Honshu, Japan

Main commodities: Cu Zn Pb Ag Au
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The Yoshino cluster of kuroko type Cu-Zn-Pb-Ag-Au orebodies are hosted by middle Miocene volcanics in Yamagata Prefecture, Honshu in the central part of north-eastern, Japan.

This part of Honshu lies within the "Green Tuff" Region that includes the Hokuroko Basin 250 km to the north which hosts most of the largest of the Kuroko type deposits.

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

Yoshino falls within the Yonezawa Basin.   The geology of the mine sequence consists of a basement of Mesozoic granitic rocks overlain by Miocene volcanics and sediments, comprising the lower 300 m thick Taro Formation - sandstone and conglomerate with local interbedded mudstone, overlain by andesite and andesitic pyroclastics and an upper unit of arkosic sandstone and interbedded tuffaceous sandstone and conglomerate.   These are conformably overlain by the Yoshino Formation - commencing with a widespread 400 m thick lower volcanic sequence of andesite, dacite, andesitic pyroclastics, with some conglomeratic tuffs, sandstone, sandy tuff and pumice tuff, overlain by an upper 300 m of rhyolite, dacite, pyroclastics, pumice tuff and mudstone, capped by andesite.

The top of the lower 400 m thick volcanic suite of the Yoshino Formation is marked by two-pyroxene andesite, andesitic pyroclastics and thin epiclastics.   The stratabound Kuroko, Oko and siliceous orebodies of the Yoshino cluster all occur immediately below these two-pyroxene andesitic rocks, while numerous vein or network ore deposits containing Cu, Pb, Zn, Ag and Au are found below this level, between the top of the Taro Formation and the base of the upper suite of the Yoshiro Formation.

The Yoshino Formation is correlated with the regional Nishikurasawa Stage that hosts the major deposits of the Hokuroko Basin.

The dimensions and grades of the main orebodies are summarised as follows:

Ebisu 150x30x100 m1.0% Cu, 4.0% Zn, 50 g/t Ag, 2 g/t Au
Daikoku   50x30x30 m1.5% Cu, NA Zn, 100 g/t Ag, 3.0 g/t Au
Hotei 100x30x20 m1.5% Cu, NA Zn, 100 g/t Ag, 3.0 g/t Au
Fukujyu 160x200x30 m1.65% Cu, 3.8% Zn, 60 g/t Ag, 1.1 g/t Au
Nirasawa   70x50x60 m0.46% Cu, 7.25% Zn, 55 g/t Ag, 1.4 g/t Au

Production from1955 to 1971 totalled:
    1.9 Mt @ 0.81% Cu, 2.75% Zn, 12.37% S.

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:
Osada T, Abe M, Daimaru K  1974 - Geology of the Yoshino Mine, Yamagata Prefecture: in Ishihara S (Ed.), 1974 Geology of Kuroko Deposits The Society of Mining Geologists of Japan   Mining Geology Special Issue 6 pp 183-187

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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