Geology and description of thorium and rare-earth deposits in the southern Bear Lodge Mountains, northeastern Wyoming

Abstract

The Bear Lodge Mountains are a small northerly trending range approximately 16 km northwest of the Black Hills in the northeast corner of Wyoming. Thorium and rare-earth deposits occur over an area of 16 km2 in the southern part of these mountains. These deposits occur in the core of the Bear Lodge dome in a large multiple intrusive body made up principally of trachyte and phonolite. Two types of deposits are recognized: disseminated deposits and veins. The disseminated deposits are made up of altered igneous rocks cut by numerous crisscrossing veinlets. The disseminated deposits contain thorium and rare-earth minerals in a matrix consisting principally of potassium feldspar, quartz, and iron and manganese oxides. Total rare-earth content of these deposits is about 27 times that of the thorium content. The general size and shape of the disseminated deposits were outlined by making a radiometric map using a scintillation counter of the entire Bear Lodge core, an area of approximately 30 km2. The most favorable part of this area, which was outlined by the 40 count's (count-per-second) isograd on the radiometric map, was sampled in detail. A total of 341 samples were taken over an area of 10.6 km2 and analyzed for as many as 60 elements. Rare earths and thorium are the principal commodities of interest in these deposits. Total rare-earth content of these samples ranged from 47 to 27,145ppm (parts per million), and the thorium content from 9.3 to 990 ppm. The amount of total rare earths of individual samples shows little correlation with that of thorium. Contour maps were constructed using the analytical data for total rare earths, thorium, uranium, and potassium. The total rare-earth and thorium maps can be used to define the size of the deposits based on what cutoff grade may be needed during mining. The size is large as the 2,000 ppm total rare-earth isograd encloses several areas that total 3.22 km2 in size, and the 200 ppm thorium isograd encloses several areas that total 1.69 km2. These deposits could be mined by open pit. The Bear Lodge disseminated deposits have one of the largest resources of both total rare earths and thorium in the United States, and although the grade of both commodities is lower than some other deposits, their large size and relative cheapness of mining make them an important future resource. Vein deposits in the Bear Lodge Mountains include all tabular bodies at least 5 cm thick. Twenty-six veins were noted in this area These veins are thin and short; the longest vein was traced for only 137 m. Minerals vary greatly in the amount present. Gangue minerals are commonly potassium feldspar, quartz, or cristobalite intermixed with varying amounts of limonite, hematite, and various manganese D2 GEOLOGY AND RESOURCES OF THORIUM IN THE UNITED STATES hydrothermal solutions. This alteration is most intense in the central part of the Bear Lodge dome, and here the various rock types are difficult to separate. Many of the outlying intrusives show little alteration, and the various rock types are mor...

A deposit model for carbonatite and peralkaline intrusion-related rare earth element deposits

“…Examples include the mineralized carbonatite at Mountain Pass, which lies within an ultrapotassic alkaline complex that includes shonkinites, melasyenites, syenites, and quartz syenites (Castor, 2008b). Carbonatite mineralization at the Bear Lodge deposit is within the Bear Lodge alkaline complex, which includes tonalites and phonolites, as well as latites, syenites, and nepheline syenites (Staatz, 1983). The Iron Hill carbonatite intruded an alkaline complex, which includes, from oldest unit to youngest: pyroxenite, uncompahgrite, ijolite, and nepheline syenite ( fig.…”

Mineral resource potential and geology of the Black Hills National Forest, South Dakota and Wyoming with a section on salable commodities

“…Iron-formation units in the Flag Rock are impossible to tell from the Homestake Formation except that they tend to be thinner and are associated with different overlying and underlying rocks. The Flag Rock is more than 3,000 ft thick and may be thicker, but effects Staatz (1983), Pillmore and Mapel (1963), Robinson, Mapel, and Bergendahl (1964), Fashbaugh (1979), Jenner (1984, O'Toole (1981), White (1980), andWilkinson (1982 Harney Peak Granite (Early Proterozoic) Coarse-grained to pegmatitic muscovite granite and pegmatite containing accessory garnet, biotite, and tourmaline. Age {recalculated using decay constants recommended by Steiger and Jaeger (1977) from Rb-Sr whole-rock isochron is 1697+33 Ma (Riley, 1970 Norton (1976), Norton (unpub.…”

“…7) is granite that is exposed in large xenoliths or screens within Tertiary intrusive rocks. The granite has a U-Th-Pb zircon minimum age of about 2.63 Ga (Staatz, 1983).…”

“…Archean rocks comprise less than 1 percent of the outcrop area of the Precambrian in the Black Hills; the other 99 percent consists of Early Proterozoic rocks. The following summary of the areas is condensed from work by Runner (1934), Woo (1952), Zartman and Stern (1967), Bayley (1972b), Ratte (1980), Redden (1980), and Staatz (1983).…”

“…Strata there include sandstone, conglomerate, and calcareous siltstone derived from the underlying Tertiary intrusive rocks. A minimum thickness of 50 ft of Ogallala Formation was measured by Staatz (1983).…”