The Kosaka kuroko type Cu-Zn-Pb-Ag-Au deposit is located within the Hokuroko Basin, Akita Prefecture, in northern Honshu, Japan. The mine closed in 1990.
For backrgound information on the geology and setting of the Hokuroko Basin see the Hokuroko Basin - Kuroko Deposits record.
The Kosaka operation comprises a cluster of deposits distributed over a north-south trending interval of approximately 3.5 km, including the Motoyama Deposit in the north, the Uchinotai Western and Eastern Deposits a little over 1 km further south and the Uwamuki First, Second, Third, Fourth and Fifth Orebodies to the east.
The deposits are in an uplifted block between two sub-parallel NNW to north-south trending faults approximately 4 km apart. The host volcanics, which are deposited on a basement of Palaeozoic phyllite and quartzite, comprise five formations, all of which are Miocene in age, as follows:
Sashata Formation - unconformably overlying basement, this 50 to 80 m thick formation comprises conglomerate, sandstone, tuffaceous mudstone and black mudstone, partially silicified and argillised.
Uwamuki Formation - unconformably over the Sashata Formation this 250 m thick unit is composed of a lower 100 m thick member of rhyolite lavas and tuffs and an upper 50 to 150 m thick tuff breccia, with "white rhyolite" and volcanic breccias developed from and through the "white rhyolite". This unit hosts all of the mineralisation in the field, with "white rhyolite" domes in the footwall of the Motoyama, Uchinotai and Uwamuki deposits.
Akamori Formation - around 150 m of mudstone and tuff with 15 to 16 m of alternating red siliceous mudstone and tuff containing clay with fine quartz, barite and hematite.
Harukizawa Formation - 200 m of conformably overlying tuff, sandy tuff and andesitic flows.
Ashinazawa Formation - a widespread volcanic breccia and tuff breccia containing breccia, pumice and thin layers of siltstone.
Intrusives in the mine area include: Rhyolite, Quartz-andesite, "Omari" Andesite and Basalt.
The individual deposits are as follows: Motoyama - which is 700x300x120 m composed mainly as Oko (yellow pyrite-chalcopyrite), with Sekko (gypsum) and lesser Kuroko (black Zn-Pb-Cu-Ag) ores. Uchinotai is made up of Oko, Kuroko and Keiko ores, and comprises i). the Western Deposit with a volume of 400x280x95 m overlying a 250x150 m "white rhyolite" dome with silicified breccia hosting Keiko ore on one flank. ii). the Eastern Deposit - 450x380x100 m overlying a 400x150 m "white rhyolite" dome with silicified breccia hosting Keiko ore on one flank. The Eastern Deposit is further divided into a northern and southern orebody by faulting. The Uwamuki Orebodies form a north-south trending, 1.5 km long string on the eastern magin of the field. They are composed of bedded Kuroko ore, with siliceous black ore and Sekko (gypsum) with minor Oko. The First and Second orebodies are directly assocaited with "white rhyolite" domes only. Of these the fourth is the largest with dimensions of 300x100 in plan area by 150 m in thickness. The Kuroko ore is in the south and occupies a volume of 140x50x50 m, overlying Keiko siliceous ore which is formed over a thickness of 100 m.
For detail of the ore types and their composition see the Hokuroko Basin - Kuroko Deposits record.
The ore deposits tonnage and grades are as follows:
Motoyama - 7.5 Mt @ 1.97% Cu.
Uchinotai - pre-mining reserve - 9.24 Mt @ 2.5% Cu, 1.3% Pb, 4.2% Zn, 18% FeS2.
Typical grades of the orebodies are as follows:
Uchinotai:
Kuroko Ore - 2.1 g/t Au, 646 g/t Ag, 1.41% Cu, 9.42% Pb, 18.29% Zn, 2.50% Fe, 24.39% S, 56.45% BaSO4.
Semi-Kuroko Ore - 1.8 g/t Au, 160 g/t Ag, 5.3% Cu, 1.49% Pb, 10.72% Zn, 17.49% Fe, 26.15% S, 36.14% BaSO4.
Oko Ore - 0.7 g/t Au, 27 g/t Ag, 5.38% Cu, 0.27% Pb, 0.15% Zn, 42.77% Fe, 49.78% S, 0.425% BaSO4.
Keiko Ore - 0.4 g/t Au, 18 g/t Ag, 2.24% Cu, 0.48% Pb, 0.16% Zn, 21.55% Fe, 24.26% S, 0.44% BaSO4.
Uwamuki:
Kuroko Ore - 3.0 g/t Au, 859 g/t Ag, 1.27% Cu, 10.42% Pb, 23.20% Zn, 11.75% Fe, 10.98% S, 31.16% BaSO4.
Oko Ore - 0.7 g/t Au, 109 g/t Ag, 8.59% Cu, 6.61% Pb, 8.82% Zn, 30.66% Fe, 38.75% S, 3.75% BaSO4.
Keiko Ore - 0.3 g/t Au, 19 g/t Ag, 1.46% Cu, 2.41% Pb, 4.13% Zn, 8.72% Fe, 3.76% S, 9.22% BaSO4.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2005)
