The Altar porphyry Cu-(Au-Mo) deposit (Argentina): A complex magmatic-hydrothermal system with evidence of recharge processes

Abstract

Altar (31° 29' S, 70° 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 middlelate 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.