| Research Abstract |
In orfer to understand the genesis of the metalliferous mudstones and shales, i.e., age, distribution, formative processes and environment, in the Green tuff region of northeastern Japan, mineralogy, chemistry and sulfur isotopes of mudstones and shales in the region had carefully been investigated. The results obtained are reported here. Important results are as follows :
Chemical analysis data for a series of whole-rock samples from the taiheizan district reveal that the uppermost Uyashiai Mudstone (Nishikurosawa stage) and the iowermost onnagawa Hard Mudstone (Onnagawa stage) are enriched in Zn, Cu, Pb and Ba compared to the upper onnagawa Hard Mudstone and Funakawa Black Mudstone (Funakawa stage). Sulfur isotopic rations (delta<@D134@>D1S<@D2CDT@>D2) of whole-rock sulfides from the Uyashinai Mudstone range from -35* to -40*, those from the Onnagawa Hard Mudstone range from +5* to -10*, and those from Funakawa Black Mudstone range from (]SY.+-。[)0* to -30*.It should be noted that rem
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arkable enrichment in Zn, Cu, Pb and Ba and abrupt change of sulfur isotopes can be observed just at the vary Nishikurosawa/Onnagawa boundary.
Ore-microscopic observation has revealed that the mudstones of the upper Nishikurosawa stage (eg. the Ohmatagawa Mudstone) commonly contains micro-grains of sphalerite and chalcopyrite in association with framboidal and euhedral aggregates of pyrite. The sphalerite often has an unique texture like a sort of chalcopyrite-diseased one and almost always fills in the tests of fossil foraminifera. The sphalerite formation is thus suqqested to be due to bacterial sulfate reduction coupled with decomposition of organic matter during the early diagenetic stage. It is worth noting here that the Ohmatagawa Mudstone with the unique occurrence of zinc and copper sulfides is stratigraphically correlatable to the currently-accepted Kuroko-bearing horizon.
On the basis of the results obtained, an attempt is made to construct a qenetical model of the metalliferous mudstones and shales, and to evaluate paleoenvironmental controls on the middle Miocene metallogensis. Less
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