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-mica 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 containing 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 as calculated from the more abundant data of Castor and others (1978). occurrence of uranium and thorium in the muscovite and hornblende suites is systematically different. Many muscovite-bearing rocks are much more enriched in uranium (>15 ppm) than they are in thorium, and have a relatively low Th:U correlation coefficient of +0.409. Many of the uraniferous muscovite-bearing rocks contain less than 20 ppm Th, probably a consequence of forming by anatexis of thorium deficient sedimentary rocks. Uranium and thorium variation is much more regular in the hornblende suite, which has a Th:U correlation coefficient of +0.780. Uranium in the muscovite suite is held primarily in magnetite and biotite, and possibly as minute uraninite grains, whereas in the hornblende suite uranium resides primarily in sphene, zircon, and allanite. Many muscovite-bearing plutons are considered fertile by the following two criteria: high uranium content, and uranium residence in labile phases. The hornblende-bearing granitic plutons are not considered fertile, regardless of uranium content, because uranium resides in refractory phases.
Twenty-one sample localities in four plutons are considered highly anomalous according to one or more of the following attributes: (1) Uranium content =115.8 ppm (total population mean plus 1 d). (2) Thorium content =30.3 ppm (mean plus 1 d) and uranium =8.8 ppm (mean). (3) Th/U lower than 1.68 (mean minus 1 d) and uranium content -18.8 ppm. The four identified highly anomalous plutons are, from southwest to northeast, (1) Cretaceous porphyritic quartz monzonite of the Midnite mine; (2) Cretaceous muscovite quartz monzonite east of Deer Lake; (3) Cretaceous quartz monzonite of Hungry Mountain, and (4) Cretaceous quartz monzonite of Granite Pass. The study area is favorable for at least five types of uranium deposits including: intragranitic hydrothermal veins as in the Massif Central, France; intragranitic supergene veins, as at the Daybreak mine; contact zone deposits, as at the Midnite mine; and basal-type sandstone deposits, as at the Sherwood mine. The fifth type is Rossing-type deposits for which the high-grade metamorphic terrane of the Kettle River Range, Ferry County, seems favorable, if sufficient volume of low-tenor rock can be located. The first four deposit types seem most likely in or adjacent to uraniferous plutons, as no other uranium source rocks have been identified in the area. 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 depositions
Additional Publication Details
USGS Numbered Series
Uranium and thorium in granitic rocks of northeastern Washington and northern Idaho, with comments on uranium resource potential
U.S. Geological Survey,
46 p., 1 leaf of plates :ill., maps (1 fold.) ;28 cm.