The sedimentary-hosted opal deposits of central Australia are located in weathering profiles elevated above the surrounding plains. The presence of potassium-bearing minerals (alunite (KAI3(S04)2(OH)6) and hollandite ((Ba1K)Mn8O16.xH2O)) within these weathering profiles permits the use of 40Ar/39Ar geochronology to assess the age of weathering and provide information on the palaeoclimatic history of Australia. The association of these potassium-bearing minerals with opal allows the relationship between opal and weathering to be established by investigating the geochemical conditions present during weathering and their possible involvement in the formation of opal.
Despite the wide geographical separation of the opal producing areas of Lightning Ridge, Coober Pedy and Andamooka, the variation in the host rock compositions, and differences in the landscapes and the potassium-bearing minerals hosted in the weathering profiles, similarities exist in both the 40Ar/39Ar geochronology results and the sequence of geochemical processes present during weathering in these three areas. 40Ar/39Ar laser step-heating analysis of one hundred and five alunite grains from Coober Pedy, nine alunite grains from Andamooka and thirty-five hollandite grains from the Lightning Ridge area yield dominantly Miocene ages. Laser step-heating plateau ages for alunite grains range from 19.34 ± 0.11 to 6.45 ± 0.17 Ma for Coober Pedy and 12.94 ± 0.17 to 7.87 ± 0.14 Ma for Andamooka. These results complement, and are consistent with, the K-Ar results of Bird et al. (1990) for alunite from the same areas. The hollandite grains from the Lightning Ridge area yield ages ranging from 29 ± 2 to 4.7 ± 0.5 Ma. The ideogram for all of the hollandite grains indicates precipitation dominantly occurred from 18.35 to 6.36 Ma.
The 40Ar/39Ar data suggests that the precipitation of the supergene minerals hosted in the weathering profiles was dominantly a result of Miocene processes. The likely earlier weathering event(s), particularly in the Coober Pedy area, which contributed to the formation of the profiles, were overprinted and reset by events in the Miocene.
Scanning electron microscopy (SEM) revealed pseudocubic alunite grains with two grain sizes (about 2 mm and 5 mm in diameter) in most samples from Coober Pedy and Andamooka. The hollandite samples also have a small grain size (averaging 5 mm in diameter) but are complicated by growth bands and cross-cutting (manganese oxide) veins. Both the alunite and hollandite were subjected to partial dissolution sometime after precipitation, indicating a change of geochemical conditions away from those conducive to the precipitation and preservation of these supergene phases. The presence of silica coatings on the alunite and hollandite grains indicates that at least one period of silicification occurred in all three areas after precipitation of these minerals.
Alunite in the Coober Pedy area dominantly forms by the metasomatism of kaolinite, a reaction that also produces monosilicic acid. Geochemical modelling of the Coober Pedy system indicates this is a plausible source for the silica required for opal formation. Textural relationships of alunite crosscut by opal veins also suggest that opal formation occurred after alunite precipitation. The intimate relationship of hollandite and opal from the Lightning Ridge area indicates that the precipitation of these two phases is closely related. The association of these supergene minerals with opal suggests that opal formation is a consequence of the development of the weathering profiles.
A distinct age with elevation trend, with older results recorded at higher elevation sites and younger ages for lower elevation minerals, was identified in the Coober Pedy area. Vasconcelos and Conroy (2003) identified this age versus elevation trend in the weathering profile at Dugald River, northwest Queensland and interpreted it as the record of a deepening groundwater table with time. For the Coober Pedy area, a best fit curve through the 40Ar/39Ar geochronology results suggests a descent rate for the groundwater table in the area of 3.48 m.My-1 This rate of groundwater table descent is compatible with the rate of groundwater table descent measured in the Dugald River area of 3.75 m. My-1. The two sets of 40Ar/39Ar geochronology results are for the same time interval and possibly record the progressive recession of the groundwater table In Australia during the Late Tertiary in response to aridification of the continent and/or possible tectonic uplift.