Sulfur isotopes of carbonatite from the giant Maoniuping REE deposit, SW China

Abstract

Sulfur is an indispensable volatile component, which generally occurs as both sulfides and sulfates in global carbonatite-hosted REE deposits. In this study, we present mineral chemistry and sulfur isotopes of the associated sulfides and sulfates from carbonatite of different settings in the giant Maoniuping REE deposit, SW China, to decipher the magmatic-hydrothermal ore-forming processes. The early-stage carbonatite (stage-I) is barely REE mineralized, with sulfide-dominated sulfur species and melt inclusions in barite. The middle-stage carbonatite (stage-II) is weakly mineralized, with both sulfides and sulfates and barite with melt-fluid inclusions. The late-stage carbonatite (stage-III) is intensively REE mineralized, with sulfate-dominated sulfur species and fluid inclusions in barite. Barite from the early- to late-stage carbonatites show a progressively 34S-enriched signature (δ34S = +1.93‰ to +8.10‰), whereas, galena has an increasing 32S-enriched trend (δ34S = −3.54‰ to −16.73‰). The estimated bulk δ34S values show an increasing from +0.31 ‰ to +1.06 ‰, and then decrease to −0.08 ‰, from the early- to late-stage carbonatites. Principally, these sulfur isotopic shifts may be controlled by the temperature dependence of the sulfur isotope fractionation between crystallizing sulfides/sulfates and magma, but it is dominated by the increasing oxidation state and intensively SO2 degassing of the residual magma in later stages. Mass-balance calculation indicates that more than 45% of gaseous sulfur has been degassed, with about 20 % of gaseous sulfur remaining in the late hydrothermal system. Such post-magmatic SO2 degassing and high SO42- activity may be the crucial controlling factors leading to the instability of REE complexes and REE precipitation and mineralization from the REE-bearing hydrous system.