SIZE AND CHARACTERIZATION OF IRON FLOCS AND THEIR SIGNIFICANCE IN BIF FORMATION

Due to the lack of a modern analogues and their antiquity, the depositional processes responsible for banded iron formations (BIF) are poorly understood. One broadly held hypothesis is that BIF were deposited as a result of iron flocculation. Flocculation is the process whereby suspended particles (flocs) coalesce to form larger particles that subsequently settle.

In this experiment, ferrous iron (Fe²⁺) solutions of 3 and 10 ppm were produced and added to either river, saline, marine or silicified artificial saltwater. These solutions were then allowed to oxidize and the resultant ferric iron (Fe³⁺) to precipitate. The size of the resultant flocs was measured using Dynamic Light Scattering, Scanning Electron Microscopy, and Sedigraph analyses. The average measured Fe floc size was then used in conjunction with Stokes' Law to determine settling velocities. Modelled settling velocities were then independently tested using a settling chamber. The flocs ranged from approximately 200 nanometres to 20 micrometres, resulting in settling rates on the order of magnitude of mm/day to cm/day. Based on these settling rates, Fe flocculation and thereby, BIF deposition could only occur in very quiescent sedimentary environments. In terms of modern analogues, currents in modern marine settings would preclude Fe-floc accumulation in most oceanic basins. This would suggest either restricted low-energy environments or attenuated paleomarine currents during periods of BIF deposition.