Hydrothermal Calcite Precipitation in Veins: Inspired by Experiments for Oxygen Isotope Fractionation between CO2 and Calcite from 1 °C to 150 °C

The oxygen isotope ratios of minerals are important for estimating the formation temperature of hydrothermal deposits. Calcite is generally found in most mining districts. The oxygen isotope ratio of hydrothermal calcite in isotopic equilibrium with a fluid depends on that of the fluid as well as the formation temperature. The isotope ratios of calcite are generally determined to measure the isotope ratios of CO2 generated via reaction with 100% phosphoric acid at 25 °C. This is why the oxygen isotope fractionation between CO2 and calcite at 25 °C is required to conduct the procedure. When using other carbonate minerals such as dolomite, ankerite, and siderite, the reaction requires higher temperatures. Therefore, the oxygen isotope fractionation between CO2 and calcite at higher temperatures should be known because the laboratory standard sample of carbonates is usually calcite. Here, the oxygen isotope fractionation in CO2 during the reaction of calcite with phosphoric acid at temperatures between 1 °C and 150 °C was thoroughly determined in the first half, and the value can be represented by the following equation: 1000 lnα = 0.494 (106/T2) + 4.64. The experiments raised questions such as the equilibrium vs. kinetics issue in isotopic reactions in the formation of hydrothermal ore deposits. The raised issue is discussed in the second half based on the findings of previous isotope studies in deposits.