Mineralogic and Stable Isotope Studies of Hydrothermal Alteration at the Jinchuan Ni-Cu Deposit, China

摘要

The Jinchuan intrusion hosts one of the largest magmatic Ni-Cu sulfide deposits in the world. Net-textured and disseminated pyrrhotite, pentlandite, and chalcopyrite are found in olivine-rich rocks (60–80 vol %) that occur as a northwest-southeast–striking dike. Olivine in the intrusion has been variably converted to mixtures of serpentine and fine-grained magnetite. Interstitial pyroxene and minor plagioclase have been altered to mixtures of chlorite, amphibole, epidote, clinozoisite, Na-rich feldspar, and lesser amounts of calcite. Although the interstitial net texture of sulfide minerals surrounding olivine grains is preserved, up to 30 vol percent of sulfide minerals are replaced by magnetite, serpentine, and chlorite. An external origin for Mg and Si needed for the isovolumetric replacement of sulfides cannot be discounted, but a local origin related to olivine and pyroxene alteration is favored. Serpentine and chlorite may be enriched in Cu and Ni, but the enrichment can account for less than 1 percent of the metals lost during the replacement of sulfide minerals by silicates and oxides. Some of the metals and sulfur that were removed from the net-textured sulfide assemblages have been redeposited on a centimeter scale in fine ribbons and veins within serpentine, chlorite, and amphibole.