Exploration in the Deeper Area of the Iwami Mine, Shimane Prefeture

Abstract

The Iwami mine is located in Isotake Machi, Ooda City, Shimane Prefecture, Western Honshu, and is producing black ore ("Kuroko") and gypsum.
The upper part of the black ore deposits is of bedded type and the lower part is of network and disseminated type. The gypsum deposits are found between these two deposits.
Since 1966 the lower network and disseminated deposits have been developed.
The following geologic features are found:
1) The Iwami deposits were embedded in the breccia pipe of the Yoneyama quartz andesite, which is believed to represent the earlier episode of volcanic activity in "Kuri" age of the Miocene.
2) Network and disseminated deposits are closely related to small bodies of rhyolite, which intruded repeatedly into the breccia pipe, and high copper concentration has been found in the periphery of the small rhyolite bodies.
The relationship of ore grade to host rocks such as small rhyolite bodies and enclosed various kinds of the brecciated rocks has been studied in detail. Based on those results, the exploration has been carried out and the new ore bodies, W No. 4 and W No. 6 deposits, have been found. Also the important areas to be explored are the periphery and deeper parts of the known ore bodies.
The mining plan and the grade control for network and disseminated deposits with extremely variable grades are successfully achieved by detailed study on character and occurrence of the host rocks, so that the stable mining operation has been maintained.