Geology And Geochemistry Of Ore Body~#1of The Jinchuan Ni-Cu Sulfide Deposit, Gansu Province

The Jinchuan Ni-Cu-(PGE) sulfide deposit is hosted by an olivine-rich ultramafic intrusion in northwestern China. It is the third-largest Ni deposit in the world. The intrusion contains three main Ore Bodies numbered24,1, and2from NW to SE. It is distinguished from other deposits of this type by a very large amount of sulfide ores (up to47vol%at～1.2wt%Ni) hosted in a relatively small intrusion. Most of the intrusion is composed of ultramafic rocks such as dunite, harzburgites, lherzolites, and plagioclase lherzolites. Rare wehrlites and gabbros occur along the margins of the intrusion. The Ni-Cu-(PGE) ores occur as net-textured and disseminated sulfides, plus minor massive sulfide, but the ore types vary between the different ore bodies.The central part of the Jinchuan intrusion(Ore Body1) has a "flame-like" shape in cross section and hosts～56.8%of the known Ni resources. It is characterized by concentrically zoned, with net-textured ore surrounded by disseminated sulfide, barren lherzolites, plagioclase lherzolites, and marginal wehrlites with significantly lower olivine content. Net-textured ore is dominant over other ore types in ore body "1. The main objective of this study is the deep portion of ore body#1. Based on field observations and geochemical data from this study, the following conclusions are made:(1) Compositional variations of the sulfide ores occur at different scales in Ore Body#1. IPGE and small Ni/Cu variations are likely due to fractional crystallization of MSS. Small Cu-PPGE rich lenses and large scale variations of Ni/Cu in the deep part of ore body#1may be the result of MSS fractionation or hydrothermal alteration.(2) The Jinchuan host rocks are very Mg-and Fe-rich. The sulfide ores at Jinchuan are generally characterized by moderate amounts of sulfides but are rich in Ni and Cu and depleted in PGE. The sulfide ores of the Jinchuan deposit are compositionally similar to that of the Pechenga and Kabanga deposits, all of them are believed to have formed from a picritic and/or ferropicritic parental magma. These deposits may have the similar petrogenesis and ore genensis.(3) The Jinchuan host rocks are slightly enriched in LREE and their isotopes and trace element geochemistry indicate contamination with the lower crust, fractional crystallization of olivine occuring in a staging magma chamber, which resulted in sulfide saturation and segregation during the formation of the Jinchuan deposit. (4) The dominant proportion of net-textured sulfide ores in Ore Body#1is interpreted to have resulted from compaction in a staging chamber and strong wetting of olivine by sulfide liquids due to high fO2in the magma that formed the central part of the intrusion.(5) It is concluded that PGM (platinum group minerals) crystalling from fractionated sulfide liquids, and the massive ores formed by accumulation of monosulfide solid solution. Injection of pure sulfide liquids which fromed the PGM-bearing and IPGE-rich massive ores. The ore bodies subsequently underwent hydrothermal alteration and tectonic-hydrothermal alteration. As a result. Pt, Pd, Cu, Au are more concentrated in some place.(6) Based on the special relationships between massive sulfide ores and Pt-Pd-rich zones, the best exploration targets for new ore bodies especially massive sulfide ores are at depth including the fractures in the footwalls of orebody-1and orebody-24.