Uranium
Uranium (U) minerals are classified as primary and secondary. Primary uranium minerals are in the same physical state as when originally deposited and secondary uranium minerals are formed by chemical weathering of the primary minerals. The most commercially important primary mineral is pitchblende. Pitchblende, a massive form of uraninite (UO2), is not known in Arkansas. Secondary uranium minerals are normally brightly colored and may be in any type of rock. Carnotite (K2(UO2)2V2O8.3H2O), a canary-yellow mineral, is the most common. Both primary and secondary uranium minerals may be detected by their radioactivity, using Geiger counters or scintillometers. Some instruments will not distinguish between radiation emitted by uranium or by other radioactive materials. Therefore, a chemical assay of the radioactive substance is necessary to determine the amount of uranium.
With the advent of the "atomic age" in July, 1945, the search for uranium in the United States began. Prospecting was stimulated by exploration and discovery bonuses provided by Federal atomic energy legislative acts.
In Arkansas, several uranium anomalies were discovered during the 1950's. Several localities yielded samples with 0.1 percent or more uranium oxide. At most localities the radioactive mineralization is secondary or related to organic matter. In most instances, the uranium-bearing minerals have not been identified.
The Potash Sulphur Springs igneous intrusion in Garland County is probably the best known and perhaps the first site where uranium was discovered in Arkansas. The mineralization is at the contact of the Cretaceous syenite complex with folded Paleozoic novaculite and shale beds. The U.S. Geological Survey identified the uranium-bearing mineral as pyrochlore ((Ca,Na)2Nb2O6(OH,F)), a primary mineral. Soil samples assaying up to 0.4 percent uranium were collected from this site. The Rankin prospect in Pike County consists of radioactive carbonized wood fragments in the lower part of the Trinity Group (Cretaceous). The fragments range greatly in size, the smallest containing the most uranium. The highest assay obtained was 0.24 percent uranium oxide. The uranium mineralization is secondary, but the uranium-bearing minerals have not been identified. At the Chandler prospect in Garland County, uranium is present in gorceixite (BaAl3(PO4)(PO3OH)(OH)6), an uncommon mineral that coats the surface of narrow fractures in the novaculite. Samples of this mineral have as much as 0.35 percent uranium oxide.
The radioactive material at the Bear Hill prospect in Marion County is a bitumen sparsely scattered through an outcrop of Paleozoic black shale. Samples of the bitumen assayed up to 2.0 percent uranium oxide, although no uranium-bearing mineral has been identified. At the Runyan prospect, just north of Magnet Cove in Hot Spring County, radioactive material is present in narrow smoky quartz veins that fill fractures in the host rock, novaculite. Samples assaying as much as 0.14 percent uranium oxide were collected from this deposit, although the individual radioactive minerals are unknown. The Uebergang prospect in Saline County contains both thorium and uranium in a granite-like quartz-feldspar rock. A select sample contained 0.019 percent uranium. Individual uranium-bearing minerals have not been identified.
Although samples from Potash Sulphur Springs and the other prospects contain uranium-bearing minerals, no economically viable deposits have been discovered.
References
Erickson, R. L., and Blade, L. V., 1963, Geochemistry and petrology of the alkalic igneous complex at Magnet Cove, Arkansas: U. S. Geological Survey Professional Paper 425, 99 p.
Stroud, R. B., 1951, The areal distribution of radioactivity in the Potash Sulfur[sic] Springs complex: Fayetteville, University of Arkansas, M. S. thesis, 42 p.
Swanson, V. E., and Landis, E. R., 1962, Geology of a uranium-bearing black shale of Late Devonian age in north-central Arkansas: Arkansas Geological and Conservation Commission Information Circular 22, 16 p.