The Fe Skarn of Vegas Peladas: Geologic and mineralogic characteristics and distribution of the mineral paragenesis

Abstract

The Vegas Peladas iron prospect (Cordillera Principal, SW de Mendoza) is hosted in Jurassic marine sedimentary siliciclastic (Puchenque Formation), calcareous (Calabozo Formation) and evaporitic (Auquilco Formation) rocks affected by Neogene magmatism (15.19 ± 0.24 Ma, Rb-Sr whole rock) and tectonism. Four intrusive events are associated with hydrothermal alteration and mineralization. Two of them produced metasomatic Fe skarn. The most important and oldest alteration event is an Fe skarn associated with a diorite pluton that contains an aureole of hornfels and marble and a zoned prograde skarn with an oxidized mineralogy rich in garnet (And_31-100Py_0,34-0Grs_68-0) and subordinate clinopyroxene (Di_24-70Jo_4,1-0,7Hd_72-29,3). The diorite has an incipient and selective alteration (actinolite ± chlorite ± calcite ± titanite ± orthoclase ± epidote ± pyrite (magnetite)) and its margins are replaced by massive and irregular orthoclase + quartz alteration. Retrograde assemblages replace early alteration and are rich in epidote and amphibole. The mineralization consists of magnetite (83 to 88% FeO_total) in the diorite and in the inner exoskarn zone, and hematite-mushketovite (93.5 to 95% FeO_total) in the intermediate and outer exoskarn zones. Iron oxides are in equilibrium with retrograde minerals. The second metasomatic event is associated with granite emplacement. It formed a zoned skarn rich in scapolite (Me_25-36), garnet (And_10-81Py_0,72-0Gr_89-19) and pyroxene (Di_42-93Jo_2,6-1,7Hd_54,8-5,1) overprinted by retrograde alteration and scarce iron mineralization, probably remobilized from the earlier skarn. The composition and distribution of the mineral assemblages in the Vegas Peladas district can be useful guides for Fe skarns exploration in other regions of SW Mendoza.