John Paul Hunt talks about Greenstone potential

By | 2019-05-31T09:55:03+00:00 May 31st, 2019|In the Stope|

The life of one of South Africa’s oldest mining regions is far from over, JP Hunt tells Leon Louw.

JP, please give us a short summary of your background in the mining industry.
I studied at the Economic Geology Institute of Wits University. Most of my research has been on ore petrology, more specifically assessing what geological processes are required to make an economic mineral deposit.

I focused on magmatic nickel initially, but then pursued other subjects including orogenic gold, including the Barberton Greenstone belt, magmatic-hydrothermal systems, such as porphyry-epithermal copper-gold or iron oxide copper gold (IOCG), pegmatite-hosted and vein-hosted mineralisation. I’ve done research on the Bushveld Igneous Complex, worked for Norilsk Nickel on several exploration projects throughout Africa and, following that, I did generative work in Scandinavia and West Africa with a junior exploration team. After a stint at the Council for Geoscience where I focused on prospectivity analysis and the residual potential of the mineral resources of South Africa, I joined SRK Exploration Services about a year ago.

You have done quite a bit of work in Barberton, and as I understand, also in Zimbabwe’s Greenstone Belt. What method did you apply in the Greenstone Belt of Barberton?
I used what is known as the rank statistical analysis, where the residual potential of a mineral belt can be estimated by looking at the size-frequency distributions of the known deposits. I used this method specifically in Barberton in 2013 as a way, firstly to motivate for continued exploration in the belt, but also to establish an expectation of what the level of mining maturity of the belt might be and what might be left to find.

I looked at Zimbabwe in a similar way – mostly to compare Barberton with a terrain of comparable geological age and also with a mining history of similar duration.

Which region of Zimbabwe did you look at and how does Zimbabwe compare to the Barberton Greenstone belts?
I focused on that part of the country where the cratonic floor rocks are exposed. Both terrains have very similar geology and they are almost of the same geological age. The area of preserved greenstone terrain and the total known gold endowment in Zimbabwe are both larger than Barberton terrain by an order of magnitude, Zimbabwe having more than 6 000 deposits and prospects versus Barberton’s 255 recorded workings. They have both been mined for at least the last 140 years, and so the expectation commonly held is that these areas have been largely depleted of their resources.

Analysis of the size-frequency distributions of both terrains provides some insight into their respective residual gold potentials. Zimbabwe can be shown to be far more mature in terms of its depletion of its potential for gold deposits of economic significance than Barberton, that is, having a greater proportion of its resources already discovered or exploited.

This implies that Barberton has more potential for discovery, but the targets are expected to be smaller. Barberton would be ideal for a small- to medium-sized business to explore, while the expected prizes still to be found would probably not interest the majors.

In other words, they would only be interested in looking for the big deposits?
Yes, the majors have much higher value thresholds that need to be met, so their focus either has to be on the acquisition of Tier 1 mineral assets or to be first-mover in new terrains and jurisdictions where the possibility of finding world-class deposits is higher, given the right geological conditions, of course.

When looking at the residual potential for the discovery of a deposit, are there other factors that can have an influence?
If an exploration company is able to change the search space, it can have a positive effect. This is, in effect, the idea of looking where others have not or could not look before. There’s an over-arching principle in mineral exploration that the largest (and by extension the most valuable) deposits are often found early in the exploration process because they tend to have the largest geological footprints. If one can be first mover on a new technology or new geological model, one can unlock unrealised potential in old districts. For example, the introduction of airborne magnetic surveying in the 1940s permitted the search for lithological and mineralogical signatures of large areas and under cover, which was followed by a flurry of discoveries.

The other ways that a company may change its search space is either by changing the conceptual understanding of the mineralisation or by changing the mining or beneficiation method. Moving from a high-grade ore-shoot emphasis to a bulk tonnage operation, for example, has been done successfully in other parts of the world, and this change in mining approach has the same effect as creating a new search space. Having said this, some ore deposit types are more amenable to bulk mining methods than others.

What has been done in Barberton historically and are there opportunities for exploration companies in the area?
Historically, mining companies working in the Barberton Greenstone Belt were only interested in deposits of, let’s say, at least 10g per tonne or higher. Often sample results yielding less than 5‒10g per tonne were ignored. In other words, there is potentially a lot of metal in the ground. So a shift in how one thinks about what constitutes an orebody involving a change in mining method or application of a different lower-cost beneficiation technique could unlock this and hence reduce the cut-off grade. That’s one of the opportunities in Barberton — look for lower grade bulk-mineable resources. Another is looking under the headframes of historic deposits which were stopped for non-geological reasons.

Is there land available for new mining companies to still explore in the Barberton area?
There is a lot of prospective land in the hands of existing operations. The proclamation of the Barberton Biodiversity Heritage Site might have carved out portions of the Barberton greenstone belt, but these areas were by and large of lower mineral potential.

Are the existing mines in Barberton still doing exploration work?
Yes, they are, and some of the best results are coming from newly discovered extensions of known deposits. For the size-frequency study that I did, I had access to historic information in which the production of each deposit during the first 100 years was tabulated. I then added the production from 1983 to 2013. Interestingly, the mines continued exploration, not to make a new discovery, but to just to keep their operations going. These companies had actually mined more gold than what was predictable in 1983. Furthermore, the expectation for more gold has also increased in real terms, although not in relative terms. This does mean that the region is slowly becoming more mature. Its residual potential has decreased; in 1983 it was 47%, while in 2013 the residual potential was 42%. Interestingly, therefore, the known gold endowment of absolute metal had nearly trebled from 1983 to 2013 even though the residual potential is decreasing.

I approached a number of the mines to determine why this is the case; I hoped they had a new geophysical technique or were doing something different to unlock all this metal. The answer was that they are actually drilling ahead of the delineated part of the orebody – and this makes sense. Once a company has delineated a decent ore body that will keep it operating for the next 20 years or so, exploration won’t be a priority. The mining companies in Barberton are fortunate in that the mineralised system simply continues, and they are able to sustain their operations incrementally.

How long do you think these mining companies will continue in Barberton?
There might be external factors which can have an influence – such as the gold price, for example. Depth is also normally a big constraint. Historically, mining beyond the water table was often a major constraint, and the introduction of technology that resolved this increased the potential. So, unless a mine develops some sort of remote mining method that doesn’t put people at risk, the current operation may then just become too dangerous and expensive, resulting in mine closure.

At Lily, for example, Vantage Goldfields changed the way they looked at the ore body historically. They didn’t have wonderful 30g per tonne payshoots, but they had enough low-grade gold that they could extract it in a bulk operation. They changed the mining method, which unlocked the metal. As a result, this became a significant gold resource in Barberton.

It won’t be possible to duplicate this at every deposit, but if you look at deposits differently in terms of economics and mining methods, there is real potential to increase their value. I think the next big mine in Barberton is one we already know about; it would have been mined historically as either a high-grade shoot which might have been blind at surface or has a mineralised halo that nobody’s really looked at.

Do the ore bodies at Barberton extend much deeper than what is currently mined?
Yes, certainly, many deposits have not been fully delineated. The mines won’t be as deep as in the Witwatersrand, but there will definitely be opportunities to mine deeper.

What are the differences between the Zimbabwean and Barberton Greenstone Belts?
At the time when the gold in these two systems were formed, there were two different continents. In most current reconstruction models, the gold mineralising events pre-date their amalgamation.

They formed in very similar environments and their volcano-sedimentary sequences being preserved in very similar ways.

When the continents collided with each other, the Archean successions were force downwards; at the same time, granitic magma forces its way upwards. The currently exposed Greenstone Belts are preserved relics of these sequences, squashed between the later magmatic granites.

The Komatiitic volcanic rocks that are found in both Barberton and Zimbabwe also show differences. Komatiite is a magnesium-rich variety of basaltic lava. Petrologically, they’re distinct as they form at much higher temperatures. At present day temperatures, the earth’s crust is too cool for the formation of komatiites. The early earth had much higher temperatures. The komatiites of Zimbabwe are associated with nickel sulphide deposits that are not as evident in Barberton. There are also two distinct ages of komatiite in Zimbabwe, one older, one younger, whereas all the komatiite in the Barberton Greenstone Belt belongs to the older group.

How expansive are the Zimbabwe greenstone belts?
Coincidentally, the limits of the Zimbabwe craton nearly match the limits of the country. The areas in the northwest and the southeast of the country are off the craton. The greenstones are present as various fragments within this craton but have an aerial extent three times greater than that of Barberton. An interesting exercise might be to compare the various Zimbabwe craton greenstone belts with each other to assess their relative endowments.

In a nutshell, what are the main opportunities in Barberton?
The main opportunities are re-looking at historical mineralisation. Of course, getting good geochemistry results when undertaking an exploration project does not automatically mean one will be able to develop a top-class mine. There are a lot of factors to consider when building a mine, including the continuity of mineralisation, the ‘mineability’ and recovery. For example, some deposits have metallurgical challenges that will have to be overcome. All these factors combined will determine the eventual economic viability of a deposit.

Are there a lot of historical mines around?
There are many small historical prospects and mines. Looking at re-opening old mines, however, is a challenge and hugely costly, when these workings have flooded or collapsed.

What are the challenges of operating mines in the Barberton area?
Underground mining in Barberton can be challenging. It is steep. Wits reefs are generally shallow-dipping, on average dipping around 25 degrees (although ranging 5-35 degrees), but the Barberton mines are significantly steeper. Mineralisation is also less continuous. Other challenges may include geotechnical issues.

Can new technology improve the way we mine gold in the Barberton area?
There is a Barberton reef system known as the Barbrook Line which has a slightly different geology and mineralogy to the active mines in the area. The Barbrook Line was discovered very early in the prospecting history. It is easily traced by its prominent siliceous outcrops. Along the ridge there are numerous little adits and pits where miners have prospected in the past. The problem is that the gold is “double refractory”, which means it is embedded in the lattice of the pyrite and processing this material is a more costly exercise. So, the mineralisation is there but it has been left for metallurgical reasons. New beneficiation technologies may increase the extraction of gold on the Barbrook Line. There are several people doing research on how to get the gold out in an economical way.

Is it a big area?
It is a relatively narrow system, which I would say is about 10m wide? However, it runs the full length of the Barberton Mountain land – probably in the region of 20‒30km; I don’t know how much of that is economically mineralised.

Has any drilling been done there?
I expect there has been some drilling, but in general if exploration companies perceive it as uneconomical, it will be a lower priority target. It’s simply too expensive to get the gold out.

Will it be mined in the future?
What’s been shown in the past is that we’re quite good at coming up with solutions to problems given the right economic incentive.