Garnet U-Pb dating and magnetite geochemistry: Constraints on the origin of Fe mineralization in the Huogeqi polymetallic deposit, Northern China

Abstract

The Huogeqi Fe polymetallic deposit, which contains 48.17 Mt Fe, occurs in tremolite-diopside rocks of the Neoproterozoic Langshan Group in the north China. This study focuses on the magnetite mineralogy, texture and chemical composition, and U-Pb geochronology of garnet associated with magnetite and zircon from the granitoid related to mineralization in order to constrain the origin of Fe mineralization in Huogeqi deposit. LA-ICP-MS U-Pb dating of magnetite-bearing garnets yields a lower intercept age of 277.5 ± 8.7 Ma, which is coeval with the emplacement age of the mineralized-related granitoid (275.3 ± 1.3 Ma). Three types of magnetite were formed in the Huogeqi deposit, including Mt-A (coarse-grained disseminated), Mt-B (stratiform and fine-grained dense disseminated), and Mt-C (massive). The Mt-A can be subdivided into high-Ti Mt-A1 and relatively low-Ti porous and fractured Mt-A2. The Mt-B can be subdivided into porous Mt-B1 with relatively high V and low Ti contents, dark porous Mt-B2 with a relatively low V contents, porous inclusion-rich Mt-B3 with low Fe and V contents, and high Si and Mg contents, and less-porous Mt-B4 with low Si and Mg contents. The Mt-C includes porous Mt-C1 with a low Ti contents and high Si, Al, Mn contents and less-porous Mt-C2 with low Si, Al, and Mn contents. The decreasing Ti and V contents in the subsequent magnetite types indicate progressive cooling of the mineralizing fluid, increasing oxygen fugacity, and more intense fluid-rock interaction. Dissolution and re-precipitation played an important role in the formation of different generations of magnetite of Mt-B and Mt-C. The magnetite from Huogeqi deposit has a high Ni/Cr mass ratio, a high Fe content, and a high (Ca + Al + Mn) value, and a low V content, a low Ni/(Cr + Mn) mass ratio, and a low (Ti + V) value. These geochemical characteristics indicate that the magnetite has a hydrothermal origin in a skarn-type deposit, which is related to Early Permian magmatism in the Langshan area in north China.