Mineralogical and geochemical constraints of Archean Baniaka Iron Formation: implication for origin and source of iron

Precambrian iron formations (IF), which typically contain more than 15% Fe, are important economic sedimentary rocks for iron resources and constitute an archive for understanding the geochemical evolution and processes of the early Earth. While iron deposits are found throughout the geological record, the origin, depositional conditions, biogeochemical cycling, and sources of geochemical components in Precambrian iron formations remain unclear.

Here, for the first time, we present mineralogical, geochemical, and Sm-Nd isotope data from two drill holes in the 2.8-2.7 Ga Baniaka iron deposits, south-eastern Gabon. The IFs are underlain by gneiss, consisting of metamorphosed silicate IF (amphibolitic facies) and oxide IF facies, from the base to the top, with minor occurrences of interbedded gneiss occurring within the amphibolitic facies. The basal silicate IFs essentially comprise biotite, stilpnomelane, magnetite, and actinolite, while the upper oxide facies are dominated by hematite and goethite with traces of magnetite. Quartz is a key component of all the samples while kaolinite and smectite are present in some of the oxide IF and the upper part of silicate IF samples, respectively. Traces of chlorite and siderite are locally observed in a few samples., The IF samples, except the kaolinite-rich ones, are rich in Fe (~20-60 wt.%) and relatively low in Al (0.1-4.0 wt.%). The Si content ranges from 1.3 to 30 wt.%, while Ca, Ti, Mg, Mn, K, and Na occur in trace amounts. The absence of a significant correlation between Fe and detrital proxies (Al and Ti) suggests that Fe enrichment is not controlled by detrital flux, indicating the involvement of Fe cycling. 

The positive Eu anomalies in the chondrite- and shale-normalized rare earth element (REE) patterns, the slight depletion of light REE relative to heavy REE in the shale normalized patterns, and the chondritic to superchondritic Y/Ho values (27-48) demonstrate the influence of hydrothermal fluids and seawater mixing in an open ocean water depositional environment. The geochemical proxies of the incompatible elements (Th, Zr, Nb, and Sc), coupled with the positive εNd isotopic values (+0.34 to +9.75), are consistent with mantle-derived mafic materials from less-differentiated juvenile crust in a volcanic arc environment were significant source of the geochemical components during the deposition of the Archean Baniaka IF. Taken together, these results suggest that a significant proportion of the Fe likely derived from oceanic crust following hydrothermal alteration and seawater percolation within the Archean greenstone belts in southeastern Gabon.