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 emission spectrometry. These methods enable rapid and extremely sensitive analyses to be made on large samples.
In the past, 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. That definitely makes Nevada a prospectors paradise.
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.
Article by: Harold Kirkemo, William L. Newman, and Roger P. Ashley