Sericite is an important constituent of Roseki clay deposits, which are hydrothermally altered pyroclastics and lavas distributed regionally in the Chugoku province of western Japan. It is usually utilized as raw materials for refractories and ceramic wares as well as kaolin and pyrophyllite. X-ray, chemical and thermal analyses have been made on sericite specimens from several Roseki clay deposits, which are classified into three types based on characteristic alteration minerals. Their X-ray properties differ considerably from deposit to deposit. Sericite in close association with andalusite and corundum in the so-called high temperature type deposit of the Uku mine (Yamaguchi Prefecture) is well crystallized, 2M 1 mica with a composition closer to muscovite. In typical Roseki deposits producing characteristic monomineralic sericite clay in the Yagi and Shinagawa-Mitsuishi mines (Mitsuishi area, Okayama Prefecture) and the Kayano deposit (Hiroshima Prefecture), however, it often occurs as interstratified mica/smectite with expandable layer less than 20%. Its polytype is determined to be predominantly 2M 2, though it is confirmed by two diagnostic reflections at 2.43Å and 2.09Å. The specimens in the Horo mine (Hiroshima Prefecture) are intermediate in crystallinity and polytype. Substitution of NH4 for interlayer K in muscovite-tobelite series is very common and extensive in the specimens from the Yagi, Shinagawa-Mitsuishi and Horo mines, which can be correlated with enlargement of basal spacing from 10.0Å (muscovite) to 10.36Å (tobelite). Because of liberation of NH4 at 400°C, two steps of dehydroxylation in the range of 600-800°C and rapid transformation to mullite at 1000°C, these NH4-bearing sericites are similar to pyrophyllite rather than to muscovite (K-mica) in thermal behavior. Sericite from typical Roseki clay deposits in the Mitsuishi area is used as excellent raw materials for ceramic wares: both adequate plasticity and high refractoriness are ascribed to interstratification with small amounts of smectite layer and interlayer NH4 composition, respectively.
A geochemical investigation has been performed on the whole rock specimens from a quartz porphyry dyke of about 10 m width which intrudes into the Tochibora area of the Kamioka Mine, central Japan. This Kamioka quartz porphyry is high in SiO2(76-77 wt.%) and K2O(>5wt.%) with per-metaluminous signature and enriched in Rb, Cs, Y, Nb, Ta, Th, U and HREE. The REE patterns are almostfl at (LREE≅HREE) with pronounced negative Eu anomalies. These features are identical to non-alkalic A-type granites or Topaz Rhyolites in the western United States of America. The concentrations of Na, K, Ba and Sr change substantially from specimen to specimen. Sodium correlates negatively with other three elements. This elemental variation may be due to the hydrothermal metasomatism which occurred after the intrusion of the quartz porphyry dyke. A specimen from open-cut region, where Pb-Zn ores are mined, shows distinct depletions of La, Sm and Y and great enrichments of Zn and Pb. The depletions of La, Sm and Y are supposed to be related to the formations of Pb-Zn ores or penetrations of fluids into the quartz porphyry. The geochemical characteristics demonstrate that the intrusion of the Kamioka quartz porphyry with hydrothermal activity may be closely related to Mo deposit rather than Pb-Zn deposits.
The geologic potential for mineralization is just as important as the past presence of a metal-bearing fluid in determining whether or not ore deposition occurs. This paper discusses the geologic setting of gold mineralization in southern and central Kyushu in order to investigate the genetic relationship between cauldrons and gold vein systems. The geology of southern and central Kyushu comprises: From Pliocene to Quaternary, mainly andesitic volcanic activity occurred on land, and many cauldrons formed. Gold mineralization was restricted to fractures on the margins of these cauldrons. Gold mineralization tends to take place at sites between two or more cauldrons, where fractures were more abundant elsewhere. In February 1991, the Metal Mining Agency of Japan discovered gold-rich guartz veins (Au 171.5 g/t, Ag 811.0 g/t) in drill holes in the Hikiji area, Oita prefecture, central Kyushu. The site was predicted to have a potentiality for gold deposits because of its location between two cauldrons. Thus, the significance of the volcanic cauldron model for gold exploration was supported. Subaerial cauldrons and their associated structures are very favorable sites for gold exploration.