Analysis of Structural Control and Results of Prospecting for Bedding-Plane Veins in the Taishu Mine

Abstract

The Taishu mine, well known as one of the high-grade lead-zinc deposits in Japan, is located in Tsushima islands, Nagasaki Prefecture, Japan. The ore deposits are of mesothermal or xenothermal vein types of the late Miocene age, consisting predominantly of galena, sphalerite (marmatite) and pyrrhotite with minor amounts of chalcopyrite, arsenopyrite and pyrite. There are two kinds of veins. The one is the veins which occur along the N-S fault, and the N-S strike slip faults dipping about 60° to the east. The other is the veins formed along the bedding-plane faults which trend. N 35°E and dip about 35° to the southeast in sandstones and alternations of sandstone and shale.
Since 1952, structural analyses of the bedding-plane veins have been made. The results are as follows.
(1) The reverse faults along the bedding plane were formed during folding of the Taishu formations of the Tertiary age, prior to N-S faulting.
(2) Ore shoots of a large scale are found in places where many structural features of faults and sandstone are favorable to the ore deposition.
(3) It seems that the mineralizing solution ascended earlier along the bedding plane faults, and then later penetrated into the N-S faults.
Since 1962, prospecting for a great number of new bedding-plane veins, based on the analyses of the structural control, has been successfully carried out. The minable ore reserves discovered since spring 1962 up to December 1967, have amounted to about 2, 369, 400 tons.