Metallic mineralization associated with pillow basalts in the Yaeyama Central Graben, Southern Okinawa Trough, Japan

Abstract

CITATIONS 13 READS 145 8 authors, including: Some of the authors of this publication are also working on these related projects: Petrography, petrology and geochemistry of magmatic enclaves and host granitic rocks from NW-part of Malayer-Boroujerd plutonic complex (M.Sc thesis, University of Tehran) View project Origin and magmatic evolution of southwest Nodooshan, west of Yazd View project Abstract Mineralogically investigated were massive sulfide samples of chimneys and mounds collected during the "SHINKAI 6500" dives (Dive 562 and Dive 567) from two basalt-hosted hydrothermal ore deposits, being at a short distance of about 1 km, occurring in the eastern end of the Yaeyama Central Graben, the Southern Okinawa Trough. In both sample series (#562-and #567-series), ore minerals are dominated by low-iron sphalerite with small amounts of chalcopyrite and galena. Sphalerite in the #567-series samples shows somewhat wide compositional variation, ranging from 0.4 to 3.8 mole%FeS, with the highest mode of about 1 to 2 mole%FeS. Significant amounts of pyrite and secondary marcasite are present in the #562-series samples with subordinate bornite, while in the #567-series samples, they are lacking, but instead tetra-hedrite-tennantite series with atomic ratios of Sb/(Sb+As) being 0.2 to 0.8 is abundant and contains Ag up to 0.7wt% with mostly less than 0.5 wt%. Neither pyrrhotite nor isocubanite is encountered in the two sample series. Microthermometries of two-phase (liquid + gas) fluid inclusions trapped in calcite (#567-series) have revealed the following results: homogenization temperatures (not corrected for pressure) of 242 to 317 °C and salinities of 5.1 to 7.6 wt.%NaCl equiv., being a little more saline than the seawater. The observed mineral assemblages and sphalerite compositions obtained for the Yaeyama Central Graben deposits lead us to indicate that the basalt-hosted mineralizations might have occurred under rather higher sulfidation state. From the mineral associations observed, it is also indicated that both hydrothermal deposits are characterized by a metal association of essentially Zn-Pb-Cu. The mineral assemblages and ore textures observed are similar to those of the Kuroko deposits rather than the Besshi-type deposits, as well as their metal association (Cu-Pb-Zn).