472 



UNITED STATES MINERAL RESOURCES 



tite, pegmatitic migmatite, and thorium-rich zones 

 in metamorphic rock. 



VEINS 



Three types of thorium veins are known. First are 

 the unique monazite-apatite veins of South Africa 

 that have been the principal producers, but are 

 largely depleted. The second type comprises urano- 

 thorite or uranothorianite deposits in Malagasy 

 and on Bokan Mountain, Alaska; these also are 

 largely depleted. The third type consists of veins 

 composed chiefly of fine-grained quartz, potassium 

 feldspar, and iron oxide. The most abundant thorium 

 mineral in these veins is thorite, but monazite, allan- 

 ite, or brockite are common in some places. Veins of 

 this type are known at 13 localities in the United 

 States. The veins are from a few feet to 5,000 feet 

 long and many are 1-2 feet thick. They occur in 

 steeply dipping fractures that cut across the struc- 

 ture of the host rocks ; many are brecciated. In many 

 areas the veins are adjacent to alkalic complexes and 

 carbonatites. The veins formed from highly mobile 

 fluids at low temperatures, and hence may be as 

 much at 20 miles from their probable source. 



The grade of the veins is commonly erratic, but 

 many contain an average of several tenths of a 

 percent thorium. A few veins, such as those in the 

 Wet Mountains, Colo. (Christman and others, 1959, 

 p. 522 and 530), and at Hall Mountain, Idaho (Staatz, 

 1972, p. 247), may contain several percent thorium. 

 The rare-earth content of the veins may equal or 

 exceed the thorium content. Inasmuch as the grain 

 size is small and the iron oxide may be abundant, 

 separation of thorium minerals by standard physical 

 methods is difficult. 



In many countries, monazite is a common acces- 

 sory mineral in placer deposits that were formed 

 from the debris of metamorphic and granitic rocks. 

 Many of these placer deposits are formed on active 

 beaches, where the monazite and other heavy min- 

 erals are concentrated in a narrow belt along the 

 shoreward facies. Many placer deposits are exten- 

 sive; some are more than 10 miles long and one- 

 quarter mile wide. Some placer deposits occur in 

 partly indurated beach sediments that now lie land- 

 ward of the active beaches. 



In tropical areas, where weathering has been ex- 

 tensive and penetrated deeply, beach placers may 

 contain more than 50 percent heavy minerals that 

 consist chiefly of ilmenite or magnetite, although 

 garnet, zircon, rutile, sillimanite, cassiterite, or 

 monazite may also be sufficiently abundant to be 



economically important. The monazite, however, gen- 

 erally occurs in minor amounts compared to many 

 of the other minerals. The ThOa content of monazite 

 in these deposits is highly variable, and commonly 

 ranges from 3.5 to 9.0 percent (Parker and Baroch, 

 1971, p. 67), but the ThOs content of the monazite 

 from any one deposit or area is fairly constant. 



Stream placers are smaller and tend to be less 

 uniform in composition than beach placers, and 

 therefore must be of higher unit value to be worked 

 successfully. Monazite generally is the principal and 

 only thorium mineral in these deposits, but other 

 thorium minerals are found in a few places. The 

 principal thorium mineral is euxenite in the placers 

 at Bear Valley, Idaho, and uranothorite in the Hailey 

 area, Idaho (Mackin and Schmidt, 1956). 



DEPOSITS IN SEDIMENTARY ROCKS 



Some sedimentary rocks consist chiefly of indu- 

 rated placer sediments, such as those of fossil beach 

 deposits. In some of these deposits, the sorting and 

 concentration of the valuable minerals has been 

 more complete than in others. In some of these 

 deposits especially in sandstone and conglomerate, 

 the heavy minerals tend to be dispersed, but even 

 here they have been concentrated because the softer 

 or more brittle minerals have been ground finer and 

 carried farther away. Monazite is commonly the only 

 thorium-bearing mineral found, but near Elliot Lake, 

 Canada, it occurs with brannerite and uraninite 

 (Roscoe and Steacy, 1958, p. 482). The latter two 

 minerals, important sources of uranium, are found 

 in conglomerates in ancient Precambrian rocks. 



Indurated sedimentary rocks require grinding to 

 liberate the thorium minerals — unless they contain 

 an important coproduct, such as uranium, they are 

 not likely to be mined until the less expensive 

 unconsolidated placers are largely exhausted. 



CONCENTRATIONS IN METAMORPHIC AND IGNEOUS ROCKS 



Thorium minerals are found in some metamorphic 

 and igneous rocks. Monazite is especially widespread 

 in metamorphic rocks of garnet to sillimanite grade. 

 In a few small areas, such as near Central City, Colo. 

 (Young and Sims, 1961), and in southern Music 

 Valley, California (Evans, 1964), monazite and 

 xenotime are concentrated, but the deposits are too 

 small to mine. 



Thorium minerals are commonly associated with 

 potassic igneous rocks, especially granitic and al- 

 kalic complexes. Also related to these igneous rocks 

 are bodies of carbonatite, which commonly contain 

 several valuable minerals. Thorium occurs in some 

 such carbonatites; for example, it might be recov- 



