Advances in the Understanding of Epithermal Gold-Silver Deposits, with Special Reference to the Western United States
Byron R. Berger, Richard W. Henley, 1989. "Advances in the Understanding of Epithermal Gold-Silver Deposits, with Special Reference to the Western United States", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Research in the past two decades on epithermal gold and silver deposits has led to the recognition of three principal varieties—volcanic-rock-related adularia-sericite(illite) type, alunite-kaolinite ± pyrophyllite type, and sedimentary-rock-hosted Carlin-type deposits. A review of the salient characteristics of each variety and the state of our understanding about the genesis is best accomplished through well-studied examples. Creede, Colorado, Bodie, California (adularia-sericite(illite) types), and Summitville, Colorado (alunite-kaolinite ± pyrophyllite type), illustrate the tectonic setting, alteration and mineralization, and environment of ore deposition of volcanic-rock-related systems. Carlin, Getchell, and Jerritt Canyon, Nevada, serve as good examples of the same characteristics in sedimentary-rock-hosted Carlin-type systems.
There are many similarities among the different varieties of epithermal deposits including the predominance of a meteoric water component throughout the ore-forming stages in the volcanic-rock-related systems and in the intermediate to later stages of the Carlin-type systems; the predominance of large-scale, near-neutral pH alteration assemblages in the deeper parts of the hydrothermal systems; and the time-space relations which indicate heat energy input from intrusive rocks at differing levels in both the volcanic-rock and Carlin-type systems.
The above factors allow a common genetic framework to be established for epithermal deposits. The general model involves interaction of deeply penetrating ground waters and magmatic vapor derived from contemporaneous intrusions. In the large-scale systems responsible for Carlin-type deposits, connate brines also play an important role. The transport capability of deep fluids in epithermal hydrothermal systems may be shown to be dependent largely on the H2S content of the deep fluid, and in turn, through a series of fluid-mineral equilibria to depend strongly on temperature and inversely on CO2 content. Both factors relate to the source strength of magmatic or other evolved fluid at depth in the convection systems. This same common geochemical and hydrodynamic framework provides a basis for the discussion of depositional regimes and the real distinctions between the types of epithermal deposits observed in the field.
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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