Platinum group minerals in chromitite bodies of the Santa Elena Nappe, Costa Rica: mineralogical characterization by electron microprobe and Raman-spectroscopy

Resumen

Forty-seven grains of
platinum group minerals (PGM) associated with small chromitite
bodies of the Santa Elena ultramafic Nappe (Costa Rica) were
mineralogically investigated with electron microscope, electron
microprobe and Raman spectroscopy. The mineralogical assemblage
includes sulfides of the laurite-erlichmanite series (RuS2 -OsS2 ),
irarsite (IrAsS), osmium, Ir-Rh sulfides containing relevant amounts
of Ni, Fe and Cu, and a Ru-As-S compound, possibly ruarsite (RuAsS).
Most platinum group element (PGE) sulfides and sulfarsenides
represent primary magmatic phases entrapped in chromite at high
temperatures, whereas native osmium is probably formed by subsolidus
exsolution. The lack of primary PGE alloys suggests relatively high
S-fugacity in the chromite forming system. This investigation
emphasizes the efficiency of Raman spectroscopy in the
identification of PGM of extremely small size, and shows how this
technique can be used in revealing distinctive compositional
differences among PGM of the laurite-erlichmanite series and
irarsite