266 



UNITED STATES MINERAL RESOURCES 



About 60 percent of current U.S. production comes 

 from lode deposits, and 40 percent is a byproduct 

 from the refining of other metals, principally cop- 

 per. Placer deposits, once a major source of gold in 

 the United States, are now only a very minor source. 

 In recent years, consumption of gold in the United 

 States has been three to four times domestic produc- 

 tion of new gold. The difference between the amount 

 consumed and the amount produced is made up by 

 reclaimed scrap gold (2.3-3 million oz annually (U. 

 S. Bur. Mines, 1972, p. 60)), by releases from U.S. 

 Treasury stocks (until 1968), and by imports, main- 

 ly from Canada. The United States thus is far from 

 being self-sufficient in gold, and it is unlikely to be- 

 come so in the foreseeable future. 



GEOLOGIC ENVIRONMENT 



GEOCHEMISTRY 



Gold is a rare element — at least 70 of the naturally 

 occurring elements are more abundant, most of them 

 at least an order of magnitude more abundant, than 

 gold (Rankama and Sahama, 1950, p. 39-40 ; Mason, 

 1966, p. 45-46; Krauskopf, 1967, p. 639-640; Lee 

 andYao, 1970, p. 782). 



Estimates of abundance of gold in the earth's 

 crust made during the last 50 years range from 

 0.001 ppm (parts per million) to 0.006 ppm; as 

 analytical methods have improved, earlier estimates 

 have been reduced. A reasonable estimate of crustal 

 abundance based on modern analyses is 0.003-0.004 

 ppm, or about 1 g (gram) per 300 metric tons 

 (Jones, 1968, p. 3; Lee and Yao, 1970, p. 782). Gold 

 is more abundant in mafic than in felsic igneous 

 rocks (Gottfried and others, 1972, p. 8, 21) and 

 apparently is somewhat more abundant in sedimen- 

 tary than in igneous rocks and in sandstone than in 

 other sedimentary rocks, although data are sparse 

 (Jones, 1969). 



Gold is much more abundant in the iron-rich me- 

 teorites than in rocks of the earth's crust, as shown 

 by the following data from Jones (1968, p. 1-3) . The 

 iron meteorites, or siderites, contain as much as 8 

 ppm gold, but the average ranges from 0.67 to 1.3 

 ppm, or 150-300 times as much as the earth's crust. 

 The siderolites, or stony iron meteorites, contain 

 about the same amount of gold as the siderites. The 

 chondritic stony meteorites contain 0.15-0.27 ppm 

 gold, 40-70 times as much as crustal rocks, whereas 

 achondritic meteorites have about the same gold 

 content as the earth's crust. The earth's core is be- 

 lieved to be chemically similar to the iron meteor- 

 ites, and the earth's mantle, similar to the chondritic 

 stony meteorites; so most of the earth's gold is 



probably locked up below the earth's crust. How- 

 ever, rocks that are believed to have been part of 

 the mantle and that are now exposed at the earth's 

 surface do not show notable enrichment in gold ; the 

 most iron rich of these, the dunites, contain only 

 about twice the average gold content of the crust 

 (Jones, 1969, p. 15, tables 4, 5) . 



The abundance of gold in the oceans appears to 

 vary appreciably from ocean to ocean and within a 

 given ocean; two recent estimates of average gold 

 content are 0.011 ppb (part per billion) (1 g per 

 90,000 tons of sea water) (Rosenbaum and others, 

 1969) and 0.05 ppb (1 g per 20,000 tons of sea 

 water) (Jones, 1970, p. 1-3). 



Bedrock deposits of gold that have been mined 

 profitably have contained gold in amounts ranging 

 from 0.05 to 1 ounce or more per ton, but the average 

 gold content appears to be in the range of 0.3-0.5 

 ounce per ton. Ore being mined at present at the 

 three largest gold mines in the United States (Home- 

 stake, S. Dak.; Carlin and Cortez, Nev.) averages 

 about 0.3 ounce per ton, or about 10 ppm. This gold 

 content represents a concentration of 2,500-3,000 

 times the average crustal abundance. This ratio of 

 concentration, which is in a very general way a 

 measure of the likelihood of occurrence of an eco- 

 nomic concentration of an element (the higher the 

 ratio of concentration, the less the likelihood that 

 concentrations of the requisite size and grade will be 

 formed), is one of the highest needed to produce an 

 economic mineral deposit. 



In ore deposits, gold is commonly associated with 

 pyrite and sulfide minerals of silver and copper even 

 though it does not form a separate sulfide mineral. 

 All gold ores contain some silver, many ores that are 

 predominantly of silver contain appreciable quanti- 

 ties of gold, and copper ores produce the bulk of by- 

 product gold. 



Gold deposits are found in so great a variety of 

 rocks as to almost preclude generalizations about 

 association of gold with particular rock types. Never- 

 theless, gold deposits occur more commonly with 

 felsic or intermediate igneous rocks than with mafic 

 or ultramafic types, and more commonly in siliceous 

 or aluminous sedimentary or metamorphic types 

 than in carbonate rocks (however, the host rock at 

 the recently discovered Carlin and Cortez gold de- 

 posits in Nevada is a silty dolomitic limestone) . 



Gold is extremely inert in natural environments 

 at the earth's surface, and during weathering and 

 decomposition of rocks it is little affected. Thus, it 

 may be residually concentrated at the weathering 

 site or transported in the native state and concen- 

 trated locally along with other highly resistant min- 



