Sulfur and oxygen isotopic record in sulfate and sulfide minerals of early, deep, pre-Main Stage porphyry Cu–Mo and late Main Stage base-metal mineral deposits, Butte district, Montana

Authors

C.W. Field, Oregon State University
L. Zhang, Oregon State University
J.H. Dilles, Oregon State University
Robert O. Rye, U.S. Geological SurveyFollow
M.H. Reed, University of Oregon

Date of this Version

2005

Comments

Published in Chemical Geology 215 (2005)

Abstract

Typical porphyry-type Cu–Mo mineralization occupies two connected domal centers, the eastern Pittsmont and western Anaconda domes, that predate and largely underlie the well-known, throughgoing, Main Stage polymetallic veins of Butte. Among the sulfur-bearing minerals recovered from deep drill core of this early pre-Main Stage hydrothermal assemblage are anhydrite, chalcopyrite, pyrite, and molybdenite in veinlets bordered by K-silicate alteration, and pyrite from slightly younger quartz–pyrite veinlets with dgray-sericiticT alteration selvages. The ranges of δ³⁴S values for minerals of the K-silicate assemblage are 9.8–18.2%◦ for anhydrite (n=23 samples), 3.0%◦ to 4.7%◦ for molybdenite (n=6), 0.4%◦ to 3.4%◦ for pyrite (n=19), and ‒0.1%◦ to 3.0%◦ for chalcopyrite (n=13). Sulfate–sulfide mineral fractionation is consistent with an approach to isotopic equilibrium, and calculated temperatures for mostly coexisting anhydrite–sulfide pairs (anhydrite–molybdenite, n=6, 545 to 630 °C; anhydrite–pyrite, n=13, 360 to 640 °C; and anhydrite–chalcopyrite, n=8, 480 to 575 °C) are broadly consistent with petrological, alteration, and fluid-inclusion temperature estimates. The δ³⁴S values for pyrite (n=25) in veinlets of the dgray-sericiticT assemblage range from 1.7%◦ to 4.3%◦. The δ³⁴S values for sulfides of the pre-Main Stage K-silicate and 'gray-sericitic" assemblages are similar to those of most Main Stage sulfides, for which 281 analyses by other investigators range from ‒3.7%◦ to 4.8%◦. Sulfide–sulfide mineral pairs provide variable (‒175 to 950 °C) and less reliable temperature estimates that hint of isotopic disequilibria. The sulfide data, alone, suggest a conventionally 'magmatic" value of about 1%◦ or 2%◦ for Butte sulfur. However, the high modal mineral ratios of sulfate/sulfide, and the isotopic systematics of the early K-silicate assemblage, suggest that pre-Main Stage fluids may have been sulfate-rich (X_{SO4 ^2‒}≈0.75) and that total sulfur was isotopically heavy (δ³⁴S_ΣS≈10%◦), which would have required an evaporitic crustal component to the relatively oxidized granitic parental magma that was the source of the hydrothermal fluids and sulfur. Modeling of brine–vapor unmixing of a 10%◦ fluid, reduction of sulfate, and vapor loss suggest that these processes may have formed the isotopically heavier (14%◦ to 18%◦) anhydrite of the western and shallower Anaconda Dome, contrasting with the lighter and more numerous values (9.8%◦ to 12.9%◦) for anhydrite of the eastern and