Controls on variations of platinum-group element concentrations in the sulfide ores of the Jinchuan Ni-Cu deposit, western China

摘要

Ongoing underground exploration in the giant Jinchuan Ni-Cu sulfide deposit in western China is beginning to emphasize the potential for Cu-, Pt-, and Pd-rich sulfide ores that may have formed by sulfide liquid fractionation. The success of such an effort relies on whether or not fractional crystallization of sulfide occurred in the Jinchuan system. In this paper, we used available PGE data to evaluate such a process. We found that about two thirds of the 126 samples analyzed to date exhibit significant decoupling not only between Pt and Pd but also between Ru, Rh, and Ir. The best explanation for the decoupling is postmagmatic hydrothermal alteration, which affected not only silicates but also sulfides. The effects of postmagmatic alteration must be considered when using metal and isotopic ratios to evaluate primary mineralization. PGE variations in the remaining one third of the samples with Ir/(Ir + Ru) = 0.3–0.7, Ir/(Ir + Rh) = 0.4–0.8, and Pt/(Pt + Pd) = 0.3–0.7 indicate variable R-factors within individual ore bodies as well as the entire deposit, consistent with the interpretation that multiple sulfide-bearing magmas from depth were involved in the formation of the Jinchuan deposit. The mantle-normalized PGE patterns of the least-altered samples from the Jinchuan deposit are similar to the picrite-related Pechenga Ni-Cu sulfide deposit in Russia. PGE variations that can be related to sulfide liquid fractionation are observed in orebody-1 and orebody-24 but not in orebody-2 at Jinchuan. Exploration for Cu-, Pt-, and Pd-rich sulfide ores that may have been expelled into fractures in the footwalls of orebody-1 and orebody-24 appears to be justified.