Thesis

Geochronological, structural, isotopes and fluid inclusion constraints of the polymetallic Domo de Yauli district, Peru

BEUCHAT, Sebastien

Abstract

L'origine des gisements polymétalliques du Domo de Yauli est liée au magmatisme Miocène ayant affecté les Andes péruviennes. Cette étude met en évidence les relations géochronologiques et structurales entre les intrusions et les corps minéralisés, ainsi que les caractéristiques des fluides hydrothermaux. Les âges obtenus sur les intrusions et les minéralisations indiquent un système pulsé et répétitif durant 9 Ma, alors que les contraintes associées à la formation des gisements montrent un champ hétérogène probablement lié à l'influence de linéaments crustaux. Les rapports isotopiques de Sr, Pb, O et H associés à l'étude des inclusions fluides, par microthermométrie, spectroscopie Raman et LA-ICP-MS, révèlent l'influence d'au moins trois fluides d'origines et de caractéristiques différentes. Leur mélange a conduit à la formation des gisements, alors que les éléments économiquement intéressants ont uniquement été apportés par le fluide s'étant séparé du magma.

Reference

BEUCHAT, Sebastien. Geochronological, structural, isotopes and fluid inclusion constraints of the polymetallic Domo de Yauli district, Peru. Thèse de doctorat : Univ. Genève, 2003, no. Sc. 3415

URN : urn:nbn:ch:unige-979762 DOI : 10.13097/archive-ouverte/unige:97976

Available at: http://archive-ouverte.unige.ch/unige:97976

Disclaimer: layout of this document may differ from the published version.

1 / 1 UNIVERSITE DE GENEVE FACULTE DES SCIENCES

Département de minéralogie Professeur L. Fontboté

Docteur R. Moritz

Geochronological, Structural, Isotopes and Fluid Inclusion

Constraints of the Polymetallic Domo de Yauli District,

Pern

THE SE présentée à la Faculté des sciences de l'Université de Genève pour obtenir le grade Docteur ès sciences, mention Sciences de la Terre

par Sébastien BEUCHAT

de Undervelier (JU)

Thèse N° 3415

GENEVE Atelier de reproduction de la Section de physique 2003 La Faculté des sciences, sur le · préavis de Messieurs L. FONTBOTÉ, professeur ordinaire (Département de minéralogie), et R. MORITZ, docteur (Département de minéralogie ) codirecteurs de thèse, U. SCHALTEGGER, professeur adjoint (Département de minéralogie), et R. TOSDAL, professeur (University of British Columbia, Department of Earth and Ocean Sciences, Vancouver - Canada), autorise l'impression de la présente thèse, sans exprimer d'opinion sur les propositions qui y sont énoncées.

Genève, le 20 janvier 2003

Thèse - 3415 -

Le Doyen, Jacques WEBER

Beuchat S.: Geochronological, structural, isotopes and fluid inclusion constrains ofthe polymetallic Domo de Yau li district, Peru. Terre & Environnement, vol. 41 , 130 pp. (2003)

ISBN 2-940153-40-X Section des Sciences de la Terre, Université de Genève, 13 rue des Maraîchers, CH-1211 Genève 4, Suisse Téléphone ++41-22-702.61 .11 -Fax ++41-22-320.57.32 http://www.unige.ch/sciences/terre/ Table of content

ABSTRACT ------1 RESUMEN 5 RÉSUMÉ DES RÉSULTATS 9 Introduction ______9

Magmatisme miocène ------11 Métallogénie du Domo de Y auli 12 Le paléo-champs de contraintes miocènes 13 Inclusions fluides 13 Description et microthermométrie des inclusions fluides 13 Analyse des inclusions fluides par LA-ICP-MS 14 Isotopes d'oxygène et d'hydrogène 14 Discussions 15 La répétition d'événements magmatiques et hydrothermaux à Morococha, indique-t-elle la présence d'une large chambre magmatique sous-jacente? 15 L'influence de linéaments crustaux sur les paléo-champs de contraintes et la formation de gisements 16 Modèle génétique des minéralisations polymétalliques de San Cristobal 16 Références ------17 CHAPTER 1: RESOLVING MIOCENE MAGMATIC AND MINERALISING EVENTS IN THE ZN-PB-AG-CU DOMO DE YAULI DISTRICT (PERU) BY HIGH-PRECISION GEOCHRONOLOGY 21 Abstract ------21 Introduction 22 Geological settin 23 Regional Geology ofthe Domo de Yauli district 23 Miocene igneous activity 23 Ore deposits of the Domo de Yauli district 28 Analytical techniques 31 Microprobe analysis 31 Cathodoluminescence (CL) and secondary electron (SE) images 31 Whole rock analysis 32 Whole rock isotope data 32 U/Pb age and Hf isotope determinations 32 Re/Os age determinations 33 40 ArP9 Ar age determinations 33 Results 34 Whole rock and microprobe analyses ofMiocene intrusions 34 Zircon cathodoluminescence (CL) images 34 U/Pb dating and Hfisotopic composition ofzircons ______37 The Anticona diorite: TIC 39 The San Francisco and the Yantac intrusions: TOR and POR 39 The Chumpe intrusion: CHU 39 Re/Os dating of molybdenite 39 40 ArP9 Ar ages of phlogopite and sericite 41 Discussion 43 Two mining districts of different ages in the Domo de Yauli area 43 Do multiple magmato-hydrothermal events reveal the presence of a large underlying magmatic chamber? 46 Are Precambrian rocks involved in the generation ofMiocene magmatism? 48 Conclusions ------49 Acknowledgments ______50 References 50 CHAPTER II: LINEAMENT CONTROL ON MIOCENE ORE FORMATION IN CENTRAL PERU, THE ZN-PB-AG-CU SAN CRISTOBAL EXAMPLE 55 Abstract ------55 Introduction 56 Regional Geology ______58 Stratigraphy 58 Tertiary igneous activity 59 Andean deformation 60 Ore deposits of the San Cristobal district 62 Vein ore type 62 Carbonate replacement ore type 66 Paleostress determinations 68 Methodology 68 Results 68 Discussion and conclusions ______69 Acknowledgments ______71 References 72 CHAPTER III: THE ZN-PB-AG SAN CRISTOBAL DISTRICT, CENTRAL PERU: ISOTOPE AND FLUID INCLUSION CONSTRAINTS 75 Abstract ------75 Introduction ______76 Geological setting 78 Regional Geology 78 V ein ore type 8 0 Carbonate replacement ore type 82 Previous S, Sr and Pb isotopie data 84 Analytical methods 86 Fluid inclusion petrography, microthermometry and Raman spectroscopy 88 Fluid inclusion LA-ICP-MS results 97 H and 0 isotopes 97 Discussion 100 Earl y vein stages 100 Late vein tage 102 Carbonate replacement ore bodies 106 Conclusions 107 Acknowledgments 108 References 108 APPENDIX I: DESCRIPTIVE LIST OF THE STUDIED ROCKS AND CITED ANALYSES ------113 40 39 APPENDIX II: AR/ AR DATING 121 APPENDIX III: MICROTHERMOMETRY AND RAMAN FLUID INCLUSION DATA 123 APPENDIX IV: LA-I CP-MS FLUID INCLUSION DATA 125 REMERCIEMENTS 129

Abstract

The Domo de Y auli contains two of the district, there are multiple small apophyses major Zn-Pb±Cu±Ag mining districts of of the very altered Chumpe intrusion. The Peru, the Morococha and the San Cristobal northern monzogranite stocks are related to districts. They are classical intrusion the formation of four different ore deposit related ore districts with high temperature, types: Cu-porphyries, Zn-Pb skarns, Zn­ carbonate-hosted base metal deposits and Pb±Ag carbonate replacement deposits, are a typical of the Miocene metallogenic and veins. In the south, only polymetallic belt of Central Peru. This study is aimed at carbonate replacements and veins are three main topics, a geochronological, a associated to the Chumpe intrusion. structural and an isotope and fluid inclusion study. These three different U-Pb dating of zircons from the northern approaches permit us to constrain the intrusions gives concordant ages of 14.11 ± factors needed to form world-class 0.04 Ma for the Anticona diorite and close deposits. Indeed, formation of such large to 9 Ma for different monzogranite stocks districts are linked to the conjunction and related to Cu-porphyry style and skarn the right timing among magmatic activity deposits. V eins of the Cu-porphyry deposit providing heat and fluids, structural stress have been dated at 7.9 ± 0.1 Ma by Re-Os permitting large fluid circulation and on molybdenite and phlogopite from a Zn­ available fluids and elements sources. Pb skarn gives a 40 Ar/39 Ar plateau age of 7.2 ± 0.2 Ma. U-Pb analyses ofzircons The Domo de Y auli is located 100 km east from the southern Chumpe intrusion result of Lima in the Western Cordillera of Peru. in discordant points defining a lower The area is mainly composed of Paleozoic intercept age about 6.6 (+1;-3.6) Ma, in phyllites of the Excelsior Group, Permo­ agreement with 40Ar/39 Ar ages of 4.90 ± Triassic volcanic and sedimentary rocks of 0.15 Ma ·and 4.78 ± 0.16 Ma obtained on the Mitu Group, Triassic-Jurassic sericite from wall rock alteration selvages. limestones of the Pucara Group, and U-Pb, Re-Os and 40Ar/39 Ar Cretaceous sedimentary rocks. Incaic age determinations reveal the existence of compression events of Eocene age have three distinct magmatic events at 14.1, 9.1 produced isoclinal folds, ramp thrusts in and 6.6 Ma, with the two later ones related the sedimentary cover rocks and a NE-SW to a phase of mineralisation. We therefore fracture system which crosscuts the entire conclude that the northern and southem ore Domo de Yauli. Major N 120° W oriented deposits bear a different age and that the lineaments are present in the basement and particularly large abundance of economie affect the morphology of the whole area. ore bodies at Domo de Y auli is the result The positions of these lineaments coïncide of successive hydrothermal systems. with the emplacement of the major ore deposits of the district. Outcrops of A continuous magmatic activity beneath magmatic rocks are more abundant in the the Morococha district, sustained by Morococha district, where numerous small repeated injections of new magma and stocks of monzogranite intrude the large subsequent melting of the host magmatic Anticona diorite. In the San Cristobal rock, IS documented by numerous 2 concordant U/Pb ages between 9 and 14 associated with the mineralisation event. Ma and petrological evidences such as In this study, the inverse method was reaction rims and plagioclase zonations. applied to determine the local stress The absence of dissolution textures in tensors of different parts of the Domo de zircons points out Zr-saturated magmas Y auli area. Determination of paleostress with temperature as low as 800°C. 1t related to the Miocene magmatic event implies a rapid cooling and a probable indicates a heterogeneous compression intermediate composition of the successive field shifting from E-W toN-S from east to injections of new magma. Isotopie west. The rotation of the main compression compositions of the magma (87 Sr/86 Sr = orientation occurred across a N 120° W 0.705627 to 0.707453; 143Nd/144Nd = oriented basement lineament. lt reveals the 0.512350 to 0.512510; 206PbP04Pb =18.698 active role of strike-slip movement along to 18.761; 207PbP04Pb = 15.635 to 15.669; such lineaments as a control for the 208PbP04Pb = 38.682 to 38.787) suggest a formation of ore deposits in the Domo de hybrid melt source of mantle plus crustal Y auli area. The origin of these lineaments origin compatible with zircon EHr around are difficult to define, nonetheless, zero. Precambrian inheritances in zircons numerous structural features of the same and Hf-depleted mantle model ages orientation in the Mitu Group indicate that of around 1.0 Ga indicate contribution of their origin date back to the Permian partial melts from the underlying Arequipa rifting. style basement in the generation of the Miocene magmatism. Fluid inclusions in sphalerite and quartz homogenise to the liquid phase between 140 and 330°C and are two-phase (0.4 and The orientation of the San Cristobal veins 6.7wt% NaCl) at room temperature; rare are highly variable and rotate from N 30° inclusions contain an additional crystal of W easterly to N 90° W westerly. Veins halite in the early stage (28 to 50wt% present a paragenesis that can be N aCl). The vein data show a decrease in subdivided into 3 phases: (a) an early homogenisation temperatures concomitant wolframite-quartz-pyrite stage, (b) a with a diminishing salinity. Contrary to the quartz-base metal stage and, (c) a late vems, the data from the carbonate quartz-carbonate-barite stage. The replacement ores show a wide variation in carbonate replacement ore bodies are salinity (3.3 to 14wt% NaCl) at constant generaliy stratiform but show clear features homogenisation temperature. This can be of discordance with respect to the explained either by mixing of the fluid carbonate host rocks. Three-dimensional related to the vein system and a hot brine, representations of ore bodies and or by boiling of the fluid migrating out of associated veins show that these discordant the veins into the carbonate. Wolframite, features and the highest grades are related galena and sphalerite from each ore type to the prolongation of veins into the yield similar lead isotopie compositions carbonate rocks. Their mineralogical 06PbP04Pb 18.676 to 18.840; assemblage is similar to that of the veins, e 207PbP04Pb = 15.615 to 15.649; with the only difference that the early 208PbP04Pb = 38.704 to 38.827) ànd wolframite-quartz-pyrite stage is absent overlap with those of the Miocene and an important iron oxide stage is intrusions 06PbP04Pb = 18.698 to 18.761; observed early in the paragenetic sequence. e 207PbP04Pb = 15.635 to 15.669; 208PbP04Pb = 38.682 to 38.787). On the Orientation data were collected for dilatant contrary, strontium isotopie compositions veins, Miocene dykes and altered striated of carbonate and barite are highly variable faults in order to define the paleostress and too radiogenic to be explained by 3

87 86 magmatic input only ( Sr/ Sr = 0.712187 three- and two-phase primary inclusions. to O. 722782). It may correspond to a The concentrations of the major ore predominantly magmatic fluid followed by elements, i.e. W, Cu, Zn and Pb, decrease incoming of 87 Sr-enriched fluids. This through the paragenesis and, W, and to a evolution in two steps is consistent with lesser extent Cu, show high variations, hydrogen and oxygen isotope data. associated to a steep decrease in Isotopie compositions of the fluid concentration with time. The decreasing associated to the first stages reveal a trend concentrations can be explained by mineral with constant 8 180 values with decreasing deposition and differences in the speed of 18 8D values (8 0 = 3.2 to 5.0 %o SMOW decrease indicate selective precipitation. and 8D = -60 to -112 %o SMOW), which is On the contrary, fluid inclusions of the last interpreted as mixing of a dominantly stages show an abrupt increase of Ba and magmatic component with minor meteoric Sr concentrations. It points out a higher water equilibrated with the host rocks. On volume of silicate alteration, probably due the contrary, ending stages bear isotopie to the larger size of the fluid flow cell and characteristics that define a trend with a is explained by the input of a fluid from a conjugated decrease of 8180 and 8D (8 18 0 different origin. LA-ICP-MS analyses = -8.1 to 2.5 %o SMOW and 8D = -57 to - show that the fluids were totally depleted 91 %o SMOW) and is rather explained by in W and Cu before reaching the large admixture of meteoric water in the carbonates, whereas Zn and Pb were still system and subsequent mixing with the present in considerable amounts. This is magmatic component. again due to the selective precipitation and tells us that the economically interesting Their different origins are confirmed by metals were dominantly introduced by laser ablation ICP-MS analyses of the magmatic fluids. 4 5

Re su men

El Domo Y auli contiene dos de los Anticona. Estos stocks monzograniticos mayores distritos mineros del Peru: estan asociados a la formaci6n de cuatro Morococha y San Crist6bal. Ambos tipos diferentes de yacimientos: p6rfidos corresponden a clasicos yacimientos de cupriferos, skam Zn-Pb, dep6sitos de metales base relacionados a intrusivos de reemplazo en rocas carbonatadas Zn-Pb± alta temperatura hospedados en rocas Ag y vetas. En el distrito San Crist6bal carbonatadas, tipicos del cintur6n (sector sur del Domo Yauli), pequefios metalogénico del Mioceno, en Peru ap6fisis del intrusivo Chumpe, Central. Este estudio incluye tres aspectas intensamente alterado, son abundantes. En principales: geocronol6gico, estructural e este distrito se reconocen solo dep6sitos inclusiones fluidas. Estos tres aspectas polimetalicos de reemplazo en rocas estudiados, permiten restringir los factores carbonatadas y vetas, asociados al necesarios para formar un dep6sito de intrusivo Chumpe. clase mundial. La formaci6n de grandes yacimientos esta unida a la coexistencia Dataciones U-Pb en zircon de los entre la actividad magmatica, que intrusivos del sector norte, reportan edades proporciona calor y fluidos, un stress concordantes de 14.11±0.04 Ma para la estructural tal que permita una importante diorita Anticona y de casi 9 Ma para circulaci6n de fluidos, y, una(s) fuente(s) diferentes stocks monzograniticos de fluidos y metales base. relacionados a dep6sitos de tipo p6rfido cuprifero y skam. Dataciones en vetillas El Domo Y auli esta localizado a 100 Km del p6rfido cuprifero, han reportado una al este de Lima, en la Cordillera Occidental edad Re-Os en molibdenita de 7 .9±0.1 Ma, del Peru. El area esta compuesta de filitas y en el skam Zn-Pb se ha obtenido una del Grupo Excelsior (Paleozoico ), rocas edad plateau 40Ar/39 Ar en flogopita de 7.2 volcano-sedimentarias del Grupo Mitu ±0.2 Ma. Dataciones U-Pb en zircones (Permo-Triasico ), calizas del Grupo Pucara para el intrusivo Chumpe, en el sector sur, (Triasico-Jutasico) y -rocas sedimentarias han arrojado valores discordantes que (Cretacico ). La compresi6n asociada a la definen un intercepta inferior de 6.6 (+1;- fase Incaica, durante el Eoceno, produjo 3.6) Ma, en concordancia con edades pliegues isoclinales y rampas de 40Ar/39 Ar de 4.90 ± 0.15 Ma y 4.78 ± 0.16 sobreescurrimiento en rocas de la cobertura Ma, estas dos ultimas ligadas a la fase de sedimentaria y un sistema de fracturas NE­ mineralizaci6rt. De lo anterior se concluye, SW el cual afecta por completa al Domo que los dep6sitos del sector norte y sur son Y auli. Lineamientos mayores de de edad diferente, y que el gran tamafio de orientaci6n 120° estan presentes en el los dep6sitos en Domo Y auli son el basamento y afectan toda el area. La resultados de sucesiVos sistemas posici6n de estos lineamientos coïncide hidrotermales. con el emplazamiento de los yacimientos mayores del distrito. En el distrito Numerosas edades concordantes U-Pb en Morococha (sector norte del Domo Yauli), el rango 9-14 Ma y evidencias petrol6gicas los afloramientos de rocas intrusivas son tales como halos de reacci6n y zonaci6n en abundantes, aqui numerosos stocks plagioclasas, documentan una actividad monzograniticos intruyen a la diorita magmatica continua bajo el distrito 6

Morococha, sostenida por repetidas importante etapa temprana de hematita se inyecciones magmâticas, seguidas de observa en la secuencia paragenética. fusion parcial de la roca de caja. La ausencia de texturas de disolucion en Datos estructurales para vetas distensivas, zircones excluye magmas sobresaturados diques miocenos y fallas, fueron en Zr e indica temperaturas inferiores a colectados para definir el paleostress 800°C. Lo anterior implica un enfriamiento asociado al evento de mineralizacion. En rapido y una composicion probablemente este estudio se ha aplicado el método intermedia, para las inyecciones sucesivas inverso para determinar el tensor local de de nuevo magma. La compos1c1on stress, en diferentes sectores en el area del 87 86 isotopica del magma ( Sr/ Sr = 0.705627 Domo Y auli. La determinacion del 143 144 to 0.707453; Nd/ Nd = 0.512350 to paleostress ligado al evento magmatico 0.512510; 206PbP04Pb =18.698 to 18.761; Mioceno, indica un campo compresivo 207 04 208 04 PbP Pb = 15.635 to 15.669; PbP Pb heterogéneo que cambia de E-W, en el = 38.682 to 38.787) sugiere un fundido sector este de Domo Yauli, a N-S, en el hibrido resultado de un magma de origen sector oeste. La rotacion de la direccion de mantélico, que ha interactuado compresion principal, ocurrio en tomo a un intensamente con la corteza continental, eje 120°, que corresponde a un lineamiento este fundido hibrido es compatible con en el basamento. Tal rotacion pone en zircon cHf cercano a cero. La participacion evidencia el roi activo de movimientos de de fundidos de fusion parcial de rocas del rumbo a lo largo de antiguos lineamientos, basamento de Arequipa en el magmatismo en la formacion de los depositos minerales del Mioceno, ha sido documen.tada a través en el area del Domo Y auli. La edad de de zircones heredados de rocas estos lineamientos es dificil de determinar, precambricas y magmas empobrecidos en no obstante, numerosas estructuras con la Hf, con edades modelo en tomo a 1.0 Ga. misma orientacion en el Grupo Mitu, indican que su origen se remonta al menos, La orientacion de las vetas en el distrito al rifting del Pérmico. San Cristobal es altamente variable y esta rotada hacia el oeste, desde N30°W a EW. En el distrito San Cristobal, inclusiones La paragénesis de estas vetas puede ser fluidas en esfalerita y cuarzo se dividida en tres fases: (a)Etapa temprana: homegeinizan en fase liquida entre 140 y wolframita-cuarzo-pirita, (b) etapa 330°C y son bi-fasicas (0.4 y 6.7 %peso intermedia: cuarzo-rnetales base, y (c) NaCl) a temperatura ambiente; algunas de etapa tardia: cuarzo-carbonato-baritina. estas inclusiones de la etapa temprana, Los depositos de reemplazo en rocas contienen un cristal adicional de halita (28- carbonatadas son generalmente 50 %peso NaCl). Datos provenientes de las estratiformes, pero muestran claras vetas, muestran un descenso en la indicaciones de discordancia respecto a la temperatura de homogeneizacion junto a roca carbonatada de caja. Representaciones una disminucion de la salinidad. A tri-dimensinales de estos cuerpos diferencia de las vetas, los datos mineralizados y de las vetas asociadas, provenientes de los depositos de reemplazo muestran que estas caracteristicas en rocas carbonatadas, muestran una discordantes y las altas leyes, estan ligadas amplia variacion en la salinidad (3.3-14 a las prolongacion de las vetas en las rocas %peso NaCl) a temperatura de carbonatadas. Su asociacion mineralogica homogeneizacion constante. Lo anterior es similar a la de las vetas, con la sola puede explicarse ya sea por mixing de un diferencia de que la etapa temprana fluido relacionado al sistema de vetas y wolframita-cuarzo-pirita esta ausente y una una salmuera caliente, o, por ebullicion del fluido que emigra de las vetas en las rocas 7 carbonatadas. Wolframita, galena y de aguas mete6ricas al sistema y esfâlerita, provenientes de cada tipo de consecuente miXmg con un componente yacimiento, reportan similares magmatico. compos1c10nes isot6picas de plomo e06PbP04Pb = 18.676 a 18.840; 207PbP04Pb Diferentes origenes para los fluidos han = 15.615 a 15.649; 208PbP04Pb = 38.704 a sido confirmados por amilisis LA-ICP-MS 38.827) y son equivalentes a los valores en inclusiones primarias trifasicas y reconocidos en los intrusivos del Mioceno bifasicas. Las concentraciones de los e06PbP04Pb 18.698 a 18.761; elementos de interés economico, i.e. W, 207PbP04Pb= 15.635 a 15.669; 208PbP04Pb = Cu, Zn y Pb decrece durante la 38.682 a 38.787). Por el contrario, las paragénesis, y tanto el W como (en menor compos1c10nes isot6picas de Sr en extension) el Cu, muestran fuertes carbonata y baritina de la etapa tardia, son variaciones asociadas a una disminuci6n altamente variables y demasiado escalonada de la concentraci6n, en el radiogénicas para ser explicadas por aporte tiempo. Las concentraciones que magmati co solamente (87 Sr/86Sr disminuyen, se pueden explicar por la O. 712187 a O. 722782). Estos val ores depositaci6n de minerales y las diferencias pueden corresponder a la evoluci6n de un en la velocidad de la disminuci6n, indican fluido predominantemente magmatico a un una precipitaci6n selectiva. Por el fluido rico en 87 Sr. Estas dos etapas son contrario, inclusiones fluidas de la etapa consistentes con los datos isot6picos de tardia muestran un abrupto incrementa en oxigeno e hidr6geno. Las composiciones las concentraciones de Ba y Sr. Esto isot6picas de los fluidos asociados a la precisa un mayor volumen de silicatos etapa temprana, revelan una tendencia con alterados, probablemente debido al mayor val ores constantes de o 180 y valores tamafio de la célula de flujo fluido, y se decrecientes de oD (o 180 = 3.2 a 5.0 %o explica por un aporte de liquido de origen SMOW yoD= -60 a -112 %o SMOW), lo distinto. Anâlisis LA-ICP-MS muestran cual se interpreta como mixing entre un que los fluidos estaban completamente componente predominante magmatico con empobrecidos en W y Cu, antes de llegar a proporciones menores de aguas mete6ricas las rocas carbonatadas, mientras que Zn y equilibradas con la roca de caja. Por el Pb, estaban presentes a-Lm en · cantidades contrario, las etapas finales presentan considerables. Esto de debe nuevamente a caracteristicas isot6picas que definen una la precipitaci6n selectiva, e indica que los tendencia con un descenso simultaneo para metales econ6micamente interesantes los valores de 6 180 y oD (o 180 = -8.1 y fueron introducidos predominantemente 2.5 %o SMOW y o D = -57 a -91 %o por fluidos magmaticos. SMOW) ~ se explica por una gran adici6n 8 9

Résumé des résultats

Introduction et Robinson-Cook ( 1987), Dalheimer ( 1990) et Kobe ( 1990) sont probablement les études les plus marquantes. Toutefois, plusieurs questions restent sans réponses. Cette partie présente un résumé étendu des Ainsi, les relations chronologiques entre principaux thèmes développés et résultats les différents événements magmatiques, obtenus lors de 1'étude du district minier de hydrothermaux et tectoniques durant le Domo de Y auli, situé dans la Cordillère Miocène n'ont jamais été réellement Occidentale du Pérou central. Cette portion élucidées. Cette méconnaissance a de la cordillère présente de nombreux notamment mené à une controverse quant à gisements hydrothermaux miocènes la genèse des minéralisations encaissées comprenant notamment des porphyres par les roches carbonatées, expliquée soit cuprifères, des corps de remplacement par un modèle syngénétique (Dalheimer, dans les roches carbonatées et des veines 1990 ; Kobe, 1990) ou par un modèle polymétalliques ; ils forment une ceinture épigénétique (Petersen, 1965 ; Bartlett, qui s'étend sur plus de 900km (Noble and 1984). D'autre part, les différentes McKee, 1999). Le district minier de Domo interprétations des données d'isotopes de Y auli est en 1'occurrence représentatif stables (Campbell et al., 1984 ; Heinrich, de cette ceinture, puisqu'il exhibe la même 1990) ont notamment contribué à ce que diversité de gisements. Ils sont l'origine des fluides impliqués dans les principalement exploités pour les métaux événements minéralisateurs et les de base tel que le zinc, le plomb et le processus de déposition restent également cuivre, certains d'entre eux présentent en controversés. De telles incertitudes sur la outre des teneurs en argent élevées. Le genèse des différents types de gisements nombre, le tonnage et la teneur des corps liés aux intrusions et la chronologie des minéralisés exploités font du district minier différents événements magmatiques et de Domo de Y auli un des plus gros hydrothermaux ne sont pas l'apanage du producteurs de métaux de base du Pérou. seul Domo de Yauli. En effet, si ces Durant le vingtième siècle, il a produit plus gisements sont généralement localisés 6 de 1.9 * 10 tonnes métriques de Zn, 6.4 * autour de systèmes porphyriques et que, 5 5 10 tonnes métriques de Pb, 4.6 * 10 par conséquent, leur lien génétique semble 8 tonnes métriques de Cu et 2.1 * 10 onces établi {Titley, 1993), savoir si les différents d'Ag. types de corps minéralisé sont réellement liés à un seul système porphyrique central (Sillitoe and Bonham, 1990 ; Hedenquist Dû à son importance et à sa longue période and Lowens.tem, 1994) ou si cette diversité d'exploitation, son début datant de gisements est plutôt le résultat de probablement de l'époque coloniale plusieurs événements se superposant (Masias, 1905), le district minier de Domo (Marsh et al., 1997; Ballard et al., 2001; de Yauli a été intensivement étudié. Gustafson et al., 2001, Munteau and McLaughlin (1924), Harrison (1943), Einaudi, 2001 ; Bendezu and Fontboté, in Wilson (1963), Petersen (1965), Bartlett press) reste à éclaircir. D'autre part, (1984), Campbell et al. (1984), Campbell l'interprétation génétique de districts 10 mmiers similaires aux Etats-Unis et au déformation associes à 1'ouverture des Mexique et, notamment, 1'importance de la structures minéralisées. La dernière partie composante magmatique dans le processus de mon travail consiste en 1' étude des minéralisateur restent débattues. Ainsi, fluides impliqués dans la formation de la différentes hypothèses sur 1'origine des veine San Cristobal et des corps de fluides minéralisateurs ont été proposées, remplacement associés. Elle comprend une telles qu'étant exclusivement magmatique étude d'inclusions fluides, contenues dans (Sillitoe, 1976), un mélange entre fluides les différentes générations de quartz, par magmatiques et saumures de bassin microthermométrie, spectrométrie Raman (Megaw et al., 1996; Smith, 1996), ou et laser-ablation ICP-MS. Un lot uniquement d'origine non-magmatique de additionnel d'analyses d'isotopes stables style Mississippi Valley mais modifié (H, 0) a été ajouté à celles obtenues par ultérieurement par des fluides Campbell et al. (1984). Ma discussion des magmatiques (Beaty et al., 1990). données d'inclusions fluides et d'isotopes stables tiendra compte des données récentes de Moritz et al. (2001). Afin de répondre aux problématiques énoncées ci-dessus, les différentes méthodes appliquées durant 1' étude du En guise de synthèse, trois discussions, district minier de Domo de Y auli ont été représentatives des trois prochains pluri-disciplinaires. Je commencerai par chapitres, sont présentées. La première, présenter le magmatisme Miocène ainsi basée sur les données géochronologiques qu'un aperçu métallogénique des différents associées aux données isotopiques et de types de gisements présents. La description microsonde, démontre la superposition de du magmatisme comprend notamment un plusieurs événements magmatiques et résumé de l'étude pétrographique, des hydrothermaux dans la région de données d'éléments majeurs et traces, des Morococha (Fig. 1.1 ). Ceci étant données isotopiques de Sr, Nd et Pb sur probablement rendu possible par la roche totales ainsi que les âges U/Pb et les présence continue de petites chambres rapports isotopiques 176Hf/ 177Hf des magmatiques réapprovisiOnnées par zircons. L'aperçu métallogénique présente injections répétées de magma moins brièvement les descriptions minéralogiques évolué. La deuxième discussion, centrée et paragénétiques de chaque type de sur la région de San Cristobal (Fig 1.1 ), gisement; la description des corps illustre l'importance cruciale qu'ont eu les minéralisés de la région de San Cristobal a linéaments crustaux dans la formation d'un bénéficié de 1' apport des travaux de paléochamps de contraintes hétérogène diplôme de Stucky (2001), Lisboa (2002) responsable de l'ouverture des failles. La et Sallier (2002). La minéralisation et dernière discussion présente le modèle 1'altération ont été datées par les méthodes génétique déduit des analyses d'inclusions Re/Os sur molybdenite et 40 Ar/39 Ar sur fluides et d'isotopes stables ; modèle qui, phlogopite et séricite. Dans l'optique de si il démontre la présence de fluides de définir les contraintes associees aux différentes ongmes, souligne que le minéralisations de San Cristo bal, j'ai mené principal apport d'éléments une analyse structurale approfondie en économiquement intéressants est dû aux plusieurs sites représentatifs. Le traitement fluides d'origine magmatique. des données par la méthode d'inversion (Angelier, 1989, 1994) a permis de définir les paléocontraintes pour chacun de ces sites et de définir ainsi les paléochamps de 11

Magmatisme miocène préférablement expliqué comme étant 1'emregistrement de plusieurs pulses magmatiques. Les âges des deux Le Domo de Yauli, d'orientation nord­ intrusions, datées à 8.81 ± 0.06 Ma pour Yantac et à 9.1 ± 0.1 Ma pour San ouest, est principalement composé de Francisco, sont contraints par la phyllites paléozoïques de l'Excelsior, de superposition des points concordants les roches volcani-clastiques penna-triasiques plus jeunes, ils représentent probablement du groupe Mitu, des roches carbonatées du des âges maximums. Alors que le groupe Pucarà ainsi que de différentes magmatisme miocène est bien représenté formations sédimentaires crétacées (Fig. 1.1 ). L'activité magmatique miocène est au nord du Domo de Y auli, au sud du dôme n'apparaissent que plusieurs principalement représentée par une large apophyses d'une intrusion fortement altéré intrusion localisée près de Morococha et surnommée l'intrusion du Chumpe. La surnommée diorite d' Anticona (Fig. 1.2). forte altération argilique qui 1'affecte rend Sa minéralogie primaire consiste en un sa minéralogie primaire obscure et ne assemblage porphyrique dominé pàr des laisse que des cristaux arrondis de quartz plagioclases, des biotites et des comme témoins de cette dernière. Un âge hornblendes. De nombreux xénolithes U/Pb de 6.6 11-3.6 Ma a été déduit de noirs, foliés et riches en corindons, sont + 1'intercept inférieur définie par dispersés et proviennent probablement de 1'alignement 1'Excelsior encaissant. Un âge U/Pb sur des points discordants (Fig. 1.9d). zircon de 14.11 ± 0.04 Ma a été obtenu pour cette diorite (Fig. 1.9a). La bordure orientale de cette diorite est recoupée par les intrusions felsiques de San Francisco, Les rapports isotopiques de Sr et Nd des Yantac, San Pablo et Gertrudis (Fig. 1.2). intrusions du Domo de Y auli définissent Leur minéralogie primaire consiste deux groupes. Le premier, contenant les principalement en orthose, plagioclase, intrusions de Morococha, a des rapports biotite et quartz et est généralement isotopiques compris entre O. 705665 et oblitérée par les altérations pervasives liées 0.706070 pour le Sr et entre 0.512459 et à la formation des gisements. Les 0.512510 pour le Nd, alors que les phénocristaux de phigioclases de la diorite intrusions proches du camp minier de San et des intrusions felsiques indiquent de Cristobal donnent des rapports 87Sr/86Sr brusques variations de la teneur en plus élevés (0.706573 et 0.707453) et anorthite (Fig. 1.7). Les datations U/Pb sur 143Nd/144Nd plus faibles (0.51231 0 et zircons de ces intrusions felsiques donnent, 0.512350) reflétant peut-être une plus d'une part, de nombreux points nettement grande assimilation crustale (Fig. 1.6). Les discordants et, d'autre part, un alignement analyses isotopiques de Sr et de Nd de points analytiquement concordants entre obtenues sur les intrusions du Domo de 9 et 14 Ma (Fig. 1.9b etc). Si les premiers, Y auli correspondent à celles obtenues par par suite de la présence de xénolithes Soler (1991) et Petford et al. (1996) sur d'Excelsior, s'expliquent par des zircons d'autres parties de la Cordillère hérités de roches assimilées par le magma, occidentale. Les rapports isotopiques du Pb cela ne peut être que difficilement le cas des intrusions proches de Morococha pour les seconds. En effet, les images en définissent un groupe homogène cathodoluminescence de zircon ne révèlent CZ06PbP04Pb = 18.698 à 18.761 ; 207PbP04Pb 208 04 pas de noyaux hérités et indiquent des = 15.635 à 15.669; PbP Pb = 38.682 à structures concentriques (Fig. 1.8). Cet 38.787) (Fig. 1.13a et b). La moyenne alignement de points concordants est donc pondérée des rapports 176Hf/177Hf des 12

40 39 zircons datés varie autour de EHf = 0 Cette dernière a été datée par Ar/ Ar à suggérant par là une source hybride de 7.20 ± 0.20 Ma dans le cas du skam de magma. Toutefois, ces analyses ne sont pas Parvenir et à 6.42 ± 0.19 Ma dans celui du homogènes, ams1 celles de zircons skam de Gertrudis. Le système de fractures indiquant un âge U/Pb discordant ont des affectant le Domo de Y auli est valeurs EHf négatives, alors que celles qui fréquemment minéralisé et d'importantes donnent des âges concordants montrent des veines comme Manuelita, Andaychagua ou valeurs positives et donc une influence encore San Cristobal sont exploitées (Fig. mantellique plus prononcée (Fig. 1.1 0). 1.3). Elles ont généralement un fort pendage, coupent les phyllites de l'Excelsior, les roches volcani-clastiques du Mitu ainsi que localement les intrusions du miocène et ne sont pas rectilignes mais Métallogénie du Domo de Yau/i généralement coudées (Figs. 2.4 et 2.6). Un halo d'altération dominé par la présence de séricite, quartz, chlorite, pyrite et argiles se forme aux épontes. La séricite Deux districts mm1ers sont encore en d'altération des épontes des veines de San activité dans le Domo de Yauli: Cristo bal et d' Andaychagua a été datée par Morococha au nord et San Cristobal au sud 40 39 la méthode Ar/ Ar à 4.78 ± 0.16 Ma et à (Fig. 1.1 ). Le district de Morococha est 4.90 ± 0.15 Ma (Figs. 1.1 Oc et d). Ces âges constitué d'un porphyre à Cu-Mo-(Au) sont dans la marge d'erreur attribuée à entouré de skams à Zn-Pb et de corps de l'âge U/Pb de l'intrusion du Chumpe (6.6 remplacement dans les carbonates associés + 11-3.6 Ma) et démontrent ainsi leur à des veines à Zn-Pb-Ag. Le district de probable lien génétique. Les importantes San-Cristobal ne comprend que des corps marges d'erreur ne peuvent toutefois pas de remplacement et des veines, aucune exclure 1'influence d'autres intrusions minéralisation économique de type n'affleurant pas à la surface. La paragenèse porphyre ou skam n'y a été découverte. des différentes veines est généralement uniforme et est dominée par un stade quartz-sulfures et un autre tardif à quartz, Le porphyre cuprifère de Toromocho est carbonates, argiles et barytine (Stucky, centré sur l'intrusion de San Francisco 2001; Lisboa, 2002). Les veines associées (Fig. 1.2), créant un halo d'altération à 1'intrusion du Chumpe contiennent un potassique et séricitique. La minéralisation stade à wolframite-quartz-pyrite prédatant économiquement intéressante est associée la minéralisation à sulfures. Les corps de à l'altération séricitique et consiste en remplacement sont encaissés par les veines à minéraux de cuivre ou carbonates du Pucara et se situent polymétalliques. De petites veines de principalement le long du flanc ouest du quartz et molybdénite sont également Domo de Y auli, leur emplacement associées à cette altération et ont été datées correspondant à la position de nombreuses à 7.87 ± 0.07 Ma par la méthode Re/Os sur veines. Ces corps minéralisés sont molybdénite. L'intrusion des granites de généralement stratiformes mais montrent Yantac et de Gertrudis dans les dolomies également des structures discordantes par du Pucara a produit des skams magnésiens rapport aux bancs de dolomie, telles que à Zn et Pb (Fig. 1.2). Leur minéralogie les brêches, les halos d'altération de hypogène est dominée par le grenat, le carbonates secondaires et d'argiles et la diopside, la magnétite, l'épidote, la pyrite, distribution des métaux (Fig. 2.1 0). A la pyrrhotine, la sphalérite, la galène, le 1'exception d'un important stade à hématite chrysocale, la chlorite et la phlogopite. prédatant les sulfures (Sallier, 2002), leur 13 minéralogie est très proche de celle des failles ainsi que de leurs stries, linéations et veines auxquelles ils sont associés. fibres minérales ont été relevées en plusieurs emplacements du district minier de San Cristobal (Fig. 2.11). L'orientation des contraintes Miocènes de chaque emplacement a été déterminée par la Le paléo-champs de méthode d'inversion (Angelier, 1989, contraintes miocènes 1994) et révèle un champ hétérogène. En effet, le champ de contraintes des emplacements localisés à 1' est de l'intrusion du Chumpe indique une Le Domo de Y auli est composé de deux compression principalement est-ouest, zones tectoniques différentes (Fig. 2.3). alors que celui des localités situées à Alors qu'au centre la structure des roches 1'ouest montre une composante cr 1 proche du groupe de 1'Excelsior est influencée par du nord-sud (Fig. 2.11 ). la déformation hercynienne (Lepry, 1981 ), au sud-ouest les roches volcani-clastiques permo-triasiques du groupe Mitu et les roches carbonatées mésozoïques montrent des plis isopaques, kilométriques, faiblement déversés vers le nord-est et Inclusions fluides associés à des failles inverses qui correspondent à la phase fini-éocène de la déformation andine (Harrison, 1943; L'étude d'inclusions fluides a Mégard, 1978). Les deux zones sont principalement été menée dans le quartz de recoupées par un Jeu de failles la veine San Cristobal et des corps de décrochantes conjuguées, également remplacement associés. La présence de associées à la déformation andine, mais qui quartz dans chacun des stades ont rejoué postérieurement à la formation paragénétiques de la veine (Fig. 3.3), des plis dont ils déplacent les axes. Cela a wolframite-quartz-pyrite, quartz-sulfures et notamment été le cas lors du magmatisme carbonates-quartz-barytine-argiles, rend miocène et a ainsi permis la formation de possible de suivre 1' évolution des nombreuses veines polymétalliques à processus minéralisateurs. Au contraire, les travers tout le Domo de Y auli. Du à corps de remplacement dans les roches l'orientation des failles héritées de la carbonatées ne possèdent qu'un seul stade déformation éocène, les minéralisations quartzeux séparant le stade à oxydes de miocènes ont toujours été associées à une celui à sulfures (Fig. 3.3). compression simple (Lepry, 1991), alors qu'un champ de contrainte totalement différent pourrait profiter des zones de faiblesse pré-existantes (Sylvester, 1988). Description et microthermométrie Afin de définir le paléo-champ de des inclusions fluides contraintes associées aux minéralisations et par là-même faciliter la découverte de nouveaux corps minéralisés, une étude Toutes les inclusions fluides analysées sont structurale détaillée a été menée. de type H20-NaCl-(KCl), le dioxyde de carbone est en quantité trop faible pour être détecté par microthermométrie, toutefois, Les orientations des veines les analyses par spectrométrie Raman en polymétalliques, des dykes miocènes et des 14 révèlent de faibles quantités variables valeurs (Th = 263 à 320 oc ; salinité = 4.6 jusqu'à 0.6 g/cm3 (Table 3.4). à 6.2 % pds éq. NaCl). Le quartz automorphe des corps de remplacement Le quartz associé à la wolframite présente dans les carbonates contient d'une part des trois différentes populations (Fig. 3.10 et inclusions primaires de grande taille (Th = Table 3.4); deux familles d'inclusions de 285 à 290 °C ; salinité = 4 % pds éq. petite taille (7 à 20 !Jill) sont primaires, NaCl) et des inclusions secondaires avec l'une formée d'inclusions tri-phasées avec des températures et des salinités montrant un cristal de halite (Th = 205 à 259 oc ; une très grande dispersion (Th = 166 à 290 salinité = 44.6 à 54 % pds éq. NaCl) et oc; salinité= 0.2 à 4.8 % pds éq. NaCl), l'autre d'inclusions bi-phasées (Th= 146 à pouvant être le résultat de phénomènes de 237 oc; salinité = 2.9 à 5.1 % pds éq. rééquilibrage. NaCl). Ces inclusions de salinité élevée suggèrent un fluide d'origine magmatique (Bodnar, 1995). Les inclusions secondaires sont également biphasées, beaucoup plus Analyse des inclusions fluides abondantes et de taille généralement par LA-/CP-MS supérieure (8 à 50 !Jill). Elles homogénéisent entre 157 et 257 °C, alors que leur salinité varie entre 0.4 et 5.4 % Les teneurs en Na, K, Fe, Cu, Zn, Sr, Ag, pds éq. NaCl. Le quartz laiteux Sn, Ba, W, Pb, Mg, Ca, Mn, As, Rb, Y, accompagnant la dépôt des sulfures Mo et Ce des inclusions fluides ont été contient des inclusions fluides bi-phasées analysées par laser ablation ICP-MS pouvant être reconnues sur la base de leur (Appendix III). Des concentrations de W morphologie (Fig. 3.13 et Table 3.4); les supérieure à la limite de détection ne se unes pseudosecondaires et de formes fines trouvent que dans les inclusions fluides du et tubulaires (Th= 328 à 337 oc ; salinité= quartz associé à la wolframite ; alors que 3.9 à 4.0 % pds éq. NaCl), et les autres, celles des éléments caractéristiques des secondaires et de formes plus irrégulières minéralisations du Domo de Y auli, comme (Th = 155 à 256 °C ; salinité = 2. 7 à 4.2 % le Cu, le Zn et le Pb, sont suffisamment pds éq. NaCl). Les données élevées pour être détectées dans chaque microthermométriques d'inclusions fluides inclusion, mais décroissent à travers la dans la sphalérite du stade à quartz­ paragenèse (Fig. 3 .18). Par contre, les sulfures obtenue par Moritz et al. (200 1) teneurs en certains éléments comme le Ba montrent des températures et salinités et le Sr obéissent exactement à la tendance équivalentes à celles obtenues dans le inverse en étant plus élevées dans le quartz. Le quartz associé à la phase tardive dernier stade de minéralisation. est automorphe et très transparent, 1'identification des trois différentes familles en est ainsi facilitée (Fig. 3.15 et Table 3.4). Les inclusions primaires le long des zones de croissance sont allongées, de Isotopes d'oxygène et larges tailles (15 à 50 !Jill), homogénéisent d'hydrogène entre 302 et 322 °C et ont une salinité variant entre 5.9 et 6.7 % NaCl. Les inclusions pseudosecondaires et secondaires ont une taille similaire mais Les analyses isotopiques de deuterium et possèdent des températures d'oxygène ont été effectuées sur le quartz, d'homogénéisation et des salinités la wolframite et leurs inclusions fluides, couvrant une plus large fourchette de sur les argiles et sur les inclusions fluides 15

contenues dans les sphalérites (Tables 3.5 indiquent que les intrusions de San et 3.6). Elles ont été complétées par les Francisco et de Y antac sont responsables analyses de Campbell et al. (1984). La de la formation du porphyre cuprifère de composition isotopique du fluide associé à Toromocho et du skam de Porvenir. Alors la déposition de la wolframite, la sphalérite qu'à San Cristobal, l'intrusion du Chumpe et du quartz des deux premiers stades occupe une position centrale par rapport paragénétiques des veines indique une aux différentes vemes et corps de grande variation de oD pour un 8 180 remplacement ; leur lien génétique est par constant (Fig. 3.19) ; elle a été interprétée ailleurs démontré par la correspondance de comme étant la signature de l'interaction l'âge U/Pb sur zircon de l'intrusion (6.6 d'un fluide météorique avec l'intrusion du + 1/-3.6 Ma) et les âges 40 Ar/39 Ar obtenus Chumpe (Campbell et al., 1984) ou par un sur les séricites de l'altération (4.78 ± 0.16 mélange entre des fluides magmatiques Ma et 4.90 ± 0.15 Ma). Ainsi, dominants et des fluides météoriques en l'accumulation de corps minéralisés au faible quantité (Heinrich, 1990). Les Domo de Yauli n'est pas le produit d'un mmeraux représentatifs des stades seul événement magmato-hydrothermal, paragénétiques tardifs, tel que les mais le résultat de deux : 1'un centré à carbonates, la barytine ou les argiles, Moro cocha et 1'autre, distinctement plus définissent une tendance oblique dans la jeune, à San Cristobal. Figure 3 .19) qui s'explique par un mélange entre fluides magmatiques et météoriques. Avec un seul et unique système magmato­ hydrothermal, le district minier de San Cristobal apparaît relativement simple comparativement à celui de Morococha où Discussions plusieurs intrusions felsiques, chacune liée à un ou plusieurs autres corps minéralisés, se côtoient et se superposent. En effet, si La répétition d'événements les deux intrusions datées par la méthode magmatiques et hydrothermaux à U/Pb, celles de San Francisco (9 .1 ± 0.1 Morococha, indique-t-elle la Ma) et de Yantac (8.81 ± 0.06 Ma), ont présence d'une large chambre approximativement le même âge magmatique sous-jacente ? maximum, il n'en va pas de même pour les corps minéralisés. En effet, 1'âge Re/Os sur molybdénite du porphyre cuprifère (7 .87 ± 0.07 Ma) ne correspond pas à l'âge plateau Les datations U/Pb des intrusions du Domo 40 39 Ar/ Ar sur la phlogopite du skam de de Y auli mettent en évidence trois Porvenir (7.2 ± 0.2 Ma). Cela implique que différentes époques de magmatisme au le modèle proposé par Sillitoe et Bonham Miocène, la première correspondant à la (1990) ou Hedenquist et Lowenstem mise en place de la diorite d' Anticona à ( 1994), proposant un seul centre 14.11 ± 0.04 Ma, la deuxième aux magmatique et plusieurs types de intrusions felsiques de San Francisco et de gisements à différentes distances, n'est pas Y antac autour de 9 Ma et, finalement, valable à Morococha où nous sommes en l'intrusion du Chumpe à 6.6 + 1/-3.6 Ma. A présence de plusieurs événements 1'exception de la première, à laquelle magmato-hydrothermaux se superposant, à aucun gisement n'est associé, chacune de l'instar d'autres systèmes porphyriques et ces époques est liée à la formation de épithermaux (Marsh et al., 1997; Ballard et gisements métallifères. En effet, à Morococha, les relations de terrain 16 al., 2001; Gustafson et al., 2001, Munteau veines ne sont pas rectilignes mais and Einaudi, 2001 ). changent leur direction (Fig. 2.5), mais également par 1'orientation des cristaux automorphes de quartz, qui diffère selon les veines subissant une composante cr 1 La présence de multiples intrusions est-ouest ou nord-sud. L'emplacement de felsiques localisées à 1'est de la diorite la rotation des contraintes correspond à la d' Anticona s'explique relativement bien position d'un linéament crustal qui par de multiples « cupolas » originaires influence la géomorphologie du Domo de d'une chambre magmatique sous-jacente Yauli et se dessine sur les images satellites (Gustafson et al., 2001), dont la présence (Fig. 2.6). L'hétérogénéité du champ de est d'ailleurs documentée par les zonations contraintes est donc expliquée par la des plagioclases (Fig. l. 7). La durée réactivation de zones de faiblesse crustale de « vie » de cette chambre magmatique ne lors du miocène, permettant ams1 se restreint probablement pas aux seules l'ouverture en trans-tension des failles intrusions felsiques mais son origine préexistantes et la création de veines pourrait remonter jusqu'à la mise en place polymétalliques (Fig. 2.12). L'influence de de la diorite d' Anticona. Les datations ces linéaments crustaux sur la formation U/Pb sur zircons des intrusions de San des gisements du Domo de Y auli est Francisco et de Yantac ont donné plusieurs également démontrée par le fait que les points analytiquement concordants entre 9 principaux corps minéralisés sont alignés et 14 Ma (Fig. 1.9). Les images par le long de ces structures (Fig. 2.6). cathodoluminescence des zircons ne L'influence de ces linéaments d'orientation montrant aucun cœur indiquant une 120°, d'âge minimum permien, peut dissolution (Fig. 1.8), cet alignement de également s'observer dans la zone côtière points concordants est préférablement (Polliand, 2002) et devrait se révéler un expliqué par la continue mise en place de utile moyen de prospection minière petites chambres magmatiques qui péruvienne. refroidissent, puis sont refondues par l'apport de nouvelles injections de magma. Ces apports de magma moins évolué étant documentés non seulement par les Modèle génétique des oscillations de la composition des minéralisations polymétalliques plagioclases mais également par les hautes de San Cristobal valeurs de E:Hf (Fig. 1.10).

Au contraire du district minier de L'influence de linéaments Morococha où les événements magmato­ crustaux .sur /es paléo-champs hydrothermaux se sont succédés et de contramtes et la formation de superposés, celui de San Cristobal est en gisements comparaison plus simple avec une unique intrusion responsable des minéralisations et des corps minéralisés dispersés autour. Cela semble d'ailleurs confirmé par la Les relevés structuraux et leur concordance de l'âge de l'intrusion du interprétation par la méthode d'inversion Chumpe (6.6 + 11-3.6 Ma) et de celui de indiquent une rotation du champ de l'altération des épontes (4.78 ± 0.16 Ma et contraintes miocène dans le district minier 4.90 ± 0.15 Ma). L'absence de minéraux de San Cristo bal (Fig. 2.11 ). Celle-ci est aptes à être datés par méthode non seulement confirmée par le fait que les radiométrique dans les corps de 17

remplacement dans les roches carbonatées dilution d'un fluide chaud et salin par un ne permet pas de contraindre leur autre plus froid et moins salin (Fig. 3.20). chronologie exacte. Mais les caractères Les isotopes de Pb des premiers stades de discordants comme les brèches et la la minéralisation sont homogènes et distribution de la minéralisation en coïncident avec la signature isotopique du prolongement des veines (Fig. 2.7), ou magma (Fig. 3.8), alors que les isotopes de encore la concordance des signatures Sr des barytines tardives indiquent une isotopiques du Pb (Moritz et al., 2001), grande dispersion ne pouvant pas soulignent leur lien génétique et réfutent s'expliquer par une signature magmatique ainsi le modèle syngénétique proposé par (Fig. 3.9). Finalement, les tendances Dalheimer (1990) et Kobe (1990). contraires des concentrations d'éléments, avec la diminution du Cu, Zn, Pb et As et l'augmentation du Sr et du Ba à travers la paragenèse, illustrent aussi cette évolution Les études précédentes d'inclusions fluides d'un fluide à dominance magmatique de (Bartlett, 1984 ; Campbell et al., 1984) plus en plus dilué par des fluides n'ont jamais documenté la présence météoriques. Cela indique également que d'inclusions fluides contenant des cristaux la majorité des éléments économiquement de halite à San Cristobal et ont ainsi réfuté importants ont été introduits dans le l'hypothèse d'une participation de fluides système par le fluide magmatique, le fluide magmatiques aux processus météorique n'ayant pour sa part minéralisateurs. La découverte de fluides qu'introduit les éléments se retrouvant de salinités élevées implique donc la dans la phase terminale de la présence d'une probable composante minéralisation. Celui-ci a cependant magmatique initiale dans le fluide et mène certainement joué un rôle important en se à une réinterprétation des modèles mélangeant au fluide magmatique et génétiques et des données d'isotopes facilitant par là la déposition des minéraux. stables. Effectivement la grande variation de 8D pour un 8 180 constant définissant une tendance verticale sur la Figure 3.19 s'explique ainsi plus facilement par l'addition mineure de fluide météorique s'équilibrant avec l'intrusion du Chumpe à Références un fluide majoritairement magmatique. La quantité d'eau météorique impliquée dans Angelier, J. (1989): From orientation to les processus minéralisateurs augmente par magnitudes in paleostress determinations la suite pour former la tendance oblique de using fault slip data. J. Struct. Geol., vol. 11, mélange observée sur la Figure 3.19. Un p. 37-50. tel modèle de mélange, avec des fluides Angelier, J. (1994): Fault slip analysis and paleostress reconstruction. Dans: magmatiques prédominant dans les Continental deformation, Hancock P .L. ( ed), premiers stades et des fluides météoriques Tarrytown, Pergamon Press, p. 53-100. prenant de plus en plus d'importance est en Ballard, J.R., Palin, J.M., Williams, I.S., accord avec la microthermométrie des Campbell, I.H. et Faunes, I.H. (2001): Two inclusions fluides et les isotopes du Pb et ages of porphyry intrusion resolved for the Sr (Moritz et al., 2001) ou encore super-giant Chuquicamata copper deposit of 1' évolution des concentrations des northem Chile by ELA-ICP-MS and éléments dans le fluide minéralisateur. La SHRIMP. Geology, vol. 29, p. 383-386. température d'homogénéisation et la Bartlett, M.W. (1984): Petrology and genesis of carbonate-hosted lead-zinc-silver ores, salinité des inclusions définissent une co­ San Cristobal district, Department of Junin, variation positive s'expliquant par la Peru. Thèse de doctorat non publiée, Oregon 18

State University, Corvallis, Oregon, U.S.A., ore deposits in the Andes, Berlin: Springer­ 272 p. Verlag. p. 267-278. Beaty, D.W., Landis, G.P. et Thompson, T.B. Lepry, L.A. ( 1981 ): The structural geology of (1990): Carbonate-hosted sulfide deposits of the Yauli dome region, Cordillera the Central Colorado mineral belt: Occidental, Pern. Unpub. MSc Thesis, Univ. introduction, general discussion, and of Arizona, Tucson, 99p. summary. Econ. Geol., Monograph 7, p. 1- Lisboa, H. (2002): Etude de la veine 722 à Zn­ 18 Pb-Ag de la mine San Cristobal (District Bendezu, R., Fontboté, L. et Cosca, M. (sous minier de Domo de Yauli, Pérou central). presse): Formation of cordilleran base Mémoire de diplôme, Université de Genève, metal lodes and replacement deposits post Suisse precious metal high sulfidation epithermal Masias, M.G. (1905): Estado ctual de la mineralisation in the Colquijirca district, industria minera de Morococha. Cuerpo de central Pern. Mineralium Deposita. ingenioros de minas del Peru, p. 1-22. Bodnar, R.J. (1995): Fluid inclusion evidence Marsh, T.M., Einaudi, M.T. et McWilliams, for a magmatic source for metals in M. (1997): 40Ar/39 Ar geochronology of Cu­ porphyry copper deposit. Mineralogical Au and Au-Ag mineralisation in the Association Short Course, series 23, p. 139- Potrerillos District, Chile. Econ. Geol., vol. 152. 92, p. 784-806. Campbell, A. et Robinson-Cook, S. ( 1987): McLaughlin, D.H. (1924): Geology and Infrared fluid inclusion microthermometry physiography of the Peruvian Cordillera on coexisting wolframite and quartz. Econ. Cordillera, Department of Junin and Lima. Geol., vol. 82, p. 1640-1645. Geol. Soc. Am. Bull., col. 35, p. 591-632. Campbell, A., Rye, D. et Petersen, U. (1984): Mégart, F. (1978): Etude géologique des A hydrogen and oxygen isotope study of the Andes du Pérou central. Mémoire San Cristobal mine, Pern: Implications of ORSTOM, vol. 86, 303p. the role of water to rock ratio for the genesis Megaw, P.K.M., Barton, M.D. et Falce, J.I. of wolframite deposits. Econ. Geol., vol. 79, (1996): Carbonate-hosted lead-zinc (Ag, Cu, p. 1818-1832 Au) deposits of northem Chihuahua, Dalheimer, M. (1990): The Zn-Pb-Ag deposits Mexico. Dans: D.F. Sangster (ed.), Huaripampa and Carahuacra in the mining Carbonate-hosted lead-zinc deposits. district of San Cristo bal, Central Peru. Dans: Society of Economie Geologists Special Fontboté, L. et al. (eds.), Stratabound Ore Publication, vol. 4, p. 277-289. Deposits in the Andes, Berlin: Springer­ Moritz, R., Beuchat, S., Chiaradia, M., Stucky, Verlag. P., Sallier, B. et Lisboa, H. (2001): Zn-Pb Gustafson, L.B., Orquera, W., McWilliams, mant os and veins at Domo de Y auli, Central M., Castro, M., Olivares, 0., Rojas, G., Peru: two products of one hydrothermal Maluenda, J. et Mendez, M. (2001): system with common Pb & S sources, but Multiple centers of mineralisation in the contrasting fluid inclusion characteristics. Indio Muerto district, El Salvador, Chile. Dans: Piestrzyi'iski, A. et al. (eds), Mineral Econ. Geol., v. 96, p. 325-350. deposits at the beginning of the 21 st Harrison, J.V. (1943): The geology of the century. Balkema Publishers, p. 173-176. Central Andes in part of the province of Munteau, J.L. et Einaudi, M.T. (2001): Junin, Peru. Bol. Soc. Geol. del Peru, vol. Porphyry-epithermal transition: Maricunga 16, p. 1-97. belt, northem Chi1e. Economie Geology, Hedenquist, J.W. et Lowenstem, J.B. (1994): vol. 96,p. 743-772. The role of magmas in the formation of Noble, D.C. et McKee, E.H. ( 1999): The hydrothermal ore deposits. Nature, vol. 370, Miocene metallogenic belt of central and p. 519-527. northem Peru. Society of Economie Heinrich, C.A. (1990): The chemistry of Geologists Special Publication, vol. 7, p. hydrothermal tin( -tungsten) ore deposition. 155-193. Econ. Geol., vol. 85, p. 457-481 Petersen, U. (1965): Regional geology and Kobe, H. W. (1990): Metallogenic evolution of major ore deposits of Central Peru. Econ. the Yauli dome, central Peru. A summary. Geol., v. 60, p. 407-476. Dans: Fontboté, L. et al. (eds.), Stratabound 19

Petford, N., Atherton, M.P. et Halliday, A.N. Geologists Special Publication 4, p. 255- (1996): Rapid magma production rates, 263. underplating and remelting in the Andes: Soler, P. (1991): Contribution à l'étude du isotopie evidence from northem-central magmatisme associé aux marges actives - Pern (9-11 °S). Journal of South American Pétrographie, géochimie et géochimie Earth Science, vol. 9, p. 69-78. isotopique du magmatisme Crétacé à Polliand, M. (2002): Zn-Pb-Ba massive Pliocène le long d'une transversale des sulfides at Pernbar, Central Pern. Thèse de Andes du Pérou Central - implications doctorat, Université de Genève, Suisse géodynamiques et métallogéniques. Thèse Sallier, B. (2002): Minéralisations stratiformes de doctorat d'état, Université Pierre et Marie à Zn, Pb, ±Ag dans les calcaires du Pucara Curie, Paris, France, 846 p. (Domo de Yauli, Andes du Pérou Central). Stucky, P. (2001): La veine de Pb, Zn, Cu, Ag Mémoire de diplôme, Université de Genève, d' Andaychagua (Domo de Y auli, Andes Suisse, 170 p. centrales, Pérou). Mémoire de diplôme, Sillitoe, R.H. (1976): Andean mineralisation: a Université de Lausanne, Suisse. model for the metallogeny of convergent Sylvester, A.G. (1988): Strike-slip faults. plate margins. Dans: Strong, D.F., ed., Geological Society of America Bulletin, vol. Metallogeny and plate tectonics, Geological lOO, p. 1666-1703. Association of Canada Special Paper, vol. Titley, S. R. (1993): Characteristics of high­ 14, p. 59-100. temperature, carbonate-hosted massive Sillitoe, R.H. et Bonham, H.F. ( 1990): sulphide ores in the United States, Mexico Sediment-hosted gold deposits: distal and Pern. Dans: Kirkham, R.V. et al. (eds.), products of magmatic-hydrothermal Mineral Deposit Modeling. Geological systems. Geology, vol. 18, p. 157-161. Association of Canada, Special paper 40, p. Smith, D.M. Jr. (1996): Sedimentary basins 585-614. and the ongm of intrnsion-related Wilson, J.J. (1963): Cretaceous stratigraphy of carbonate-hosted Zn-Pb-Ag deposits. Dans: the cetal Andes of pern. Am. Ass. Petrol. D.F. Sangster (ed.), Carbonate-hosted lead­ Geol. Bull., vol. 47, p. 1-34. zinc deposits. Society of Economie 20 21

CHAPTER I: Resolving Miocene Magmatic and Mineralising

Events in the Zn-Pb-Ag-Cu Domo de Yauli district (Peru)

by High-Precision Geochronology

1 1 2 1 1 3 1 S.Beuchat , U. Schaltegger • , R. Moritz , M. Chiaradia , M. Cosca and D. Fontignie

l Section des Sciences de la Terre, University of Geneva, 1211 Genève 4, Switzerland

2 Institute for Isotope Geology and Mineral Resources, Federal Institute ofTechnology ETH, 8092 Zürich, Switzerland

3 Institute ofMinera1ogy and Geochemistry, University of Lausanne, 2015 Lausanne, Switzerland

Abstract

The Domo de Yauli is one of the major Zn-Pb±Cu±Ag mining districts of Peru and a classical Miocene intrusion related ore district with high temperature, carbonate-hosted base metal deposits. Felsic stocks are related to the formation of four different ore deposit types: Cu-porphyries, Zn-Pb skarns, Zn-Pb±Ag carbonate replacement deposits, and veins. The U-Pb, Re-Os and 40 ArP9 Ar dating methods allow the definition of a sequence of magmatic and mineralising events and the duration of mineralisation in the Domo de Yauli area. U-Pb dating of zircons from the northern intrusions yields concordant ages of 14.11 ± 0.04 Ma for the Anticona diorite and close to 9 Ma for different monzogranite stocks related to Cu-porphyry style and skarn deposits. Veins of the Cu-porphyry deposit have been dated at 7.9 ± 0.1 Ma by Re-Os on molybdenite, and phlogopite from a Zn-Pb skarn gives a 40Ar/39 Ar plateau age of 7.2 ± 0.2 Ma. U-Pb analyses of zircons from the southern Chumpe intrusion result in discordant points defining a lower intercept at 6.6 (+1/-3.6) Ma, in agreement with 40Ar/39 Ar ages of 4.90 ± 0.15 Ma and 4.78 ± 0.16 Ma obtained on sericite from wall rock alteration selvages. U-Pb, Re-Os and 40Ar;J9Ar age determinations reveal the existence of three distinct magma tic events at 14.1, 9.1 and 6.6 Ma, with the two later ones related to a phase ofmineralisation. We therefore conclude that the northern and southern ore deposit districts bear a different age and that the particularly large abundance of economie ore bodies at the Domo de Yauli is the result of successive hydrothermal systems.

A repeated magmatic activity beneath the Morococha district, sustained by several pulses of new magma and subsequent melting of the magmatic host rock, is documented by 22

numerous concordant U/Pb ages between 9 and 14 Ma and petrological evidences such as reaction rims and plagioclase zonations. Absence of dissolution textures in zircons indi~ates Zr-saturated magmas with temperature as low as 800°C. It implies a rapid coohng and a probable intermediate composition of the successive injections of new magma. Isotopie compositions of the magma (87 Sr/86 Sr = 0 705627 to 0 707453· 143 144 . . ' Nd/ Nd= 0.512350 to 0.512510; 206PbP04Pb =18.698 to 18.761· 207Pb/204Pb = 15 635 . 208 204 ' • to 15.669,_ . Pb/ Pb~ 38.682 to 38.787) suggest a hybrid melt source ofmantle plus crustal ongm compatible with DHf values of zircons around zero. Precambrian inheritances in zircons and Hf depleted mantle model ages of around 1.0 Ga indicate co~tributions of partial melts from the underlying Arequipa style basement during the M10cene magmatism.

Introduction style deposits, little attention has been paid to the timing and the duration of hydrothermal circulation in ore districts presenting different intrusion related ore Magmatic-hydrothermal systems are probably one of the most efficient setting deposit types. for hydrothermal circulation. They create The Domo de Y auli is located 11 Okm east not only numbers of different economie of Lima in the Western Cordillera of ore deposit types but also supply a large central Peru (Fig. 1.1 ). The Morococha and diversity of base and precious metals. For San Cristobal districts are two major Zn-Pb example, low-grade Cu-porphyry deposits mining districts of Peru with Cu and Ag as pro vide more than half of the world supply by-products. Their cumulate metal of copper and molybdenum. Recent pro ductlon . dunng . the 20 th century attains geochronological studies have found 6 5 evidences for multiple magmatic­ 1.~ * 10 MT Zn, 6.4 * 10 MT Pb, 4.6 * 10 MT Cu and 2.1 108 Oz Ag (Bendezu, hydrothermal events in Cu-porphyries and * pers. comm.). They are two classical high­ epithermal environments such as temperature, carbonate replacement ore Chuquicamata, Indio Muerto or Maricunga districts but are also the host of other (Marsh et al., 1997; Ballard et al., 2001; deposit types, including a Cu-porphyry, Gustafson et al., 2001, Munteau and Einaudi, 2001; Bendezu et al., in press). skams and veins. The location of the carbonate-hosted ore deposits nearby Such evidences raise two questions. The Miocene intrusions was interpreted as first, how multiple magmatic events can evidence for replacement processes be generated in such a short period of time (Petersen, 1965). A different model, based and if this implies the presence of a major on the stratabound to stratiform underlying magma chamber. The second, morphologies, invokes synsedimentary whether overprinting hydrothermal events processes, as for instance in the Domo de associated to discrete intrusive pulses are a Yauli area (Dalheimer, 1990; Kobe, 1990). pre-requisite to form a world-class district Moritz et al. (200 1) have shown that or if a single intrusion event is sufficient to carbonate replacement deposits, commonly produce the necessary volume of called mantos, and veins have similar hydrothermal circulation (Sillitoe and sulfur and lead isotopie compositions Bonham, 1990). In order to answer these suggesting that they were deposited from a questions, knowledge of timing and fluid with a similar isotopie composition. duration of magmatic and mineralisation Moreover, this isotopie ore composition is events is necessary. If previous studies concordant with the Miocene intrusion have been principally focused on porphyry 23 one. Therefore, the regional geological this area, but in the Eastern Cordillera, setting of Domo de Y auli pro vides an Harrison (1943) estimated its thickness to adequate environment for investigating be around 2000m. The first deformation chronological relationships among that affected the Excelsior Group is related Miocene magmatic activity, ore formation to the Variscan orogeny (Lepry, 1981). and a possible underlying magma chamber. Two main lithologies are recognised in the In this study, we present zircon U-Pb ages Mitu Group; they consist of red beds with in order to define the crystallization ages of conglomerates, sandstones and shales, and the Miocene intrusions, a molybdenite Re­ of intercalated alkaline lavas and Os age that precises the mineralisation age pyroclastic flows of probable continental of the Cu-porphyry, and 40Ar/ 39 Ar ages of origin (Mégard, 1987). Extreme lateral skarn phlogopite and sericite from wall variation in thickness can be related to the rock alteration. They allow the definition Permo-Triassic extensional tectonics as of a sequence of overprinting magmatic suggested by Kontak et al. ( 1985). The and mineralising events and point out the Triassic-Jurassic Pucara Group limestones difficulty of estimating the duration of rest uncomfortably on the Mitu Group fluid circulation in the Domo de Y auli (Rosas, 1994) and is overlain by different area. Additional whole rock chemistry, Cretaceous carbonate formations (Szekely, microprobe analysis, Sr, Nd, Pb and Hf 1967). isotopie compositions and zircon cathodoluminescence imagery help in the Many different stages of volcanism are interpretation of the magma evolution and recorded in the Domo de Yauli area point out a repeated magmatic process over including the Permian volcanism of the 5 My at the Morococha district. Mitu Group, the Montero basait hosted by Pucara limestone (Rosas, 1994) and the early Cenozoic volcanism which is developed on the western flank of the Geological setting Domo de Yauli. As mentioned previously, ore formation is directly linked to the Miocene igneous activity that create by far the most important volume of magmatic Regional Geology of the Domo rocks in the area (Table 1.1, Petersen, de Yauli district 1965; Noble and McKee, 1999).

The northwest-trending Domo de Yauli is Miocene igneous activity mainly composed of Paleozoic phyllites of the Excelsior Group, Permo-Triassic volcaniclastic rocks of the Mitu Group, Triassic-Jurassic limestones of the Pucara Miocene igneous activity in the Morococha Group, and Cretaceous sedimentary rocks area is mainly expressed by a large stock (Fig. 1.1 ). The Excelsior Group is the of quartz diorite that occupies the western oldest rock unit exposed in the district and portion of the mining district and is locally it crops out in the core of the Domo de known as the Anticona diorite (Fig. 1.2). Yauli (Fig. 1.1 ). This is a heterogeneous This quartz diorite has a porphyritic texture group that includes shales, phyllites, with a groundmass composed of limestones, quartzites and basaltic flows plagioclase, quartz and oxides. Phenocrysts where pillow lavas can be recognized in consist of plagioclase, biotite and places (Kobe, 1990). The thickness of the hornblende whereas accessory minerais Excelsior Group cannot be estimated in include apatite and zircon (Fig. 1.4a). 24

STUDY AREA 0•• • ••••••10km i+++ + + +j l__±_±_-1:: Tertiary Intrusions

1 1 Tertiary formations

l:l;i;i;i;i;il Cretaceous formations f {tl Pucarà group lv v v 1 Mitu group

Excelsior group

Fig. 1 1: Simp1ified geo1ogica1 map of the Domo de Yauli area.

The plagioclase phenocrysts are euhedral tourmaline, clay minerais and pyrite. and their diameter can reach up to 2.5mm. Crosscutting relationships between the Biotite and hornblende are partly replaced Anticona diorite and the intrusion located by an assemblage of chlorite, epidote on its eastern side indicate a younger age andcalcite and can be as large as 1.5mm for the latter. (Fig. 1.4a). Rare sphene and rutile have been found. Development of pervasive sericite in plagioclase is highly variable but More felsic intrusions occur east of the is generally higher in the eastern part of the Anticona diorite; they include the San intrusion, close to the Morococha mining Francisco, Y an tac, Gertrudis and San center. A fine-grained dark breakdown rim Pablo stocks (Fig. 1.2). The primary consisting predominantly of oxides and mineral assemblages are in sorne cases accessory pyroxene occurs around difficult to determine due to the intense amphibole and in sorne cases biotite (Fig. pervasive alteration. The least altered 1.4a). The Anticona diorite contains samples of the Y antac intrusion show numerous black foliated xenoliths that can phenocrysts of K-feldspar, plagioclase, vary in diameter from a few centimeters to biotite, hornblende and quartz set in a one meter (Fig. 1.4b ). They are generally microgranular assemblage of quartz and composed of microgranular assemblages of plagioclase. plagioclase, epidote, corundum, quartz, 25

SIMPLIFIED GEOLOGICAL MAP OF THE MOROCOCHA DISTRICT

1 km

Simplified geology: Ore deposit types:

1 Quaternary ._.. Cu-(Mo) porphyry style deposit ~~ ft;+; 4Tertiary fel sic intrusions Zn-Pb skarn type deposit 1 1 Tertiary formations .... Zn-Pb-Ag Carbonate replacement (manto) 1 : 1: 1: 1l Cretaceous sediemnatry formations Zn-Pb-Cu-(Ag) veins R \' 1 Pucarà carbonate Group

1 v 1 Mitu vocaniclastic Group - Excelsior phyllites Group

~ Fault and thrusts

Fig. 1 2: Geological map of the mining districts ofMorococha with the location ofthl bodies. See locations on figure 1.1. 26

SIMPLIFIED GEOLOGICAL MAP OF THE SAN CRISTOBAL DISTRICT ,

N

1 km

Simplified geology: Ore deposit types:

1 Quaternary ~ Zn-Pb-Ag Carbonate ~~ f~+ ~ 4 Tertiary felsic intrusions replacement (manto) 1; 1: 1: 1! Cretaceous sediemnatry formations Zn-Pb-Cu-(Ag) veins R X 1Pucarà carbonate Group

1 v 1 Mitu vocaniclastic Group - Excelsior phyllites Group

~ F ault and thrusts

Fig. 1 3.: Geological map of the mining districts of San Cristobal with the location of the major intrusions and ore bodies. See locations on figure 1.1.

Accessory minerais include euhedral chlorite and senc1te alteration affecting apatite and zircon. Phenocrysts of K­ biotite and plagioclase of the Yantac feldspar can reach up to 2 cm and, in intrusion (Fig. 1.4c), to different pervasive places, include smaller crystals of alteration assemblages consisting sericite plagioclase and biotite (Fig. 1.4c). Quartz and kaolinite on plagioclase sites and is commonly rounded and shows clear shreddy biotite on rare hornblende sites in dissolution features. The alteration is the San Francisco intrusion. Eyzaguirre et highly variable, it can range from minor al. (1975) reported wh ole-rock KI Ar ages 27

Fig. 1 4: A. Photomicrograph of Anticona diorite, showing altered crystals of hornblende (hbl) and biotite in chlorite (chi) and carbonate (carb.). Note the oxide reaction rim. By contrast phenocrysts of plagioclase appear to be "fresh". Sample TIC-15. B. Black foliated xenolith hosted by the Anticona diorite. C. Photomicrograph of the Yantac intrusion showing the typical texture with K-feldspar embracing plagioclase and biotite crystals. Sample POR-5. D. Photomicrograph showing the pervasive alteration affecting the Chumpe intrusion with relies of biotite and plagioclase. Sample CHU-18. of 8.2 ± 0.2 Ma for a dyke cross-cutting t23 ± 0.3 Ma for the Yantac intrusion. At the south end of the Domo de Yauli district, Miocene magmatic activity is represented by two intrusive bodies, named the San Cristobal and the Chumpe intrusions (Fig. 1.3). The third Carahuacra intrusion is affected by lncaic folding (Lyons, 1968) and have been dated by K­ Ar method at 43.5 ± 1.6 Ma by Bartlett (1984); it is therefore much older than the studied Miocene magmato-hydrothermal systems. Faults that crosscut both intrusions produce a displacement of tens of meters on the San Cristobal intrusion and the associated dykes, no displacements of such amplitude have been found for the Chumpe intrusion. This may indicate an older age for the San Cristobal intrusion in comparison to the Chumpe one. The San Cristobal intrusion consists of a small body, which is largely covered by alluvial sediment and its shape is principally known from exploration drilling. The primary magmatic mineral assemblage is composed of plagioclase, quartz and biotite with accessory hornblende and apatite. This rock is pervasively altered; the alteration mineral assemblage consist of sericite, illite, chlorite and tremolite. Dykes crosscutting limestones of the Pucani Group and rocks of the Mitu Group are associated to this intrusion. The Chumpe intrusion consists of series of small apophyses and dykes parallel to the axis of the Domo de Yauli anticlines. The width of 28

Table 1.1: Miocene magmatism in the Domo de Ya uli area

Intrusion, location Magmatic mineral Tentative of Alteration Ore deposit structurally Previous Figs. 1.2 and 1.3 assemblage classification related KJ Ar age

Anticona diorite Plagioclase. hornblende, Quartz Diorite Biotite and Hornblende --> Uneconomic magnetite 8.2 ± 0.2 Ma 'l biotite, quartz (apatite, Chlorite, epidote, ca lcite; weak skarn type carbonate zircon) sericrtisation on plagioclase, replacement close ta the pyrite dissemination Ticlio pass

San Pablo intrusion Quartz, Feldspar?, biotite? Granite? Very intense sericitisation, Victoria Cu, Zn, Pb chlorite carbonate replacement and Potosi skarn

San Francisco intrusion Plagioclase, quartz, K­ Gran rte? Potassique alteration in places Cu-(Mo) Toromocho feldspar, biotite, rare with formation of K feldspar porphyry hom blende (zircon) and shreddy biotite, overprin t by strong quartz-sericite alteration

Gertrudis intrusion Quartz. Feldspar?, biotite? Granite? Very intense sericitisation, Gertrudis skarn and San chlorite Antonio Zn, Pb , Ag veins

Yantac intrusion Orthose, Plagioclase, Granodiorrte Biotite and Hornblende --> Porvenirskarn and Zn, Pb, 8.3 ± 0.3 Ma" quartz. hornblende and Chlorite, calcite; weak Ag veins biotite (sphene, zircon) sericrtisation on feldspar, pyrite dissemination

San Cristobal intrusion Plagioclase, quartz, biotite Granodiorrte lili te, chio rite, pyrite and Uneconomic skamoïds (hornblende, apatite, zircon) trema lite

Chumpe intrusion Quartz, K-feldspar, Granite? Intense pervasive atteration San Cristobal and 5.4 ± 0.3 Ma'' plagioclase, biot le characterise by illite, kaolinite, Andaychagua Zn, Pb, Ag, W (hornblende, apatite. zircon) pyrite, sericite and sorne veins system, Huaripampa alunite and Toldorumi Zn. Pb, Ag carbonate replacement

a) Eyzaguirre et al. (1975) b) Bartlett (1984)

these dykes ranges from severa! decimeters Ore deposits of the Domo de to tens of meters. The largest dykes show Yauli district assimilation of large xenoliths from the host Excelsior phyllites. The primary mineralogical composition is obscured due With the exception of the small and to intense alteration. Primary minerais uneconomic magnetite-bearing carbonate include quartz eyes and highly altered replacements, the Anticona quartz diorite phenocrysts of plagioclase and K-feldspar is not linked to any type of mineralisation. set in an aphanitic groundmass consisting Major ore bodies are ali related to the last predominantly of quartz, sericite, kaolinite, stages of the Miocene magmatic activity. illite, pyrite and very fine alunite (Fig. The ore deposits can be classified in four 1.4d). The !east altered samples are types (Table 1. 1): characterized by a chlorite and epidote alteration. In such sampies , relies of biotite A Cu-(Mo) porphyry style deposit is and sorne amphibole can be recognized. centered on the San Francisco intrusion (Fig. 1.2), producing a large potassic and sericitic alteration halo affecting the surrounding rocks. The potassic alteration, characterized by the formation of K­ feldspar and shreddy biotite, is 29 accompanied by discontinuous granular The crosscutting fracture system that quartz A veinlets (following the affects the entire length of the Domo de terminology of Gustafson and Hunt, 1975), Yauli is commonly mineralised and hosts which contain only trace amounts of pyrite important Zn-Pb-Cu-Ag veins such as the and chalcopyrite. These A veinlets are Andaychagua, Manuelita- Sulfurosa, San crosscut by quartz-pyrite-molybdenite Antonio and San Cristobal veins systems veins up to lem width (Fig. 1.5a), a halo of (Figs. 1.2 and 1.3). The veins are steeply­ sericite alteration is not always present. dipping and crosscut rocks of the Mi tu and Quartz-sericite alteration and severa! D Excelsior Groups, and locally Miocene veins (Gustafson and Hunt, 1975) intrusions. The wall rock alteration obliterate almost all traces of the earlier outwards from a vein in rocks of the Mitu assemblage on the site of the mining Group and of the Miocene dykes consists activity. D veins can be separated in two mainly of an assemblage of sericite, generations on the basis of their secondary quartz, carbonate, chlorite, mineralogy and crosscutting relationships. pyrite and clay minerais. X-ray analyses The first (Dl) small sulfides veins (

31

the mantos. A complete description of the Fig. 1 5: A. Quartz-pyrite-molybdenite vein of the carbonate replacement ore bodies is given Toromocho porphyry. Sample DY-17F. B. Narrow in chapter III. Alteration of limestone of pyrite vein (D 1) crosscut by pyrite-chalcopyrite­ the Pucara Group consists mainly of bomite D2 vein, halo of sericite alteration, secondary Fe-rich carbonates associated to Toromocho porphyry. C. Decimetric polymetallic kaolinite, illite and chlorite (Sallier et al., vein very rich in enargite, Toromocho porphyry. D. 2000). Tuff horizons intercalated in the Reflected light image of late base metals veins Pucara Group are generally silicified but crosscutting the Toromocho porphyry (cp: still show their initial texture. They seem chalcopyrite, enr: enargite, sl: sphalerite, td: to have played a role of impermeable layer tetrahedrite). Sample TOR-7. E. Prograde and have therefore controlled fluid assemblage of diopside and gamet replacing the circulation during mineralisation dolomite of the Pucara Group. Small veinlets rich processes. in galena (ga) brecciate the dolomite and the prograde assemblage. Sample GER-8. F. Massive phlogopite of Zn-Pb skam associate to quartz. Sample GER-3. G and H. Transmitted light image of wall rocks alteration outwards the Zn-Pb-Ag veins in the San Cristobal district (722 vein). The Analytical techniques rim of the biotite is altered in muscovite but the core is still unaltered. On the other hand, plagioclases are entirely recrystallized in an Microprobe analysis aggregation of carbonate and sericite. Sample SC-5. 1. Brecciation of the main sulphide stage, consisting in that case of quartz (qz), sphalerite (si) and galena Electron microprobe analyses were (gn) cemented in that case by late ankerite (ank). performed on carbon coated thin section Sample SC-1. J. Reflected light image of a using 5 spectrometers SX-50 Cameca at concretion showing crystals of pyrite (py) set in a the University of Lausanne. Instrumental chalcopyrite (cp) matrix and concretion texture conditions were set to an accelerating showing spahlerite (sl), galena (gn) and very fined voltage of 15 kV, a bearn current of20 nA, layer of tetrahedrite. Sample SC-11. and a magnification of200'000 K.

Zn-Pb-Ag high-temperature carbonate replacement deposits, usually called Cathodoluminescence (CL) and mantos, are hosted by the basal limestones secondary electron (SE) images of the Pucara Group along the contact with the underlying Mitu Group. The emplacement of the mantos coïncides with Cathodoluminescence and secondary locations where base metal veins crosscut electron images of zircons were the carbonate rocks. These ore bodies are photographed on a CamScan CS 4 generally parallel to the bedding but show scanning electron microscope at ETH in clear features of discordance (chapter 11). Zürich operating at 13 kV. The SEM is Mantos have a mineralogical composition equiped with an ellipsoïdal mirror which is and paragenetic sequence close to that of located close to the sample within the the veins, although with a higher contents vacuum chamber and can be adjusted by of hematite preceding the sulfide stage, electro-motors. The sample can thus be early hematite is commonly replaced by located in one focal point while the second later pyrite, sphalerite and magnetite. focal point lies outside the sample Barite tends also to be more abundant in chamber. The CL-light enters a highly 32 sensitive photo multiplier through a quartz Lead isotope analyses were determined, glass-vacuum window and a light channel. using the methods of leach and residue The signal of the multiplier is then used to fractions described in Chiaradia and produce the CL-picture via a video­ Fontboté (in press). Fractions of the amplifier. The SE-pictures were produced purified lead (residue) were loaded onto a simultaneously with the CL-pictures using MAT-Finnigan 262 mass spectrometer at a different detector. In general, strong CL the University of Geneva. Lead isotope mean high abundance of trace elements. ratios were corrected for fractionation SE images were performed to define the based on more than 100 analyses of the shape of cracks and ho le of the zircons. NBS981 standard. The analytical uncertainties (2cr) are 0.05% for 206PbP04Pb, 0.08% for 207PbP04Pb and 208 04 Who/e rock analysis 0.10% for PbP Pb.

Major and trace elements were analysed by U/Pb age and Hf isotope X-ray fluorescence at the University of determinations Lausanne using a Philips PW 1400. Rare earth elements were analyzed by atomic absorption at the University of Geneva, Investigated rock samp1es consisted of 3 to following the procedure of Voldet (1993). 8 kg of material. Zircon and other heavy The relative 2cr precision of REE analyses minerais were separated after crushing and ranges between 5 to 10% depending on the sieving using a Wilfley table, heavy liquids concentration of the elements. and magnetic separators. Ten to thirty suitable grains (transparent, inclusion and crack-free) were selected from a non­ Who/e rock isotope data magnetic zircon fraction, mostly according to morphological and color criteria, and air abraded to remove marginal zones of lead loss. Groups of 3 to 6 abraded grains were Sr and Nd were extracted from the same washed in warm 4N nitric acid and rinsed sample using dissolution and extraction several times with distilled water and procedures following the method described acetone in an ultrasonic bath. Dissolution by Schilling et al. (1994). The Sr and Nd and chemical extraction of U and Pb were isotope measurements were undertaken in performed following the procedure of a dynamic mode at the University of Krogh (1973), using miniaturized bombs Geneva on a seven-collector Finnigan and anion exchange columns. Spiking with MAT 262 thermal ionization mass mixed 205Pb-235U was done prior to spectrometer using double Re filaments. dissolution. Total procedural blanks were 2 The Sr isotope ratios are normalized to the ± 1 pg Pb. Pb and U were loaded with Si­ 87 86 E&A standard with Sr/ Sr = 0.708000 gel and phosphoric acid on single Re using an average of O. 708028 ± 5 (2SE, filaments and measured at ETH Zürich on n=52) measured during the period of a Finnigan MAT 262 mass spectrometer analyses, whereas Nd isotope ratios are equipped with an ion counting system. The normalized to the La Jolla Nd standard performance of the ion counter was 143 144 ( Nd/ Nd = 0.511830) using an average continuously monitored by measuring a of0.511838 ± 6 (2SE, n=28). NBS 982 standard solution. Regressions were computed using the program of Ludwig (2000). 33

The Hf fraction was isolated usmg an the residual solution of the first Os Eichrom™ Ln-spec resin, and measured in distillation step by solvent extraction. Re static mode on a NuPlasma™ multi­ and Os isotopie compositions and collector ICP-MS using a MCN-6000 concentrations were measured on a Nu microconcentric nebulizer for sample Plasma multicollector ICP-MS at the introduction. Zircons are commonly Institute of Geology of the University of characterized by extremely low 176Lu/177Hf Bem (see Schoenberg et al., 2000). In-run 185 187 ratios ofless than 0.005. Hf isotopie values fractionation correction of the Re/ Re were therefore not corrected for in-situ ratio was performed by measuring the radiogenic ingrowth from 176Lu, because 191 Ir/93 Ir ratio of an Ir standard solution corrections for young zircons are within added to the sample (Schoenberg et al., the analytical uncertainty of the measured 2000). Re was measured in time-resolved 176Hf/177Hf ratios. The Hf isotope ratios mode in order to limit memory effects. were corrected for mass fractionation using a 179Hf/177Hf value of 0.7325 and Besides measuring ali Os isotopes, the 12 normalized to 176Hf/177Hf= of0.282160 of Faraday collector Nu Plasma ICP-MS the JMC-475 standard (Blichert-Toft et al., allows simultaneous monitoring of the 176 177 isotopes of potentially interfering 1997b). Errors of the measured Hf/ Hf 182 186 186 185 ratios are either given as extemal 2a elements: W for W-7 0s, Re for 187Re-71870s, I94Pt for t9oPt-7t9oOs and reproducibility of standard measurements 1nPt~--, t920 s. The 192 s;t880 s rat. 10 was (± 0.3 G units) or individual 2a errors, 0 whatever is larger. Epsilon Hf values were used for in-run fractionation correction of 76 177 calculated with e Hf/ Hf)cHUR(O) the other Os isotope ratios measured. Ali 0.282772 (Blichert-Toft and Albarède, isotopes were measured in Faraday 1997). Mean ages and mean Hf isotopie collectors. Total procedure blanks were < values are given at the 95% confidence 20 pg for Re and < 1 pg for Os. level.

40Arl 9Ar age determinations Re/Os age determinations

Samples from altered ·wall-rocks For Re/Os age determinations, a few containing abundant hydrothermal sericite milligrams of molybdenite and appreciable and from ore skam with abundant amounts of a 185Re spike and an ICP phlogopite were crushed. Different grain standard Os solution (Johnson & Matthey, size fractions were separated by sieving Karlsruhe, Germany) were weighed into and centrifugation. After electrostatic Cari us tubes and wetted with 1Oml of separation, hand picking and ultrasonic inverse aqua regia. Sample dissolution, cleaning, the purity of the samples was oxidation of Re and Os, as weil as controlled by X-ray diffraction. The 20- homogenization was achieved by heating 125flm fraction for sericite and the 125- the closed Carius tubes in an oven at 250f.!m fraction for phlogopite where 225°C for three days. After cracking the packed in copper foil and placed in Cu Carius tubes, Os was directly distilled from disks along with Fish Canyon sanidine the digestion solution into HBr, using the (FCs) as neutron fluence monitor. Samples techique described by Nagler and Frei were irradiated for 18 MWH in the Triga ( 1997), and puri fied, using the reactor at Oregon State University. microdistillation described by Roy-Barman Analyses were made using a low blank, and Allègre (1995). Re was separated from double vacuum resistance fumace and 34 metal extraction line connected to a Mass 1.6). Pb isotope determinations have only Analyzer Products-215.5 mass been performed on the three least altered spectrometer at the University of samples of the Anticona diorite and the Lausanne. Y antac and San Francisco intrusions (Table 1.3). The three residue results are in agreement with data obtained from other Miocene intrusions of the Peruvian Western Cordillera (Gunnesh et al., 1990; Results MacFarlane and Petersen, 1990; Soler, 1991; Fontboté and Bendezu, 2001).

In most intrusions, phenocrysts are Who/e rock and microprobe generally too altered to be analyzed by analyses of Miocene intrusions microprobe (Fig. 1.4). However, sorne samples of the Anticona diorite and the Yantac intrusion only show uniquely a The Miocene intrusions in the Domo de weak propylitic alteration that affects Yauli area are highly altered and elements principally the hornblende and biotite su ch as K, Na, Ca and Sr, easily mobilized phenocrysts and leaves the feldspar by hydrothermal fluids, show large unaffected. The feldspar compositions of variations in concentration when plotted the Anticona diorite range between An52 against Si02. Unaltered or slightly altered and An37, and their K content remains samples indicate a high K calc-alkaline between 1.2 and 2.4 wt %. Biotites of the magmatism with Si02 between 63.82 % same sample have mg-numbers ranging and 66.82 %, Na20 between 2.96 % and from 0.55 to 0.57 with low Al contents 4.31 % and K20 between 2.60 % and 3.23 (Ahot 2.50 - 2.57). Plagioclases of the % {Table 1. 2). At Morococha, crosscutting Yantac intrusion range in composition relationships indicate an increase in silica between An35 and An23 and biotites have content through time with the evolution similar Al contents than those from the from the Anticona quartz diorite to the Anticona diorite but have a slightly higher more felsic Yantac granodiorite. The Sr mg-number between 0.60 and 0.62. and Nd isotopes {Table 1. 3) define two Analyses of plagioclases from both groups, one group contains the intrusion of intrusions reveal perturbed evolution with the Morococha district with lower Sr and several discrete steps in the An content higher Nd isotope ratios compared to the followed by normal zoning (Fig. 1.7). One second group containing the intrusions of of those is associated with a spike of the the San Cristobal district (Fig. 1.6). Ba content (0.024), whereas this element is Strontium isotope ratios of the Chumpe generally below limit of detection for the and San Cristobal intrusions should be rest of the profile (Fig. 1. 7). taken with caution since no samples without alteration features have been found. However, the observed alteration Zircon cathodoluminescence may not have any influence on the (CL) images 143 N d/144Nd raho. and therefore do es not affect the distinct signature between the two groups of intrusion. The isotopie The zircons of Miocene intrusions are composition of both groups fall on the typically 50 to 200 J.lm long, pink, s?s r-enne. h e d and 144 Nd-depleted end of prismatic, euhedral and long except in the the Miocene intrusion trend observed by case of the Anticona diorite where they are Soler ( 1991) and Petford et al. (1996)(Fig. generally shorter. Zircons from the 35

Table 1.2: Representative whole-rock geochemistry of the Miocene intrusion

Anticona diorite San Francisco intrusion Yantac intrusion San Cristobal intr. Chumpe intrusion TIC-1 TIC-10 TOR-10 TOR-13 TOR-14 TIC-6 POR-2 SC-12 SCW2-340 CHU-2 CHU-3 CHU-9

Si02 65.86 63.82 71.32 65.26 66.82 66,00 66.65 60.21 62.46 68.22 67.48 75.56 Ti02 0.54 0.52 0.61 0.51 0.60 0.58 0.56 0.69 0.40 0.43 0.44 0.47 Al203 16.49 15.92 15.67 16.16 16.01 16.22 16.14 15.67 14.13 15.64 15.95 14.34 Fe203 2.39 5.05* 1.24* 4.58* 1.78 1.65 4.03 5.64* 7.60* 1.60 2.34 1.64 FeO 2.39 n.d. n.d. n.d 1.78 1.59 0.35 n.d. n.d 5.05 4.95 0,00 MnO 0.05 0.20 0.01 0.06 0.03 0.02 0.02 0.02 1.10 0.12 0.09 0,00 MgO 1.66 1.60 0.80 1.63 1.68 1.63 1.51 1.49 0.64 1.36 1.15 0.33 CaO 4.04 3.42 0.05 3.87 3.38 2.80 2.75 2.09 0.32 0.12 0.18 0.03 Na20 3.57 3.62 0.17 3.44 2.96 4.31 4.21 1.43 0.20 0,00 0,00 0.03 K20 2.60 2.97 7.71 2.78 3.17 3.23 3.11 5.25 6.68 3.84 4.15 4.37 P205 0.20 0.19 0.07 0.19 0.23 0.22 0.21 0.23 0.13 0.12 0.15 0.05 LOI 0.38 1.18 2.20 1.10 1.70 0.76 0.68 4.80 5.92 3.01 3.39 3.41 Cr203 0.00 0.00 0.00 0.00 0.00 0.00 0 00 0.01 0.00 0.00 0.00 0.00 NiO 0 00 000 0.00 0.00 0.00 0.00 0.00 000 0.00 0.00 0.00 0 00 Somme 100.17 98.48 99.84 99.58 100.13 99.01 100.22 97.53 99.57 99.5 100.28 100.23

Rb 135 113 376 124 116 114 112 265 187 253 279 230 Ba 654 693 430 622 758 899 686 564 170 278 328 561 Th 13 13 16 14 14 13 12 9 <2 3 <2 6 u 4 2 3 6 2 <2 <2 7 5 3 5 4 Nb 9 9 8 10 12 11 11 9 8 5 5 9 Pb 14 18 308 16 21 25 20 61 43 15 17 24 Sr 364 346 94 404 640 712 652 202 32 7 5 26 Zr 111 103 161 113 138 110 121 143 126 140 149 165 Hf 4 5 <1 4 5 5 4 6 <1 3 3 3 y 18 19 <1 20 15 11 12 16 28 3 3 4 Sc 13 11 5 11 11 8 7 10 6 <2 3 4 v 73 81 58 BD 92 81 73 105 32 58 59 57 Cr 14 14 13 15 19 17 19 55 15 10 9 10 Ni 4 4 3 3 5 3 4 5 12 2 <2 <2< Co 109 36 192 58 41 62 48 29 32 34 29 172 Cu 7 6 146 17 57 113 21 189 81 11 7 7 Zn 51 129 53 66 56 57 55 87 111 113 90 17 As 4 7 119 7 12 8 5 28 27 4 16 12 s 65 <3 6243 131 1424 234 171 16077 10659 70 519 7781

La 19.9 25 15 25 28.7 27.1 25 25.6 13.5 21 20.5 19.6 Ce 41.9 38 32 46 58.1 55.6 35 58.2 25.8 42 46.1 44.6 Pr 4.8 6.1 6.4 6.9 3.4 5.5 5.3 Nd 18.1 14 21 22 25.2 25 17 26.7 14 19 2.2.4 21.4 Sm 3.9 4.8 4.6 5.5 3.7 4.5 4.5 Eu 0.98 1.08 1.11 0.9 0.78 1.25 0.98 Gd 3.1 3.2 2.7 2.9 2.7 2.3 2.5 Tb n.d. n.d. n.d. n.d. n.d. n.d. n.d. Dy 2.8 2.4 1.8 2.1 2.8 1.5 1.8 Ho 0.53 0.46 0.36 0.43 0.6 0.31 0.32 Er 1.4 1.2 0.8 1.1 1.6 n.d. n.d. Tm 0.2 0.17 0.15 0.16 0.22 0.11 0.13 Yb 1.2 1 0.8 0.8 1.3 0.6 0.6 Lu 0.17 0.13 0.1 0.09 0.15 0.06 0.07

*total Fe as Fe 3' Major elements as oxydes and in wt%. Trace elements in ppm.

Anticona diorite show an uniform central patterns m the centre but are on the domain (TIC 30, 34 and 37; Fig. 1.8), contrary characterized by continuous sorne with sector zoning (TIC 13; Fig. 1.8), oscillatory zoning (TOR 30, POR 32; Fig. that are surrounded by a thin layer showing 1.8). None of the Anticona, Yantac and oscillatory zoning. Zircons from the San San Francisco zircons photographed show Francisco and Yantac intrusions never embayment or patterns that could be have a pronounced homogeneous CL interpreted as dissolution textures, with 36

Table 1.3: Whole rock isotopie composition

Br Sr/a6Sr 143Nd/44 Nd

TIC-1b Anticona diorite 0.705665 (7) 0.512459 (15) 18.698 15.635 38.682 TOR-14 San Francisco intrusion 0.705979 (8) 0.512487 (8) TOR-14b San Francisco intrusion 0.706070 (13) 0.512468 (14) 18.761 15.645 38.787 TIC- 6 Yantac intrusion 0.705627 (9) 18.739 15.669 38.787 POR-2 Yantac intrusion 0.705764 (7) 0.512510 (6) SC-12 San Cristobal intrusion 0.707453 (14) 0.512350 (20) CHU-3 Chumpe intrusion 0.706573 (9) 0.512310 (40) DY-31 A Mttu detrictic 0.713030 (15) 0.512419 (10) 19.563 15.643 39.071 DY-31C Mttu volcanic 0.713559 (12) 0.512443 (10) 18.961 15.627 39064 DY-31 E Mttu volcanic 0.725776 (11) 0.512460 (6) 19.095 15.678 39.018 DY-31 1 Excelsior Phyllttes 0.755418 (14) 0.512036 (14) 25.241 15.992 45.648 DY-31 L Excelsior Phyllttes 0.740893 (9) 0.512057 (26) 22.317 15.847 41.111 0.5134

OMM 0.5130 Andean mantle •• • Il Hl 0.5124 Miocene intrusions

0.5126 [) -o z .__.,. -o z ";!: 0.5122

0.5118

0.5114 0.70 0.71 0.72 0.73 0.74 0.75

Morococha intrusions San Cristobal intrusions Literature data • Anticona diorite L San Cristobal intrusion 1$ Miocene intrusions, • San Francisco intrusion o Chumpe intrusion Western Cordillera (Soler, 1991) ..a. Yantac intrusion ~'» Miocene intrusions, Eastern Cordillera (Soler, 1991) " Cordillera Blanca (Petford et al., 1996)

Fig. 1 6: 143Nd/144Nd vs 87Sr/86Sr from Domo de Yauli intrusions compared to literature data of Miocene intrusions from Soler (1991) and Petford et al. (1996). Precambrian rock (grey square) is from James (1982), analyses of the Mitu group and the Excelsior2tudy (Table 1. 3). 37

36

32

c <( 28 l re ~0

24

50 J..lm 20

Fig. 1 7: Anorthite content transverse deduced from microprobe analysis of a plagioclase set in a K-feldspar matrix, Yantac intrusion. Note the coïncidence between a major increase of%An and a spike ofBa.

maybe one exception (TIC 37; Fig. 1.8). U/Pb dating and Hf isotopie Additionally, no older cores were detected. composition of zircons Zircons of the Chumpe intrusion indicate similar thin oscillatory zoning than the Y an tac and San Francisco intrusions, but show partly dissolved cores that may be Since zircon may reflect the crystallization interpreted as inherited zircons (CHU 30 age of an intrusion, U-Pb ages have been and 31 ; Fig. 1.8). used to establish the magmatic succession. Relatively low radiogenic/common lead ratios, due to the elevated common lead concentration in the zircons and the very small amount of radiogenic Pb (due to low U and young age), causes large uncertainties on the 207PbP35U ratio. 38

Fig. 1 8: Cathodoluminescence (on the left) and secondary electron (on the right) pictures of zircon from the Anticona diorite (TIC 13, 30, 34 and 37), the San Francisco intrusion (TOR 30), the Yantac intrusion (POR 32) and the Chumpe intrusion (CHU 30 and 31). 39

Therefore, only the 206PbP38U ages are and Yantac intrusions, respectively. The significant. This leads to apparently San Francisco and Yantac intrusion yield concordant analysis even in cases of lead similar weighted mean cHf within error loss or inheritance. Repeated analyses have than the Anticona diorite, -0.1 ± 0.5 and been carried out to reproduce the age and 0.0 ± 1.4 respectively (Fig. 1.1 0). to insure that the 206PbP38U ratios represent However, due to inheritances, their mean the crystallization age. The U/Pb ages values do not have a geological reveal the existence of three distinct significance since low cHf correspond to intrusion groups. inherited zircons and high cHf to concordant points (Fig. 1.1 0). As a consequence for the Y an tac intrusion, the best estimate of the magmatic EHt{T) is The Anticona diorite: TIC given by the multigrain analyses 2 at cHf= 3.1 ± 1.2 (Table 1.4). Three multigrain analyses yield well reproducible concordant points and define a mean 2o6Pb/23s U age of 14.11 ± 0.04 Ma The Chumpe intrusion: CHU for the Anticona diorite (Table 1.4, Fig. U/Pb analyses on zircon from the Chumpe 1.9a). These multigrain analyses yield intrusion yield discordant points defining 176 177 uniform initial Hf/ Hf ratios, with a two discordia lines (Table 1. 4, Fig. 1.9d). weighted mean cHfof0.6 ± 0.2 (Fig. 1.10). The upper intercept ages of these lines are at 57 ± 25 Ma and 216 ± 42Ma. The age of the Chumpe intrusion (6.6 + 11-3.6 Ma) is The San Francisco and the Yantac defined by the lower intercept; the + 1 Ma intrusions: TOR and POR errors is also constrained by the youngest concordant point at 7.6 Ma, which is Analyses of zircons from the San considered as a maximum age and the -3.6 Francisco and the Y antac intrusions show Ma error is defined by the intercept variable ages and degrees of discordance uncertainty. On the contrary to the San (Table 1. 4, Figs. 1.9b and c). They define Francisco and the Y an tac intrusion, lines with upper intercept showing multiple inheritance does apparently not influence inherited components of different ages. 177 the initial 176Hf/ Hf of zircons (Fig. 1.1 0). Two zircon analyses from the San The weighted mean cHf is -0.1 ± 0.5 (Fig. Francisco intrusion show a strong 1.1 0). discordance with upper intercepts at 777 ± 170 Ma and 1706 ± 150 Ma, which indicates the presence of old cores. Re/Os dating of molybdenite Analyses on zircons from the Y an tac intrusion indicate an inheritance of ca. 240 Ma which may correspond to the Permian Two molybdenite samples from the quartz­ volcanism. The majority of analyses are molybdenite veins of the Toromocho Cu­ situated along the concordia between 9 and porphyry were dated by the Re/Os method. 187 14 Ma, nonetheless the large 207PbP35U Re and Os concentrations and the Re­ uncertainties and high-angle interceptions Os ages are shown in Table 1.5. Analyses with mixing lines from young inherited on these two samples overlap and give a components, render the "quality of weighted mean value of 7.9 ± 0.1 Ma. concordance" statement very difficult. The analytically concordant results of 9.1 ± 0.1 Ma and 8.81 ± 0.06 Ma represent therefore the maximum ages for the San Francisco 40

.0025 A

Anticona Diorite Concordant Mean Age= .0020 14.11 ±,0.04 Ma

ro:J 12 "' "'..0 a_--

0.00216 ...__ ___.______,

0.012 0.014 0.016

.0010 .008 .010 .012 .016 .018 .020

.0025 ~B

.0020

San Francisco intrusion .0015 Concordant Age= 9.11 ±. 0.10 Ma lntersects at: 8.94 ±. 0.38 Ma & 1706 ±. 150 Ma 8.7 ±. 1.2 Ma & 777 ±. 170 Ma

.0010 .008 .010 .012 .014 .016 .018 .020 207Pbt235u

.006 c Yantac intrusion Concordant Mean Age= 8.81 .±. 0.06 Ma 30 lntersects at: 5:!:,11 Ma &240:!:,150Ma :J .004 "" "'<::L...... 0 a_ 8 0.00136 "' .002

0.00132 0.006 0.008 0.010 0.012

0

0 .01 .02 .03 .04 .OS 2a1PbF35U 41

0.006 ./~ ~-·~~ -·· Chumpe intrusion .-" lntersects at: " · · · . · · 6 (+4.7/-12) Ma & 57 (+30/-25) Ma / .<- · .· 0.004 6.6 :!:,3.6 Ma & 265 ±57 Ma • .(.-: .·· ::::> "' /~:. · · · "'<::!..... 20 .,.:1·:>0.00130 .. ..0 ,--:: .· a.. /~.,.... 0 "'N A ..~ ••~.: --··' 0.002 ,. :· 0.00122 10 , /(•' .... 7•.. 7-6' ,r 0.00114 ,__.:..._/ ______J -'/ 0.0074 0.0082 0.009 ..:r .., • .:/ 0 .::y 0 0.01 0.02 0.03 0.04 207Pbf35u

Fig. 1 ~: U/~b Concordia diagram of zircons from the Anticona diorite (A), the Toromocho porphyry (B), Yantac mtrus10n (C) and the Chumpe intrusion (D).

40Arf3 9Ar ages of phlogopite and 16b, Fig. 1.11a). The age of 6.42 ± 0.19 sericite Ma obtained on the phlogopite sample in the Gertrudis skarn is a minimum age since it does not fulfill requirement for a plateau 40Ar /39 Ar spectra from phlogoplte. samp1ed (Fig. 1.11 b ). in the Morococha ore skarns indicate an increasing apparent age during the low Sericite samples from wall rock alteration of the San Cristo bal and the Andaychagua temperature steps which stabilize more or 40 39 1ess quick1y (Figs. 1.11 a and b ). Phlogopite veins indicate more complex Ar/ Ar spectra (Figs. 1.11 c and d). As for the grains are pristine and homogeneous under . 40 /39 the microscope and have uniform major p hl ogoplte Ar Ar spectra, the apparent and minor element concentrations (Table age of sericite samples increase with 1.6). No chlorite nor any mixed phases increasing temperature (Figs. 1.11 c and d). have been detected by microprobe (Table An intermediate plateau can be observed 1.6) and X-ray diffraction. Nevertheless, before the apparent age decreases slightly; one microprobe traverse (Fig. 1.12) on an important increase of the apparent age phlogopite from sample DY-16b exhibits occurs at the end of the gas release. The "hump-shaped" spectra of sample SC-5 higher F values and lower K20 value towards grain edges. This variation of F (Fig. 1.11 d) and less prominent in sample and K20 could indicate crystal overgrowth 810-lm (Fig. l.llc) is probably due to or diffusion pro cesses (Phillips, 1991) by recoil effects (Dong et al., 1995). Indeed, if thermal resetting or fluid alteration. Due to the smallest fraction (<20)-lm) was the homogeneous habits of crystal in thin removed during mineral separation, very section, diffusion processes are a preferred thin interlayered clay minerais can be mterpretatwn.. . An 40 Ar/ 39 Ar plateau age of observed in the sericite crystal lattice. The 7.2 ± 0.2 Ma for 80% of the released Ar steep increase of apparent ages at the end can be defined for the Parvenir skarn (DY- of the gas release is probably due to the presence of relie K-bearing phases of the .j:::.. Table 1.4: Analytical results of U-Pb age determinations. N

N urn , De$ription " Weight Na of Conœnl"i ions Ttv U ' A lamie ratios A ppa·ent '!J65 [ M ~ error Hf isotopes [mg] gn;ins U Al ra:l . Al n ra:l , 2061204 '2061235 a. Error 2071235 • Error 21)71206 u En-or 206123& '207/ 235207/ 206' corr. 1761177 epsHf epsHf ± 2sT(CM) [ppm] [ppm] [pg] 2s[%] 2s[%] 2s[%] norm. (0) (T) (Ga)

A ni ica na diorite, TIC pr incl 0 .0264 5 198 0.44 1.84 0.9 411 0.00219 0.44 0.01388 1.25 0 .04589 1.12 14.13 14,00 - 0.46 0 .28278 0.2 0.5 0 .5 0.99 2 Ige frags 0 .0362 356 0 ,8 11.34 0.19 176 0.00219 0.41 0.01396 2.28 0 04632 2 .21 14.07 14 08 14.28 0.26 0.28278 0.4 0.7 0 .5 0 .98 3 pr ind 0 0115 5 915.7 1,96 44.31 0.1 52 0 0022 1.1 0.01409 10.28 0 .04649 10.22 14.15 14.21 23.17 0.11 4 pr ind 0.28278 0.1 0 .5 0.5 0 .99

Sen Frmci~o intruSon, TOR

2.1 "''ta a~b 0 .0076 6 358 0.53 1.58 0.07 171 0 .00141 0.72 0.00916 5.46 0.04694 5.22 9.11 9.25 46.21 0.39 0 28275 -0.9 -0.7 0 .5 1.06 2,2 pr 0.013 5 517 1,07 7.97 0,01 128 0 ,00206 0 56 0.01855 3,28 0 .06519 3 .11 13.29 18.67 780.63 0.38 0.28274 -1,0 -0.8 1 .8 1 .06 16,1 pr 0.28279 0.6 0 .7 1.1 0 ,97 16.2 lpr la prisn 0.0192 5 405 0.85 2 ,14 0.05 491 0 .00207 0. 6 001359 2.17 0 .04767 2 .02 13.32 13 .71 82.73 0.38 0 ,28279 0.8 1,0 2.8 0 ,96 16.3 pr 0.0156 5 630 2.54 27 .6 0.03 112 0.00413 0.59 0.02916 3.75 0 .05117 3 .66 26.59 29.19 248.6 0.23 0.28275 -0.9 -0.7 0 .5 1 .05 16.4 lpr 0.0229 3 470 1 01 15.46 01 112 0 .00214 0.55 0.01378 3,61 0 .04665 3.6 13.8 13.9 31.41 0,09 0 .28278 0 .1 0.3 0 .5 0.99 16 5 pr 0 0118 4 452 0 89 2.24 0.34 311 0 .00196 0.53 0.01242 2.94 0 .04606 2.42 12.6 12.53 0.98 0 28277 0,0 0.2 0 .5 1,00 16 6 lgepr 0 0382 4 423 0 74 11.2 0.09 177 0 ,00176 0.44 0.01125 231 0 .04646 2.24 1132 11 .36 21.42 0.25 028278 0 .1 0 .3 0 .5 0 .99

Yanta::: intruSon, FOR 1 lprind 0 .0032 4 919 4.98 18.82 0.05 76 0.0056 0.73 0.03802 5.69 0 .04923 5.64 36.01 37.89 158.59 0.13 0 .28274 -1.1 -0.9 0 .5 1 .07 2 snalllpr 0 .0029 186 0.27 1.2 0.07 57 0.00136 1.86 0.00887 22.86 0.04718 21.94 8.79 8.97 58.25 0.52 0 .28285 2.9 3 .1 1.2 0 .83 3 lprind 0.0041 5 637 1,04 1,65 0.15 176 0.0016 0.76 0.01016 5.62 0 .04617 5 .36 10.28 10.26 6.3 0.4 0.28281 1.4 1.6 0 .5 0 .92 4 lpr 0 .0062 854 2.07 0.75 0.02 1134 0 ,00251 1 09 0.01692 1.1 0.04895 1 .02 16.144 17.03 145.3 0.57 0.28276 -0.6 -0.4 0.5 1 .04 5 lpr 0 0055 6 2748 5.28 2.8 0.02 84 0 .00192 1.02 0.01312 9 .2 0 .04955 8 .95 12.37 13.23 173.7 0.3 0.28275 -0.7 -0.5 0.5 1 .04 6 snalllpr 0 0018 3 390 0.81 0.6 0.02 167 0 .00203 0.69 001494 5.4 0 .05331 5 .12 13.09 15.06 341.96 0.5 7 snalllpr 0 .0086 4 278 0.38 2.69 0.08 94 0.00137 0,76 0,00872 5.74 0.04626 5.6 8.8 8.81 11.24 0.25 lpr ind 0 .0045 5 387 1.64 0.41 0.03 1157 0.00428 0.44 0.03072 0.82 0 .05201 0 .71 27.55 30.55 285.79 0.5

Chumpe intruSon, CHU lpr euh 0.0102 6 1545 3.78 5.62 0.07 465 0.00253 0,34 0.01633 1.01 0,0439 0 .94 16.26 16.45 44.01 0.37 0.28277 -0.1 0,0 0 .5 1 .01 0.0099 5 1053 3.03 1.43 0.1 1359 0 .00291 0.39 0.01883 0.76 0.0469 0 .63 18.75 18.95 44.38 0.56 0 28275 -0.7 -0,5 0 .5 1 ,04 3 frags"'' of lpr 0.012 687 0.8 1.14 0.05 549 0 ,00118 0.5 0.00755 1.84 0.04655 1.72 7.58 7.63 26.07 0.37 0 28277 -0.2 -0.1 0.5 1.02 4 lpr euh 0 .0186 4 1019 2.67 6.9 0.03 481 0 .00252 0.44 0.01704 1.15 0.04901 1.03 16.23 17.15 148.4 0.45 0.28277 0 .1 0 .2 0 .5 1,00 5 lprlr 0 0069 6 883 1.14 0.65 0.02 768 0 ,00128 0 92 0 00849 1.87 0.04823 1 .75 8.23 8.59 110.35 0.37 6 lp1 ind 0,0154 4 861 5.14 1,0 0,09 4566 0.0056 0.42 0.03914 0,51 0.05065 0 .33 36.03 38 99 225,00 0.76

a) lpr long priSTlatic; euh euhedra; frags fragments; spr S'lort priSTlatic; Ige large; pr priSTl; incl indus on; wb SJbround; tr transparent b) Calcul

ANTICONA SAN FRANCISCO YANTAC CHUMPE DIORITE INTRUSION INTRUSION INTRUSION protoliths overprinted by sericite alteration. 14.11 ±0.04 Ma 9.10 ~ 0.10 Ma 8.81 ~ 0.06 Ma -6 Ma If the volcanic rocks of the Mi tu Group are 4 generally entirely altered into an ,0. assemblage of sericite and quartz, sorne 3 .a.. discrete relicts of the former rocks have 3 1. been detected (Figs. 1.5g and h). The dtsturbed. appearance of the 40 Ar/39Ar .a. spectra does not allow determination of 4 plateau ages, nevertheless flat portions of 2.~ o.. the spectra indicate ages of 4. 78 ± 0.16 Ma 6 (65% gas release) and 4.90 ± 0.15 Ma ~ (25% gas release) that agree with .1. integrated ages (Figs. 1.11 c and d). 0 .8

0 4

I - 0 t (/) ~~~ -- o. Mun""0.6.±02 (l) .Q.4

.Q. 8 ~~ Discussion ~ ~ ~ 12.. ~ Mean =·0,1.;t.05

1.6 Mai!n 2 0 1 ± 1.-'

Two mining districts of different 20 ages in the Domo de Yauli area 24...

Mean =-0.1j;O 5 2.8 !),0

YANTAC This study allows distinction of at least 4.0 INTRUSION three Miocene rnagrnatic pulses in the 9.81 ±. 0.06 Ma Domo de Y auli area. The structurally 3.0 o 1dest Antlcona. d.wnte . y1e . ld s a 206Pb/23su - 2.0 I mean age of 14.11 ± 0.04 Ma. This early ~B li 1.0. rnagrnatic stage is followed by several (l) 0.0.. more felsic stocks (San Francisco and 8 8 Y antac). The age of the later· intrusions is -1 o_ 8 poorly constrained because of the multiple -2.n 1 1 inheritances and the apparent analytical 0 10 20 30 40 Age '"'Pbf3"U (Ma) concordance of U/Pb analyses. Nevertheless the few possible concordant Fig. 1 10: Compilation of EHr values of zircons from points associated to the lower intercepts the dated Miocene intrusions. allow determination of maximum U/Pb ages around 9 Ma (Fig. 1.9), whereas the and Yantac (Table 1) and since ore-bearing minimum age of the San Francisco veins of the San Cristobal area crosscut intrusion is on the other hand defined by the Churnpe intrusion, the ore deposits of the Re/Os age of rnolybdenite et 7.9 ± 0.1 both rnining districts are therefore not of Ma. Analyses of zircons from the southem the same age. This rneans that ore deposit Churnpe intrusion result in a lower from Morococha and San Cristobal belong intercept age of 6.6 (+ 1/-3.6) Ma (Fig. to different hydrothermal systems. This 1.9). Since major ore deposits from the difference in age between the two rnining Morococha area are structurally related to districts is also supported by 40 Ar/39 Ar the intrusions intrusions of San Francisco 44

Table 1.5: Re/Os data for molybdenites from quartz evins of the Toromocho porphyry

N échantillon weight(mg) Re (ppm) 187 Os (ppb) 187 Os*/ 187 Re Age (Ma) 2s (Ma)

TOR-15 151 .8 139.9 11.34 5 .07E-05 7.77 0.11 DY-17 F 381 .7 156.2 12.98 5.19E-05 7.97 0.11

data, i.e. 7.2 ± 0.2 Ma for the Porvenir comparison of duration generally admitted skarn and ages of about 4.78 Ma in veins since numerical modeling and of the San Cristo bal district (Fig. 1.11 ). geochronological studies point out that The difference in ages could therefore be hydrothermal cells are not expected to last one cause of sorne differences in for more than a couple of 1OO'OOOs years mineralisation types, such as presence of (Cathles et al., 1997; Ballard et al., 2001). an earl y W stage and absence of porphyry This can be explained by overestimating mineralisation and economie skarn in the the magmatic age, as explained in the southern San Cristobal area. description of the results section (see above). Nonetheless, the formation of the Porvenir skam is somewhat younger than Crosscutting relationships indicate that the the Toromocho porphyry. The Morococha Anticona diorite is the oldest Miocene mining district is not restricted to these two intrusion in the Domo de Y auli district and intrusion-ore bodies systems, sorne is not related to any type of mineralisation, intrusions and related ore bodies have not except formation of hornfels and small been dated largely due to their intense magnetite bodies in the host limestone. alteration, the San Pablo-Victoria system This absence of ore-bearing skam and (Table 1. 1), or the small scale of the associated hydrous mineral alteration outcrops, the Gertrudis intrusion. These suggests the absence of large fluid multiple successive events may also circulation during the emplacement of the explain the typical diffusion profile of the diorite body at 14 Ma. On the contrary, Y antac and Gertrudis phlogopites emplacement of the felsic intrusions of San interpreted as Ar loss (Fig. 1.11). The Francisco and Y an tac are accompanied by felsic magrÙa bodies of the Morococha abundant hydrothermal fluid circulation mining district were then emplaced in revealed by considerable alteration around temporally distinct pulses, driving the different ore bodies. Our data point out successive ore-forming hydrothermal successive magmato-hydrothermal events systems. Such pulsed, temporally and including a porphyry style system, spatially linked intrusive centers and constrained by the 9.1 MaU/Pb maximum hydrothermal systems have already been age of zircon and the 7.9 ± 0.1 Ma Re/Os demonstrated in porphyry-style deposit age of molybdenite, and a skarn deposit, districts, such as the Potrerillos district bracketed by the 8.8 Ma U/Pb maximum (Marsh et al., 1997), the Indio Muerto age of the intrusion and the 7.2 ± 0.2 Ma district (Gustafson et al. 2001) and the 40 ArP9Ar plateau age obtained on Chuquicamata mine (Ballard et al., 2001 ). phlogopite. The U/Pb ages very likely It suggests that overprinting hydrothermal reflect magmatic activity, and Re-Os and events is, at least in Morococha, an 40 ArP9Ar ages reflect age of the associated important condition to produce a mining mineralising event. The differences district with large abundance of economie between the magmatic and the mineralising ore bodies. Even if Morococha do not look ages are definitively too long m like these giant porphyry-style deposit by 45 the small s1ze of Toromocho and the deposits in the limestones of the Pucani abundance of Zn-Pb ores, its economie Group. 40 Ar/39 Ar age determinations on attractiveness has also be increased by the sericite from wall-rock alteration indicate pulsed magmo-hydrothermal activity. an age of 4.78 ± 0.16 Ma and 4.90 ± 0.15 Ma (Fig. 1.11) which overlap within errors with the U/Pb age of the Chumpe intrusion. However, caution on the relation To our knowledge, the younger San between the Chumpe intrusion and the Cristobal system is to the contrary to mineralisation has to taken since the large Morococha an example where overprinting error obtained on the U/Pb age of the magmato-hydrothermal did not occur and Chumpe intrusion do not permit to where ali mineralisation are linked to the decipher between dyke of different ages. It same Chumpe intrusion. Indeed, the could therefore be that the so called Chumpe intrusion (6.6 + 1/-3 Ma) is at the Chumpe intrusion is in fact a mix of felsic origin of the hydrothermal event that dykes close in age and that the magmatic created veins and by inference high­ was also pulsed. temperature carbonate replacement

10~------, 10~------, A B 7.2 2:. 0.2 8 .. , 8 co l"' co 6.42 2:.0.19 :2 ~ l" "'1 &6 ...... Cil c c ~ 4 ~ 4 Cil co o. c.. o. c.. <( <( DY-16b GER-3

lntegrated Age= 6.9 :t 0.2 lntegrated Age= 5.78 ~ 0.17 0 0 0 20 40 60 80 100 0 20 40 60 80 100 Cumulative %39Ar released Cumulative %39Ar released 10 10 c D 8 Cil co ~ ~ Cl.> 4.78 2:. 0.16 4.90 + 0.15 0> &6 4 co 1 - "'1 .....co ..... c c ~ ~ 4 o.Cil o.Cil o. o. <( <( 2 810-1 m SC-5

lntegrated Age= 4.87 ~ 0.15 lntegrated Age= 4.87 ~ 0.15 0 20 40 80 100 20 40 60 80 100 Cumulative %39Ar released Cumulative %39Ar released

Fig. 1 11: 40Ar/39Ar spectra from phlogopites from the Yantac (DY-16b) and the Gertrudis (GER-3) skams and sericites fi:om vein alteration of the San Cristobal vein (SC-5) and the Andaychagua vein (810-lm). 46

F (wt%) probably in the Y erington district, where 2.0 Tertiary tectonics and subsequent erosion allowed exposition of the vertical 2 dimension to more than 6 km paleodepth 1.5 2 2 (Dilles and Profett, 1995). To avoid rapid ~ ~ ! solidification, such magma chambers are 1.0 2 ~ dependent of their sustaining by injection ! ofnew magma (Marsh et al., 1997). These injections are docurnented in all intrusion 0.5 type by fine-grained dark breakdown rirns consisting of predominant oxides and accessory pyroxene occurring around 0.0 1 amphibole and in sorne cases biotite (Fig. 0 100 200 300 1.4a), but also by rnicroprobe traverses of distance (~--tm) plagioclases revealing perturbed evolutions with several discontinuities in the An content followed by normal zoning (Fig. 1.7). If these two petrological 12.0-r----=:------, characteristics may also be produced by other processes such as magma degassing, the association of the increase of the An content with a high Ba spike (Fig 1. 7) 11 .0 argues strongly for at least one injection of new magma during the crystallization of the plagioclase analyzed. The presence of an active parental magma charnber lasting a few 100 Ka, during the intrusion of San 1 o. o+-----..--r------r,-~--.-"""'iÏr=-....,.--' Francisco and Y antac stocks, is then more 0 100 200 300 than probable. distance (!l-m)

Fig. 1 12; : Variation of K20 and F content along traverses in phlogopite microprobe analysis, open U/Pb analyses of zircon from the San square traverse bl2, black square traverse b4. Francisco and the Y an tac intrusion indicate nurnerous analytically concordant points between 9 and 14 Ma (Figs. 1.9b and c), Do multiple magmato­ which may be interpreted as zircons hydrothermal events reveal the inherited from the Anticona diorite body or presence of a large underlying continuously cristallising in a long-living magmatic chamber? rnagmatic system. Therefore, the question rernains whether the source of the Anticona diorite is the sarne magma charnber as for the later felsic intrusions or Multiple felsic intrusions on the eastern not. It would suggest a long living magma flank of the Anticona diorite (Fig. 1.2), charnber of over 5 My, akin to the Indio each with related hydrothermal alteration Muerto district where Gustafson et al. and polyrnetallic ore bodies (Table 1), are (200 1) proposed a magma chamber lasting best explained as multiple cupolas rising 3 My. In contrast to zircon of the Churnpe above a parental magma charnber. The best intrusion, catholurninescence images do analogy of such a composite system is not reveal any dissolution textures or cores 47 in the zircons of the northem intrusions 207Pbl04 Pb (Fig. 1.8). This indicate first that, in case of 16.0 Anticona diorite partial melting, its zircons have not been dissolved. However the 15.9 Anticona diorite zircons have a distinctive 15.8 feature in their uniform center (Fig. 1.8), o! tn,~ which does not appear anymore in zircon 15.7 from the Yantac and San Francisco intrusions (Fig. 1.8). Therefore, the 15.6 interpretation of massive zircon inheritance from the Anticona diorite can be rejected. 15.5 16 18 20 22 24 26 206Pbl04 Pb The absence of dissolution textures in the 20sPb/'04p b San Francisco and Yantac zircons indicate 42 also that zircons had no contact with a magma undersaturated in Zr between 14 41 ph ji ,<2,z;. and 9 Ma ago. Due to the chemical composition of the Domo de Yauli intrusions (Table 2), and especially their 40 low Zr concentration, the temperature of zircon saturation remained below 800°C 39 (Watson and Harrison, 1983). Such low temperatures of magma imply a very rapid 38 cooling. To keep a magma chamber above 16 18 20 22 24 26 the solidus temperature but below 800°C, 216 P b/'04 Pb continuous injection of new magma is necessary, which is probably not reasonable. The hypothesis of a long-living Fig. 1 13: 207PbP04Pb vs 206PbP04Pb and 207PbP04Pb magma chamber lasting from the Anticona vs 206PbP04Pb plot summarizing Pb data of Peru. diorite to the Y an tac events is therefore Precambrian rocks and Quatemary volcanic rocks doubtful. The multiple concordant ages from the Arequipa Massif data are from Tilton and between 9 and 14 Ma (Figs. 1.8b and c) is Barreiro (1980), Coastal Batholith data are from then best exp1ained by inheritance from Mukasa (1986b), Miocene data are from Gunnesh successive magmatic bodies partially et al. (1990), MacFarlane and Petersen (1990), molten by temporally distinct pulses, one Soler (1991) and Fontboté and Bendezu (2001). of them resulting in the formation of the Data from the Mitu Group, Excelsior phyllites and magma chamber and its related San the Domo de Y auli intrusions (black square) are Francisco and Y an tac cupolas. The from this study (Table 1. 3). Orogene and Upper resulting magma has never attained a Crus t curves are from Zartman and Doe (19 81). temperature above 800°C, suggesting incoming of an intermediate to acidic component. 48

Are Precambrian rocks involved Neodymium and strontium isotope data in the generation of Miocene from Domo de Y auli intrusive rocks lie magmatism? along the trend defined by analyses from Soler (1991) and Petford et al. (1996) for other Miocene intrusions of the Western Cordillera (Fig. 1.6). These authors have The cathodoluminescence (CL) images of explained this trend either by modification the Chumpe zircon indicate presence of of mantle derived magmas by AFC-MASH inherited cores (Fig. 1.8), that must processes or heterogeneities from the probably linked to the U/Pb analyses subcontinental lithospheric mantle. indicating zircons with inherited However, mixing trend defined between components of different age range from 57 the depleted mantle (DMM), or the to 265 Ma (Fig. 1.9d and Table 1.4). "Andean mantle" pole as defined by However, U/Pb analyses of the Chumpe Lucassen et al. (2002) on one hand, and intrusion are not the only ones analyses of "South American Precambrian demonstrating presence of inherited cores crust" (James, 1982) on the other hand, since sorne analyses of the San Francisco plot close to our analyses (Fig. 1.13). A and Y an tac intrusions also reveal mixing trend between mantle and granulite discordias with upper intersect ages up to rocks as described by DePaolo et al. (1982) 1706 Ma (Figs. 1.9b and c). These cores would even fit closer with the analyses. were not detected by the The three Pb isotopie analyses of Miocene cathodolumiscence, it must be due to the magmatic rocks plot closely together into small number of CL images (Fig. 1.8) in the field of "Miocene intrusions" (Soler, comparison to the number of analysed 1991; Peford et al., 1996). Therefore, they zircons (Table 1A). Paleozoic ages can be do not indicate any influence of explained by zircon incorporated from the Precambrian rocks in the generation of surrounding rocks, as evidenced by Domo de Yauli magmatism (Fig. 1.13), foliated xenoliths in the Anticona diorite which can be due to the very low (Fig. 1.4b ), probably originating from the concentration of Pb in Precambrian rocks metamorphosed detrital sequence of the (15 ppm; James, 1982) in comparison of Paleozoic Excelsior Group. However a the upper crust rocks. On the other hand, Precambrian inheritance remains more an influence of the highly radiogenic enigmatic. Indeed, a Precambrian phyllites from the Excelsior Group can be basement has never been documented in excluded in view of the isotope data shown the Western Cordillera of Central Peru and in Figure 1.13. Precambrian rocks the closest Precambrian basement known influence is moreover confirmed by initial to date is located in the Eastern Cordillera Hf isotopie compositions of the dated (Dalmayrac et al., 1980). However, the zircons, which scatter around an ~::Hf of upper intercept of discordia lines and zero (Fig. 1.1 0); they thus suggest a hybrid decreasing initial 176Hf/177Hf ratios with melt source of mantle and crust increasing 206PbP38U ages (Fig. 1.1 0) argue components, which remained virtually strongly in favor of a Precambrian origin identical throughout 8 million years of for sorne zircon cores. Old cores can be magmatism. Despite high-~::Hr melt inputs, explained in two ways, either rocks of the contribution of partial melts from Precambrian age exist underneath the the underlying Proterozoic basement in the Domo de Y auli or the cores represent generation of the Miocene magmatism of detrital zircons derived from the Excelsior the Domo de Y auli is documented by Group. relatively uniform Hf depleted mantle modelages of around 1.0 Ga (Table 1. 4). 49

1140 ~ '" Ma (U/Pb)

525 '' Ma,16Q4 •60:H Ma :~~ (U/Pb inter.) ------'~ ... . 395 '' ' ·'• Ma, 1147 •., _.,. Ma (U/Pb ••• •• 1198 ·•.-• Ma, ca. 1900 Ma (U/Pb into•r""'r"""

1918'" Ma (Rb/Sr iso.) 720 ' ~ Ma , 1910 , ,. Ma (U/Pb inte 970 ~ 23 Ma, ca. 1900 Ma (U/Pb 152' ' Ma, 1697 "''"·"r Ma (U!Pb inter.)

Fig. 1 14: Map of Peru showing Precambrian outcrops of the Eastern Cordillera and the Arequipa Massif, the Coastal batholith and Cenozoic intrusions, modified after Pitcher and Cobbing (1985). Precambrian age data from Shackleton et al. (1979), Dalmayrac et al. (1980), Mukasa (1986a), Mukasa and Henry (1990) and Wasteneys et al. (1995). inter: intercept, iso: isochrone.

Conclusions intermediate compositiOn and relatively low temperature ( <8ûû0 C) as indicated by the absence of dissolution textures on zircon. Petrological features, such as The intrusion of the Domo de Yauli area reaction rims or plagioclase zonations, and their related ore deposits have different point out the presence of a magmatic ages indicating successive short-lived chamber sustained by repeated injections magmato-hydrothermal systems. Those of new magma which may have generated pulses are the results of a continuous the successive mineralising felsic cupolas. magmatic activity, characterized by We therefore conclude that the particular successive injections of new magma of 50 high abundance of economie orebodies at Morococha is the result of overprinting Acknowledgments hydrothermal systems of different ages. As multiple centers of intrusion and mineralising fluid have already been We gratefully acknowledge the Volcan demonstrated in other porphyry-style Compania Minera which provided access deposits (Marsh et al., 1997; Ballard et al., to underground exposures and support 200 1; Gustafson et al., 2001 ), it seems to during fieldwork. We thank also Les be a important condition to form districts Oldham and the Anglo-Peruana company with high abundance of economie ore for advice, criticism and help during bodies. fieldwork. The help of I. Ivanov during separation of zircon, A. von Quadt for U/Pb analyses and Sr, Nd and Hf isotopes TIMS maintenance, M. Frank for Hf evidence that old inherited zircon cores analyses and D. Gebauer for zircon have their origin in Precambrian rocks cathodoluminescence are kindly likely present underneath the Domo de acknowledged. This study was supported Yauli area. The question remains if this by the Swiss National Science Foundation underlying basement is part of the (Grant n°2000-062000.00). Arequipa or the Amazonian craton. Extensive regions of old basement are exposed immediately south of the A ban cay deflection but do not crop out to the north (Fig. 1.14). However, an extremely thin References layer of Precambrian crust is indicated beneath the Lima segment by crustal models (Couch et al., 1981; Jones, 1981 ). Atherton, M.P. and Sanderson, L.M. (1987): Our analyses indicate inherited The Cordillera Blanca Batholith: a study of components of up to 1.7Ga, such old ages granite intrusion and the relation of crustal thickening to peraluminosity. Geolog. being characteristic of the Arequipa Massif Rund., vol. 76, p. 213-232. (Fig. 1.14). Precambrian rocks of outcrops Ballard, J.R. , Palin, J.M., Williams, I.S., from the Eastern Cordillera indicate much Campbell, I.H. & Faunes, I.H. (200 1): Two younger ages (Fig. 1.14). In conclusion, ages of porphyry intrusion resolved for the zircon cores observed in the Domo de super-giant Chuquicamata copper deposit of Yauli intrusions may be derived from an northern Chile by ELA-ICP-MS and Arequipa-type Precambrian basement. SHRIMP. Geology, vol. 29, p. 383-386. Since no Precambrian inheritance was Bartlett, M.W. (1984): Petrology and genesis discovered more northerly in the Cordillera of carbonate-hosted lead-zinc-silver ores, Blanca or more westerly in the Coastal San Cristobal district, Department of Junin, Peru. Unpublished Ph.D. thesis, Oregon Batholith (Mukasa, 1986b; Polliand, 2002), State University, Corvallis, Oregon, U.S.A., the Domo de Y au li location may defi ne the 272 p. northern limit of the Arequipa Massif as Beuchat, S., Moritz, R., Sartori, M., Chiaradia, proposed by Haeberlin (2002). M. and Schaltegger, U. (2001): High precision geochronology and structural constraints on the Zn-Pb-Ag-Cu Domo de Yauli district, Central Peru. Extended abstract, 6th biennial meeting of the SGA, Krakow, Poland, 26-29 August 2001. Blichert-Toft, J. and Albarède, F. (1997): The Lu-Hf isotope geochemistry of chondrites and the evolution of the mantle-crust 51

system. Earth and Planetary Science Letters, sulfidation system. Proexplo 2001, Lima, vol. 148, p. 243-258. Peru, April2001, CD-ROM, doc. 18 p. Campbell, A., Rye, D. and Petersen, U. (1984): Fournier, R.O. (1999): Hydrothermal processes A hydrogen and oxygen isotope study of the related to movement of fluid from plastic San Cristobal mine, Peru: Implications of into brittle rock in the magmatic-epithermal the role of water to rock ratio for the genesis environment. Econ. Geol., vol. 94, p. 1193- ofwolframite deposits. Econ. Geol., vol. 79, 1211. p. 1818-1832. Frei, R., Nagler, T.N. and Meise!, T. (1996): Campbell, A.R. and Robinson-Cook, S. Efficient N-TIMS rhenium isotope (1987): Infrared fluid inclusion measurements on outgassed tantalum microthermometry on coexisting wolframite filaments: very low filaments blanks and quartz. Econ. Geol., vol. 82, p. 1640- determined by a 'standard addition' 1645. approach. Int. Journal Mass. Spectrom., vol. Cathles, L.M., Erendi, A.H.J. and Barrie, T. 153, p. 7-10. (1997): How long can a hydrothermal Gunnesh, K.A., Baumann, A. and Gunnesh, M. system be sustained by a single intrusive (1990): Lead Isotope variations across the event? Econ. Geol., vol 92, p. 7 66-771 Central Peruvian Andes. Econ. Geol., vol. Chiaradia, M. and Fontboté, L. (in press): 85, p. 1384-1401. Separate lead isotope analyses of leachate Gustafson, L.B. and Hunt, J.P. (1975): The and residue rock fractions: implications for porphyry copper deposit at El Salvador, metal source tracing in ore deposit studies. Chile. Econ. Geol., vol. 70, p. 857-912. Mineralium Deposita. Gustafson, L.B., Orquera, W., McWilliams, Dalheimer, M. (1990): The Zn-Pb-Ag deposits M., Castro, M., Olivares, 0., Rojas, G., Huaripampa and Carahuacra in the mining Maluenda, J. and Mendez, M. (2001): district of San Cristo bal, Central Peru. In: L. Multiple centers of mineralisation in the Fontboté et al.(eds.), Stratabound Ore Indio Muerto district, El Salvador, Chile. Deposits in the Andes, Berlin: Springer­ Econ. Geol., v. 96, p. 325-350. Verlag, p. 281-291. Haeberlin, Y. (2002): Geological and structural Dalmayrac, B., Laubacher, G. and Marocco, R. setting, age, and geochemistry of the (1980): Géologie des Andes Péruviennes. orogenie gold deposits at the Pataz province, Travaux et documents de l'ORSTOM, 122, eastern Andean cordillera, Peru. Terre et 501 p. Environnement, University of Geneva, DePaolo, D.J., Manton, W.I., Grew, E.S. and Switzerland, vol. 36, 182 p. Halpern, M. (1982): Sm-Nd, Rb-Sr and U­ Harrison, J.V. (1943): The geology of the Th-Pb systematics of granulite facies rock Central Andes in part of the province of from Fyfe Hills, Enderby Land, Antartica. Junin, Peru. Bol. Soc. Geol. Del Peru, vol. Nature, vol. 298, p. 614-618. 16, p. 1-97. Oilles, J.H. and Proffett, J.M. (1995): James, D.E. (1982): A combined 0, Sr, Nd, Metallogenesis of the Y erington Batholith, and Pb isotopie and trace element study of Nevada. Arizona Geological Digest, vol. 20, crustal contamination in central Andean p. 306-315. lavas, 1. Local geochemical variations. Earth Dong, H., Hall, C.M., Peacor, D.R. and Planet. Sei. Lett., vol. 57, p. 47-62. Halliday, A.N. (1995): Mechanisms of Kobe, H. W. (1990): Metallogenic evolution of Argon Retentio in clays revealed by laser the Yauli dome, central Peru. A summary. 40Ar-39Ar dating. Science, vol. 267, p. 355- In: L. Fontboté et al. (eds.), Stratabound ore 359. deposits in the Andes, Berlin: Springer­ Eyzaguirre, V.R., Montoya, D.E., Silberman, Verlag. p. 267-278. M.L. and Noble, D.C. (1975): Age of Kontak, D.J., Clark, A.H., Farrar, E. and igneous activity and mineralisation, Strobg, D.F. (1985): The rift associated Morococha district, Central Peru. Econ. Permo-Trias sic magmatism of the eastern Geol., vol. 70, p. 1123-1126. Cordillera: a precursor to the Andean Fontboté, L. and Bende:zU, R. (2001): The orogeny. In: Pitcher, W.S. et al. (eds), carbonate-hosted San Gregorio and Magmatism at a plate edge: the Peruvian Colquijirca (Zn-Pb-Ag) deposits (central Andes. Blakie, Glasgow and Halsted Press, Peru) as products of an epithermal high Newyork, p. 36-44. 52

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Rosas, S. (1994): Facies, diagenetic evolution, de doctorat d'état, Université Pierre et Marie and sequence analysis along a SW-NE Curie, Paris. 846 p. profile in the southern Pucara basin (Upper Soler, P. and Rotach-Toulhoat, N. (1990): Sr­ Triassic-Lower Juras sic), Central Pern. Nd isotope compositions of Cenozoic Heidelberg Geowiss. Ab., Band 80, granitoids along a traverse of the Central Germany, 337 p. Peruvian Andes. Geological Journal, vol. Roy-Barman, M. and Allègre, C.J. (1995): 25, p- 351-358. 1870s/1860s in oceanic island basalts: Stacey, J.S. and Kramers, J.D. (1975): tracing oceanic ernst recycling in the Approximation of terrestrial lead isotope mantle. Earth Planet. Sei. Lett., vol. 129, p. evolution by a two-stage model. Earth 145-161. Planet. Sei. Lett., vol. 26, p. 207-221. Sallier, B., Moritz, R. and Beuchat, S. (2000) : Stein, H.J., Sundblad, K., Markey, R.J., The genesis of mantos at Domo de Y auli, Morgan, J.W. and Motuza, G. (1998): Re/Os Central Pern isotopie (C, 0, S, Pb) and fluid ages for Archean molybdenite and pyrite, inclusions constraints. Abstract of X Kuittila-Kivisuo, Finland and Proterozoic Congreso Peruano de Geologia, Lima, Pern. molybdenite, Kabeliai, Lithuania: testing the Schilling, J.-G., Hanan, B.B., McCully, B., chronometer in a metamorphic and Kingsley, R.H. and Fontignie, D. (1994): metasomatic setting. Mineralium Deposita, Influence of the Sierra Leone mantle plume vol. 33, p. 329-345. on the equatorial Mid-Atlantic Ridge: a Nd­ Stucky, P. (2001): La veine de Pb, Zn, Cu, Ag Sr-Pb isotopie study. J. Geophys. Res., vol. d' Andaychagua (Domo de Y auli, Andes 99, p. 12005-12028. centrales, Pérou). Unpublished MSc thesis, Schoenberg, R., Nagler, T.F. and Kramers, University of Lausanne, Switzerland. J.D. (2000) Precise Os isotope ratio and Re­ Szekely, T. S. (1967): Geology near Os isotope dilution measurements down to Huallacocha lakes, central high Andes, the picogram level using multicollector Pern. Am. Assoc. Petrol. Geol. Bull., vol. inductively coupled plasma mass 51, p. 1346-1353. spectrometry. International Journal of Mass Tilton, G.R. and Barreiro, B.A. (1980): Origin Spectrometry 197, 85-94. of Lead in Andean Calc-Alkaline Lavas, Sébrier, M. and Soler, P. (1991): Tectonics and Southern Pern. Science, 210, p. 1245-1247. magmatism in the Peruvian Andes from late Volcan (2000): Annual report 2000. 149 p. Oligocee time to the Present. Geological Voldet, P. (1993): From neutron avtivation to Society of America, Special Paper, vol. 265, inductively coupled plasma-atomic emission p. 259-277. spectrometry in the determination of rare­ Shackleton, R.M., Ries, A.C., Co ward, M.P. earth elements in rocks. Trends in analytical and Cobbold, P.R. (1979): Structure and chemistry, vol. 12, p. 339-344. geochronology of the Arequipa Massif of Wasteneys, H.A., Clark, A.H., Farrar, E. and coastal Pern. Journal geol. Soc. Lond., vol. Langridge, · R.J. (1995): Grenvillian 136, p. 195-214. granulite-facies metamorphism in the Sillitoe, R.H. (1976): Andean mineralisation: a Arequipa Massif, Pern: a Laurentia­ model for the metallogeny of convergent Gondwana link. Earth Planet. Sei. Lett., vol. plate margins. In : Strong, D.F., ed., 132, p. 63-73. Metallogeny and plate tectonics, Geological Watson, E.B. and Harrison, T.M. (1983): Association of Canada Special Paper, vol. Zircon saturation revisited: temperature and 14, p. 59-100. composition effects in a variety of crustal Soler, P. (1991 ): Contribution à 1' étude du magma types. Earth Planet. Sei. Lett., vol. magmatisme associé aux marges actives - 64, p. 295-304. Pétrographie, géochimie et géochimie Zartman, R.E. and Doe, B.R. (1981): isotopique du magmatisme Crétacé à Plumbotectonics the model. Pliocène le long d'une transversale des Tectonophysics, vol. 75, p. 135-162. Andes du Pérou Central - implications géodynamiques et métallogéniques. Thèse 54 55

CHAPTER II: Lineament control on Miocene ore

formation in Central Peru, the Zn-Pb-Ag-Cu San

Cristobal example

S.Beuchat and R. Moritz

Section des Sciences de la Terre, University of Geneva, 1211 Genève 4, Switzerland

Abstract

Due to several overprinting tectonic events, lineaments are striking features of the Andean Cordillera of Central Peru and their influence as channel-ways for rising magma or fluids is documented from the Coastal to the Subandean Zones. However, structural models of the Miocene metallogenic belt of Central Peru fail to show their control on ore formation. The Zn-Pb-Ag-Cu San Cristobal district, localised in the Domo de Yauli structure, is typical of this belt with numerous polymetallic veins and carbonate replacement ore bodies. The area is mainly composed of Paleozoic phyllites of the Excelsior Group, Permo-Triassic volcanic and sedimentary rocks of the Mi tu Group, Triassic-Jurassic limestone of the Pucara Group, and Cretaceous sedimentary rocks. Incaic compression everits have produced isoclinal folds, ramp thrusts in the sedimentary cover rocks and a NE-SW fracture system which crosscuts the entire Domo de Yauli. This fault system was mineralised in association to the intrusion of the Chumpe magma during Late Miocene. The orientation of the veins is highly variable and rotates from N 30° W easterly toN 90° W westerly. The carbonate replacement ore bodies are generally stratiform but show clear features of discordance. Three-dimensional representations of ore bodies and associated veins show that these discordant features and the highest grades are related to the prolongation of veins into the carbonate rocks. Finally major lineaments are present in the basement and affect the morphology of the who le area. The position of a 120 oriented lineament coïncides with the emplacement of the major ore deposits of the San Cristobal district.

Orientation data were collected for dilatant veins, Miocene dykes and altered striated faults in order to define the paleostress associated with the mineralisation event. In this study, the inverse method was applied to determine the local stress tensors of different parts of the Domo de Yauli area. Determination of paleostress related to the Miocene magmatic event indicates a heterogeneous compression field shifting from E-W to N-S 56

from east to west. The rotation of the main compression orientation occurred across a 120° ~riented basement lineament. lt reveals the active role of strike-slip movement along su~h.lmeaments as a control for the formation of ore deposits in the Domo de Yauli area. Ongms of these lineaments are difficult to defme, nonetheless, numerous structural features of the same orientation in the Mitu Group indic ate that their origin probably date at least back to the Permian rifting.

Introduction This study is focused on the San Cristobal district located in the Domo de Y auli ' 11 Okm east of Lima in the Western Cordillera of Peru (Fig. 2.1 ). This mining The Andean Cordillera has been subject to district is part of the Mio cene metallo genie numerous orogenie periods from the belt of Central Peru (Noble and McKee, Proterozoic to the present (Mégard, 1984 1999) and is close to other polymetallic and 1987; Jaillard et al., 2000). These districts such as Morococha and Casapalca multiple periods of deformation have (Petersen, 1965). It is a classical vein and formed lineaments that are believed to be carbonate replacement polymetallic ore the surface expression of deep-crustal deposit district. Initially, Petersen ( 1965) structures, which may provide channels for and later Bartlett (1984) suggested that magmas and fluids (Richards, 2000). In the veins and carbonate replacement deposits Andes, lineaments of numerous were contemporaneous and Miocene in orientations have played a role during age. Due to the stratiform geometry of the geodynamic processes and are generally ore bodies, also called mantos a revealed by alignments of volcanoes ' synsedimentary model was more recently (Matteini et al., 2002) or intrusive bodies invoked, suggesting a Mesozoic syngenetic (Petford and Atherton, 1992). In Central origin for the mantos followed by Tertiary Peru, structures with a 120° orientation are vein formation (Dalheimer, 1990; Kobe, often documented. During the Late 1990a). However, recent studies in the Cretaceous, normal faults of this Domo de Yauli area (Moritz et al., 200 1) orientation may have formed channels for have shown that carbonate replacement rising magmas in the Coastal Zone (Fig .. deposits and veins have similar sulfur and 2.1; Polliand, 2002). Whereas, on the lead isotopie compositions suggesting that eatem side of the Cordillera, in the they were deposited from the same Subandean Zone (Fig. 2.1 ), faults with the hydrothermal fluid. This contrasts with the same orientation have controlled the previous synsedimentary interpretations by deposition of sedimentary rocks of the Dalheimer (1990) and Kobe (1990a). Pucara Group (Hasler, 1998) and displaced Polymetallic veins yield 40 Ar/39 Ar ages of ore bodies of the MVT type deposit of San 4.90 ± 0.15 Ma obtained on sericite from Vicente (Davila et al., 2000). Therefore wall rock alteration selvages (Beuchat et 120° lineaments may exist across the al., 2001 b) that attribute the formation of whole Andean Cordillera of Central Peru ' ore deposits to the late Miocene however they have never been documented compressional tectonic phase named in the Western Cordillera and their Quechua 3 or F5 according to Sébrier et al. influence on Miocene ore formation has (1988). never been demonstrated. 57

only based on the orientation of the different faults, which, in most cases, were not created during mineralisation processes but only reactivated preexisting faults.

This study uses a detailed structural analysis to constrain the paleostress associated to the formation of ore deposits in the San Cristobal district and helps to discriminate between the Coulomb­ Anderson model of pure shear and the Riedel model of simple shear (Fig. 2.2). It also points out the influence of the 120° lineament on ore formation and shows how such a structural analysis can help in identifying predominant structural features in ore formation. Finally, the lineaments origin and their influence are discussed.

Fig. 2.1: Structural zoning of the Peruvian Andes after Mégard (1987) and location of the study area.

The San Cristobal district and the Domo de Y au li area have been the subject of numerous stratigraphie (McLaughlin, 1924; Harrison, 1943; Wilson, 1963; Szekely, 1967; Rosas, 1994) and metallogenic studies (Petersen, 1965; Bartlett, 1984; Campbell et al., 1984; A. Coulomb-Andeson model of pure shear Kobe, 1990a; Dahleimer, 1990; Moritz et al., 2001; Stucky, 2001; Lisboa, 2002; Sallier, 2002). In contrast, the structural p evolution of this region and the paleostress associated to the ore formation have been poorly studied. Lepry (1981) studied the Devonian deformation in Paleozoic rocks 1 located in the central part of the Domo de Y au li and the Eocene deformational pulses that affected the entire Domo de Yauli • region producing north-northwest trending folds and ramp thrusts in the sedimentary cover rocks. Nevertheless, this author paid less attention to the stress regime B. Riedel madel of simple shear associated to ore formation. The structural Fig. 2.2: A. Coulomb-Anderson madel of pure model associated to the mineralising period shear, proposed by Pastor (1970) and Rivera and generally admitted is a Coulomb-Anderson Kobe (1983) for the San Cristaobal district. B. model of pure shear (Pastor, 1970; Rivera Riedel modelof simple shear. Modified after and Kobe, 1983). However, this model is Sylvester (1988). 58

76°10'

o••••• 5km

~ampthrust E:+: + :J Tertiary Intrusions 1 1 Tertiary Formations 11; 1; 1; 1; l; 1li Cretaceous Formations

Pucarà Group

Mitu Group

Excelsior Group

Fig. 2.3: Simplified geological map of the Domo de Yauli area with location of the two mining districts and the ramp thrusts of the fold and thrust belt. Location of Figure 2.4 is marked with blue dashed line.

Regional Geology The thickness of the Excelsior phyllites cannot be estimated in this area. At Tarma, Harrison (1943) estimated the thickness to Stratigraphy be close to 2000m. The oldest identified deformation that affected the Excelsior phyllites is related to the Variscan orogeny The Domo de Y au li is mainly composed of which "transformed portions of the Paleozoic phyllites of the Excelsior Group, Devonian phyllites into tectonites pervaded Permo-Triassic volcaniclastic rocks of the by passive folds, mullion structures and Mitu Group, Triassic-Jurassic dolomitic crenulations cleavage" (Lepry, 1981). A limestones of the Pucara Group, distinct angular unconformity exists Cretaceous carbonate rocks and Miocene between the Excelsior phyllites and the felsic intrusions (Fig. 2.3). The Excelsior overlying Mitu volcanic rocks. The Mitu Group is the oldest rock unit exposed in Group is divided in two contrasting the district and it crops out in the core of lithologies, it consists of red beds with the Chumpe Anticline (Fig. 2.4). This is a conglomerates, sandstones and shales, heterogeneous group, which includes intercalated with alkaline lavas and shales, phyllites, limestones, quartzites, pyroclastic flows of probable continental and basaltic flows where pillow lavas can origin (Mégard, 1978). A stratigraphie be recognized in places (Kobe, 1990b ). sequence of the Mitu Group can be established in the San Cristobal area 59 showing an evolution from breccias with Tertiary igneous activity numerous Excelsior clasts to andesite lava flows with large phenocrysts, dacitic and Igneous rocks of Tertiary age are well rhyolitic ash flows and finally aphanitic exposed in the Domo de Y au li area and are lava flows (C. Astorga, pers. corn.). best represented in the Morococha mining However this sequence cannot be district (Fig. 2.3). At San Cristobal, three recognized in other places of the Domo de different intrusions have been recognized Y auli area. Extreme lateral variations in namely the Carahuacra, San Cristobal and thickness can be related to the Permo­ Chumpe intrusions (Fig. 2.4). The Triassic extensional tectonics as suggested Carahuacra intrusion bas been described by by Kontak et al. (1985). The Triassic­ Lyons (1968) and Bartlett (1984) and Jurassic carbonate of the Pucara Group consists in a quartz monzonite, which rests uncomfortably on the Mitu Group and shows seriate and porphyritic textures. is composed of the Chambara, Aramachay Phenocrysts of plagioclase and pyroxene and Condorsinga Formations (Rosas, are set in a groundmass of plagioclase, 1994 ). The thickness of the Pucara Group pyroxene, quartz and orthoclase. The is extremely variable, in the Domo de Carahuacra intrusion is altered to a Y auli area it is close to 400m along its propylitic mineral assemblage with eastern flank and can be as thin as 120m in formation of calcite, chlorite, sericite, the San Cristobal district (Rosas, 1994). epi dote and clay minerais. Lyons ( 1968) This difference in thickness bas raised the postulated that the Carahuacra intrusion controversy whether the two lowest fed the flows of the Permian lava flows of formations of the Pucarâ Group are present the Mitu Group, this assumption was at the Domo de Y auli or not. According to contradicted by Lepry (1981) who found Szeleky and Grose ( 1972) and Lepry crosscutting relationship between the ( 1981 ), the supposed absence of the intrusion and volcanic rocks. The unique Chambara Formation could be explained radiometrie age existing on this intrusion is by the result of non-deposition and/or post a whole rock K-Ar age of 43.5 ± 1.6 Ma Chambara Formation erosion. Due to its obtained by Bartlett (1984) and is difficult more clastic composition, the Aramachay to interpret due to the alteration. The San Formation bas usually a higher content of Cristobal intrusion crops out in the vicinity titanium than the two other formations of the mining camp (Fig. 2.4) and bas also (Rosas, 1994). Therefore, the low titanium been recognized below the mining camp concentrations in the carbonate rocks on by a recent drilling program. It consists of the western flank of the Domo de Y auli an assemblage of plagioclase phenocrysts may confirm the absence of the and relies of biotite and hornblende; Aramachay and, by inference, the intense silicification and sericitization Chambara Formations (Sallier, 2002). affect the whole intrusion. The Chumpe Stratigraphie markers such as tuffs and intrusion consists of a "series of small plugs basalts are hosted by the Pucara Group and and dykes which have been intruded in the can be recognized over the whole Domo de core of the Excelsior Group (Fig. 2.4). By Yauli area (Bartlett, 1984; Rosas, 1994; crosscutting relationship, it is the Rosas et al., 1997). The Pucani Group is structurally youngest intrusion of the overlain by different Cretaceous units of district and also the most altered. The carbonate composition (Wilson, 1963). primary mineralogy cannot be defined due to the intense sericitic alteration. Less altered samples show rounded crystals of quartz and relies of biotite and amphibole entirely altered to chlorite, epidote and 60 carbonate. The Chumpe intrusion has been and usually with a normal displacement. dated at 6.6 (+ 11-3.6) Ma by the U/Pb These major transverse faults are method on zircon (Beuchat et al., 2001 ). associated to a set of conjugate faults with See Chapter I for more detailed strike-slip motion (Fig. 2.4). The descriptions of the Miocene intrusions. development of faults is highly influenced by the rheology of the host rock. For example, transverse faults are more Andean deformation common in competent lithologies such as andesites of the Mitu Group than in the ductile phyllites of the Excelsior Group. The orientation of these faults is highly During the Eocene, the Incaic pulses of variable and rotates from N 30°E easterly deformation affected the Domo de Y auli to N 90°E westerly (Figs. 2.4 and 2.5). area producing north-northwest trending Deformation of the Domo de Y auli may folds and ramp thrusts in the sedimentary either be influenced by old structures cover rocks (Harrison, 1943; Mégart, inherited from the Variscan Orogeny 1978). The crosscutting fracture system (Lepry, 1981 ), from the Permo-Triassic that affects the entire length of the Domo extensional tectonics (Kontak et al., 1985) de Y auli may have been formed during the or from the Andean deformation. The pre­ same period. Two structural domains can existence of such structures are revealed by be distinguished on Figure 2.3: (1) a fold the presence of regional scale lineaments. and thrust belt located on the southwestem The more obvious ones are flank of the Domo de Yauli, and (2) a geomorphologie features with a 35°, 50° domain within the core of the Domo de and 120° orientation that can be clearly Y auli, characterized by an overprint of the traced on satellite images (Fig. 2.6). Faults pre-existing Paleozoic fabrics. The fold with these orientations are rarely and thrust belt is oriented N 25° W and encountered on the ground and are only deformation affects all formations younger found in Paleozoic rocks; the 120° ones are than the Excelsior Group. This belt generally accompanied by a meter large contains a large number of ramp thrusts alteration halo. The relative motion of that can be traced throughout the whole these 120° faults cannot be determined in region and are named Ticlio, Poma, Ruri most cases, except east of Oyama (Fig. and Pancar according to Lepry (1981) (Fig. 2.4) where slickenside. lineations indicate 2.3). They trend north-northwest and dip left lateral strike-slip. The importance of steeply ( 50-80°) to the southwest. The these 120° structures as fluid channel-ways displacement measured along the fault is emphasized by their alteration and by the planes are generally on the order of 0.5km, fact that major ore bodies of the Domo de with the exception of the Pancar thrust Y auli area are aligned along this lineament which shows a displacement of up to 2km (Fig. 2.6). (Lepry, 1981 ). Such displacements have resulted in the juxtaposition of the Pucara limestones and locally the Mitu volcanic rocks in the upper plate against Cretaceous carbonate rocks in the lower plate (Fig. 2.3). Folds are parallel to thrust faults. Most anticlines and synclines generally display near-vertical axial planes. The folds are intimately associated to the major Fig. 2.4: Simplified geological map of the San transverse faults of the region, which are Cristobal district and location of the principal ore steeply dipping, perpendicular to fold axes bodies. -1 ·"1 ,., --1 •1 ·1

'1 -1 "1 -1 ...., -1 '1 '1...,

~ #:2.. '1/ _, "~~- '1 ,, -1 -1 '1 " -1 '1 ~ ... ·· ~ /, __/ ' -7v -~ -1 '1 -:;;;;.- '1 " ~ ! '1 ·'1 '1 ·1 ;/;~u-:. -"1 r ··: ·"' ·'1 -'1 -1 .-1 ..... __.... __. ,;•.;p_C:> -L '1 "-~ _,· _.. - -1 ., -~.., ,, ' -1 •1 •1 .

...., -'1 '1 ·1

'1 -1 ...., ". '1 " ·1 ' 1 '1 "'1 •1" ·1 ·1 '1 -, -1 •1 ' '1 '1

-; ·1 -1 ....,

"1 '1 ~ - ..... •1 .,.,. + ....."' ...... ~--A~-- ....x ~ ...... A A~P "' 'tJ.c .._ N ' ' -?<-.;t;;... ' ..... "' 1 k ..... "' ..v$

" " " 1 '1 -- 1 1 '1 1 1 / 1 1 1 1 1 1 km 1 1 1

/;;_ç_ô:> Tertiary Intrusions Pucan:\ Group (Tr,Ju) Mineralized faults Name of principal ore bodies ~~~ ~ \~'>-""'-"''''/' / Mitu Group (Permian) ----- Chulec Formation (Cret inf) 1 ~ \ ''1 Unmineralized faults Na me of Tertiary intrusions ~~- RS> --.:. .--:.; .,j ~j_~f..··(:.f Goyllarisquizga Group (Cret inf) l______j Excelsior Group (Pal. inf.) Carbonate replacement ore bodies ~ A-B: Figure 2.7 0\ 62

and, and (3) a final quartz-carbonate-barite stage (Fig. 2.8). Whereas faults crosscut rocks of ali lithologies (Figs. 2.3 and 2.6), Excelsior phyllites, Mitu volcanic rocks and Tertiary intrusions host the large majority of ore grade mineralisation. The San Cristobal district can be subdivided in two zones displaying veins with economie importance: the Andaychagua and the San Cristobal vein systems (Fig. 2.4). The Andaychagua system is located on the eastern flank of the Chumpe anticline, immediately southeast of the intrusion (Fig. 2.4). It is mainly composed of the Fig. 2.5: Diagram (lower hemisphere, equal Andaychagua vein, which crosscuts projection area) with major transverse fault poles. volcanic rocks of the Mitu group with a Note their variable orientation from the Chumpe mean orientation· of 120/80. Toward the anticline western flank (white circle), to its crest southwest, the vein, which can be as large (gray circle) and toits eastern flank (black circle). as four meters in volcanic rocks, is hosted by the Excelsior phyllites and tends to split in numerous centimetric veinlets. This horse-tail structure of the vein is not only Ore deposits of the San associated with a change in the host Cristoba/ district lithology but also with a small change in orientation, since its orientation in phyllites is doser to 150/70. Other accessory veins, such as the Veta Prosperidad or Veta Puco Two ore deposit types are recognized in Urco, are part of the Andaychagua vein the San Cristobal district: Zn-Pb-Ag veins system (Fig. 2.4). By contrast, the San and carbonate replacement bodies that are Cristobal vein system is located on the distributed around the Chumpe intrusion western flank of the Chumpe Anticline and (Figs. 2.3 and 2.6). See Chapter III for a northwest in comparison to the complete description of the different ore Andaychagua vein system (Fig. 2.4). It is types. composed of severa} sub-parallel veins, where the most important are the San Cristobal and the 722 veins (Fig. 2.1 ). Due Vein ore type to the higher competence of the volcanic rocks, these veins have been preferably exploited in the latter, where veins form Nearly ali-major transverse faults present large and straight parallel structures. in the San Cristobal district are mineralised However, old mining activity and surface to sorne extent and the larger are exploited mapping in the Excelsior phyllites reveal for Zn, Pb, Cu and Ag. The central part of changes in vein orientation in the central the mining activity, where tungsten ore is part of the Chumpe Anticline (Figs. 2.3 prevalent, is nowadays no more exploited and 2.6). This orientation change may be and inaccessible. Each vein has a similar related to the different vein opening ore and alteration mineralogy. The direction that is indicated by crystal paragenetic sequence can be separated in orientation of late stage (Fig. 2.8). The late three main stages: (1) an earl y wolframite stage are antitaxial massive veins of stage, (2) an important base-metal stage carbonate and quartz III with sharp 63

DIST!liTO MINERO YAULI - TICLIO

Fig. 2.6: Satellite imagery of the Domo de Yauli with the position of one of the 120° geomorphologie features and the location of the principal ore deposit of the sou them Domo de Yauli. 64

A B sw NE sc:_oom !~UY._êL 120 , , _NIVEL 170 ' , NIVEL 220 ' ,. NIVEL 270 NIVEL 320 NIVEL 370 NIVEL 430 • !:Jl'((OL 500 NIVEL 580 NIVEL 630

NIVEL 730 SAN CRISTOBAL INTRUSION CHUMPE INTRUSION

E:~=~~ Tertiary Intrusions 1,. ,. " "1 Mi tu Group 1·- ·· · ) 1 Quaternary

Pucara Group Excelsior Group Mineral extracted

Fig: 2,-7 : ~on~itudinal section of the San Cristobal vein and composite map of the minèd levels revealing the vanahon m onentation of the San Cristobal vein. See location on Figure 2.4.

boundaries. Unfortunatly the large amount model of pure shear (Fig. 2.2), which does of secondary fluid inclusion has strongly not explain such a variation on vein disturbed the crystals and no trails orientation. Moreover, the vein orientation representing the crack seal events have change is not the only features unexplained been discovered. Quartz III oblique by the model proposed by Pastor (1970) crystals in comparison of vein selvage and Rivera and Kobe (1983). For example, orientations indicate constant direction in the lithology displacements along the each studied vein. They are then thought to major transverse faults display contrary represent the opening direction of the vein. sinistral and dextral sense of motions A probable N-S opening for the depending on the side of the Chumpe Andaychagua vein and an approximately Anticline (San Cristobal vein, Fig. 2.3). E-W opening for the Veta 722 is therefore Accesory small vein systems occur at the deducted (Fig. 2.9). Data are scarce and are SE of the San Cristobal-Andaychagua only indicative on the quartz crystallization district; they are named Aerolito and that pertains to the last stage of the Soraya (Fig. 2.6). paragenesis (Fig. 2.8). No data are available for the earlier stages of mineralisation due to the irregular grain fabrics. However, it raises doubts about the 65

lia lib Ill Quartz ...... ___ _ ._ ....

WoJran'ite MU$è(Wite Chabopyrte

Arseoo pyrtte Sp hal erite - Gal ena Frelleqjte Il Pyrarwrite

Bismulhinite

Argentite

MIHioh siderite, ankerte,nhôdochnJSie

DicWt~ lill Modifiad aftar CampbaJI (1984), ·-· Bari te stuoky (2001) and Lisboa (2002)

EARLY 1 BASE METAL LATE STAGE STAGE STAGE Fig. 2.8 : Paragenetic sequence of the San Cristobal vein.

ANDAYCHAGUA VEIN

722VEIN

Fig. 2 9: Diagram (lower hemisphere, equal projection area) showing the orientation of la te automorphous quartz from the Andaychagua vein (crosses) and Veta 722 (black diamonds). 66

· Distribution of Zn grade Distribution of Pb grade

2to 5 %Zn 0.25 to 0.50 %Pb 5to 10%Zn 0.50 to 1.00 %Pb > 10 %Zn 1.00to 2.00 %Pb - - >2.00%Pb - + + 9•0! 00

i=' J:::tn!l!f~lJIJ.:Oeo!. ::!~14::,. c=::=Joom llmJJI .___ _.__ __.__ ...__ ..... __ ..._ _ _.__ ___.

lw:llOJ» ltlŒU» 1rxo.œ

Distribution of Cu grade Distribution of Ag grade

0.1 to 0.2 %Cu 1 to 2 Oz/TM Ag CJ 0.2 to 0.3 %Cu - 2to50ziTMAg - >0.3%Cu - >50z!TMAg - + +

Ptl'YH.''t iMr ~.-.rf~• )':i-11:

16Qû

Fig. 2.10: Plan view illustrating the position of carbonate replacement ore bodies with respect to the orientation of the major Toldorumi vein. Isovalues of the carbonate replacement ore (Toldorumi Sur, see location on Fig. 2. 1) are obtained by krigeage on 315 drillcore samples (black dots). The orientation of the Toldorumi vein system is extrapolated from surface mapping information and exploration drillholes intersection.

veins. The emplacement of mantos Carbonate replacement ore type coïncides with the location where veins crosscut the carbonate rocks (Figs. 2.3 and 2.6). Ore bodies are generally bed-parallel Zn-Pb-Ag carbonate replacement deposits but show clear features of discordance are hosted by the basal limestone of the such as breccia pipe-like characteristics Pucani Group along the contact with the (Sallier, 2002). Three-dimensional underlying Mitu Group (Fig. 2.1), and have representations of ore bodies and a close mineralogical composition as the 67

associated veins show that these discordant Y auli area, indicating that carbonate features and the highest grades are related replacement deposits and veins have to the prolongation of veins into the similar sulfur and lead isotopie carbonate rocks (Fig. 2.1 0). It may indicate compositions suggesting that they were the role played by veins as fluid channel­ deposited from the same hydrothermal ways during mineralisation. This confirms fluid (Moritz et al., 2001 ). recent isotopie studies m the Domo de n n ~ ü A/\ /}:"'--, / 0 i /l' '·\ // f..... ~-;"'"1:'>., -;, ·-, --- ' -,, : 1 / \ /// - -~- : , ~:-~-- ~>·. ., ("-0 '',.__, '-.... ,' 1 / / \ l:~;;J;{/:;; --\ 1 ' ~' ·~> 'Jl' ! / 1 \i-:/:·1_ - '-f // /A::::> \ -\... ,_Y;:/1\ // ~ i f(/ \ -t;//f/ _____.- ' -\ \ ~'l~y \ .,. 9;:·(;,(~(-- j <~ \~ \, ----l~, \,\ « --;-/ '-.!; \ / ~ ,::r - -~:-- -. // \, Y; //, \ • /' '-. \,.--// " ).(1/ /; '<.\_ / .><.:>-~ _____/ \ '!?' '~r ------/ y · ·-..::- ""- ...... _J' 1 0 \ //. :::r ... ___ -- Tf - Hua ri ~m a

_,

0 a1 D a2 .6. o-3

Fig. 2.11: Map showing orientation of compression during the Miocene in the San Cristo bal district. Fault slip data: diagrams (lower hemisphere, equal area projection) with fault as thin curves and slickensides lineations as dots with double arrows (left or right lateral) or simple ones (normal or reverse fault). Filled and open arrows filled or not indicate high and medium confidence levels, respectively. Direction of compression and extension are indicated as large gray arrows. Map symbol as in Fig. 2.4. 68

Paleostress determinations indicators as a function of the geologie unit and the location about the Chumpe anticline and the 120° lineament (Fig. 2.11 ). Thus, we have obtained Miocene Methodology paleostress information on a surface of 20 km E-W by about 10 km N-S, covering geological units from Excelsior Group to Orientation data were collected from Mesozoic carbonate rocks and crosscutting dilatant mineralised veins, dykes and the 120° lineament (Fig. 2.1 0). Three striated faults. They were collected from locations are located far away from the underground workings at San Cristobal and mineralised area (East of Oyama, East of Andaychagua (Fig. 2.4) and they were Andaychagua, Pomacocha; Table 2.1 ); compared to data collected on surface. In they permit to test whether the paleostress order to evaluate only the paleostress calculated in the mining area are perturbed regime associated with Miocene by the magmatic and mineralising systems. mineralisation processes and avoid data The location Soraya is one of the small associated to earlier tectonic regime, data vein system located at the SE of the San were only collected on fault planes Cristobal-Andaychagua district on the 120° showing the same mineralogical lineament. Its determined paleostress characteristics as the major polymetallic permits, first, to test again the consistency veins. Indeed, faults opened during the of the paleostress at a large scale and, Miocene mineralising event are secondly, to control the influence of the recognizable due to the presence of 120° linement in an other location than the mineral infill (especially quartz and the San Cristobal-Andaychagua district. carbonate) and the alteration halo in the adjacent wall rock (principally chlorite and/or kaolinite, illite assemblages). Resu/ts Particular attention was also paid to the orientation of mineral development and their relationship with brittle deformation. The analysis is based on the concepts of Numerical results of paleostress mechanical relationships between brittle determinations are summarized in Figure features and paleostress orientation. In this 2.11 and Table 2.1. Miocene paleostress study, regional paleostress orientations located on the eastern flank of the Chumpe were determined on the basis of fault slip anticline, such as Oyama, east of Oyama data using the inverse methods of Angelier and east of Andaychagua, corresponds to (1989, 1994), after a solution of Sperner et an E-W compression (Fig. 2.11 and Table al. (1993). The principle of the method is 2.1 ). The Soraya paleostress, which is the to find the best fit between observed easternmost location on the 120° lineament directions and senses of slip on faults and (Fig. 2.6) indicates a similar compression. theoretical shear stress induced on these In contrast, the Miocene paleostress planes by a common stress tensor. This located on the western flank indicates a cr 1 method has been applied in 11 different orientation close to N-S. This N-S locations (Table 2.1 ); the selection of these compression is recognised regionally oon locations has been principally based on the the western flank of the Domo de Y auli as accessibility and the quality of the stress evidenced by analysis on the remote tensor indicators. They have also been Pomacocha area (Fig. 2.3 and Table 2.1 ), selected m order to provide a located 10 km west of San Cristo bal in the representative network of paleostress fold and thrust belt. However, paleostress calculations based on the Pomacocha data 69

Table 2.1: Paleostress results determined at the San Cristobal district

Location Rock Number of data a1 a2 a3 cp Angle

East of Oyama Mitu 29 079"/04" 185"/76" 348"/13" 0.28 4 East of Andaychagua Mitu 14 075" /01" 344"/46" 166"/34" 0.34 17 Laguna Chumpe Excelsior 14 218"/"12 125"/16" 343"/70" 0.41 14 Huaripampa Pu ca ra 17 176"/11" 038"/76" 268"/09" 0.55 11 Moises Pu ca ra 13 226"/19" 317"/02" 053"/71" 0.30 18 Veta San Cristobal Mitu 12 197"/11" 090"/56" 293"/31" 0.40 12 Pomacocha Cretaceous 12 008"/09" 271"/37" 110"/51" 0.42 7 Oyama Mi tu 11 056"/30" 158"/21" 278"/52" 0.67 12 Soraya Mitu 11 097" /05" 006"/07" 224"/81" 0.17 4 Veta 722 Mitu 10 356" /18" 127"/64" 259"/18" 0.47 24 Toldorumi Mitu and Pucara 10 191"/17" 068"/60" 289"/24" 0.07 3

Location as on Figure 7,

The rotation of the cr 1 orientation is probably the most distinctive feature on The paleostress orientations calculated Figure 2.11. In addition the data in Table 1 from the collected data over the whole San also show: ( 1) the permutation of the cr2 Cristobal district indicate a distinct change and cr3 axes at Oyama, Laguna Chumpe in cr 1 orientation wh ether they are located and Soraya (Fig. 2.11 and Table 2.1) and, on the western (cr1 ~ N-S) or eastern flanks (2) ~ values, (cr2-cr3)/(cr1-cr3), are higher (cr 1 ~ E-W) of the Chumpe Anticline (Fig. for data collected close to the Chumpe 2.11 ). This paleostress rotation is intrusion (Figs. 2.3 and 2.1 0). It indicate an corroborated by the fact that the vein increase of stress component parallel to the structures are not straight, but show orientation of the Chumpe related dykes changes in their direction (Figs. 2.4 and (Fig. 2.11) and may correspond to regional 2.6). Based on the regional geology (Fig. effect due to the perturbation of the 2.4), this paleostress heterogeneity may paleostress field by the intrusion of the have two different ongms: (1) the 70 difference in rheology between volcanic zone (Lacombe et al., 1993). From a rocks of the Mitu Group and phyllites of metallogenic point of view, the importance the Excelsior Group and, (2) the presence of such preexisting structures is confirmed of crustal lineaments. As explained above, by the location of the principal ore deposits if the change in cr 1 orientation is the most of the southern Domo de Yauli (Fig. 2.5) obvious feature (Fig. 2.11 ), paleostress and also by the fact that faults parallel to determinations also indicate a cr2 and cr3 this structure display an alteration halo. permutation for the Laguna Chumpe, This may indicate that these structures Oyama and Soraya location (Fig. 2.11 and have played an important role as fluid Table 2.1 ). At the Laguna Chumpe channel-ways during mineralisation location, the rocks are Exce1sior phyllites processes. Finally, the 6.6 Ma old Chumpe (Table 1), therefore the cr2 and cr3 intrusion is strongly structurally oriented permutation may effectively be imputed to and is mainly located on the western side the lower ductility of such rocks. of the 120° lineament (Fig. 2.4). Alternativelly, it coul be a result to a local Paleostress relative to this compartment heterogeneity due to the emplacement of indicates a cr3 orientation concordant to the the Chumpe intrusion. However, these two Chumpe intrusion dykes. It may indicate explanations cannot be applied to the two InJection of the magma along an other places, since they are composed of extensional fault. The presence of this volcanic rocks of the Mitu Group and far intrusion close to the lineament may also away from Miocene intrusion. On the other tell us that the magma has taken advantage band, the three locations of axe of the high crustal permeability channels permutations correspond roughly to the for ascent. location of the 120° lineament and may be interpreted as a result of its influence. As mentioned previously m the Finally, this compression change across the introduction, lineaments of 120° lineament is also documented by variations orientation are a common feature in in orientation of quartz III of the late ore Central Peru. They have played a major stage (Figs. 2.9 and 2.12), which indicates role as channels for Late Cretaceaous aN-S opening at its eastern side and an E­ magma emplacement in the Coastal Zone W opening at its western side. However, (Fig. 2.1; Polliand, 2002) or as control on this vein opening direction is only the deposition of sedimentary rocks of the significant for the late ore stage and cannot Pucara Group (Hasler, 1998). Therefore, be confirmed for the previous principal ore their influence can be shown over the stages (Fig. 2.8). The calculated whole Andean Cordillera from the Coastal paleostress from fault slip data being in zone to the Eastern Cordillera and from the good agreement with the vein opening Cretaceous to the present. However, their direction (Fig. 2.11 ), polymetallic ore of age and ongm remain difficult to the San Cristobal district have been formed determine. While a study on the San in a heterogeneous field of stress. Since the Cristobal district cannot answer alone to rotation of the later coïncide to a 120° these questions, sorne constraints can be lineament observed on satellite image (Fig. drawn. First of all, faults with a 120° 2.6), we therefore interpret the paleostress orientation are common in the Mitu Group heterogeneity as a consequence of but absent in the younger formations. It reactivation of this preexisting zone of may indicate a minimum age for this brittle weakness. Such an influence of preexisting deformation. On the other side, such an fractures or zones of weakness on strength orientation has not been documented in the anisotropies have already been ductily deformed rocks of the Excelsior demonstrated in other geological settings, Group (Lepry, 1981) suggesting at least a such as the in the Rhine-Saone transform negligeable role of these structures during 71 the Variscan Orogeny. This age bracket districts are needed before drawing any may suggest an origin of these 120° conclusion. In addition, such studies would structures during the Permian rifting also define the compressional stress axis of (Kontak et al., 1985; Sempere et al., 2002) the Quechua F5 phase and may help to that propagated southward to Bolivia until understand the different shortening the Dogger, but an older origin cannot be direction proposed for the W estem excluded. Cordillera and the Altiplano, i.e. E-W and N-S respectively (Sébrier et al., 1988). This difference can have its ongm m underlying deeper structures.

In conclusion, the influence of a lineament, at least Permian in age, resulted in a strong paleostress heterogeneity during Miocene tectonics, which is both indicated by a rotation of the cr 1 compression and by the opening vein direction across the 120° lineament (Fig. 2.12). This heterogeneous Miocene paleostress is in agreement with the other structural features, such as orientations of vein and quatz III growth, and, tends to indicate that the San Cristobal Mean paleostress of each compartment represented as focal mechanism,calculated from data in Table 2 1 veins and carbonate replacement systems

Shearing sense as indicated by fault slip data (Fig 2.11), were formed during a trans-tensional lineament sense of shearing is deduced from faults parallel to this structure stress. It reveals the active role of sinistral 1.--- strike-slip movement along such Fig. 2.12: Structural sketch indicating principal lineaments as a control for the formation of structural components involved in the San Cristobal ore deposits in the Domo de Y auli area. Miocene paleostress. Not in scale. This contrasts with the previous structural Based on its mineralogy and age, the San model of simple shear by Pastor (1970). Cristobal district belongs to the Miocene Influence of these 120° structures is metallogenic 'belt of Central Peru ·(Noble common in Central Peru and date back at and McKee, 1999), it differs with respect least to the Permian rifting. to the majority of other deposits by the fact that it is structurally located deeper. Indeed, the majority of the other Acknowledgments polymetallic are hosted by much younger rocks than the phyllites of the Excelsior Group (Petersen, 1965). This deep We gratefully acknowledge the Volcan structural setting, due to the dome structure Compania Minera, which provided access of the Domo de Yauli (Mégard, 1978), to underground exposures and support may have favored the influence of deep during fieldwork. We thank also Les crustal structures on the formation of the Oldham and the Anglo-Peruana for the San Cristobal ore deposits. Therefore it is excellent mapping and help during not certain that these 120° lineaments fieldwork. Discussions and reviews by M. influence is a common feature of the Sartori are kindly acknowledged. Miocene metallogenic belt of Central Peru and detailed structural studies on other 72

References Hasler, C.A. ( 1998): Facies Characterization and sedimentary model of the carbonate host rock (Upper Triassic-Lower Liassic) at Angelier, J. (1989): From orientation to the Mississippi Valley-Type Zn-Pb ore magnitudes in paleostress determinations deposit of San Vicente, Central Peru. using fault slip data. J. Struct. Geol., vol. 11, Unpublished M.Sc. thesis, University of p. 37-50. Geneva, Switzerland, 118 p. Angelier, J. (1994): Fault slip analysis and Jaillard, E., Hérail, G., Montfret, T., Diaz­ paleostress reconstruction. In: Hancock P.L. Martinez, E., Baby, P., Lavenu, A. and ( ed), Continental deformation, Tarrytown, Dumont, J.F. (2000): Tectonic evolution of Pergamon Press, p. 53-100. the Andes of Ecuador, Peru, Bolivia and Bartlett, M.W. (1984): Petrology and genesis northernmost Chile. In: Cordani, U.G. et al. of carbonate-hosted lead-zinc-silver ores, (eds), Tectonic evolution of South America, San Cristobal district, Department of Junin, Rio de Janeiro, p. 481-559. Peru. Unpublished Ph.D. thesis, Oregon Kobe, H. W. (1990a): Metallogenic evolution State University, Corvallis, Oregon, U.S.A., of the Y auli dome, central Peru. A 272p. summary. In: L. Fontboté, G.C. et al. (eds.), Beuchat, S., Moritz, R., Sartori, M., Chiaradia, Stratabound ore deposits in the Andes, M. and Schaltegger, U. (2001a): High Berlin: Springer-Verlag. precision geochronology and structural Kobe, H. W. (1990b): Stratabound Sulfide constraints on the Zn-Pb-Ag-Cu Domo de occurrences in the Paleozoic of the Yauli Y auli district, Central Peru. Extended Dome, Central Peru. A summary. In: L. abstract, 6th biennial meeting of the SGA, Fontboté, G.C. et al.(eds.), Stratabound ore Krakow, Po land, 26-29 August 2001. deposits in the Andes, Berlin: Springer­ Beuchat, S., Schaltegger, U., Moritz, R., Verlag. Chiaradia, M., Cosca, M. and Fontignie, D. Kontak, D.J., Clark, A.H., Farrar, E. and (200 1b ): High-precision geochronology Strobg, D.F. (1985): The rift associated constrains on Miocene magmatic and Permo-Trias sic magmatism of the eastern mineralising events in the Pb-Zn-Ag-Cu Cordillera: a precursor to the Andean Domo de Yauli district, Peru. GSA Annual orogeny. In: Pitcher, W.S. and al., eds, Meeting, Boston. A-358. Magmatism at a plate edge: the Peruvian Bendezu, Rand Fontboté, L. (in press): Andes. Blakie, Glasgow and Halsted Press, Campbell, A., Rye, D. and Petersen, U. (1984): New-York, p. 36-44. A hydrogen and oxygen isotope study of the Lacombe, 0., Angelier, J., Byme, D. and San Cristobal mine, Peru: Implications of Dupin, J.M. (1993): Eocene-Oligocene the role of water to rock ratio for the genesis Teconics and kinematics of the Rhine-Saone ofwolframite deposits. Econ. Geol., vol. 79, cohtineri.tal transform zone. Teconics, vol. p. 1818-1832. 12, p. 874-888. Dalheimer, M. (1990): The Zn-Pb-Ag deposits Lepry, L.A. (1981 ): The structural geology of Huaripampa and Carahuacra in the mining the Yauli dome regwn, Cordillera district of San Cristobal, Central Peru. In: Occidental, Peru. Unpub. MSc Thesis, Univ. Fontboté, L. et al. (eds.), Stratabound Ore of Arizona, Tucson, 99p. Deposits in the Andes, Berlin: Springer­ Lisboa, H. (2002): Etude le veine 722 à Zn-Pb­ Verlag. Ag de la mine San Cristobal (District minier Davila, D., Febres, 0., Fontboté, L. and de Domo de Yauli, Pérou central). Oldham, L. (2002) : Exploraci6n y geologîa Unpublished M.Sc. thesis, University of del yacimiento San Vicente. In: Instituto de Genève, Switzerland. ingenieras de minas del Peru ( ed), Primer Lyons, W.A. (1968): The Geology of the volumen de monografias de yacimientos Carahuacra Mine, Peru. Econ. Geol., vol. minerales peruanos, p. 305-328. 63, p. 247-253. Harrison, J.V. (1943): The geology of the Matteini, M., Mazzuoli, R., Omarini, R., Cas, Central Andes in part of the province of R. and Maas, R. (2002) : The geochemical Junin, Peru. Bol. Soc. Geol. Del Peru, vol. variations of the upper cenozoic volcanism 16, p. 1-97. along the Calama-Olacapato-El Toro transversal fault system in central Andes 73

(24°S): petrogenetic and geodynamic Richards, J.P. (2000): Lineaments revisited. implications. Tectonophysics, vol. 345, p. SEG Newsletter, vol. 42, p. 1-20 211-227. Rivera, G.N. and Kobe, H.W. (1983): McLaughlin, D.H. (1924): Geology and Evoluci6n geol6gica de Domo de Yauli. physiography of the Peruvian Cordillera Bol. Soc. Geol. Pero, vol. 72, p.l59-175. Cordillera, Department of Junin and Lima. Rosas, S. (1994): Facies, diagenetic evolution, Geol. Soc. Am. Bull., col. 35, p. 591-632. and sequence analysis along a SW-NE Mégart, F. (1978): Etude géologique des profile in the southem Pucarâ basin (Upper Andes du Pérou central. Mémoire Triassic-Lower Jurassic ), Central Peru. ORSTOM, vol. 86, 303p. Heidelberger Geowissenschaftliche Mégard, F. (1984): The Andean orogenie Abhandlungen, Band 80, 337 p. period and its major structures in central and Rosas, S., Fontboté, L. and Morche, W (1997): northem Peru. J. Geol. Soc. London, vol. Vulcanismo de tipo intraplaca en los 141, p. 893-900. carbonatas del grupo Pucara (Triasico Mégard, F. (1987): Structure and evolution of superior - Jurasico inferior, Peru cetral) y su the Peruvian Andes. In: Schaer, J.P. and relacion con el vulanismo del grupo Mitu Rodgers, J. (eds), The anatomy of mountain (Permico superior- Triasico). IX Congreso ranges, Princeton Univ. Press, NJ, United Peruano de Geologia, Resumenes States, p. 179-209 Extendidos, p. 393-396. Megaw, P.K.M., Barton, M.D. and Falce, J.I. Sallier, B. (2002): Minéralisations stratiformes (1995): Carbonate-hosted lead-zinc (Ag, Cu, à Zn, Pb, ±Ag dans les calcaires du Pucara Au) deposits of northem Chihuahua, (Domo de Yauli, Andes du Pérou Central). Mexico. In: D.F. Sangster (ed.), Carbonate­ Unpublished M.Sc. thesis, University of hosted lead-zinc deposits. Society of Genève, Switzerland, 170 p. Economie Geologists Special Publication 4, Sébrier, M., Lavenu, A., Fomari, M. and p. 277-289. Soulas, J.P. (1988): Tectonics and uplift in Moritz, R., Beuchat, S., Chiaradia, M., Stucky, Central Andes (Peru, Bolivia and northem P., Sallier; B. and Lisboa, H. (2001): Zn-Pb Chi le) from Eocene to present. mantos and veins at Domo de Y auli, Central Géodynamique, vol. 3, p. 85-106. Peru: two products of one hydrothermal Sempere, T., Carlier, G., Soler, P., Fomari, M., system with common Pb and S sources, but Carlotto, V., Jacay, J., Arispe, 0., contrasting fluid inclusion characteristics. Néraudeau, D., Cardenas, J., Rosas, S. and Extended abstract, 6th biennial meeting of Jimenez, N. (2002): Late Permian-Middle the SGA, Krakow, Poland, 26-29 August Jurassic lithospheric thinning in Peru and 2001. Bolivia and its bearing on Andean-age Noble, D.C. and McKee, E.H. (1999): The tectonics. Tectonophysics, vol. 345, p. 153- Miocene metallogenic belt of central and 181. northem Peru. In: Geology and ore deposits Spemer, B., Ott, R. and Ratschbacher, L. of the central Andes, B.J. Skinner eds., Soc. (1993). Fault-striae analysis: a turbo pascal Econ. Geol. Spec. Pub!. Vol. 7., p. 155-193. program package for graphical presentation Pastor, J.A. (1970): The mneralization in San and reduced stress-tensor calculation. Cristobal mine. Unpublished MSc thesis, Computers and Geosciences, vol. 19, p. University of Arizona, 116p. 1361-1388. Petersen, U. (1965): Regional geology and Stucky, P. (2001): La veine de Pb, Zn, Cu, Ag major ore deposits of Central Peru. Econ. d' Andaychagua (Domo de Yauli, Andes Geol., v. 60, p. 407-476. centrales, Pérou). Unpublished MSc thesis, Petford, N. and Atherton, M.P. (1992): University of Lausanne. Granitoid emplacement and deformation Sylvester, A.G. (1988): Strike-slip faults. Geol. along a major crustal lineament: the Soc. An1 . Bull., vol. 100, p. 1666-1703. Cordillera Blanca, Peru. Tectonophysics, Szekely, T. S. (1967): Geology near vol. 205, p. 171-185. Huallacocha lakes, central high Andes, Polliand, M. (2002): Zn-Pb-Ba massive Peru. Am. Assoc. Petrol. Geol. Bull., v. 51, sulfides at Perubar, Central Peru. PhD p. 1346-1353. thesis, University of Geneva, Switzerland Szeleky, T.S. and Grose, L.T. (1972): Stratigraphy of he carbonate, black shale, 74

and phosphate of the Pucara Group (Upper Wilson, J.J. (1963): Cretaceous stratigraphy of Triassic- Lower Juras sic) central Andes, the cetal Andes of peru. Am. Ass. Petrol. Pern. Geol. Soc. Am. Bull., vol. 83, p.407- Geol. Bull., vol. 47, p. 1-34. 428. 75

CHAPTER III: The Zn-Pb-Ag San Cristobal district,

Central Peru: Isotope and Fluid inclusion constraints

1 1 2 S.Beuchat , R. Moritz and T. Pettke

1Section des Sciences de la Terre, University ofGeneva, 1211 Genève 4, Switzerland

2 lnstitute for Isotope Geochemistry and Mineral Resources, Federal Institute ofTechnology ETH, 8092 Zürich, Switzerland

Abstract

The Zn-Pb±Ag±Cu San Cristobal district is located 100 km east of Lima in the Western Cordillera of Peru. It is centred around the Chumpe intrusion (U/Pb age of 6.6 + 1;-3.6 Ma) and is composed of a vein type and a carbonate replacement ore type. Veins present a paragenesis that can be subdivided into 3 phases: (a) an early wolframite-quartz-pyrite stage, (b) a quartz-base metal stage and, (c) a late quartz-carbonate-barite stage. The carbonate replacement ore bodies have a mineralogical assemblage that is similar to that of the veins, with the only difference *at the early wolframite-quartz-pyrite stage is absent and an important hematite stage is observed earl y in the paragenetic sequence.

Fluid inclusions in sphalerite and quartz are two-phase at room temperature, homogenise to the liquid phase between 140 and 330°C and their salinities range between 0.4 and 6. 7wt% NaCl; rare inclusions contain an additional crystal of halite in the earl y stage and yield a salinity of 44 to 50wt% NaCI. The vein data show a decrease in homogenisation temperatures concomitant with a salinity decline. Additionally, the data from the carbonate replacement ores show a wide variation in salinity of 3.3 to 14wt% NaCl at constant homogenisation temperature. This can be explained either by mixing of the fluid related to the vein system and a hot brine, or by boiling of the fluid migrating out of the veins into the carbonate. Wolframite, galena and sphalerite from each ore type yield 06 04 207 04 similar lead isotopie compositions e PbF Pb = 18.676 to 18.840; PbF Pb = 15.615 to 15.649; 208PbF04Pb = 38.704 to 38.827) and overlap with those of the Miocene 06 04 207 04 208 04 intrusions e PbF Pb = 18.698 to 18.761; PbF Pb = 15.635 to 15.669; PbP Pb = 38.682 to 38.787). On the contrary, strontium isotopie compositions of carbonate and barite are highly variable and too radiogenic to be explained by magmatic input only 76

87 86 ( Sr/ Sr = 0.712187 to 0.722782). lt may correspond to a predominantly magmatic fluid followed by incoming of 87Sr-enriched fluids. This evolution in two steps is consistent with hydrogen and oxygen isotope data. Isotopie compositions of the fluid associated to 18 18 the first stages reveal a trend with constant & 0 values with decreasing &D values (& 0 = 3.2 to 5.0 %o SMOW and &D = -60 to -112 %o SMOW), which is interpreted as a mixing of a dominantly magmatic component with minor meteoric water equilibrated with the host rocks. By contrast, ending stages bear isotopie characteristics defining a trend with a conjugated decrease of &180 and &D (& 180 = -8.1 to 2.5 %o SMOW and &D =-57 to -91 %o SMOW) and that is rather explained by large admixture of meteoric water in the system and subsequent mixing with the magmatic component.

These different fluid origins are confirmed by laser ablation ICP-MS analyses of the three- and two-phase primary inclusions. The concentrations of the major ore elements, i.e. W, Cu, Zn and Pb, decrease through the paragenesis and, W, and to a lesser extent Cu, show high variations, associated to a steep decrease in concentration. The decreasing concentrations can be explained by mineral deposition and differences in the rate of decrease indicate selective precipitation. Fluid inclusions of the last stages show an abrupt increase of Ba and Sr concentrations. lt points out a higher volume of silicate alteration, probably due to the larger size of the fluid flow cell and is explained by the input of a fluid from a different origin. LA-ICP-MS analyses show that the fluids were already depleted in W and Cu before reaching the carbonates, whereas Zn and Pb were still present in considerable amounts. This is again due to the selective precipitation and tells us that the economically interesting metals were dominantly introduced by magmatic fluids.

Introduction these deposits can be clearly shown to be carbonate replacement deposits at the periphery of igneous centres and, therefore, they have been interpreted to be High-temperature, carbonate-hosted genetically related to each othei- (Megaw et massive sulfide ore deposits constitute a al., 1988; Bookstrom, 1990; Sillitoe and distinctive genetic class recognized in Bonham, 1990; Titley, 1996). This Cordilleran orogenie systems in the magmatic model has been disputed by Western United States, Mexico and Peru sorne authors. For instance, Beaty et al. (Einaudi, 1977; Megaw et al., 1988; Beaty (1990) have suggested that sorne et al., 1990; Titley, 1996). Their carbonate-hosted base metal deposits ofthe metallogenic characteristics are similar; Central Colorado Mineral Belt, U.S.A. most consist of pyritic-rich ore with base­ were initially formed by MVT-like and precious-metal sulphide, they are processes and were later modified by typically invariably hosted by carbonate igneous hydrothermal processes. A rocks and are stratabound in character, but different model invokes synsedimentary with clear features of stratigraphie processes based on the stratabound to discordance, such as veining and breccia stratiform geometry of the carbonate­ pipes. Their genetic interpretation has been hosted deposits, as for instance in Central a matter of debate, and a variety of Peru (Dalheimer, 1990; Kobe, 1990a). different models have been invoked. As in Recent geochronological, isotopie and the San Cristobal district, Peru, sorne of structural studies has demonstrated that the 77

SIMPLIFIED GEOLOGICAL MAP OF THE SAN CRISTOBAL DISTRICT

N -/ ./ / / / / / / / / / / / 1 km

Tertiary Intrusions Mineralised veins

Chulec Formation Unmineralised faults

Goyllarisquizga Group Carbonate replacement ore bodies

Pucara Group Name of principal ore bodies

Mitu Group Na me of Tertiary intrusions

Excelsior Group A-B: Section of Figure 3.2

Fig. 3.1: Location of the San Cristo bal district, simplified geological map of the San Cristo bal district and location of the vein systems and principal ore bodies.

entire San Cristobal district in Peru is product of a single hydrothermal fluid related to Miocene magmatic activity evolving with distance from an intrusive (Beuchat et al., 2001ab; Moritz et al., center (Sillitoe, 1976). However, recent 2001), syn-sedimentary and MVT-like field and geochemical data from a number processes can then be excluded in this of North and Central American districts case. have been interpreted in terms of magmatic fluid and basinal brine mixing (Megaw et al., 1996; Smith, 1996). Mineral precipitation mechanisms have also been In settings with an obvious magmatic subject to controversy as previous stable genetic link, such deposits have isotopie data can be interpreted in terms of traditionally been considered to be a mixing or fluid-rock interaction (Campbell 78

A 8 sw NE ~ <- 5000m :;;~e;=~::t~•_.:::r~ ,,~ ~ ~:0 ~~ ~ ~g ~~:::::==~:0..:.,, NIVEL 220 :"' ' , KWEl 270 •. NIVEL320 "i"""'f'""'"'-- KWEL370 """""~..__ !:'!'{~L430 ~------N['i!':LSOO _NI '{~L580 4500m. hJ'{i;_L 630

NN EL 730 SAN CRISTOBAL INTRUSION CHUMPE INTRUSION

Tertiary Intrusions Mitu Group ::. :: F. ; 1 Quaternary

Pucarà Group Excelsior Group ,~ Mined mineralization 1 km

Fig. ~.2: Synthetic longitudinal section of the San Cristobal vein with location of fluid inclusion samples. Locatton of the cross section on Figure 3.1.

et al., 1984; Heinrich, 1990). The San sedimentary rocks and Miocene intrusions Cristobal district in Peru presents an (Fig. 3.1). The Excelsior Group is the excellent opportunity to test whether fluids oldest rock unit exposed in the district and of different origins are present or not, and, it crops out in the core of the Chumpe on the other hand, the depositional Anticline (Fig. 3.1). This is a heterogeneous unit, which includes shales mechanism involved in the precipitation of ' ore. In this paper, we present a large set of phyllites, limestones, quartzites, and new isotopie and microthermometric and basaltic flows where pillow lavas can be LA-ICP-MS fluid inclusion data that argue recognized in places (Kobe, 1990b). A strongly in favor of multi fluid-mixing distinct angular unconformity exists processes. However, this study shows that, between the Excelsior phyllites and the if fluids from different origins have overlying Mitu volcanic rocks. The affected the entire area, base metals in that Triassic-Jurassic carbonate of the Pucara district were mainly provided by a Group rests unconformably on the Mitu magmatic fluid. Group, it is intensively brecciated at its base. The Pucani Group is overlain by different sedimentary Cretaceous units. See Chapter II for a more detailed description Geo/ogical setting of the different units. Igneous rocks of Tertiary age are well exposed in the Domo de Y auli area and are Regional Geology best represented in the Morococha district. At San Cristobal, three different intrusions have been recognized: these are the The San Cristo bal district is located Il 0 Carahuacra, San Cristobal and Chumpe 40 39 km east of Lima in the Western Cordillera intrusions (Fig. 3.1). Ar/ Ar and (Fig. 3.1 ). It is mainly composed of structural data indicate that ore processes Paleozoic phyllites of the Excelsior Group, are temporally related to the Chumpe Permo-Triassic volcaniclastic rocks of the intrusion dated at 6.6 (+ 11-3.6) Ma by Mitu Group, Triassic-Jurassic carbonate U/Pb of zircons (Beuchat et al., 2001 ). It rocks of the Pucani Group, Cretaceous consists of a series of small plugs and 79

lia lib Ill Quartz 1 •••1111 ------.ça----~-· ____ Pyrite

Wolf ra mite

Muscovite

Chalcopyrite

Arse no pyrite Sphalerite - Gale na

Freibergite Il

Pyrargyrite

Bismuthinite

Argentite

Mn-rich siderite, ankerite, rhodochrosite ~-- Dickite Modified after Campbell (1984), 1111 Ba rite Stucky (2001) and Lisboa (2002)

EARLY 1 BASE METAL LATE STAGE STAGE STAGE

Mn-rich siderite

Hematite

Quartz

Pyrite Fe rich Fe poor Sphalerite 11111111

Chalcopyrite Gale na ~~---- 1 Pyra rgirite 1111111

Tertahedrite 1111111

Marcasite

Magnetite

Ba rite Dickite 1111111111111111- Native silver

Covellite Modified after Bartlett (1984) Mn oxides and Sallier (2002)

EARLYI BASE METAL THIRD SUPERGENE 1 1STAGE STAGE STAGE 1 STAGE Fig. 3.3: Paragenetic sequences from veins (up) and carbonate replacement ore bodies (down) 80

Fig. 3.4: Ore sample of the veins. (a) Early stage showing vein of quartz and wolframite crosscutting massive pyrite. Sample CHU-1. (b) Small crystals ofwolframite (wf) hosted in chalcopyrite (cp) ofthe base metal stage. Sample DY-603. (c) and (d) Typical concretion texture on two different scales indicating repetitive mineral deposition. Samples 722-1, SC-11 . (e) Breccia texture with clast of the base metal stage consisting of sphalerite, gal ena and quartz, set in a matrix of carbonate and trace amount of barite. Sample SC-1 . (f) Breccia texture on a small scale showing becciation event during the time of the base metal stage. Sample CARMEN-2. dykes which have been intruded along the 105 MT Pb, 4.6 * 105 MT Cu et 2.1 * 108 axis of the Chumpe anticline (Fig. 3.1). oz Ag (R. Bendezu, pers. comm.). See Chapter 1 for a more detailed description of the Tertiary intrusions. Vein ore type Two ore deposit types are recognized in the San Cristobal district: Zn-Pb-Ag veins and carbonate replacement bodies. Their Nearly ali-major transverse faults present cumulative production during the 20th in the San Cristobal district are mineralised century attained 1.9 * 106 MT Zn, 6.4 * to sorne extent and the larger ones are 81

presently exploited for Zn, Pb, Cu and Ag. pyrite, sphalerite, chalcopyrite, galena and Whereas faults crosscut rocks of ali quartz (Figs. 3.3 and 3.4c-f). Accessory lithologies (Figs. 3.1 and 3.2), vein ore minerais include marcasite, pyrrhotite, type is restricted to Excelsior phyllites, arsenopyrite, freibergite, boulangerite, Mitu volcanic rocks and Tertiary allabandite, bismuthinite, argentite, intrusions. The San Cristobal district can pyrargirite and stannite (Stucky, 2001; be subdivided in two zones displaying Lisboa, 2002). Brecciation and concretion veins with economie importance, the textures are common (Figs. 3.4c-f), and the Andaychagua and the San Cristobal vein sequence of precipitation within the base systems (Fig. 3.1). These vein systems are metal stage is partly repetitive (Fig. 3.3). composed of series of sub-parallel veins, During the year 2000, ore grades were see Beuchat and Moritz (in prep.) for a around 6.1 wt% Zn, 1.1 wt% Pb, 0.24 wt% detailed description of the vein system and Cu and 3.5 oz/T Ag (Volcan, 2001). These the structural features. V eins from the ore grades are highly variable, since the Andaychagua and the San Cristobal ore minerais are not equally distributed. systems have a similar mineralogie and For example, Cu ore is highly enriched at paragenetic sequence (Fig. 3.3). However the contact between the volcanic rocks of studies on specifie veins reveal small the Mitu Group and the Excelsior phyllites. discrepancies in the accessory phases, such However, zonation of ore grade, as pointed as presence of wolframite or Ag-bearing out by Pastor (1970), was not confirmed by sulfosalts (Campbell et al., 1984; Stucky, detailed studies on the 722 and 2001; Lisboa, 2002). This is probably due Andaychagua veins (Stucky, 2001; Lisboa, to the position of the studied locations 2002). Finally, the last stage is mainly relative to the Chumpe intrusion. Detailed composed of large crystals of investigations of hand-specimen, polished automorphous quartz associated to the and thin sections indicate a depositional deposition of carbonates such as sequence that can be subdivided in three rhodochrosite, Mn-rich siderite and main stages (Fig. 3.3). The early stage, ankerite (Figs. 3.3, 3.4c and 3.4e). Close to known as tungsten ore, is characterized by the contact between volcanic rocks and tabular crystals of wolframite and limestones, they crystallize in vugs and automorphous crystals of pyrite, generally develop nice rhomboedric crystals and may enclosed in quartz (Fig. 3.4a). The quartz be followed by deposition of barite. In is milky and shows undulatory extinction other locations they form rather colloform in thin section. Sorne crystals in pressure textures and, instead of barite,' small shadows of pyrite and/or wolframite show pockets of dickite are present. no sign of deformation. Trace amounts of sericite and augelite can be observed in places (Campbell et al., 1984). The Hydrothermal alteration of the volcanic distribution of this early stage is directly rocks of the Mitu Group and the Miocene linked to the proximity with the Chumpe intrusion adjacent to the veins consist intrusion, as already observed by Pastor principally in two types of alteration, (1970). For example, tungsten ore is totally defined sericite-argillic and chloritic. The absent in veins that do not crosscut the sericite-argillic alteration is recognized in a Chumpe intrusion. In the rare places where zone of a few meters and results in the very small wolframite grains are observed, conversion of primary plagioclase, biotite they show brecciation textures and are and amphibole into sericite, kaolinite and surrounded by the base metal stage (Fig. dickite (Fig. 3.4g). Sericite has been dated 3.4b ). The base metal stage is by Beuchat et al. (200 1b) at 4. 78 ± 0.16 volumetrically by far the most important Ma. This zone is also correlated with the and is dominated by the precipitation of 82 pervasive development of pyrite. As already noted by Pastor (1970), the volume Carbonate replacement ore type of the sericite-argillic alteration decreases westwards. The chloritic alteration is up to tens of meters wide and characterized by Zn-Pb-Ag high-temperature carbonate the development of chlorite and carbonate. replacement deposits are hosted by the carbonate rocks of the Pucarâ Group and 14 occur along the contact with the underlying Mitu Group; their position coïncides with 300 the location where veins extend from the Mitu rocks into the Pucarâ limestones (Fig. 250 3.1) (Bartlett, 1984; Dalheimer, 1990). Ê Even if the early tungsten ore present in 0.. 200 8 ë veins crosscutting the Chumpe intrusion is

2 This base metal stage is preceded by the deposition of specular hematite and crystals of automorphous quartz up to 1 cm

0 5 10 15 long (Figs. 3.3 and 3.6a). Bodies of Distance from vein (m) hematite occur preferably close to the 1~ •1~ •1 contact with the volcaniclastic rocks of the SERICITE- CHLORITE ARGILLIC ALTERATION Mitu Group. This early stage is associated ALTERATION to the pervasive alteration of the dolomitic­ Fig. 3.5: Elements variation in the volcanic rocks of limestones of the Pucarâ group converted the Mitu Group at contact of the Andaychagua vein. into manganosiderite (Bartlett, 1984; Sallier, 2002). This alteration is best These two alteration zones are less developed close to the major ore bodies developed m phyllites but are still and decrease outward producing a zoning observable. Additionally strong with an hematite center and an outer zone silicification can occur in phyllites. of manganosiderite altertation. Chemical analyses confirm this zonation Beginning of the base metal stage is with a strong increase of iron and rubidium marked by the replacement of hematite by contents and decreasing contents of pyrite (Figs. 3.6a and b), Bartlett (1984) strontium, calcium and sodium content has also observed replacement by towards the vem (Fig. 3.5). This pyrrothite. Following this replacement, the corresponds to the crystallization of major sulfide stage began with sericite and pyrite and the dissolution ·of precipitation of dark brown, iron-rich feldspar. sphalerite. This sphalerite replaces early hematite and pyrite and fi11s voids in the ore. Bartlett (1984) noted a decrease of iron content of sphalerite with the paragenesis. The early, iron-rich sphalerite contains numerous chalcopyrite blebs (Fig. 3.6c) whereas the later, iron-poor sphalerite is free of them (Sallier, 2002). 83

Fig. 3.6: Ore sample of the mantos. (a) Early stage of massive hematite and automorphous quartz filling voids, replacement by pyrite. Sample S04-76.8. (b) Pyrite replacing needles of hematite. Sample DY-443. (c) Chalcopyrite blebs in iron-rich sphalerite.Sample DY-463. (d) Botryoidal pyrite. Sample DY-444. (e) Mineralised tuff with hematite, pyrite, sphalerite and dickite filling vugs. Sample LYD-2. (f) Polished section in reflected light showing needles of hematite replaced by magnetite at the contact of carbonate. Sample DY-473.

The presence of chalcopyrite completely with iron-poor sphalerite (Bartlett, 1984). replacing sphalerite may explain the blebs Severa! events of brecciation occur and by replacement processes as suggested by sorne cracks in iron-poor sphalerite and Barton and Bethke (1987). Only minor galena are filled by later iron rich galena is associated to the iron-rich sphalerite indicating repetitive stages. Due sphalerite, whereas it systematically to this observation and the texture of the accompanies the later iron-poor sphalerite. deposited mineral, i.e. void filling or The galena contains a few inclusions of colloform texture (Fig. 3.6d), Sallier sulfosalt, but the majority of pyrargirite (2002) proposed two principal sulfide and tetrahedrite is located at grain contacts stages. Sallier (2002) also pointed out that 84 the succession from void filling to colloform textures are inverse in sorne Previous S, Sr and Pb isotopie locations. However, in our opinion, this data separation cannot be applied to an entire ore body and may reflect local phenomena. Dickite crystallizes as vug filling during Previous studies (Bartlett, 1984; Campbell, the end of the base metal and the third 1987; Moritz et al., 2001; Stucky, 2001; stage (Fig. 3.3 and 3.6e). By contrast with Lisboa, 2002; Sallier, 2002) have produced the carly hematite-manganosiderite stage, a considerable amount of S isotopie data no zoning has been pointed out in the base on the San Cristobal district. The study of metal stage. Campbell (1984) was restricted to the San Cristobal vein and principally pointed out an increase of 834S values with time. The A third mineralisation event is recent studies have done a representative characterized by predominant precipitation sampling over the whole district (Tables of carbonate, barite and marcasite. Siderite 3.1 and 3.2), which indicate that if 834S and marcasite were deposited as small values obtained in the vein system are veinlets crosscutting massive ore and restricted to a narrow range (8 34 S between filling vugs within the ore (Bartlett, 1984). 0.5 and 9.2 %oCDT), that range becomes Barite was deposited slightly later and its wider in carbonate replacement ore bodies, distribution is more irregular; it seems to especially toward lower 834S values (834S be more important in distal ore bodies between -3.5 and 7.9 %oCDT) (Fig. 3.7). relative to the Pucani-Mitu contact. Lead isotopie data have been obtained Crystallization of barite ceased before mostly on galena and accessorily on botryoidal carbonate was deposited. sphalerite and wolframite from vein and Magnetite pseudomorphically replaces carbonate replacement ore bodies (Table specular hematite; Barlett ( 1984) pointed 3.3). Their Pb isotopie compositions are out that this occurs mostly in contact with homogeneous 06PbP04Pb = 18.676 to siderite (Fig. 3.6f). Ail phase depositions e 18.840; 207PbP04Pb = 15.615 to 15.649; are associated with minor precipitation of 208PbP04Pb = 38.704 to 38.827) and sphalerite. Finally a supergene event took overlap with those of the Miocene place with precipitation of native silver, intrusions 06PbP04Pb = 18.698 to 18.761; covellite, manganese oxides and e 207PbP04Pb = 15.635 to 15 .669; 208PbP04Pb rhodochrosite. As mentioned previously, = 38.682 to 38.787) (Fig. 3.8). This is the alteration affecting the Pucara consistent with a predominant magmatic limestones consist principally of secondary lead input from the Miocene intrusions into carbonates. Abundant layers of the early and base metal stages of ore volcaniclastic tuffs in the Pucara Group deposits (Moritz et al., 2001). underwent alteration to dickite, kaolinite, illite and chlorite (Sallier, 2002)(Fig. 3.6e). Unlike lead, Sr isotopie ratios of late barite are highly variable (87Sr/86Sr = 0.712187 to 0.722782) and too radiogenic to invoke Miocene magmatism as a main source of Sr, and by inference Ba (Fig. 3.9). The influence of the host rocks is particularly evident since the most radiogenic barites are hosted by phyllites of the Excelsior Group (Fig. 3.9). 85

Table 3.1: Sulfur isotopie values of sulfides

834S 834S 834S Sample Mineral ("'oo COD Sample Mineral (%o CDT) Sample Mineral (%o CDT)

San Cristobal vein system Andaychagua vein system Carbonate replacement

ML-3 Pyrite 5.8 810 Om Arsenopyrite 5.7 DY-444 Pyrite -1.8 SC-7 Pyrite 7.3 869 A rs:nopyrite 6.9 DY-459 Pyrite 7.0 DY-5A Pyrite 3.8 816 c A rs=nopyrite 6.2 DY-473 Pyrite 5.5 DY-5 B Pyrite 6,0 DY37C Pyrite 4.5 DY-12 B Pyrite 5.3 DY-7 C Pyrite 6.7 815 Pyrite 4.7 DY-12 F Pyrite 6.2 DY-7 0 Pyrite 6.8 816 c Pyrite 5.2 DY-12 Hb Pyrite 7.3 DY-7 E Pyrite 5.5 816 b Pyrite 7.3 TOL-2 Sphalerite 5.4 DY-7 F Pyrite 5.8 825 Pyrite 8.3 TOL-1 Sphalerite 1.7 DY-7G Pyrite 6.0 853 Pyrite 7.0 504-91.5 Sphalerite 5.7 DY-8 Pyrite 6,0 859 Pyrite 6.1 DY-494 Sphalerite 7.0 DY-15A Pyrite 5.3 816 e Pyrite 5.9 DY-470 Sphalerite 7.6 DY-15 B Pyrite 5.8 824 cd Chalcopyrite 9.0 DY-91S Sphalerite 5.6 DY-15 C Pyrite 5.9 861 Chalcopyrite 9.2 DY-2 Sphalerite -0.8 DY-626 Pyrite 4.3 871 f Chalcopyrite 6.6 DY-4 Sphalerite 5.3 DY-610a Pyrite 4.5 870 Chalcopyrite 8.8 DY-12 F Sphalerite 7.3 DY-629 Pyrite 4.7 810 Om ~halerite 6.5 DY-12 G Sphalerite 7.5

DY-7A Chalcopyr~e 5.7 821 Om ~halerite 5.6 DY-12 Hb Sphalerite 6.1

DY-7 E Chalcopyr~e 5.1 815 ~halerite 6.0 DY-121 Sphalerite 6.9 DY-7G Chalcopyrrte 7.3 814 ~halerite 0.5 DY-4 Sphalerite 5.8 DY-14 Chalcopyrrte 5.6 816 e ~halerite 7.1 DY-533 Gal ena 2.6 DY-15 B Chalcopyrrte 6,0 871 b ~halerite 9.2 DY-411 Gal ena 3.1 DY-603 Chalcopyrrte 7.8 825 ~halerite 8.9 DY-2 Gal ena -3.5 DY-5 B Sphalerite 6.1 825 ~halerite 7.9 DY-121 Gal ena 4.1 DY-7A Sphalerite 5.8 840 ~halerite 8.1 DY-7 C Sphalerite 6.7 824 cd ~halerite 9 .7 DY-8 Sphalerite 6.3 824 cd ~halerite 9.7 DY-10A Sphalerite 5.9 853 ~halerite 7.9 DY-11 Sphalerite 6.6 856 ~halerite 7 .8 DY-14 Sphalerite 7.7 861 ~halerite 7.6 DY-15 C Sphalerite 5.7 871 f ~halerite 6.9 DY-677 Sphalerite 5.5 871 d ~halerite 9.4 DY-8 Sphalerite 6.3 871 c ~halerite 9.2 DY-605 Galena 3.2 870 ~halerite 8.5 DY-646 Gale na 4,0 DY-37 C ~halerite 5.1 DY-5 A Gal ena 4.5 DY-13 A ~halerite 6.2 DY-6 Galena 3.3 DY-13 B ~halerite 5.9 DY-8 Galena 4.4 832 ~halerite 9.3 DY-10 A Galena 3.3 DY-13 A ~halerite 5.9 DY-11 Galena 3.6 832 ~halerite 9.7 OY-14 Galena 3.6 DY-13 A Ga ena 3.5 821 Om Ga ena 2.7 825 Ga ena 7.9 825 Ga ena 4 .9 832 Ga ena 6.7 869 Ga ena 5.3 871 d Ga ena 5.5 871e Ga ena 5.9 871 b Ga ena 5.9 data are from Moritz et al. (2001 ), Stuc ky (2001 ), Lisboa (2002) and Sa !lier (2002); uncertainities are equal to.:!: 0.2 %o 86

Table 3.2: Barite isotopie compositions

0180 34S s7Sr/sssr o Sample Location Host rock (%o SMOW) (%o CDT)

Vein type

M-1 Veta Prosperidad Excelsior 0.720721 (7) 9.5 24.6 M-2 Veta Prosperidad Excelsior 0.719662 (7) 11 .1 24.9 DY-681 Veta 722 Mi tu 0.718703 (8) 8.3 14.6 OYA5 Veta San Cristobal Excelsior 0.722782 (9) 16.6 23.4 OYA6 Veta San Cristobal Mi tu 0.712187 (8) 9.4 11 .9

Carbonate replacement type

M-3 Manto Toldorrumi Pucara 0.722311 (10) 12.7 22.7 M-4 Manto Toldorrumi Pucara 0.719459 (8) 6.3 29.4 Esc 1 Manto Escondida Pucara 0.719593 (7) 6.9 13.6 Esc 2 Manto Escondida Pucara 0.719523 (7) 12.6 17,1 Moises A Manto Moises Pucara 0.715302 (8) 3.6 17.5 Moises B Manto Moises Pucara 0.715648 (11) 3.6 16.8 Gavilan R-1 Manto Gavilan Pucara 0.712867 (5) 11.4 17.7 Gavilan R-1 b Manto Gavilan Pucara 0.713281 (8) 11.7 17.5 data are from Moritz et aL (2001); uncertainties are equal to::. 0.7 %o for o'4 S and ::t 0.4 o/oo for o"O

Analytical methods The C02 content of the fluid inclusions was determined using the method of Rosso and Bodnar (1995). Our experiment was For the microthermometric fluid inclusion performed on a Labram Raman microprobe with a modified Olympus microscope. study, 100 !-lm thick double polished wafers were prepared for quartz samples. Raman spectra were recorded using the Measurements were performed on a 534 nm line from a Coherrent DPSS 534 Linkam THMSG-600 heating-freezing Nd-Y AG laser. stage mounted on a DMLB Leica microscope equipped with a Nikon 100x long working distance lens. The system The used LA-ICP-MS consists of an Elan was calibrated with synthetic fluid 6000 ICP-MS instrument (Perkin-Elmer) inclusions at - 56.6°C, 0.0°C and 374.1 oc combined with a 193-nm excimer laser (Stemer and Bodnar, 1984). Low (Compex 110 I, ArF, Lambda Physics; temperature measurements have a Günther and Heinrich, 1999). Fluid precisiOn of 0.1 °C, whereas high inclusions of this study tended to temperature ones have a precision of 1°C. decrepitate during analysis, as one tried to Salinities were calculated from the final ice ablate them directly with a large bearn size. melting temperature for bi-phase H20- This was avoided by opening the analyzed NaCl-(KCl) inclusions (Potter et al., 1978). inclusion with a pit much smaller than the In the case of tri-phase inclusions, inclusion and then by increasing the size of salinities were calculated either by halite the pit in severa! steps as soon as the first dissolution (Stemer et al., 1988) or by signal appeared. It allowed us to improved volume ratios (Shepherd et al., 1985), for reproducibility for elements that are cases where fluid inclusions decrepitate present in daughter crystals (Günther et al., prior to the final dissolution ofhalite. 1998). Pits of 4, 10, 20, 40 and 60 !-lm in 87

diameter were used. The resulting signais Hydrogen and oxygen isotope analyses on were integrated and compared to signal minerais and fluids were performed at intensities from reference material (NBS Queen's University, Kingston, Canada. 61 0 glass from NIST), allowing calculation Sphalerite samples in which fluids were of relative elements abundances in the analyzed, have been carefully selected in inclusion (Longerich et al., 1996). The order to avoid as much as possible elemental ratios were transformed into secondary fluid inclusion trails. The values absolute concentration by using an internai are reported as per mil deviations relative standard with the aid of the equivalent to the SMOW standard. Errors reported are salinity determined by microthermometry. ± 0.2 %o for oxygen in minerais and ± 2 %o The NaCl equivalent value was corrected in fluids. The error for hydrogen is ± 5 %o. for other major cations present in the inclusion (K, Fe and Mg) according to

Heinrich et al. (1992). In fluids that are 208Pbl04Pb dominated by Na, the uncertmmty 41.3 ....------: associated with this correction is estimated to be around 10 to 20 % (Audétat et al., 41 .1 Excelsior 2000). phyllites

py cp si gn bar i 30 / San Cristobal 1 20 vein system 38.9 ' R-~niary iY1agïral;ç.l/ rocks / / [J ,. ..., / Mitu volcanic rocks 10 38.7 '------··-----

38.5 -~--~-~--.--..--~---.t•~----1 18.6 18.8 19.0 22.3 22.5 206Pb/:

A Wolframite of the San Cristobal vein

Base metal vein stage

.t. Galena of the San Cristobal vein system • Ga lena of the Andaychagua vein

Base metal stage from the carbonate replacement o Sphalerite of the carbonate replacement ore bodies • Gale na of the carbonate replacement ore bodies

Fig. 3.8: 208PbP04Pb vs 206PbP04Pb diagram for =c: Minimum- Maximum galena, sphalerite and wolframite from ore deposits D 25%-75% of the San Cristobal district. Tertiary magmatism, • Median value Mitu volcanism and Excelsior Group fields are Fig. 3.7: Box plot of D 34S values from the San from Moritz et al. (2001). Cristobal vein system, the Andaychagua vein system and the carbonate replacement ore bodies. py: pyrite, cp: chalcopyrite, si: sphalerite, gn: galena and bar: barite. Data are from Moritz et al. (2001), Stucky (2001), Lisboa (2002) and Sallier (2002). 88

Table 3.3: Lead isotopie compositions

Sample Ore type Mineral 205Pb/204 Pb 207Pb/204Pb 20sPb/204Pb

Early vein stage

CHU-1 San Cristobal vein wolframite 18.840 15.642 38.793 SC-13 San Cristobal vein wolframite 18.737 15.649 38.824

Base metal vein stage

DY-13 Andaychagua vein ga lena 18.676 15.642 38.707 DY-13 A Andaychagua vein ga lena 18.760 15.640 38.783 DY-14 Andaychagua vein ga lena 18.753 15.631 38.760 DY-OSA San Cristobal vein gal ena 18.777 15.634 38.794 DY-06 San Cristobal vein ga lena 18.742 15.641 38.786 DY-08 San Cristobal vein ga lena 18.772 15.647 38.818 DY -10a San Cristobal vein ga lena 18.771 15.646 38.825 DY -11 San Cristobal vein gal ena 18.783 15.644 38.827

Base metal stage from carbonate replacement

DY-02 Carbonate replacement ga lena 18.726 15.636 38.761 DY-121 Carbonate replacement ga lena 18.722 15.637 38.767 804-91,5 Carbonate replacement sphalerite 18.727 15.615 38.704

data are from Moritz et al. (in prep); unœrtainties are 0.07% for '""Pb!"'' Pb, 0 14% for '"'Pb!"" Pb and 0.10% for ·o:'Pb!''"Pb

Fluid inclusion petrographv, Chumpe microthermometry and Raman intrusion spectroscopy --+ Condorsingua Excelsior phyllites formation 0.741 -0.755

The analyzed fluid inclusions of this study are trapped in quartz from vein and carbonate replacement ore bodies. The 0.706 0.710 0.714 0.718 0.722 paragenetic sequence of the veins indicates " Sr/'' Sr three successive quartz stages related to a different type of mineralisation; quartz 1 is f--] Hosted in Mitu volcanic rocks associated to the early W stage, quartz II to the base metal stage and quartz III to the ~ Hosted in Pucara limestones late stage (Fig. 3.3). Unfortunately, quartz Il Hosted in Excelsior phyllites is rare in the carbonate replacement paragenesis and only present at the end of the early stage (Fig. 3.3); therefore it may Fig. 3.9: Histogram ofbarite 87Sr/86Sr ratios (Moritz just give information of the beginning of et al., 2001). Data of the Chumpe intrusion, Mitu this mineralisation process. Table 3.4 gives volcanics and Excelsior phyllites are from Moritz et an overv1ew of the fluid inclusion al. (2001). 87Sr/86Sr ratio of the Condorsingua characteristics. formation is from Moritz et al. (1996). 89

1 mm

Type Ail: Tri-phase primary fluid inclusions, n° 1-7

Type Alli: Bi-phase primary fluid inclusions, n° 8-24

.,0

Fig. 3.10: Petrography of the early vein stage representing the five types of inclusion present in quartz I. Typical large inclusions of type Ail, Alli and AIV are represented by photomicrograph. Inclusions are not to scale. \0 0

Table 3.4: Summary table of the fluid inclusions recognized in this study, including the descriptive properties, microthermometry and C02 content deduced from Raman spectroscopy.

Family Number Location and terminology* Morphology Size N Salinity Homog. T.** co2 of phases (~~m) (wt % NaCI equiv.) (oC) (g/cm' )

Early vein stage (wolframite-pyrite-quartz /), samp/e FPE-4a

Ail 3 Clusters of few inclusions within grains; prima ry rounded 7 to 15 7 44 to 54 205-259 Alli 2 Clusters of few inclusions within grains; primary rounded 8 to 20 17 2.9to 5.1 146-237 Ota 0 .6 Al V 2 along !rails crosscutting grain edges; secondary irregular 8 to 50 35 0.4 to 5.4 154-257 Ota 0 .5

Base-metal vein stage (base metal su/fides-quartz//), samples DY-37C, 816c, OYA-1

BI 2 along trail within grains; pseudosecondary thin and tubular 8 to 20 3 3.9to 4.0 328-337 Ota 0 .3 Bll 2 along !rails crosscutting grain edges; secondary elongated, irre gualr 5 to 15 15 2.7to 4.2 155-256

Late vein stage (carbonate-barite-quartzJ/1), sample 722-10

Cl 2 along grow zone; primary elongated 15 to 50 8 5.9 to 6.7 302-322 0 to 0.4 Cil 2 Sm a Il cluster within grains; primary rounded 20 to 60 3 5.4to6.2 263-275 Cl li 2 along crosscutting ribbon; secondary irregular 15 to 50 11 4.6 to 6.2 274-320 0 to 0.4

Carbonate replacement, samp/e, samples DY-554 and TOL-2

Dl 2 along grow zone; primary elongated and rounded 30 to 70 2 4.0 285-290 Ota 0.1 011 2 Small !rails within gra in; pseudosecondary irregular 7 to 25 8 0.2 to 4.8 166-290 Ota 0 .1

* Terminology according to Roedder (1984). **Ali inclusions homogenize to the liquid phase. 91

The quartz I associated to the early W An, but they are bi-phase and have stage is generally milky and it contains a more angular morphology (Fig. fluid inclusions along grow zones that are 3.10). Their homogenization generally decrepitated and form series of temperatures cover a larger range dotted black lines (Fig. 3.10, Type AI); few (146 to 238°C) and salinities vary inclusions have preserved their fluid from 2.9 to 5.1 wt% NaCl equiv. content but since they are connected by C02 was detected by Raman thin tubes and present variable phase ratios spectroscopy in two 20 J..lffi sized at room temperature, they were probably inclusions (inclusions n°11 and 17, necked down and therefore avoided. Intact 0.6 g/cm\ but the majority of the fluid inclusions could only be found in Raman analyses did not reveal any pressure shadows of coexisting wolframite presence of C02. and pyrite crystals. In such zones, four different types of H20-NaCl-(KCl) Type Aw inclusions are by far the inclusions can be recognized (Fig. 3.10, most abundant. They are bi-phase Table 3.4): and clearly secondary since they form large ribbons which crosscui Type AII are tri-phase rounded grain edges. They are strongly inclusions and have a diameter irregular and their size is generally between 7 to 15 J..lffi. They form larger, from 8 to 50 J..lffi, than the isolated small clusters of few previous types (Fig. 3.10). inclusions within grains, never Homogenization temperatures vary form trails and their locations are from 155 to 257 °C, while salinities free of secondary inclusions; they are between 0.4 to 5.4 wt% NaCl are therefore interpreted as primary equiv. Raman spectra of type A1v inclusions. Homogenization inclusions indicate variable temperatures range between 206 amounts of co2 with a density and 259 oc and the salinity varies 3 ranging from 0 to 0.5 g/cm , from 44.6 to 54.0 wt% NaCl equiv. inclusions where no co2 was Total dissolution of halite in order detected are the smallest ones, to calculate the salinity was below 15 J..lffi in diameter (Fig. obtained for only one inclusion 3.11 ). It indicates that the mass of prior to decrepitation (Appendix co2 contained in the smallest III, inclusion n°3 at 436°C). For the inclusions is too low to be detected other inclusions (Appendix III, and this must therefore not be inclusions n° 4 to 7), the salinity interpreted as an absence of calculated on the basis of carbonic gas. decrepitation temperature represents only a minimum value. Finally ·type Av inclusions consist In order to preserve sorne of numerous trails that crosscut ail inclusions for LA-ICP-MS other inclusion assemblages. They analyses, salinity were determined were not analyzed due to their very on the basis of phase ratios in two small size, below a few microns. cases (Appendix III, inclusions no 1 and 2). Raman spectroscopy did not permit to detect any co2 in the tri­ The wolframite and quartz fluid inclusions phase inclusions. of this early vein stage were already Type Am inclusions have a similar studied by Campbell and Robinson-Cook size and location as those of type (1987). As discussed below, their data on Am and Arv types indicate similar salinity 92 but higher homogenization temperature base metal stage samples. They than those measured in this study (Fig. have a size between 5 to 15 ~-tm and 3.12). if they are also elongated as type B 1, they are more irregular in 0,6 .------, • shape. They were only analyzed in • samples contammg the largest ~E 0.4 • -..u • inclusions, homogenization ~ temperature cover the whole range ON 0.2 • • ü • • between 155°C and 256°C and 1 0,0 salinities from 2.7 to 4.2 wt% NaCl 0 10 20 30 40 50 60 eqmv. Size (~-tm) Fig. 3.11: C02 content of AIV fluid inclusions versus their size A large set of fluid inclusion data exists on sphalerite associated with quartz II deposition, where criteria for primary The base metal stage is accompanied by origin are easier to determine (Stucky, abundant quartz II deposition (Fig. 3.3). 2001; Lisboa, 2002; Moritz et al., 2001). If Probably due to the successive brecciation primary inclusions in sphalerite cover more events, this quartz is milky and intensively or less the same range of temperatures than fractured with the consequence that few secondary inclusions observed in quartz II, inclusions survived and none of them are their salinity appears to be slightly higher unequivocally primary. The few of them (Fig. 3.14). The quartz secondary remaining can be divided in two types of inclusions probably post-date ore bi-phase and H 0-NaCl-(KCl) inclusions 2 deposition. (Table 3.4). With one exception no C02 was detected in fluid inclusions of quartz II, this may be due to its absence or to the Salinity wt% Na Cl equ. smaller size of these inclusions as observed 8,------. for type A v inclusions (Fig. 3.11): 1 • 6 Type B 1 corresponds to thin tubular inclusions of a maximum length of 4 (' 10 ~-tm (Fig. 3.13). They form trails R but do not crosscut grain edges, hence they are classified as 2 pseudosecondary inclusions • • according to Roedder (1984). They 0~--~0---~---~--~ have only been found in sorne 150 250 350 samples of the Andaychagua vein Homogenisation temperature (° C) and their shapes rend their microthermometric measurement ('· Type A, fluid inclusions difficult. The very few data 11 o Type A,v fluid inclusions obtained indicate homogenization e Quartz data, Campbell and Robinson-Cook (1987)

temperatures between 328°C and • Wolfram~e data, Campbell and Robinson-Cook (1987) 337°C and salinities ranging around 4 wt% NaCl equiv. Fig. 3.12: Sa1inity versus homogenization temperature of type Alli and AIV fluid inclusions Type Bff inclusions are numerous, in comparison with data obtained by Campbell and secondary and are found in each Robinson-Cook ( 1987). 93

They are isolated, far away from secondary inclusion trails and growth zones, and may then also be considered as pnmary. Their homogenization temperatures range from 263°C to 275°C and salinities vary between 5.4 to 6.2 wt% NaCl equiv. (Table 3.4).

Fig. 3.13: Photomicrograph of pseudosecondary Type C111 inclusions are located tubular inclusions of sample 816c. along trails starting from the crystal border (Fig. 3.15) and are therefore secondary m ongm. They homogenize between 253 and The latest stage of the vein paragenesis 319°C and have a salinity ranging consists of quartz III, Mn-rich siderite, from 4.6 to 6.2 wt% NaCl equiv. rhodochrosite, ankerite and barite (Fig. (Table 3.4). 3.3). Unlike quartz I and II, quartz III is automorphous and very clear, permitting Salinity wt% NaCI equ. reliable distinction among different types 8~------~ of inclusions (Fig. 3.15, Table 3.4). All inclusions of this stage are bi-phase, are 6 .. .. composed of H20-NaCl-(KCl) fluids and contain variable amounts of dissolved co2 4 Il.. with a density of 0 to 0.5 g/cm3:

Type Cr consists of large elongated 2 inclusions (up to 50 mm) along primary grow zones (Fig. 3.15). 0+-----~------r------~----~ 150 250 350 Numerous of the inclusions located m these grow zones are Homogenisation temperature CC) decrepitated and it may be • Pseudosecondary fluid inclusions in quartz Il, type ~ interpreted as possible o Secondary fluid inclusions in quartz Il, type B, reequilibration of inclusions. .A Sphalerite data (Moritz et al., 2001; Stucky, 2001 ; Careful observations reveal cracks Lisboa, 2002) close to the decrepitated inclusions. Fig. 3.14: Salinity versus homogenization However, part of the growth zone temperature of fluid inclusions contained in quartz are far from such breaks and have II in comparison with primary inclusions in escaped such resetting since phase associated sphalerite (Stucky, 2001; Lisboa, 2002; ratios are constant. Furthermore Moritz et al., 2001) homogenization temperatures and salinities are within a narrow range, from 302°C to 323°C and from 5.9 Inclusions in quartz III appear to indicate to 6.7 wt% NaCI equiv., very variable data, Lisboa (2002) report respectively (Table 3.4). They are primary inclusions of quartz III estimated to be primary in ongm homogenizing between 150°C and 246°C and not necked down. with salinities ranging from 2.4 to 4.5 wt% NaCl equiv. Whereas no fluid inclusions of Type Cu consists of three large sufficient size have been found m rounded fluid inclusions located at associated carbonates, barite crystals the center of a crystal (Fig. 3.15). 94

Type Cl: Bi-phase primary fluid inclusions n' 1 to 8

/""' /. .

•

T • •

-,- • ,. .: 301-'m •

f - Type CUl: ' Bi-pnase secondary , 1 mm fluid inclusions n• 12 to 22 If: /l

Fig. 3.15: Petrography of the late vein stage representing the three different types of inclusion. Largest inclusions of each types are shown on photomicrograph. Inclusion are not to scale. Qz: quartz, Rho: rhodochrosite, Anlc ankerite. 95

Fig. 3.16: Petrography of the earl y stage of carbonate replacement ore bodies representing the three different types of inclusion. Typical inclusions of two first types are shown on photomicrograph. Qz: quartz, Sph: sphalerite.

contain large inclusion (up to tens of microns) that homogenize between 136°C and 159°C and have a salinity ranging from 1.6 to 3.5 wt% NaCl equiv. (Lisboa, 2002). However, none of these inclusions . . are pnmary m ongm.

The paragenetic sequence of the carbonate replacement ore bodies contains only one / quartz generation which is associated to the early hematite stage and prior to sphalerite (Figs. 3.3 and 3.16). This quartz contains three different types of fluid 0.5 mm inclusions; all of them being bi-phase, with an H20-NaCl-(KCl) compositiOn and Type Dl: contain very low densities of co2 (0 to 0.1 Bi.phase primary fluid inclusion g/cm3) compared to A1v and C inclusions: n•1 and 2

Type D1 inclusions are rounded and very large, up to 70 J.liD in size, and are located along growth zones of automorphous crystals (Fig. 3.16). They are therefore primary in ongm. Homogenization temperatures range between 285°C and 290°C and salinities are around 4 wt% NaCl equiv.

Type D11 are aligned irregular inclusions located within grains (Fig. 3.16). These trails contain only few inclusions and do not crosscut grain edges nor typical secondary fluid inclusions trails. They are then interpreted as pseudosecondary fluid inclusions. Their homogenization temperatures

Type 011: and salinities are however very pseudosecondary ftuid inclusion variable within the same cluster, n•3 to 6 166 to 290°C and 0.2 to 4.8 oc 30 respectively. Due to this large 96

range, reequilibration cannot be As already mentioned above, fluid excluded, even if liquid/vapor inclusions in carbonate replacement quartz ratios are constant. are only representative of the early part of the hydrothermal system (Fig. 3.3). Type D111 consists of trails of However we can compare the data secondary inclusions. These obtained in quartz to those obtained in the inclusions are very small in size, a later sphalerite (Moritz et al., 2001 ). The few microns, and were therefore homogenization temperature display more not analyzed. or less the same range, but the vertical trends observed in clusters of sphalerite Salinity wt% NaCI equ. fluid inclusion and interpreted as 16 -,------, intermittent boiling of the moderately saline fluids (Moritz et al., 2001) is not 12 observed in the earlier quartz-stage.

8

4 0 0 [J As pointed out by Hedenquist and Henley ( 1985), salinity estimates based on final ice 0 150 250 350 melting measurements are frequently Homogenisation temperature CC) biased by the presence of volatile solutes such as C02. The amount of C02 dissolved + Primary fluid inclusions in quartz from carbonate in the fluid inclusions of this study is very replacement, type D, low and variable, but certainly led to u Secondary fluid inclusions in quartz from carbonate replacement, type D, misjudge the sodium chlorine content. L Sphalerite data (Moritz et al., 2001) However Raman spectrometry do not Fig. 3.17: Salinity versus homogenization allow us to detect co2 in the smallest temperature of fluid inclusions contained in quartz inclusions, as pointed out by the example of carbonate replacement ore type (primary of A1v inclusions (Fig. 3.11). Correcting inclusions: black square, secondary inclusion: white salinity on the basis of Raman square) in comparison with primary inclusions in spectrometry would therefore add an associated sphalerite (grey triangle)(Moritz et al., additional factor of error, due to the 2001). different sizes of the fluid inclusions.

Table 3.5: Oxygen isotope compositions

Sample Mineral Ore type

CHU-1 Wolframite Ear!y vein stage 2.8 CHU-1 Quartz 1 Early vein stage 13.8 722-4 Quartz Il Base metal vein stage 12.3 DY-644 Dickite Late vein stage 9,0 SC-3 Dickite Late vein stage 8.7 722-10 Quartz Ill Late vein stage 17.7 DY-494 Quartz manto Ear!y stage from carbonate replacement 13.9 LYD-2 Dickite Base metal stage from carbonate replacement 13.4 DY-498 Kaolinite Third stage from carbonate replacement 6.6

Uncertainties are equal ta .:. 0.2 %o for ô"O 97

Fluid inclusion LA-/CP-MS to the vein ore type and have therefore results excluded the carbonate replacement ore bodies. The isotopie compositions of wolframite, quartz and sphalerite of Campbell et al. (1984) show a well defined An-tv, Bn, Ct-m, Dt-Il inclusion types were vertical trend, which is confirmed by our analyzed for Na, K, Fe, Cu, Zn, Sr, Ag, Sn, analyses (Fig. 3.19). Indeed, the Ba, W and Pb whereas Mg, Ca, Mn, As, wolframite and quartz I samples are within Rb, Y, Mo, and Ce were only determined error close to the field defined by for a minority of fluid inclusions Campbell et al. (1984) and quartz II has (Appendix IV). Y, Mo and Ce were even lower bD values, which tend to generally below limit of detection (LOD) increase the vertical trend (Fig. 3.19). and are therefore not reported in Appendix However, hydrogen isotope analyses of IV. Inclusions with significant W content fluid inclusions remain difficult to are exclusively hosted by quartz I (Fig. interpret, while samples of quartz I and III 3.18). On the other hand, Cu, Zn, Pb and could be selected in order to avoid a As, distinctive elements of the base metal maximum of secondary fluid inclusion stage, are detected in all inclusion types; (Figs. 3.10 .and 3.15), this was nearly their content decreases through the impossible for quartz II, in which paragenetic sequence and secondary secondary inclusions form the great inclusions indicate lower concentrations majority. The bD values from inclusions in than their primary counterparts (Fig. 3 .18). quartz II should be taken with care; it may Sorne elements, such as Ba and Sr, obey be interpreted as resulting predominantly the exactly inverse tendency since their from secondary fluid inclusions. concentration is higher in quartz III than in quartz from earlier paragenetic stages (Fig. 3.18). K, Fe, Mn and Rb are elements that were detected in all paragenetic stages, In comparison to quartz I and II, quartz III their concentrations do not show any and quartz of the carbonate replacement 18 obvious variations between the different appear to be heavier in b 0 (Fig. 3. 19). quartz stages. However these elements can Hydrogen isotopie signatures of quartz are show a disparity in a single quartz stage, as systematically lower than the ones of for example the Mg concentrations in associated sphalerite and wolframite; it inclusion hosted by quartz III where C1 could be due to the influence of type inclusions contain thousands of ppm structurally bound water within quartz Mg whereas this element was not detected (Simon, 2001) but the vertical trend in secondary Cm inclusions (Appendix IV). observed for the two early stages remains Finally, numerous elements, such as Ca, valid even without the quartz data. Ag and Sn, were in the majority of the Hydrogen isotopie compositions of fluid cases below LOD. inclusions hosted by sphalerite are in the same range as those of Campbell et al. (1984). The oxygen isotopie compositions could not be determined since the amount H and 0 isotopes of water (0.0 1 to 0.2 mole fraction of water) was too low to permit precise analyses. This is principally due to the Hydrogen and oxygen isotope analyses selection of small amounts of sphalerite to (Tables 3.5 and 3.6) were performed in insure that it contained a large majority of order to complete the isotopie study of primary inclusions. Fluid in equilibrium Campbell et al. (1984) that was restricted with late dickite and kaolinite are close to 98

Mante ore type Qz 1 Qz Ill D, A, c, c, 10000

1000

100

10

• • 1000 e. 100

10

0 1000

100

10 • 1000

100 •

10

1000 ~:::: 0

0 u 0 100 0 []

10

1000

100

10 •

1000

Cl 0 100

10 99

the field of paragenetically associated Fig. 3.18: Evolution of the fluid W, Cu, Zn, Pb, Rb, carbonate as defined by Campbell et al. Sr and Ba concentration reconstructed from LA­ (1984) and by Sallier (2002). No hydrogen ICP-MS analysis and microthermometric study of analyses have been performed on inclusions of the three successive quartz of the vein carbonate fluid inclusions since in our and the single quartz stage of the carbonate samples they are principally filled with replacement ore type. Plain lines represent LOD secondary inclusions. and dotted line represent mean value of secondary fluid inclusion of each stage. Inclusions are sorted according interpreted petrographie timing and decreasing homogenization temperature.

80 (o/oo SMOW)

0 1 1

-40 2 1 ·= / Wolframit;

; carbonate•testage -80 :2 ,.,b. • u:::> ü: •

"0 -120 Quartz

~~ ro m -a~1 ..a <> cc V ' o ._ u (]) (]) -160 -20 -1 0 Carbonate 0 1 20

Ba rite Qz Ill 0180 (o/oo SMOW)

Early vein stage Early stage from the carbonate replacement e Wolframite of the San Cristobal vein <> Quartz of the carbonate replacement ore bodies + Quartz 1 of the San Cristobal vein Third stage from the carbonate replacement Base metal vein stage [:,_ Dickite and kaolinite of the carbonate replacement ore bodies <) Quartz Il of the San Cristobal vein

Late vein stage .Â. Dickite of the San Cristobal vein Fig. 3. 19: Hydrogen vs oxygen isotopie composition offluids in equilibrium with wolframite, quartz, sphalerite, bari te, carbonate and die kite according fluid inclusion data and equation of Clayton et al. ( 1972), Kusakabe and Robinson (1977), Land and Dutton (1978), Rosenbaum and Sheppard (1986), Carothers et al. (1988), Zhang et al. (1994) and Gilg and Sheppard (1996). Baritt'l 8 180 bar is from Moritz et al. (2001) and carbonate one from Sallier (2002). Grey fields of wolframite, quartz, carbonate and sphalerite and barite 8D barite bar are from Campbell et al. (1984). 100

Table 3.6: Hydrogen isotope compositions

Sam pie Mineral Ore type 60 (%oSMOW)

CHU-1 Fluid in wolframite Early vein stage -97 CHU-1 Fluid in quartz 1 Early vein stage -107 722-4 Fluid in quartz Il Base metal vein stage -137 DY-677 Fluid in sphalerite Base metal vein stage -79 DY-644 Dickite La te ve in stage -110 SC-3 Dickite La te ve in stage -101 DY-494 Fluid in quartz Early stage from carbonate replacement -109 LYD-2 Dickite Base metal stage from carbonate replacement -57 DY-12 Fluid in sphalerite Base metal stage from carbonate replacement -72 SCW2-302.2 Fluid in sphalerite Base metal stage from carbonate replacement -65 DY-494 Fluid in sphalerite Base metal stage from carbonate replacement -62 S17-87 Fluid in sphalerite Base metal stage from carbonate replacement -87 S04-91.5 Fluid in sphalerite Base metal stage from carbonate replacement -75 DY-470 Fluid in sphalerite Base metal stage from carbonate replacement -65 DY-498 Kaolinite Third stage from carbonate replacement -91

Uncertainties are equal to ~ 5%o quartz I (Au and A111), with more or less similar homogenization temperatures but Discussion different salinities (Fig. 3.20). The coexistance of these high and low salinity phases is not demonstrated since these two inclusion types have not been found in the Early vein stages same growth zone. However, the fact that these inclusions are both isolated in center of quartz I crystals (Fig. 3.1 0) suggest a With the exception of the diagenetic model close timing between the high and low of Dalheimer (1990) and Kobe (1990a) salinity fluids. It may suggest splitting that was discarded by several authors within the immiscibility field (Bodnar et (Moritz et al., 2001; Sallier, 2002), the al., 1985), and thus a probable initial genetic model associated to the formation magmatic fluid component (Bodnar, 1995). of the San Cristobal district have However, such interpretation of these two traditionally been explained by a single different primary inclusion types is not fluid migrating outward from the Chumpe without contradiction. First, no intrusion. It was suggested to be either a significative change in the liquid vapor dense magmatic brine mixed with cooler phase has been discovered as it may be meteoric water at distance (Bartlett, 1984) expected in the case of separation in the or a single meteoric fluid with isotopie immiscibility field (Bodnar et al., 1985). exchange with a granite at low water to Afterwards, type Au inclusions halite rock ratios (Campbell et al., 1984). Such melting temperatures indicating with models were principally based on the fact salinities between 44 to 54 wt% NaCl that fluid inclusion data indicated only low equiv indicate fluid trapped at high salinity fluids. The only inclusions with pressure and temperature over 400°C higher salinities detected previous to this (Stemer et al., 1988; Bodnar, 1994; Fig. study were secondary inclusions hosted by 3.21) or post-entrapment changes. High quartz-eyes in the Chumpe intrusion pressure (> 3kbars) and temperature (Bartlett, 1984). However, this study reveal (>400°C) can be rejected since these two different primary inclusion types in conditions imply a ductile environment 101

(Fournier, 1999; Fig. 3.21), that is not admixture of meteoric water to a compatible whith vein ore filling large dominantly magmatic fluid and opened structures. Additionally, post­ reequilibration of this mixture with the entrapment changes were not detected. granite (Fig. 3.23), rather than by Therefore, the proposed explanation is that equilibration of a meteoric fluid with the these two fluid inclusion types reflect an Chumpe intrusion. The influence of a law­ heterogeneous trapping. Effectively, Na/K temperature and low-salinity meteoric fluid and Fe/K ratios of An and Am fluid is also strongly documented by the general inclusions are very variable (Fig. 3.22); it trend of microthermometric data (Fig. could therefore indicate fluid already 3.20). Homogeneous lead and sulfur saturated in chlorine solids before quartz isotope data from Moritz et al. (200 1) point crystallization. These two fluid inclusion out a predominant magmatic source and types (Au and Am) reflect therefore a are consistent with the model proposed by probable unique fluid of bulk salinity Heinrich (1990) (Figs. 3.7 and 3.8). ranging between the eutectic of the NaCl­ water system (26.2 wt% NaCl equiv) and the lowest salinity of the three phases fluid LA-ICP-MS results from the fluid inclusion (46.1 wt% NaCl equiv, FPE-4a- inclusions show that this model is probably 2), that was saturated in chlorine solids due valid for at least the two earliest vein to temperature and/ or pressure decrease stages where concentrations of W, Cu and before quartz I growth. Such an high Zn, the major mineralising components, salinity fluid has probably been created by and 8D values decrease whereas 0, S and the passage between a lithotastic to an Pb isotopie compositions remain constant hydrostatic pressure as proposed by throughout the two paragenetic stages Fournier (1999). If there is no evidence of (Figs. 3.7, 3.8, 3.18 and 3.19). While W, fluid inclusion trapping the steam in the Cu and Zn concentrations decrease vein, it is due to its escape before the throughout the paragenesis due to mineral quartz 1 started to cristallise. However, precipitation and vary relative to the total sorne relies of this fluid can be the salinity of the fluid, they do not decrease at secondary vapor-rich inclusion described the same rate and W deposition seems to by Bartlett (1984) in quartz eyes of the be more efficient compared to the base Chumpe intrusion. Na/K and Fe/K ratios metal ones (Fig. 3.18). By contrast, Na, do not vary only in Au fluid inclusions but Mg, Mn, Rb, Sr, Ba and Pb are relatively also in the two phases fluid inclusions, it is nonreactive elements (Ulrich et al., 2002) probably due to undetected solids during during the two first stages. This the microthermometric measurments. demonstrates selective mineral precipitation (Ulrich et al., 1999) and that large quantities of ore metals were flushed The input of a saline magmatic fluid into out of the vein system and introduced in the San Cristobal system lead to a the carbonate replacement ore bodies. The reinterpretation of the hydrogen and early fluid composition, with thousands of oxygen isotope data proposed by Campbell ppm of W and base metals (Fig. 3.18), et al. (1984) and favors the one suggested indicates that the economically interesting by Heinrich (1990). Indeed, the vertical metals were dominantly introduced by trend A in Figure 3.19, with a constant magmatic fluids. The same conclusion was 8 180 value consistent with a high­ deduced from porphyry system data temperature magmatic source, but variable (Heinrich et al., 1992; Bodnar, 1995; 8D extending to ones that are lower than Ulrich et al., 1999). magmatic values, is explained by minor 102

60

50

• Primary inclusions in quartz 1 IEl Primary inclusions in quartz 11 D Primary inclusions in quartz Ill :::i 40 e Primary inclusions in quartz C'"

4

2

x

100 200 300 400 Homogenisation temperature (° C)

Fig. 3.20: Salinity vs Homogenization temperature of quartz hosted fluid inclusions determined by microthermometric measurements. Salinities are calculated with equation ofPotter et al. (1978).

On the contrary to what might be expected of degrees. Therefore it is preferably in the model described above, fluid interpreted as temperature inversions inclusion homogenization temperatures do caused by high level flows as shown in not decrease through the paragenesis, they active geothermal systems (Hedenquist et even increase from the earl y to. the al., 1992). Such an interpretation cannot be intermediate stage (Fig. 3.20). Moreover tested in the early paragenetic stage since variation of tens of degree occur in the this part of the mining activity is totally same stage. For example in the early one inaccessible at present, however it 1s where data from Campbell and Robinson­ clearly revealed by the late vein stage. Cook ( 1987) indicate homogenization temperatures approximately 50°C higher than data of this study (Fig. 3.12). Late vein stage Temperature disparity due to the difference in elevation (Fig. 3.2), and by inference in pression, can be excluded since altitude Fluid inclusions in quartz III of sample variation do not exceed a thousand meters 722-10, located at the Mi tu - Pucara and can at most exp lain a difference of tens contact (Fig. 3.2), have homogenization 103 temperatures between 253 and 323°C and W and base metals in success1ve salinities ranging from 4.6 to 6. 7 wt% fluid inclusion generations can be NaCl equiv.(Fig. 3.20). Nevertheless, as explained by mineral deposition, described previously, fluid inclusion data the abrupt increase of Ba and Sr of this late quartz stage are not constant. concentrations in the last stage can Lisboa (2002) reported thermometrie data only be explained by the input of a on two samples with homogenization fluid from a different origin or, at temperatures between 150°C and 246°C least, migrating through a different and salinities from 2.4 to 4.5 wt% NaCl pathway. Such an increase in Ba equiv., which are both distinctly lower than and Sr concentrations was also data obtained on sample 722-1 O. Samples observed in the porphyry described by Lisboa (2002) come from the environment by Ulrich et al. same level as 722-1 0 and are located close (2002), which interpreted this as to the contact between the volcanic rocks the response to the destruction of of the Mitu Group and the phyllites of the Ca and Mg silicates but with no Excelsior Group (Fig. 3.2). Therefore evidence of meteoric water. In the variation in homogenization temperatures case presented here, increasing Ba and salinities may be interpreted as a and Sr concentrations may gradient originating at the Mitu - Pucara probably be linked to the contact. A high temperature anomaly along destruction of silicates such as the Mitu - Pucara contact is consistent plagioclase and amphibole as is with field observations since the base of demonstrated by profiles through the Pucara Group is composed of arenites the wall rock (Fig. 3.5). (Sallier, 2002) which have a higher permeability than surrounding rocks and, The homogenisation temperatures its intensively brecciate texture may be a and salinities measured in quartz III favourable channel-way for the circulation fluid inclusions are much higher of a hot fluid. than the low-temperature and low­ salinity meteoric fluid (Fig. 3.20).

And major influx of fluid from a A distinct upflow zone at the end of the different origin than the previously paragenesis, and therefore an influx of a called meteroic and magmatic third fluid from a different pathway, is fluids is also suggested by highly consistent with the geochemical data of the variable 87 Sr/86Sr ratios of vein fluid for four reasons: barite (Fig. 3.9). The 87Sr/86 Sr ratios of barite appear to be The fluids associated to the correlated with the host rock, since deposition of the late quartz stage barites hosted by the Excelsior are enriched in co2 in comparison Group are generally more to earlier fluids. radiogenic than the ones hosted by the Mitu Group (Fig. 3.9). The very LA-ICP-MS fluid inclusion 18 analyses of the two first stages, the high 8 0 value of quartz III may early and the base metal ones, are be related to the same phenomenon. characterized by high concentrations of base metals, whereas high Sr ~nd Ba However, magmatic and meteoric fluid concentrations are rather typical for influences are still strongly suggested by the late vein stage (Fig. 3.18). isotopie data, since, in contrast to the two While decreasing concentrations of first stages, oxygen and hydrogen isotopie 104 composition of dickite and late stage It must also be pointed out that fluid carbonates define an oblique trend (trend inclusions hosted by quartz III do not have B; Fig. 3.19). This oblique trend may the same composition as secondary fluid correspond to mixing of magmatic and inclusions of the earlier stages. For meteoric fluids, as defined by ()D values of example, secondary fluid inclusions of secondary fluid inclusions trapped in barite quartz I contain lower W, Cu or Zn (Campbell et al., 1984). Nevertheless, concentrations than primary inclusions pf mixing with a third fluid of a unknown quartz I (Fig. 3.18), but these origin can not be rejected for the reasons concentrations remain definitively higher exposed above (Fig. 3. 23). than primary inclusions of quartz III. It may indicate that secondary inclusions in quartz I are earlier in the paragenesis than 5 the late stage, and therefore, that hydrothermal activity has totally ceased in the central part of the vein system whereas it was still active along the Mitu-Pucani contact. 4 3,0,.....------. FPE-4a-2• 3

FPE-4a-16 FPE-4a-9 a.. ~ ~ 2 6,0 8,0 10,0 12,0 14,0

Na/K + Primary three phases fluid inclusions

in early quartz, type A, 1 ô Primary two phases flu id inclusions

in early quartz, type A, 11 Fig. 3.22: Fe/K vs Na/K diagram for primary fluid inclusion of quartz 1. 0 0 200 400 600 Whereas cooling and slight mixing for the first two stages of mineralisation and then Fig. 3.21: Temperature-pressure diagram. L-V-H important mixing for the last stage were and L-V curves after Bodnar (1992); liquidus probably the most efficient mechanism of curves after Bodnar (1994); isochore field after deposition in the San Cristobal vein Bodnar and Vytik (1994); change in isochore slope system, a subsequent mechanism may be across the liquidus not evaluated; brittle-ductile C02-degassing during the last stages. As limit after Fournier (1999). mentioned before, fluid inclusions hosted in quartz III generally indicate higher C02- contents, however this content is highly variable and is linked for example to the Mg content of the fluid (Fig. 3.24). The 105

Meteoric f/uid Early vein stage

T < 100°C Low salinity liquid phase (< 2 wt %Na Cl equiv.), Low water/rock ratio 8'80- -18 (%oSMOW), 50--140 (%oSMOW)

Magmatic fluids

T = 200 to 250°C Coexistance? of a low salinity liquid phase (4 to 6 wt %NaCI equiv.), a high salinity liquid phase (- 30 to 40 wt% NaCI equiv) halite saturated 5'80 - 4 (%oSMOW), 80 - -40 (%oSMOW) 206Pbi""'Pb -18.7-18.8, 207Pbt"'Pb -38.7-38.8 87Srf"Sr -0.707 High W. Cu, Zn and Pb contents

Meteoric f/uid Late vein stage

T < 100°C Low salinity liquid phase (< 2 wt %NaCI equiv.), 8'80- -18 (%oSMOW), 50- -140 (%oSMOW)

Magmatic fluids

Thirdf/uid T = 200 to 250°C Coexista nee? of a low salinity liquid phase (4 to 6 wt T =300 to 320°C %NaCI equiv.), a high salinity liquid phase (- 30 to 40 Low salinity liquid phase (4 to 6 wt %NaCI equiv.) wt% NaCI equiv) halite saturated 5'8 0 - 4 (%oSMOW), 5D - -40 (%oSMOW) 206Pbf"'Pb 87 Variable Srt"Sr- 0.712-0.724 -18.7-18.8, 207Pbf"'Pb -38.7-38.8 87Srf"Sr -0.707 High Ba and Sr contents High W. Cu, Zn and Pb contents

Fig. 3.23: Schematic sketch of the different fluids involved in the early and the late vein stage. San Cristobal vein cross section as on figure 3.2.

paragenetic sequence of the last stage ankerite grow on the periphery of the large indicates deposition of carbonates slightly automorphous crystals of quartz (Fig. later than the associated quartz (Fig. 3.3), it 3 .15). Primary fluid inclusions contain is particularly obvious in sample 722-100 higher C02 and Mg contents than where crystals of rhodochrosite and secondary ones (Fig. 3.15), it is therefore 106 concluded that co2 degassing has played a observed in fluid inclusion trails hosted by local influence and may have acted an sphalerite (Fig. 3.17). Such vertical trends important role during the deposition of the were interpreted by Moritz et al. (200 1) as carbonate by pH increase. Unfortunately, intermittent boiling of the moderately other carbonate cation-related, i.e. Ca or saline fluids. The absence of similar Fe, are generally below LOD to control vertical trends in the earlier quartz stage this assumption (Appendix IV). implies that these processes seem to be likely restricted to the later base metal stage. In contrast to the early vein stage, Mg content (ppm) fluid inclusions from carbonate replacement minerais do not indicate any 100000 presence of W but still present similar • 10000 concentrations of Zn and Pb (Fig. 3. 18). Fluid inclusion metal concentrations is 1000 •• then representative of the mineralogy as it

100 was already underlined by Ulrich et al. (1999). ~ 10

1 0.0 0.1 0.2 0.3 0.4 0.5 Hydrogen and oxygen isotopie 3 C02 (g/cm ) compositions of the fluid in equilibrium • Primary inclusion (Qz Ill) with quartz are close to the ones observed o Secondary inclusion (Qz Ill) during the two early vein stages (Fig. 3. Limit of detection of inclusion with Mg content 19). Fluids associated to deposition of Dto low to be detected quartz in the carbonate replacement ore

Fig. 3.24: Concentration of Mg vs C02 dissolved in bodies may then be close to fluids of the fluid inclusions from quartz III. According Rosso early and base metal vein stage vein stages, and Bodnar (1995) error on C02 content is equal to except that they were already depleted in ± 0,25 (g/cm3) and error on Mg content figure in W and to a smaller extent in Cu. The base Appendix IV. metal stage, where boiling processes were pointed out (Moritz et al., 2001 ), remains dominated by the influx of magmatic fluid as shown by S and Pb isotope systematics Carbonate replacement ore (Figs. 3.7 and 3.8). However o34S values of bodies carbonate replacement ore bodies are not as homogeneous as the ones from the veins. Indeed, sulphides and barite sulphur On the contrary to the vein where isotopie compositions tend to have lower successive generations of quartz permit to o34S values in comparison to the veins trace the entire mineralising process, (Fig. 3.7). This would be expected for a carbonate replacement ore shows a unique mineralising system evolving toward more quartz event, which is indicative of the oxidising conditions (Ohmoto and early stage before the main sulfide ones Goldhaber, 1997). Nevertheless such an (Fig. 3.3). Microthermometric data on assumption is contradicted, on the first quartz fluid inclusions indicate similar hand, by the paragenetic sequence that homogenisation temperatures than the later moves from hematite to pyrite and finally sphalerite inclusions (Fig. 3.17). However magnetite (Fig. 3.3), and on the other hand, no vertical trend with large salinity by the vein sulphur isotopie composition variation has been put forward as it is that tends toward higher values through the 107 paragenesis (Campbell, 1987). Carbonate determination of the fluid ongm that is replacement sulphides with the lowest 834S hidden by the meteoric and magmatic values belong to the end of the paragenesis components. (lib in Fig. 3.3) or are located in ore bodies remote from the Mitu-Pucara contact. It may suggest the premise of the late stage 30 • o " O (%o SMOW) • with influence of different S sources. The 0 o"'S (%o CDT) • • late stage is effectively strongly influenced 20 •• by three different fluids. First, hydrogen •• • • 0 • 0 and oxygen isotopie data on dickite 10 ~ oo ~ 0 0 indicate a probable major mixing of @ 0 magmatic and meteoric waters (Fig. 3.19). 0 But, as for the vein system, growing 0.712 0.716 0.720 0.724 importance of a third fluid is also indicated 87Sr/6Sr by the augmentation of the Ba content in secondary fluid inclusions (Fig. 3.18) and by barite samples revealing large ranges of Fig. 3.25: 8 180 and o34S vs 87Sr/86Sr ratio for barite 0, S and Sr isotopie compositions (Figs. from vein and carbonate replacement ore. 3.7, 3.9 and 3.19). For example, 87Sr/86Sr barite ratios from the carbonate replacement ore bodies range from 0.712867 to 0.722311 (Table 3.2), which Conclusions are ali too radiogenic in comparison with the Chumpe intrusion signature (Fig. 3.9). However, this large range of 87Sr/86Sr The two different ore types of the San ratios is not observed in each ore bodies, Cristobal district, i.e. vein and carbonate on the contrary barite from specifie ore replacement ore bodies, are linked to the bodies indicate much more homogeneous Chumpe intrusion and present a similar 87Sr/86Sr ratios. Barites from ore bodies paragenetic sequence. Their principal located at the periphery of the San mineralogie differences can be observed in Cristobal district, such as Gavilan or their respective early stage; the early vein Moises (Fig. 3.1), have low 87Sr/86Sr ratios, stage is principally composed of while the ones from the principal ore wolframite, quartz and pyrite, which is bodies at the centre of the district, such as totally absent in the carbonate replacement Toldorumi or Escondida (Fig. 3.1), yield ore. Nonetheless, these two different ore definitively higher 87Sr/86Sr ratios (Table types present similarities in their fluid and 3.2). Since ail these four ore bodies are metal origins. located in the Pucara limestones and close to the contact with the Mitu volcanic rocks, these variations cannot be attributed to the influence of the carbonate rocks. In Highly saline fluid inclusions and the contrast it indicates equilibration with the isotopie compositions of the fluids, 18 more radiogenic underlying formations, i.e. variable 8D values with constant 8 0 Mitu volcanic rocks or Excelsior phyllites. values and Pb isotope ratios overlapping However this equilibration has probably with the isotopie composition of the been partial and not homogeneous since intrusion, argue strongly in favor of a correlation or distinctive patterns are major magmatic input and a slight difficult to reveal in multi-isotope diagram meteoric fluid admixture in the two earliest (Fig. 3.25). Hydrogen and oxygen isotope vein stages. LA-ICP-MS analyses of fluid data (Fig. 3.19) do not help in the inclusions indicate that the bulk of the 108 economically attractive elements are during fieldwork. We thank also Les provided by the t1uid exsolved from the Oldham and the Anglo-Peruana for advice, intrusion. The fluids associated to the late criticism and help during fieldwork. C. stage have very different characteristics. Heinrich is kindly acknowledge for the With variable 8 180 and 8D values, this access to his laboratory at ETH Zürich. stage can probably still be interpreted as a This study was supported by the Swiss magmatic-meteoric mixing, but with a National Science Foundation (Grant larger increase of the meteoric component. n°2000-062000.00). Variation in 87Sr/86Sr ratios of barite and enrichment in Ba and Sr may be attributed to the incoming of a the third fluid from a References undetertnined origin.

Audétat, A., Günther, D. and Heinrich, C.A. (2000): Causes for Large-scale metal Isotopie data of the carbonate replacement zonation around mineralised plutons: fluid ore bodies tell us that the fluid responsible inclusion LA-ICP-MS evidence from the for their formation of the carbonate Mole Granite, Australia. Econ. Geol., vol. replacement has followed a similar 95, p. 1563-1581 evolution. While no W ore exists in the Bartlett, M.W. (1984): Petrology and genesis carbonate rocks, and while Cu contents are of carbonate-hosted lead-zinc-silver ores, strongly depleted in comparison to the San Cristobal district, Department of Junin, Peru. Unpublished Ph.D. thesis, Oregon veins, it is mainly due to the selective State University, Corvallis, Oregon, U.S.A., mineral precipitation with the result that 272 p. the fluids reaching the carbonate rocks Barton, P.B. and Bethke P.M. (1987): were already depleted in these elements. Chalcopyrite disease in sphalerite: Mineralisation processes, as happened in Pathology and epidemiology, American the San Cristobal district where fluid Mineralogist, vol. 72, p. 451-467 m1xmg was the principal deposition Beaty, D.W., Landis, G.P. and Thompson, T.B. mechanism, indicates selective (1990): Carbonate-hosted sulfide deposits of precipitation with a far higher efficiency the Central Colorado mineral belt: introduction, general discussion, and for W minerais in comparison to base summary. Econ. Geol., Monograph 7, p. 1- metal ones. 18. Beuchat, S. and Moritz, R. (in prep): The Zn­ Pb-Ag-Cu San Cristobal district, Central Large vanatton in homogenization Peru: A case of lineament influence on Miocene ore formation. temperatures is thought to be the result of Beuchat, S., Moritz, R., Sartori, M., Chiaradia, temperature inversions caused by high M. and Schaltegger, U. (2001a): High level flows in particular locations. This is precision geochronology and structural notably revealed at the Mitu-Pucani constraints on the Zn-Pb-Ag-Cu Domo de contact during the influx of a large quantity Y au li district, Central Peru. Extended of the third fluid. abstract, 6th biennial meeting of the SGA, Krakow, Poland, 26-29 August 2001. Beuchat, S., Schaltegger, U., Cosca, M., Moritz, R. and Chiaradia, M. (2001b): High­ Acknow/edgments precision geochronology constrains on Miocene magmatic and mineralising events We gratefully acknowledge the Volcan in the Pb-Zn-Ag-Cu Domo de Yauli district, Peru. GSA Annual Meeting, Boston. A-358. Compania Minera which provided access Bodnar, R.J. (1994): Synthetic fluid inclusions: to underground exposures and support XII. The system H20-NaCl. Experimental 109

determination of the halite liquidus and into brittle rock in the magmatic-epithermal. isochores from a 40 wt% NaCl solution. Econ. Geol., vol. 94, p. 1193-1212. Geochim. Cosmochim. Acta, vol. 58, p. Gilg, H.A. and Sheppard, S.M.F. (1996): 1053-1063. Hydrogen isotope fractionation between Bodnar, R.J. (1995): Fluid inclusion evidence kaolinite and water revisited. Geochim. for a magmatic source for metals in Cosmochim. Acta, vol. 60, p. 529-533. porphyry copper deposit. Mineralogical Günther, D. and Heinrich, C.A. (1999): Association Short Course, series 23, p. 139- Comparison of the ablation behaviour of 152. 266nm Nd:YAG and 193 nm ArP excimer Bodnar, R.J., Burnham, C.W. and Sterner S.M. lasers for LA-ICP-MS analysis. Journal of (1985): Synthetic fluid inclusions in natura1 analytical atomic spectrometry, vol. 14, p. quartz. III. Determination of phase 1369-1374. equilibrium properties in the system H20- Günther, D., Audétat, A., Frischknecht, R. and NaCl to 1000°C and 1500 bars. Geochim. Heinrich, C.A. (1998): Quantitative analysis Cosmochim. Acta, vol. 49, p. 1861-1873. of major, minor and trace elements in fluid Bookstrom, A.A. ( 1990): Igneous Rocks and inclusions using laser ablation-inductively carbonate-hosted ore deposits of the Central coupled plasma-mass spectrometry (LA- Colorado mineral belt. Econ. Geol., , ICP-MS). Journal of Analytical Atomic Monograph 7, p. 45-65. Spectrometry, vol. 13, p. 457-481. Campbell, A. (1987): A sulfur isotopie study Hedenquist, J.W. and Henley, R.W. (1985): of the San Cristoba1 tungsten-base metal The importance of C02 on freezing point mine, Peru. Mineral. Deposita, vol. 22, p. measurements of fluid inclusions: evidence 42-46 from active geothermal systems and Campbell, A. and Robinson-Cook, S. (1987): implications for epithermal ore deposition. Infrared fluid inclusion microthermometry Econ. Geol., vol. 80, p. 1379-1406. on coexisting wolframite and quartz. Econ. Hedenquist, J.W., Reyes, A.G., Simmons, S.F. Geol., vol. 82, p. 1640-1645 and Taguchi, S. (1992): The thermal and Campbell, A., Rye, D. and Petersen, U. (1984): geochemical structure of geothermal and A hydrogen and oxygen isotope study of the epithermal systems: a framework for San Cristobal mine, Peru: Implications of interpreting fluid inclusion data. Eur. J. the role of water to rock ratio for the genesis Mineral., vol. 4, p. 989-1015. ofwolframite deposits. Econ. Geol., vol. 79, Heinrich, C.A. (1990): The chemistry of p. 1818-1832 hydrothermal tin( -tungsten) ore deposition. Carothers, W.W., Adami, L.H. and Econ. Geol., vol. 85, p. 457-481 Rosenbauer, R.J. (1988): Experimental Heinrich, C.A., Ryan, C.G., Mernagh, T.P. and oxygen isotope fractionation between Eadington, P.J. (1992): Segregation of ore siderite-water and phosphoric acid liberated metals between magmatic brine and vapor: a COrsiderite. Geochim. Cosmochim. Acta, fluid inclusion study using PIXE vol. 52, p. 2445-2450. microanalysis. Econ. Geol., vol. 87, p. 1566- Clayton, R.N., O'Neil, J.R. and Mayeda, T.K. 1583 (1972): Oxygen isotope exchange between Kobe, H. W. (1990a): Metallogenic evolution quartz and water. Journal Geophys. Res., of the Y auli dome, central Peru. A vol. 77, p. 3057-3067. summary. In: L. Fontboté et al. (eds.), Dalheimer, M. (1990): The Zn-Pb-Ag deposits Stratabound ore deposits in the Andes, Huaripampa and Carahuacra in the mining Berlin: Springer-Verlag. p. 267-278. district of San Cristobal, Central Peru. In: L. Kobe, H. W. (1990b): Stratabound Sulfide Fontboté, G.C. Amstutz, M. Cardozo, E. occurrences in the Paleozoic of the Yauli Cedillo and J. Frutos (eds.), Stratabound Ore Dome, Central Peru. A summary. In: L. Deposits in the Andes, Berlin: Springer­ Fontboté, G.C. Amstutz, M. Cardozo, E. Verlag. Cedillo and J. Frutos (eds.), Stratabound ore Einaudi, M.T. (1977): Environment of ore deposits in the Andes, Berlin: Springer­ deposition at Cerro de Pasco, Peru. Econ. Verlag. Geol., vol. 72, p. 893-924 Kusakabe, M. and Robinson, B.W. (1977): Fournier, R.O. (1999):Hydrothermal processe Oxygen and sulfur isotope equilibria in the

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centrales, Pérou). Unpublished MSc thesis, University of Lausanne, Switzerland. Titley, S.R. (1996): Characteristics of high temperature, carbonate-hosted replacement ores and sorne comparisons with Mississippi valley-type ores. In: D.F. Sangster (ed.), Carbonate-hosted lead-zinc deposits. SEG Special publication, vol. 4, p. 244-254. Ulrich, T., Günther, D. and Heinrich, C.A. (1999): Gold concentrations of magmatic brines and the metal budget of porphyry coèèer deposits. Nature, vol. 399, p. 676- 679. Ulrich, T., Günther, D. and Heinrich, C.A. (2002): The evolution of a porphyry Cu-Au deposit, based on LA-ICP-MS ana1ysis of fluid inclusions: Bajo de la Alumbrera, Argentina. Economie Geology, vol. 96, p. 1743-1774. Volcan compafia minera S.A.A. (2001): Memoria annual2000. 149 p. Zhang, L.-G., Liu J.-X., Chen, Z.-S. and Zhou H.-B. (1994): Experimental investigations of oxygen isotope fractionation in cassiterite and wolframite. Econ. Geol., vol. 89, p. 150-157. 112 SamJJie · ····· ······ 'locatÏoÏ1 Unit (or host) DescriJltion Mineralo!JV ,)1. w date hy , Cl (\) c (/) :::s >. ·- Cl ' Q) ë.. .c: Cl '(;! c ~ ~ a. c.. ' ..c c "C ·- ' Kl Kl Kl Kl · Kj '(;! c.. c.. Kl Kl .S · cC:. ~ c.. Q Q c.. c.. x· i: Q c.. c.. c, ~ <( "C Q Q Q Q Q "' u "' c.. "C ...... '"l' Q Q ë ë ... tl E ~ ~ ë Q Q Q ë H L.J.. ..c .! 0 .!!! .!!! .!!! · ~ ~ 1§ 1 ~ 1 :::i ë ti c.. <( ' .!!! .!!! .c: c:: 1 ~ ' "C ..c : ·- ·- c.. x ::i :::, M 0 1816 e oct-98 PS :Andaychagua vein, Nv 770, Tj · Mrtu volcanics Breccia w rth colcanics clasts n 1680,Pz 4 en 1821 Om oct-98 PS ;!l.ndaychagLta vein, Nv 770, Tj · Mrtu volcanics Small veinlets in sericrtized " 1 Q 680, Pz 5 volcanics :::s '824 cd oct-98 PS ' Andaychagua vein, Nv 720, Tj Excelsior phyllrtes Breccia w rth phyllrtes clasts a. I171,Pz 10 825 ocl-98 PS ; AndaychagLta vein, Nv 720, Tj Gabbro Small veinlets n AOO, Pz 5 =+ a.(\) 832 oct-98 PS AndaychagLta vein, Nv 720, Tj Gabbro ·Veinlets Q 400, Pz 5 :::s 840 oct-98 PS Anclaychagua xein,NxJ 20, TL Mrtu volcanics Contact between volcanics and Q 853 oct-98 PS ' AndaychagLta vein, Nv soo, ...... •E ~celsiorp~yUrtes Small veinlets 856 ocl-98 PS 'AndaychagLta vein, Nv 720, Tj Mrtu volcanics Veinlets -

w Cl >. CFJ .: ·-= Cl ci. (1) Cl '(;! = rs:l rs:l VI ~ 1:1. .c rs:l rs:l rs:l rs:l rs:l =· -= ·-'(;! 1:1. 1:1. 1:1. 1:1. ..s c.. ~ = ·- 1:1. 1:1. 1:1. =~ u Cl , ... '(;!.Q:-= .9 = = .9 =... c.. "C C') t.n .s = .9 u E = .!!!--= .!!! = = 0 ~

DY-8 01 ID611998 , RM Vein 658, Nv 820, YI 020 : Mttu volcanics •Brecciated host rocks in massive •qz++, si++, py+, ore gn+ iDY-10 i'' o1 Aï6/1ssà ' RM Vein 722, Nv820 Mttu volcanics :Banded sulfides-carbonate vein :car++, si+, gn, • ···· ·············································· py Vein 722, Nv820 :Mttu volcanics ,Massive galena ore, voids filled wtth,gn++, car+, qz+,

DY-12 F Cara!)uacra open-p~ Pucara limestones 'Manto ore py+, st

DY-1 2 Carahw'lcra open-ptt Pucar~ iirÏÏeStones 'Hemattte ore in manto :hm, si G DY-12 CarahLtacra open-ptt Pucara timèS!onês .. •orê in rmirù) near tüiï iqz, hm, py Hb DY-121 021D6t1998 RM Carahuacra open-ptt TPucarti litneStënïes rv ein close to marrtos si

'DY -13 ' 021D611998 RM Andaychagua vein, Nv 770, Tj : Mrtu volcanics , 4l35,Pz6 DY-13 021D611998 RM Andaychagua vein, Nv 770, Tj ; M~Lt volcanics :Vein ore si A 465, Pz 6 DY-13 021D6!1998 RM .i\ndaychagua vein, Nv 770, Tj Gabbro · srïïan veinleis ·· si+, gn B ,...... 465,Pz 6 DY-14 . 031D6t1998 RM ??? ??? ???

DY-15 San Cristobal vein iA 'DY-15 San Cristobal vein 'B 'DY-15 '??? 'Small veinlets py, si, qz c DY-16 PLicari.lirÏÏeStones 'Skarn in limestones phi B DY-17F Toromocho porphyry 'Miocene . = C) V) 0\ .c Q) C) i :::! c.. ..c "1:3 = f!l f!l = i c.. c.. c:. e i "1:3 Q Q "'c, c.. "1:3 !d Q $ ë ë .s ..c "'• 1/) Q LJ.. ti .!:2 :3 .c c:: c,. cl: ·-"1:3 c. x ~ ::::1 Oi z: DY -31 E 081'1 Ot1998 RM Victoria Mitu volcanics wr w r

DY-311 0811011998 RM ,Victoria Excelsior phyllites w r w r ...... DY-31 L 08t1011998 [ RM [Victoria Excelsior phyllites • w r wr

DY-37 16!D5t1999 RM ;Prosperidad vein Exeelsior phyllites c DY-91 ??? RM ??? ·??? ~ ??? ??? s DY-411 ·1SID911998 BS :carahuacra open-pit ,Pueara limeStones ·Manto ore [hm+ , gn+, sid, si, [ [epy DY-443 23!D811998 BS [Carahuacra open-pit Pucar.Ï iim~Stëines :Manto ore x

DY-444 23>D811998 BS :carahuacra open-pit P~c ara iimeStones ;Manto ore, massive pyrite in ribbon Py++, si+, sid, x .texture, sphalerite intercalation, [gn, epy ~ breccias ~ ·~~· DY-459 29!D811 998 . BS :Hu aripampa, Nv -400 ~ Pucara lim·~~ on·~; -· : Ma~t~ ore si+, gn+ ·- - ···- DY-470 0111011998 BS [Huaripampa, Nv -400 PucarélimeStones. Manto ore, replacement of hematite ma+, hm, py, ..•.. . .•...... •.... ;by magnetite sph, kao DY-473 02t1011998 BS .Hua ripampa, Nv -400 .Pueara limestones ;Manto ore, replacement of hematite ma++, hm, py, by magnetite gn,.sl,.qz DY-494 0611011998 BS [Huar ipampa, Nv -300 Pucara limestones Marrto ore hm+, si+, gn, py, ca DY-498 05t1 011998 ' BS ;Huaripampa, Nv -350 PLÏcarâ iimesïëiiïes. [Manto ore, early carbonate hm++, sid+, qz+ ,replacement stage, replacement of siderite by quarü DY-533 18.oD911998 · BS :carahuacra open-pit Pueara limestones Manto ore hm+, gn+ , sid, si, epy DY-554 19!D911998 BS 'Huaripampa, Nv -400 PcÏcarà limeStones ;Large aLrtomorphous cryslals of hm++, qz+, si+, [quartz in a matrix of hematite oct-98 HL V ein 722, Nv 580, Tj 230E . Excelsior phyllites ' P\'r~ i c ore ··············· · ·· · ····· ·· · ·· ··· py++,wo,bis, , X qz, epy, apy, fh, [ pÇJ,pol oct-98 ' HL ,Vein 722, Nv 580, Tj 230E Excelsior phyllites 'Brecci w ith sphalerite veins sl++,gn+,st,py, : fh oct-88 HL Nein 722, Nv 580, Tj 231 E Excelsior phyllites Phyllite breceia, eemented by sulfide si++ , epy+, gn+, 'quartz veins . ipy+ oct-98 HL Vein 722 , Nv 580, Tj 261 Excelsior phyllites ;Massive pyrite 'py++ DY-629 [ oct-98 , HL Vein 722 , Nv 580, Tj 262 ,Excelsior phyllites ·Pyllite w ith intercalations of pyrite py++, qz++, sid DY-644 oct-88 HL [Vein 722 , Nv 820, Tj 020 ~~~~~ voiëanics ;Ciastes of pyr~e and sulfide qz++, car++, . : ~er~ented by .latecarbonate py+,sl, gn, kao oct-98 HL Vein 722, Nv 820, Tj 325 Mitu volcanies Massive banded sphalerite crosscLrt ,si++, gn++, qz, by laie quartz-carboante vein

oct-98 HL 'Vein 722, Nv 680, Tj 110 ··· 'éxëelsior pl1yllites 'Breeeia w ith centimetric :sphalerite w Cl c (Il >. : ·- Cl ' (!) è.. ..c i Cl VI Q.~ c ~ ~ · ~ ~ .., . .: ~ ~ ~ c. c. c. ~ ~ ~ ..c c c...... 0 i "' c. 0 c. c. c. ëi u Cl .9 ... .S · s 0 ... c. -= 0 0 0 (.,) E 0 -= VI 0 .9 ...0 .9 "' .9 .: 0 VI VI 0 0 :± 0 u.. tl .!!! .!!! ï~ i ~ ....j ..c c:: c. ~ ·­ ·- .: .... .!!! .!!! .!!! c. >c: :::! :::) c:: l tll -=z c. ::1:: (Il 0 ::1:: ii 1 ii ii DY-68·1 ocl-98 HL :Vein 722, Nv 580, Tj 231 E Excelsior phyll~es 'Centimetric crystals of bar~e ba++, si+, gn ba ba Esc 1 RM 'Escondida Pucara limestones ba Esc 2 · RM ·Escondida F>ué:arâ 1irneSiones

FPE-4a LF San Cristobal vein, Nv 180 Excelsior phyll~.es Wo++, py++, qz+, ser RM 'Gavilan Pucara limestones

sB :oertrLïëiis Skârii Puëarâ limestones i Skarn ore SB 'Gertrudis skarn Pucara lime stones~ : Skarn ore ...... Pucarâ iimestones -~ÏanÏo ore at the contact w~h tuff SB

RM Prosperidad 'Excelsior phyll~es

RM Prosperidad Excelsior phyll~es M-3 RM Toldorumi Pucara limestones· M-4 RM Toldorumi rliëârâ limeSiones - . ML-3 071DSt1 999 1 SB Maria Laura vein, Nv 250 Mnu volcanics 'Small veinlets of pyrne crosscutting [the host silicified Milu Moises RM Moises ·Pucara ltméstones · A Moises RM Moises F>liëtirâlirneSiaiïes ··· B OYA-1 SB Oyama prospect Milu volcanics OYA-5 SB Mnu volcanics 'Small veinlets OYA-6 i 29105tl 999 SB Oyama prospect 'Mnu volcanics 'Breccia w~h barile crystals

POR [ 06.

Small veinlets Cvu rich ore 'SC-12 San Cristobal vein, Nv 630 at 50 m :Miocene Porphyrrtic texture from Mrtu-Pucarâ contact : magmatism SC-13 San Cristobal vein, Nv 270 :Miocene Vein ore magmatism SCV\11- 18J05!'1 999 SB Drillcore SCWl , uncler San 'Miocene .. Porphyrrtic texture plg+ , (\Z+, kfd, 21,4 Cristobal mining _camp .... rnaÇJmêitism . bio, zr, ser,ill __ SON2- 18!0511999 SB Drillcore SON2, under San :Miocene Porphyrrtic texture plg+, qz+, kfd, 302.2 ù isjob€11_mining camp ...... i rnaflmatism bio, _zr, s~r. _ ill __ _ SON2- 18!0511 999 SB Drillcore SON2, under San Miocene Porphyrrtic texture plg+, qz+, kfd, 340 C:ristobal mining camp magmajism bio, __zr, ser, ill TIC 26!0511 999 SB Anticona diorrte, Laguna :Miocene Porphyrrtic texture bio+, plg+, hbl, !HLJacracocha magmatism qz, ap, zr, chi, epi, ca !TIC-6 26!0511999 .. SB Ticlio Pass Miocene Monzogranrte intruding Antivcona iTIC-10 06t1 0!200'1 SB Laguna Huacracocha Miocene ' Anticona diorrte wrth foliated .. mal)tfl_atism ___ _ !TOL-1 12J0511999 : SB Toldorumi Mrtu volcanics contact ,TOL-2 12!05/1999 SB Toldorumi · Pucarâ lirnestones 'Manto ore 'TOR 17!0511999 : SB :San Francisco intrusion Mio cene Porphyrrtic texture maflrnatisrn !TOR-10 31!0511999 SB Toromocho poprhyry 'Miocene Sericutized intrusion imagmatism 'TOR-13 3'1J05t1999 : SB 'Anticona diorrte Miocene Potassic atteration ''TI"I.I'natistn epi,CI11 !SON2- 18!05!'1999 SB Drillcore SON2, Lmder San :Miocene Porphyrrtic texture plg+, qz+, kfd, !302.2 Cristobal mining camp 'magrnatistn bio, zr, ser, ill SON2- 1810511899 SB ,Drillcore SON2, under San Miocene Porphyrrtic texture plg+, qz+, kfd,

340 Cristobal rnining camp magmatisrn __ ...... bio1 zr, ser, ill TIC 26!0511999 SB )l.nticona diorrte, Laguna Miocene . Porphyrrtic textLtre bio+, plg+, hbl, : Huacracocha i magrnatism qz, ap, zr, chi,

TIC-6 26!0511998 SB 'Ticlio Pass !Miocene Monzogranrte intruding Antivcona :magmatism diorrte Laguna Huacracocha 'Miocene Anticona diorrte wrtl1 toliated magmatism xenolrth volcanics srilan veinleï i:.iïiïe ~lrtu:Pl;;;a;~ w C) c (1) . >. C) ' :i. ; ..c Cl) C) 'i ::! ..c c "CS .: &1 &1 &1 &1 .c · =-(<:~ 't;Sc.=-=-c. &1 &1 &1 "'c Q (<:l c:. E 'i , -:: 0 0 c. c. c. u c, c. . -c s .s Q Q ~ tl) ë c 0 ë .... s .... ti • E < := ~ .c Q Q Q (<:l sQ u.. "'• C? . .!a .!a .!a .!a ~ -~ i .~ .!a 'ë c: ...J ..c c: .!::! ~ ~ Q) .... "'C ..C: ...... c... x ::! ::::1 ~ c: (1) z c... ::c (1) 0 ::c u:: : u:: u:: TOL-2 12JlJ5fl999 SB To ldorumi Pucara lime stones , Manto ore si++, dik, hm, gn si qz qz TOR 17JlJ511999 SB San Francisco intrusion Miocene Porphyrrtic texlure qz++, plg+, kfd, zr .. ' ''"' TOR-10 3'1ID511999 SB Toromocho poprhyry Mio cene : SericLrtized intrusion

TOR-13 31 JlJS/1999 SB Anticona diorrte Mio cene , Potassic a~eretion kfd+, plg, bio,

TOR-14 3110511999 SB San Francisco intrusion Miocene 'Propylrtic a~eretion kfd+, bio+, qz+, wr w r wr ; TOR-15 31 JlJS/1999 SB Toromocho poprhyry Miocene Small veinlet of molybdenrte qz+,mo X mo

.Sampling Codes: ;Mineralogy Codes .AC .A.nfbal Chavez al alabandite Jh fa hl ore 'po i pyrrh_otite ;88 Benjarnin SaUier ap __ apatite , : gn ! galf311~ pol :polybasite :HL :Henri Lisboa :apy arsenopyrite hbl hornblende py ;pyrite 'LF . Lluis F ontboté :bar ba rite hm hematite' qz · q~ artz PS Phillippe Stucky :bio biotite ill 'illite rho 'rhodochrosite' RM Robert Moritz bis bismuthinite : l<.ao : kaolinit e: •ser,§eri cite SB bou: boulangerite 1 ..... L~td . l<:fel d spar sid :siderite ca calcite _ ma _magnetite si 'sphalerite location Codes car carbonate ·mc :marcassite .. sp~ · sphene Nv Nive! ch! chlorite mo lmoly!Ïdenite si stannite Tj Taj

\0 120 121

Appendix II: 40Ar/ 39 Ar dating

Temperature (0 C) Ca/K "'Ar!" Ar ,,.Art"' Ar "'Ar (x 10·" mole) "Ar (x 10·" mole) %'.-Ar Apparent ages± 1cr (Ma)

DY-16b, Phlogopite, J =0.00155, wt. =16 .9 mg. 825 0,9779 0,03343 0,366 6,3 0,6 3,6 1 ± 0,2 850 0,6089 0,03035 0,914 7,9 0,8 9,3 2,6 ± 0,2 875 0,2436 0,02339 1,684 11,8 1,4 19,6 4,7 ± 0,2 900 0,0867 0,01222 2,141 11,6 2 37,3 6 ± 0,2 925 0,0357 0,00669 2,383 13,4 3,1 54,7 6,6 ± 0,2 950 0,0506 0,00474 2,537 15 3,8 64,5 7,1 ± 0,2 975 0,0386 0,00389 2,537 17,1 4,6 68,8 7,1 ± 0,2 1000 0,0329 0,00337 2,526 17 4,8 71,7 7 ± 0,2 1025 0,0405 0,00311 2,547 16,9 4,9 73,5 7,1 ± 0,2 1050 0,0412 0,00294 2,583 19,1 5,5 74,9 7,2 ± 0,2 1075 0,0267 0,00249 2,628 20 ,6 6,1 78,2 7,3 ± 0,2 1100 0,0295 0,00206 2,597 19 5,9 81,1 7.2 ± 0,2 1150 0,1015 0,0018 2,616 18 5,7 83,2 7,3 ± 0,2 1300 3,0109 0,00356 2,554 13 3,7 73,3 7,1 ± 0,2 1549 1,236 0,00446 2,687 7,9 2 68 7,5 ± 0,2

810-1m, Seri cite, J = 0.00151, wt. = 20.7 mg. 600 2,9354 0,04059 0,767 21 ,3 1,7 6,1 2,1 ± 0,3 625 1,7142 0,02845 1,344 26 ,1 2,7 13,9 3,7 ± 0,2 650 1,5428 0,02077 1,664 22,9 3 21,5 4,5 ± 0,2 675 1,2658 0,01703 1,584 20,8 3,2 24,1 4,3 ± 0,2 700 0,868 0,01442 1,801 18 3 29,9 4,9 ± 0,2 725 0,6764 0,01364 1,933 14,9 2,5 32,6 5,3 ± 0,2 750 0,7592 0,01519 1,905 11,2 1,8 29,9 5,2 ± 0,2 775 0,7848 0,01758 2,721 8,4 1,1 34,5 7,4 ± 0,3 800 1,394 0,0256 3,127 6,5 0,6 29,4 8,5 ± 0,3 825 1,7636 0,03905 3,934 6,1 0,4 25,5 10,7 ± 0,4 850 2,2192 0,05519 4,354 5,5 0,3 21,1 11 ,8 ± 0,5 875 2,6239 0,0735 3,01 4,8 0,2 12,2 8,2 ± 0,5 900 3,6508 0,09317 1,46 4,6 0,2 5,1 4 ± 0,6

GER-3, Ph/ogopite, J = 0.00156, wt. ':' 10.6 mg. 900 0,8279 0,01552 1,123 5,9 19,8 3,2 ± 0,1 925 0,4701 0,00755 1,567 5,3 1,4 41,5 4,4 ± 0,1 950 0,3618 0,00593 1,551 5,1 1,6 47,2 4,4 ± 0,1 975 0,2901 0,00421 1,898 5,5 1,8 60,6 5,3 ± 0,2 1000 0,1783 0,00387 2,024 6,3 2 64,1 5,7 ± 0,2 1025 0,1361 0,00397 2,074 7,5 2,3 64 5,8 ± 0,2 1050 0,0806 0,00296 2,218 9,5 3,1 71,8 6,2 ± 0,2 1075 0,0487 0,00239 2,275 12,1 4,1 76,4 6,4 ± 0,2 1100 0,0622 0,00167 2,346 11 ,7 4,1 82,7 6,6 ± 0,2 1150 0,133 0,00216 2,281 9,4 3,2 78,3 6,4 ± 0,2 1300 21,1888 0,03988 1,344 8,4 0,7 10,9 3,8 ± 0,3 1549 37,0743 0,05794 1,628 8,4 0,5 9,3 4,6 ± 0,4 122

Temperature (°C) Ca/K "'Ar/"Ar "'Ar!"Ar "'Ar (x 10-" mole) "Ar (x 10-" mole) %'~Ar Apparent ages ± 1a (Ma)

SC-5, Sericite, J =0.00147, wt. =19.5 mg. 600 2,8907 0,01196 0,084 6,2 1,8 2,4 0,2 ± 0,1 625 1,8224 0,01069 0,385 7,9 2,3 11,1 1 ± 0,1 650 1,349 0,00805 0,84 8,7 2,7 26,5 2,2 ± 0,1 675 0,8859 0,00056 1,024 8,6 3,3 38,9 2,7 ± 0 ,1 700 0,5026 0,00373 1,373 10,1 4,1 55,9 3,6 ± 0,1 725 0,282 0,00289 1,572 11 4,6 65,1 4,2 ± 0,1 750 0,2435 0,00288 1,616 10,6 4,3 65,8 4,3 ± 0,1 775 0,1672 0,00265 1,822 10,1 3,9 70,1 4,8 ± 0,2 800 0,1955 0,00299 1,876 9 3,3 68,2 5 ± 0,2 825 0,2246 0,00367 1,882 8,2 2,8 63,6 5 ± 0 ,2 850 0,2578 0,0047 1,789 6,9 2,2 56,5 4,7 ± 0,2 875 02819 0,00041 1,736 5,8 1,7 52,2 4,6 ± 0,1 900 02751 0,00007 1,671 5,4 1,6 48,4 4,4 ± 0,1 925 0,2056 0,00071 1.557 5 1,4 44,1 4,1 ± 0,1 950 0,2205 0,0073 1,439 4,6 1,3 40,1 3,8 ± 0,1 975 0,1869 0,00812 1,326 4,4 1,2 35,7 3,5 ± 0 ,1 1000 02326 0,00858 1,289 4 33,8 3,4 ± 0 ,1 1050 0,1436 0,00846 1,816 5.4 1,3 42 ,1 4,8 ± 0 ,2 1200 0,0672 0,0089 3,78 26,2 4,1 59 %10.0 ± 0,3 1551 02104 0,00089 5,662 22,3 2,9 73,6 14,9 ± 0 ,5 123

Appendix III: Microthermometry and Raman fluid inclusion data

co, co, C02 Sample n°fam. UV T, T HA me phase Sali nit y L.band U.band density

FPE-4a 1 Ali n.d. n.d. n.d. 206.4 n.d. L 54.0b) 0 FPE-4a 2 Ali n.d. n.d. n.d. 205.8 n.d. L 46.1 b) 0 FPE-4a 3 Ali n.d. -22.2 n.d. 259.4 436 L 51.7 n.d. n.d. n.d. FPE-4a 4 Ali n.d. n.d. n.d. 235.5 400.2a) L 47.6 n.d. n.d. n.d. FPE-4a 5 Ali n.d. n.d. n.d. 227.4 427.7a) L 50.8 n.d. n.d. n.d. FPE-4a 6 Ali n.d. n.d. n.d. 225.1 370.5a) L 44.6 n.d. n.d. n.d. FPE-4a 7 Ali n.d. n.d. n.d. 259 454a) L 53.9 n.d. n.d. n.d. FPE-4a 8 Alli n.d. -21.6 -2.5 239 L 4.18 0 FPE-4a 9 Alli n.d. -21.6 -2.5 239 L 4.18 0 FPE-4a 10AIII 1.3 n.d. -1 .7 237.8 L 2.9 n.d. n.d. n.d . FPE-4a 11 Alli n.d. -22.1 -2.2 1.54.2 L 3.7 1292 1396.1 0.6 FPE-4a 12 Alli n.d. -21 .9 -2.6 156.1 L 4.34 n.d. n.d. n.d. FPE-4a 13 Alli 4 n.d. -2.3 146.2 L 3.9 0 FPE-4a 14 Alli n.d. n.d. -3.1 218.8 L 5.11 n.d. n.d. n.d. FPE-4a 15 Alli 3.3 n.d. -2.8 218 L 4.6 0 FPE-4a 16 Alli n.d. -29 -2.6 n.d. n.d. 4.3 n.d. n.d. n.d. FPE-4a 17 Alli 4.9 -22.1 -2.2 154.2 L 3.7 1292 1396.1 0.6 FPE-4a 18AIII 4 -21.9 -2.6 156.1 L 4.3 0 FPE-4a 19 Alli 2.5 n.d. -2.2 148.9 L 3.7 n.d. n.d. n.d. FPE-4a 20 Alli 1.8 n.d. -2.3 146.2 L 3.9 0 FPE-4a 21 Alli 1.8 n.d. -2.8 193.9 L 4.6 n.d. n.d. n.d. FPE-4a 22 Alli 1.9 n.d. -2.9 196.1 L 4.8 n.d. n.d. n.d. FPE-4a 23 Alli 1.9 -22.1 -1.8 228.4 L 3.1 n.d. n.d. n.d. FPE-4a 24 Alli n.d. n.d. -2 n.d . n.d. 3.4 0 FPE-4a 25 AIV n.d. -21 .6 -2.5 239 L 4.2 0 FPE-4a 26 AIV n.d. n.d. -2.3 236 L 3.9 0 FPE-4a 27 AIV n.d. n.d. -1.5 214 L 2.6 1289.3 1392 0 FPE-4a 28 AIV n.d. n.d. -1.6 232 L 2.7 1289.3 1392 0 FPE-4a 29 AIV n.d. -20.7 -1.9 227 L 3.2 1289.2 1392 0 FPE-4a 30 AIV n.d . -20 .7 -1.9 227 L 3.2 n.d. n.d. n.d. FPE-4a 31 AIV n.d. n.d. -1.9 237 L 3.2 1289.4 1392 0 FPE-4a 32 AIV n.d . n.d. -1.9 225 L 3.2 1289.4 1391.9 0 FPE-4a 33 AIV n.d. n.d. -1.9 257.4 L 3.2 0 FPE-4a 34 AIV n.d. n.d. -1.8 272.4 L 3.1 0 FPE-4a 35 AIV n.d. n.d. -1.7 229.3 L 2.9 0 FPE-4a 36 AIV n.d. n.d. -1.7 228.1 L 2.9 0 FPE-4a 37 AIV n.d. n.d. -1.7 228.1 L 2.9 0 FPE-4a 38 AIV n.d. n.d. -1.7 n.d. L 2.9 n.d. n.d. n.d. FPE-4a 39 AIV n.d. n.d. -1.7 237.8 L 2.9 0 FPE-4a 40 AIV n.d. n.d. -2.3 154.8 L 3.9 1292.2 1396.2 0.5 FPE-4a 41 AIV n.d. -27 -2.1 191.9 L 3.5 1292.3 1395.9 0.4 FPE-4a 42 AIV n.d. n.d. -1.9 218 L 3.2 n.d. n.d . n.d . FPE-4a 43 AIV n.d. n.d. -1 .9 220.2 L 3.2 1286.8 1390.8 0.5 FPE-4a 44 AIV n.d. n.d. -1.9 219.8 L 3.2 n.d. n.d. n.d. FPE-4a 45 AIV n.d. <-19 -3.1 218.8 L 5.1 0 FPE-4a 46 AIV n.d. <-19.3 -1.2 200.2 L 2.1 1293.4 1396.6 0.2 FPE-4a 47 AIV n.d. n.d. -1 .4 194 L 2.4 1293.2 1395.8 0 FPE-4a 48 AIV n.d. n.d. -1.3 208.1 L 2.2 1293.2 1397 0.5 FPE-4a 49 AIV n.d. n.d. -0.2 201.7 L 0.4 0 FPE-4a 50 AIV n.d. -21.8 -1.4 -180 L 2.4 1292.3 1395.7 0.3 FPE-4a 51 AIV n.d. n.d. -1.8 195 L 3.1 1293.8 1396.9 0.2 FPE-4a 52 AIV n.d. n.d. -1.6 199.7 L 2.7 n.d. n.d. n.d. FPE-4a 53 AIV n.d. -21.2 -3.3 208.9 L 5.4 1283.6 1386.9 0.2 FPE-4a 54 AIV n.d. -20.8 -2.8 207.4 L 4.6 0 FPE-4a 55 AIV n.d. n.d. -2.4 214.8 L 4 n.d. n.d. n.d. FPE-4a 56 AIV n.d. n.d. -2.4 202.8 L 4 n.d. n.d. n.d. FPE-4a 57 AIV n.d. -23.2 -2.4 211.9 L 4 1283.5 1386.3 0.1 FPE-4a 58 AIV n.d. n.d. -2.5 218 L 4.2 1283.6 1386.5 0.1 FPE-4a 59 AIV n.d. n.d. -2.5 210.7 L 4.2 1283.6 1386.4 0 124

co, co, C02 Sample n• fam. l/V T, T"'u" phase Salin il y L. band U.band density

816c 1 BI n.d. n.d. -2.4 328 L 4 1289.4 1392 0 816c 2 BI n.d. n.d. -2.3 328 L 3.9 1289.4 1392 0 816c 3 BI n.d. n.d. -2.4 337 L 4 n.d. n.d. 0 DY-37 C 1 Bll n.d. n.d. -1.6 155 L 2.7 n.d. n.d. n.d . DY-37 C 2 Bll n.d. n.d. -1.7 242.9 L 2.9 n.d. n.d. n.d . DY-37 C 3 Bll n.d . n.d. -1.7 249.3 L 2.9 n.d. n.d . n.d . DY-37 C 4 Bll n.d. n.d. -1 .6 213 L 2.7 1289.2 1392.6 0.3 DY-37 C 5 Bll n.d. n.d. -1 .6 213 L 2.7 1289.4 1392 0 DY-37 C 6 Bll n.d. n.d. -1 .7 239.7 L 2.9 n.d. n.d. n.d. DY-37 C 7 Bll n.d. n.d. -1.7 256.3 L 2.9 0 DY-37 C 8 Bll n.d. n.d. -1.8 233 L 3.1 0 OYA-1 1 Bll n.d. n.d. -2.3 201 L 3.9 1289.3 1392 0 OYA-1 2 Bll n.d. n.d. -2.2 206 L 3.7 n.d. n.d. n.d. OYAc1 3 Bll n.d. n.d. -2.2 206 L 3.7 1289.3 1392 0 OYA-1 4 Bll n.d. n.d. -2.1 221 L 3.5 0 OYA-1 5 Bll n.d. n.d. -2.5 225 L 4.2 1289.3 1392 0 OYA-1 6 Bll n.d. n.d. -2.2 223 L 3.7 n.d. n.d. n.d. OYA-1 7 Bl l n.d . n. d. -2.3 238 L 3.9 0 722-10 1 Cl 1.3 -20.4 -3.6 32 1.6 L 5.9 1282.4 1386 0.4 722-10 2 Cl 1.7 -23.5 -3.8 322.7 L 6.2 n.d. n.d. n.d. 722-10 3 Cl 1.1 -20.4 -3.6 310 L 5.9 1282.6 1386 0.3 722-10 4 Cl 1.5 -22.4 -3.8 312 L 6.2 1282.5 1385.9 0.3 722-10 5 Cl 0.9 -23.7 -3.8 311 .1 L 6.2 1282.3 1385.8 0.3 722-10 6 Cl 1.3 -20.2 -4.2 311 L 6.7 1282.3 1384.9 0 722-10 7 Cl 1.3 n.d. -4.2 310.3 L 6.7 n.d. n.d . n.d. 722-10 8 Cl n.d. n.d. -3.6 302.2 L 5.9 1282.4 1385.9 0.4 722-10 9 Ci l 1.1 n.d. -3.8 263.4 L 6.2 1283.2 1385.7 -0.1 722-10 10 Cil 0.8 n.d. -3.7 266 L 6 1283.2 1386 0 722-10 11 Ci l 1.5 n.d. -3.3 275 L 5.4 1283.2 1385.9 0 722-10 12 Gil l -22.1 -3.3 n.d. n.d. 5.4 1283.8 1387.1 0.2 722-10 13 Gil l 1.8 n.d. -3.4 281.7 L 5.6 1283.7 1387.1 0.3 722-10 14 Gil l 1.3 n.d. -3.4 276.6 L 5.6 n.d . n.d. n.d. 722-10 15 Gill 0.7 -22.2 -3.6 295.2 L 5.9 1283.3 1386.7 0.3 722-10 16 Gill 1 n.d. -3.6 284.4 L 5.9 1283.5 1387 0.3 722-10 17 Gill 0.9 -19.7 -3.6 295.3 L 5.9 n.d. n.d. n.d. 722;10 18 Gill 1.8 -21.6 -3.5 274.1 L 5.7 1283.3 1387.1 0.4 722-10 19 Gill 0.7 n.d. -3.8 250a) n.d. 6.2 1283.2 1386.6 0.3 722-10 20 Gill 0.8 n.d. -2.8 297.8 L 4.6 1283.7 1386.7 0.1 722-10 21 Gill 1 n.d. -2.9 318.9 L 4.8 n.d. n.d. n.d. 722-10 22 Gill n.d . -21.2 n.d. 252.9 L n.d. 1283.4 1387.1 0.4 DY-554 1 Dl n.d. n.d. -2.4 290.9 L 4 1289.1 1391 .9 0.1 DY-554 2 Dl n.d. n.d. -2.4 285.2 L 4 1289.2 1391.9 0 DY-554 3 Dll n.d. n.d. -0.1 238.7 L 0.2 0 DY-554 4 Dll n.d. n.d. -1 .7 243.1 L 2.9 n.d. n.d. n.d . DY-554 5 Dll n.d. n.d. -1.7 244.1 L 2.9 0 DY-554 6 Dll n.d. n.d. -2 .5 286.2 L 4.2 1283.3 1386.3 0.1 DY-554 7 Dll n.d. n.d. -2.6 269.1 L 4.3 n.d. n.d . n.d . DY-554 8 Dll n.d . -23.2 -2.9 290.7 L 4.8 1284 1387.1 0.1 TOL-2 1 Dll n.d. -33 -1.8 166.6 L 3.1 0 TOL-2 2 Dll n.d. -30.2 -1 .7 173.9 L 2.9 0

a) Temperature of inclusion decrepitation b) Salinity calulated by volume ration of inclusion, liquid, gaz and salt 125

Appendix IV: LA-ICP-MS fluid inclusion data

Sample n• lam. Na23 Mg25 K39 Ca42

FPE-4a 1 Ail 161843"' 760 14956 7 3034 90760 137 4108

Firsl number is the concentration of lhe element, number as supersrcipt is lhe 3" errer and the limit of detection (LOO) is after lhe semicolon

Mn 55 Fe 57 Cu65 Zn66 As 75 Rb85

2169 51 1775 37269 11 19885 5080 . 696 7645 11 1889 7690 15 1097 61 1 4 65 6074" 975 34734 11 7628 725 5 397 9789' 550 4081 15 437 802 5 37 1481 23 1036

Sr88 Ag 107 Sn 118 Ba 137 W182 Pb208

17S 7 82

Remerciements

A 1'heure de clore ce travail, je tiens à remercier toutes les personnes et institutions qui, de près ou de loin, m'ont aidé dans sa réalisation. Tout d'abord, je souhaiterais remercier profondément mes deux co-directeurs, Lluis Fontboté et Robert Moritz, qui m'ont, d'une part, procuré un sujet de recherche extrêmement intéressant et, d'autre part, encouragé par leurs conseils et leurs suggestions. Je salue également la souplesse et l'ouverture d'esprit dont ils ont su faire preuve au fur et à mesure que le projet évoluait. D'autre part, j'aimerais remercier profondément Urs Schaltegger, non seulement pour son apport non négligeable dans ce travail, mais également pour m'avoir «ouvert>> les yeux à plein d'autres facettes de la géologie.

Plusieurs mois au Pérou ont été nécessaires à l'échantillonnage et aux levers de terrain, ceux­ ci se seraient révélé bien plus pénibles sans les multiples soutiens dont j'ai bénéficié. Ce projet a été réalisé grâce aux soutiens logistiques et financiers de la société Volcan à Lima, ce pour lesquels elle est vivement remerciée. D'autre part, ma plus grande gratitude va à Les Oldham pour les nombreuses discussions, à Carlos Astorga pour ses connaissances de la géologie régionale, à Dora Carnac pour avoir résolu maints petits problèmes administratifs ainsi qu'à toute l'équipe de l'Anglo-Peruana. Je souhaite également dire un grand merci au personnel de toute l'équipe géologique de Volcan, et plus précisément à Edwin Salinas, Anibal Chavez, Jesus Ylazaca, Jaime Calla et Oscar Cuba. Mes remerciements les plus vifs vont également à Edgar Roman de Centromin, pour les nombreuses journées passées ensemble à me faire découvrir le district minier de Morococha. Un grand merci également à Sylvia Rosas et Ulrich Kobe pour avoir partagé leurs connaissances du Domo de Yauli.

Les nombreuses méthodes analytiques appliquées durant cette étude ont été rendues possible grâce à plusieurs collaborations. Je voudrais tout spécialement remercier Thomas Pettke pour les analyses LA-ICP-MS et Massimo Chiaradia pour les mesures isotopiques du Pb et les datations Re-Os. Un grand merci également à Chris Heinrich pour l'accès à son laboratoire, à Mike Cosca pour les datations 40 Ar/39 Ar, à François Bussy pour son support lors des analyses de microsonde, à Fabio Capponi pour les mesures XRF, à Phillipe Thélin pour son assistance lors de la séparation des argiles et, finalement, à Denis Fontinie et à Marcelle Falcheri pour les rapports isotopiques de Sr et Nd.

J'aimerais d'autre part remercier les diplômants ayant travaillé au Domo de Yauli, Henri Lisboa, Benjamin Sallier et Phillipe Stucky ainsi que toute l'équipe du Département de Minéralogie pour l'excellente ambiance dont j'ai bénéficié durant ces quelques années. Finalement, je dirais merci du fond du coeur à ma petite femme, Alexandra, pour son soutien et tout le reste. 130