Open File Report 35
Exploration geochemistry about the Mt Gibson Gold Deposits, Western
Australia. Progress to 31st March 1989
Anand, R.R., Smith, R.E., Innes, J. and Churchward, H.M.
A regolith, landform, and geochemical orientation study about the
S, C, and N lateritic Au deposits at Mt Gibson clarifies landscape
evolution and geochemical dispersion in terms of the dynamics of
formation, preservation, and dismantling of the undulating lateritic
The relatively complex regolith and vegetation patterns are explained
in terms of the distribution of (i) sub-areas of erosion of the
lateritic mantle to the level of saprolite, (ii) sub-areas of essentially-complete
lateritic mantle, and (iii) sub-areas characterized by depositional
accumulation of detritus provided by the dismantling of the lateritic
mantle up-slope, commonly burying the essentially complete laterite
weathering profile in the local foot slopes and lowlands.
The regolith units were mapped over the central 3 km by 5 km area,
the regolith stratigraphy established, and units of the upper regolith
were characterized in field profiles petrographically, mineralogically,
and chemically. An idealized regolith-landform facies model has
been erected for use in predictions in appropriate terrain, and
for planning and integrating follow-up research. Geochemical analyses
of samples of the loose pisolitic, nodular laterite unit collected
systematically both from surface and from pit walls, where the unit
occurred sub-surface, document the characteristics of the lateritic
Au deposits. These and earlier results of the orientation study
show that the lateritic Au ore, and the area peripheral to it, is
a multi-element, chalcophile, geochemical anomaly, measuring 1-1.5
km across and greater than 4 km in length, with a Au, Ag, Pb, As,
Bi, Sb, W association.
Within the loose lateritic unit, and in the underlying duricrust,
coincident highs of several of these elements in centres within
the overall anomaly suggested a close genetic link with bedrock
sources, now verified by occurrences of gold-bearing quartz-hematite
veining in saprolitic bedrock revealed by exploration and mining.
Geochemical results of 37 samples of the underlying lateritic duricrust
show similar strengths and associations of elements as seen for
the unit consisting of loose lateritic pisoliths and nodules. Close
comparisons of the geometry of the dispersion patterns of these
two closely related regolith units await the result of current research
on more extensive sampling of the duricrust.
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