THE LENGSHUIQING Ni-Cu DEPOSIT, SICHUAN, SOUTHWESTERN CHINA: ORE CHARACTERISTICS AND GENESIS

摘要

Lengshuiqing is a Ni-Cu sulfide deposit related to the Neoproterozoic Panxi arc, in southwestern China. The mineralization is associated with five separate but genetically related intrusions. The magmatic rocks from Lengshuiqing range from peridotite and olivine pyroxenite to gabbro, diorite, quartz diorite, granodiorite and granite. The sulfide ore occurs in the ultramafic rocks and in four distinct facies: (1) disseminated to net-textured (the dominant type), (2) blebby (disseminated globular), (3) breccia matrix ore, and (4) rare veins of massive sulfides. We present the first geochemical data on the individual types of ore from Lengshuiqing. The disseminated to net-textured ore has Ni/Cu = 3, and average Ni tenor of 4.9%. The fractionation of monosulfide solid-solution induced the occurrence of high Ni tenors in the massive ore (6-8% Ni). The PGE tenors of all ore types are less than 1 ppm. The mantle-normalized values of the chalcophile elements show PGE depletion relative to Ni and Cu, which can be explained by a pre-emplacement equilibration of the magma in the presence of immiscible sulfides. The IPGE and Pt are depleted relative to Pd and Rh, suggesting the retention of some PGE as alloys in the mantle source. The formation of the ore is explained in conjunction with the conduit-related origin of the host intrusions. Sulfide immiscibility probably occurred in a staging chamber as a result of the assimilation of Si02-rich crustal material. The segregated sulfide droplets, together with olivine crystals, were transported to the shallower intrusions at Lengshuiqing, which were likely to have been wide magmatic conduits. The decrease of magma velocity in the intrusions at Lengshuiqing determined the accumulation of sulfide melt and olivine crystals, and the formation of the disseminated to net-textured ore. Late pulses of volatile-rich magma produced breccias and led to the spatial redistribution of the sulfide melt, which generated breccia matrix ore.