The emergence of life on the juvenile Earth is still poorly understood and remains one of the major questions in geobiological research. Some of our planet´s most ancient rocks contain carbonaceous matter (CM) that may represent a valuable archive to trace earliest life.

However, it is often difficult to prove the origin and syngeneity of such CM. Here we report on CM preserved in ∼3.5 Ga old barites from the Dresser Formation (Pilbara Carton, Western Australia). On outcrop scale, spatial associations between bedded and vein-hosted barites suggest that the bedded barite may have formed from hydrothermal fluids discharging into subaquatic caldera environments [1]. Bedded barites associated with stromatolites contain abundant CM (total organic carbon = 0.3 wt% [2]) whose nature has been investigated further. Three populations of CM were recognized by means of light microscopy and high-resolution Raman mapping: (i) CM flakes at the edges of single growth bands of barite crystals, (ii) CM dispersed within barite crystals, and (iii) CM in 50–300 µm wide secondary quartz veins that cross-cut barite crystals. Raman spectra of the CM indicate peak metamorphic temperatures of approximately 300 ± 50 °C, corresponding to lower greenschist-facies conditions which are consistent with the metamorphic overprint by granitic intrusions in the area ∼3.3 Ga ago [3]. Near edge X-ray absorption fine structure (NEXAFS) and solid-state nuclear magnetic resonance (NMR) measurements revealed a highly aromatic nature of the CM which is in line with relatively high thermal maturity. As all three CM populations experienced the major metamorphic overprint ∼3.3 Ga ago, a syngenetic formation of the CM with the host barite can be assumed or, in case of the vein-hosted secondary CM, an emplacement soon after barite growth.