Early Archean Hot Springs above Epithermal Veins, North Pole, Western Australia: New Insights from Fluid Inclusion Microanalysis

Abstract

The world’s most ancient biogenic structures are found in the North Pole Dome of Western Australia, where 3.47-Gyr-old algal mats and stromatolites are closely associated with bedding-conformable and discordant laminar quartz, chalcedony, and barite. Barite-rich quartz hydrothermal veins with similar mineralogy occur throughout the stratigraphy below the conformable biogenic structures. With the exception of the large volume of barite, these bedding-conformable and discordant laminar quartz veins exhibit textures and associated hydrothermal alteration (quartz-chalcedony-chlorite-illite ± calcite-adularia-pyrite) typical of epithermal deposits formed from near-neutral pH fluids. We characterize the physical and chemical conditions of the ancient water responsible for depositing both the discordant and conformable quartz-chalcedony-barite as it passed through the upper parts of the Archean crust. Field relationships, combined with new fluid inclusion data, suggest that the best documented stromatolites in the North Pole Dome occur adjacent to quartz-chalcedony bands formed from cool (120°C), low-salinity (<3 wt % NaCl equiv) waters. Higher temperature (up to 300°C), more saline (up to 10 wt % NaCl equiv) and CO2-H2S-rich (±CH4) aqueous fluids occur in deeper level veins. Rare inclusions that are unusually rich in CO2 (containing liquid and gaseous CO2 and liquid H2O) support the existence of multiple batches of hydrothermal fluids (with variable densities and gas contents). Oxygen isotope data (8.7-3.7) suggest that the causative fluids comprised admixtures of deeply circulated surface water with variable input of magmatic components. Our findings reveal that the earliest life known on Earth lived in and around a hydrothermal system with temperatures from ~300°C at depth to 120°C near the paleosurface, in an environment closely analogous to modern hot springs, developed above epithermal veins. Evidence exists for the introduction of different batches of hydrothermal fluids (with variable densities and gas contents) during the development of veins. These findings support previous studies that demonstrate that the processes that form epithermal deposits have been active throughout geologic time, and the present-day distribution of epithermal deposits is dominantly a result of preservation, not process