The Kushikino Mine, one of the largest gold-silver mine in Japan, is located in the City of Kushikino on the northwestern coast of Satsuma Peninsula, and is involved in the Southwestern Kyushu Gold Field. Ore deposits of the mine consist of more than twenty veins of epithermal type emplaced in the andesitic volcanic rocks of Miocene age.
The exploration and development have long been done essentially on the Kushikino vein group, headed by the champion vein called Vein No. 1, extending laterally for 2, 500 m with the width of 40 m and explored for 400 m downwards. As is well known through several papers ever published, the exploration on this mineralized zone has been conducted by the structural analy-sis of fissure systems with considerable discoveries of new veins and ore shoots.
Upon the recent demand to expand the prospecting area outwards the known ore zone, the regional ground survey of geology was carried out and the re-examination was attempted on the various exploration data accumulated through various sources, for the purpose to re-establish the principle to conduct more efficient exploration. The results have revealed such interesting facts as stated below on the localization of ore deposits.
1) The Miocene andesitic formations, the host rocks, are divided stratigraphically into three units; lower lava, middle alternation of lavas and tuffaceous rocks, and upper lava. Most of the ore deposits are emplaced limitedly in the middle formation and the upper part of the lower andesite.
2) Thick accumulation of the volcanic formations is found in the area covering the Kushikino mineralized zone and Arakawa district involving north of Soora, which corresponds to an arch-like depression of basement formation surrounding a dominant palaeorelief in the southwestern part of the mine area. Veins are developed mostly in the depression zone, and especially, the scale of the veins seems approximately in proportion to the thickness of volcanic formations.
3) Among fissures and fractures, those of ENE-WSW and NW-SE systems are most important. The former includes the majority of productive veins, while the latter bears local high grade concentrations, and enriched ore shoots are often formed at the intersections of the two systems. These fissures and fractures are developed in a mesh-like pattern especially in the zone of structural depression.
Taking the above control of ore localization for guidance, the drive of exploration on the Arakawa ore zone since 1970 has led to the discovery of the Arakawa No. 3 and No. 4 Veins, as well as verifying the downward persistence of the Arakawa No. 2 Vein.

View full abstract

The writer would like to stress the importance of the detailed geological mapping on primary porphyry copper or porphyry molybdenum deposits in the inconclusively explored area. It is a useful tool for delineating irregular zones of concentrated mineralization for targets to be checked in the advanced exploration stage and also, for revealing the structural control for the purpose of a more accurate evaluation.
The case of the Ingerbelle deposit is discussed in this paper as an example of timesaving and efficient exploration work for the assessment of areal mineral potential by means of the writer's special mapping technique, as well as the geologically interesting features of the deposit.

View full abstract

Tsushima Island is covered largely by Tertiary Taishu Group consisting mainly of thick alternation of major amount of shale and thin sandstone beds, which shows abundant NE-SW trending folds throughout the island. The group is intruded by many igneous dikes and stocks having the same trend. To the west offshore of the island large N-S trending faults are recognized (after Tomita et al., 1974, 1975).
Based upon field data, the authors examined a distributional characteristics of wavelength, amplitude and plunge of the folds, and found the following facts; (1) the island can be divided into two tectonic provinces of a short-period folding province in the northwest and a long-period folding province in the southeast which are bounded by a NE-SW trending tectonic line. (2) the short-period folding province can be further divided into two tectonic units of a medium-period folding sub-province in the west and a long-period folding sub-province in the east which are bounded by a N-S trending tectonic line. (3) the first order trend surface of the folds in these three tectonic units have different dip and strike each other. Because any remarkable fault is never found along the boundaries among each unit, the tectonic lines must represent the axis of two different flexures of the Taishu Group.
Based upon these results, the authors reprocessed and reinterpreted the gravity data, and came to the conclusion that the NE-SW trending axis of the flexure reflects a step-like structure of high-density basement, which is inferred to consist of gneissose rocks or weak-metamorphosed Palaeozoic System, and the N-S trending axis of the other flexure corresponds to the axial zone of the basement flexure, a inclination of which is nearly horizontal in eastern side and about 20 degrees toward west in the western tectonic units.
The complicated fold system of the Taishu Group is an reflection of structural movement resulted from the older NE-SW folds and younger N-S flexures of the Taishu Group and the basement.

View full abstract

The geologic history of the Proterozoic Erathem in the Hamersley area is successively interpreted in terms of the uplift of a large scale dome of the Archaean basement with the approximate centre between. Wittenoom and Marble Bar (Figure 5).
This concept well define geology in this area, especially in the southern half of it; migration of the Proterozoic sedimentary basin from north to south with age, distribution pattern of folds and faults, and the Proterozoic tectonical history.
There is no available Proterozoic outcrop to indicate detailed geology in the northern part of the dome. However, if the concept of a large scale dome-up is acceptable, the northern part is not probably expected to have a thick accumulation of sediments and then to have structural features due to the tectonical stress occurred in the hinge area between the sinking basin and uplifting dome in the southern part.
The favorable sedimentary environment for banded iron formation has been considered to be appeared by establishment of a closed basin to the ocean. In the case of the Hamersley area, the appearence of the imperfectly closed basin was probably caused by uplifting of the Archaean basement.

View full abstract