The Nurukawa kuroko ore deposits was discovered on the west of Lake Towada on the border of Aomori and Akita Prefectures, northern Honshu, Japan, as the result of systematic exploration program from 1979 to 1984. This report is a summary of story of the discovery, the geology and the ore deposits.
In the east of this area, the old Towada Ginzan deposits which mined from late 19th century to early this century has been known, and also the Namariyama Ginzan deposits were discovered in 1968. Both deposits are of the kuroko type.
In 1982, a possible existance of the new kuroko deposits was indicated by the study of old Towada Ginzan deposits, detailed geological survey, geochemical study of the alteration of low Na₂O content in pre-mineralization acidic volcanics, and geophyisical exploration of IP method.
Based on the information obtained by these study, drillings for investigation on the geologic structure and a systematic drilling program was planned and carried out. Consequently Nurukawa No. 1 to No. 5 ore bodies were discovered by underground and surface drillings between late in 1982 and September 1984.
The geology of the area is mainly characterized by active dacite lava and dacitic pylocastics of the Miocene Tertiary, which are divided into four units named Nabekurazawa, Lower Hayasemori, Upper Hayasemori, and Tobe formations in ascending order.
The ore deposits occur at the top of the Lower Hayasemori formation and consists of five ore bodies of black ore. The ore is also divided into two types on the basis of their mode of occurtences. The one is bedded, and associated with silicious network ore, disseminated ore and gypsum ores. The other is fragmental, and associated with gypsum ores.

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Promising exploration margins around known kuroko deposits can well be evaluated in terms of the Na₂O anomaly reported previously (HASHMOUCHC et al., 1981). In the present study, using 662 samples from 61 drill holes in the Fukazawa mine area, an attempt has been made to figure out statistically the metallic contents and their variations in the footwall mineralization zone, which is essentially included within the promising margin delimited from the Na₂O anomaly. The results obtained are as follows.
1) Two economically significant thresholds in metallic distribution can be set at Cu 0.07%, Pb 0.18% and Zn 0.29% as a lower limit and at Cu 0.5%, Pb 1.4% and Zn 3.1% as a higher one.
2) The anomalous zones delimited by the lower thresholds are about two times as large as the presently known deposits in scale and are quite similar to them in both distribution and shape.
3) The higher thresholds obtained are virtually the same as the experiential cut-off values for the "siliceous ores" of the mine.
4) The "siliceous yellow ores" and the "siliceous black ores" appear to be different in their spatial distribution. The former tends to be predominated in the central portion of the mineralization zone and the latter in the peripheries of the zone.
5) The footwall mineralization zone thus defined extends rather regionally along the NW-SE direction, which is in harmony with what are given from our knowledges of magnetic susceptibility anomaly, alteration zoning and distibutions of so-called "Fe-chert" and post-ore basalt flows.

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In order to recognize the chronological transition of the volcanism related to the formation of Kuroko deposits, authors examined the variation of major chemical compositions on the felsic and basaltic volcanic rocks that belonged to pre-/post-Kuroko mineralization at the Hokuroku district in northern Japan. The felsic volcanics are classified, based on field observation, into D1 dacite, D2 dacite, and footwall dacite group. The footwall dacite group is further subdivided into Green dacite, Shako dacite, White rhyolite (D3 dacite) and definite footwall dacite(D4 dacite) in descending order of the stratigraphy.
By the preliminary examination on AL₂O₃-(CaO+Na₂O)-K₂O and K₂O-Na₂O-CaO diagrams, it was suggested that about a half of specimens in the footwall dacite group (35/72) could be reliable to discuss the primary difference in chemical compositions without considering the influence of alteration, and that the various dacite in the footwall dacite group could have different origins from the petrological viewpoint.
Post-Kuroko dacite (Green dacite and D2 dacite) and the dacite (Shako dacite and White rhyolite) formed contemporaneously with Kuroko deposits are clearly separated by the AL₂O₃-SiO₂ diagram; the former is plotted in the low Al and slightly high Si domain, and the latter is plotted in the opposite side. On the other hand, D4 dacite (pre-Kuroko activity) and D1 dacite (later in the post-Kuroko volcanism) are dispersed between former two domains. However, they can be clearly separated by CaO-TiO₂ diagram; the D1 group dacite is represented in the high CaO and high TiO₂ domain.
Authors investigated the chronological transition in mean values of major elements by rock type. Data in pre-Kuroko dacite show constant AL₂O₃ and SiO₂ contents. However, the contemporaneous dacite (White rhyolite, except for Shako dacite) is characterized by low MgO/(MgO+FeO^*) and high Na₂O+K₂O value. On the contrary, post-Kuroko dacite (Green dacite and D2 dacite) is characterized by high SiO₂ and low AL₂O₃ contents and high MgO/(MgO+FeO^*) value. Felsic volcanic rocks in the hanging-wall, such as T2 tuff, Porphyry, Granitoid and D1 dacite, show constant AL₂O₃ value with slightly ascending SiO₂ value and slightly descending MgO/(MgO+FeO^*) value from bottom to top stratigraphically. Those indicate that there were two volcanic cycles, and that the Kuroko mineralization took place related to more differentiated magma at the end of the first volcanic cycle, and then another felsic volcanism commenced immediately after Kuroko mineralization.
Basaltic rocks are called B3 (pre-Kuroko), B2-b (just above Kuroko horizon), B2-a (early in the hanging-wall rock units) and B1 (later in the units) basalt respectively in ascending order. It is recognized that TiO₂ contents gradually decrease from B3 (high Ti contents) via B2-b(middle Ti) to B2-a basalts (low Ti), and that TiO₂ contents become high in B1 (high Ti) again on the TiO₂ -SiO₂ diagram. This suggests that B3 basalt would be deeper origin and that the magma chamber would have gradually become shallower until the period of post-Kuroko activity.
Both felsic and basaltic volcanic rocks indicate that the magma chamber changed from deeper level at the pre-Kuroko stage to shallower level at the post-Kuroko volcanism. It appears from the above that the Kuroko mineralization was caused at the transitional period of volcanism that had changed from deeper to shallower origin.

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Previous studies of geochemical prospecting in the Hokuroku region have revealed that acid lavas underlying kuroko deposits are in general significantly depleted in Na ₂O in comparison with those on which no kuroko deposits exist. The Na ₂O content of the footwall acid lavas may thus provide a practical geochemical indicator which enables us to delimit a promising area for prospecting around the known kuroko deposits.
The Na ₂O variation in the footwall acid lavas has extensively been examined in the vicinity of the Uchinotai-Uwamuki deposits, Kosaka mine and of the Fukazawa deposits, Hanaoka mine. The following results have been obtained:
1) The most economical threshold to delimit the promising area can be set at Na ₂O = 0.36 wt% in the Uchinotai-Uwamuki area and at Na ₂O = 0.32 wt% in the Fukazawa area.
2) The promising areas thus defined are two or three times as large as the presently known extents of these deposits.
3) Some new orebodies have successfully been discoverd by this method.

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Felsic volcanism taken place pre- and post-Kuroko mineralization in the vicinity of the Hokuroku district have been chronologically studied based upon dating data and microfossil data. The evolution of the volcanic activities was compared with those in northern part of Honshu. It is suggested that the volcanic sequence in the Hokuroku district is correlated to those of Northern part of Honshu in relation to the general evolution of the arc tectonics and Kuroko deposits were generated in the transitional stage from the volcanism under back-arc environment to that of island-arc environment. The evolution history of the Neogene volcanic activities in northern part of Honshu including the Hokuroku district is summarized as follows:
∑The preceding period to 17Ma is characterized by the terrestrial volcanism composed of the former basaltic activity (25-22Ma) and the later granitoid intrusions (22-17Ma).
∑The second period during 17-8 Ma is subdivided into intermediate to mafic lava and the pyroclastics in the pre- to initial rifting stage (17-16Ma), submarine dacitic lava and massive pyroclastics in the back-arc depression prior to the Kuroko generation (16-13Ma), and intermittent felsic pyroclastics under the up heaving environment of the Kuroko basin and contemporaneous shallow to terrestrial felsic volcanism in the surrounding area (13-8Ma).
∑The following period to 8Ma is characterized by a limited extended acidic volcanism under shallow water environment along the marginal part of Kuroko basin and extensive andesitic volcanism under terrestrial condition in the surrounding mountains.

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