Monazite and xenotime U-Pb geochronology constraints on the genesis of the Paleoproterozoic Yuanjiacun banded iron formation-hosted high-grade hematite ores of the North China craton

Abstract

High-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. It was once viewed that the lack of high-grade hematite ores in the North China was due to the lack of prolonged epigenetic weathering-leaching conditions, but that could not explain the fault-controlled characteristics of high-grade hematite orebodies in Yuanjiacun. On the basis of field contact relationship, petrological and mineralogy analysis, the in-situ U-Pb dating of monazite and xenotime and the fluid inclusion analysis of quartz intergown with hematite was both carried out. The results show that, the timing of high-grade hematite mineralization was at 1.41 to 1.34 Ga, revealing that the deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO2-rich, and highly saline (about 20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. By comparing to the typical iron deposits worldwide, and combining with the views of the predecessors, we argue that the high-grade hematite ores formed in lower metamorphic-grade BIFs at shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton. The determination of ore genesis of the Yuanjiacun high-grade hematite ores  is of theoretical and practical significance to enriching the mineralization process of high-grade iron ores and futher prospecting exploration for high-grade iron ores in the Yuanjiacun area.