Uranium and thorium in granitic rocks of northeastern Washington and northern Idaho, with comments on uranium resource potential
This study was undertaken to document the distribution of uranium or thorium in plutonic rocks as a possible general guide to uranium deposits within the plutons, adjacent to the plutons in contact zones, or in nearby sedimentary rocks. Northeastern Washington and northern Idaho is a uranium province in which many Cretaceous and Tertiary granitic plutons contain abnormal amounts of uranium. Mean uranium content of 108 samples of granitic rock is 8.8 parts per million (ppM), more than twice normal for rocks of this composition. The mean thorium content, 20.3 ppM, and mean Th/U, 3.19, are normal. The most uraniferous and fertile rocks are the peraluminous two-mica granitic suite, although not all two-mica plutons are enriched in uranium. The muscovite-bearing suite has mean uranium content of 22.3 ppM, mean thorium content of 22.8 ppM, and mean Th/U of 2.82. Porphyritic quartz monzonite of the Midnite mine, which I interpret to be a two-mica granitic rock, is especially radioactive with mean U of 14.7 ppM, mean Th of 32.1 ppM, and mean Th/U of 2.72. Mean uranium and thorium contents of the two-mice granitic plutons are significantly different from those of the calcalkaline hornblende granitic suite, which are mean U, 5.0 ppM; mean Th, 17.6; and mean Th/U, 3.78. Biotite granitic rocks containg no hornblende or muscovite appear to be an intermediate suite in terms of U and Th, or possibly are variants of both hornblende and muscovite type; mean U is 3.88 ppM, mean Th is 14.4 ppM, and mean Th/U is 3.03. The study area is favorable for at least 5 types of uranium deposits. Contact zone deposits and intragranitic hydrothermal veins, both possibly supergene-enriched, and basal-type deposits in Tertiary sandstone and conglomerate, seem most likely to be of economic importance. Exploration and discovery of potential uranium deposits in the area is severely hampered by deep weathering, glacial cover, and poorly exposed structure. 5 figures, 4 tables.
- Research Organization:
- Geological Survey, Denver, CO (USA)
- OSTI ID:
- 6181487
- Report Number(s):
- USGS-OFR-79-233; ON: DE83902370
- Country of Publication:
- United States
- Language:
- English
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580400 -- Geochemistry-- (-1989)
ACTINIDES
CHEMISTRY
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