FORMATION OF ALUNITE, JAROSITE AND HYDROUS IRON-OXIDES IN A HYPERSALINE SYSTEM - LAKE TYRRELL, VICTORIA, AUSTRALIA

Alunite (KAl3(SO4)2(OH)6) and jarosite (KFe3(SO4)2(OH)6) are common weathering products of aluminosilicates and pyrite. Long-range transport of the constituents forming these minerals and the subsequent deposition of alunite and jarosite in an evaporite setting have not been previously documented. Such conditions for the occurrence of alunite and jarosite were investigated in a hypersaline system where acid groundwater enriched in K, Al, Fe(III) and SO4 enter a salt-playa lake. Sediment cores were studied by thin section, XRD and SEM-EDS. Groundwater and pore-water chemistries were also analyzed. The results show that alunite and jarosite occur together or as individual layers and cements within the top 20 cm of the sediments, where the groundwater is most concentrated due to evaporation. Hydrous Fe-oxides also occur as cements or vein fillings with the alunite and jarosite, but are distributed throughout the 70-cm sediment column studied. The results are consistent with a model in which alunite and jarosite precipitate as a result of the evaporation of water. Thermodynamic modeling indicates that the pore water in the playa sediments may be in equilibrium with alunite, jarosite and hydrous Fe-oxides.