Tectonically Induced Hydrothermal Activity and Gold Mineralization Adjacent to Major Fault Zones
Published:January 01, 1989
D. Craw, P. O. Koons, 1989. "Tectonically Induced Hydrothermal Activity and Gold Mineralization Adjacent to Major Fault Zones", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Southern Alps mountain range, New Zealand, is a collisional orogenic belt in which rapid uplift (near 10 mm/yr) has caused high near-surface geothermal gradients which commonly exceed 50°C/km. Thermal modeling suggests that conductive heat flow perturbations reach a steady state after about 2.5 Ma, with temperatures as high as 300°C within 5 km of the surface. The high heat flow results in enhanced fluid flow in the upper 6 km of the crust. Uplifting greenschist facies rocks release gold-bearing metamorphic water produced during metamor-phism by dehydration of chlorite. Gold precipitation occurs due to cooling and/or dilution where this>300°C fluid with salinities up to 4 equiv wt percent NaCl mixes with convecting lower salinity <250°C meteoric water. Uplifting amphibolite facies rocks release relatively small quantities of an aqueous fluid which is CO2-bearing but contains little or no gold. Influx of this fluid results in alteration of biotite to chlorite, causing progressive decrease in XH2 0 to values as low as 0.6. The resulting CO2-rich fluid mixes with meteoric water while migrating to the surface, where CO2-bearing hot springs emanate. Minor base metal mineralization occurs throughout the zone penetrated by meteoric water. The mineralization model can be extended to the Archean, when higher geothermal gradients prevailed. For an initial geothermal gradient of 40°C/km, uplift rates as low as 1 to 2 mm/yr extended over about 5 Ma could have produced near-surface thermal anomalies similar to those observed in the Southern Alps.
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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