Geology, Geochemistry, and Genesis of Mesothermal Lode Gold Deposits of the Canadian Cordillera: Evidence for Ore Formation from Evolved Meteoric Water
Bruce E. Nesbitt, Karlis Muehlenbachs, 1989. "Geology, Geochemistry, and Genesis of Mesothermal Lode Gold Deposits of the Canadian Cordillera: Evidence for Ore Formation from Evolved Meteoric Water", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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To date, mesotlierma] lode Au deposits have constituted the principal source of lode and placer Au in the Canadian Cordillera. In addition, these deposits are significant to the understanding of the genesis of Au deposits in general, since they are young analogues of the larger, more enigmatic Archean deposits.
Mesothermal lode Au deposits in the Canadian Cordillera are situated in allochthonous terranes of the cordillera and show a strong spatial association with major strike-slip faults. The deposits are hosted by rock types of the greenschist facies varying from clastic and chemical sedimentary units to felsic and mafic volcanics and plutons. Age dating and structural relations indicate a timing for the emplacement of the ores which is subsequent to the peak of metamorphism. The ores are generally composed of subvertical, continuous quartz veins with minor amounts of carbonate, pyrite, arsenopyrite, and scheelite. Au is paragenetically late and is associated with minor amounts of galena or sphalerite. Hydrothermal alteration zones are dominantly comprised of Fe-Ca-Mg carbonates, albite, quartz, or sericite. Sb and Hg deposits in the Canadian Cordillera possess many of the geologic characteristics described above and are believed to represent distal portions of the Au system.
In mesothermal Au deposits of the cordillera, Au/Ag ratios are typically greater than 1.0, and enrichments in As, B, Ba, W, Sb, Hg, and S are common. Fluid inclusion studies indicate formation temperatures of 250° to 350°C and pressures of 1.0 ± 0.3 kbars, with high CO2 contents and low salinities in the ore fluids. Stable isotope studies indicate that the ore fluids were enriched in 18O (delta;18On,lid = 6-10‰). The 5D values of the ore fluids are strongly depleted in D and latitudinally dependent indicating the involvement of evolved meteoric water in the ore- forming fluid.
A model for the origin of mesothermal lode Au deposits in the Canadian Cordillera invokes deep convection (12-15 km) and chemical evolution of meteoric water in the brittle crust. The fluids ascend in highly permeable zones associated with major strike-slip faults. At approximately 10-km depth, Au-bearing quartz veins are formed. At shallower levels, Sb and Hg deposits are formed.
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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