Gold and silver

Silver and gold have played important roles in human economies, industries, and finances for thousands of years. They have served widely in applications as diverse as medicine and jewelry, and remain two of the most useful and sought-after metals.

Natural Forms

Gold, a deep yellow metal, is chemical element number 79, with an atomic weight of 196.967. Natural gold consists of a single isotope, gold-107, and in pure form exhibits a density of 19.3 grams per cubic centimeter. Silver, a brilliant white metal, is chemical element number 47, with an atomic weight of 107.87. Natural silver consists of two isotopes, silver-107 (51.4 percent) and silver-109 (48.6 percent), and in pure form exhibits a density of 10.5 grams per cubic centimeter. Both metals are very malleable, and this property has led to the common use of gold in gilding, a process by which extremely thin films are applied to surfaces of metal, ceramic, wood, or other materials for decorative purposes. Gold is so malleable that it can be pounded into translucent films so thin that more than 300,000 of them would be required to form a pile 1 inch (2.5 centimeters) high (the sheets are about 100 atoms thick, in other words).

Gold and silver occur in a variety of minerals. Native gold and electrum, the gold-silver alloy, are the most commonly occurring natural forms of gold in ore deposits. Although laboratory studies indicate that all compositions of gold-silver alloys may be synthesized, naturally occurring gold and electrum compositions are nearly always between 50 and 95 weight percent gold. The most commonly encountered compositions contain about 92 percent gold.

Economic Value

Although gold and silver are commonly thought of as precious metals, they differ greatly in their chemical properties and economic value. Gold is a true noble metal; that is, it is almost totally resistant to chemical attack and dissolution. Gold forms few chemical compounds and minerals. By comparison, silver is less noble, meaning that it is considerably easier to dissolve and forms several compounds and minerals. Gold is about sixty times more costly than silver, though relative values fluctuate; this difference is partly because of gold’s superior chemical properties but mostly because of its rarity relative to silver. These differences affect the geochemical and geological behavior of these metals as well as the intensity of the mining and mineral exploration activities directed at their recovery.

The average abundance of gold in the earth’s crust is about 0.004 part per million, and the average abundance of silver is 0.08 part per million, about twenty times that of gold. These amounts of gold and silver are far too small to pay for the extraction of these metals from common rock. In some places in the earth’s crust, however, hydrothermal solutions have circulated, collected some of the gold and silver from the rocks, and transported them to sites of deposition. The rocks at these sites contain gold and silver values many times greater than those of average rocks and are sought out as ore deposits. In addition, the weathering of gold deposits and transportation of gold particles by streams creates placer deposits, where gold can be extracted from sand and gravel by simple techniques. In order for gold deposits to be mined economically, the concentrations of the gold generally must be on the order of 1 to 3 parts per million (about 0.1 troy ounce per ton), but some of the richest deposits contain ores with 8 to 10 parts per million (about 0.2 troy ounce per ton). Because silver is worth much less, deposits must generally contain 150 to 300 parts per million (5 to 10 troy ounces per ton) if they are mined solely for silver. Much silver, however, is extracted as a by-product from copper, lead, and zinc ores in which the silver is present in concentrations of 1 to 50 parts per million.

Because economic concentrations of gold are so low, very sensitive chemical, analytical techniques must be used to determine the amount of gold present. One of the oldest but still widely used is fire assaying. In this method, lead oxide is mixed with a powdered ore sample and melted until the lead oxide is converted to metallic lead that picks up the gold and silver and settles to the bottom of the crucible. The lead, gold, and silver bottom is melted in a porous cup where the lead is again converted to lead oxide that is absorbed by the cup, leaving a gold and silver mixture known as doré metal. The doré metal is weighed and the silver is dissolved by nitric acid. The remaining pure gold is reweighed to determine the amount of gold.

Mining and Extraction

When economic gold-bearing deposits are found, the method of mining the ore and extracting the metals (some silver is always present) varies, depending on the location of the deposit and its mineralogy. If the gold occurs as large enough grains, it is readily concentrated by gravity techniques, and the high density of the gold (15 to 19 grams per cubic centimeter) allows it to be physically separated from other minerals that have much lower densities. The gold pans and sluices used by early miners and some mines today are small-scale examples of gravity methods. Conversely, if the gold is present in very small, sometimes submicroscopic grains, chemical solvent techniques are utilized. The most commonly employed method applies a sodium cyanide solution that dissolves the gold. The gold is removed from the solution by activated charcoal, and the solution is reused. The gold is redissolved from the charcoal and then electrochemically precipitated on steel wool. The steel wool is mixed with silica, borax, and niter and melted to produce a doré metal of the gold and silver and a slag with all the impurities. The doré metal is sent to a refinery to separate the gold and silver.

Silver-rich ores usually contain much higher concentrations of silver-bearing minerals than do gold mines. After grinding the ores to 0.01 to 0.05 inch (0.2 to 1.0 millimeter), the silver minerals are separated by flotation techniques in which small bubbles generated in soaplike solutions are used to pick up and concentrate the small grains. The concentrated materials are shipped to refineries for chemical separation of the metals and sulfur.

As of 2023, the two largest silver mines in the United States—Greens Creek Mine and Red Dog Mine—are both in Alaska, while the three largest gold mines—Carlin Mine, Cortez Mine, and Turquoise Ridge Mine—are in Nevada. The world’s largest gold mine was South Africa’s South Deep Mine, which produced 322,000 ounces of gold in 2023.  The Penasquito Mine in Mexico was the world’s largest silver mine, producing more than 30 million ounces in 2020. Gold and silver are produced in many other areas of the world as well. In addition, significant amounts of gold and silver are recovered as by-products during the processing of the ores of other metals, such as copper.

Hydrothermal Systems

There are many types of hydrothermal systems that produce deposits of gold and silver. Many of these hydrothermal systems are ancient analogues of modern geothermal systems such as the one at Yellowstone National Park, Wyoming. Geologists deduce the nature of the hydrothermal system that produced a particular deposit by studying its size, shape, mineralogy, ore grade, and tectonic setting. The size and shape of the ore body are determined from maps of the surface outcrop and from geological cross-sections of the subsurface extent of the ore body, prepared by consulting drill core logs and maps of outcrops in underground workings. Observations of hand samples, microscopic observation (transmitted and reflected light), X-ray diffraction analysis, and electron microprobe analysis are common processes used to determine the mineralogy of the ore (mineralized rock from which metals can be economically extracted), the gangue (minerals that have no economic value and that were deposited by the hydrothermal solutions), the country rock (the unmineralized rock surrounding the deposit), and the alteration zone (country rock that was changed chemically or mineralogically by reactions with the hydrothermal solutions). The types, compositions, and associations of these minerals are used to infer the chemical conditions at the time of the ore deposit’s formation. Fluid inclusions, or small bubbles of the hydrothermal solution trapped in growing mineral grains, are commonly observed features that are particularly useful in determining the nature of the fluids from which the ore minerals precipitated. The gold and silver content of the rocks (ore grade) is determined by chemical analysis of systematically selected samples from various parts of the ore body. Some ore bodies show sharp cutoffs of grade between the mineralized rock and the country rock. Others show a gradual decrease from the mineralized zone to the background levels of the country rock. The tectonic (deformational crust) and structural setting of the ore body give evidence of the general geologic framework for the ore-forming process. Nearly all hydrothermal ore deposits are associated with tectonically active areas, where the rate of heat flow from the earth’s interior to the surface is quite high. It is this heat flow that increases the temperatures of the hydrothermal solutions and drives them through the rocks. Midocean ridges and rises and island arcs associated with subduction zones (where the edge of one crustal plate descends below the edge of another) are recognized as tectonically active areas that have high rates of heat flow and are thus settings for the potential formation of hydrothermal ore deposits.

Weathering breaks down rocks both chemically and physically. The products of weathering are ions that are carried away by solutions and solids that are eroded from the surface by rainwater. When hydrothermal ore deposits weather, most of the ore minerals are destroyed by oxidation, and their components are carried away in solution. If these solutions penetrate downward into the outcrop, they sometimes encounter more reducing conditions at depth, and the ore elements reprecipitate, or separate out again, to form an enriched zone. This process is called secondary enrichment. The grade of gold and, particularly, silver deposits can be improved by this process. Most of the time, however, gold is simply too unreactive to be destroyed by chemical weathering and instead concentrates in the soil horizon to form an eluvial placer. As the soil erodes, the gold grains are carried into streams and rivers where they are transported until they encounter areas of slower water velocity. There, they are dropped by the stream and accumulate, because these grains have a much greater density than common silicates. Gold and electrum grains have densities ranging from 15.5 to 19.3 grams per square centimeter, whereas common silicate minerals have densities ranging from 2.6 to 3.0 grams per square centimeter. Thus, the common silicates simply are washed away, leaving the gold and electrum grains behind. The Witwatersrand deposits are paleoplacer deposits; that is, they are placer deposits that formed about 2 billion years ago when rivers and streams carrying gold grains flowed into a large basin. As the waters of these rivers and streams entered the basin, their velocity slowed and they deposited their load of gold-bearing gravels and sands. These deposits were eventually covered by other rocks and became lithified, or changed to stone. The very large extent and relatively high grade of these deposits (8 to 10 parts per million) make the Witwatersrand district the largest gold district in the world.

One of the more interesting problems surrounding the formation of silver and lode gold deposits is determining how these elements were transported by aqueous solutions. Gold is a noble metal, meaning that it is very resistant to the natural chemical attacks that would dissolve it. Silver is not as noble as gold and therefore tends to form compounds (minerals) with a wide variety of elements. These silver minerals, as well as native silver, have low solubilities. The solubility of gold and silver and their minerals does seem to increase with temperature; however, even at very high temperatures, their solubility in pure water is much too low to account for the amounts of the metals transported into ore deposits. The most likely explanation for the ability of hydrothermal solutions to transport these metals is that they react with other ions in the solution, such as chloride, bisulfide, and perhaps others, to form very stable complexes. The formation of complexes increases the stability of the gold or silver species in the aqueous solution and therefore makes gold and silver soluble enough to account for their transport to form ore deposits. When solutions containing these complexes are cooled, mixed with solutions containing lower concentrations of complexing ions, oxidized by reactions with other minerals, or boiled to remove chloride or bisulfide as vapors, the complexes tend to break down, lowering the solubility of gold and silver and causing these elements to precipitate in metallic form or as constituents of minerals.

Continuing Demand

Gold and silver were among the earliest metals used by humans. They were employed as amulets and jewelry as early as 5000 BCE, and their use as money and in coinage had begun in Asia Minor and Greece by about 600 BCE Although these practices continue today, gold and silver have found many additional uses and have diverged in their principal applications. In the United States, gold today is used primarily in jewelry and in the arts (about 55 percent of the total usage); other major uses include solid-state electronic devices (34 percent), dental supplies (12 percent), and investment products (0.1 percent). In contrast, silver is primarily an industrial metal, with 43 percent being used in photography and 35 percent being used in electrical products. Other important silver uses include jewelry (6 percent), sterling ware (6 percent), and coinage (4 percent). Gold and silver coins were widely minted in many countries, especially in the 1700s and 1800s, but were generally dropped from use in the early 1900s. The United States minted gold coins from 1849 until 1933 and silver coins from 1794 until 1964. In 1964, the price of silver for photography exceeded the value of the silver in coins, and silver coins were discontinued. Since 1986, the United States has minted gold and silver bullion coins that do not have assigned denominations but rather contain specified amounts of gold or silver and vary in value as metal market prices fluctuate.

Throughout history, the quest for gold and silver has played a very important role in the development and expansion of human civilization, culture, and enterprise. By the time of the Egyptian empire, humankind had already developed sophisticated mining and metallurgical processes for producing precious metals. The wealth and prosperity of Athens was based in large part upon the silver production of the mines at Laurion in southeastern Attica (Greece), and the Roman Empire flourished as a result of gold and silver obtained from the Iberian Peninsula. Christopher Columbus’s encounter with gold-bearing Native Americans on his first voyage provided a powerful stimulus for the Spanish to explore and exploit the New World. The new gold and silver stocks (181 metric tons of gold and 18,000 metric tons of silver) shipped from the New World to Europe from 1500 to 1650 produced a revitalization of the economic system that helped fuel the later stages of the Renaissance. Paradoxically, Spain, glutted by such money, became impoverished as a result. The search for gold and silver also played a major role in the development of the United States, especially the gold rush of 1849 that lured thousands to California. Although less known, the first gold rush in the United States was in the Carolinas and Georgia in the 1830s, and concerned rocks quite similar to those of the Sierra Nevada foothills of California. Subsequent gold discoveries in the Black Hills of South Dakota in 1874 and in the Klondike and Yukon in 1896 drew additional treasure hunters. The quest for gold also led to the opening of Australia in 1851 and South Africa in 1886. Currently, gold exploration and gold rushes in Brazil have produced an invasion of much of the tropical rain forests.

Gold and silver are expensive metals, and for this reason there is a constant search for less expensive substitutes. Many pieces of jewelry and electronic products can be made by alloying other metals with gold and silver, or by merely coating base metals. Other products can be redesigned to maintain their utility while using smaller amounts of gold. In many cases, less expensive palladium and silver can be substituted for gold. Although more expensive, platinum is sometimes used instead of gold for coins, jewelry, and electrical products. Aluminum and rhodium are less expensive substitutes for silver in mirrors. Surgical plates, pins, and sutures can be made of tantalum instead of silver. Silver tableware can be replaced by stainless steel products. Video cameras, silverless black-and-white film, and xerography have reduced the silver demand of copying and photography. Even with these substitutions, it is unlikely that the demand for these metals will diminish.

Principal Terms

amalgam: an alloy of mercury and another metal; gold and silver amalgams occur naturally and have been synthesized for a variety of uses

electrum: a term commonly used to designate any alloy of gold and silver containing 50 to 80 weight percent gold

fineness: a measure of the purity of gold or silver expressed as the weight proportion of these metals in an alloy; gold fineness considers only the relative proportions of gold and silver present, whereas silver fineness considers the proportion of silver to all other metals present

karat: a unit of measure of the purity of gold (abbreviated “k”); pure gold is 24 karat

lode deposit: a primary deposit, generally a vein, formed by the filling of a fissure with minerals precipitated from a hydrothermal solution

placer deposit: a mass of sand, gravel, or soil resulting from the weathering of mineralized rocks that contains grains of gold, tin, platinum, or other valuable minerals derived from the original rock

troy ounce: approximately 31 grams; there is about 14.5 troy ounces per pound, compared to the 16 ounces in everyday (avoirdupois) ounces.

Bibliography

Boyle, Robert W. The Geochemistry of Silver and Its Deposits. Geological Survey of Canada Bulletin 160. Ottawa: Queen’s Printer, 1968.

‗‗‗‗‗‗‗‗‗‗. Gold: History and Genesis of Deposits. New York: Van Nostrand Reinhold, 1987.

Brooks, Robert R., ed. Noble Metals and Biological Systems: Their Role in Medicine, Mineral Exploration, and The Environment. Boca Raton, Fla.: CRC Press, 1992.

Cotton, Simon. Chemistry of Precious Metals. London: Blackie Academic and Professional, 1997. Illustrates the procedures and protocols used to determine the chemical makeup and properties of precious metals. Illustrations, bibliography, and index.

Evans, Anthony M. An Introduction to Economic Geology and Its Environmental Impact. Malden, Mass.: Blackwell Science, 1997.

"The Five Largest Gold Mines in Operation in US." Mining Technology, 18 June 2024, www.mining-technology.com/marketdata/five-largest-gold-mines-the-us/. Accessed 25 July 2024.

"The Five Largest Silver Mines in Operation in US." Mining Technology, 18 June 2024, www.mining-technology.com/marketdata/five-largest-silver-mines-the-us/. Accessed 25 July 2024.

Gasparrini, Claudia. Gold and Other Precious Metals: From Ore to Market. New York: Springer-Verlag, 1993.

Gee, George E. Recovering Precious Metals. Palm Springs, Calif.: Wexford College Press, 2002. Focuses on gold and silver recovery and recycling processes. Discusses many precious metal sources, including silver from photography solutions, and mirror-maker’s solutions. Covers the separation of gold and silver from platinum.

Gray, Theodore. The Elements: A Visual Exploration of Every Known Atom in the Universe. New York: Black Dog & Leventhal Publishers, 2009.

Laznicka, Peter. Giant Metallic Deposits. 2d ed. Berlin: Springer-Verlag, 2010.

Marsden, John O., and C. Iain House. The Chemistry of Gold Extraction. 2d ed. Littleton, Colo.: Society for Mining, Metallurgy and Exploration, 2006.

St. John, Jeffrey. Noble Metals. Alexandria, Va.: Time-Life Books, 1984.

"Mineral Commodity Summaries 2023." U.S. Geological Survey, 31 Jan. 2023, www.usgs.gov/publications/mineral-commodity-summaries-2023. Accessed 25 July 2024.

"National Minerals Information Center." U.S. Geological Survey, 2024, www.usgs.gov/centers/national-minerals-information-center. Accessed 25 July 2024.

Watkins, Tom H. Gold and Silver in the West: The Illustrated History of an American Dream. Palo Alto, Calif.: American West, 1971.

White, Benjamin. Silver: Its History and Romance. Detroit, Mich.: Tower Books, 1971.