A brief history of gold uses, prospecting, mining and production
Republished from a USGS general interest publication by Harold Kirkemo, William L. Newman, and Roger P. Ashley
Uses of Gold in the Ancient World
Gold was among the first metals to be mined because it commonly occurs in its native form, that is, not combined with
other elements, because it is beautiful and imperishable, and because exquisite objects can be made from it. Artisans
of ancient civilizations used gold lavishly in decorating tombs and temples, and gold objects made more than 5,000
years ago have been found in Egypt. Particularly noteworthy are the gold items discovered by Howard Carter and Lord
Carnarvon in 1922 in the tomb of Tutankhamun. This young pharaoh ruled Egypt in the 14th century B.C. An exhibit of
some of these items, called "Treasures of Tutankhamun," attracted more than 6 million visitors in six cities during a
tour of the United States in 1977-79.
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.
Ancient Gold Sources
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.
Gold as a Medium of Exchange
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, like silver, is used in jewelry and allied wares, electrical-electronic applications, dentistry, the
aircraft-aerospace industry, the arts, and medical and chemical fields.
Gold Price Regulation and Variability
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.
Properties of Gold
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.
Fineness, Karats and Troy Ounces
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."
Formation of Primary Gold Deposits - Lode Gold
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 igneous 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.
Concentration of Gold in Placer Deposits
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."
Prospecting for Placer Deposits
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 gemstones 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.
Gold Books and Panning Supplies
Looking for gold? We have over 50 different gold books and gold maps that show where gold has been found in the past and provide instruction on gold prospecting methods. Also available are gold pans in a variety of sizes and gold panning kits that include everything that you will need.
Free Gold Assay
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.
Early Gold Finds and Production
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
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.
Disseminated Deposits and By-Product Gold
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.
Role of a Geologist in Gold Prospecting
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.
of ancient civilizations used gold lavishly in decorating tombs and temples, and gold objects made more than 5,000
years ago have been found in Egypt. iStockPhoto image © Akhilesh Sharma.
|As a highly valued metal, gold was used as a financial standard and has been used in coinage for thousands of years. United States ten dollar gold coin from 1850. . iStockPhoto image © Brandon Laufenberg.
|The discovery of gold triggered numerous gold rushes in the United States and around the world. iStockPhoto image © Duncan Walker.
|Small nuggests of gold obtained by panning. Prospectors worked stream sediments to find tiny nuggets that they would sell or trade for supplies.
|Fortitude Mine in Nevada produced about 2 million ounces of gold from a lode deposit between 1984 and 1993. USGS image.
|Hydraulic placer mining at Lost Chicken Hill Mine, near Chicken, Alaska. The firehose blasts the sediment outcrop, washing away sand, clay, gravel and gold particles. The material is then processed to remove the gold. USGS image.
|Portable gold sluice. Miners place the sluice in the stream and dump sediments in the upstream side. The current transports the sediments through the sluice and the heavy gold particles become lodged in the sluice. One miner can process a lot more sediment through a sluice than through a gold pan. iStockPhoto image © LeeAnn Townsend.
|A scuba diver vacuums sediment to be processed by a portable gold dredge. Scuba gear allows the prospector to carefully get access to cracks and crevaces on the stream bed where gold nuggets might be lodged. iStockPhoto image © Gary Ferguson.
|A portable dry washer used to sift gold nuggets from soil where water is not available. Soil is dumped into the top pan and is shaken through the bottom pan. Heavy gold nuggets are mechanically separated from lighter materials. iStockPhoto image © Arturo M. Enriquez.
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