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: 2014 vol. 109 p. 621 - 641

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 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 (fO2= 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.