MERCURY 



407 



though there is much variation, typical amounts 

 can be given (table 81). 



The average mercury content of continental rocks 

 has been calculated at about 80 ppb (parts per bil- 

 lion) (Taylor, 1964), and a similar figure vv^as cal- 

 culated for the earth's crust (Saukov, 1946). More 

 recent vi^ork (Ehmann and Lovering, 1967) sug- 

 gests that these values may be a little too large, but 

 they are certainly the right order of magnitude. In 

 contrast, the content of a minable mercury ore de- 

 posit must be at least 2 pounds to the ton, or 

 1,000,000 ppb (0.1 percent), representing a mer- 

 cury concentration 10,000 times greater than the 

 usual content of most rocks. 



For additional details regarding the occurrence 

 of mercury compounds in nature and their be- 

 havior during the weathering cycle, see Tunell 

 (1970). 



Table 81. — Mercury content of various natural substances 



[Most data from U.S. Geol. Survey (1970)] 



Parts per billion 



Igneous rocks: 



Peridotite, dunite, basalt 5-20 



Andesite, granite 15 - 100 



Sedimentary rocks: 



Sandstone 20 - 75 



Shale 100 - 1,000 



Limestone 15 - 50 



Coal 10 - 1,000 



Metamorphic rocks 20 - 200 



Soil 10 - 100 



Natural fluids: 



Rain water .2 



River water .1 



Ocean water .03- 1 



Petroleum 2,000 -20,000 



Air .01 



Organisms : 



Plants <500 



Fish 20 - 200 



Mammals <20 



ORE DEPOSITS 



All major mercury deposits are mined for their 

 mercury content alone, and all contain the red mer- 

 curic sulfide, cinnabar, as the chief ore mineral. 

 Liquid mercury metal and a black mercuric sulfide, 

 metacinnabar, are locally of some economic im- 

 portance, and in one medium-size deposit a mercury- 

 antimony sulfide, livingstonite, was the main ore 

 mineral. The more than a dozen other known mer- 

 cury minerals are relatively rare and inconsequen- 

 tial as sources of mercury. 



Within the past few years, mercury has been pro- 

 duced as a byproduct at the Gortdrum copper mine 

 in Ireland (1,300 flasks per yr), the Rudnany zinc- 

 copper mine in Czechoslovakia (1,000 flasks per yr), 

 and the Carlin gold mine in Nevada (60 flasks per 

 yr). Cinnabar has been identified at all of these 

 deposits, but other minerals probably also contain 



small amounts of mercury. The St. Joe Minerals 

 Corp. smelter in Monaca, Pa., also recovers mer- 

 cury (300 flasks per yr) from zinc concentrates 

 from deposits in New York, but the mercury- 

 bearing mineral is unknown. 



Silica and carbonate minerals are the common 

 introduced gangue minerals in mercury deposits, 

 but pyrite and marcasite may be abundant in de- 

 posits formed in iron-bearing rocks. Stibnite is rare 

 but is more common than orpiment. Other metals, 

 such as gold, silver, or base metals, are generally 

 present in only trace amounts. An exception, how- 

 ever, is a base-metal assemblage containing mer- 

 curian tetrahedrite, from which mercury has been 

 recovered in a few places. 



The ore-forming mercury minerals, and intro- 

 duced gangue minerals, are deposited from hot solu- 

 tions at temperatures from perhaps 50°C to 200°C, 

 and they thus form relatively low-temperature, epi- 

 thermal deposits (White, 1967; Tunell, 1970). The 

 deposits are characteristically shallow, and in the 

 United States only half a dozen have been mined at 

 depths greater than 1,000 feet. Even in the major 

 mercury deposits of the world, ores have not been 

 mined more than about 2,000 feet below the sur- 

 face, although at the largest, the rich Almaden mine 

 in Spain, ore is reported to have been drilled at 

 depths of 3,000 feet. The greatest depth at which 

 cinnabar has been found is reported to be 5,900 

 feet, in a well drilled for geothermal steam in the 

 Geyser area, Sonoma County, Calif. (G. H. Gaul, 

 written commun., Feb. 22, 1969). 



Mercury ore deposits have doubtless been formed 

 throughout geologic time; but owing to their shal- 

 lowness, those deposited long ago have been either 

 eroded away or covered over so they cannot be seen. 

 Very young deposits, still uneroded and uncovered, 

 constitute the discoverable and minable ore bodies. 

 Because they were deposited in preexisting rocks, 

 almost any kind of rock might be the host, though 

 some varieties have unusually favorable chemical or 

 physical properties. However, the distribution of 

 mercury deposits about the world is restricted be- 

 cause the volcanic and tectonic activity responsible 

 for generating the heated pregnant solutions that 

 deposited the mercury has in recent time taken 

 place only in limited belts (Bailey, 1959). One 

 highly productive belt extends northward along the 

 western part of South and North America, passes 

 through central Alaska, extends southward through 

 Japan, then turns westward across central China 

 and through the southern part of the Soviet Union 

 and Europe. Other shorter and much less productive 

 belts extend from New Zealand up through Papua- 



