Fossil hydrothermal systems theoretically inverted with oxygen isotopes of constituent minerals hydrothermally reequilibrated with internally 18O-enriched fluids: Cooling and mineralizing implications

A barren hydrothermal system provides a useful baseline for understanding mineralized systems, offering detailed insights into the hydrothermal ore-forming process. Given that the unique initial oxygen isotopes of water (hereafter referred to as δ18OWi values), hydrothermal reequilibration temperatures, and water-to-rock (W/R) ratios cannot be constrained by traditional forward modeling, we thermodynamically and kinetically studied fossil magmatic and metamorphic hydrothermal systems across the Dabie orogen in central-eastern China. These systems, unassociated with economic mineralization, were analyzed using recently proposed theoretical inversion and diffusive modeling to provide a complete understanding. Given the covaried oxygen isotopes observed from hydrothermally altered minerals of the Early Cretaceous postcollisional granitoids and Triassic gneissic country rock, the δ18OWi values of magmatic or metamorphic water—ranging from 6.57‰ ± 0.05‰ (one standard deviation [1SD]) to 8.16‰ ± 0.01‰—were theoretically inverted from the closed hydrothermal systems. Oxygen diffusive modeling further suggests that the cooling rates of alkali feldspar (≥1810 ± 170 °C Myr−1) are systematically higher than those of quartz (≥32.7 ± 7.7 °C Myr−1) and zircon (≥2.6 ± 0.8 °C Myr−1), and the ancient hydrothermal systems studied here could have been active for a duration of 0.07 Myr to 19 ± 8 Myr. Collectively, these conditions imply that the mobile fluids were not dynamically expelled from their host rocks until they had hydrothermally reequilibrated with the constituent minerals via oxygen diffusion. Given the following factors, the formation of hydrothermal ore deposits across the Dabie orogen seems less likely: (1) the hydrothermal systems were less frequently open, (2) the closed hydrothermal systems experienced less heat loss, (3) the studied hydrothermal systems were less saturated with water, and (4) the cooling processes were dominated by thermal conduction with variable lifetimes. The application of theoretical inversion to hydrothermal systems by this study will help advance our quantitative understanding of the thermal history and mineralization potential of both active and fossil hydrothermal systems worldwide.