The origin and significance of carbonaceous matter within Palaeoarchaean marine environments of the Barberton Greenstone Belt, South Africa

Palaeoarchaean (3.6-3.2 Ga) cherts are commonly associated with hydrothermal activity and retain evidence for an early biosphere. They are rich in disordered carbonaceous matter (CM) whom origin cannot be readily associated to biological activity. This project investigate CM-rich material from the BARB3 core obtained from the c. 3.4 Ga old Buck Reef Chert (BRC) of South Africa, one of the best preserved volcano-sedimentary succession on Earth. CM from the core is characterized through a multiple analytical approach based on in situ complementary techniques. The aim was to reconstruct the CM origin and evaluate its biogenicity. Samples from the BARB3 shallow-platformal lithofacies, are rich in CM occurring within: (i) crinkly laminated chert; (ii) massive black chert; (iii) laminated black chert; and (iii) granular carbonaceous chert. These four facies bear a specific CM microtextures related to their depositional history. The crinkly laminated chert has a planar stromatolitic-like fabric and include CM mat-like laminae and grains. The massive black chert is structureless and include “cloudy” diffuse CM, CM grains and bitumen-interstitial CM originated as a fluid phase. The laminated black chert include CM grains and the granular carbonaceous chert is a mixture of carbonaceous detritus generated by the hydrothermal brecciation of soft crinkly laminated sediments and it is associated to botryoidal-quartz stratiform veins. The structural characterization of such CM microtextures by means of Raman spectroscopy and HRTEM analyses have confirmed their consistency with the BGB regional metamorphic imprint. However a between-facies and, more importantly, a between-microtexture structural heterogeneity occur guiding the reconstruction of the CM and of the depositional facies history. The result is a revisited picture of the BRC as a shallow-water hydrothermal field characterized by complex microbial communities and carbon remobilization now expressed by multiple CM generations.