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Open File Report 56

Petrology and geochemistry of surface materials overlying the Bottle Creek Gold Mine, WA

Robertson, I.D.M. and Wills, R.

The tectonically-related mineralisation at Bottle Creek is a chalcophile, pathfinder-rich Au-Ag deposit. Gossans contain anomalous Ag, As, Au, Cu, Pb, Sb and Zn. The mineralised zone at Bottle Creek strikes for 5 km and traverses a complex geomorphic environment consisting of residual lateritic duricrust, partly covered by colluvium, parts where the duricrust has been partly or wholly stripped to saprolite, areas where laterite and stripped profiles are buried beneath sheet-flood colluvium-alluvium and numerous, more recent drainage channels, now filled with fluvial gravels. Most profiles have been hardpanised to a varying extent. The study area lies in an arid area of mulga scrub, very close to, but north of, the Menzies Line.

Within the broad catchments, regolith-landform relationships have been integrated with both the original exploration data and with CSIRO orientation geochemistry to test the effectiveness of surficial sampling in residual, erosional and depositional areas. The two study areas, Emu and VB Boags, lie in contrasting residual-erosional and depositional regimes respectively. In the former area, geochemical sampling of laterite and lag gives broad indications of mineralisation typical of lateritic areas. Indications from the latter are very dependent on localised thinning of the cover, on mechanical dispersion and on bioturbation carrying ferruginised saprolite and hardened, ferruginous mottles to the surface.

The best indicator elements are As > Sb > Au > Pb. The width of the As anomaly is 1200 m, that of Pb is 400 m. A phyllic halo surrounds the Emu mineralisation and this is detectable mineralogically as remnant muscovite in the lag by both XRD and petrography and geochemically as a K halo with minor Na and Ba. At VB-Boags, the phyllic halo is very much reduced in size and patchy, as are halos in As, Sb, Au and Pb. A sediment-filled fluvial channel, between VB and Boags, has truncated all geochemical anomalies.

A very brief investigation indicated that the coarse soil fraction would be as effective as the lag; the choice between these sampling media would be one of cost and convenience.

Regolith-landform mapping provides an essential basis for planning geochemical prospecting. In the residual-erosional lateritic terrain, surrounding the Emu Pit, wide-spaced (up to 1 km) lag sampling on a triangular grid, using As, Sb, Au and Pb as indicator elements will delineate the mineralised zone. In thinly-covered depositional regimes (for example VB-Boags), lag may be used at a much reduced sample spacing (e.g., 50-100 m) but there are significant attendant risks, in that success is very dependent on the cover being thin. Thin cover may, in part, be due to induration of the weathered mineralisation which tends to form positive features in the palaeotopography.

Last updated: Thursday, January 06, 2000 09:04 AM


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