624 



UNITED STATES MINERAL RESOURCES 



smaller deposits have been mined in the eastern 

 serpentine belts and in California, Texas, and other 

 States. 



Talc deposits that formed by intrusion of sodic 

 pegmatites into serpentinite are mined in Harford 

 County, Md. (Pearre and Heyl, 1960, p. 795). Ap- 

 parently the igneous intrusions provided solution 

 and heat for the reaction with serpentinite to pro- 

 duce a thin contact-zone rind of vermiculite and 

 actinolite, and farther into the country rock, an 

 irregular zone of massive talc rock. 



Some talc deposits were formed by pervasive 

 steatitization of mafic rocks associated with ultra- 

 mafic rocks. At Schuyler, Va., tabular or lenticular 

 bodies of hypersthene gabbro have been altered to 

 greenish-gray soapstone (Hess, 1933). Steatitization 

 here is related either to regional metamorphism or 

 to igneous activity deep in the crust. The altered 

 rock consists of varying proportions of amphibole, 

 chlorite, carbonate, talc, and magnetite and there- 

 fore varies from hard to soft. The rock is tough 

 because of an interlocked felted texture of the min- 

 erals and is desirable for sawed shapes such as 

 laboratory tables and sinks. 



TALC DEPOSITS FORMED BY CONTACT 

 METAMORPHISM OF DOLOMITIC SEDIMENTS 



Granite plutons and diabase dikes that intrude 

 favorable dolomitic strata provided the heat and 

 solutions for the steatitization of the sedimentary 

 rocks, and in some places granite itself has been 

 replaced by talc. Southern California contains ex- 

 cellent examples of talc deposits formed by such 

 contact metamorphism. A mining district that ex- 

 tends from the Panamint Range eastward across 

 southern Death Valley to the Kingston Range in 

 Inyo and San Bernardino Counties contains talc 

 deposits that occur mainly along the upper contacts 

 of diabase sills with cherty dolomite of Precambrian 

 age. These ore bodies average 10-25 feet in width 

 and are 1,000 feet to at least 1 mile in length. Most 

 of the talc produced in California comes from them. 

 Wright (1968) presented a detailed description of 

 more than 40 deposits in this district. 



Talc deposits of northeastern San Bernardino 

 County in the Silver Lake-Yucca Grove district are 

 genetically associated with granitic plutons. The 

 deposits occur as lenses in a terrane composed of 

 lower Precambrian metasedimentary and intrusive 

 rocks. Dolomitic strata were selectively tremolitized 

 when granitic rocks were emplaced nearby. Subse- 

 quently, during retrograde metamorphism, the 

 tremolite was altered to talc schist along the borders 

 of the deposits. The industrial talc in these deposits 



is snowy white and medium to coarse grained. 

 Minable talc bodies are about 10 feet wide and as 

 much as 800 feet long (Chidester and others, 1964, 

 p. 25). 



RESOURCES 



IDENTIFIED AND HYPOTHETICAL RESOURCES 



The United States is self-sufficient in most grades 

 of talc and soapstone. In the past, we have imported 

 most of our block steatite and cosmetic-grade talc. 

 Because of technical refinements in the use of 

 phosphate-bonded domestic ground talc of steatite 

 grade and die-formed steatite as a substitute for 

 insulator forms machined from block steatite, we 

 are no longer dependent on foreign supplies, and 

 stockpiled material probably will be sold. About half 

 the cosmetic-grade talc consumed (Industrial min- 

 erals, 1971) is still imported, but selective-flotation 

 techniques using domestic talc have eliminated much 

 of our dependence on foreign sources. 



Chidester, Engel, and Wright (1964) estimated 

 our identified resources at a minimum of 89 million 

 tons, with the possibility of additional discoveries 

 being good to excellent in all mining districts. Re- 

 sources should be adequate to meet the demands for 

 many decades. The Bureau of Mines, in its Com- 

 modity Data Summary of January 1972, estimated 

 United States reserves at 150 million tons and world 

 reserves at 360 million tons. Projected demand for 

 talc in the United States for the year 2,000 by the 

 U.S. Bureau of Mines is 2.28 to 3.36 million tons 

 (Cooper and Hartwell, 1970). Worldwide demand 

 for talc is increasing faster than demand in the 

 United States as societies become more complex, but 

 the world's identified resources appear adequate to 

 meet demand. 



SPECULATIVE RESOURCES 



Most talc deposits are found in areas of high- 

 grade or contact metamorphism. The deposits in 

 Talledega County, Ala., however, are in relatively 

 unmetamorphosed Cambrian and Ordovician dolo- 

 mite. Similar dolomites exist at many other places 

 in the United States. The Alabama deposits should 

 be carefully studied to determine their genesis so 

 that similar deposits might be predicted elsewhere. 

 Many areas of dolomitic marble in the southeastern 

 Piedmont, such as the Wakefield and Cockeysville 

 Marbles in Maryland, might contain talc deposits. 

 Another intriguing possibility is the dolomite and 

 marble units in areas of Precambrian rocks in Wis- 

 consin and Michigan. For example, the Randville 

 Dolomite in Dickinson County, Mich., locally con- 

 sists of white tremolitic marble (James and others, 



