An Empirical Model for the Formation of Archean Gold Deposits: Products of Final Cratonization of the Superior Province, Canada
A. C. Colvine, 1989. "An Empirical Model for the Formation of Archean Gold Deposits: Products of Final Cratonization of the Superior Province, Canada", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Superior province of Canada hosts hundreds of gold mines which have recorded production ranging from less than 1 metric ton up to 1,000 metric tons of gold. All of them occur within large-scale, transcurrent and oblique slip-shear deformation zones, which were active during the latest Archean. The deformation zones constitute a conjugate set to a north-northwest-directed compression of the Canadian Shield. Within these deformation zones gold camps are localized in extensional structures; many of these are pull-apart structures which were the loci of fluvial-alluvial sedimentation, a suite of felsic intrusions, and alkaline volcanism. The localization of gold at all scales, from individual veins up to camp scale, is attributed to the dilation zones produced by the shear deformation.
Although mineralization and attendant wall-rock alteration are the products of auriferous hydrothermal fluids of similar composition ascending along deformation zones, they are manifested in many styles in different deposits and even within a single deposit. Wall-rock alteration assemblages vary according to metamorphic grade of host rocks and reflect alteration mineral stabilities under elevated ambient pressure and temperature of regional metamorphism; metamorphism does not overprint alteration mineral assemblages. Thus mineralization formed during, not only late Archean regional shearing, but also regional metamorphism; wall-rock metamorphic grade is therefore an indicator of depth of formation of mineralization within deformation zones. Wall rocks range in metamoprhic grade from subgreenschist to amphibolite facies, indicating that gold deposition took place over a substantial range of depths, possibly in excess of 10 km.
The depositional model is based on observations of more than 30 deposits in various terrains in the Superior province. Distribution and style of mineralization and alteration are a function of fluid access to wall rocks through permeability generated by shearing; with increasing depth brittle, brittle-ductile, and ductile deformation corresponds to breccia-style, veining, and foliation-parallel mineralization, respectively. Alteration mineralogy also reflects depth of formation. The most prominent aspects of the mineralogy are the predominance of pyrrhotite over pyrite as the characteristic alteration sulfide and the absence of ankerite carbonatization in deposits which formed under higher pressure and temperature conditions. Physical and chemical characteristics of varied lithologies also affect response to shearing and alteration. The model therefore provides a degree of predictability to the distribution and styles of mineralization and alteration expected in any specific lithologic and metamorphic environment. The correspondence of the observations with the model suggests that Archean gold mineralization is one single deposit type.
At several widely separated locations, the mineralizing event took place around or after 2,680 Ma, at least 20 Ma after the cessation of greenstone volcanism. The mineralization is somewhat younger than the felsic intrusions, which are spatially associated with many gold deposits, and may be, temporally, more closely related to slightly younger, minor magmatism of a more alkaline or a lamprophyric affinity. In the Canadian Shield, the timing of the mineralization roughly corresponds to a late batholith emplacement and granulitization and accompanying magmatism in the lower crust. Thus, the mineralization, which involves heat and mass transfer, is a manifestation in the upper crust of the widespread cratonization processes in the very late Archean. The latest Archean was the most important metallogenic epoch for this style of mineralization.
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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