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Open File Report 104

Geochemical expression of concealed gold mineralization, Safari Prospect, Mt Celia, Western Australia

Bristow, A.P.J., Lintern, M.J. and Butt, C.R.M.

The distribution and solubility of Au and the distribution of other elements in the upper regolith and the nature and distribution of regolith materials has been studied at and around the Safari deposit, 200 km NE of Kalgoorlie, Western Australia. The deposit has a resource of 1.08 Mt @ 3.3 g/t Au associated with quartz veins within sheared metavolcanic rocks (mainly quartz-chlorite-sericite schists) in the southern extension of the Laverton Tectonic Zone. The mineralised and country rocks are deeply weathered, and subsequently eroded to the lower saprolite and, in places, fresh rock. They are now covered by up to 9 m of colluvium/alluvium derived from eroding greenstone and granite regolith several km to the ENE. Groundwater at the deposit is approximately 40 m below surface. Much of the regolith, and particularly the upper few metres, shows evidence of widespread and intensive modification associated with arid conditions (namely calcification from approximately 0.5-4.0 m below surface). Samples of regolith (transported overburden and the upper two metres of saprolite) from a drill traverse across the deposit, and selected samples of primary mineralisation and vegetation have been examined to determine element distributions, their relationship to regolith evolution and their significance in exploration. Elements associated with Au mineralisation are W, As, Zn, S, Pb, Si, heavy REE and possibly Sb, though only Pb displays a direct correlation with Au. Of these, (excluding Pb, S and Si, not analysed in the regolith), only Au shows evidence of remobilisation in the regolith, the others being closely confined to the mineralised zones in the weathered Archaean. Plant materials were found to be ineffective in detecting mineralisation.

The distribution and solubility of Au in the regolith suggest that Au has been dissolved from mineralisation in the top of the saprolite and re-precipitated in the transported overburden close to the surface, in association with carbonate minerals. Gold is strongly enriched at the top of the saprolite (10-l000 ppb for 500 m peaking over mineralisation), and solubility is generally high, although appears lower close to mineralisation (<30% iodide soluble), increasing with lateral distance from mineralisation (up to 80% iodide soluble), suggesting secondary lateral dispersion around the unconformity. In contrast, Au is anomalous (7-40 ppb), and highly soluble (>70% iodide soluble) throughout the calcareous horizon, and close to surface, for up to 800 m across strike, peaking directly over mineralisation, suggesting dominantly vertical chemical dispersion. This may be the result of two phases of Au dispersion: the first, prior to sedimentation, resulted in lateral dispersion of Au down the relatively steep palaeo-surface; the second, after sedimentation, resulted from solution of Au from the carbonates in the upper saprolite and vertical dispersion by capillary action and/or evapo-transpiration, resulting in evaporative precipitation with carbonate 0.5-4 m from the surface.

It is suggested that a combination of a shallow overburden and a source at the top of the saprolite have contributed to the formation of Au anomalies in the transported overburden. These factors need to be considered if carbonate sampling is to be applied to other depositional landform regimes.

Last updated: Sunday, August 05, 2001 13:55:33


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