Open File Report 54
Geochemical background, Mt Percy, Kalgoorlie, Western Australia
In an earlier study, the distribution of gold and over 40 other
elements in the regolith was determined over the mineralized Mystery
Zone at Mt Percy. The site is typical of supergene Au deposits in
the Kalgoorlie area. However, most of the samples were from the
pit, so that only those from the western margin, including some
drill cuttings, were in unaltered and unmineralized rocks or their
weathered equivalent. None were distant from mineralization and
could provide an adequate measure of the background. A further study
has been undertaken to determine the geochemical background, using
samples from diamond and percussion drilling approximately 1000
mN of the Mystery pit, across the same stratigraphy. The site was
not ideal because, unlike the Mystery Zone, the regolith has been
partly eroded and there is no lateritic duricrust or gravel horizon.
Nevertheless, it provides useful comparative data for much of the
regolith over barren or weakly mineralized equivalents of the rocks
of the Mystery Zone.
The unweathered rocks in the background site consist of talc-chlorite-carbonate
rocks of the Hannan's Lake Serpentinite, intruded by felsic porphyries.
Unlike the Mystery Zone, there is no fuchsite-quartz-carbonate alteration
and the rocks are essentially barren, with a maximum Au content
of 8 ppb. The weathering front is at about 60 m but the full regolith
profile could not be sampled as all percussion holes were terminated
in saprolite at about 34 m vertical depth. The regolith consists
of strongly leached saprolite that becomes softer and increasingly
clay-rich towards the surface, merging with plasmic and mottled
clays at about 10 m depth. These form a transitional zone, up to
4 m thick, between the saprolite and an almost uniform cover of
non-calcareous and calcareous red clay soils. The regolith samples
are from a traverse about 150 m south of the barren unweathered
rocks of the diamond drill section and contain minor Au mineralization,
associated with now-weathered fuchsite-altered ultramafic rocks.
For the unweathered rocks and saprolites, comparisons between the
background site and mineralization in Mystery Zone indicate:
- In both fresh and weathered ultramafic rocks, alteration is
indicated by fuchsite (chromian muscovite) and by elevated concentrations
of K, Ba and V. Altered porphyries can possibly be distinguished
by their higher K contents.
- Unmineralized talc chlorite ultramafic rocks in the Mystery
Zone are relatively enriched in As and Sb compared to their equivalents
in the background area. This may indicate widespread weak primary
dispersion from mineralization into apparently barren wallrocks.
The enrichment continues into the saprolite.
- Unweathered mineralized porphyries have enhanced K, V, Au, As,
W and Sb contents relative to unaltered background porphyries.
However, although Au, As and W abundances are greater in the saprolite
in the Mystery Zone, there appears to be no significant difference
in Sb content compared to the minor background mineralization.
- Weathered fuchsitic ultramafic rocks of the Mystery Zone are
enriched in Au, As, W and Sb relative to background.
- Overall, in addition to Au, mineralized saprolite is indicated
by As (contrast x10 compared to barren talc-chlorite ultramafic
rocks), Sb and W (contrasts x2, but x6 in porphyries). Regional
scale thresholds are 20 ppb Au, 10 ppm As, 4 ppm Sb and 3 ppm
W; local thresholds are rather greater, i.e., 90 ppb Au, 35 ppm
As, 8 ppm Sb, 6 ppm W.
In the clay-saprolite and mottled clay horizons
broadly similar conclusions apply, except that Au may be leached
and that W and, particularly, Sb tend to be more strongly concentrated
than in the saprolite, particularly over the porphyries. However,
data from these horizons are too few to provide an adequate comparison.
The red clay soils at the background site do not retain a clear
indication of their parent lithologies, unlike immediately underlying
horizons, and are, therefore, probably transported. The data indicate
that Au becomes concentrated in the calcareous surface horizons,
although it is not certain whether the anomaly represents a widespread
enrichment (local threshold about 30 ppb) or is related directly
to the concealed minor mineralization deeper in the regolith. The
occurrence of Au anomaly in calcareous soils overlying buried mineralization
is consistent with the findings of several other investigations
in the southern Yilgarn Block.
Last updated: Thursday, January 06, 2000 09:02 AM