Sulfur isotopes and pyrite-anhydrite equilibria in a volcanic basin hydrothermal system of the middle to lower Yangtze River valley

Anhydrite-pyrite mineral pairs from a paleohydrothermal system of the volcanic area of the middle to lower Yangtze River valley have been quantitatively interpreted within the constraints of sulfur isotope and thermochemical data on relevant mineral assemblages. The delta (super 34) S values of pyrite in this deposit range from -11 to +9 per mil and those of anhydrite from 14 to 20 per mil. The fractionation between coexisting pyrite and anhydrite (delta (super 34) S anhydrite-pyrite) ranges from 9 to 25 per mil. If sulfur isotopes equilibrated between pyrite and anhydrite, then these fractionations imply mineralization temperatures of 550 degrees to 310 degrees C. These temperatures are roughly 100 degrees C higher than fluid inclusion homogenization temperatures for the same anhydrites, suggesting probable sulfur isotope disequilibrium between coexisting pyrite and anhydrite. The chemistry of the ore-forming fluid is constrained by thermodynamic data and observed mineralization. For example, early-stage pH ranged from 6.4 to 5.0, Sigma m (sub s) was roughly 0.02, and m (sub Ca (super +2) ) 0.17, whereas late-stage values of the same variables were 6.0 to 3.0, 0.02 to 0.01, and 0.17 to 0.1, respectively. The sulfur isotope composition of the ore-forming fluid remained relatively constant, with delta (super 34) S = 15.5 to 16.0 per mil from early- to late-stage deposition. The heavy sulfur isotope signature of the ore fluid was presumably acquired by interaction of isotopically lighter (more magmatic) fluids with basal sediments, including anhydrite. The delta (super 34) S values of pyrite decrease from roughly 10 per mil in the early ore stages to roughly -10 per mil in the late stages; this is attributed primarily to increasing oxygen fugacity and decreasing temperature throughout ore deposition. At the same time, the delta (super 34) S values of coexisting anhydrites increase from roughly 15 to 20 per mil, apparently in response to continuing fractionation of heavier sulfur into later stage sulfates, which would also decrease the delta (super 34) S value of coprecipitated sulfides.