MINERALOGY AND DISTRIBUTION OF PLATINUM-GROUP MINERALS (PGM) AND OTHER SOLID INCLUSIONS IN THE NEYRIZ OPHIOLITIC CHROMITITES, SOUTHERN IRAN

Abstract

High-Cr podiform chromitites hosted by upper mantle depleted harzburgite and cumulative dunite from crustal sections of the Neyriz ophiolite, southern Iran, were investigated for PGM and other solid inclusions. The chemical composition of chromian spinels, Cr# [100*Cr/(Cr + Al) = 59–83], Mg# [100*Mg/(Mg + Fe 2+ ) = 58–72], TiO 2 ≤ 0.36 wt.%, and the presence of abundant primary hydrosilicates included in the chromian spinels indicate that the deposits were formed from aqueous melt generated by a high degree of partial melting in a suprasubduction zone setting. Solid phases hosted by chromian spinel grains from the Neyriz ophiolite can be divided into three categories: PGM, base-metal minerals, and silicates. Most of the studied PGM occur as very small (generally less than 25 μm in size) primary single or composite inclusions of IPGE-bearing phases associated with or without silicates and base metal minerals. The PGM were divided into three subgroups: sulfides, alloys, and sulfarsenides. Spinel-olivine geothermometry gives the temperatures of 1089–1187 °C for the formation of the studied chromitites. At these temperatures, f S 2 values ranged from 10 −3 to 10 −1 and provided a suitable condition for Ru-rich laurite formation in equilibrium with Os-Ir alloy. Progressive crystallization of chromian spinel was accompanied by increase of f S 2 in the melt. The formation of Os-rich laurite, erlichmanite and then sulfarsenides occurred by increase of f S 2 and slight decrease in temperature of the milieu. The compositional and mineralogical determinations of PGM inclusions with respect to their spatial distribution in chromian spinels show that the minerals are regularly distributed within the chromitites, reflecting cryptic variation consistent with magmatic evolution during host chromian spinel crystallization.