Segregation and Fractionation of Magmatic Ni-Cu-PGE Sulfides in the Western Jinchuan Intrusion, Northwestern China: Insights from Platinum Group Element Geochemistry

摘要

The Jinchuan Ni-Cu-(PGE) deposit is one of the world's largest magmatic sulfide deposits. Recent studies have suggested that the Jinchuan intrusion originally comprised two individual intrusions, the Eastern and the Western intrusions. The Western intrusion consists of an Upper and a Lower unit. The Upper unit is largely free of sulfides with wealdy disseminated sulfides only at the base of the unit; these sulfides are characterized by moderate Cu/Pd ratios of 5,500 to 14,900 and relatively high PGE tenors (PGE in 100% sulfides) in the range of 510 to 1,000 ppb Ir_(sul), 420 to 1,200 ppb Ru_(sul), and 2,900 to 8,000 ppb Pd_(sul). Disseminated, net-textured and minor massive sulfides in the Lower unit comprise the No. 24 orebody, the third largest at Jinchuan. The disseminated ores have remarkably high Cu/Pd ratios (24,200-85,600) and low PGE tenors in the range of 120 to 560 ppb Ir_(sul), 100 to 480 ppb Ru_(sul), and 430 to 3,600 ppb Pd_(sul). The net-textured and massive ores contain high IPGE contents (240-820 ppb Ir_(sul) and 170-630 ppb Ru_(sul)) and Ni/Cu ratios (1.7-11.3), low Pd/Ru ratios (0.53-3.1), and exhibit significant Pt depletion (<300 ppb Pt_(sul)). Additionally, small Cu-rich orebodies occur at the base of the No. 24 orebody or in small intrusions within footwall schists, marbles, and granites beneath the Western intrusion. The Cu-rich sulfides have very low IPGE tenors (1.9-16 ppb Ir_(sul) and 6.4-21 ppb Ru_(sul)), and Ni/Cu ratios (0.3-2.8), high Pd/Ru ratios (37.3-378), and highly variable Pt_(sul) between 43 and 11,000 ppb. The PGE geochemistry and model calculations indicate that the disseminated sulfides of the Upper and Lower units segregated from PGE-undepleted and -depleted parental magmas, respectively. The PGE-depleted magma had undergone a minor amount of earlier sulfide liquid extraction at depth. Furthermore, this PGE-depleted parental magma had slightly higher PGE contents than the magma from which the sulfides of the Nos. 1 and 2 orebodies in the Eastern intrusion segregated. The significant variations in base and precious metal ratios of the net-textured and massive sulfide ores are attributed to fractionation of the sulfide melts that formed these ores. This led to both migration of the residual Cu-rich sulfide melts away from the early-formed monosulfide solid solution (mss) to form Cu-rich ores as well as back intrusion of the Cu-rich ores into the mss. The negative Pt anomalies of the net-textured and massive sulfides resulted from preferential extraction of Pt into the Cu-rich residual melts due to the presence of As, Te, and Bi.