Water involved in chloritization and sericitization in and around the Rendaiji mine, the Izu Peninsula, Japan

The gold- and silver-bearing quartz veins of the Rendaiji mine (now closed) in the southern part of the Izu Peninsula are associated with sericitization and silicification in the Miocene volcanic and pyroclastic formations which have also suffered from a regional chloritization. δD values of the sericite and chlorite together with XFe values of chlorite were determined.Based on the average measured δD value of -64.5‰ of chlorite samples with the average XFe (=Fe''/(Fe''+Mg)) value of 0.32 and the dependence of the D/H fractionation factor between water and chlorite on XFe, -4‰/0.1XFe (Kuroda et al., 1976), we estimated the δD value for the pure magnesian chlorite to be -52‰. The estimated value of -52‰ is precisely the same as the average δD value of sericite of -52‰. Since there is no D/H fractionation between sericite and magnesian chlorite according to Kuroda et al. (1986b) and the formation temperature of the sericite and chlorite is estimated to be the same, the water responsible for both the sericitization and chloritization in the Rendaiji mine should be the same so far as the δD value is concerned.Using the δD value of sericite (average -52‰), we calculated the δD value of the water participated in both the sericitization and chloritization to be -12‰ on the basis of the D/H fractionation factor experimentally determined by O'Neil and Kharaka (1976), or -25_??_-28‰ based on that estimated by Marumo et al. (1980) from the natural samples. The δD value of the water estimated above is definitely higher than that of the contemporary local meteoric water. The δD value of the water extracted from fluid inclusions in the quartz associated with sericitization, -53_??_-63‰, is much lower than that of the water associated with sericitization.