Geochemistry of selected areas in the southern Black Hills, Fall River County, South Dakota

Abstract

Geologic and radiometric studies and chemical analyses of soils, rocks, and water were made north of an area between Edgemont and Hot Springs, South Dakota. The soil is shallow, poorly developed, and not present in some areas. The median pH of the soil from three areas ranges from 6.7 to 7.0. Rocks are more alkaline and have a median pH from several areas of 7.3 to 8.9 and some mineralized rocks have a pH of 9.4. The pH of shallow perched high-radon bearing waters varies from 3.5 to 6.8 as contrasted to deeply originating low-radon bearing thermal waters with very small ranges of from 6.9 to 7.0. Sediments are oxidized down to the perched water table, a depth of more than 50 feet below the surface of the ground. The uranium deposits are mostly in this zone of oxidation and are characterized by the minerals carnotite and tyuyamunite. In the oxidized zone sandstones are most commonly colored a very pale orange in contrast to the underlying yellowish gray colored sandstones. Mudstones, in the oxidized zone, are most commonly colored a grayish orange in contrast to the underlying olive gray colored mudstones. The characteristic mineral in the oxidized zone is hematite, and below, pyrite. Sediments in the oxidized zone contain not only less iron and uranium than do the underlying sediments but also the joints and contacts between strata in this oxidized zone contain relative large amounts of arsenic, barium, calcium, chromium, cobalt, copper, manganese, nickel, vanadium, and zinc. No pattern was apparent in the size and position of mineralized zones with respect to ground water levels: Highly uraniferous beds occurred above, at, and below the uppermost perched water levels. Gamma ray logs were made of 43 drill holes that totalled about 4,300 feet. The number and intensity of radioactivity peaks was a function of increasing grain size in the sediments. About 60 percent of the gamma ray radioactivity peaks occurred within beds rather than at their contacts. Contacts between coarser-grained beds contained a higher incidence of radioactivity peaks than contacts between finer-grained beds. Between beds of dissimilar lithology such as between mudstones and sandstones or very fine-grained sandstones and medium-grained sandstones, the coarser the bed at the contact in relation to the other bed at the contact the greater the chance for a radioactivity peak. An apparently impermeable bed above or below a contact seemed to have little influence on the number and magnitude of radioactivity peaks. Over 70 percent of the radioactivity peaks occurred, not as isolated peaks, but in groups containing from two to ten peaks. Most of these groups were in sandstones rather than in mudstones. The average vertical range of each group of radioactivity peaks is 15 feet, and for isolated peaks, one foot. Radiometric anomalies apparently are not detected by hand-carried scintillometers unless there is a subsurface anomaly greater than about 1,000 counts per second within a few feet of the surface of the ground. Larger radioactivity anomalies occurred in the higher sloping areas. The chances for uranium anomalies in soil were best where the soil was underlain by an uraniferous coarse non-calcareous sandstone which was intersected by joint or faults.