Girvanella and its iron mineralization in ferruginous microbialites of the Mantou Formation Member Ⅲ(Cambrian Miaolingian Series) of Northern Henan

Key words: 
• hematite /
• Girvanella /
• ferruginous microbialites /
• microbial iron-mineralizatio

Abstract: [Objective] Girvanella, a cyanobacterial fossil commonly found in Cambrian carbonates, plays a crucial role in the formation of many microbialites. Despite considerable advancements in elucidating calcifying Girvanella and its biomineralization mechanisms, the understanding of iron-mineralized Girvanella and its iron-mineralization mechanism is still relatively vague. [Methods]This paper explores the structural characteristics of Girvanella and its iron mineralization in ferruginous microbialites from the Mantou Formation Member Ⅲ (Cambrian Miaolingian Series) of Weihui area, northern Henan through an integrated analysis of sedimentology, mineralogy and geochemistry. [Results]The results indicate that the lamina or clots of ferruginous microbialites in study area are dominated by densely distributed Girvanella. The tube core of Girvanella is primarily composed of crystalline calcite crystals, whereas the tube wall of Girvanella is composed of a large number of sheet-like hematite crystals with diameter of 0.1-0.5 μm and random distribution. [Conclusions]Through CO2-concentrating mechanism, Girvanella generates a large number of anions in its surroundings，which adsorb Fe3? cations along with Al、Ca 、Mg and Si, facilitating nucleation on Girvanella EPS, leading to the formation of cryptocrystalline hematites. This kind of microbially induced hematite is significantly much complex in mineral morphology and elemental composition than chemically precipitated hematite. The co-occurrence of cryptocrystalline hematite minerals formed by the iron mineralization of Girvanella and idiomorphic chamosite crystals formed by chemical precipitation within ferruginous microbialites indicate that there are fluctuating changes in the redox gradient and iron mineral phase, as well as involving the coexistence of ferric and ferrous minerals within the cyanobacterial microbial mat（film）.