CRC LEME
Open File Report 94
ABSTRACT
The regolith geology and geochemistry of the area around the
Harmony Gold Deposit, (Baxter Mining Centre), Peak Hill, Western
Australia
Robertson, I.D.M., Phang, C. and Munday, T.J.
Primary Au mineralisation at Harmony is associated with quartz
veins but is low in sulphides. Gold and W (scheelite) are the most
significant indicators in mineralised fresh rock at Harmony, supplemented
by Ta and Nb. Elements associated with a phyllic alteration halo
are K, Rb and Ba. Some REE, namely Eu, Lu and Yb, increase in abundance
and range near mineralisation for reasons that are not clear yet.
Although As and Sb are elevated, their abundances are not as great
as for other, sulphide-rich mineralisation styles.
The Harmony Au deposit was completely covered by a blanket of soil
and colluvium. Drilling beneath this revealed a complex regolith
of weathered and partly lateritised Proterozoic basement, clay-rich
valley fill sediments and colluvium. Logging the main regolith units
produced a 3D regolith model which provided a valuable guide to
sampling and later interpretation. The basement has been eroded
and weathered and consists of mafic and ultramafic metavolcanics
and fine-grained metasediments. The higher parts of this basement
are of ferruginous saprolite, the axes of the palaeovalleys are
largely of saprolite and mottled zone and are deeply weathered.
Lateritic residuum occupies the flanks of the palaeo-relief.
The colluvium varies from 0.5 m over parts of Harmony deposit to
20 m over the palaeovalleys and probably contained some alluvium
where the cover was at its deepest. It presents a significant hindrance
to exploration. The base of the colluvium is complex in places,
being a mixture of saprolite blocks included in what appears to
have been a palaeosol.
The palaeovalleys have been infilled with smectite-kaolinite sediments,
probably derived from the surrounding saprolites. Hematitic, manganiferous
and dolomitic mega-mottles have developed in these sediments and
the tops of some valley-fill sediments contain pisolitic structures.
All this indicates intense post-depositional weathering both at
the surface and at oxidation fronts within the sedimentary pile.
Parts of the valley-fill sediments were eroded prior to deposition
of the colluvium.
Sampling of the top of the basement (ferruginous saprolite, lateritic
residuum and mottles washed from the mottled zone) on a 250 m sample
spacing showed significant Au and W anomalies in the vicinity of
the Harmony mineralisation. Au dispersion in the ferruginous saprolite
is restricted and requires close-spaced sampling (50 m). Some elements
(Si, Fe, Cr, Zr, Hf, V, Rb, Ba, As and Sb) are influenced to some
extent by the distribution of regolith types. Data normalisation
to the modes of background populations removed most of this dependency.
Gold and W anomalies were unchanged.
The unconformity between the stripped basement and the colluvium
was tested, using a 150 m sample spacing as an alternate to ferruginous
saprolite sampling in the vicinity of mineralisation. This capitalised
on any mechanical dispersion, soil dispersion or hydromorphic permeation
along the unconformity that may have developed during or since deposition
of the colluvium. Dispersions of Au, W, Ta and Nb along this interface,
indicating mechanical down-slope migration, produced better anomalies
than the basement sampling in this stripped environment.
The geochemistry of the valley fill sediments indicated probable
leaching of Au, even where a small clay-filled palaeovalley had
drained the Harmony Au deposit; however W was mechanically dispersed
here. The soil fine fraction (<75 µm) may not be relied
upon to locate mineralisation, even though some very weak Au anomalies
appeared where the colluvial cover over the Harmony mineralisation
was extremely thin (~0.5 m).
Last updated: Friday, July 21, 2000 04:24 PM
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