Iron Skarns of the Vegas Peladas District, Mendoza, Argentina

Abstract

The Andean belt southwest of Mendoza, Argentina, hosts 23 Fe, Fe-Cu, and Cu (Ag) deposits classified variously in the literature as skarn, iron oxide-copper-gold (IOCG), and manto-type Cu deposits. The Vegas Peladas deposit is one of the best exposed Fe skarns with mineral assemblages and hydrothermal features similar to many other calcic Fe skarns of the world. The plutonic rocks of Vegas Peladas consist of a series of diorite to granite stocks, dikes, and sills. The major-, trace-, and rare earth-element geochemistry analyses of these igneous rocks indicate they were derived from subarc mantle sources. The Vegas Peladas deposit formed by the overprinting of two different metamorphic and metasomatic events associated with early diorite and later granite intrusions. Alteration associated with the early diorite intrusion consists of a metamorphic halo (800 m wide) and a zoned calcic skarn with inner garnet (Ad31–89 Py0–2) + clinopyroxene + magnetite + quartz, intermediate garnet (Ad38–51 Py1–2) ± clinopyroxene, and distal veins of garnet (Ad92–100) ± clinopyroxene (Hd72–29 Jo1–4). The latest alteration consists of widespread albite (Ab96–98) ± epidote ± quartz ± calcite ± chlorite ± pyrite ± titanite. Magnetite and hematite are the main iron ore minerals and occur as massive orebodies and veins associated with retrograde epidote and amphibole. Alteration of the diorite consists of early orthoclase + quartz followed by later amphibole ± quartz ± magnetite ± epidote ± feldspar. The granite-related skarn overprints the earlier diorite-related skarn and consists of garnet + clinopyroxene + scapolite (Me28–36) ± quartz ± alkali feldspar endoskarn and a zoned exoskarn with proximal garnet ± clinopyroxene ± quartz, intermediate green garnet (Ad30–81 Py0–1) + clinopyroxene (Di82–93 Jo4–2), and distal scapolite (Me25–36) ± ferroactinolite ± pyrite veins. Based on fluid inclusion, stable isotope, and REE data, the prograde skarn formed at depths of ~ 3.5 km under lithostatic pressure of ~1 kbar, from high temperature (670°–400°C), saline and iron-rich (>50 wt % NaCl equiv, NaCl ± KCl ± FeCl2) magmatic fluids (garnet δ18OH2O = 7.2–8.5‰) with intermediate oxygen fugacity. Iron ore and retrograde exoskarn assemblages formed under hydrostatic condition after the fracturing of early skarn. Fluids in this stage had lower temperature (T 550°C) and also generated saline (30.3 to 45.3 wt % NaCl equiv, H2O-NaCl-FeCl2) + vapor fluids by immiscibility that redistributed some of the iron from the previous skarn.