STABLE ISOTOPE VARIATIONS DURING VOLATILIZATION AND INFILTRATION PROCESS IN THE CARBONATE CONTACT AUREOLE OF THE MUAMSA GRANITE IN THE HWANGGANGRI MINERALIZED ZONE, SOUTH KOREA

 The Muamsa granite in Hwanggangri Mineralized Zone, South Korea intruded the Cambro-Ordovician Samtaesan Formation which is mainly composed of calcitic marble and dolomitic marble with minor intercalations of slate and amphibolite, and caused isotopic depletions in the carbonate rocks. Stable isotopic compositions of calcitic carbonates vary from noncrystalline fresh limestone (d¹⁸O=20.9–25.1‰, d¹³C=3.5–5.5‰) and calcitic marble (d¹⁸O=13.4–21.7‰, d¹³C=-2.9–2.5‰) to skarn (d¹⁸O=9.6–10.2‰, d¹³C=-5.1–-4.2‰). Isotopic depletions of calcitic carbonates were initiated with volatilization accompanying calc-silicate-forming reactions and then, with increased permeability, the H₂O-rich siliceous fluids of igneous origin actively infiltrated the system to promote fluid-rock interactions, up to 25.6 of fluid/rock ratio in skarn for closed system, and to increase ¹⁸O depletion rates compared to volatilization stage. More than 70% of isotopic depletion amounts in calcitic marbles were attributed to infiltration process. With this effect such mineral assemblages as phlogopite in the absence of K-feldspar, phlogopite + diopside, and wollastonite which are rarely produced in poor fluid flux are commonly observed in calcitic marbles. As for dolomite, noncrystalline fresh dolomites have isotopic composition ranges of d¹⁸O=19.2–21.7‰ and d¹³C=4.5–5.6‰, and dolomitic marbles of d¹⁸O=14.5–19.4‰ and d¹³C=2.3–6.5‰. The dolomitic marbles comparatively less depleted in isotope composition compared to calcitic marbles are plotted on generally below the curve with 0.6 in fluid/rock ratios and show wide range of XCO₂ conditions in the decarbonation models of different XCO₂ conditions. Active decarbonation reaction in dolomitic marble was restricted by silica-poor dolomite protolith, and this led to limited fluid infiltration and resulted in the high XCO₂ mineral assemblages. In contrast, the occurrence of dolomite + calcite + tremolite + forsterite assemblage stable at high H₂O-activity condition with more depleted d¹⁸O value than carbon isotope depletion indicates that there was a localized fluid infiltration in dolomitic marbles.