Green tuff

These different sites of hydrothermal and ore-forming activity may have resulted from the mode of subduction of the Pacific Plate. Mariana-type subduction (characterized by a steep angle of subduction and back-arc basin formation Uyeda and Kanamori, 1979) during middle Miocene caused WNW-ESE extension, submarine hydrothermal activity, thick accumulation of bimodal (basaltic and dacitic) volcanic activity (Green tuff) and Kuroko-type formation (Shikazono and Shimizu, 1993). Plio-Pleistocene Chilean-type subduction (shallow-dipping subduction zone, E-W compression Uyeda and Kanamori, 1979) and oblique subduction of the Pacific Plate beneath the North American Plate led to uplift and expansion of land area, subaerial hydrothermal activity accompanied by meteoric water circulation, subaerial andesitic volcanic activity and formation of vein-type deposits. 

Kuroko deposits occur in the Green tuff region which is characterized by thick altered volcanic and sedimentary piles of Miocene age. 

It is clear in Fig. 1.10 that the distribution of Kuroko deposits is restricted in a narrow zone in the Green tuff region which was called a Kuroko belt by Inoue (1969). This belt was formed by rapid subsidence under the extensional stress regime and is thought to have been a back-arc depression zone at middle Miocene age. The relationship between tectonic setting and formation of Kuroko deposits is discussed in section 1.5. 

Kuroko-type Gypsum or Barite Green Tuff Beit of Japan Clusters of Kuroko Deposits... 

Figure 1.10. The distribution of the Green Tuff belt of Japan and the Kuroko-type massive sulfide deposits within it. Major mining districts are labeled and ore deposit clusters outlined (Cathles, 1983a).

Epithermal base-metal vein-type deposits are distributed in the Green tuff region (Southwest Hokkaido, Northeast Honshu) (Fig. 1.62). The distribution area of this type of deposits is nearly same as that of Kuroko deposits. For example, large deposits (Osarizawa Cu-(Au) Ani Cu-Au Hosokura Pb-Zn deposits) occur in Northeast Honshu, but are more widely distributed in the Green tuff region than Kuroko deposits. 

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.

The difference in selenium content of acanthite from the different types of ore deposits can, of course, also be explained by the difference in ESe/ES in the ore fluids i.e., the ore fluids responsible for the formation of epithermal Au-Ag vein-type deposits may have had a higher ESe/ES ratio than that for epithermal Pb-Zn vein-type deposits. It is likely that considerable amounts of sulfur were derived from marine rocks (Green tuff) and were incorporated into ore fluids for base-metal veins. 

Some characteristic features (S S, S C, Ag/Au total production ratio, metals produced, gangue minerals) of epithermal Au-Ag vein-type deposits of the Green tuff-type and the Non-Green tuff-type in Japan (after Shikazono, 1996)... 

Green tuff-type Non-Green tuff-type... 

Figure I.IIO. Frequency of values of sulfides from the Green tuff-type and the Non-Green tuff-type deposits (Shikazono, 1999b).

Figure 1.112. Sulfur isotopic composition of pyrrhotite-bearing (solid) and hematite-bearing (open) samples from base-metal-rich deposits in Green tuff region (Shikazono, 1987b).

Figure 1.114 demonstrates that the deposits in northeastern Hokkaido, central Honshu, Sado Island and Kyushu deposits are Non-Green tuff type and those in southwestern Hokkaido, Northeast Japan (Tohoku), the Izu Peninsula, and San-in are Green tuff-type. 

Sedimentary rocks often occur as host rocks, footwall rocks and basement rocks in the Non-Green tuff mine area. For example, in southern Kyushu, the Shimanto Supergroup shale is dominant as basement and a host rock for epithermal Au-Ag vein-type deposits (e.g., Hishikari). 

Total metal (Au, Ag, Cu, Pb, Zn, Mn) production during the past, and Ag/Au total production ratio of major epithermal Au-Ag deposits (Green tuff-type and Non-Green tuff-type) are summarized in Table 1.17. 

Large amounts of Au and Ag have been produced from the Non-Green tuff-type (e.g., Hishikari, Sado, Kohnomai, Kushikino). Ag/Au total production ratio of the Non-Green tuff-type (average 10.7) is lower than for the Green tuff-type (average 19.1). 

These differences can be explained by the HSAB principle by Pearson (1963, 1968). This principle indicates that HS and H2S are likely to form complexes with the metals enriched in the Non-Green tuff-type (Au, Hg), whereas Cl prefers to form complexes with the metals concentrated in the Green tuff-type (Ag, Pb, Mn, Fe, Cu). 

Occurrence of gangue minerals in both types of deposits is different. For example, Mn minerals (Mn carbonates, Mn silicates) occur abundantly in the Rendaiji, Yugashima, Yatani, and Todoroki epithermal Au-Ag vein-type deposits in the Green tuff region but not in the Non-Green tuff-type. values of barite from these deposits are high (-1-18%o... 

This mechanism as a main cause for epithermal-type Au deposition is supported by sulfur isotopic data on sulfides. Shikazono and Shimazaki (1985) determined sulfur isotopic compositions of sulfide minerals from the Zn-Pb and Au-Ag veins of the Yatani deposits which occur in the Green tuff region. The values for Zn-Pb veins and Au-Ag veins are ca. +0.5%o to -f4.5%o and ca. -l-3%o to - -6%c, respectively (Fig. 1.126). This difference in of Zn-Pb veins and Au-Ag veins is difficult to explain by the equilibrium isotopic fractionation between aqueous reduced sulfur species and oxidized sulfur species at the site of ore deposition. The non-equilibrium rapid mixing of H2S-rich fluid (deep fluid) with SO -rich acid fluid (shallow fluid) is the most likely process for the cause of this difference (Fig. 1.127). This fluids mixing can also explain the higher oxidation state of Au-Ag ore fluid and lower oxidation state of Zn-Pb ore fluid. Deposition of gold occurs by this mechanism but not by oxidation of H2S-rich fluid. 

Contributions both from sulfide sulfur leached from volcanic rocks and marine sulfate in Green tuff. 

Geology of the province is composed of Paleozoic basements. Tertiary altered submarine volcanic and sedimentary rocks (Green tuff) and Quaternary volcanic rocks. The basements are shale, tuff, limestone and chert of unknown ages. A simplified geologic map is shown in Fig. 1.148. 

Tertiary rocks are distributed widely. They are composed of alternations of sandstone, mudstone, andesitic and dacitic tuff, tuff breccia and lava. These rocks are intensively and extensively altered and are called as Green tuff. Tertiary volcanic rocks are variable in composition. Andesite, dacite and basalt are found. Quaternary volcanic rocks are dominantly andesite lava and are abundantly distributed in the northern part of the province (Fig. 1.148). 

According to Ishihara (1977), these granitic rocks are ilmenite-series while granitic rocks in Green tuff region are magnetite-series. ... 

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