NUCLEAR FUELS 



471 



covered principally from pegmatites; and in 1958, 

 650 tons of thorite concentrate was obtained from 

 thorite-bearing quartz-carbonate veins in Colorado 

 (Kelly, 1962, p. 3). 



The principal placers that have been mined for 

 monazite in the southeastern United States are small 

 fluvial placers in the Piedmont of North and South 

 Carolina. The value of the land for agricultural pur- 

 poses is commonly greater than for monazite placer 

 mining. The changes in land use and the small size 

 of many of the deposits deter the operation of most 

 fluvial placers. In some of the larger beach placers 

 from South Carolina to northern Florida, the value 

 of residential property would deter their use as 

 beach placers. For example, Hilton Head Island, 

 S. C, where 8,266,000 tons of heavy minerals con- 

 taining approximately 2,880,000 tons of ilmenite and 

 94,600 tons of monazite were reported (McCauley, 

 1960, p. 5-6), is covered by many fine estates, golf 

 course, and summer residences. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Recent estimates of the thorium content of the 

 earth's crust range from about 6 to 13 ppm. Most of 

 the thorium is widely disseminated in small quanti- 

 ties which vary in amount in different rocks. In its 

 geochemical cycle, thorium becomes concentrated 

 during several episodes, for example: 



1. In magmatic intrusion and differentiation, be- 



cause of its relatively large ionic radius and 

 high valence thorium tends to become concen- 

 trated in the residual solutions as crystalliza- 

 tion progresses. Thus it is most abundant in 

 the younger members of rock series such as 

 granitic and alkalic rocks and other rocks 

 formed from residual magmatic fluids. The 

 thorium content of granitic rocks that ap- 

 proach true granite in composition is likely to 

 be higher than that of granodiorite or quartz 

 monzonite, and it is notably high in some small 

 alkalic rock complexes with which carbonatite 

 and fenite are associated. During magmatic 

 and metamorphic processes, therefore, tho- 

 rium may be concentrated in veins, carbona- 

 tite, pegmatite, and certain alkalic rocks, 

 granites, and metamorphic rocks. 



2. In magmatic and metamorphic phases, the type 



and grade of metamorphism are factors in 

 determining whether monazite, the principal 

 ore mineral of thorium, is formed, and in de- 

 termining the thorium content of the mona- 

 zite. Thus the higher grade metamorphic and 

 related igneous terranes are the best source 



areas for monazite with a high thorium con- 

 tent. 

 3. During weathering and erosion, monazite tends 

 to remain as detrital grains owing to its in- 

 solubility and high specific gravity. The weath- 

 ered mantle thus becomes enriched in this and 

 other residual heavy minerals, which gradu- 

 ally move downslope and downstream, forming 

 local placer concentrations. Monazite and other 

 heavy minerals that reach the sea are de- 

 posited chiefly near shore at the mouths of 

 rivers, but some thorium is also adsorbed on 

 clay minerals during weathering and becomes 

 disseminated in clayey sediments. During 

 weathering and erosion, therefore, thorium 

 may be concentrated in residual, stream (val- 

 ley) placer, or beach placer deposits. Detrital 

 mineral deposits formed in these ways may 

 also be preserved in ancient sedimentary rocks 

 under proper conditions, or on erosion of them 

 the monazite may be reconcentrated to form 

 rich, second-generation placer deposits. 



ORE MINERALS AND ASSOCIATED MINERALS 



The chief ore minerals of thorium are monazite 

 [(Ce,La,Nd,Th)P04], thorite, (ThSiOi), uranothor- 

 ite, and brannerite (a multiple oxide of Ti, U, Ca, 

 Fe, Th, and Y). Other thorium-bearing minerals 

 present in some deposits include allanite; multiple 

 oxides such as euxenite, samarskite, fergusonite, and 

 betafite; and thorianite (ThOz). Thorium is com- 

 monly associated, in minerals and in deposits, with 

 the rare earths, niobium, titanium, and zirconium; 

 thorium may occur with these elements as important 

 coproducts or byproducts. In placer deposits, poten- 

 tial source minerals for thorium include monazite 

 and several thorium-bearing multiple-oxide minerals. 

 In veins or lodes the principal thorium-bearing min- 

 erals are thorite, monazite, and uranothorianite, and 

 less commonly allanite, brockite, and brannerite. 

 Thorium-bearing pyrochlore is found in some car- 

 bonatites. 



TYPES OF DEPOSITS 



Thorium concentrations of possible economic in- 

 terest occur in the following types of deposits: (1) 

 Veins, (2) beach and stream placers and residual 

 deposits, (3) deposits in sedimentary rocks — ancient 

 placer deposits, thorium-bearing dolomite (as found 

 in Canada), and conglomerates or quartzites en- 

 riched in thorium and in some places uranium (as 

 at Elliot Lake, Ontario), and (4) concentrations in 

 igneous and metamorphic rocks, mostly of low grade, 

 which include carbonatite, fenite, dikes or layers of 

 alkalic igneous rock, thorium-rich granite, pegma- 



