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