Soil, weathered rocks and shallow sediments (regolith) have a long history of providing clues to buried mineralisation, writes IAIN COPP
Diggers on the Australian goldfields would dry-blow their 'patches', revealing 'colour' as it fell from a plume of dry dirt and dust.
Although such easy pickings are now a rarity, explorationists continue to sample the regolith for subtle signs of gold and other metals that may point to mineralisation at greater depths.
Any techniques that help identify prospects prior to expensive drilling campaigns are highly regarded, particularly as exploration moves to greenfields areas with poor outcrop, deep weathering profiles and a thick succession of cover rocks.
Gold beneath the regolith
CSIRO's long history of regolith research has recently changed focus to aid exploration in these areas with thick cover.
Partnering with Gold Road Resources, CSIRO chose to study the Yamarna Terrane in the eastern Yilgarn in Western Australia, where the company had recently discovered the world-class 6.2 Moz (million ounces) Gruyere granitoid-hosted gold deposit.
Although the terrane is poorly explored because of sand dunes, deep weathering, increasing thicknesses of the Permian sedimentary cover to the east and no mining activities, the discovery has highlighted the great potential of this region.
CSIRO's Senior Research Scientist, Dr Walid Salama said the project was designed to understand the geochemical dispersion processes that may form anomalies within thick sediments and the aeolian sand cover.
"For CSIRO, it is the first time that we have worked on an area where the host rock, mostly basalt, is covered by weathered Permian glacial sediments and overlying sand dunes, and we wanted to understand how to explore in a terrane like this."
From this partnership has emerged a fascinating story of an evolving palaeolandscape, changing climate and tectonic history, and the movement of metals from buried mineralisation into the overlying regolith.
Looking for geochemical anomalies
Extensive research by CSIRO over more than four decades has shown that concealed mineral deposits can impart subtle geochemical anomalies in the overlying weathered bedrock (saprolite) and soils.
Targeted sampling media such as ferruginous nodules and pisoliths, termite mounds, pedogenic carbonates and vegetation have been shown to give a response through shallow transported cover in certain environments.
"Such models cannot be applied everywhere, and where there is deep transported cover over the bedrock, and with different landscape histories, we need to find new techniques and materials to sample that will give us the best vectors towards buried mineralisation," Dr Salama explains.
The primary methodology used in the Yamarna project was to reconstruct the evolving palaeolandscape using existing drillhole intersections over the Toppin Hill, Santana and Smokebush prospects, as well as over the Gruyere deposit.
Discovering vectors to buried gold
The best regolith units that might provide vectors towards the buried gold deposits, such as ferruginous nodules and pisoliths formed in aeolian sand, were then sampled.
"Through this process we then aimed to understand the various metal dispersion mechanisms responsible for the formation of gold anomalies in the transported cover" said Dr Salama.
"The main idea was to look for the most probable locations and sample media for metal anomalism."
"We found evidence of the underlying mineralisation formed by hydromorphic and mechanical dispersion of both gold and arsenic at the unconformities (or interfaces) between the different transported and regolith layers, and in authigenic ferruginous nodules and pisoliths formed in the Permian sediments and aeolian sand."
Pisoliths provide clues to mineralisation
Pisoliths that form within the sand dunes have proven to be one of the best vectors towards underlying mineralisation.
"The porous Permian sandstone has allowed groundwater to easily move through it and transport gold and arsenic upwards from the mineralised bedrock into the sand dunes above" remarked Dr Salama.
"The pisoliths are only about 1–3m below the surface, so there are a lot of savings in drilling costs to sample these".
"Once we found anomalism in the pisoliths, we started to test the <75 µm soil as well. We used different partial extraction techniques for gold and arsenic. The arsenic is associated with organic matter, whereas gold is dispersed in particulate and soluble forms and had moved away from the arsenic. They behaved quite differently from each other, which is important to know".
"So, we tested the hypothesis that the vegetation might be picking up this arsenic, creating yet another anomaly above the mineralisation. And this is what we found in the foliage of Eucalyptus trees at Smokebush.
However, at other prospects where the gold in bedrock is not associated with arsenopyrite, there is no arsenic to form vegetation anomalies, so explorers must be careful with how they interpret a negative arsenic response in vegetation".
The Yamarna project has been very successful in identifying anomalous zones in the transported cover over known mineralisation, but Dr Salama cautions that care needs to be taken with this strategy. It is important to know how they formed — they can be transported from far or from very close, or be formed in situ, so understanding the paleolandscape is critical."
So as explorers move to areas of increasing cover in the eastern Yamarna, these improved strategies — landscape reconstruction, shallow augering and vegetation sampling — will be cost effective exploration methods needed to find deeper, more elusive buried deposits.