A Genetic Link Between Paleoproterozoic Yuanjiacun BIF and the Great Oxidation Event in North China Craton

Abstract

The Paleoproterozoic (~2.38–2.21 Ga) Yuanjiacun banded iron formation (BIF), located in Shanxi Province, is a Superior-type BIF in the North China Craton (NCC). This BIF is within a metasedimentary rock succession of the Yuanjiacun Formation in the lower Luliang Group. The clastic metasediments associated with the BIF are mainly represented by well-bedded meta-pelites (chlorite schists and sericite-chlorite phyllites) and meta-arenites (sericite schists), which have undergone greenschist-facies metamorphism. The Yuanjiacun Formation had been originally deposited in a passive margin setting, most probably on a stable continental shelf. Iron oxide (magnetite and hematite), carbonate, and silicate facies are all present within the iron-rich layers. Integration of petrographic and isotopic evidence indicates that the most likely precursor materials were comprised predominantly of probably hydrous, Fe-silicate gels of stilpnomelane-type composition, amorphous silica gels, and ferrihydrite. The \({\text{P}}_{{{\text{O}}_{2} }}{-} {\text{P}}_{{{\text{CO}}_{2} }}\) and pH-Eh fields of the mineral assemblages (and/or their precursors) in the Yuanjiacun BIF indicate anoxic and near-neutral to slightly alkaline conditions for the original depositional environment except for the hematite precursor field (that of Fe(OH)3), which is very small and exists only at relatively high \({\text{P}}_{{{\text{O}}_{2} }}\) values. The eastward transition from carbonate- into oxide-facies iron formations is accompanied by a change in mineralogical composition from siderite in the west through magnetite-ankerite and magnetite-stilpnomelane assemblages in the transition zone to magnetite and then hematite in the east. These distinct lateral facies are also observed vertically within the BIF, i.e., the iron mineral assemblage changes up section from siderite through magnetite into hematite-rich iron formation. The oxide-facies BIF formed near shore, whereas carbonate (siderite)- and silicate-facies assemblages formed in deeper waters. Based on detailed analyses of these variations on a basinal scale, the BIF precipitated during a transgressive event within an environment that ranged from deep waters below storm-wave base to relatively shallow waters. The BIF samples display distinctively seawater-like REE + Y profiles that are characterized by positive La and Y anomalies and HREEs enrichment relative to LREEs in Post-Archean Australian shale-normalized diagrams. Consistently positive Eu anomalies are also observed, which are typical of reduced, high-temperature hydrothermal fluids. In addition, slightly negative to positive Ce anomalies, and a large range in ratios of light to heavy REEs and Y/Ho, are present in the oxide-facies BIF. These characteristics, in combination with consistently positive δ56Fe values, suggest that deposition of the BIF took place along the chemocline where upwelling of deep, anoxic, iron- and silica-rich hydrothermal fluids mixed with shallower and slightly oxygenated seawater. The ankerite displays highly depleted δ13C values and the carbonate-rich BIF has a high content of organic carbon, suggesting dissimilatory Fe(III) reduction of a ferric oxyhydroxide precursor during burial of biomass deposited from the water column; that same biomass was likely tied to the original oxidation of dissolved Fe(II). The fact that the more ferric BIF facies formed in shallower waters suggests that river-sourced nutrients would have been minimal, thus limiting primary productivity in the shallow waters and minimizing the organic carbon source necessary for reducing the hematite via dissimilatory Fe(III) reduction. By contrast, in deeper waters more proximal to the hydrothermal vents, nutrients were abundant, and high biomass productivity was coupled to increased carbon burial, leading to the deposition of iron-rich carbonates. The deposition of the Yuanjiacun BIF during the onset of the Great Oxidation Event (GOE; ca. 2.4–2.2 Ga) confirms that deep marine waters during this time period were still episodically ferruginous, but that shallow waters were sufficiently oxygenated that Fe(II) oxidation no longer needed to be tied directly to proximal cyanobacterial activity.