Graduate Thesis Or Dissertation
 

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

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  • 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 (öD = -100 to -120%) has partly to totally reset the hydrogen isotopic compositions of hornblende (D = -45 to -l26%) and biotite (öD -61 to -l35%) in the fresh Butte Quartz Monzonite host rock, biotite (D = -47 to -131%o) of the early high temperature K-silicate alteration assemblages, and locally reset the öD values (-25 to -1 l7%o) of sericite (muscovite) of gray-sericite alteration assemblages. The effect of argillic alteration on these minerals was to produce D-depleted isotopic composition (öD = -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 (34S = +0.4 to +4.7 %) presented here are similar to those of Main Stage sulfides reported previously and suggest a conventional "magmatic" value (ö345 about 2 %o) for Butte sulfide-sulfur in the hydrothermal fluid. However, the anhydrite from the K-silicate alteration yields a much heavier 634S value (+ 12.9 %o), therefore, total sulfur (&4Szs %) of the early K-silicate assemblage was likely as heavy as 10 per mil, suggesting a possible crustal component to this relatively oxidized system.
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