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 dgray-sericiticT alteration selvages. The ranges of d 34 S values for minerals of the K-silicate assemblage are 9.8-18.2x for anhydrite (n=23 samples), 3.0x to 4.7x for molybdenite (n=6), 0.4x to 3.4x for pyrite (n=19), and À0.1x to 3.0x 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 8C; anhydrite-pyrite, n=13, 360 to 640 8C; and anhydrite-chalcopyrite, n=8, 480 to 575 8C) are broadly consistent with petrological, alteration, and fluid-inclusion temperature estimates. The d 34 S values for pyrite (n=25) in veinlets of the dgray-sericiticT assemblage range from 1.7x to 4.3x. The d 34 S values for sulfides of the pre-Main Stage K-silicate and dgray-sericiticT assemblages are similar to those of most Main Stage sulfides, for which 281 analyses by other investigators range from À3.7x to 4.8x. Sulfide-sulfide mineral pairs provide variable (À175 to 950 8C) and less reliable temperature estimates that hint of isotopic disequilibria. The sulfide data, alone, suggest a conventionally bmagmaticQ value of about 1x or 2x 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 2Àc0.75) and that total sulfur was isotopically heavy (d 34 S AS c10x), 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 10x fluid, reduction of sulfate, and vapor loss suggest that these processes may have formed the isotopically heavier (14x to 18x) anhydrite of the western and shallower Anaconda Dome, contrasting with the lighter and more numerous values (9.8x to 12.9x) for anhydrite of the eastern and

Sulfur Isotope Geochemistry of Sulfide Minerals

“…Data from El Salvador, Chile (Field and Gustafson 1976), Gaspé, Quebec (Shelton and Rye 1982), Papua New Guinea (Eastoe 1983), and Butte, Montana (Field et al 2005) are plotted in Figure 16. In general, the paired data plotted in Figure 16 suggest that equilibrium conditions were approximated by these hydrothermal systems.…”

Geochemistry of Flooded Underground Mine Workings Influenced by Bacterial Sulfate Reduction

“…Nor could the H 2 S have been generated abiotically by dissolution of hydrothermal sulfide minerals such as ZnS under anaerobic, initially acidic conditions, as this process would have produced H 2 S with an isotopic composition similar to that of the precursor sulfide minerals. Published values for d 34 S of pyrite and other sulfide minerals from the Butte ore bodies were recently compiled and discussed by Field et al (2005). These data, summarized in Fig.…”

Estimate of mole sulfate ratios in magmatic rocks

“…A detailed discussion about sulfur isotopic fractionation at magmatic to hydrothermal conditions is beyond the scope of this study, but can be found in the literature (e.g., Field and Gustafson, 1976;Ohmoto and Rye, 1979;Hoefs, 1987;Ishihara and Sasaki, 1989;Rollinson, 1993;Ohmoto and Goldhaber, 1997;Field et al, 2005;Li and Liu, 2006). However, it is noted that sulfate species tend to be enriched in 34 S relative to sulfide species at magmatic temperatures.…”

“…T, pH (Field and Gustafson, 1976;Sakai et al, 1982Sakai et al, , 1984Ueda and Sakai, 1984;Zheng, 1990;Poulson et al, 1991;Alt et al, 1993;Ohmoto and Goldhaber, 1997;de Hoog et al, 2001;Ripley and Li, 2003;Field et al, 2005). This helps the establishment of genetic models for mineral deposits, which bears implications for mineral exploration.…”