COMPOSITION AND MINERALOGY OF PGE-RICH CHROMITITES IN THE NURALI LHERZOLITE-GABBRO COMPLEX, SOUTHERN URALS, RUSSIA

Abstract

We have investigated two subeconomic bodies of chromitite in the Nurali Iherzolite-gabbro complex, in the southern Urals, Russia, with regard to the composition of the chromian spinel and the distribution and mineralogy of the platinum-group elements (PGE). The bodies of chromitite, referred to as CHR-I and CHR-II, occur as small concordant lenses located at two stratigraphic levels within layered wehrlite and clinopyroxenite, overlying the Iherzolitie mantle tectonite. The chromian spinel is Al-rich, showing an increase of Cr/(Cr + Al), Fe2+/(Fe2+ + Mg) and TiO2 from CHR-I to CHR-II. The total PGE contents vary from 1.26 to 11.61 ppm, and show increase in (Pt + Pd)/(Os + Ir + Ru) from 0.1 to 52.2 as a result of the appearance of magmatic sulfides in the upper chromitite. The PGM assemblage shows a drastic change from laurite-erlichmanite-dominated to enriched in Pt Pd sulfides and alloys. Laurite is the first PGM to crystallize, and its composition typically reflects the Ru/Os ratio of the primitive mantle, indicating that the parent melt of the chromitite did not undergo fractionation during ascent. The Nurali chromitites are rather unusual as they have characteristics in common with chromitites associated with ophiolitic cumulates, layered intrusions, Alaskan-type complexes, and the subcontinental orogenic mantle.