The reconstruction of the 2.41 Ga Hotazel BIF paleo-depositional environment and mechanism of iron and manganese cycling in a stratified basin.

Abstract

The Hotazel formation is the uppermost part of the Paleoproterozoic Transvaal Supergroup and is characteristic for having three distinct Mn-oxide layers interbedded with banded iron formations (BIFs). The depositional age of the Hotazel Fm. is estimated at 2.41 Ga, right at the onset of the Great Oxidation Event (GOE) which represents the transition from an anoxic ocean and atmosphere to a full oxidative cycle in the oceans at around 2.22 Ga. The Hotazel formation’s stratigraphy shows an antithetic thickness relationship between the BIF and the Mn-oxide lithology’s of which the primary controls remain somewhat unknown. Determining what controls this relationship is of major importance and could imply that under the right conditions, compositionally different styles of chemical sedimentation can be produced at the same time. Optical microscopy, X-ray diffraction, XRF, LA-ICP-MS and carbon isotope measurements were carried out on two different drill-cores showing this antithetic relationship a) to construct a clear correlation between the two different sections and b) to obtain in much detail the mineralogical and chemical composition, redox conditions and the paleo-depositional environment of the first BIF in the Hotazel Fm. Trace element data presented an increase in HFSE towards the base of both sections, related to allochthonous input in the water column. The REE + Y distribution showed depleted LREE, positive La, Gd and Y anomalies and no negative Ce anomaly in both sections. Apart from the absence of a negative Ce anomaly, indicating an anoxic marine environment, the water column resembled that of modern (and ancient) seawater. The carbon isotopes are largely depleted (δ13C of -20.66 to -11.35 ‰) and showed a gradual increase towards the top of the MP-BIF section and a rather sharp increase towards the top of the G7BF related to differences in subsidence rate. The G7BF section is correlated to be the stratigraphic equivalent of the top 13 meters of the MP-BIF section, indicating that precipitation rates were nearly equal during time of deposition. This would subsequently imply that the bottom of the MP-BIF section had been deposited during the same time interval as the Mn-oxide intercalation of the G7BF, showing that different types of chemical sediments, under the right condition, can indeed precipitate simultaneously at laterally different parts of the basin.