CONCENTRATIONS OF PLATINUM-GROUP ELEMENTS IN CHROMITITES

Abstract

Average concentrations of platinum-group elements (PGE) are reported for the stratiform chromitites of layered massifs and the podiform chromitites of ophiolitic and zonal massifs of the Ural-Alaska type. The level of PGE concentrations in all genetic types of chromitites is one-two orders of magnitude higher than the analogous values for genetically related ultramafic rocks. Higher and lower PGE concentrations occur in, respectively, stratiform and podiform chromitites. All chromitites are characterized by strong PGE fractiunation in terms of both their concentrations and the character of their distribution. The PGE proportions of stratiform chromitites of layered massifs vary from Pt-dominated in the UG-2 unit of the Bushveld and Imandra massifs, Pd-rich in the Unit A of the Stillwater Massif and the Sopchinskii Unit of the Monchegorsk Massif, and Ru-Os-Ir-dominated in the Main Chromite Unit of the Burakovskii Massif to mixed types in oilier chromitite units of the Stillwater and Bushveld massifs. Podiform chromitites in ophiolitic massifs and tectonized ultramafic rocks are enriched in Os, Ru, and Ir, and the analogous rocks in the layered complexes of these massifs bear PGE dominated by Pd and Pt. Chromitites hosted by zonal massifs of the Ural-Alaska type commonly have Pt-dominated PGE, with the single exception of the Guli Massif, whose PGE are dominated by Ir and Os, as is typical of podiform chromitites of ophiolitic massifs.