The Hanaoka group of kuroko type Cu-Zn-Pb-Ag deposits has historically been the largest metal producer in Japan. It is located just north of Ohdate City on the western side of the Hokuroko Basin, Akita Prefecture, in northern Honshu, Japan.
The Hanaoka cluster includes deposits that were mined by Dowa Mining at the Doyashiki and Matsumine mining units (collectively referred to as the Dowa Hanaoka mine) and the Shakanai mine of Nippon Mining immediately to the east, as well as the Mitsubishi Takadate and Matsuki deposits to the south-west, all of which are distributed over an area of approximately15 x 6 km.
For background information on the geology and setting of the Hokuroko Basin see the Hokuroko Basin - Kuroko Deposits record.
The geology of the Hanaoka district can be summarised as follows:
Basement - Strongly folded black phyllite and chert of possible Permian age and probable Cretaceous granitoids.
Meneichizawa Formation - altered andesitic pyroclastic flows with intercalated lavas, overlain by sandstones. The unit is around 200 m thick.
Hotakizawa Formation - lower basaltic pillow lavas and upper mudstones which commenced with shallow water facies.
Hanaoka Formation - comprising two units, the upper of which hosts the ore deposits. The lower unit is approximately 250 m thick and is composed mostly of pale green to pale grey volcanic breccias developed from blocky, flow banded rhyolitic lava. The breccia overlies massive, un-brecciated rhyolitic lavas at the base of the unit. The upper unit comprises around 200 m of pale green tuff, lapilli tuff and tuff breccia, of submarine pyroclastic flow origin,with locally developed rhyolite lava domes. Nine such lava domes are outlined in the Hanaoka mine area, bearing a close genetic relationship to ore. The ores are all in the upper section of this upper unit.
Tsutsumizawa Formation - mudstones and submarine tuffs overlie the ore deposits.
Younger Rhyolite - locally present, mostly at the same stratigraphic position as the Tsutsumizawa Formation.
Dolerite - as sheets, dykes or irregular massive bodies throughout the mine area.
The Hanaoka mine has exploited clusters of orebodies of varying sizes over an area of 3 km N-S by 2 km E-W which incorporates both the Doyashiki and Matsumine mining units. Doyashiki alone includes more than 20 named lenses/bodies. The following provides details of the larger bodies:
Orebody Depth Dimensions Ore types
Matsumine 215-325 m 600x400x110 m Kuroko, Oko, Keiko, Sekkoko
Doyashiki 20-135 m 270x200x100 m Kuroko, Oko, Keiko, Sekkoko
Tsutsumizawa 10-165 m 150x80x110 m Kuroko, Oko, Keiko, Sekkoko
Kamiyama 75-130 m 60x40x55 m Kuroko, Oko
Nanatsudate 20-75 m 100x40x50 m Kuroko, Oko
Kannondo 20-65 m 50x35x45 m Kuroko, Oko, Keiko
Ochiaizawa 60-137 m 60x40x45 m Kuroko, Oko, Keiko, Sekkoko
Oishizawa 20-90 m 50x35x20 m Oko, Sekkoko
Inarizawa 170-190 m 60x15x20 m Oko
For detail of the ore types and their composition, occurrence and relationships see the Hokuroko Basin - Kuroko Deposits record.
There are/were at least 11 orebodies in the Shakanai mine with largest being No. 4 which is 400x300x40 m with a grade of 1.7% Cu, 0.70% Pb, 2.9% Zn, 22% S, while the highest grade lens is No. 1 which is 300x150x12 m and has 2.3% Cu, 3.2% Pb, 14.6% Zn, 14.0% S. The geology is the same as at the Hanaoka mine, although different nomenclature is employed, with the ore bearing unit being the Shakanai Formation, with ore occurring in altered tuffs above the "white rhyolite" that is traced into the neighbouring Hanaoka mine.
The Takadate and Matsuki deposits are similarly in the same stratigraphic setting as the other mines in the Hanaoka group of deposits, again with a varying nomenclature - the ore bearing unit being the 1000 m thick Hotakizawa-Okuzu Formation. The orebodies are composed of Kuroko, Oko, Keiko and Sekkoko ores. The Takadate deposit is composed of 7 sulphide orebodies and one 600x200x20 m gypsum deposit. Matsuki is made up of six sulphide and one gypsum deposit, the latter being 900x300x40 m with associated disseminated sulphides (mainly pyrite and chalcopyrite).
Each of the ore bearing units with differing nomenclatures at the various mines, corresponds to the regional Nishihurasawa stage of the Green Tuff Region as detailed in the Hokuroko Basin - Kuroko Deposits record, with the ore being deposited at a position equivalent to the top of that stage.
Tonnage and grade figures include:
Hanaoka Mine - production to 1975 - 17 Mt @ 2.2% Cu, 0.7% Pb, 3.5% Zn, 20% FeS2.
Hanaoka - Matsumine - reserve in 1990 - 10 Mt @ 2% Cu, 1% Pb, 3.5% Zn.
Shakanai - production to 1975 - 1.821 Mt @ 2.15% Cu, 0.90% Pb, 3.3% Zn, 24.5% S.
Takadate & Matsuki - reserve, 1975 - 3 Mt @ 3.0% Cu, 1.0% Pb, 3.0% Zn.
The total tonnage in the Hanaoka group of deposits is believed to have been approximately 80 Mt.
(Source: Porter Geoconsultancy, http://www.portergeo.com.au/, 2006)