Sulfur isotopic reconnaissance of Butte, Montana

Abstract

The immense amount of geologic information available for interpreting geochemical data makes Butte, Montana, ideal for a sulfur isotopic study.The mean per mil values for the different Main Stage minerals show the following order of S 34 enrichment.Peripheral Zone: pyrite (3.6) > sphalerite (0.5)Intermediate Zone: pyrite (1.3) > chalcopyrite (--0.1) = enargite (--0.1) >bornite (--0.3) > chalcocite (--1.3)Central Zone: pyrite (0.7) > chalcocite (--1.7) > enargite (--2.0)Deep Level Zone: pyrite (1.0) > chalcopyrite (--0.9) >chalcocite (-1.0) > bornite (--1.2)The order of S 34 enrichment among the minerals is proportional to their bonding strength and similar to the equilibrium results between coprecipitated sulfides of other districts reported by other authors. However, the copper sulfides are somewhat later than pyrite and sphalerite. The above isotopic dissimilarities indicate that the copper sulfides probably do not represent replacement or arrested copper metasomatism of earlier pyrite. The approach toward sulfur isotopic equilibrium between the non-arsenic-bearing copper sulfides possibly may have been achieved during exsolution from a single, high-temperature parental sulfide in the bornite solid solution field. Depositional isotopic equilibrium between sulfides may have been preserved in the Peripheral Zone of the Main Stage mineralization and in the molybdenite-bearing pre-Main Stage mineralizations, where molybdenite is more enriched in S 34 than pyrite.The outward increase in per mil values is best illustrated by pyrite and is the reverse of other published examples of sulfur isotopic zoning. The geological evidence and Sakai9s (1968) calculated isotopic effects suggest that the isotopic trend is primarily due to an increase in the pH of the hydrothermal solutions as they moved upward and outward from the Central Zone. Thus the vein mineralogy at Butte may not be so much due to a decrease in temperature outward as to the chemical evolution of the hydrothermal solutions.The apparent isotopic similarity of the minerals in the E-W and NW vein systems within the same zone is compatible with the geologic evidence that the vein systems were mineralized nearly simultaneously. Preliminary data suggest that the pre-Main Stage mineralizations have a different sulfur isotopic composition than the Main Stage.The mean (0.4ppm) and standard deviation (1.6ppm) of all the samples are similar to other magmatic hydrothermal Cordilleran ore deposits. Our sulfur isotopic data and published lead isotopic data indicate that the ore deposits are genetically related to the Butte quartz monzonite. Published lead, strontium, oxygen, and hydrogen isotopic data permit the speculation that at Butte the sulfur was of crustal rather than subcrustal origin.