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Open File Report 105

Geochemical dispersion in transported and residual regolith, Fender Gold Deposit, Cue, Western Australia

Butt, C.R.M.

Fender is a small Au deposit (248 000t @ 2.4 g/t Au) approximately 2 km south of Big Bell, WNW of Cue, on the margin of a colluvial-alluvial plain. The deposit itself is entirely overlain by a thin (2-5 m) cover of transported overburden and does not outcrop. The overburden consists of two units, fine- to coarse-grained sandy clay, sand and gravel, overlying silty clays. Both the sands and the silty clays locally contain detrital lateritic gravels. The sands are weakly cemented in the top metre to form hardpan and some deeper sediments are mottled; there is no pedogenic carbonate. The sediments contain feldspar grains (0.5-1.0 cm) and are probably derived from the granites to the west. There are two principal regolith situations beneath the overburden. In the south, the lateritic profile appears largely complete and a small Au resource is hosted by lateritic residuum and ferruginous saprolite. In the north, the lateritic profile is truncated and the sediments are deposited on saprolite which, in some places, is depleted in Au but, in others, has Au at ore-grade concentrations immediately beneath the unconformity. Similar situations are present 200 m to the west, where lateritic residuum and saprolite outcrop. There is no surface geochemical expression of the deposit in soils (15-30 cm depth), determined by conventional total analysis or by bulk cyanide leach (BLEG) analysis, nor in composite samples (4 m) of the sediments, except where drilling has penetrated into the concealed lateritic residuum. Possible geochemical dispersion into the sediments has been investigated by careful sampling of the sediments and uppermost residuum by RAB drilling, with care taken to avoid cross-hole contamination. The primary mineralization (Au >1000 ppb) is characterized by enrichment in Ag (mean 1.4 ppm), As (145 ppm), Sb (450 ppm), W (130 ppm), Cd (1.2 ppm) Mo (37 ppm), Tl (7 ppm), Zn (475 ppm) and Hg (100 ppm). However, of these, only As, Sb and Ware detectable in the near-surface samples. In the residuum, the W distribution indicates the weathered primary mineralization, even where Au has been depleted in saprolite or enriched and dispersed in lateritic residuum. In comparison, As and Sb are both enriched and widely dispersed in the nodular ferruginous clays and ferruginous saprolite to give broad anomalies. Concentrations are homogeneous and remain anomalous (>50 ppm As, 70 ppm Sb) in shallow ferruginous saprolite and outcropping lateritic gravels for at least 200 W of the subcropping mineralization. Gold abundances are generally <5 ppb in the sediments over saprolite, except for some spot concentrations (80-245 ppb) immediately above subcropping mineralization, and an associated weak enrichment (5-16 ppb) extending 50 m down slope. However, they are significantly anomalous (60 ppb Au) for over 100 m east of the subcropping lateritic residuum. In comparison, As (40-120 ppm), Sb (12-50 ppm) and, over saprolite, W (5-17 ppm) are anomalous in the clays for at least 200 m down slope to the east. Essentially all of the As and Sb in the silty clay unit is hosted by mechanically transported ferruginous nodules (80 to over 100 ppm Sb, 300-450 ppm As). Neither Au nor W are concentrated in the nodules. The sands and soils contain background concentrations of Au, As, Sb and W except where directly overlying lateritic residuum. It is concluded that a sampling strategy that targets lateritic residuum, whether outcropping or buried, would locate this deposit. Where the regolith is truncated, restricted dispersion in residual and transported units implies that analysis for Au alone is unsatisfactory in the top 20 m. Multi-element analysis would reveal broad, low-order As+Sb+/-W anomalies in the silty clays, which can be markedly enhanced by selective sampling of ferruginous nodules. Such selective sampling is preferable to the common practice of compositing samples over intervals of 2-6 m. However, composites of ferruginous nodules from the lowermost 2-4 m of the sediments may be suitable, especially if they are scarce. The preferential concentration of As and Sb in detrital ferruginous nodules, rather than in the matrix, of the silty clays, implies that there has been little or no post-depositional chemical dispersion.

Last updated: Sunday, August 05, 2001 13:59:33


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