Furutobe - Ainai

Honshu, Japan

Main commodities: Cu Zn Pb Ag Au
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The Furutobe and Ainai group of kuroko type Cu-Zn-Pb-Ag-Au deposits are within the north-eastern part of the Hokuroko Basin, some 9 and 7 km respectively to the NNW of Kosaka in Akita Prefecture, northern Honshu, Japan.

Furutobe and Ainai were under different ownership and were separate operations in the same district.   Each is composed of a number of separate deposits.   Furutobe includes the Higashimatazawa, Yunosawa, Daikokuzawa-Higashi, Magariyazawa and Daikokuzawa-Nishi orebodies, while Ainaiincludes the Kanayamazawa, Yunosawa-Suehiro, Daikoku, Benten, Yokodawara-Hagoromo and Yunosawa-north orebodies.

For background information on the geology and setting of the Hokuroko Basin area see the Hokuroko Basin - Kuroko Deposits record.

The geology of the Ainai - Furutobe district can be summarised as follows:
Basement - Pre-Miocene phyllite and chert, surmised from fragments from the lower parts of the sequence.
Lower Formation - which is correlated with the regional Nishikurosawa Stage.   This unit consists of acid igneous rocks and pyroclastics, subdivied as follows:
 i). Rhyolite-dacite lava flow - white grey or pale green, from a compact lower to a porous upper section.   More than 200 m thick.
 ii). Rhyolite-dacite tuff breccia - overlies the Rhyolite-dacite lava flow and comprises both lapilli tuff and/or volcanic breccia and a pumiceous lapilli tuff - totals 350 to 400 m in thickness.
 iii). Rhyolite lava domes - which are generally a compact lava, although it is brecciated in the marginal sections with disseminated pyrite and chalcopyrite.   Where these domes are found in the footwall of the orebodies they are generally altered to "white rhyolite".
 iv). Rhyolite pyroclastics - 200 m of tuff breccia and lesser fine to coarse tuff.   The tuff breccia is greyish to white rhyolitic breccia and is usually intensely mineralised and silicified.   The coarse tuff is greyish to white to pale green and occupies the uppermost part of the formation near the ore horizon where it contains gypsum mineralisation at some of the orebodies.
 v). Rhyolite-dacite pyroclastics - which are generally more than 250 m thick, occurring mainly to the north where they are found as either rhyolitic tuff breccia or as pumiceous tuff breccias.
Middle Formation - which occurs above the ore deposits and is composed of the following five pyroclastic members with overlying sediments:  i). Andesite-basaltic tuff, 5 m thick,  ii). Basalt lava flow and tuff, 350 to 400 m thick,  iii). Pumice tuff, approximately 350 m thick,  iv). Dacite,  v). Basaltic lava flow and dolerite, around 220 m thick,  vi). Mudstone, approximately 50 m thick.
Upper Formation - seen in the western part of the area, composed of:  i). Pumiceous tuff, 150 m thick,  ii). Dacite - 60 m thick.
Tobe Formation - an approximately 100 m thick sequence of grey to yellow-grey, non-foliated tuff.

The Middle and Upper Formation together correspond to the regional Onnagawa Stage, while the Tobe Formation is correlated with the regional Funakawa Stage.

The Furutobe mine has exploited a cluster of orebodies of varying sizes over a NW-SE elongated area of 1500x700 m while the Ainai group lies within an area which is approximately 800 m in diameter.   The following provides details of the larger bodies:

OrebodyDimensionsOre typeGrade
Daikokuzawa-Higashi 180x120x20 m  
  Kuroko3.74% Cu, 5.6% Pb, 20.87% Zn, 286 g/t Ag, 7.5 g/t Au
  Han-Kuroko5.89% Cu, 2.81% Pb, 9.51% Zn, 215 g/t Ag, 5.0 g/t Au
  Oko6.91% Cu, 0.70% Pb, 2.74% Zn, 87 g/t Ag, 2.2 g/t Au
Daikokuzawa-Nishi 300x100x50 m  
  Kuroko2.74% Cu, 5.51% Pb, 20.07% Zn, 243 g/t Ag, 3.7 g/t Au
  Han-Kuroko4.47% Cu, 2.78% Pb, 9.89% Zn, 197 g/t Ag, 3.6 g/t Au
  Oko5.25% Cu, 0.66% Pb, 2.46% Zn, 67 g/t Ag, 2.2 g/t Au
Magariyazawa 200x80x15 m  
  Kuroko3.61% Cu, 6.61% Pb, 18.46% Zn, 223 g/t Ag, 4.7 g/t Au
  Han-Kuroko4.15% Cu, 3.88% Pb, 9.90% Zn, 240 g/t Ag, 3.9 g/t Au
  Oko4.84% Cu, 1.25% Pb, 2.45% Zn, 107 g/t Ag, 2.0 g/t Au
Yunosawa 250x100x15 mKuroko/OkoNot quoted
Higashimatazawa 250x100x15 mKuroko/OkoNot quoted
Kanayamazawa 50x20x? mClosed 1860'sNot known
Yunosawa-Suehiro 100x50x30 m  
 30% Kuroko3.5% Cu, 4.8% Pb, 15.0% Zn, 870 g/t Ag, 3.2 g/t Au
 70% Keiko1.5% Cu, trace% Pb, trace1% Zn, NA g/t Ag, NA g/t Au
 0.5 Mt of Sekkoko20% SO3
Daikoku 100x200x40 m  
  15%Kuroko2.1% Cu, 4.5% Pb, 14.7% Zn, 900 g/t Ag, 3.5 g/t Au
  13% Oko3.9% Cu, 1.9% Pb, 6.0% Zn, NA g/t Ag, NA g/t Au
  72% Keiko1.9% Cu, 0.8% Pb, 2.5% Zn, NA g/t Ag, NA g/t Au
  1.5 Mt of Sekkoko25% SO3
Benten 200x100x30 m  
  15% Kuroko2.0% Cu, 3.0% Pb, 10.5% Zn, 850 g/t Ag, 2.5 g/t Au
  15% Oko3.5% Cu, 1.0% Pb, 2.0% Zn, NA g/t Ag, NA g/t Au
  70% Keiko1.2% Cu, 0.3% Pb, 0.7% Zn, NA g/t Ag, NA g/t Au
  1.0 Mt of Sekkoko20% SO3
Yokodawara-Hagoromo 300x150x5 m  
  65% Kuroko1.9% Cu, 4.1% Pb, 12.1% Zn, 950 g/t Ag, 3.2 g/t Au
  25% Oko3.6% Cu, 1.9% Pb, 2.1% Zn, NA g/t Ag, NA g/t Au
  15% Keiko2.0% Cu, 0.1% Pb, 2.5% Zn, NA g/t Ag, NA g/t Au
  0.7 Mt of Sekkoko15% SO3
Yunosawa-north 100x150x30 m  
  100% Stockwork1.5% Cu, trace Pb, trace Zn, NA g/t Ag, NA g/t Au

For detail of the ore types and their composition, occurrence and relationships see the Hokuroko Basin - Kuroko Deposits record.

Tonnage and grade figures quoted include:
    Ainai - total production to 1975:   2 Mt @ 1.58% Cu, 1.58% Pb, 4.03% Zn, 76 g/t Ag, 1.6 g/t Au, 10.4% S as pyrite.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1975.    
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
   Selected References:
Tanaka T, Kuroda H, Kusaka H, Odashima Y  1974 - Geology of the Furutobe Mine, Akita Prefecture: in Ishihara S (Ed.), 1974 Geology of Kuroko Deposits The Society of Mining Geologists of Japan   Mining Geology Special Issue 6 pp 67-78

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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