Relationships between Archean Gold Quartz Vein-Shear Zone Mineralization and Igneous Intrusions in the Val d’Or and Timmins Areas, Abitibi Subprovince, Canada
D. R. Burrows, E. T. C. Spooner, 1989. "Relationships between Archean Gold Quartz Vein-Shear Zone Mineralization and Igneous Intrusions in the Val d’Or and Timmins Areas, Abitibi Subprovince, Canada", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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An initial investigation of the geochemistry of the igneous intrusions which commonly occur spatially associated with lode gold deposits in the southwest part of the Abitibi subprovince shows four main types of intrusion with distinctive geochemical compositions: (1) synvolcanic quartz diorites to tonalites showing characteristics of recent island-arc calc-alkaline suites, (2) feldspar ± quartz porphyries similar in general to tonalite-trondhjemite suites external to the greenstone belts, (3) small plugs of monzodiorite to granodiorite composition which most closely resemble the geochemistry of modern high K calc-alkaline intrusions, and (4) calc-alkaline (shoshonitic) lamprophyre-diorite dikes. Only group 3 shows anomalous primary Au concentrations (G = 7± 24/7 ppb) in contrast to the other groups (G = l±2/1 ppb). Intrusions in groups 1 and 2 played a passive role in the generation of lode gold deposits and host mineralization as a result of the intrusions’ mechanical behavior during subsequent shearing. Zones of dilation within larger deformation zones can be controlled by the positions and sizes of such intrusions. Group 3 intrusions, represented by three plugs in the Lamaque mine, exhibit an extremely close intrusion-gold mineralization association (hosting 90% of the ore) and show an anomalous primary gold enrichment in the least altered samples. This finding suggests that the dikes which coalesced to form these plugs were derived periodically from a parental dioritic magma at depth in which gold (initially ∼6 ppb) was progressively enriched through fractional crystallization processes into the most evolved late melt (∼30 ppb), and therefore, possibly, into a coexisting magmatic fluid.
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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