Fluid inclusion and stable isotope studies of gold- and silver-bearing vein deposits, South Korea: Geochemistry of a te-bearing Au - Ag mineralization of the Imcheon mine

Mesohypothermal, tellurium-bearing gold - silver vein mineralization of the Imcheon mine was formed in mineralogically complex quartz - sulfide veins that filled fault fractures in Precambrian gneiss and Mesozoic granite. Ore grades average 15 g/ton gold with a gold/silver ratio of 1 : 3. Mineralization was deposited in three stages (I to III) of veins: stage I is ore-bearing, whereas stages II and III are barren. Major ore mineralization during stage I is composed of three mineralization phases which show a progressive change in ore mineralogy and mineral composition with increasing paragenetic time: early Fe - sulfide mineralization → middle base-metal sulfide mineralization accompanying active gold deposition → late telluride mineralization. This shift in mineralogy reflects a progressive decrease in temperature with a concomitant increase in fugacity of tellurium. Fluid inclusion and stable isotope data indicate a complex history of boiling and later cooling and dilution of ore fluids. Stage I mineralization occurred from dominantly aqueous (but containing little CO₂) fluids with a wide range of temperature (200° -430 °C) and salinity (0.5-10.0 wt % NaCl equiv.) under a depth of about 1 km below the paleosurface. Early Fe - sulfide was deposited at temperatures of 350°-430 °C from boiled magmatic fluids (δ³⁴S_Σs = 3 ‰, δ¹⁸O_water = 7.1 to 8.5 ‰, δD_water = - 70 to - 74 ‰). Middle base-metal sulfides + Au mineralization resulted from lower temperature fluids (260°-350 °C, δ¹⁸O_water = 2.3 to 4.2 ‰, δD_water = - 76 to - 8.2‰) through continued boiling and concomitant meteoric water mixing. Gold precipitation was mainly a result of a decrease of activity of reduced sulfur by sulfide precipitation and/or H₂S loss accompanying boiling. Late telluride mineralization resulted from the continued cooling (down to 200°-260 °C) and concomitant increase in f_Te2 of ore fluids, likely due to the meteoric water inundation. This meteoric water mixing is evidenced by decrease in oxygen and hydrogen isotope compositions of hydrothermal fluids (down to δ¹⁸O_water = - 0.1 to 1.2 ‰ and δD_water = - 85 to - 88 ‰). Comparison of Imcheon with other Au - Ag ( - Te) deposits indicates that the complex sequence of Fe - sulfide, base-metal sulfide, and telluride mineralization is a natural consequence of repeated inundation of a hydrothermal system by progressively cooler meteoric groundwaters.