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Open File Report 94

The regolith geology and geochemistry of the area around the Harmony Gold Deposit, (Baxter Mining Centre), Peak Hill, Western Australia

Robertson, I.D.M., Phang, C. and Munday, T.J.

Primary Au mineralisation at Harmony is associated with quartz veins but is low in sulphides. Gold and W (scheelite) are the most significant indicators in mineralised fresh rock at Harmony, supplemented by Ta and Nb. Elements associated with a phyllic alteration halo are K, Rb and Ba. Some REE, namely Eu, Lu and Yb, increase in abundance and range near mineralisation for reasons that are not clear yet. Although As and Sb are elevated, their abundances are not as great as for other, sulphide-rich mineralisation styles.

The Harmony Au deposit was completely covered by a blanket of soil and colluvium. Drilling beneath this revealed a complex regolith of weathered and partly lateritised Proterozoic basement, clay-rich valley fill sediments and colluvium. Logging the main regolith units produced a 3D regolith model which provided a valuable guide to sampling and later interpretation. The basement has been eroded and weathered and consists of mafic and ultramafic metavolcanics and fine-grained metasediments. The higher parts of this basement are of ferruginous saprolite, the axes of the palaeovalleys are largely of saprolite and mottled zone and are deeply weathered. Lateritic residuum occupies the flanks of the palaeo-relief.

The colluvium varies from 0.5 m over parts of Harmony deposit to 20 m over the palaeovalleys and probably contained some alluvium where the cover was at its deepest. It presents a significant hindrance to exploration. The base of the colluvium is complex in places, being a mixture of saprolite blocks included in what appears to have been a palaeosol.

The palaeovalleys have been infilled with smectite-kaolinite sediments, probably derived from the surrounding saprolites. Hematitic, manganiferous and dolomitic mega-mottles have developed in these sediments and the tops of some valley-fill sediments contain pisolitic structures. All this indicates intense post-depositional weathering both at the surface and at oxidation fronts within the sedimentary pile. Parts of the valley-fill sediments were eroded prior to deposition of the colluvium.

Sampling of the top of the basement (ferruginous saprolite, lateritic residuum and mottles washed from the mottled zone) on a 250 m sample spacing showed significant Au and W anomalies in the vicinity of the Harmony mineralisation. Au dispersion in the ferruginous saprolite is restricted and requires close-spaced sampling (50 m). Some elements (Si, Fe, Cr, Zr, Hf, V, Rb, Ba, As and Sb) are influenced to some extent by the distribution of regolith types. Data normalisation to the modes of background populations removed most of this dependency. Gold and W anomalies were unchanged.

The unconformity between the stripped basement and the colluvium was tested, using a 150 m sample spacing as an alternate to ferruginous saprolite sampling in the vicinity of mineralisation. This capitalised on any mechanical dispersion, soil dispersion or hydromorphic permeation along the unconformity that may have developed during or since deposition of the colluvium. Dispersions of Au, W, Ta and Nb along this interface, indicating mechanical down-slope migration, produced better anomalies than the basement sampling in this stripped environment.

The geochemistry of the valley fill sediments indicated probable leaching of Au, even where a small clay-filled palaeovalley had drained the Harmony Au deposit; however W was mechanically dispersed here. The soil fine fraction (<75 µm) may not be relied upon to locate mineralisation, even though some very weak Au anomalies appeared where the colluvial cover over the Harmony mineralisation was extremely thin (~0.5 m).

Last updated: Friday, July 21, 2000 04:24 PM


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