Transport and Deposition of Gold, Uranium, and Platinum-Group Elements in Unconformity-Related Uranium Deposits
A. R. Wilde, M. S. Bloom, V. J. Wall, 1989. "Transport and Deposition of Gold, Uranium, and Platinum-Group Elements in Unconformity-Related Uranium Deposits", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Gold is ubiquitous in the unconformity-related uranimum deposits of Australia’s Alligator Rivers uranium field and in some cases constitutes an economic resource, Palladium and platinum accompany gold in significant amounts at the Coronation Hill and Jabiluka deposits. Spatial association of uranium, gold, and the platinum-group elements and textural relationships between gold and uraninite suggest that all these metals together with magnesian chlorite (amesite) were deposited during the same mineralizing event. The most likely ore-transporting solution would have been an oxidized, slightly acidic, chloride-rich brine (or brines), derived from within the mid-Proterozoic hematitic terrestrial clastic cover sequence to the deposits. In addition to carrying ore-forming amounts of U, Au, Pd, and Pt as chloride complexes, such brines are predicted to have been capable of transporting Se and Te as acid oxyanions. The reaction path history of these brines probably involved initial equilibration with atmospheric oxygen and subsequent equilibration with rocks with a high intrinsic oxidation state, namely a hematitic clastic sedimentary sequence (Kombolgie Formation), poor in organic detritus and ferrous iron. It is proposed that this reaction path history is more important in ore formation than interaction with specific, metal-enriched source rocks.
Preliminary calculations of irreversible mass transfer are presented which support the interpretations of previous workers who have suggested that reduction is the principal mechanism of ore deposition. These calculations show that either mixing of an oxidized metal-bearing solution with a reduced CH4-bearing fluid, derived from lower Proterozoic graphitic schist host rocks, or direct interaction of the oxidized metal-bearing solution with lower Proterozoic graphitic and ferrous iron-bearing schistose host rocks could have produced the association of uranium, gold, and platinum-group elements. Some involvement of ferrous iron-bearing phases in the host rocks is suggested by hematitic alteration around the deposits. The predicted sequence of ore and associated gangue mineral deposition is magnesian chlorite and uraninite, followed by native gold (as is actually observed), followed by palladium telluride (kotulskite), palladium sulfide (vysotskite), and finally platinum sulfide (cooperite). Thus, spatial association of uranium, gold, and platinum-group elements deposition with the unconformity reflects a major difference in ambient oxidation state between mid-Proterozoic cover rocks and underlying lower Proterozoic metasediments, coupled with fault permeability. Similar depositional mechanisms are believed to occur in deposits hosted by younger rocks, the Kupferschiefer of Poland being one possible example.
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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