the conventional name for any of the natural mineral formations containing thorium in compounds and concentrations that make commercial use technically feasible and economically practical. Deposits of thorium itself are unknown; the element is extracted as a by-product of the dressing of complex ores. Here, Th is extracted together with Nb, Ta, Zr, U, and rare-earth elements. The content of Th varies in such ores from hundredths to tenths of a percent. The principal thorium-bearing minerals are monazite (3.5–10 percent ThO2), thorite (up to 77 percent ThO2), and thorianite (Th, U)O2 (45–93 percent). The minerals bearing thorium in the ores include silicates, simple and multiple oxides (titanotantaloniobates), phosphates, and carbonates; thorium is present in these minerals as an isomorphous admixture.
A distinction is made between endogenous and exogenous thorium deposits. The endogenous group includes magmatic, pegmatite, carbonatite, albitite, skarn, and hydrothermal deposits, all of which are related to alkalic magmatic rocks. The largest pegmatite deposits are in the vicinity of Bancroft, Canada. A significant thorium-bearing rare-earth carbonatite deposit is located in Mountain Pass, Calif. The maximum content of thorium (3–6 percent) is found in the rare-earth hydrothermal deposits at Steenkampskraal, in the Republic of South Africa. Exogenous deposits include various types (eluvial, alluvial, coastal) of monazite placers, some of which are buried. The exogenous deposits constitute the major source of thorium. The largest coastal deposits are in India (with monazite content in the sand averaging 0.75 percent) and Brazil (2–5 percent).
REFERENCEGeologiia postmagmaticheskikh torievo-redkometal’nykh mestorozhdenii. Moscow, 1972.
V. A. NEVSKII