The Deformed Iron-Formation-Hosted Starra and Trough Tank Au-Cu Mineralization: A New Association from the Proterozoic Eastern Succession of Mount Isa, Australia
Published:January 01, 1989
Garry J. Davidson, Ross R. Large, Greg L. Kary, Robert Osborne, 1989. "The Deformed Iron-Formation-Hosted Starra and Trough Tank Au-Cu Mineralization: A New Association from the Proterozoic Eastern Succession of Mount Isa, Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Two massive to banded strata-bound magnetite-rich ironstones (Fe2O3total + SiO2 = 96%, Fe2O3total > 60%) host Au-Cu mineralization in an intracratonic rift setting within the Mount Isa eastern succession. The deposits are of international interest because of the present divergent views on exhalative versus epigenetic genesis; these ores have features which support both origins.
Starra is the main orebody cluster, consisting of four geographically separate lodes, totaling 5.3 million metric tons at 5.0 g/metric tons Au, and 1.98 percent Cu. The origin of the Starra ores is complicated by an intense deformational history. The lodes lie on the margin of a major D1 decollement, which was subsequently reactivated during D2 and D4. Starra ores are deformed by all recognizable stages of deformation, occurring both in folded and unfolded segments. The footwall is extensively altered to albite-magnetite-pyrite-bearing assemblages, whereas the hanging wall shows only sporadic albite alteration overprinted by D4 calcite gash veining not spatially related to ore.
Ores are massive to banded, characterized by Fe, Si, Au, Cu, W, and Sn enrichment and by Pb, Zn, Ag, and Ba depletion. Au shows good correlations with Si, W, and Cu but is inversely correlated to Fe in the only lode studied in detail. Trough Tank, 40 km southeast of Starra, a similar but less highly strained deposit, is characterized by Co, Mo, and P enrichment in addition to the above elements.
A syngenetic exhalative origin with transport of Au as Au chloride complexes at 280° to 380°C into a low S oxidizing environment is invoked. This best explains local and regional features such as high background Au levels in banded iron-formation, a lack of replacement textures in massive ore, zoning of geochemistry and mineralogy, high Cu/Au ratios, and location at the boundary between a basic-acid sequence and calcareous metasediments.
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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