In situ sulfide saturation induced by geometric changes of magma conduit in the world-class Jinchuan magmatic Ni-Cu sulfide deposit, China

Abstract

When and how sulfide saturation occurred in the Jinchuan magmatic Ni-Cu sulfide deposit are poorly constrained. To address this issue, we investigated compositional variations and crystallization temperatures of olivine in various ores and rocks in orebody 1 of the Jinchuan deposit. The forsterite (Fo) values and Mn contents in olivine vary respectively from 79.6 to 85.2 and 1270 to 2463 ppm in net-textured ores, 76.7 to 84.2 and 1425 to 2865 ppm in patchy net textured ores, 78.7 to 84.1 and 1440 to 2649 ppm in disseminated ores, and 69.7 to 81.8 and 1495 to 3191 ppm in sulfide-barren lherzolites. The estimated crystallization temperatures of olivine in these ores/rocks range from 803 to 1163 ℃, 871 to 1314 ℃, 821 to 1507 ℃, and 929 to 1652 ℃, respectively. These data revealed two distinct groupings in both olivine Fo-Mn correlations and crystallization temperatures cluster splitting at a Fo of 79.5 and a temperature of 1100 ℃. The wider range of Mn content with a given Fo value higher than 79.5 compared to those with Fo values lower than 79.5 indicates the onset of in situ sulfide saturation at a Fo of 79.5. The termination of sulfide saturation is epitomized by olivine in disseminated ores with a peak Fo content of 84. The abrupt temperature drops from ca. 1400–1500 °C to ca. 800–1100 °C in the parental magma likely occurred when the magma flowed through the westernmost portion of this segment, which resembles a de Laval nozzle. This geometry could have induced rapid cooling and decompression of the magma, consequently promoting degassing through decompression. These reductions in temperature and H2O content of the magma would collectively result in a decrease in the sulfur concentration at sulfide saturation (SCSS), which would exceed any SCSS increase due to decompression. Our findings emphasize the critical role of magma conduit geometry in physicochemical changes of the parental magma for sulfide saturation and subsequent mineralization in the Jinchuan deposit, which may also be applicable to other magmatic deposit worldwide.