The Altar porphyry Cu-(Au-Mo) deposit (Argentina): a complex magmatic-hydrothermal system with evidence of recharge processes Maydagan, L., Franchini, M., Chiaradia, M., Dilles, J., & Rey, R. (2014). The Altar porphyry Cu-(Au-Mo) deposit (Argentina): A complex magmatic-hydrothermal system with evidence of recharge processes. Economic Geology, 109(3), 621-641. doi:10.2113/econgeo.109.3.621 10.2113/econgeo.109.3.621 Society of Economic Geologists Accepted Manuscript http://cdss.library.oregonstate.edu/sa-termsofuse The Altar porphyry Cu-(Au-Mo) deposit (Argentina): a complex magmatic-hydrothermal system with evidence of recharge processes Laura Maydagána,b,c*, Marta Franchinia,b, Massimo Chiaradiad, John Dillese, Roger Reyf a CONICET; b Departamento de Geología y Petróleo, Facultad de Ingeniería, Universidad Nacional del Comahue, Buenos Aires 1400, 8300 Neuquén, Argentina. c Departamento de Geología, Universidad Nacional del Sur, San Juan 670, 8000 Bahía Blanca, Argentina d Department of Mineralogy, University of Geneva, Rue des Maraîchers 13, 1205 Geneva, Switzerland e College of Earth, Ocean and Atmospheric Sciences, Oregon State University, 104 CEOAS Admin Bldg, Corvallis f Peregrine Metals Ltd. Santa Fe (Oeste) 117, Piso 5, Edificio Derby, Ciudad San Juan, Argentina *Corresponding author: [email protected] Abstract 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. Introduction Detailed studies of porphyry Cu-Au-Mo systems have highlighted the complexity of their magmatic-hydrothermal evolution which comprises successive phases of intrusion, hydrothermal fluid release, and metal deposition (e.g., Skewes and Stern, 1996; Proffett, 2003; Harris et al., 2004, Seedorff et al., 2005; Vry et al., 2010; Longo et al., 2010; Stern et al. 2011). These complex systems can be developed episodically over time spans that may last up to several million years. During this period magmas and parental plutons associated with the porphyries may experience processes such as recharge or mixing (e.g., Keith, 1997; Hattori and Keith, 2001; Maughan et al., 2002; Halter et al., 2004; Audétat and Pettke, 2006; Chiaradia et al., 2009a; Chiaradia et al., 2012). These processes influence the metal budget and evolution of the magmatic-hydrothermal system. Altar (31º 29’ S, 70º 28’ W) is a large porphyry Cu-(Au-Mo) deposit with associated epithermal Au-(Ag-Cu) veins (802 Mt @ 0.42% Cu and 0.059 g/t Au, with a cut-off grade of 0.30% Cu equivalent, Table 1; Peregrine Metals Ltd., 2010). It is located in the Cordillera Principal of SW San Juan Province, Argentina, which corresponds to the southern portion of the Andean flat-slab segment (28-33ºS). The world-class porphyry Cu-Mo deposits of Los Pelambres and Pachón occur 25 km to the south (Fig. 1). The importance of this region has been highlighted by the recent discovery of the porphyry Cu districts of Piuquenes, Rincones de Araya, Quebrada de la Mina, Yunque, Los Azules, and Los Bagres (Fig. 1; Mpodozis and Cornejo, 2012). Maydagán et al. (2011) presented the first geological mapping and petrographic and geochemical studies of the Altar magmatic rocks. The Altar upper subvolcanic suite which intruded volcanic rocks of the lower volcanic complex is genetically linked to the porphyry Cu-(Au-Mo) and epithermal Au-(Ag-Cu) veins. The lower volcanic complex is part of an early Miocene magmatic arc formed above a normal dipping subduction zone whose magmas underwent fractional crystallization and crustal contamination. The upper subvolcanic suite represents a late-middle Miocene magmatic arc formed above a shallow subduction zone (Maydagán et al., 2011). This study describes the Altar subvolcanic porphyritic stocks, presents constraints on duration of magmatism, and processes that occurred in parental magma chambers during the evolution of the magmatic-hydrothermal system. Our work is based on a detailed petrographic study coupled with analyses of mineral chemistry, new LA-ICPMS U-Pb dating and Sr, Nd, and Pb radiogenic isotopes of the different porphyritic intrusions. 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 plasma- emission 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, degree of alteration and mineralization, and cross-cutting relationships (Fig. 2; Table 2). Hydrothermal alteration and Cu-(Au-Mo) and Au-(Ag-Cu) mineralization in the project are associated spatially and temporally with the porphyries and breccias emplacement. A late undeformed volcanoclastic breccia contains fragments of both the lower volcanic complex and upper subvolcanic suite. It crops out in the northern edge of the district over an area of ~0.2 km2 (Fig. 2). Pre-mineralization and syn-mineralization subvolcanic plutons and magmatic-hydrothermal breccias Porphyry 1 crops out on the eastern ridges of the district at elevations of 4000 m, where it has intruded tuffs of the lower volcanic complex and is cut by Au-(Ag-Cu) epithermal veins (Fig. 2, Table 2). It is the largest porphyry with a surface area of ~1 km2. Phenocrysts of plagioclase (30-60 vol. %, 0.1-7mm), edenite (2-15 vol. %), accessory spinel minerals (―magnetite-ulvöspinel solid solution‖, 1-1.5 vol. %), apatite, and zircon are set in a fine- grained (<40 μm) quartz + feldspar + magnetite groundmass (Fig. 3a). Some plagioclase phenocrysts have compositional zoning documented by cores of oligoclase (An ) with 28-32 FeO concentrations between 0.10 and 0.18 wt. percent (Fig. 4; digital appendix A1), middle sections of oligoclase (An and An ), and an outer edge (light colored; Fig 4a) of 31-34 21-33 andesine (An ) that also exhibits an increase in FeO (0.39 to 0.45 wt.%) content. Other 39-42 crystals show alternating zoning patterns with cores of labradorite (An ) and margins of 64 andesine (An ; Figs. 4b; digital appendix A1). Porphyry 1 has been affected by weak to 39 moderate propylitic alteration and, at the contact with the epithermal veins, by intense argillic and phyllic alteration. Porphyry 2 crops out in the east valley over an area of ~0.5 km2 (Fig. 2) where it has intruded porphyry 1 and the volcanic host rocks. Porphyry 2 has undergone phyllic alteration (quartz + white mica + tourmaline; Fig. 3b). The porphyritic texture has been preserved with relict phenocrysts of plagioclase (50 vol. %, 0.4-3 mm) and zircon set in a groundmass that has been altered to microgranular quartz aggregates (100-200 μm). Porphyry 2 has been cut by a stockwork of early quartz veinlets (2-10 vol. %) and contains Cu-(Au-Mo) mineralization below the oxidation zone. Polymictic matrix supported breccia 1 was intersected by drill-holes in the east valley (Fig. 2), in the contact between porphyry 2 and the volcanic wallrocks. The breccia consists of subrounded fragments (10-70 vol. %) of porphyry 1, porphyry 2, tuff, quartz veins, and plagioclase crystals in a matrix consisting of mud-sized fragments. It has been affected by phyllic alteration and weak Cu-(Au-Mo) mineralization. At shallow levels it has cut porphyry 2 and presents an hydrothermal cement of quartz + tourmaline + pyrite (2-10 vol. %; Fig. 3c). Porphyry 3 crops out on the central ridge (Fig. 2) where occurs as a stock (~0.3 km2) and related dikes that cut tuffs of the lower volcanic complex. Porphyry 3 contains phenocrysts of plagioclase (45-50 vol. %), edenite (3-10 vol. %), biotite (1 vol. %) and accessory quartz, magnetite, and zircon set in a coarse-grained (100-200 μm) hydrothermal quartz + feldspar + biotite groundmass (Fig. 3d). Most of the phenocrysts of plagioclase analyzed have oscillatory compositional zoning with cores of andesine (An ) and 39 intermediate zones of oligoclase (An ; Fig. 5; digital appendix A1). An increase in the 25-31 anorthite molar content (An ) in the outer edge of the phenocryst is accompanied by an 31 increase in FeO (0.25 wt. %; Fig. 5a). Porphyry 3 contains 2-10 volume percent quartz veins and has undergone moderate potassic, sodic, and locally intense phyllic alteration and moderate Cu-Au mineralization. Porphyry 4 crops out on the southern portion of the central ridge (Fig. 2) where it has intruded the volcanic wallrocks. Phenocrysts of plagioclase (60-70 vol. %) have been altered to hydrothermal albite (An ; digital appendix A1) and accessory zircon occurs in a 1-4 groundmass altered to an aggregate (100-400 µm) of hydrothermal quartz. Porphyry 4 has been affected by pervasive potassic alteration at depth and phyllic alteration at shallow levels. It has less groundmass compared to porphyry 3 and a higher proportion of quartz veinlets (10-25 vol. %; Fig. 3e). Porphyry 4 has the highest copper grades of the district and contains Mo and Au mineralization. Polymictic clast- to matrix-supported breccia 2 crops out in the central ridge as an halo around porphyry 4. It also occurs as dikes that have intruded porphyries 3-4 and the volcanic wallrocks. This breccia contains sub-rounded fragments (55 vol. %) of porphyry 3, porphyry 4, tuff, and quartz veins set in a mud-sized matrix. It has been affected by intense phyllic alteration, quartz veining (10-25 vol. %; Fig. 3f), and contains Cu-Au mineralization. At shallow levels in the central ridge the breccia presents an hydrothermal cement of quartz + tourmaline + pyrite (20 vol. %) and cuts porphyry 4. Post-mineralization subvolcanic plutons and magmatic breccia Several post-mineralization intrusions have been recognised based on crosscutting relationships, low metal contents, and weak veining and alteration. An andesite porphyry crops out on the east and north-east ridges and has intruded porphyries 1 and 2 (Fig. 2). This rock has phenocrysts of plagioclase of variable size (40-70 vol. % ), amphibole (1-7 vol. %), accessory quartz (1 vol. %), and fine-grained opaques (<1 mm, magnetite) in a microcrystalline (0.025-0.05 mm) quartz + feldspar groundmass (Fig. 6a). This porphyry differs from the other porphyries because of the different size of the plagioclase phenocrysts and the finer grained groundmass with higher proportion of plagioclase. It is weakly quartz-veined (1-2 vol. %) and has been affected by moderate potassic and propylitic alteration. This intrusion is barren.