SULFUR 



609 



plant, wood, or animal, and both liquid and gaseous 

 hydrocarbons as a source of carbon and hydrogen; 

 at the same time it uses sulfate ions to provide it 

 with oxygen and sulfur. It expels waste carbon di- 

 oxide and hydrogen sulfide. It thus converts sulfate 

 waters to carbonate waters and should calcium be 

 available a biogenic variety of limestone may be 

 generated. The hydrogen sulfide can leak into the 

 atmosphere, be retained in rocks at depth, combine 

 with organic material to form organic sulfur mole- 

 cules, or combine with available metallic ions to form 

 sulfide minerals. 



The activities of sulfate-reducing bacteria are 

 probably responsible for most of the iron sulfide 

 minerals in sedimentary rocks of all ages. They 

 probably generated most of the sulfur compounds in 

 petroleum, asphalt, kerogen, and coal, and the hydro- 

 gen sulfide contained in sour natural gas (Thode and 

 Monster, 1965). The tonnage of sulfur in the world's 

 beds of gypsum and anhydrite may be exceeded by 

 the total tonnage of sulfur in other sedimentary 

 rocks and fossil fuels. The hydrogen sulfide now be- 

 ing generated by sulfate reducers provides a major 

 source of the sulfur compounds emitted into the 

 atmosphere. 



Biogenic hydrogen sulfide also has formed several 

 types of ore deposits. It has been suggested that 

 bacteria generated the hydrogen sulfide that reduced 

 soluble salts and precipitated minable concentrations 

 of uranium. Where these bacteria had a source of 

 hydrocarbons and ample anhydrite rock, they cre- 

 ated masses of biogenic limestone impregnated with 

 elemental sulfur, which form the principal elemental 

 sulfur deposits of the world. 



ATMOSPHERIC CIRCULATION 



Sulfur compounds are constantly circulating in 

 lithosphere, the biosphere, the atmosphere, and the 

 hydrosphere. Some 550 million tons of sulfur com- 

 pounds, calculated as sulfate-ion equivalent, are 

 thought to be emitted into the atmosphere and re- 

 turned to the earth's surface annually (Kellogg and 

 others, 1972). A comparable tonnage of dissolved 

 sulfate is carried annually by the world's rivers to 

 the oceans. 



More than half the sulfur emitted into the atmos- 

 phere results from biologic decay, and about one- 

 quarter each is attributed to windblown sea salt 

 and to manmade pollution ; only some 2 million tons 

 of sulfate-ion equivalent is attributed to volcanic 

 activity. About 60 percent of the total emission, some 

 330 million tons, is thought to fall back on the 

 world's landmasses. Atmospheric fallout of sulfur 

 compounds replenishes the sulfur supply needed by 



plants, but it settles unevenly on landmasses. The 

 available statistics also indicate that substantially 

 more sulfur is being carried by streams and rivers 

 to the seas than is being replenished by atmospheric 

 fallout. Soils in many parts of the world thus are 

 being depleted of the sulfur needed to sustain opti- 

 mum plant growth. 



DEPOSITS AND RECOVERABLE ACCUMULATIONS 



INORGANIC SULFUR 



Excluding the very small tonnage of elemental sul- 

 fur extracted from volcanic deposits and an even 

 smaller tonnage extracted for only a brief period 

 from one gypsum deposit, sulfur in deposits of in- 

 organic origin has been recovered in the United 

 States only as a byproduct or a coproduct of the 

 mining and smelting of the sulfide ores of metallic 

 minerals. 



VOLCANIC DEPOSITS 



Gases and acids escaping from volcanoes convert 

 silicate minerals of lava and ash to clay, silica, and 

 sulfate minerals. Any reaction between escaping 

 hydrogen sulfide and sulfur dioxide produces ele- 

 mental sulfur, which is deposited as replacements, 

 impregnations, and vein fillings in such altered ma- 

 terial. The resulting sulfur-bearing masses are apt 

 to be irregular in size, shape, and grade. The gangue 

 of altered material must be removed, a relatively 

 costly process, so volcanic sulfur is mined in the 

 United States only when the price of sulfur is high. 



Pyrite is generated in volcanic vents by a reaction 

 between hydrogen sulfide and iron. Pyrite may be 

 deposited as disseminations or masses in the altered 

 rock and is minable as a sulfur ore in some parts of 

 the world. 



Elemental sulfur also is deposited by hot 

 springs, probably the result of near-surface oxida- 

 tion of hydrogen sulfide emissions. Gangue consists 

 of altered rock and various minerals precipitated by 

 the springs. Most such deposits are low in grade and 

 have rarely been mined. 



Volcanic deposits of sulfur are known in Cali- 

 fornia, western Nevada, and Alaska, and hot-spring 

 deposits occur in various Western States. 



HYDROTHERMAL SULFIDE DEPOSITS 



When molten rocks crystallize at depth, some of 

 their metals and sulfur are concentrated in hydro- 

 thermal solutions and ultimately deposited as dis- 

 seminations, masses, or veins either within or out- 

 side the cooling masses, thus forming sulfide ore de- 

 posits of copper, molybdenum, lead, zinc, and other 

 metals. During the smelting of such ores, the sulfide 

 is converted into gaseous sulfur dioxide which, with 



