Median Tectonic Line

Figure 1.2. Distribution of the stratiform Cu-sulfide and chert-hosted Mn deposits in Japan (Sato and Ka.se, 1996). MTL Median Tectonic Line TTL Tanakura Tcetonic Line ISTL Itoigawa-Shizuoka Tectonic Line BTL Butsuzo Tectonic Line.

In Southwest Japan, two styles of vein-type mineralization (Hg and Sb) formed from middle Miocene to the present. Many Hg and Sb deposits are present along the Median Tectonic Line, associated with the Setouchi andesites and ilmenite-series granitic rocks (Fig. 1.3). 

Figure 1.62. Location of epithermal-type deposits in Japan (Shikazono and Shimizu, 1988a). 1 Green tuff and subaerial volcanic region of Tertiary/Quaternary ages, 2 Main Paleozoic/Mesozoic sedimentary terranes, 3 Main metamorphic terranes. TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median tectonic line. Open circle epithermal Au-Ag vein-type deposits, solid circle epithermal base metal vein-type deposits, open triangle epithermal Au disseminated-type deposits.

In contrast, in Southwest Japan, polymetallic veins (so-called xenothermal-type deposits in the sense of Buddington (1935) or subvolcanie hydrothermal type in the sense of Cissartz (1928, 1965) and Schneiderhohn (1941, 1955) occur. Examples of these deposits are Ashio, Tsugu, Kishu and Obira. All these vein-type deposits have formed at middle Miocene age in western part of Tanakura Tectonic Line under subaerial environment. In these deposits, many base-metal elements (Sn, W, Cu, Pb, Zn) and small amounts of Au and Ag are concentrated. These deposits are associated with felsic volcanic and plutonic rocks along the Median Tectonic Line (MTL) or south of MTL. 

Hg deposits are distributed along the MTL (Median-Tectonic Line) in Southwest Japan and in Northeast Hokkaido (Fig. 1.184). The deposits are vein or disseminated in form. The deposits are hosted by sedimentary and igneous rocks. No K-Ar age data on the deposits are available. However, from the age of host rocks, the age of igneous activities along MTL and the studies on the movement of MTL and K-Ar ages of Au-veins associated with Hg mineralization in Northeast Hokkaido (e.g., Khonomai), it is likely that these deposits formed at middle Miocene age. However, mercury mineralization in Kitami Province (north Hokkaido) occurred at approximately the same age as the epithermal gold-silver mineralization in the same district (4.5-5.3 Ma) (Maeda, 1997). 

Fig. 2.36. Tectonic setting of Kyushu, Japan, showing location of (north) Hishikari and Nansatsu (south) deposits in Kirishima arc. Triangle volcanoes younger than Middle Pleistocene stipple accretionary prism of Cretaceous and Tertiary age W-B zone isobath of deep Wadati-Benioff seismic zone MTL Median Tectonic Line BTL Butsuzo Tectonic Line K-P Ridge Kyushu-Palau Ridge (Mitchell and Leach, 1991).

MTL Median Tectonic Line TTL Tanakura Tectonic Line Legend I Hida Terrane 2 South Kitakami Terrane 3 Akiyoshi Terrane 4 Suo Terrane 5 Maizuru Terrane 6 Ultra-Tanba Terrane 7 Mino Terrane 8 Sanbagawa Terrane 9 Shimanto Terrane. 

Figure 38.5 Distribution of pre-Neogene Terranes of Japan and reconstruction of Early Cretaceous Japan (modified and adapted from Tazawa, 1993). Asterisk shows the fossil locality of Okutadami. Solid black arrow shows the direction of left-lateral strike-slip motion along the Tanakura-Median Tectonic Line in Early Cretaceous time.

Figure 1.171. Map showing location of the Tsugu gold-antimony deposit. 1 Green-tuff region, 2 outer zone of southwestern Japan, TTL Tanakura tectonic line, ISTL Itoigawa-Shizuoka tectonic line, MTL Median...

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