Fe Skarn, Iron Oxide-Copper-Gold, and Manto Cu-(Ag) Deposits in the Andes Cordillera of Southwest Mendoza Province (34–36°S), Argentina

MARTA B. FRANCHINI1-2, RAÚL E. DE BARRIO3, MARÍA JOSEFINA PONS1-2, ISIDORO B. SCHALAMUK1-3, FRANCISCO JAVIER RÍOS4, AND LAWRENCE MEINERT5

EXPLORATION AND MINING GEOLOGY

J. Richards

Año: 2007 vol. 16 p. 233 - 265

0964-1823

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 km by 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, and Las Choicas and several other Cu-(Ag) prospects.             The main features of the Fe skarns include: association with mantle-derived middle Miocene (~15 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 20g/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 the Miocene diorite 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 alteration of 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 ore body, through chalcopyrite-rich breccia with Zn anomalies, and chalcopyrite ± tetrahedrite vein network 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.