Sulfide remobilization and trace element redistribution during metamorphism and deformation at the Xitieshan Pb-Zn deposit, NW China

Abstract

The effects of post-ore metamorphism and deformation on trace element concentrations in sulfides have recently received increasing attention. In this study, we report comprehensive petrographic observations combined with laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) trace element analysis data of pyrite, sphalerite, and chalcopyrite to evaluate the influence of regional metamorphism and deformation on sulfide remobilization and trace element distribution in the metamorphosed volcanic-sediment-hosted Xitieshan deposit, NW China. The Xitieshan deposit, located in the central segment of the North Qaidam orogenic belt along the northeastern margin of the Tibetan Plateau, is one of the largest Pb-Zn producers in China. Marble-hosted mineralization accounts for approximately 80% of the total mineral resources with the rest hosted in schist in this deposit. Petrographic studies coupled with detailed field work revealed that sulfide minerals in marble-hosted mineralization generally preserve the primary ore textures well with respect to the original mineralization. In contrast, sulfide minerals in schist-hosted ores exhibit various brittle and ductile deformation textures, including cataclasis, recrystallization, annealing, Durchbewegung, fracture healing textures, and veinlets, indicating that sulfide remobilization during metamorphism and deformation was most likely achieved by the combination of mechanical and chemical processes.LA-ICP-MS trace element data for pyrite grains with inclusion-rich cores (Py1b) inside “clean” inclusion-free overgrowth (Py3) indicate that the recrystallization of pyrite both expels trapped inclusions and liberates trace elements such as Cr, Mn, Ni, Cu, Zn, Ag, In, Sn, and Sb. Py3 contains more As than Py1, suggesting that the enrichment of As into recrystallized pyrite occurred during metamorphism. Compared to primary sphalerite (Sp1), the deformed counterpart (Sp2) is characterized by the depletion of In, which could be attributed to (a) remobilization of In into recrystallized chalcopyrite, and/or (b) remobilization and diffusion of In out of primary sphalerite and subsequent formation of the indium-bearing mineral during metamorphism. Indium, Sn, and Ag concentrations were significantly higher in recrystallized chalcopyrite grains than in other analyzed sulfide minerals, indicating that these elements are preferentially incorporated into chalcopyrite under metamorphic conditions.Trace element signatures for deformed and recrystallized pyrite, sphalerite, and chalcopyrite resemble previously reported data from other massive sulfide deposits that have undergone metamorphism and deformation elsewhere. Original mineralization at Xitieshan formed in a high-temperature, magmatically-driven hydrothermal system, which is indicated by the high Co/Ni ratios in pyrite, elevated concentrations of Sn in chalcopyrite and indium in sphalerite, and high calculated temperatures (350–420 ℃) by using the novel “Ga-Ge-In-Mn-Fe in sphalerite” geothermometer.