Hydrothermal origin of platinum-group minerals during serpentinization of the podiform chromitites from the Kızıldağ ophiolite in southern Türkiye

Abstract

Platinum-group minerals (PGMs) in podiform chromitites usually occur in the interior and/or edge of chromite. However, the origin of PGMs in podiform chromitites has long been a matter of debate. Here we examined sub-micro to nanoscale textural features, morphologies, and compositions of PGMs from the disseminated, banded, massive and nodular chromitites in the Kızıldağ ophiolite in southern Türkiye, and found both primary and secondary PGMs. The aim of this study is to reveal the transformation processes from primary to secondary PGMs, thereby taking a thorough examination of the origin of these PGMs. Primary PGMs include laurite and Os-Ir alloy, which are prevalent in all samples. They are typically enclosed within or located at the edge of chromite, and formed either prior to or contemporaneously with the crystallization of chromite at temperature of 1100–1200 °C and logƒS2 values of -2 to -1. In contrast, PGE-bearing pentlandite are commonly present at the edge of chromite, corresponding to an increase of fS2 with the progressive crystallization of chromite. These primary PGMs and PGE-bearing pentlandite in the intergranular space of chromite are susceptible to alter and transform into secondary PGMs and base metal mineral assemblages, which include Os-Ru nanophases (Os-Ru nanoparticle and OsRu3 nanoalloy) + awaruite (FeNi3) + trevorite (Fe2NiO4) in nodular chromitite, Os-rich laurite + Os-Ir(Ru) alloy/oxide + pentlandite + millerite (NiS) in banded and massive chromitite, and Ru(Ir) oxide + heazlewoodite (Ni3S2) in disseminated chromitite. The development of these diverse assemblages can be attributed to the degrees of serpentinization of chromitites. The nodular chromitite underwent weak serpentinization and had low water/rock ratios (<∼1), fS2 and fO2, leading to the conversion of IPGE (Os, Ir and Ru)-bearing pentlandite into Os-Ru nanoparticle- and OsRu3 nanoalloy-bearing awaruite. The massive and disseminated chromitites had high water/rock ratios and high fS2 and fO2 relative to those of the nodular chromitite during serpentinization, and consequently the corresponding pentlandite was transformed into heazlewoodite and/or millerite associated with S loss. Meanwhile, Ir, Os and possibly Ru were released from laurite to form Os-Ir(Ru) alloy/oxide at the edge of laurite. Our observation highlights that primary PGMs and pentlandite in the chromitites of the Kızıldağ ophiolite have been modified under different physical-chemical conditions during serpentinization, resulting in the formation of complex secondary PGMs and base metal mineral assemblages.