Fe Skarn And Iocg-Type Deposits In The Andes Cordillera Of Sw Mendoza, Argentina,

FRANCHINI, M., DEBARRIO, PONS, J., R., MEINERT, L., RÍOS, J., AND SCHALAMUK, I.,

EXPLORATION AND MINING GEOLOGY

The Geological Society of Canadain Institute of Mining, Metallurgy and Petroleum

Lugar: Montreal, Canada; Año: 2007 vol. 16 p. 233 - 265

0964-1823

Abstract ? Several Fe, Fe-Cu, and Cu-Ag prospects hosted by Mesozoic carbonate-rich sedimentary rocks that were intruded by diorite stocks define an approximately 20 × 200 km belt along the Andes Cordillera of southwest Mendoza province, Argentina (34°?36°S). This belt includes the Hierro Indio and Vegas Peladas Fe skarns, Las Choicas, and several other Cu-(Ag) prospects. The main features of the Fe skarns include: association with mantle-derived middle Miocene (~15?11 Ma) diorite stocks and sills; widespread alteration including epidote ± amphibole ± magnetite endoskarns, and zoned garnet (Grs0?66 Adr32.5?100) ± magnetite ± pyroxene (Di24?50 Jo2?9 Hd74?41) exoskarns formed from oxidized, saline, high-temperature brines (530º?660ºC; 60?70 wt.% NaCl equiv.); and magnetite-hematite orebodies associated with quartz ± epidote ± calcite ± actinolite formed at lower temperatures (290º?436ºC) from saline fluids (32?50 wt.% NaCl equiv.) of magmatic origin. West of the Fe skarn belt, the Valle Hermoso district covers more than 300 km2 and hosts several poorly known Cu-(Ag) prospects (estimated 30 Mt with 1.5% Cu and 20 g/t Ag). These prospects share many characteristics with the iron oxide copper-gold-type systems and manto-type Cu deposits of northern and central Chile. One of the prospects (Las Choicas) is located at the apex of a regional 015°-trending anticline, along the contact of stratified calcareous sandstone and limestone (Neocallovian-Oxfordian) with a diorite pluton. This pluton has positive Ta and Nb anomalies (on normalized diagrams) and a high Ti content, features uncommon in Miocene diorites associated with Fe skarns and of magmas derived from sub-arc mantle sources. Alteration at Las Choicas includes early widespread chlorite + calcite ± albite ± scapolite ± ilmenite or titanite ± apatite, and local actinolite ± hematite ± biotite alterationof diorite and sedimentary rocks along contacts. Later Cu mineralization (1%?10% Cu) is associated with calcite ± albite ± quartz alteration in fractures, vein networks, and crackle breccias. Mineralization is zoned from bornite ± chalcopyrite ± millerite with Zn, As, Mo, Ag, and U anomalies in the main orebody, through chalcopyrite-rich breccias with Zn anomalies, and chalcopyrite ± tetrahedrite vein networks with Sb, As, Ag, and Zn anomalies in the uppermost zone, to a Cu-depleted pyrite-rich envelope with Co anomalies, and finally to late, barren calcite veins in distal zones.