Iron biomineralization and biometallogenesis in the ancient-wood buried zone from coast of Zhoushan Island, Zhejiang Province

In the present study, we describe the formation of iron ores collected in the intertidal zone of the Zhoushan Island in Zhejiang Province in the East China Sea, where ancient wood layers were buried. Morphological, mineralogical and geochemical analyses were performed on the iron ores and the surrounding geological material. The results show that the iron ores are not only composed of spherical and fibre-like aggregates of goethite, but also contain the dead bacterial sheaths, which present morphological characteristics reminiscent of bacterial activity. Similar present day biomineralization characteristics were also observed in an iron seepage system near the studied intertidal zone. The presence of Leptothrix like sheaths and Gallionella like stalks in the present-day environment promoted the oxidization of Fe²⁺ to Fe³⁺ and the rapid precipitation of biogenic iron oxides around the bacteria. The role of bacteria in mineral formation in the seepage area is believed to represent an analogue mechanism for the formation of the iron ores. It is hypothesized that the degradation of the ancient wood provided humic substances which accelerated the leaching process of iron from the surrounding bedrock and soils, and then created local biogeochemical conditions which led to the biomineralization of the iron ores. The present findings help elucidate the role of bacteria and humic substances in the formation of iron ores in the history time.