Figure 1.

Sampling location maps (a,b) and petrographic context of the microstructures of interest; irregular clots (c,d) and volcanic particle coatings (e,f) sourced from the 3.33 Ga Josefsdal Chert (Barberton greenstone belt, South Africa). (a) Map of southern Africa showing the location of the Barberton greenstone belt. (b) Map of the southern portion of the Barberton greenstone belt; white box indicates the location of the Josefsdal Chert, near the border of South Africa and Eswatini, from which all studied samples were collected. (c) Thin section of weakly laminated clotted carbonaceous chert corresponding to black massive chemical chert observed in the field (see Fig. S2). Carbonaceous clots are distributed throughout a silica matrix; other phases include scattered microminerals including pyrite and chromite spinel (see Fig. 2). (d) High-resolution photomicrograph showing irregular, stellate carbonaceous clots of up to ~1 mm diameter. (e) Thin section of laminated black and white banded chert corresponding to laminated chert observed in the field (see Fig. S2). (f) High-resolution photomicrograph showing sub-rounded and sub-angular pseudomorphed and silicified volcanic particles coated and impregnated with carbonaceous material (dark regions).