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