ORIGIN OF PLATINUM-GROUP MINERALS IN MAFIC AND ULTRAMAFIC ROCKS: FROM DISPERSED ELEMENTS TO NUGGETS

摘要: Based on the studies of paragenetic assemblages platinum-group minerals (PGM) from placers and chromitite ores associated with zonal-ring dunite intrusions in Aldan Shield, Siberian platform, a novel model for PGM formation is offered, which implies separation Cr-rich oxide liquid significant concentration elements (PGE) picritic magma. The presence alloys sulfide-poor layered zonal gabbro–dunite massifs indicates that PGE mineralization genetically related not just to sulfide but chromite as well. A hypothetical (MgO + FeO CaO) – SiO2 (Cr2O3 Fe2O3 Al2O3) triangular diagram shows fractional crystallization olivine results enrichment residual melt Cr, PGE, range metals, easily fusible volatile elements, magma into light silicate-rich dense PGE–Cr-rich liquid. In fluid-saturated magma, Cr-enriched completely separates silicate melt, its temperature lower, solubility higher. It likely takes form sybotactic domains, clusters surrounded by ligands prevents removal via an untimely micronuggets. Crystallization PGE-, can be traced schematic Os Pt ternary proposed, wide field immiscibility existing between PGE. Owing low chromite, accumulation interstitial With IPGE prevailing over PPGE, trend does reach field, ending synchronous small grains Pt–Ru–Ir–Os alloys. case dominance, reaches gradually accumulates final occurring at triple eutectic point, creates favorable conditions large nuggets tens grams few kilograms weight, are characteristic only PGE–chromite deposits Uralian–Alaskan Aldanian types.