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

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 ‘gray-sericitic’ alteration selvages. The ranges of δ 34S 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 δ 34S values for pyrite ( n=25) in veinlets of the ‘gray-sericitic’ assemblage range from 1.7‰ to 4.3‰. The δ 34S 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 SO 4 2− ≈0.75) and that total sulfur was isotopically heavy ( δ 34S 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 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 δ 34S ΣS compositions of either 2‰ or 10‰ for these later fluids. Oxygen isotopic data for late Main Stage barite (−0.3‰ to 12.4‰, 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 δ 34S values for sulfate–sulfur of barite are markedly variable (4.4‰ to 27.3‰), and the unusual 34S depletion indicates sulfur formed by oxidation of H 2S.