Assessing the link between Earth’s oldest stromatolites and hydrothermal fluids: The c. 3.5 Ga Dresser Formation, North Pole Dome, Pilbara Craton, Western Australia

摘要

Extensive mapping, petrological data and geochemical analyses shed new light on the environment of deposition of cherty sedimentary rocks that contain Earth’s oldest stromatolites in the c. 3.5 Ga Dresser Formation, North Pole Dome, Western Australia. Some researchers have interpreted a quiet shallow water evaporitic setting and others a tidal flat, but detailed stratigraphic and petrographic data supports deposition within a developing volcanic caldera that was flushed by voluminous hydrothermal fluids. A series of stratigraphic profiles measured on either side of the Dresser (barite) Mine over a distance of seven kilometres display true sediment thickness variations across active growth faults, as previously noted. Rapid lateral facies variations and diverse depositional settings, as well as multiple newly discovered eruptive layers of felsic volcaniclastic material support a volcanic caldera setting. Importantly, the first discoveries of geyserite and tourmaline-bearing ferruginous laminates interpreted as radiogenic, B-rich hot spring crusts are documented, providing evidence of emergence of the volcanic landsurface. Geyserite consists of alternating K-Al clay-rich (light) and anatase-rich (dark) laminae, 20 µm thick, that appear identical to modern geyserite formed from alternating acidic and alkaline fluids. Morphologically variable stromatolites, including domical, stratiform and coniform varieties, are restricted to shallow water environments, but are widespread within the lower parts of the succession, suggestive of phototrophs. Spatially more restricted occurrences of dendritic microbialites are overlying large hydrothermal veins, suggestive of chemoautotrophs. The deposition of stromatolitic rocks is inferred to have developed as a response to uplift of the surface during emplacement of a subvolcanic magma system that drove geysers and erupted ash. Subsequent caldera collapse formed deeper basins accompanied by coarse clastic sedimentary rocks in which there are no visible signs of life. Results support a diverse microbial community in these ancient rocks that were able to utilise energy sources from a variety of habitats within stages of a developing volcanic-hydrothermal system.