Differential platinum group elements (PGE) re-mobilization at low fS2 in Abdasht and Soghan mafic-ultramafic complexes (Southern Iran)

The Abdasht and Soghan ultramafic complexes, Southern Iran, host major actively exploited chromitite bodies, variably enriched in PGE. The Platinum Group Minerals (PGM) – Base Metal Minerals (BMM) assemblage was studied in order to assess PGE remobilization during post-magmatic processes. Studied chromitites have variable textures: massive, banded, nodular and disseminated. Primary silicates are almost completely replaced by serpentine. Ferrian chromitization is present but not widespread. BMM are mostly found in the silicate matrix, while PGM are more common as inclusions within chromites. Primary BMM (pentlandite and bornite) within the silicate matrix are poorly preserved, while primary PGM (laurite) are more abundant. Secondary BMM (heazlewoodite, millerite and Ni[sbnd]Fe alloys) and PGM (PGE-alloys) are the dominant mineralogical species. During serpentinization the pervasive percolation of low fS2 fluids induced the desulfurization of the metallic assemblage. Primary sulfides were replaced by S-poor (heazlewoodite) or S-free (Ni[sbnd]Fe alloys, PGE alloys, PGE-BM alloys) phases. Low fS2 differentially mobilized PGE. The release of IPGE (Ir, Os, Ru) into the fluids follows the order: Ru> > Os>Ir. PPGE (Pt, Pd, Rh) mobility is more difficult to assess, due to their small concentration, but the evidence suggests that Rh is more mobile than Ru, and that Pd and Pt were partially added to the system by fluids. A mass balance calculation on a polyphasic grain, showing differential stages of desulfurization, allowed a semi-quantitative analysis on the desulfurization degree of Abdasht and Soghan complexes, estimated, in the more serpentinized portions, between 75 and 100%.