Gold was among the first metals to be mined

Throughout 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.

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The graves of nobles at the ancient Citadel

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.

Tutankhamun’s gold mummy case (photo by Lee Boltin).

Sketch of the so-called mask of Agamemnon, one of many gold
items from the tombs at Mycenae

Location of the Mahd adh Dhahab gold mine.

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The ancient civilizations

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.).

(Below)
An air view of the Mahd adh Dhahab gold mine in Saudi Arabia.
Swarms of gold-bearing quartz veins (seen as long irregular
trenches at a) have been mined for gold and silver for more than
3,000 years. Some of the veins have been followed downward to
depths as much as 300 feet. Similar quartz veins lace the hill to the
right (b), but these veins are not rich enough to mine.
Lumps of charcoal (the remains of wood fires used to smelt the
metals) were recovered from ancient slag piles and dated by scientists
of the U.S. Geological Survey using the carbon-14 method.
Some of the charcoal is as much as 3,000 years old indicating that
the mine was active during the reign of King Solomon.
The Saudi Arabian Mining Syndicate worked the mine from 1939 to
1954, getting ore from below the ancient workings as well as from
an open cut (c) and from old surface dumps. The mill (far left) and
buildings were erected by the Syndicate.

Mahd adh Dhahab gold mine.

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The gold in the Aztec

The gold in the Aztec and Inca treasuries of Mexico
and Peru is 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.

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Gold in the Depository

Gold in the Depository consists of bars about the
size of ordinary building bricks (7 x 3.6 x 1.75 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.

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The changes in demand for gold

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.

Stacks of gold bars in the Fort Knox Bullion Depository
(photo courtesty of the U.S. Mint).

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Gold is called a “noble” metal

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.

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Impure gold

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.

Recovering gold in a long sluicebox. Gold-bearing gravels are
shoveled into the upper end of the sluiceway; a series of riffles set
across the bottom of the box traps the gold. Myrtle Creek, Alaska,
circa 1900.

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Gold dissolves in aqua regia

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.

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The degree of purity of native gold

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.

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The basic unit of weight used in dealing with gold

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.

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No article having a gold alloy

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.

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One widely accepted hypothesis proposes

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

Mineralizing solutions travel upward along fractures and form
primary deposits.

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.

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Another hypothesis suggests that gold-bearing

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 wateris expelled from the rocks and migrates upward,
precipitating ore materials as pressures and temperatures
decease. The ore metals are thought to originate
from the rocks undergoing active metamorphism

The gold content of rocks is commonly determined by means of a
fire assay, a method known to metalworkers for 3,000 years or
more. In modern practice, a weighed sample of pulverized rock is
melted in a flux consisting of lead oxide, soda, borax, silica, and
flour or potassium nitrate, along with a measured amount of silver
as lead-silver alloy, in a furnace at a temperature of 1000° Celsius
(1800°F). The lead fraction contains the gold and added silver and
settles to cool as a button. The button is then remelted and oxidized
in a bone-ash cupel, which absorbs the lead oxide, leaving
behind a bead consisting of precious metals in the silver collector.
The bead is dissolved in acid and usually analyzed by atomic
absorption spectrometry.

Gold nugget weighing 81.9 troy ounces from the Union Placer
mine near Greenville Plumas County, California (photo courtesy of
Smithsonian Institution).

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The primary concerns of the prospector

The primary concerns of the prospector or miner
interested in a lode deposit of gold are to determine
the 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.

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Gold is extremely resistant to weathering

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.”

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In gold-bearing country

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

 Laboratory researchers develop new methods of analyzing rocks
for gold content.

 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.

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Through laboratory research

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 emission
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 rapdly. 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. By1905 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.

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During World War

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

Mining rich gold-silver at Goldfield, Nevada, circa 1905.

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. Since1980 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.

The main pit at the Carlin mine, Nevada, July 1974 (photo by
R.P. Ashley).

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Total world production of gold

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.

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Where most byproduct gold has come from

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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