Quaternary Gold Mineralization and Its Geologic Environments in Kyushu, Japan
Elji Izawa, Yukitoshi Urashima, 1989. "Quaternary Gold Mineralization and Its Geologic Environments in Kyushu, Japan", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The late Cenozoic volcanic activity in Kyushu is characterized by large-scale volcano-tectonic depressions. The sites of these depressions together with associated volcanism and gold mineralization migrated southeastward in northern Kyushu and eastward in southern Kyushu. Thus, Quaternary gold deposits in Kyushu occur within 30 km west from the present volcanic front; the Pliocene gold deposits occur farther away from the active volcanoes.
Typical Quaternary gold mineralization in Kyushu is the quartz vein type with associated adularia and minor calcite. Although host rocks are predominantly andesitic volcanic rocks and sometimes rhyolite and lacustrine sediments, the major portion of high-grade quartz veins of the Hishikari deposit discussed here is hosted in basement sedimentary rocks close to the unconformity between the basement and overlying Quaternary andesites. Five distinct alteration types are recognizable on the basis of mineral assemblages for Quaternary gold deposits. Two are the deeper propylitic alteration and the shallower smectite-zeolite alteration of the widespread and temperature-controlled type; the rest are mica-chlorite alteration, argillic alteration, and siliciflcation of the fracture-controlled type.
Most Quaternary and some Pliocene gold deposits in Kyushu are located near small Bouguer anomaly highs in areas of regional gravity anomaly lows. In the case of the youngest deposits the gravity anomaly highs are overlapped by low electrical resistivities. The small gravity anomaly highs have been ascribed to underlying uplifted blocks of basement. The low resistivity anomalies are caused by the presence of argillic alteration of the shallow portion of the mineralized systems.
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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