ABSTRACT

Sulfur isotopic analyses on vein pyrite and associated sulfides in a short section of a vein in the Intermediate Zone, Butte, Montana, show remarkable isotopic uniformity. The standard deviation (0.5 ‰) and range of pyrite values (1.4 to 3.9 ‰) are attributed to ore formation in a fO₂-pH regime where the isotopic composition was insensitive to large changes in the physico-chemical conditions. The δ³⁴S of the hydrothermal solution responsible for pyrite deposition is estimated to be +3.1 ‰. Large per mil differences and textural relationships between pyrites and associated copper and zinc sulfides show that the different sulfides were not deposited simultaneously and/or in isotopic equilibrium. Disseminated wall rock pyrites are depleted in ³⁴S by as much as 2.4 ‰ relative to the adjacent vein and veinlets. The isotopic relationship between vein and disseminated pyrites may be due to a number of phenomena including diffusional fractionation, the effects of wall rock on the physico-chemical properties of the solution but are probably largely due to the interaction of pre-Main Stage and Main Stage hydrothermal processes. Delineation of these mechanisms is confounded by the presence of many veinlets in the wall rock.