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, lherzolite, 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.