Contrasting Geochemistry of Apatite from Peridotites and Sulfide Ores of the Jinchuan Ni-Cu Sulfide Deposit, NW China

Abstract

Abstract Apatite is present within both the hosting lherzolite and sulfide ore at the Jinchuan magmatic Ni-Cu sulfide deposit of northwest China. Apatite grains within the lherzolite are generally large and hexagonal (>200 μm) and are associated with interstitial phlogopite and amphibole. These apatite grains contain ~0.9 wt % F, ~1 wt % Cl, 6,800 to 14,500 ppm rare earth elements (REE) and have in situ δ18OV-SMOW values of 5.10 to 6.38‰, all of which are indicative of crystallization from an evolved silicate magma. In comparison, the massive and disseminated sulfide ores contain fine-grained apatite (<200 μm) that is associated with sulfide minerals, phlogopite, and albite. These apatite grains contain sulfide inclusions that are indicative of crystallization almost coevally with or slightly later than the sulfide minerals. They are Cl-rich apatite with an average Cl of 5.6 wt % but F concentrations are below the limit of detection. They contain 1,860 to 2,300 ppm REE and have in situ δ18OV-SMOW values of 5.62 to 6.47‰. These data suggest that the sulfide-associated apatite formed from F- and REE-depleted, Cl-bearing sulfide melts. The apatite within the lherzolite was overprinted by later hydrothermal fluids as evidenced by the presence of abundant rounded and needle-like monazite and rare allanite inclusions within the apatite that formed as a result of a coupled metasomatism-reprecipitation process shortly after crystallization. Altered and fresh apatite domains have similar δ18O values, suggesting that this alteration was induced by postmagmatic hydrothermal fluids. The apatite within the lherzolite and sulfide ore crystallized from two conjugate immiscible silicate and sulfide melts, respectively. Rare earth elements and F were preferentially partitioning into silicate melts, whereas most volatile components were mainly partitioned into the sulfide melts. The silicate magmas from which apatite crystallized were rich in light REE (LREE) relative to heavy REE (HREE). Volatile components in the sulfide melts changed the physicochemical conditions to enable such high-density melts to migrate upward and finally settle in the shallow chamber with silicate rocks.