Gold, Lamprophyres, and Porphyries: What Does Their Association Mean?
Nicholas M. S. Rock, David I. Groves, Caroline S. Perring, Suzanne D. Golding, 1989. "Gold, Lamprophyres, and Porphyries: What Does Their Association Mean?", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Extremely intimate space-time associations between caic-alkaline (shoshonitic) lamprophyres and mesothermal gold deposits are now confirmed worldwide and from Archean to Tertiary times. They include the late Archean gold deposits of the Superior province (Canada) and the Norseman-Wiluna belt (Western Australia), which hosts the world’s most golden square mile (Kalgoorlie) and almost certainly one of the richest gold deposits outside South Africa, (Porgera, Papua New Guinea). In an increasing number of areas, lamprophyres are found to be the only igneous rocks emplaced at the same time as the gold, and economic status has been found to correlate quantitatively with the presence of lamprophyres. Many lamprophyric rocks appear to be enriched in Au relative to other igneous rocks, with contents of tens of ppb Au being common. Evidence that such enrichments could be primary includes the following, although this will remain impossible to prove for lamprophyres in major gold fields: (1) the persistence of high Au contents in calc-alkaline lamprophyres which lie outside the alteration halos of large-scale gold systems (e.g., British Caledonides) and more especially in lamprophyric rocks which are not associated with gold deposits at all (e.g., lamproites); (2) the plausible explanation for Au enrichment that exists in the lamprophyres’ exceptionally deep origins in presumed Au-rich regions of the earth (>150 km), high F, K, Ba, and Rb, moderate S contents, and H2O/(H2O + CO2 ratios, and fluidized condition, which make them uniquely similar to auriferous ore fluids in their element abundances and possibly in their physical state, and thus, well suited to transporting gold into the crust; and (3) detailed statistical analysis of Au data, notably for the Superior province, comparing fresh versus altered and proximal versus remote samples from gold mineralization to rule out any overall addition of Au to these lamprophyres from external sources. The widespread presence of lamprophyres suggests a much more significant role in gold deposition for large-scale crust-mantle events and for certain (e.g., oblique subduction) tectonic regimes than has hitherto been generally recognized. A part-genetic, part-structural interpretation is preferred, in which lamprophyres may contribute at least some Au or fluids from deep sources into mesothermal systems, which then redeposit the Au according to the metamorphic model in its broadest sense. This is in accord with stable isotope evidence, which argues against a direct relationship between lamprophyric and gold-depositing fluids but by no means precludes an indirect one.
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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