Abstract

Broad areas within and adjacent to the Idaho batholith of Cretaceous age are characterized by radiometric dates of Tertiary age, distinctive oxygen-hydrogen isotope values, and high background radioactivity. Reconnaissance mapping in the Atlanta lobe of the batholith has defined several plutons of Tertiary age that are exposed because of large-scale block faulting. Similar rocks occur extensively in the Bitterroot lobe and along the eastern edge of the batholith where they have been exposed mainly by erosion.These plutons are remarkably similar. Common characteristics include quartz monzonitic to granitic compositions, pink color, the presence of miarolitic cavities containing smoky quartz crystals, and a well-defined vertical joint system that results in the development of a rugged terrain. Smoky quartz crystals are useful for identifying and delineating these plutons. The Tertiary granitic intrusions contain twice the background radiation of the Cretaceous batholith units, as measured with a gamma ray spectrometer.Many of these plutons contain disseminated molybdenum, tungsten and tin mineralization and are spatially related to blacksand placer deposits that contain radioactive minerals. These intrusive rocks may be an important guide in exploring for deposits of sedimentary uranium and epigenetic sulfides of Tertiary age.

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