Archean Gold Mineralization in Zimbabwe: Implications for Metallogenesis and Exploration
R. P. Foster, 1989. "Archean Gold Mineralization in Zimbabwe: Implications for Metallogenesis and Exploration", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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Historically, and in recent years, most of Zimbabwe’s gold production has been derived from lode deposits, which range from pervasively silicified and sulfidized schistose zones and shear zone vein arrays to ribbon-textured and massive quartz veins. Stable isotope and fluid inclusion data are consistent with those for hypothermal lode deposits in other cratons, although complex Au-Te mineralization at the Commoner mine was deposited below 200° C. Precipitation of gold and sulfide minerals was constrained largely by fluid-rock interaction and less commonly by fluid-cooling, phase separation within local areas of dilation and explosive hydraulic fracturing. The lodes formed in predominantly compressional environments, during a major, late Archean, ca. 2.7 to 2.6-Ga, tectonic event, but generally they postdate at least one phase of deformation. A close spatial association and general temporal equivalence is evident with trondhjemite-tonalite-granodiorite stocks which intruded the greenstone belts.
Auriferous orebodies in iron-formation are mostly structurally controlled, and the size and tenor of the deposits are defined in a complex manner by the ductile or brittle behavior of the metasediments during deformation and concurrent hydrothermal activity. Sulfidation of the ferruginous units was an important depositional mechanism. Gold enrichment to 50 ppb or more is evident in some sulfide iron-formations and geochemical data suggest a bimodal fumarolic and/or ambient marine origin for some of these sulfidic metasediments. At the Athens gold-copper mine the orebodies have been interpreted as volcanogenic sulfide deposits by Fabiani (1987).
Gold deposits in late Archean rhyodacitic volcaniclastic rocks exhibit an important epigenetic component, but a gold-enriched, mixed chemical, clastic sedimentary protolith is indicated for some deposits. Most deposits are relatively small, but the Shamva mine has produced more than 52 metric tons of gold from a complex chemogenic sediment-lode deposit which evolved initially as a porphyry (Au-Mo-As)-linked exhalative system.
The lode deposits, and in some cases their exhalative equivalents, developed in response to regional deformation and metamorphism which marked the culmination of komatiitic → tholeiitic → calc-alkaline volcanism and preceded the voluminous granitic magmatism which marked final stabilization of the craton. Near-synchronous deformation, metamorphism, and trondhjemite-tonalite-granodiorite magmatism provided optimum conditions for crustal dewater-ing, focused fluid-fluid, and eventually gold precipitation.
By far the greatest proportion of gold produced from lode deposits in the late Archean greenstone belts has come from the volcanic-dominated western succession which was also the locus of the intrusive (trondhjemite-tonalite-granodiorite) and extrusive calc-alkaline magmatism. This is tentatively identified as a zone of rifting, perhaps multiback-arc spreading, in which subsequent crustal foundering (subduction?), anatexis, and compressional tectonic activity facilitated crustal dewatering.
With this framework a number of first-order exploration targets can be identified, in particular lode deposits in tholeiitic (not calc-alkaline) sequences, high-strain zones at the margins of some greenstone belts, porphyry-intruded rhyodacitic sequences, and regional zones of rifting which became the focus of subsequent deformation and magmatism.
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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