Abstract

Silica sinters deposited from hot springs have been found associated with volcanic rocks of Late Devonian to late Carboniferous age in north Queensland, Australia. These deposits have many features in common with modern hot-spring sinters, such as those of the Taupo Volcanic Zone in New Zealand. They occur associated with subaerial volcanic rocks and are locally found with air-fall tuffs and fluvial and lake deposits in which hydrothermal eruption breccias are common. The associated rocks are extensively hydrothermally altered and are cut by silica veins showing characteristic epithermal vein textures; the silica veins contain low-salinity fluid inclusions trapped at epithermal temperatures, and there is evidence of boiling.

Textures preserved in the sinters are identical to those found in modern sinters; they include columnar structures similar to bacterial stromatolites described from Yellowstone National Park, as well as striated surfaces apparently resulting from silica deposition on filamentous algae. Plant fossils, including Oxroadia gracilis, are abundant. Criteria applied to identify these ancient sinters may be applied to other possible sinter deposits and may provide evidence indicating the level of exposure of the former geothermal system, which may be an important guide in exploration for epithermal precious-metal deposits.

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