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Open File Report 50

Laterite geochemistry for detecting concealed mineral deposits, Yilgarn Craton, Western Australia. P240 Summary Report

Smith, R.E., Anand, R.R., Churchward, H.M., Robertson, I.D.M., Grunsky, E.C., Gray, D.J., Wildman, J.E. and Perdrix, J.L.

The objective of this four-year AMIRA study was to develop new and improved methods for mineral exploration based upon the use of multi-element laterite geochemistry. The project focussed upon using the knowledge that geochemical dispersion haloes in laterite greatly enlarge the target size in the search for concealed ore deposits. Laterite geochemistry was known to be applicable to a wide range of commodity types, including precious-, base-, rare-, and strategic-metal deposits. A multidisciplinary team was established with skills in regolith geology, geomorphology, soil science, exploration geochemistry, bedrock geology, numerical geology, hydrogeochemistry, and, through collaboration, remote sensing.

Orientation studies

Considerable emphasis was placed on establishing a regolith-landform understanding of four carefully chosen orientation districts, within which geochemical dispersion from concealed Au deposits was studied. The orientation districts, Mt Gibson, Bottle Creek, Boddington, and Lawlers range from 120 km2 to 500 km2 in area, represent a range of regolith and geomorphic settings distributed across the present-day rainfall gradient of the Yilgarn Craton. The Beasley Creek Au deposit was the focus of a subsidiary orientation study.

Specific themes

Research was also directed at the following specific themes: establishing (a) a workable scheme for terminology and classification of laterites for the Yilgarn Craton, (b) the siting and bonding of ore-associated elements in laterites, (c) processes of geochemical dispersion, (d) anomaly recognition methods for use with multi-element laterite geochemistry, (e) knowledge of regional variations in laterite geochemistry, (f) geochemical dispersion models, and (g) participating in exploration feasibility tests based on the research findings.

Exploration applications

Study of the chosen orientation districts allowed a substantially-improved understanding of the distribution of regolith units, regolith stratigraphy, the characteristics of regolith units, regolith evolution, and of geochemical dispersion patterns in laterite. This understanding is fundamental to effective exploration and has progressively been transferred to industry users through field trips, sponsors' meetings, project reports, and workshops.

Laterite classification and sampling methods

The major findings of the project concern the characteristics, origin and use of the lateritic residuum as a sample medium in exploration. The project's volume Laterite types and associated ferruginous materials - terminology classification and atlas was an important practical outcome and is in widespread use amongst the sponsorship both in Australia and overseas.

Considerable attention was given to the question of where best to sample within the regolith stratigraphy. The spacing of samples and sampling patterns with regard to anomaly size have been integrated into general procedures. Knowledge of which parts of laterite blankets to sample is now well established from the results of this project. This knowledge has been extended, at pilot scale, to ferruginous material immediately underlying lateritic residuum, (materials such as mottled zone, ferruginous saprolite, and various iron segregations). Such media are important alternatives in areas where erosion has removed the lateritic residuum.

Data interpretation

The geochemical dispersion anomalies, studied within the orientation areas, have been comprehensively described. Sample type and geochemical characteristics are linked to the position within the regolith stratigraphy. These studies provide well-controlled reference geochemical data sets and thus provide important information on the geochemical characteristics for the settings and ore types studied. When coupled with knowledge of the background variation and thresholds for the target pathfinder elements, as provided in the CSIRO-AGE database, these data sets provide the essential components of a powerful exploration system. A discussion document on anomaly recognition methods for multi-element laterite geochemistry, complete with worked-through examples from the project, was produced and distributed to sponsors.

Areas of sedimentary cover

The use of laterite geochemistry in areas of sedimentary cover, particularly for exploration of substantial colluvial and alluvial outwash plains, was developed during the project. Application of these techniques, in collaboration with sponsors, proved the feasibility of exploration in sediment-covered areas by drilling for geochemical haloes in buried laterite, capitalizing on their far larger size than the source ore deposits. An important outcome of this phase of the project has been the recognition that, in the semi-arid and arid parts of the Yilgarn Craton, buried laterite profiles are widespread, and can occur even in areas where lateritic residuum has been largely stripped from the uplands.

Overall, it is clear that the level of expertise in laterite geochemistry used by industry is now substantially greater than when the project commenced in mid-1987. In collaboration with other researchers, project staff have played a major role in improving this expertise, developing application capabilities, and transferring these skills to sponsors.

Last updated: Thursday, January 06, 2000 08:54 AM


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