Metal Mining Agency of Japan (MMAJ) had carried out a systematic exploration program for regional mineral resources in the Bantan district, western Japan, using geologic, gravimetric, aeromagnetic and drilling methods, during a period of about seven years from 1971. The district embraces one of the major metallogenic provinces in this country, where famous polymetallic vein deposits of Ikuno and Akenobe are known.
Although there are many exposures of magnetic body and high density body which are expected to cause magnetic and gravimetric anomalies in the district, the solutions derived from the conventional analysis of the data could not disclose the proper geologic boundaries because many anomalies of different wave-length and amplitude interfere each other to form composite anomalies.
The authors reexamined the gravimetric data with the manual bandpass filtering process (TERASHIMA and TsucHIYA, 1976a, b; TERASHIMA and YOSHIZAWA, 1976; TERASHIMA et al., 1979) and clarified the subterranean geologic structure that is quite consistent with the surface geology. A remarkable conclusion of the new analysis is the existence of a relatively shallow, mostly hidden emplacement of granitic batholith in the central district, along the periphery of which major Sn- and W-bearing polymetallic mineralizations such as Akenobe-Ohmidani, Kawakami and Ikuno seem to occur.
The aeromagnetic data were also reexamined with special attention to the distribution pattern of pair (positive and negative) anomalies of the second vertical derivatives and of pair (high and low) anomalies of the total magnetic field. The detailed magnetic structure thus obtained succeeded to delineate the distribution of granodioritic intrusives which have genetic relation to Mo-bearing mineralization in the district.

View full abstract

The purpose of this study is to deliniate geological background of coalification and to interprete characteristic nature of coal rank variation in the Ishikari coalfield from a view point of orgnic metamorphism in determining and representing rank of coalification by vitrinite reflectance (R ₀). In order to elucidate the coalification variation covering the entire coalfield, numerous vitrinite samples were collected both from coal seams of the major coal-bearing formations and from phytoclasts of non-coal-bearing formations of selected sampling routes.
The measured R ₀ values are provided to analyze both stratigraphic coalification pattern within the sucsession and regional variation pattern within the major coal-bearing formations. The stratigraphic coalification patterns in the selected sampling sucsessions reveal rather low reflectance gradient with some remarkable retardation near the bottom horizon of the Noborikawa coal-bearing formation. Regional rank variation pattern of the major coal-bearing formations is characteristically concordant with the variation of overlying thickness, and is cut and shifted by major faults. In comparing these rank variation patterns with sedimentary and structural data and with R ₀-gradient of the deep oil exploring bore holes that penetrate the same horizons, the geological processes of coalification in the Ishikari coalfield are summarized as follows. The coalification process underwent through the diagenetical alteration during sedimentation of the Ishikari and overlying groups which accumulated in a paleogeothermal condition of low heat flow, and the consequent coalification pattern was modified by differential sinking and upheaving movements of the basin which acted as a precursor of the Hidaka orogeny. The present pattern was constructed during the latest stage of the orogeny resulting in physical deformation of the original pattern by faulting and folding movements under a condition of falling temperature in coalification reaction due to the total uplift of the basin.
The vitrinite reflectance gradient in the Cenozoic groups of the present high heat flow region in the continent side (inner zone) of the Japanese Islands is relatively high compared with that of the Ishikari coalfield. This contrasting coalification condition is attributed to the contrasting crustal thermal structure of both sides of the island arc in the Cenozoic Era, which suggests a paired organic metamorphism in the tectonic development as the pair of the higher grade of metamorphism formed by the older orogenic movements in the island arc. The feature of the coalification of the Ishikari coalfield can be designated as a typical example of the lower temperature and/or higher pressure organic metamorphism in the Pacific side of the Japanese Islands during the Cenozoic development of the arc.

View full abstract

In order to accomplish a quick and objective handling of a very large number of data obtained in the exploration work of kuroko deposits in the Hokuroku district, a computer system has been developed and is ready to use routinely. As its first trial the system examines the rock alteration data of the footwall dacite lava of the Ezuri ore deposits.
On the basis of 5 measurable elements (magnetic susceptibility, concentrations of Na, Ca, Kand Mg) which have been measured on some 1, 500 specimens from more than 50 drill holes in an area of 5×6km, 11 variables(magnetic susceptibility, Na, Ca, K, Mg, Alteration Index=K+Mg/Na+K+Mg+Ca, K/Na, Mg/Ca, Na/Na+K+Ca, K/Na+K+Ca, Ca/Na+K+Ca) have been examined. After the calculation of basic statistics, halo maps of these variables are drawn by means of line printer and X-Y plotter. Except for Mg which has no returned a distinct halo, the halo maps obtained are generally quite consistent with the distribution of orebodies. The magnetic susceptibility halo has a bigger extention that embraces nearly all the chemical haloes. Among the chemical haloes those related to"leached elements"(Na, Ca, Alteration Index, Na/Na+K+Ca, K/Na+K+Ca, Ca/Na+K+Ca)show somewhat different patterns from those related to "additional element"(K, K/Na, Mg/Ca).
Usefulness of the alteration halo in the kuroko exploration is quite clear. The filed data processing system saves much of our time and is very useful for future exploration works.

View full abstract

¹⁸O/ ¹⁶O ratios and homogenization temperatures of fluid inclusions were determined for five quartz samples collected from veins in the Akenobe polymetallic deposits, southwest Japan. The δ ¹⁸O values (relative to SMOW) of quartz range from +0.4 to +1.8‰. The homogenization temperatures of fluid inclusions, which are estimated to be primary, are in a range of 260 to 320°C. δ ¹⁸O values of -8.0 to -4.5‰ are calculated for thermal waters equilibrated with the quartz by utilizing the above data and quartz-water isotopic fractionation. The low δ ¹⁸O values of thermal waters indicate that the waters are primarily of meteoric origin.
A positive correlation between oxygen isotopic composition of thermal waters and homogenization temperatures of fluid inclusions is observed. This correlation suggests mixing of ¹⁸O-poor, low-temperature meteoric water with ¹⁸O-rich, high-temperature magmatic water. If a simple mixing model is assumed, the proportion of meteoric waters in the thermal waters is as high as 80 to 100%. Some modification of isotopic composition of waters may also have been produced by reaction with hot rocks.

View full abstract