Effect of Mixing and Boiling of Fluids on Isotopic Compositions of Quartz and Calcite from Epithermal Deposits

Abstract

Oxygen and carbon isotope effects due to boiling and mixing of fluids were examined for epithermal vein systems. The magnitude of oxygen isotope change of fluid which could be produced by boiling is not greater than 2‰ at temperatures higher than 100°C. In case of carbon isotopes, boiling could produce an increase in δ ¹³ C values of precipitating calcite (up to 2 to 3‰, ) due to increasing pH of the fluid. Mixing of a deep hydrothermal fluid with a low-temperature groundwater could provide a wide range of δ ¹⁸O values in a relatively small temperature interval, depending upon variable initial compositions of the deep hydrothermal fluid. The data for quartz and calcite from the Kushikino gold-bearing epithermal veins suggest either boiling or mixing with a low-temperature groundwater of a slightly ¹⁸O-enriched (by 3 to 4‰) deep hydrothermal fluid.