###### Abstract  
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 \({}^{\circ}\)C. This is why the oxygen isotope fractionation between CO2 and calcite at 25 \({}^{\circ}\)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 \({}^{\circ}\)C and 150 \({}^{\circ}\)C was thoroughly determined in the first half, and the value can be represented by the following equation: 1000 Ina = 0.494 (10\({}^{6}\)/T\({}^{2}\)) + 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. M +
Footnote †: journal: Accepted: 28 April 2023  
To determine the isotope ratios of calcite, CO\({}_{2}\) is generated from the calcite, and its carbon and oxygen isotope ratios are measured via IRMS. CO\({}_{2}\) is directly liberated from calcite via treatment with 100% phosphoric acid in an evacuated reaction tube in a water bath at 25 \({}^{\circ}\)C [2]. To calculate the oxygen isotope ratio of calcite from CO\({}_{2}\) analysis, the oxygen isotope fractionation between CO\({}_{2}\) and calcite should be known. The oxygen isotope fractionation between CO\({}_{2}\) and calcite depends on the temperature of the CO\({}_{2}\) formation, and it has been obtained at 25 \({}^{\circ}\)C (e.g., [3,4]). For carbonate minerals other than calcite, higher temperatures and more time are required to complete the CO\({}_{2}\) generation reaction. Only two-thirds of carbonate oxygen converts into CO\({}_{2}\) during the reaction, and one-third of it remains in the reactant fluid. This may cause an isotope fractionation, which is a function of temperature and differences in carbonate minerals. Therefore, the reaction should be kept at a certain temperature that is determined for each carbonate mineral to ensure that the fractionation factor between CO\({}_{2}\) and the carbonate remains unchanged. Additionally, a laboratory working standard of calcite is used to calibrate the oxygen isotope ratio of various carbonates at various experimental temperatures. A possible temperature error in the reaction is almost canceled if an unknown carbonate sample and the working standard calcite are at the same temperature in the same reaction bath. Thus, the oxygen isotope acid fractionation factor (AFF) of the working standard calcite is experimentally obtained by measuring the oxygen isotope ratio of the CO\({}_{2}\) that is produced using phosphoric acid at various temperatures. When using calcite as a working standard for the isotope ratio measurements of carbonate, the oxygen isotope AFF between CO\({}_{2}\) and calcite at the reaction temperature should be known. Here, the oxygen isotope ratio of CO\({}_{2}\) produced using phosphoric acid with a calcite working standard was experimentally measured between 1 \({}^{\circ}\)C and 150 \({}^{\circ}\)C to determine the oxygen isotope AFF between CO\({}_{2}\) and calcite at the reaction temperature. Since the AFF is controlled by kinetics, the AFF is not exactly equal to the equilibrium fractionation factor (EFF).