Author: Richard Brian Wice

Publisher:

Published: 1982

Total Pages: 0

ISBN-13:

Rhyolitic volcanic and volcaniclastic rocks found in or on the margins of Tertiary basins that contain sandstone-type uranium deposits are considered by many workers to be the source rocks for the uranium. In southwestern Montana and adjacent Idaho three volcanic areas were mapped and evaluated by geochemical analysis. X-ray diffraction and petrographic studies to determine if the volcanics are good source rocks for uranium deposits in nearby Tertiary basins. Area I volcanics, south of Dillon, Montana, have radiometric ages, uranium, thorium and fluorine contents and petrography similar to the Post-Lowland Creek Volcanics in the Boulder Batholith region and are tentatively correlated with them. Area II volcanics, south of Salmon, Idaho, were locally erupted and are a small satellite field separate from the main Challis Volcanic field. Area III volcanics, south of Salmon, Idaho, along the gorge of the Salmon River, lie on the northeast edge of the main Challis field. Comparisons of the average uranium, thorium and fluorine contents of rhyolites and tuffs indicate that the uranium and thorium may have been lost as volatile fluoride complexes during eruption. Although some uranium may have been lost from tuffaceous rocks soon after their eruption, geochemical data suggest that uranium is not mobile during diagenetic alteration of the tuffs to clays and zeolites. The formation of the clays and zeolites may inhibit the migration of uranium by either lowering the permeability of the rock or by adsorption of uranium. This conclusion is supported by the observation that present-day groundwaters associated with tuffaceous sediments contain low uranium concentrations and are strongly undersaturated with respect to uranium. Hydrothermal solutions leached significant amounts of uranium from the welded tuffs near Ennis, Idaho. This uranium apparently was deposited in lignite-bearing beds within a nearby tuffaceous sandstone.