No abstract

Also included is a map showing the location of gravity stations used in the models.

The U.S. Geological Survey evaluated the mineral resource potential of the Custer National Forest in the Pryor Mountains of south-central Montana. The study area comprises approximately 122 square miles (316 km2) of National Forest lands, located 40 miles (64 km) south of Billings, Montana. Five uplifted and tilted, fault-bounded blocks form the Pryor Mountains. The National Forest study area includes most of Big Pryor Mountain and East Pryor Mountain, which represent the southwestern and southeastern blocks of the uplift, respectively. These blocks are bordered by high-angle faults on their north and east flanks. Mississippianaged Madison Limestone is exposed in half of the National Forest. In the southern Pryor Mountains region, including the study area, a paleokarst horizon in the upper 190-240 ft of the Madison Limestone Group hosts uranium-vanadium deposits. Host structures are shallow chaotic breccia bodies that fill solution caverns in the paleokarst. The breccias formed by the collapse of cavern roof and wall rocks accompanied by inflow of overlying Amsden Formation sediments. Typical "collapsed caverns" hosting uranium-vanadium deposits are about 100 ft or less in diameter, 20-25 ft in height, and often circular in plan view. The primary ore minerals are silt-size, bright yellow tyuyamunite and metatyuyamunite. Uranium was mined from about 40 small (median size of 154 metric tons), relatively high-grade (median grades of 0.26% U3O8 , 0.23% V2O5) deposits in the region during 1956-70. During 1956 to 1964, 21 properties in the Big Pryor Mountain district of Montana (including at least 3 mines in the study area) produced more than 45,000 pounds of uranium oxide (U3O8) and 30,000 pounds of vanadium oxide (V2O5). Similar deposits in the Little Mountain district of northern Wyoming, about 10 miles to the southeast, produced about 250,000 pounds of uranium oxide and 205,000 pounds of vanadium oxide from 1956 to 1970. Geologic analysis suggests that any undiscovered uranium-vanadium deposits of the Custer National Forest are likely to be similar in character to those found in the 1950's and 1960's. About 80% of the study area is permissive for undiscovered uranium-vanadium deposits of this type to a maximum depth of 550 ft. A grade and tonnage model was constructed for the uranium-vanadium deposits using ore data obtained in 1956-70 from the producing mines of the Big Pryor Mountain and Little Mountain districts. Using a computer simulation, estimates of numbers of undiscovered uranium-vanadium deposits in the study area were combined with the grade and tonnage model. The computer simulation generated a probability distribution representing the likelihood of a given amount of ore or metal potentially present in undiscovered deposits of the study area. The results using this method suggest that the Custer National Forest may contain about five times as much ore as was produced from the Big Pryor Mountain district and about as much as the combined total production from the Big Pryor Mountain and Little Mountain di...

The Blacktail-Snowcrest uplift in Beaverhead County, Montana, is a northeasttrending structure of the Rocky Mountain foreland. Models of a detailed gravity traverse across the steep southeastern limb of the structure require the presence of a major thrust-fault system, only subsidiary parts of which are exposed at the surface. Gravity models, consistent with mapped surface structure, are based on density logs and thicknesses of Paleozoic and Mesozoic stratigraphic intervals penetrated in nearby drillholes. Interpretation of the models requires a thicker sequence of low-density Cretaceous rocks southeast of the uplift than was encountered in the wells, and also suggests that the thrust system beneath the uplift involves rocks of both the sedimentary sequence and the crystalline basement with at least 3,700 m (12,000 ft) of vertical separation. The models require the assumption of unusually low densities for some of the Precambrian and Paleozoic rocks; these densities are interpreted as being related to pervasive fracturing of these rocks during thrust deformation.Gravity and aeromagnetic trends associated with the Snowcrest-Greenhorn fault system extend westward beneath the Cordilleran thrust belt, indicating that the fault system is present there in the subsurface. A reentrant in the Medicine Lodge thrust sheet, the termination of the Four Eyes Canyon thrust sheet, and a change in strike of the Cabin thrust sheet, all in the frontal thrust belt, overlie the inferred westward extension of the Snowcrest-Greenhorn fault system, and probably are caused by the impingement of east-directed Cordilleran thrust faults on the older Rocky Mountain foreland structure. Cordilleran thrust sheets appear not only to overlap but also to crosscut older foreland faults, uplifts, and basins. Where crosscutting occurs, both upper and lower plates may exhibit rapid changes in stratigraphic level along strike, and these changes may be used as field evidence of overlap in the subsurface.