The Chengchao Fe skarn deposit (280 Mt @ 45% Fe; nearly half of the total Fe reserve has an ore grade higher than 53 wt %) is the largest high-grade magnetite skarn deposit in the Middle-Lower Yangtze River metallogenic belt. The magnetite orebodies and skarns occur mainly along the contact zones between granitic (129 ± 2 Ma) rocks and diorite (133 ± 1 Ma) and Triassic strata. The paragenesis of the Chengchao deposit includes the following five stages: Na-K alteration stage (albite + K-feldspar), prograde skarn stage (garnet + pyroxene), retrograde skarn stage (magnetite + phlogopite + amphibole), sulfate-sulfide stage (anhydrite + pyrite ± garnet ± magnetite), and carbonate stage (calcite).
Four generations of garnet and magnetite with distinct textures are recognized. The elemental compositions and fluid inclusion data of the three generations of garnet from the prograde skarn stage indicate fluctuating changes in the oxygen fugacity during their precipitation. Some magnetite grains from the retrograde skarn stage are characterized by well-developed zonation with respect to Al, K, Ca, and Si, with concentrations of up to hundreds of parts per million. The higher contents of (V + Ti) and (Al + Mn) in Mt2 relative to Mt1 (0.8 and 2,321 ppm for Mt1, and 182 and 10,315 ppm for Mt2, respectively) may be indicative of increasing fluid temperatures from episodic magmatic events. Fluid inclusion data show that the fluids responsible for the prograde skarn stage have high temperature (>750°C), high salinity (>50 wt % NaCl equiv) and high Fe concentration (e.g., magnetite and pyrite daughter crystals within garnet and pyroxene). Oxygen and hydrogen isotope data suggest that magmatic water was dominant during the prograde skarn stage and that increasing percentages of meteoric water were involved during later stages. Multiple episodic incursions of Fe-rich fluids, which were probably related to the emplacement of granitic rocks and diorite, were crucial for controlling the formation of the high-grade Chengchao Fe skarn deposit.