The Last Chance group> Brown Bear and Shady Tree claims in Beaverhead County, Mont., were explored for thorium under a Defense Minerals Exploration Administration Contract in 1951 and 1952.
The project was undertaken to explore northwest-trending moderately to steep dipping, thorite-bearing quartz-barite-hematite veins. The veins are wall-rock replacements and fissure fillings in faults and shears that cut rocks of the Precambrian Belt series. Recurrent movement along the faults has intense fractured the veins. Quartz iron-oxide minerals, and thorite have been deposited in these fractures. The iron oxides and thorite are intimately associated and were among the last minerals deposited. Because no rare earth or uranium minerals have been found in the veins, it is thought that the small amounts of these elements reported in the analyses must substitute for thorium in the thorite.
Under the D. M. E. A. contract the Last Chance vein was traced on surface for a distance of about 1,300 feet; the thickness ranges from about 35 feet to a few inches. Two diamond drill holes cut the vein 240 and 290 feet below the outcrop.
Additional Publication Details
USGS Numbered Series
Some thorium prospects, Lemhi Pass area, Beaverhead County, Montana
Trace Elements Memorandum
31 p.; 3 figs. (one in envelope), 3 tables; 27 cm.