Recent Exploration of the Arakawa Veins, Kushikino Mine, Kagoshima Prefecture

Abstract

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.