Formation of analcime and moganite at Lake Lewis, central Australia: significance of groundwater evolution in diagenesis

Abstract

Abstract Lake Lewis basin in the Northern Territory is a hydrologically closed intermontane basin in which clay-rich palaeolacustrine sediments are infused with highly evolved brines. Acquisition of dissolved silica and sodium by groundwaters from the Proterozoic Arunta granite-gneiss terrain, and from arkosic alluvium and weathering products sourced from these bedrocks, is important to diagenesis at the salt lake. Supersaturation of groundwaters with respect to SiO 2 and the availability of catalytic ions, coupled with high rates of evaporation, favour neoformation of opal and chalcedonic silica from solution near the groundwater discharge zone. Moganite, a distinctive silica polymorph that is commonly associated with evaporites, has been identified amongst precipitated silica species. Down-gradient from the zone of silica precipitation, beneath the playa, brines are SiO 2 -deficient. Long periods of weathering and diagenesis of lacustrine clays immersed in semi-stagnant brines has transformed detrital minerals into substantial proportions of amorphous or gelatinous aluminosilicate material. Aluminosilicate phases have become metastable in the presence of evolved SiO 2 -deficient interstitial brine. Analcime, a sodium-bearing zeolite group mineral, is stable in the presence of the Na-rich brine and is crystallizing authigenically below the watertable from the amorphous aluminosilicate material in the lacustrine sediment. No volcanic rocks or pyroclastic sediments are involved in this diagenetic occurrence of analcime. The appearance of analcime in the evolution of the salt lake system probably commenced within the last 100 000 years and is occurring in the present-day hydrologic and climatic regime of this arid zone area.