Open File Report 58
Geochemical exploration in complex lateritic environments of
the Yilgarn Craton, Western Australia. Final Report
Anand, R.R., Smith, R.E., Phang, C., Wildman, J.E., Robertson,
I.D.M. and Munday, T.J.
The objectives of this two-year extension project were to develop
and improve methods of finding mineral deposits (particularly gold
and base metals) in lateritic environments of the Yilgarn Craton,
using geochemistry within a sound regolith-landform framework. The
project focussed on more complex lateritic environments than its
precursor, by including a study of the Kalgoorlie region and areas
of transported cover overlying variably truncated laterite profiles
elsewhere. The multidisciplinary project team augmented its skills
in remote sensing and regolith mapping through collaboration with
the newly formed Co-operative Research Centre in Australian Mineral
The Kalgoorlie region
A regolith-landform framework for the Kalgoorlie region was established
over an area of 10 000 km2. Landscapes were shown to be characterised
by extensive stripping of upland areas, very shallow to deep weathering,
patchy development of lateritic residuum and extensive occurrence
of calcareous and acid, red clays, which commonly contain ferruginous
granules. Erosional stripping of the upper, more weathered, parts
of the regolith appear to be important factors, coupled with the
distribution of mafic and ultramafic rocks, influencing the gross
distribution of carbonates in the regolith. Results were then compared
and contrasted with other parts of the Yilgarn Craton.
Eight additional geochemical orientation studies were completed
and set within their district-scale regolith-landform setting. In
the Kalgoorlie region these were: Ora Banda (Matt Dam), Kanowna
Belle and Wombola; and elsewhere: Boddington, Mt McClure, Lawlers
(Waroonga and Genesis), Bottle Creek, Golden Grove, Beasley Creek
and Lights of Israel. All are Au deposits except Golden Grove, a
Cu-Zn-Au volcanic-hosted massive sulphide deposit.
Systematising regolith-landform mapping units
A framework for classification of regolith-landform mapping units
was established which links landform with regolith stratigraphy.
Most regolith-landform mapping units so far encountered in the Yilgarn
are accommodated and can be compared with each other. An accompanying
atlas aids recognition of regolith units being mapped or sampled.
Developments in new sample media
Several new sample media were demonstrated to have application
in exploration, particularly where lateritic residuum is missing.
These include ferruginous saprolite, iron segregations, ferruginous
mottles in upper saprolite, ferruginous granules in soils (including
soils on transported overburden) and ferruginous mottles in cover
Ferruginous granules in soil. It was shown at Kanowna Belle, that
near-surface ferruginous granules hold trace element indicators
(As, Sb, Au) of the Au deposit forming a broad (>400 m wide)
anomaly in the thin, transported cover overlying the deeply leached,
partly truncated profile. These granules are a distinctive sample
medium, quite separate from lateritic nodules. Their use holds considerable
promise in exploration for Au and base metals in areas of deeply
leached, partly truncated profiles, with or without the presence
of transported cover, situations which are common in the Kalgoorlie
Ferruginous saprolite. At Matt Dam, ferruginous saprolite gave
a strong multi-element anomaly (Au, As, W) over the deeply leached,
partly truncated profile and the concealed Au mineralisation are
similar to Lawlers. Ferruginous saprolite is common, in both the
Kalgoorlie and Leonora-Wiluna regions, and forms a useful sampling
medium for reconnaissance. It is a useful medium for target definition
by sampling at surface or by drilling through sedimentary cover.
Ferruginous mottles in cover sediments. It was shown by the research
at the Kanowna deep leads and the Matt Darn prospect that such ferruginous
mottles in the vicinity of concealed Au deposits can carry anomalous
As, Sb, W, and Au, apparently generated through hydromorphic dispersion
(As, Sb, and Au) during weathering after sediment deposition.
Dispersion models and exploration procedures
Regolith and geochemical dispersion findings have been integrated
into new exploration models for the following settings: (i) full
lateritic profile, high seasonal rainfall, no surficial cover (based
on Boddington); (ii) full lateritic profile, semi-arid to arid climate,
little or no cover (based on Golden Grove, VHMS Cu-Zn; Mt Gibson,
Bottle Creek and Beasley Creek). Updated models were generated for:
(iii) buried, complete profile, arid terrain (based on Mt McClure
and Waroonga), (iv) buried, partly truncated profile, arid terrain
(based on Kanowna Belle, Matt Dam, Kanowna deep leads, Bottle Creek,
Boags, Mt McClure, Waroonga and Genesis).
Systematic differences in the distribution of Au between the high
and low rainfall areas were revealed by the Boddington orientation
study, in comparison with the Mt McClure and Lawlers Au orientations.
The surface depletion of Au in the upper ferruginous part of the
profile at Boddington is opposite to that found at Mt McClure and
Lawlers. However, despite surface depletion of Au at Boddington,
Sn, W, Mo, and As are strongly anomalous throughout the profile.
Research findings were transferred to sponsors through production
of research reports, review meetings, workshops and a field trip.
Research projects for nine Bachelors Degree Honours students, in
regolith geology, geochemistry or geophysics, were an integral part
of project strategy, providing an additional, effective mechanism
of experience transfer to industry.
Last updated: Thursday, January 06, 2000 09:10 AM