DIVERSITY OF PGE DEPOSITS IN BASIC-ULTRABASIC INTRUSIVES - SINGLE MODEL OF FORMATION

摘要

Three types of Pt and Pd deposits are defined on the basis of host-rock association and nature of the predominant platinum group minerals (PGM) in basic-ultrabasic intrusions: oxide, sulphide, and silicate associations. (1) The oxide association comprises Pt- and Pd- rich chromitites hosting PGE alloys and/or sulphides. The Pt/Pd ratio is usually above 1; extreme Pt fractionation (ratio up to 60) occurs in alloy-type (isoferroplatinum) chromitite having PGM included in chromite. (2) The sulphide association comprises most of the economic platinum group elements (PGE) deposits. There is a great variety of PGM comprising sulphides, arsenides, antimonides, bismuthides, tellurides, intermetallic compounds and alloys. These PGM are associated with disseminated interstitial base-metal sulphides (BMS) (< 2%) in reef-type deposits (Merensky Reef and J-M reef of the Bushveld and Stillwater complexes, respectively), or with massive and zoned marginal-type BMS deposits (Norilsk, Sudbury). Significant Pd contents are largely controlled by pentlandite (Pt/Pd about 0.3 in pentlandite-type ore). Extreme Pd fractionation is observed in late Cu-rich ore (Pt/Pd < 0.3). (3) The silicate association is dominated by various PGM assemblages which occur in interstices of silicate rocks (dunite, pyroxenite, gabbro) devoid of significant concentration of chromite and BMS. The Pd/Pt ratio is variable - about 1 for arsenide-type deposits. The three rock-type associations occur at distinct ore horizons forming a 'critical zone' in layered, zoned or ophiolitic complexes. The strikingly diverse stratigraphic distribution of PGE reflects the distinct paragenetic assemblages which evolved as temperature decreased from magmatic to hydrothermal conditions. A fluid-driven multistage process is responsible for the successive deposition of Pt- and Pd- barren chromite deposits at depth, Pt-rich chromitite in the uppermost chromite layers, Pt- and Pd- rich sulphide concentration and late PGE-rich silicate rocks. The roie of the intercumulus melt is particularly significant for the concentration and deposition of noble metals.