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Lithologic and Geochemical Constraints on the Genesis of a Newly Discovered Orebody in the Jinchuan Intrusion, NW China


 The Jinchuan Ni-Cu-platinum group element (PGE) sulfide deposit is one of the world’s major magmatic sulfide deposits. The Jinchuan intrusion originally consists of two independent bodies, named the Western and Eastern intrusions. Recently, an underground exploration program discovered a large economic orebody in the Fine-Grained unit of the Western intrusion, independent of the orebody No. 24 that occurs at the base of the Coarse-Grained unit. The newly discovered orebody dips to the southwest with thicknesses of 20–150 m and is composed of olivine-sulfide cumulates with net-textured sulfide overlain by lherzolite with disseminated sulfide. The net-textured olivine-sulfide cumulates have average grades of 2.1 wt % Ni and 1.2 wt % Cu, whereas the disseminated ores in the lherzolite have average grades of 0.6 wt % Ni and 0.4 wt % Cu. This makes this orebody, named orebody No. 3 and hosted in segment III, the fourth largest at Jinchuan. The sharp lithologic and chemical variation between the net-textured olivine-sulfide cumulates and the disseminated sulfide-bearing lherzolite indicates that they were formed by different processes. Evident differences in sulfidefree whole-rock compositions reflect the dunite forming as olivine adcumulate and the lherzolite as olivine orthocumulate with variable trapped liquid abundances. The olivine-sulfide cumulates were formed by percolation of sulfide liquid into the olivine adcumulates, expelling the interstitial silicate melts upward, whereas the disseminated ores in the lherzolite were formed by settling of finely dispersed sulfide droplets in the olivine crystal mush, within which pyroxene crystallized from the interstitial melts. Variations of PGE tenors of the net-textured sulfides demonstrate fractionation of the sulfide liquids. The relatively high PGE tenors of the disseminated sulfides compared with the net-textured sulfides are associated with upgrading of the sulfides due to reaction with fresh magma.