Gold forms in close association with volcanoes or is hosted in volcanic rocks. Three environments/styles are most common: gold in greenstone belts, gold in porphyry deposits, and gold in epithermal deposits.
Gold in Greenstone Belts
Gold is found in Archean (rocks older than 2.5 billion years) greenstone belts in Australia, southern Africa, and Canada. Greenstone belts are volcanic-sedimentary sequences, which include ultramafic rocks, dolerite, basalt, chert, sandstone, shale, tuff, banded iron-formation and other rock types. These rocks are very complex, having undergone metamorphism, folding, faulting, and shearing. Gold is most commonly found along the edges of greenstone belts and associated with structural features. Intensely altered and fractured basalt is a common host rock. The gold is though to be mobilized by hydrothermal solutions during regional metamorphism. The solutions probably contain only a few parts-per billion gold but great volumes of solution can precipitate their gold in a small zone with favorable chemical conditions. The deposit itself is usually a quartz vein that carries the gold or adjacent altered rock.
A classic example of gold hosted in greenstone is the Golden Mile in Kalgoorlie, Western Australia. The top diagram is a simple map that shows folds, faults, rock type, and mineralization. The bottom diagram is a cross-section of the deposit. Dolerite refers to dike rocks with plagioclase crystals in pyroxene crystals. By 1993, 40 million ounces of gold has been mined from the Golden Mile. Diagram from ITAM Gold by the Minerals Council of Australia.
Another example of gold hosted in greenstone is the St. Ives deposit near Kambalda, Western Australia. Three mines have produced more than 2.1 million ounces of gold from 1980-1993. Another 5.4 million ounces of gold remains in the deposits. The gold is found in altered rocks in all parts of the stratigraphic sequence. Diagram from ITAM Gold by the Minerals Council of Australia.
Gold in Porphyry Deposits
Gold and copper are found in ore bodies associated with porphry. Porphry is a general term applied to igneous rocks of any composition that contain conspicuous phenocrysts (crystals) in a fine-grained groundmass. The term is from a Greek word for purple dye and was first applied to a purple-red rock with phenocrysts of alkali feldspar that was quarried in Egypt. Diagram from ITAM Copper by the Minerals Council of Australia.
This type of deposit forms beneath stratovolcanoes and is associated with subduction zones. Erosion strips off overlying rocks to expose the mineralization. Gold and copper are found in sulfide minerals disseminated throughout the large volumes of intrusive rock (strictly speaking, this ore is associated with volcanic systems, usually not the volcanoes themselves). This requires large amounts of rock to be mined, often in open pits. The deposits are commonly 3-8 km across and copper may be less than 1% of the rock. Porphyry deposits are zoned in alteration (potassic ® sericitic ® argillic ® propylitic) and mineralization.
Gold in Epithermal Deposits
Epithermal refers to mineral deposits that form in association with hot waters. The deposits form within 1 km of the surface and water temperatures are about 50-200 degrees C. Shallow bodies of magma supply heat. The rising hot water carries dissolved gold and other elements. The water boils about 300 m below the surface and hydrogen sulfide gas escapes. This causes the gold to precipitate. The boiling zone is the target for mineral exploration. Veins commonly host the economic minerals. Diagram from ITAM Gold by the Minerals Council of Australia.
Lihir Island in Papua New Guinea
Lihir Island is an epithermal deposit discovered in 1982. The island is made of three volcanoes including Luise caldera, where the deposit formed. The rocks are trachybasalt lava flows, breccia, and tuffs.The mineralized rocks are highly altered. Most of the ore is in breccia thought to have been a boiling zone for rising fluids. The deposit formed between 350,000 and 100,000 years ago. It is estimated that the deposit contains 21.3 million ounces proven and probably another 42 million ounces as a geological resource. Most of the gold is fine particles in pyrite (FeS) grains. Hot springs and fumaroles are still active on the caldera floor.
Uses for Gold
Gold is rare, durable, chemically inert, and beautiful. These qualities make it useful for monetary exchange and investment, jewelry, and art.Gold’s high electrical conductivity, malleability, and ductility make it useful as an industrial metal. Gold is alloyed with silver, copper, nickel, palladium, zinc, and titanium to increase its tensile strength. Note: for gold, 1 carat = 1/24 part. Pure gold is 24 carat.
World Supply of Gold
Below is a pie diagram of major gold producers. In 1994, 2,296 tonnes of gold was produced. South Africa is the largest producer, followed by the USA, Australia, and the CIS (former Soviet Union). Diagram from ITAM Gold by the Minerals Council of Australia.