Hokkaido, Japan

Main commodities: In Zn Pb Ag Cu Au
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The Toyoha epithermal zinc, lead, silver, copper, tin and indium (Zn-Pb-Ag-Cu-Sn-In) deposit is located ~43 km SW of Sapporo, on the Oshima Peninsula of southwestern Hokkaido, Japan.

The Toyoha deposit is within the Green Tuff region of Japan, and is located in the Sapporo-Iwanai district, at the intersection between the northern end of the northeast Japan and the Kuril arcs, associated with the subduction zone. The area is characterised by Miocene submarine volcanism followed by Plio-Pleistocene subaerial magmatism.

The stratigraphy the Toyoha district comprises:
Pre-Tertiary basement - sandstone and mudstone, which is not exposed around Toyoha;
Miocene successions - mainly basaltic andesite of the early Miocene Shiramizugawa Formation; mudstone, felsic tuff, and andesite of the middle Miocene Shiraigawa Formation; dacite of the middle to late Miocene Takinosawa Formation; and andesite of the late Miocene Asari Group;
Pliocene Muineyama Lava - composed of andesites which unconformably overlie these Miocene units.

The Shiramizugawa and Shiraigawa Formations host the Toyoha deposit, although the Pliocene andesite eruption and the subsequent latent magmatic intrusions may be genetically related to the Toyoha mineralisation (Ohta, 1995).

The deposit is made up of a series of steeply dipping Zn-Pb- Ag-Cu-Sn-In veins that occur over a 3500 m long by 2500 m wide area, with the depth below surface to the veins generally increasing to the southeast. Veins have been mined underground to a depth of 700 m up until 2006. The maximum thickness of veins is 30 m (Kanbara et al., 1989).

Three groups of faults and fissures are recognised in the mining area:
  i). normal faults with a general east-west strike, dip of 60°N and throw of~60 m;
  ii). faults with no discernable throw, that generally strike NW-Se and dip at 60°N and cut the east-west structures with a 5 m horizontal displacement;
  iii). small scale faults that generally strike NNE-SSW and dip steeply.
All of these faults and fissures may be mineralised and filled with ore.

The host rocks have undergone extensively hydrothermal alteration, with both acid and neutral-pH alteration assemblages (e.g., Sawai, 1984; Shimizu and Aoki, 2011). The acid alteration, is predominantly composed of dickite and kaolinite with lesser alunite, mainly found near the surface and locally at depth in southeast Toyoha. Neutral alteration is mainly present as quartz, chlorite, sericite, illite/smectite, smectite, calcite and pyrite, and occurs along ore veins at depth.

Cooling ages indicate the earliest hydrothermal activity occurred ~2.9 Ma in the NW (K-Ar dating of sericite in altered host rocks and veins; Sawai et al., 1989; Masuta et al., 1996; Shimizu and Aoki, 2011), although the system generally youngs to the SE, where it coincides with the site of the present geothermal system (Kanbara et al., 1989). The age of the ore vein associated sericite in host-rock at the Izumo vein in the south (sericite in alteration zone ā€œCā€; Sawai, 1984) ranges from 1.8 to 1.4 Ma (Sawai et al., 1989).

Two major stages of mineralisation are recognised (Yajima and Ohta, 1979):
  The early stage veins are characterised by Zn and Pb mineralisation to the NW and contain little or no In and are mainly composed of sphalerite, galena, pyrite, hematite, magnetite and quartz, with lesser amounts of chalcopyrite.
  In contrast, late stage veins are characterised by Zn-Pb-Ag-Cu-Sn-In mineralisation to the SE, where the Shinano, Izumo, Iwami, Sorachi, and the southern half of the Soya veins, and No. 1 and No. 2 veins, are relatively rich in In, mainly consisting of pyrrhotite, pyrite, sphalerite, galena, chalcopyrite and arsenopyrite, with lesser amounts of tetrahedrite, tennantite, wolframite, and various other sulphide minerals containing Ag, Sn, Cu, In, and Bi. Pyrrhotite and sphalerite are the first minerals deposited in the later veins (Ohta, 1991).

Indium is generally concentrated in Zn- and Cu-rich ores (Kanbara et al., 1989; Ohta, 1989), where the indium-bearing minerals are typically sphalerite and an unnamed Zn-In mineral (CuZn2InS4), with lesser amounts of stannite, kesterite, roquesite, sakuraiite, and chalcopyrite (Ohta, 1989), and laforêtite (Meisser et al., 1999). Indium-bearing sphalerite is commonly occurs as thin growth bands within In-free sphalerite. Electron microprobe analysis shows that In-bearing sphalerite contains more than 10 wt.% In in places (Ohta, 1989). Chalcopyrite contains up to 1.7 wt.% In (Ohta, 1989).

The total production of ore from the mine was:
      >16 Mt @ an average grade of 2.6% Pb, 7.4% Zn, 140 g/t Ag (Ono and Sato, 1994), and
      33.7 Mt @ 138 g/t Ir (Ishihara et al., 2006).

The most recent source geological information used to prepare this summary was dated: 2012.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Shikazono N,  1975 - Mineralization and Chemical Environment of theToyoha Lead-Zinc Vein-Type Deposits, Hokkaido, Japan: in    Econ. Geol.   v.70 pp. 694-705
Shimizu T and Morishita Y,  2012 - Petrography, Chemistry, And Near-Infrared Microthermometry Of Indium-Bearing Sphalerite From The Toyoha Polymetallic Deposit, Japan: in    Econ. Geol.   v.107 pp. 723-735
Tokunaga M,  1970 - Lead-zinc veins of the Toyoha mine: in Tatsumi T (ed.), 1970 Volcanism and Ore Genesis, University of Tokyo Press, Tokyo    pp. 247-257

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