PLATINUM POTENTIAL OF NORILSK-TYPE INTRUSIONS

Abstract

Soviet and foreign authors repeatedly have used the distribution patterns of platinum-group elements (PGE) to determine more accurately the origin of ore-bearing igneous rock associations. We analyzed the distribution of PGE in the main rocks and ores of Noril'sk-type intrusions for the same purpose. Results show that the geochemical distribution of PGE reveals the discreteness in their concentrations in ultramafic and mafic rocks, as well as the geochemically different PGE specialization of these rocks. The taxitic gabbro-dolerite layer underlying the picritic is intermediate in content of PGE between the gabbroic and ultramafic rocks. Pyrrhotite ore is more enriched than the chalcopyrite in refractory PGE and, in terms of PGE as a whole, is less fractionated. The identical distribution of PGE and particularly Os, Ir, Pt and Pd in the picritic gabbro-dolerite (disseminated ore) and massive pyrrhotite ore indicates that such dolerite and ore are genetically closely related and initially were associated with ultramafic rocks formed at considerable depths. The concentrations of refractory PGE in Noril'sk-type ultramafic rocks are commensurate with those in concentrically zoned and Alpine-type ultramafics and suggest that both are similar in origin and related to deep mantle layers. The above data may be explained if we assume that PGE were differentiated upon partial melting of the mantle substrate, although their accumulation in the restite was obscured by intensive superposed processes within the corresponding fluid-magma systems.