Origin of Platinum Group Minerals (PGM) Inclusions in Chromite Deposits of the Urals

Abstract

This paper reviews a database of about 1500 published and 1000 unpublished microprobe analyses of platinum-group minerals (PGM) from chromite deposits associated with ophiolites and Alaskan-type complexes of the Urals. Composition, texture, and paragenesis of unaltered PGM enclosed in fresh chromitite of the ophiolites indicate that the PGM formed by a sequence of crystallization events before, during, and probably after primary chromite precipitation. The most important controlling factors are sulfur fugacity and temperature. Laurite and Os–Ir–Ru alloys are pristine liquidus phases crystallized at high temperature and low sulfur fugacity: they were trapped in the chromite as solid particles. Oxygen thermobarometry supports that several chromitites underwent compositional equilibration down to 700 °C involving increase of the Fe3/Fe2 ratio. These chromitites contain a great number of PGM including—besides laurite and alloys—erlichmanite, Ir–Ni–sulfides, and Ir–Ru sulfarsenides formed by increasing sulfur fugacity. Correlation with chromite composition suggests that the latest stage of PGM crystallization might have occurred in the subsolidus. If platinum-group elements (PGE) were still present in solid chromite as dispersed atomic clusters, they could easily convert into discrete PGM inclusions splitting off the chromite during its re-crystallization under slow cooling-rate. The presence of primary PGM inclusions in fresh chromitite of the Alaskan-type complexes is restricted to ore bodies crystallized in equilibrium with the host dunite. The predominance of Pt–Fe alloys over sulfides is a strong indication for low sulfur fugacity, thereby early crystallization of laurite is observed only in one deposit. In most cases, Pt–Fe alloys crystallized and were trapped in chromite between 1300 and 1050 °C. On-cooling equilibration to ~900 °C may produce lamellar unmixing of different Pt–Fe phases and osmium. Precipitation of the Pt–Fe alloys locally is followed by an increase of sulfur fugacity leading to crystallize erlichmanite and Ir–Rh–Ni–Cu sulfides, occurring as epitaxic overgrowth on the alloy. There is evidence that the system moved quickly into the stabilization field of Pt–Fe alloys by an increase of the oxygen fugacity marked by an increase of the magnetite component in the chromite. In summary, the data support that most of the primary PGM inclusions in the chromitites of the Urals formed in situ, as part of the chromite precipitation event. However, in certain ophiolitic chromitites undergoing annealing conditions, there is evidence for subsolidus crystallization of discrete PGM from PGE atomic-clusters occurring in the chromite. This mechanism of formation does not require a true solid solution of PGE in the chromite structure.