The Paleozoic Huoshenmiao iron skarn deposit in the Tongbai area of North Qinling Orogen, China: Insights from garnet U-Pb dating and geological constraints

The Qinling Orogen is a composite orogenic belt that can be subdivided into the North and South Qinling, broadly separated by the Shangdan suture zone. These two orogenic belts were generated by subduction-collisional processes in the Early Paleozoic and Late Triassic, respectively. During the Late Jurassic to Early Cretaceous, the eastern portion of the Qinling Orogen was tectonically reactivated due to westerly subduction of the Izanagi plate underneath the East China continental margin. The Qinling Orogen is well-endowed with numerous Au, Mo, Ag-Pb-Zn deposits that predominantly formed in the Late Triassic to Early Cretaceous, with rare Paleozoic varieties documented. In this study, we present garnet and zircon U-Pb dating results to show that the Huoshenmiao iron skarn deposit in the Tongbai area of North Qinling orogenic belt formed in the Early Silurian and is genetically related to subduction-related magmatism. The Huoshenmiao deposit consists of lenticular and stratiform orebodies that contain massive or densely disseminated magnetite variably associated with garnet, diopside, epidote, hornblende, and actinolite. Garnets from iron ores are andradite and grossular in composition (Ad83.1Gr13.3-Ad86.5Gr10.2) and characterized by enrichment in light rare earth elements (ΣLREE=57.85–103.82 ppm) and depletion in heavy rare earth elements (ΣHREE=5.50–11.34 ppm), with significantly positive Eu and Ce anomalies (δEu of 1.09–1.89 and δCe of 1.39–1.69). These compositional signatures are distinctly different from those of garnets in the ore-hosting metamorphic rocks that are typically dominated by almandine, spessartine and grossular (Al47.4Sp30.4Gr13.8-Al51.9Sp24.8Gr17.6), depleted in LREE (0.14–0.69 ppm), enriched in HREE (95.68–841.60 ppm) and have pronounced negative Eu anomalies (δEu=0.24–0.51). In addition, garnets from iron ores of the Huoshenmiao deposit contain abundant daughter mineral-bearing, multiphase fluid inclusions, further confirming their hydrothermal origin. Two samples of those hydrothermal garnets yield U-Pb dates of 437±9 Ma and 437±10 Ma (2σ), revealing a Paleozoic mineralization event that has long been ignored and rarely documented. These garnet dates agree well with zircon U-Pb dates of 438±4 Ma to 436±3 Ma for a gabbroic diorite and 430±4 Ma for a granite intrusion in close proximity of the mine, supporting a possible genetic link between the iron mineralization and Early Silurian magmatism. The Paleozoic intrusions are enriched in large ion lithophile elements (LILEs: Ba, K, Sr) and LREEs, depleted in high field strength elements (HFSEs: Nb, Ta, P, Ti), have whole-rock (87Sr/86Sr)i, eNd(t), and zircon eHf(t) values of 0.7039–0.7042, 3.32–4.33, and 13.0–14.9, respectively. These geochemical and isotopic characteristics suggest that the Paleozoic intrusions were affiliated with arc magmatism triggered by subduction of the Shangdan oceanic plate in the Early Paleozoic. Recognition of the Silurian Huoshenmiao iron skarn deposit opens a new window for exploration of Paleozoic mineral resources in the Tongbai area and other portions of the North Qinling Orogen.