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Fukazawa - Kanayamazawa, Manjakuzawa, Tsunokakezawa |
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Honshu, Japan |
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Cu Zn Pb Ag Au Ba
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Super Porphyry Cu and Au
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The Fukazawa group of kuroko type Cu-Zn-Pb-Ag-Au deposits are located within the southern part of the Hokuroko Basin, Akita Prefecture, in northern Honshu, Japan and comprise three main orebodies, Kanayamazawa, Manjakuzawa and Tsunokakezawa distributed over a north-west elongated area of around 1600 x 900 m (#Location: Manjakuzawa - 40° 16' 35"N, 140° 40' 22"E; Tsunokakezawa - 40° 15' 32"N, 140° 41' 8"E).
For backrgound information on the geology and setting of the Hokuroko Basin see the Hokuroko Basin - Kuroko Deposits record.
The geology of the Fukazawa district can be summarised as follows:
Basement - no basement is exposed or had been encountered in drilling in 1975.
Meneichizawa Formation - alternating altered andesitic pyroclastic flows and sandstones of the Monzen Stage.
Yukisawa Formation - unconformably overlying the Meneichizawa Formation, this formation corresponds to the regional Nishikuraswa Stage and represents the mineralised unit. It is characterised by abundant volcanic material with a number of lithologic associations, namely:
• Yukisawa dacite lava - identical to the footwall dacite (or when altered the "white rhyolite") of the standard Kuroko deposits, deposited as several submarine flow pulses of porphyritic (feldspar phenocryst) dacite which is generally pale green, but altered to a white sericitic rock near the orebodies.
• Uwamuki tuff breccia - which is the "ore horizon tuff" and has a maximum thickness of 160 m at Fukazawa where it only occurs in the vicinity of the orebody cluster. It consists of alternating lapilli tuff, volcanic breccia and pumice tuff. The lower parts are a lithic tuff with fragments of Yukisawa dacite lava, while the upper section is a layered tuff with pumice fragments. In the vicinity of ore it is stongly sericite altered and pyritised,
• Basalt lavas - up to 140 m thick of submarine lava resting on the Uwamuki tuff breccia, with fragments of kuroko ore in its basal flows. It has been overprinted by pervasive chlorite and epidote and carries stockwork hematite and disseminated pyrite.
• Intrusives - mostly andesitic or dacitic in composition and feldspar porphyritic.
Kagoya Formation - which conformably overlies the Yukisawa Formation is composed of alternating mudstone and pumice flows, with mudstone predominating in its upper sections.
Shigenai Formation - conformably overlies the mudstones of the upper Kagoya Formation and comprises layers of andesitic tuff breccia, pumice tuff and mudstone.
The main orebodies of the Fukazawa cluster may be summarised as follows:
| Orebody | Dimensions | Ore types | Grade |
| Kanayamazawa | 210 x 90 x 5 m | Kuroko | 1.6% Cu, 6.8% Pb, 17.0% Zn, 219 g/t Ag, 1.5 g/t Au, 4.0% Fe, 35.7% BaSO4 |
| | | Keiko | 1.1% Cu, 0.2% Pb, 1.0% Zn, 21 g/t Ag, 0.3 g/t Au, 11.2% Fe |
| Manjakuzawa | 190 x 190 x 6 m | Kuroko | 1.1% Cu, 1.5% Pb, 10.1% Zn, 66 g/t Ag, 0.7 g/t Au, 9.1% Fe, 9.3% BaSO4 |
| Tsunokakezawa | 480 x 400 x 8 m | Kuroko | 1.1% Cu, 3.2% Pb, 15.9% Zn, 163 g/t Ag, 1.3 g/t Au, 4.8% Fe, 18.2% BaSO4 |
The kuroko ore occurs in three forms, namely compact, brecciated and disseminated (in argillised tuffs occuring on the margins of the main orebodies and in the satellite lenses). Seiko predominates in the Kanayamazawa orebody as disseminations or stockworks in silicified and pyritic lithic tuff and brecciated dacite and forms a funnel shaped zone, with the intensity of mineralisation decreasing downwards.
For detail of the ore types and their composition see the Hokuroko Basin - Kuroko Deposits record.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
Manjakuzawa Tsunokakezawa
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Selected References:
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Tanimura S, Shimoda T, Sawaguchi T 1974 - On the Fukazawa ore bodies, Akita Prefecture: in Ishihara S (Ed.), 1974 Geology of Kuroko Deposits The Society of Mining Geologists of Japan Mining Geology Special Issue 6 pp 147-155
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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