SA’s fields of gold

By | 2019-04-04T12:30:30+00:00 April 4th, 2019|Mining in Focus|

Contrary to popular belief, great opportunities still exist for gold miners in South Africa. In fact, the country might be sitting on the world’s next great gold deposit. But we won’t know until the drill rigs are unleashed, writes Leon Louw.

Historical head gear on the West Rand of the Wits Basin. Image credit: Leon Louw

Historical head gear on the West Rand of the Wits Basin. Image credit: Leon Louw

For more than a century, the origin of the Witwatersrand gold in South Africa has been the subject of vociferous arguments and debates. Generations of geological researchers have burnt the midnight oil to explain the phenomenon that continues feeding a ravenous global demand for yellow metal. Despite these years of research, and knowledge gained by more than 120 years of non-stop mining, the origin of South Africa’s gold deposits remains uncertain. Today, the shallower portions of the Witwatersrand Basin, where most of the country’s gold is found, is a warren of tunnels and stopes. Nonetheless, they continue producing the goods.

More gold to be mined

Many believe that the Witwatersrand Basin is reaching the end of its productive life, and that it won’t be able to continue delivering the gold on which the foundations of South Africa’s most prosperous city, Johannesburg, and the country’s economy were built.

The mainstream narrative is that gold mining in South Africa is a sunset industry. Most geologists with a good knowledge of the Witwatersrand Basin, nonetheless, say that half of the in situ gold has not been brought to surface yet. The problem is not so much the depth of the remaining reefs (which is true, especially on the West Rand) or the low grades of what is left (which is also true), but most importantly, the lack of drilling holes in areas previously not explored to the full.

According to an article authored by three legendary South African geologists — Morris Viljoen, Richard Viljoen, and Rodney Tucker — the Witwatersrand contains six times more gold than the world’s second-largest goldfield. In an article titled “A review of the Witwatersrand Basin — the world’s greatest goldfield” that appeared in the June 2016 issue of Episodes Journal of International Geoscience (Vol. 39 No. 2: The great mineral fields of Africa), they write that the Witwatersrand is a mature goldfield with declining production, but that there is still a significant amount of gold left to mine.

“Much of the remaining resource occurs at considerable depths. However, there are still opportunities for extracting lower-grade deposits at moderate to shallow depths. The Basin thus remains a major exploration target,” the article states.

Pursuing the low-hanging fruits

That there are shallow, low-hanging fruits in and around the traditional mining areas of the Witwatersrand Basin is a fact. One doesn’t need to look much further than the many artisanal workings that pockmark areas previously deemed mined-out. The presence of companies like West Wits Mining, Gold One, and White Rivers Exploration is proof that there is life in the reefs yet. Many potentially shallow, productive deposits were sterilised during the past 50 or so years, and West Wits has showcased what can be done if a mining company changes its mindset and pursues lower-grade deposits, which are easier to mine than deep and complex underground reefs. White Rivers, on the other hand, is going after deeper, potentially higher-grade deposits in the Free State.

According to John Paul (JP) Hunt, senior exploration geologist at SRK Exploration Services, the challenge is that most of the remaining deposits are getting deeper, which makes them more difficult to access. “Furthermore, the grades become lower the further one moves away from the primary source areas into the more distal parts of the depositional fan,” he says.

But apart from these deeper deposits, did historical gold mining companies not leave us with at least some pickings after their insatiable feasts? (Not referring only to their tailings, which DRDGold is competently taking care of.)

Mark Wanless, partner and principal geologist at SRK Consulting (SA), says that there are indeed reefs that contain gold, and which nobody has really exploited before, mostly because of their low grades. To mine these lower-grade reefs would be expensive, and it would be a big ask to expect junior mining companies to carry the cost burden of developing the necessary infrastructure to get the gold out. Mining companies have historically mined through these lower-grade reefs to get to the high-grade areas without even reporting the lower-grade reefs. “So, I guess that although we can say there is still plenty of gold in the Witwatersrand Basin, we have to ask whether it would make economic sense to liberate the gold,” says Wanless.

Wits Basin has plenty to offer

Manie Swart and Rob Handley, geologists at consultancy firm Shango Solutions, agree that many gold-bearing reefs of the Witwatersrand Basin are not yet depleted, as is often reported. Both men have worked extensively throughout the Witwatersrand Basin on different projects for many years and are convinced that there are reefs that still have plenty to offer. There is a false perception that the Witwatersrand Basin is now depleted, and that there is hardly any gold left. Improved technology, and better government legislation, will make the country more attractive for junior mining companies. “Unfortunately,” says Swart, “the regulatory uncertainty and political instability over the past decade or so have kept the exploration companies and drill-rigs at bay.”

“There could be just as much gold left in the ground in South Africa as what has already been mined in the country over the past 40-odd years,” says Handley. “Of course, the great producing reefs like the Main Reef and the Main Reef Leader have virtually been mined out, but there are a number of lower-grade narrow reefs out there that are ready for the taking,” he adds.

The main concern is that, apart from the Modder East gold mine, close to Springs on the East Rand, and the Burnstone project, near the town of Balfour in Mpumalanga, on what is referred to as the South Rand Goldfield of the Witwatersrand Basin, not many new gold mines have come online since the early 1990s. Moreover, mining companies in South Africa have basically stopped investing in exploration, and that reinforces the belief that the gold has run out.

Great geology presents opportunity

Hennie Theart, corporate consultant, partner, and geologist at SRK Consulting (SA), says that geologically speaking, there is still huge opportunity in South Africa, but warns that the political risks have increased and that the operational challenges are greater. “There is definitely potential for small-scale shallow gold operations if the regulations allow. Unfortunately, companies shy away from investing money in exploration because of concerns that have nothing to do with mining. The South African government needs to encourage exploration more than ever before to ensure the future of the gold mining sector in South Africa,” says Theart.

Many ounces of gold are still locked up in the old mining dumps of Johannesburg. Image credit: Leon Louw

Many ounces of gold are still locked up in the old mining dumps of Johannesburg. Image credit: Leon Louw

Exploration, of course, would have to include drilling, not only at current mining operations, but ideally in new areas not previously considered, which is an expensive exercise. But there are areas where more drilling could lead to unexpected results. Over the past 100 years or so, geologists have been able to delineate what they refer to as ‘goldfields’ in and around the Witwatersrand Basin. Today, these goldfields are well established. However, there are what Theart calls ‘gaps’ between these existing gold provinces that need closer scrutiny.

“The Potchefstroom Gap and the Bothaville Gap, for example, might have potential. Furthermore, there is definitely mineralisation in some of these gaps to the north and north-east of the Free State Goldfield. This is where Harmony Gold has developed the Target Mine, one of the later additions to gold mining in that region. Immediately north of Target is a zone of gold enrichment that could possibly be described as a new ‘fan’, the Paradise Fan, where some initial drilling has been undertaken,” Theart explains.

As far as Mining Mirror could establish, Anglovaal (later AVGold) explored this area in the past and discovered the Target and Paradise Fan. Similarly, there are known lower-grade gold deposits on the south-eastern and eastern side of the Basin, north-east of the Free State Goldfield.

Wanless agrees but adds that there are a number of challenges in the Bothaville Gap. “For one, the cover is much thicker here. So those reefs would be deeper than for example at Harmony’s Target Mine, and the company’s Loraine Mine to the south of Target,” says Wanless. Both Target and Loraine initially mined the Basal Reef and the B-Reef, which are the bottom economic reefs in the Free State Goldfields. Today, Target is also mining the Elsburg Reef, a set of stacked reefs that require massive mining methods, similar to what Goldfields has to deal with, at times, at its beleaguered South Deep Mine on the Far West Rand Goldfield of the Witwatersrand Basin. The Basal and B-Reefs are very deep, and although the Elsburg Reef occurs near the top of the sequence, they are extremely challenging to explore and to find the areas that contain the high-grade mineralisation.

Adding more goldfields

Up to now, geologists have identified eight established goldfields, namely the South Rand Goldfield; Evander Goldfield; East Rand Goldfield; Central Rand Goldfield; West Rand Goldfield; Carletonville Goldfield; Klerksdorp Goldfield; and the Free State Goldfield. To the north-east of the Free State Goldfield possibly lies another goldfield, which could be referred to as the Ventersburg Goldfield.

The potential Ventersburg Goldfield to the east of the Free State Goldfield was explored extensively by AngloGold Ashanti and much more recently by Gold One. Unfortunately, the excitement seems to have waned, again possibly because of the increased risk of committing to a long-term exploration and mining development project.

Are we looking in the right places?

According to Professor Terence McCarthy, emeritus professor of geology at Wits University and principal geologist at Shango Solutions, we are probably still not looking in the right places. Although the Witwatersrand Basin is punted to be the area where the next big gold discovery will be made, there are other regions in South Africa that could be prospective. These include the Barberton Greenstone Belt in Mpumalanga, which is the oldest gold region in the country and offers, according to Hunt, potential for shorter-term shallow operations (Ed’s note: Barberton is a subject for another time and Mining Mirror will run an extensive feature on the Greenstone Belt in Barberton in the near future. Hunt is an expert on the Barberton Greenstone Belt.)

“We have done several studies and looked at the complete distribution of the Witwatersrand-type rocks, and it turns out that they are much more extensive than originally thought,” says McCarthy. He adds that there are potentially two new areas containing Witwatersrand rocks. “We’ve mapped out the full distribution of upper and lower Witwatersrand rocks using information available in the public domain. They stretch from Johannesburg all the way to Colesberg in the Karoo and eastwards to Bethlehem, where there are possibly other gold-bearing basins,” says McCarthy. A few holes were drilled in and around the Bethlehem area in the early 1990s, when the mining industry decided there was no longer a future for deep-level mining, and no exploration work has been done since. Moreover, no work has been undertaken in the Colesberg Basin. According to Theart, there is another sub-basin in the Koster area of the North West Province that is worth mentioning. The Koster sub-basin has been explored by Goldfields and other exploration companies.

McCarthy is at the forefront of a movement that challenges traditional thinking about the Witwatersrand Basin, and has proposed, in a recent presentation at the Africa Down Under Conference in Perth, Australia, that South Africa’s gold was transported into the Witwatersrand Basin by glaciers, during a previous ice age, and that therefore, the gold could have been carried much further south than was originally thought. Traditional theory postulates that water and hydrothermal events, or a combination of the two, were the principal transportation agents of the abundant gold deposit in the Witwatersrand Basin.

According to McCarthy, there are three gold basins in South Africa: the well-known Witwatersrand Basin, the Bethlehem Sub-Basin, and the Colesberg Sub-Basin. “Research indicates that the distribution of the Witwatersrand-type rocks is much wider than what was previously thought,” says McCarthy. “There are definitely Witwatersrand-type rocks outside of the Witwatersrand Basin in the Bethlehem and Colesberg areas,” says McCarthy.

It is important to distinguish between goldfields and gold basins. Where there are clusters of deposits within a Basin, they are referred to as goldfields, like the Central Rand Goldfield and the Klerksdorp Goldfield, for example, each of which supported numerous gold mines. Potential new goldfields have been identified by several top geologists within the traditional Witwatersrand Basin, and those, according to McCarthy, include an area near Kroonstad in the Free State, and close to Ventersburg, also in the Free State, both of which look extremely promising. Both are also very shallow. McCarthy is of the opinion that the Evander Goldfield has been misunderstood. ‘We have only really mined half of that Goldfield, the other half is untouched,” he says. In addition, McCarthy’s model of glacial movement might add another dimension and could put the Bethlehem and Colesberg basins on the map.

Resuscitating gold exploration

There is evidence of artisanal mine workings throughout the Witwatersrand Basin. Image credit: Leon Louw

There is evidence of artisanal mine workings throughout the Witwatersrand Basin. Image credit: Leon Louw

To revitalise the interest in gold mining and exploration in South Africa, it is imperative that we start to critically re-examine existing theories on the formation of the Witwatersrand gold deposits (like McCarthy and others have done). We need to re-assess the geological record contained in the rocks, and then develop new theories that might lead us to virgin ground that has never been on the map. That will require out-of-the-box thinking.

After years and years of studying the Witwatersrand Basin, many questions remain unanswered. According to Professor McCarthy, we do not fully understand the Basin and all the processes that resulted in its formation. There might be another gold basin in South Africa waiting to be discovered, apart from the other opportunities that current theories have identified within what we know as the Witwatersrand Basin. A better understanding of how the gold got there in the first place might shed more light on where the rigs should be deployed.

“The arguments and controversies about the origin of the gold have been raging since mining started in the Witwatersrand Basin, and we don’t have all the answers,” says Swart. “What we do know is that there are more complex reefs out there waiting to be discovered,” he adds.

Theories like the one proposed by Professor McCarthy need more attention. Most South African geologists, and Witwatersrand Basin specialists, are familiar with the great debate and heated discussions about placer gold, hydrothermal sources, and a combination of the two, called the modified placer-theory. Placer theory proponents believe that the Witwatersrand gold was deposited in gravel-bed streams by means of flowing water. McCarthy, a placer gold advocate all his life, believes that the gold was delivered, along with copious sediment, by ice age glaciers and deposited as water from the melting ice, redistributed the sediment, and concentrated the gold. If this is the case, it could open a whole new world for geologists, gold miners, and explorers in South Africa.

Glacier theory

According to Professor McCarthy, a strange rock type is associated with all the goldfields in the Witwatersrand Basin, referred to by most geologists as ‘puddingstone’.

Puddingstone is basically an unsorted conglomeration of rocks that consists of distinctly rounded pebbles whose colours contrast sharply with the colour of the much finer-grained, often muddy matrix or cement surrounding them. The size of the pebbles in the puddingstone found in the Witwatersrand Basin varies considerably, and they are of diverse types. “It is an accepted geological fact that in most other parts of the world, ‘puddingstone’ (correctly known as tillite) is deposited by ice,” says Professor McCarthy.

“The idea of glacial transportation of material in the Witwatersrand Basin was first proposed in the 1950s by Dr J H Wiebols, a Union Corporation geologist, and was then, without reason, abandoned, and it never gained traction again. We came across it while working on the Evander Goldfield, trying to understand how its gold-bearing reef was deposited. We continued running into these puddingstone rocks and I vaguely remembered this weird theory about the glacial origin of the Witwatersrand conglomerates. We had access to, and read, many excellent internal company reports about the Evander Goldfields, as well as Wiebols’s articles. From those reports it was clear that the only way those rocks could have originated was by glacial deposition. So, the whole notion of long-distance transport of pebbles and gold by ice began to make sense. The melting of the ice generated lots of water, which concentrated the gold into the gravel (conglomerate) beds we see today,” Professor McCarthy explains.

He continues: “The flowing water washed away the less dense and smaller particles and concentrated the pebbles and dense gold grains. The critical step was the initial ice transport. Advance of ice sheets across the developing Witwatersrand Basin dispersed and deposited glacial debris in the form of puddingstone (tillite). This material was washed and reworked by melt water during glacial retreat. Repeated cycles of ice advance and retreat, lasting perhaps millions of years, spread gold-bearing gravel over the vast Witwatersrand Basin, which at that time extended at least from Polokwane in the north to Colesberg in the south-west and to Swaziland in the south-east.

“This is actually only a fragment of the area actually covered by the glacial deposits, as other parts have since broken away and are now located in Australia and India, and possibly elsewhere.

The Central Rand has been a prolific producer of gold for more than 100 years. Image credit: Leon Louw

The Central Rand has been a prolific producer of gold for more than 100 years. Image credit: Leon Louw

“The notion of ice transport may sound far-fetched, but this is exactly what happened in North America during the last ice age, which began about two million years ago and ended only 10 000 years ago,” Professor McCarthy continues.

The glacial theory could potentially be a revolutionary paradigm shift for gold exploration in South Africa. What it means is that the concept of a distinct source away from which the gold concentration gets poorer, doesn’t apply. Glaciers can move massive quantities of rock over huge distances; in other words, they can spread gold over vast areas, so vast that the original sources become indistinct and impossible to trace. In fact, repeated cycles of sediment transport by ice, deposition of gold-bearing gravel, re-transport of the gravel during renewed ice advance, re-concentration of gold during ice melting, and so forth, lasting hundreds of thousands to millions of years, would create a multiplicity of secondary gold sources, and the original source would be lost.

We see a modern example in North America. Most of the alluvial gold in the northern states of the US and Alaska came from multiple sources in Canada, was brought south by glaciers, and became concentrated in river gravels by melt water. The mining of this gold was dramatised in the recent TV series Gold Rush. The same gold concentrating processes appear to have happened in South Africa, but many millions of years earlier.

A lot of gold is left in the age-old source rocks in the northern parts of South Africa, and it is possible that source rocks in the far north, which supplied some of the gold, no longer exist, having been transported away by earth movements. According to Professor McCarthy’s theory, the glaciers moved from the north and north-west to the south and spread alluvial gold everywhere. There might even be more gold in the south than in the north, but the theory, of course, remains completely untested. Nobody has ever done any drilling in the Karoo and very little on the potentially shallow deposits on the southern edge of the Witwatersrand Basin. But according to Professor McCarthy, these deposits will be discovered.

“It is a slow, progressive effort to gather information and interpret it. Eventually, gold will be found; it is just a question of sentiment, commitment, and the motivation to keep trying. Under the present mining regulations in South Africa, however, I fear that it might take a long time before the theory is tested,” says Professor McCarthy.

A lack of funding, regulatory and political constraints, and rising costs have prevented exploration companies from taking on the risk and venturing into unknown territory. We haven’t seen many drill rigs searching for gold deposits in the Karoo lately. It’s about time we do. For once, we should listen to and believe in geologists.

Gold mining is not a sunset industry in South Africa; in fact, its rebirth might be imminent.