Abstract

The opal mining areas of inland Australia have exposures in which a systematic association between near-surface silcrete and one or more silicified horizons at depth is displayed. In the Stuart Creek opal field, the deepest horizons are opalite and glassy quartzites in which all primary sedimentary structures are conserved. Higher in the section, lenses and nodules of quartzite occur in bleached, unconsolidated sands. Near the top of the section, a nodular and columnar silcrete displays numerous illuviation features. At the base of the section, the earliest phase of silicification is the pseudomorphic replacement of sedimentary clay minerals by opal. Subsequently, void linings of micro-laminated opal were formed, and fibrous silica was precipitated in residual cavities. In the middle part of the section, sedimentary clay minerals were replaced by microcrystalline quartz, while silicification of clay-free sands was achieved by overgrowth of detrital quartz grains. In both cases, residual voids were filled with chalcedony and euhedral quartz. In the upper part of the section, silicification produced microcrystalline quartz in the matrix of the host sediment and in titania-rich illuviation laminae at the base of voids and channels. The near-surface silcrete displays many features relating to infiltration and downward percolation of water. Variable rates of water percolation, as well as alternating periods of leaching and deposition, are inferred from macro- and micro-scale structures and fabrics. The presence of microquartz indicates that solutions contained comparatively low silica concentrations, but enough impurity elements to restrict the growth of large crystals. In the deepest horizons, the preservation of sedimentary structures, the occurrence of micro-laminated void cutans of silica, and the horizontal disposition of silicified pans or lenses suggest a relationship with former groundwater tables. Secondary silica is mainly opal, indicating that the precipitating solutions had high silica concentrations. In the middle part of the section, structures are similar to those at depth except that opal is lacking in the matrix and appears to have recrystallized to microquartz. Silicification may have commenced at a groundwater level, but it later proceeded in response to dissolution and recrystallization in the unsaturated zone above the water table. The different silicification processes occurred in the same landscape in response to different mechanisms. Near the surface, in the pedogenic silcrete, solutions appear to have dissolved silica during infiltration and concentrated it through evaporation during dry periods. At depth, there appears to have been a general acidification of the environment leading to destruction of sedimentary clay minerals and the consequent production of silica phases that had a comparatively high solubility. Opal was precipitated in these groundwater environments. During landscape dissection, the water table was lowered and former silicified horizons were stranded in the unsaturated zone. Here, percolating waters with relatively low concentrations of silica dissolved opal and precipitated microquartz. Meanwhile, groundwater silicification proceeded at a deeper level at the new water table.

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