Fingerprinting the metal source and cycling of the world’s largest antimony deposit in Xikuangshan, China

Abstract

The Xikuangshan antimony (Sb) deposit is the largest Sb deposit in the world; however, the metal source and cycling and a holistic understanding of the deposit genesis remain equivocal. Sulfur isotope signatures offer a means of fingerprinting different sources in a hydrothermal deposit, although one must be careful to rule out subsequent isotope fractionation during fluid ascent, mixing, and ore precipitation. Here, we investigated the sulfur isotope composition of stibnite occurring at depth in the Xikuangshan deposit to distinguish the isotopic signals from the source and the superimposed imprint from near-surface–derived sulfur mixing or isotopic fractionation. All stibnites from the deep orebodies displayed limited δ34S variation from +6.8‰ to +8.4‰, despite their widely varying depths. These results provide direct evidence that δ34S values measured in deep orebodies are representative of the isotopic composition of initial fluids. The most likely factor controlling the variation of the sulfur isotopes in shallow stibnites (+3.5‰ to +16.3‰) is a series of hydrothermal processes, including Rayleigh fractionation during ore precipitation, fluid boiling induced by pressure release, and/or local input of pyrite from wall rocks via fluid-rock interaction. Accordingly, we conclude that the Neoproterozoic basement served as the metal source. We propose a holistic genetic model wherein we envisage that Sb and S were leached from the basement rocks, and the ore-bearing fluids ascended along a deep fault and eventually precipitated beneath the Devonian shale cap. Thus, sulfur isotopic systematics represent a powerful repository for interrogating the metal source and cycling in the hydrothermal ore system.