Geologic, Fluid Inclusion, and Stable Isotope Studies of Granitoid-Hosted Gold-Bearing Quartz Veins, Charters Towers, Northeastern Australia
S. G. Peters, S. D. Golding, 1989. "Geologic, Fluid Inclusion, and Stable Isotope Studies of Granitoid-Hosted Gold-Bearing Quartz Veins, Charters Towers, Northeastern Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Charters Towers gold field lies within the mid-Paleozoic Lolworth-Ravenswood block where pre-Ordovician metasedimentary and meta-igneous rocks were intruded by the I-type Hogsflesh Creek and Towers Hill Granodiorites in the Late Ordovician, followed by north- and northeast-trending dike swarms in the Silurian. Early Devonian Millchester Creek Tonalite and Alabama Diorite intruded all these rock types and were in turn intruded by Carboniferous mafic dikes.
East- and northwest-trending mylonites cut the Late Ordovician granitoids and older rocks but are truncated by the undeformed Early Devonian granitoids. Gold-bearing quartz veins are contained within shallow east- and north-dipping brittle shear zones, mainly in the Day Dawn, Brilliant and Queen, and Towers Hill vein systems, with depths up to 1,000 vertical meters. Ore shoots are localized along granitoid contacts and locally in brittle reactivations of mylonite. Alteration selvages consist of an inner phyllic assemblage dominated by muscovite and an outer propylitic assemblage dominated by montmorillonite-illite. No direct relationship between any of the granitoid phases and the mineralization has been established.
Fluid inclusion studies indicate a fluid salinity of 5.3 to 11 equiv wt percent NaCl with local CO2 and formation temperatures of between 240° and 300°C. Oxygen isotope values of quartz (12.3-15.6‰), muscovite (8.3-8.6%,), and calcite (11.0-14.8%,); hydrogen isotope values of muscovite (— 57 to — 42%) indicate a fluid composition at these temperatures of 3.2 to 8.4 per mil for δ18O and —62 to —42 per mil for δδD which overlaps the range of metamorphic and magmatic waters. Sulfide δ34S values (-1.1 to +4.2%) and carbonate δ13C values (-4.7 to -3.6%) are consistent with derivation of sulfur and carbon from juvenile fluids or by dissolution and/or thermal decomposition of magmatic sulfides and carbonates during metamorphism.
The mineralization and alteration style, combined with stable isotope and fluid inclusion data, is interpreted to indicate that the Charters Towers gold field is the product of deep-seated Devonian magmatism and crustal ± mantle outgassing.
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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