Elliptical textures preserved in Lake Superior-type Hamersley banded iron-formations and its genetic significance

Abstract

Mineralogical and microtextural features of Lake Superior-type banded iron-formations(BIFs)from the Hamersley group(2.5Ga), western Australia, are reported. BIBs samples are divided into five types according to their colors and mineralogy; black(maginetite, quartz, hematite, ankerite), gray(quartz, magnetite, riebeckite, stilpnomelane), yellow(quartz, magnetite, stilpnomelane), red(quartz, hematite, magnetite) and white(quartz) ones.
In thin sections, striking elliptical textures(30-40μm in diameter)with hematite core and quartz rim are exclusively recognized in red BIFs. Scanning electron microscopic observation of the red BIFs reveals that the hematite core is composed of aggregates of filamentous and tube-like materials, 1-7μm in length. These microtextures are very similar to those preserved in modern hydrothermal sediments and soils formed by biomineralization. Mineralogical and microtextural evidence suggests that the biomineralization is responsible for the elliptical textures and that the formation of the Hamersley BIFs is likely to be related to the biological activity of Precambrian era.