Lithophile Element Systematics of Gold Vein Deposits in Archean Greenstone Belts: Implications for Source Processes
R. Kerrich, 1989. "Lithophile Element Systematics of Gold Vein Deposits in Archean Greenstone Belts: Implications for Source Processes", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Potassic alteration domains of greenstone belt lode gold deposits, typified by the Kerr Addison mine, Ontario, are characterized by systematic partitioning between different groups of incompatible elements such that Al, Ga, Th, U, Ti, and V are decoupled from K, Rb, Ba, Cs, Li, and Tl. The former group of elements have concentrations and interelement ratios in alteration domains which reflect host-rock control, implying isochemical behavior. In contrast, the lithophile elements K, Rb, and Ba are generally coenriched and linearly correlated over three orders of magnitude in abundance, where K/Rb = 230 to 380, and K/Ba = 35 to 100. K/Rb and K/Ba ratios trend toward higher values with respect to increasing concentrations of Rb and Ba, possibly as a result of mixing between host rock and hydrothermal reservoirs of the lithophile elements. K/Cs and K/Tl are weakly correlated, and lithium abundances and Rb/Sr ratios are erratic in altered rocks. These interelement trends, collectively, are present in deposits variously hosted by ultramaflc, mafic, or felsic volcanic rocks, and sediments or granitoids.
Magmatic processes involving crystal fractionation of biotite, K feldspar, and plagioclase generate trends to systematically diminished K/Rb (≤50), K/Li, K/Cs, K/Tl, and Al/Ga but enhanced K/Ba (≥8 x 103) and Rb/Sr in most late-stage differentiates. Such late-stage trends are the rule in magmatophile deposits including the Archean Cadillac molybdenite deposit, Phanerozoic Cu, and Mo porphyry deposits, Sn-W greisens, and most pegmatites. Accordingly, magmatic processes of this type can be ruled out as the dominant source of volatiles for gold-forming systems. In most granulites, especially Archean examples, lithophile element depletion is a primary feature rather than being acquired during carbonic metamorphism. Consequently there is no complementarity between large ion lithophile element-depleted granulites and the K, Rb enrichment characteristic of gold deposits. Moreover, the high K/Rb, K/Cs, and Rb/Cs ratios but low K/Ba ratios of most depleted granulites are not reflected in the gold deposits, where K/Rb and K/Ba ratios approximate average crustal values. The compliance of K/Rb and K/Ba ratios in potassic alteration domains of Au deposits with values characteristic of main trend igneous rocks, or average crust, implies that K, Rb, and Ba were partitioned into the hydrothermal ore-forming fluids in approximately the same ratios as in the source rocks. Dehydration reactions in the source, or equilibration of fluids with source rocks under conditions of low water/rock ratio, rather than purely magmatic or granulitization processes, may satisfy the requirement for proportional K, Rb, and Ba coenrichment.
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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