THESIS RONNER BENDEZU - Colquijirca PDF [PDF]

Zitiervorschau

Ronner BENDEZÚ JUAREZ

Shallow polymetallic and precious metal mineralization associated with a Miocene diatreme-dome complex: the Colquijirca district in the Peruvian Andes.

2007

Volume 64

Déjà paru /already published : Previous volumes are listed under http://www.unige.ch/sciences/terre/mineral/publications/ter_env.html Vol. 30 (2001) F. Parat: Contemporaneous magmatic differentiation of S-rich trachyandesitic and high-K calcalkaline andesite in an intracontinental setting, San Juan Volcanic Field, Colorado, U.S.A. (121 pages). ISBN 2-940153-29-9 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1211 Genève 4; 30.- CHF Vol. 31 (2001) J. Viszkok: Subsurface fluid flow simulation with finite element method in the East Pannonian Basin. (232 pages). ISBN 2-940153-30-2 Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 32 (2001) T. Ton-That: 40Ar/ 39Ar dating of late Pleistocene marine and terrestrial tephra from the Tyrrhenian and Ionian Seas, Mediterranea; some implications for global climate changes. (99 pages). ISBN 2-940153-31-0 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1211 Genève 4; 30.- CHF Vol. 33 (2001) S. Girardclos: Sismostratigraphie et structure sédimentaire en 3D d'un bassin lacustre, du retrait glaciaire à nos jours (Lac Léman, Suisse). (182 pages). ISBN 2-940153-32-9 Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 34 (2001) P. Rosset: Evaluation de l'aléa sismique dans les vallées alpines par des méthodes détermi-nistes. (133 pages). ISBN 2-940153-33-7 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1211 Genève 4; 30.- CHF Vol. 35 (2002) R. Gilbin: Caractérisation de l'exposition des écosystèmes aquatiques à des produits phytosanitaires: spéciation, biodisponibilité et toxicité. (192 pages). ISBN 2-940153-34-5 Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 36 (2002) Y. Haeberlin: Geological and structural setting, age, and geochemistry of the orogenic gold deposits at the Pataz Province, eastern Andean Cordillera, Peru. (182 pages). ISBN 2-940153-35-3 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1211 Genève 4; 50.- CHF Vol. 37 (2002) S. Mosquera Machado: Analyse multi-aléas et risques naturels dans le département du Chocó, nord-ouest de la Colombie. (159 pages). ISBN 2-940153-36-1 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1205 Genève; 30.- CHF Vol. 38 (2002) I. Baster: Holocene delta in western Lake Geneva and its palaeoenvironmental implications: seismic and sedimentological approach. (159 pages). ISBN 2-940153-37-X Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 39 (2002) Ö. F. Çelik: Geochemical, petrological and geochronological observations on the meta-morphic rocks of the Tauride Belt Ophiolites (S. Turkey). (257 pages). ISBN 2-940153-38-8 Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1205 Genève; 50.- CHF Vol. 40 (2002) A. Carnelli: Long term dynamics of the vegetation at the subalpine-alpine ecocline during the Holocene: comparative study in the Aletsch region, Val d'Arpette, and Furka Pass (Valais, Switzerland). (324 pages). ISBN 2-940153-39-6 Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 41 (2003) S. Beuchat: Geochronological, structural, isotopes and fluid inclusion constrains of the polymetallic Domo de Yauli district, Peru. (130 pages). ISBN 2-940153-40-X Commande à : Département de Minéralogie, 13 rue des Maraîchers, 1205 Genève; 50.- CHF Vol. 42 (2003) Z. El Morjani: Conception d'un système d'information à référence spatiale pour la gestion environnementale; application à la sélection de sites potentiels de stockage de déchets ménagers et industriels en région semi-aride (Souss, Maroc). (300 pages). ISBN 2-940153-41-8 Commande à : Institut F.-A. Forel, 10 route de Suisse, 1290 Versoix (Suisse); 30.- CHF Vol. 43 (2003) C. Pellaton: Distribution of sedimentary organic matter (palynofacies) with respect to palaeoenvironmental conditions: two case histories from the Miocene of the USA. (185 pages). ISBN 2940153-42-6 Commande à : Département de Géologie, 13 rue des Maraîchers, 1205 Genève; 30.- CHF

suite page III de couverture

UNIVERSITÉ DE GENÈVE Département de minéralogie

FACULTÉ DES SCIENCES Professeur Lluís Fontboté

Shallow polymetallic and precious metal mineralization associated with a Miocene diatremedome complex: the Colquijirca district in the Peruvian Andes THÈSE présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention sciences de la Terre

par

Ronner BENDEZU du Pérou

Thèse N°3779

GENÈVE Atelier de reproduction de la Section de physique 2006 

Bendezú Juarez, R.: Shallow polymetallic and precious metal mineralization associated with a Miocene diatreme-dome complex: the Colquijirca district in the Peruvian Andes. Terre & Environnement, vol. 64, IV + 221 pp. (2007) ISBN 2-940153-63-9 Section des Sciences de la Terre, Université de Genève, 13 rue des Maraîchers, CH-1205 Genève, Suisse Téléphone ++41-22-702.61.11 - Fax ++41-22-320.57.32 http://www.unige.ch/sciences/terre/

TABLE OF CONTENTS

TABLE OF CONTENTS Abstract

1

Résumé étendu

5

Generalities, historical sketch and production Chapter 1a. The geology of the Colquijirca district local context. Some observations and contributions

16 in a regional and

21

Paleozoic 21 Stratigraphy 21 Paleogeographic and tectonic evolution 23 Magmatism 23 Mesozoic 25 Stratigraphy 25 Paleogeography, tectonic evolution and structures 27 Magmatism 27 Cenozoic 29 Stratigraphy 29 Paleogeography, tectonic evolution and structures 32 Magmatism and Mineralization 32 References 32

Chapter 1b. The Marcapunta diatreme-dome complex

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Main volcanic facies and spatial configuration 35 General Chemistry 38 References 39

Chapter 2 Description of the mineralization types

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Introduction 43 Field and laboratory methods of study 43 Main terminology 43

Chapter 2a: The Smelter deposit

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Host rock 45 Spatial configuration of the mineralized bodies and controls on mineralization 45 Mineral deposition history: stages and zoning 51 Early silica – pyrite stage 51 Main ore stage 55 

TABLE OF CONTENTS Late ore stage 64 References 64

Chapter 2b. The Colquijirca deposit

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Host rock 73 Spatial configuration of the ore bodies and controls on mineralization 73 Mineral deposition history: stages and zoning 76 Early silica – pyrite stage 76 Main ore stage 78 Late ore stage 86 Interpretation on the observed patterns of relative ages of mineral assotiations in the Smelter and Colquijirca deposits 87 References 89

Chapter 2c. The central sector. “Disseminated” Au-(Ag) mineralization in the Oro Marcapunta prospect 93 Host rock 93 Breccias 93 Dome and lava flows 95 Spatial configuration of the orebodies and controls on mineralization 95 Mineral deposition history: stages and zoning 95 Early barren stage 97 Gold stage 99 Late stages 102 Massive oxide veins 103 References 103

Chapter 2.d The San Gregorio deposit

105

Host rock 105 Spatial configuration of the mineralized bodies and controls on mineralization 106 Mineral deposition history: stages and zoning 109 Ore stage 109 Other manifestations of hydrothermal activity in the San Gregorio deposit 114 Pre-Ore stage dolomitization and dissolution 114 Post-ore stage quartz-alunite-kaolinite alteration and silicification 114 Main differences with the Colquijirca deposit 115 References 115

ii

TABLE OF CONTENTS

Chapter 2.e Similarities, differences and spatial relationships between the Au–(Ag) disseminated mineralization at Oro Marcapunta and the Cordilleran base metal deposits of Smelter and Colquijirca. A distinction base on field and mineralogical observations

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Similarities 117 High level emplacement 117 High sulfidation mineral assemblages and acid-sulfate gangue mineral assemblages 117 Differences 117 Strong ore zoning 119 Economical metal suite 119 Maximum depth of formation 119 Decreasing vs. increasing oxidation state 120 Crosscutting relationships 120

Chapter 3a. Relative age of Cordilleran base metal lode and replacement deposits and high sulfidation Au-(Ag) epithermal mineralization in the Colquijirca mining district, central Peru 121 Abstract 121 Introduction 122 Geologic setting and mineralization 122 The Colquijirca Mining district 122 Dated samples 124 Analytic procedure 127 Results 128 Discussion and conclusions 131 Acknowledgements 132 References 132

Chapter 3b. Infra-red (CO2) laser 40Ar/39Ar analysis of alunites from The Miocene Colquijirca District, central Peru.

139

Abstract 139 Introduction 140 Previous geochronologic data and scope of the present work 143 Dated samples 143 Analytic procedure 145 Results 148 Discussion 149 References 150 iii

TABLE OF CONTENTS

Chapter 4 Basic features of the hydrothermal fluids in the Colquijirca District. A record from fluid inclusions and stable isotopes. 157 I. The Oro Marcapunta Au–(Ag) epithermal high sulfidation fluids 157 a. Fluid inclusion microthermometry 157 b. Stable isotopes analysis 163 Discussion 168 Early barren stage 168 Gold stage 172 Late stages 172 II. Polymetallic Cordilleran fluids 172 a. Fluid inclusions microthermometry 173 b. Stables isotopes 176 Discussion 185 b. Early silica-pyrite stage 186 Main ore stage 186 References 191

Chapter 5: Summary and final discussion

195 Geological setting 199 General genetic model 206 References 208

General Appendix

211

Remerciements

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iv

ABSTRACT

Abstract

The Colquijirca District is located on the central Andes of Perú and displays two main mineralization types: (i) basically carbonate-hosted Cordilleran sulfide-rich Cu-(Au-Ag) and Zn-Pb-(Ag) deposits (Smelter, Marcapunta Oeste, Colquijirca, and San Gregorio), and a (ii) volcanic-hosted Au-(Ag) disseminated high sulfidation epithermal prospect (Oro Marcapunta). The two mineralization types are in close spatial relation to the Marcapunta Miocene diatrema-dome complex, which occupies the central sector of the district. The Marcapunta complex consists of multiple porphyrytic dome-lava intrusions which pre- and post-date several episodes of phreatomagmatic breccias typical of diatreme conduits. Geochemical data obtained in this study show that the lavadome facies of the Marcapunta volcanic complex are of dacitic high-K subalkaline composition. In addition, trace element concentrations of Sr, Y, and REE indicate that Marcapunta dacites approach “adakite-like” signatures. 40Ar/39Ar dating conducted on magmatic biotite gave inverse isochron ages of 12.9±0.1 and 12.7±0.1 for two slightly propylitized dome-lava bodies and a well defined plateau age of 12.43±0.06 for an unaltered dome. The last age is the available best approximation to a magmatic episode related to the hydrothermal activity which started at the most ~0.5 My later (~11.9 Ma, see below). Volcanic-hosted Au-(Ag) disseminated mineralization at Oro Marcapunta was formed by a recurrent succession of events consisting each of an early barren acid alteration followed by Au-(Ag) deposition. The barren acid alteration generated cores of vuggy silica surrounded by quartz-alunite assemblages. Au-(Ag) was largely deposited as millimetric wide sulfide- and oxide-bearing veinlets mainly in the highly permeable altered ledges of vuggy silica. Portions including sulfide-bearing veinlets are made up of pyrite, covellite, enargite, chalcocite, and sphalerite in amounts that collectively are 300 in the sphaleritegalena zones at Colquijirca. The Ag/Au ratio of the bulk Smelter-Colquijirca corridor is about 250 to 300. Cross cutting relationships indicate that during the productive stage, the inner Cu zones encroached on the Zn-Pb zones recording a progressive advancing of at least 1.5 km of the sulfide-rich mineralization front with time. Conversely, the late chalcocite-bearing and tennantite-bearing assemblages are interpreted in terms of retraction of the mineralization front, possibly during the waning stages of the hydrothermal system activity when meteoric water/magmatic water ratios were higher. Seven alunite samples from the Au-(Ag) 

ABSTRACT disseminated high sulfidation epithermal system (Oro Marcapunta) gave furnace and laser derived 40 Ar/39Ar plateau ages in the range of 11.9±0.07 to 11.10±0.06 Ma at ±2σ. This is a particularly long lifespan and it is interpreted in terms of several shorter episodes of mineralization sustained by multiple intrusive and/or thermal episodes related to a large magma reservoir. The existence of several episodes of mineralization finds support by linking individually the 40Ar/39Ar ages with observed crosscutting relationships between dated alunite and periods of gold deposition defined by the sulfideand oxide-bearing veinlets. At least two recurrent episodes of acid alteration-gold deposition took place during the activity of the Au-(Ag) epithermal system. Seven laser and furnace derived 40Ar/39Ar ages obtained on alunite that predate, are coeval, and post-date different generations of enargite-bearing and sphalerite-bearing assemblages, indicating that the sulfide-rich Cordilleran Smelter and Colquijirca deposits were formed several hundred thousand years later than the Au-(Ag) disseminated mineralization at Oro Marcapunta which lasted from 11.9 to 11.1 Ma. The obtained values (plateau ages) from Cordilleran ores in Smelter and Colquijirca define a relatively narrow period of hydrothermal activity between 10.83±0.06 and 10.56±0.08 Ma (±2σ). Additionally, three K/Ar ages were obtained from the Cordilleran San Gregorio deposit by means of conventional whole rock analysis. Two ages resulted similar at 13.1-13.2±2.2 Ma (±2σ) and the third at 13.9±4.0 Ma (±2σ). Ruled out an excess of inherited argon caused by detrital mica in the alunite-rich concentrates and supported by the concordance of two of the ages at 13.1-13.2±2.2 Ma, it is not excluded that the San Gregorio deposit formed earlier than the other Cordilleran ores of the district. Alternatively, given the large errors on the K-Ar ages, the San Gregorio deposit could also form coetaneously with Smelter and Colquijirca. Fluid inclusions from the Au-(Ag) disseminated mineralization at Oro Marcapunta show that vuggy silica and quartz-alunite alteration entrained fluids that reached temperatures in the range of 210°C and 280°C. These fluids were of low salinity with typical values ranging from ~0 to ~4 NaCl wt % equivalent. These data examined on the basis of mineralogical and zonal patterns suggest that the early barren stage fluids derived from a mixing process which involved variable amounts of two end member “cold” fluids (210°C-240°C) of nearly zero salinities with “hot” slightly saline fluids (250°C-280°C and 3 %-4 NaCl wt % equivalent). Stable isotope data are in agreement with this conclusion. The calculated isotope compositions of water (δD and δ18OSO4) in equilibrium with alunite define an elongated field whose extreme values are on the one side (with δD and δ18O values from -45 ‰ to -55 ‰ and from 7 ‰-7.5 ‰ ) typical of magmatic signatures, respectively and on the other (with δD from around -75 ‰ to -90 ‰ 

and δ18O from-3 ‰ to -6 ‰) indicative of meteoric input. The elongated field fits with the meteoric water-magmatic water mixing trend defined at the contemporaneous Julcani system, located 300 km to the south. The magmatic end-member was most likely a magmatic vapor plume, from which SO2 interacted with water and condensed to form H2SO4, the main acid agent in the formation of the vuggy silica and quartz-alunite alteration. S isotope compositions of alunite (δ34S from 21.5 ‰ to 22.4 ‰) support this origin in which sulfate has equilibrated with H2S formed through disproportionation of SO2 within a condensing vapor plume. Limited fluid inclusion data from the productive Au-(Ag) stage define a mixing trend that broadly fits with that of the early acid barren stage suggesting similar end-member fluids. The available data cover a temperature range between 190°C to 260°C and salinities from ~1 to ~5 NaCl wt % equivalent. A number of fluid inclusions hosted in quartz from veinlets located 90 m below the summit of Marcapunta record boiling. The microthermometric results indicate that the boiling curve adjusts for a 200-300 m depth equivalent, indicating that erosion of the system was minor. Most of the fluid inclusion and stable isotope data on the polymetallic Cordilleran deposits were obtained on the productive stage. Fluid inclusions gave homogenization temperatures from 165.9°C to 298.2°C and salinities between ~0 and ~7 NaCl wt % equivalent. Significant variations of Th are correlated with the distance to the Marcapunta volcanic complex. In the inner enargite zones, homogenization temperatures reached 300°C whereas in more distal parts (i.e., at Colquijirca) temperatures declined to as low as ~170°C. In contrast to temperature, no systematic variation in salinities from Smelter to Colquijirca is distinguished. The gradual temperature decrease from Smelter to Colquijirca is in agreement with drilling correlation indicating that these deposits are part of a single and continuous mineralized corridor which was formed from a single process of mineralization during the productive stage. In the Smelter-Colquijirca corridor, the strong zonal pattern, including the notable decrease in the sulfidation states of sulfides assemblages from high (enargite-pyrite) to intermediate (chalcopyritepyrite) seems to respond to mainly a temperature decrease. Such temperature decrease could probably be reinforced by local mixing with meteoric fluids particularly in the external sphalerite-galena zones. Two relatively contrasting Th vs. salinity trends may be discriminated from the available fluid inclusion data of the productive stage. A first trend defined by quartz-hosted inclusions from alunite-bearing assemblages comprises salinity values between ~0 and ~4 NaCl wt % equivalent and homogenization temperatures in the range of 221°C and 273°C. This trend is nearly identical to that delineated by the early barren acid stage during the Au-(Ag) disseminated mineralization at Oro Marcapunta. The

ABSTRACT mineralogical observations combined with stable isotope data allow the interpretation that this trend reflects mixing between a magmatic end-member, likely a SO2-bearing vapor plume, with meteoric water. The estimated δ18O and δD compositions of water in equilibrium with Main ore stage alunite range from values around-41 ‰ δD and 7 ‰ δ18O to progressively isotopically lighter fluids down to around -99 ‰ δD and -6 ‰ δ18O along a linear trend that suggests mixing of magmatic fluids and unexchanged meteoric fluids. These values fit with the mixing line deduced for the acidic fluids that generated the vuggy silica and quartz-alunite zones at Oro Marcapunta. By projecting the regression mixing line representing the entire population of alunite δD and δ18O values of the Colquijirca district it is possible to estimate a composition of around 140±10 ‰ δD and -18±1 ‰ δ18O for the 10.6-11.9 Ma Colquijirca meteoric water. These values are similar to meteoric water composition estimated for other Miocene systems of central Perú. Entrainment of a SO2 magmatic plume in the formation of the alunite fluids, is also suggested by the δ34S values for alunite (21.1 ‰ to 26.4 ‰) from the Main ore stage quartzalunite assemblages which are typical of hydrothermal magmatic fluids in which sulfate has equilibrated with H2S formed through disproportionation of SO2 within a condensing vapor plume. The second Th vs salinity trend defined by fluid inclusions representing enargite-bearing assemblages, largely devoid of alunite, covers a larger salinity interval between 0.35 and as high as 7.2 NaCl wt % equivalent and temperatures from 218°C to 286°C. These data combined with stable isotope data suggest mixing of magmatic fluids with exchanged meteoric waters. The entrainment of exchanged meteoric waters is indicated by the estimated δ18O and δD compositions of water in equilibrium with kaolinite and dickite, minerals recognized to have coprecipitated with the ores. These compositions plot following a pattern typical of meteoric waters that have exchanged isotopically with host rocks displaying a positive δ18O shift and a minimal shift in δD. Modeling of isotopic equilibrium through convection suggests that 10.5-11.9 Ma Colquijirca meteoric water exchanged with host rocks, predominantly with sedimentary rocks. The exchanged meteoric waters then most likely experienced mixing with magmatic waters. δ34S compositions of sulfides of the whole Smelter-Colquijirca corridor, with average values close to zero, support such magmatic component. The modeling predicts that the isotopic exchange could take place only if the integrated water/rock mass ratio attained values between 0.3 and 1. A genetic model based on the integrated geological, mineralogical, and microanalytical data may be broadly proposed. The model supposes the coexistence at depth, near the brittle-ductile transition, of hypersaline fluid and single-phase vapor. Because of the probable shallow emplacement of

the main degassing cupola(s) of the magma chamber (suggested by the shallow nature of porphyry Cu mineralization related to other Cordilleran deposits of Perú), the single-phase vapor fluids during their ascent could prevent vapor contraction permitting breaching of the two phase field below 450°C. The resulting two fluids, independently of the mechanism of phase separation (i.e., boiling or condensation), are possibly the precursor fluids responsible for the generation of the acid alteration-gold deposition recurrent pulses recognized during the Au-(Ag) high sulfidation epithermal mineralization at Oro Marcapunta. The low Ag/Au ratios (10 to 40) from the Au-(Ag) disseminated bodies similar to values obtained from quantitative microanalysis of singlephase vapor fluid inclusions in other porphyry-related systems are in agreement with this view. While the vapor phase ascended, the brine (hypersaline fluid) probably remained largely within the brittle-ductile transition and then gradually liberated as this transition retracted downward with time. It is in these instances, perhaps up to tens of thousands of years later, when meteoric waters that had exchanged isotopically with mainly sedimentary rocks of the Excelsior and Mitu Groups through convection, could mix with the brines. At Colquijirca, modeling based on the isotopic composition of ore-related kaolinite and dickite predicts that these exchanged waters could acquire the obtained isotopic compositions at moderate to high total mass water/rock ratios (between 0.3 and 1). On the other hand, stable isotope evidences (D, O, S) pointing to extensive mixing between magmatic and exchanged meteoric water have the implication that the 2 to 7 NaCl wt % equivalent salinities of the Cordilleran ore fluids at Colquijirca can be best explained by dilution of magmatic brines. The hypothesis that diluted magmatic brines originated the Cordilleran ore-forming fluids is also supported by key element concentration ratios of the ore samples and of the bulk Smelter-Colquijirca mineralized corridor notably Ag/Au (~50->300), Cu/Au (~7x104-~9x104), and Fe/Au (~2x107-~5x107), which are similar to ratios observed in brines from microanalysis of fluid inclusions from other porphyry-related localities and different from the ratios measured in single-phase vapor fluid inclusions coexisting with the brine fluid inclusions. As mentioned before, the Ag/Au, Cu/ Au, and Fe/Au ratios of the Au-(Ag) disseminated bodies at Oro Marcapunta are, in turn, similar to those measured on single-phase vapor fluid inclusions in several porphyry-related systems. Acidic fluids were common during the period of Cordilleran mineralization. They manifested most prominently through assemblages exempt of ore minerals, though less pronounced, they also precipitated alunite accompanied by enargite(sphalerite). The isotopic evidence indicates that, as at Oro Marcapunta, these acidic fluids formed from SO2 disproportionation by reaction with water, 

ABSTRACT suggesting that multiple vapor plumes were released during the lifespan of the Colquijirca hydrothermal history. Most likely, SO2 would have derived from the rising single-phase vapor fluids. Cordilleran fluids could, subsequent to dilution, potentially mix with the acidic fluids at different levels of the system resulting most probably in oxidized acidic ore fluids. Alternatively, Cordilleran fluids and acidic fluids could enter the epithermal environment separately. The mineralogical evidence suggest that this second scenario was possibly more common.



RESUME ETENDU

RÉSUMÉ ÉTENDU

Le District de Colquijirca se situe dans les Andes du Pérou central (Figure a) et comporte deux types de minéralisations (Figures a et b): (i) des gisements à Cu-(Au-Ag) ainsi que Zn-Pb-(Ag) de type «Cordilleran» riches en sulfures, encaissés dans des roches carbonatées et localisés à Smelter, Marcapunta Oeste, Colquijirca et San Gregorio (e.g., Ahlfeld, 1932; Lindgren, 1935; McKinstry, 1936; Bendezú, 1997 ; Vidal et al., 1997) et (ii) le prospect épithermal disséminé de type «high sulfidation» à Oro Marcapunta encaissé dans des roches volcaniques. Ces deux types de minéralisations ont une relation spatiale avec le complexe de dôme-diatrème de Marcapunta, d’âge Miocène, occupant une place centrale dans le district.

LE COMPLEXE VOLCANIQUE DE MARCAPUNTA ET TYPES DE MINERALISATIONS Le complexe de Marcapunta est composé de plusieurs dômes et coulées pyroclastiques précédant et succédant plusieurs épisodes phréatomagmatiques donnant lieu à des brèches typiques de conduits de diatrème (Figure b). Les données géochimiques obtenues lors de cette étude révèlent que le faciès de «lava-dome» du complexe de Marcapunta possède une composition dacitique subalkaline enrichi en K. De plus, la concentration des éléments traces tels que Sr, Y et les terres rares suggère que les dacites de Marcapunta s’approchent d’une signature adakitique. Une étude géochronologique 40Ar/39Ar sur de la biotite magmatique de deux affleurements de «lava-dome» légèrement propyllitisés ainsi que d’un dôme frais, donne des ages d’isochrones inverses à 12.9±0.1 et 12.7±0.1 et à un âge plateau bien défini à 12.43±0.06 respectivement (Figure e). Ce dernier âge est la meilleure estimation à disposition d’un épisode magmatique lié à l’activité hydrothermale (Bendezú et al., 2003) qui s’est manifestée au maximum 0.5 My plus tard (~11.9 Ma, voir ci-dessous). La minéralisation à Au-(Ag) encaissée dans les roches volcaniques à Oro Marcapunta (Figure b) a été formée par une succession récurrente d’événements consistant chacun en une altération acide stérile suivie par la précipitation d’Au-(Ag). L’altération acide stérile a engendré des cœurs de «vuggy silica» entourés par des assemblages quartz-alunite (Figure c). L’Au(Ag) a largement précipité dans les zones altérées très

perméables du «vuggy silica» dans des veinules millimétriques riches en sulfures et oxides (Figure c). Les veinules riches en sulfures sont composées de pyrite, enargite, chalcosine et sphalérite en quantités inférieures à 5 % vol. Contrairement au stade précoce stérile, la présence de kaolinite-(smectite-illite-sericite) associée à la précipitation d’Au-(Ag) indique que les fluides n’étaient pas extrêmement acides. Les valeurs typiques d’Au et Ag sont de l’ordre de 0.1-0.5 ppm et 5-15 ppm respectivement (rapports Ag/Au de ~10 à ~30; Figure h). Les veinules riches en oxides sont composées principalement d’hématite et goetite botryoïdale (Figure c) contenant de l’Au et de l’Ag dont les concentrations varient de 0.5 à 2 ppm et de 20 à 80 ppm respectivement (rapport Ag/Au de ~10 à ~30; Figure h). Les gisements polymétalliques riches en sulfures (30-60 % vol.) de type Cordilleran en remplacement dans les roches carbonatées constituent le second type de minéralisation dans le District de Colquijirca (Figure b). Ce type de minéralisation constitue des ressources globales estimées à non moins de 300 Mt de minerai de Cu-Zn-Pb-(Au-Ag). Ce minerai polymétallique résulte d’une succession de processus débutant par un stade précoce de remplacement étendu des roches carbonatées bordant le complexe de Marcapunta et précipitant essentiellement de la silice et de la pyrite (Figure d). Le stade productif s’est ensuivit et a télescopé le remplacement de silice-pyrite et même parfois au-delà. Ce stade productif a engendré une zonation minérale et métallique bien définit qui est observé tout le long du corridor minéralisé de Smelter-Colquijirca (Figure b). Ce corridor minéralisé possédait, avant son exploitation, environ 70 Mt à ~1.9 % Cu, ~ 0.3 g/t Au (gisement de Smelter) et 30 Mt à ~6 % Zn, ~ 3 % Pb, ~ 5 oz/t Ag, et 0.03-0.05 % Bi (gisement de Colquijirca). La zone centrale est constituée par l’assemblage enargite-(luzonite) accompagné par endroits par l’assemblage abondant alunite-(zunyite)(Figure c). La grande majorité de la zone centrale est généralement dépourvue de minéraux de gangue excepté le quartz, mais la présence de kaolinite-(dickite, illite, smectite) peut être observée localement. La zone à enargite(luzonite) est entourée d’une zone à chalcopyrite qui est à son tour entourée d’une zone externe riche en Ag dominée par de la sphalérite et de la galène, constituant ainsi le minerai historique de la mine de Colquijirca. Un stade tardif, généralement à caractère sub-économique, consiste principalement en 

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GEOLOGICAL UNITS Colquijirca

B

Dacite diatreme-dome complex (Miocene) Pocobamba Formation (E ocene), mainly limestones and marls Goyllarizquizga Group (C retaceous), sandstones Pucara Group (Upper T riassicL ower Jurassic), limestones Mitu Group (Permian-T riasssic), sandstones E xcelsior Group (Devonian), phyllites

A Marcapunta

MINERALIZATION TYPE Au-(Ag) disseminated epithermal

San Gregorio

Polymetallic Cordilleran deposits

Figure a

Figure a: Géologie générale du secteur Cerro de Pasco-Colquijjirca incluant les deux types principaux de mineralisations. Il est à noter que la couverture sédimentaire quaternaire a été volontairement ôtée.



Figure b: Section longitudinal Nord-Sud du secteur Nord du district de Colquijirca d’après la Figure a. La section inclut les gisements de Oro Marcapunta, Smelter et Colquijirca

Figure 5.2

ORO MARCAPUNTA

S

100 m

SMELTER

200 m

Zn-Pb-(Ag) zone

Zn-Pb-Cu-(Ag-Bi) zone

Cu-(Au-Ag) zone

Cordilleran ores

MINERALIZATION Quartz-alunite + vuggy silica containing Au-bearing veinlets

Au-(Ag) disseminated ores

Mitu Group (Permian-Triasssic), sandstones

Shuco Member (Eocene), calcareous conglomerates

Calera Formation (Eocene), mainly limestones and marls

Dacite diatreme-dome complex (Miocene)

GEOLOGICAL UNITS

COLQUIJIRCA

N

RESUME ETENDU



RESUME ETENDU

quartz-alunite

I

1 cm

II

III

1 cm

IV

1 cm

PARAGENETIC SEQUENCE IN THE DISSEMINATED Au-(Ag) OCCURRENCE OF ORO MARCAPUNTA

Figure c: Partie superieure, images representatives de differents assemblages minéralogiques correspondants aux corps polymétalliques du type “Cordilleran”. Partie inférieure, séquence paragénétique pour ce type de minéralisation, tel qu’il est observé à Smelter et Colquijirca. 

RESUME ETENDU

enargite dickite

1 cm

I

II

1 cm

enargite telluride goldfieldite pyrite III

tennantite

enargite 0.5 cm

IV

50 µm

PARAGENETIC SEQUENCE IN THE CORDILLERAN POLYMETALLIC DEPOSIT OF SMELTER

Figure d: Partie supérieure, images representatives des differents assemblages minéralogiques des corps à Au-(Ag) du type épithermal “high sulfidation”. Partie inférieure, séquence paragénétique pour ce type de minéralisation, tel qu’il est observé à Oro Marcapunta. 

RESUME ETENDU un assemblage à chalcocine-tennantite superposant de larges portions du corridor minéralisé de Smelter-Colquijirca. Les rapports Ag/Au de la totalité du corridor de Smelter-Colquijirca atteignent des valeurs entre 30 et 50 dans les zones internes (Figure h) et >250 dans les zones externes.

DURÉE DE VIE DU SYSTÈME MAGMATO-HYDROTHERMAL Les relations de recoupement indiquent que lors du stade productif, les zones internes à Cu ont empiété sur les zones à Zn-Pb, enregistrant ainsi un avancement progressif avec le temps du front de minéralisation riche en sulfure sur au moins 1.5 km. A l’inverse, les assemblages tardifs à chalcosine et ceux à tennantite sont interprétés comme résultant de la rétraction du front minéralisateur, probablement lors du déclin du système hydrothermal lorsque le rapport eau météorique/eau magmatique était plus élevé Sept échantillons d’alunite provenant du système épithermal Au-(Ag) disséminé de type «high sulfidation» ont été analysés par la méthode 40Ar/ 39 Ar «furnace» et «laser» et donnent des âges plateau de 11.9±0.07 à 11.10±0.06 Ma à ±2σ (Figure e). Ce laps de temps est particulièrement long et est interprété comme résultant de plusieurs épisodes courts

de minéralisations liés à plusieurs intrusions et/ou d’épisodes thermiques, le tout associé à un large réservoir de magma. L’existence de plusieurs épisodes de minéralisation est indiquée part la mise en relation individuelle des ages 40Ar/39Ar avec les relations de recoupement entre les analyses datées et les périodes de précipitation de l’or définit par les veinules à sulfures et oxides. Au moins deux épisodes récurrents d’altération acide-déposition de l’or a eu lieu pendant l’activité du système épithermal Au-(Ag) (Figure e). Sept âges 40Ar/39Ar four et laser obtenus sur de l’alunite précédent, contemporaine et postdatant différente générations d’assemblages à enargite et d’assemblages à sphalérite, indiquent que les minéralisations de type Cordilleran riches en sulfures à Smelter et Colquijirca ont été formés quelques centaines de milliers d’années plus tard que la minéralisation épithermale de type «high sulfidation» à Oro Marcapunta, persistant de 11.9 à 11.1 Ma (Figure e). Les âges plateau sur alunite du gisement de type Cordilleran à Smelter et Colquijirca définissent un temps d’activité hydrothermale relativement court, allant de 10.83±0.06 à 10.56±0.08 Ma (±2��������������� σ�������������� ) (Bendezú et al., 2003). De plus, trois âges K/Ar ont été obtenus sur roche totale dans le gisement de type Cordilleran à San Gregorio (Figure e). Deux âges ont été obtenus à 13.1-13.2±2.2 Ma (±2������������������������� σ������������������������ ) et un troisième âge à 13.9±4.0 Ma (±2������������������������������������� σ������������������������������������ ). En éliminant la possibilité d’un Late Miocene

Middle Miocene a on biotite

a on sanidine

a on alunite

Probable periods of gold deposition

on alunitea

Dome emplacement

12

Age (Ma)

11 Au-(Ag) disseminated epithermal mineralization

Previous surveys K/Ara 39 40 Ar/ Ar (furnace)b This survey: 39 Ar/ 40Ar (infrared laser) K/Ar whole rock

15

Cordilleran ores of San Gregorio 14

13

Age (Ma)

Cordilleran base metal deposits 12

11

10

Figure e: Diagramme résumant les données géochronologiques (K/Ar et 40Ar/39Ar) disponibles dans le district de Colquijirca. 10

RESUME ETENDU excès d’argon hérité causé par des grains de mica détritique présents dans les concentras riches en alunite, ainsi qu’en supposant correcte la concordance des deux âges à 13.1-13.2 ±2.2 Ma, il n’est pas exclu que le gisement de San Gregorio se soit formé avant les autres gisement de type Cordilleran dans le district. Alternativement, étant donné les larges erreurs sur les âges K/Ar, le gisement de San Gregorio a également pu être synchrone à Smelter et Colquijirca.

FLUIDES MINERALISATEURS Les inclusions fluides de la minéralisation épithermale de type “high sulfidation” à Oro Marcapunta montrent que l’altération de type vuggy silica et celle de type quartz-alunite ont été formés à partir de fluides à des températures allant de 210 à 280°C (Figure f). Ces fluides ont une salinité faible avec des valeurs typiques allant de ~0 à 4% poids NaCl équivalent. Ces données examinées sur la base de la composition minéralogique et de la zonation existante suggèrent que les fluides stériles du stade précoce ont été issus d’un processus de mélange impliquant une quantité variable d’un pôle caractérisé par un fluide “froid” (210°C-240°C) à salinité proche de zéro, avec un pôle défini par un fluide “chaud”, légèrement salin (250°C-280°C et 3 % -4 % poids NaCl équivalent) (Figure f). Les données d’isotopes stables sont en accord avec cette interprétation. En effet, la composition isotopique calculée de l’eau (������� δ������ D et �δ18OSO4) en équilibre avec l’alunite définit un champ allongé avec des valeurs extrêmes qui sont d’un côté (avec δ����� ������ D et δ18O values de -45 ‰ à -55 ‰ et de 7 ‰ à -7.5 ‰) typique de signature magmatique et de l‘autre (avec δ������������������������ D de ~-75 ‰ à -90 ‰ et δ�18O de -3 ‰ à -6 ‰), caractéristique d’une contribution d’eau météorique (Figure g). Le champ allongé est en accord avec une droite de mélange d’eau magmatique et météorique la composition de celle-ci étant définie à Julcani, gisement contemporain au District de Colquijirca, situé à 400 km vers le sud (Deen et al., 1994). Le pôle d’eau magmatique était vraisemblablement un panache de vapeur magmatique, duquel SO2 a interagi avec de l’eau puis condensé pour former H2SO4, l’agent majeur d’acidité dans la formation de vuggy silica et de l’altération quartz-alunite (e.g., Rye, 1993). La composition isotopique du soufre dans l’alunite (�δ34S de 21.5 ‰ à 22.4 ‰) est en accord avec une origine dans laquelle le sulfate a équilibré avec H2S formé à travers la disproportionation de SO2 au sein d’un panache de vapeur subissant une condensation (e.g., Rye, 1993). Des données limitées d’inclusions fluides du stade productif à Au-(Ag) sont définis par une droite de mélange qui est grossièrement en accord avec celle du stade précoce stérile, suggérant des pôles de fluides similaires (Figure f). Les données disponibles couvrent une variation allant de 190 à 260 °C et des salinités de ~1 à ~5 % poids NaCl équivalent (Figure f). Un certain nombre d’inclusions fluides dans

le quartz des veinules situées à 90 m en dessous du sommet de Marcapunta enregistre la présence d’ébullition. La courbe d’ébullition issue des résultats microthermométriques correspond à des profondeurs de 200-300m, témoignant ainsi une érosion mineure du système. La plupart des données d’inclusions fluides et d’isotopes stables sur les gisements de type Cordilleran ont été obtenus dans les stades productifs. Les inclusions fluides mesurées dans le quartz donnent des températures d’homogénisation allant de 165.9 à 298.2 °C et des salinités variant entre ~0 à ~7 % poids NaCl équivalent (Figure f). Les variations significatives des températures d’homogénisation sont corrélées avec la distance jusqu’au complexe de Marcapunta. Dans les zones centrales à enargite, les températures d’homogénisation atteignent 300°C alors que dans les parties plus distales (i.e. Colquijirca), les températures diminuent jusqu’à 170°C. Contrairement aux températures, aucune variation systématique de salinités de Smelter à Colquijirca n’est établie. La diminution graduelle de température de Smelter à Colquijirca est en accord avec les corrélations de forages indiquant que ces gisements font partie d’un corridor minéralisé unique et continu, qui a été formé par un processus de minéralisation pendant le stade productif. Dans le corridor de Smelter-Colquijirca, la forte zonation incluant la diminution des états de sulfidation des assemblages de sulfures allant de haut (enargite-pyrite) à intermédiaire (chalcopyrite-pyrite), semble répondre principalement à une diminution de température. Ce genre de diminution de température peut être probablement amplifié localement par un mélange avec de l’eau météorique, plus particulièrement dans les zones externes à sphalérite-galène. Deux tendances de la température d’homogénisation versus salinité peuvent être mises en évidence à partir des données d’inclusions fluides disponibles des stades productifs. Une première tendance se dessine par les inclusions fluides dans le quartz des assemblages à alunite-quartz, caractérisée par des salinités entre ~0 à ~4 % poids NaCl équivalent et des températures d’homogénisation allant de 221 à 273°C (Figure f). Cette tendance est presque identique à celle définie par le stade précoce stérile acide lors de la minéralisation Au-(Ag) disséminée à Oro Marcapunta (Figure f). Les observations minéralogiques combinées aux résultats d’isotopes stables permettent d’interpréter cette tendance comme le résultat d’un mélange entre un pôle magmatique, vraisemblablement un panache de vapeur riche en SO2, avec de l’eau météorique. Les compositions de l’eau en équilibre avec l’alunite du stade principal de minéralisation ont des valeurs allant de -41 ‰ ����������� δ���������� D et 7 ‰ �δ18O à progressivement des fluides isotopiquement plus légers ayant des valeurs autour de -99 ‰ ������������ δ����������� D et -6 ‰ �δ18O le long d’une tendance linéaire suggérant un mélange de fluides magmatiques avec des fluides météoriques 11

RESUME ETENDU

A

350

300

a

Th°C

Th°C

300 250

200

150

150

100

100 1.0

2.0 3.0 Salinity (wt % NaCl eq.)

4.0

Type of Early barren stage: primary in quartz from vuggy silica zones primary in quartz from quartz-alunite zones secondary in quartz from vuggy silica zones secondary in quartz from quartz-alunite zones primary in alunite from quartz-alunite zones Type of First gold stage fluid inclusions: primary in quartz from First gold stage secondary in quartz from First gold stage

b

a

250

200

0.0

B

350

0.0

1.0

2.0

3.0 4.0 5.0 6.0 Salinity (wt % NaCl eq.)

7.0

8.0

Type of Early silica-pyrite stage fluid inclusions: Isolated in quartz Fracture-controlled in quartz

Type of Main ore stage fluid inclusions: Primary in quartz from enargite-bearing assemblage Secondary in quartz from enargite-bearing assemblage Primary in sphalerite from sphalerite-bearing assemblage Secondary in sphalerite from sphalerite-bearing assemblage Primary in quartz from alunite-bearing assemblage Secondary in quartz from alunite-bearing assemblage Secondary in quartz from sphalerite-bearing assemblage Primary in barite from tennantite-bearing assemblage Primary in enargite from enargite-bearing assemblage

Figure f: Diagramme montrant les resultats des mesures microthermométriques effectuées dans des inclusions fluides (salinité versus temperature d’homogénisation). A, dans les inclusions fluides appartenant à la minéralisation à Au-(Ag) épithermale de type “high sulfidation”. B, dans les inclusions fluides appartenant à la minéralisation polymétallique de type “Cordilleran”. non-échangés (Figure g). Ces valeurs sont en accord avec la ligne de mélange déduite des fluides acides qui ont donnés lieu aux zones à vuggy silica et à quartzalunite à Oro Marcapunta. Par projection régressive de la ligne de mélange représentant la totalité de la population des valeurs δ������ ������� D et �δ18O de l’alunite présente dans le District de Colquijirca, il est possible d’estimer la composition de l’eau météorique présente à Colquijirca entre 10.6-11.9 Ma caractérisée par des valeurs de -140±10 ‰ ��������������� δ�������������� D et -18±1 ‰ δ�18O (Figure g). Ces valeurs sont similaires à la composition de l’eau météorique pour d’autres systèmes du Miocène au Pérou central. L’implication d’un panache de SO2 magmatique dans la formation des fluides en équilibre avec l’alunite est aussi suggérée par les valeurs de �δ34S de l’alunite (21.1 ‰ à 26.4 ‰) présent dans l’assemblage quartz-alunite du stade principal. Celui-ci est typique d’un fluide hydrothermal d’origine magmatique où le sulfate a équilibré avec H2S formé lors de la disproportionation de SO2 au sein d’un panache de vapeur en phase de condensation (e.g., Rye, 1993). La seconde tendance caractérisée par la température d’homogénisation versus la salinité est définie par les inclusions fluides représentant les assemblages à enargite, largement dépourvus d’alunite. Cette 12

tendance couvre un intervalle de salinité plus large entre 0.35 à 7.2 % poids NaCl équivalent et des températures d’homogénisation de 218 à 286 °C (Figure f). Ces données combinées aux isotopes stables suggèrent un mélange de fluides magmatiques avec des eaux météoriques isotopiquement échangées avec les roches encaissantes. L’entraînement de ces fluides météoriques échangés est indiqué par les estimations des compositions en �δ18O et δ��������������������� ���������������������� D de l’eau en équilibre avec la kaolinite et la dickite, ces dernières ayant co-précipités avec la minéralisation (Figure d). Ces compositions suivent un patron typique d’eaux météoriques ayant subit un échange isotopique avec les roches encaissantes, traduit par un décalage en �δ18O et un décalage minimal en ������������������������� δ������������������������ D (Figure g). La modélisation d’un équilibre isotopique par convection suggère que l’eau météorique à 10.5-11.9 Ma de Colquijirca à subit un échange avec les roches encaissantes, principalement des roches sédimentaires. Les eaux météoriques échangées ont subit vraisemblablement un mélange avec des eaux magmatiques. Les compositions isotopiques en �δ34S des sulfures dans la totalité du corridor Smelter-Colquijirca, avoisinant des valeurs proches de zéro, confirment une composante magmatique. La modélisation prédit que l’échange isotopique a eu lieu seulement si la valeur du rapport eau/roche se situe entre 0.3 et 1 (Figure g).

RESUME ETENDU Au-(Ag) disseminated mineralization fluids alunite fluids

-20

Cordilleran mineralization fluids

-60

300°C

W L

dickite fluids kaolinite fluids Minerals alunite

400°C 300°C

200°C

kaolinite

100°C

300°C

exchange with igneous rocks

10

15

1

400°C

300°C

200°C

100°C

-140

1

200°C

100°C

-120

exchange with sedimentary rocks

0.15

100°C

dickite

-100

0.3

200°C

d tren g in mix

mix ing

-80

PMW

alunite fluids

M

D (‰, SMOW)

-40

VV

COLQUIJIRCA METEORIC WATER

-20

-15

-10

-5

0

5

δ18O (‰, SMOW) Figure g: Diagramme résumant les données d’isotopes stables de la minéralisation Au-(Ag) de type high sulfidation et la minéralisation de type Cordilleran dans le district de Colquijirca. Les compositions isotopiques de δD et δ18O(SO4) des fluides en équilibre avec l’alunite ont été calculées à partir des équations de Stroffregen et al. (1994) pour des températures obtenues à l’aide de la microthermométrie Figure 5.5 des inclusions fluides. Les compositions isotopiques de δD et δ18O des fluides en équilibre avec de la kaolinite et dickite ont été calculées avec les équations de Gilg and Sheppard (1996) et Sheppard and Gilg (1996). Les températures utilisées ont été obtenues par la microthermométrie des inclusions fluides. Le diagramme comprend également la modélisation isotopique des eaux météoriques de Colquijirca à 11.9-10.6 Ma échangée isotopiquement par convection avec des roches sédimentaires et ignées. Plus de détails sont donnés dans le texte.

500

Ag (ppm)

400 300 200 100 0

0

1

2

3 Au (ppm)

4

5

Disseminated epithermal A u-(A g) mineralization Polymetallic C ordilleran deposits samples from supergene zone samples from hypogene zone

Figure h: Rapports Ag-Au caractéristiques pour les deux types de minéralisations du district de Colquijirca. Le carré à fond Figure h de Smelter (Marcapunta Norte). blanc represente le rapport de tout le gisement 13

RESUME ETENDU

MODELE GENETIQUE PROPOSÉ Un modèle génétique basé sur des données géologiques, minéralogiques et microanalytiques peu être proposé. Le modèle suppute la coexistence en profondeur, proche de la transition ductile-cassante, d’un fluide hypersalin et d’une vapeur monophasée (e.g., Heinrich, 2005). Du fait de la probable mise en place peu profonde d’une coupole de dégazage de la chambre magmatique (suggéré par la nature de la minéralisation de porphyre cuprifère lié à d’autres gisements de types Cordilleran au Pérou), la vapeur monophasée n’a pas pus se contracter, permettant ainsi à la vapeur d’empiéter dans le champ biphasé au dessous de 450°C lors de sa remontée. Les deux fluides en résultants, indépendamment du mécanisme de séparation de phase (i.e. ébullition ou condensation), sont probablement les précurseurs des fluides responsables de la génération de l’altération acidedépôt d’or issu de pulses récurrents reconnus lors de la minéralisation épithermale disséminée de type Au-(Ag) disseminated ores

«high sulfidation» à Oro Marcapunta. Les faibles rapports Ag/Au (10 à 40) des corps de Au-(Ag) disséminés sont similaires aux valeurs obtenues à l’aide de méthodes microanalytiques quantitatives sur la vapeur monophasée dans d’autres systèmes liés aux porphyres cuprifères et sont en accord avec cette prise de position (e.g., Ulrich et al., 1999). Alors que la vapeur remonte, la saumure magmatique reste très probablement en retrait dans la transition ductile-cassante et est libérée graduellement lors de la rétraction de cette transition en profondeur avec le temps. Dans ce cas, quelques dizaines de milliers d’années plus tard, lorsque les eaux météoriques ont subit un échange isotopique par convection avec les roches sédimentaires du groupes de l’Excelsior et du Mitu, ces eaux ont pu être mélangées avec la saumure magmatique. A Colquijirca, la modélisation, basée sur la composition isotopique de la kaolinite et de la dickite liés à la minéralisation, prédit que les eaux échangées peuvent obtenir des compositions isotopiques calculées, lorsque le rapport total eau/roche Au-(Ag) disseminated ores

Cordilleran ores

Cordilleran ores

Marcapunta diatreme-dome complex Pocobamba carbonate rocks

Pocobamba carbonate rocks mixing of diluted brines with acidic fluids derived from SO2 disproportionation

Mitu red beds diluted brine

Mitu red beds

isotopic exchange

diluted brine

mixing

isotopic exchange

mixing

brine

brine brine refluxing

magma chamber

Excelsior slates

brine refluxing

magma chamber

Brittle-ductile transition

a. Scenario 1

Excelsior slates

Brittle-ductile transition

b. Scenario 2 1 Km

Fluids brine vapor plume ascent meteoric water

Figure i: Schéma présentant le modèle génétique global proposé pour les minéralisations du district de Colquijirca. Les possibles processus impliqués dans chaque scénario sont décrits dans le texte. 14

RESUME ETENDU est élevé (entre 0.3 et 1). Les isotopes stables (H, O et S) indiquent un mélange intense entre des eaux magmatiques et des eaux météoriques échangées impliquant ainsi que les salinités des fluides de la minéralisation de type Cordilleran à Colquijirca, entre 2 et 7 % poids NaCl équivalent, peuvent être expliquées par une dilution de saumures magmatiques. L’hypothèse selon laquelle de saumures magmatiques sont les précurseurs des fluides ayant formé la minéralisation de type Cordilleran est soutenue par les rapports clés des concentrations dans le corridor minéralisé de Smelter-Colquijirca, principalement Ag/Au (~ 150~ 350), Cu/Au (~ 7x104 - ~ 9x104), and Fe/Au (~ 2x107 - ~ 5x107). Ceux-ci sont similaires aux rapports mesurés dans les saumures magmatiques par microanalyses dans les inclusions fluides dans d’autres lieux liés à un porphyre ainsi qu’aux rapports mesurés dans des inclusions vapeur monophasées coexistant avec des inclusions fluides salines (e.g., Ulrich et al., 1999). Comme déjà mentionné précédemment, les rapports Ag/Au, Cu/Au, et Fe/Au dans les corps à Au-(Ag) disséminés à Oro Marcapunta, sont similaires à ceux mesurés dans des inclusions vapeur monophasée dans plusieurs systèmes liés à un porphyre. Des fluides acides étaient fréquents lors de la période de minéralisation de type Cordilleran. Ces fluides acides ont produit des assemblages exempts de minéraux économiques, toutefois avec parfois une faible précipitation d’alunite accompagné par de l’enargite-(sphalerite). Les évidences isotopiques indiquent qu’à Oro Marcapunta, ces fluides acides se sont formés à la suite de la disproportionation de SO2 par réaction avec de l’eau, montrant ainsi que des panaches multiples de vapeur ont été libérés tout au long de l’histoire hydrothermale de Colquijirca. SO2 a le plus probablement été derivé d’une vapeur monophasée. Les fluides de la minéralisation de type Cordilleran, à la suite de dilution, ont été potentiellement mélangés à différents niveaux du système, résultant ainsi en fluides acides oxydants (scénario 1 dans Figure i). Alternativement, les fluides de la minéralisation de type Cordilleran et les fluides acides ont pu s’incorporer dans l’environnement épithermal séparément (scénario 2 dans Figure i). Les évidences minéralogiques indiquent que ce second scénario a été également important.

Bendezú, R., Fontboté, L., and Cosca, M. 2003, Relative age of Cordilleran base metal lode and replacement deposits and high sulfidation Au-(Ag) epithermal mineralization in the Colquijirca mining district, central Peru. Mineralium Deposita, 38, p. 683-694. Heinrich, C.A., 2005, The physical and chemical evolution of low-salinity magmatic fluids at the porphyry to epithermal transition: a thermodynamic study. Mineralium Deposita, 39, p. 864-889. Lindgren, W., 1935, The silver mine of Colquijirca, Perú. Economic Geology, 30, p. 331-346. McKinstry, H.E., 1936, Geology of the silver deposit at Colquijirca, Peru. Economic Geology, 31, p. 619-635. Rye, R.O., 1993, The evolution of magmatic fluids in the epithermal environment; the stable isotope perspective. Economic Geology, 88, p. 733-752. Ulrich, T., Günther, D., and Heinrich, C.A., 1999, Gold concentrations of magmatic brines and the metal budget of porphyry copper deposits. Nature (London), 399, 6737, p. 676-679. Vidal, C., Proaño, J., and Noble, N., 1997, Geología y distribución hidrotermal de menas con Au, Cu, Zn, Pb y Ag en el Distrito Minero Colquijirca, Pasco. IX Congreso Peruano de Geología, p. 217-219.

BIBLIOGRAPHIE Ahlfeld, F., 1932, Die Silberlagerstätte Colquijirca, Perú. Zeitschrift Praktischer Geologie, 40, p. 81-87. Bendezú, R., 1997, Características geológicas mineralógicas y geoquímicas de los yacimientos de Zn-Pb (±Ag) de San Gregorio y Colquijirca emplazados en unidades sedimentarias en los bordes del sistema epitermal de alta sulfuración de Marcapunta. Tesis ����������������������������� Ingeniero, Universidad Nacional de Ingeniería, Lima, 60 p.

15

GENERALITIES

16

GENERALITIES

Generalities, historical sketch and production

Colquijirca is one of the numerous examples of mining districts related to Miocene intrusions of the high Cordillera of central and northern Perú. It contains several deposits which together constitute one of the largest accumulations of Zn-Cu-Ag-AuPb ores of the world. The district contains two main ore types: (1) “disseminated” Au-(Ag) high sulfidation epithermal mineralization, similar to the Yanacocha and Pierina deposits of northern Perú and Rodalquilar in Spain and (2) large zoned sulfide-rich polymetallic deposits only comparable in tonnage to the ores of the nearby giant Cerro de Pasco system. This second type is economically important and similar in many respects to one of the most prominent deposit types of the porphyry-related family in the American cordilleras, the so called and characterized by Einaudi (1977) “Cordilleran” deposits. Classical papers on this second ore type by noted geologists such as Ahlfeld (1929), Lindgren (1935) and McKinstry (1936) reflected the major economic importance of this district during the first decades of the 20th century. It is from these works that we have the first epigenetic postulations on the genesis of the mineralization, all of them linking the Marcapunta volcanic complex with the source of the ore fluids. In the seventies, and diametrically opposed, Lehne (1977) and Lehne and Amstutz (1982) proposed a syngenetic model for the mineralization. More recently, however, several workers including Arroyo (1983), Vidal et al. (1984, 1997), Bendezú (1997), Fontboté and Bendezú (1999) and Bendezú et al. (2003), among others, argued, based on new data, for a magmatic-related epigenetic origin. The ores of the Colquijirca district occur in the following four distinctive types or styles: 1. Disseminations, patches and veinlets of Au-(Ag)-bearing oxide minerals exclusively hosted in the volcanic rocks of the Marcapunta diatreme-dome complex. This style represented by the Marcapunta prospect, is a characteristic example of high sulfidation Au epithermal mineralization. 2. Cu±(Au-Ag-Sn) sulfide-rich replacements essentially in carbonate rocks of the Pocobamba Formation and diatreme breccias. This type of mineralization consists of enargite and chalcocite-rich veins and mantos. Several projects and prospects of this style form a large funnel-shaped

body surrounding the Marcapunta volcanic center. Projects and/or prospects include the so-called here Cobre Marcapunta “Sur” and “Oeste” and the Cobre Marcapunta (“Norte”) or Smelter deposit. 3. Zn-Pb-Ag±(Cu-Bi) sulfide-rich replacements in carbonate rocks of the Calera Member. Represented by the historic Colquijirca deposit, this style of mineralization displays the strongest resemblance to many of the classic polymetallic Zn-Pb-Cu-Ag deposits of central and northern Perú. 4. Zn-Pb±(Ag-Bi) fine-grained sulfiderich replacements in carbonate rocks of the Pucará Group. This prominent style of mineralization was virtually unknown in other parts of the world prior to the discovery of San Gregorio. It is, indeed, the economically most important in the district and one of the largest Zn-Pb deposits of any type in the Andes. Colquijirca is one of the earliest known silver mining centers of the Americas. According to several archeological surveys (e.g., Atalaya, 2000), the pre-incaic Tinyahuarco ethnic group lived on the Colquijirca hill and worked silver between the 5th and 8th centuries. It is believed that some of the silver goods transported by hundreds of llamas to pay the rescue of the Inca king Atahualpa, and found in 1533 by Spanish conquerors near Colquijirca (Miguel de Astete, 1533 in Markham, 1872) had their origin in the Colquijirca mines. Some decades after and following the route of Miguel de Astete, the Spaniards rediscovered Colquijirca and began mining as early as 1567 (Fisher, 1977). Mining since then, and until the end of the 19th century was intermittent, mainly using small-scale open-pit techniques. Unfortunately, there is scarce available documentation for this period. In the 1890s the mine passed to Mr. Eulogio Fernandini who started underground workings and an economically very successful production period from the early 1900s. For several years the mine was the most productive in silver of the continent, with an annual peak of nearly 7.5 million ounces in 1925 (the “Bonanza Epoch”). From 1905, bismuth mining was at San Gregorio, some 6 km south of Colquijirca. For several decades it was the most important mine of this metal in Perú before its closure in 1959. From 17

GENERALITIES the end of the 1950s, mining was concentrated at Colquijirca and production not only of silver but also of zinc and lead increased considerably. This period of polymetallic production ceased in 1976, and with it the underground workings in the historic Colquijirca mines. From 1972 until the late seventies, mining in the district was almost exclusively for copper (at Smelter, 2 km south of Colquijirca). Since 1981, once again, mining for zinc, lead and silver characterizes the Colquijirca district, this time through modern open-pit operations. The lack of information during the Colonial and early Republican periods precludes a precise evaluation of the total historical silver production from the Colquijirca district. A rough and conservative figure for the pre-Fernandini period is estimated to be 30 million ounces. The registered data for the 20th account for 100 million ounces (compiled from Hutchinson, 1920 and available government statistics), which indeed represent an underestimate since the production of a significant number of years is undocumented. The total silver production of the Colquijirca district may therefore approximate no less than 150 million ounces. Although it is also difficult to calculate the total zinc and lead production, the figure for the tonnage mined is simpler. It can be estimated that some 20 to 25 Mt of ore at 6% Zn, 3% Pb and 3-4 oz/t Ag were extracted since the beginning of the 20th century. The bismuth production at San Gregorio probably exceeded 200000 tones (t) at 1 to 3 % Bi. The amount of copper ore treated from Smelter was insignificant (100 m (e.g., drill holes 22G, 41G). There follows an interval of more than 50 m of beige laminated dolostones with cinerite intercalations in the central portions. Chert tends to increase towards the upper parts of the interval, where laterally discontinuous monogenic breccias are present. The upper 30 m of this sequence consists of cream micritic dolostones with abundant chert. The Chambará sequence of the Formation at Cerro Lachipana was measured by Angeles (1993) to be near 360 m thickn; nevertheless, from exploration drilling carried out since 1996 in the San Gregorio area, it is known to exceed 400 m towards the south and southeast. Goyllarisquizga Formation Clastic continental sedimentary rocks of the Goyllarisquizga Formation disconformably overlie the carbonate rocks of the Pucará Group. These deposits crop out over relatively large areas throughout the region, though in the Cerro de PascoColquijirca sector, they are restricted to the western and eastern borders of the mineralized areas (Figures 1a.1 and 1a.2). The deposits consist of cross-bedded, medium-grained sandstones and some thin pebblesized beds of mostly chert. Thick beds of red to black shales are intercalated with the sandstones at several levels. Coal horizons are common in the Goyllarisquizga Formation (Broggi, 1922). Chulec-Pariatambo-Jumasha Formations Marine limestones of the Chulec, Crisnejas, Pariatambo and Jumasha Formations (Albian to Turonian) are exposed in the northwest and northeast sectors of the region. They have not been recognized in the Cerro de Pasco and Colquijirca districts. Paleogeography, tectonic evolution and structures The presence of detrital sediments, including Excelsior clasts, in the “basal breccia” (the terrigenous basal unit of the Pucará Group, see above) implies exposure of the basement due to uplift during the Upper Permian-Lower Triassic. The uplift was possibly related to extensional tectonism active when the Triassic sea (in which the Pucará Group formed) transgressed intracontinental basins (e. g., Mégard, 1978). According to Rosas et al. (2004), the Pucará

transgression is attributable to regional subsidence that marked the transition from the Upper PaleozoicLower Triassic Mitu rift to post-rift tectonism and fault-controlled subsidence. The Pucará Group succeeded the Mitu continental sediments and subordinate alkaline volcanic rocks as a carbonatedominated blanket late in the rift stage, when the various Mitu depocenters became yoked. The carbonate cover grades landwards into clasticevaporitic continental deposits. The Pucará Group was largely deposited in shallow water, with no evidence for deep-water paleoenvironments during deposition of the Chambará and Condorsinga Formations. Only the Aramachay Formation (Hettangian-Sinemurian) records generalized flooding of the basin. The presence of several intercalations of alkaline volcanic rocks supports the idea of a late-rift character for the Pucará basin. The very thick (nearly 3000 m) “eastern Chambará domain”, recognized west of the Longitudinal fault in the Cerro de Pasco-Colquijirca area, corresponds to one of the most important tectonically controlled depocenters recognized in the basin. According to Mégard (1978), extensional tectonism occurred in post Bajocian and preNeocomian times, at the end of the Pucará sedimentation, and represents the northern equivalent of the Nevadian epirogeny widespread in southern Perú (Jenks, 1951; and Newell, 1949). At Rancas, in the Cerro de Pasco-Colquijirca sector (Figure 1a.1), an angular discordance between the western domain sequences of the Pucará Group and the Cretaceous Goyllarisquizga Formation could reflect these movements. Paleocurrent determinations in the region indicate that the terrigenous sediments of the Lower Cretaceous Goyllarisquizga Formation were deposited from an emergent segment (Marañón geanticline), along what is now the Eastern Cordillera (Angeles, 1993, 1999). A marine transgression starting in the Albian gave rise to the Chulec, Pariatambo and Jumasha Formations deposed. As stated by Angeles (1993; 1999) variations in the thickness of this unit are controlled by the distance to the source blocks, and hence, the distance to the Longitudinal fault (Figure 1a.4). The approximate thickness of 150 m at Quivlacocha (Angeles, 1993) close to the Longitudinal fault, combined with measured thicknesses of 80 to 100 m farther west (at Colquijirca, from logging of exploration drill holes SD-88 and SP1-218), is in general accordance with such a concept. Magmatism Important magmatic activity in the region during the Mesozoic is restricted to Cretaceous volcanism in the Atacocha mining district, some 10 km northeast of Cerro de Pasco and, less importantly at Cacuán, 4 km north-norwest (Figures 1a.1 and 1a.2). 27

28

4000

4500

4000

4500

R. San Juan

b

Rio San Juan

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Colquijirca

Zone of complicated block system due to collapse of the Marcapunta volcanic complex

Marcapunta

San Gregorio

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4000 m.

4500 m.

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CHAPTER 1

Figure 1a.4: General geological sections of the Colquijirca district as indicated in Figures 1a.2 and 1a.3. Slightly modified from Angeles (1999).

CHAPTER 1 At these places, basaltic flows up to several hundred meters thick are intercalated with the Goyllarisquizga sandstones (Johnson, 1955). Soler (1991) studied some of these intercalations and concluded that the basalts may represent early stages of rifting.

Cenozoic

Stratigraphy Over large areas of the central Peruvian Andes, a relatively thick continental sequence composed mostly of conglomerates, sandstones, sedimentary breccias, marls and limestones of Upper CretaceousEocene age rests unconformably on the Paleozoic to Lower Cretaceous rocks (e.g., Mégard, 1978; Figure 1a.4;). Part of this sequence constitutes the Pocobamba Formation in the Cerro de Pasco and Colquijirca districts. Pocobamba Formation The Pocobamba Formation was first described by McLaughlin (1924). Jenks (1951) divided the Pocobamba Formation into three members: Lower Member (or Cacuán Member; Angeles, 1999) at the bottom, Shuco Member in the middle and Calera Member at the top (Figure 1a.4). Cacuán Member. The Cacuán Member, named after the type locality near Cacuán (Angeles, 1999), consists of a sequence of up to 300 m thick of shales, sandstones, conglomerates and limestones. In the Colquijirca district, the Cacuán Member is poorly exposed and limited to the western block between the Lachipana and Puca Ingenio hills (Figure 1a.2). It can, in general, be characterized as a typical red bed sequence. Its lower parts are composed of reddish sandstones, siltstones, and mudstones, often displaying cross-bedding. Channel-fill conglomerates, breccias and other coarse-grained detrital rocks also typify these intervals. The clasts, 20 cm in diameter (Figure 1a.5). The Shuco Member is considered as a fanglomerate or piedmont deposit (Jenks, 1951; Angeles 1993, 1999), with the above mentioned characteristics being consistent with this interpretation. The Longitudinal is the eastern limit of the main uplifted block, largely made up of the Chambará Formation which is the erosional source for most of the Shuco material. As stated by Angeles (1993, 1999), variations in the thickness of this unit are also controlled by distance from the uplifted blocks, and hence, from the Longitudinal fault (Figure 1a.4). Calera Member. McKinstry (1936), Noble (1992) and Angeles (1999) assigned the status of Formation to the carbonate and detrital rocks resting on the Shuco Member (Figure 1a.4). In this review, use of the classical sub-division proposed by Jenks (1951) is preferred which refers, and refer to this sequence as part of the Pocobamba Formation, namely the Calera Member. According to Jenks (1951), the Calera Member is estimated to be composed of about 70 per cent argillites, limestones, siltstones and sandstones and about 30 per cent limestones. 29

CHAPTER 1

A

B

C

D

Pocobamba Formation

E Figure 1a.5: Images of the main rocks of the Colquijirca-Cerro de Pasco area. A. Arkosic sandstone from the Upper Permian-Lower Triassic Mitu Group red beds on the southern flank of the Marcapunta volcanic complex. B. Finely laminated dolostone from the Upper Triassic-Lower Jurassic Pucará Group at Lachipana hill. C. Calcareous conglomerate from the Upper Cretaceous Shuco Member of the Pocobamba Formation, west of Colquijirca. D. Intercalations of limestones, shales, marls and tuffs of the Eocene Calera Member of the Pocobamba Formation. These intercalations are exposed in the Principal pit at Colquijirca. E. View from the north of the Marcapunta volcanic complex. An outcrop of the Pocobamba Formation is observed which farther south is intruded by the volcanic complex. Numerous beds, other than true limestones, have a calcareous component, many of them marls. Angeles (1993) divided the Calera Member into three members, here sub-members, based mainly on differences in lithofacies: Lower, Middle and Upper sub-members. The following is an abstract of his work. Lower sub-member: The first 10-20 m above the Shuco Member are not exposed. There follows a sequence dominated by detrital sediments, including tuffs, sandy siltstones and lesser conglomerates. The conglomerate beds are of decimetric thickness and 30

consist of pebbles of black limestones in an argillic matrix. Similarly, several intervals with no exposure complete this first 50-m-thick section. The submember is completed by a thin interval characterized by marls, calcareous argillites and limestones, the last with characteristic relicts of rhizoconcretions and roots. This interval is also interbedded with detrital sediments, similar to the lower part, although in this case subordinate. The total thickness of the lower sub-member is estimated to be 64 m. Middle sub-member: This part of the Calera

CHAPTER 1 Member is the best exposed, both as natural outcrops in several parts of the district in the open pits (Figure 1a.5). It is characterized, in contrast to the other submembers, by an abundance of nearly pure carbonate rocks. In the old La Calera quarry, limestones grade from mudstone to coarse grainstone, usually with biogenic structures and, in some cases, fossiliferous. Intercalations of large concretions of chert and thin beds of argillite complete the interval. There follows a generally finer-grained interval consisting of an alternation of multicoloured argillites and limestones, generally mudstones to wackestones. Exposure of this sub-member in the open pits reveals that it is much richer in chert compared with the unaltered rocks beyond the limits of mineralization. This suggests that at least part, if not probably most, of the chert in this part of the district has a hydrothermal origin. Angeles (1993) estimated the thickness of the middle sub-member to be nearly 55 m. Upper sub-member: In contrast to the lower sub-members, it appears to be present only south of the Cuchis Grande lake (Figure 1a.2), and thus, restricted to the Colquijirca district. The most complete stratigraphic interval is exposed in the open pits where intercalations of argillites, siltstones, marly dolostones and some volcanosedimentary beds, including tuffs, occur. According to Angeles (1993), this sub-member exceeds 150 m in thickness. Considering the measurements made by Angeles (1993), the total thickness of the Calera Member attains a minimum of 250 m. However, exploration drilling immediately northeast of Marcapunta hill reveals an up to 500 m thick sequence. The Calera Member is thought to be of Upper Eocene age on the basis of K-Ar age of ~36-37 Ma for biotite from a tuff layer located near the base of the Lower submember (Noble et al., 1999). Paleogeography, tectonic evolution and structures Mégard (1978) stated that the abundant Lower Cenozoic red beds of central Perú were products of extensive erosion from essentially two emerged blocks. One was located along the present coast and isolated the Andean platform from marine influences. The second passed approximately through Cerro de Pasco, parallel to the general alignment of the Andean Cordillera. Much of the continental material present in the Cerro de Pasco-Colquijirca sector, including the deposits of the Cacuán and Shuco Members, was formed by erosion of this second block. This same scenario apparently existed since the Upper Cretaceous, although at that time the presence of marine deposits suggests an ephemeral marine incursion. According to Mégard (1978), major faults strongly influenced the spatial distribution of these high-energy Eocene deposits, as in the Cerro de Pasco

and Colquijirca districts. In the Colquijirca district, Angeles (1999) linked the Cacuán Member deposits to tectonic movements related to the Longitudinal fault. The abundance of coarse sediments in the lower intervals of this member, and the predominance of calcareous material at the top, have been attributed to waning of tectonism, possibly linked to this major structure. A similar process could have occurred during deposition of most of the Shuco Member with variations in lithofacies and thickness being related to distance from the Longitudinal fault. The formation of a lacustrine basin during the Upper Eocene marked the beginning of a period of relative tectonic quiescence. The interpretation by Angeles (1999) implies an early ephemeral basin, with periodic incursions of fluvial material (Lower submember), followed by the establishment of a more stable basin occupied by the Calera lake. The Calera lake, according to the same author, evolved from a shallow holomictic to shallow ectogenic meromictic lake, in which sediments either poor and rich in organic matter and typical of the Middle and Upper sub-members, respectively, accumulated. Variations in facies were controlled by climatic changes and by a possible extension of the hydrographic net rather than by tectonic processes (Angeles, 1999) Tectonics in central Perú during Cenozoic were essentially compressive (e.g., McLaughlin, 1929; Mégard, 1978), and consisted of discrete, short, intense periods of deformation with probable peaks at 100-38 (not well defined; e.g., Myers, 1975; Cobbing et al., 1981), 26, 17, 10, 7 and 2 Ma (e.g., Mégard, 1984). Indeed, most of the structural features currently observable in the Cerro de PascoColquijirca sector formed or were active during this Cretaceous-Tertiary interval (Jenks, 1951; Angeles, 1993). Mégard (1984) stated that the major Andean structures in central and northern Perú are related to the Inca (50-38? Ma), Quechua I (17 Ma) and Quechua III (7 Ma) tectonic pulses. At Colquijirca, the major tectonic phases occurred subsequent to Pocobamba Formation sedimentation, that during the Aymara (26 Ma) or Quechua I (17 Ma, Angeles (1993, 1999) events. The major north-south Longitudinal and FRSJV faults display evidence for reverse motion coherent with the nearly east-west main shortening direction for the region at 26 and/or 17 Ma (Figures 1a.1, 1a.2, and 1a.3). A strike-slip fault system present throughout the Cerro de Pasco-Colquijirca sector (F1 and F2 system in Figure 1a.3) is directionally consistent with first-order breaks in a similar stress regime, suggesting that this system was also formed as a response to compression. Both the folding and F1-F2 fault system occurred prior to hydrothermal mineralization and as will be shown later, constituted important controls on the channeling of mineralizing solutions, particularly in the northern part of the Colquijirca district. 31

CHAPTER 1 Magmatism and Mineralization As in large parts of the Peruvian high Cordillera, the most important period of magmatism took place during Cenozoic. Several periods of magmatic activity are recorded in the Junin-Goyllarizquizga region, with which virtually all the recognized hydrothermal mineralization is related. On the basis of radiometric dating, mostly using the K/Ar method (Soler and Bonhomme, 1988; Cobbing et al., 1981), a long early event from the Upper Eocene at ~ 39 Ma and Upper Oligocene at ~ 29 Ma can be roughly defined. The apparently unmineralized dacitic to rhyodacitic stocks at Huangoc (38.5 Ma), Raco (35.2 Ma) and Huancayán (33 Ma), in the central and northern part of the region belong to this early period. Those east of Cerro de Pasco at Mariac and Sunkullo (31 Ma; Soler and Bonhomme, 1988; Figure 1a.1) are of rhyodacitic composition and younger. The earliest recognized hydrothermal event in the region, possibly related to the Huangoc and Raco intrusions, generated a 37.5 Ma epithermal Au deposit within a volcanic dome at Quicay (Noble and McKee, 1999; Figure 1a.1). The nearby Pacoyán Au prospect as well as tuff deposits of the Calera Member dated at ~36 Ma may be related to this same ~37 Ma pulse (Figure 1a.1). At the end of this major magmatic event, at ~ 29 Ma, a dacitic intrusive center was active in the vicinity of Atacocha. An important hydrothermal system associated with it produced polymetallic veins and skarn-related deposits of the Milpo-Atacocha district. At ~28 and ~15 Ma there is no evidence of magmatic activity in the region, although it was intense elsewhere in Perú, including nearby parts of the Western Cordillera. A second major magmatic event occurred between 15 and 11 Ma in the Cerro de PascoColquijirca districts, and was one of the richest and most mineralized in the world. In contrast to the earlier magmatism in the region, there was important development of several diatreme-dome volcanic complexes emplaced along the major north-south reverse faults (Figure 1a.1). This fault-controlled magmatic-hydrothermal belt seems to extends at least 15 km north of Cerro de Pasco and includes the El Aguila polymetallic prospect. From north to south, this belt includes the volcanic complexes at Cerro de Pasco, Yanamate and, in the Colquijirca district, Marcapunta. The Cerro de Pasco diatreme-dome volcanic complex (15-14 Ma; Silberman and Noble, 1977) is the largest volcanic center in the belt. It consists of a multistage dike-dome intrusion emplaced both prior and subsequent to a major diatreme. Substantial collapse of large portions of the volcanic edifice, as well as development of a relatively complicated fault block system, characterizes this magmatic center. Compositionally, the rocks are chiefly dacitic and, less importantly, trachydacitic and andesitic in 32

composition, all of them texturally porphyritic. The largest hydrothermal sulfide concentration related to a single intrusive center took place at Cerro de Pasco. There, even after extensive erosion, the mining camp still records a minimum of 2.5 billion tones containing more than 50 volume % of sulfides, largely pyrite, but also rich zoned polymetallic ores of Cordilleran base metal lode type (Einaudi, 1977). South of Cerro de Pasco is the smaller Yanamate dacitic volcanic center (~15 Ma; Soler and Bonhomme, 1988). A survey carried out by Angeles (1993) revealed similar characteristics to those of Cerro de Pasco, including development of a diatreme associated with a dome-dike system. Hydrothermal alteration is recognized in the vicinity of the volcanic center, although its economic significance has not been evaluated in any detail. Further to the south, about 10 km from Cerro de Pasco, is the Marcapunta volcanic complex (~12 Ma; Vidal et al., 1984), in the center of the Colquijirca district (Figure 1a.5). As at Yanamate, it also shares many similarities with Cerro de Pasco including diatreme development, multistage superimposition of a dome-lava complex and important collapse of the sedimentary pile surrounding the main diatreme neck. Geochemical reconnaissance of unaltered and slightly altered rocks, presented in the next Chapter, reveals homogeneous dacitic composition. Several lines of evidences, discussed in detail in this thesis, indicate that the mineralization is related to the Marcapunta magmatic center. Mineralization consists of disseminated high sulfidation epithermal Au-(Ag) mineralization and zoned base metal bodies similar to those at Cerro de Pasco, i. e., Cordilleran base metal lodes. Perhaps the last important magmatic expression in the region are the widespread late Miocene (~6.4~5.2 Ma) rhyolitic ash-flow tuffs of Huayllay (40 km southwest of Colquijirca) deposited along the western border of the high plateau of the central Peruvian Andes (Harrison, 1943; Cobbing et al., 1981; Figure 1a.1).

REFERENCES Angeles, C., 1993, Geología de Colquijirca y alrededores. Informe privado Sociedad Minera El Brocal S.A., 39 p. Angeles, C., 1999, Los sedimentos cenozoicos de Cerro de Pasco: estratigrafía, sedimentación y tectónica, Sociedad Geológica del Perú. Volumen Jubilar N° 5, p. 103-118. Benavides, V., 1999, Orogenic evolution of the Peruvian Andes: The Andean cycle, in Skinner, B.J., ed., Geology and mineral deposits of the central Andes. Society of Economic Geologists Special Publication No. 7, p. 61107.

CHAPTER 1 Boit, B., 1949, Sobre la edad de la Formación Caliza Triásica del Perú Central. Publicaciones del Museo de Historia Natural “Javier Prado”, serie C, 2.

Jenks, W.F., 1951, Triassic to Tertiary stratigraphy near Cerro de Pasco, Perú. Geological Society of America Bulletin, 62, p. 203-220.

Boit, B., 1962, Revisión de la estratigrafía en varias regiones de las provincias de Pasco y Junín. Memorias del Museo de Historia Natural “Javier Prado”, 41 p.

Johnson, R.F., 1955, Geology of the Atacocha mine, department of Pasco. USGS, 50, p. 249-270.

Boit, B., 1964, Extensión en el Perú de la estratigrafía centroandina. Memorias del Museo de Historia Natural “Javier Prado”, serie C, 14. Boit, B., 1966, Fauna de la facies occidental del Noriano al Oeste de Colquijirca. Publicaciones del Museo de Historia Natural “Javier Prado”, serie C, 11. Bowditch, S.I., 1935, The geology and ore deposit of Cerro de Pasco, Perú. Unpubl. PhD Thesis, Harvard Univiversity, 160 p. Broggi, J., 1922, Carácter sensiblemente lenticular de los depósitos de carbón de Goyllarizquizga. Archivos de la Asociación Perúana por el Progreso de la Ciencia. Lima, 2, fasc. 1, p. 25-41. Cobbing, E.J., Pitcher, W.S., Wilson, J.J., Baldock, J.W., Taylor, W.P., McCourt, W.J., and Snelling, N.J., 1981, The geology of the western Cordillera of Northern Perú. Overseas Memoir of the Institute of Geological Science London, 5, 143 p. Couch, R., Whitsett, R., Huehn, B., and Briceno-Guarupe, L., 1981, Structures of the continental margin in Perú and Chile, in Kulm, L.D., Dymond, J., Dasch, E.J., and Hussong, D.M., eds., Crustal formation and Andean convergences. Geological Society of America Memoir, 154, p. 703-726. Cox, L.R., 1949, Moluscos del Triásico Superior del Perú. Boletín de la Sociedad Geológica del Perú, 12, 48 p. Dalmayrac, B., 1978, Géologie des Andes Péruviennes. ORSTOM, Paris, 93, 161 p. Einaudi, M.T., 1977, Environment of ore deposition at Cerro de Pasco, Perú. Economic Geology, 72, p. 893924. Gutscher, M.A., Aslanian, D., Eissen, J.P., Olivet, J.L., Maury, R., 1999, The «lost Inca Plateau»; cause of flat subduction beneath Perú?. Earth and Planetary Science Letters, 171; 3, p. 335-341.

McKinstry, H.E., 1936, Geology of the silver deposit at Colquijirca, Perú. ��������������������������������� Economic Geology, 31, p. 619-635. McLaughlin, D.H., 1924, Geology and physiography of the Peruvian Cordillera, Departments of Junín and Lima. Geological Society of America Bulletin, 35, p. 591– 632. Mégard, F., 1968, Geología del cuadrángulo de Huancayo. Boletín Servicio de Geología y Minería, Lima, 18, 123 p. Mégard, R., 1978, Etude géologique des Andes du Pérou central. Contribution a l’étude géologique des Andes No 1, Mémoires ORSTOM, 86, 310 p. Mégard, F., 1984, The Andean orogenic period and its major structures in central and northern Perú. Geological Society of London Journal, 125, p. 893-900. Myers, J.S., 1975, Vertical crustal movements of the Andes in Perú. Nature, 254, p. 672-674. Newell, N.D., 1949, Geology of the Lake Titicaca region, Perú and Bolivia. Geological Society of America Memoir, 36, 111 p. Newell, N.D., Chronic, B.J., et Roberts, T.G., 1953, Upper Paleozoic of Perú. Geological Society of America Memoir, 58, 276 p. Noble, J.A., 1931, Colquijirca examination, 1930-31. Informe privado Cerro de Pasco Copper Corporation, ��� 34 p. Noble, D.C., McKee, E.H., Farrar, E., and Peterson, U., 1974, Episodic Cenozoic volcanism and tectonism in the Andes of Perú. Earth and Planetary Science Letters, 21, p. 213-220. Noble, D.C., McKee, E.H., and Mégard, F., 1979, Early Tertiary “Incaic” tectonism, uplift, and volcanic activity, Andes of central Perú. Geological Society of America Bulletin, 90, p. 903–907.

Haas, O., 1953, Mesozoic invertebrate faunas of Perú. Bulletin of the American Museum of Natural History, 101, 328 p.

Noble, D.C., Sebrier, M., Mégard, F., and McKee, E.H., 1985, Demonstration of two pulses of Paleogene deformation in the Andes of Perú. Earth and Planetary Science Letters, 73, p. 345-349.

Harrison, J.V. 1943, The geology of Central Andes in part of the province of Junin, Perú. Quarterly Journal of the Geological Society of London, 99, p. 1-36.

Noble, D.C., McKee, E.H., Mourier, T., and Mégard, F., 1990, Cenozoic stratigraphy, magmatic activity, compressive deformation, and uplift in northern Perú. Geo-

33

CHAPTER 1 logical Society of America Bulletin, 102, p. 1105-1113.

of America, Special Paper, 241, p. 161-172.

Noble, D.C., and McKee, E.H., 1999, The Miocene metallogenic belt of central and northern Perú, in Skinner, B.J., ed., Geology and mineral deposits of the central Andes. Society of Economic Geologists Special publication, 7, p. 155-193.

Stanley, G.D., 1994, Early Mesozoic carbonate rocks of the Pucará Group in northern and central Perú. 1994. Paleontographica Abt. a., 233, p. 1-32.

Noble, D.C., 1992, Informes privado varios. Compañía de Minas Buenaventura. Pilger, R.H., 1981, Plate reconstructions, aseismic ridges, and low-angle subduction beneath the Andes. Geological Society of America Bulletin, 92, p. 448-456. Prinz, P., and Hillebrandt, A., 1994, Stratigraphy and ammonites of the north Peruvian Pucará Group. Paleontographica Abt. a., 233, p. 33-42. Rosas, S., 1994, Facies, diagenetic evolution, and sequence analysis along a SW-NE profile in the southern Pucará basin (Upper Triassic-Lower Jurassic), central Perú. Heidelberger Geowiss. Abh., Ruprecht-Karls-Universität, Heidelberg, Germany, 80, 337 p. Rosas, S., Fontboté, L., and Tankard, A., 2007, Tectonic evolution and paleogeography of the Mesozoic Pucará basin, central Perú. Journal of South American Earth Sciences, v. 23. Silberman, M.L., and Noble, D.C., 1977, Age of igneous activity and mineralization, Cerro de Pasco, central Perú. Economic Geology, 72, p. 925-930. Soler, P., 1987, Sur l’existence d’un épisode de métamorphisme régional d’age Miocène inférieur dans la Cordillère Occidentale des Andes du Pérou central. Comptes Rendus de l’ Académie des Sciences, Paris, 304, p. 911914. Soler, P., 1991, Contribution à l’étude du magmatisme associé aux marges actives – pétrographie, géochimie isotopique du magmatisme Crétacé à Pliocène le long d’une transversale des Andes du Pérou Central – Implications géodynamiques et métallogéniques. Thèse de Doctorat des Sciences Naturelles, Paris, Université Pierre et Marie Curie, Paris VI, 832 p. Soler, P., and Bonhomme, M.G., 1988, New K-Ar age determinations of intrusive rocks from the Cordillera Occidental and Altiplano of central Perú. Identification of magmatic pulses and episodes of mineralization: Journal of South American Earth Sciences, 1, p. 169-177. Soler, P., and Rotach-Toulhoat, N., 1990, Implications of the time-dependent evolution of Pb- and Sr-isotopic compositions of Cretaceous and Cenozoic granitoids from the coastal region and the lower Pacific slope of the Andes of central Perú, in Kay, S.M., et Rapela, C.W., eds., Plutonism from Antarctica to Alaska. Geological Society

34

Steinmann, G., 1929, Geologie von Perú. Carl Winters Universitätsbuchhandlung, Heidelberg, p. 1-448. Szekely, T.S., and Grose, L��������������������������� .T., 1972, Stratigraphy of the carbonate black shale and phosphate of the Pucará Group (Upper Triassic-Lower Jurassic), Central Andes, Perú. Geological Society of America Bulletin, 83, p. 407428. Vidal, C., Mayta, O, Noble, D.C., and McKee, E. H., 1984, Sobre la evolución de las soluciones hidrotermales dentro del centro volcánico Marcapunta en ColquijircaPasco. Volumen Jubilar Sociedad Geológica del Perú, 10, p. 1-14.

CHAPTER 1

1b. The Marcapunta diatreme-dome complex

The Marcapunta volcanic complex, as introduced above, is an intrusive center (Figure 1b.1) consisting of multiple dome-lava bodies of mainly dacitic composition, which pre- and postdate several episodes of explosion brecciation and pyroclastic accumulation typical of diatremes. The overall internal construct of the volcanic complex, including the vent structure, is as yet poorly defined. Only recently, an intense exploration diamond drilling campaign along the western flank of the complex, coupled with similar exploration data from the northern sector, is providing the first constraints on the three-dimensional internal configuration. Some of these data are preliminarily presented here. Also preliminary is the reconnaissance study of the chemical composition of the volcanic rocks presented in this section.

Main volcanic facies and spatial configuration Topographically, the Marcapunta diatremedome complex resembles a centralvolcano, with typical gently dipping flanks and the center of the complex as a topographic high (Figure 1b.1). The complex is exposed within a slightly elliptical area and has major axis north-south and nearly parallel to the main direction of Cenozoic compressive stress. Most of the outcrops are strongly altered due to a widespread and intense hydrothermal alteration; however, a broad lithological distinction can be drawn on the basis of remnant textures and evidences provided by limited areas of less-altered rocks (Figure 1b.1). The early work by Noble (1931) was the first reconnaissance mapping carried out at Marcapunta and his nomenclature is often used in the present work. More detailed work, although focused on hydrothermal features, as conducted by Barba (1992) and Vidal (1992), and some additional details of the Marcapunta volcanic complex can be found therein. The outcrops of the Marcapunta volcanic complex can be divided into two main units: one constituted by a variety of fragmental rocks (according to the nomenclature of Schmid, 1981), largely pyroclastic, known as Unish Tuff and a second and more important in terms of volume, dominated by dacitic lava domes and flow rocks. In addition,

drill holes have revealed a large body of polymictic breccias below the summit of Marcapunta hill (Figure 1b.2). In accordance with Noble (1992), Barba (1992), and Sillitoe (2000) these breccias represent the upper fingers of a major diatreme conduit at depth. There is no detailed map of the original volcanic facies of the Marcapunta volcanic complex, however, based on integration of available maps, including those currently in progress by Brocal-Buenaventura geologists, the fragmental rocks of the Unish Tuff unit can be assigned to two main sectors, north and east of the center of the volcanic complex (Figure 1a.3). In both places the pyroclastic deposits comprise lapillituff and less abundant lapilli-tuff breccias (Figure 1b.2). Outcrops within the high parts of Marcapunta hill comprise lapilli-tuff, commonly displaying welldeveloped internal stratification (including crossbedding), considerable variation in the grain size and, although rare, accretionary lapilli. These features are typical of base surge deposits which are generally near-vent pyroclastic manifestations, suggesting that at Marcapunta the main vent was located below the present-day topographic high (Figure 1b.1). Drill holes located in the upper portions of Marcapunta (e.g., Brocal-548), intercepted, besides lapilli-tuff deposits, important intervals of breccias. Such breccias consist of monomictic and polymictic angular clasts, up to several decimeters in sizes, a fine-grained matrix composed of milled dacitic rock and juvenile material. These breccias are interpreted as products of phreatomagmatic explosions. Also typical of the central outcrops of the volcanic complex are thin dike-like bodies of massive fine-grained tuffaceous material. According to several workers (e.g., Corbett and Leach, 1998), these are “tuffisite” injections, i.e., another phreatomagmatic product. Drill hole SD11 intercepted several intervals of milled polymictic breccias up to nearly 100 m of vertical extension. In contrast to the phreatomagmatic breccias described above, these contain smaller clasts, generally several centimeters across. Although strongly leached, it is still possible to recognize original clast lithology which includes laminated limestone, milky quartz and phyllite from the Pocobamba Formation, Mitu Group and Excelsior Group respectively. Such varied lithology suggests a major phreatomagmatic process originating at depths of at least 1 km, which accords with the idea that a relatively large diatreme conduit exists in the inner parts of the Marcapunta volcanic 35

CHAPTER 1

S

N

D

C

B SD-11

100 m 200 m

Unconformity Collapse-related fault Diamond drill hole

A Brocal-548

CM2-604

3800 masl

Figure 1b.1

Figure 1b.1 Possible internal configuration of the Marcapunta diatreme-dome volcanic complex along a north-south ABCD section as indicated in Figure 1.4. Data from drill holes and outcroppings. complex (e.g., Noble, 1992). The SD-11 hole exited the breccia body at 450 m depth, but considering other holes located in the surroundings, the diatreme conduit may have a ramified, flared geometry and gain continuity downward, with only its upper parts encountered. The phreatomagmatic breccia also developed laterally to the north and south, with offshoots recognized as far as 600 m from the main vent. In the northern block (Figure 1b.1), beyond the main explosive center, extensive drilling has intercepted lapilli-tuff accumulations of ill-defined origin, which are often associated with, volumetrically more important, block and ash deposits up to 200 m thick. The block and ash deposits are topographically controlled and consist of monomictic, subangular to subrounded dacitic porphyry clasts in an ash-sized matrix of feldspar and pyroxene? grains, and aphanitic lithic fragments. A relatively common feature of these deposits is their reverse grading. The block and ash deposits in this sector are tentatively interpreted as formed by collapse of lava-domes, more likely by gravitational instability than by explosive triggering. The majority of the exposures of the Marcapunta volcanic complex comprise lava flows and lava-dome facies (according to the nomenclature of Self, 1982). Again, despite the fact that the rocks are strongly altered, it is still possible to recognize many of the original characteristics of these facies. Macroscopically, virtually all these rocks are homogeneously porphyritic in texture (Figure 1b.2). 36

As shown below, the chemistry of the scarce unaltered rocks indicates an exclusively dacitic composition. However, comparatively higher abundances of relict resorbed quartz in some rocks indicates that besides dacite, rhyodacitic or less probably rhyolitic rocks may have been originally present (Yacila, pers. comm., 2002). These less common rocks were mainly observed within the volcanic units of the central diatreme neck. Mineralogically, all the unaltered dacitic rocks consist of blocky sanidine crystals, as large as 12 cm (Figure 1b.2), besides smaller plagioclase (probably andesine; Harvey, 1936 in McKinstry, 1936) and resorbed quartz (Figure 1b.2). Also distinctive are euhedral biotite, hornblende and clinopyroxene. The aphanitic matrix is mainly composed of orthoclase, quartz and minor biotite. Accessory minerals include apatite, zircon and magnetite. Harvey (1936) also reported rutile. Lava flows dominate over lava domes at surface. Flow banding (Figure 1b.2) is thus much more common than blocky spherulitic lava typical of the central portions of domes (Self, 1982). Drilling information indicates that lava flows are present throughout the volcanic pile that is from the erosional discordance over the Eocene Calera sequence up to the current top of the Marcapunta complex. The flows overlie either lapilli or block and ash deposits. Recognized thicknesses of the lava flows attain 100 m. Based on well-preserved features, such as flow banding and spherulitic textures, lava dome diameters vary from a few tens of meters to as large as 500 m

CHAPTER 1

A

C

B

D

E

F

2 cm

200 Pm

3 cm

Figure 1b.2 Images of the Marcapunta volcanic complex. A. General view of the Marcapunta complex looking south. Note the marked peneplanation of the Altiplano beginning immediately south of Marcapunta. B. A satellite dome showing flow banding. C. Typical porphyrytic texture of the Marcapunta dacitic domes and lava-domes facies. In this case, dacite underwent extreme hydrothermal leaching with large voids after former K-feldspar phenocrysts D. Thin section image of a typical dacite displaying resorbed quartz, plagioclases and ferromagnesian minerals in a quartz-rich matrix. E. Well-preserved flow banding in an eroded altered dome. F. Typical fragmental rocks of the northern flank of Marcapunta, composed mainly of angular to subrounded dome clasts in a lapilli tuff matrix. 37

CHAPTER 1

General Chemistry The scarcity of unaltered rocks makes petrologic characterization of the Marcapunta volcanic rocks difficult. However, seven samples were found to be exempt from alteration and were analyzed for major and trace elements using the classic XRF technique (Appendix 1b.1). These samples were collected exclusively from flanking domes where hydrothermal alteration was minor or non-existent. Neither pyroclastic rocks nor breccias were analyzed in the present reconnaissance, mainly because all 38

those observed microscopically are considered too strongly altered. Four of the six unaltered samples were analyzed for rare earth elements (REE) (PBR-148, PBR-185c, PBR-216 and PBR-216a; Appendix 1b.2). In addition, in order to obtain REE patterns representative of the central portions of the volcanic complex, the diatreme itself, two samples from the outermost haloes of mineralized zones were also selected (PBR-145 and PBR-199). Although these last samples display a weak to moderate propylitic alteration under the microscope, they were analyzed and included in the study following several papers in which it has been shown that REE concentrations in igneous rocks affected by such alteration remain virtually unchanged and faithfully represent the original values in their unaltered equivalents (e.g., Fulignati et al., 1999). Major and trace element compositions: Major and trace element compositions are broadly homogeneous in all the analyzed samples. Major element contents are invariably high in silica (>65 %) and alumina (>15 %), whereas contents of MgO (0.76-1.25 %) and CaO (1.16-2.77 %) are low. At the level of trace elements, the most remarkable characteristics are the high concentrations of Sr (up  A seventh analyzed sample, not included in this set, from a dome affected by more advanced hydrothermal alteration (moderate to strong phyllic alteration) displays a nearly identical profile, although with slightly higher values (Sample PBR-215, Appendix 1b.2). This sample was finally not taken into account due to the possibility of REE enrichment by hydrothermal fluid introduction.

14 12 Trachyte

Na2O+K2O wt %

east of Smelter, although most of those recognized are generally 15% and MgO22, Sr>400 ppm, Y20. Indeed, this particular anomalous composition is present in the whole Cerro de Pasco-Colquijirca belt including Yanamate (Baumgartner, in prep.; Soler, 1991). The first genetic interpretation of these particular magmas was given by Kay (1978), who proposed

0 0

10

20

30

40

50

YbN

Figure 1b.5: Dacites Figure 1b.5 of the Colquijirca district plotted on a discrimination diagram for the adakitic and typical modern island arc dacitic field (after Martin 1999). N: normalized to chondrite values of Nakamura, 1981 (in Potts et al., 1981). direct melting of young subducted oceanic crust as their source. However, more recently, some of these adakite-like signatures have also been found in rocks at several latitudes in the Andes where the isotopic evidence suggests an important crustal contribution to their formation. Detailed geochemical and isotopic studies in conjunction with geotectonic reconstructions, along various segments of the southern Andes led to some researchers (e.g., Kay et al., 2005) to propose that these Chilean adakite-like rocks could have been formed from mantle-derived magmas with an important involvement of crustal material, most likely by processes of subduction and erosion of forearc crust and/or tectonic thickening and assimilation of Andean crust. The adakite-like rocks of the Cerro de PascoColquijirca volcanic belt are included in this latter category if we take into account the non MORBlike 87Sr/86Sr isotopic composition (>0.705) from Yanamate (Soler, 1991) and Cerro de Pasco (Noble and McKee, 1999). It remain to be shown if similar or alternate processes can explain the genesis of these anomalous dacitic rocks in the Cerro de PascoColquijirca magmatic belt, where two of the largest polymetallic deposits occur.

REFERENCES Angeles, C., 1999, Los sedimentos cenozoicos de Cerro de Pasco: estrastigrafía, sedimentación y tectónica. Sociedad Geológica del Perú, Volúmen Jubilar N° 5, p 103-118. Barba, G., 1992, Mapeo geológico a escala 1:2000 del Cerro Marcapunta. Informe privado Sociedad Minera El Brocal S.A., 91 p. Corbett, G.J., and Leach, T.M. 1998, Southwest Pacific

39

CHAPTER 1 Rim gold-copper systems: structure, alteration and mineralization. Society of Economic Geologists, Special Publication No. 6, 237 p. Defant, M.J., and Dummond, M.S., 1990, Derivation of some modern arc magmas by melting of young subducted lithosphere, Nature, 347, p. 662-665 Defant, M.J., and Dummond, M.S., 1993, Mount St. Helens; potential example of the partial melting of the subducted lithosphere in a volcanic arc. Geology, 21, 547550. Fulignati P., Gioncada A., and Sbrana A., 1999, REE behaviour in the hydrothermal alteration facies of the active high-sulfidation system of Vulcano (Aeolian Islands, Italy). Journal of Volcanology and Geothermal Research, 88, p. 325-342. Kay, R.W., 1978, Aleutian magnesian andesites-melts from subducted pacific ocean crust. Journal of Volcanology and Geothermal Research, 4, p. 117-132. Kay, S.M., Godoy, E., and Kurtz, A., 2005, Episodic arc migration, crustal thickening, subduction erosion, and magmatism in the south-central Andes. Geological Society of America Bulletin, 117, p. 67-88 Le Maitre, R.W., Bateman, P., Dudek, A., Keller, J., La Meyre Le Bas, M.J., Sabine, P.A., Schmid, R., Sorensen, H., Streckeisen, A., Wooley, A.R., and Zanettin, B., 1989, A classification of igneous rocks and glossary of terms. Oxford Blackwell Scientific Publications, 193 p. Lipman, P.W., 1997, Subsidence of ash-flow calderas: relation to caldera size and magma-chamber geometry. Bulletin of Volcanology, 59, 198-218. McKinstry, H.E., 1936, Geology of the silver deposit at Colquijirca, Perú. ��������������������������������� Economic Geology, 31, p. 619-635. Noble, J.A., 1931, Colquijirca examination, 1930-31. Informe privado Cerro de Pasco Copper Corporation, 34 p. Noble, D.C., and McKee, E.H., 1999, The Miocene metallogenic belt of central and northern Perú, in Skinner, B.J., ed., Geology and mineral deposits of the central Andes. Society of Economic Geologists Special publication No. 7, p. 155-193. Noble, D.C., 1992. Private report. Compañía de Minas Buenaventura. Peccerillo, A., and Taylor, S.R., 1976, Geochemistry of Eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey. Contribution to Mineralogy and Petrology, 58, p. 63-81. Potts, P.J., Thorpe, O.W., and Watson, J. S., 1981, Determination of the rare earth element abundances in 29 international rock standards by instrument neutron-ac-

40

tivation analysis a critical appraisal of calibration errors. Chemical Geology, 34, 331-352. Rickwood, P.C., 1989, Boundary lines within petrologic diagrams which use oxides of major and minor elements. Lithos, 22, p. 247-263. Schmid, R., 1981, Descriptive nomenclature and classification of pyroclastic deposits and fragments: Recommendations of the IUGS Subcommission on the Systematics of Igneous Rocks. Geology, 9, p. 41–43. Self, S., 1982, Lava flows and domes, in Pyroclastic Volcanism, L.D. Ayres, ed., Geological Association of Canada, Short Course Notes, 2, p. 53-57. Sillitoe, R. H., 2000, Zinc Exploration at Colquijirca, Central Perú. Private report for Compañía de Minas Buenaventura S. A., 9 p. Soler, P., 1991, Contribution à l’étude du magmatisme associé aux marges actives-pétrographie, géochimie isotopique du magmatisme Cretacé a Pliocene le long d’une transversale des Andes du Pérou Central-Implications géodynamiques et métallogeniques. Unpublished thèse de Doctorat des Sciences Naturelles, Paris, Université Pierre et Marie Curie, Paris VI, 832 p. Vidal, C., 1992, Exploraciones por oro en el Suroeste del Cerro Marcapunta. Distrito Minero de Colquijirca y Cerro de Pasco. Informe privado Sociedad Minera El Brocal S.A., 26 p.

CHAPTER 1 Appendix 1b.1. Major element composition of unaltered lava-dome rocks from the Marcapunta diatreme-dome complex. Analysis by XRF. Sample PBR-148 PBR-185 c PBR-199 PBR-216 PBR-216 a PBR-216 b

SiO2 67.00 65.59 67.37 67.15 66.97 66.80

Al2O3 15.51 17.72 15.57 15.35 15.23 15.18

Fe2O3 4.51 3.47 4.71 3.51 3.54 3.56

MnO 0.08 0.02 0.09 0.04 0.04 0.04

MgO 0.90 0.73 0.87 1.25 1.25 1.25

CaO 1.93 1.16 1.95 2.66 2.66 2.67

TiO2 0.74 0.92 0.67 0.72 0.72 0.73

Na2O 3.11 2.96 2.87 4.15 3.76 3.78

K2O 3.09 4.06 3.10 3.45 3.42 3.42

P2O5 0.27 0.36 0.26 0.61 0.59 0.58

LOI Sum 2.14 99.28 3.03 100.02 2.25 99.72 1.09 99.98 1.10 99.28 1.15 99.15

Appendix 1b.2: Rare earth element concentrations in dacitic domes and lava-domes of the Marcapunta volcanic complex. Sample

La

Ce

Pr

Nd

Sm

Eu

Gd

Tb

Dy

Ho

Er

Tm

Yb

Lu

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

ppm

PBR-145

33.5

75.8

9.3

41.8

6.5

1.37

2.8

n.d.

1.6

0.34

0.9

0.14

0.8

0.08

PBR-148

33

74.2

9.1

37.2

6.8

1.46

2.8

n.d.

1.9

0.39

0.9

0.13

0.7

0.1

PBR-185c

38.6

83.1

9.5

39.6

6

1.28

2.4

n.d.

1.6

0.32

0.8

0.11

0.7

0.09

PBR-200

35.4

77.8

9.3

38.3

5.7

1.43

2.7

n.d.

1.8

0.36

1

0.15

0.8

0.09

PBR-215

59.3

121.2

14.5

68.3

10.3

2.44

5

n.d.

2.9

0.62

1.5

0.21

1.2

0.15

PBR-216

42.7

90.5

10.8

45.7

6.6

1.65

2.9

n.d.

1.8

0.39

0.9

0.13

0.7

0.08

PBR-216a

37.8

82.6

9.7

42.4

6.2

1.54

2.8

n.d.

1.9

0.42

1

0.12

0.6

0.09

41

CHAPTER 1

42

CHAPTER 2

Chapter 2 Description of the mineralization types

Introduction The goal of Chapters 2a, 2b, 2c, 2d, and 2e is to document physically, mineralogically and geochemically the main characteristics of the two mineralization types in the district: (a) sulfide-rich polymetallic deposits (Chapters 2a, 2b, and 2d) and (b) disseminated Au-(Ag) mineralization (Chapter 2c). Additionally, these two mineralization types are compared in Chapter 2e. Field and laboratory methods of study The field part of the present work was carried out for more than 18 months while the writer was a mine geologist at Colquijirca (1997-1998). During that time and for an additional period of six months as a PhD student (this study), the writer worked in the different deposits and exploration projects of Sociedad Minera El Brocal S.A.A. in the Colquijirca district. Most of the time was spent logging 15000 m of drill core, and revising a similar number of meters of core from each of the main deposits in the district. Mapping of selected parts of the open pits in the Colquijirca deposit and key areas in different parts of the district was also undertaken. Another important part of the study was integration of available mine data as well as discussions with other mine geologists (see acknowledgments at the end of the manuscript). Laboratory work comprised the study of nearly 450 representative samples from throughout the district. Although the majority of them correspond to mineralized areas, a number of them are fresh rocks. Of this total, some 300 resulted in thin and polished sections, which were studied by microscopy, infrared microscopy, X-ray diffraction, electron microprobe and scanning electron microscopy. Major and trace element characterization of ores and unaltered rocks were made using a combination of several quantitative chemical techniques, basically X-ray fluorescence, inductively coupled plasma and neutron activation analysis. Details on each individual analytical technique, such as operating parameters and standardization are given in tables.

Main terminology The following terms are employed in the sense of Barton (1963, 1970), Hemley and Hunt (1992), and Einaudi et al. (2003). Mineral assemblage: A grouping of minerals occurring in direct contact and displaying no evidence of reaction with one another (Einaudi et al., 2003). Although Hemley and Hunt (1992) applied this term when chemical equilibrium is implied, in this work, if not specifically discussed, the term mineral assemblage is strictly descriptive. Mineral association: A grouping of minerals occurring together but which are not necessarily all in contact nor necessarily deposited at the same time (Einaudi et al., 2003). The common non-equilibrium conditions under which a mineral association was often formed is evident here (Barton et al., 1963; Hemley and Hunt, 1992). Stage or single stage of mineralization: This term is adopted in the sense of Barton (1970) for a grouping of phases representing an interval of deposition during which there is no discernable chemical or physical change. In this work, single stages of mineralization are discriminated from space-integrated, crosscutting relationships. Paragenesis: This term is used, following a discussion by Hemley and Hunt (1992), for a group of minerals implying contemporaneous formation. It is used as an alternative term when referring to a mineral assemblage in which two or more minerals are believed to have precipitated from a single stage of mineralization. “crystallization reversal”: This informal term refers to a particular mineral assemblage texture consisting of two or more phases displaying one or more repetitive sequences of reversed crystallization.

43

CHAPTER 2

44

CHAPTER 2

2a. The Smelter deposit

The Smelter deposit is one of the sulfide-rich polymetallic deposits of the Colquijirca District. It is located immediately north of the Marcapunta volcanic complex (Figure 2a.1). As described here, corresponds to the mineralized area comprised between grid lines 524 and 628 (Figure 2a.2). These limits are informally used by Colquijirca geological staff for resource calculation purposes. This work, however, adds the sector comprised between grid lines 628 and 700, this last the conventional southern limit of the Colquijirca deposit (Figure 2a.2). The Smelter deposit consists essentially of enargite-rich bodies with significant gold concentrations close to the Marcapunta volcanic complex. The Smelter deposit contains approximately 50 Mt at 1.9 % Cu, 24 g/t Ag, and 0.35 g/t Au. Using Ag/Au ratios as a characterizing parameter, the whole Smelter deposit show a Ag/Au ratio of about 75. However, lower values (30-50) characterize the internal parts of the deposit and, in strong contrast, values of up to 500-1000 are present in more external positions.

Host rock Drill hole correlations indicate that the enargite-bearing silica-pyrite replacements are largely hosted by the carbonate rocks of the Pocobamba Formation (Figures 2a.2 and 2a.3). Surrounding the main diatreme conduit and the central volcanic and subvolcanic domes of the Marcapunta complex, replacement was extensive and affected the entire Pocobamba Formation, including 150 meters of the Calera and apparently also the Shuco Member (Figure 2a.3). With increasing distance toward the north, the silica-pyrite bodies became progressively restricted to intermediate stratigraphic positions within the Pocobamba Formation. This is particularly evident northward from line 596 (Figure 2a.3) where the lower portions of the Shuco Member and/or Calera Member are not mineralized (e.g., DDH, CM5596, CM2-604), and northward from approximately the 612 line where the mineralized bodies occur exclusively within the Calera Member (Figure 2a.3). A matter of controversy is whether the lower mineralized interval belongs to the Shuco Member or to the lower part of the Calera Member. This  DDH: Diamond drill hole. General Appendix 1 presents

UTM location coordinates for all drill holes cited in this work.

controversy is mainly due to the fact that the two units contain, though in different proportions, conglomeratic sediments, and to the intense replacement that considerably masks and in part strongly obliterates the original lithological fabrics. Numerous metric mineralized intervals display clastsupported conglomerate of cobble size, often with clasts > 10 cm long (DDH-CM2-604, CM5-596). This feature has only been described previously for two decimetric intervals in the lower series of the Calera Member (Angeles, 1993), whereas it constitutes the fundamental characteristic of the Shuco Member (Angeles, 1993). The silica-pyrite replacements also occur in the volcanic rocks of the Marcapunta complex, though in minor proportion. The drill holes located within or in the vicinity of the diatreme conduit (e.g., DDH Brocal 524, DDH CM9-548, and DDH CM7-564 in Figure 2a.3) show largely lava domes and block and ash deposits intensely replaced by silica and pyrite. Finally, an apparently insignificant fraction of the silica-pyrite mineralization affected the uppermost beds of the Mitu Group red beds, particularly south of line 588 (Figure 2a.3).

Spatial configuration of the mineralized bodies and controls on mineralization The following description is based almost entirely on relatively wide-spaced diamond drilling evidence. Only in a few places is a three dimensional perspective obtained from limited data provided by still accessible underground workings. The spatial configuration of the early silicapyrite replacements is distinctly different from that of the subsequent copper bodies (mainly enargite-rich). At a district scale, the replacements can be roughly depicted as a funnel-shaped structure surrounding at least 75 per cent, if not entirely, the main volcanic conduit (Figures 2a.1 and 2a.3). Collectively, the silica-pyrite replacement geometry is controlled in parts close to the Marcapunta volcanic complex by the configuration of the contact between the main intrusive conduit and the carbonate host sequence. In more external sectors, bedding was the determinant. In the Smelter sector, within an area of about 700-800 m radius from the approximate contact between the conduit and the intruded sedimentary rocks, replacement of the host rock was virtually total. 45

E-360 000

CHAPTER 2

Condorcayán

4300

45°

70°

A

25° 22° 50° 46°

COLQUIJIRCA

4448

25°

The northern block

N-8 811 000 4458

33°

35°

4250

4200

SMELTER OR COBRE MARCAPUNTA 85°

Huacchuacaja

60°

4300

A

4336

4458

26°

MARCAPUNTA OESTE Lachipana

ORO MARCAPUNTA

4150

44°

N

500 m ? Sulfide-rich mineraliaztion

4179

SAN GREGORIO

4184

A Figure 2a.1: Geological map of the Colquijirca district showing the extension of the sulfide-rich polymetallic mineralized area. Except for the recently defined Marcapunta Oeste project, all deposits are included in this work. 46

0

8 4,4

E-362000

4,320

4,340

4,360

4,380

4,400

4,420

4,440

4,460

4,50

0

E-361000 Marcapunta

524 line

4,280

4,300

4,500

60

N-8809000

D

C 612 line Outer limit of the Zn-Pb-(Ag) zone

Cu-(Ag-Sn-Bi) zone

Smelter

N-8810000

644 line

Cu-(Ag-Bi) zone

MINERALIZATION

a

cac

580 line 4,3

Cu-(Au-Ag) zone

A

548 line

rau

0

40

ua

To H

meters

B

708 line

500

700 line

workings

676 line 4,3

E-360000

740 line

772 line

Colquijirca

N-8812000

804 line Condorcayan

Mercedes-Chocayoc pit

Colquijirca

Principal pit

4,400

o a sc eP d o r Cer To

CHAPTER 2

Figure 2a.2: Plan view of the Smelter-Colquijirca mineralized corridor showing the main ores zones as described in the text and old underground workings.

47

N-8811000

48

SORO MARCAPUNTA

A

100 m

SMELTER

200 m

COLQUIJIRCA

N

Zn-Pb-(Ag) zone

Zn-Pb-Cu-(Ag-Bi) zone

Cu-(Au-Ag) zone

Cordilleran ores

MINERALIZATION Quartz-alunite + vuggy silica containing Au-bearing veinlets

Au-(Ag) disseminated ores

Mitu Group (Permian-Triasssic), sandstones

Shuco Member (Eocene), calcareous conglomerates

Calera Formation (Eocene), mainly limestones and marls

Dacitic diatreme-dome complex (Miocene)

GEOLOGICAL UNITS

B

CHAPTER 2

Figure 2a.3: North-South composite cross section of the Smelter and Colquijirca deposits along the main axis of the deposit as indicated in Figure 2a.2. Because of the complexly ramified pattern of the manto-like ore bodies, only the external limits of the silica-pyrite replacements and ore zones are drawn.

CHAPTER 2 Thus, the entire section of the Pocobamba Formation (up to > 150 m thick) including poorly reactive beds of sandstone, argillite, and siltstone, was extensively replaced. The great intensity of mineralization in this area is also reflected by the fact that, in addition to the Pocobamba Formation, intervals up to several tens of meters thick, of the underlying Mitu Group red beds and overlying volcanic units of the Marcapunta complex were also replaced. It is likely that normal faults related to the subsidence-collapse of the sedimentary pile surrounding the volcanic center played a major role as fluid channelways. In contrast to the virtually total replacement in areas close to the intrusive center, northward from line 580 (Figure 2a.3) replacement is largely controlled by the receptivity of the original lithology. Preferred lithologies are fine-grained limestones, highly permeable coarse-grained sediments such as packstones, several types of coarse sandstones, and monomictic to polymictic breccias of granule to pebble size, all of them typical of the lower series of the Calera Member. The non-replaced intervals are dominated by massive, fine-grained argillites and marls, usually rich in organic matter. The lithological contrast between replaced non-replaced intervals suggests that permeability exerted a primary control in channeling of the early hydrothermal fluids

responsible for the silica-pyrite replacement. From these permeable horizons, fluids spread into the immediately over- and underlying beds. As shown in Figure 2a.3, the silica-pyrite replacements separate into two or, in places, three branches. With increasing distance from the diatreme conduit, narrowing of the replaced interval is recognized, and north of line 644 replacement is confined to the middle portion of the Pocobamba Formation. In this sector, the Shuco Member is not replaced (Figure 2a.3). Normal faults probably channeled the fluids into higher stratigraphic positions of the Pocobamba Formation, as suggested by abrupt “jumps” of the replaced intervals coinciding with projected faults between lines 588 and 628 (Figure 2a.3). Several thinly mineralized (silica and pyrite), high-angle normal faults recognized in old underground workings at Smelter support this view (e.g., SD CM4-588). On the basis of drill core correlations, it can be stated that the fluids that formed the silica-pyrite replacements migrated outwards from the inner parts of the Marcapunta volcanic complex, where they formed essentially veins. As the fluids flowed upward, they encountered a thick pile of mainly carbonate rocks where they formed mantos but also cross-cutting structures (Figure 2a.3). In this

W

E

4,400

4,400 CM4-588-95 CM9-588-05 CM8-588-96 CM6-588-96 CM10-588-06 4,348.257 227.30m. 4,344.225 218.00m. 4,336.851 238.65m. 4,333.439 95.00m. CM11-588-06

CM1-588-95

4,350.479 242.05m.

4,348.732 261.55m.

0.00

4,321.905 60.60m.

CM7-588-95 4,284.458 195.80m.

108.50

4,200

4,200

0

10

50

METERS

MINERALIZATION

100

E - 361,400

4,000

E - 361,000

221.95

GEOLOGICAL UNITS

Silica-pyrite replacement

Calera Formation carbonates (Eocene)

Cu-(Au-Ag) zone

Shuco Member (Eocene)

Zn-Pb-(Ag) zone

Dome Mitu Group (Permian-Triassic)

Figure 2a.4: East-West cross section of the Smelter deposit as indicated in Figure 2a.2. 49

CHAPTER 2

A

1 cm

B

Jasperoidal silica 1 cm

C

1 cm

E

Opaline silica

1 cm

D

1 cm

F sheelite

rutile zircon

G

pyrite

50 µm

rutile

H

30 µm

Figure 2a.5: Images of typical occurrences of the Early silica-pyrite stage at Smelter. A. Nearly massive pyrite replacement of a conglomeratic bed. B. Silica-pyrite replacement of laminated limestones of the Calera Middle sub-member. Possible biogenic textures now replaced by pyrite. C. Silica-pyrite veinlets cutting silicified limestone. Jasperoidal silica on the borders of the thick veinlet. D. Pyrite-poor silicified limestone. Part of the silica in the opaline variety. E. Typical sequence of silica-pyrite precipitation from nearly pyrite-free replacement to younger pyrite-rich veinlets and replacements. F. Typical dark silica produced by argillite replacement by “chert”. G. Polished section photomicrograph showing typical fine-grained pyrite disseminations within a quartz matrix. Rutile and zircon as accessory phases. H. Polished section photomicrograph showing besides rutile, scheelite.

50

CHAPTER 2 northern sector, from Marcapunta to Colquijirca, fluid movement was essentially from south to north, possibly controlled by faults related to the compressive tectonism (Chapter 1a). The pronounced flatness of the manto-like silica-pyrite replacements in cross and longitudinal sections (Figures 2a.3 and 2a.4) demonstrates that hydrothermal fluids, once introduced into the carbonate unit, were restricted to lateral migration within the Pocobamba sequence. A similar general longitudinal migration pattern is recorded in the southern sector of the district where the fluids seem to have been controlled by the northsouth fold axes toward the Zn-Pb-(Ag) deposit at San Gregorio (Figure 2a.1). The available drill core data indicate that besides the main longitudinal fluid flow to the north and south, fluids also spread in all directions from the central portion of the volcanic center, possibly, as early postulated by McKinstry (1936), following the margins of the sub-volcanic conduit. Paraphrasing his statement “since no mineralized fractures that could have served as feeders were found (despite ample crosscutting under the ore bodies) the conclusion is that the ore-solutions entered along the beds from the margins of the stock, traveling gently upward along the pitch”. Only recently, has it been proved, using old drill-hole evidence that the entire western sector of the Marcapunta center hosts a significant Cu-(Au) resource (Vidal et al., 2004). The copper orebodies are mostly hosted by the early silica-pyrite replacements. Because of the widely spaced nature of the drilling, delineation of individual structures is difficult (Figure 2a.3). Available drill hole and underground evidence indicates that the orebodies are distributed irregularly within the silicapyrite replacements and that they commonly display crosscutting geometry. Veins mostly reopen the early silica-pyrite veins within the volcanic complex (DDH BROCAL 524, 450-500 m depth). Copper mantos also occurs within the silica-pyrite replacements, apparently constrained by inherited barriers. Relict bedding in the Calera Member appears determinant in the upper mineralized sequence, whereas it seems less important in the lower sequence where the probable main barriers were the Calera-Shuco and Shuco-Mitu contacts. The silica-pyrite replacements themselves, which developed extensive intergrain microcavities (up to > 1 mm wide), provided secondary permeability along which fluids flowed, enhancing the lateral fluid migration which controlled the shape of the mantos.

Mineral deposition history: stages and zoning The spatial configuration of ore bodies, crosscutting relationships at different scales and the study of more than 180 polished and thin sections distributed systematically along the northern block

of the district, has allowed discrimination of three mineral stages. They developed their own zoning patterns, each of which consists of distinct and contrasting zones. These stages are: (i) an Early silicapyrite stage which produced the large silica-pyrite replacements; (ii) a Main ore stage which deposed arsenical Cu-(Au) minerals surrounding the volcanic complex and rich Zn-Pb-(Ag) ores in their external parts (Figures 2a.2 and 2a.4); and (iii) a Late ore stage which generated gold-free copper minerals. In general, a given mineralized portion is constituted by a sequence of overlapping mineral associations and/or assemblages corresponding to at least two of these three stages. For example, the Late ore stage is found to overprint internal or external zones of the Main ore stage, which in turn may overprint early silica-pyrite replacements. More complex associations may be related to processes of zonal migration typical of these classes of sulfiderich deposits (e.g., Sales and Meyer, 1949). Early silica-pyrite stage In virtually any portion of the Smelter deposit, an early pre-ore stage composed essentially by silica and pyrite is evident. This stage, although economically uninteresting, is by far the most important in terms of volume. An estimated 800-1,000 millions tones containing an average of 40-50 volume % pyrite exist in the Smelter deposit, and a similar volume probably exists in the recently defined Marcapunta Oeste resource. The Early silica-pyrite stage displays distinct textures depending basically on the lithofacies of the host rock. In carbonate rocks of the Pocobamba Formation, the silica-pyrite stage formed a wide range of textures. In terms of volume, the most important are those formed by simple replacement of limestones and/or dolostones, marls, calcareous argillites and conglomerates, in which some of the original fabric of the host rock is preserved (Figure 2a.5). Finely laminated textures composed of alternate pyrite- and quartz-rich bands are characteristic of many intervals, particularly toward the top of the mineralized sequence (Figure 2a.5). Lamination is observed even at a microscopic scale with individual bands as thin as a few tens of microns. Judging by their stratigraphic position above the top of the Mitu Group, these banded intervals correspond to varved carbonaceous dolostones and limestones, typical of the upper sub-member of the lacustrine Calera Member. Other textures also mimic the original fabric of the carbonate rocks, and it is relatively common to observe biogenic textures completely replaced by silica-pyrite. Strongly silicified and pyritized conglomeratic beds characterize the sequence below the Calera Member. This sequence consists of alternating clast- and matrix-supported breccias, both with well51

CHAPTER 2

A

2 cm

B

C

2 cm

D

2 cm

alunite

pyrite II pyrite E

pyrite I

2 cm

enargite 0.5 cm

F

enargite dickite

G

enargite

1 cm

H

dickite

0.5 cm

Figure 2a.6: Representative occurrences of the Main ore stage. A. Enargite and pyrite II cutting silica-pyrite replacement. B. Massive sulfide sample consisting of pyrite from the Early silica-pyrite stage and enargite and pyrite II from the Main ore stage. Note repetitive sequences of crystallization between bands of pyrite II and enargite. C. Enargite replacing matrix of a conglomeratic rock. Note that enargite in this sample, as well as in those shown in images A and B, show visually insignificant amounts of gangue minerals other than pyrite and quartz. This type of occurrence represents more than half of the total enargite resource in the Smelter Deposit. D. Main ore stage veinlets cutting dacitic rocks of a lava-dome. E. Undulant bands of pyrite II-alunite replacing massive pyrite from the Early silica-pyrite stage. F. Enargite intergrown with alunite and pyrite in a small geode-like cavity. G and H. Enargite in contact with dickite and kaolinite. In both cases no reaction effects are observed. 52

CHAPTER 2 rounded monomictic clasts of various sizes. Most of the brecciation is observed in this sequence, ranging from relatively rare hydraulic breccias to more common crackle breccias (Figure 2a.5). Another kind of breccia consists of sulfide-poor breccias that show internal sediments filling interclast spaces, suggesting karstification pre-silica-pyrite formation (Figure 2a.5). These latter breccias are characterized by monomictic to heterolithic clasts consisting of centimetric clasts of original carbonate and/or detrital rocks texturally identical to the hanging-wall rock. Matrix accounts for 40-60% of the breccia. Replacement mineralogy The main form of silica in the silica-pyrite replacements is quartz, which typically accounts for > 60 %. The quartz occurs, depending on the grain size of the original protore, in 30-2,000 μm, subhedral to subordinate euhedral grains. Coarser quartz grains are rare and always coats cavities. Other forms of silica are cryptocrystalline and, in order of decreasing abundance, chalcedonic “chert”, jasperoidal silica and much rarer opaline masses. Chalcedonic chert appears dark in color, commonly almost black (Figure 2a.5). Under the microscope, dark chalcedonic chert, which accounts for about 10 to 20% of the total, is composed of quartz crystals 5-20 µm in size along with variable amounts of amorphous material. The fine grain size and impurities derived from the replaced rock are probably responsible for the dark color. Jasperoidal silica can be locally important (Figure 2a.5). From microscopic observations, hematite inclusions give the typical reddish color. Rarer is the opaline silica occurring with banded colloform textures and as alternating colourless and white bands (Figure 2a.5). Pyrite is most commonly octahedral, ranging in size from 20 to 1000 μm, but mainly between 50 and 400 μm (Figure 2a.5). Grains are mainly appreciably corroded, but the octahedral shape is still recognizable. Other less common forms are cubes and, more rarely, pentagonal dodecahedra. SEM imagery reveals that pyrite is commonly finely zoned. Arsenic and, to a lesser degree, copper concentrations are responsible for the zoning. Microprobe analysis reveals As concentrations up to 1.2 wt. percent, typically between 0.2 and 0.6 (Table 2a.1). Measured copper values do not surpass 0.3 wt percent and are typically 90 volume %), up to >2 m thick, are composed of multigenerational anhedral grains in which dissolution and reprecipitation of enargite are common. The enargite precipitates in most instances with minor pyrite. It should be stressed that if the main style of the enargite is filling pyrite clusters and geodes, the pyrite generally belongs to the Early silica-pyrite stage (pyrite I). Luzonite is almost always observed intergrown with coarse anhedral-subhedral grains of enargite. In contrast to enargite, luzonite is generally anhedral and is rarely found as well-formed crystals. In polished section, luzonite is distinctly pinkish where in contact with enargite. In most examples, the luzonite displays fine polysynthetic twinning under crossed nicols. Irregular masses of luzonite without evident twinning also almost completely replace enargite (Figure 2a.7). In some places, enargite clearly replaces luzonite along twinning, more rarely luzonite infiltrates along enargite grain contacts. In a single grain, both directions of replacement were also noted. Infrared observations show that enargite commonly underwent dissolution and recrystallization (e.g., Figure 4.2). Although much less abundant than in the Early silica-pyrite stage, pyrite also precipitated during formation of the Cu-(Au-Ag) zone. The main style of this “pyrite II” filling veinlet is up to a few centimeters thick, which partly grade into open spaces following bedding. These open spaces and veinlets are commonly poor or devoid of enargite and contain, in order of decreasing abundance, alunite, quartz, and zunyite. Pyrite II occurs also as repetitive thin bands with enargite (Figure 2a.6). Pyrite II, in general, is largely subhedral to euhedral and coarsegrained (up to 1 cm in size). Pyrite II commonly forms pyritohedra (Figure 2a.7), whereas octahedral and cubic forms, typical of pyrite I, are only locally important. In polished section, pyrite II displays a remarkable zoned appearance of two types. The first and most prominent is due to the alternation of dense and porous bands, which contain minute voids thought to be remnants of fluid inclusions. In some places such voids reach several hundreds of microns long. The second type, as seen below, is a product of compositional variations, and can be observed using SEM imagery or as a result of surface oxidation, the latter occurring extremely fast. Much of the pyrite II occurs in spatial association with enargite-rich zones. Subordinately, pyrite II occurs directly overprinting considerable parts of early silica-pyrite bodies which are essentially enargite-free. In such cases, only quartz is macroscopically observed accompanying the pyrite II. Around 2-5 volume % of the pyrite II is found as tiny subhedral inclusions in enargite, which is commonly a partial replacement product. Gold deposition is recognized in two paragenetic positions in the Cu-(Au-Ag) zone. Early deposition occurred as irregular inclusions, usually 55

CHAPTER 2

quartz alunite

alunite 2 mm

A

50 µm

B woodhouseite

svanbergite

50 µm

C

50 µm

D

zunyite

luzonite E

alunite

200 µm

F

100 µm

200 µm colusite enargite

G H 100 µm Figure 2a.8: Typical habits and assemblages of non-sulfide gangue minerals. A. Intimate intergrowth between alunite, quartz, and pyrite revealed by backscattered electron image. B. Backscattered electron image showing detail of reversed crystallization between alunite and quartz. Note quartz growing on alunite and alunite on quartz. C. EDS image showing grains of APS group minerals consisting of corroded cores of the svanvergite series and rims of alunite. D. EDS image of APS grains made up of cores of the woodhouseite series and rims of alunite. E. Polished section image showing a alunite-zunyite assemblage accompanied by luzonite filling intergrain spaces. F. Polished section photomicrograph of alunite grains as inclusions in enargite. G. Photomicrograph in transmitted light of a quartz-alunite assemblage in contact with anhedral enargite. H. Alunite postdating enargite. Note that colusite appear to have formed as a product of the reaction between enargite and alunite. 56

CHAPTER 2 Table 2a.1. Composition of selected sulfur-bearing phases from the Smelter deposit based on electron microprobe analysis. Sample

Mineral

stage/zone

S

Fe

As

Ag

Sb

Bi

Zn

Total

PBR-391

pyrite I

Early silica-pyrite stage

54.72

46.07 0.54

Cu

0.07

0.02

0.05

0.00 0.00

Sn

n.a.

101.47

PBR-391

pyrite I

Early silica-pyrite stage

54.85

45.76 1.31

0.12

0.00

0.00

0.00 0.00

n.a.

101.94

PBR-247

pyrite I

Early silica-pyrite stage

54.68

45.71 0.32

0.21

0.01

0.00

0.00 0.00

n.a.

100.72

PBR-247

pyrite I

Early silica-pyrite stage

54.66

45.88 0.41

0.13

0.02

0.00

0.02 0.02

n.a.

101.05

PBR-132

pyrite II

Main ore stage/Cu-(Au-Ag)

54.71

45.58 0.04

0.02

0.04

0.00

0.00 0.00

n.a.

100.37

PBR-132

pyrite II

Main ore stage/Cu-(Au-Ag)

54.40

46.02 0.09

0.12

0.01

0.01

0.00 0.00

n.a.

100.54

PBR-132

pyrite II

Main ore stage/Cu-(Au-Ag)

54.77

45.94 0.19

0.04

0.01

0.00

0.00 0.00

n.a.

100.95

PBR-137

pyrite II

Main ore stage/Cu-(Au-Ag)

54.35

45.43 0.08

0.08

0.01

0.00

0.11

0.02

n.a.

99.98

PBR-137

pyrite II

Main ore stage/Cu-(Au-Ag)

54.02

45.16 0.17

0.01

0.01

0.00

0.00 0.01

n.a.

99.36

PBR-137

pyrite II

Main ore stage/Cu-(Sn-Bi-Ag)

53.70

44.89 0.16

0.08

0.01

0.00

0.00 0.00

n.a.

98.77

PBR-137

pyrite II

Main ore stage/Cu-(Sn-Bi-Ag)

53.38

44.62 0.16

0.12

0.01

0.00

0.00 0.00

n.a.

98.18

PBR-132

enargite

Main ore stage/Cu-(Au-Ag)

32.64

0.06

49.27 17.68 0.00

0.42

0.00 0.08

n.a.

100.08

PBR-132

enargite

Main ore stage/Cu-(Au-Ag)

32.12

0.05

49.65 18.22 0.00

0.27

0.00 0.18

n.a.

100.32

PBR-132

enargite

Main ore stage/Cu-(Au-Ag)

32.55

0.25

48.49 17.33 0.00

0.45

0.00 0.00

n.a.

99.07

PBR-132

enargite

Main ore stage/Cu-(Au-Ag)

32.89

0.05

48.66 16.80 0.31

0.74

0.00 0.30

n.a.

99.46

PBR-132

enargite

Main ore stage/Cu-(Au-Ag)

33.36

0.00

48.88 17.32 0.09

0.26

0.00 0.67

n.a.

99.91

PBR-263

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.18

0.05

48.36 17.11 0.10

0.40

0.25 0.34

n.a.

99.20

PBR-263

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.08

0.01

49.02 17.11 0.03

0.89

0.00 0.23

n.a.

100.14

PBR-263

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

32.98

0.00

49.22 17.07 0.03

0.83

0.00 0.26

n.a.

100.13

PBR-263

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

32.77

0.03

48.59 17.01 0.09

0.81

0.00 0.43

n.a.

99.29

PBR-265

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

32.53

0.08

48.70 17.00 0.03

0.54

0.00 0.72

n.a.

98.88

PBR-265

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.19

0.02

48.82 17.22 0.07

0.50

0.00 0.27

n.a.

99.82

PBR-265

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.20

0.10

49.27 17.67 0.07

0.02

0.20 0.00

n.a.

100.33

PBR-265

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.25

0.12

48.66 17.38 0.11

0.44

0.00 0.47

n.a.

99.96

PBR-265

enargite

Main ore stage/Cu-(Sn-Bi-Ag)

33.01

0.27

48.73 17.61 0.05

0.10

0.00 0.04

n.a.

99.77

PBR-137

enargite

Main ore stage/Cu-(Au-Ag)

33.11

0.27

48.87 17.67 0.05

0.10

0.00 0.04

n.a.

100.07

PBR-137

enargite

Main ore stage/Cu-(Au-Ag)

33.21

0.27

49.02 17.72 0.05

0.10

0.01 0.04

n.a.

100.37

PBR-137

enargite

Main ore stage/Cu-(Au-Ag)

33.31

0.27

49.17 17.77 0.05

0.10

0.01 0.04

n.a.

100.68

PBR-137

enargite

Main ore stage/Cu-(Au-Ag)

33.41

0.27

49.32 17.83 0.05

0.10

0.01 0.05

n.a.

100.98

PBR-132

luzonite

Main ore stage/Cu-(Au-Ag)

31.97

0.14

47.78 16.25 0.00

1.16

0.00 1.61

n.a.

97.30

PBR-132

luzonite

Main ore stage/Cu-(Au-Ag)

31.54

0.21

48.72 15.48 0.01

1.21

0.00 3.15

n.a.

97.18

PBR-132

luzonite

Main ore stage/Cu-(Au-Ag)

32.34

0.00

49.03 17.04 0.01

1.81

0.00 0.06

n.a.

100.24

PBR-265

luzonite

Main ore stage/Cu-(Sn-Bi-Ag)

32.61

0.05

48.39 17.02 0.00

0.86

0.00 0.36

n.a.

98.93

PBR-265

luzonite

Main ore stage/Cu-(Sn-Bi-Ag)

33.03

1.09

47.19 16.37 0.10

1.24

0.00 0.91

n.a.

99.01

PBR-265

luzonite

Main ore stage/Cu-(Sn-Bi-Ag)

33.36

1.10

47.66 16.53 0.10

1.25

0.00 0.76

n.a.

100.00

PBR-265

luzonite

Main ore stage/Cu-(Sn-Bi-Ag)

33.69

1.11

48.14 16.69 0.10

1.26

0.00 0.59

n.a.

101.00

PBR-265

luzonite

Main ore stage/Cu-(Sn-Bi-Ag)

34.03

1.12

47.71 16.86 0.10

1.28

0.00 0.54

n.a.

101.09

PBR-132

colusite

Main ore stage/Cu-(Au-Ag)

30.60

0.64

54.25 7.41

0.00

0.02

0.00 7.03

n.a.

99.95

PBR-131

colusite

Main ore stage/Cu-(Sn-Bi-Ag)

29.81

2.74

52.25 4.82

0.00

0.00

0.02 10.96

n.a.

100.61

PBR-131

colusite

Main ore stage/Cu-(Sn-Bi-Ag)

30.73

0.58

55.00 7.42

0.00

0.00

0.00 7.34

n.a.

101.07

PBR-131

colusite

Main ore stage/Cu-(Sn-Bi-Ag)

31.04

0.87

53.94 7.17

0.00

0.00

0.00 7.88

n.a.

100.91

PBR-131

colusite

Main ore stage/Cu-(Sn-Bi-Ag)

30.85

0.59

54.51 8.10

0.00

0.00

0.00 6.55

n.a.

100.59

PBR-131

colusite

Main ore stage/Cu-(Sn-Bi-Ag)

30.62

0.86

52.37 8.32

0.00

0.02

0.01 7.34

n.a.

99.54

PBR-430

sphalerite

Main ore stage/Zn-Pb-(Ag)

32.64

0.48

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

66.43

99.55

PBR-430

sphalerite

Main ore stage/Zn-Pb-(Ag)

32.81

0.62

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

66.02

99.45

Sulfides were analysed using a Cameca SX50 electron microprobe at the University of Lausanne. Instrumental conditions were: accelerating voltage of 12 kV, beam current of 30 nA, and spot size of 10 mm.

57

58

Cu-(Au-Ag) 27.84

Cu-(Au-Ag) 28.99

PBR-263

PBR-263

Cu-(Au-Ag) 29.61

PBR-132

Cu-(Au-Ag) 30.62

PBR-132

Cu-(Au-Ag) 0.03

PBR-263

Cu-(Au-Ag) 25.70

PBR-263

n.d.

n.a.

n.a.

0.04

0.04

0.01

0.03

0.86

2.53

8.35

5.85

8.59

6.86

2.53

0.59

Fe (wt%)

n.d.

8.40

8.38

45.90

45.90

0.75

0.79

47.37

67.93

41.30

47.38

42.87

44.35

67.93

54.51

Cu (wt%)

n.d.

n.a.

n.a.

0.00

0.00

0.00

0.02

0.55

0.07

2.10

0.27

1.09

0.02

0.07

0.06

Zn (wt%)

n.d.

0.00

0.00

5.52

5.52

0.00

0.01

8.32

0.03

0.99

2.34

1.36

18.97

0.03

8.10

As (wt%)

n.d.

0.52

3.35

0.08

0.08

57.52

60.37

0.00

0.39

0.00

0.00

0.00

0.00

0.39

0.00

Ag (wt%)

n.d.

0.00

0.00

0.05

0.05

0.00

0.00

7.34

0.01

16.83

15.77

15.40

0.07

0.01

6.55

Sn (wt%)

0.42

0.28

0.24

1.52

1.52

0.13

0.09

2.13

0.02

0.00

0.52

0.00

0.17

0.02

1.11

Sb (wt%)

n.d.

n.a.

n.a.

0.03

0.03

0.00

0.03

0.00

0.00

0.00

0.00

0.00

0.00

0.00

0.00

Bi (wt%)

n.d.

n.a.

n.a.

n.a.

n.a.

n.a.

n.a.

0.02

0.05

0.01

0.00

n.a.

n.a.

0.05

0.00

Cd (wt%)

n.d.

n.a.

n.a.

n.a.

n.a.

0.00

0.24

n.a.

n.a.

n.a.

n.a.

0.15

0.00

n.a.

n.a.

V (wt%)

0.17

25.06

24.68

n.a.

n.a.

n.a.

n.a.

0.12

0.01

0.02

0.02

n.a.

n.a.

0.01

0.00

Au (wt%)

101.59

67.23

65.81

20.90

19.90

42.21

39.25

n.a.

n.a.

n.a.

n.a.

0.01

0.03

n.a.

n.a.

Te (wt%)

102.18

101.49

102.51

99.74

98.74

100.64

100.87

97.33

99.45

99.20

101.72

98.46

98.31

99.45

101.76

sum

Sulfides were analysed using a Cameca SX50 electron microprobe at the University of Lausanne. Instrumental conditions were: accelerating voltage of 12 kV, beam current of 30 nA, and spot size of 10 mm.

PBR-263

n.d.

Cu-(Au-Ag) 0.00

PBR-263

Native tellurium

Cu-(Au-Ag) 0.05

PBR-263

kostovite

Cu-(Au-Ag) 25.70

PBR-263

golfieldite

Cu-(Au-Ag) 0.05

PBR-263

hessite

Cu-(Au-Ag) 28.41

PBR-132

covellite

Cu-(Au-Ag) 29.57

PBR-132

stannoidite

Cu-(Au-Ag) 28.41

PBR-132

S (wt%)

Cu-(Au-Ag) 30.85

zone

PBR-132

tennantite

Sample/mineral

Table 2a.2. Composition of some selected accesory minerals from the Smelter deposit.

CHAPTER 2

CHAPTER 2 elongate and 4000 ppm in Appendix 2a.2). Other accessory minerals precipitated during the Main ore stage are bismuthinite, which may be locally abundant (up to 0.5 volume %), and more rarely, emplectite and stannoidite. They occur as The term electrum is used here following the definition of ����������������������������������������������������������� Barton and Toulmin (1964) for natural Au-Ag alloys between pure gold and pure silver end members. 

minute inclusions, exclusively in enargite. In similar form, unidentified Bi and Ag minerals, shown by EDS tests, may have been introduced coevally. Economic argentiferous intervals may in part be explained by these Ag-bearing phases. The observations indicate that all of these extremely fine-grained phases tend to be more abundant in late enargite generations. As mentioned above, alunite, quartz and zunyite are also found in pyrite II veins. Alunite occurs largely as platy euhedral grains intimately intergrown with quartz and zunyite (Figure 2a.8). The best developed and largest grains, up to 5 mm in size are pinkish and translucent and found in small geodes and fractures. Whitish grains are also common but usually do not exceeds 1 mm in size. Massive friable blades of finegrained alunite and variable amounts of minute euhedral quartz grains, typically less than 2 mm across are locally volumetrically important. Reversals in crystallization sequences without evidence of reaction between pyrite II-alunite and pyrite II-quartz pairs, are common. Euhedral pyrite II grains are encapsulated in alunite and vice versa (Figure 2a.8). These observations suggest that alunite, pyrite II and quartz were precipitated essentially coetaneously and in chemical equilibrium (see also chapter 4). Approximately 40 % of the enargite in the Cu-(Au-Ag) zone is found in direct contact with alunite. Commonly, alunite accompanied by quartz and subordinate pyrite comprises veinlets that cut and/or replace enargite. A reversed paragenetic sequence is also common and mainly occurs as enargite filling intergranular spaces within alunite. Most of the enargite in contact with the alunite that precipitated paragenetically late displays the effects of strong corrosion and recrystallization using infrared imagery, commonly with no evidence of the original grain morphology. Subordinately, enargite occurs as intimate intergrowths with euhedral grains of alunite in open spaces (Figure 2a.6). Under the microscope, alunite displays highly variably birefringence with a relatively well defined pattern depending on the distance to the main Marcapunta magmatic center. The birefringence attains second-order blue in the innermost parts, with a tendency to lower first-order grey to yellow in the rest of the zone. Observation with the SEM shows that alunite usually consists of two discernable phases of APS (aluminum phosphate sulfate) group minerals (Figure 2a.8). Cores identified with microprobe analysis are dominated by the woodhouseite series whereas rims are composed largely by the alunite series, essentially pure alunite. The cores, although sometimes corroded in appearance, generally displays well-developed oscillatory zoning. Seen in detail, the core is made up of dense bands, which became progressively less abundant towards the borders of the grains (Figure 2a.8). Zunyite is widespread throughout the entire Cu-(Au-Ag) zone. It consists of euhedral tetragonal pyramids (Figure 2a.8), sometimes truncated with an 59

CHAPTER 2 octahedral form. The grains can reach 2 mm wide and are translucent. Occurrence of zunyite without alunite is uncommon. Mutual relations of the pyrite II-zunyite pair are similar to those of pyrite II-alunite. For example, euhedral pyrite grains are encapsulated within zunyite but the reverse, although rarer, has also been noted. Quartz occurs mostly as an accompaniment to alunite-(zunyite). In common with the distinction made between Early stage pyrite I and Main ore stage pyrite II, quartz deposited during the Main ore stage is called quartz II. This quartz II is grossly euhedral and highly variable in size. Tiny grains occur in intricate intergrowths with alunite and zunyite as part of pyrite II veinlets. Larger quartz grains are observed intergrown with enargite but lacking the alunite-zunyite pair. Minor amounts of clays are observed locally accompanying this quartz. Quartzenargite lacking alunite characterizes up to 60 % of the enargite resource of the entire Smelter deposit. Quartz with enargite is typically corroded where in contact with later alunite. Small amounts of dickite, kaolinite and, in places, smectite and illite are present throughout the Cu-(Au-Ag) zone, particularly in portions devoid of alunite-bearing assemblages. These clays typically fill intergranular spaces in enargite, pyrite II, and quartz II. Where kaolinite and dickite are in contact with enargite, no reaction is observed (Figure 2a.6). Furthermore, kaolinite, dickite, and apparently also illite as inclusions in enargite have been observed to display no reaction effects. A reconnaissance study of clay minerals carried out by Cominco in 1999 suggests that most of the “alunite-free” enargite bodies contain minor amounts of clays including, besides kaolinite and dickite, some pyrophyllite, illite and smectite. Observed in detail, again virtually no reaction effects are observed between enargite and clays and no alunite is observed to have coprecipitated with the enargite-clay assemblages. It is therefore likely that important parts of the enargite bodies macroscopically exempt of alunite (~60 % of the Smelter deposit) contain trace to minor amounts of clays. Significant amounts of hematite are observed partially replaced by pyrite II. However, as explained later and based on observations at district-scale, the hematite is thought to be a remnant mineral from external parts of the system, i.e., from Zn-Pb-(Ag) parts, that have been almost entirely replaced by mineral associations from the internal Cu-(Au-Ag) zone. Composition of minerals of the Cu-(Au-Ag) zone

A selection of representative microprobe analyses of enargite, luzonite, pyrite II, tennantite, goldfieldite and accessory minerals is given in Table 2a.1. 60

Enargite compositions are nearly stoichiometric (Table 2a.1). All measured antimony contents are below 1.5 wt. %, but in general between 0.5 and 1.0 wt. %. Sn displays similar concentrations (0.3-1.5 wt. %). Less abundant elements are Zn and Fe (100 m grading 8% Zn, and 2.5% Pb (Figure 2d.3). In addition, strongly anomalous values in this hole (up to 3% Zn+Pb) extend vertically down to the upper beds of the Mitu Group red beds where multiple mineralized fractures and faults can be recognized. A generalized scheme showing the composite

100 m

Cordilleran ores

200 m

San Gregorio deposit

Zn-Pb-(Ag) zone

Zn-Pb-Cu-(Ag-Bi) zone

Cu-(Au-Ag) zone

Mitu Group (Permian-Triasssic), sandstones

Pucara Group (Upper Triasssic-Lower Jurassic), Limestones

Shuco Member (Eocene), calcareous conglomerates

Calera Formation (Eocene), mainly limestones and marls

Dacite diatreme-dome complex (Miocene)

GEOLOGICAL UNITS

Quartz-alunite + vuggy silica containing Au-bearing veinlets

Au-(Ag) disseminated ores

N

S MINERALIZATION

A

B

CHAPTER 2

Figure 2d.2: AB geological cross section of the southern sector of the Colquijirca district as indicated in Figure 2d.1. The section pass through the middle of the San Gregorio deposit. Correlations based on drill hole data and surface geology.

107

108

39G

3800 masl

SW

a'

quaternary deposits

14G

example of ore body

drill hole

37G

fault-fracture

discordance

2N

34F

100 m

Mitu Group red beds

basaltic dike

Post-Ore Stage quartz-alunite-kaolinite alteration

Post-Ore Stage silicification

economical interesting Zn-Pb-(Ag) interval

intermediate dolostones basal breccia

Zn-Pb-(Ag) zone, largely sphalerite-galena or "sulfide rock" subzone dissolution breccias

Zn-Pb-Cu-(Ag-Bi) zone

50 m

lower limit of oxidation

old workings

?

NE

a

CHAPTER 2

Figure 2d.3: aa’ geological cross section as indicated in Figure 2d.1. The section is indeed part of section AB and gives more details on the mineralization at San Gregorio.

CHAPTER 2 geometrical character of a mineralized body is shown in Figure 2d.3. In this figure, it is interpreted that “sulfide rock” intervals grade into “sulfide rock” veins or vein sets. External parts of sulfide rock partly replace along bedding and form roughly conformable manto-like shapes (up to >150 m long). Examples of manto-like shapes are the ones encased above, below, or in between various units of “dissolution breccias” with argillaceous matrix, as shown in Figure 2d.3. Permeability, and consequently, lithology was the main parameter controlling replacement along bedding. In a few cases, some manto-like bodies appear to be limited by dolostone units, as several orebodies located immediately above the “basal dolostones” or between relict “intermediate dolostones” (Figure 2d.3). Parts in contact with relict dolostones are constituted mainly by sphalerite, galena, and MnFe-Zn carbonates and are external relative to the acidically-formed “sulfide rock” intervals. The less acidic character of the fluids in external zones probably favored selective replacement of more reactive lithologies, for example limestone rather than dolostone and resulted in flat manto-like bodies. In contrast, in the internal zones, where more acidic fluids prevailed, bulk replacement of the carbonate rocks derived in irregular bodies of sulfide rock. Veins are volumetrically less important than mantos. There was no attempt to correlate the different vein intercepts due to the wide-spaced drilling. They are generally narrow (