Garwin Appendix***

©2005 Society of Economic Geologists, Inc. Economic Geology 100th Anniversary Volume pp. 891–930 Supplement to Tectonic Setting, Geology, and Gold and Copper Mineralization in Cenozoic Magmatic Arcs of Southeast Asia and the West Pacific STEVE GARWIN, ROBERT HALL, AND YASUSHI WATANABE (Note: Figure and table numbers correspond to those cited in the printed part of the paper) APPENDIX 1 Descriptions of the Geologic Settings and Mineral Deposit Styles for Major Cenozoic Magmatic Arcs of Southeast Asia and the West Pacific Southwestern Kuril intrusions and domes of the Miocene bimodal assemblage. The host rocks of these deposits are Cretaceous to Paleogene Geologic setting: The Miocene to Recent Kuril magmatic sedimentary rocks and Miocene sedimentary and volcanic arc extends approximately 2,200 km from the northeastern rocks. These deposits occur mainly as gold-bearing quartz- Kamchatka peninsula to southwestern Hokkaido, where it adularia veins in the east-northeasterly strike-slip faults, connects to the Aleutian and northeastern Japan arcs, respec- whereas some of them are disseminated in the host rocks tively (Fig. 11; Table 1). The southwestern portion of the (Watanabe, 1995). The timing of the epithermal gold miner- Kuril arc is associated with the Kuril backarc basin, which alization ranges from 14 to 4 Ma, with a hiatus from 12 to 8 formed before the middle Miocene, due to northeast-south- Ma, which corresponds to the period of the backarc basin vol- west rifting (Baranov et al., 2002). The basement rocks of the canism. A few 3 to 1.5 Ma low-sulfidation gold deposits are southwestern Kuril arc consist of a Mesozoic accretionary also located near the present andesitic volcanic front (Yahata complex with a cover of Cretaceous and Paleogene sedimen- et al., 1999). Representative deposits in northeast Hokkaido tary rocks. Eocene to middle Miocene ilmenite-series grani- are Konomai (73.2 t Au, 1,243 t Ag), Sanru (6.7 t Au, 46.4 t toids intrude the basement rocks (Ishihara et al., 1998). The Ag), and Itomuka (3,086 t Hg). volcanism of the southwestern Kuril arc has changed from Small volcanic islands in the middle and northeastern parts middle Miocene andesitic activity to middle to late Miocene of the Kuril arc have not been well explored. The middle and bimodal basalt and rhyolite, including a period from 12 to 8 northeastern parts contain polymetallic base metal vein-type Ma with basalt-only volcanism. The andesitic and bimodal prospects of middle and late Miocene age, which are associ- volcanic activity migrated trenchward during the middle ated with intrusive rocks (Ishihara, 1994). Miocene (Watanabe, 1995). The middle to late Miocene bimodal and basalt-only volcanism occurred mainly in a north- Japan (northeastern and southwestern) south–trending graben perpendicular to the arc trend Geologic setting: The Japan arc extends approximately (Watanabe, 1995). The basalts of the Miocene bimodal as- 1,800 km from southwest Hokkaido to north Kyushu, where semblage changed from island-arc type at 13 to 11 Ma to it connects to the Kuril and Ryukyu arcs, respectively (Fig. backarc basin basalt at 9 to 7 Ma and again changed into is- 11; Table 1). The arc has a concave configuration toward the land-arc type at 5 to 4 Ma (Ikeda et al., 2000). Since the Pacific Ocean, consisting of a north-trending northeast seg- Pliocene, bimodal volcanism in the backarc has disappeared ment and east-trending southwest segment. Presently these and andesitic volcanic activity at the volcanic front has be- segments are bounded by a major fault zone (Itoigawa- come dominant. This Plio-Pleistocene activity was associated Shizuoka tectonic line), which also marks the boundary be- with formation of calderas several to ten kilometers in diam- tween the North American and Eurasian plates (Uyeda, eter, which erupted large amounts of felsic ignimbrite (Ikeda, 1991). The Izu-Bonin arc is connected with the Japan arc 1991). near the joint between the northeast and southwest segments, East-northeasterly trending right-lateral strike-slip faults and the Pacific and Philippine Sea plates subduct beneath the were active during the late middle Miocene nearby the vol- northeast and southwest segments, respectively. canic front of the southwestern Kuril arc due to oblique sub- The basement rocks of the Japan arc consist of continental duction of the Pacific plate (Watanabe, 1995). This fault blocks of Permian-Triassic gneiss (the Hida belt, central movement led to the westward migration and collision of the Japan) and high pressure-temperature schist, and a Jurassic Kuril forearc sliver with the northeastern Japan arc at south- accretionary complex (Isozaki, 1997a, b). Mesozoic and ern Hokkaido, forming the present concave joint between the Paleogene I-type granitoids related to oceanic plate subduc- Kuril and northeastern Japan arcs (Kimura et al., 1983). tion intruded into these basement rocks (Ishihara and Sasaki, Mineral deposit styles: More than 40 low-sulfidation ep- 1991). The Japan arc was a part of the Eurasian continent ithermal gold and mercury deposits and prospects are distrib- until latest Oligocene but was separated from the continent uted in northeast Hokkaido at the southwestern Kuril arc due to backarc spreading mainly along the Japan and Yamato (Fig. 11, App. 2). They are associated mainly with rhyolitic basins during the early Miocene, with about 60º clockwise 1 2 and counterclockwise rotation of the southwest and northeast Some Kuroko-type gypsum deposits are distributed in the segments, respectively (Otofuji et al., 1985; Hoshi and Taka- backarc in the southwest segment of the Japan arc (Sato, hashi, 1999). 1974). An Re-Os age of the Wanibuchi deposit in the south- Since the early Miocene, arc volcanism has been active in west segment, 18.4 ± 0.6 Ma (Terakado, 2001), is older than both segments. This volcanism is divided into rift-related ac- the range of ages (15.4–12.4 Ma; Sawai and Itaya, 1993) for tivity including bimodal volcanism of basalt and rhyolite in Kuroko mineralization in the northeast segment, suggesting the backarc during the early-middle Miocene and subduc- that Kuroko-style settings occurred slightly earlier in the tion-related andesite-dacite activity during the late Miocene, southwest segment than in the northeast segment of the Pliocene, and Quaternary (Tsuchiya, 1990, Nakajima et al., Japan arc. 1995; Kimura et al., 2003). Rift-related basaltic activity Some middle (or early) Miocene epithermal Au-Ag de- occurred during the Plio-Pleistocene in northern Kyushu, at posits occur in the northeast segment of the Japan arc and are the western end of the southwest segment of the Japan arc. related to felsic volcanism (Watanabe, 2002). These include This rifting is related to the backarc spreading along the Oki- the Sado deposit (72.7 t Au, 2,278 t Ag), which has character- nawa trough (Kamata and Kodama, 1999). istics of an intermediate-sulfidation type. Because different The tectonic regime of the northeast segment of the arc sets of mineralization ages, 24.4 to 22.1 Ma (Ministry of In- since the Miocene has changed from early to middle Miocene ternational Trade and Industry, 1987a) and 14.5 to 13.4 Ma extension, characterized by arc-parallel normal faulting with (Shikazono and Tsunakawa, 1982), are reported for the de- rifted basins, to late Pliocene to Quaternary shortening with posit, the relationship between the Kuroko and epithermal arc-parallel thrusting and folding, through a late Miocene to mineralization is not clear. early Pliocene transitional regime without significant tectonic Late Miocene to Pleistocene Cu-Pb-Zn-Ag epithermal and deformation (Sato, 1994). The middle Miocene rifting and re- Au-Ag epithermal deposits are mainly distributed in the lated bimodal volcanism is best developed in the middle part northeast segments of the Japan arc, associated with calc-al- of the northeast segment. Bimodal volcanism is not clear or kaline andesite-dacite volcanism in a terrestrial environment. mixed with andesite-dacite volcanism in the northern and These deposits are mostly of vein type and hosted by southern margins of the northeast segment, where the Okhotsk transtensional faults. Although there are numerous mineral continental block and the Izu-Bonin arc have collided with prospects of late Miocene age, economic deposits were the Japan arc during the middle Miocene, respectively (Kimura mainly formed during the Pliocene or Pleistocene in an arc et al., 1983; Amano, 1991. East-northeasterly trending right- setting (Watanabe, 2002). Epithermal deposits during this pe- lateral strike-slip faulting occurred during the Pliocene in the riod are classified into intermediate- or high-sulfidation types, northern part of the northeast segment due to the westward but the high-sulfidation deposits are small (Watanabe, 2002). migration of the frontal Kuril arc (Watanabe, 1990). Representative deposits are the Chitose intermediate-sulfida- Significant faulting and folding have not been recognized in tion (18 t Au, 83 t Ag), Teine high- and intermediate-sulfida- the southwest segment of the Japan arc during the Miocene tion (9 t Au, 130 t Ag), Toyoha polymetallic epithermal (1.8 and Pliocene. Since the latest Pliocene, east-trending right- Mt Zn, 0.5 Mt Pb, 3,000 t Ag, 10 t Au), Takatama intermedi- lateral strike-slip faulting has occurred along the Median tec- ate-sulfidation (28.7 t Au, 280 t Ag), and Ashio xenothermal tonic line and other tectonic zones, as well as thrusting along (0.6 Mt Cu) deposits. the north-northwest–trending Itoigawa-Shizuoka tectonic An epithermal gold province occurs in northern Kyushu, ine. These tectonic movements are ascribed to the oblique where backarc rifting along the Okinawa trough has extended subduction of the Philippine Sea plate beneath the southwest into the arc. The province contains about 20 low-sulfidation segment of the Japan arc (Itoh et al., 2002) and convergence deposits of Plio-Pleistocene age, associated with intermediate between the North American and Eurasian plates (Uyeda, to felsic volcanism in and around the Beppu-Shimabara 1991), respectively.

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