Interpretation Of Formation Condition In Superior-Type Banded Iron Formation (Bif) Of Yuanjiacun In North China Craton Using Magnetite

The superior-type Yuanjiacun banded iron formation (BIF) was formed during the Paleoproterozoic (2.38-2.21 Ga) and distributed in the Shanxi Province of the North China Craton. The studied BIF is within the Yuanjiacun Formation of the lower part of the Luliang Group and precipitated along a stable continental shelf. The iron ore of the Yuanjiacun BIF has a layering structure that consists of magnetite-rich dark layers and quartz-rich light layers. The major minerals are magnetite and quartz with subordinate hematite, pyrite, amphibole, etc., varying in grain sizes. The grain size of magnetite is 20-180 mu m and that of quartz is 50-500 mu m. 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 (fO(2)) through Raman analysis of magnetite as a representative ore mineral in the BIF. Hematite spectra occurred at the edge of the some magnetite grains, representing change of oxidation level during the BIF formation. In addition, lack of negative Ce anomalies of iron ores from Yuanjiacun BIF indicates that fO(2) during their formation was enough for Fe2+ oxidation, but not enough for Ce3+ oxidation. Combined with previous geochemical data, the results of this study suggest that the Yuanjiacun BIF has close relationship with Great Oxidation Event.