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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 Exploration Technologies.

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.

Orientation studies

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 sequences.

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 region.

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


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