Origin of oscillatory zoned garnets from the Xieertala Fe–Zn skarn deposit, northern China: In situ LA–ICP-MS evidence

• In situ LA–ICP-MS analyses of oscillatory zoned garnets are conducted.
• Garnet core and rim sections demonstrate distinct chemical characteristics.
• Fluid evolution and PT conditions of the skarn system are constrained.
• Changes of fluid properties are important for trace element distributions in garnets.

Oscillatory zoned garnets are able to record numerous geological processes including variations of physicochemical conditions and hydrothermal fluid evolution. In this contribution, we present the mineralogy, petrography, and systematic major, trace and rare earth element (REE) concentrations of zoned garnets from the Xieertala Fe–Zn skarn deposit. The in situ analytical results of oscillatory zoned garnets from core to rim demonstrate that garnet cores are remarkably HREE-rich, with relatively high ∑REE and HREE/LREE ratios as well as negative Eu anomalies. REE patterns of garnet rims are typically LREE-rich, with relatively low ∑REE and HREE/LREE ratios and positive Eu anomalies. Estimates of PT conditions suggest that the early fluid formed in relatively high temperature, pressure and salinity conditions, while the later fluid formed in relatively low temperature, pressure and salinity conditions. This transition in P, T and salinity conditions is interpreted to be caused by the dominance of magmatic and meteoric waters in early and late stages, respectively. Based on optical and textural characteristics, REE patterns, Eu anomalies and trace element variations in zoned garnets, it can be shown that during skarn formation, Xieertala hydrothermal fluids shifted from near neutral pH and oxidizing conditions with relatively high ∑REE, low LREE/HREE ratios, and U-poor and Y-rich characteristics to acidic and reducing conditions with relatively low ∑REE, high LREE/HREE ratios, and U-rich and Y-poor characteristics. We infer that variations of fluid compositions and physicochemical conditions may have a major control on incorporating trace elements and REEs into different sections of oscillatory zoned garnets.

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