Country rocks in the environs of the Shakanai Mine are composed chiefly of rhyolite and related pyroclastic rocks.
The horizon of ore bodies of economic value of the Shakanai ore deposits lies in the boundary between rhyolite and pyroclastic rocks in the upper part of the Shakanai formation (Nishikurosawa stage correlated to lower Burdigalian), but horizon of another mineralizations can also be observed to be situated in the Sainokami formation (Onnagawa stage, upper Burdigalian to Helvetian).
The hydrothermal alteration which took place during the formation of deposits has the following characteristics.
1. Elongated texture of pumice in the tuffaceous country rocks.
2. Existence of illcrystalline sericite group.
3. Impregnation of fine-grained pyrite in the country rocks.
4. Natural radioactive anomaly.
5. High concentration of mercury.
6. High concentration of various trace elements.
Zone of montmorillonite-zeolite facies as the diagenetic alteration is highly disturbed close to the ore, deposits. This evidence should be thought to have been mainly caused by repetition of post-mineralization alterations.

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Abundant copper vein deposits of the mine occur in the Itaya formation composed of sandstone and intercalating shalebeds of the Miocene Miyai group. Principal ore mineral is chalcopyrite, and it is associated with pyrite, sphalerite, galena, quartz, calcite, chlorite, sericite, etc. Chloritization, silicification and sericitization of wall rocks are very distinct. Vein fissures of the mining district can be grouped in three sets of strike direction: N-S, E-W and NW-SE groups. Fissures of N-S and E-W group are shear fractures, while those of NW-SE group are tension cracks. The ore-bearing veins of the E-W and NW-SE groups have been mined chiefly for copper ore. Fissures of the N-S group contain a large amount of pyrite but are rather barren of chalcopyrite.
Depth zone that comprises the workable extent and ore shoots of each veins is remarkably limited within the sandstone-rich members of the Itaya formation.
Th0e Miyai group surrounding the main ore-bearing area has general strike of N20-4°E and dips 10-30°eastward. The main ore-bearing area is situated in a, dome within the northwestern part of a large basin of the. Miyai group. The long axis of the dome is orientated parallel to the general strike of the Miyai group. The ore-bearing area is divided into four districts by the difference of local structures: northeastern, northwestern, southeastern and southwestern districts. Structural and mineralogical features of ore-bearing fissures in each district reflect the difference of geologic structure. A notable difference is that the fissures in the northeastern district run at nearly right angles to the strike of the strata, while those in the southwestern district are parallel to them.
Four stages of mineralization can be recognized: the first stage (Cu-Py), the second (Pb-Zn), the third (Au-Ag) and the fourth (calcite).
Distribution of the metals deposited in the area are expressed as assay grade multiplied by the width of veins, and iso-metal quantity contours are illustrated on the maps. It seems possible that mineralization centers of each four stages overlap one after another in the southwestern district, while no evidence of coincidence is observed in the northeastern district. Fissures of the N-S group can be regarded as the channelways of mineralizing solutions.
Cu and Pb-Zn zones can be depicted successively outwards from centers where two sets of fissures of the N-S group and the E-W or NW-SE group cross each other. These features might be useful for further exploration.

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The Furutobe mine, one of the most productive Kuroko deposits in Japan, is located inthe northern part of the so-called "Hokuroku Mineralized Zone" in Northeast Japan. The area is composed of preTertiary, Miocene, and Pleistocene formations. The Miocene formation is subdivided into three stratigraphic units, namely lower, middle and upper formations. All of the ore deposits of the mine are found in the upper part of lower formation which is mainly composed of lavas and pyroclastic rocks of acidic composition. Based on modes of occurrence and mineral assemblages, each ore deposit is divided into several zones. Some deposits are bedded and others are of fissure-filling or dissemination type. As the result of field and microscopic work, the writers conclude that the main parts of these ore deposits were formed by submarine exhalative and hydrothermal sedimentary processes, genetically related to volcanic activities. It is also suggested that the ring-and vent-type structures which are found in the deposits were formed by the mineral solution ascending after the formation of the main sequence of the ore deposits. Later sulfide veins are also recognized.

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A considerable amount of gold is contained inthe ores of "Kuroko"-type ore deposits. In the case of Uchinotai deposits, the gold content is 1.6g/ton in average, but in some parts it exceeds 10g/ton in "Kuroko" (polymetallic ore).
In this study, electrum was examined with the ore microscope and EPMA, and the following results were obtained.
(1) Electrum is concentrated in the upper part of the "Kuroko" ore body and found generally associated with galena. The peak of its grain size distribution exists at 3 to 15μ.
(2) In larger electrum grains, marginal part (2-3μ wide) is generally 10-20% higher in silver than the core. Composition of smaller electrum grains (less than 10μ in diameter) ranges between 65-75wt % Au, though the existence of the silver-rich zone is undetectable.
(3) Electrum is rarely observed in the specimens with Au content of less than 1g/ton, while many electrum grains can be found in the specimens of more than 2g/ton Au.

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