Trace element composition of stibnite: Substitution mechanism and implications for the genesis of Sb deposits in southern China

Abstract

Though stibnite is among the most predominant Sb-bearing mineral phases in most Sb deposits, little is known about the trace elements and their substitution mechanism in stibnite that are valuable to better decipher the genesis of Sb deposits. This study determined quantitatively the trace element compositions of stibnite from two representative Sb deposits (Xikuangshan and Woxi) in southern China using high-resolution LA-ICP-MS. Results show that stibnite at Xikuangshan contains measurable Hg and As (typically 10–100 ppm); however, As, Cu, and Pb are the most abundant trace elements at Woxi (generally 100–500 ppm). Furthermore, the relatively smooth LA-ICP-MS profiles indicate As, Cu, and Pb primarily occurs as solid solution in stibnite. Hence the substitution 2Sb3+↔Cu++Pb2++As3+ is likely responsible for the enrichment of Cu, Pb, and As in stibnite. Other trace elements, including Au, Ag, Bi, In, Mo, Sn, Co, Cr, V, Zn, Ni, Ga, Ge, Rb, Sr, Pd, Cd and U, are generally at concentrations of 0.01–1 ppm, with spiky LA-ICP-MS depth profiles, which are indicative of their presence as sparse micro-inclusions within stibnite. The apparent differences in trace element concentrations of stibnite suggest that the stibnite from two deposits may have been generated by different processes, or may reflect differences in fluid chemistry. The relative enrichment of As, Cu, and Pb indicate that the original Sb-bearing fluids for the Woxi deposit were likely enriched in these elements in the source region, but a geochemical segregation led to the formation of stibnite-dominated ores. In contrast, the monomineralic ores at Xikuangshan was likely originated from an initial absence of these elements in their source regions. The Hg/(Cu + Pb) ratio for the Xikuangshan Sb deposits is typically greater than 1 but the Woxi deposit tends to be much lower than 1. Therefore, the differences in trace element composition in stibnite can be useful to discriminate between Sb deposit types.