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Sulphur isotopic fractionation between sulphate and sulphide in hydrothermal ore deposits: disequilibrium vs equilibrium processes
Author(s) -
Zheng YongFei
Publication year - 1991
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1111/j.1365-3121.1991.tb00186.x
Subject(s) - disequilibrium , sulfur , geology , fractionation , geochemistry , hydrothermal circulation , equilibrium fractionation , mineralogy , isotope fractionation , metallurgy , chemistry , paleontology , materials science , medicine , organic chemistry , ophthalmology
Correlative fractionation relationships of sulphur isotope data for coexisting sulphate and sulphide pairs from hydrothermal ore deposits on δ 38 S versus Δ 34 S diagrams are deciphered theoretically. Taking into account dissolved H 2 S and SO 4 2‐ in hydrothermal fluids during precipitation of both sulphate and sulphide minerals, a 4‐species closed system is suggested for describing the conservation of mass among all sulphur‐bearing species on the δ‐Δ diagrams. The covariation in the δ 34 S values of both sulphate and sulphide is ascribed to isotopic exchange between oxidized and reduced sulphur species during mineral precipitation. The isotopic exchange could be a thermodynamic process due to simple cooling of high temperature fluids, which results in an equilibrium fractionation, or a kinetic process due to mixing of two sulphur reservoirs, which leads to a disequilibrium fractionation. The δ 34 S value of total sulphur in a hydrothermal system could change due to the precipitation of minerals, or due to the escape of H 2 S and/or SO 4 2‐ . Sulphur isotope data for anhydrite and pyrite pairs from the Luohe porphyrite iron deposit in the Yangtze River Valley is used to illustrate the mixing responsible for the disequilibrium fractionation.