1978 Volume 28 Issue 152 Pages 373-384
Skarn-type tungsten ore deposits of the Kiwada mine are located at Futashika, Iwakuni City, Yamaguchi Prefecture, western Japan. The area is underlain by the Triassic Kuga formation which consists of mudstone, chert, sandstone and limestone. The formation is intensely folded into anticlinorium and synclinorium trending east-westerly, and is metamorphosed to biotite or spotted biotite-cordierite hornfels by the late Cretaceous granitic intrusion.
The mineralization which replaces the limestone of the Kuga formation is thought to be related to the granitic activity. Some fifteen orebodies of varying sizes from 200t to 80, 000t, in which the grade ranges from 0.5 to 7.0% in WO 3, have been found to date. Tungsten mineral is exclusively scheelite with a minute exceptional occurrence of wolframite which appears to be pseudomorph after scheelite. The skarn of the deposits consists of such minerals as garnet, clinopyroxene, wollastonite, vesuvianite, epidote, plagioclase, muscovite, chlorite, quartz and calcite, and is rather commonly cut by quartz veins. Scheelite occurs both in the skarn and the quartz vein, but the highest concentration is seen in and around the quartz vein that runs amid the skarn body. In larger orebodies there remains some limestone in the core which is surrounded successively by the zones of wollastonite-vesuvianite, quartz-calcite and clinopyroxene-garnet. Major sulfide minerals found in the orebodies include pyrrhotite, .chalcopyrite, arsenopyrite, pyrite and sphalerite.
Localization of orebodies seem to be controlled by two structural factors: (1) folding axes of the Kuga formation which define the possible site of limestone lenses and (2) tensional fractures crossing the folding axes at nearly right angles which provide the paths for ascending ore fluids. The limestone lenses cut by quartz veins filling the above fractures are therefore the targets of search for high grade ores. As the limestone in the area is generally very small in scale and is seldomly exposed, bore hole exploration based on the detailed geologic data seems to be the sole promising way of searching new orebody and has been quite successful in the past few years. Underground S. P. measurement and the magnetic survey with a proton magnetometer have also been found useful in the case of locating the hidden orebodies within relatively short distances (5-10 meters).
