Field Stop: Geology and Geophysical Expression of the Yamagano Low-Sulfidation Epithermal Au-Ag Deposit, Southwest Kyushu, Japan
Hiroyasu Murakami, Craig A. Feebrey, 2001. "Field Stop: Geology and Geophysical Expression of the Yamagano Low-Sulfidation Epithermal Au-Ag Deposit, Southwest Kyushu, Japan", Epithermal Gold Mineralization and Modern Analogues, Kyushu, Japan, Craig A. Feebrey, Toshihiko Hayashi, Sachihiro Taguchi
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Turn off the main road and move toward field stop 1. Participants can observe variations in alteration starting with patchy occurrences of zone I, followed by zone II, and finally by zone iii assemblages, representing a progressive increase in paleohydrothermal fluid temperatures associated with mineralization. Field stop 1 is located above the southern vein system in the vicinity of Takasuka vein and within zone III alteration, which is characterized by the assemblage quartz + interstratified illite/smectite ± kaolinite (Fig. 1). Analyses from these veins have been as high as 27.7 g/t, and fluid inclusions from the same veins indicate temperatures of about 200° to 250°C (figs. 4 and 5, Murakami and Feebrey, 2001). Also at this location silicified Nagano Formation rocks essentially cap and have preserved mineralization. Sedimentary bedding planes of the Nagano Formation are also still evident in some localities (Figs. 2 and 3).
Also of particular interest in terms of exploration for this style of mineralization is the excellent correspondence between the distribution of mapped zone III alteration, characterized by interstratified illite/smectite, and the area of extremely high K counts obtained during a high-resolu- tion airborne radiometric survey. Points of interest include the amount of vegetation that exists and covers zone III alteration, indicating that significant anomalies are still evident in areas with moderate to high vegetation cover.
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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.