Reconstruction of the genesis of banded iron formation in North China Craton: Evidences from geochemical studies on the iron ore, related wall rock and magnetite

Abstract

My Ph.D. studies focus on the petrological, mineralogical and geochemical studies of iron ore and magnetite from banded iron formations (BIFs) and their related wall rock (amphibolite). I performed integrated studies on ore deposit geology, mineralogy, petrology and geochemistry on the Yishui BIF. Yishui BIF is located in North China Craton (NCC) and considered as the representative area for revealing an association between the genesis of BIF and tectonic setting. The genesis of Yishui BIF had never been studied before. The data constrained the genesis of these BIFs and provided information about the tectonic evolution of the Archean NCC. Additionally, the data provided an improved understanding of the petrogenesis of the BIF–related wall rocks. It can also be used to interpret the relationship between the BIFs and wall rock and the tectonic evolution of the NCC. Rare earth elements (REEs) and trace elements were used as geochemical proxies. The results of these studies suggest that the Yishui BIFs were precipitated from a mixture of less than 1% high–temperature hydrothermal fluids (> 250 °C) and seawater. The abundant ore–forming materials, such as iron and silica, formed under anoxic and suboxic submarine conditions. In addition, the protolith of the related wall rock (Yishui amphibolite) was alkali basalts reminiscent of ocean island basalt (OIB) and formed in an intraplate setting with no crustal assimilation during magma ascent. On the basis of these results, mantle plume model was preferred, which can explain the geochemical signatures of both the Yishui amphibolites and the Yishui BIFs. This model is also very consistent with the tectonic evolution of the NCC.<br/> And, I determined trace element contents of magnetite from Superior–type BIFs and Algoma–type BIFs by in–situ LA–ICP–MS analyses. Combining the data obtained in my study with those from existing literatures, new systematic differences in magnetite composition between Archean Algoma–type and Paleoproterozoic Superior–type BIFs to understand the influence of their genetic conditions on magnetite chemistry were proposed. Magnetite is a representative ore mineral in BIFs and its composition can be a powerful indicator of petrogenesis and provenance. Previous studies classified BIFs into Algoma– and Superior–types based on their tectonic setting, size and lithologic associations. Our results, together with a compilation of previously reported trace element data on magnetite from other regions, indicate that there are systematic differences in trace element compositions of magnetite between Algoma– and Superior–type BIFs due to differences in their depositional environments. The magnetite from Algoma–type BIFs is more enriched in Al, Ti, Ni, and V than the magnetite from Superior–type BIFs. The former precipitated dominantly from high–temperature hydrothermal fluid under low oxygen fugacity conditions, whereas the latter formed mainly from low–temperature and relatively oxidized seawater. The results of the study demonstrate that the trace elemental composition trends of magnetite from Algoma– and Superior–type BIFs are well consistent with previously suggested geochemical characteristics of these BIFs, further indicating the close relationship of BIFs with their genetic environments. <br/> In addition, magnetite from the superior–type Yuanjiacun BIF in NCC was studied using Raman spectroscopy. The main purpose of the study was to clarify the relationship between the timing for the formation of Yuanjiacun BIF and the coeval oxygen fugacity (fO2) through Raman analysis of magnetite. Combined with previous geochemical data, the results of this study suggest that the Yuanjiacun BIF has close relationship with Great Oxidation Event (GOE).<br/>