Bifs

Banded Iron-Formation:

A Continuing Enigma of Geology

Jelte P. Harnmeijer, University of Washington

March 2003

Abstract

Banded Iron Formations (‘BIFs’) are highly controversial chemical precipitates characterized by the presence of alternating layers of iron-rich and amorphous silica-rich layers. This dichotomous compositional layering is usually expressed on several scales at any given outcrop, from fine sub-millimetre-scale varve-like laminae to metre-scale bands. Even on a microscopic scale, the boundary between the ferruginous and siliceous layers is distinctly abrupt.

This paper sets out to explore the key aspects of BIFs pertinent to their role in the study of early earth evolution, and outline the major controversies surrounding their interpretation. The focus rests on BIF classification, temporal- and spatial-distribution, behavior in solution, depositional environment, and origin.

An understanding of the origin of BIFs provides useful insight into conditions existing on, and governing, the early lithosphere, hydrosphere and atmosphere. BIFs have proved particularly useful to studies of the early earth due to their intimate coupling with oxygen concentration. Researchers have benefited tremendously from the narrow temporal-, depositional-, and tectonic- niches inhabited by BIFs in our geological record.

In the context of early earth evolution, it is the possible link to an evolving biosphere that makes BIFs particularly interesting. Iron is used as a metabolic agent by numerous microorganisms. Some of these, including specific species of oxygenic- and anoxygenic- photoautotrophs and chemoferrotrophs, lend credit to the theory of BIF-deposition being, at least in part, a microbially mediated process. Direct evidence for a microbial role in Archaean BIF deposition remains elusive.

The dominant enigma surrounding BIFs is the spectacular alternating BIF banding. A distal hydrothermal source for iron is favored, while recent work