610 



UNITED STATES MINERAL RESOURCES 



appropriate smelting techniques and installations, 

 can be recovered and converted to sulfuric acid or, 

 at a greater cost, to elemental sulfur. Sulfide ore 

 deposits thus are actual or potential sources of recov- 

 erable sulfur compounds. 



The geology of metallic-sulfide deposits is de- 

 scribed in other chapters of this volume, but mention 

 should be made here of masses of pyrite and pyrrho- 

 tite deposited from hydrothermal solutions, general- 

 ly containing only small amounts of metals other 

 than iron. In some parts of the world, such iron sul- 

 fides are roasted to provide sulfur dioxide used in 

 sulf uric-acid manufacture ; iron and any accompany- 

 ing metals may be recovered as coproducts. Only one 

 such mass of pyrite in the United States (Ducktovi^n,. 

 Tenn.) is exploited as a large supplier of sulfuric 

 acid, but it contains sufficient values of copper and 

 other metals to be classed as a base-metal deposit 

 (Kinkel and others, 1968). The resources of sulfur 

 contained in other masses of pyrite that have little 

 or no base metals occur mainly in Eastern States, 

 less commonly in Western States, and in Alaska (Es- 

 penshade and Broedel, 1952). The deposits are rela- 

 tively large and probably will be utilized after lower 

 cost sources of sulfur are exhausted. 



Gypsum deposited during the evaporation of sea 

 water forms beds that may contain layers of calcite 

 or dolomite, and may contain beds of clay, silt, or 

 sand derived from surrounding arid lands. When 

 buried, gypsum (CaS04*2H20) loses its water of 

 crystallization and is converted to anhydrite 

 (CaSOj). In contact with ground water, anhydrite 

 reconverts to gypsum. The transformation has been 

 noted at depths ranging from several tens of feet to 

 about 1,000 feet. 



Gypsum and anhydrite are widespread in the 

 United States. (See chapter on "Evaporites and 

 Brines".) Reserves and resources are vast, but it is 

 not known what fraction is sufficiently pure to be 

 used as a raw material for production of sulfuric 

 acid or elemental sulfur. The one plant designed to 

 produce elemental sulfur from gypsum, near Van 

 Horn, Tex., was supplied from a large reserve of 

 very pure gypsum, virtually free of interbedded car- 

 bonates and clastic material. The explored block, still 

 nearly intact, is in the upper part of the Castile Gyp- 

 sum, a unit of Permian age in the Delaware Basin of 

 west Texas (Jones, 1954; and Adams, 1969). 



The cost of converting gypsum or anhydrite to sul- 

 furic acid or elemental sulfur is well above the cost 

 of mining elemental sulfur or recovering sulfur from 

 hydrocarbons or metallic ores. It can nonetheless be 



a future source of our sulfur supply after lower cost 

 raw materials now available are mined out. 



ORGANIC SULFUR 



The conversion of sulfate ions to sulfide ions by 

 anaerobic bacteria produced the chief sources of sul- 

 fur now minable at lowest cost, particularly the ele- 

 mental sulfur deposits in accumulations of evaporite, 

 the organic sulfur contained in petroleum, and the 

 hydrogen sulfide associated with sour natural gas. 

 Even greater tonnages of sulfur compounds of the 

 same derivation are locked in coal, oil shale, and tar 

 sands (Theobald and others, 1972; and Duncan and 

 Swanson, 1965). 



ELEMENTAL SULFUR DEPOSITS 



Bacterial attack on anhydrite produces lenses of 

 limestone impregnated with elemental sulfur. Such 

 deposits are formed in two stages : the first resulting 

 in the formation of masses of porous, vuggy medium- 

 grained gray limestone and the second in the forma- 

 tion of coarsely crystalline white calcite and ele- 

 mental sulfur deposited within the pores and open- 

 ings in the limestone (Hanna, 1934). Very small 

 amounts of selenite, barite, and celestite may be 

 deposited later. The reactions theoretically produce 

 1 ton of sulfur for every 3 tons of calcite, or an ore 

 containing nearly 25 percent sulfur, but no lime- 

 stone lenses contain this theoretical amount because 

 hydrogen sulfide is so highly mobile and tends to 

 escape from its zone of generation. The grade, porosi- 

 ty, and permeability of deposits diifer over a sub- 

 stantial range. 



Elemental sulfur deposits of this type are associ- 

 ated with anhydrite caprocks of probable Cenozoic 

 age overlying salt diapirs in the gulf coast and also 

 occur in widespread sequences of anhydrite of Per- 

 mian age in west Texas. In both areas, deposits range 

 from small pockets containing several thousand tons 

 to large accumulations containing tens of millions of 

 tons of sulfur. The Boling Dome deposit, Texas, the 

 largest in the gulf coast, underlies an area of 1,500 

 acres and has yielded 70 million tons of sulfur to 

 date ; the Culbertson property deposit, the largest in 

 west Texas, is said to contain about 80 million tons 

 of sulfur underlying an area of 1,200 acres. The 

 depth of minable sulfur deposits has ranged from 

 400 to 3,200 feet. Some 30 deposits are known in the 

 gulf coast, many of them mined out, and perhaps 10 

 have been found as the result of recent exploration 

 in west Texas (Hawkins and Jirik, 1966; Ellison, 

 1971; and Zimmerman and Thomas, 1969). Twelve 

 such deposits currently being mined supply about 75 

 percent of the sulfur produced in the United States. 



