226 



UNITED STATES MINERAL RESOURCES 



mites, phosphorites, and, especially, volcanic glass 

 give rise to high fluoride waters. 



Surface waters vary greatly in fluoride content 

 depending upon source of the water and amount of 

 rainfall, the fluoride content generally being higher 

 in dry periods. Fleischer and Robinson (1963) gave 

 0.2 ppm as the average fluoride content of rivers. 

 This figure also is probably close to that for fresh- 

 water lakes. Highly saline lakes, on the other hand, 

 have much higher fluoride contents: 14 ppm is re- 

 ported for Great Salt Lake, Utah, and as much as 

 1,627 ppm for lakes in Kenya. 



Individual species of plants contain variable quan- 

 tities of fluorine depending upon the amounts they 

 need for growth and the amounts available in the 

 soil or air. Some plants can be classed as fluorine 

 accumulators, and some plants increase in fluorine 

 content with age. Other plants accumulate fluorine 

 from the air only. All these factors must be con- 

 sidered if fluorine in plants is to be used in explora- 

 tion for ore deposits. 



Fluorine deposits of past and present significance, 

 of probable future significance, and of possible fu- 

 ture significance are classified into the following 

 four groups, which, although they overlap and inter- 

 grade somewhat, provide a convenient basis for 

 further discussion. 



1. Fluorine deposits associated with igneous rocks. 



Includes disseminated deposits, deposits in 

 pegmatite and carbonatite, and deposits in con- 

 tact aureoles. 



2. Fluorine deposits associated with sedimentary 



rocks. Includes those in volcaniclastic and la- 

 custrine sedimentary rocks and those in evapo- 

 rite, marine carbonate, and phosphorite rocks. 



3. Fluorine deposits associated with regionally 



metamorphosed rocks. 



4. Fluorine in hydrothermal (hot water) deposits. 



Includes those in veins and mantos, in pipes 

 and stockworks, and in zones of alteration. 



FLUORINE DEPOSITS ASSOCIATED WITH 

 IGNEOUS ROCKS 



Fluorine deposits associated with igneous rocks 

 contain fluorine minerals that are considered to be 

 genetically related to the igneous rocks. These de- 

 posits include accessory fluorine minerals dissemi- 

 nated through the igneous rock and fluorine minerals 

 in pegmatites, carbonatites, and contact aureoles of 

 intrusive rocks. Fluorine-bearing igneous rocks of 

 possible future economic significance are hypabys- 

 sal bodies and extrusive rocks, generally of alkalic 

 or silicic-alkalic composition, and plutonic rocks of 

 silicic composition. Topaz and fluorite occur in some 



silicic volcanic rocks of the Western United States 

 (Shawe, 1966, p. C209), where they are dispersed 

 through the rock and concentrated within vugs. 

 Stocks of alkalic rocks in the Western United States 

 and in many other parts of the world contain fluorite 

 and, in places, topaz as accessory minerals. Fluorite 

 also is a common constituent of batholithic rocks of 

 granite composition in South Africa, Newfoundland 

 (Van Alstine, 1944), the United States, Eastern 

 Mongolia (Khasin and Kalenov, 1965), and many 

 other parts of the world. Although large amounts of 

 fluorine occur in these rocks, the tenor is low, gen- 

 erally less than 2 percent CaFg. 



Some pegmatites and carbonatites are mined for 

 their fluorine content. Pegmatites commonly contain 

 local concentrations of fluorite and other fluorides, 

 but not in large volume. Most fluorite mined from 

 pegmatites has been used as specimens or for optical 

 purposes. However, the fluorspar deposits at Crystal 

 Mountain, Mont., and the cryolite deposits at Ivig- 

 tut, Greenland, are both considered to be almost 

 monomineralic pegmatites. The Crystal Mountain 

 deposits are mainly fluorite but have minor amounts 

 of biotite, quartz, feldspar, sphene, rare-earth-bear- 

 ing apatite, amphibole, fergusonite, thorianite, and 

 thortveitite (Parker and Havens, 1963). This de- 

 posit is exceedingly rich and has yielded a substan- 

 tial amount of fluorspar since production began in 

 1952. The deposit at Ivigtut, Greenland, is largely 

 cryolite but also contains complex fluorides, fluorite, 

 microcline, topaz, columbite, siderite, sphalerite, 

 galena, and chalcopyrite. 



Carbonatite complexes are notably enriched in 

 fluorine, which is mostly dispersed through the rock 

 in silicates and apatite. Economic fluorine deposits 

 associated with carbonatite complexes are contact- 

 zone deposits and late-stage hydrothermal veins. 

 Deposits at Okorusu in South West Africa and at 

 Amba Dongar, India, are of this type (Deans and 

 others, 1972). Apatite-bearing carbonatite rock is 

 the principal source of phosphate in southern and 

 eastern Africa. The apatite rocks, of which there are 

 considerable reserves, contain 1-3 percent fluorine 

 (Deans, 1966, p. 393). Apatite-bearing carbonatite 

 rock near Jacupiranga, Sao Paulo, Brazil, being 

 mined for phosphate, contains 4-5 percent fluorine 

 (Abreu, 1960, p. 123), and apatite-bearing carbona- 

 tite rocks in the U.S.S.R. are a present source of 

 fluorine ; the fluorine is recovered as a byproduct of 

 fertilizer production. Carbonatite at Mountain Pass, 

 Calif., contains at least 1 percent fluorine, mostly in 

 bastnaesite (Olson and others, 1954). 



Fluorite, topaz, and apatite are common minerals 

 in contact aureoles around many intrusive bodies. 



