Abstract

Geochemical dispersion in the regolith was investigated at the Bronzewing gold deposit in a variety of geomorphological environments. A series of economic gold deposits are concealed by 5–35 m of sediments, and a thick residual profile about 100 m thick. Prior to mining, samples of regolith materials were collected from surface and drill cuttings, along several lines over the north–south trending, elongate zone of mineralization. These samples were examined in detail to determine element distributions, their relationship to regolith evolution and their significance to exploration.

The residual materials (lateritic residuum and ferruginous saprolite) have Au, W and Cu anomalies in the vicinity of the deposit. Elements associated with the Au mineralization in the lateritic residuum of the Laterite Pit are Ag, Ba, Ce, W, Mo, As, Sb and Cu. Gold anomalies in lateritic residuum are less consistent in the Discovery Pit and, at both the Central and Discovery Pits, are enriched in Cu and W only close to the weathered primary mineralization. Three-dimensional modelling indicates that Au in saprolite is localized and largely residual. Geochemical dispersion from primary mineralization into the sediments is only minor, except immediately overlying the lateritic residuum or saprolite. This dispersion is, however, almost entirely mechanical and there appears to have been little, if any, post-depositional chemical dispersion. Ferruginous mottles developed in palaeochannel sediments have very low Au contents (<5 ppb) but are slightly enriched in Cu. There is a significant concentration of Au (up to 300 ppb) in authigenic smectites formed at the residual-transported interface. The deposits are blind to soil sampling (10–30 cm depth) using total and several partial extraction techniques.

The results demonstrate that dispersion of ore-related elements at Bronzewing is mostly confined to the lateritic residuum and ferruginous saprolite and that sampling of these materials, even if buried beneath transported overburden, will readily locate deposits of this type. However, it is important to distinguish between residual and transported regolith, especially nodules and pisoliths. In the absence of residual materials, sampling of the lowermost sedimentary units (gravelly sediments), possibly including the unconformity, is recommended because it may show the geochemical dispersion that is wider than the primary anomaly in saprolite or bedrock. The results also provide strong support for the use of multi-element geochemistry, with elements such as Cu and W being enriched close to primary mineralization.

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