Garnet U-Pb and O isotopic determinations reveal a shear-zone induced hydrothermal system.

The absolute crystallization ages of minerals from hydrothermal fluids measured in situ can unravel the timing of key events leading to the formation of, for instance, ore deposits and hydrothermally derived geological terrains. In this study, a skarn iron deposit from northwest (NW) China is shown to have U-Pb garnet and U-Pb zircon ages of 254.2 ± 1.7 Ma and 255.5 ± 1.0 Ma, respectively, which are both significantly younger than magmatism and metamorphism of the region. This skarn age instead correlates with the occurrence of strike-slip and thrust faulting in the region. The water/rock mass ratio of 0.065~0.115 suggests the δO garnet composition is ~1‰ at temperatures ranging from 250-450 °C. The low oxygen isotopic composition indicates the role of meteoric water in the garnet formation. These measurements can be interpreted as the shear along faults circulating meteoric water ~10 km below the hanging wall of meta-volcanic sedimentary rock. Meteoric water in this hydrothermal system would leach cations from the meta-volcano-sedimentary rocks necessary for mineralization. Silica-rich hydrothermal fluid reacts with calcic-rich materials in the meta-volcano-sedimentary rocks, depositing the garnet and magnetite. Our work suggests that the shear zone is rich in ores, rendering this deposit for NW China a prospective source for future mineral resource exploration.