GEOCHEMISTRY OF MAGMATIC NI-CU-PGE DEPOSITS

Much of our understanding of the petrogenesis of magmatic Ni-Cu-PGE ores is based on a very limited range of elements. Utilization of a wider range of major (S, Fe, Ni, Cu), minor (Cr, Co, Zn), and trace (Au, Pd, Pt, Rh, Ru, Ir, Os, Pb, Ag, Cd, Hg, Se, Te, Tl, As, Sb, Bi, Mo, Sn, W) elements, which behave differently during mantle melting, magma crystallization, sulfide segregation, sulfide crystallization, metamorphic modification, and hydrothermal modification, provides better constraints on ore genesis. Differences in behaviour are most evident when normalized to abundances in primitive mantle and plotted in order of increasing compatibility during partial melting, rather than in order of melting point with Ni and Cu arbitrarily added to each end of the plots, as normally done. When plotted this way, bulk ore composites, mill feeds, and many disseminated ores exhibit relatively smooth patterns, indicating that they have sampled original magmatic compositions, whereas grab samples, most massive, semi-massive, and breccia ores, and veins do not, indicating that they represent variably modified magmatic compositions. When the bulk compositions of the world's major ore deposits are plotted this way, they define 3 major groups: Group I deposits (e.g., Noril'sk, Sudbury, Duluth, Thompson, Mt. Keith, Kambalda, Perseverance, and Raglan) exhibit slightly to moderately fractionated, but relatively smooth patterns, with the degree of fractionation decreasing with the MgO content of the parental magma. Group II deposits (e.g., Jinchuan, Pechenga, and Voisey's Bay) are depleted in PGE relative to Cu-Ni-Co, a signature of a low magma:sulfide ratio (R factor). Group III deposits (e.g., UG-2, Merensky Reef, and Plat Reef, Great Dyke, and Stillwater) are enriched in PGE, a signature of a high R factor, but are less enriched in Cu and Ni, supporting suggestions that the parental magmas were anomalously enriched in PGE. Variations in the abundances of other metals, semi-metals, and volatile metals of similar primary compatibility are superimposed on these patterns and reflect the influence of late-post magmatic processes, including MSS fractionation, hydrothermal mobilization, and tectonometamorphic mobilization.