Controls on High-Grade Gold Mineralization at Tennant Creek, Northern Territory, Australia
M. Richard Wedekind, Ross R. Large, Brian T. Williams, 1989. "Controls on High-Grade Gold Mineralization at Tennant Creek, Northern Territory, Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Gold, bismuth, and copper mineralization at Tennant Creek is hosted by magnetite-hematite replacement bodies in lower Proterozoic sediments of the Warramunga Group. The sediments have been folded about east-west axes, are characterized by a pervasive axial-plane slaty cleavage, and are intruded by pre- and postfolding granites.
Marked structural and stratigraphic control yields lines of lode (ironstones) that can be traced for distances of up to 40 km. Ironstone lodes are restricted to the magnetite-rich Black Eye Member of the Carraman Formation and concentrate adjacent to argillaceous banded iron-formations. They are aligned parallel to the regional axial-plane cleavage, commonly lying in the cores of third-order folds, especially in areas of fold hinge plunge reversal. Faulting and shearing parallel to the cleavage may also play a role in the localization of some lodes.
Gold, bismuth, and copper mineralization and associated alteration form a late-stage overprint on the magnetite lodes, with gold typically concentrated toward the footwall of the ironstone or at its margins in distinct pods associated with chlorite and muscovite. Copper and bismuth mineralization occurs in overlapping zones around these pods, and this zonation is complemented by gangue mineralogy, and trace element and sulfur isotope zonation patterns.
A model for the formation of the ironstone lode involves the movement of hot connate brines into developing fold axes during regional deformation of the Warramunga Group. The fluids in equilibrium with the magnetite in the sedimentary pile reacted with more oxidized horizons (e.g., hematite shales), resulting in the deposition of hematite (or a hydrated precursor) that was subsequently converted to magnetite as equilibrium was restored. Economic mineralization is associated with faulting and fracturing of the ironstone lodes and introduction of hot, saline, relatively reduced and sulfur-bearing solutions. Reaction of these solutions with chlorite in the lodes resulted in its replacement by muscovite with a consequent increase in pH and reduction in fO2 of the fluid. This reaction is likely to have controlled gold, bismuth, and copper deposition. The relative availability of sulfur, metals, and fluid between ironstone lodes is thought to be responsible for the spectrum from unmineralized to copper- and gold-rich ironstone lodes.
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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