The Altar Porphyry Cu-(Au-Mo) Deposit (Argentina): A Complex Magmatic-Hydrothermal System with Evidence of Recharge Processes

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 southwest 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 premineralization porphyry (11.75 ± 0.24 Ma), three mineralized porphyries (11.62 ± 0.21 and 11.68 ± 0.27 Ma, 11.13 ± 0.26 Ma, 10.35 ± 0.32 Ma) related to hydrothermal breccias, two postmineralization intrusions, and a postmineralization breccia (8.9 ± 0.4 Ma). The three mineralized porphyries (porphyries 2, 3, and 4) were emplaced within ~0.7 to 1.3 m.y. Amphibole phenocrysts from the porphyries crystallized from oxidized magmas (fO2 = NNO +1 to +2) at temperatures of 780° to 850°C and pressures between 0.9 and 1.8 kbars corresponding to depths of ~4 to 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

Apatite (U–Th)/He thermochronology and Re–Os ages in the Altar region, Central Andes (31°30′S), Main Cordillera of San Juan, Argentina: implications of rapid exhumation in the porphyry Cu (Au) metal endowment and regional tectonics

“…Zircon U-Pb crystallization ages of the mineralized porphyries (12.3 to 10.5 Ma) together with the Re-Os molybdenite ages (11.8 to 10.1 Ma) in the Los Pelambres-Frontera system (Bertens et al 2003(Bertens et al , 2006Perelló et al 2012, Fig. 6a) are similar to those of the Altar porphyries and mineralization (Maydagán et al 2014; this study). Available Re-Os molybdenite ages from El Pachón deposit are, however, younger (9.16 and 8.43 Ma, Bertens et al 2006, Fig.…”

“…These ages are nominally different even when including full analytical and decay constant uncertainties (Table 1). As such, although only two ages, the Re-Os molybdenite data suggest that the Altar East and Altar Central deposits were associated with two magmatic hydrothermal centers, as suggested by Maydagán et al (2014) that were active during two different time periods (Fig. 5a).…”

“…The subvolcanic intrusions of the Altar porphyry deposit were emplaced at depths of 3.5-5 km based on the following evidence: 1) amphibole phenocrysts from porphyry 1 (Altar East) are estimated to have crystallized at 800 °C and pressures between 0.9 and 1.2 kbar that reflect depths of ~4 km for pluton emplacement (Maydagán et al 2014); 2) the early quartz veins have fluid inclusions of intermediate density (Maydagán et al 2015) Considering the temporal data together with the available geological and metallogenetic information of the Altar system, we interpret that the rapid cooling recognized in the Altar intrusions reflects an exhumation pulse that occurred immediately after the emplacement of the subvolcanic intrusion, and that the cooling and related exhumation was contemporaneous with the hypogene mineralization stage of the hydrothermal system. An alternative interpretation for the (U-Th)/He ages of the subvolcanic stocks sampled close to the porphyry deposit (A-22, A-16, A-17, Fig.…”

“…Cristallini and Ramos 2000;Giambiagi et al 2003). Crustal thickening during uplift of the Frontal Cordillera and eastward migration of the magmatic front seems to have created the favorable conditions for the generation of water-rich, high Sr/Y "adakitic" magmas at Altar and Los Pelambres (Perelló et al 2012;Maydagán et al 2014;Bergoeing 2016) whose presence has been shown to be critical for the formation of most of the porphyry copper deposits around the world (Kay and Mpodozis 2001;Chiaradia et al 2012;Richards 2011).…”

Geology of the Josemaría Porphyry Copper-Gold Deposit, Argentina: Formation, Exhumation, and Burial in Two Million Years

“…The age of older volcanic rocks west of the Los Helados fault is uncertain, but between Paleocene and Oligocene. Deposit age assignments based on Sillitoe et al (1991Sillitoe et al ( , 2013Sillitoe et al ( , 2016, Kay et al (1994), Mpodozis et al (1995), Bissig et al (2001), Mpodozis and Kay (2003), Perelló et al (2012), Maydagán et al (2014), Cáceres (2015), Y. Kapusta in Rode et al (2015), Yoshie et al (2015), Holley et al (2016), Astorga et al (2017), and NGEx Resources Inc. (unpub. data).…”