Ideally, evaluating drillholes on the fly would provide faster and better-informed drilling decisions, thereby reducing unnecessary drilling and its associated costs.
CSIRO has been working towards better understanding Australia’s regolith and how metals move and concentrate in the regolith.
CSIRO’s Exploration Though Cover group uses geophysics, data science, regolith and groundwater to help solve these problems, and provides expert advice to companies exploring undercover by using cost-effective exploration methods.
Much of this knowledge has been gained by CSIRO’s engagement with Junior Exploration Companies and SMEs.
These small companies are the driving force of mineral exploration activity. While they may not have internal technical expertise to support them in their growth, partnering with CSIRO enables their R&D requirements to be met while also contributing valuable data for understanding the regolith.
CSIRO’s Senior Research Scientist Dr Walid Salama explains that some of his team’s present work aims is to identify vectors towards mineralisation in a regolith setting, dominated by a gold-depleted residual weathering profile, a thick leached zone and transported cover.
“If you can find a signature of buried mineralisation in the cover sequence and then follow it, rather than exploring further away, then this would be a very important tool for a mining company to use,” said Dr Salama.
“What we aim to do is to find anything in the regolith that could be sampled so as to reduce the cost of the drilling and help guide mining companies exploring their tenements.”
In 2020, CSIRO teamed up with Great Boulder Resources on a collaborative research project for gold exploration at their Mulga Bill prospect in the Wydgee–Meekatharra Greenstone Belt in Western Australia’s Murchison Mineral Field.
Mulga Bill is a very deep and structurally complex prospect, which has a very thick leached zone where most of the gold and pathfinder elements have already moved to the bottom of the weathering profile.
As Great Boulder Resources’ Managing Director Andrew Paterson explains, “Although there is mineralisation in the lower saprolite and fresh rock, there is no convincing surface response that we could find. So, we engaged CSIRO with the hope of identifying some indicators of this buried mineralisation in the transported cover.”
Using CSIRO’s UltraFine+ soil analysis process, Dr Salama’s team first tested the surface soils, but it similarly didn’t reveal any significant anomalies, confirming Great Boulder Resources’ results.
Expanding the program, they started looking deeper using logged regolith profiles from 43 drillholes across three drilling traverses, as well as HyLogger spectral scans of 25 RC drillholes, and selected petrography, x-ray diffraction (XRD) and scanning-electron microscopy (SEM).
“With these datasets we first began looking at the unconformity between the transported cover and the underlying saprolite, but again it didn’t show anything. When we looked deeper into the saprolite, however, that’s when we started to see something very significant in the HyLogger data,” Dr Salama said.
White mica is present in the fresh felsic rocks at Mulga Bill, and because of its resistance to weathering it is also found throughout the saprolite. Although mica is too common to be a pathfinder mineral, Dr Salama’s team found that its compositional variability was the vector to mineralisation that they were after.
“We found that in the saprolite, sodium-rich white mica, or paragonite increased towards the mineralized zone, whereas potassium-rich white mica, or muscovite was dominant on either side. And importantly this was not due to differences in lithology of the fresh rock, which remained the same across the drilling traverses,” said Dr Salama.
Along with white mica, chlorite also showed significant differences.
Dr Salama’s team discovered that on either side of the mineralized zone, you see only magnesium-rich chlorite, which is similar to the underlying fresh rock.
And the iron-rich variety is only found within the mineralised zone.
Identifying white mica and chlorite alteration is not new in exploration, though analysing their composition in the regolith is.
As Dr Salama explains, “Even if you don't see gold or any pathfinder elements in the weathered zone, you may see the associated alteration and have vectors to mineralisation.”
Dr Salama emphasises that if companies include sodium and potassium in geochemistry datasets when investigating leached zones, they potentially could find a white-mica vector if they understand the white-mica abundance and composition patterns in the saprolite.
By plotting sodium oxide at Mulga Bill, Dr Salama identified a sodium-rich anomaly over the mineralized zone within the first meter of the saprolite below the unconformity.
The implications of this methodology for exploration are clear as Dr Salama emphasises that by drilling very shallow holes just below the transported cover, a vector to mineralisation can very quickly identified and substantial drilling costs saved.
Dr Salama’s team also found something completely unexpected and extremely significant on the HyLogger data.
“We found a very specific spectral water-feature that could be traced back to fluid inclusions in quartz veins, and importantly they refer to high-grade gold mineralisation, both in fresh rock and regolith,” Dr Salama said.
Such narrow, mineralized intervals would typically be missed by highly diluted composite sampling. But by identifying them early on during spectral scanning, sampling intervals can be reduced, and the anomalous gold detected.
The future of exploration
Paterson sees great benefit in CSIRO’s methodology at Mulga Bill.
“Although the initial UltraFine+ sampling program was unsuccessful, we ended up learning a lot about the regolith mineralogy, which was hugely beneficial. It also gave us an increased understanding of what elements remain behind after weathering — which are reliable and maybe which aren't,” said Paterson.
“Until CSIRO’s involvement we'd been steered almost entirely by geochemistry, and we weren't really using mineralogy as a vector, particularly in the regolith.
“Probably the greatest potential for this methodology would be when drilling a new target, and by identifying those vectors straight away we would know that we were getting pretty close to mineralisation,” Patterson said.
Paterson is extremely satisfied with the project and its outcomes.
“Getting access to CSIRO’s equipment and expertise was just incredible. And I just can’t recommend it highly enough to anyone interested in increasing their technical understanding of their ore body.”