An evaluation of phases in banded iron formation of the 3.25 Ga Fig Tree Group (Barberton Greenstone Belt) suitable as a seawater archive

Banded iron formations (BIFs) are authigenic marine sedimentary rocks that formed in Precambrian oceans. They may record the chemical composition of the ambient seawater and are thus important archives for reconstructing ancient marine environments. The ca. 3.25 Ga Algoma-type BIF of the Fig Tree Group in the Barberton Greenstone Belt, South Africa, provides insights into the Palaeoarchaean marine environments and seawater chemistry during the early development of the Kaapvaal Craton [1,2]. However, it remains incompletely understood, which mineral phases within this BIF most reliably preserve primary seawater-derived signatures and therefore represent the most suitable archives for palaeo-environmental reconstructions.

We present trace and major element concentrations of 28 individual layers of Fig Tree Group BIF. These layers are dominated by either magnetite, chert, or siderite. In addition, mudstones intercalated with BIF were also analysed. All samples originate from the BARB 4 drill core and were digested using HF–HNO₃–HCl digestion combined with ICP-MS and OES analyses to investigate the geochemical composition of the different mineral phases and their reliability as archive for ancient seawater chemistry.

Immobile element (Zr, Th) concentrations are in the ppb to ppm level range and vary over four orders of magnitude between the BIF samples. Samples with the highest immobile element concentrations show non-seawater-like shale-normalised (subscript SN) rare earth element and yttrium (REY) patterns and a positive correlation of REY and immobile element concentrations (e.g. Zr), in indicating detrital contamination. However, cherts and five of the magnetite samples with the lowest immobile element concentrations 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. Positive Eu_SN anomalies indicate contributions of high-temperature hydrothermal fluids. The lack of negative Ce_SN anomalies indicates anoxic depositional conditions with respect to the Ce³⁺-Ce⁴⁺ redox couple. The chert layers, however, show Th/U fractionation compared to the value of the continental crust, suggesting redox-dependent uranium mobilization, indicative of slightly oxic conditions.

We identified chert and magnetite, if devoid of detrital contamination, to be the most suitable phases in Fig Tree Group BIF for obtaining information to reconstruct their depositional environment. The remaining layers, on the contrary, do not reflect pure seawater precipitates and have to be excluded for interpretations regarding ancient seawater chemistry.

 

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

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