Multiphase Mineralization in Concordant and Discordant Gold Veins, Dome Mine, South Porcupine, Ontario, Canada
Published:January 01, 1989
David C. Proudlove, Richard W. Hutchinson, Dean S. Rogers, 1989. "Multiphase Mineralization in Concordant and Discordant Gold Veins, Dome Mine, South Porcupine, Ontario, 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 Dome mine, in the Porcupine district of the Abitibi greenstone belt, has produced about 360 metric tons of gold from a variety of ore types in a block of ground 2.5 km long, 1.2 km wide, and 1.4 km deep. The ore zones are hosted in regionally metamorphosed, greenschist facies Archean metavolcanic and metasedimentary rocks. Attention is focused here on two structurally and morphologically different types of gold veins: one concordant with its metavolcanic host rocks, and the other discordant. These veins are exposed together in several parts of the mine, and the latter clearly crosscuts the former.
Veins of the first type consist of layers along flow contacts within the metavolcanic sequence; they occasionally extend into facies-equivalent metasedimentary rocks. These corcordant veins can be traced along strike for up to 500 m, downdip for 1,000 m, and are up to 2 m thick. Their mineralogy changes along strike from ankerite + quartz + sulfides to quartz + tourmaline + sulfides ± ankerite. The veins are often well laminated and predate regional metamorphism.
Veins of the second type consist of en echelon sets of discordant, lenticular quartz veins. Individual veins are generally less than 5 m in length, have a vertical extent of 2 to 5 m, and are up to 30 cm thick. Their mineralogy consists of approximately 93 percent quartz + 5 percent sulfides + 2 percent ankerite. These discordant lodes were emplaced during, or immediately after, regional metamorphism.
Geochemical studies comparing the two types of veins reveal many differences. These results, combined with evidence from field relationships, indicate a multistage and multiprocess origin for these differing veins. The concordant veins formed as auriferous chemical sediments deposited on the sea floor which underwent lithification and burial prior to regional metamorphism. During metamorphism, auriferous metamorphogenic fluids were channeled upward along major structural breaks, depositing gold in a single pulse and filling extension fractures to form unlaminated, quartz-dominated discordant veins.
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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