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Oily old ores: Evidence for hydrothermal petroleum generation in an Archean volcanogenic massive sulfide deposit

¹ Centre for Strategic Mineral Deposits, Department of Geology and Geophysics, University of Western Australia, Nedlands, Western Australia 6907, Australia
² School of Geosciences, Edgeworth David Building FO5, University of Sydney, Sydney, New South Wales 2006, Australia

Oil is formed in some modern seafloor vent systems after rapid thermal maturation of sedimentary organic matter by hydrothermal fluids. Although most of this petroleum escapes into the ocean, some hydrocarbons may be trapped as fluid inclusions in hydrothermal precipitates or as bituminous residues in cavities. However, the geological record lacks evidence of this phenomenon, which is unfortunate because it could be significant for understanding the history of petroleum genesis, the processes of hydrothermal mineralization, and perhaps the origin and early evolution of life. From the Pilbara craton of Australia, we report the discovery of pyrobitumen and oil in an Early Archean (ca. 3235 Ma) deep-sea volcanogenic massive sulfide deposit. The oil occurs in fluid inclusions within hydrothermal barite, and pyrobitumen is intergrown with polymetallic sulfides. Petrographic textures show that the oil was emplaced and thermally altered (coked) before late-stage sulfide mineralization. The results demonstrate that subseafloor hydrothermal petroleum generation was active during the Early Archean, possibly providing an energy and carbon source for a subsurface microbiota metabolizing hydrothermal sulfur species.

Key Words: hydrothermal oil • volcanogenic massive sulfide mineralization • pyrobitumen • Archean • Pilbara craton

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