Microbial structures in oolitic iron formations

Ferriferous ooids and microoncoids occur in many sedimentary iron formations. These ferriferous-coated grains are found in fine-grained carbonate-rich groundmasses. Iron mineral-encrusted microbiota are observed in both the coated grains and the groundmass. The branching nature and other morphological features of the microorganisms suggest fungal origins for the oolitic iron ores particularly of the Lower Jurassic (Lorraine Minette). The similarity of the microbial structures in coated grains and their groundmasses suggests that both had developed within microbial mats growing under calm environmental conditions. The contribution of stromatolitic marine fungal mats to the fast extraction and immobilization of iron and thus to the genesis of iron ores is demonstrated. Observations on well-preserved freshwater-derived fungal stromatolites of the Tertiary and laboratory experiments with organotrophic fungal mats confirm the findings on the Jurassic and imply a general role of microorganisms in the formation of such deposits.