About 106 kg (>30 Moz) of gold have been recovered from skarn deposits. These occurrences can be subdivided into five major skarn types: gold, iron, copper, porphyry copper, and lead-zinc. Each skarn type has distinctive characteristics and can be differentiated on the basis of geochemistry, mineralogy, pedogenesis, and tectonic setting. The most attractive exploration targets are the skarn deposits which can be mined for their gold content alone.
Most gold skarns contain only small amounts of copper, lead, and zinc and are associated with relatively mafic diorite and granodiorite plutons. They are also distinct in that they typically contain major amounts of arsenic, bismuth, and tellurium, elements which are not normally abundant in other skarn types. Gold grades are high, usually in the 5 to 15 ppm range. Gold skarn host rocks usually contain a significant clastic or volcaniclastic component. Pyroxenes from gold skarns tend to be more aluminum and iron rich (up to pure hedenbergite), amphiboles tend to be more aluminum rich, and garnets tend to be more aluminous grandites than are typical of many skarn types. Potassium feldspar, biotite, prehnite, idocrase, scapolite, apatite, and sphene are locally abundant. Arsenopyrite, pyrrhotite, and marcasite are the most common sulfide minerals with surprisingly large amounts of bismuth minerals (native bismuth, maldonite, wittichinite, hedleyite) and telluride minerals.
Most copper skarns are large but have relatively low gold grades; the highest grades are associated with zones of high sulfide content and intense retrograde alteration, especially low-temperature, silicified zones. Epidote, amphibole, and chlorite are the main retrograde alteration products of garnet and pyroxene and also occur with massive sulfide replacement of skarn. The highest average Ag and Au grades occur in skarn that is both sulfide rich and has abundant retrograde alteration. Calcic Fe skarns are also very large but appear to contain gold only in spotty high sulfide and high copper zones. These zones can be very high grade and might be attractive for small-scale mining, but the potential for larger targets, such as gold disseminated in the main magnetite skarn bodies, is limited. Pb-Zn skarns are small to moderate in size and often contain significant silver, but very little gold.
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The Geology of Gold Deposits: The Perspective in 1988
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