas K-Ar Bl 219 5 Mpodozis and Cornejo, 1990 Cochiguas K-Ar Bl 235 6 Pankhurst et al., 1996 Cochiguas Rb-Sr Rb-Sr 256 10 Martin et al., 1995 Cochiguas U-Pb U-Pb 311 19 Tosdal, unpublished Cochiguas U-Pb U-Pb 296 3 Nasi et al., 1985 Guanta K-Ar Bl 245 4 Mpodozis and Cornejo, 1990 Guanta K-Ar Bl 250 4 Ribba, 1985 Guanta K-Ar Bl 252 6 Nasi et al., 1985 Guanta K-Ar Bl 258 4 Ribba, 1985 Guanta K-Ar Bl 260 6 Nasi et al., 1985 Guanta K-Ar Hbd 256 7 Nasi et a!., 1985 Guanta K-Ar Hbd 266 7 Nasi et al., 1985 Guanta K-Ar Hbd 297 9 Nasi et al., 1985 Guanta K-Ar Hbd 303 9 Herve In Ribba, 1985 Guanta K-Ar Hbd 310 18 Parada et al., 1981 Elqui Rb-Sr Rb-Sr 328 21 Pankhurst et a!., 1996 Guanta U-Pb U-Pb 285.7 1.5 Tosdal, unpublished Guanta U-Pb U-Pb 298 2 Rodriguez Fernandez et al., 199: Granodiorite Las Vizcachas K-Ar WR? 200 7 Sato and Kawashita, 1988 Granite Chita Rb-Sr Isochrone 247 15 Sato and Llambias, 1995 Granite Los Lavaderos Rb-Sr Isochrone 259 2 Sato and Llambias, 1995 Granite Las Openas Rb-Sr Rb-Sr WR-BI 256 2 Sato and Llambias, 1995 Granite El Fierro Rb-Sr Rb-Sr WR-BI 256.5 0.5 Sato and Llambias, 1995 Granite Los Puentes Rb-Sr Rb-Sr Bl 257 0.5 Sato and Llambias, 1995 Granodiorite Tocota Rb-Sr Rb-Sr WR-BI 268 1 Linares and Llambias, 1974 Granodiorite Tocota K-Ar Bl 283 15 Esplna et al., 1998 Granodiorite Las Vacas and Casposo? K-Ar WR? 261 6 Esplna et al., 1998 Granodiorite Las Vacas and Casposo? K-Ar WR? 250 8 Sato and Llambias, 1995 Granodiorite Las Piedritas Rb-Sr Rb-Sr WR-Bi 261.5 1.5 Sato and Llambias, 1995 Granodiorite Romo Rb-Sr Rb-Sr WR-Bi 264 1.1 Sato and Llambias, 1995 Granodiorite Tocota Rb-Sr Rb-Sr WR-Bi 268 1 Sato and Llambias, 1995 Granodiorite Los Leones Rb-Sr Rb-Sr WR-Bi 270 2 Sato and Llambias, 1995 Granodiorite Tabaquito Rb-Sr Rb-Sr WR-Bi 327.5 1.5 Hurtado 1 Martin et al., 1995 Hurtado K-Ar WR 254 8 List of abbreviations: GS= Guanaco Sonso Formation; WR= Whole-rock; Bi= Biotite; Musc= Muscovite; Hbd= Hornblende; Rb-Sr WR-Bi= Whole-rock and Biotite Isochrone 28 Volcanic and volcaniclastic rocks Rio T a g u a s P a c k a g e Late Paleozoic rocks along the Taguas River in the eastern part of the study area define a 750 m wide by 6 km long north-northeast trending belt. The lower contact is a thrust fault, and the upper contact is a slight angular unconformity with Tertiary volcanic rocks. Light grey and yellow rhyolitic to dacitic ignimbrites with varying degrees of welding and interbedded green volcaniclastic sandstones that dip to the west form the structural package (Figure 2-6). Ignimbrites dominate the package, but volcaniclastic rocks account for as much as 2 0 % of the outcrop. Embayed quartz phenocrysts are very common and differentiate the late Paleozoic volcanic rocks from Tertiary volcanic rocks (see chapter 3). ® Tertiary Volcanic Sequence 4 0 0 0 — ® Explanation Lithophyseae Spherulite Fiamme Columnar Joints Sandstone lenses Green horizon ' Conglomerate Bedding A Alteration halo M Covered m Sample location 3 8 0 0 — Taguas River valley ~259 Ma Intrusive Figure 2-6: East-west cross-section of the late Paleozoic rocks exposed along the Rio Taguas. I, II and III represent outflow, pyroclastic units separated by beds that are predominantly clastic. No vertical exaggeration. 29 Sample DC-119 is from an advanced argillic altered, matrix-supported, quartz-rich, lapilli-tuff with small fiammes located at the top of the Rio Taguas late Paleozoic structural package (Figure 2-6). No accessory lithic fragments are present and all the material is representative of a primary pyroclastic deposit. The age of the package, determined by U-Pb geochronology, is 263.7 ± 0.7 Ma (Table 2 - 3 , Figure 2 -7 , see Appendix for detailed discussion of U-Pb systematics). G u a n a c o Z o n z o P a c k a g e Isolated outcrops of late Paleozoic rocks define the 900 meters wide, north-northeast trending, Guanaco Zonzo structural package in the centre of the study area (Figure 2-5). To the east, a west-dipping thrust fault forms the contact with Tertiary rocks. Towards the west, a west-dipping normal fault brings the late Paleozoic rocks in contact with Tertiary sequences and, in the northern part of the map, with intensely silicified dacites assigned to the late Paleozoic. Light grey and purple dacitic to rhyolitic spherulitic welded lapilli-tuffs, intensely altered lithic-rich dacitic (?) tuff-breccias, fine conglomerates and sandstones are the predominant rock types in the Guanaco Zonzo area. Tertiary and late Paleozoic dacitic porphyries intrude the package. A sample of advanced argillic altered, poorly sorted, matrix-supported, (volcanic) lithic-poor dacite with fiamme-like and feldspar vughs (sample DC-239) was dated using U-Pb method (Figure 2-5). Accessory volcanic lithic fragments form less than 5 % of the rock volume and were removed during the sampling stage in order to collect volcanic material only. U-Pb geochronology yields an age of 262.6 ± 0.7 Ma (Table 2 - 3 , Figure 2-7, see Appendix for discussion of U-Pb systematics). 30 i n cr n m > * • n t W "> rV ?ro <" =r , n S » > ° m m i 3 O c o o ro -^ ST o> B ro — r - S o Q n O J S Q f? ° o-S ro Q. 3 O CL r. -f, 3 S -1 n o 3 oi 3 S, ui r r a o S g o L 2 O I ll o ' oi rj T o °- 0j r t [ . Q O ' - S 5!. i rt- ro 3 j a a & 5 ro n ui -• ui _ n o g ; n o OJ o o 3 : o i 3 o 5 5 3 a. i—1 cr P°c? OJ w Mco gin cr ~~ II o 2 II O i_i OJ U l o O J c ? ^ 3 °" cr ~~ II g O J 52. g c? D W 3 - i °- cr C» w n m D o ° 1 / 1 2. 5r s = ga -o 2 <" -o cr O J Q J o~ • 3- 3 -» P - °-s s ° => o 9 u O J a n rS-a: " 3 3 ° . o o n „ 5 o o ° t ro !2. £. b - Q - c r o — ° O CO j g ? 3 ui °~ o T 3 O J u, c X ro ,, o. H-o N J 3 o £3 a Pi o 3< 6 S8 n °- = n c » o c r 3 w. g § O to 3 O O Qj_ O & 2.? H-NJ OJ _^ n- Oj o 3 o n> ro 3 • • < " * O J r - — ^ 3 c r t o ro O J U l 3 o n II J J 2. » II J, R " 3 3 3 o O oj 0) 3 3 Q a. n ' 3 o - 3 n> 3 3 01 Cj~. 0 » 3 O 5 I ™§ n o . " 0 ' =i 3 u, O J ™ ; a S 5 3 ro N ™ 3 S2. 3 ro o ° O J 3 ^ ro => O J cr ? S P " T ° - 0 S °- P-CJS 0 1 cn O J 3 - l -1 3 r r ro O J o 3- W fO r t n r j . ? 3 ro if a . S " w (/) S ; fO V r t 3 - ii w r o O J ro J > 3 ro ro II i -r^ ,-, A co c " c 3 3 F T Ul r t O J 3 ftl t/1 y ^ g g § 3 S < 3 3 • O O J o =i g o ! « 11 ™ , 11 A r t 3 S 9 o cn m n > o " 3 - n 3 3 ? " I * 3 ' ^ h* 3 3 N I 3 3 en - M S J ' r^o (o j " •? P y ^  T3 T3 3 T3 ^ m r n D n o > 0 '..-n - » < < < < < o n n ri o o i 3 3 ~ ~ ~ ~ - P-> N J N J 3 3 3 3 3 0 0 , ~ - W M W M N M ' 3 q " ~ - ~ - _ , - .? ui tn 3 ui U l v^-ci -o -a 3 ui ui n n n g n g 3 g g g 9 w W M ii ^ i 3 3 3 3 3 U 1 -o -a -o 3 w •n a n co > o 3 3 3 .° ? 3 - - - 3 M W 3 3 3 ji K -3 = 3 3 . 3 -3 = • - 0 - 0 " ° ^ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 O O 0 O b b b 0 0 0 U) SJ I- 1 0 0 0 I—1 I—1 r-1 0 NI 0 0 0 LO NJ LO LO M U l M UJ I—1 UJ cn 01 to LO U l NJ VO I—1 LO Ln VI cn NJ CO cn J> 0 CO U l 01 01 0 J> 01 1—1 to v j CO 0 CO 10 r-1 cn 0 0 Ln 0 U l O l VI LO LO Q\ cn Ln Ln cr> 4^ 1—1 cn cn un vO 4^ o co co W W W M P OJ O l O ^ U l U l ir j O OJ NJ Cn cn Cn CO S j N j J > O . V j N j U l t ^ NJ M A OJ OJ J > V D o 00 on O J H 1 co cn O J P O J C O C O U l I—1 LO H * l_t 1—. I—. H i Ol LO J> )—i Ul to 01 cn VI 0 NJ Ln CO Ln I—1 J> 0 LO to VI Ln NJ J> LO I-* cn 01 CO M O J> 0 to 0 CO Ul 01 J> NJ VI to Ln LO CO H * J> to to CO VO N) CO LO NJ O to H 1 VI cn J> J> VI VI 0 CO NJ NJ O LO to LO O tO Ol CO LO N J CO N J CO N J v D N J O J O J L n o J c n c n OJ r—1 N J OJ H P > CO U) J> CO J> CO O i P v l A J> cn ui cn 00 NJ NI K J> H CO CO CO J> OJ I—1 O OJ v£) U l U l cn i—1 NJ OJ NJ OJ NJ OJ NJ P* NJ NJ NJ O U l t—' U l U l U l CO cn t—1 CO r—t I-1 CO I—4 V J V J cn •vj cn OJ cn CO CO CO \D v j CO 0 U l OJ NJ U l NJ VD CO U l OJ 0 0 0 0 CD 'CD 'CD 'O J > O J O J J > O -vJ UD O M O H U J s j J> J> N> NJ O l s i J> O O O O O O O O b b b b b b b b 0 J 0 J 0 J 0 J 4 ^ 0 J 0 J 0 J N ) J > O J N J C O N ) H U l V j D U 3 O J J > 0 J C n J > N J O C O U J O U U I M C O 0 0 0 0 0 b b b b b J > J > J > J > J > O O t—' NJ O ID O P CO O P O P O NJ O J J > NJ O O O O O b b b b b A J A A A A O P* P* P* o ID U l U l N J P cn ui VJO o O W O U l N l J A O O O O O O G O Cf O J > J > J > J > O J O O P* P* KO J> s j O v J U l cn S J co M P O J cn J> ui kD O O O O O O O O O O O O O O O O O N J O J N J N J Ul O O O O O O O O O O O O O O O J N ) N J N J o cn co cn O J U J U D cn p* ui O J P* P* N J O O O O O O O O N J N J N J N J O J N J N J N J u i u i c n v j o j c n v j c o U l C O C O N J v J O J v J O J > U 1 N U J C O O J O O J o u j u i c o c n c o c o o O O O O O NJ NJ NJ OJ N) ID ID UJ O CO P N l O W s j CO NJ OJ CO vJ CO M CO P J> O O O O O NJ NJ NJ NJ N ID UJ O CO NJ SI U l OJ P J> VD OJ U l U D NJ NJ P J> O O O O O O N J N J N J N J N J C O C O I D I D s j cn I D N J cn ^ D s i U l U D JA N J N J Ul N J o cn P* O O P* O O O O O O O O O O O O O O O O O O O O O O O O O O O b b b b ui ui ui ui J > p* p* p* ui cn s j O J Cft P P P O O O O O O O O b b b b b b b b U 1 U 1 U 1 U 1 U 1 U 1 U 1 U 1 P P W P C O P P P O O J P * S I U 1 N J O J J > j>.ix> o s j c o c n o J O J O O O O O b b b b b ui ui ui ui ui P P P NJ P 01 s j O J O J P* J A p* ui O J O J O O O O O b b b b b ui ui ui ui ui P N) P P O s i o J > cn V D s j O U J C O O O O O O P* O O P* O O O O O O O O O O O O O ui O J J A N J cn CO J A O J V D N J O O O O O P* P* j> P 1 Ul U D J> P U l S i O O O O O " S r i ™ ^ Sa 0 . 0 o V ro r: ui 3 „° - D J -h D II CL A NJ s l o NJ NJ NJ NJ U l OJ J > U l s l J > s l CO cn ui ui cn N J N J N J N J N J I N J N J N ) N J O J O J J > c n o j J > J > i O p C h p i > u i s i u j C O P ^ O J v D N j b u i b N J N J N J N J N J u i u i u i cn u i ID vO ID U l s l b o p i o i o N J N J N J N J N J u i cn cn cn u i CO N J N J O O J CO J A s ) O J U l N J N J N J N J N J ui ui ui cn J A U l s j U J OJ VD s i cn cn ui U J 0 0 0 0 co ui cn s i O O O O O O O O U 1 U 1 C 0 J > U 1 U 1 S J U 1 O O O O O cn cn cn cn s i O O O O O ui ui cn ui s i O O O O O ui s i cn s i U J N J N J N J N J S l O J U l U l P S l p C O cn cn b N J N J N J N J N J N J N J N J N J O J 0 J J > J A U J 0 J J > U l P J > P U 1 U 1 S I C O O O O J O J P U I ' J A O J U J NJ NJ M NJ NJ cn cn ui cn on 0 0 co vo cn b O J vD J A CO NJ NJ NJ NJ NJ cn cn cn cn ui o J A NJ P* NJ U l CO s l OJ NJ N J N J N J N J N J u i u i cn cn u i cn co o O J o b N J uj cn P * CO P NJ U l s l U l p* s l P P U P P P P P N J U 1 N 1 N J 0 J P O P NJ P NJ P OJ CO UJ NJ s l o P P NJ P NJ NJ O P O CO OJ U) P NJ NJ b cn ui j> b O J N J N J N J U J cn s i ui J A 00 N J J> O J P 1 cn s i NJ OJ NJ J > N J N J N J N J U 1 N J N J N ) J A U l U J S l U l U l U l C n M C 0 O U 1 P M U 1 O C O N j b u i O J c n u j N J O O O P* s l N J N J N J O J N J cn s i ui 0 J A U J N J cn o cn s i s i cn b U J s i 00 s i ui cn N J N J N J N J N J s i co cn s i O J U l U l N J O U D ui ui U J cn U J co cn P * s i N J CO U l ^ 0 ^ P ^ 1 U l N J N J N J N J N J ui cn s i cn ui CO O J N J J A N J j> J A cn N J p* OJ UJ CO N J O 264 X DC-119 Rhyolitic flow 263.5 ± 0.7 Ha 260 f • 25 6 <r ) 252 f Assumed age ; C x A of crystallization (Single fraction) 0.275 O.285 O.295 O.305 207 235 Pt>/ u O . 043 DC-239 Rhyolitic Flow 262.6 ± 0.7 Ma 270 H i 1*1 < \ | -260 ^^^&^r " 1 ! O Q . Q39 250 jf' 240 Assumed age c7\ of crystallization ^ (2 KPb/2 MU Age of considered fractions) o. 16 O . 30 207 235 Pb/ O Figure 2-7: Concordia diagrams from volcanic rocks in the Veladero North area. Reported age of crystallization is 2MPb/2"U age of indicated fractions. Uncertainty in age is based upon range of uncertainty of considered fractions and reflects maximum and minimum probable ages at 2 sigma level. See appendix for age determination and Table 2-3 for analytical data and results. Uncertainty ellipses shown at 2 sigma level. P o t r e r i l l o s - C a n i t o P a c k a g e Late Paleozoic rocks exposed in the western part of the study area underlie the international border and extend to the east down the Canito creek forming the largest structural package in the map (Figure 2-5). In this area, the rocks crop-out between 5000 m a.s. l . and 3900 m a.s. l . and extend into Chile where it links with mapping of Martin et al. (1995, 1999). North-striking normal faults drop the blocks to the east thereby repeating the sequence. The minimum stratigraphic thickness is 650 m. The strata dip between 30° and 40° to the southeast or east and are unconformably overlain by Tertiary volcanic and sedimentary rocks along a gently angular unconformable to disconformable contact. Brown and purple dacitic to rhyolitic welded tuff, quartz-phyric dacites and minor lenses of coarse sandstones are the common rock types. A diorite porphyry that yielded a biotite(?) K-Ar age of 31 Ma (Thompson, 2000; personal communication) and four mapped small dacitic domes with vertical flow banding (see below) intrude the package. Martin et al. (1995, 1999) proposed the name Guanaco Sonso Sequence to refer to the late Paleozoic volcaniclastic rocks that crop-out in Chile, immediately west to the rocks previously described. U-Pb geochronology studies determined that the age of the Guanaco Sonso Sequence is 265.8 ± 5.6 Ma (Martin et al., 1999). Intrusive rocks Small (< 1 km diameter) intrusives cut the late Paleozoic volcanic and volcaniclastic rocks. The plagioclase and quartz-bearing dacite-rhyolite porphyry intrusions are commonly altered with advanced argillic mineral assemblages. Quartz phenocrysts are embayed and plagioclase is altered to clays. Where less altered, the phenocrysts average 0.5 cm but reach up 1 cm in length. The groundmass is fine grained and it is normally completely recrystallized to quartz and feldspars (?). Flow foliation and spherulitic devitrification are common, but not unique, textures in the late 33 Paleozoic intrusive rocks. Tertiary intrusives have the same overall aspect and, in the absence of cross-cutting relationships, are practically identical. Monomict, clast-supported dacitic to rhyolitic breccias with very angular fragments are often associated to the intrusives, indicating near-surface magmatic activity. 4* Figure 2-8: View of the Guanaco Zonzo creek exposures looking south. The shallow dacitic dome with spherulites forcefully intrudes the volcanic succession. Approximately 250 m (vertical) exposed from the creek to the summit. The Guanaco Zonzo creek, in the south central part of the map area, cuts across a spherulitic dacitic dome that forcefully intruded late Paleozoic rocks of the Rio Taguas package. In the 250 m of vertical exposure along the creek (Figure 2-8) , no evidence that the dome vented to the surface has been observed. The sampled dome rock, DC-142, is a clay and silica (?) altered, porphyritic, intensely recrystallized dacite with spherulites. U-Pb geochronology studies determined that the age of the dome is 259 ± 0.7 Ma (Table 2 - 3 , Figure 2-9). Although the common occurrence of spherulites in the lithology of the dome is similar to one ash-flow tuff located in the upper section of the late Paleozoic package (sample DC-043 in Figure 2-6) , field relationship and geochronological data argue that the dome is younger and not part of the same volcanic event that formed the spherulitic tuff. O . 043 DC-142 Guanaco Zonzo Dome 259.0 ± 0.7 Ma 260 b CO \ O, 041 260 2 ^ o" y A ^ > 0 Assumed age of crystallization ("'Pb/^U Age of considered fractions) O . 2 207 P i ? / 235 u 254 ± 4.2 Ma Assumed age of crystallization is the upper intercept of regression line DC-181 Canito Creek Dome 254 ± 4.2 Ma O . 25 O.27 0.29 C 207 235 Pb/ 0 280 0 . 043 DC-162 Agostina Sur Dome 258 ± 1.25 Ma 270 b CD <-> 260 = » . i PI X a, 'a 0 . 039 25 O X G 0 (\ 0. 035 O. 230 Assumed age C7] of crystallization (""Pb/^U Age of considered fractions) 24 207 ° 235 Pb/ V O. 35 Figure 2-9: Concordia diagrams from intrusive rocks in the Veladero North area. See appendix for age determination and Table 2-3 for analytical data and results. Uncertainty in age is based upon range of uncertainty of considered fractions and reflects maximum and minimum probable ages at 2 sigma level. Uncertainty ellipses shown at 2 sigma level. Other rhyolite-dacite intrusives, such as the domes in the northern part of the study area, have less clear crosscutting relationships with their host rocks. Two porphyries were sampled for geochronological study. Coarse grained, matrix-supported, porphyritic rhyolite-dacite with clay altered feldspars and rounded quartz crystals larger than 5 mm form the intrusive dome located in the northern part of the map where sample DC-181 has been collected (Figure 2-5). The U-Pb age of the intrusive is 254 ± 4.2 Ma (Figure 2 -9 , Table 2-3). In the central part of the Veladero North area, also known as Agostina Sur, advanced argillic altered, quartz-phyric porphyritic dacite with relict flow-banding texture characterize the late Paleozoic intrusive facies sampled as DC-162. Preliminary U-Pb data indicate that the maximum age of this dome is 258 ± 1.25 Ma (Figure 2 -9 , Table 2 -3 , see Appendix for detailed discussion of U-Pb systematics). Both domes have been previously mapped as Miocene intrusions and related to the Miocene high-sulfidation alteration system at Veladero North. Geochemistry of late Paleozoic rocks of the Veladero North area Geochemistry is used to classify volcanic and plutonic rocks and to compare magmatic units at a regional scale. Rock geochemistry, by analogy to modern examples, is also an effective approach to constrain the tectonic setting where the magmatism occurred. In this study, new geochemical data of pre-Tertiary rocks are presented and compared to available regional information (Tables 2 - 1 and 2-4). Hydrothermal alteration affects nearly all rocks in the study area and the data reported in Table 2-4 are from fresh and altered rocks. The alteration effects are quantified comparing data from the literature (Bissig, 2001; Malizia et al., 1997; Sato and Llambias, 1993) to the data presented in this study (see Appendix IV). The REEs from all volcanic samples are considered but only some samples are taken into account for the major and trace element characterization. Table 2-4 outlines which elements from the dataset are used to describe the lithogeochemistry of the Veladero North area. a o o oi x x 0 1 CL OL a a -7 V) Ul 2 o> o 3 = 2 -» a. 3 1*3 SLJBS' c ? 3 3. < « m z it m n a S 2 «• S, 2 ° ° 3 M |?§ si* a i 5-•n " S. I ID « • O n O c 3 rr 5' ID r o in w o * 3 3 m o " 3 3 3 . °- E 4 ro O » " " fl). o " ^ ™ 3 1 ro i 3 3 ! ra3 cr s ° ? §§ £ n fD o C [ tf) QJ C L cr. fu ° 3 V) o. 3 O » 5 ? c s r - S o r* H - n r ^ H m l O H O m W z - o n r B Z ^ 3 - o i - i , c t T 3 - i o < o - a c 3 a - i ( t n i a p i T ^ 3 73 m H * bo ui In In Co Lo •fc K CO K W CO D D U D D D n D t J D D D D D D D U O t J D D D O 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 o i p m o i o f c K U i o u i H t t N J O f c w ^ r t i - t r t f t r t r t r t r t f t f t r f r t ^ ^ ^ ^ J ^ ^ ^ s P s P ^ ^ pi o o o o o O b o I—1 M O JO bo b V l O CO b U l o pi CO U l NJ b in 10 1 - 1 M K oo n P 1 J» P O Ul ' , „,.., PiPiOCTiOOOOOOOpi • • • • • • • • . . • • . r : ^ ' ™ O K * 0 1 U 1 M g 0 1 0 V J P W P O * 0 1 p * H K H ^ U l C ^ L n s J W O l O U N J ^ U l l O O S O D M W C O ^ l D C o O h j ^ ^ U H U H C O L n N U l l D o ,_ . P 4 CO O l O M M u ^ ? ? ? P P P P P P ^ O c o p i - P i n i j i p i p ^ i j i b f c p i M p i b J ^ J ^ p i L o - p o co ui M ^ !° P ^  P P P P P !° ° 0 0 6 O f c O W W K Q y u f l f J U U y ^ y n ^ y y y o p- v j n A U) M p v l O O O O O N O O ^ OJKUiuo g b s b k i b o H b c o f c O H j " N J O l C O l D O U l O C O ^ O L n H f f i W s l K S J S j y ) o O M l - ' P i P i p U 1 0 0 0 0 0 p i O p i _ uih-'^oocn g i / i b A b b M b i o v i b i ^ i n o P . vi n VI A l - 1 n A O O O O O O P f J l P h ' ^ Cftp»NJp»cocf t2Kjb^Lnki i fcb lobbf>JCr i l puuipouQuoo^ P O f c p y i u i i i N O D p - co n A M M k N O U i p p p O O P p p , ! . u i H U i o w p O o o b b H u b b i b i b f o u i V O O l U K V J O O Q C O I i O O l O P O l O O O l D U l U l P W P U f f i P * bo b b b un Lo Lo 73 • p i A m S p p 4 i U ) i o p u i ' v l U l v O A l O O C O p i p i U O l W n h ^ M O M M O O O P i O i " P y , ? * ? * r ' ? r ? P ? P > l ^ P S p A O W L n g w b l o b p w b p - j O C * C 0 v J v J v l U ) - P V l O O U ) U ) s 0 s 0 O v l v l U ) p i - p M C 0 O A L n C r i . p U l C 0 M U I M p i U l M U ^ 1 _ . p l M O - p U I P > O O M . f c p p ^ p ^ p ^ r ^ r i ' r ^ s ^ i ' y ' S p i S ^ D p p u o i f c i b u i b S I C O ^ ^ ^ O ^ O O W W O W W I f l s l O I N P I O ^ O U I U l ^ O p p i y o i r o s l C O V O f c 7) m - P 00 O M pi M Js. Lo M io * i-i Lo bo pi . b p- O . p . , k . P - 0 - p M O O O M * p ? ? * ? * r > l r ^ r ! 0 y | ! j ! j y , 2 p U , S * u g ^ . b ^ p ? p b « P P W C O O l P O l W U l O l N I I O P A W ^ P W L I W N I l D O l D O w J i U l p p y i N O O W p * C T i O M O O O O p i - P co n P w ^  b Lo co CTl p i U l a vi n O N ^  Lo Lo co \ 0 M -fc a vi n i-i Lo cnl M vO C0| co n o o 73 ra O N O v l v l P * Ln '-fc Ln Lo Lo co i - i N P U P W U g L n O O I O W U O W U I I p i M V I 4 i t D L n Q c f l O O V I C O P i p i p i U I O ( p- vi n p i ui aiui r o p * M O - f c M O O o o p i O x O P M ^ ? ! » P * ° * r P r > l P ! I * ! s ^ S p u i p 5 w O N b f f l p b b b f f l f f l b f f l J N J W s J S I W s J O > 0 > W W C i U O N J N J O O C O U l U O O O O o i | i O ) y i ( ? * l P 0 1 0 0 0 1 v J l C D , P . V I ? £ K H u N p ^ N J O O O O K O l D r O l O H A C O U ^ Q w O K ^ H W o i B O o i - ' ^ f 3 I o 3-ft 3 o 5" o s o "tl sr & 3 % o Bl 3 B) O I P O ^ U O * Lo co M Ln bo co co p i 00 M O U l p i U l Ln Lo b vi cn vi Lo ol i pi co O ^ 1 - 1 p ' i n p ' C o u i n . k.oio o o o o p i j ^ o o ' ° ? ? r ? r P H ! » ! - ^ ^ ^ ^ p y ' S p S t i ^ g ^ p ! J o b L , b ^ W p £ D ! I CO Ol • • H H U l W u l y A O W O O O O . O l O M ^ U l O O u i a i r ^ N j M O i s l i O N i O W U I ^ ^ H a i O ^ O v j O i D o, CO 0 _ ^ * f-* ,n ,_. p ^ L o o r o O r - ' o o O v O o - ^ ' . ^ ^ • • ' • • - • • • • • • • • • ^ ^ O C O W O i O ^ U l U P N j i D ^ I N J O W M ^ O O M W W V O T O I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I Phonolite Rhyoiite L | m | t Q f a | t e r a t i o n Figure 2-10a: TAS rock classification for late Paleozoic rocks from the Veladero North area. Limit of alteration defined by unaltered rocks from the literature (Bissig, 2001; Malizia et al., 1997 and Sato and Llambias, 1993). Samples DC-060 and DC-184 are from the least altered rocks in the study area used to characterize major oxide concentrations. i b -.01 .001 1 .01 65 70 75 Si03 (wt%) 1 1—I I I I I I | Limit of alteration l 1—I I I I 11 Andesite/Basalt SubAlkaline Basalt J I i i i i i 11 I I I Alk-Bas I I I I I I I I Figure 2-10b: Geochemical classification of Veladero North late Paleozoic rocks based on partially immobile elements, from Winchester and Floyd (1977). 10 FeO* Nb/Y * Rio Taguas structural package A Guanaco Zonzo structural package * Potrerillos-Canito structural package f> Intrusive (Guanaco Zonzo creek) • Intrusive (undifferentiated) Tholeiitic Calc-Alkaline Figure 2-10c: AFM diagram presents the calk-alkaline affinity of the late Paleozoic rocks in the Veladero North area. Alk MgO 38 6 £ I j j o, 5T j * 3 £ | | in 3 I T Ql i? "> is a * * sr 2 Q. O (V £ Ii -Is 0 » 1 O 05 in o n 3 o sr I 3 ai a. m 5 h 3 ° fj. rn a 5 c o 2 . * 5 o _ 4i « • *f§ ?83 Sn 01 o 3& 8" a II . v»«» 3- a> Si. 2 i t s 0. a 01 J« * 5. Is* Ell 211 Ol 0 _ III 3 (o a Z. Kl 3 v ~ lis J J 70 0 Q 3 5 -3 o HL l I I I I I 4 s J I J 2 L H C T o TTT1 1 1—I I I I I I JO o n s | 3 2 J I 1 J I I I I I 11 M a j o r and t r a c e e l e m e n t g e o c h e m i s t r y Late Paleozoic volcanic rocks include mainly calc-alkaline rhyolites and dacites with K 2 0 contents over 4 wt% and subordinate andesites (Figure 2-10, Table 2-4). Late Paleozoic intrusives plot in the rhyolite field of Winchester and Floyd (1977) and their trace element concentrations suggest that their composition, compared to that of volcanic rocks, tends towards intermediate rocks (Samples DC-099 and 184 in Figure 2-10). Data from unaltered samples suggest that the volcanic rocks are slightly peraluminous (molar A/CNK > 1) which could be a primary compositional feature or perhaps due to alkali loss during devitrification or weathering. Strontium concentrations are low in the analysed rocks (7 to 144 ppm) and Rb has medium to high concentrations (104 to 411 ppm). The Rb/Sr ratio is variable but greater than one. Compared to Tertiary rocks, late Paleozoic rocks have high Rb/Sr ratios that may indicate upper crust affinity. Yttrium values are high and average 40 ppm reflecting that high-pressure minerals in the source have not retained this element during magma genesis (e.g. Mpodozis and Kay, 1992). Rare e a r t h e l e m e n t g e o c h e m i s t r y REE abundances of late Paleozoic rocks are overall one to two orders of magnitude greater than primitive mantle Taylor and McLennan (1985) and HREE display moderately flat patterns (Figure 2-11). The three volcanic packages are geochemically indistinguishable. In detail, LREE (La to Sm) are slightly enriched and have a moderately steep slope averaging 3.2 log units. HREE (Tb to Lu) show little fractionation (slope ~1). Negative europium anomalies are moderate to strong in the volcanic rocks. Late Paleozoic intrusives show similar HREE patterns and moderate to strong negative europium anomalies but exhibit flatter LREE patterns. The geochemical characteristics have been interpreted as resulting from melting of a garnet-free, plagioclase-bearing source located in a relatively low-pressure 40 environment (e.g. Mpodozis and Kay, 1992). These magmas may be related to anatexis of felsic granitoids and/or pelitic material. Discussion Summary of Veladero North Sequence Volcanic and volcaniclastic rocks of the Cordillera Frontal are assigned to the Permian-Triassic Choiyoi Group. In the Veladero North area, late Paleozoic rocks are exposed in three structural packages that represent time equivalent pyroclastic and volcaniclastic deposits and shallow intrusive facies emplaced under similar conditions. In terms of lithology and age, these volcanic rocks are comparable to the Guanaco Sonso Sequence in Chile (Martin et al., 1995, 1999), herein referred as Guanaco Sonso Formation to fulfill stratigraphic nomenclature standards. Because of those similarities, the Guanaco Sonso Formation is extended into Argentina and intends to designate the volcanic and volcaniclastic rocks emplaced during the Permian in this portion of the Cordillera Frontal of Argentina and Chile. Uranium-lead and 4 0 A r - 3 9 A r geochronological studies in the region (Figures 2-5 and 2-12) show that the volcanic rocks of the Guanaco Sonso Formation were emplaced in a relatively short period between 265 Ma and 260 Ma. Geochronological data and field relationships indicate that the shallow volcanic porphyries in the Veladero North area intruded the Guanaco Sonso rocks immediately after their emplacement. Uranium-lead geochronology constrains their age to between 260 Ma and 255 Ma. The Guanaco Sonso intrusives, although relatively small and lithologically similar to Tertiary and other Paleozoic to Mesozoic porphyries, form an individual map unit previously unrecognized in regional surveys. Whereas the intrusives are slightly younger than the Guanaco Sonso Formation, their spatial relationship and similar lithology imply they are part of the same magmatic episode. How far the Guanaco Sonso Formation extends eastward is not precisely known yet, but it is very 360 340 320 300 CHOIYOI GROUP 280 260 240 220 200 180 160 Ma INGAGUAS SUPERUNIT Colorado U Choliay-EI Leon Unit Los Carricitos ELQUISUPERUNIT El Volcan Unit <1 i - ® - i Cochiguas Unit Guanta Unit COLANGUIL BATHOLITH • • Tabaquito Granodiorite O • 1 Vizcachas Granites Granodiorites Hurtado Formation H U-Pb Zircon • "Ar-MAr Biotite ^ Rb-Sr Biotite O Rb-Sr W-R isochrone • Rb-Sr Biotite-W-R ® K-Ar Biotite ® K-Ar Hornblende ® K-Ar Muscovite C K-Ar Whole-rock 2 2 2 3 3 8 3 15 3 2 1: Nasi et al., 1985; 2: Mpodozis and Cornejo, 1990; 3: Martin et al., 1995; 4: Rodriguez Fernandez et al., 1997; 5: Linares and Llambias, 1974; 6: Llambias and Sato, 1995; 7: Sato and Kawashita, 1988; 8; Pankhurst et al., 1996; 9: Bissig, 2001; 10: Herve in Ribba, 1985; 11: Malizia et al., 1997; 12: Cardo et al., 2000 13:Parada et al., 1981 14:Espina et al., 1998 15: Tosdal, unpublished 16: This study 360 340 320 300 280 260 240 220 200 180 160 Ma Figure 2-12: Compilation of late Paleozoic and Mesozoic ages from the Cordillera Frontal of Argentina and Chile. Ages from the Veladero North area are 2 sigma uncertainty, remaining ages are reported as originally published. Shaded boxes outline the expected age by magmatic unit, see also Figure 2-13. Arrow indicates single sample analyzed by different methods. likely that it extends farther east than the Colangiiil plutons. The age of the Guanaco Sonso Formation, considering volcanic and intrusive facies together, is comparable to the Rb-Sr cooling dates obtained in the Colanguil Granodiorites and Granites. Whether the Colanguil plutons are time equivalent to the Guanaco Sonso Formation or wheter they are older, unrelated rocks that have been thermally disturbed during the emplacement of the extensive rhyolitic sequence, needs to be defined using higher precision dating techniques. Los Tilos Formation (modified name after Los Tilos Sequence of Martin et al., 1999) yielded a biotite K-Ar age of 235 ± 5 Ma that U-Pb geochronological method fails to replicate (Martin e t a / . , 1999). However, the 2 0 8 P b / 2 0 6 P b isotopic compositions of Los Tilos and Guanaco Sonso zircons are noticeably dissimilar (0.055 to 0.067 and 0.065 to 0.159 respectively; Martin et al. (1999) data), supporting the likelihood of diverse sources and consequently different age for the two formations. Regional corre lat ion The Cordillera Frontal of Chile and Argentina is dominated by north-trending Tertiary thrust faults and back-thrusts that juxtapose late Paleozoic and Miocene or younger rocks and obscure pre-Tertiary structures (Martin et al., 1995; Mpodozis and Ramos, 1989). In a generalized cross-section, Tertiary deformation and subsequent erosion results in rocks packages that decrease in age towards the east whereas deeper portions are exposed towards the west. Consequently, crosscutting relationships and relative stratigraphy are best established along strike. Recognition of the intrusive units in the region relies on textural and compositional similarities, implying that defined lithological characteristics are unique and representative of each magmatic episode (for example Nasi et al., 1985). Potassium-argon and Rb-Sr geochronological studies indicate that magmatism in this part of the Andes was apparently continuous for 130 m.y. Recent studies, relying on the precision of U-Pb and 4 0 A r - 3 9 A r geochronometers, suggest that the late Paleozoic to Mesozoic magmatic activity was not continuous, but rather occurred during distinct episodes much like the history that characterizes the modern Andes (e.g. Martin etal., 1999). Different geochronological methods have been used to define the timing of magmatism throughout the Cordillera Frontal of Argentina and Chile. The results by unit and method are summarized in Table 2 -2. Whole-rock and biotite K-Ar geochronological studies (Nasi et al., 1985; Mpodozis and Cornejo, 1988; Martin et al., 1995, 1999; Heredia et al., 2002) were used in initial geochronological research and constitute the majority of the available data. The assumption that the age produced by whole-rock K-Ar analysis is the age of crystallization needs to be evaluated. In the same way, and because of closure temperatures near 300°C (Harrison et al., 1985), biotite K-Ar dates in intrusive rocks must be interpreted with caution in regions of multi-episodic intrusive history. Moreover, hornblende closure temperatures in the range from 450°C to 500°C (Harrison, 1981) might explain the observation that older K-Ar ages are commonly produced by hornblende than biotite from the same rock (e.g. NBT-302: hornblende 256 ± 7 Ma vs. biotite 245 ± 4 Ma, Nasi et al., 1995; see Table 2-2 and Figure 2-12 for other pairs). Uranium-lead geochronological studies using magmatic zircons reproduce within uncertainty amphibole K-Ar ages but fail to duplicate biotite data from the same unit. Of interest is the fact that biotite is commonly the same age as that of younger intrusive and volcanic rocks dated by the U-Pb zircon method (Figure 2-12) . Similar patterns that reveal a discrepancy between K-Ar and U-Pb data have been seen in other convergent margin batholiths, for example in the Sierra Nevada (Chen and Moore, 1982) or the Whitsunday Volcanic Province in Australia (Bryan eta/ . , 2000; Allen et al., 1998). In some of those cases, the ages of the micas may not always be correct and geochronometers that are more robust are required to establish a precise magmatic timing. In the Cordillera Frontal between 28° 30'S and 31° 30'S, revision of available U-44 Pb and Ar- Ar geochronologic data suggest three peaks of intrusions and two diachronous volcanic periods (Figure 2-13). The following interpretation assumes that, although some rocks may have been inadequately assigned to a specific unit, petrology and field evidence sustain the unit subdivision. From oldest to youngest the plutonic episodes are represented by: Late Carboniferous (320 Ma to 280 Ma) Guanta and Cochiguas Units (Elqui Superunit), Late Permian-Early Triassic (270 Ma to 235 Ma) Chollay-EI Leon Unit (Ingaguas Superunit) and Late Triassic - Early Jurassic (220 Ma to 190 Ma) Los Carricitos and Colorado Units (Ingaguas Superunit). No U-Pb or 4 0 A r - 3 9 A r data are available from the Colanguil Batholith. However, as the dates of the Granodiorites and Granites are 270 Ma to 262 Ma and 261 Ma to 255 Ma respectively, those units have been preliminary assigned to the Chollay-EI Leon Unit. The Permian (275 Ma to 250 Ma) Guanaco Sonso Formation and the poorly constrained Triassic (225 Ma to 210 Ma) Los Tilos Formation constitute two apparently independent volcanic episodes. Biotite K-Ar dates form a group around 260-250 Ma, including plutons with Carboniferous U-Pb and hornblende K-Ar ages. Moreover, the Late Carboniferous sedimentary Hurtado Formation yields a whole rock K-Ar date of 254 + 8 Ma (Martin et al., 1995). These observations suggest the likelihood of a widespread Permian thermal event that disturbed the low temperature chronometers, and the initial isotopic composition of older rocks. Probably the same event partially mobilized Rb-Sr isotopes and therefore the dates determined by Rb-Sr method may reflect cooling ages if the isotopic system was completely homogenized at that time. The age of the thermal resetting event is similar to the age of the widespread volcanic rocks from the Guanaco Sonso Formation, and suggests a causal link to the widespread Permian magmatism. The reassessment of the geochronological data suggest that plutonic units with distinct geochemical characteristics might have been emplaced in a relatively short period of time (e.g. Guanta and Cochiguas Units or Los Carricitos and Chollay-EI Leon 45 Units; see Figure 2-12) . Conversely, plutonic units with similar geochemical signature occur from Late Carboniferous to Late Triassic (e.g. Guanta and Chollay Units; data Mpodozis and Kay, 1992), implying a complicated petrotectonic interpretation. Pb Age or4 0Ar-3 9Ar Ar Age • U • Rl?-Sr Age Colorado Unit Los Carricitos Unit OOO Vizcachas ? ^ i vi V 0004! Los Tilos Formation ? 3 J Chollay-Et Leon Unit ] Guanaco Sonso Formation Cochiguas Unit Guanta Unit; L Colanguil Granite j Colanguil Granodiorite 1 i Tabaquito Granodiorite? 340 320 300 280 260 240 220 2 0 0 180 Ma E L Carboniferous E L Permian E M L Triassic E M Jurassic Figure 2-13: Summary of late Paleozoic to early Mesozoic magmatic episodes from the Cordillera Frontal of Argentina and Chile. Conclusions In the Veladero North area, the Permian volcanic sequence is assigned to the Guanaco Sonso Formation, the oldest formation in the Choiyoi Group. The sequence is formed by a succession of rhyolitic to dacitic out-flow tuffs, interfingered volcaniclastic rocks and shallow intrusives lithologically similar to the volcanic rocks. All these rocks were emplaced between 275 Ma and 250 Ma. Although K-Ar geochronometers indicate that the Guanaco Sonso Formation is coeval to the (270 Ma to 235 Ma) Chollay-EI Leon plutonic event, U-Pb studies suggest that both units are diachronous. 4 6 Revision of geochronologic studies in the Cordillera Frontal indicates that the Los Tilos Formation (Choiyoi Group) was emplaced in the 225 Ma to 210 Ma range. Available data also suggest that a 25 m.y. volcanic gap exists between the two volcanic units that form the Choiyoi Group implying that the emplacement of the volcanic rocks from the granite-rhyolite province was probably discontinuous. The intrusive rocks from the Granite-rhyolite province have been grouped into the Elqui and Ingaguas Superunits. Plutons from the Guanta and Cochiguas Units represent the largest part of the Elqui Superunit and, according to U-Pb geochronology, they have been emplaced between 300 Ma and 280 Ma. Chollay-EI Leon intrusives are attributed to the oldest section of the Ingaguas Superunit. The Los Carricitos and El Colorado Units are also part of the Ingaguas Superunit and the plutons correspond to a younger and unrelated magmatic episode that spans from 220 Ma to 190 Ma. The Rb-Sr ages of the Granites and Granodiorites from the Colanguil Batholith overlap with the emplacement of the Guanaco Sonso Formation but it is otherwise older than the estimated age of the Ingaguas Superunit. The single Early Carboniferous Rb-Sr date from the Tabaquito Granodiorite may indicate that this Unit groups the oldest plutons in the Cordillera Frontal. Future geochronologic studies are required to establish a precise timing of the emplacement of the Colanguil Batholith. References Bell, C M . , 1987, The late Paleozoic evolution of the Gondwanaland continental margin in northern Chile, in McKenzie, G.D., ed., Gondwana Six; Structure, tectonics, and geophysics. Volume 40: Geophysical Monograph: Washington, DC, United States, American Geophysical Union, p. 261-270. 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Malizia, D., Limarino, C O . , Sosa Gomez, J . , Kokot, R., Nullo, F.E., and Gutierrez, P.R., 1997, Hoja Geologica Cordillera del Zancarron (Provincia de San Juan) N° 3169-26 y 25: Buenos Aires, Servicio de Geologia y Mineria de Argentina (SEGEMAR), 197 p. Martin, M.W., Clavero, J . , Mpodozis, C , and Cuitino, L., 1995, Estudio geologico regional de la franja El Indio Cordillera de Coquimbo, Servicio Nacional de Geologia y Mineria, Compania Minera San Jose, 238 p. Martin, M.W., Clavero, R.J., and Mpodozis, C , 1999, Late Paleozoic to Early Jurassic tectonic development of the high andean Principal Cordillera, El Indio region, Chile (29-30°S): Journal of South American Earth Sciences, v. 12, p. 33-49. 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Nasi, C P . , Moscoso, R.D., and Maksaev, V.J . , 1990, Hoja Guanta, IV Region de Coquimbo: Santiago, Servicio Nacional de Geologia y Mineria, 140 p. Nasi, P.C., Mpodozis, M.C., Cornejo, P.P., Moscoso, D.R., and Maksaev, J.V., 1985, El Batolito Elqui-Limari (Paleozoico Superior-Triasico); caracteristicas petrograficas, geoquimicas y significado tectonico: Revista Geologica de Chile, v. 25 -26 , p. 77-111. Pankhurst, R J . , Millar, I.L., and Herve, F., 1996, A Permo-Carboniferous U-Pb age for the part of the Guanta unit of the Elqui-Limari batholith at Rio del Transito, Northern Chile: Revista Geologica de Chile, v. 23, p. 3 5 - 4 2 . Parada, M.A., 1990, Granitoid plutonism in central Chile and its geodynamic implications; a review, in Kay, S . M . , and Rapela, C.W., eds., Plutonism from Antarctica to Alaska. , Volume 241: Special Paper - Geological Society of America: Boulder, CO, United States, Geological Society of America (GSA), p. 5 1 - 6 6 . Parada, M.A., Levi, B., and Nystrom, J.O., 1991, Geochemistry of the Triassic to Jurassic plutonism of central Chile (30 to 33 ° S) ; petrogenetic implications and tectonic discussion, in Harmon, R.S., and Rapela, C.W., eds., Andean magmatism and its tectonic setting, Volume Special Paper 265: Boulder, CO, Geological Society of America (GSA), p. 99 -112. Petersen, U., 1999, Magmatic and metallogenic evolution of the central andes, in Skinner, B.J., ed., Geology and ore deposits of the central andes, Volume Special publication number 7, Society of Economic Geologists, p. 109-153. Poma, S . , and Ramos, V.A., 1994, Las secuencias basicas iniciales del Grupo Choiyoi, Cordon del Portillo, Mendoza, sus implicancias tectonicas, in Campos, E., and Cecioni, A., eds., 7 congreso geologico chileno; actas, Volume 2: Actas -Congreso Geologico Chileno. 7, Vol: Antofagasta, Chile, Universidad del Norte Chile, Departamento de Geociencias, Facultad de Ciencias, p. 1162-1166. Ramos, V.A., 1988, Late Proterozoic-early Paleozoic of South America: a collisional history: Episodes, v. 11, p. 168-174. Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Kay, S . M . , Cortes, J .M . , and Palma, M.A., 1984, Chilenia: un terreno aloctono en la evolucion Paleozoica de los Andes centrales, Actas del Noveno Congreso Geologico Argentino, Volume 2: Bariloche, Asociacion Geologica Argentina, p. 84 -106 . Ramos, V.A., Jordan, T.E., Allmendinger, R.W., Mpodozis, M.C., Kay, S . M . , Cortes, J .M. , and Palma, M., 1986, Paleozoic terranes of the central Argentine-Chilean Andes: Tectonics, v. 5, p. 855-880. Rapalini, A.E. , 1989, Estudio paleomagnetico del volcanismo permotriasico de la region andina de la Republica Argentina. Consecuencias tectonicas y geodinamicas [Ph. D. thesis]: Buenos Aires, Universidad de Buenos Aires. Rapalini, A .E . , and Vilas, J.F.A., 1991, Tectonic rotations in the late Palaeozoic continental margin of southern South America determined and dated by palaeomagnetism: Geophysical Journal International, v. 107, p. 3 3 3 - 3 5 1 . Ribba, L., Mpodozis, C , Herve, F., Nasi, C , and Moscoso, R., 1988, El basamento del valle del Transito, Cordillera de Vallenar; eventos magmaticos y metamorficos y su relacion con la evolucion de los Andes chileno-argentinos: Revista Geologica de Chile, v. 15, p. 129-149. Rodriguez Fernandez, L.R., Heredia, N., Espina, R.G., and Cegarra, M.I., 1997, Estratigrafia y estructura de los Andes centrales argentinos entre los 30° y 31° de latitud sur, in Busquets, P., Colombo, F., Perez, E.A., and Rodriguez, F.R., eds., Geologia de los Andes centrales argentino-chilenos, Volume 32: Acta Geologica Hispanica: Barcelona, Spain, Instituto Nacional de Geologia, p. 5 1 - 7 5 . Rolleri, E.O., and Criado Roque, P., 1969, Geologia de la provincia de Mendoza, Actas de las Jornadas Geologicas Argentinas, Volume 2, Asociacion Geologica Argentina, p. 1-46. Sato, A . M . , and Llambias, E.J., 1993, El Grupo Choiyoi, Provincia de San Juan: Equivalente efusivo del Batolito de Colanguil, in Anonymous, ed., Actas del Decimo Segundo Congreso Geologico Argentino y Segundo Congreso de Exploracion de Hidrocarburos, Volume 4: Actas del Congreso Geologico Argentino. 12, Vol, Asociacion Geologica Argentina, p. 157 -165. Stipanicic, P., Rodrigo, F., Baulies, O.L., and Martinez, C.G., 1968, Las formaciones presenonianas en el denominado Macizo Nordpatagonico y regiones adyacentes: Revista de la Asociacion Geologica Argentina, v. 23, p. 6 7 - 9 8 . Taylor, S.R. , and McLennan, S . M . , 1985, The continental crust: its composition and evolution: Oxford, Blackwell, 312 p. Thiele, C.R., 1964, Reconocimiento geologico de la Alta Cordillera de Elqui: Santiago, Universidad de Chile, Departamento de Geologia, 73 p. Vilas, J.F.A., and Valencio, D.A., 1982, Implicaciones geodinamicas de los resultados paleomagneticos de formaciones asignados al Paleozoico tardio-mesozoico temprano del Centro-Oeste Argentino, Quinto congreso latinoamericano de geologia, Volume 3 : Buenos Aires, Argentina, Serv. Geol. N a c , p. 743-758. Winchester, J.A., and Floyd, P.A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. Chapter 3 Geological framework of the Veladero North area, Cordillera Frontal, Argentina. Abstract Located in the northern part of the El Indio-Pascua Belt, the Veladero North area is underlain by a north-trending fold and thrust belt that superposes late Paleozoic and Oligocene rocks over early Miocene rocks. The deformed rocks are unconformably covered and intruded by middle Miocene rocks. Field mapping and U-Pb geochronology indicate that the Tertiary volcanic rocks are equivalent in age and lithology to region-scale units previously defined in Chile (Bissig et al., 2001). In the study area the recognized units include: dacite, andesite and volcaniclastic rocks from the (26 Ma to 23 Ma) Tilito Formation; heterolitic breccia, tuff, sandstone and dacite from the (17 Ma to 14 Ma) Cerro de las Tortolas Formation and Infiernillo Unit; biotite-phyric dacites from the (12 Ma to 11 Ma) Vacas Heladas Formation and rhyolite from the (2 Ma) Cerro de Vidrio Formation. Geologic and geochronologic data together with the position of regionally defined erosional surfaces indicate multiple episodes of deformation in the study area. At least two distinct periods of shortening, one pre- and other post- ~ 1 6 Ma, are characterized by folding and thrusting. Shortening is followed by a post-11 Ma deformation, which reactivated pre-existing faults. Gold mineralization is largely hosted in volcanic and volcaniclastic rocks from the Cerro de las Tortolas Formation. Limited published information suggests that the age of the hydrothermal alteration (and likely the mineralization) of the Veladero North deposit is 5 m.y. younger than the ~16 Ma host-rock. 52 Introduction The Veladero North and Pascua-Lama high-sulfidation deposits at the northern end of the El Indio Belt contain gold reserves over 30 million ounces that were deposited in association with Miocene volcanic and plutonic activity. Together, the deposits represent one of the largest undeveloped gold and silver mining districts in the world (Barrick data, 2002). The deposits lie along a 7-km long west-northwest trending zone that straddles the Argentina-Chile international border at 29°30' S in the Cordillera Frontal morphostructural province of the Central Andes (Figure 3-1) . The stratigraphy, geologic and tectonic evolution of the Chilean part of the El Indio Belt is well documented (Maksaev et al., 1984; Nasi et al., 1990; Martin et al., 1995, 1997a,b; Bissig et al., 2001). Furthermore, known deposits such as El Indio, Tambo and Pascua-Lama have been studied in detail (Jannas et al., 1999; Deyell, 2001; Chouinard, in prep.). In stark contrast, the geologic and tectonic history of the Argentinan side of the El Indio Belt is poorly defined (Malizia et al., 1997; Marin and Nullo, 1988; Nullo and Marin, 1990), and the setting of the Veladero North deposit is only known from a reconnaissance study (Jones etal., 1999). This chapter, which builds upon the Paleozoic framework in Chapter 2, provides the first well constrained Miocene geologic and tectonic history of the area. The work utilizes previous geologic studies (Bissig, 2001; Jones et al., 1999; Malizia etal., 1997; Martin et al., 1995) and incorporates data from the Chilean side of the Belt. The discussion is focused on major late Tertiary volcanic and volcanosedimentary units that surround the Veladero North high-sulfidation deposit. Permian rocks, although volumetrically important in the study area, are not fully described in this chapter. Understanding the local geology is critical in order to evaluate the relationship, if any, between gold mineralization and Tertiary magmatism. In detail, the diatreme model currently being used in mining exploration (e.g. Jones e t a / . , 1999) needs to be tested against new geological information. 53 r2b 2JB-S-S-o ,A68°W . Bolivia Nazca Plate 4) I Juan Fernandez Ridge Antartic Plate 74°W 24°S .Cj 70 ! •Col lahuas i (34) • El Abrai(16 -39) v. II, y p Antofagasta • Chuquicamata (31-34) NAZCA PLATE 28° Cordillera] Frontal/ La/Serena 1 • Esc^nbiaai(.35,) Central Volcanic Zone j / El/5al*ad6r M * # 3 ) U \ \ \A \lOTucuman • La Coipa ( 2 2 3 2 4 ) • Marte- l iood (\13-14) , J . .Bajb.de la Alumbrera / / /• k/eWsl U 0 6 - 0 7 ) ' Pa'mpeahas* • — /•. " i .\" .\'\. •. • . E L I N D I O - P A S C U A . . B E L T (6^9.5,) U " . " J h32° • Figure 3-2. O'San 3uan- \Xi Ul Chilean Flat-Slab ""Santiago |o Cordillera] Principah i ii y.Jl # i o s Pelambrej^PKch^n (.09-10) l^ endoza . . 1 -j southern Li? S / Bronces (4.9-)" . " : . / 2 0 0 k m | Vo/can/c HyjEI/Tianiente (4<74).:.• 1 1* Zone Figure 3-1: Southern South America Andes. Location of the El Indio-Pascua Belt and major Tertiary mineralization districts. Age of mineralization is shown in million years between brackets. Based on Bissig (2001) and Dilles and Camus (2001). Represented depth contour lines (25 Km interval) of the Wadati-Benioff zone are from Cahill and Isacks (1992) and Andean Magmatic segments from Kay et al. (1991). Morphostructural elements are simplified from Herve etal. (1987). Geologic setting of the El Indio Belt Mining, exploration and new mineralization discoveries have successively extended the north-south dimension of the El Indio Belt to an approximately 150-km long strip that has recently been referred as the El Indio-Pascua Belt (e.g. Deyell, 2001). The belt is confined between two reverse faults, the Banos del Toro fault in the west and the Colanguil fault in the east (Figure 3-2) . The opposite dipping Miocene thrust faults place late Paleozoic rocks over late Paleozoic to early Mesozoic igneous and sedimentary units which are unconformably overlain by a thick succession of Tertiary volcanic and volcaniclastic rocks. Late Paleozoic to pre-Miocene rocks form the effective basement to 54 the Au deposits and prospects, which are genetically related to the Miocene volcanic complexes (e.g. Martin etal., 1995). 30° S Simplified geology Late Pliocene Cerro de Vidrio Formation Late Miocene Vallecito Formation Middle to late Miocene Vacas Heladas Formation Late Eocene to Middle Miocene Cerro de las Tortolas Formation Escabroso Formation Tilito Formation Bocatoma intrusive unit Late Paleozoic to Jurassic Choiyoi Group, Elqui-Limari Batholith and Colanguil Batholith •tS Faults >*4 Reverse faults » » * Lineaments ^ Mine -"jfer Exploration prospects ^ _ International border " N Chile-Argentina 10 km Figure 3-2: Regional Geology of the El Indio-Pascua Belt and location of major deposits and mines modified from Bissig et al. (2001). Abbreviations: BdTF = Bahos del Toro Fault, P-V L = Pascua-Veladero Lineament, Co F = Colanguil Fault. The El Indio-Pascua Belt lies within the Cordillera Frontal morphostructural unit of Argentina and Chile, as defined by Herve et al. (1987). Geological studies of the Cordillera Frontal of Chile and Argentina have proceeded at a different pace in each country after the initial reconnaissance of Groeber (1946, 1951). Regional scale Tertiary 55 rock units were first outlined in Chile (Thiele, 1964) where further mapping and geochronological studies established a more detailed volcanic stratigraphy (Maksaev et al., 1984; Martin et al., 1995, 1997a). In Argentina, the geology of the eastern flank of the Cordillera Frontal is poorly known, as geological studies are usually focused on relatively small and separate areas (Ramos eta/. , 1987, 1989; Litvak eta/. , 2002; Nullo, 1988; Godeas et al., 1993; Jones et al., 1999). The lack of geochronological studies (Marin and Nullo, 1988; Nullo and Marin, 1990) or imprecise K-Ar whole-rock determinations (Malizia et al., 1997) hindered the development of a coherent regional stratigraphy. Instead, the rock units were either correlated to Chilean formations according to their composition and lateral continuity, or new formation names were assigned to poorly time-constrained or isolated rock entities. Bissig et al. (2001) refined the volcanic stratigraphy on the Argentinan side of the El Indio Belt in the first attempt to reconcile Chilean and Argentinan stratigraphic columns in the region (Figure 3-4). These formation names and paleosurfaces or unconformities separating them have been adopted herein as the geologic framework recognized at Veladero North supports the sequence established by Bissig etal. (2001). The El Indio Belt lies in a region of active magmatism and shortening deformation from the Eocene to late Miocene (e.g. Kay and Mpodozis, 2001). Magmatism continued to early Pliocene (Bissig etal., 2001). The limited Eocene to early Oligocene magmatism and deformation in the 29°S-30°S is attributed to a relatively slow, oblique convergence tectonic setting between the Farallon and South American plates (Pilger, 1984; Pardo-Casas and Molnar, 1987; Somoza, 1998 and Figure 3-3) . The pattern of magmatism and deformation have changed over time, and have been proposed to reflect changes from normal to a "flat" subduction stage over almost 25 m.y. (Kay and Abruzzi, 1996; Kay, 1991; Kay eta/ . , 1987, 1988; Kay and Mpodozis, 2001, 2002; Isacks, 1988; Mpodozis and Ramos, 1989) (Figure 3-3) . 56 15 c m / y 20 10 0 Ma Tbo Tt Te T c t T v h Tv Tcv • UHJU • o i M -20 20 10 0 Ma Tbo Tt Te T c t T v h Tv Tcv • D D D • D D E H M i P j i Figure 3-3: Mean convergence rate (a) and obliquity (b) between the Nazca and South America plates during the last 40 m.y. Positive values indicate dextral convergence. (Somoza, 1998). Grey boxes represent the time span of the Tertiary volcanic and intrusive units of the El Indio-Pascua Belt. Abbreviations: E=Eocene, 0=Oligocene, M=Miocene, P=Pliocene, Tbo=Bocatoma Intrusive Unit, Tt=Tilito Formation, Te=Escabroso Formation, Tct=Cerro de las Tortolas Formation, Tvh=Vacas Heladas Formation, Tv=Vallecito Formation and Tcv=Cerro de Vidrio Formation. With the break-up of the Farallon plate into the Nazca and Cocos plates around 25 Ma, an increase in the relative subduction rate (~15 cm/year) and the beginning of normal convergence between the Nazca and South American plates coincided with the onset of the most current phase of Andean volcanism and deformation (Pilger, 1984; Pardo-Casas and Molnar, 1987; Somoza, 1998; Mpodozis and Ramos, 1989 and Figure 3-3). Crustal thickness increased from 40-45 km at the moment of emplacement of the voluminous Tertiary volcanic units, to approximately 65 km at the waning stages of volcanism in response to shortening in the upper Miocene (Kay and Abbruzzi, 1996, Ramos et al., 1989; Allmendinger et al., 1990). At ca. 12 Ma, a shallow-subduction geometry (<<30°) beneath the Andes resulted in the formation of a magmatic gap between the Central and Southern Volcanic Zone between 28°S and 33°S (Barazangi and Isacks, 1976; Kay e t a / . , 1999 and references therein). The present amagmatic gap and shallow subduction geometry is interpreted to have resulted from either of two mechanisms. One, the geometry resulted from a progressive and continuous flattening of the oceanic slab since 26 Ma (Kay eta/ . , 1987, 1988, 1991, 1999; Kay and Abbruzzi, 1996) related to the shape of the overriding South American plate (Isacks, 1988; Cahill 57 and Isacks, 1992) or, two, it is linked to the subduction of the Juan Fernandez Ridge (Pilger, 1981, 1984; Yanez e ta/ . , 2001; Kay and Mpodozis, 2002). The El Indio-Pascua Belt occupies the centre of the present-day amagmatic segment of the Andes between the volcanically active Central and Southern Volcanic Zones (e.g. Jordan eta/. , 1983). Late Paleozoic to Jurassic basement rocks Late Paleozoic to middle Mesozoic volcanic, sedimentary and plutonic units are the dominant rocks in the composite basement of the El Indio Belt (Martin eta/ . , 1999; Jannas e t a / . , 1999; Bissig e ta/ . , 2001). Carboniferous (?) shallow marine sandstones, schist and gneisses of restricted occurrence are the oldest rocks and lie in the hanging wall of the Banos del Toro Fault. East of that fault, Permian to Early Jurassic granitoids from the Elqui-Limari Batholith and volcanic rocks of the Choiyoi Group form the basement to the Tertiary volcanic sequence. The Choiyoi Group, subdivided into the Permian rhyolitic Guanaco Sonso Formation and the Middle Triassic-Early Jurassic bimodal Los Tilos Formation, represent the oldest volcanic-related products in the region. The rhyolitic to dacitic ash-flow tuff and interbedded volcaniclastic rocks of Guanaco Sonso Formation are a major component of the El Indio Belt. These rocks, especially where hydrothermally altered, are locally very difficult to distinguish from altered Miocene volcanic rocks (Martin eta/. , 1995; Bissig etal., 2001). Tertiary stratigraphy of the El Indio Belt E o c e n e to e a r l y O l i g o c e n e r o c k s The Eocene to early Oligocene Bocatoma Intrusive Unit (Mpodozis and Cornejo, 1988) and the Tobas del Valle del Cura Formation (Malizia et al., 1997) form the oldest Tertiary units of the El Indio Belt. 58 Regional Erosional Surfaces Volcanic and Intrusive Rocks Description Ma 0 1.65 5.30 11.2 16.6 V u ra t t s t— 4_J U ra is « o in «•> O ra 2 £ o ^ h i o •o ro ra s 92 J> ra « 2 f ^ o r l h ! 5 Mr 23.8 20 h 2 5 Cerro de Vidr io Format ion (2.0 ± 0.2 Ma) Val lec i to Format ion (5.5 ± 0.1 Ma to 6.1 ± 0.4 Ma) Pascua Format ion (7.8 ± 0.3 Ma) Vacas Heladas Format ion (11.0 ± 0.2 Ma to 12.7 ± 0.9 Ma) Cerro de las Torto las Fo rmat ion 1 and In f iern i l lo Unit 1 1 (14.9 ± 0.7 Ma to 16.0 ± 0.2 Ma) Escabroso Format ion (17.6 ± 0.5 Ma to 21.9 ± 0.9 Ma) Ti l i to Format ion (23.1 ± 0.4 M to 25.1 ± 0.4 Ma) Bocatoma In t rus ive Unit (30.0 ± 1.9 Ma to 35.9 ± 1.2 Ma) Los T i los Format ion (230 Ma to 245 Ma) Guanaco Sonso Fo rmat ion (250 Ma to 270 Ma) o o. Rhyolitic dome of undevitrified glass and scarce volume of plagioclase, quartz, sanidine and biotite phenocrysts. Poorly to moderately welded, biotite and plagioclase-phyric rhyolitic tuffs with bipyramidal quartz euhedra and phenocrystic sanidine. Coarse-grained, biotite, quartz and plagioclase-phyric dacitic dike lacking hornblende phenocrysts. Amphibole and quartz-bearing, biotite and plagioclase-phyric dacitic to andesitic welded ignimbrites, air-fall tuffs and domes. Biotite, augite, hornblende and plagioclase-phyric andesite flows and coarse plagioclase-porphyritic and equigranular, and fine-grained porphyritic granodiorites and diorites with hornblende and biotite. Massive or flow-banded, augite and plagioclase-phyric andesitic to dacitic lava flows, autoclastic breccias and volcaniclastic rocks. Variably welded dacitic, andesitic and rhyolitic ash-flow and lithic-crystal tuffs and volcaniclastic rocks. Fine-grained to coarsely porphyritic granodiorites and diorites with poikilitic hornblende and biotite. Variably welded rhyolitic, dacitic and andesitic ash-flow and lithic tuffs, volcaniclastic rocks and hypabyssal domes. Figure 3-4: El Indio-Pascua Belt stratigraphy compiled from Martin et al. (1999) and Bissig et al. (2001). Abbreviations: P=Permian, Tr= Triassic, E-Eocene and 0=early Oligocene. Choiyoi Group Formation names as in Chapter 2. The relative position of the Erosional Surfaces of the region is represented from higher altitude in the leflt side to lower altitude in the right side. Lithology from each unit is from the most representative rock types. Absolute ages extracted from Geology (1983) Time Scale. 09 o7 JL «o ui * to in o « g. r* a iB 5- ? » » § 3 3 9 1 3 " 3 B02BBD j s g g 2 ra 3 7 Ol 3T o ra 3 01 9L 3 CP o - 1 CL ra CD ra Q. 9-3 ' 1£> O 3 " 5 3 in oi 0, «• 3 N O 3 o o O 3-o 2. 3 5T o.< Ii n o If 01 _ n O O 3 W§ o o n • 3 =! o Ul 01 o n oi o 3 0 1 o n o -T 3 01 3 a The Bocatoma Unit consists of fine to coarse-grained, hornblende-biotite-rich granodiorite and diorite stocks and dacite to andesite porphyries (Mpodozis and Cornejo, 1988; Martin et al., 1995; Bissig et ai, 2001). The 36 Ma to 30 Ma small stocks, less than 2 km in diameter, intrude Paleozoic rocks. They are compositionally and texturally similar to intrusive phases of the Miocene Escabroso Formation and Infiernillo Unit and can be confused with those rocks in the field. The Eocene to Oligocene (45 Ma to 34 Ma) Tobas Valle del Cura Formation, found only in Argentina, consists of andesitic flows interbedded within a predominately clastic sequence formed by sandstone and conglomerate (Malizia etal., 1997). Late O l i g o c e n e to la te P l i o c e n e r o c k s Miocene volcanic and subvolcanic intrusions dominate the Tertiary stratigraphic sequence. The Tilito Formation (Martin et al., 1997) is the most voluminous Cenozoic unit of the El Indio Belt, being locally as thick as 1200 m. The variably welded dacitic to rhyolitic ash-flow tuffs, volcano-sedimentary rocks and basaltic to andesitic lava flows were erupted between 25.1 ± 0.4 Ma and 23.1 ± 0.4 Ma (Bissig etal., 2001). The Escabroso and Cerro de las Tortolas Formations unconformably overlie the Tilito Formation whereas the Infiernillo Unit intrude these rocks. These rocks represent the magmatic products of a volcanic arc active during that time span. The age of the Escabroso Formation is between 21.9 ± 0.9 Ma and 17.6 ± 0.5 Ma whereas the Cerro de las Tortolas Formation and Infiernillo Unit is 16.0 ± 0.2 Ma to 14.9 + 0.7 Ma (Bissig et al., 2001). Andesitic to dacitic lava flows, autoclastic breccias, conglomerates, sandstones and minor ash-flow tuff form the extrusive phases of the Escabroso and Cerro de las Tortolas Formations. Andesitic to dacitic porphyries that grade into equigranular stocks form the intrusive phases. Similar intrusive rocks form the (16.0 Ma to 14.9 Ma) Infiernillo Unit and these rocks represent the subvolcanic equivalent of the volcanic rocks of the Cerro de las Tortolas Formation. Volcanic rocks from these two 61 units are petrologically and geochemically indistinguishable; they are assigned to the respective unit based on their ages (Martin etal., 1995, 1997). The 12.7 + 0.9 Ma to 11.0 ± 0.2 Ma Vacas Heladas Formation unconformably overlie the older rocks in the El Indio-Pascua Belt ( 4 0 Ar - 3 9 Ar data, Bissig et al., 2001). Amphibole- , biotite- and quartz-phyric dacitic to andesitic welded ignimbrite, subvolcanic rocks of the same composition and reworked tuffs form the Vacas Heladas Formation (Martin et al., 1995; Bissig et al., 2001). The flat-lying volcanic rocks commonly form the highest ridges, and are associated with small eruptive centres scattered throughout the region. Age equivalent dacitic to andesitic dome complexes and contemporaneous ignimbrites extend to the eastern flank of the Cordillera Frontal and in the Precordillera, they record the eastward widening of the magmatic arc (Kay et al., 1988; Kay and Abbruzzi, 1996), a common phenomenon that extended virtually as far north as southern Peru (e.g. Jordan etal., 1983; Clark etal., 1992). Magmatic activity in the region markedly decreased after ca. 12 Ma and ceased by late Pliocene. A series of small units formed isolated entities along the El Indio-Pascua Belt. The Pascua Formation, as defined by Bissig eta/. (2001), includes 7.8 ± 0.3 Ma dacitic dykes that crosscut the Vacas Heladas Formation. Dacitic tuffs of the same age, previously assigned to the Vallecito Formation at the southern end of the Valle del Cura in Argentina, are also included in the Vacas Heladas Formation. This rhyolitic tuff has ages of 6.1 ± 0.4 Ma to 5.5 ± 0.1 Ma (Bissig et al., 2001; Ramos et al., 1989) and contain subordinate sandstone and conglomerate. The latter tuff unit is related to a single volcanic centre although sedimentary rocks of the same age are very likely present throughout the El Indio-Pascua Belt. A 2.1 ± 0.5 Ma (Bissig et al., 2001) rhyolite dome located 4 km to the east of the Veladero North area (Figure 3-5 and Plate 1) is the youngest volcanic feature of the region. Dark grey glass with scarce plagioclase, quartz, sanidine and biotite phenocrysts form the dome that constitutes the Cerro de Vidrio Formation. Bissig and others (2002) 62 document the petrography, geochronology and goechemistry of the 3 - k m 2 obsidian dome that form this unit. Structure Multiple episodes of deformation during the Oligocene to Recent (Andean Orogeny) formed the north striking, east- and west-dipping high angle reverse faults, associated folds and subordinate normal faults that define the structural architecture of the El Indio-Pascua Belt (Martin etal., 1995; Maksaev e t a / . , 1984; Nasi e t a / . , 1990; Mpodozis and Cornejo, 1988; Moscoso and Mpodozis, 1988, Malizia et al., 1997). Late Paleozoic to early Mesozoic structures, such as the northern portion of the Chollay fault in Figure 3 - 5 , are strongly overprinted by younger deformation events (Mpodozis and Ramos, 1989; Heredia et al., 2002; Martin et al., 1995). Triassic and Jurassic (?), northwest trending extensional structures controlled the position of relatively small pull-apart basins such as the Cuyo Basin, which are best preserved in southern part of the Cordillera Frontal (Ramos and Kay, 1991, Uliana et al., 1989). In the El Indio Belt, tectonic inversion of Mesozoic faults is represented by northwest striking lineaments such as the Pascua-Veladero lineament (Jones et al., 1999; Figure 3-2) . Shortening deformation appears coeval with the initial stages of the Andean orogeny, and very likely limited the emplacement of the Tertiary volcanic units. The deformation front migrated progressively from west to east to the present location at the base of the Sierras Pampeanas, 600 km east off the trench, east of the Veladero North area (Jordan et al., 1983; Ramos eta/ . , 1996; Cristallini eta/. , 2000; Martin eta/ . , 1995). Tertiary volcanic units provide important temporal constraints on the timing of the episodic deformation within the El Indio Belt. East of the Banos del Toro Fault, an angular unconformity and a regolith horizon between the (25.1 Ma to 23.1 Ma) Tilito and (21.9 Ma to 17.6 Ma) Escabroso Formations record regional Early Miocene deformation (Martin e ta/ . , 1997; Bissig etal., 2001). High-angle reverse faults and associated folds developed in the Tilito and Escabroso Formation rocks and unconformably overlain by sub-horizontal strata from the (16.0 Ma to 14.9 Ma) Cerro de las Tortolas Formation demonstrate a middle Miocene, east-west shortening event (Martin etal., 1997). Remanent late Miocene landforms Bissig eta/. (2002) defined a succession of geomorphological features throughout the El Indio-Pascua Belt as remnants of extensive pediplains. The moderately dissected planar landforms formed throughout the late Miocene when semi-arid climatic conditions dominated the region. Three erosional surfaces, vertically separated by 200 m to 400 m resulted from the episodic uplift of the Cordillera Frontal. From oldest to youngest, thus from highest to lowest in altitude, the surfaces are: Frontera-Deidad, Azufreras-Torta and Los Rios. The 17 Ma to 15 Ma Frontera-Deidad Surface eroded intrusive bodies assigned to the Escabroso Formation, whereas volcanic rocks of the Cerro de las Tortolas Formation were deposited over the erosional surface. The same relationship is documented for the 14 Ma to 12.5 Ma Azufreras-Torta surface as the Vacas Heladas Formation unconformably overlie along the surface cut across the Infiernillo Unit intrusives. The Los Rios Surface is the youngest geomorphic feature dissected after the (~11 Ma) Vacas Heladas Formation and before the emplacement of the (~6 Ma) Vallecito Formation ignimbrites (Figure 3-4) . It commonly lies in the present valleys within the Cordillera. Miocene volcanic framework of the Veladero North area The Veladero North area is underlain by a Miocene fold and thrust system that superposes late Paleozoic Guanaco Sonso Formation and Oligocene Bocatoma Unit rocks over early Miocene rocks. The deformed rocks are unconformably covered and intruded by middle Miocene rocks (Plate 1 and Figure 3 -5) ; The Tilito, Cerro de las Tortolas-Inifernillo, Vacas Heladas and Cerro de Vidrio Formations have been recognized in the area. Rocks of the same age as the Escabroso Formation have not been identified. Tilito Formation Volcanic, intrusive and sedimentary rocks assigned to the Tilito Formation underlie as much as 5 0 % of the map area on Figure 3 -5 . Three packages bounded by thrust and normal faults or unconformities with underlying Paleozoic rocks are recognized (Plate 1). Each package consists of a unique stratigraphic sequence not repeated in adjacent packages. Andesitic porphyry, dikes and plugs intrude the volcanic and volcaniclastic rocks, and may represent subvolcanic equivalents to the extrusive volcanic rocks. From east to west, the structural packages are located along and east of the Rio Taguas valley (Rio Taguas structural package), at the eastern flank of the Veladero deposit (East Veladero structural package) and at the base and south of the Cerro Pelado (West Veladero structural package). Rio T a g u a s s t r u c t u r a l p a c k a g e Dark green andesitic rocks, brown volcaniclastic conglomerate and breccias and minor block and ash deposits form a sequence that crops out in the Taguas valley, along the access road to the Veladero North project. The rocks extend towards the east and south outside of the map area. The Fabiana fault defines the western limit of the structural package. It is a steep, west-dipping thrust fault that superposes late Paleozoic rocks over Miocene andesite along the Rio Taguas. Middle Miocene Cerro de las Tortolas and Vacas Heladas Formations unconformably cover the andesitic rocks east of the Fabiana fault (Plate 1). The volcanic rocks are plagioclase- and biotite-phyric, hornblende, clinopyroxene (augite ?) and quartz-bearing, matrix-supported andesites that form coherent packages without visible bedding or foliation. The matrix is generally altered to clay minerals and partially devitrified. In hand specimen, the rocks are weakly magnetic due to the abundant fine-grained opaque minerals that are a common component of the matrix. Dark green monomict breccias with andesitic matrix and andesite blocks larger than 1 m 65 in diameter are subordinate to the andesitic lava previously described. Volcaniclastic conglomerate dominate the sequence (Figure 3-6a). The sequence likely represents the remanents of a dome field and surrounding volcaniclastic apron deposits. Figure 3-6: Tilito Formation rocks from the Rio Taguas structural package, a) Volcaniclastic beds interbedded within monomict andesitic conglomerates, b) and c) Andesitic matrix in thin-section: Crystals and fragments of plagioclase (P), Pyroxene (Augite?) (Px) in a fine-grained, altered matrix, b) Transmitted cross-polarized light, c) Transmitted plane-polarized light. Dark green porphyritic andesite intrudes the volcaniclastic sequence. The texture of these rocks varies from aphanitic to coarsely porphyritic. Their composition and irregular contacts suggest a syn-volcanic intrusive origin. Magmatic zircon geochronology indicates that the age of one of those porphyritic andesites is 25 ± 1.4 Ma (Figure 3-10a Sample DC-128b, SHRIMP data, Table 3-1) . That age is consistent with the structural package being equivalent to the oldest part of the Tilito Formation rocks. Previously reported whole-rock K-Ar date of 20 ± 1 Ma from a sample located 500 m east of DC-128b (Malizia et al., 1997) is a minimum age, most likely due to the overimposed propylitic alteration. 66 E a s t V e l a d e r o s t r u c t u r a l p a c k a g e Green and brown andesitic lithic tuffs, dark green volcaniclastic flows volcaniclastic conglomerates, and minor dacitic tuffs compose this unit. Welding defined by pumice fiammes with an aspect ratio of 10:1 (Figure 3-7) is a distinct feature of this sequence around Veladero North. The western margin of the East Veladero package is represented by a west-dipping thrust that places Permian Guanaco Sonso Formation over Tertiary units. The eastern boundary is an angular unconformity with late Paleozoic rocks. The Tertiary rocks dip more steeply than the underlying rocks. The tuff and conglomerate form a northerly striking unit that extends from the northern to the southern limit of the map area (Plate 1). Figure 3-7: a) Tilito Formation rocks in the eastern flank of the Veladero North area. Fiammes and flow features are characteristic of this structural package. Fiammes are sub-vertical in the photographed outcrop. Welded dacite: Crystals and fragments of plagioclase (P), biotite (B) and recrystallized, deformed lithic fragments (F). b) Transmitted cross-polarized light, c) Transmitted plane-polarized light. In detail, the volcanic rocks are carbonate-, clay- and chlorite- altered, crystal-rich andesites with as much as 4 0 % lithic volcanic fragments. Plagioclase (40%), biotite and amphibole (30%) , rare relict clinopyroxene (?), opaque minerals (30%) and small 6 7 amounts of quartz form phenocrysts. In the clay-altered aphanitic matrix, some flow or welding textures have been preserved (Figure 3-7) . In the southern part of the study area, dark green, microphenocrystic plagioclase-bearing andesitic rocks with a preferred orientation of crystals and clasts are interpreted as volcaniclastic debris-flows. Very fine-grained to aphanitic intrusive andesites cannot be clearly distinguished from those flows except where contact relations are exposed. Outcrops of welded andesitic tuff with sparse lithic fragments located 10 m above the unconformity with late Paleozoic rocks represent the lowest stratigraphic exposures of the Tilito Formation in the study area (Sample DC-120, Plate 1). Conventional U-Pb geochronology indicates that the 2 0 5 P b - 2 3 8 U age of the rock is 24.5 ± 0.2 Ma (Table 3 -2 , Figure 3 - l l a and Appendix for discussion). An east-dipping thrust places dacitic tuffs over the welded andesitic rocks at the Cerro Blanco, east of the Veladero mineralization area. Those dacitic rocks yield a conventional U-Pb age of 22.8 ± 1.7 Ma (Sample DC-I l l , Table 3 -2 Figure 3 - l l b and Appendix). W e s t V e l a d e r o s t r u c t u r a l p a c k a g e Light brown and purple welded dacitic to andesitic tuff, quartz-phyric dacite, red conglomerate and sandstone and minor dark grey basaltic andesite flows form the west Veladero structural package of the Tilito Formation. Most rocks assigned to this package lie south of the Potrerillos River valley. On the northern flank of that valley, the rocks that underlie the Cerro de las Tortolas Formation (see below) in the Cerro Pelado area and that comformably cover the Guanaco Sonso Formation are also included in the same stratigraphic sheet. In both areas, dacitic ash-flow tuffs overlie red conglomerates or sandstones, and very likely represent similar stratigraphic levels. They provide an important stratigraphic link.across the Potrerillos River valley. Two oppositely dipping faults that strike north define the east and west limits of the structural package (Plate 1). Along both faults, the observed stratigraphic relationship consists of younger rocks 68 over older, implying a normal displacement. Previous history of the fault is poorly constrained but the steep dip observed in the rocks is elsewhere seen in association with reverse faults (e.g. Martin eta/ . , 1995). Tight folds in the sedimentary rocks adjacent to the eastern fault require a component of shortening along the package-bounding faults (see below). Thus, although the younger-over-older stratigraphic relationship suggests normal displacement, it seems likely that the faults also had significant reverse movement. The dominant rock types of this package are advanced argillic altered, welded, crystal-rich dacitic to andesitic ash-flow tuffs. Plagioclase, biotite and amphibole are the dominant phenocrysts; clinopyroxene is a common mineral in the andesitic rocks. Quartz phenocrysts are reabsorbed and in hand sample they appear as rounded, clear eyes (Figure 3-8) . The felsic rocks from the Tilito Formation are very similar to the Permian Guanaco Sonso rhyolitic tuffs, although in the Permian rocks the quartz phenocrysts are more abundant and clinopyroxene is lacking. Where both Tertiary and Paleozoic rhyolitic tuffs are moderately to intensely altered it is very difficult to differentiate between them. Red conglomerates and sandstones with interbedded volcaniclastic horizons are relatively common in the Tilito Formation. In the Portezuelo Sur area, the conglomerates contain rounded clasts, less than 25 cm in diameter, derived from volcanic rocks similar to the Guanaco Sonso Formation. Cross beds and channels are common features. Dacitic ash-flow tuffs (e.g. sample DC-163) overlie the sedimentary rocks. In the area located to the south of the Potrerillos Valley, folded sandstones and subordinate conglomerates with clasts compositionally identical to the Permian rocks, form a ~ 2 km long, north-south striking belt. The sandstones grade transitionally into purple volcaniclastic rocks and dacitic volcanic flows (e.g. sample DC-249) . Isolated, dark grey basaltic andesite lava flows with columnar joints are a minor rock type of the Tilito Formation. The rock is glassy and only plagioclase phenocrysts are visible. In thin section, the texture is microporphyritic; the matrix is very fine and represents 6 0 % of the rock. Plagioclase (80%), clinopyroxene (20%) and opaque minerals are the only phenocrysts. Figure 3-8: Tilito Formation rocks from the West Veladero structural package, a) Welded, dacitic flow tuffs cover volcaniclastic conglomerates. Dacitic tuff in b) transmitted cross-polarized light and c) transmitted plane-polarized light: Intensely altered crystals and fragments of plagioclase (P) and embayed quartz phenocrysts (Q) in a fine-grained, altered matrix. Relict welding is preserved. The age of the West Veladero structural package on either side of the Potrerillos River is defined by samples DC-163 and DC-249 (Plate 1). The first sample (DC-163) is from the dacitic ash-flow tuffs located in the western margin of the mineralization zone in Veladero North area and immediately above the conglomerates. The second sample (DC-249) is from rocks of similar composition that overlie a sequence of tuffaceous sandstones and red conglomerates south of the Potrerillos River valley. Numerous zircon crystals and fragments were recovered from sample DC-163. SHRIMP analysis of elongate grains (typically magmatic) yield weighted mean 2 0 6 p b - 2 3 8 U age of 23 ± 1 Ma (Table 3 -1 and Figure 3-10b) . Preliminary results from conventional U-Pb geochronology show that the age of the same sample is constrained in the 22 Ma to 25 Ma range and that very likely the rock age is 23.5 ± 0.9 Ma (Table 3 - 2 , Figure 3 - l l c and Appendix), 70 consistent with the limited data produced by SHRIMP. Sample DC-249 yield a preliminary U-Pb conventional age of 23.9 ± 0.2 Ma (Table 3 - 2 , Figure 3 - l l d and Appendix) that is comparable to the age of the first sample. A n d e s i t i c i n t r u s i v e r o c k s e q u i v a l e n t to t h e T i l i t o F o r m a t i o n Volcanic and volcaniclastic rocks from the Tilito Formation are intruded by dark green microporphyritic to aphanitic andesites, which likely represent the subvolcanic equivalent to the volcanic rocks. The intrusives range in plan view from several metres to as much as 100 metres in diameter. These rocks contain predominantly plagioclase phenocrysts, which are commonly altered to clays, as well as amphibole, biotite and minor clinopyroxene. The latter mineral is more abundant in these rocks than in the Infiernillo Intrusive Unit (see below), but otherwise both units have the same overall composition. The matrix is fine grained and opaque minerals are very abundant (Figure 3-9) . Figure 3-9: a) Hand specimen from the intrusive phase of the Tilito Formation. Porphyritic andesite in b) transmitted cross-polarized light and c) transmitted plane-polarized light: Altered crystals and fragments of plagioclase (P) and pyroxene (PX) phenocrysts in a fine-grained, altered matrix. 71 Table 3-1: Summary of SHRIMP analyses in the Veladero North area Sample 2 0 6 p b 1 U Th 2 3 2 T h 2 0 6 p b 2 206 p b 2 l a Apparent Age (Ma, la) Spot Name % (ppm) (ppm) 2 3 8 ( J (ppm) 2 3 8 u % 2 0 6 P b / 2 3 8 U 3 DC-128b DC128-1.1 0.0 116.1 134.5 1.196 0.36 0.003604 7.1 22.73 (1.7) DC128-2.1 0.0 230.1 70.7 0.318 0.83 0.004222 5.4 26.49 (1.5) DC128-3.1 2.8 2407 686.7 0.295 8.70 0.004089 3 1 27.08 (0.8) DC128-4.1 0.0 400.5 355.4 0.917 1.38 0.004001 4 7 25.80 (1.2) DC128-5.1 0.0 359.3 200.8 0.577 1.18 0.003819 4 6 24.26 (1.2) DC128-6.1 0.0 305.5 205.0 0.693 0.94 0.003598 4 7 23.18 (1.1) DC128-7.1 0.0 1013 643.7 0.657 3.19 0.003662 3 6 23.55 (0.9) DC128-8.1 0.0 359.3 300.6 0.864 1.22 0.003943 5 4 25.25 (1.4) DC-163 DC163-1.1 29.4 237.1 387.3 1.688 0.91 0.003138 27 3 27.1 (1.7) DC163-2.1 0.0 294.5 292.0 1.024 0.91 0.003590 5 4 21.9 (1-2) DC163-3.1 31.3 388.6 662.4 1.762 2.06 0.004232 30 0 22.3 (2.8) DC163-4.1 57.7 302.2 260.0 0.889 1.09 0.001778 75 1 23.5 (1.6) DC163-5.1 0.0 592.0 519.0 0.906 1.80 0.003532 5 9 22.7 (1.3) DC163-6.1 42.9 211.8 177.6 0.866 0.69 0.002177 20 3 21.2 (1.6) DC163-7.1 34.1 689.9 1179.0 1.766 2.36 0.002620 22 3 21.8 (1.1) DC163-8.1 0.0 212.6 190.4 0.925 0.76 0.004180 6 4 25.6 (1.7) DC-375 DC375-1.1 112.4 120.5 132.0 1.131 0.47 -0.00056 301 3 20.31 (2.1) DC375-2.1 27.6 913.4 954.9 1.080 3.11 0.00287 5 3 15.01 (1.7) DC375-3.1 0.0 389.1 379.0 1.006 1.00 0.00299 4 8 17.18 (0.9) DC375-4.1 0.0 82.8 60.7 0.758 0.25 0.00352 12 8 17.77 (2.8) DC375-5.1 0.0 131.2 121.3 0.955 4.63 0.04107 3.4 257.3 (8.8) DC375-6.1 69.5 597.0 766.1 1.326 1.67 0.00099 93.7 15.03 (0.9) DC375-7.1 0.0 292.7 352.0 1.242 0.69 0.00276 6.0 16.53 (1.0) DC375-8.1 0.7 555.1 365.8 0.681 19.66 0.04094 2.7 259.0 (7.1) 1 Common lead 2 Radiogenic lead 3 2 0 6 p b / 2 3 8 u a g e u s i n g 2 0 7 p b t o c o r r e c t f o f C O m m 0 n | e a d 72 2 a error bars DC-128b Andesite 25 ± 1.4 Ma mswd: 2.4 Probability: 0.02 a) 2aerror bars DC-163 Dacite 23 ± 1.1 Ma mswd: 1.68 Probability: 0.11 b) DC-375 Dacite 16.4 ± 1.0 Ma mswd: 1.45 Probability: 0.2 2 O"error bars c) Figure 3-10: SHRIMP data of Veladero North samples. 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The Cerro de las Tortolas Formation is present in three separate areas, Veladero, Turbio River and Fabiana (Plate 1). A tabular package along the east and northeast flank of the Cerro Pelado and next to its summit is the largest extent of the formation. Most of the Veladero North epithermal deposit is hosted in these rocks and consequently these rocks are referred hereafter as the Veladero section. A second section of the formation comprises a series of outcrops in the southern flank of the Turbio River and west of the Rio Taguas. These rocks, known as the Turbio River section, are located to the northeast but are separated from the mineralized zone by rocks of the Tilito Formation. Flat-lying rocks in the Fabiana prospect area at the east side of the Rio Taguas form the third section, these are referred to as Fabiana section (Plate 1). Age-equivalent shallow intrusive, domes and their vent products, although intimately related, form the Infiernillo Intrusive Unit and will be described in a separate section. V e l a d e r o S e c t i o n A diatreme model has been proposed to explain both the mineralization characteristics and the origin of the Veladero North deposit and the Veladero section (Jones et al., 1999; Corbett, 1999). In this model, a volcanic pipe (diatreme) with sub-vertical margins cut through the basement rocks as a result of near-surface, phreato-magmatic eruptions localized along structural anisotropies. In the Veladero North area, two types of breccias were recognized. Bedded heterolitic breccias are considered to represent the subaerial volcanic products emplaced during and immediately after the explosive eruption. Milled breccias composed of rounded to sub-angular fragments in a fine-grained predominant matrix (tuffisites) are considered to represent the vent facies. Figure 3-12: a) Cerro de las Tortolas Formation rocks from the Veladero Section. Bedded heterolitic breccias and interfingered fine-grained beds of volcaniclastic sandstone and tuff, b) DDH042 core (-125m) intercepts very well bedded sandstone and heterolitic breccias. Note the volcanic (VI) and juvenile dacitic fragments (JI) embedded in a fine-grained matrix. East of Cerro Pelado, strongly altered heterolithic breccias in coarsely to well-bedded horizons form the upper part of the Cerro de las Tortolas Formation. Breccia fragments are angular to subrounded, derived from quartz-bearing volcanic rocks, porphyritic intrusives or textureless sedimentary (?) rocks. The original composition and texture are, in most of the cases, completely obscured by superposed alteration. Near Amable, the matrix of the fragmental rocks is ash to lapilli-size material, and the breccias range from clast-supported to matrix-supported assemblages. Very well bedded volcaniclastic sandstones lacking fragments are also common (Figure 3-12a). Tabular clasts are aligned parallel to bedding, and in strongly silicified rocks, their presence can be used to define bedding. In general, the rocks are non-graded or they show a relatively narrow zone of normal grading, which suggests deposition during fluid flow. Along the Filo Federico ridge, the fragments are completely replaced by silica and they 79 are often difficult to distinguish from the matrix. There, the breccia texture appears to be massive, but whether this is primary or an effect of secondary silicification is uncertain. As a general rule, the fragment size increases towards the west and south, reaching a maximum of 45 -50 cm near Cerro Pelade Tuff, volcaniclastic siltstone and sandstone form beds 1 to 10 cm thick throughout the bedded heterolitic breccias. The beds are also visible in drill core (DDH040 and DDH042) between 114 and 125 m below the surface (Figure 3-12b). These horizons set the minimum thickness of the Veladero section of the Cerro de las Tortolas Formation. A thin (<3 cm) sandstone layer interbedded with heterolithic breccias was also observed 245 m below the surface (DDH040), but pervasive superimposed advanced argillic alteration obscures the primary protolith of these rocks. The rocks could be tuffaceous rocks in the Veladero section, or be part of the composite Permian Guanaco Sonso and Tilito basement that underlies the Veladero section. N / 7 8 % of Planes / fli^ v • IE / ' a +b s Equal angle projection Figure 3-13: Poles to bedding measured in a) the Cerro de las Tortolas Formation located between the Limite and Fabiana Faults (Veladero Section) and b) in the Fabiana prospect area. Average dip of the Veladero Section is 16°E whereas the Fabiana Section dips ~12°E. Steeper dips are associated with shallow domes and their flanking coarse-grained sandstone, conglomerate and volcanic breccias. 80 Bedding strikes northerly and dips an average of 16° to the east (Figure 3-13). The occurrence of very fine sandstone and siltstone layers of limited lateral continuity and non-erosive base to beds suggests a very low-energy deposition environment. Shallow intrusives have domed the rocks and steepen the bedding over 16° around younger intrusives at Cuatro Esquinas and on Cerro Pelade In addition, steeper bedding is locally associated with the deposition of block-size material. For example, at the margins of the Cerro Pelado dome, the volcaniclastic rocks are subvertical, and the beds are contorted, and contain slump features. These textures and sedimentology of the volcaniclastic rocks suggest that deposition, deformation and dome intrusion likely occurred synchronously. In the absence of large volumes of magmatic material to be sampled (pumice fragments or porphyritic matrix), geochronological studies have been carried on very fine grained, non-bedded, massive tuff horizons (Samples DC-374 and DC-375). The presence of an irregular basal contact (non-erosive) and of accretionary lapilli suggests that the sampled material is a primary ash-fall deposit. SHRIMP studies yield a 16.4 ± 1 Ma age (DC-375, Table 3 - 1 , Figure 3-10c) that is statistically similar to a 15.8 ± 1.0 Ma age obtained by conventional U-Pb (DC-374, Table 3 - 2 , Figure 3 - l l e ) . T u r b i o R i v e r S e c t i o n Isolated outcrops of conglomerates and pyroclastic rocks located in the southern margin of the Turbio River valley represent the second section assigned to the Cerro de las Tortolas Formation (Plate 1). The conglomerates are clast-supported, coarsely bedded, and polymictic. Advanced argillic alteration is superposed on the rocks from this unit. The clasts are subrounded and they are derived from dacitic rocks similar to the domes intruding the Veladero section, green andesites similar to the underlying Tilito Formation, and massive (sedimentary?) units. An assemblage of quartz and alunite, which obscures the original texture, largely replaces the yellowish matrix of the 81 conglomerates (Figure 3-14) . Feldspar-rich sandstone lenses are the subordinate volcaniclastic rocks of this unit. Unlike the Veladero section, where volcanic processes dominate, sedimentary processes dominate the environment of deposition. Tuff occurs as very thin (<5 cm), massive beds that most likely represent fall deposits and evidence subordinate, contemporary volcanism, thus indicating that the sequence represents the transition from the volcanic complex to the volcaniclastic sedimentary apron. Figure 3-14: Cerro de las Tortolas Formation south of Turbio river, a) Coarsely bedded, polymict conglomerate with silicified and rounded clasts. b) Similar rock with advanced argillic alteration assemblages replacing the matrix. F a b i a n a S e c t i o n Light yellow and grey tuffaceous sandstone, conglomerate and heterolitic breccia crop out at the Fabiana prospect (Plate 1). Based on texture and composition similarities, the less than 50-meter thick sequence is also included in the Cerro de las Tortolas Formation and is referred to as the Fabiana section. The volcaniclastic rocks unconformably overlie the Tilito Formation. The sedimentary rocks consist of bedded volcaniclastic sandstones that grade into massive, moderately sorted, reworked tuffs (Figure 3-15) . Discontinuous horizons of very well sorted, fine ash-size material with mud-cracks and evidence for bioturbation (?) are interbedded within the section; these 82 may represent deposition in ephemeral standing water in ponds or small lakes. The rocks gently dip east (<15°) or are almost horizontal. Clasts in the matrix-supported, heterolitic breccias do not exceed 20 cm long in dimension. The clasts are subangular to subrounded, and they consist of dacites and porphyritic andesites similar to those found south of the Turbio River and in the Veladero section. Textureless rocks are also present. The transition from coarsely bedded breccias to bedded conglomerates is gradual. The rocks from the Fabiana prospect are interpreted as the volcaniclastic, distal equivalent of the Veladero section. Figure 3-15: Cerro de las Tortolas Formation in the Fabiana Prospect area, a) Coarsely bedded heterolitic breccias similar to the Veladero Section rocks, b) Reworked tuff and ash-size, laminated material very likely deposited in shallow water. Infiernillo Intrusive Unit Green weathering porphyritic intrusives of dacitic to andesitic composition and light grey to orange altered dacitic domes with associated pyroclastic vent products form the Infiernillo Unit. The intrusives are generally elongated in a north-south direction, and range from a few hundred of meters to as much as to 2 km in length. The widespread subvolcanic stocks are typically less than one kilometre in diameter. Some of the domes form prominent topographic features such as the Cerro Colorado, the Cerro Pelado, the 83 Cerro Libori and a small hill in the Canito Creek (Plate 1). In the Cerro Libori area, 350 m of vertical exposure shows the transition from crystalline intrusive rocks in the lower part to near-surface and probably sub-aerial flow-banded dacitic domes at the top (Figure 3-16a) . Figure 3-16: Infiernillo Intrusive Unit, a) Outcrops of flow-banded andesites in the upper part of the Cerro Libori. In the background to the left, altered rocks of similar composition form the summit of the Cerro Pupa. Andesitic porphyry in b) transmitted cross-polarized light and c) transmitted plane-polarized light: Crystals and fragments of plagioclase (P), altered biotite (B) in a fine-grained, slightly altered matrix. Green porphyritic, plagioclase-, hornblende- and biotite-phyric intrusive andesite outcrops are concentrated on the east flank of Amable and the lower part of the Cerro Libori. Similar rocks intercepted in DDH042 beneath west Amable are also included in this unit. Plagioclase phenocrysts are 0.5 to 1 cm long and they compose up to 3 5 % of the rock volume. Clinopyroxene is less common than in intrusive rocks of the Tilito Formation. The fine-grained matrix is generally altered to clays and composed of 1 5 % to 2 5 % opaque minerals (Figure 3-15b and c). Within the extrusive facies of the Cerro de las Tortolas Formation, clasts lithologically similar to the porphyritic andesites very likely represent juvenile lithic fragments derived from the erupting magma. Dacite, andesite and subordinate rhyolites with varying alteration assemblages form dome structures in the Veladero North area. The subvolcanic facies consist of quartz-poor porphyritic dacites with characteristic banding and flow textures. Monomict breccias are commonly associated with the subvolcanic rocks. Those breccias present a jigsaw-fit texture formed by closely packed blocky fragments of dacitic composition immersed in a microporphyritic matrix. Matrix-supported, monomict breccias with angular and sub-rounded clasts represent the transition towards the heterolithic breccias included in the Cerro de las Tortolas Formation. Conventional U-Pb geochronology from the Cerro Libori intrusive rocks yields an age of 16.6 ± 0.3 Ma (Sample DC-265, Table 3 - 2 , Figure 3 - l l f and Appendix) that is slightly older than the minimum age (16.2 ± 0.2 Ma, 4 0 A r - 3 9 A r data, Bissig e ta/ . , 2001) of the Infiernillo Unit elsewhere in the El Indio Belt. Bissig (2001) reports an age of 15.3 ± 0.3 Ma ( 4 0 Ar - 3 9 Ar data, plagioclase) from a dioritic intrusive related to the dome in the Canito Creek. Preliminary U-Pb geochronology is not conclusive on the age of the intensely altered dacite from the Cerro Colorado dome. Available data show that these rocks were emplaced between 12 Ma and 16 Ma (Sample DC-304, Table 3 -2 and Figure 3 - l l g ) . The maximum calculated alteration age of 10.7 ± 0.9 Ma for hydrothermal alunite ( 4 0 Ar - 3 9 Ar data from Bissig etal., 2001) places only a minimum age of the host rocks. Elsewhere in the El Indio-Pascua Belt, rocks that are age-equivalent to the minimum age (Vacas Heladas Formation) are lithologically and geochemically different from the Cerro Colorado rocks. Additionally, a 16 Ma whole-rock K-Ar date (Ramos eta/ . , 1998) from a sample collected near the intrusive (Plate 1) is interpreted as a complete resetting age. The lithological evidence as well as the interpretation of the geochronological data suggest that the age of the Cerro Colorado dome is closer to 16 Ma (see Appendix I). Except for most of the Cerro Libori rocks, the Infiernillo Unit dome-related rocks are altered to advanced argillic or pervasive silicification alteration assemblages, and cut 85 by hydrothermal breccias. Heterolitic breccias from the Cerro de las Tortolas Formation, particularly the Veladero section, are also intensely silicified. Hydrothermal breccias that crosscut the volcanic units are often difficult to distinguish from their host-rock. The precise timing of the hydrothermal alteration in the Veladero North area is not known. Vacas Heladas Format ion Brown, biotite-phyric dacitic domes and flows and subordinate andesite form this unit. The rocks are distinctly fresh when compared to older units of the region. Flows partially cover the Cerro de las Tortolas Formation at Fabiana, but in the southern part of the mapped area, they discordantly overlie or intrude volcanic rocks from the Tilito Formation (Plate 1). The Vacas Heladas Formation rocks are commonly porphyritic. Plagioclase (60%), hornblende (20%), biotite (15%) and reabsorbed quartz (5%) phenocrysts represent less than half of the rock volume. The matrix is fine-grained, partially recrystallized and contains small plagioclase laths. In the Fabiana prospect area, the rocks have a fragmental texture defined by lithic clasts that form less than 1 0 % of the volume. Most of those clasts are preferentially oriented and have the same composition as the matrix; none is pumiceous. The rest (<3% volume) is formed by subrounded accessory lithic fragments of diverse composition (Figure 3 -17) . The age of the Vacas Heladas Formation is well documented in the study area although the ages obtained by different methods do not overlap within their analytical uncertainties. Disagreement in biotite K-Ar data of 12.3 + 0.3 Ma (Minera Rio Frio, 2000) and 4 0 A r - 3 9 A r data 11.0 ± 0.2 Ma (Bissig et al., 2001) from samples collected in the Fabiana prospect area likely reflects the combination of post-crystallization isotopic remobilization and discrepancies between the two methods. The 12.7 ± 0.9 Ma to 11.0 + 0.2 Ma emplacement interval proposed in Bissig et al. (2001) for the Vacas Heladas Formation has been determined regionally, and will be used in this study. 86 Figure 3-17: Vacas Heladas Formation, a) Fragmental dacite in the Fabiana Prospect area. Lithic fragments and matrix have the same composition, no accessory lithic fragments have been recognized. Dacitic matrix in b) transmitted cross-polarized light and c) transmitted plane-polarized light: Megacrysts of plagioclase (P), biotite (B), amphibole (A) and partially reabsorbed quartz (Q) in a fine-grained, devitrified matrix Geochemistry Trace and rare earth element concentrations and the isotopic compositions of igneous rocks have been related to the physico-chemical conditions prevailing at the origin of the magmas. Bissig et al. (2003) and Kay and Mpodozis (2002) review a series of petrotectonic models derived from geochemical data. In detail, these models are beyond the scope of this contribution. Only some general interpretations regarding the major difference between units will be discussed here. Whole-rock geochemistry in the El Indio-Pascua Belt has been used to characterize volcanic formations of known age and as a preliminary correlation method when geochronological data are not available (Maksaev etal., 1984; Martin etal., 1995; Bissig et al., 2001; Malizia et al., 1997; Litvak et al., 2002; Godeas et al., 1993). In particular, the relative abundance of REEs and the signature displayed in normalized plots have proven to be an effective tool for discriminating between late Paleozoic to 87 Triassic and Tertiary units. Three parameters can be used to quantify this signature. First, the average slope of the curve in these plots reflects the overall REE fractionation (or the relative fractionation of light versus heavy REE) and can be approximated by the La n /Lu n ratio. Second, the HREE fractionation is calculated as the average of the normalized concentration of Er, Tm, Yb and Lu. Finally, the Eu anomaly (Eu* in Table 3 -3) expressed as the normalized concentration of Eu. The characteristic REE pattern of late Paleozoic to Triassic volcanic rocks exhibits a prominent negative Eu anomaly and similar fractionation of light and heavy REE (e.g. Mpodozis and Kay, 1992). Tilito, Escabroso and Cerro de las Tortolas Fromations as well as Bocatoma and Infiernillo Units present minor Eu anomalies and moderate HREE fractionation. In the Vacas Heladas, Pascua and Vallecito Formations the Eu concentration is normal and they show strong HREE fractionation (e.g. Bissig et al., 2003). Figure 3-18 outlines the geochemical difference between late Paleozoic to Triassic and Tertiary rocks recognized in the El Indio-Pascua Belt as well as in the Veladero North area. I 1 I i — I 0 10 20 30 40 Lan/Lu„ Figure 3-18: Whole-rock geochemistry of the Veladero North lithostratigraphic units. Ratio and average of normalized REE concentration. Normalization factors are those of the Primitive Mantle from Taylor and McLennan (1985). Late Paleozoic to Triassic field outlined using data from Bissig (2001) and from samples described in Chapter 2. Remaining fields outlined from Bissig et al. (2003) data and analyses in Table 3-3. Abbreviation: Pzi, Pzv; Permian volcanic, intrusive; Ttt, Tte, Ttw, Tti; Tilito Taguas, East, West, intrusive; Tto; Cerro de las Tortolas; Tin; Infiernillo; Tvh: Vacas Heladas. See text for description. 88 68 2 3 3 a o o 5J. 2. 2. 7 « ui o OJ n 3 = o Si s o c i 5 - 3 3. •< ui < - - i ro Z 3 e r a OJ OJ Z OJ ro ro n . 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Late Paleozoic Guanaco Sonso Formation and late Oligocene-early Miocene Tilito Formation are superposed in a northerly striking fold and thrust belt. From east to west, Fabiana, MAGSA and Limite faults, three north-striking faults, bound the major structural packages within the fold and thrust belt in the study area (Figure 3-5) . Within these packages, similar structures are also present, but the rocks are very different. The middle Miocene Cerro de las Tortolas Formation unconformably overlies the fold and thrust system and dips homoclinally eastward. Faults within the fold and thrust belt do not usually outcrop and their location is inferred from older-over-younger stratigraphic relationships, abrupts changes in bedding dips and the presence of tight to open folds. Exploration trenches or roads locally uncover the fault zones for closer examination. The Fabiana fault, in the eastern part of the mapped area, strikes northerly and places Permian over late Oligocene-early Miocene rocks. In an exploration trench, the fault zone locally dips ~75° to the west. Asymmetric fractures bounded between thrust parallel microfaults imply a reverse displacement of the fault, which is consistent with the older over younger structural relation along the fault. On the eastern flank of the Rio Taguas valley, andesitic porphyries from the Infiernillo Unit (?) intrude and have steepened the Fabiana fault plane. Folding associated with the fault is preserved in the upper plate where Permian rocks form an anticline that plunges to the north (Figure 3 -19). 91 Z6 In the central part of the study area, the north-striking MAGSA fault superposes Permian over late Oligocene-early Miocene rocks. It is covered by ~16 Ma breccias of the Cerro de las Tortolas Formation. Middle Miocene dacitic and andesitic intrusives emplaced along the fault obscure the sense of displacement. Bedding of Permian rocks that occur near the fault is consistent with a fault that dips to the west (Figure 3 -20) . In the northwestern part of the study area, the Limite fault and the Muneco fault represent two branches that form a subvertical, north-striking structure. Field evidence records reverse and normal displacement along the structure that separates two blocks with contrasting characteristics. The western block is formed by late Paleozoic units whereas the eastern block includes a late Oligocene-early Miocene volcanic sequence. Middle Miocene rocks that dip to the east unconformably overlie older units on both sides of the Lfmite fault. The projection of the discordance plane from both sides of the fault as well as geomorphological features interpreted as scarps record a normal sense of displacement along the fault plane. However, it is very likely that, following the same structural style as the MAGSA and Fabiana faults, the Limite fault had initial reverse displacement, which superposed late Paleozoic rocks in the west over younger rocks in the east. The area between the MAGSA fault in the east and the Limite fault in the west represents the most intensely deformed portion of the fold-and-thrust belt. Thrusts that dip steeply (>70°) to the west and back-thrusts that dip steeply (>70°) to the east involve the Tilito (late Oligocene-early Miocene) and the Guanaco Sonso (Permian) Formations. Within each thrust panel, a specific rock sequence is exposed and is not repeated in other panels. The faults strike generally in a north-south to NNE direction. The traces of the axial planes of the folds are parallel to the faults, however, the fold and thrust vergence is unclear from the field exposures. 93 Lithologic association of the epithermal deposit The Veladero North epithermal deposit is hosted in intensely altered subaerial heterolitic breccias, volcaniclastic rocks and subordinate fall tuffs of the (16 Ma to 15 Ma) Cerro de las Tortolas Formation and underlying dacitic flow tuff and sandstone of the (25 Ma to 23 Ma) Tilito Formation and dacitic flows and tuffs of the (270 Ma to 250 Ma) Guanaco Sonso Formation. The latter two formations are structurally superposed in a fold-and-thrust system that unconformably underlies the Cerro de las Tortolas Formation. Only a small portion of the Au ore body is apparently hosted in these rocks. Mineralized, hydrothermal breccias of presumed Miocene age cut the Cerro de las Tortolas Formation as well as older units. Geologic mapping demonstrates that volcanic and volcaniclastic rocks from the Cerro de las Tortolas Formation are spatially linked to a series of dacitic to andesitic domes assigned to the Infiernillo Unit. Monolithologic, clast-supported breccias represent proximal facies near the intrusives. The proximal breccias grade outwards to heterolithic, bedded volcanic breccias and volcaniclastic rocks. Well-bedded volcaniclastic rocks, sandstones and siltstones characterize the distal facies of the system. The low proportion of ash pyroclasts and vesiculated fragments suggest that non-explosive conditions prevailed at the time of emplacement of the sequence. Although sub-surface data are needed to corroborate the hypothesis, available information suggests that the Cerro de las Tortolas Formation in the Veladero North area represents a dome field that was active ca. 16 Ma. Explosive vent-clearing eruptions were the source of fragmental juvenile material as well as the common, thin ash beds. Elsewhere in the El Indio-Pascua Belt, high-sulfidation style alteration has been thought to have formed contemporary with the (16 Ma to 15 Ma) Infiernillo Unit and the Cerro de las Tortolas Formation (Nasi e ta/ . , 1990; Maksaev e ta/ . , 1984; Clavero eta/ . , 1997; Martinez et al., 1993; Bissig et al., 2001). However, Bissig et al. (2001, 2002) demonstrated that the bulk of the currently known economic Au in the El Indio-Pascua 94 Belt was deposited between 9.5 Ma and 6 Ma and represents the youngest hydrothermal event in the belt. They further suggest that the Miocene high-sulfidation style epithermal alteration may have been accompanied by some gold. However, erosion has stripped the upper parts of those systems, and the gold, if deposited, has been removed. At Veladero North area (Filo Federico), limited chronologic data suggest that the advanced argillic alteration presumably associated with gold deposition occurred in the 11.0 Ma to 10.5 Ma interval (Bissig etal., 2001). This data imply that the Au is somewhat older than gold at Pascua-Lama and El Indio. Conversely, it confirms that the volcaniclastic rocks from the Cerro de las Tortolas Formation and the intrusions from the Infiernillo Unit are considerably older than the mineralization. Thus, at the moment, there is no concrete evidence to support a direct relationship between the genesis of the host-rock package and the epithermal gold. Timing of deformation Late Paleozoic to Mesozoic structures in the high Andes are difficult to identify since they have been reactivated by younger deformation events, eroded or covered by younger rocks. Northwest trending lineaments such as the Pascua-Veladero lineament (Figure 3-2) are very likely the expression of Triassic extensional faults (Ramos and Kay, 1991) that were inverted during Tertiary tectonism (Jones et al., 1999). Furthermore, west of the Banos del Toro Fault (Figure 3-2) , high-angle reverse faults and associated folds appear to be pre- to syntectonic with (~40 Ma to 31 Ma) Bocatoma Unit intrusions (Martin et al., 1997, 1995). Many of these faults may be reactivated margin parallel faults formed in the Paleozoic, coincident with the formation of the Carboniferous and Permian arcs (Mpodozis and Kay, 1992). Because of the relative youthfulness, the timing and complexity of Miocene deformation is more apparent. Multiple episodes of deformation occurred in the El Indio-Pascua Belt throughout the Neogene (e.g. Martin et al., 1995) in sharp contrast with 95 observations made farther south in the Cordillera Frontal (e.g. Heredia et al., 2002). Specifically, the regional relations in the El Indio and Pascua-Lama Veladero areas indicate two Miocene deformation events older than 16 Ma. The angular unconformity and the regolith horizon that separate the Tilito Formation from the overlying (22 Ma to 17 Ma) Escabroso Formation rocks set the minimum age of the first early Miocene regional shortening event to between 23 Ma and 22 Ma (Martin et al., 1995; Bissig et al., 2001). The Escabroso Formation rocks are not found in the Veladero North area. The second phase of reverse faulting and folding involves rocks as young as the early Miocene (22 Ma to 17 Ma) Escabroso Formation. These structures likely represent the main phase of deformation, and is constrained to have occurred between 18 Ma and 16 Ma (Martin etal. 1995, 1997). In the Veladero North area, the fold and thrust belt is constrained between the emplacement of the (~24 Ma) early Miocene rocks from the Tilito Formation and the deposition of the (~16 Ma) middle Miocene flows that constitute the Cerro de las Tortolas Formation and intrusion of the (~16 Ma) domes that form the Infiernillo Unit. In the absence of the Escabroso Formation andesites deposited during the 24 Ma to 16 Ma period, the deformation event that shaped the fold and thrust belt in the study area cannot be more precisely dated. It may have formed largely during one or the other, or is a combination of both Miocene episodes of shortening. The three regional erosional surfaces recognized in the El Indio-Pascua Belt, also found in the Veladero North area Figure 3-21) , provide additional constraints on deformation episodes that are younger than 16 Ma. In the study area, ~16 Ma volcaniclastic rocks from the Cerro de las Tortolas Formation discomformably overlie a regional low-relief unconformity, which is equivalent to the Frontera-Deidad surface. Based on relationships throughout the El Indio-Pascua Belt, the surface formed between 17 Ma and 15 Ma (Bissig et al., 2001). At Veladero North, the Cerro de las Tortolas Formation rocks and subjacent Frontera-Deidad erosional surface dip shallowly (<20°) to 96 Lb the east at an elevation close to 4200 m a.s. l . At Fabiana the surface lies at slightly lower elevations. To the west along the Argentina-Chile frontier and east on the Cordillera de la Ortiga, the Frontera-Deidad surface lies at approximately 5000 m elevation (Figure 3-21) . Deformation therefore must have postdated the Cerro de las Tortolas Formation and either folded or faulted the erosional surface after 16 Ma. A younger regional low-relief pediment cut the ~16 Ma tilted rocks, the fold and thrust belt and is locally covered by ~ 1 1 Ma volcanic rocks. The pediment is comparable to the Azufreras-Torta surface, a region-scale erosional feature incised between 14 Ma and 12.5 Ma (Bissig et al., 2002). In the Portezuelo Matias area, the Azufreras-Torta surface which is at 4400 m a.s. l . lies below the Frontera-Deidad surface. Conversely, in the Fabiana area the Azufreras-Torta surface is above the Frontera-Deidad surface at approximately 4200 m a.s. l . (Figure 3-21) and it is covered by 11 ± 0.2 Ma dacitic tuff (Bissig et al., 2001). The relative position of the surfaces is inconsistent with regional relations throughout the El Indio-Pascua Belt where the Frontera-Deidad surface is 300 to 450 m topographically higher than the Azufreras-Torta surface (Bissig et al., 2002). Interestingly, the top of the ore zone in Veladero North defines a flat horizon that lies at 4200 m a.s . l . , approximately 200 vertical metres below the Azufreras-Torta surface. A similar spatial relation between the mineralization and the Azufreras-Torta surface was noted also at Pascua-Lama and El Indio. Gold deposition in those areas occurred between 9.5 Ma and 6 Ma, and was influenced by the incision of a younger (10 Ma to 6 Ma) planar landform (Bissig eta/ . , 2002). A similar timing could be inferred at Veladero North. In the study area, tilting of the Frontera-Deidad surface and the superjacent rocks post-dates the 17 Ma to 15 Ma erosive surface but occurred prior to the Azufreras-Torta surface incision and burial beneath Vacas Heladas Formation at ~ 1 1 Ma. The resulting geometry could be explained by large amplitude folds (Charchaflie eta/ . , 2002) as shown by the broad antiformal shape of the Frontera-Deidad surface west from 98 Veladero North (Figure 3-21) . The relative position of the Azufreras-Torta surface, expressed by approximately 150 m of vertical separation on each side of the Rio Taguas valley, impies that post-11 Ma deformation disrupted the Miocene fold and thrust belt and regional erosional surfaces (Figure 3-21) . Available data fail to determine the nature of the tectonic event. However, as many of the older, northerly-striking thrust faults are reactivated as normal slip faults, it seems likely that the deformation was largely extensional. Alternatively, the deformation could reflect a renewed shortening and thrusts reactivation that uplifted the structural blocks located to the west of the faults. Conclusions In the Veladero North area, Permian dacites and rhyolites from the Guanaco Sonso Formation and late Oligocene to early Miocene andesites and dacites from the Tilito Formation form a north-south striking fold and thrust belt. Shortening necessarily predates deposition of rocks older than the overlying middle Miocene Cerro de las Tortolas Formation. Unfortunately, evidence in the Veladero North area is not conclusive as to the precise age of the fold and thrust system; it may have been formed either during an (23 Ma to 21 Ma) early Miocene or a (18 Ma to 16 Ma) middle Miocene deformation event, or be a composite of both events. Tilted volcanic and volcaniclastic rocks from the (16 Ma to 14 Ma) middle Miocene Cerro de las Tortolas Formation cover the Frontera-Deidad pediplain, an erosional surface that truncates the fold and thrust system. Syn-volcanic domes of the Infiernillo Intrusive Unit appear to be the source of the pyroclastic and volcaniclastic rocks. Near the domes, the volcaniclastic rocks dip steeply but, in general, the sequence dips gently (<20°) to the east. Tilting of the Cerro de las Tortolas Formation rocks corresponds to a deformation event that, in the study area, is constrained between 16 Ma and 11 Ma. Region-scale relations suggest that the deformation occurred between 15 Ma and 13 Ma 99 (Martin etal., 1995; Bissig etal., 2001). Sub-horizontal volcanic rocks and intrusives that represent the (12 Ma to 11 Ma) Vacas Heladas Formation unconformably overlie and intrude units of every age in the Veladero North area. The volcanic rocks overlie the (14 Ma to 12.5 Ma) Azufreras-Torta surface in the eastern part of the study area. The same erosion surface is recognized in the west, approximately 150 m higher, supporting a post-Vacas Heladas deformation. References Allmendinger, R.W., Figueroa, D., Snyder, D., Beer, J . , Mpodozis, C , and Isacks, B.L., 1990, Foreland shortening and crustal balancing in the Andes at 30 degrees S latitude: Tectonics, v. 9, p. 789-809. Barazangi, M., and Isacks, B.L., 1976, Spatial distribution of earthquakes and subduction of the Nazca Plate beneath South America: Geology (Boulder), v. 4, p. 686-692. Bissig, T., 2001, Metallogenesis of the Miocene El Indio-Pascua gold-silver-copper Belt, Chile/Argentina: geodynamic, geomorphological and petrochemical controls on epithermal mineralization [Ph.D. thesis]: Kingston, Queen's University. Bissig, T., Lee, J . , W, Clark, A., H, and Heather, K., B, 2001, The Cenozoic history of volcanism and hydrothermal alteration in the Central Andean Flat-Slab Region: New 40Ar-39Ar constraints from the El Indio-Pascua Au (-Ag, Cu) Belt, 29°20'-30°30' S : International Geology Review, v. 43 , p. 312-340. Bissig, T., Clark, A .H . , and Lee, J .K.W., 2002, Cerro de Vidrio rhyolitic dome: evidence for Late Pliocene volcanism in the central Andean flat-slab region, Lama-Veladero district, 29°20'S, San Juan Province, Argentina: Journal of South American Earth Sciences, v. 15, p. 571-576. Bissig, T., Clark, A., H, Lee, J . , W, and Hodgson Jay, C , 2002, Miocene Landscape Evolution and Geomorphologic Controls on Epithermal Processes in the El Indio-Pascua Au-Ag-Cu Belt, Chile and Argentina: Economic Geology, v. 97, p. 971 -996. 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M . , and Abbruzzi, J .M. , 1996, Magmatic evidence for Neogene lithospheric evolution of the central Andean "flat-slab" between 30 degrees S and 32 degrees S, in Dewey, J.F., and Lamb, S .H . , eds., Geodynamics of the Andes., Volume 259: Tectonophysics: Amsterdam, Netherlands, Elsevier, p. 15 -28. Kay, S . M . , Mpodozis, C , and Coira, B., 1999, Neogene magmatism, tectonism, and mineral deposits of the Central Andes (22 degrees to 33 degrees S latitude), in Skinner, B.J., ed., Geology and ore deposits of the Central Andes, Volume 7: Special Publication - Society of Economic Geologists: Littleton, CO, United States, Society of Economic Geologists, p. 2 7 - 5 9 . Kay, S . M . , and Mpodozis, C , 2001, Central Andean ore deposits linked to evolving shallow subduction systems and thickening crust: GSA Today, v. 11, p. 4 - 9 . Kay, S . M . , and Mpodozis, C , 2002, Magmatism as a probe to the Neogene shallowing of the Nazca Plate beneath the modern Chilean flat-slab: Journal of South American Earth Sciences, v. 15, p. 39 -57. Litvak, V.D. , Page, S . , and Kay, S . M . , 2002, La Cordillera del Zancarron en el Valle del Cura, Provincia de San Juan: un centro eruptivo Mioceno?, Actas del XV Congreso Geologico Argentino, Asociacion Geologica Argentina. Maksaev, J.V., Moscoso, D.R., Mpodozis, M.C., and Nasi, P.C., 1984, Las unidades volcanicas y plutonicas del Cenozoico superior en la Alta Cordillera del Norte Chico (29°-31°S); geologia, alteracion hidrotermal y mineralizacion: Revista Geologica de Chile, v. 21 , p. 11 -51 . Malizia, D., Limarino, C O . , Sosa Gomez, J . , Kokot, R., Nullo, F.E., and Gutierrez, P.R., 1997, Hoja geologica Cordillera del Zancarron (Provincia de San Juan) N° 3169-26 y 25: Buenos Aires, Servicio de Geologia y Mineria de Argentina (SEGEMAR), 197 p. Marin, G., and Nullo, F.E., 1988, Geologia y estructura al oeste de la Cordillera de la Ortiga, San Juan: Revista de la Asociacion Geologica Argentina, v. 43 , p. 153-162. Martin, M.W., Clavero, J . , Mpodozis, C , and Cuitino, L., 1995, Estudio geologico regional de la franja El Indio Cordillera de Coquimbo, Servicio Nacional de Geologia y Mineria, Compania Minera San Jose, 238 p. Martin, M.W., Clavero R., J . , and Mpodozis, C , 1997, Eocene to Late Miocene Magmatic Development of El Indio Belt, ~30° S , North- Central Chile, Actas del Octavo Congreso Geologico Chileno, Volume 1: Actas del Congreso Geologico Chileno, p. 149-153. Martin, M.W., Clavero R., J . , and Mpodozis, C , 1997, Eocene to Late Miocene Structural Development of Chile's El Indio Gold Belt, ~30° S, Actas del Octavo Congreso Geologico Chileno, Volume 1: Actas del Congreso Geologico Chileno, p. 144-148. Martin, M.W., Clavero, R.J., and Mpodozis, C , 1999, Late Paleozoic to Early Jurassic tectonic development of the high andean Principal Cordillera, El Indio region, Chile (29-30°S): Journal of South American Earth Sciences, v. 12, p. 33 -49 . Martinez, R.D., Grassi, J.I., and Hernandez, M.B., 1993, Consideraciones Estructurales sobre las Alteraciones Epitermales de la Region del Valle del Cura, San Juan: Sus Implicancias en Prospeccion Metalifera, in Anonymous, ed., Actas del Decimo Segundo Congreso Geologico Argentino y Segundo Congreso de Exploracion de Hidrocarburos, Volume 4: 12, Asociacion Geologica Argentina, p. 202 -210 . Minera Rio Frio, S.A. , 2000, Edades K-Ar (Biotita) de Intrusivos Terciarios: San Juan, p. 3. Moscoso, R., and Mpodozis, C , 1988, Estilos estructurales en el Norte Chico de Chile (28-31° S), regiones de Atacama y Coquimbo: Revista Geologica de Chile, v. 15, p. 151-166. Mpodozis, C , and Cornejo, P., 1988, Hoja Pisco Elqui, IV Region de Coquimbo: Santiago, Servicio Nacional de Geologia y Mineria, 164 p. Mpodozis, C , and Kay, S . M . , 1992, Late Paleozoic to Triassic evolution of the Gondwana margin; evidence from Chilean Frontal Cordilleran batholiths (28° S to 31° S) ; with Suppl. Data 9 2 - 2 2 : Geological Society of America Bulletin, v. 104, p. 999-1014. Mpodozis, C , and Ramos, V.A., 1990, The Andes of Chile and Argentina, in Ericksen, G.E., Pinochet, M.T.C., and Reinemund, J.A., eds., Geology of the Andes and its relation to hydrocarbon and mineral resources.: Circum-Pacific Council for Energy and Mineral Resources, Earth Science Series: Houston, TX, United States, Circum-Pacific Council for Energy and Mineral Resources, p. 5 9 - 9 0 . Nasi, C P . , Moscoso, R.D., and Maksaev, V.J . , 1990, Hoja Guanta, IV Region de Coquimbo: Santiago, Servicio Nacional de Geologia y Mineria, 140 p. Nullo, F., 1988, Geologia y Estructura del area de Guanaco Zonzo y Veladero, Oeste de Cordillera del Sancarron, San Juan, in Anonymous, ed., Actas del Tercer Congreso Nacional de Geologia Economica, Volume 2: Olavarria, Pcia. de Buenos Aires, p. 503-515. Nullo, F.E., and Marin, G., 1990, Geologia y estructura de las quebradas de La Sal y de La Ortiga, San Juan: Revista de la Asociacion Geologica Argentina, v. 45, p. 323-335. Pardo, C.F., and Molnar, P., 1987, Relative motion of the Nazca (Farallon) and South American plates since Late Cretaceous time: Tectonics, v. 6, p. 233 -248. Pilger, R.H., Jr., 1981, Plate reconstructions, aseismic ridges, and low-angle subduction beneath the Andes: Geological Society of America Bulletin, v. 92, p. I 448-1 456. Pilger, R.H., 1984, Cenozoic plate kinematics, subduction and magmatism; South American Andes: Journal of the Geological Society of London, v. 141, p. 793-802. Ramos, V.A., 1998, Analisis Geocronologico de la region de Veladero Norte (Valle del Cura), p. 21. Ramos, V.A., Kay, S . M . , Page, R.N., and Munizaga, F., 1989, La ignimbrita Vacas Heladas y el cese del volcanismo en el Valle del Cura, Provincia de San Juan: Revista de la Asociacion Geologica Argentina, v. 44, p. 3 3 6 - 3 5 2 . Ramos, V.A., Cegarra, M., and Cristallini, E., 1996, Cenozoic tectonics of the high Andes of west-central Argentina (30-36 degrees S latitude), in Dewey, J.F., and Lamb, S .H . , eds., Geodynamics of the Andes., Volume 259: Tectonophysics: Amsterdam, Netherlands, Elsevier, p. 185-200. Ramos, V.A., and Kay, S . M . , 1991, Triassic rifting and associated basalts in the Cuyo Basin, central Argentina, in Harmon, R.S., and Rapela, C.W., eds., Andean magmatism and its tectonic setting., Volume 265: Special Paper - Geological Society of America: Boulder, CO, United States, Geological Society of America (GSA), p. 7 9 - 9 1 . 103 Ramos, V.A., Page, R.N., Kay, S . , Lapido, 0 . , and Delpino, D.H., 1987, Geologia de la region del Volcan Tortolas, Valle del Cura, Provincia de San Juan, in Acenolaza, F.G., ed., Actas del Decimo Congreso Geologico Argent ine, Volume 4: Buenos Aires, Argentina, Asociacion Geologica Argentina, p. 2 6 0 - 2 6 3 . Somoza, R., 1998, Updated Nazca (Farallon)-South America relative motions during the last 40 My; implications for mountain building in the Central Andean region: Journal of South American Earth Sciences, v. 11, p. 211 -215. Taylor, S.R. , and McLennan, S . M . , 1985, The continental crust: its composition and evolution: Oxford, Blackwell, 312 p. Thiele, C.R., 1964, Reconocimiento geologico de la alta cordillera de Elqui: Santiago, Universidad de Chile, Departamento de Geologia, 73 p. Uliana, M.A., Biddle, K.T., and Cerdan, J . , 1989, Mesozoic extension and the formation of Argentine sedimentary basins, in Tankard, A .J . , and Balkwill, H.R., eds., Extensional tectonics and stratigraphy of the North Atlantic margins., Volume 46: AAPG Memoir: Tulsa, OK, United States, American Association of Petroleum Geologists, p. 599-614. Winchester, J.A., and Floyd, P.A., 1977, Geochemical discrimination of different magma series and their differentiation products using immobile elements: Chemical Geology, v. 20, p. 325-343. Yanez, G.A., Ranero, C.R., von Huene, R., and Diaz, J . , 2001, Magnetic anomaly interpretation across the southern Central Andes (32° -34° S) ; the role of the Juan Fernandez Ridge in the late Tertiary evolution of the margin: Journal of Geophysical Research, B, Solid Earth and Planets, v. 106, p. 6325-6345. 104 Chapter 4 Conclusions and recommendations for future research Conclusions Based on field mapping, geochronology and geochemistry, this study defines the geologic framework of the Veladero North epithermal deposit within the El Indio-Pascua Belt in the Cordillera Frontal of Argentina and Chile. The l:20,000-scale map i n Plate 1 summarizes the geology of the study area and represents the first attempt to correlate locally defined, lithological units with time-constrained, regional stratigraphic units. Prior to this investigation, available geochronological data were scarce and often inconclusive on the stratigraphic position of the sampled unit. From a region-scale viewpoint, the revision of published and new information in this portion of the Andean C o r d i l l e r a contributes to the comprehensive model of the Cordillera Frontal and represents the foundation u p o n which to define the geology of the eastern flank of the Andes. Geology Six lithostratigraphic units, equivalent to regional units defined in the El Indio-Pascua Belt (Bissig etal., 2001), are recognized in the Veladero North area. From oldest to youngest these units are: 1) the Guanaco Sonso Formation, consisting predominantly of rhyolitic to dacitic flows and shallow level intrusives emplaced between 259 Ma and 254 Ma; 2) the Bocatoma Unit, formed by dioritic stocks intruded between 36 Ma and 30 Ma; 3) the Tilito Formation, comprising volcanic rocks that range in composition from andesite to dacite as well as volcaniclastic rocks formed between 24.5 Ma and 22.8 Ma; 4) the Cerro de las Tortolas Formation, including volcanic and sedimentary rocks intimately associated to the dacitic to andesitic domes from the Infiernillo Unit emplaced 105 around 16 Ma; 5) the Vacas Heladas Formation, consisting of 12.7 Ma to 11 Ma biotite-rich dacites; and 6) the 2.1 Ma Cerro de Vidrio Formation represented in the type-locality by a rhyolitic dome that is the youngest volcanic feature of the El Indio-Pascua Belt. Late Paleozoic volcanic-plutonic arc Late Paleozoic to middle Mesozoic volcanic and plutonic units are widespread in the Cordillera Frontal where they are referred as granite-rhyolite province. Plutonic episodes are represented by the (320 Ma to 280 Ma) Guanta and Cochiguas units, the (270 Ma to 325 Ma) Chollay-EI Leon Unit and the (220 Ma to 190 Ma) Los Colorados and Carricitos units. Volcanism is characterized by the extensive Choiyoi Group, tentatively subdivided into the (275 Ma to 250 Ma) Guanaco Sonso Formation and the (225 Ma to 210 Ma) Los Tilos Formation. The Permian rocks in the Veladero North area are equivalent in age and lithology to the (275 Ma to 250 Ma) Guanaco Sonso Formation, the oldest member of the Choiyoi Group. Revision of available geochronologic information suggests that, between late Paleozoic and Triassic, episodes of magmatic activity alternate with 15 m.y.- to 25 m.y.-long quiescence periods in a similar way as the Tertiary arc. In addition, the reassessment of the geochronological data implies that rocks with contrasting geochemical characteristics were emplaced simultaneously and also, that chemical signatures are not unique features of any magmatic episode. Petrotectonic interpretations, largely derived from geochemical data, indicate a complex scenario that will require future studies to fully elucidate. Miocene volcanism and deformation A north-trending fold and thrust belt characterized by steeply dipping faults and tight to open folds underlies the Veladero North area. Permian Guanaco Sonso rhyolitic to dacitic volcanic and volcaniclastic rocks intruded by Oligocene Bocatoma Unit dioritic porphyries are superposed over Miocene Tilito Formation andesitic to dacitic lavas, pyroclastic and sedimentary rocks. Unique portions of the stratigraphic column, not repeated in other sheets, form each structural panel precluding a shortening estimation. Shortening predates the emplacement of middle Miocene (ca. 16 Ma) pyroclastic rocks and porphyries of the Cerro de las Tortolas and Infiernillo Unit that unconformably overlie and intrude the fold and thrust belt. Available data, however, is inconclusive as to the precise age of the fold and thrust system that might have formed during one or both of regionally defined shortening episodes between 23 Ma and 21 Ma or 18 Ma and 16 Ma. Rocks from the Cerro de las Tortolas Formation overlie the (17 Ma to 15 Ma) Frontera-Deidad pediplain, a regional erosional surface that truncates the fold and thrust belt. The Frontera-Deidad surface and the superjacent rocks dip gently (<20°) to the east. Tilting necessarily predates the incision of the (14 Ma to 12.5 Ma) Azufreras-Torta surface and the emplacement of the ca. 11 Ma volcanic rocks from the Vacas Heladas Formation that horizontally overlie the Cerro de las Tortolas Formation. The deformation is compatible with a shortening event that, in El Indio-Pascua Belt, has been constrained between 15 Ma and 13 Ma (Martin etal., 1995; Bissig etal., 2001). Sub-horizontal volcanic rocks and intrusives that represent the (12 Ma to 11 Ma) Vacas Heladas Formation were emplaced over the Azufreras-Torta surface. The erosional surface is cut on volcanic and intrusive rocks of dissimilar age in the Veladero North area. The relative position of the Azufreras-Torta surface and overlying rocks suggests that post-11 Ma deformation disrupted the Miocene fold and thrust belt as well as the regional erosion surfaces. Litholoqic association of the epithermal deposit Strongly altered, subaerial heterolithic breccias in coarsely to well-bedded packets and interbedded volcaniclastic sandstones and subordinate fall tuffs form the 107 Veladero section of the middle Miocene Cerro de las Tortolas Formation in the study area. These rocks host the greater part of the epithermal mineralization. Only a small portion of the Au ore body is apparently hosted in Permian and Miocene rocks in the subjacent fold and thrust belt. These lesser host rocks are formed by dacitic flow tuff and sandstone of the Oligocene Tilito Formation and dacitic flows and tuffs of the Permian Guanaco Sonso Formation. Volcanic and volcaniclastic rocks from the Veladero Section are spatially and genetically linked to a series of dacitic to andesitic domes assigned to the coeval Infiernillo Intrusive Unit. Field observations indicate that scattered, vent clearing eruptions occurred during the emplacement of the sequence but that non-explosive conditions prevailed at that time. Limited chronologic data further suggest that the advanced argillic alteration presumably associated with the gold mineralization occurred in the 11.0 Ma to 10.5 Ma interval (Bissig et al., 2001). This data confirm that the volcanic and volcaniclastic rocks from the Veladero Section and the intrusions from the Infiernillo Unit are at least 4 m.y. older than the epithermal mineralization, thus precluding a direct relationship between the genesis of the host-rock package and the gold. Recommendations The geologic framework established in this study represents the first step towards a comprehensive model of the Veladero North epithermal deposit. The lithologic association and the structural geometry, as defined here, are consistent with regional scale units and the overall tectonic style of the El Indio-Pascua Belt. Moreover, this research proves that the epithermal mineralization and the host-rock are genetically unrelated. In contrast to the Pascua-Lama deposit where the Miocene Pascua Formation is of the same age as the advanced argillic alteration and the gold (Bissig et al., 2001, 2002), this study fails to identify a magmatic unit that is coeval to the epithermal 108 mineralization in the Veladero North area. High-sulfidation epithermal deposits are, however, closely related to degassing magmas (e.g. White and Hedenquist, 1990; Sillitoe, 1993; Arribas, 1995; Hedenquist and Arribas, 1999; Cooke and Simons, 2000). Thus, it seems likely that the coeval igneous rocks must be present, but they are yet unidentified. Future geochronologic studies should constrain the precise timing of the mineralization, for example, by dating, hydrothermal alunite associated with gold within the ore body. Furthermore, determining the age of sub-economic epithermal mineralization, as found in the alteration haloes around late Paleozoic porphyries, may test hypotheses such as: 1) favourable periods for mineralization, 2) magmatic association of epithermal systems or 3) lithologic and textural control on the mineralization. Geochronology may also help to elucidate the age of the dacitic flows that underly the Veladero section in an attempt to clarify the contact between the Tilito and Cerro de las Tortolas Formations within the mineralized body. In terms of regional geology and paleotectonic reconstructions, the Argentinean flank of the Cordillera Frontal in the Central Andes remains largely undocumented. It is, however, the place where future research may find new evidence regarding the evolution from the Gondwana margin to the Cenozoic arc. References Arribas, A J . , 1995, Characteristics of high sulfidation epithermal deposits, and their relation to magmatic fluids, in Thompson, J .F.H. , ed., Magmas, Fluids and Ore Deposits, Volume 23, Mineralogical Association of Canada Short Course Notes, p. 419-454. Cooke, D.R., and Simmons, S.F. , 2000, Characteristics and genesis of epithermal gold deposits, in Hagemann, S .G . , and Brown, P.E., eds., Reviews in Economic Geology, Volume 13, p. 221-244. Bissig, T., Lee, J . , W, Clark, A., H, and Heather, K., B, 2001, The Cenozoic history of volcanism and hydrothermal alteration in the Central Andean Flat-Slab Region: New 40Ar-39Ar constraints from the El Indio-Pascua Au (-Ag, Cu) Belt, 29°20'-30°30' S : International Geology Review, v. 43 , p. 312-340. 109 Bissig, T., Clark, A., H, Lee, J . , W, and Hodgson Jay, C , 2002, Miocene Landscape Evolution and Geomorphologic Controls on Epithermal Processes in the El Indio-Pascua Au-Ag-Cu Belt, Chile and Argentina: Economic Geology, v. 97, p. 971 -996. Hedenquist, J.W., and Arribas, A.J . , 1999, Epithermal gold deposits: I. Hydrothermal processes in intrusion-related systems, and II. Characteristics, examples, and origin of epithermal gold deposits, in Molnar, F., Lexa, J . , and Hedenquist, J.W., eds., Epithermal Mineralization of the Western Carpathians, Volume 31, Society of Economic Geologists, Guidebook Series, p. 13 -63 . Sillitoe, R.H., 1993, Epithermal models: Genetic types, geometric controls, and shallow features, in Kirkham, R.V., Sinclair, W.D., Thorpe, R.I., and Duke, J .M. , eds., Mineral Deposit Modelling, Geological Association of Canada Special Paper 40, p. 403-417. White, N.C., and Hedenquist, J.W., 1990, Epithermal environments and styles of mineralization: variations and their causes, and guidelines for exploration: Journal of Geochemical Exploration, v. 36, p. 445 -474 . n o Appendix I Geochronology Thirteen samples from surface exposures were prepared for U-Pb geochronological studies. Sample preparation and mass spectrometry were performed at the Geochronology Laboratory of the University of British Columbia. Zircons from samples DC-128, DC-163 and DC-375 were analyzed at the Stanford-U.S. Geological Survey SHRIMP-RG because the amount of material was insufficient for conventional dating techniques or to corroborate the possible effects of inheritance. Zircon was separated from 15-20 kilogram samples by crushing, grinding and heavy-mineral concentration using a conventional Wilfley table followed by heavy liquid and magnetic separation. Zircon crystals and fragments were grouped into fractions according to their magnetic susceptibility, grain size and morphology. Conventional U-Pb Most fractions were abraded before dissolution to minimize post-crystallization lead loss following the technique of Krogh (1982). Fine zircon needles were not abraded. Fractions were dissolved in a mixture of concentrated hydrofluoric acid, nitric acid and 233-235y . 205 p L j t r a c e r i (j a n c j pb were separated using an ion exchange column techniques (Parrish eta/ . , 1987), eluted separately and loaded together on a Re filament with a phosphoric acid silica-gel emitter. Isotopic ratios were measured with a single j collector VG-54R thermal ionization mass spectrometer equipped with a Daly photomultiplier. Uranium fractionation was determined on every run using the 2 3 3 " 2 3 5 u tracer. Daly runs of Pb isotopic ratios were corrected for a fractionation of 0.43%/amu determined by replicate analyses of the NBS-981 Pb standard and values recommended by Thirlwall (2000). Analytical results with their corresponding uncertainties propagated through the age calculations following the numerical technique of Roddick (1987) are summarized at the 2a level in Tables 2 -3 and 3 -2 . i n Guanaco Sonso Formation Zircon crystals and fragments were recovered using gravity and magnetic separation, then grouped into fractions (A, B, etc.) that include morphologically similar individuals. Published common-Pb isotopic compositions are preferred for data reduction (see Table 2-3) , assuming that samples from the Veladero North area have an isotopic composition similar to units defined regionally. D C - 1 1 9 : R h y o l i t i c a s h - f l o w t u f f f r o m t h e Rio T a g u a s la te P a l e o z o i c s t r u c t u r a l p a c k a g e This sample is from an advanced argillic altered, matrix supported quartz-rich lapilli-tuff with small fiamme located at the top of the Rio Taguas structural package (Figure 2-5). The sample was selected from the tuff horizon because it lacked accessory lithic fragments, which are common in all other ignimbrites. Zircon grains are pale pink prisms with simple tips, abundant fractures and no visible cores or elongate (length-width ratios > 7) "needles". Their sizes range from larger than 104 pm to 74 pm. The 2 0 6 P b / 2 3 8 U age of the concordant fraction B is interpreted as the igneous crystallization age of the rock at 263.7 ± 0.7 Ma. The other fractions contain zircon that are interpreted to have lost Pb from their rims during younger hydrothermal alteration or weathering. D C - 2 3 9 : D a c i t i c a s h - f l o w t u f f f r o m the G u a n a c o Z o n z o a r e a l a t e P a l e o z o i c s t r u c t u r a l p a c k a g e Advanced argillic altered, poorly sorted, matrix-supported, (volcanic) lithic-poor dacite with fiamme-like and feldspar vugs was sampled in the Guanaco Zonzo area (Figure 2-5). Accessory volcanic lithic fragments form less than 5 % of the rock volume and were removed during the sampling stage. Abundant colourless to pale pink zircon grains and fragments were recovered. Fractions B, E, F and G combine crystals or fragments that range from smaller than 74 pm to coarser than 104 pm. Fraction A 112 groups fine, elongate zircon "needles" smaller than 104 urn. The crystallization age of the rock is calculated from the 2 0 5 Pb/ 2 3 8 U age of fractions E and F, of which the former is concordant. Zircon in fractions B, G and particularly A, very likely lost Pb after crystallizing. Rhyolitic flows in the Guanaco Zonzo area are interpreted to have a crystallization age of 262.6 ± 0.7 Ma. D C - 1 4 2 : R h y o l i t i c - d a c i t i c d o m e i n t r u d i n g t h e Rio T a g u a s la te P a l e o z o i c s t r u c t u r a l p a c k a g e The sampled rock is a clay and silica (?) altered, porphyritic, intensely recrystallized dacite with spherulites. The sample is representative of the Guanaco Zonzo creek intrusive dome. Colourless to pale pink elongate prismatic zircon fragments and grains with simple faceted tips and no visible cores were recovered. Pale pink prismatic crystals with length-width ratio of 3 constitute fraction A. Fractions D, E and F include small fragments and grains that are concordant or slightly discordant. Elongate zircon coarser than 104 um with fractures form fraction C. Fraction C is discordant in the Concordia diagram of Figure 2 -9 , most likely because of some inherited zircon. The crystallization age of the rock is indicated by the overlap of fractions D, E and F at 259.0 ± 0.7 Ma ( 2 0 6 Pb/ 2 3 8 U age and uncertainty take into account the maximum and minimum possible ages at a 2 sigma level). D C - 1 8 1 : R h y o l i t i c - d a c i t i c d o m e i n t r u d i n g t h e L a t e P a l e o z o i c r o c k s of P o t r e r i l l o s and C a n i t o a r e a s Coarse grained, matrix-supported, porphyritic rhyolite-dacite with clay altered feldspars and rounded quartz crystals form the intrusive dome that crop out in the northern part of the study area (Figure 2-5). The sampled rock has a moderate advanced argillic superposed alteration that is more intense outside the intrusion. Abundant colourless to pale pink zircon grains were recovered. Based on size and morphological characteristics, nine fractions were picked, of which only fractions C and I ("needles") were not abraded. The discordant character of fraction D is probably related to a small amount of inherited zircon. Lead loss accounts for the discordant character of the rest of the fractions in a Concordia plot (Figure 2-9). Fractions A, B, C, H and I have been used to calculate the age of the sample. The upper intercept of the regression line forced through the origin and fractions A, B, C, H and I with Concordia at 254.5 + 4.2 Ma is interpreted as the crystallization age. D C - 1 6 2 : R h y o l i t i c d a c i t i c d o m e i n t r u d i n g t h e G u a n a c o Z o n z o a r e a la te P a l e o z o i c s t r u c t u r a l p a c k a g e Advanced argillic altered, quartz-phyric porphyritic dacite with relict flow-banding texture characterize the intrusive rocks sampled as DC-162 (Figure 2-5). Abundant pale pink, elongate zircon grains with rods, bubbles and no visible cores were recovered and grouped into eight fractions of which only four yielded reliable isotopic data (Table 2-3). The age of the intrusion is preliminarily reported as the 2 0 6 P b / 2 3 8 U age of fractions A and G. Other fractions very likely lost Pb after crystallization. Tertiary rocks Zircon crystals and fragments were recovered from volcanic and intrusive rocks using gravity and magnetic separation techniques and then grouped into fractions (A, B, etc.) that include morphologically similar individuals. The common Pb isotopic composition from Bissig et al. (2003) is preferred for data reduction in conventional analyses, assuming that, samples from the Veladero North area have a similar isotopic composition as the same units defined elsewhere in the region. Tilito Formation D C - 1 2 0 , A n d e s i t i c f low t u f f , 2 4 . 5 ± 0 . 2 Ma The reported age of the sample considers the 2 0 6 p b - 2 3 8 U age of four concordant 114 fractions (A, C, E and F) and their respective uncertainties. Four discordant fractions (D, G, H and I) indicate the likelihood of inheritance. Although fraction B has 2 0 6 P b - 2 3 8 U and 2 0 7 P b - 2 3 5 U apparent ages consistent with the rock age, it is not considered in the age estimation as it reversely discordant. D C - I l l , A n d e s i t i c f low t u f f , 2 2 . 8 ± 1.7 Ma Four concordant fractions (A, B, F and G) and a slightly discordant fraction (D) define the age of the sample. The associated uncertainty is relatively large as it considers the whole range of 2 0 6 p b - 2 3 8 U ages from fractions A, B, F and G. Inheritance and lead-loss processes affected the zircon population. D C - 1 6 3 , D a c i t i c f l ow t u f f , 2 3 . 5 ± 0 . 9 Ma The reported age of the sample represents the 2 0 6 p b - 2 3 8 U age of two concordant fractions (C and F) and agrees, within uncertainty, with the age obtained from SHRIMP analyses from the same rock. Fraction G yields a relatively younger age that results from post-crystallization lead-loss. Inherited zircon is very common in this sample, as shown by the isotopic composition of fractions A, B, D and E. D C - 2 4 9 , D a c i t i c f l ow t u f f , 2 3 . 9 ± 0 . 2 Ma ( p r e l i m i n a r y ) Zircon is not abundant in this sample and the recovered grains are of poor quality. The age of the sample is calculated from a single concordant fraction (A) formed by multifaceted, prismatic and elongated grains, a morphology that most likely represents magmatic zircons. Fractions B, E and C yield discordant ages, and represent the inherited population. Isotopic data from three fractions are expected to resolve the age of sample DC-249. 115 Cerro de las Tortolas Formation and Infiernillo Intrusive Unit D C - 2 6 5 , A n d e s i t i c p o r p h y r y , 1 6 . 6 ± 0 . 3 Ma The age of the sample is constrained by the 2 0 6 p b - 2 3 8 U age of three concordant fractions (A, B and E). Fraction D is discordant and it is not considered in the age calculation but yields a similar 2 0 6 P b - 2 3 8 U age than the rock. Only zircon with morphological characteristics as fraction C is likely to be inherited. D C - 3 7 4 , D a c i t i c f a l l t u f f , 1 5 . 8 i 1 Ma Three concordant fractions (B, C and F) overlap to define the age of the sample. The reported age is a conservative estimate that takes into account the extreme 2 0 6 P b -2 3 8 U ages from fractions B and C. The age is indistinguishable from the SHRIMP age yield by sample DC-375 collected less than 5 metres away from DC-374. Both conventional and SHRIMP analyses reveal zircon inheritance. D C - 3 0 4 , D a c i t i c i n t r u s i v e , 1 6 . 4 ± 0 . 3 Ma ( p r e l i m i n a r y ) Very abundant zircon grains were recovered from this sample. However, isotopic data are not conclusive as to the age of the sample. Four concordant fractions range in age between 16 and 12 Ma. Alteration age from Bissig eta/. (2001) is younger than the youngest possible U-Pb age. In agreement with a 16 Ma whole-rock K-Ar date (Ramos et al., 1998) interpreted as complete resetting, the age of the sample is preliminarily reported as the 2 0 5 p b - 2 3 8 U age of fraction G. Additional fractions are expected to resolve the age of sample DC-304. SHRIMP Three samples were analyzed using the Stanford-U.S. Geological Survey SHRIMP-RG (Sensitive High-mass-Resolution Ion MicroProbe-Reverse Geometry) by Richard Tosdal (written communication, 2002). Reviews of the ion microprobe technique and 116 data interpretation are given in Ireland (1994) and Compston (1999). Operation conditions for the Stanford-U.S. Geological Survey SHRIMP-RG have also been recently summarized (Bacon et al., 2000; Ayuso et al., in press) and a short description is provided below. The SHRIMP-RG differs from the earlier SHRIMP instruments built at the Australian National University because of its reverse geometry design. The design uses an electrostatic mass analyzer downstream of the magnet and permits the SHRIMP-RG to be capable of third order focusing resulting in improved mass resolution compared to conventional SHRIMP designs (Williams, 1998; Bacon eta/ . , 2000). Common Pb used for age corrections is from the model by Cumming and Richards (1975). The zircon and Pb isotope data were reduced using the programs PRAWN and LEAD (Ireland, 1994) and ISOPLOT/EX (Ludwig, 1999). Table 3-1 shows the analytical results of the geochronology study. Tilito Formation D C - 1 2 8 b , A n d e s i t e , 25 ± 1.4 Ma The reported age is a weighted mean of eight 2 0 6 p b - 2 3 8 U ages determined from elongate, prismatic zircon crystals, which are most likely to be magmatic. Age, uncertainty, mean standard weighted deviation (mswd in Figure 3-10a) and probability values do not vary significantly if zircon grain 1.1 is excluded from the analysis. Cerro de las Tortolas Formation and Infiernillo Intrusive Unit D C - 1 6 3 , D a c i t i c f low t u f f , 23 ± 1 .1 Ma The reported age is a weighted mean of eight 2 0 6 p b - 2 3 8 U ages determined in elongate, prismatic zircon crystals, which are most likely to be magmatic. Age, uncertainty, mean standard weighted deviation (mswd in Figure 3-10b) and probability values do not vary significantly if zircon grain 1.3 is excluded. 117 D C - 3 7 5 , D a c i t i c f a l l t u f f ( ? ) , 1 6 . 4 ± 1.0 Ma The reported age is a weighted mean of six 2 0 6 P b - 2 3 8 U ages determined near the rim of clear, elongate and prismatic zircon crystals, which are assumed to be magmatic. The age of Permian, inherited zircons (sampling points 1.5 and 1.8) is not considered in the statistical analysis. Age, uncertainty, mean standard weighted deviation (mswd in Figure 3-10c) and probability values do not vary significantly if zircon grain 1.4 is excluded. References Bacon, C.R., Persing, H.M., Wooden, J.L., and Ireland, T.R., 2000, Late Pleistocene granodiorite beneath Crater Lake caldera, Oregon, dated by ion microprobe: Geology (Boulder), v. 28, p. 467-470. Bissig, T., Clark, A., H, Lee, J . , K, W, and Von Quadt, A., 2003, Petrogenetic and Metallogenic Responses to Miocene Slab Flattening: New Constraints from the El Indio-Pascua Au-Ag-Cu Belt, Chile/Argentina: Mineralium Deposita, (submitted). Compston, W., 1999, Geological age by instrumental analysis; the 29th Hallimond Lecture: Mineralogical Magazine, v. 63, p. 2 9 7 - 3 1 1 . Cumming, G.L., and Richards, J.R., 1975, Ore lead isotope ratios in a continuously changing Earth: Earth and Planetary Science Letters, v. 28, p. 155-171. Ireland T, R., 1994, Ion Microprobe mass spectrometry: Techniques and applications, in Hyman, M. and .R., M., eds., Cosmochemistry, geochemistry and geochronology: Advances in analytical Geohemistry, JAI Press, p. 1 -118. Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique: Geochimica et Cosmochimica Acta, v. 46, p. 637-649. Ludwig, K., R, 1999, ISOPLOT: A plotting and regression program for radiogenic-isotope data: U.S. Geological Survey Open File, v. 9 1 - 4 4 5 , 41p. Parrish, R.R., Roddick, J.C., Loveridge, W.D., and Sullivan, R.W., 1987, Uranium-lead analytical techniques at the Geochronology Laboratory, Geological Survey of Canada, Radiogenic age and isotopic studies; Report 1. , Volume 8 7 - 2 : Geological Survey of Canada: Ottawa, ON, Canada, Geological Survey of Canada, p. 3 -7 . Roddick, J.C., 1987, Generalized numerical error analysis with applications to geochronology and thermodynamics: Geochimica et Cosmochimica Acta, v. 51, p. 2129-2135. Thirlwall, M.F., 2000, Inter-laboratory and other errors in Pb isotope analyses investigated using a 2 0 7 P b - 2 0 4 P b double spike: Chemical Geology, v. 163, p. 299-322. Williams, I.S., 1998, U-Th-Pb geochronology by ion microprobe, in McKibben, M.A., Shanks, W.C., III, and Ridley, W.I., eds., Applications of microanalytical techniques to understanding mineralizing processes., Volume 7: Reviews in Economic Geology: Socorro, NM, United States, Society of Economic Geologists, p. 1-35. 118 Appendix II Sample description and location The following table is a compilation of field observations recorded during two campaigns to the Veladero North area. Abbreviations used in table OCoherent, V=Volcaniclastic Alteration type p-propylitic a-argyllic aa-advanced argillyc s-silica sh-steam-heated Components x-crystal fragments l-lithic fragments Iv-volcanic lithic li-intrusive lithic Is-sedimentary lithic j-juvenile p-pumice fragments s-shards f-fiamme c-cryptocrystalline f-very fine grained m-microcrystalline g-glassy r-rcrystallized d-devitrified Texture: phenocrysts, qroundmass p-porphyritic a-aphanitic g-glassy v-vesicular s-spherulitic f-fine <1 mm m-medium c-coarse >5 mm fm-ferromagnesian op-opaque Grain Size b->64 mm a-<2 mm fl-fine lapilli 2-34 mm cl- 34-64 mm Lithofacies w-welded ff-flow foliated fb-flow banded ps-poorly sorted fs-fairly sorted ws-well sorted ma-massive xg-non graded ng-normal graded rg-reverse graded cs-clast supported ms-matrix supported Composition and-andesite dac-dacite dio-diorite 119 rtWinjkOOOOOOOO 0 1 0 j S l 0 1 0 1 0 » M M M I I i u l _ _ _ _ . _ , . _ , . . , O U ^ I O K O O i e i n « £ ^ 0 1 N O O l O U O I O ) l » « U N \ l t n « J > U N 9 l \ l 0 1 U I ^ O I f l J l N l l O l l ) 0 1 N l | o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o o W m < J i L n L n J > J > J > J > J > J > U W W W W U W S ) K ) S ) N M K R h ' O O O O l ^j. ,_i ,_i I—1 U l U ) H* U l O l O l U l 01 O l o o CO o o 00 00 VO o o o VO VO VO VO VO VO 00 00 VO 00 00 00 CO H* H 1 00 O l CO 00 01 VO NI U l l-» I—. N) NI O l NI VO 00 U l o o o o o 1—I 1—1 oo NI O l O l U l VO 00 VI O l O l J> VI VO VI CO VI O VO VO 1—1 N) O NI VI 01 01 VO I—1 I—1 00 O l I—1 O l O l O l o v l 00 O l VO VI U l VO O O O l NI N) VO UJ U> U l U l U l 00 U ) O l O l U l U l NI 00 00 o v l o U l O l v l O l 00 o NI oo NI U l U l J> J> U l U l U l J> JA •u J> J> J> J> U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l U l o o 00 00 o o o v l v l v l v l VI VO VO 00 00 v l o o o NI NI NI NI U l U l U) NI NI NI NI NI o O v l H 1 1—1 NI NI NI UJ O v l J> NI NI CO O l O l U l NI VO NJ VI VI CO J> U l NI VO O l O O l O l O l VO 00 VO U l VI VI U) U) VO U l VI v l VI J> O l CO U l NI U l NJ O J> U l NI O J> v l J> O l UJ VO 00 VO J> U l U l 00 00 o VO 00 O l U l U l UJ U l J> J> U l U l U l U l O O l VI J> vo O UJ VI O l O l O OO O l VO VO O l J> UJ oo _ CL i?vQ<9. S IQ ^ ™ ^ IQ TT 7T 3 fD fD '? mm ' IQ IO ID ! -1 - i < I . Q. — — ' Oj IQ lO . ~L IX 3" _ , ;*T i-t rt Fr i-r id IQ IQ ID _ 3? "< IQ ^ f D t Q l C ) rt-XJ 0) f D ^ 5 c O r r t Q rt (O ID ~ - - i - i / * — 3 3 ^ T | O J - - - -fl) " J T 3 CT1] ! vo vo < ° ! 3 3 3 3 3 I— J > h ^ N J I ^ A U ) N ) U J U J N I U l N I N I U l J > M 4 ^ 4 ^ U ) U I U I U ) U ) A J > U J J > U J U J U l U l U l U l N ) N I U ) U l U l M OJ "O ^ OJ QI OJ QI ™ T 3 T 3 T 3 ° J T 3 T 3 T 3 T j T l i n T 3 T 3 g g T 3 v / l v / l l / l i n O J O J O j g ~ (Ji -< ^ i/i i/i ui in T3 3 VI o J> n O n < n n O O o T J TD TD T J T J T J T J T J •T* O 3 "D "D T J T J T J T J 0) OJ T J Ol Oi- OJ T J OJ IQ iQ IQ lO iQ OJ LO V l IQ VO VO CO <Q V l O O O O O o cr g ; O O 13 O c r J> Ul cn 1—*• NJ NJ 00 o o o O o O o o (Q —h o —n —h —h —h —ti Qj X < n < < < < < < < < < o < x < x o < < x x o < n < < TO T3 cr 3 . - Q J £» o QJ 0. S- QI 3 oo OJ ° 3 "S, o 33=RE?3 5 ! 3 = ' ! 3 3 3 3 = f ! 3 g : 3 3 3 3 v c r S QJ Ul QJ 0) 0 J 0 J D J U 1 0 ) = ; U 1 Q ) 1 / 1 U 1 ^ * -— T J ={ Ul OJ 01 ui - -3 3 Qi Qi n o 3 x vo <"2. • • Ql Q . VO 01 ~ n 3 Pf o •."> - O 3 OJ - i x" cr X an TJ OJ fD t o cry CO O m < <" n 3 3 r< < 3 OJ 01 i n 0J r r r t = S r t _. _ 3 S 0 1 QJ -T-- ' 3 ) 3 Ol A - X o X ID - . IO 3 3 r 3 n i n S 3 S S 3 n 3 3 Ul Ul 3 3 QJ 0> q fD SJ R 1 3 3 y i v i 3 3 QJ Ql g X X X 3 3" °J "2. "5. X X , n x ' <3 O l 3 J 3 T .2! - Q • . Ql Ql Ql ; j ; ' vo vo vo 3 "-t, O l 1„ I 3 ° 3 1 0 iff .. c _ - o - o ^ c: 11 2 . 3 57 57-< 3. "2. ftT < P P 9! fi> 3 9_ vo ^ . 3 3 OJ ui 3 01 QJ VO VO QJ Q| 3> g u o ^ . 3 3 c QI o i 3 Ql Ql 3 3 CL CL Q J Q I Q I O . Q I Q I Q I Q I 0 J Q . D J Q 1 3 3 3 0 J 3 3 3 3 3 Q I 3 3 a a a n 0 . 0 . 0 . 0 . 0 . 0 a. a. 0 *o <3 S * 3 01 QJ =3 50. 3 Qi ™ i i 6-3- U l Ol Ql O O 3 " ' 7f X X 01 Ql CL CL 3 3 Ql Ql a a n n 0 w 01 n a. a. CL OJ Ql Q) - - n Ql Ql Ql Q| O" Qi 3 3 3 3 01 3 CL CL CL CL in o . 1 Ql ~ P- rr ro 3 J 3 C L (II Ql Ql Ql Q . 3 3 3 Ql 0 . 0 . 0 . 0 "1 J > 01 2 tt + o 3 01 o 3 < o H ffi s C ft o 3 TJ O a u 5' (A ft O 3 •a M r» h* H r* r* r* l-» l-» r» h» h* h* H* H r» r» r» r* (-» r* h* h* h* l-» h» h» M H» h* h» h» h* r* h> h» O 10 10 00 00 00 0 0 M Ot 0 1 Ol Ol Ol Ol Ul Ul A A A U U U U U M M Nl M M M h> h> O O o o Ul U 00 4>> W h» KJ 10 00 NJ U M o 10 Nl 00 Ol Ul Nl H Nl Ul M K O VO Ol U M O 10 00 Nl Ul cr O 00 Nl 0 1 Ul U l VI U l U l U l U l CO CO CO N l O N l N l N l O O I—1 H* o CO o O CO O O CO CO 00 00 CO A U l U l 01 U l VI CO CO VO u> U l VI CO U l NJ v l J -1 VI CO O O l CO o O l I- 1 O l NJ N l CD NJ O O l CO CO CO VI o CO VI CO CO U l U l O A CO CO 01 I—1 NJ U l VI CO CO O l v l 00 O U l CO O t-t o O l CO U l A U l v l CO N l U l U l U l o CO v l CO VI CO VI U l CO O l VI N» 1-1 O CO t- 1 CO U l O CD N l CO O l A NJ o U l I- 1 U l CO NJ NJ U) VI CO CO N l A 1-* o CO O l O l U l U l U l U l U l U l U l U l A U l U l A A U l U l U l U l U l U l U l U l U l U l U l U l VI CO CO CO NJ N l N l I-* CO VI VI CD CO CO v l VI CO U l U l NJ NJ CO 00 00 CO o CO O O O o o o o o o O l U l U l N l N l U l U l U l U l U l VI NJ VI CO CO O l U) O O l v l U l CO CO O l UJ O o N l 00 VI CO U l U l NJ A A 00 VI U l CO A CO U l NJ O CTl O l O l CO O l CO U l O CO CO U l CTl U l U l U l U l v l v l N l o o CO N) U l U l A O l o O l U l CTl U l O CO O 01 U l O O l VI N l N l U l U l 1-1 CO 1— v l O v l U) N l 00 00 U l CO CO A t- 1 1—1 o O l U l CO VI 1 0 -r-i _ -I T J t o ro cu . 31= <B ro to . 3 3 OJ ro. 3 C L QJ t o -1 7T TJ t o l O ZT —• - i => n> -< - i fD fD 3 CL CL Q. — CL UJ TJ QJ QJ to OJ -1 >ale to -l zr - l 7s >ale ZT 7s 7s 7s LO >ale r-T CO tO LQ l O ~1 i-f —\ -l -i -i ~i fD QJ ro ft) ft) fD n> ro 3 Q. ro fD fD fD 3 3 3 3 3 ro 0J "D OJ "Cl o O ^ r o r o r o r o r o Z S r o 1 " ^ m s a ' s s n a z i r a ^ ^ ro U I U I U I U I U I A A U I U I U I A U I A U I A I - ^ N I N J U I U I A A N I N I U W U I U I U I I ^ U I N J U I N J N J N I U I A A A H ^ N J U I U I *-3 t/1 in in n < < < < n n < n < n < n r > n n n r i < n n < CT n ui ~ Q QJ •o 3 ro Ql J3 M <x 2 D O l o. yi "O "O X ) X ) X3 T j 3 3 3 — " * ' t l " ' T3T3 -° b7 p "2. QT QT o co OJ to to =L N j|L>° VI VI •< o $ oo p p 5 S V ! a a O cn 3>x « 3 « CO < t/l q ro -3 Q) - n Ico < « x < Ql NJ ro _ ui ^ - i -tj ^ v " > < - • - , =' E.<Q I D 1 = 1 1 < < < < i n n i 3 3 Ql C/l rt 3 ' L? 3 3" 3 S> 3 L0 QJ ^ QJ n n < n n n T3 T3 n T3 n 3 3 3 ^ i - TD T J •o X J OJ 01 OJ 01 Ol CO CO LO CO CO v l CTl CO CO CO O O O o CJ "* " OJ o LO U l U l O o o o o - 1 CO 3 < n < < n n n n n n n n < ^ . CO QJ O LO -TJ T J 3 3 TJ_ TJ_ OJ QJ LO l O TJ T J T J n n "2. "2 .x . Q) 0) ~-(Q LQ OJ 3 3 3 "° oi y OJ QI 3 3 3 3> 3 S x ro 3 cr Ql ro o. n to" 3 3 3 3 3 3 _ QJ OJ If! QJ QJ QJ 3 i p o < Z 3 rt" fD II -< X3 in ro Ql ro o 3 S 3 3 3 3 3 0) Ul Q) QJ L0 OJ 3 0 1 yi OTS co cr 3 3 3 QJ L0 OJ 3 3 OJ QJ CT TJ i i zn n OJ QJ o) CL OJ a a a OJ a a Q. 3 3 3 Q J 3 Q J Q J 0 J 3 Q J Q J Q ) C L C L C L n Q . n n n a _ n r ) n . Q J O . Q . Q ) 0) 0) 0) Q_ CL UJ OJ Qj Qj Qj QJ QJ OJ Q . 0 J Q J CL QJ QJ QJ Q) QJ CL CL CL CL QJ 3 Q ) Q J 3 _ 3 3 3 Q ) Q J 3 3 3 3 3 3 3 3 Q ) - 3 D Q J 3 3 3 3 3 Q J o o o ' 3 CL n n a a a a n o a a a a a c i a a n a a n a a a a a n ^ ^ Q _ EU n = fD 5 > <o H a s c n o 3 •o o 3 ro 3 01 5' (A N n n o 3 •a H ) ' i ' M O o a o ? £ : k O i s g g « c o « ^ o < o i « » J > u u K i K H U l J> U l J> *. J> J> J> J> J> J> J> J> J> J> CO CO CO CO O VI U l *. .» U l U l J> U l O l O l U l U l U l U l O l U l O l U l O l U l O l CO U l U l VI CO NJ CO o CO J> NJ o VI U l U l U) O l O l O l J> U) CO U l CO VI I—1 o CO NJ J> NJ UJ O l UJ u i U) U l VI O o N) t-» VI VI c n U) CO NJ H * VI U l o U l O l CO U l O l H* CO U l U l U l U l CO o U l CTI U l U) NJ o CO CO M O l CTl o U l CO CO CO , „ , T T J (j CO 111 5 S S ' S (1 CO _ CO -1 CL Q . CO S -3 n> => ro S . •< sr 1 1 3 => ro •a 5. ro ro co' & 3: n.c S — - , CTco ™ ro n- X J -c ^ ro D ro ^ o ro D T T S ro A N J N J A U l A A N J N J N J A U l N J N J U J N J U l l - ^ l - ^ A U l A U J N J A U J U J N J U ) Q J T 3 ' o c u J Q j g g ' a ' a - a ' g J T 3 T 3 Q j o j Q J T 3 T 3 g J T 3 Q j Q j - o Q j Q j o j ' a T 3 < <-> n < T3 "a 3 3 -k " O O J a T " CO CO VI P 0 _ 01 O i o o 3 3 5 3 3 3 O J O J cn < < n n n n < n n n < < n n < n < < n < < < n n T3 T J T J T l 3 3 3 3 OJ T J X ) T3 T3 3 3 pm, pm, •o-g. 'a plag plag lag 9i T3 T3 T3 T3 DJ OJ QJ Ql CO CO CO CO plag plag lag 9i OJ OJ CO CO lag lag CO vj v l v l o o o o ° 00 v l - » o o O l O l O O v l o VI p U l O l O l A o p o o IS ... A A O l O l p p o O l O l O O 150, (60, r-r o n m, 90% p c, with op c, altered n 3 c ? 3 ! 3 3 3 3 , 3 S Q J t-r t-r O J y> Q J Q J S 3 3 QJ QJ Q J 3 3 Ol QJ -o 3 ro •2 ro 2 3 3 Lfi LO L J - D P N J O s s s X X TJ TJ (T> T J 3 oT CT cO ^ o o y — - -t, c . D J D J D J D D D a a a C L Q Q J Q J n n a w a a Q i Q J Q J Q ) Q J — QJ QJ o m ni m m r*> m m m m r» rti m n o o m fti - Q J D D D D Q J D D D D Q J D D D D D D D Q J Q J Q J o n 0 . 0 . 0 . 0 . 0 a a Q. Q n Q Q Q . Q . Q . Q Q D n n "O "O i i T3 I 5j J> U 3 3 > a. =; <o X O -fc n n o 3 •o o 3 rc 3 rt in m —r o> 5' (fl n o 3 13 Appendix III XRF and ICP-MS Geochemistry Whole-rock geochemistry Eighteen samples from the late Paleozoic sequence and 25 Tertiary samples were processed and analysed for major and six trace elements (Sr, Zr, Rb, Y, Nb and Ba) using X-ray fluorescence (XRF) at Bondar-Clegg, North Vancouver BC, Canada. From that group, nine samples were selected for further REE and trace elements analysis based on petrography and previous geochemistry. The new measurements were done by Inductively Coupled Plasma Mass Spectroscopy (ICP-MS) at Memorial University, St. Johns NF, Canada. Major oxides were extracted by borate fusion and their concentration was measured by XRF. Trace element abundance measured by Bondar-Clegg was done by XRF on pressed pellets. The same elements were measured at Memorial University by XRF and ICP-MS after sodium peroxide sinter decomposition. The replicate data agree for key elements such as Y, Ba and Ce. This suggests no dissolution problems that may be associated to samples containing refractory minerals such as zircons. Detection limits The following table summarizes the detection limits reported by the laboratories where the Veladero North area samples were analyzed. 123 Element XRF ICP-MS Si0 2 0.01 % Ti0 2 0.01 % Al 2 0 3 0.01 % Fe 2 0 3 0.01 % MnO 0.01 % MgO 0.01 % CaO 0.01 % Na 20 0.01 % K 2 0 0.01 % P 2 O s 0.01 % LOI 2 % Sr 2 ppm Zr 2 ppm Rb 2 ppm Y 1 ppm Nb 2 ppm Ba 1 ppm 0.138 ppm La - 0.022 ppm Ce - 0.024 ppm Pr - 0.002 ppm Nd - 0.011 ppm Sm - 0.005 ppm Eu - 0.004 ppm Gd - 0.005 ppm Tb - 0.001 ppm Dy - 0.002 ppm Ho - 0.001 ppm Er - 0.003 ppm Tm - 0.001 ppm Yb - 0.003 ppm Lu - 0.002 ppm Hf - 0.012 ppm Ta - 0.003 ppm Th - 0.011 ppm Appendix IV Alteration Present-day concentration of major oxides is largely influenced by superposed post-crystallization alteration. Geochemically, the alteration can be characterized by silica addition and alkali leaching as shown by a general displacement of the Veladero North samples towards the bottom-right corner in Figure 2-10 a. Nb, Y and Zr are generally considered immobile elements under weak alteration conditions. However, in the study area, Nb, Y and Zr were partially remobilized. Although for most of the collected samples the original concentration of those elements is very close to the present-day concentration, only the samples that plot within the non-altered field will be considered (Figure 2-10 B). REE mobility in a rock unit is principally controlled by the stability of the mineral phases that concentrate them, the distribution coefficients between hydrothermal fluids and rock and the stability of secondary minerals that may fractionate them (Humphris, 1984). REE patterns of the Tertiary volcanic rocks in contact with late Paleozoic rocks are identical to the patterns of unaltered samples documented in the region (see chapter 3 for Tertiary rocks geochemistry). Since the samples collected from Tertiary and late Paleozoic volcanic rocks are similarly altered, it is very likely that the measured REE abundance reflects the original concentration in both groups. In that sense, the effect of advanced argyllic alteration is negligible as long as the feldspars or the secondary clays are not removed by weathering or by hydrothermal alteration. Conversely, most of the late Paleozoic intrusive rocks are pervasively altered and feldspar molds are common suggesting that the original magmatic REE concentration has been modified. 125 

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