When conglomerate gold nuggets were discovered in the Pilbara in 2017, it sparked an old time gold rush. Geologists are now focused on understanding how the gold formed to determine whether or not the region, best known for its iron ore, could be further developed for gold. TIM TREADGOLD reports

 

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Pilbara gold rush

Despite early uncertainty, there is growing confidence that a unique gold discovery has been made in Pilbara region of Western Australia (WA).

But, knowing how the gold was deposited in an area better known for its iron ore, is the subject of an intense debate among geologists.

Unlike traditional gold-bearing rocks that have led to the development of a number of mines in the Pilbara over the past 100 years, the latest discovery is in conglomerate sedimentary rock – material made up of rocks ranging in size from gravel to boulders.

"Nuggety" gold can be difficult to mine


This lack of uniformity is one of the challenges being tackled by companies, including Perth-based Artemis Resources and their Canadian partner, Novo Resources.

Their work has included trenching a few metres from the surface and drilling to obtain deeper samples. The near-surface work has allowed a large number of nuggets to be collected.

Artemis Resources executive director Ed Mead has an extensive collection of Pilbara nuggets in his Perth office, but it’s the conglomerate-hosted nuggets that are a key part of the puzzle.

These nuggets are different shapes depending on the location where they were found. They range from quite large and smooth and weighing several ounces, to small and flattened, leading to their comparison with “watermelon seeds”.

Understanding how the conglomerate gold nuggets formed


The differences could mean that the nuggets have more than one source or have been subject to different forces. This adds a layer of complexity to the challenge of unravelling the puzzle of their genesis, which might lead to finding a solution to their eventual mining.

“We need to know a lot more about the formation of the gold-bearing conglomerate on our tenements before we can devise a mining method,” Mr Mead says.

“Gathering data is not easy. While the surface material is easy to see – and trenching for several metres reveals encouraging signs – it is difficult to drill because a conventional drill bit can easily miss nuggets.

“Even a wide-diameter drill of the sort used in diamond exploration might not do the job.”

As explorers try to find ways to extract meaningful samples to calculate the potential size of the conglomerate gold discovery south of Karratha, they have been seeking scientific help in answering the most basic question of all; how did the gold get into the conglomerate?

Mark Pearce, CSIRO geologist and project lead, says work so far on the genesis of the Pilbara’s conglomerate gold has yielded more questions than answers.

“We’re dealing with a number of theories that range from offshore marine deposition to a hydrothermal event,” Dr Pearce says. 

“But at this stage, I would have to say the jury is out.”

One man who believes he is close to an answer is the chairman of Novo, Quinton Hennigh, an experienced economic geologist.

Although Canadian-based, Dr Hennigh is credited with igniting a rush by gold explorers to the Pilbara in 2017 when first reports were published about the region’s conglomerate gold.

Traces back to a three-billion-year-ago event


What generated most interest in the early stages of the search was a proposed comparison by Dr Hennigh with the gold found in the Witwatersrand of South Africa from which a large proportion of the world’s gold has been mined.

It’s in the conglomerate that a significant number of gold nuggets have been found. These are perfect for prospectors, but a challenge for mining companies because “nuggetty” gold is difficult to mine on a commercial scale. It can be rich in one place and barren in another.

Dr Hennigh’s theory is that about three billion years ago the Earth’s original continent called Ur comprised part of southern Africa (the Kaapvaal craton), part of India and the Pilbara craton of WA, which includes the Fortescue Basin.

“It was upon the primitive platform of Ur that Earth’s earliest supra-crustal basins formed, and it is within these basins that the origins of the gold cycle can trace its roots,” Dr Hennigh said in a paper delivered at a recent seminar organised by the Australian Institute of Geoscientists.

It was into these ancient basins that sedimentary and volcanic rocks were deposited, then later covered by incursions of the ocean and layered with living organisms in a microbial mat that was responsible for the oxidative precipitation of the gold from seawater.

Gold-bearing conglomerate, which contains much of South Africa’s gold, has been dated to an era between 2.7 billion and 3 billion years ago – a time when the Pilbara sequence was also being formed.

Dr Hennigh said that in the Pilbara at least two gold-bearing conglomerate beds have been identified, each marking significant transgressive (overlapping) events during the early Fortescue Basin evolution.

He said the lower bed, called the Cannonball conglomerate, is about three metres thick. The second, the Upper Cannonball, is about 15 metres higher and is between one-and-two metres thick.

“Gold nuggets in the Cannonball conglomerate range in size from about 0.1 to 2 grams, whereas those in the Upper Cannonball conglomerate are typically larger at 0.5 to 8 grams.”

Formation of Pilbara conglomerate gold still a mystery


CSIRO’s Dr Pearce says the search for an answer to the forces that created the Pilbara conglomerates, which lie between the very old Archaean basement and the later Fortescue group of rock units, have narrowed to focus on what must have been a very wet and high energy environment.

But, even that general hypothesis can not explain the presence of chlorite, a soft mineral, in a halo-like pattern around some of the nuggets.

“The presence of chlorite is a puzzle that is yet to be explained,” Dr Pearce says.

Mark Creasy, one of Australia’s most successful prospectors and an early participant in the Pilbara gold hunt, agrees with the theory that the gold was deposited in a marine environment.

“It’s got to be alluvial with some kind of input with gold precipitated from water,” Mr Creasy says.

“But exactly how it was formed and whether there is a connection to events in South Africa is harder to say.

“We’re talking about a period of time covering between 2.8 and 3 billion years ago, and a lot can happen in 200 million years.”

Fully understanding the origin of the gold, and then using that information to go the next step to mining, will require intensive scientific analysis.

A wider industry research collaboration on conglomerate gold in the Pilbara is something that CSIRO has suggested could help find the answers.

Many towns in the Pilbara rely on mining for employment, so identifying new gold reserves and turning these into mines would boost regional development if successful.

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