The redox conditions and C isotopes of magmatic Ni-Cu sulfide deposits in convergent tectonic settings: The role of reduction process in ore genesis

Abstract

Reduction process might play a critical role in the genesis of magmatic Ni-Cu sulfide deposits in convergent tectonic settings. To test this hypothesis, we have determined the oxygen fugacities (fO2) and C isotopes of several typical magmatic Ni-Cu sulfide deposits in convergent tectonic settings in western China. Two popular spinel-olivine-orthopyroxene (Spl-Ol-Opx) oxybarometers, one from Wood (1990) and the other from Ballhaus et al. (1991), are evaluated for their accuracy for this type of deposit by comparing the results with those from an independent method, the olivine-sulfide (Ol-Sul) oxybarometer of Barnes et al. (2013). We find that only the results from the Spl-Ol-Opx oxybarometer of Wood (1990) are consistent with those from the independent method. The fO2 of the sulfide-mineralized samples from the selected deposits, estimated using these two methods, are from QFM–0.56 to QFM+1.84 (quartz-fayalite-magnetite buffer). Using the Spl-Ol-Opx oxybarometer of Wood (1990), the estimated fO2 values of the sulfide-unmineralized ultramafic rocks from primitive arc cumulates in the same region, which have higher olivine Fo contents than the sulfide-mineralized rocks, are significantly higher (~QFM+2.42). The decrease of fO2 from the sulfide ore-barren to ore-bearing magmatic systems indicates that reduction process indeed played a critical role in the genesis of the studied magmatic sulfide deposits. The δ13C values of the sulfide-bearing ultramafic rocks are from –16.5 to –37.2‰ (n = 52), which are significantly lower than the primitive arc cumulates (−7.1 to −9.6‰) but similar to those of organic matter in sedimentary rocks worldwide (–15 to –35‰). The isotope data indicate assimilation of organic matter from the country rocks by the parental magmas for the studied magmatic Ni-Cu sulfide deposits, supporting the interpretation that sulfide saturation in the parental magmas were mainly induced by the reduction process associated with the addition of organic matter to the magmas.