FELDSPAR 



221 



miles and so significant tonnages of nearly pure 

 potash feldspar may be present in California in the 

 Barstow Formation of Miocene Age, San Bernardino 

 County, and in Lake Tecopa, Inyo County (Sheppard 

 and Gude, 1965, 1968). 



RESOURCES 



IDENTIFIED AND HYPOTHETICAL RESOURCES 



Identified and hypothetical resources of feldspar 

 in the United States are large enough to meet any 

 anticipated demand, but quantitative data are not 

 available for calculations on reserves. A recent esti- 

 mate by the Bureau of Mines (Cooper and Wells, 

 1970, p. 985) of our feldspar reserves is 500 million 

 long tons of material similar to that being mined in 

 1970. Reserves in the Spruce Pine district of North 

 Carolina alone exceed 200 million tons of material 

 vi^ithin 50 feet of the surface (Brobst, 1962, p. A15). 

 Immense hypothetical resources of feldspar exist in 

 granitic rocks exposed in the igneous and meta- 

 morphic terranes of the United States and many 

 other countries. Although almost any granite can 

 be used, those that contain few ferromagnesian 

 minerals, such as the alaskite of Spruce Pine, N.C., 

 are the most satisfactory. The average granite body 

 is generally thousands of feet to several miles wide 

 and may contain 60 percent or more feldspar plus 

 potential byproduct quartz and flake mica. 



Reserves and resources of feldspar in pegmatites 

 are difficult to estimate. Many deposits of high-grade 

 potash feldspar in zoned pegmatites have been 

 worked out, but most of these were in pegmatite 

 bodies exposed at the surface. It is safe to assume 

 that the present surface is a random sample of 

 what exists at shallow depths, less than 500 feet, 

 and therefore our undiscovered resources of feld- 

 spar in zoned pegmatites not exposed but within 

 minable depth are equal to the amount already 

 mined, that is, more than 8 million tons. Prospecting 

 for these deposits will not be easy, but detailed geo- 

 logic mapping, geophysical techniques, drilling, and 

 possibly geochemical methods can be used. Because 

 of the availability of resources of fine-grained feld- 

 spar and the low prices of crude ore, the additional 

 expense in exploring for more zoned pegmatites is 

 probably not justified at this time. 



Domestic and foreign reserves and resources of 

 feldspar in feldspathic sands are also large. Such 

 deposits may contain from 20 to at least 50 percent 

 feldspar, at least 50 percent quartz, and only minor 

 amounts of other minerals. Deposits may cover many 

 acres or even square miles. Unlike nearly all other 

 mineral deposits, alluvial sands dredged from stream 



channels are renewable. Such deposits are becom- 

 ing more desirable, not only because they can be 

 mined inexpensively, but also because they can be 

 used as feldspar-silica mixtures with very little 

 beneficiation. A few deposits of this type have been 

 studied and several are being mined, but many more 

 probably exist. Recent mapping by the U.S. Geo- 

 logical Survey in southern Delaware and adjacent 

 Maryland has outlined a large outcrop area of the 

 Beaverdam Sand, a feldspathic marine sand of Pleis- 

 tocene age (J.P. Owens, written commun., 1972). 

 Preliminary studies indicate a feldspar content as 

 high as 50 percent of equal amounts of plagioclase 

 and microcline. The sand has a known thickness of 

 80-90 feet and is exposed in an area of about 1,800 

 square miles. 



Resources of authigenic feldspar are very large, 

 but until further study is made of the uses of such 

 material, these deposits are only a potential resource. 



SPECULATIVE RESOURCES 



Identified and hypothetical resources of feldspar 

 are so large that little thought has been given to 

 speculative resources. The basic types of igneous 

 feldspar deposits, the coarse-grained feldspar in peg- 

 matite and the finer grained in granite, occur in 

 igneous and metamorphic terranes, most of which 

 are probably identified in much of the world. A few 

 additional districts may be found in areas not yet 

 mapped. The distribution of detrital and authigenic 

 feldspar in sediments is not so well known, and the 

 possibility of discovery of new districts is probably 

 greater than for the igneous deposits. Speculative 

 resources of both types are probably large, but 

 prospecting and resource study will be concentrated 

 more on the known districts or in areas nearest to 

 markets because of the high cost of transportation 

 and the low value of the commodity. 



PROBLEMS FOR RESEARCH 



Even though existing sources of feldspar are large, 

 continued evaluation of potential sources should be 

 made during regional geologic mapping. Because 

 transportation costs make up a major part of the 

 delivered cost of feldspar, the most pressing need 

 in the United States is for discovery of supplies 

 closer to consuming areas, especially in the Central 

 States. Possible sources in that area — in feldspathic 

 sands and granites — have been outlined, but addi- 

 tional work is needed on technical feasibility, eco- 

 nomic evaluation of markets, and byproduct recov- 

 ery. Geologic mapping may outline additional or 

 more economic sources in the Central States. 



