Stable isotope geochemistry of clay minerals from fossil and active hydrothermal systems, southwestern Hokkaido, Japan

Abstract

Miocene submarine to Quaternary terrestrial volcanism in southwestern Hokkaido, Japan, is associated with hydrothermal clay alteration and mineralization, including Kuroko-type deposits at Kagenosawa (14.2 Ma, Cu > Zn, Pb > Au) and Minamishiraoi (12.5 Ma, Ba > Zn, Pb, Cu), vein-style mineralization at Date (5.2 Ma, Au-Ag-Cu-Pb-Zn) and Chitose (3.6 Ma, Au-Ag), and geothermal activity at Noboribetsu (≤1.8 Ma). The δD and δ 18O values of mica (sericite), mica-smectite, chlorite, chlorite-smectite, nacrite, dickite, kaolinite, and smectite were used to deduce the type(s) of hydrothermal fluid at each locality. Calculated compositions for Minamishiraoi and Kagenosawa fluids suggest that seawater was dominant, but some mixing with magmatic water is also indicated, particularly for the polymetallic Kagenosawa deposit. Hydrothermal fluids at Date, Chitose, and the Noboribetsu geothermal area were dominated by meteoric water. Minor involvement of magmatic water during mineralization at Date cannot be ruled out, but evolution of local meteoric water along an evaporation trend and/or an 18O-shift due to hydrothermal rock-meteoric water interaction also could have produced appropriate fluid compositions. The δD and δ 18O values of modern hot-spring waters at Noboribetsu closely parallel fluid compositions calculated for the clay alteration at Date, Chitose, and Noboribetsu. Because relatively poor reproducibility was obtained for the δD values of the swelling clays, additional tests were conducted. Stepwise heating showed that, for some smectitic clays, water evolved between 200 and 300°C had anomalously high δD values because of residual interlayer water. This error can be minimized by sufficiently long preheating (in vacuo) at ≤200°C. In vacuo TG patterns of other smectitic clays suggested gradual loss of hydroxyl-groups beginning near 200°C, rather than the more typical distinct separation between interlayer water at <200°C and hydroxyl-groups at > 400°C. This behaviour constrains the maximum temperature that can be used for in vacuo preheating. Furthermore, shifts to lower δD values (by as much as 19‰) were obtained when this smectite was dispersed in low-D water for three weeks, perhaps indicating isotopic exchange. However, with appropriate care, δD values obtained by conventional procedures (including preheating to ≤200°C) normally reproduced natural compositions of the smectitic clays with acceptable accuracy and precision.