Abstract

The high gold fineness, geographic distribution, and general lack of bedrock lode sources make the Ninemile district of west-central Montana one of the most interesting gold placer localities in Montana. Analysis of the distribution of the gold-bearing gravels and the petrography and geochemistry of the gold grains leads to the conclusions that the gold is detrital, of very high original purity, and derived from lode deposits hosted by Proterozoic-age Belt Supergroup rocks located within the Ninemile Valley. The limited known lode occurrences, type, and metamorphic grade of bedrock in the Ninemile Valley, and gold grain characteristics are compatible with slate hosted-type, gold-bearing quartz veins.The lode deposits, based upon relationships noted in the Ninemile Valley and elsewhere in the region, were probably emplaced during Laramide-age felsic plutonism. The gold, however, may ultimately have been derived from the Prichard Formation, the basal unit of the Belt Supergroup, which underlies the entire area and hosts gold-bearing quartz veins elsewhere in the Belt basin. The stratigraphic interval encompassing the middle Belt carbonate through the lower part of the Missoula Group may also have been the source for some of the vein gold. We speculate that the distribution of placer deposits resulted from dextral offset of lode sources by the northwest-striking Ninemile fault during the Tertiary. This fault may have displaced the northeast half of the formerly circular- to ellipically shaped lode district up to 15 km to the southeast.

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