The isotopic composition of oxygen and carbon in hydrothermal minerals at Butte, Montana

Abstract

The O¹⁸/O¹⁶ ratios in vein quartz at Butte are found to vary by 13 permil, the quartz in manganese-bearing veins of the Peripheral Zone commonly being depleted in O¹⁸ relative to the quartz in centrally located iron- or copper-bearing veins. This trend is the reverse of that expected from temperature considerations alone, and implies that the isotopic composition of the hydrothermal fluid varied by 13ppm or more. The variation is possibly due to mixing of magmatically derived hydrothermal fluid with O¹⁸-poor meteoric water.Wall-rock alteration envelopes adjacent to O¹⁸-poor veins have undergone isotopic exchange with the hydrothermal fluid and are consequently depleted in O¹⁸ relative to unaltered host rock. Quartz has experienced much less isotopic exchange than microcline in these alteration envelopes.Isotopic fractionations between coexisting quartz and carbonate are highly irregular, ranging from +5 to -3ppm, and suggest that these two minerals seldom crystallized contemporaneously. C¹³/C¹² ratios in three calcite samples are a few permil larger than those in nine rhodochrosite samples, but there is no obvious correlation between the O¹⁸/O¹⁶ and C¹³/C¹² ratios.

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