CHARACTERISTICS OF NI-CU-PGE MINERALIZATION AND GENESIS OF THE JINCHUAN DEPOSIT, NORTHWEST CHINA

The jinchuan Ni-Cu sulfide deposit is hosted by a northwest-striking ultramafic body located at the southwest margin of the Sino-Korean platform, China. Sulfide mineralization occurs in the lower part of the intrusion and comprises three main orebodies, 24 and 1 in the west, and 2 in the east of the intrusion. Three types of sulfide ores are identified: net textured, disseminated, and a small amount of massive ore which occurs only in orebody 2. The principal ore minerals are pyrrhotite, pentlandite, violarite, chalcopyrite, and cubanite. A systematic study has been made of the S, Ni, Cu, Co, platinum-group elements (PGE), and Au of the ore samples. A lateral zonation is defined by these elements in the two western orebodies. The concentrations of Ni, Cu, and PGE increase from the margins toward the center of the orebodies. This trend is also present when the metal elements are normalized to the S content of the samples. A vertical variation of elements is present in the east orebody where the amounts of Ni, Cu, and PGE, as well as S, decrease upward. However, when normalized to the S content, PGE increase upward, but Ni and Cu remain more or less constant. A plot of PGE versus S clearly shows three sample sets: (1) samples from orebody 24 have lower S contents, mostly less than 5 wt percent, and their PGE contents roughly increase with that of sulfur; (2) orebody 1 has higher PGE contents, but the PGE plots are more scattered, suggesting multiple processes during PGE mineralization; and (3) orebody 2 has very low PGE concentrations. Ore samples studied show an average Ni/Cu ratio of 2, and (Pt + Pd)/(Ru + Ir + Os) ratios of 5 to 30, with most values falling in the range of 1 0 to 15. These are typical features of sulfide ores that relate to mafic magma. There is little difference in the (Pt + Pd)/Os + Ir + Ru) ratios and the chondrite-normalized PGE distribution patterns between the PGE-rich and PGE-poor ores, suggesting that they originated from magmas with similar major element compositions. The close association of PGE enrichment with sulfide mineralization and the strong correlations exhibited in all ore types between S and Ni, Co, and PGE indicate that the ores are the consequence of the segregation of sulfide from silicate magmas. The high PGE contents of ore-bodies 24 and 1 and the low PGE contents of orebody 2 suggest that two batches of sulfide liquid were involved. The PGE-rich sulfide liquid equilibrated with a relatively primitive silicate magma whereas the PGE-poor sulfide liquid equilibrated with a silicate magma from which PGE had been depleted by prior (probably fractional) sulfide segregation. The low values of Ni/Cu ratios (about 2) and high values of (Pt + Pd)/(Ru + Ir + Os) ratios (10-15) of the samples, together with the chemical composition and rare earth element data of the intrusive rocks provide convincing evidence that the initial magma of the intrusion was gabbroic rather than ultramafic.