Digging for gold

Musings on:
Anand RR, Cornelius M, Phang C, 2007. Use of vegetation and soil in mineral exploration in areas of transported overburden, Yilgarn Craton, Western Australia: a contribution towards understanding metal transportation processes.Geochemistry-Exploration Environment Analysis, 7: 267-288.

The use of soils and vegetation as sample media for geochemical exploration is not new. In recent years there seems to have been a resurgence of interest in sampling soils, and particularly plants, with the expectation that these media carry a signature of underlying ore bodies. In many cases the rock or regolith which hosts the ore body may be masked by transported sediments, so that even the weathered ghostly imprint of an ore may lie well below the surface.

Several mechanisms may allow the formation of signatures of ore bodies in surface soils, even in a transported overburden. In addition to purely chemical or physical processes, it is possible that plants can access ore-forming elements at depth, as deep roots search the regolith for water. Not only can this give anomalous concentrations of trace elements in plant tissues, but biological re-cycling can subsequently enrich surface soils with a signature of what lies beneath.

These ideas are the basis for Anand et al's. article. They took vegetation (mulga; Acacia aneura and related species) and soil samples across known but unexploited gold and/or base-metal (Cu, Zn, Ag) deposits in Western Australia, where much of the bedrock is covered by deeply weathered regolith profiles and, in many cases, transported sediments as well. They found that analysis of soils did not clearly delineate the vertical projection of underlying ores (even using selective extractions for trace elements which commonly improve signal:noise ratios). In contrast the living plant tissues, and particularly the litter layer, showed clear signatures of underlying mineralization, with peak concentrations in both media vertically over the ore body.

These results are of obviously practical importance to exploration geochemists, demonstrating that (at least in some instances) vegetation sampling and analysis offers an additional tool to "see through" weathered and/or transported material to ore bodies beneath. It is possibly more generally significant, from an "understanding the Earth" point of view, that vegetation may be important in redistributing trace elements in earth surface environments, particularly in arid regions where plants' search for water requires root growth to great depth. In such environments, geochemical signatures in soils may well be the result of millennia of plant uptake.

Image of the Moolart Well prospect from www.regisresources.com