Gold by Harold Kirkemo, William L. Newman, and Roger P. Ashley:
Here is another detailed aticle discussing 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."
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, gold masks, gold cups, gold diadems,
and gold jewelry, plus hundreds of decorated gold beads and gold buttons. These
elegant works of art were created from gold 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
Artisans in Mesopotamia and Palestine probably obtained
their supplies of gold 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
The gold in the Aztec and Inca treasuries of Mexico and Peru
believed to have come from Colombia, although some gold 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
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
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
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 for gold is the pennyweight (dwt.) which is equivalent to 1.555
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 of gold content 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
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
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
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
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
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
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.
This publication is one of a series of general interest
publications prepared by the U.S. Geological Survey to provide
information about the earth sciences, natural resources, and the
environment. To obtain a catalog of additional titles in the
series "General Interest Publications of the U.S. Geological
U.S. Geological Survey
Denver, CO 80225
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