Timing of weathering-induced alteration of manganese deposits in western Australia: Evidence from K/Ar and Ar-40/Ar-39 dating

Under particular climatic conditions, tetravalent K-Mn oxides can be formed during terrestrial chemical weathering Mn ores. In this study the results of K/Ar and Sr-40/Ar-39 dating of weathering-related K-Mn oxides (mainly cr yptomelane (K,Ba)Mn8O16. xH(2)O) from supergene Mn deposits in the Pilbara and Peak Hill Mn provinces (NW Australia) and from scattered occurrences of K-Mn oxides found in deeply weathered rocks in Western Australia are prese nted. In ideal circumstances, K/Ar and Ar-40/Ar-39 ages will reflect the time sin ce the formation of K-Mn oxides. However, 40Ar/39Ar and K/Ar dating of K-Mn oxides which formed as a result of chemical weathering may be complicated by admixture of mica inherited from bedrock and by mixing of different gene rations of weathering-related K-Mn oxides. To correct for admixture of inhe rited mica a two-stage K/Ar dating technique was applied. Some samples of dated cryptomelane had finely banded textures, and therefor e might represent several generations. The age differences between these ge nerations were investigated using microsampling and laser microprobe 40Ar/3 9Ar dating. Total fusion age determinations of several groups of growth ban ds of a late Oligocene densely layered cryptomelane from the Woodie Woodie Mn deposit (NW Australia) suggest an averaged growth rate of 0.8 +/- 0.3 mm /m.y. However, the growth rate calculated for much of the band sequence is 1.5 mm/m.y., which is five times greater than for the remaining youngest gr oup of bands (0.3 mm/m.y.), thus suggesting uneven growth rates. The low gr owth rate might be the result of very slow growth of cryptomelane and/or th e presence of hiatuses in cryptomelane precipitation. Total fusion ages measured on void-filling banded and colloform cryptomelan e in samples from a vertical profile through a mesalike deposit near Horses hoe (NW Australia) show peaks at 52 to 48, 44 to 43, 40 to 39, 35, 30 to 29 , and 7 Ma. These results are interpreted as reflecting formation of weathe ring products in the deposit over a remarkably extended period, possibly in an episodic manner related to alternation of more humid and drier climatic periods during the Tertiary. The results from this study show some evidence of episodic formation of wea thering-related manganese oxides within the deposits studied, although modi fication of age patterns by local hydrological and geomorphological factors allows only a limited degree of correlation of these periods on a regional scale.