PLATINUM GROUP ELEMENTS (PGE) GEOCHEMISTRY OF BAIMAZHAI NI-CU SULFIDE DEPOSIT AND ITS CONSTRAINTS ON THE ORE GENESIS

Abstract

Concentrations of platinum group elements (PGE) of ores and related rocks from Baimazhai Ni-Cu sulfide deposit were determined by using an ICP-MS with nickel sulfide fire assay pre-concentration. The results show that the total PGE (∑PGE) in the Baimazhai ores and the around rocks are quite low, which decreases gradually from central massive ores (78.15 × 10-9 ~ 555.92 × 10-9) to the mineralized peridotite (6.77 × 10-9 ~ 70.95 × 10-9) and gabbro (3.02 × 10-9). The lamprophyre dykes also show low ∑PGE (6.86 × 10-9 ~ 17.03 × 10-9). Besides, the ∑PGE show obvious direct correlation with Ni contents. Primitive mantle normalized PGE patterns of various rocks and ores from Baimazhai mine are quite different, the massive ores possess Pt-Pd type with relatively Ir depleted patterns, while the mineralized peridotite show relatively flat with positive Ru patterns. The gabbro and lamprophyre dykes show nearly same PGE patterns with marked positive Ru anomaly. Cu/Pd ratios of the Baimazhai rocks and ores are greatly higher than that of the primitive mantle, implying that the parental magmas might have undergone strong sulfide segregation. (Pd+Pt)/(Os+Ir+Ru) and Pd/Ir ratios of the massive ores are 138.24 and 24.7, indicating that its parental magma is of gabbroic or basaltic and underwent exhaustively complete sulfide segregation. On the contrary, the corresponding ratios of the mineralized peridotite are 3.02 and 16.09 respectively, which are similar to those of komatiitic magma, implying that the massive ores has no genetic relationship with the mineralized peridotite. Before intrusion of the sulfide liquid of massive ores, its parental magma might have undergone long-term and strong sulfide segregation, and the sulfide liquid might have intruded into the core of the rock body No. 3 of the Baimazhai mine as an isolated intrusive phase. The PGE parameters of Baimazhai Ni-Cu deposit suggest that evolution and mineralization of the late part of the Emei mantle plume might have undergone highly S saturation, and crustal contamination might have played an role during mineralization of the Baimazhai deposit.