ASSOCIATION OF PGE MINERALS IN MASSIVE CHROMITE ORES OF THE KEMPIRSAI OPHIOLITIC COMPLEX (THE SOUTHERN URALS): EVIDENCE FOR MANTLE METASOMATISM

摘要

Kempirsai ultramafic massif is located in the southernmost Urals as part of the Great Ophiolite Belt of Ural. The massif has become famous by its gigantic chromite deposits. Recently two types of PGE (platinum group elements) mineralizations included in the Cr-spinels, have been found within the massive chromitites. This study predominantly aims on the investigation of the so-called "secondary" PGM association and its origin. Apart from a suggested primary PGM (platinum group mineral) association represented by Os-Ir-Ru alloys and sulphides, the secondary assemblage is formed by Ir-Ni, Ni-Ir and Ir-Cu sulphides and sulpharsenides. Based on microprobe analyses, these PGM phases are compositionally characterized by (Ni, Cu, Fe) : (Ir, Os, Ru, Rh) : S = 1:2:4. This base-metal/PGE/sulphur ratio indicates a thiospinel structure of these PGM similar to natural spinels (Rudashevsky e. a., 1985). Abundant silicate inclusions such as amphibole, phlogopite, serpentine, chlorite, as well as H_2O-CO_2-dominated fluid inclusions in close association with secondary PGM inclusions, clearly indicate the importance of fluids in the course of PGM-forming processes. Geochemical data, as evidenced by enrichment of LREE (light rare earth elements) in dunites enveloping the massive chromitites hosting the PGE mineralization, as well as relatively high oxygen fugacities in these chromitites, are considered further support for the significant role of fluids. Mantle metasomatism as one possible fluid-dominated process is discussed.