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 rare, durable, chemically inert, and beautiful. Hot springs and fumaroles are still active on the caldera floor. Most of the gold is fine particles in pyrite (FeS) grains. It is estimated that the deposit contains 21.3 million ounces proven and probably another 42 million ounces as a geological resource. The deposit formed between 350,000 and 100,000 years ago. Most of the ore is in breccia thought to have been a boiling zone for rising fluids.
The rocks are trachybasalt lava flows, breccia, and tuffs.The mineralized rocks are highly altered. The island is made of three volcanoes including Luise caldera, where the deposit formed. Lihir Island is an epithermal deposit discovered in 1982. Diagram from ITAM Gold by the Minerals Council of Australia. Veins commonly host the economic minerals. The boiling zone is the target for mineral exploration. The water boils about 300 m below the surface and hydrogen sulfide gas escapes. The rising hot water carries dissolved gold and other elements. The deposits form within 1 km of the surface and water temperatures are about 50-200 degrees C. Porphyry deposits are zoned in alteration (potassic ® sericitic ® argillic ® propylitic) and mineralization.Įpithermal refers to mineral deposits that form in association with hot waters. The deposits are commonly 3-8 km across and copper may be less than 1% of the rock. This requires large amounts of rock to be mined, often in open pits. 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). Erosion strips off overlying rocks to expose the mineralization. This type of deposit forms beneath stratovolcanoes and is associated with subduction zones. Diagram from ITAM Copper by the Minerals Council of Australia. 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. Porphry is a general term applied to igneous rocks of any composition that contain conspicuous phenocrysts (crystals) in a fine-grained groundmass. Gold and copper are found in ore bodies associated with porphry. The gold is found in altered rocks in all parts of the stratigraphic sequence. Another 5.4 million ounces of gold remains in the deposits. Three mines have produced more than 2.1 million ounces of gold from 1980-1993. Ives deposit near Kambalda, Western Australia. Diagram from ITAM Gold by the Minerals Council of Australia.Īnother example of gold hosted in greenstone is the St. By 1993, 40 million ounces of gold has been mined from the Golden Mile. Dolerite refers to dike rocks with plagioclase crystals in pyroxene crystals. The bottom diagram is a cross-section of the deposit. The top diagram is a simple map that shows folds, faults, rock type, and mineralization. The deposit itself is usually a quartz vein that carries the gold or adjacent altered rock.Ī classic example of gold hosted in greenstone is the Golden Mile in Kalgoorlie, Western Australia. 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 gold is though to be mobilized by hydrothermal solutions during regional metamorphism. Intensely altered and fractured basalt is a common host rock. Gold is most commonly found along the edges of greenstone belts and associated with structural features. These rocks are very complex, having undergone metamorphism, folding, faulting, and shearing. Greenstone belts are volcanic-sedimentary sequences, which include ultramafic rocks, dolerite, basalt, chert, sandstone, shale, tuff, banded iron-formation and other rock types. Gold is found in Archean (rocks older than 2.5 billion years) greenstone belts in Australia, southern Africa, and Canada. Three environments/styles are most common: gold in greenstone belts, gold in porphyry deposits, and gold in epithermal deposits. Gold forms in close association with volcanoes or is hosted in volcanic rocks. Photo shows gold jewelry recovered from the ash deposits of the 79 A.D.
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