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The Kushikino gold mine (Gold Park) is located 30 km northwest of Kagoshima city (Fig. 1) and historically has been the fourth largest gold mine in Japan. The mine was opened in 1660 by a lord of the Satsuma Province (a province in southern Kyushu during the Edo era) and development took place mainly on the Serigano vein group (Fig. 2) in the south central part of the area (Miyahisa and Wakabayashi, 1972). In the late Meiji era, 1905, Mitsui Mining and Smelting Co., Ltd. acquired the central part of the Kushikino area including the Kushikino no. 1 vein. In 1913, an all-slime cyanidation plant, the first of its kind in Japan at that time, began treating ore at a rate of 150 (t) per day. Since 1660 the mine has produced 55 t of gold and 497 t of silver, of which more than 80 percent was produced from the Kushikino no. 1 vein. Approximate average Au and Ag grades have been reported as 6.7 g/t and 61 g/t, respectively (MITI, 1979).
In 1988, the Kushikino Gold Park was opened adjacent to the operating gold mine. In the park, the western part of the champion vein (Kushikino no. 1 vein) around the “New” inclined shaft (Shin-shako) can be seen in the adit (mine level 2L, 60 m above sea level), though the mining operation is no longer active. This article compiles previous studies and revises our understanding of the epithermal gold mineralization at the Kushikino deposit.
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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 silicification 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.
The importance of circum-Pacific volcanic belts as a source of rich gold deposits has been highlighted by recent discoveries of deposits such as McLaughlin, California; El Indio, Chile; Lihir, Papua New Guinea; and Hishikari, Japan. In particular, epithermal gold deposits related to Quaternary volcanoes have become the focus of exploration in island-arc systems of the western Pacific rim, in which Kyushu lies. Kyushu is the principal gold-producing area in Japan. There are several important gold deposits which are related to waning or extinct geothermal systems of Quaternary age. The erosion level in the Quaternary volcanic areas is shallow so that some of the surface expressions are different from those of older deposits. The purpose of this paper is to outline the characteristics of gold mineralization of Quaternary volcanic areas in Kyushu on the basis of detailed geologic and geophysical investigations undertaken over the last ten years. Major epithermal gold deposits in Kyushu occur in late Cenozoic volcanic areas which were formed by the subduction of the Philippine Sea plate beneath the Eurasian plate.