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, 40Ar-39Ar) 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, 40Ar-39Ar) 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 Ill Conventional U-Pb Ill 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.