Silicification of calcrete in palaeodrainage Basins of the Australian arid zone

Quaternary calcrete deposits in palaeodrainage basins of the arid zone of Western and central Australia have developed near the surface within a remarkably short span of time. Silicification of the calcrete, involving the filling of voids and replacement of calcite by opaline and quartzose silica, has taken place in both the vadose and phreatic hydrological zones as a late diagenetic event. Diagenetic changes in the textures of silicified calcrete commonly correspond with silica-phase transformation and reflect replacement of carbonate through dissolution and reprecipitation. Silicified calcrete horizons in ancient terrestrial (lacustrine and groundwater) carbonate deposits may be related to the migration history of the silica in palaeohydrological systems. The presence of silica as thin grain coatings, veins and pore and cavity lining'˜; is a manifestation of the intricate balance between the solution of host calcrete and precipitation of silica under prevailing arid/semi-arid hydrological conditions. Such cogenesis, although spatially highly variable, has an important bearing on preservation and redistribution of economic mineral ores, and on the occurrence of fresh groundwater aquifers in the internal drainage basins of Australia.