Stable isotope investigation of a hydrothermal alteration system: Butte porphyry copper deposit

Typical porphyry-type Cu-Mo mineralization predates and underlies the well-known Main Stage polymetallic veins of the Butte district, Montana. This thesis presents the first systematic study of the isotopic characteristics of the pre-Main Stage K-silicate and sericitic wallrock alteration related to the porphyry Cu-Mo stage.; Oxygen and hydrogen isotopic compositions were obtained from hydrous and other silicate minerals of sixty-two samples from the unaltered host rock and ores: four Butte Quartz Monzonite; seven samples from K-silicate alteration; twenty samples of K-silicate alteration affected by late argillic alteration; and twenty-seven samples of gray-sericite alteration. These data support current porphyry Cu-Mo models that the associated hydrothermal fluids that produced pre-Main Stage K-silicate and gray sericite were dominantly magmatic in origin. Butte differs from other porphyry Cu-Mo districts because the widespread Main Stage or younger argillic alteration dominated by meteoric water (δD = –100 to –1200‰) has partly to totally reset the hydrogen isotopic compositions of homblende (δD = –45 to –126‰) and biotite (δD = –61 to –135‰) in the fresh Butte Quartz Monzonite host rock, biotite (δD = –47 to –131‰) of the early high temperature K-silicate alteration assemblages, and locally reset the δD values (–25 to –117‰) of sericite (muscovite) of gray-sericite alteration assemblages. The effect of argillic alteration on these minerals was to produce D-depleted isotopic composition (δD –80 to –140‰).; Sulfur isotope analyses have been applied to sulfides and anhydrite from forty-seven samples selected from deep drill cores. These include sulfate-sulfide assemblages in veinlets associated with K-silicate alteration selvages and slightly younger quartz-pyrite veinlets associated with gray-sericite alteration selvages. The K-silicate and gray-sericite sulfide values (δ 34S = +0.4 to +4.7‰) presented here are similar to those of Main Stage sulfides reported previously and suggest a conventional “magmatic” value (δ34S about 2‰) for Butte sulfide-sulfur in the hydrothermal fluid. However, the anhydrite from the K-silicate alteration yields a much heavier δ34S value (+12.9 ‰), therefore, total sulfur (δ34S_Σs ‰) of the early K-silicate assemblage was likely as heavy as 10 per mil, suggesting a possible crustal component to this relatively oxidized system.