The Isotopic Composition of Strontium in Barites and Anhydrites from Kuroko Deposits

Abstract

The isotopic composition of strontium in sulphate minerals from the Fukasawa and Kosaka mines has been measured to evaluate the importance of seawater in the development of Kuroko deposits. The ⁸⁷Sr/⁸⁶Srr ratios in. samples of bedded gypsum and anhydrite fall in a narrow range (0.70827-0.70854) whose upper limit approaches that of Miocene seawater. The ⁸⁷Sr/⁸⁶Srr ratios of barites fall in a somewhat wider range (0.70677-0.70820) between the upper limit of the ⁸⁷Sr/⁸⁶Srr ratio of the Miocene volcanics and the lower limit of the ⁸⁷Sr/⁸⁶Srr ratio in the anhydrite and gypsum samples.
The ⁸⁷Sr/⁸⁶Srr data are most readily explained if the ore forming fluid was normal Miocene seawater which acquired isotopically lighter strontium from the underlying Miocene volcanics prior to precipitation of the sulphates. None of the ⁸⁷Sr/⁸⁶Srr ratios are higher than that of Miocene seawater suggesting that the contribution of isotopically heavy strontium from the Paleozoic basement rocks was minor. Mixing of the hydrothermal solution with seawater during deposition of the sulphate minerals appears to have been insignificant.