Genesis of platinum-group minerals in the Las Aguilas mafic-ultramafic rocks, San Luis Province, Argentina: Textural, chemical and mineralogical evidence

Abstract

Palladium bismuthotellurides (merenskyite-michenerite-moncheite-melonite) of variable composition and platinum arsenides (sperrylite) are documented in several drill cores of the Las Aguilas mafic-ultramafic intrusion, San Luis province, central Argentina. The mafic-ultramafic rocks ranging in composition from harzburgite to dunite, Iherzolite, norite, and gabbro, intruded an amphibolite facies basement causing locally granulite facies metamorphism. The platinum - group minerals (PGM) identified in these rocks occur in (1) unaltered orthopyroxene-rich, plagioclase- and spinel-bearing rocks which carry abundant base-metal sulphides (BMS), such as chalcopyrite, pentlandite, and pyrrhotite; and in (2) serpentinized olivine-rich zones along cracks, but always associated with BMS, spinel, serpentine, and secondary magnetite. Textural evidence suggests a primary magmatic origin as well as late-stage hydrothermal remobilization, transport, and deposition of the PGM. Geophysical exploration and geological field work have delineated the extent of the Cu-Ni sulphide mineralization in the mafic-ultramafic intrusion underlain by a crystalline basement of Ordovician age. It is estimated that platinum-group-element (PGE) - enriched zones of several meters thickness could be present at depth in the mafic-ultramafic intrusion. Chondrite - normalised plots reveal that the ultramafic rocks have a similar PGE fractionation trend as the Bushveld and Stillwater layered complexes. This contrasts with the ophiolite-type ultramafic associations of the Sierras Pampeanas in Cordoba Province, which occur about 400 km to the northeast of the study area.