SULFUR ISOTOPE SYSTEMATICS OF MESOPROTEROZOIC HYDROTHERMAL PYRITE TUBES, BLACK BUTTE DEPOSIT, MEAGHER COUNTY, MONTANA

Millimeter-scale irregular tubes of isopachous and porous colloform pyrite may represent a unique style of hydrothermal sulfide mineralization in the Mesoproterozoic Black Butte copper deposits (formerly Sheep Creek) in the Helena Embayment of the Belt Basin, MT. McGoldrick & Zieg (2004) proposed that these structures represent tubes formed around microbial filaments. While the Black Butte deposit is interpreted as a SEDEX-style ore body, the tubes we observe have similar textures to modern black smoker hydrothermal systems. This suggests precipitation around a concentrated vent of sulfide-rich hydrothermal fluid. Pyrite and chalcopyrite precipitation along an increasing thermal gradient would have been followed by lower-temperature mineralization characterized by Pb (and minor Zn) sulfides. In support of this model, Graham et al. (2012) and White et al. (2013) found evidence for multiple mineralization stages at different temperatures in the Black Butte paragenetic sequence.

To further explore tube origin, we measured the sulfur isotopic composition of sulfide minerals using secondary ion mass spectrometry, and of gangue minerals (barite and carbonate-associated sulfate) using MC-ICP-MS. Sulfur isotope data constrain the following stages of the Black Butte paragenesis: (1) early diagenetic barite rosettes, (2) first-generation porous/colloform pyrite and other pyrite cements, (3) chalcopyrite and second-generation coarse pyrite overgrowths and infilling barite and dolomite, and (4) lower-temperature base metal sulfides that replaced earlier pyrite/chalcopyrite as flow pathways occlude.

Our results exhibit an unexpectedly large range of sulfide-sulfur isotopic composition, from -17‰ (VCDT) to +44‰ with a mean of 9.6‰. If early diagenetic barite δ³⁴S (~15‰, Lyons et al., 2000) represents seawater slightly ³⁴S-enriched by closed-system bacterial sulfate reduction, then the sulfide minerals analyzed here represent a wide range of Rayleigh distillation products. The range of isotopic compositions measured in Black Butte has not been observed with abiotic mixing and removal of thermally-reduced sulfides in modern black smoker systems (~0-5‰). Thus, thermophilic microbial communities may have also oxidized organic material in the surrounding shale to drive sulfate reduction.