10 miles N of Sundance (T. 51-52 N., R. 63 W.), includes Petersen deposits (secs. 15 & 22, T. 52 N., R. 63 W.), Hutchinson prospect, and Royal Purple Nos. 1 & 2 claims (sec. 30, T. 52 N., R. 63 W. & sec. 25, T. 52 N., R. 64 W.); siliceous fluorite veins and limestone replacement associated with andesite sills intruded into limestone.
The REE-thorium deposits and nearby gold mineralization of the southern Bear Lodge Mountains are hosted by middle Tertiary alkaline intrusions. These intrusions are Eocene (38.3−50 million years old) and consist primarily of phonolite and trachyte (Staatz, 1983). They intruded Paleozoic and Mesozoic sedimentary rocks, forming a dome about 13 km (8 mi) long by 10 km (6 mi) wide. Breccia bodies are associated with the igneous intrusions, such as a heterolithic diatreme breccia near Bull Hill. Rare earth elements–bearing carbonatite dikes intruded near the Bull Hill diatreme; the dikes are surrounded by a large zone of low-grade REE mineralization that fills thin, narrow stockwork fractures within the large alkaline intrusions. These thorium and REE deposits crop out throughout an area of about 16 km2 (6 mi2) (Staatz, 1983). The igneous core of the dome is microfractured and altered, thereby forming disseminated deposits. The REE and thorium mineralization precipitated within thin fractures as coatings and veinlets as much as 6 mm thick. The coatings and veinlets consist predominantly of iron and manganese oxide minerals, along with potassium feldspar and quartz. The REE and thorium are incorporated into the minerals monazite, thorite, and brockite.
References
Rocks & Minerals 76:385
USGS Open File report 2010-5220
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