While there is no doubt that conglomerate gold has been discovered in the Pilbara, the broader understanding of this style of gold mineralisation in the Pilbara remains incomplete and, therefore, any comparisons with the Witwatersrand (Wits) basin are premature, states SRK Consulting geologist Mike Cunningham.
Commenting on the furore surrounding the recent discoveries of conglomerate gold in the Pilbara region, in Western Australia, he notes that some of the excitement is understandable, as the discoveries represent “a new and exciting style of gold mineralisation in a hitherto underexplored part of the country traditionally known for its world-class iron endowment”.
This excitement started in July last year, when Canadian-listed miner Novo Resources discovered nuggetlike gold at the Purdey’s Reward prospect, a joint venture project with Australian miner Artemis Resources. This initial discovery was seemingly further supported by Australian gold miner De Grey Mining discovering more nuggets at its Louden’s Patch licence, about 100 km east of Purdey’s Reward.
By November last year, more than 25 publicly listed companies and “legions” of small prospectors had flocked to the Pilbara looking for gold, according to a report by Reuters. The media organisation further noted that a record 1 896 gold prospecting licences were granted by the Western Australia Department of Mines in 2017.
Some of the hype surrounding this discovery is based on speculation and the hypothesis by prospecting luminaries that the South African Kaapvaal Craton (which hosts the Wits basin) and the Pilbara Craton are geologically linked.
The theory posits that the Pilbara contains ‘Wits-style gold’, which prospectors find particularly appealing, because the Wits basin has produced more than a third of the world’s mined gold to date.
“Wits-style gold refers to gold deposited in palaeoplacers within coarse sediments in the Wits basin. The gold distribution and grade in these deposits can be directly related to sedimentary controls. The source of the gold is believed to be the Kaapvaal Craton and greenstone belts within it . . . However, there is a contrasting model proposing a hydro- thermal origin to the gold in the Wits basin, so the origin and mineralisation models for the Wits basin remain somewhat contentious,” states Cunningham.
He says ‘palaeoplacer gold’ is markedly different from the styles of gold mineralisation traditionally mined in Australia. Moreover, Cunningham notes, while there are some similarities between the gold found in the two basins, there are also inconsistencies, and that the lack of sufficient evidence either way makes it difficult to form a conclusive opinion.
“The Pilbara and Wits gold deposition both predate the Great Oxygenation Event, when there was less oxygen in the atmosphere. In both cases, deposition of gold is closely related to the deposition of graphitic carbon, pyrite and uraninite under anoxic conditions. A palaeoplacer model has been suggested for both Wits and Pilbara conglomerate gold deposits, but there are tantalising hints to suggest significant differences between the two systems,” he states.
Cunningham notes that Pilbara and Wits gold both have a relatively high nugget content, but that most of the conglomerate gold in the Pilbara has been found in locations – in the lower part of the Fortescue Group formation – of an age equivalent to the Ventersdorp Supergroup, which is younger than the Central Rand Group hosting most of the gold in the Wits basin.
“It is true that some conglomerate gold hosted in basins of similar age to the Central Rand Group has been recognised in the Pilbara, the Lalla Rookh basin for example, but these basins are comparatively small in volume,” Cunningham says.
Additionally, he points out that the clasts in the reef conglomerates of the Central Rand Group mainly comprise siliceous material and, at the level of the main mineralisation, the conglomerates are regarded as mature. “Clasts in the conglomerates are mostly mafic in character, and the sequence is interpreted to be immature.”
Cunningham says the high-nugget- mineralisation deposits, as in the case of the known gold occurrences in the Pilbara, require unconventional sampling approaches to overcome the “nugget effect”.
Exploration work in the Pilbara is still at the stage of solving sampling issues, with only limited work on processing or mining studies undertaken to date.
He adds that, given that most projects are based on outcropping and subcropping prospective stratigraphy, any potential mining of Pilbara gold will most likely involve openpit or shallow mining scenarios, in contrast to the kilometres- deep mines located in the Wits basin.
Only Time Will Tell
Although the excitement surrounding the initial Pilbara discoveries has diminished somewhat, partially because of the lacklustre drilling results released by Novo Resources in November, investment in companies with Pilbara- based assets, and prospecting and exploration programmes continue.
One such company is Western Australia-based exploration and mining company Kairos Minerals, which raised A$7-million to expand its Pilbara exploration programme in October last year.
Kairos’s Pilbara licence comprises 1 158 km2, and is “highly prospective for conglomerate-hosted gold discoveries”, says Kairos chairperson and MD Terry Topping.
While the focus is currently on Kairos’s existing assets, namely the Mount York gold/lithium project and the Roe Hills gold project, the miner, having fast- tracked its Pilbara-based exploration, has released some “encouraging” results, notes Topping.
Having identified three high-priority areas at its Croydon project, Kairos has completed exploration work, comprising reconnaissance outcrop mapping, stream sediment, and soil and rock sampling, as well as gold panning. Topping notes that the stream sediment sampling produced particularly high grades.
Further, he notes that the sampling programme demonstrates that most of the target zones on Kairos’s tenements remain prospective for conglomerate- hosted gold – “an exciting development”. Topping believes that the company’s methodical approach to assessing the conglomerate gold potential will continue to pay dividends for Kairos and its shareholders.
Meanwhile, De Grey Mining continues to develop what it believes is a “low-risk, near-term 1.2-million-ounce project” and is currently compiling a prefeasibility study.
De Grey has identified three target areas, including Louden’s Patch, which has progressed to the bulk sampling stage, and the Jarret Well and Steel Well prospects.
De Grey technical director and operations manager Andy Beckwith reports that the project has suffered from the ‘nugget effect’, noting that “overall, the results have been highly variable, with the repeatability of the assay results being very difficult at each site and therefore not considered as a representative sample”.
The company mitigated this challenge by collecting a series of nominal 15 kg samples from the conglomerate targets, as well as a 100 kg sample from the waste dump material from the previous Louden’s Patch workings. The samples were sieved and then processed through a gravity-based, mini gold concentrator sluice, followed by panning of the concentrates to highlight any visual gold particles.
While this method is not intended to produce an analytical and qualitative result, De Grey’s technical team believes that this sampling method, if carried out “diligently”, offers a practical solution to reconnaissance sampling, proving consistency and repeatability.
De Grey’s next phase of exploration comprises detailed mapping, stream sediment sampling and metal detecting along the remaining 12 km target and other areas.
Both the Kairos and De Grey prospects are at a very early stage of development, and it is too soon to determine whether they will become profitable operations. Despite this, the companies are cautiously optimistic about the projects’ potential.
Cunningham points out that gold has been discovered and mined in the Pilbara for over 100 years. He says that, while the current finds have not been at the same level or scale as the Wits or other Archaean gold provinces, only minor prospecting and limited exploration have been carried out on these Pilbara deposits to date.
He concludes that SRK looks forward to seeing the results of the various exploration companies in this space, and that, in time, this will improve the understanding of these Pilbara occurrences.