Decoding the oxygen fugacity of ore-forming fluids from garnet chemistry, the Longgen skarn Pb-Zn deposit, Tibet

•High spatial resolution trace element mapping has more potential for recording multistage garnet growth zoning.•The incorporation of REE in a single garnet crystal is co-controlled by coupling substitution and fluid properties.•Garnet with REE fluctuation demonstrates the fO2-pH evolution of ore-forming fluids.•The redox-sensitive elemental composition of garnet could distinguish different types of skarn deposits.