CRC LEME
Open File Report 26
ABSTRACT
Multi-element dispersion in the saprolite at the Beasley Creek
Gold Mine, Laverton, Western Australia
Robertson, I.D.M.
A fence of 11 inclined percussion drill holes, which crossed from
the hangingwall to the footwall of the Beasley Creek Orebody, was
selected for study of geochemical dispersion in the near surface
materials and in the deep saprolite. Of these, four drillholes intersected
the ore, its immediate foot- and hangingwall rocks and overlying
calcretes, another four intersected amphibolitic saprolites and
clay-rich materials proximal to the ore (40-160 m distant) and a
further three cut amphibolites and amphibolitic saprolites distal
to the ore (250-400 m distant). Samples were selected from these
to maximise geochemical detail near the surface and to give adequate
information to bedrock or to the limit of the drilling (60-100 m).
They were analysed for eight major and 28 trace elements.
The zone around the orebody has been intensely weathered to below
the depth of current diamond drilling (230 m) but amphibolites 400
m to the west are only weathered to a depth of 40 m. This locally
intense weathering appears to be due to acid conditions generated
by the weathering of sulphides in the orebody and its sulphidic
host rock. The surficial materials immediately to the east of the
ore host unit (to a distance of at least 75 m) contain calcrete
and some gypcrete.
Some Rb- and muscovite-rich rocks in the footwall, enriched in
Au, As, Cd, Cu and Pb and sporadically enriched in Ag, Bi, Mo, Sb,
Sn and W, form an ill-defined zone about 100 m wide, 150 m to the
west of the orebody, at a depth of 20-50 m. This zone, which straddles
the saprolite-saprock boundary, appears to be a partly weathered
phyllic alteration halo which apparently has not been fully investigated
by drilling.
Apart from Au, the orebody is characterised by elevated concentrations
of Ag, As, Cd, Cu, Pb, Sb, W and Zn. The lateritic duricrust and
mottled zone are weakly enriched in Ag, Nb and W but are generally
strongly enriched in As, Bi, In, Pb, Sb, W and Sn which enhances
the value of these elements as pathfinders. Cobalt, Zn and Cu tend
to be depleted near the surface, reducing their effectiveness as
pathfinders but they show some enrichment below the mottled zone.
Bismuth, Ge and In are at low abundances and have only sporadic
anomalies so, despite some surficial enrichment, they are not very
effective near the surface. Both Pb and Sb are also at low abundances
but they tend to be strongly concentrated a little further below
the surface, so their surficial anomalies are not readily interpreted.
High concentrations of Al, Fe, Ba, Ce, Cr, Ga, Mn, Ni, Rb, V and
Y reflect the composition of the host lithology.
Last updated: Tuesday, January 04, 2000 03:29 PM
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