Bulk Mineable Gold Deposits of the Western United States
Bulk mineable gold deposits in the western United States contain reserves of more than 55 million oz (1,710 metric tons) Au and produce more than 3 million oz (93,300 kg) Au/yr. They can be subdivided into five main deposit types: porphyry-related, sediment-hosted (includes deposits of the Carlin type), metamorphic-hosted, volcanic-hosted, and hot spring gold deposits. Each deposit type can be further divided into several deposit subtypes.
Examples of porphyry-related gold deposits are Zortman-Landusky in Montana and the Fortitude and McCoy skarn-hosted gold deposits in Nevada. Major sediment-hosted gold deposits in the western United States occur mainly in Nevada and include the world-class gold deposits located within the Carlin and Getchell trends. Metamorphic deposits include Mother Lode and Mesquite types, located principally in California and Arizona. Volcanic-hosted deposits can be subdivided into three main deposit subtypes: high sulfur deposits such as Summitville, Colorado, and Paradise Peak, Nevada; low sulfur deposits such as Round Mountain, Nevada; and alkalic deposits such as Cripple Creek, Colorado. Hot spring gold deposits are typified by the McLaughlin gold deposit in northern California.
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When the price of gold rose from about $200 (U.S.) an ounce in 1979 to nearly $700 an ounce by the end of the same year, the gold rush of the 1980s was under way. Gold production in the western world rose dramatically; from 1981 to 1986 production increased by 300 to 1,282 metric tons per year. Annual production may reach 1,500 to 1,600 metric tons by 1990 (Woodall, 1988). The major contributors to the increased stream of gold have been Australia, Canada, Brazil, and the United States together with other circum-Pacific countries. The increased price of gold and new methods of extraction have allowed many older deposits to be reopened, but the most important factor has been the high success level of exploration. This success has resulted in large part from the application of new genetic models and from the development of new exploration techniques.
There are hundreds of thousands of reported gold occurrences around the world. The majority are alluvial placers, but large numbers of bedrock occurrences have also been discovered. Most of these occurrences prove to be very small and are relatively unimportant in the overall world production level. Most mined gold has come from a small number of giant deposits, which were found by prospectors. It is becoming increasingly clear, however, that the discovery of giant deposits in the future will involve more than the sharp eyes and persistence of the old prospector. The use of sound geologic principles, and exploration programs based on those principles, is what the future holds. An example can be seen in the successful search for gold deposits in the South Pacific. There, exploration models have been based on principles developed in the study of modern geothermal systems. Giant deposits such as Lihir and Porgera have been the reward. Another example is the giant copper-gold-uranium deposit at Olympic Dam, South Australia, discovered beneath 300 m of cover using an exploration program based on models developed by Western Mining Corporation geologists for Zambian copper belt-type deposits.
Gold deposits are widely dispersed throughout many geologic settings and in virtually all kinds of rocks, but they do not seem to have formed at a uniform rate throughout geologic history. On the contrary, two very distinct metallogenic periods have been defined. The first is the Archean era, when most of the great deposits in greenstone belts were formed and the vast Witwatersrand basin deposits in