PETROGRAPHY AND PARAGENESIS OF PLATINUM-GROUP MINERALS IN JINCHUAN ULTRAMAFIC INTRUSION, NORTHWEST CHINA

The Proterozoic ultramafic complex of Jinchuan is located in Gansu Pro vince, northwest China. It forms a dyke-like body composed of sulphide -bearing dunite, lherzolite, plagioclase lherzolite and olivine-bearin g pyroxenite. The olivine composition varies from Fo86 to Fo75, the mo st Mg-rich olivine occurring in the dunite and the most Fe-rich olivin e in the olivine-bearing pyroxenite. Olivine may show up to 5 mol% var iation in forsterite content from grain to grain within a single thin section. The Mg numbers of clinopyroxene and orthopyroxene lie, respec tively, in the ranges 80.9-84.7 and 79.1-84.9. The Mg/(Mg + Fe2+) rati os of the chromite vary from 0.15 to 0.31 and the Cr/(Cr + Al) ratios vary from 0.54 to 0.74. The compositions of olivine, pyroxene and chro mite from the jinchuan ultramafic complex are analogous to those of cu mulate ultramafic rocks of tholeiitic origin, suggesting that the pare nt magma of the jinchuan ultramafic complex may have been tholeiitic, not komatiitic, in composition. The complex contains economically sign ificant Ni-Cu sulphide mineralization. The high sulphide/silicate rati o is best explained in terms of the segregation of sulphides from a la rge volume of magma of which the present complex represents only the u ltramafic cumulate fraction, the remaining parts of the original compl ex now having been eroded. The platinum-group minerals (PGM) that have been identified include sperrylite, moncheite, native platinum, frood ite, merenskyite, michenerite and Pd-rich pentlandite lamellae exsolve d from pyrrhotite. The PGM are characteristically found in close assoc iation with chalcopyrite, either along the contacts between hydrosilic ate and sulphide grains (chalcopyrite, pyrrhotite, pentlandite) or in cracks within pentlandite and chalcopyrite. They may also be intergrow n with galena. These PGM parageneses are interpreted as representing f ormation by crystallization from a Cu-rich residual magmatic liquid in which Pt, Pd, As, Sb, Bi, Te, Zn and Pb were also enriched.