Transported regolith has the capacity to mask underlying mineralisation by restricting the migration of most trace elements to the surface. Oxidation of sulphides generates highly mobile H+ which may migrate to surface, resulting in alteration of minerals and redistribution of elements within transported regolith cover. A detailed geochemical and geophysical study has been conducted at the Mandamah Cu-Au deposit in central-western New South Wales, where sub-economic mineralisation is covered by ~50 m of transported regolith and ~30 m of in situ regolith. A shallow-penetration electromagnetic survey was undertaken on nine transects and detailed mineralogical and selective extraction/ICP-MS geochemical analysis performed on regolith samples obtained from 107 soil cores and 16 pits spanning three extensive traverses across buried mineralisation. The selective extractions used were ammonium acetate, hydroxylamine.HCl and aqua regia. A distinct vertical zonation exists in the upper two metres of the transported regolith cover across the site and is related to soil mineralogy, soil pH, electrical conductivity and the amount of selectively extractable elements using the different geochemical extractions. The upper zone of near-neutral soil pH contains organic material but little carbonate; the intermediate high-pH zone has up to 2% Mg-calcite; the underlying low-pH zone displays Fe mottling. This zonation results from precipitation of salts due to evaporation, changes in redox potentials and accumulation of organic materials, in an otherwise relatively homogeneous quartz-clay alluvium. Ground conductivity measurements and selective extraction geochemistry display a strong response to parts of the underlying mineralisation. The principal signature is the depletion of Ca, S and Na, a reduction in the cation exchange capacity, the presence of non-carbonate alkalinity and a low electrical conductivity. A model to account for these patterns has been developed and involves a "prograde" stage of alteration of clay mineralogy and a redistribution of carbonates and various trace elements due to the development of an "acid chimney" above the oxidizing mineralisation during a period of elevated water tables and a "retrograde" stage involving a redistribution of some mobile elements back into the former acid chimney zone following the onset of more arid conditions. The results of this research demonstrate that the effects of sulphide mineralisation on the upper transported regolith at Mandamah can be detected using a combination of selective extraction geochemistry and shallow depth conductivity measurements. This technique has potential application in similar arid to semi-arid terrains.
| Identifer | oai:union.ndltd.org:ADTP/215611 |
| Date | January 2007 |
| Creators | Mokhtari, Ahmad Reza, School of Biological, Earth & Environmental Sciences, UNSW |
| Source Sets | Australiasian Digital Theses Program |
| Language | English |
| Detected Language | English |
| Rights | http://unsworks.unsw.edu.au/copyright, http://unsworks.unsw.edu.au/copyright |
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