Intrusion history of the Altar porphyry Cu-(Mo-Au) deposit (Argentina): a complex magmatic-hydrothermal system with evidence of recharge processes

LAURA MAYDAGÁN; FRANCHINI, MARTA; MASSIMO CHIARADIA; JOHN DILLES; ROGER REY

ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS

SOC ECONOMIC GEOLOGISTS, INC

Lugar: Litletton; Año: 2012

0361-0128

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. 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 magmatic-hydrothermal breccias, two post-mineralization intrusions and a post-mineralization magmatic breccia (8.9 ± 0.4 Ma). These LA-ICP-MS U-Pb ages of zircon from the intrusions suggest a period of magmatic activity of ~1.5-2 Ma. We propose that the middle-late Miocene Altar porphyries (pre-, syn- and postmineralization) were derived from a single magma chamber or from several similar magma chambers located in the upper crust that experienced both episodic magmatic recharge events by a less evolved magma and incipient magmatic differentiation. The Injection of less evolved magmas into the chambers may have provided heat to allow long-lived magmatic activity, and also led to episodical volatile saturation and porphyry ore formation. The hydrous and oxidized parental magmas of the ore-related intrusions may have previously evolved at deep crustal levels by MASH (mixing, assimilation, storage and homogeneization) processes and hydration. These processes may explain the "adakite-like" signatures recognized in Altar porphyries and in other fertile porphyries of the southern Andean flat-slab segment that were emplaced in temporal and spatial coincidence with periods of increased compression in the crust.