Abstract

Analyses of mineral separates from granites from the Granite Mountains, Wyoming, suggest that most of the thorium and at least half of the uranium are associated with biotite, epidote, and accessory minerals. Biotite and epidote contain most of the uranium and thorium in a drill core sample of granite that did not incur any appreciable amount of uranium loss during Tertiary time. These two minerals also form the dominant sites for thorium in a surface sample of granite that incurred a large uranium loss during Tertiary time but only account for 35 percent of the whole-rock uranium in this sample. Concentration and isotopic data for biotite and epidote from the surface sample suggest that these minerals lost enough uranium in Tertiary time to account for the uranium loss by the whole-rock system. None of the mineral ages, calculated from the uranium-lead, lead-lead, and thorium-lead systems, seem to be geologically significant.

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