IRON DIAGENESIS IN WESTERN AUSTRALIA ACID SALINE SEDIMENTS: OBSERVATIONS FROM REFLECTANCE SPECTROSCOPY AND MICROSCOPY

The Yilgarn Craton in Western Australia hosts landscapes illustrating pervasive iron diagenesis caused by extensive weathering and highly acidic groundwater. This research explores the spatial patterns of iron oxides and iron sulfates in regolith and sediments. Mineralogical data was collected with visible-to-shortwave-infrared reflectance spectroscopy and optical microscopy from stratigraphic cores, field samples, and aerial and satellite remote sensing from Lake Aerodrome, Lake Brown, and the Twin Lakes. Reflectance spectra of core slabs, thin sections, and field samples show a dominance of phyllosilicates, iron oxides, and gypsum. Optical microscopy highlighted the complex weathering and depositional history, as heterogeneous textures, immature grains, significant porosity, and many diagenetic features were commonly observed. The samples show extensive iron oxide cements, iron oxide nodules, dissolution cavities, bottom-growth, swallowtail and acicular gypsum crystals, and irregular cementation boundaries indicative of reaction fronts. Sediment cores show patterns of iron oxides concentrated spatially as horizontal bands and isolated spherical zones. The iron oxide bands typically contain weakly cemented concretions and vertically diffusive branches of iron oxides radiating into the phyllosilicate matrix. Microscopic observations show isolated networks of iron oxide cementation with many nodes of dense iron oxides. Remote sensing products also show spatial patterns of concentrated iron oxides isolated in bulbous zones around the lake boundaries. This work compares the differences between the various sites with differing fluid geochemistry and host rock composition, as well as the differences in mineral assemblages that are observed from subsurface cores versus aerial or satellite observations. The combination of depositional, diagenetic, and weathering features, most overprinted at scales of a cm or smaller, makes distinction of the various phases challenging. These observations add to the overall understandings of formation and preservation of mineralogical products that are indicative of extreme groundwater chemistries associated with acidic and saline lakes and has implications for interpretation of past extreme environments on Earth and on Mars.