# Gold

> Through the ages men and women have cherished gold, and many have had a compelling desire to amass great quantities of it--so compelling a desire, in fact, that the frantic need to seek and hoard gold has been aptly named "gold fever."

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The graves of nobles at the ancient Citadel of Mycenae nearNauplion, Greece, discovered by Heinrich Schliemann in 1876,yielded a great variety of gold figurines, masks, cups, diadems,and jewelry, plus hundreds of decorated beads and buttons.  Theseelegant works of art were created by skilled craftsmen more than3,500 years ago.

The ancient civilizations appear to have obtained theirsupplies of gold from various deposits in the Middle East.  Minesin the region of the Upper Nile near the Red Sea and in theNubian Desert area supplied much of the gold used by the Egyptianpharaohs.  When these mines could no longer meet their demands,deposits elsewhere, possibly in Yemen and southern Africa, wereexploited.

Artisans in Mesopotamia and Palestine probably obtainedtheir supplies from Egypt and Arabia.  Recent studies of the Mahdadh Dhahab (meaning "Cradle of Gold") mine in the present Kingdomof Saudi Arabia reveal that gold, silver, and copper wererecovered from this region during the reign of King Solomon(961-922 B.C.).

The gold in the Aztec and Inca treasuries of Mexico and Perubelieved to have come from Colombia, although some undoubtedlywas obtained from other sources.  The Conquistadores plunderedthe treasuries of these civilizations during their explorationsof the New World, and many gold and silver objects were meltedand cast into coins and bars, destroying the priceless artifactsof the Indian culture.

Nations of the world today use gold as a medium of exchangein monetary transactions.  A large part of the gold stocks of theUnited States is stored in the vault of the Fort Knox BullionDepository.  The Depository, located about 30 miles southwest ofLouisville, Kentucky, is under the supervision of the Director ofthe Mint.

Gold in the Depository consists of bars about the size ofordinary building bricks (7 x 3   5/8 x 1  3/4 inches) that weighabout 27.5 pounds each (about 400 troy ounces; 1 troy ounceequals about 1.1 avoirdupois ounces.) They are stored withoutwrappings in the vault compartments.

Aside from monetary uses, gold is used in jewelry and alliedwares, electrical-electronic applications, dentistry, theaircraft-aerospace industry, the arts, and medical and chemicalfields.

The changes in demand for gold and supply from domesticmines in the past two decades reflect price changes.  After theUnited States deregulated gold in 1971, the price increasedmarkedly, briefly reaching more than $800 per troy ounce in 1980. Since 1980, the price has remained in the range of $320 to $460per troy ounce.  The rapidly rising prices of the 1970'sencouraged both experienced explorationists and amateurprospectors to renew their search for gold.  As a result of theirefforts, many new mines opened in the 1980's, accounting for muchof the expansion of gold output.  The sharp declines inconsumption in 1974 and 1980 resulted from reduced demands forjewelry (the major use of fabricated gold) and investmentproducts, which in turn reflected rapid price increases in thoseyears.

Gold is called a "noble" metal (an alchemistic term) becauseit does not oxidize under ordinary conditions.  Its chemicalsymbol Au is derived from the Latin word "aurum." In pure formgold has a metallic luster and is sun yellow, but mixtures ofother metals, such as silver, copper, nickel, platinum,palladium, tellurium, and iron, with gold create various colorhues ranging from silver-white to green and orange-red.

Pure gold is relatively soft--it has about the hardness of apenny.  It is the most malleable and ductile of metals.  Thespecific gravity or density of pure gold is 19.3 compared to 14.0for mercury and 11.4 for lead.

Impure gold, as it commonly occurs in deposits, has adensity of 16 to 18, whereas the associated waste rock (gangue)has a density of about 2.5.  The difference in density enablesgold to be concentrated by gravity and permits the separation ofgold from clay, silt, sand, and gravel by various agitating andcollecting devices such as the gold pan, rocker, and sluicebox.

Mercury (quicksilver) has a chemical affinity for gold. When mercury is added to gold-bearing material, the two metalsform an amalgam.  Mercury is later separated from amalgam byretorting.  Extraction of gold and other precious metals fromtheir ores by treatment with mercury is called amalgamation.  Gold dissolves in aqua regia, a mixture of hydrochloric andnitric acids, and in sodium or potassium cyanide.  The lattersolvent is the basis for the cyanide process that is used torecover gold from low-grade ore.

The degree of purity of native gold, bullion (bars or ingots ofunrefined gold), and refined gold is stated in terms of goldcontent.  "Fineness" defines gold content in parts per thousand. For example, a gold nugget containing 885 parts of pure gold and115 parts of other metals, such as silver and copper, would beconsidered 885-fine.  "Karat" indicates the proportion of solidgold in an alloy based on a total of 24 parts.  Thus, 14-karat(14K) gold indicates a composition of 14 parts of gold and 10parts of other metals.  Incidentally, 14K gold is commonly usedin jewelry manufacture.   "Karat" should not be confused with"carat," a unit of weight used for precious stones.

The basic unit of weight used in dealing with gold is thetroy ounce.  One troy ounce is equivalent to 20 troypennyweights.  In the jewelry industry, the common unit ofmeasure is the pennyweight (dwt.) which is equivalent to 1.555grams.

The term "gold-filled" is used to describe articles ofjewelry made of base metal which are covered on one or moresurfaces with a layer of gold alloy.  A quality mark may be usedto show the quantity and fineness of the gold alloy.  In theUnited States no article having a gold alloy coating of less than10-karat fineness may have any quality mark affixed.  Lowerlimits are permitted in some countries.

No article having a gold alloy portion of less thanone-twentieth by weight may be marked "gold-filled," but articlesmay be marked "rolled gold plate" provided the proportionalfraction and fineness designations are also shown.  Electroplatedjewelry items carrying at least 7 millionths of an inch (0.18micrometers) of gold on significant surfaces may be labeled"electroplate." Plated thicknesses less than this may be marked"gold flashed" or "gold washed."

Gold is relatively scarce in the earth, but it occurs inmany different kinds of rocks and in many different geologicalenvironments.  Though scarce, gold is concentrated by geologicprocesses to form commercial deposits of two principal types:lode (primary) deposits and placer (secondary) deposits.

Lode deposits are the targets for the "hardrock" prospectorseeking gold at the site of its deposition from mineralizingsolutions.  Geologists have proposed various hypotheses toexplain the source of solutions from which mineral constituentsare precipitated in lode deposits.

One widely accepted hypothesis proposes that many golddeposits, especially those found in volcanic and sedimentaryrocks, formed from circulating ground waters driven by heat frombodies of magma (molten rock) intruded into the Earth's crustwithin about 2 to 5 miles of the surface.  Active geothermalsystems, which are exploited in parts of the United States fornatural hot water and steam, provide a modern analog for thesegold-depositing systems.  Most of the water in geothermal systemsoriginates as rainfall, which moves downward through fracturesand permeable beds in cooler parts of the crust and is drawnlaterally into areas heated by magma, where it is driven upwardthrough fractures.  As the water is heated, it dissolves metalsfrom the surrounding rocks.  When the heated waters reach coolerrocks at shallower depths, metallic minerals precipitate to formveins or blanket-like ore bodies.

Another hypothesis suggests that gold-bearing solutions maybe expelled from magma as it cools, precipitating ore materialsas they move into cooler surrounding rocks.  This hypothesis isapplied particularly to gold deposits located in or near massesof granitic rock, which represent solidified magma.

A third hypothesis is applied mainly to gold-bearing veinsin metamorphic rocks that occur in mountain belts at continentalmargins.  In the mountain-building process, sedimentary andvolcanic rocks may be deeply buried or thrust under the edge ofthe continent, where they are subjected to high temperatures andpressures resulting in chemical reactions that change the rocksto new mineral assemblages (metamorphism).  This hypothesissuggests that water is expelled from the rocks and migratesupwards, precipitating ore materials as pressures andtemperatures decrease.  The ore metals are thought to originatefrom the rocks undergoing active metamorphism.

The primary concerns of the prospector or miner interestedin a lode deposit of gold are to determine the average goldcontent (tenor) per ton of mineralized rock and the size of thedeposit.  From these data, estimates can be made of the deposit'svalue.  One of the most commonly used methods for determining thegold and silver content of mineralized rocks is the fire assay. The results are reported as troy ounces of gold or silver or bothper short avoirdupois ton of ore or as grams per metric ton ofore.

Placer deposits represent concentrations of gold derivedfrom lode deposits by erosion, disintegration or decomposition ofthe enclosing rock, and subsequent concentration by gravity.

Gold is extremely resistant to weathering and, when freedfrom enclosing rocks, is carried downstream as metallic particlesconsisting of "dust," flakes, grains, or nuggets.  Gold particlesin stream deposits are often concentrated on or near bedrock,because they move downward during high-water periods when theentire bed load of sand, gravel, and boulders is agitated and ismoving downstream.  Fine gold particles collect in depressions orin pockets in sand and gravel bars where the stream currentslackens.  Concentrations of gold in gravel are called "paystreaks."

In gold-bearing country, prospectors look for gold wherecoarse sands and gravel have accumulated and where "black sands"have concentrated and settled with the gold.  Magnetite is themost common mineral in black sands, but other heavy minerals suchas cassiterite, monazite, ilmenite, chromite, platinum-groupmetals, and some gem stones may be present.

Placer deposits have formed in the same manner throughoutthe Earth's history.  The processes of weathering and erosioncreate surface placer deposits that may be buried under rockdebris.  Although these "fossil" placers are subsequentlycemented into hard rocks, the shape and characteristics of oldriver channels are still recognizable.

The content of recoverable free gold in placer deposits isdetermined by the free gold assay method, which involvesamalgamation of gold-bearing concentrate collected by dredging,hydraulic mining, or other placer mining operations.  In theperiod when the price of gold was fixed, the common practice wasto report assay results as the value of gold (in cents ordollars) contained in a cubic yard of material.   Now results arereported as grams per cubic yard or grams per cubic meter.

Through laboratory research, the U.S. Geological Survey hasdeveloped new methods for determining the gold content of rocksand soils of the Earth's crust.   These methods, which detect andmeasure the amounts of other elements as well as gold, includeatomic absorption spectrometry, neutron activation, andinductively coupled plasma-atomic emissionon spectrometry.  Thesemethods enable rapid and extremely sensitive analyses to be madeof large numbers of samples.

Gold was produced in the southern Appalachian region asearly as 1792 and perhaps as early as 1775 in southernCalifornia.  The discovery of gold at Sutter's Mill in Californiasparked the gold rush of 1849-50, and hundreds of mining campssprang to life as new deposits were discovered.  Gold productionincreased rapidly.  Deposits in the Mother Lode and Grass Valleydistricts in California and the Comstock Lode in Nevada werediscovered during the 1860's, and the Cripple Creek deposits inColorado began to produce gold in 1892.  By 1905 the Tonopah andGoldfield deposits in Nevada and the Alaskan placer deposits hadbeen discovered, and United States gold production for the firsttime exceeded 4 million troy ounces a year--a level maintaineduntil 1917.

During World War I and for some years thereafter, the annualproduction declined to about 2 million ounces.  When the price ofgold was raised from $20.67 to $35 an ounce in 1934, productionincreased rapidly and again exceeded the 4-million-ounce level in1937.  Shortly after the start of World War II, gold mines wereclosed by the War Production Board and not permitted to reopenuntil 1945.

From the end of World War II through 1983, domestic mineproduction of gold did not exceed 2 million ounces annually. Since 1985, annual production has risen by 1 million to 1.5million ounces every year.  By the end of 1989, the cumulativeoutput from deposits in the United States since 1792 reached 363million ounces.

Consumption of gold in the United States ranged from about 6million to more than 7 million troy ounces per year from 1969 to1973, and from about 4 million to 5 million troy ounces per yearfrom 1974 to 1979, whereas during the 1970's annual goldproduction from domestic mines ranged from about 1 million to 1.75 million troy ounces.  Since 1980 consumption of gold hasbeen nearly constant at between 3 and 3.5 million troy ounces peryear.  Mine production has increased at a quickening pace since1980, reaching about 9 million troy ounces per year in 1990, andexceeding consumption since 1986.  Prior to 1986, the balance ofsupply was obtained from secondary (scrap) sources and imports.     Total world production of gold is estimated to be about 3.4billion troy ounces, of which more than two-thirds was mined inthe past 50 years.  About 45 percent of the world's total goldproduction has been from the Witwatersrand district in SouthAfrica.

The largest gold mine in the United States is the Homestakemine at Lead, South Dakota.  This mine, which is 8,000 feet deep,has accounted for almost 10 percent of total United States goldproduction since it opened in 1876.  It has combined productionand reserves of about 40 million troy ounces.

In the past two decades, low-grade disseminated golddeposits have become increasingly important.  More than 75 suchdeposits have been found in the Western States, mostly in Nevada. The first major producer of this type was the Carlin deposit,which was discovered in 1962 and started production in 1965. Since then many more deposits have been discovered in thevicinity of Carlin, and the Carlin area now comprises a majormining district with seven operating open pits producing morethan 1,500,000 troy ounces of gold per year.

About 15 percent of the gold produced in the United Stateshas come from mining other metallic ores.  Where base metals--such as copper, lead, and zinc--are deposited, either in veinsor as scattered mineral grains, minor amounts of gold arecommonly deposited with them.  Deposits of this type are minedfor the predominant metals, but the gold is also recovered as abyproduct during processing of the ore.     Most byproduct gold has come from porphyry deposits, whichare so large that even though they contain only a small amount ofgold per ton of ore, so much rock is mined that a substantialamount of gold is recovered.  The largest single source ofbyproduct gold in the United States is the porphyry deposit atBingham Canyon, Utah, which has produced about 18 million troyounces of gold since 1906.

Geologists examine all factors controlling the origin andemplacement of mineral deposits, including those containing gold. Igneous and metamorphic rocks are studied in the field and in thelaboratory to gain an understanding of how they came to theirpresent location, how they crystallized to solid rock, and howmineral-bearing solutions formed within them.  Studies of rockstructures, such as folds, faults, fractures, and joints, and ofthe effects of heat and pressure on rocks suggest why and wherefractures occurred and where veins might be found.  Studies ofweathering processes and transportation of rock debris by waterenable geologists to predict the most likely places for placerdeposits to form.       The occurrence of gold is not capricious; its presence invarious rocks and its occurrence under differing environmentalconditions follow natural laws.  As geologists increase theirknowledge of the mineralizing processes, they improve theirability to find gold.



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