Re-examination of the Metallogenic Epoch of the Ikuno-Akenobe Province in Japan

Abstract

Polymetallic vein-type deposits of tin, tungsten, copper, zinc, lead, silver and gold are found at Akenobe (SAIGUSA, 1958; ABE, 1963; MURAOKA and IKEDA, 1968) and Ikuno (MARUYAMA, 1957; TANAKA et al., 1971). These are among the best-known of Japanese deposits and have been cited as type examples of a xenothermal deposit (e. g., PARK, Jr. and MACDIARMID, 1964). The ore deposits at both mines are quite similar in the assemblage of vein-forming minerals but differ in their host rocks. The host rocks at the Akenobe mine are Permo-Car-boniferous sedimentary rocks and the early Triassic dioritic rocks of the Yakuno mafic complex; while those of the Ikuno deposits are the Cretaceous rhyolitic pyroclastic rocks of the Ikuno group. No granitic rocks younger in age than the host rocks have been found very close to these deposits (Fig.1). However, a number of dikes, basaltic to rhyolitic in composition, have intruded before (especially at the Ikuno mine) and after (especially at the Akenobe mine) the mineralization.
There has been no definite evidence reported about the geologic age of the mineralization in and around these deposits. But most Japanese geologists have followed T. KATO (1927) in arguing for a late Tertiary metallogenic epoch (NISHIWAKI and WATANABE, 1956; SEKINE, 1956; TSUBOYA et al. 1956; SEKINE et al., 1960; TATSUMI et al., 1970; NAKAMURA, 1970). During the last five years, this conclusion has been questioned by some geologists studying the regional geology and ore deposits of the province, who have suggested rather a much older stage of late Cretaceous to Paleogene (e. g., IMAI, 1966; IMAI et al., 1970). This dispute came to the annual meeting of the Society of Mining Geologists of Japan in February 1967 when IMAI et al. (1967) presented a paper about the metallogenic province of the western Kinki district. In Imai's opinion the ore solution derived from a cryptobatholith of the late Cretaceous to Paleogene granitic rocks, similar rocks of which are widely distributed in the Inner Zone of Southwest Japan.
Such was the general climate of opinion about this subject when we began to collect specimens for isotopic dating. Direct dating of these deposits is difficult because of the lack of suitable minerals, either among the vein-forming material or alteration products. We finally decided therefore to determine the age of the post-ore dike by the potassium-argon method, which thus yields the minimum age of the mineralization.