The “Kurokô” or black ore named for its black colour is massive ore, composed of sphalerite, galena, chalcopyrite, tetrahedrite, pyrite, barite, gypsum etc. The “Kurokô” deposit occurs in Miocene formation rich in volcanic products mostly in Northeast Japan. Some of the “Kurokô” deposit are irregular massive shape, but others are lenticular and intercalated between the Miocene pyroclastics or sedimentaries. is noticed that many “Kurokô” deposits are accompanied by the dacitic dome in their underlying parts.
The genesis of the “Kurokô” deposit is a problem now in discussion among the Japanese geologists. is Interpreted in the three ways (i) hydrothermal replacement origin, (ii) submarine exhalative sedimentary origin, (iii) the combination of (i) and (ii).
The similar problem on the genesis of the strata-bound deposit is also a topic of the dispute among the geologists throughout the world.
Since 1955, many large ore bodies of the “Kurokô” deposit have been found in the northern part of Akita Pref., as shown in Table 1.
Now the large scaled prospecting project of the “Kurokô” deposit is being carried out in many promising places throughout Japan in co-operation with the private mining companies and Japanese goverment.

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In this paper, the results of the stratigraphical and geo-stractural surveies, geo-chemical observations, and geophysical prospects by gravity, magnetic, seismic and electric methods in the northern part of Akita Prefecture known as the so-called Hokuroku Kurokô Area are described.
As the results of the above-mentioned observations, the lateral and vertical changes in lithofacies of the sedimentary and igneous rocks, their relations, and the stratigraphic positions of the Kurokô are deposits were established and particularly that the geo-structure and the igneous activity of the Tertiary strata, and the forming-place of the Kurokô are were intimately related with the pre-Tertiary basement structures was recognized.

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The Promoting Agency for Metallic Minerals Prospecting was established in May, 1963. The business of the Agency is to promote the base-metal minerals prospecting both by loan and by geological survey.
Since 1964, geological survey by grid drilling (1km-2km span) has been carried on in the Hokuroku black are district, Akita Prefecture.
We adopted the wire-line drilling method which is characterized by high speed and high core recovery.
The writer introduces the outline of the grid drilling and its results, one of which is that 47 of 136 logs with 61 kilometers total depth during three years hit the underlying rhyolite (dacite) of the “Kurokô”-type mineral deposits.

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“Kurokô” are deposit, the most productive mineral resources in Japan, such as copper, lead, zinc, iron sulphide, gold, silver and barite, is now considered genetically that ores are formed by the low-tem perature hydrothermal or exhalative sedimentary origin related to the submarine volcanic activities in the middle Miocene age. Peculiar genetic conditions of submarine sedimentary environment formed various type of the ores characterized by complexly intergrown mineral assemblage.
In this paper, hypogene minerals described by the Japanese investigators on the kuroko ores are summarized. Except the common mineral constituents, outlines of the occurrence and paragenesis of some uncommon sulphides, such as enargite-luzonite group, tetrahedrite-tennantite series and Cu-Fe-S series minerals including, pyrite, colloformed pyrite, chalcopyrite, bornite, idaite and chalcocite are described.
The occurrence of extraordinary ores associating with magnetite is presented: galena-magnetitehematite ores intimately intergrown by bornite, chalcocite, stromeyerite and electrum found between the bariteferruginous quartz zone and the uppermost zone of the kuroko ore body in the Furutobe mine, and pyritic yellow ores characterized by the formation of magnetite and pyrrhotite caused by the thermal metamorphism associated with later volcanic intrusives as dolerite and dacite at the Hanaoka and Hanawa mines.
It is also shown that gold and silver concentrate in the black ores intimately associating with chalcopyrite and galena at the Ainai and Shakanai mines. On the banded structure of the kuroko ores resembling the sedimentary structure, which have been considered by epigenetic metasomatism preserving the primary structure of beddings and laminations of sedimentary pyroclastics, the considerations based on the syngenetic theory are presented.

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Mineral compositions of ores from the Hanaoka and Kosaka mines are calculated from the bulk chemical analyses of “Kurokôo” (black ore), “Ôkô” (yellow ore), and “Keikô” (siliceous ore) which are the most typical are types of the so-called “Kurokô deposits”. The “Kurokô” is rich in sphalerite, galena and barite with minor tetrahedrite group mineral, chalcopyrite and pyrite.The “Ôkôo” is characterized by high concentration of pyrite and chalcopyrite and is rather poor in gangue minerals. The “Keiko” is highly siliceous ore with some chalcopyrite and pyrite. These natures in mineral composition of ores from the “Kurokô deposits” are compared with those of ores from Cu veins, Pb-Zn veins and cupriferous pyrite deposits. There is found very similar tendency both in mineral composition and in Cu-Pb-Zn ratios of the “Ku rokô” with ores of Pb-Zn veins, and the “Ôkô” and “Keikô” with ores of Cu veins and cupriferous pyritic deposits.

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The ores are devided into three types. Keikô(siliceous ore) and Ôkô(yellow ore) show simple textures and mineral assemblages. The former consists of pyrite, chalcopyrite and quartz and the latter of pyrite and chalcopyrite. In both ores, pyrite is euhedral or subhedral. On the contrary, mineral assemblages and textures of Kurokô(black ore) are very complicated. Main constituents are barite, pyrite, chalcopyrite, sphalerite, galena and tetrahedrite. Under the microscope, diaspore, native gold and unknown Ni-bearing mineral are found. These minerals are intimately associated with each other making colloform and mottled textures. A cupola of keikô, located near the center of the deposits, differs from the keikô above-mentioned in showing colloform textures and bearing minor barite, sphalerite and galena.

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The ore in the adit 130 level, No.10 line is classified into three types in the texture, i. e., black ore, yellow ore and siliceous ore.
The black ore is composed of sulphide bands with several millimeter wide, and its mineral assemblage consists of sphalerite and galena with pyrite, chalcopyrite, tetrahedrite and small amount of bornite. Pyrite ore seam in the ore is characterized by several types of colloform texture and euhedral orsubhedral pyrite grain.
The yellow ore is weakly banded, but where it is highly enriched with chalcopyrite and clay minerals, fine banding of pyrite ore and chalcopyrite ore is observed. In the lower and middle part of the ore, ore is compact and cataclastic texture is observed, but in the upper part of the ore, fragments of yellow ore showing the colloform texture are often observed.
The siliceous ore is composed of pyrite, chalcopyrite, quartz and volcanic fragments. Pyrite of the ore in the southern part occurs as the principal ore mineral, disseminated between quartz grains or as pools of aggregates. It is characterized by euhedral or subhedral from. But pyrite of the ore in theno rthern part differs from it in the southern part in type, and fine banding of quartz-pyrite-chalcopyriteore is observed. Fine banding of the ore cuts the boundary between yellow ore and siliceous ore, and in the upper part fragments of yellow ore Showing the colloform texture are observed.

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The Furutobe are deposits are constituted with five orebodies, namely, Yunosawa, Higashi-and Nishi-Daikokuzawa, Higashi-and Nishi-Magariyazawa ore body.
In this paper, the descriptions are mainly focused to both the Daikokuzawa orebodies which are most exploited and have a typical zonal arrangement of ore and other minerals constituted by the mineral assemblages and textural features.
Macroscopically, some specific structures in addition to the zonal arrangement are found in the orebodies, such as breccia or “schlieren” of are belonging to other ore zone, compositional banding, fissure-filling vein, and ring ore.
Microscopic characteristic features such as framboidal, spheroidal, colloform banding, exsolution texture are widely spread throughout each zone, accompanying with the very complicated fine-grained general texture.
Some results of statistical microscopic analyses of the are are described for illustration of grain size of the main components.

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The Daiichi ore-deposits of the Shakanai Mine are of the “Kuroko” type, having a zonal arrangement of Kuroko (black ore), bko (yellow ore), siliceous ore and gypsum ore.
The Kuroko is classified into three types, fine grained compact Kuroko, coarse grained Kuroko and breccia-like Kuroka. Galena, grey copper minerals and colloform texture in pyrite are characteristic in fine grained compact Kuroko. Conversely, they are seldom observed in coarse grained Kuroko. Breccia-like Kuroki5 is composed of fragments of ores and clayey matrix. The mode of occurence of it is very interesting. Main ore minerals in Oko, siliceous ore and gypsum ore are pyrite and chalcopyrite, of which grain sizes are, generally, large, and in which colloform texture is seldom found.
It is surmised from microscopical observation and chemical analyses of the ores, that the grey copper minerals range from tetrahedrite to tennantite, and that the tetrahedrite mainly bears silver.

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1. 花輪鉱山には層状の黒鉱鉱床と, 塊状-レンズ状の石膏硫化鉄鉱床とがある。前者に属する鉱床には明通鉱床があり, 後者に属する鉱床には本山・三平・安代鉱床がある。
2. 同鉱山から産出する金属鉱物には, 黄鉄鉱・黄銅鉱・閃亜鉛鉱・方鉛鉱・四面銅鉱・斑銅鉱・輝銅鉱・方輝銅鉱・銅藍 (2次成), Cu-Fe-S系未知鉱物 (ほぼ (Cu-Fe) S2, Cu/Fe≒3)・磁鉄鉱・磁硫鉄鉱・赤鉄鉱・白鉄鉱などがあり, 粘土鉱物を除いた非金属鉱物としては石膏・硬石膏・重晶石・石英・螢石などがある。
3. 黒鉱鉱床の黄鉄鉱・黄銅鉱・閃亜鉛鉱・方鉛鉱にはコロフォーム組織がしばしば認められ, また石膏硫化鉄鉱床でも石膏鉱体中の黄鉄鉱・黄鉱鉱体中の黄鉄鉱・黄銅鉱・閃亜鉛鉱にはコロフォーム組織が認められることがある。硫化鉱物に見られるコロフォーム組織は, これらの鉱物が比較的低温の多成分膠状溶液かち晶出したことを暗示している。
4. 黒鉱鉱石中の閃亜鉛鉱のFeS含有量はほぼ1.7-6.0mol%であり, もつともFeS含有量の多いものは黒色の繊維状閃亜鉛鉱であり, もつとも少ないものは黒鉱を脈状に切る淡黄褐色の閃亜鉛鉱である。
5. 四面銅鉱には安四面銅鉱と砒四面銅鉱の両種が見られる。西安代-45mL産の砒四面銅鉱のAs, Sb, Zn含有量はそれぞれほぼ18.8%, 0.4%, 5%± であるが花輪鉱山産四面銅鉱の As: Sb の量比, Ag・Zn の含有量にはかなりの変動が見られるものと考える。
6. 磁鉄鉱・磁硫鉄鉱・赤鉄鉱は石英安山岩脈が火山砕屑岩質の母岩, および石膏硫化鉄からなる鉱床を貫く付近にのみ認められ, その成因は石英安山岩脈の迸入に密接な関係があるものと考える。ここに見られる磁硫鉄鉱は単斜型の磁硫鉄鉱である。

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1) The Wanibuchi Gypsum Deposit is a hydrothermal replacement deposit in the late tertiary period.
2) The Gypsum Deposits consist of several bedded ore bodies and few irregular massive ores, accompanied with small amount so-called “Blaek Ore” (Kuro-kô).
3) The wall rocks of are deposits are black shale and dacitic tuff or tuff breceia (so-called Green Tuff).
The alteration products in the wall rocks are clay minerals such as Mg-chlorite, talc and montmorillonite.
4) The ore minerals are anhydrite, alabaster, selenite and satiuspar.
5) The occurence of gypsum is spheroidal, massive, or veined type.
6) The, anhydrite almost has changed to the alabaster by hydration in the later stage.
7) The crystallization stage of the anhydrite is earlier than that of the metallicminerals.

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Distribution of the minor elements, silver, lead, zinc, arsenic, barium, manganese, molybdenum, strontium, nickel and cobalt in some ore-bodies and their country rocks of “Kurokô” deposits in the northern part of Akita Prefecture, is indicated by the spectroscopic analysis (Spectrograph-Quartz, G-170, Shimazu) of the samples more than 2000 from boring-cores.
Behavior of some minor elements indicating remarkable differences between the tuffs of Nishikurosawaformation and Onnagawa-formation, plays a part in correlation of these formations.
Characteristic distribution of vanadium and manganese in the Matsumine-tuff, is useful for the key-bed in correlation.
Silver, lead, zinc and others seem to be effective for the prospecting of Kuroko deposits and distribution of silver in the Matsumine-tuff is the best index.
Vanadium which is not found in a part of the tuff of Nishikurosawa-formation seems to be effective for correlation of the bed in the formation.
One of characteristic minor elements in the acidic volcanic rocks in Nishikurosawa horizon is vanadium, and some minor elements in the basic volcanic rocks are nickel, cobalt chromium, vanadium and manganese.

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In the Green Tuff region, detailed comparison of published and new analyses leads to the following conclusions about the distribution of minor elements in ore minerals, especially sulfide minerals:
(1) In xenothermal type deposits such as the Ashio mine, Bi, In, Sn and Co show high contents, and in epithermal vein deposits the contents of these elements decrease, while in Kuroko type deposits they show extremely low Contents.
(2) In Kuroko type deposits, As, Sb, Ge, Ga, Hg, TI, Ag, and Mo, which are rarely detected in vein deposits, show high concentration. Besides, enrichment of W, U, and V is in the oxide bed of Kuroko type deposits.
(3) These elements are closely similar to the elements which form volatile compounds of volcanic emanations at low temperature.
(4) If the distribution of minor elements in ore minerals has close connection with the conditions of ore formation, it is considered that enrichment of these elements in Kuroko type deposits deals with volatiles in volcanic emanation.
During the recent years many are becoming to believe that the Kuroko type deposit is of a submarine exhalative sedimentary origin by detailed field observations in the Kuroko area.Conclusions about the distribution of minor elements in ore minerals in the Kuroko type deposits support the exhalative sedimentary origin.

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As practically all the black are deposits in shallow places have already been discovered, explorations are conducted for those occurring in deeper places in recent years. The exploration of are reserves in deeper places is conducted in three different stages namely an areal survey ofgeological structure aimed at clarifying basin structure and grasping igneous movement, precise surveyof geological structure to find out places of occurrence of are deposits and prospecting of are deposits themselves with an object to find out the special characteristics of the property of matter.
The geophysical exploration methods adopted for the survey of geological structure are such as gravity survey, seismic and ground magnetic survey, while in the case of prospecting of ore deposits, electromagnetic method (Turam method) is used for reconnaisance prospecting whereas induced polarization method is used for precise prospecting. The explanation on each of these are given hereunder using two or three examples in each case.

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Geochemical prospecting of kurokei are deposits are generally done using Zn, Cu, Pb, Ca, Ba, SO₄ etc., as the indicator elements. This method is most effective to the deposits of comparatively shallow depth in the mountinous areas. The are deposits in the plain are, in most cases, deep and had many artificial contaminations, which make the geochemical prospecting impossible just now. As for the surveys of the wide area, where kuroko are deposits and vein type are deposits are expected geochemical prospecting by natural water can assume the area of the mineralized zone.

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