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

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 'gray-sericitic' alteration selvages. The ranges of delta(34)S values for minerals of the K-silicate assemblage are 9.8-18.2 parts per thousand for anhydrite (n = 23 samples), 3.0 parts per thousand to 4.7 parts per thousand for molybdenite (n = 6), 0.4 parts per thousand to 3.4 parts per thousand for pyrite (n = 19), and -0.1 parts per thousand to 3.0 parts per thousand 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 degrees C; anhydrite-pyrite, n = 13, 360 to 640 degrees C; and anhydrite-chalcopyrite, n = 8, 480 to 575 degrees C) are broadly consistent with petrological, alteration, and fluid-inclusion temperature estimates. The delta(34)S values for pyrite (n = 25) in veinlets of the 'gray-sericitic' assemblage range from 1.7 parts per thousand to 4.3 parts per thousand. The delta(34)S 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 parts per thousand to 4.8 parts per thousand. Sulfide-sulfide mineral pairs provide variable (-175 to 950 degrees C) and less reliable temperature estimates that hint of isotopic disequilibria. The sulfide data, alone, suggest a conventionally magmatic value of about 1 parts per thousand or 2 parts per thousand 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 (XSO42- approximate to 0.75) and that total sulfur was isotopically heavy (delta(34)S(Sigma S) approximate to 10 parts per thousand), 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 parts per thousand fluid, reduction of sulfate, and vapor loss suggest that these processes may have formed the isotopically heavier (14 parts per thousand to 18 parts per thousand) anhydrite of the western and shallower Anaconda Dome, contrasting with the lighter and more numerous values (9.8 parts per thousand to 12.9 parts per thousand) for anhydrite of the eastern and deeper Pittsmont Dome. Such a process might also have been able to produce the sulfide isotopic compositions of the younger 'gray-sericitic' and Main Stage zones, but the limited data for sulfates permit delta(34)S(Sigma S) compositions of either 2 parts per thousand or 10 parts per thousand for these later fluids. Oxygen isotopic data for late Main Stage barite (-0.3 parts per thousand to 12.4 parts per thousand, n = 4 samples) confirm variable meteoric water contributions to these fluids, and the data support either the absence of, or limited, sulfate-sulfide isotopic equilibrium in these samples. The delta(34)S values for sulfate-sulfur of barite are markedly variable (4.4 parts per thousand to 27.3 parts per thousand), and the unusual S-34 depletion indicates sulfur formed by oxidation of H2S. (c) 2004 Elsevier B.V All rights reserved.