Exploring for conglomerate gold in the Pilbara

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SRK News | Issue 57Exploration Geology: Keys to success

Stuart Munroe, Structural Geologist 
Mike Cunningham, Geologist

Conglomerate-hosted gold and alluvial gold draining conglomerate sequences have been discovered by multiple explorers over an extensive area of the Pilbara region of Western Australia. Most of the conglomerate-hosted gold in the region has been recognised in the conglomerates of the Hardey Formation at the base of the Mount Roe Basalt in the lower part of the Fortescue Group.

Like the Witwatersrand gold deposit to which they have been compared, the conglomerate contained in the Pilbara was deposited prior to the Great Oxygenation Event, where low levels of oxygen in the atmosphere allowed detrital gold to be co-deposited with pyrite, graphitic carbon and uraninite.

Understanding sedimentary features and controls on gold distribution are key tools for exploration. Numerous researchers and explorers have suggested a sedimentary or paleoplacer deposition model for the Pilbara conglomerate-hosted gold deposits, a different style of gold mineralisation than traditionally mined in Australia. The paleoplacer model refers to gold deposited within coarse sediments, where distribution and grade can be directly related to sedimentary controls.

Gold discovered in the Pilbara appears to have a relatively high nugget. ‘High nugget’ refers to the large statistical differences between closely spaced samples of the same material in the mineral system, making a resource difficult to estimate.

High nugget is a common problem in gold deposits and may be exacerbated in the Pilbara deposits by the relative immaturity of the host conglomerates, but this has not been extensively tested. Early indications are that the Pilbara conglomerate-hosted gold is typically coarse, producing watermelon seed-sized gold grains. How these gold nuggets are formed remains a contentious topic, and a lot more work is needed to fully understand this phenomenon and the implications for sample assay statistics.

Results from conglomerate-hosted gold at the similarly challenging Pardo deposit in Ontario highlight the variability in sample grade in this style of mineralisation. A recent 985 tonne bulk sample of exposed pyritic conglomerate returned a head grade of 4.2 g/t gold in a pilot plant program. The volume of rock excavated had an estimated average grade of 1.3 g/t gold from 11 diamond drill holes. The variability in grade from drill core to bulk sample is typical of high nugget gold deposits and creates a challenge when estimating a Measured or Indicated Resource required for preliminary economic assessment.

Because high-nugget mineralisation requires customised sampling to address the statistical variance, more work is required to determine exactly how the nugget effect will influence the accuracy of resource estimation of Pilbara conglomerate-hosted gold deposits.

Stuart Munroe: smunroe@srk.com.au

Mike Cunningham: mcunningham@srk.com.au

 

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