The depths and dangers of digging for gold

Throughout June, The Naked Scientists have been bringing you science at the extremes. And this week, Chris is going extremely deep by exploring one of the deepest gold mines in the world with members from the company, Sibanye-Stillwater.

Chris - This is seriously fast already though. My ears are beginning to go.

Len - Yeah. It's because you're dropping from sea level to below sea level at a high speed. Make a few yawns so that your ears can pop open.

Chris - People pay a lot of money for a ride on a fairground though!

Len - Yeah, we go on a merry-go-round everyday.

Chris - Today I’ve been lucky enough to descend into one of the deepest working mines in the world. It’s operated by the company Sibanye-Stillwater, in the Witwatersrand gold basin near Johannesburg in South Africa, one of the world’s richest sources of gold. In fact, about a third of the world’s entire gold reserve is thought to reside in South Africa, sitting beneath the surface. But why is that? Well, it’s because about 3 billion years ago, this area - in what’s now the northeast of South Africa - was an inland ocean. And surrounding it were mountains built from gold-rich magma brought to the Earth’s surface by volcanoes, and then eroded and washed into the sea by rivers, where they formed a gold reef, as Sibanye-Stillwater’s James Wellsted explains...

James - The Witwatersrand gold basin was essentially an inland water body, or lake, or sea, that had a number of rivers flowing into it. It was in quite a volcanically active period where you had a lot of old granites that had been produced from volcanic activity. The rivers were eroding those granites with the gold that was contained in those granites, and then as these rivers ran into the ocean you've got these big alluvial fans forming. And obviously you've got a river rushing into a still body of water, the water starts to slow down, and what happens is that the heavier and bigger particles drop out first. So in the early part of the fan you get a lot more gold concentrated, and it’s associated with conglomerates which are the bigger rock particles or rock pebbles. And then in the distal parts obviously you get the finer gold, which is probably at a lower grade, and that's what we’re finding over time: that the grade is getting less as we get deeper and deeper.

Chris - Are those sort of fans at multiple depths? In other words, have there been a succession, over millions of years, of deposits; and therefore you just dig through each of them to get to the gold in each case?

James - Yeah. There would have been a number of periods of, you know, dryer and wetter periods over the thousands of years that it took to deposit these ore bodies. So what you would have had is the inland water body growing and shinking. And as it did so obviously you got different phases: of more active deposition, and less active deposition. So you get stages where you’ve got the reefs, which tend to be conglomerates and associated with high fluvial activity; and then the more still periods which are dryer, where you get more muds and things being deposited, which don't have gold associated with them.

Chris - And what form is the gold in, in those deposits? Is it tiny particles? Are we talking nice big nuggets? What does it look like?

James - It's not nuggets like we find in some of the typical gold deposits overseas. Because they've been in river streams, they eroded, and they’re quite rounded particles. In some instances you can't see them with the naked eye at all, in fact most instances. So the typical grade of the ore bodies in South Africa is about 5 to 7 grams a ton - which, you know, that’s 5 to 7 grams of gold per ton of rock. It's not as rich as some of the big nuggetty ore bodies that you find overseas, but certainly it’s much more consistent and over a much bigger area.

Chris - So if I give you my wedding ring for a second, assuming I can get it off...I'm not asking you to marry me or anything. There you go, a wedding ring. How much do you reckon that weighs? That's 18 carat gold.

James - You tell me! That's a couple of ounces of gold, I guess.

Chris - And how much rock would you need to move in your plant to make that?

James - You'd have to move a couple of tons of rock to get this.

Chris - It's a lot isn't it?

James - It's very labour intensive work, capital intensive, dangerous work. So yeah, the price of gold is that for a reason.

Chris - It's worth its weight in gold even?

James - Indeed.

Chris - And with that, I went to explore this incredible setup. There are literally hundreds of kilometres of tunnels below ground, and I met James’ colleague, Len Zore, who accompanied me down. Now this is a very dangerous industry, although thankfully very few accidents now happen because safety is taken incredibly seriously. A poignant reminder was someone training me to use a rebreathing apparatus that would hopefully keep me alive for a while if we got buried. Then, kitted out in our hard hats, boiler suits, and boots, we stepped forward to await our ride - or as it turned out, near-freefall - down into the mine.

Len - We are now standing on the bank waiting for the conveyance to arrive. It is a vertical shaft that drops about 1 1/2 km, and you'll see the conveyance moves quite fast. It's not like a lift in a building, it's like 12 metres a second.

Chris - These are just sort of open cages, aren't they really?

Len - There's four compartments. You get your man winders, and you get your rock winders on this side. This we use for people and material; the other side is for rock.

Chris - We were joining one of the mining shifts heading to work, and this was the first of a number of lifts that descend into the mine, which extends kilometres down into the ground. In fact, it’s so deep - it’s the third or fourth deepest mine in the world - that the men working in the farthest depths take more than an hour to reach their destination. And that’s despite the dizzying speed of the descent...

Chris - This is seriously fast already though. My ears are beginning to go.

Len - Yeah. It's because you're dropping from sea level to below sea level at a high speed. Make a few yawns so that your ears can pop open.

Chris - People pay a lot of money for a ride on a fairground though!

Len - Yeah, we go on a merry-go-round everyday.

Chris - Once our stomachs caught up with us, we emerged from the lift at one of the main underground stations where the rock walls were painted a very tasteful cream colour. Now this area is big, and it's got a train track running through it to bring piles of rock containing the gold from the faces where they're being drilled and blasted out, so they can go back to the surface. Tunnels like this come off horizontally at different levels of the mine. They are called crosscuts and they're intended to intersect the gold reef, as it's known, on what would have been the floor of that ancient ocean way back in time. Pretty quickly though, the tunnels narrow down to just a couple of metres, and Len and I followed a team of miners out to where they were extracting ore from one part of the reef.

Len - We're standing here now at an 8 metre reef and the other side is 12 metres high. And in this reef you can see the various rocks that were transported with the sediment where you get your gold in.

Chris - Oh, I see. So you can use the fact that in amongst this, it's almost like a cake with raisins in, isn't it. Those are the pebbles - that tells you this is a layer which must've been washed into this primitive basin by the rain?

Len - That's it, correct.

Chris - And the geologists know what sort of configurations go with where the gold is?

Len - Yes, yes they do. Also with past experience, as the mines have started mining 50/60 years ago, they intersected the top bands of reef first, which is this one we’re talking to now, VCR. As you go lower down, you get your main reef and you get a carbon leader, carbon meaning it's heavier, so it's much deeper, which is a much older reef. You talk about older - this is millions of years of difference between the reefs. But the carbon leader is the one that's most richest, it’s got the most gold in it obviously, because gold is heavier than most substances. This VCR one has also got lots of reef in it, but you have to mine much more of it to get the same amount of gold as you would mine carbon leader.

Chris - Now this is of course a working mine and before I could ask Len any more questions, a gentleman turned up with the most incredible drill, more than a metre long, and he began to make holes so he could plant charges to blast out more ore for processing...

Chris - But how do the team know where the reef is and how to get to it? I walked a bit further along the tunnel with company Vice President Karel de Lange…

Karel - We do a lot of drilling beforehand, so we anticipate, we know already beforehand, how far we are from the reef; and we will know exactly when we intersect reef as well. The drilling goes ahead, so we all know within 150 metres exactly where the reef is, although we've already got delineation of the reef body through surface boreholes that we've drilled previously.

Chris - So the team know where the reef is, but because the deposit is sloping, they need to extract it in a series of manoeuvres that progressively remove more material each time, and eventually open up a larger space, one of which we were standing in…

Karel - In this instance - what we're looking at here - we’re actually right on the level and we mined into the reef on the level itself. So the mining that we do here, we actually mine from the crosscut, we mine into the reef and we've got these wide excavations. So we’ll take a top cut first, install our support in the hanging wall by means of long tendons, long anchors, and they will start to dredge down, so we will start to take it out. Eventually we'll have this wide open excavation of about 10 metres which we are standing in.

Chris - Between these opened up “paddocks”, as they call them, the team initially leave supporting pillars of rock to prevent collapse. But once the ore has been cleared from each paddock space, the open area can then be re-filled with processed ore tailings, which are brought back from the surface and mixed with cement; and then the pillars, which still contain gold ore, are themselves removed…

Karel - Now we don't take out the large excavation either, we run about 10 metres wide paddocks, and then we leave a pillar and we take out another paddock 10 metres further, and so we carry on. And then we will fold these open excavations by means of cemented tailings, and once these have settled and hardened we will come back and we will take out the pillars in between these cemented back fold paddocks.

Chris - Oh, I see. So you use the waste from the mining, cement it back together, shove it into the hole you've made, and then the bits you had to leave behind before, you then take those out. So you really do clear out the area and then fill it back in.

Karel - Exactly, exactly like that. We try to optimise the resource.