Geological and geochemical characteristics of deep and shallow ore bodies in the Mengjiagou BIF iron deposit in the Qian'an area of the eastern Hebei, China and their geological significancs

The Mengjiagou iron deposit is located at the northern end of the Mengjiagou-Beitun-Caiyuan synclinorium fold belt in the western ore belt of the northern Qian′an iron orefield in the eastern Hebei. Its orebodys ocurred in form of the gentle and open syncline in the metamorphic rocks of the Santunying Formation of the Archan Qianxi Group. In recent years, with the improvement of exploration level, a second layer of iron ore body has been discovered in the deep part of the Mengjiagou iron deposit. This leads to the opening up of a new space for prospecting iron ore in the Qian′an area of the eastern Hebei. However, the relatively low research level of iron orebodies in depth of the area restricted the understanding of regional mineralization theory. This article aims to explore the similarities and differences between the deep and shallow orebodies of the Mengjiagou iron deposit, to distinguish the sources of ore-forming materials and their ore-forming environment through the analysis of geological and geochemical characteristics of those two layering orebodies. The results show that ores of both the deep and shallow orebodies in the Mengjiagou iron deposit are mainly pyroxene magnetite quartzite ore, with a small amount of dipyroxene magnetite quartzite ore. They have insignificantly different contents of magnetite and basically same texture and structure. However, the dipyroxne magnetite quartzite ore in the shallow layer contains slightly high content of magnetite relative to the deep layer. The lithology of wallrocks of the shallow layering orebodies is relatively consistent, while that of the deep layering orebodies is relatively complicated. Ores of the deep and shallow layering orebodies contain main chemical composition of TFe2O3 and SiO2, with relatively low contents of Al2O3 and TiO2, and extremely low contents of high field strength elements (Hf, Sc, Rb, Th), indicating that no continental debris material was added in the sedimentation process. Ores of both the deep and shallow layering orebodies have left-declined REE distribution patterns, with La and Y positive anomalies and Eu positive anomalies. The ratios of various elements indicate that both layers of orebodies in the Mengjiagou iron deposit are characterized with the dual superposition of seawater and hydrothermal fluids. The lack of negative Ce anomalies indicates that the Mengjiagou iron deposit was formed in an anoxic environment. Results of the comparative analysis of geochemical characteristics of various BIF iron deposits hosted in different metamorphic rocks in the eastern Hebei show that they have consistant source of ore-forming materials.