STRATIGRAPHIC AND SPATIAL CHANGES IN MINERALOGY IN ACID SALINE LAKES: IMPLICATIONS FOR FLUID CHEMISTRY AND MINERAL STABILITY

While natural modern environments that have a combination of extremely low pH and high salinity are rare on Earth, ephemeral salt lakes with extreme acidity are common in the semi-arid landscape of southern Western Australia (WA). These lakes and their associated groundwaters have unusual and complex brine geochemistries, which affect the mineralogy of the associated sediments. This extreme chemistry allows for both the mobility and enrichment of uncommon elements as well as the subsequent precipitation of some minerals not commonly associated with evaporite sedimentary systems. X-ray diffraction analysis of surface sediments (0 to ~2 m deep) from two contrasting acid saline lakes and the surrounding mud/sandflats indicates the presence of several complex mineral assemblages with extreme spatial heterogeneity that reflect the complex geochemistry of these lakes. In addition to halite and other soluble chloride and sulfate salts, and coarse-grained detrital components, the fine-grained (<62 and <2 mm) fraction include unique combinations of authigenic and detrital Al- and Fe/Mg-phyllosilicates, sulfates (e.g. alunite, jarosite, rozenite, and gypsum), Fe-oxides (e.g. hematite and goethite), and other silicates (mullite and heulandite-clinoptilolite). In addition to surface and shallow subsurface sediment samples, deeper (<60 m) sediment cores were obtained from several lakes in 2009. Detailed reflectance spectroscopy of the sediment cores from two of these acid saline lakes reveals the presence of similar mineral assemblages at depth. Petrographic observations will help to constrain which minerals formed under similar syndepositional/early diagenetic conditions in ancient acid saline lake environments and which minerals are a more recent result of acid brine fluid flow beneath the modern lakes. Geochemical modeling of observed minerals, trace elements, and fluids may be useful in constraining how conditions in these environments vary spatially and have changed over time. This detailed study of the sedimentary mineralogy in acid saline lakes will expand our understanding of mineral formation and stability for the observed mineral assemblages and will allow for better identification of these types of systems in the geologic record – both on Earth and on Mars.