紀伊半島南部における中期中新世熱水活動

抄録

Many vein type mineralization including the Kishu and Myoho deposits are known to occur in the southern and southeastern part of Kii Peninsula, where Middle Miocene volcanic products are underlain by the pre-Miocene and Miocene sedimentary complex of the Shimanto Belt. Vein type ore deposits that are related to felsic volcanism mainly occur in the Miocene sedimentary rocks except few in the pre-Miocene sedimentary rocks. Calcite veins are found with or without sulfide minerals in these sedimentary rocks. We have studied isotopic composition of the vein calcites in drilled cores obtained from these sedimentary rocks. The total ranges of δ¹⁸O and δ¹³C for calcites are from +6 to +17‰, and from -14 to -4‰, respectively. These value are divided into two groups based on their oxygen isotopic values: δ¹⁸O≤10‰ and δ¹⁸O>10‰, respectively. These data, together with homogenization temperature of fluid inclusions and strontium isotope data indicate that the relatively low δ¹⁸O value in calcites (δ¹⁸O<-10%0) formed under hydrothermal condition, however, the relatively high δ¹⁸O value in calcites (δ¹⁸O>10‰) formed from solution trapped in sedimentary rocks. The calcite veins with relative-ly high δ¹⁸O value are hosted mainly in the pre-Miocene sedimentary rocks, which suggests that the pre-Miocene sedimen-tary rocks might restrict free hydrothermal circulation. That resulted in lesser number of vein deposits in the rocks.