Abstract

Under particular climatic conditions, tetravalent K-Mn oxides can be formed during terrestrial chemical weathering of Mn ores. In this study the results of K/Ar and 40 Ar/ 39 Ar dating of weathering-related K-Mn oxides (mainly cryptomelane (K.Ba)Mn 8 O 16 .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 presented. In ideal circumstances, K/Ar and 40 Ar/ 39 Ar ages will reflect the time since the formation of K-Mn oxides. However, 40 Ar/ 39 Ar 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 generations of weathering-related K-Mn oxides. To correct for admixture of inherited mica a two-stage K/Ar dating technique was applied. Some samples of dated cryptomelane had finely banded textures, and therefore might represent several generations. The age differences between these generations were investigated using microsampling and laser microprobe 40 Ar/ 39 Ar dating. Total fusion age determinations of several groups of growth bands of a late Oligocene densely layered cryptomelane from the Woodie Woodie Mn deposit (NW Australia) suggest an averaged growth rate of 0.8+ or -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 group of bands (0.3 mm/m.y.), thus suggesting uneven growth rates. The low growth rate might be the result of very slow growth of cryptomelane and/or the presence of hiatuses in cryptomelane precipitation. Total fusion ages measured on void-filling banded and colloform cryptomelane in samples from a vertical profile through a mesalike deposit near Horseshoe (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 weathering 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 weathering-related manganese oxides within the deposits studied, although modification of age patterns by local hydrological and geomorphological factors allows only a limited degree of correlation of these periods on a regional scale.

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