Thesis
Reference
Magmatic and hydrothermal fluid processes at the origin of the giant porphyry-related epithermal polymetallic deposit of Cerro de Pasco
(Central Peru)
ROTTIER, Bertrand
Abstract
Le district du Miocène moyen de Cerro de Pasco, situé au centre du Pérou, contient le second plus grand gisement épithermal polymétallique lié à un système porphyrique. Ce travail consiste en une étude du magmatisme, des évènements hydrothermaux de hautes températures consistant de minéralisation de type porphyrique, ainsi que de l'étude des évènements hydrothermaux de basse température formant le gisement géant épithermal polymétallique de Cerro de Pasco. Les nouvelles données obtenues permettent de contraindre le lien entre le magmatisme et les différentes minéralisations hydrothermales, d'un point de vue temporel, géochimique et pétrographique.
ROTTIER, Bertrand. Magmatic and hydrothermal fluid processes at the origin of the giant porphyry-related epithermal polymetallic deposit of Cerro de Pasco (Central Peru). Thèse de doctorat : Univ. Genève, 2016, no. Sc. 5086
URN : urn:nbn:ch:unige-955694
DOI : 10.13097/archive-ouverte/unige:95569
Available at:
UNIVERSITÉ DE GENÈVE FACULTÉ DES SCIENCES Section des Sciences de la Terre et de l’environnement Directeur : Prof. Lluís Fontboté Département des Sciences de la Terre Co-directeur : Dr. Kalin Kouzmanov
Magmatic and hydrothermal fluid processes at the origin of the giant porphyry-related epithermal polymetallic deposit of Cerro de
Pasco (Central Peru )
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
Bertrand Rottier
GENÈVE
Atelier d’impression “ReproMail”, Université de Genève 2017
De France
Thèse n°5086
Table of Contents
Abstract ix
Résumé xiii
Chapter 1: Crystallization and rejuvenation cycles of a silicic magmatic chamber at the origin of the magmatic-hydrothermal system of Cerro de Pasco, Peru: geochronological and petrological evidence
Abstract 1
Introduction 3
General geology 4
District geology and mineralization 4
Description of the dated porphyry-type mineralization occurrences 7
Porphyry-type mineralization PM1 and PM2 7
Porphyry-type mineralization PM3 9
Methods 9
Results 15
Timing of magmatic and high-temperature hydrothermal events 15
Trace element signatures of zircon 17
Mineral composition and textures of magmatic rocks from the Cerro de Pasco district 19
Dacitic tuff 19
Dacitic domes 21
Rhyodacitic domes 21
Quartz-monzonite dykes 21
Porphyritic trachyte intrusions 22
Feldspar chemistry 23
Whole rock geochemistry 24
Silicate melt inclusions 31
Composition 31
Zircon saturation 33
Ti-in quartz thermobarometry 34
Discussion 35
New geochronological constraints on the hydrothermal mineralization at Cerro de Pasco 35
Magmatic evolution at deep crustal levels 37
Magma storage and rejuvenation 37
Conclusions 41
Acknowledgements 42
References 42
Supplementary tables 48
Chapter 2: Hydrothermal evolution of hidden porphyry-type mineralization related to the large epithermal polymetallic deposit of Cerro de Pasco (Peru)
Abstract 78
Introduction 80
Geological settings 81
Recently recognized porphyry mineralization at Cerro de Pasco 85
Analytical methods 87
Sample selection and SEM-CL petrography 87
Microthermometry 87
Raman microspectroscopy 88
SIMS analyses 88
LA-ICP-MS analyses 88
Mineral thermobarometers 89
Ti in quartz 89
Zr in rutile 90
Zr in titanite 90
Results 90
Mineralized clast petrography 90
HT1 veins 92
HT2 veins 92
Pre-diatreme polymetallic mineralization 94
Trace element analysis of quartz, rutile and titanite 94
Quartz 94
Rutile 95
Titanite 96
In-situ SIMS oxygen isotope analyses of different quartz generations 96
Fluid inclusions 97
Petrography, microthermometry and Raman analysis 97
Fluid composition 102
Melt inclusions 104
Petrography and description 104
Melt inclusions composition 104
Discussion 107
Comparison of the mineral thermobarometers 107
Pressure temperature reconstruction 108
HT1 veins 108
HT2 veins 110
Low temperature fluids 110
Origin and evolution of the fluids 111
Oxygen isotope signatures of the different events 112
Significance and origin of silicate melt inclusions in hydrothermal veins 114
Cerro de Pasco mineralizing system 115
Summary and conclusions 117
Acknowledgements 118
References 118
Supplementary figures 123
Supplementary tables 127
Chapter 3: Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the magmatic-hydrothermal transition (Cerro de Pasco district, Peru)
(Published as Rottier, B., Kouzmanov, K., Bouvier, A.-S., Baumgartner, L., Wälle, M., Rezeau, H., Bendezú, R., Fontboté, L., 2016, Heterogeneous melt and hypersaline liquid inclusions in shallow porphyry type mineralization as markers of the
magmatic-hydrothermal transition (Cerro de Pasco district, Peru): Chemical Geology, doi:
10.1016/j.chemgeo.2016.10.032)
Abstract 146
Introduction 147
Geological setting 147
Porphyry mineralization at Cerro de Pasco 149
Analytical methods 149
Results 152
Trachyte porphyry and associated alteration 152
Vein petrography and SEM-CL quartz textures 152
In-situ SIMS oxygen isotope analyses of quartz 153
SIMS and LA-ICP-MS trace element analyses in quartz 154 Petrography and microthermometry of HSMIs and fluid inclusions 155
LA-ICP-MS analyses of fluid inclusions 157
Hypersaline liquid inclusions 157
Aqueous L-V inclusions 158
Composition of heterogeneous silicate melt inclusions (HSMIs) 158 Characterization HSMIs after heating using back-scattered electron imaging 158
EPMA 158
LA-ICP-MS analyses 158
Cation diffusion through quartz 161
Discussion 163
Origin of HSMIs in porphyry veins: supercritical melt versus heterogeneous entrapment 163
Fluid evolution 164
Origin of the fluid and ore precipitation 165
Cu and Ag diffusion through quartz 166
Implications for ore formation processes at Cerro de Pasco 166
Summary and conclusions 167
Acknowledgements 167
References 167
Supplementary tables 170
Chapter 4: Sulfide replacement processes revealed by textural and LA-ICP-MS trace element analyses: example from the early mineralization stages at Cerro de Pasco, Peru
(Published as Rottier, B., Kouzmanov, K., Wälle, M., Bendezú, R., and Fontboté, L., 2016, Sulfide replacement processes revealed by textural an LA-ICP-MS trace element analyses: example from the early mineralization stages at Cerro de
Pasco, Peru: Economic Geolology, v.111, p. 1347-1367.)
Abstract 179
Introduction 179
Geology and mineralization at Cerro de Pasco 180
Analytical methods 183
Pyrrhotite pipes and their rims (Stage A) 183
Pyrite-quartz body (Stage B) 184
Mineral textures at pyrrhotite pipe - pyrite-quartz body contacts 184
Trace element analysis by LA-ICP-MS 188
EMP analyses of sphalerite 192
Discussion 192
Textures 192
Trace elements 193
New interpretation of fluid evolution during mineralization at Cerro de Pasco 196
Conclusions 196
Acknowledgments 197
References 197
Supplementary tables 201
Chapter 5: Cyclic tapping of metal-rich hypersaline magmatic fluids at the origin of the giant Miocene epithermal polymetallic deposit of Cerro de Pasco, Peru
Abstract 209
Introduction 211
Geology and mineralization 212
Analytical methods 217
Results 221
Fluid inclusion petrography and microthermometry 221
P-T conditions 232
Element concentrations in fluid inclusions 233
Trace elements in sulfides 237
Discussion 243
Fluid mixing and origin of the mineralizing fluids 243
Fluid evolution and nature of the mineral assemblages 248
Trace elements in sulphides 249
Fluid inclusions in ore and gangue minerals 253
Conclusions 253
Acknowledgments 254
References 254
Supplementary tables 261
Chapter 6: Various fluid sources at the large epithermal polymetallic deposit of Cerro de Pasco (Peru): constraints from radiogenic and stable isotope (Sr, Pb, Cl) tracing on inclusion fluids and in-situ δ18O analyses of hydrothermal quartz
Abstract 313
Introduction 315
Geological setting 316
Porphyry-type mineralization 319
Epithermal mineralization 319
Analytical methods 321
SEM-CL 321
SIMS 322
LA-ICP-MS 323
Crush-leach analyses 323
Results 325
Trace element ICP-MS analyses of inclusion fluids 325
Anion content and Cl isotopic composition 328
Sr and Pb isotopic compositions 329
In-situ oxygen isotope and trace element analyses in hydrothermal quartz 332
In-situ oxygen isotopes 332
Stage A 332
Stage B1 333
Stage B2 334
Stage C2 336
Trace elements in quartz 337
Discussion 340
Trace elements in hydrothermal quartz 340
Application of Ti-in-quartz thermobarometer at low temperatures and pressures 340
Trace element and hydrogen distribution in quartz 340
Various fluid sources at the large epithermal polymetallic deposit of Cerro de Pasco 342 Role of meteoric water in the magmatic-hydrothermal system at Cerro de Pasco 342
Equilibrated fluids 346
Metal content of the mineralizing fluids 348
Chlorine isotopic composition 350
Conclusions 350
Acknowledgement 351
References 351
Supplementary figures 357
Supplementary tables 361
Appendix A.1 377
Chapter 7: Trace element diffusion and incorporation in quartz during heating experiments
(Published as Rottier, B., Rezeau, H., Casanova, V., Kouzmanov, K., Moritz, R., Schlöglova, K., Wälle, M., Fontboté, L.
(2017). Trace element diffusion and incorporation in quartz during heating experiments. Contributions to Mineralogy and Petrology, v.172, 20p.)
Abstract 381
Introduction 381
Sample and analytical procedure 382
Analytical methods 384
Results 385
Phase transition during heating experiments 385
EPMA profiles and maps 386
Magmatic quartz 386
Hydrothermal quartz from porphyry vein HVA 386
Hydrothermal quartz from porphyry vein HVB 389
LA-ICP-MS results 390
Trace element composition of magmatic quartz and melt inclusions 390
Trace element composition of quartz and HSMI from porphyry vein HVA 390 Trace element composition of hydrothermal quartz from porphyry vein HVB 392
Discussion 393
Discrepancy between EPMA and LA-ICP-MS measurements 393
Diffusion of elements into SMIs and HSMIs 395
Cations incorporation in the quartz structure 395
Source of the diffusive elements 396
Conclusions and implications 398
Acknowledgements 398
References 398
Supplementary figures 401
Supplementary tables 405
Acknowledgements
431Abstract
The mid-Miocene Cerro de Pasco district, central Peru, hosts the second known largest porphyry-related epithermal polymetallic ("Cordilleran") deposit after Butte in Montana, USA. The epithermal polymetallic mineralization mainly replaces carbonate rocks of the Mesozoic Pucará Group along the eastern border of a large diatreme-dome complex. Previous works had proposed an affiliation to a porphyry-system, but no direct evidence of porphyry mineralization was found.
During the present study, three porphyry-type mineralization styles were identified as clasts in the diatreme (PM1), outcropping at surface (PM2), and as clasts in quartz-monzonite dykes (PM3).
Large pervasive alteration zones (pyrophyllite-quartz-pyrite; illite-smectite-muscovite-pyrite;
chlorite-calcite-pyrite) have been also identified in the central and the northern parts of the diatreme- dome complex probably resulting from interaction with relatively acidic magmatic vapors, generated by degassing of the deeper part of the porphyry system. These new findings give the opportunity to study the porphyry system from its deeper to shallower parts. The broad aim of the present contribution is to bring a new comprehensive study of the porphyry system at the origin of the large Cerro de Pasco epithermal polymetallic deposit, starting with its magmatic evolution, then studying the fluid processes leading to the porphyry-type mineralization, and to the formation of the large epithermal polymetallic deposit.
Chapter 1 is devoted to the magmatic system and its relationship with the different mineralization styles. The newly obtained CA-ID-TIMS U–Pb ages of magmatic zircons from the porphyry manifestations suggest continuous magmatic activity during more than 300 ka punctuated by high-temperature porphyry-type mineralization events. The epithermal polymetallic mineralization occurs after the first one but to some extent could also overlap it, a maximum gap of 0.9 Ma between the two events has been estimated. The sub-volcanic and volcanic rocks of Cerro de Pasco were studied using petrographic, geochemical (whole-rock and mineral), isotopic (Pb, Sr, Nd) methods combined with the study of silicate melt inclusions hosted in quartz phenocrysts. All magma batches underwent a period of evolution in the deep crust (> 8 kbar) as reflected by their typical subduction-related and garnet-fractionation chemical signatures. Prior to their emplacement at shallow levels, magmas were stored at upper crustal level at high degree of crystallinity (up to 90% crystals) at pressures between 2 and 4.4 kbar and temperatures between 675° and 750°C. At such conditions magmas past the point of rheological lock-up, therefore several events of rejuvenation of this high-crystallinity silicic magmatic body were necessary to allow ascent of the parental magma for the various subvolcanic and volcanic bodies at Cerro de Pasco. The energy necessary to rejuvenate this high-crystallinity silicic magmatic body was provided by circulation of hot magmatic volatiles in the not fully crystallized magma chamber, as suggested by the presence of different high-temperature porphyry-type mineralization styles. Our results show complex transient
regimes of crystallization and rejuvenation of a stored high-crystallinity silicic magmatic body where episodes of rejuvenation could be as short as 85 ka.
Chapters 2 and 3 are dedicated to the study of newly found porphyry type mineralization.
Chapter 2 is focused on the high-temperature porphyry-type veins in hornfels and magmatic clasts in the diatreme breccia (PM1). Such veins allow studying porphyry-type mineralization originated from deep portion of the magmatic-hydrothermal system. They contain hydrothermal quartz that hosts silicate melt inclusions as well as fluid and solid mineral inclusions. Two types of quartz- molybdenite-(chalcopyrite)-(pyrite) veins were identified an early HT1-type veins crosscut by HT2- type veins. Fluid inclusion microthermometry and the various mineral thermobarometers applied reveal that HT1 veins were formed at temperatures >700°C and at pressures between 500 and 1300 bars, and that HT2 veins were formed at temperature ~600°C and pressure between 710 and 850 bars. Both vein types are locally crosscut and/or reopen by a late polymetallic event consisting of pyrite, sphalerite with “chalcopyrite disease”, galena, chalcopyrite, fahlore group minerals, and minor quartz. Crosscutting relationships indicate that the polymetallic event recognized in the clasts took place before the one that formed the giant post-diatreme polymetallic deposit. The silicate melt inclusions trapped in the veins represent melt droplets transported in the ascending hydrothermal fluids. LA-ICP-MS analyses of the mineral inclusions and the fluid inclusions hosted in HT1 and HT2 veins, and in-situ SIMS oxygen isotope analyses of vein quartz reveal a strong magmatic signature of the mineralizing fluids with minor meteoric water input, and allow reconstructing the chemical evolution of fluids forming these veins. Chapter 3 focuses on the stockwork of banded quartz-magnetite-chalcopyrite-(pyrite) porphyry-type veinlets (PM2) crosscutting the trachyte porphyritic intrusions cropping out at surface in the central part of the diatreme-dome complex. This porphyry-type mineralization is emplaced at shallow level (depth < 1 km, P < 270 bars), implying rather unusual low-pressure and high-temperature environment for the formation of porphyry-style mineralization. The banded porphyry-type veinlets record a multiphase history of formation with two successive high-temperature (>600°C) stages, followed by a lower-temperature (<350°C) stage.
Stage 1 is characterized by the entrapment of heterogeneous inclusions containing variable proportions of evolved silicate melts and hypersaline liquid. These unusual inclusions result from fluid exsolution at low pressure (270 bars) and high temperature (>600°C) from an evolved hydrous rhyolitic melt ascending to shallow levels. During stage 2 only magmatic hypersaline liquid inclusions are trapped. The low-temperature stage 3 (<350°C) is associated with the precipitation of a chalcopyrite-pyrite-sphalerite-(pyrite) assemblage possibly favored by mixing of minor meteoric water with magmatic-dominated fluids, as indicated by the in-situ SIMS 18O/16O isotope analyses.
The hypersaline liquid inclusions found in the porphyry-type mineralization PM1 and PM2 have similar Pb/Zn ratios to those of the bulk ore extracted from the epithermal polymetallic ore bodies at Cerro de Pasco, thus suggesting a common source for the fluids associated with the different mineralization styles at Cerro de Pasco.
Chapters 4, 5, and 6 are focused on the large epithermal polymetallic mineralization. The latter results from the following three successive mineralization stages characterized in general terms by an evolution from low- to high-sulfidation mineral assemblages: (i) pyrrhotite pipes grading outwards to Fe-rich sphalerite and galena replacement bodies (stage A), (ii) deep quartz-pyrite veins (stage B1) and a massive funnel-shaped pyrite-quartz replacement ore-body (stage B2) with quartz- sericite alteration, and (iii) well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement ore-bodies (stage C1) and E-W- trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins (stage C2); stages C1 and C2 are spatially associated with zones of advanced-argillic alteration. Chapter 4 is focused on the relative chronology of stages A and B. A detailed study of mineral replacement textures combined with LA- ICP-MS analyses shows that the pyrrhotite pipes and associated polymetallic mineralization of stage A predates the pyrite-quartz-body (stage B) and not the contrary as proposed in previous works. The mineral textures and compositions indicate progressive increase of fS2, fO2, and decrease of pH of the mineralizing fluids from stage A to B and within stage B. Chapter 5 deals with the evolution (temperature, pressure and composition) of the mineralizing fluids during mineralization stages A, B, and C. It is based on a detailed microthermometric and LA-ICP-MS study of fluid inclusions hosted in gangue and opaque ore minerals and available bulk and in-situ SIMS oxygen isotope analyses of hydrothermal quartz. Mineralizing fluids of the three stages were trapped at moderate temperatures (~150°–280°C) and display wide range in salinity (1.2 to 19 wt % NaCl equiv.) without evidence of phase separation; in stage C2, FIAs display a more restricted salinity range, mainly between 1.2 and 2.7 wt% NaCl eq. Fluids forming mineralization stages A, B1, B2 and C1 result from a mixing between high-salinity metal-rich (> 1 wt% Mn, Fe, Zn, and Pb and up to several 1000s of ppm Sb) magmatic fluids and low-salinity less metal-rich fluids also of magmatic origin. Our results suggest the existence of a homogeneous reservoir of hypersaline metal-rich porphyry-type fluids stored at depth, that has been repeatedly remobilized by ascending newly exsolved condensed magmatic vapor-like fluids. The resulting mixed fluid rose to form mineralization stages A, B1, B2 and C1, whereby in this latter stage fluids were also strongly mixed with meteoric water as indicated by bulk and in-situ (SIMS) oxygen isotope analyses on quartz. The enargite-pyrite veins of stage C2 were formed by the ascent of CO2-bearing contracted vapor, subsequently mixed with meteoric water; no evidence of mixing with the hypersaline metal-rich magmatic fluids has being found for this
mineralization stage. The repeated remobilization of metal-rich high-salinity magmatic fluids stored at depth recognized in stages A, B1, B2 and C1 is a distinctive feature of the formation of the giant polymetallic deposit of Cerro de Pasco. Chapter 6 is dedicated to the definition of the various fluid sources and the relative proportion of different fluid types involved in the formation of the
epithermal polymetallic mineralization at Cerro de Pasco. Application of Sr, Pb and Cl isotope analysis, in combination with ion chromatography and ICP-MS analyses of fluid inclusion leachates from 2 samples of porphyry-type mineralization and 12 samples of epithermal mineralization allows recognizing radiogenic Sr and Pb signatures and high Br/Cl ratio (> 2 x 10-3) in the fluid forming the
epithermal polymetallic deposit. These signatures have been interpreted as interaction of the ascending mineralizing fluids with the Excelsior shales and phyllites. Potential mixing between this upcoming magmatic fluids and meteoric water at shallower level was tracked using in-situ δ18O SIMS analyses of quartz crystals from stages A, B1, B2 and C2. Only minor input of meteoric water was recorded by quartz from stages A, B1 and B2, and no correlation with the obtained δ18O, trace element content and the SEM-CL textures of the quartz crystals was observed. In contrast, quartz crystals from the enargite-veins of stage C2 registered involvement of meteoric water as suggested by growth zones associated with low δ18O, low trace element content and dark CL intensity.
Chapter 7 investigates the diffusion and incorporation of trace elements (derived from surrounding minerals and inclusions, as well as from the heating stage itself) in quartz and quartz-hosted melt inclusions during high-temperature heating experiments using a Linkam stage TS1500. Heated magmatic and hydrothermal quartz crystals reveal significant modification of their Cu, Li, Na, and B content and modification of the Cu, Li, B, Na, Ag, K, Cs, and Rb concentrations of their melt
inclusions. Heated magmatic quartz crystals display only discrete Cu and Na enrichments (few ppm), mostly incorporated by substitution for Li. In contrast, heated hydrothermal quartz shows enrichment in Cu, Li, and Na up to several hundreds of ppm. These elements are incorporated by substitution of H+ cations present in the quartz structure prior to heating. Our results suggest that the composition of quartz and quartz-hosted melt inclusions may significantly be modified upon heating experiments, which may subsequently lead to erroneous elemental concentration determinations. In addition, this study reveals the existence of a sub-surface layer being significantly enriched in Cu during heating.
We propose that this sub-surface enrichment, easily detectable by EMPA, can be used to track Cu diffusion in quartz from an external source.
Résumé
Le district du Miocène moyen de Cerro de Pasco, situé au centre du Pérou, contient le second plus grand gisement épithermal polymétallique lié à un système porphyrique. Ce dernier se situe à la bordure Est d’un complexe dôme-diatrème et remplace principalement les roches carbonatées du Trias supérieur – Jurassique inferieur appartenant à la formation du Pucará. Des travaux antérieurs ont proposé une affiliation de ce gisement à un système porphyrique mais aucune évidence directe d’un tel lien n’avait été trouvée. Lors de cette étude, trois différentes occurrences de minéralisation de type porphyre ont été identifiées. Elles consistent en : des clastes trouvés dans la brèche de diatrème (PM1), affleurant à la surface (PM2) et en des clastes inclus dans un dyke de quartz-monzonite. Une large altération (pyrophyllite-quartz-pyrite; illite-smectite-muscovite-pyrite;
chlorite-calcite-pyrite) a aussi été identifiée dans la partie centrale et nord du complexe dôme- diatrème. Cette altération est probablement liée à l’interaction des roches avec de vapeurs magmatiques acides générées par un dégazage des parties profondes du système porphyrique. Ces nouvelles découvertes nous donnent l’opportunité d’étudier le système porphyrique des parties les plus profondes jusqu’à celles les plus proches de la surface. Le thème de cette étude est d’apporter une compréhension nouvelle du système porphyrique lié au grand gisement épithermal polymétallique de Cerro de Pasco, en commençant par l’étude du système magmatique, puis par celle des minéralisations de type porphyre et enfin par l’étude du gisement épithermal polymétallique.
Le Chapitre 1 est dédié au système magmatique et à sa relation avec les minéralisations de type porphyres. Les nouvelles datations U–Pb des zircons magmatiques par CA-ID-TIMS suggèrent une activité magmatique continue sur plus de 300 ka, ponctuée par différents évènements de minéralisation de type porphyre. Les minéralisations épithermales polymétalliques se forment suite à cette première période mais dans une certaine mesure peuvent temporellement chevaucher cette dernière, un écart maximum de 0.9 Ma entre les deux périodes. Une étude pétrographique, géochimique (roches totales et minérales) et isotopique (Pb, Sr, Nd) a été conduite sur les roches volcaniques et sub-volcaniques du district, ainsi qu’une étude des inclusions vitreuses contenues dans le quartz. Cette étude montre que l’ensemble des magmas ont subi une première période d’évolution dans la croûte inférieure (> 8 kbar) comme illustré par leurs signatures chimiques typiques de magma de subduction ayant fractionné du grenat. Avant leurs emplacements, les magmas ont été stockés à un haut degré de cristallinité, à une pression entre 2 et 4.4 kbar et à basse température entre 675° et 750°C. Dans de tels conditions, les magmas ne peuvent pas être mobilisés.
Ainsi, plusieurs évènements de refonte de cet amas de cristaux ont été nécessaires pour permettre ponctuellement l’ascension du magma formant les roches magmatiques visibles aujourd’hui.
L’énergie nécessaire à la refonte a été fournie par de nouvelles intrusions et la circulation de volatiles magmatiques chauds comme suggéré par la présence des minéralisations de type porphyre de haute
température. Nos résultats montrent un régime transitoire complexe alternant des périodes de cristallisation et de refonte du corps magmatique fortement cristallisé stocké en profondeur, celle-ci pouvant être rapides (< 85 ka).
Les chapitres 2 et 3 sont consacrés à l’étude des minéralisations de type porphyre. Le Chapitre 2 est dédié aux veines de haute température recoupant des clastes de cornéennes et de roches magmatiques incorporées dans la brèche de diatreme (PM1). Ces veines offrent l’opportunité d’étudier les minéralisations de porphyres de haute température mises en place dans les parties profondes du système. Elles contiennent des cristaux de quartz présentant des inclusions vitreuses et ainsi que des inclusions fluides et solides. Deux types de veines à quartz- molybdenite- (chalcopyrite)-(pyrite) ont été identifiées : des veines précoces « HT1 » recoupant des veines
« HT2 ». L’étude par microthermométrie des inclusions fluides et l’application de différents géothermobaromètres ont révélé que les veines HT1 ont été formées à des températures > 700°C et des pressions entre 500 et 1300 bars, et que les veines HT2 ont été formées à des températures
~600°C et des pressions entre 710 and 850 bars. Ces veines (HT1 et HT2) sont parfois recoupées par une minéralisation polymétallique formée de pyrite, de sphalerite avec des exsolutions de chalcopyrite, de galène, de chalcopyrite, de fahlore et de quartz en de moindre proportion. Les relations de recoupement indiquent que la minéralisation polymétallique observée dans les veines est différente et plus ancienne que celle constituant le large gisement épithermal polymétallique. Les inclusions vitreuses piégées dans les veines représentent des gouttes de magmas transportées par les fluides hydrothermaux ascendants. L’analyse par LA-ICP-MS des différents inclusions fluides et solides présente dans les veines HT1 et HT2 ainsi que les analyses in-situ des isotopes d’oxygène du quartz montrent une origine magmatique des fluides minéralisateurs et permettent de reconstituer leurs évolutions chimiques. Le chapitre 3 se concentre sur le stockwork de veinules de type porphyre à quartz-magnétite-chalcopyrite-(pyrite) recoupant une intrusion de trachyte porphyrique affleurant à la surface au centre du complexe dôme-diatrème. Cette minéralisation de type porphyrique s’est formée proche de la surface à une profondeur inférieure à 1 km (P < 270 bars), impliquant des conditions inhabituelles pour ce type de minéralisation qui consistent en une faible pression et une température élevée. Les veinules de type porphyre enregistrent une histoire polyphasée comprenant deux épisodes de haute température (>600°C) suivit par un épisode de plus faible température (<350 °C). Plus de 90% du quartz des veinules s’est formé durant les épisodes de haute température. La période 1 se caractérise par le piégeage dans les quartz hydrothermaux d’inclusions contenant, en différentes proportions, un magma silicaté et un liquide hypersalin riche en métaux (>90 wt % NaCl eq. ; nommées HSMIs). Ces inclusions inhabituelles sont formées par le piégeage hétérogène d’un magma rhyolitique et d’un fluide hypersalin à basse pression et haute température (270 bar, >600°C). Durant la deuxième période, seules des inclusions hypersalines sont piégées. La troisième période (< 350°C) est associée à la précipitation de chalcopyrite, de pyrite, et
de sphalerite, potentiellement favorisée par le faible mélange d’eau météorique au fluide magmatique comme suggéré par les analyses in-situ des isotopes d’oxygène du quartz associés à cette période.
Les inclusions hypersalines observées au sein des veines des minéralisations de type porphyres PM1 et PM2, présentent un ratio Pb/Zn similaire à celui du minerai extrait des corps minéralisés du gisement polymétallique de Cerro de Pasco. Cette observation suggère une source commune des fluides minéralisateurs à l’origine des différents types de minéralisations présentes à Cerro de Pasco.
Les chapitres 4, 5 et 6 sont dédiés à l’important gisement épithermal polymétallique. Il est formé par trois épisodes minéralisateurs successifs caractérisés en général par une évolution allant d’associations minérales de basse à haute sulfidation : (i) plusieurs pipes de pyrrhotite entourés de corps de remplacement dominés par de la sphalerite et de la galène (épisode A) ; des veines profondes de quartz et de pyrite (épisode B1), ainsi qu’un important corps de remplacement formé de pyrite et de quartz (épisode B2), de plusieurs corps de remplacement (épisode C1) bien zonés à Zn- Pb-(Bi-Ag-Cu) ainsi que des veines E-W riche en Cu-Ag-(Au-Zn-Pb) formées de pyrite et d’énargite (épisode C2). Les deux derniers épisodes sont associés à une altération argilique avancée. Le chapitre 4 est concentré sur l’étude de la relation chronologique des épisodes A et B. Il consiste en une étude détaillée des textures de remplacement minérales associées à l’analyse par LA-ICP-MS des minéraux remplacés et nouvellement formés. Les observations montrent que les pipes de pyrrhotite et leurs minéralisations polymétalliques (épisode A) sont précoces par rapport au corps de pyrite et de quartz (épisode B) contrairement à ce qui avait été proposé dans les travaux antérieurs.
Les textures minérales ainsi que leurs compositions indiquent une progressive augmentation de fs2, fo2, et une diminution du pH des fluides minéralisateurs de l’épisode A à l’épisode B, ainsi que au sein de l’épisode B lui-même. Le chapitre 5 est consacré à l’étude de l’évolution (température, pression et composition) des fluides minéralisateurs formant les différents épisodes de minéralisations A, B and C. Il consiste en une étude microthermométrique détaillée et en l’analyse par LA-ICP-MS des inclusions fluides présentes dans les minéraux de gangues et les sulfures opaques des différents épisodes minéralisateurs utilisant la microscopie classique et proche infrarouge. Cette étude se base notamment sur les résultats précédemment des isotopes stables (O and H) obtenus sur cristaux entiers et des isotopes d’oxygènes obtenus in-situ par SIMS sur des cristaux de quartz. Le fluide minéralisateur à l’origine des trois épisodes était à une température modérée (~150°–280°C) mais présentait une large gamme de salinité (entre 1.2 et 19 wt % NaCl equiv.). Aucune évidence de séparation de phase n’a été observée. Pour l’épisode C2, les assemblages d’inclusions fluides sont caractérisés par des salinités entre 1.2 et 2.7 wt % NaCl equiv.
Les fluides formant les épisodes A, B1, B2 et C1 sont le résultat d’un mélange entre un fluide magmatique de forte salinité et riche en métaux (> 1 wt% Mn, Fe, Zn, and Pb et jusqu’à plusieurs milliers de ppm de Sb) et un autre fluide magmatique peu salin et moins riche en métaux. Nos
résultats suggèrent l’existence d’un réservoir de fluide hypersalin, riche en métaux de type porphyre stocké en profondeur. Ce réservoir a été mobilisé de façon répétée par l’ascension de vapeurs magmatiques condensées nouvellement exsolvées. Le fluide résultant de ce mélange remonte et forme les épisodes de mineralizations A, B1, B2 et C1. Pour l’épisode C1, le fluide ascendant est aussi mélangé à de l’eau météorique comme indiqué par les isotopes d’oxygène précédemment obtenus.
Les veines d’énargite et de pyrite de l’épisode C2 ont été formées par l’ascension de vapeurs magmatiques condensées contenant du CO2 mélangées avec de l’eau météorique proche de la surface. Aucune interaction avec les fluides hypersalins de porphyres n’a été observée. La mobilisation répétée de fluide hypersalin, riche en métaux de type porphyre stocké en profondeur, identifié pour les épisodes de mineralizations A, B1, B2 et C1,est une caractéristique distinctive dans la formation du gisement épithermal polymétallique géant de Cerro de Pasco. Le chapitre 6 est dédié à la détermination des sources potentielles des fluides minéralisateurs ainsi qu’une estimation de leurs relatives importances. Dans cet objectif, nous avons déterminé les signatures isotopiques du Sr, Pb et du Cl, ainsi que le contenu en anions et éléments traces de deux lessivages d’inclusions fluides, de deux échantillons de minéralisations porphyriques et de 12 échantillons provenant des différents épisodes de la minéralisation épithermale. Les analyses montrent un contenu en métaux plus important dans les lessivages d’inclusions fluides des épisodes de la minéralisation épithermales que dans les minéralisations porphyriques. Elles indiquent que le fluide formant le gisement épithermales polymétallique de Cerro de Pasco présente une signature isotopique de Sr et Pb plus radiogéniques et un ratio Br/Cl élevé (> 2. 10-3). Ces signatures ont été interprétées comme résultant de l’interaction de fluides minéralisateurs ascendants avec les phyllites et les argiles noires de l’Excelsior. Le possible mélange entre ce fluide et une eau météorique dans la partie supérieure du système minéralisé a été investigué grâce à plusieurs analyses in-situ d’isotope d’oxygène par SIMS sur les cristaux de quartz des épisodes minéralisateurs A, B1, B2 and C2. Seulement de faibles additions d’eaux météoriques ont été enregistré dans les cristaux de quartz des épisodes minéralisateurs A, B1, and B2, et aucune corrélation entre les signatures isotopiques, le contenu en éléments traces et la texture (SEM-CL) des cristaux de quartz a été observée. En contraste, les cristaux de quartz provenant des veines à enargite et pyrite de l’épisode C2, présentent des zones caractérisées par de faible signature en δ18O, un faible contenu en élément traces et couleur sombre de luminescence (SEM-CL). Nous interprétons ces parties de cristaux comme étant formées par un fluide dominé par de l’eau météorique.
Le chapitre 7 consiste en l’étude de la diffusion et de l’incorporation de certains éléments traces (provenant des minéraux voisins, des inclusions du quartz et aussi de la platine elle-même) dans les cristaux de quartz ainsi que de leurs inclusions, à travers un chauffage séquentiel avec une platine Linkam TS1500. Les cristaux de quartz hydrothermaux et magmatique et leurs inclusions ayant été chauffés présentent respectivement une modification de leurs concentrations en Cu, Li, Na, and B et
en Cu, Li, B, Na, Ag, K, Cs, et Rb. Les quartz magmatiques chauffés présentent seulement un faible enrichissement en Cu et Na, majoritairement incorporé par substitution du Li. En revanche, les quartz hydrothermaux chauffés présentent un fort enrichissement en Cu, Li and Na, jusqu’à quelques centaines de ppm. Ces derniers sont incorporés par remplacement de l’hydrogène présent initialement dans la structure du quartz avant chauffage. Nos résultats montrent que la composition du quartz et de leurs inclusions vitreuses peut être modifiée de manière significative durant leurs chauffages. De plus, notre étude démontre l’existence d’une couche supérieur au sein des cristaux pouvant être enrichie en Cu de manière significative pendant le chauffage. Nous suggérons que l’enrichissement de cette couche supérieure peut être utilisé pour identifier la diffusion du cuivre d’une source externe vers les cristaux de quartz.
Chapter 1
Crystallization and rejuvenation cycles of a silicic magmatic chamber at the origin of the magmatic-
hydrothermal system of Cerro de Pasco, Peru:
geochronological and petrological evidence
Bertrand Rottier*, Kalin Kouzmanov*, Maria Ovtcharova*, Alexey Ulianov** Markus Wälle***
†, Dave Selby****, and Lluís Fontboté*
*Departement of Earth Sciences, University of Geneva, 1205 Geneva, Switzerland ([email protected])
**Institute of Mineralogy and Geochemistry, University of Lausanne, 1015 Lausanne, Switzerland
*** Institute of Geochemistry and Petrology, ETH Zürich, 8092 Zürich, Switzerland
†
present address: Memorial University of Newfoundland, CREAIT, CRC and CFI Services (CCCS), Bruneau Centre for Research and Innovation, St. John’s, NL, Canada, A1C 5S7
**** Department of Earth Sciences, University of Durham, Durham DH1 3LE, United Kingdom
Corresponding author: [email protected]
Abstract
Cerro de Pasco district in central Peru hosts the second world largest porphyry-related epithermal polymetallic deposit. The district is centered onto a large diatreme-dome complex crosscut by numerous dacite to rhyodacite bodies showing domal structures and dykes.
Recently, three temporally distinct high-temperature porphyry-type mineralization events
have been established (PM1, PM2, and PM3). New dating of the first porphyry-type
mineralization event PM1 (15.59 ± 0.12 Ma, molybdenite Re-Os) indicates that it slightly predates volcanic activity (15.36 ± 0.03 and 15.16 ± 0.04 Ma, zircon U-Pb, published previously). Crosscutting relationships and new and available zircon U-Pb ages of magmatic rocks allow bracketing the two other porphyry-type mineralization events: PM2 was formed between 15.286 ± 0.018 and 15.158 ± 0.035 Ma, and PM3 between 15.165 ± 0.043 and 15.158 ± 0.035 Ma. These dates suggest continuous magmatic activity during more than 400 ka punctuated by three high-temperature porphyry-type mineralization events that are precursor (maximum gap of 0.9 Ma) of the large epithermal polymetallic mineralization at Cerro de Pasco.
New petrographic, geochemical (whole-rock and mineral), isotopic (Pb, Sr, Nd) data
and the study of silicate melt inclusions hosted in quartz phenocrysts indicate that the magmas
at the origin of the sub-volcanic and volcanic rocks of Cerro de Pasco underwent a period of
evolution in the deep crust, as indicated by typical subduction-related and adakite-like
signatures. Our results suggests that magmas prior to their emplacement at shallow level were
stored at depth with a high degree of crystallinity (up to 90% crystals) at pressures between 2
and 4.4 kbar and at temperatures between 675° and 750°C. At such conditions, the magmas
are beyond the point of rheological lock-up and are not eruptible. Several events of
rejuvenation of this high-crystallinity silicic magmatic body were necessary to allow ascent of
the parental magmas of the various subvolcanic and volcanic bodies at Cerro de Pasco. Our
results point to complex transient cycles of crystallization and rejuvenation of a stored high-
crystallinity silicic magmatic body with circulation of hot magmatic volatiles. Episodes of
rejuvenation and porphyry-type mineralization could be as short as 85 ka.
Introduction
Vertical extent of porphyry systems, from deep underlying plutons to near-surface, where epithermal deposits are formed, is between 5 and 15 km (Sillitoe 2010). Reconstruction of the spatial and temporal connection between the different parts of the system (i.e., plutonic intrusions, volcanic bodies, and porphyry-type and epithermal mineralizations) is complex and requires good knowledge of the magmatic system and geochronological data. Classical models imply that magmatic volatiles are exsolved from a deep crystallizing and convecting large silicic magma chamber (e.g. Burnham 1997, Shinohara 1995, Shinohara and Hedenquist 1997). Volatile saturation is reached by primary boiling due to decompression and by secondary boiling linked to crystallization, and when the fluid percolation threshold is reached, fluids are able to migrate upwards and potentially to form porphyry and epithermal deposits (Burnham 1997, Shinohara 1995, Shinohara and Hedenquist 1997, Sillitoe 2010).
Growing evidence that silicic magmas in upper crustal chambers are stored at high- crystallinity (> 60 % crystals) degree and under a fundamentally immobile, non-convective state as a mush (e.g. Bachmann and Bergantz, 2004; Buret et al., 2016 and 2017: Cashman and Blundy, 2013; Cashman et al., 2017; Cooper and Kent, 2014; Cooper 2017; Deering et al., 2016; Schoene et al., 2012; Spark and Cashman 2017) brings new challenges in the comprehension of magmatic systems associated to porphyry copper formation (Buret et al., 2016 and 2017; Tapster et al., 2016). Addition of energy, generally provided by new magma injections, is required to rejuvenate the silicic mush and to form the volcanic products (e.g.
Cashman et al., 2017; Halter et al., 2004; Hattori and Keith, 2001). Cooling of these magmas at the contact of the silicic reservoir provides large amounts of magmatic fluids having the potential of generating porphyry-type mineralization and to trigger volcanic eruptions (Buret et al., 2017; Tapster et al., 2016).
This study is part of a comprehensive study on the world-class porphyry-related
epithermal polymetallic deposit of Cerro de Pasco (Baumgartner et al., 2008 and 2009; Rottier
et al., 2016a, 2016b, submitted). The present work focuses on the temporal and genetic link of
volcanic and subvolcanic rocks, the recently found porphyry-type mineralization events, and
the epithermal polymetallic mineralization at Cerro de Pasco. New CA-ID-TIMS U-Pb zircon
dating of mineralized magmatic rocks and molybdenite Re-Os age of porphyry-type
mineralization complement previous geochronological work and demonstrates the existence
of successive short-lived episodes (<0.1 Ma) of near-surface magmatic activity and porphyry-
type mineralization. The dynamics of the upper-crustal silicic magma chamber has been
reconstructed by petrographic and geochemical studies of the volcanic and subvolcanic rocks including LA-ICP-MS analyses of silicate melt inclusions hosted in quartz phenocrysts. Our findings support a close causative relationship between rejuvenation of the upper-crustal magma chambers and the high-temperature porphyry-type mineralization events.
General geology
The Cerro de Pasco district is part of the Miocene metallogenic belt of Peru which includes a large number of polymetallic epithermal, Au-Ag high-sulfidation epithermal and Cu-Mo porphyry deposits (Noble and Mckee 1999; Bissig et al., 2008; Bissig and Tosdal, 2009, Bendezú and Fontboté 2009; Catchpole et al., 2015). The central Peruvian segment of the belt (~10.2-12°S) is part of the flat slab subduction segment of the Andes associated with the subduction of the Nazca ridge that started at 15 Ma (Gutscher et al., 1999, Hampel et al., 2002, Rosembaum et al., 2005). Cenozoic volcanism in the area is scattered from ~41 Ma to
~5 Ma (Bissig et al., 2008; Bissig and Tosdal, 2009) and is characterized by the emplacement of isolated and small shallow-level intrusions, subaerial domes and volcanic deposits (Bissig and Tosdal, 2009). Igneous rocks are high-K calc-alkaline and range from basalt to rhyolite, evolved compositions (dacite to rhyolite) being strongly dominant (Bissig and Tosdal, 2009).
Central Peruvian ore-deposits are genetically associated with this magmatism and show a similar age range from ~39 Ma to ~5 Ma, with a more intense Mid- to Late-Miocene productivity between 16 and 5 Ma (Noble and McKee, 1999; Bissig and Tosdal, 2009).
District geology and mineralization
The geology of the Cerro de Pasco district is dominated by a thick deformed sedimentary sequence consisting of Palaeozoic slates and phyllites (Excelsior Group), overlain by sandstones and conglomerates of the Middle-Late Triassic Mitu Group (Fig. 1;
Rosas et al., 2007; Spikings et al., 2016), covered by up to 1000 m of carbonate rocks belonging to the Late Triassic Chambará Formation, part of the Pucará Group (Angeles 1999;
Baumgartner et al. 2008). Sandstones of the Goyllarizquizga Group (Cretaceous) and limestones and marls of the Pocobamba formation (Eocene) overlie discordantly the other formations. At Cerro de Pasco, a regional N15° W-striking reverse fault, named Longitudinal Fault juxtaposes the Excelsior Group and the Pucará Group (Fig. 1; Angeles 1999;
Baumgartner et al. 2008).
Figure 1: Geological map and cross-section of the diatreme dome complex and different epithermal mineralization styles at Cerro de Pasco; slightly modified from Rottier et al. 2016b compiled from field observations and previous work of Rogers (1983), Baumgartner et al. (2008) and the Volcan’s geological staff. A) Location of the Cerro de Pasco district; B) Geological map; C) Porphyritic trachyte outcrop, affected by porphyry-type mineralization PM2.
The diatreme-dome complex of Cerro de Pasco, directly emplaced to the west of the Longitudinal Fault, is 2.5 km in diameter and was formed by a succession of phreatomagmatic, magmatic, and phreatic events (Fig. 1; Einaudi 1968; Rogers 1983;
Baumgartner et al. 2009; Rottier et al., submitted). An early phase of explosive activity produced a diatreme-breccia known locally as Rumiallana Agglomerate, which is the most common lithology in the diatreme-dome complex. The exact geometry of the breccia is not known, the deepest level reached by exploration drilling is at 3300 m.a.s.l. corresponding to a vertical extent superior to 1150 m; the horizontal extent is close to 2000 m. Three different facies form the diatreme breccia: i) a lower facies of non-stratified breccia, ii) an upper facies of stratified breccia, and iii) a top facies of fined-grained stratified lapilli tuff (Rottier et al, submitted). One sample of lapilli tuff was dated at 15.36 ± 0.03 Ma (zircon U-Pb ID-TIMS) by Baumgartner et al. (2009). The phreatomagmatic activity is followed by a period of volcanic and subvolcanic magmatism characterized mainly by emplacement of: i) dacite to rhyodacite bodies showing domal structures and emplaced along the western margin of the diatreme breccia, dated at 15.40 ± 0.07 Ma (zircon U-Pb ID-TIMS; Baumgartner et al. 2009);
ii) small (each less than 10x10 m) porphyritic trachyte intrusions cropping out in the central part of the diatreme breccia and iii) E-W trending quartz-monzonite porphyry dykes cutting the diatreme breccia and the dacitic to rhyodacitic magmatic domes (Fig. 1A), two of them being dated at 15.35 ± 0.05 Ma and 15.16 ± 0.04 Ma (zircon U-Pb ID-TIMS; Baumgartner et al. 2009). Following the magmatic activity, numerous, 20 cm to 3 m-wide, E-W trending, milled-matrix fluidized breccia dykes were emplaced in various parts of the diatreme-dome complex crosscutting the diatreme breccia, the dacitic and rhyodacitic domes and the quartz- monzonite dykes (Rottier et al., submitted). Based on the morphology of the diatreme-dome complex and the occurrence of lapilli tuff, the total erosion since the formation of the diatreme-dome complex has been estimated at < 1 km (Baumgartner 2007, Baumgartner et al., 2008; Rottier et al., 2016a).
The large Cerro de Pasco epithermal polymetallic mineralization is mainly hosted by
carbonate rocks along the eastern margin of the diatreme-dome complex (Einaudi, 1977,
Baumgartner et al., 2008; nomenclature and event succession according to Rottier et al.,
2016b). It is formed during three main successive mineralization stages: A) pipe-like
pyrrhotite-dominated bodies grading outwards into Fe-rich sphalerite (up to vol. 80%) and
galena (stage A), B) quartz-pyrite veins with minor chalcopyrite, hematite, magnetite, pale-
brown sphalerite, galena and tennantite-tetrahedrite (stage B
1) and by a large N-S trending
funnel-shaped massive pyrite-quartz body (stage B
2), and C) high-sulfidation mineralization
consisting of large well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate replacement ore bodies in the eastern part of the deposit (stage C
1) and a set of E-W–trending Cu-Ag-(Au-Zn-Pb) enargite- pyrite veins hosted by the diatreme breccia (stage C
2).
Large pervasive pyrophyllite-quartz-pyrite alteration grading to illite-smectite-muscovite- pyrite and more externally to chlorite-calcite-pyrite alteration affects the central and the northern part of the diatreme-dome complex (Fig. 1). This pervasive alteration is spatially disconnected from the epithermal polymetallic mineralization and is probably linked to degassing in a deeper part of the porphyry system (Rottier 2017).
Description of the dated porphyry-type mineralization occurrences
A genetic relationship between the large epithermal polymetallic mineralization and a porphyry system had been already proposed by Baumgartner et al (2008). Recently, three temporally distinct occurrences of minor porphyry type mineralization have been discovered at Cerro de Pasco. A brief description of the dated porphyry type mineralization occurrences (PM1, PM2 and PM3) recently discovered at Cerro de Pasco is given in this section. Detailed studies of PM1 (Rottier et al. submitted), and PM2 (Rottier et al., 2016a) have already been published and only their main characteristics are summarized below. Porphyry-type mineralization PM3 has not been described so far and the relevant characteristics are reported here.
Porphyry-type mineralization PM1 and PM2
PM1 consists of porphyry-type veins crosscutting hornfels and porphyritic igneous clasts
incorporated in the diatreme-breccia (Fig. 2A, B and C). Quartz-molybdenite-(chalcopyrite)-
(pyrite) are formed at high temperatures (>600°C) and pressures (> 500bar) and host highly-
evolved rhyolitic silicate melt inclusions in hydrothermal quartz (Rottier et al., submitted). A
molybdenite separate obtained from one large hornfels clasts crosscut by numerous quartz-
molybdenite-(chalcopyrite)-(pyrite) veins (CP-16-BR-X4) has been dated by the Re-Os
method.
Figure 2: A-C) Magmatic (A) and hornfels (B-C) clasts found in the diatreme breccia crosscut by HT1-type (A and C) and HT2-type (B) sulfide-poor quartz-molybdenite-(chalcopyrite)-(pyrite) veins belonging to the porphyry-type mineralization PM1. D) Magmatic clast found in a quartz-monzonite dyke and affected by porphyry-type mineralization PM3 (sample CP-14-BR-X1). The clast is affected by pervasive silicifiation and crosscut by A- and B-type quartz-magnetite- chalcopyrite-pyrite veins with potassic alteration halo. Sample selected for U-Pb zircon dating; E) Sample of the dated porphyritic trachyte intrusion crosscut by quartz-magnetite-chalcopyrite-pyrite veinlets (porphyry-type mineralization PM2).
