School of Engineering and Science - Jacobs University

and preservation of oxide-facies IF was common throughout the first 2 billion years of
Earth’s geologic history. Oxide-facies IF in Archean marine basins is frequently associated
with Fe-rich shales, with the latter commonly assumed to represent mixtures between detrital
clay minerals and Fe(III) oxide minerals.
The Archean (2.85-2.95 Ga) Pongola Supergroup in southern Africa consists of the
predominantly volcanic Nsuze Group, which is unconformably overlain by the predominantly
sedimentary Mozaan Group. Previous studies have concluded that the Pongola Supergroup
was deposited in near-shore shallow marine waters on a stable cratonic margin 10,11 , and
subsequently experienced lower greenschist facies regional metamorphism 10 .
This study discusses samples from the Sinqeni Formation of the Mozaan Group, that
were obtained from the White Mfolozi inlier where a shallow-water sequence of BIF and
ferruginous shale is bounded top and bottom by non-ferruginous shales 12 . The sample set
includes the non-ferruginous shales deposited immediately below and above the BIF and the
ferruginous shales within the BIF. Contacts between the ferruginous shales and the pure
oxide-facies BIF layers can be sharp, but some BIF beds are very impure chemical sediments
due to the co-deposition of detrital aluminosilicates.
Major element, carbon, nitrogen, and trace element data for the samples are provided in
Table 1. Variation in major elements such as Al, Fe, and K is related to the relative position
of the samples in the stratigraphic profile. The non-ferruginous shales located below and
above the BIF display a major element composition typical for Archean shales (Fig. 1). They
are indistinguishable from fine-grained clastic rocks described in a comprehensive study of
the Pongola Supergroup conducted by Wronkiewicz and Condie 13 (see Supplementary
Table), and represent sediment weathered from the Kaapvaal craton ~2.85-3.0 Ga ago.
In contrast, Fe in the ferruginous shales is inversely correlated with Al content (Fig. 1),
a relationship that reflects mixing between Al-rich clastic sediment and the Fe-rich chemical
precipitate that produced the BIF. On average, the ferruginous shales display half the Al 2 O 3
content and a five-fold increase in Fe 2 O 3 compared to the non-ferruginous shales. Similar to
Al and Fe, the alkali metals (with the exception of Na) display variable concentrations and a
distinct bimodal distribution with strong depletions for K, Rb, and Cs in the ferruginous
shales (Fig. 1). Potassium and Rb average 6.3% and 205 ppm, respectively, in the nonferruginous
shales, whereas the ferruginous shales contain less than 0.05% K 2 O and typically
less than 1 ppm Rb. Barium concentrations, unlike the other alkaline earth elements, are also
distinct between the non-ferruginous and ferruginous shales, averaging 567 ppm and less than
22 ppm, respectively (Fig. 1).
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