This transcript was generated automatically and lightly edited for readability.

Oliver Strimpel: This is Geology Bites with Oliver Strimpel. So much of what we make starts with materials we extract from the Earth. Some of these materials make up only a tiny proportion of our planet, but fortunately for us, they are concentrated in certain places, which makes it possible to extract them in economically viable quantities. So how exactly do these materials become concentrated?

Laurence Robb is a visiting professor at the Department of Earth Sciences at Oxford University. By analyzing the relationship between mineral deposits, principally in Africa and Asia, and our latest understanding of plate tectonics and mountain building, he has unraveled the processes that form some of our most important mineral deposits. Laurence Robb, welcome to Geology Bites.

Laurence Robb: Hello, Oliver. It's a pleasure to be here.

Oliver Strimpel: The abundance of gold in the Earth's crust is only about four parts per billion. So the processes that concentrate gold, to enable us to obtain decent-sized nuggets or pan gold from streams, must be extraordinarily effective. What's happening to concentrate that gold?

Laurence Robb: So a river forms. It starts up in a highland area somewhere, and as it flows downhill it accumulates into a larger river. And one assumes, then, that there must be gold in the source area itself. We'll talk a little bit more about how gold forms in the primary ore deposits in just a moment. If we assume that the gold is there, it's being eroded, it's being concentrated into river systems, and then it's able to be transported downstream and concentrated further downstream, in the river itself, or in the delta, or in some cases even in shallow-marine and beach-type environments.

Oliver Strimpel: OK. So there's a source region that contains the gold, and then a river erodes that region, picking up grains of the gold in the process, and then the river sorts and concentrates the gold, just as it does with any other material that is suspended in the water. But you mentioned that the source region is also rich in gold. So how does that get concentrated even before the river gets to work?

Laurence Robb: Probably one of the most common ways is, again, through the action of water. But this time not fluvial, not streams, not cold water that's running on the surface of the Earth, but hot water that's actually circulating deep in the Earth's crust. We refer to hot water circulating in the Earth's crust as a hydrothermal solution; hydrothermal simply refers to hot water. So hot waters, or hydrothermal solutions, circulate through the Earth's crust. These aqueous solutions could be circulating at anywhere from 200, 300, 400, even 500 degrees centigrade, depending on depth. The second thing to remember about these solutions is that they're not pure water. They've normally got other material dissolved in them, particularly highly soluble salts like sodium chloride and potassium chloride. And in particular, it's the chloride ion, Cl⁻, which is important, because it's the availability of the chloride, which bonds with the gold to make, for example, a gold chloride complex, that is able to go into solution. In hot water, gold itself doesn't go into solution, but gold chloride does. So as soon as you've got a little bit of chlorine in hot water, it has the ability to dissolve gold that may be present in a disseminated form in the Earth's crust. That hot, aqueous hydrothermal solution will pick up the gold, transport it maybe to a high level in the Earth's crust and concentrate it, and as the aqueous solution cools, the gold reprecipitates and forms what we call a hydrothermal gold deposit.

Oliver Strimpel: So it's a two-step process. First, you have hot water, which dissolves gold at depth through the action of chloride ions in the water, and then concentrates and deposits the gold as it rises to the surface. And second, rivers erode the deposited gold, concentrating it still further somewhere downstream. Are there other types of source regions? What about diamonds, for example?

Laurence Robb: That is a very fascinating geological topic. Diamonds are simply carbon, a high-pressure polymorph of carbon, and diamonds only form deep in the Earth, really right down in the Earth's mantle. You need to have very high pressures to be able to form diamond. So getting the diamonds to the surface requires some form of volcanic activity. You need to tap the deep mantle, melt it and extrude those melts onto the Earth's surface as a volcano, which brings with it the diamonds and whatever other minerals happen to be present down in the Earth's mantle. The magmas which bring diamonds to the surface are called kimberlites, after the town of Kimberley in South Africa, which is where the very first kimberlite pipe was discovered, back in the mid to late 19th century. This is a volcanic, pipe-type structure. Its source is way down, probably two to three hundred kilometers below the surface, and the kimberlite magma has brought diamonds right up to the surface, and the diamonds occur as crystals in the kimberlite rock. So when you mine diamonds, you'll be mining the kimberlite and extracting the diamonds from it. The secondary process is when you erode the kimberlite and wash the diamonds, like you would gold, into a river system and concentrate the diamonds further downstream, and that's exactly what we've just talked about for gold.

Oliver Strimpel: A diamond pipe is fascinating. Are the diamonds already concentrated at depth, at the bottom of the crust or in the upper mantle, before they get tapped by a kimberlite pipe?

Laurence Robb: The question is interesting. How much diamond is there deep down in the Earth's upper mantle or lower crust? Probably not a lot would be my gut feel. And the reason I say that is that for every kimberlite pipe that is discovered that has diamonds in it, there are probably 100 or so kimberlite pipes that are discovered that don't have diamonds in them, or so few diamonds that they can't be seen. So the distribution of diamonds in kimberlites, as we've discovered them over the past, say, 50 or 80 years, is a somewhat haphazard distribution, which tells you that the diamonds themselves are probably fairly rare in the upper mantle to start off with. Southern Africa was well known for its huge concentration of very rich kimberlite pipes, and it's not the only part of the world. Parts of Canada, the Kimberley province in Western Australia, and also parts of Russia are very rich in diamonds. So there are many parts of the world that have primary diamond deposits, but there are many parts of the world that don't have them as well.

Oliver Strimpel: So a kimberlite pipe is a source region that is rather localized. By contrast, some metal ores extend over rather enormous regions, such as those of nickel, copper, platinum and chromium. How are those formed?

Laurence Robb: Taking, let's say, chromium as an example: chromium is a metal which is reasonably abundant in the Earth's upper portions, particularly in mafic and ultramafic rocks. The chromium content of those kinds of rocks is quite high compared to, say, a granite, where the chromium content would be very low.

Oliver Strimpel: Just to explain, for those who might not be familiar with the term, mafic refers to rich in magnesium and iron.

Laurence Robb: Yeah. And so if you're extruding a mafic magma towards the Earth's surface and it starts to crystallize, it has quite a lot of chromium in it. What would initially happen is that as the mafic magma starts to crystallize, the very first minerals to crystallize would be something like olivine and also orthopyroxene. As the magma cools down, you would get other minerals starting to crystallize, like clinopyroxene and plagioclase. And depending on how much chromium there is in the magma to start off with, as you crystallize these rocks, the chromium content in the magma that's left behind increases, to such an extent that at some point the chromium contents are sufficiently high for the magma to start to crystallize chromite itself. It might be fairly rare; there might be just a little chromite grain distributed through the rock. But because chromite is a heavy mineral, if you allow those chromite grains to settle in the magma chamber, they accumulate at the base of the magma chamber to make a layer of chromite grains, and then you have what's called a chromitite seam forming. That becomes of interest, because there's enough chromite in there to constitute what might well be an economically viable chromium deposit. And this is what you see in the Bushveld Complex, in the Stillwater Complex, in Greenland, and in many other layered mafic intrusions around the world: you find areas where there are discrete chromite layers that have actually crystallized from the magma.

Oliver Strimpel: So this sounds like a process that's very typical of the way igneous rocks form. There's differentiation in the magma chamber, where the first rocks that form are different from the ones that form out of the material that's left behind in the magma, and so on, until you get the most differentiated rocks. And so some of the metals we get are really just a result of normal rock formation processes, but there must be others that are quite different. For example, I've heard of major mineral deposits around the so-called black smokers at the mid-ocean spreading ridges, such as the Mid-Atlantic Ridge or the East Pacific Rise. What's involved in those contexts?

Laurence Robb: If you look at what's going on on the ocean floor, it's quite extraordinary how much metal is potentially being concentrated there. Oceanic crust can sometimes be covered by several kilometers of seawater, or, in some of the deep trenches, 10 kilometers. So you've got a huge volume of seawater sitting above a fairly thin volume of oceanic crust. That seawater percolates down into the oceanic crust. There are cracks in it, there are faults, and as it goes deeper down, it gets heated up. So the seawater, originally cold on the ocean floor, gets heated up. There's chlorine in seawater, and the combination of the chlorine in the seawater and the temperature allows that seawater to start to scavenge and dissolve metals that are already in the oceanic crust, particularly metals like copper and zinc, and to a lesser extent lead and gold and silver: a whole range of metals that might be present in very low abundances in the oceanic crust. So the seawater percolates through and dissolves these metals, and as the seawater comes back up to the surface of the ocean floor, now heated up to perhaps 300 or 350 degrees centigrade, these very hot, very metal-charged fluids pump out onto the ocean floor in what we call exhalative vents, or, to use a more colloquial term, black smokers.

These things were first discovered using submersibles that were diving down exploring the ocean floor, by the well-known oceanographic institution at Woods Hole, and the scientists were obviously totally amazed to see them initially, because they were very unusual. But they very soon realized what they were: simply hot hydrothermal solutions venting onto the ocean floor and almost instantaneously precipitating the metals that were being carried in the aqueous solution. So around these black smoker vents, you're getting phenomenal concentrations of metal. These metals can be preserved and covered by later, younger material, and these form what are typically referred to as massive sulfide deposits. Massive sulfide deposits occur all over the world, where you have bits of ocean crust preserved and where you can access them and see them.

Oliver Strimpel: So does that mean that around these black smokers, at these spreading ridges, we have economically interesting deposits of all these metals that you mentioned forming at the present time?

Laurence Robb: That's right. That's the amazing thing. One of the very few places where you can see an ore deposit forming in real time is on the ocean floor. If you've got a submersible, you could dive down and witness the formation of metal sulfide deposits in real time. Having said that, there are more accessible environments where you can see mineral deposits forming in real time as well, such as the geothermal areas of the North Island of New Zealand. These fluids are typically concentrated in and around volcanic vents again. Again, this is just groundwater that's been heated up and is coming back up to the surface, and a lot of these fluids have interacted with the continental crust, in this case picking up things like gold and silver, and then reprecipitating them on the Earth's surface. So these geothermal hotspots, like Old Faithful and other well-known geysers around the world, are areas where metals are actually being concentrated by hot, aqueous hydrothermal solutions at the present time.

Oliver Strimpel: Is this concentration of the minerals in these hydrothermal vents, in these hot areas, similar to what's happening when we extract salt from seawater by evaporating brine on a salt flat?

Laurence Robb: Yeah, it's very analogous. There's a lot of talk these days about the so-called battery metals: metals like lithium, the rare earth elements, which are used for making magnets, and a bunch of other so-called high-technology metals, like scandium and yttrium and cobalt and tungsten. A lot of these metals are in demand as a result of the whole energy revolution, the need to move away from burning fossil fuels and polluting the atmosphere with carbon dioxide. We need to switch to different forms of energy, wind energy and solar energy and so on, and these technologies need a whole swath of different metals, of which something like lithium is a key element.

So where does lithium come from? Most of the lithium at the moment is produced in Chile, but not from conventional ore deposits at all. What they're doing in Chile, and in other parts, in Bolivia and Peru as well, is that they have these huge salt flats where, because of very high rates of evaporation, you get salt precipitated in these huge flat-lying regions. A lot of the water which circulates in and around the salt flats is very saline, very rich in chlorine, in potassium, in sodium, and in certain places in lithium. If you drill a shallow hole and pump that groundwater onto the surface in these very hot, dry environments, so that evaporation takes place very quickly, you get the salts precipitated out of the normal groundwater. A lot of those salts, as I say, are simply sodium chloride, halite, and potassium chloride, but in amongst those salts there are also very strong concentrations of lithium. So most of the world's lithium is being derived by evaporating normal groundwater in these huge salt pans, particularly in Chile and Bolivia and Peru.

Oliver Strimpel: You mentioned these metals that we need now that we're in the information age, the lithium that we get by evaporating solutions that have lithium chloride. But what about the other ones, like cobalt and indium and scandium and gallium and so on, all these things that are in smartphones? Where do these get concentrated?

Laurence Robb: Some of these metals are a byproduct of conventional mining, things like scandium and indium. If you go back to the normal massive sulfide deposits, which we talked about earlier, which are typically copper and zinc, as a byproduct of some of that mining you get some of these other metals coming out. They have to be extracted very specifically from the residues of the ore that is normally being extracted for copper and zinc. So some of these metals are coming out of conventional processes, but others occur in more exotic magmas, like carbonatites. A carbonatite is a very unusual thing. Going back to the discussion we had about kimberlites, kimberlites and carbonatites are somewhat related, but carbonatites very often have a very unusual suite of metals concentrated within them, including the rare earth elements, elements like dysprosium, indium, holmium, lutetium, lanthanum and cerium. A lot of these rare earths, which are being used particularly for the manufacture of smartphones, come from the mining of carbonatites, and although they're not common, there are some very rich and very big carbonatite mines in the world. Probably the best known one is in China, at a place called Bayan Obo, which is in fact the largest rare earth element deposit in the world.

Oliver Strimpel: So let's talk a bit about how we find mineral deposits. Ancient man was evidently able to find plenty of iron, as well as copper and tin, which are needed to make bronze. How do we find these economic resources?

Laurence Robb: A hundred years ago and further back, mineral deposits were located because people simply tripped over them. You could see a very rich accumulation of copper because it usually oxidizes to a very bright green color, so it sort of hits you in the eye. And so most of the early mineral deposits were located and found because they were obvious on the surface. But a lot of these deposits were very quickly depleted, and there are very few undiscovered mineral deposits on the Earth's surface at the present time, which means that people have to look below the surface. This is where the science of exploration geology, exploration geochemistry and exploration geophysics comes in, utilizing the indications of hidden mineral deposits on the surface to try and locate them at depth.

One of the tools that exploration geologists use is geochemistry. If you have, for example, a gold or a copper deposit several hundred meters below the surface that is not evident on the surface at all, there is a dispersion of metals, particularly the more volatile metals associated with that deposit, that makes its way to the surface, so that there's always, or pretty much always, some kind of indication on the surface of a hidden deposit at depth. For example, gold often occurs together with elements like arsenic and mercury and antimony. These metals are fairly volatile; they diffuse very rapidly and easily in the Earth's crust. So if you've got a gold deposit 500 meters below the surface, you might not see the gold on the surface, but you might well see the mercury or the antimony or the arsenic signature of that deposit on the surface. So if you systematically analyze the surface soils for arsenic and antimony and you suddenly find a structure which is elevated in terms of antimony and mercury contents, you then have every justification to suspect that somewhere below the surface there may be something of interest, like a gold deposit.

Oliver Strimpel: OK. So you mentioned the geochemical fingerprint. What about geophysical methods?

Laurence Robb: Yes. Take a deposit like a massive sulfide deposit. These are just concentrations of massive sulfide minerals, which means that the resistivity of that deposit is very different from that of the surrounding rocks. If you set up a bunch of electrodes around where you think a deposit might be and insert a very high-voltage electrical current into the Earth's surface, that electrical current will pass through your orebody, if it's there, because of its high conductivity and very low resistivity. The way in which the electrical potentials are set up would be very different if there is an ore deposit there than if there isn't one there at all. That process is called induced polarization, and the technique is used around the world. Another tool you could use: a massive sulfide deposit is going to be very dense. Its density will be substantially greater than the density of the surrounding rocks. So if you measure the gravity field over your deposit, you should pick up a high gravity anomaly over an occurrence of very dense rock.

Oliver Strimpel: What about magnetic properties?

Laurence Robb: Exactly the same. Gravity, various electrical methods and magnetic properties are the three key geophysical techniques that are used around the world in exploration.

Oliver Strimpel: Petroleum exploration is a very advanced art. Can we borrow techniques from that to find minerals?

Laurence Robb: Yes, we definitely can. The search for petroleum is very different in many respects from the search for solid minerals, but there are techniques which are common to both. The petroleum industry is somewhat more advanced than the minerals industry in terms of the way it has developed geophysical techniques. They don't use geochemistry in nearly the same way as minerals exploration does, but geophysical techniques have been advanced a great deal by the petroleum industry. The reason for that is that there's no geochemical signature of an oil trap way below the surface, but there are geophysical signatures. And it's very often so expensive to drill a borehole for an oil deposit, particularly if you're drilling offshore. You can imagine the cost of drilling one borehole in the North Sea off a platform is probably tens or even hundreds of millions of dollars, compared to the cost of drilling a borehole in the Sierra Nevada of the United States, where it might only cost you a hundred thousand dollars. There's a huge discrepancy in the cost. So the petroleum industry is very, very good at predicting where its boreholes should be located. They've taken the science of exploration, particularly geophysics, to a very advanced level, and those techniques and that know-how are continuously being borrowed for the exploration of mineral deposits.

Oliver Strimpel: As you mentioned earlier, the demand for certain metals or other elements that we really had no use for before has rocketed as we've developed the semiconductor industry and smartphones and lithium-ion batteries and so on. Do we have enough of these to satisfy the demand?

Laurence Robb: In general, I guess the answer is yes; by and large, we have fairly adequate supplies. The challenge is that the supply is not evenly distributed. Here in the UK, for example, we're not awash with mineral deposits, so we rely on external suppliers for whatever we might need, and the same is true in Europe. Whereas in South America, in countries like Chile and Peru and Bolivia, which are awash with mineral deposits, supply, of course, is less of a problem. And likewise, if you take the rare earth elements, 80% of which come from China, there are discrepancies in the supply. So it's not the abundance in terms of global supply that is so much the problem; it's more the local distribution of these deposits, and access to them, which is a potential problem. It's the geopolitical issue, in terms of where the deposits are and how people use access to those deposits in a political sense, that is potentially a problem.

Oliver Strimpel: What is your own research focus at the moment?

Laurence Robb: We're very interested in tin and in the related metals associated with tin, metals like tungsten and the rare earth elements. Tin is a metal that used to be in very abundant supply 50 years ago, and a lot of the big tin mines, for example in Southeast Asia, particularly in Malaysia, closed down, for many reasons: a drop in the price of tin, a drop in the demand for tin, environmental reasons. And then there was a resurgence in more recent years in the demand for tin, particularly because it's used so extensively as a solder to replace lead, and it has a lot of other high-tech uses as well. There are big disparities in where tin deposits occur. There's a lot of tin in South America, there's a lot of tin in Burma, but elsewhere in the world there are very few deposits. So researching tin and tin mineralization is one of the things that I've been very interested in.

To that end, we have a group here in Oxford that has been working over the past few years looking at tin deposits in Southeast Asia, and more recently particularly in Burma, Myanmar. Myanmar is an extraordinarily rich and well-endowed country in terms of its mineral deposits, but very little exploration and modern study of the geology of Myanmar has been carried out in the past 20 or 30 years. So there's a big gap there in terms of our understanding, and it's a fascinating country to work in. That has been a focus of my research, certainly over the past six or seven years.

Oliver Strimpel: For my last question, I'd just like you to speculate a bit about the future. It's often said that we've entered an information age, where value has shifted from atoms to bits. But from our discussion, it certainly seems as if we rely as much as ever on atoms, only perhaps ones that are arranged in different elements from those that drove the Industrial Age. How do you see our demand for atoms in the future?

Laurence Robb: We are in the information age, and information technology is all about bits, but if you're going to have bits, you need silicon, you need germanium, you need semiconductors. As you said, you need natural resources to drive the information technology. So you're right, there's a shift in the demand for types of metals. We don't need a lot of these metals to make a semiconductor; you only need a few micrograms. Maybe you have different types of metals there, and the amount of metal that's locked up in your smartphone, for example, is a matter of grams, not tons. But nevertheless, it's a very different type of metal, a very different range of metals that we seek. Having said that, at the same time, global population is now, what, seven or eight billion, and we're heading to maybe 10 or 11 billion by the end of the century. So there is, and always will be, a continuing demand for the basic natural resources: iron, aluminum, copper, lead, zinc, and even metals like gold and silver, as well as the rare earth metals, tin, tungsten and all the other strategic metals that are now increasingly in demand. The whole custodianship of natural resources is still incredibly important. We as Earth scientists need to be aware that ultimately we are the custodians of global natural resources, and we need to look after them responsibly for future generations, which is why Earth sciences and geology are such a great career for young people in the future.

Oliver Strimpel: Laurence Robb, thank you very much.

Laurence Robb: A pleasure. Good talking to you.

Oliver Strimpel: For more about Geology Bites, as well as pictures and diagrams that illustrate this podcast, you can go to geologybites.com.