Resource Evaluation and Some Genetic Aspects of Indium in the Japanese Ore Deposits

Abstract. There have been two primary sources for industrial indium; one from massive sulfides, while the other is dissemination‐veins and skarns, related to felsic igneous rocks. The latter group of the In‐bearing deposits is abundant in the Japanese Islands. Indium occurs as In‐minerals such as sakuraiite, roquesite, laforetite and many unidentified minerals, but the majority is contained as an impurity in sphalerite, and tin and copper sulfides. Average grades of the ores from which indium has been extracted vary from a few ppm (e.g., Kosaka mine) to more than 300 ppm (Toyoha mine). The amount of indium in all the major basemetal deposits is estimated by analyzing representative samples. The main indium deposits are subvolcanic and tin‐poly‐metallic vein types. The largest one is Toyoha mine (4,700 tons hi) and the Ashio mine (ca. 1,200 tons In) was found to be the second largest. Many small occurrences, were recognized in the Miocene magnetite‐series belt, besides the classic occurrences in the ilmenite‐series granitic terrains of SW Japan, including the Ikuno and Akenobe tin(‐tungsten) polymetallic veins, located in the northern margin of the late Cretaceous Sanyo ilmenite‐series province. Magnetite‐series magmas with deep source are necessary to concentrate sulfur in the magma chamber but sedimentary source rocks and their reducing agents are needed to collect and to precipitate indium. The Japanese islands are essentially accretionary terrains intruded by various deep oxidized magmas; thus forming magnetite/ilmenite‐series paired belts, which are sometimes mixed. This unique geologic setting may be the most fundamental reason why indium is rich in vein‐type deposits of the Japanese Island arcs.