Direct evidence for hydrothermal fluid mineralization in the Velkerri Formation, Northern Australia
thesisposted on 2022-03-28, 15:27 authored by Peter Targett
The presence of reactive iron and sulfur within marine sediments is routinely used as a proxy for marine anoxia in ancient sediments, assuming these elements were sourced from the overlying water column. Iron and sulfur in sediments of the 1.42 Ga Velkerri Formation in the McArthur Basin, northern Australia serve as one of the key records of Earth's oxygenation history. This thesis shows that much of this iron and sulfur was introduced by hydrothermal fluids long after deposition. Mineralized veins and breccias cut across sedimentary laminae at micro and macro scales. They are composed of vein lining dolomite, ankerite, pyrite, chalcopyrite, quartz, rutile, and iron oxides (hematite and magnesioferrite) indicating multiple fluid generations. Nanometer - scale mineral mapping combined with high resolution electron imaging identifies pervasive influence of later fluids in veins and micro pores through the sediment. 20 - 70% of Fe and S bearing pyrite can be attributed to later fluid sources using this approach, which would go unrecognized using averaged bulk sample analyses attributing this fraction to a seawater source. Clumped isotope palaeothermometry identifies a range of precipitation temperatures; 88.25 - 201.61 C consistent with hydrothermal mineralization observed throughout the McArthur Basin.