REE Behavior During Anhydrite and Gypsum Formations of the Kuroko Type Massive Sulfide-Sulfate Deposits

摘要

The Sr and rare earth elements (REEs) concentrations and Sr isotopic composition (87Sr/86Sr) of anhydrite and gypsum have been determined for samples from the Matsumine, Shakanai and Hanaoka Kuroko-type massive sulfide-sulfate deposits in the Hokuroku District in Akita Pref. in northern Japan. The anhydrite samples exhibit two styles of chondrite-normalized REE patterns; one with a decrease from light to heavy REEs (Type I), and another with a light REE-depleted pattern with peak in Sm (Type II). The Sr content of Type I anhydrite is higher than that of Type II. The 87Sr/86Sr ratios of all anhydrite samples are intermediate between that of middle Miocene seawater and that of country rocks. 87Sr/86Sr of Type I anhydrite is closer to Miocene seawater, and that of Type II is closer to the country rocks. Based on the Sr data obtained by this study and previous reports on geochemical data on the Kuroko mineralization, geochemical environment of anhydrite formation was investigated. However, from that investigation, it is inferred that REE fractionation would not occur during the primary anhydrite precipitation. Gypsum, which partially replaces anhydrite in the Kuroko deposits, also exhibits two distinct chondrite-normalized REE patterns. Because LREEs are likely to be more easily mobilized during the secondary process such as dissolution and re-crystallization, it is hypothesized that LREE release from Type I anhydrite, resulting in the formation of secondary Type II anhydrite with LREE depleted profiles.