The suitability of ca 3.25 Ga banded iron formations from the Fig Tree Group (Barberton Greenstone Belt, South Africa) as archives for Palaeoarchaean seawater chemistry

Banded iron formations (BIFs) are authigenic marine sedimentary rocks that may record the chemical composition of ancient seawater and are widely used as geochemical archives to reconstruct Precambrian marine environments. In this regard, the approximately 3.25 Ga old BIFs of the Fig Tree Group, Barberton Greenstone Belt, have been shown to be prime recorders for Palaeoarchaean marine environmental conditions and seawater chemistry during the early stage of Kaapvaal Craton formation [1, 2].

To further evaluate the suitability of this BIF and its individual mineral phases as geochemical archives, we present trace and major element concentrations of 13 layers dominated by magnetite, chert, and siderite from the BARB 4 drill core. Immobile element (Zr, Th) concentrations vary over four orders of magnitude between the samples. In combination with non-seawater-like shale-normalised (subscript SN) rare earth element and yttrium (REY) patterns, this indicates variable degrees of detrital contamination. However, cherts and two of the magnetite samples show typical Archaean seawater-like signatures with positive La_SN Gd_SN, and Y_SN anomalies as well as a depletion of light REY relative to heavy REY_SN, indicating a seawater-derived origin. The lack of a negative Ce_SN anomaly and positive Eu_SN anomalies indicate anoxic environmental conditions and contributions of high-temperature hydrothermal fluids, respectively.

Our results show that chert in BIF is the most reliable mineral phase for obtaining information to reconstruct BIF depositional environments through deep time.

[1] Hofmann, 2005, Precambrian Res. 143, 23-49.

[2] Satkoski et al., 2015, EPSL 430, 43-53