The Sakoshi-Ôdomari mine is a recently discovered gold mine of quartz vein type, which is located in Akô city, Hyôgo Prefecture, southwestern Japan. The mine is of highly profitable, though it is small in size.
The discovery was made in 1974 by the claim owner of a so-called "pyrophyllite" mine, who happened to assay samples taken from outcrops within the claims that had been neglected as "silicified rocks". Subsequent to the discovery, he offered a joint development programme to Sumitomo Metal Mining Co., Ltd., and the writers were assigned to assess the property by the latter.
The mine area is situated within one of the most famous "pyrophyllite" district in Japan, and mainly comprises widely distributed "pyrophyllitized" welded tuff. Therefore, the region had long been regarded as somewhat barren in metallic mineralization and few people had believed occurrence of a promising gold deposit in the region.
Without such a preconception, the writers carried out detailed geological mapping along with trenching several times, and studied the mode ofe occurrence of ores and wall rock alteration. Their findings were as follows: (1) So-called "pyrophyllitized" rocks in the mine actually consist of kaoline-minerals and sericite, and are, consequently, different from "pyrophyllite" ores in such deposits as Mitsuishi, Hiraki, etc. in the adjacent areas, where the mineralogical pyrophyllite is a major consituent. The assemblage in this mine rather resembles what is commonly observed in a gold deposit than that in a pyrophyllite deposit. (2) Four ore bodies occur en echelon within a strike length of 500 m at the surface. (3) Gold ores are apparently controlled by a quartz porphyry stock. And, therefore, it may be expected that low grade ores at the surface will turn to higher grade at the depths, where the vein approaches to the stock.
From these observations, the writers concluded that a profitable gold deposit could be anticipated to occur, and warranted to participate to the programme.
Based on the conclusion, the company decided to develop the property in 1976. Since then, the mine has being successfully operated up to date. The total production during a period between April 1977 and March 1982 is as follows: Ores mined 34, 400 t at 30.7 g/t Au and 283 g/t Ag.
This paper describes the case study of the geological assessment and development of the mine, as well as general description on geology.

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Several kuroko-type ore deposits occur in the central part of western Sulawesi, Indonesia. The state corporation P.T. Aneka Tambang is prospecting the Kuroko-type deposits in the Sangkaropi kuroko area where the Sangkaropi (Batu Marupa), Rumanga and Bilolo deposits are now exploring. The area is situated in the western Sulawesi arc which at that time acted as the volcanic arc.The volcanic activity seems to have been almost contemporaneous with that in the Japanese "Green Tuff" region in Miocene, where all kuroko-type deposits in Japan are distributed in a relatively narrow zone.
The formation of deposits distributed in the Sangkaropi area is closely associated with the extensive submarine volcanism. The deposits are composed of stratiform. or broken stratiform and stockwork ore bodies of kurokotype, containing no gypsum ore. It can be seen at outcrops in the Sangkaropi area that the stratiform ore bodies are covered with a thin layer of barite at the top.
Sulfide minerals constituting the ore deposits are sphalerite, galena, chalcopyrite, pyrite, tetrahedrite, bornite, chalcocite and covelline with quartz, barite and clay minerals. A colloform texture is frequently observed in the fine grained stratiform ore of the Sangkaropi deposits.
The feature of ores and mineral assemblages of ore deposits in the Sangkaropi area resemble to those of the kuroko-type ore deposits in Japan, excluding a marked distinction of the absence of gypsum ore.
The filling temperatures of fluid inclusions in sphalerite and quartz from druse and vein range from 160°to 346°C. This is almost the same as that of the kuroko-type deposits in Japan.

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The modes of occurrence, associated mineral assemblages, and chemical compositions of minerals from the No.11 ore deposit and from the samples of bore holes in the north area of the Shakanai mine, are described on the basis of microscopic observation, X-ray diffraction analysis and electron probe microanalysis. As a first step to estimate the activity ratio of Mg ⁺⁺ to Fe ⁺⁺ in hydrothermal solution, exchange isotherm between chlorite and chloride solution has been made.
Though principal constituent ore minerals of the No.11 ore deposit are silmilar to those of other kuroko deposits, they have wide varieties in mode of occurrence, texture and chemical compositions. Complexmixtures of black ore and tuff breccia, and a paragenesis of various sphalerites support that the No.11 ore deposit has formed at various stages.
Chlorite minerals and interstratified sericite/montmorillonite are extensively distributed around the ore deposits. Contribution of magnesium-rich hydrothermal solution is primary factor for the formation of Mg-chlorite. It is possible that the mixed layer clay zone closely associated with the ore has formed in a different process from the formation of the mixed layer clay zone regarded as a transitional zone. This zone associated with the ore is considered to be influenced by the deposition of sulfide minerals with decreasing of temperature.

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The magnetite-series and ilmenite-series granitoids situate in the San-in and Sanyoh belts respectively, which are bounded by the Ikuno-Tsuyama line, in the eastern Chuhgoku district (KANAYA and ISHIHARA, 1973). Some ilmenite-series granitic bodies, however, such as Wadayama, Hikihara, Mochigase, Chizu and Suriba-chiyama, situate in the northern side of the boundary, while some magnetite-series bodies, such as Siso, My-ohkenzan, Yanahara, Takakurayama and Sugiyama, in the southern side (KANAYA and ISHIHARA, 1973; ISHI-HARA et al., 1981).
TERASHIMA et al. (1980) re-examined the airborne magnetic and gravitational data in the Ikuno area(M.I.T.I., 1971-1974), and clarified that the Wadayama body of the ilmenite-series situates in the area of remarkable, low gravitational anomaly but of no magnetic anomaly, while the Siso body of the magnetite-series, in the area of no gravitational anomaly but of remarkable magnetic anomaly, in the area of Paleozoic system such as Maizuru group and Tamba group.
The author tried to re-examine the airborne magnetic and gravitational data in the Tsuyama area (M.I.T.I.; 1979), and confirmed that the ilmenite-series granitoids, which intruded into Paleozoicsystem, situate only in the area of the low gravitational anomaly, while the magnetite-series granitoids, in the area showing no remarkable gravitational anomaly. This phenomena indicates that the former is of lesser density than the Paleozoic system, and the latter is of almost equal density to Paleozoic system.
The author also clarified the possible distribution of the ilmenite-series and magnetite-series granitoids in the Ikuno-Tsuyama area, and considered that the latter intruded into the area of the former to make a composite body, and to give some thermal affection to the radiometric age of the former. This presumption could be powerful to explain reasonably the unordinary radiometric ages of K-Ar method from the Myohkenzan, Yanahara, Tenguyama, Sugiyama bodies of the magnetite-series and the Suribachiyama, Chizu, Mochigase bodies of the ilmenite-series.

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The authors have demonstrated, in the preceding paired paper (SUGAWARA et al., 1982), our careful observations on the mode of occurrence of the Shakanai kuroko deposits with special reference to some sedimentological and diagenetic phenomena fossilized in them. In this paper, an attempt is made on the basis of those observations to reconstruct the possible diagenetic evolution history of the deposits and thereby to make an approach to their origin.
Our conclusions derived from this study are enumerated in the following.
(1) The ore deposits were, as a whole, primarily of sedimentary origin. In other words, the initial proto-kuroko deposits are considered to have constituted a certain exogenic "bed" or "stratum" in sedimentological sense.
(2) The proto-kuroko deposits were similar in physical dynamic nature to a "viscous liquid" or something like that. In other words, they were not of hard and dense masses as what they are at present.
(3) Since the initial deposition up to present, they have been buried in sedimentary piles for probably about 13 to 15 million years, during which the maximum burial depth has reached to about 1, 000 to 1, 500 meters and the maximum burial temperature attained has exceeded at least 150°C.
(4) Important diagenetic events are as follows.
Water-escape and compaction: Extensive development of a variety of sedimentological "water-escape" structures in the ores strongly suggest that the proto-kuroko ores have also undergone similar diagenetic compaction process-es to what are well known in other normal sedimentary rocks. Significant secondary migration or mobilization of the ore-forming materials must have taken place during these processes.
Ore-textural evolution and modification: The most primitive texture so far observed reveals that the ore minerals were initially of very minute particles of about 1 micron or less in size. The minute particles were coagulated together in the earliest stage of diagenesis to form a series of aggregates with various size and shape, which were then progressively modified, during certain subsequent diagenetic stages, by some possible repeated processes of dissolution, reprecipitation, replacement and recrystallization to complete a variety of other complicated textures.
Formation of ore-zoning: The formation of ore zoning in the deposits is also considered to have principally completed in the early stage of the above ore-textural evolution and modification processes.
Formation of "ore-lumps": The "ore-lumps" constituting the deposits are considered to have been formed on and after the completion of ore-zoning mentioned above, probably due to some consolidation process of the deposits.
(5) In some previous literatures, the fact of "size-grading of ore constituents" has been recognized to be evidence of an idea that the deposits have slided down on the submarine slopes of seafloor. However, this structure may be more likely due to the diagenetic water-escape processes rather than due to such a submarine sliding.
(6) The above conception of the diagenetic evolution history of kuroko deposits is generalized as follows.

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Petrographical properties of dyke rocks observed in the Kamioka Pb-Zn Mining area, Hida metamorphic region, central Japan, are described, based on their mode of occurrences. These are classified into andesites, quartz porphyries, Tochibora and Mozumi granites (so-called granite porphyries), metabasic dykes (synplutonic dykes associating with granites) and so-called aplites (aplitic to leuco-granitic facies of the Inishi-type migmatitic rocks).
So-called granite porphyries have been assumed to be the ore-bringer dykes occurring within the core of the concentric zonation of the Kamioka deposits and closely associated with the late Cretaceous igneous activities, such as the Nohi Rhyolites. Petrographical analysis, however, shows that they are not porphyrires but granitic rocks having holocrystalline equigranular and hypidiomorphic to granoblastic granitic texture and usually granodioritic to granitic mineral compositions. The geological situations, mode of occurrences, petrological characteristics, x-ray properties of potash-feldspars and magnetic susceptibility suggest that the Tochibora and Mozumi granites are lithologically correspondent with the leuco-granitic facies of the adjacent Funatsu granites (170-180 m.y.) and different from the Shirakawa granites of the late Cretaceous to Paleogene ages. The Rb-Sr isochron age (90 m.y.; which has been presumed to be the age of so-called granite porphyries) is defined by the quartz porphyries not by the granite porphyries, and that of the Tochibora and Mozumi granites seems to be 140-160 m.y. or much older.
It will be assumed that the age of mineralization of the Kamioka deposits is not the late Cretaceous but the Funatsu-stage (pre-middle Jurassic) or much earlier.

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