Exploration and development of the Hiraki kaolin mine, Hyogo prefecture, Japan.
The Hiraki mine, situated about 50 km northwest of Osaka, is one of the main "roseki" deposits in Japan (in Japan, "roseki" deposits include hydrothermal kaolinite, sericite and pyrophyllite). The ore deposit is a bedded, hydrothermal kaolinite-ore deposit, altered from dacitic tuff and tuff breccia of Cretaceous age. It is roughly concordant with the bedding plane of the host volcanic rocks. The mine produces 50,000 tonnes per year of kaolinite concentrates. The dominant minerals are quartz and kaolinite with minor amounts of interstratified mineral and dickite. The ore deposit was discovered in 1960 and an open-pit mining started in 1961. First, a systematic drilling survey was carried out from 1961 to 1966. The number of drilling reached 111 holes of 6,400 m in total length. Based on the results, exploration tunnels with a 1,700m total length were constructed from 1965 to 1972. These exploration works have clarified the nature of hydrothermal alteration (kaolinite mineralization) and extension of the mineralized zones. On the basis of the results, it was possible to adopt large-scale underground mining methods, such as the sublevel-stoping method and mechanized cut and fill method. This is the first time that large-scale underground mining methods have been O.K. to clay deposits in Japan. The Hiraki ore is low in alkaline elements and iron and has a simple mineral assemblage (quartz+kaolinite), all of which make it suitable as raw materials for glassfibre. Because of this, the Hiraki mine is one of the leading suppliers of glass-fibre clay in Japan. Exploration at the Hiraki mine is still continuing. To date, 149 holes have been drilled for a total of 8,570 m of core, and its exploration tunnels total 3,850 m in length.