Everything You Ever Wanted to Know About Gold

by Travis Daniel

Image taken from San Diego Muesum of Natural History Field Guide:
http://www.sdnhm.org/fieldguide/minerals/gold.html

This webpage was created to fulfill an assignment for mineralogy class at Emporia State University. The purpose of this web page is to tell you everything you ever wanted to know about the fascinating mineral, gold. This page will be divided into the following sections: characteristics, occurrence, and use. So pull up a chair, and learn a thing or two about the wonderful world of gold.


Basic Characteristics

The basic characteristics to be discussed include crystallography, physical properties, composition, and structure. The crystallography of gold is not often talked about because gold is rarely found in a perfect crystal form.  Gold is more often found in irregular plates, scales and masses.  Although crystals are rare, gold is a member of the isometric crystal system and will display 4/mBar32/m external crystal symmetry.  Gold crystals are most commonly found as octahedral and very rarely found as a dodecahedron, cube and trapezohedron (Klein 336).

Since gold is so valuable, the physical properties of this mineral are fairly well known. People are always on the look out for gold and need to learn to distinguish it from other similar minerals.  Gold's physical properties are as follows:  hardness 2.5-3; specific gravity of 19.3 when pure (when other elements are included in gold, the specific gravity can be much lower, as low as 15); no cleavage, but has a hackly fracture; malleable and ductile tenacity; completely opaque transparency; metallic luster; and a color which includes many different shades of yellow (Klein 336).

Physical properties may be used to distinguish gold from the many impostors.  The most common impostor is pyrite and although they often have identical colors, other physical properties are very different.  The most distinct differences are in hardness, malleability and specific gravity.  Pyrite is a much harder mineral at about 6-6.5 and will scratch glass unlike gold.  Pyrite is not malleable, which means it will break when force is applied, whereas gold might dent or bend.  Finally gold is much heavier.  Pyrite has an average specific gravity for metallic minerals, about 5, which is much lighter than the 19.3 of gold (Kirkemo 7).

More information on the physical properties of gold may be found at: http://www.sdnhm.org/fieldguide/minerals/gold.html.


Occurrence

The occurrence of gold is interesting, in part because although it is rare, gold is mined in many different localities throughout the world.  The highest gold producing country is the Republic of South Africa.  South Africa alone is responsible for about one half the gold produced in the world.  The Commonwealth of Independent States (C.I.S.) produces about 20% of the world's total gold production.  After these two giant producers, China comes in at a distant third followed by Canada, United States and Brazil.  In the U.S. the highest gold producing state is Nevada, followed by South Dakota, Utah, Arizona, and Montana.  In Utah and Arizona, the gold is a byproduct of large copper mines (Klein 336).

Geologically, gold is mined commercially from two different types of deposits, lode and placer deposits.  Lode deposits are the "big dogs" as far as mining goes.  These are what the gold prospectors dream of finding, a nice large lode deposit of nearly pure gold.  Gold is found insitu or at the original location of deposition from mineralizing solutions in lode deposits.  The source of these mineralizing solutions are widely disputed among geologists today, but there are three major hypotheses (Kirkemo 11).

One hypothesis on the origin of gold lode deposits is related to the fact that gold may be found in volcanic and sedimentary rocks.  The hypothesis states that water, in the form of rain, enters the ground through different cracks, fissures and permeable rock/soil layers; once this water enters the ground it is drawn into hotter regions, that are heated by magma or molten rock.  The pressure forces the water upward once again, and most importantly, the heated water dissolves many of the rocks around it.  As the water is pushed upward by the hot magma, along with the dissolved minerals from the surrounding rock, it eventually reaches cooler and shallower rocks.  At some point, the different metals that were dissolved in the hot water are then precipitated out in cracks called "veins" or sometimes, flatter and larger ore bodies (Kirkemo 13).

A second gold origin hypothesis has to do with gold deposits that are found in granitic rocks, which suggests that gold is in the magma that moves up to the Earth's.  The magma cools and solidifies, with the gold bearing solutions expelled from the magma and immediately precipitated (Kirkemo 13).

A final hypothesis deals with gold veins that are found mainly in metamorphic rocks and specifically in mountainous regions of continental margins where mountain building occurs.  In this mountain building process, different rocks of all types are often buried and forced under a continent where they often experience very high pressures and temperatures.  These high temperatures and pressures often result in chemical reactions that rearrange the rock's mineral constituents.  This hypothesis suggests that as rocks are undergoing this metamorphism, ore metals are dissolved in water that was contained in the original rocks.  Metamorphism causes the solution to be expelled from the rocks and move to the surface.  During this journey, they reach lower pressures as well as lower temperatures and again the dissolved metals precipitate (Kirkemo 14).

Another type of gold deposit is a placer deposit.  Placer deposits come from preexisting lode deposits that are exposed at the surface of the earth.  As these lode deposits are exposed and weather, the gold is released from the surrounding rock and exposed veins are transported by rivers in the form of dust or flakes.  Because of gold's high specific gravity, it moves downstream usually in times of high water when everything that rests on the stream bottom is moved downstream.  When the stream slows gold will collect in pockets of sand and gravel bars.

Image taken from The South
Carolina Gold Museum Gallery,
which is no longer an active
link but still a museum located
in Pickens, SC.
When people "panned" for gold, they were looking for the placer deposits or concentrations of gold in gravel called "pay streaks." Today there are many places throughout the world where you can still pan for gold.  One could even plan a vacation around that very activity, at gold prospecting trips (Kirkemo 15). Here is a site about gold panning in South Carolina, http://www.goldmaps.com/east/south_carolina_gold_mines.htm.
The largest gold mine today is found in South Africa.  All throughout earth history it is believed that these large deposits were formed in the Precambrian times.  Unfortunately the Precambrian is a relatively long period of time and we know very little about it compared to more recent periods of earth history.  In a broad sense, these large African deposits occurred somewhere between about 3,000 to about 544 million years ago (Klein 336).

The South African deposits are a type of placer deposit, although the Precambrian deposits are called "fossil" placers.  These deposits are placers that at one point were flowing through streams but eventually the streams ran dry.  Over time the gold was buried under other sedimentary rocks and are consequently cemented into hard rock.  Evidence for ancient river channels and banks is seen, with one of the most famous of these deposits being the Precambrian Witwatersrand conglomerate nicknamed the "Rand."  This conglomerate is found in the Transvaal area of South Africa and other similar conglomerate fossil placers are found in the Orange Free State of South Africa (Klein 336). 

Uses of Gold

Gold was one of the very first metals to be mined in all of human history.  One reason for this is the fact that gold occurs so often in its native form which means it lacks significant amounts of other metals with it.  Along with its purity, the first miners, which can be traced all the way back to the first century, used gold because of its unmistakable beauty and durability.  Gold's ductility and malleability lend the metal to uses in decorations, jewelry, masks, cups, diadems, beads, buttons and use on the tombs of famous rulers, such as Tutankhamun (Kirkemo 2).

Gold is valued for its beauty and rarity, but also as bullion to be used in international currency settlements.  Gold is used as allied wares, electrical-electronic applications, dentistry, the aircraft-aerospace industry, the arts, medical, and chemical fields (Klein 336).  Gold's many uses, rarity, and beauty keep the price for this noble metal.


Bibliography

This page was created 12/9/99; updated links February 29, 2004. If you have any questions or comments please email me at danieltr@esuvm.emporia.edu. Return to Student Webpages.