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

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•ñ at temperatures higher than 100•Ž. In case of carbon isotopes, boiling could produce an increase in ƒÂ13C values of precipitating calcite (up to 2 to 3•ñ,) due to increasing pH of the fluid. Mixing of a deep hydrothermal fluid with a low-temperature groundwater could provide a wide range of ƒÂ18O 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•ñ) deep hydrothermal fluid.  Mixing and boiling of hydrothermal fluids, which produce a temperature decrease and solubility changes of chemical species in solution, are considered a principal mechanism to precipitate minerals in epithermal gold-silver deposits (e.g. HEDENQUIST and HENLEY, 1985a, b; CUNNINGHAM, 1985). On the other hand, origin of thermal waters, chemical characteristics of solutions and formation temperatures of minerals can be estimated on the basis of isotopic compositions of the minerals. In the present paper, I will discuss by means of oxygen and carbon isotopes how the isotopic compositions of hydrothermal fluids change due to mixing and boiling, and what isotopic compositions the precipitating minerals have. MATSUHISA et al. (1985) discussed the oxygen isotope variations of quartz and calcite from the Kushikino gold-bearing hydrothermal veins, Japan, by taking a simple cooling and mixing model of hydrothermal fluids. Their results will be re-evaluated particularly in view of boiling of hydrothermal fluids. Boiling Model Boiling of hydrothermal fluids responds to the preferential loss of gases to the vapor phase as well as a temperature decrease. The loss of CO2 from solution may lead to an increase in pH and the deposition of calcite and adularia (HEDENQUIST and HENLEY, 1985a). In a temperature range 200•‹ to 350•Ž and near neutral pH conditions, gold is predominantly transported as thiocomplexes in epithermal fluids (SEWARD, 1973), and may deposit in response to the loss of H2S and drop in temperature caused by boiling (HEDENQUIST and HENLEY, 1985a). This scenario seems to be supported by the occurrence of calcite and adularia in epithermal gold deposits with alteration mineral assemblages ascribed to near neutral pH conditions. If boiling of hydrothermal fluids takes place, changes in isotopic compositions of the fluids may also be induced by isotopic fractionation between vapor and liquid phases. The magnitude of the isotopic change of hydrothermal fluid depends upon boiling temperature and vapor to liquid ratio in the system. The vapor to liquid ratio depends upon the initial fluid enthalpy, the temperature interval over which boiling occurs, and the mechanism of vapor separation. TRUESDELL et al. (1977) discussed the possible isotope effects caused by different mechanisms Received on August 11, 1986, Accepted on October 31, 1986 *Geological Survey of Japan , 1-1-3 Yatabe-Higashi, Ibaraki-ken, 305 Japan.

Field of study: Geology