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The Precambrian Oatman Creek granite exposed in Gillespie County, central Texas, contains 5 to 10 times more uranium than that of an average granite. Samples of this granite, collected from outcrops and quarry openings, were studied by petrographic, delayed neutron counting, fission track, and gamma-ray spectrometry methods. Experiments of leaching uranium from disaggregated samples were also made.
The granite is medium grained with an average composition of 36% quartz, 25% K-feldspar, 38% plagioclase, and 1% biotite and others. In an 80-acre (32 ha.) outcrop area 32 samples, most of which have some uranium removed from weathering, show an average uranium content of 25 ppm; relatively unweathered samples have 50 to 100 ppm uranium. Most uranium occurs between grain boundaries which is called intergranular uranium; some occurs in microfractures developed during late, hydrothermal stages. A portion of the uranium also occurs in discrete minerals, particularly oxides of iron or iron-titanium, and accessory minerals such as zircon, sphene, garnet, and others. This distribution indicates that much of the uranium mineralization was a result of deuteric or hydrothermal activities.
Selected acids of various concentrations were used in experimental leaching of uranium from Oatman Creek granite. Other variables in the experiments were degree of disaggregation and duration of leaching. The results indicate that more than two thirds of the uranium can be leached in a few hours time from the granite without excessive grinding, when a 5N acid is used.
This study shows that the Oatman Creek granite may be a long-term source of uranium in the future.
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