Hokuroku

Other Names: Kosaka, Furutobe, Fukuzawa, Shakanai
District: Green Tuff, Nonshu
Commodities :   Copper, Lead, Silver, Zinc

The Kuroko-type volcanic hosted massive sulphide copper-zinc-lead-silver deposits of Japan are characterised by those in the 500 sq. km Hokuroko Basin of Akita Prefecture in northern Honshu where the largest examples are located.   These include the Kosaka, Hanaoka - Matsumine/Doyashiki/Shakanai, Fukazawa, Furutobe-Anai and Hanawa mines. 

All of the Kuroko-type deposits occur within the "Green Tuff" region of western Japan - the Sea of Japan side, where a thick (3000 m) volcanic and sedimentary pile was developed during the latest Oligocene to Miocene and Pliocene as a result of subsidence and violent submarine volcanism.   The region gets its name from the pervasive diagenetic and hydrothermal alteration to which the rocks have been subjected imparting a green colouration.   The region contains numerous Kuroko-type deposits, as well as vein type base metals occurrences and Au-Ag veins.   This mineralisation is developed over a range of time and stratigraphic levels generally within the span of middle to late Miocene. 

The generalised stratigraphic sequence in the Green Tuff region can be summarised as follows:
Basement of pre-Tertiary granite and slate.

Lower Miocene - commences with local developments of altered andesite to dacite, the Akashima Stage, followed by the start of the main period of intensive and extensive volcanic activity with the deposition of a sequence of trachytic rocks, alkali-rhyolite, dacite, altered andesite, olivine basalt and sediments which make up the 150 to 500 m thick Monzen Formation which represents a regional shallow sea.

Early to Middle Miocene - of the Daijima Stage which represents the commencement of mineralisation accompanied by the deposition of rhyolite-dacite, basalt and intercalated sediments.   This unit is variably developed and in many areas is represented by regression and a hiatus.

Middle Miocene - the next unit is the Nishikurosawa Stage which is the main host to ore, and corresponded to the onset of rapid regional subsidence and further rhyolite-dacite-andesite volcanism, basalts, quartz-diorite intrusion and marine sediments, and the main pulse of the continuing mineralisation responsible for the more important deposits listed above.   The Nishikurosawa Stage (locally including the Daijima Stage) is 300 to +600 m thick.

Middle to Late Miocene - intensive and extensive volcanism, subsidence and mineralisation continued into this period as the Onnagawa Stage (250 to 350 m thick) with the deposition of rhyolite, andesite-dacite, basalt, dolerite and hard siliceous marine shale.   The top of this unit marked the end of the mineralising phase.

Late Miocene - The next sequence, the variably developed Funakawa Stage, corresponds to upheaval and the formation of smaller basins.   It is composed of further rhyolite, andesite-dacite and black marine mudstones.   During the latest phase of the Late Miocene to Pliocene, the Tentokuji Stage, volcanism began to wane although regional subsidence continued, accompanied by rhyolite, dacite, andesite, mudstone and marine sandstones. 

The main ore deposits appear to have been formed in the Middle Miocene at sea depths of 100 to 200 m in basins bounded by a barrier to the open sea.   The volcanics in the ore district comprise pyroclastic flows deposited from turbidity currents accompanying submarine eruption, generally commencing with lithic fragment rich massive tuff breccias and ending in intercalated tuffs and mudstones with double grading.   Repeated lava flows and lava domes accompanied this pyroclastic sedimentation.   The submarine lavas are commonly auto-brecciated, after steam explosions, and produced strongly altered rhyolite lavas under the deposits which are phenocryst poor.   In particular, quartz-sericite altered lavas known as "white rhyolites" commonly occur below the Kuroko deposits, many representing small lava domes several hundred metres in diameter.   The kuroko type deposits commonly, but not always, show a close relationship to such lava domes.   Much of the Miocene pile has been regionally and pervasively altered to zeolite facies divided into 5 zones, the boundaries of which transgress stratigraphy. 

A number of alteration styles are associateed with the mineralisation, as follows:
Zone I - Montmorillonite and zeolites found above and peripheral to ore as a transtion to unaltered country rock.   Directly above deposits montmorillonite predominates.
Zone II - Sericite, chlorite and pyrite which occurs immediately above the orebody.   In areas of intense mineralisation this zone may be 30 up to 200 m thick, being thickest over the ore and thinning towards and away from the lateral margins.
Zone III - Sericite, chlorite and quartz which are found within the ore zone.
Zone IV - Silicification with some sericite and chlorite, found in the footwall and central parts of the orebody. 

The ores of the Kuroko type deposits take a number of forms.   In any given deposit all or only a few may be represented.   These include:
Ferruginous Quartz or Tetsusekiei zone chiefly composed of hematite, quartz and some pyrite, occuring as a bed which is generally above ore and up to 1 m thick.   It often has associated barite and some sulphides, usually pyrite.   In some instances discontinuous lenese may occur below the stratabound ore.

Barite Ore zone - usually occuring as well stratabound and stratified, monomineralic, finely crystalline barite with a sandy texture which occurs above kuroko ore and may have considerable lateral extent.

Kuroko, or Black Ore zone - composed of conformable polymetallic sulphide ore with Zn, Pb, Cu, Ag and barite as compact black sulphides, comprising in decreasing order of abundance from sphalerite, galena, barite, chalcopyrite, pyrite and tetrahedrite.   Chalcopyrite and pyrite decreases upwards, while silver and tetrahedrite increase in the same direction.   Kuroko ore commonly exhibits well developed banding parallel to bedding in the enclosing and enclosed sediments.

Han-Kuroko zone - a dense hard ore that is 'semi-kuroko', which is a yellowish to dark grey, composed of almost equal amounts of sphalerite, chalcopyrite and pyrite with lesser galena, tetrahedrite, barite, quartz and sericite.   It is an overlap between Oko and Kuroko style ores.
Oko, or Yellow Ore zone - comprising massive cupriferous pyrite ore which is poor in barite and quartz and is a transition between Keiko and Kuroko ore.   Variants include siliceous Oko a transitional ore from Keiko, pyritic Oko with little chalcopyrite, and a type which is transitional between Oko and Kuroko, where variable amounts of sphalerite, galena and tetrahedrite are also present.   Oko occurs below the Kuroko Zone, sometimes with a gradational boundary, while at others the two are separated by a thin band of altered tuffs.   Like the Kuroko ore, Oko is also often banded.
Keiko zone - which is essentially a zone of silica replacement of the underlying volcanics with accompanying sulphides, mainly chalcopyrite and pyrite as disseminations and stockworks/stringer zones.   Silificied footwall volcanics are termed Keiko when they contain sufficient chalcopyrite to be economically exploited.   This style is commonly brecciated, with silica and associated sulphides deposited between the clasts.   Keiko occurs below the Oko Zone.   In some cases only Keiko ore is found, usually occuring in rhyolites and having a general pipe like shape with a siliceous core and increasing sericite and chlorite outwards to montmorillonite in the surrounding marginal country rock.   Large stockwork bodies usually only have minor associated Kuroko ore.
Sekkoko zone - composed of banded conformable masses of anhydrite-gypsum and pyrite which may occur below the Kuroko Zone, or in place of Keiko or Oko Zone mineralisation. 

The host sequence and ores have not been strongly deformed and in general have reasonably shallow dips, although in places they are steep, in part related to the original depositional topographic slopes. 

The Hokuroko Basin is estimated to have originally contained a mineable tonnage and averaged grade of approximately:
125 Mt @ 8% Zn, 2.5% Cu, 3% Pb, 25 g/t Ag, 0.4 g/t Au.
within around 40 orebodies.   Of these, the Hanaoka group of deposits, spread over an area of 4 x 2 km is believed to have contained around 80 Mt of ore, while the Kosaka group of deposits contained around 32 Mt over a 3x1 km.   See the records on Hanaoka Group, Hanawa, Furutobe-Ainai, Fukuzawa, Kosaka and other individual deposits found within the district for more information. 

(Source: Porter GeoConsultancy, www.portergeo.com.au, 2006)

DM Sample Photographs

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