Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to t... more Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to the north of the giant Los Pelambres-El Pachón porphyry copper cluster, at the southern portion of the Pampean flat-slab segment of the Andes. Although this region hosts telescoped porphyry-epithermal deposits, the precise temporal relationship between porphyry emplacement, mineralization, cooling, and regional orogenic uplift are still poorly understood. New Re-Os molybdenite ages indicate that Altar orebodies are associated with two magmatic hydrothermal centers: Altar East (11.16 ± 0.06 Ma) and Altar Central (10.38 ± 0.05 Ma) formed at temporally distinct periods. New (U-Th)/He ages from the Early Permian and Late Eocene plutons, and the Middle Miocene subvolcanic stocks associated with Cu-Au mineralization of the Altar region reflect a rapid cooling pulse during the Middle Miocene (15.02 to 10.66 Ma) coeval with a major phase of tectonic shortening and regional uplift. The main pulse of rapid cooling and related tectonic uplift in the Altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment (Au-Cu grades) due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system. This 11-10 Ma tectonically triggered exhumation event coincides with the collision of the Etrending segment of the Juan Fernández Ridge with the Peru-Chile trench, at this latitude. Collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the Main Cordillera that has not been well documented to the north or south of the Altar-Los Pelambres region.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Chlorite, white mica and clay minerals as proximity indicators to ore in the shallow porphyry environment of Quebrada de la Mina deposit, Argentina
Journal of Volcanology and Geothermal Research, 2017
We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the N... more We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the Navidad world-class Ag + Pb epithermal district located in the North Patagonian Massif, Patagonia, Argentina. These volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of Gondwana and represent the mafic to intermediate counterparts of the mainly silicic Jurassic Chon Aike Volcanic Province. Lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions, whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation. LA-ICPMS U-Pb ages of zircon crystals from the volcanic units yielded Middle Jurassic ages of 173.9 ± 1.9 Ma and 170.8 ± 3 Ma. In the Navidad district, the volcanic rocks of the Cañadón Asfalto Formation show high-K basaltic-andesite to high-K dacite compositions, Rb, Ba and Th enrichment relative to the HFS elements, enrichment in light rare earth elements (LREE), Y-Ti depletion, and high Zr contents. These arc signatures could be explained by assimilation of crustal rocks in the Jurassic magmas, which is also supported by the presence of zircon xenocrysts with Permian and Middle-Upper Triassic ages (281.3 Ma, 246.5, 218.1, and 201.3 Ma) and quartz xenocrysts recognized in these volcanic units. Furthermore, Sr and Nd isotope compositions suggest a contribution of crustal components in these Middle Jurassic magmas. High-K basaltic-andesite has the highest isotopic 87 Sr/ 86 Sr ratios (0.7112-0.7117) and ξNd values of-6 and-6,9. High-K andesite and dacite have lower 87 Sr/ 86 Sr ratios (0.706-0.708) and ξNd values of-3.2 and-6.1. The range of Pb isotope values (206 Pb/ 204 Pb = 18.28-18.37, 207 Pb/ 204 Pb = 15.61-15.62, and 208 Pb/ 204 Pb =38.26-38.43) of Navidad volcanic rocks and ore minerals suggest mixing Pb sources with contributions of mantle and crust. 206 Pb/ 204 Pb isotopic ratios of Jurassic volcanic rocks of the Chon Aike
Porphyry-tpye ore deposits are characterized by superposition of multiple vein generations, each ... more Porphyry-tpye ore deposits are characterized by superposition of multiple vein generations, each with different vein and alteration assemblages and different ore minerals. The superposition of multiple mineralization events obscures relationships between ore minerals, vein minerals, and alteration, complicating studies that attempt to decipher the formation of these deposits using isotopic tracers and fluid inclusions. Quartz cathodoluminescent (CL) textures in porphyry-type deposits relate vein minerals, alteration minerals, and fluid inclusions to specific vein forming events. Porphyry-type deposits from around the world are characterized by a consistent pattern of CL textures and quartz trace elements that, when combined with fluid inclusion analysis, demonstrate the progressive changes in the pressure and temperature conditions of the hydrothermal system.
is a world-class Cu (Mo-Au) deposit located in Catamarca, Argentina, in which the porphyry and hi... more is a world-class Cu (Mo-Au) deposit located in Catamarca, Argentina, in which the porphyry and high sulfidation epithermal stages are juxtaposed due to the telescoping of the mineralizing system. Pyrite is the most abundant sulfide in the analyzed section of the deposit and shows variations in textures and trace metal content (determined by LA-ICPMS), between the porphyry and epithermal stages. Pyrite from the porphyry stage is fine grained and depleted in most trace elements analyzed, except for traces of Co (up to 276 ppm) and Ni (up to 131 ppm). Pyrite from the epithermal stage is texturally complex, compositionally heterogeneous, and the trace metal content varies with depth and within sub-stages of mineralization. At an intermediate depth (2625 m), epithermal pyrite from the cement of the jigsaw and clast-supported hydrothermal breccias are enriched in Cu (up to 2961 ppm) that correlates with the highest Cu grades in the section. This pyrite contains micro-inclusions of sulfosalt minerals as inferred by LA-ICPMS elemental mapping and individual spot ablation profiles. They are zoned and show a Co-rich core, an intermediate zone enriched in Cu, and an outer rim rich in Zn. At shallower levels (3000 m), epithermal pyrite cements in the heterolithic hydrothermal breccia are unusually rich in trace metals that correlate with the highest Pb, Zn, Au, and Ag grades. The ore-stage pyrite occurs as either successive colloform bands on earlier Co-bearing cores or as veinlets infill. The colloform pyrite bands and veinlets are As-poor (b30 ppm) and enriched in Pb (up to 4528 ppm), Cu (up to 3900 ppm), Zn (up to 1078 ppm), Ag (up to 136 ppm), Au (up to 6.7 ppm), Bi (up to 1077 ppm), and Te (up to 3.1 ppm). In LA-ICPMS elemental maps, arsenic concentrates in a thin inner band within the thicker, trace element-rich rims. The colloform banding in pyrite is interpreted to reflect rapid crystallization during fluid boiling at a hydrothermal fluid-meteoric water interface, creating intense fluctuations in temperature and producing undercooling in the mixed fluid. This late and shallow fluid was depleted in As and Cu and also precipitated alunite, Fe-poor sphalerite, and marcasite enriched in trace metals. Maximum Au and Ag inputs into the system occurred towards the end of the epithermal cycle and are expressed by the Au-Ag-rich rims in hydrothermal pyrite. Based on Au-As data in pyrite, ore fluids forming early pyrite were undersaturated with respect to native Au (solid solution incorporation), while later fluids precipitating colloform pyrite were supersaturated with respect to native Au forming Au nanoparticles. This study provides evidence that pyrite records chemical changes at the porphyry to epithermal transition that can be used to monitor hydrothermal fluid evolution, constrain different mineralization stages, and vector towards undiscovered ore zones.
Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Ne... more Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Neuquén (Paleogene Campana Mahuida, Caicayén and Cerro Nevazón) and SW Mendoza (Neogene Vegas Peladas and Hierro Indio) provinces of Argentina (34-38°S) are reviewed to demonstrate that geochemical signatures of igneous rocks can be used to predict the metal potential of skarn prospects of the Andes Cordillera of Argentina. These igneous rocks are calc-alkaline, metaluminous, and derived from a sub-arc mantle source without residual garnet. They were emplaced at shallow depths. The Vegas Peladas, Hierro Indio, and Cerro Nevazón igneous rocks have similar major and trace element contents, and are typical of primitive, I-type plutons associated with Fe skarns worldwide. The Vegas Peladas and Hierro Indio plutons are, however, less reduced and they have lower Ni concentration than the Cerro Nevazón rocks, whose skarns have higher Au concentrations. In the Caicayén district, small skarns with sub-economic Cu concentrations and abundant pyrite are associated with porphyry copper style alteration in igneous rocks that underwent amphibole fractionation. Their chemical compositions are intermediate between the most primitive plutons associated with Fe skarns and the most evolved plutons associated with Cu skarns, both with high fO 2. At Campana Mahuida a small porphyry copper deposit and associated Cu skarns are linked to andesite dikes and pluton with similar SiO 2 contents but richer in incompatible trace elements (K, Rb, Sr, Ba, La, Ce, and Th) than the Mendoza, Cerro Nevazón and Caicayén igneous rocks. This suggests that their parental magmas evolved in an open system and were contaminated with crustal material, thus resulting in the strongest fractionation of amphibole and accessories phases. These intrusions share many features with typical Cu skarns-related plutons.
Abstract The Altar porphyry Cu-(Au-Mo) deposit (31 29'S, 70 28'W) is lo... more Abstract The Altar porphyry Cu-(Au-Mo) deposit (31 29'S, 70 28'W) is located in the Andean Main Cordillera of San Juan Province (Argentina), in the southern portion of the flat-slab segment (28-33 S), 25 km north of the world-class porphyry Cu-Mo deposits of Los ...
ABSTRACT Sulfides and sulfosalts from the porphyry, transitional, and epithermal high sulfidation... more ABSTRACT Sulfides and sulfosalts from the porphyry, transitional, and epithermal high sulfidation stages of the Altar Cu-(Au) deposit (San Juan, Argentina) were investigated by electron microprobe, laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS), and S isotopic analysis. Compositional differences found in the trace-element content in pyrite, chalcopyrite, and enargite from different alteration zones and depths of the system have implications in the physicochemical evolution of the hydrothermal fluids and might have potential for exploration in similar porphyry-high sulfidation epithermal environments. Low contents of most analyzed trace elements were found in chalcopyrite ± pyrite from the potassic alteration, except for Zn, Sn, and Ag in chalcopyrite, and Co and Ni in pyrite. During the phyllic alteration, cooling of the fluids and a decrease of its pH (<4.5) produced the leaching and re-precipitation of the sulfides from the previous stage. The reprecipitated chalcopyrite and pyrite show micro-inclusions of Au (Ag alloys) and sulfosalts with As, Ag, V, Zn, Sn, Bi, and Sb that are interpreted to correlate with changes in the composition of the fluids.In the transitional veins and their selvages, early precipitation of bornite + pyrite and the weak kaolinite + quartz alteration envelope indicate a decrease in the temperature (<200 °C) and pH (<4) of the fluids and an increase of the fS2. This stage was followed by a sudden increase in the fO2 of the fluid and a slight decrease in the fS2, and bornite was replaced by tennantite (vein type E1). A trend toward higher fS2 favored enargite precipitation (vein type E3). Traces of Au found in tennantite and enargite therefore indicate that fluctuations in the oxidation state of the fluids promoted gold precipitation in the Altar porphyry system. Negative S isotope values (δ34S: –1.7 to –3.0‰) obtained for the sulfides and sulfosalts from the potassic, phyllic, and transitional stages at Altar are consistent with oxidizing (SO42– dominant) mineralizing fluids. The precipitation of sphalerite (δ34S = –4.4‰ and low concentration of FeS) and galena associated with calcite + anhydrite in late fractures (vein type 8) reflect cooling of the fluids and a slight increase of its pH at higher fO2, as the system moved to greater depths. The typical advanced argillic assemblages are not developed at the Altar porphyry deposit, indicating that fluids did not attain very low pH conditions. In contrast, enargite from the epithermal siliceous ledges located above the porphyry deposit precipitated with low pH mineral assemblages (quartz + alunite) and has a distinctive lighter δ34S value of –5.4‰, consistent with a decrease of temperature, pH, and/or an increase in the oxidation state of the fluids during its deposition. Enargite from the transitional stage veins of the porphyry deposit is enriched in Zn, Sb, Ag, and Bi, whereas enargite from the epithermal environment has much lower concentrations of trace elements, except for isolated inclusions of Au-Bi.
Altar (31º 29' S, 70º 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au... more Altar (31º 29' S, 70º 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au-(Ag-Cu) veins located in the Cordillera Principal of SW San Juan Province (Argentina). Altar is a complex magmatic-hydrothermal system formed from several magmatic and hydrothermal pulses during the middle-late Miocene. New LA-ICPMS U-Pb ages in zircons from the Altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca. 3 m.y. It comprises a pre-mineralization porphyry (11.75 ± 0.24 Ma), three mineralized porphyries (11.62 ± 0.21 Ma and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two post-mineralization intrusions, and a post-mineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries (porphyries 2, 3 and 4) were emplaced within ~0.7-1.3 m.y. Amphibole phenocrysts from the porphyries crystallized from oxidized magmas (fO 2 = NNO +1 to +2) at temperatures of 780 to 850°C and pressures between 0.9 and 1.8 kbar corresponding to depths of ~4-7 km. Anorthite and Fe-rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma. The middle-late Miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust. The focused magmatic output to shallow levels during a period of a few million years in the Altar area has been a main requirement in the formation of this large porphyry copper deposit. Tectonic setting Altar is located in the southern portion of the flat-slab segment (~27°-33° 30'S) of the Central Andes. In this region, the Nazca plate is subducting nearly horizontally beneath the South American plate at ~100 km depth (Gans et al., 2011). The flat-slab segment presents a smooth transition to the north, toward the Central Volcanic Zone (CVZ), and an abrupt transition to the south, to the Southern Volcanic Zone (SVZ; Fig. 1; Cahill and Isacks, 1992; Anderson et al., 2007; Gans et al., 2011). Several studies have documented the Miocene to recent evolution of the flat-slab segment (Allmendinger et al. 1990; Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002). During the Early Miocene (27-20 Ma) this segment had a slab geometry similar to that currently observed in the normal-slab segment at 35ºS, and a crustal thickness of 35-40 km (Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002). The shallowing of the subduction zone progressed from Middle to Late Miocene (20-5 Ma). It was accompanied by crustal thickening, subduction of the Juan Fernández ridge (e.g., Yáñez et al., 2001), a substantial decrease in the astenospheric wedge thickness and eastward migration, and broadening of the arc (Kay et al., 2005). Cessation of magmatic activity over the Miocene flat-slab occurred at 5 Ma. At this time, magmatism occurred in the back-arc, in the Farallón Negro, Pocho and San Luis magmatic centers (Kay and Mpodozis, 2002). Local Geology The Altar area was poorly known geologically until the mid-1990s, when CRA Exploration Argentina outlined a broad zone of alteration and mineralization. Between 1999 and 2003, Río Tinto tested this target with 2,841 m of diamond drilling. Since 2005, Peregrine Metals Ltd. has completed 140 diamond drill holes (55,688 meters). Recently, the project has been acquired by Stillwater Mining Company, which is conducting a comprehensive drilling campaign, metallurgical testwork program, environmental baseline study, and preliminary economic evaluations for a copper-gold mining and concentrating project at Altar. Maydagán et al. (2011) presented the first field mapping and data on magmatic rocks of the Altar region. The authors grouped the igneous rocks into two sequences: the early Miocene lower volcanic complex and the middle-late Miocene upper subvolcanic suite. The lower volcanic complex is an early Miocene arc in which mantle-derived magmas evolved at low pressures through plagioclase-and pyroxene-dominated fractional crystallization and assimilation of crustal rocks (AFC; Maydagán et al., 2011). The Altar porphyry Cu-(Au-Mo) deposit is partially hosted and related to the upper subvolcanic suite, a series of porphyritic stocks, dikes, and breccias that intruded the lower volcanic complex in the central and east ridges of the Altar district (Fig. 2). Magmas of the upper subvolcanic suite require a hornblende-bearing residual mineral assemblage that is interpreted to reflect their higher water contents (Maydagán et al., 2011). Analytical Techniques and Sampling Methodology Over 200 samples representatives of the different porphyritic rocks and breccias of the upper subvolcanic suite were collected from surface exposures and drill core from the Altar region. The samples were studied by transmitted and reflected light petrography at the Universidad Nacional del Comahue (Neuquén, Argentina) and twenty samples were selected for analysis. Three radiometric ages of the subvolcanic stocks were determined by U-Pb LA-ICPMS on zircons at the PCIGR-Pacific Center for Isotopic and Geochemical Research at the University of British Columbia (Canada). Another radiometric age of the late breccia was determined by U-Pb LA-ICPMS on zircons at the Arizona LaserChron Center, University of Arizona, U.S.A. Details of the dating methods and results of LA-ICPMS analyses of zircons from the porphyries and late breccia are provided in digital appendices A5-A10. Major, trace and rare earth elements were analyzed by inductively coupled plasmaemission spectrometry and ICP mass spectrometry (Group 4A-4B combined package) at Acme Analytical Laboratories Ltd., Canada (Table 4). Seven samples were analyzed for Sr and Nd isotopes and ten for Pb isotopes at the Mineralogy Department at the University of Geneva (Switzerland) following the method of Chiaradia et al. (2009a; Table 5). Chemical compositions of magmatic minerals (plagioclase (n = 74), ulvöspinel (n = 17), amphibole (n = 20), biotite (n=16), and magnetite (n = 8) from different subvolcanic intrusions were determined with electron microprobe at three different laboratories with some samples analyzed in duplicate at more than one laboratory. The full data set of microprobe analysis is provided in the digital appendices A1-A4. Altar subvolcanic intrusions At least five porphyritic intrusions and three breccias have been recognized in the Altar district. They are distinguished based on their textures, phenocryst abundances, related veins,
We investigate the geology of Altar North (CueAu) and Quebrada de la Mina (Au) porphyry deposits ... more We investigate the geology of Altar North (CueAu) and Quebrada de la Mina (Au) porphyry deposits located in San Juan Province (Argentina), close to the large Altar porphyry copper deposit (995 Mt, 0.35% Cu, 0.083 g/t Au), to present constraints on the magmatic processes that occurred in the parental magma chambers of these magmatic-hydrothermal systems. Altar North deposit comprises a plagioclase-amphibole-phyric dacite intrusion (Altar North barren porphyry) and a plagioclaseamphibole-biotite-phyric dacite stock (Altar North mineralized porphyry, 11.98 AE 0.19 Ma). In Quebrada de la Mina, a plagioclase-amphibole-biotite-quartz-phyric dacite stock (QDM porphyry, 11.91 AE 0.33 Ma) crops out. High Sr/Y ratios (92e142) and amphibole compositions of Altar North barren and QDM porphyries reflect high magmatic oxidation states (fO 2 ¼ NNO þ1.1 to þ1.6) and high fH 2 O conditions in their magmas. Zones and rims enriched in anorthite (An 37e48), SrO (0.22e0.33 wt.%) and FeO (0.21e0.37 wt.%) in plagioclase phenocrysts are evidences of magmatic recharge processes in the magma chambers. Altar North and Quebrada de la Mina intrusions have relatively homogeneous isotopic compositions (87 Sr/ 86 Sr (t) ¼ 0.70450e0.70466, ε Nd (t) ¼ þ0.2 to þ1.2) consistent with mixed mantle and crust contributions in their magmas. Higher Pb isotopes ratios (207 Pb/ 204 Pb ¼ 15.6276 e15.6294) of these intrusions compared to other porphyries of the district, reflect an increase in the assimilation of high radiogenic Pb components in the magmas. Ages of zircon xenocrysts (297, 210, 204, 69 Ma) revealed that the magmas have experienced assimilation of Miocene, Cretaceous, Triassic and Carboniferous crustal rocks. Fluids that precipitated sulfides in the Altar deposit may have remobilized Pb from the host rocks, as indicated by the ore minerals being more radiogenic (207 Pb/ 204 Pb ¼ 15.6243e15.6269) than their host intrusions. Au/Cu ratio in Altar porphyries (average Au/Cu ratio of 0.14 Â 10 À4 by weight in Altar Central) is higher than in the giant Miocene porphyry deposits located to the south: Los Pelambres, Río Blanco and Los Bronces (Chile) and Pachón (Argentina). We suggest that the increase in Au content in the porphyries of this region could be linked to the assimilation of high radiogenic Pb components in the magmas within these long-lived maturation systems.
Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to t... more Altar is a large porphyry Cu (Au) deposit located in the Main Cordillera of Argentina, 20 km to the north of the giant Los Pelambres-El Pachón porphyry copper cluster, at the southern portion of the Pampean flat-slab segment of the Andes. Although this region hosts telescoped porphyry-epithermal deposits, the precise temporal relationship between porphyry emplacement, mineralization, cooling, and regional orogenic uplift are still poorly understood. New Re-Os molybdenite ages indicate that Altar orebodies are associated with two magmatic hydrothermal centers: Altar East (11.16 ± 0.06 Ma) and Altar Central (10.38 ± 0.05 Ma) formed at temporally distinct periods. New (U-Th)/He ages from the Early Permian and Late Eocene plutons, and the Middle Miocene subvolcanic stocks associated with Cu-Au mineralization of the Altar region reflect a rapid cooling pulse during the Middle Miocene (15.02 to 10.66 Ma) coeval with a major phase of tectonic shortening and regional uplift. The main pulse of rapid cooling and related tectonic uplift in the Altar region was synchronous with the formation of the hydrothermal systems and resulted in an increased focused metal endowment (Au-Cu grades) due to the telescoping of epithermal mineralization over the rapidly uplifted porphyry system. This 11-10 Ma tectonically triggered exhumation event coincides with the collision of the Etrending segment of the Juan Fernández Ridge with the Peru-Chile trench, at this latitude. Collision and ensuing ridge subduction may have driven a localized pulse of rapid cooling and exhumation of the Main Cordillera that has not been well documented to the north or south of the Altar-Los Pelambres region.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service... more This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. Chlorite, white mica and clay minerals as proximity indicators to ore in the shallow porphyry environment of Quebrada de la Mina deposit, Argentina
Journal of Volcanology and Geothermal Research, 2017
We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the N... more We present the first study of the volcanic rocks of the Cañadón Asfalto Formation that host the Navidad world-class Ag + Pb epithermal district located in the North Patagonian Massif, Patagonia, Argentina. These volcanic and sedimentary rocks were deposited in a lacustrine environment during an extensional tectonic regime associated with the breakup of Gondwana and represent the mafic to intermediate counterparts of the mainly silicic Jurassic Chon Aike Volcanic Province. Lava flows surrounded by autobrecciated carapace were extruded in subaerial conditions, whereas hyaloclastite and peperite facies suggest contemporaneous subaqueous volcanism and sedimentation. LA-ICPMS U-Pb ages of zircon crystals from the volcanic units yielded Middle Jurassic ages of 173.9 ± 1.9 Ma and 170.8 ± 3 Ma. In the Navidad district, the volcanic rocks of the Cañadón Asfalto Formation show high-K basaltic-andesite to high-K dacite compositions, Rb, Ba and Th enrichment relative to the HFS elements, enrichment in light rare earth elements (LREE), Y-Ti depletion, and high Zr contents. These arc signatures could be explained by assimilation of crustal rocks in the Jurassic magmas, which is also supported by the presence of zircon xenocrysts with Permian and Middle-Upper Triassic ages (281.3 Ma, 246.5, 218.1, and 201.3 Ma) and quartz xenocrysts recognized in these volcanic units. Furthermore, Sr and Nd isotope compositions suggest a contribution of crustal components in these Middle Jurassic magmas. High-K basaltic-andesite has the highest isotopic 87 Sr/ 86 Sr ratios (0.7112-0.7117) and ξNd values of-6 and-6,9. High-K andesite and dacite have lower 87 Sr/ 86 Sr ratios (0.706-0.708) and ξNd values of-3.2 and-6.1. The range of Pb isotope values (206 Pb/ 204 Pb = 18.28-18.37, 207 Pb/ 204 Pb = 15.61-15.62, and 208 Pb/ 204 Pb =38.26-38.43) of Navidad volcanic rocks and ore minerals suggest mixing Pb sources with contributions of mantle and crust. 206 Pb/ 204 Pb isotopic ratios of Jurassic volcanic rocks of the Chon Aike
Porphyry-tpye ore deposits are characterized by superposition of multiple vein generations, each ... more Porphyry-tpye ore deposits are characterized by superposition of multiple vein generations, each with different vein and alteration assemblages and different ore minerals. The superposition of multiple mineralization events obscures relationships between ore minerals, vein minerals, and alteration, complicating studies that attempt to decipher the formation of these deposits using isotopic tracers and fluid inclusions. Quartz cathodoluminescent (CL) textures in porphyry-type deposits relate vein minerals, alteration minerals, and fluid inclusions to specific vein forming events. Porphyry-type deposits from around the world are characterized by a consistent pattern of CL textures and quartz trace elements that, when combined with fluid inclusion analysis, demonstrate the progressive changes in the pressure and temperature conditions of the hydrothermal system.
is a world-class Cu (Mo-Au) deposit located in Catamarca, Argentina, in which the porphyry and hi... more is a world-class Cu (Mo-Au) deposit located in Catamarca, Argentina, in which the porphyry and high sulfidation epithermal stages are juxtaposed due to the telescoping of the mineralizing system. Pyrite is the most abundant sulfide in the analyzed section of the deposit and shows variations in textures and trace metal content (determined by LA-ICPMS), between the porphyry and epithermal stages. Pyrite from the porphyry stage is fine grained and depleted in most trace elements analyzed, except for traces of Co (up to 276 ppm) and Ni (up to 131 ppm). Pyrite from the epithermal stage is texturally complex, compositionally heterogeneous, and the trace metal content varies with depth and within sub-stages of mineralization. At an intermediate depth (2625 m), epithermal pyrite from the cement of the jigsaw and clast-supported hydrothermal breccias are enriched in Cu (up to 2961 ppm) that correlates with the highest Cu grades in the section. This pyrite contains micro-inclusions of sulfosalt minerals as inferred by LA-ICPMS elemental mapping and individual spot ablation profiles. They are zoned and show a Co-rich core, an intermediate zone enriched in Cu, and an outer rim rich in Zn. At shallower levels (3000 m), epithermal pyrite cements in the heterolithic hydrothermal breccia are unusually rich in trace metals that correlate with the highest Pb, Zn, Au, and Ag grades. The ore-stage pyrite occurs as either successive colloform bands on earlier Co-bearing cores or as veinlets infill. The colloform pyrite bands and veinlets are As-poor (b30 ppm) and enriched in Pb (up to 4528 ppm), Cu (up to 3900 ppm), Zn (up to 1078 ppm), Ag (up to 136 ppm), Au (up to 6.7 ppm), Bi (up to 1077 ppm), and Te (up to 3.1 ppm). In LA-ICPMS elemental maps, arsenic concentrates in a thin inner band within the thicker, trace element-rich rims. The colloform banding in pyrite is interpreted to reflect rapid crystallization during fluid boiling at a hydrothermal fluid-meteoric water interface, creating intense fluctuations in temperature and producing undercooling in the mixed fluid. This late and shallow fluid was depleted in As and Cu and also precipitated alunite, Fe-poor sphalerite, and marcasite enriched in trace metals. Maximum Au and Ag inputs into the system occurred towards the end of the epithermal cycle and are expressed by the Au-Ag-rich rims in hydrothermal pyrite. Based on Au-As data in pyrite, ore fluids forming early pyrite were undersaturated with respect to native Au (solid solution incorporation), while later fluids precipitating colloform pyrite were supersaturated with respect to native Au forming Au nanoparticles. This study provides evidence that pyrite records chemical changes at the porphyry to epithermal transition that can be used to monitor hydrothermal fluid evolution, constrain different mineralization stages, and vector towards undiscovered ore zones.
Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Ne... more Five mineralized Cu, Au, and Fe skarns and related igneous rocks of the Andes Cordillera of NW Neuquén (Paleogene Campana Mahuida, Caicayén and Cerro Nevazón) and SW Mendoza (Neogene Vegas Peladas and Hierro Indio) provinces of Argentina (34-38°S) are reviewed to demonstrate that geochemical signatures of igneous rocks can be used to predict the metal potential of skarn prospects of the Andes Cordillera of Argentina. These igneous rocks are calc-alkaline, metaluminous, and derived from a sub-arc mantle source without residual garnet. They were emplaced at shallow depths. The Vegas Peladas, Hierro Indio, and Cerro Nevazón igneous rocks have similar major and trace element contents, and are typical of primitive, I-type plutons associated with Fe skarns worldwide. The Vegas Peladas and Hierro Indio plutons are, however, less reduced and they have lower Ni concentration than the Cerro Nevazón rocks, whose skarns have higher Au concentrations. In the Caicayén district, small skarns with sub-economic Cu concentrations and abundant pyrite are associated with porphyry copper style alteration in igneous rocks that underwent amphibole fractionation. Their chemical compositions are intermediate between the most primitive plutons associated with Fe skarns and the most evolved plutons associated with Cu skarns, both with high fO 2. At Campana Mahuida a small porphyry copper deposit and associated Cu skarns are linked to andesite dikes and pluton with similar SiO 2 contents but richer in incompatible trace elements (K, Rb, Sr, Ba, La, Ce, and Th) than the Mendoza, Cerro Nevazón and Caicayén igneous rocks. This suggests that their parental magmas evolved in an open system and were contaminated with crustal material, thus resulting in the strongest fractionation of amphibole and accessories phases. These intrusions share many features with typical Cu skarns-related plutons.
Abstract The Altar porphyry Cu-(Au-Mo) deposit (31 29'S, 70 28'W) is lo... more Abstract The Altar porphyry Cu-(Au-Mo) deposit (31 29'S, 70 28'W) is located in the Andean Main Cordillera of San Juan Province (Argentina), in the southern portion of the flat-slab segment (28-33 S), 25 km north of the world-class porphyry Cu-Mo deposits of Los ...
ABSTRACT Sulfides and sulfosalts from the porphyry, transitional, and epithermal high sulfidation... more ABSTRACT Sulfides and sulfosalts from the porphyry, transitional, and epithermal high sulfidation stages of the Altar Cu-(Au) deposit (San Juan, Argentina) were investigated by electron microprobe, laser ablation-inductively coupled plasma-mass spectroscopy (LA-ICP-MS), and S isotopic analysis. Compositional differences found in the trace-element content in pyrite, chalcopyrite, and enargite from different alteration zones and depths of the system have implications in the physicochemical evolution of the hydrothermal fluids and might have potential for exploration in similar porphyry-high sulfidation epithermal environments. Low contents of most analyzed trace elements were found in chalcopyrite ± pyrite from the potassic alteration, except for Zn, Sn, and Ag in chalcopyrite, and Co and Ni in pyrite. During the phyllic alteration, cooling of the fluids and a decrease of its pH (<4.5) produced the leaching and re-precipitation of the sulfides from the previous stage. The reprecipitated chalcopyrite and pyrite show micro-inclusions of Au (Ag alloys) and sulfosalts with As, Ag, V, Zn, Sn, Bi, and Sb that are interpreted to correlate with changes in the composition of the fluids.In the transitional veins and their selvages, early precipitation of bornite + pyrite and the weak kaolinite + quartz alteration envelope indicate a decrease in the temperature (<200 °C) and pH (<4) of the fluids and an increase of the fS2. This stage was followed by a sudden increase in the fO2 of the fluid and a slight decrease in the fS2, and bornite was replaced by tennantite (vein type E1). A trend toward higher fS2 favored enargite precipitation (vein type E3). Traces of Au found in tennantite and enargite therefore indicate that fluctuations in the oxidation state of the fluids promoted gold precipitation in the Altar porphyry system. Negative S isotope values (δ34S: –1.7 to –3.0‰) obtained for the sulfides and sulfosalts from the potassic, phyllic, and transitional stages at Altar are consistent with oxidizing (SO42– dominant) mineralizing fluids. The precipitation of sphalerite (δ34S = –4.4‰ and low concentration of FeS) and galena associated with calcite + anhydrite in late fractures (vein type 8) reflect cooling of the fluids and a slight increase of its pH at higher fO2, as the system moved to greater depths. The typical advanced argillic assemblages are not developed at the Altar porphyry deposit, indicating that fluids did not attain very low pH conditions. In contrast, enargite from the epithermal siliceous ledges located above the porphyry deposit precipitated with low pH mineral assemblages (quartz + alunite) and has a distinctive lighter δ34S value of –5.4‰, consistent with a decrease of temperature, pH, and/or an increase in the oxidation state of the fluids during its deposition. Enargite from the transitional stage veins of the porphyry deposit is enriched in Zn, Sb, Ag, and Bi, whereas enargite from the epithermal environment has much lower concentrations of trace elements, except for isolated inclusions of Au-Bi.
Altar (31º 29' S, 70º 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au... more Altar (31º 29' S, 70º 28' W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au-(Ag-Cu) veins located in the Cordillera Principal of SW San Juan Province (Argentina). Altar is a complex magmatic-hydrothermal system formed from several magmatic and hydrothermal pulses during the middle-late Miocene. New LA-ICPMS U-Pb ages in zircons from the Altar porphyries indicate four discrete events of intrusions over an extended magmatic life time of ca. 3 m.y. It comprises a pre-mineralization porphyry (11.75 ± 0.24 Ma), three mineralized porphyries (11.62 ± 0.21 Ma and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two post-mineralization intrusions, and a post-mineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries (porphyries 2, 3 and 4) were emplaced within ~0.7-1.3 m.y. Amphibole phenocrysts from the porphyries crystallized from oxidized magmas (fO 2 = NNO +1 to +2) at temperatures of 780 to 850°C and pressures between 0.9 and 1.8 kbar corresponding to depths of ~4-7 km. Anorthite and Fe-rich rims in the plagioclase phenocrysts suggest that the magmatic chambers were episodically recharged by a less evolved magma. The middle-late Miocene intrusions are interpreted to have been derived from a deeper and relatively large magmatic reservoir that supplied magmas to smaller chambers located in the upper crust. The focused magmatic output to shallow levels during a period of a few million years in the Altar area has been a main requirement in the formation of this large porphyry copper deposit. Tectonic setting Altar is located in the southern portion of the flat-slab segment (~27°-33° 30'S) of the Central Andes. In this region, the Nazca plate is subducting nearly horizontally beneath the South American plate at ~100 km depth (Gans et al., 2011). The flat-slab segment presents a smooth transition to the north, toward the Central Volcanic Zone (CVZ), and an abrupt transition to the south, to the Southern Volcanic Zone (SVZ; Fig. 1; Cahill and Isacks, 1992; Anderson et al., 2007; Gans et al., 2011). Several studies have documented the Miocene to recent evolution of the flat-slab segment (Allmendinger et al. 1990; Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002). During the Early Miocene (27-20 Ma) this segment had a slab geometry similar to that currently observed in the normal-slab segment at 35ºS, and a crustal thickness of 35-40 km (Kay and Abbruzzi, 1996; Kay and Mpodozis, 2002). The shallowing of the subduction zone progressed from Middle to Late Miocene (20-5 Ma). It was accompanied by crustal thickening, subduction of the Juan Fernández ridge (e.g., Yáñez et al., 2001), a substantial decrease in the astenospheric wedge thickness and eastward migration, and broadening of the arc (Kay et al., 2005). Cessation of magmatic activity over the Miocene flat-slab occurred at 5 Ma. At this time, magmatism occurred in the back-arc, in the Farallón Negro, Pocho and San Luis magmatic centers (Kay and Mpodozis, 2002). Local Geology The Altar area was poorly known geologically until the mid-1990s, when CRA Exploration Argentina outlined a broad zone of alteration and mineralization. Between 1999 and 2003, Río Tinto tested this target with 2,841 m of diamond drilling. Since 2005, Peregrine Metals Ltd. has completed 140 diamond drill holes (55,688 meters). Recently, the project has been acquired by Stillwater Mining Company, which is conducting a comprehensive drilling campaign, metallurgical testwork program, environmental baseline study, and preliminary economic evaluations for a copper-gold mining and concentrating project at Altar. Maydagán et al. (2011) presented the first field mapping and data on magmatic rocks of the Altar region. The authors grouped the igneous rocks into two sequences: the early Miocene lower volcanic complex and the middle-late Miocene upper subvolcanic suite. The lower volcanic complex is an early Miocene arc in which mantle-derived magmas evolved at low pressures through plagioclase-and pyroxene-dominated fractional crystallization and assimilation of crustal rocks (AFC; Maydagán et al., 2011). The Altar porphyry Cu-(Au-Mo) deposit is partially hosted and related to the upper subvolcanic suite, a series of porphyritic stocks, dikes, and breccias that intruded the lower volcanic complex in the central and east ridges of the Altar district (Fig. 2). Magmas of the upper subvolcanic suite require a hornblende-bearing residual mineral assemblage that is interpreted to reflect their higher water contents (Maydagán et al., 2011). Analytical Techniques and Sampling Methodology Over 200 samples representatives of the different porphyritic rocks and breccias of the upper subvolcanic suite were collected from surface exposures and drill core from the Altar region. The samples were studied by transmitted and reflected light petrography at the Universidad Nacional del Comahue (Neuquén, Argentina) and twenty samples were selected for analysis. Three radiometric ages of the subvolcanic stocks were determined by U-Pb LA-ICPMS on zircons at the PCIGR-Pacific Center for Isotopic and Geochemical Research at the University of British Columbia (Canada). Another radiometric age of the late breccia was determined by U-Pb LA-ICPMS on zircons at the Arizona LaserChron Center, University of Arizona, U.S.A. Details of the dating methods and results of LA-ICPMS analyses of zircons from the porphyries and late breccia are provided in digital appendices A5-A10. Major, trace and rare earth elements were analyzed by inductively coupled plasmaemission spectrometry and ICP mass spectrometry (Group 4A-4B combined package) at Acme Analytical Laboratories Ltd., Canada (Table 4). Seven samples were analyzed for Sr and Nd isotopes and ten for Pb isotopes at the Mineralogy Department at the University of Geneva (Switzerland) following the method of Chiaradia et al. (2009a; Table 5). Chemical compositions of magmatic minerals (plagioclase (n = 74), ulvöspinel (n = 17), amphibole (n = 20), biotite (n=16), and magnetite (n = 8) from different subvolcanic intrusions were determined with electron microprobe at three different laboratories with some samples analyzed in duplicate at more than one laboratory. The full data set of microprobe analysis is provided in the digital appendices A1-A4. Altar subvolcanic intrusions At least five porphyritic intrusions and three breccias have been recognized in the Altar district. They are distinguished based on their textures, phenocryst abundances, related veins,
We investigate the geology of Altar North (CueAu) and Quebrada de la Mina (Au) porphyry deposits ... more We investigate the geology of Altar North (CueAu) and Quebrada de la Mina (Au) porphyry deposits located in San Juan Province (Argentina), close to the large Altar porphyry copper deposit (995 Mt, 0.35% Cu, 0.083 g/t Au), to present constraints on the magmatic processes that occurred in the parental magma chambers of these magmatic-hydrothermal systems. Altar North deposit comprises a plagioclase-amphibole-phyric dacite intrusion (Altar North barren porphyry) and a plagioclaseamphibole-biotite-phyric dacite stock (Altar North mineralized porphyry, 11.98 AE 0.19 Ma). In Quebrada de la Mina, a plagioclase-amphibole-biotite-quartz-phyric dacite stock (QDM porphyry, 11.91 AE 0.33 Ma) crops out. High Sr/Y ratios (92e142) and amphibole compositions of Altar North barren and QDM porphyries reflect high magmatic oxidation states (fO 2 ¼ NNO þ1.1 to þ1.6) and high fH 2 O conditions in their magmas. Zones and rims enriched in anorthite (An 37e48), SrO (0.22e0.33 wt.%) and FeO (0.21e0.37 wt.%) in plagioclase phenocrysts are evidences of magmatic recharge processes in the magma chambers. Altar North and Quebrada de la Mina intrusions have relatively homogeneous isotopic compositions (87 Sr/ 86 Sr (t) ¼ 0.70450e0.70466, ε Nd (t) ¼ þ0.2 to þ1.2) consistent with mixed mantle and crust contributions in their magmas. Higher Pb isotopes ratios (207 Pb/ 204 Pb ¼ 15.6276 e15.6294) of these intrusions compared to other porphyries of the district, reflect an increase in the assimilation of high radiogenic Pb components in the magmas. Ages of zircon xenocrysts (297, 210, 204, 69 Ma) revealed that the magmas have experienced assimilation of Miocene, Cretaceous, Triassic and Carboniferous crustal rocks. Fluids that precipitated sulfides in the Altar deposit may have remobilized Pb from the host rocks, as indicated by the ore minerals being more radiogenic (207 Pb/ 204 Pb ¼ 15.6243e15.6269) than their host intrusions. Au/Cu ratio in Altar porphyries (average Au/Cu ratio of 0.14 Â 10 À4 by weight in Altar Central) is higher than in the giant Miocene porphyry deposits located to the south: Los Pelambres, Río Blanco and Los Bronces (Chile) and Pachón (Argentina). We suggest that the increase in Au content in the porphyries of this region could be linked to the assimilation of high radiogenic Pb components in the magmas within these long-lived maturation systems.
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