Abstract
Thermal water systems at ocean coasts are mixtures of hydrothermal oceanic and local meteoric waters. They are characterized isotopically by delta 18 O and delta D values intermediate between oceanic and local meteoric values and chemically by dominance of Na (super +) , Ca (super 2+) , Cl (super -) , and low concentrations of SO (super 2-) 4 and Mg (super 2+) . The chemical features are due to hydrothermal interaction between sea water and host rocks, involving precipitation of anhydrite, and are comparable to those of sea-water brines at Reykjanes, Iceland (Bjoernsson et al., 1970), at Matupi Harbor, New Britain (Ferguson and Lambert, 1972), and at Anatolia, Turkey (Brinkmann and Kuehn, 1973). Dissolved sulfates retain, in most cases, the original isotopic values of sea-water sulfates during these reactions.Highly saline brines at Arima and Takarazuka, Hyogo, and Ishibotoke, Kawachinagano, Osaka, are significantly enriched in the heavy isotopes of oxygen and hydrogen and are considered to be mixtures of up to 80 percent of a saline, CO 2 -rich brine of delta 18 O = +8 per mil and delta D = --25 to --30 per mil and local meteoric water. The brines may be magmatic in origin and associated with the upper Cretaceous to Paleogene igneous and metamorphic activity. Alternatively, the brines may be meteoric saline formation waters which acquired their chemical and isotopic characteristics by interaction with marine evaporitic sediments as discussed by Clayton et al. (1966), Graf et al. (1965), and Hitchon and Friedman (1969). The choice between the models was left for further study.Thermal waters in the Green Tuff regions may be classified into two types: Green Tuff-type and volcanic thermal waters. Most thermal waters of the first type are simply meteoric circulation waters, having neutral Na-Cl-SO 4 -type or Na-Ca-Cl-SO 4 -type chemistry. Thermal waters containing "fossil" sea waters and their alteration products also are occasionally found at certain oil fields in the Tertiary formations. Dissolved sulfates in Green Tuff-type waters are isotopically similar to sulfates in Kuroko ore deposits and are considered to be "fossil" Miocene sea-water sulfates leached from Green Tuff formations.In volcanic areas, acid chloride-sulfate and acid sulfate-type volcanic thermal waters occur in close association with each other. Although the two types of acid waters are essentially meteoric in origin, sulfates in acid chloride-sulfate waters are similar in delta 34 S to the "fossil" sulfates, while acid sulfate-type volcanic waters contain only supergene sulfates of volcanic origin. The "fossil" sulfates are probably leached from the Tertiary formations underlying most volcanic areas of Japan.The ore solutions responsible for Kuroko ore deposits were similar to both coastal thermal waters and to some of the marine connate waters. The ore solutions were nearly neutral Na-Ca-Cl-type brines of low sulfate concentration, but of higher salinity than average sea water. Their delta 18 O values were close to zero, but their delta D values varied from 0 to --30 per mil, depending on mixing ratios with meteoric water or on the original values of the connate waters. The delta 34 S values of dissolved sulfates were close to those of Miocene sea-water sulfates. The sulfate-deficient saline ore solution would have been high in Ba (super 2+) . Barite was precipitated from this solution at the top zone of the stratified Kuroko ore deposits, where sulfate ions were replenished from overlying sea water.
