BIOMEDIATION IN IRONFORMATION GENESIS SINCE THE EARLY ARCHEAN

Ironformation beds of all ages that are not highly metamorphosed have primary features, oolites, granules, micro bands, beds, coatings on siliceous clasts and bioforms, that are composed of discrete iron-rich micro ovoid units. Micro particles of iron oxide, silicate, sulphide or carbonate minerals give pigment to the micro ovoids and mark the primary distribution of iron in the beds. The micro ovoids are similar in form to bacterial cultures that secrete iron in biofilms in modern sediments. The development of micro ovoid features has been attributed in the past to biomediated processes that acquired oxygen from the atmosphere.

New insight into the origin of ironformation sediments through time is gained from the discovery of bacterial cells in biofilms that take their energy directly from iron and manganese and deposit these metals in fine particles of siderite, iron oxide, silicate and sulphide minerals in micro ovoids in anaerobic environments. Bacterial cultures with similar properties may have had an essential role in the genesis of many features in ironformation sediments since the Early Archean that are difficult to explain by physical-chemical processes alone, such as the distribution of iron in micro ovoids in primary sedimentary features, ubiquitous mixtures of ferrous and ferric iron oxide, siderite, silicate or sulphide mineral particles, the apparent selective deposition of iron and manganese from hydrothermal solutions, and the genesis of extensive ranges of ironformation in Early Archean seas before significant distribution of oxygen in the atmosphere.