Iron isotope constraints on the genesis of magnetite ore in the Huogeqi deposit of Inner Mongolia Autonomous Region in northern China

Abstract The Huogeqi deposit is a major Cu–Fe–Pb–Zn deposit in China. Numerous studies have been conducted on the genesis of Cu–Pb–Zn sulfides of this deposit; but detailed researches on the genesis of magnetite ore bodies and its relationship to sulfide ores are few. This study utilizes field and microscopic techniques to showcase the hydrothermal characteristics of magnetite ore and tremolite rocks, including net-veined magnetite ore, which cuts across carbonate rock and quartzite, and intrusive contacts between magnetite-bearing tremolite rocks and quartzite, carbonaceous slate, and carbonate rock. Furthermore, calcite with metasomatic relict texture could be widely observed near magnetite and tremolite. Magnetite-bearing tremolite rocks with low contents of trace elements, rare earth elements, and obvious positive anomalies of Eu differed from the mica schist and carbonaceous slate, but were consistent with the carbonate rocks, revealing that the magnetite ore and tremolite rocks have a hydrothermal origin and are closely related to carbonate rocks. The Fe isotopic compositions of 22 magnetite and 17 wall samples were determined using MC-ICP-MS methods. δ56Fe values of magnetite-bearing tremolite rocks varied from -0.57 to -0.15‰ (average, -0.38‰), which differed from those of the mica schist (0.27–0.33‰), quartzite (-0.12 to 0.57‰), and carbonaceous slate (0.30–0.38‰), but were similar to the carbonate rocks (-0.38 to -0.51‰). δ56Fe values of magnetite varied from -0.48 to 0.36‰ (average, -0.11‰), which differed from banded-iron formation and magmatic magnetite deposits but were like skarn deposits. The δ56Fe value of the initial ore-forming fluid was -0.09‰ at 350 °C, which is consistent with the fluid that likely originated from the fluid exsolution of granite. According to theoretical calculations, δ56Fe values of the residual fluid after precipitation of magnetite ores ranged from -1.01 to -0.94‰, and that for pyrite precipitation ranged from -0.02 to 0.07‰, which is consistent with the measured Fe isotopic data of pyrite. Hence, we suggest that the hydrothermal magnetite and Cu–Pb–Zn sulfides of the Huogeqi skarn deposit could be derived from similar hydrothermal fluids.