Geology: Magnetic Separation & Hydrothermal Orebodies

Hello and welcome to The Con episode 35. Today I'm going to talk about who is doing exploration and the different approaches of juniors and majors, as well as providing an introduction to understanding concentration factors through mineral systems. But it is just the introduction. Consider much of this episode as a warmup to a more technical deep dive into the constituent parts of a mineral system, which will come up in The Con 36.

But before I dive into the theory behind academic and applied exploration, I first want to touch on the dollars behind applied exploration. What is the current status of the exploration industry? We all know by now that exploration is a difficult and expensive business. Difficult and expensive as it is, however, the industry keeps going. It continues. Resource companies keep raising investment capital and they keep spending it on exploration.

They keep going in the hunt for the next mineral deposit and for the chance to create wealth. I return to The Wealth of Nations by Adam Smith, who defined wealth creation as requiring the successful combination of ideas, people, capital and land. There is data, of course, which reveals the ideas people have in the form of exploration, strategies and on the land on which the capital is deployed in the form of mineral concessions.

Some of the best data available is collated and interpreted by an Australian consultant called Richard Schoder. He tracks global exploration trends within his company, MinEx Consulting. MinEx is a great source of information, and I recommend that you check out Richard's presentations and publications, which he updates regularly. His most recent presentation estimates that four billion Australian dollars were invested by Australian companies into exploration last year in 2022.

A vast majority of that money will have been spent in Australia itself. Richard also keeps track of discovered rates, exploration spend by metal and the process of value creation through exploration over the years. I will take these Australian MinEx consulting numbers as being a proxy for the industry as a whole in 2022 and where applicable for the decade prior. According to MinEx consulting over the past 10 years, junior companies in Australia made 75%, three quarters, of all the reported discoveries and generated $2.10 of value calculated from project NPVs for every expiration dollar spent. This, on paper at least, is a good return. You spend $1 and you return $2.1. Not bad. In reality, there are many failures and a few winners, so the returns are quite lumpy. And also a theoretical net present value as defined in a technical report is very different from actual return on invested capital from profitable production. But that's an entirely separate area of discussion.

So moving on, we've had a look at the junior companies. In contrast, the larger companies only accounted for a quarter of the reported discoveries and their returns were lower, only generating 58 cents on the dollar.

It's not quite as black and white as junior exploration good, major exploration bad though. Remember that the majors are only focused on the biggest deposits and so they will write off or keep private within their portfolio or just ignore anything that doesn't reach a critical internal company threshold. And it's harder to find the really big deposits, the deposits that move the dial, move the needle for the major companies. And this means that the investment return on exploration at first sight is lower, but the threshold criteria are so different that it's not quite an apples versus arrow apples comparison.

Incidentally, there are critics of the exploration work carried out by large companies, by the majors, with a few regular complaints always being aired. Detractors say that the top-down and corporate safety and process culture in the majors means that no real field work can be carried out anymore. Head into any pub or bar in a mining town and you can find grizzled geologist-as-hero figures laughing at how the corporate geologists from the majors haven't even been able to head into the field because the roll bar specification in the vehicles aren't up to spec or similar. Generic head office rules, often designed for active mine sites where there are lots of big, heavy, potentially deadly bits of machinery moving about, prevent actual exploration activity from taking place. And executives who eventually leave the majors to start independent life in the junior sector often complain about the excessive safety briefings that accompany policy even when having a meeting in HQ offices downtown, for example, in the majors that is. The... the... the buccaneering, self-reliant, rugged, individual junior mining executive happily lampoons the complacent, siloed, cosseted, sheltered big company executive by asking if they have had the corporate chip removed from the base of the skull. But I digress.

There are criticisms of politics and empire building within the bigger companies, which inhibit good quality exploration. Top-down decisions to leave commodities at a stroke or countries on a whim can also leave good quality exploration teams work programmes suddenly stranded or hanging.

And of course, there is always the criticism that the lack of accountability on exploration within a well-funded major means that not enough useful money actually gets spent on the ground in the right way.

The counter argument is that juniors are always running out of money and can be poorly disciplined. Any money raised, a lot goes on GNA and very little goes in the ground. Budgetary constraints and sometimes unclear organisational structure means that reporting can be haphazard. QAQC occasionally slipshod and processes and procedures not as rigorous as are needed to have full confidence or to give full confidence in the data. Sometimes basic work needs to be done again when the project is much further advanced, adding cost and time delays at an always inopportune moment. Majors, on the other hand, have the budget to carry out the large geochemical and geophysical surveys that help capture regional data sets. When the work is done, the bigger companies make sure it is usually thorough, well-funded and extensive.

Pros and cons with both juniors and majors.

Regardless of who does the better exploration, the underlying fact is that exploration geologists working in companies of all different shapes and sizes need ways to improve their success rates. In fact, what we're seeing at the moment is that there's a tendency for the big companies who have the money, but the inability to actually do exploration on the ground because of safety restrictions are funding the junior companies who are nimble and are able to actually get out and do the work but don't have the money. So there's a kind of a symbiosis which is in progress at the moment between the major companies between the major companies and the junior companies. And in all of this the concept of mineral systems is useful.

Previously in the con I have spoken about natural concentration processes that are required to take background levels of metal content up to the enriched levels of an economic deposit. The discussion of mineral systems begins to lift the lid on the recognition and classification of those natural concentration processes and how they are used as an exploration tool in the resources industry. Mineral systems are a way of explaining how all deposits are formed. They are the means to explain how background crustal abundances of certain elements are concentrated in various processes, resulting in a highly localized enrichment of value mineral - an autoposite.

The mineral systems approach has its roots in the oil industry. The old boys have long known that there are some key components in any commercial hydrocarbon deposit. One, a source rock rich in hydrocarbons. Two, a fluid pathway along which the molecules can travel. Three, a reservoir rock suitable for hosting an accumulation of oil or gas or both. And four, a trap rock or trapping mechanism such as a fold or a salt dipyre to keep the oil and gas in place or indeed an anticline or a fold structure.

Obviously, there is a time component to this and a temperature and pressure component as well. The oil kitchen. Hot enough to mature the buried organic matter into crude, not so hot as to overcook the molecules and overcrack the chains. Before oil or gas wells are drilled, I'll do that again, edit. Before oil or gas wells are drilled, a vast amount of work is done to establish

Cut, cut, cut.

Before oil or gas wells are drilled, a vast amount of work is done to establish the probability of oil or gas being in the place that the geoscientists think it will be. Much of that work is estimating the presence and quality of the source, the pathway, the reservoir, the trap and the working of the oil kitchen. All of the right ingredients in the system are needed if there is to be a commercial discovery. And the same applies for hard rock deposits.

In 1994 scientists Wyborn, Heinrich and Jacks published a paper bringing mineral systems to the attention of hard rock geologists. They wrote, and I quote: “that the processes involved in creating mineral deposits are mappable on a district to regional scale and constitute a mineral system in which the ore deposit is the central feature.” They proposed that, and again I quote:

“A mineral system can therefore be defined as all geological factors that control the generation and preservation of mineral deposits.”

They stressed that the processes that are involved in mobilizing all components from a source, transporting them and accumulating them in more concentrated form and then preserving them throughout the subsequent geological history can be understood and therefore predicted. Even though the deposit itself may be small in size relative to the geography of an area, the total system of fluid rock interactions that led to all formation can extend over a distance of tens to hundreds of kilometers around the deposit. This mineral system usually provides a far larger exploration target than the actual ore deposit itself.

Now, the concept of providing a framework for exploration models proved to be appealing. It's much easier to get a budget from a board of directors or from new investors when exploration managers or CEOs can explain in a succinct manner why they need the money.

Fast forward 29 years, and there are now many ways that mineral systems are presented, but the fundamental principles are similar. Throw all the right ingredients together in the right order, in the right conditions, and you will get a predictable outcome. More than that, you will get an inevitable outcome. And furthermore, this is an inevitable outcome with the value object in the form of a deposit surrounded by a much wider and larger halo of clues pointing, if they can be adequately interpreted, to the center of the system.

Mineral deposits this away.

We all know that water pipes can burst in a cold snap, right? The mineral system analysis of the problem would be able to predict that if you have a standing water pipe at atmospheric pressure which is then exposed to temperatures well below zero degrees Celsius the water will turn to ice, expand and the pipe will rupture if the pressure exerted by the expanding ice exceeds the failure limits of the pipe. The laws of science are clear.

In this instance, mineral system analysis would also be able to point you in the direction of where the action is at its most intense by following the large network of pipes to the place where the outdoor tap, for example, is exposed to the elements. This is the vectoring element of the process. It may sound silly for that particular metaphor, but the concept is the key.

Another example. We all know that if you mix flour, butter and sugar in appropriate quantities and then bake it, you will inevitably end up with cake. Follow the recipe to the letter and the cake will be predictable. Shake it up a bit and go cross country by changing some of the parameters. This is my cooking style. Oh I think it'll be good with a few of those thrown in and a bit of this as well. And the results will vary. With my cooking there are some experimental successes. At other times it's less cordon bleu and more cordon bleurrgh.

But the mineral system, the mineral systems approach is a bit more complicated than these illustrations, given that there are often millions or even billions of years of geological history to handle. But the idea is the same. If all of the ingredients and conditions are met, then there has to be a mineral deposit formed. Important geological factors defining the characteristics of any mineralizing system include certain key features just like the way that the oil boys described. And those key features are:

1. A mineralizing fluid with the energy and chemistry to carry metals.
2. A source rock with an endowment of metals and other ore components.
3. Mechanisms to drive the fluid and pathways to enable the transport of metals from source. A trapping mechanism that causes the value mineral to be preferentially concentrated in a particular spot, the deposit site. Whether the process is gradual or sudden, an economic deposit is formed through that localised enrichment, which is the final stage of a mineral system.

In the Con 36, I will provide extra detail on those key features of a mineral system. Consider it a bonus episode for the proto-geeks among you. Until then, however, thank you for listening to this talk about exploration and mineral systems, and goodbye.

NOTES

Magnetic Separation

This is where you get separation of all minerals by fractional crystallisation and related processes during magmatic differentiation. What this means is that in your magma mix, you might have an elevation of PGNs of Nickel, and what happens is the first minerals that precipitate out as this thing cools down or changes pressure or temperature may not be rich in Nickel, for example, of PGNs, but as more and more crystals crystallise, precipitate out, the remaining melt is richer and richer and richer in Nickel until finally when the Coper and the Nickel do precipitate out, they form a significant portion at that stage of the fractionated crystal part.

You can get Copper-Nickel orebodies in Sudbury, Canada, in the USSR. You can get Titanium deposits such as in Allard Lake in Quebec in Canada. These can be very extensive. Think of the Bushveld or the Norilsk-Talnakh orebodies of central northern Russia. Those are valuable and key producers of Nickel, Copper, and PGNs.

Hydrothermal Orebody

These are the depositions from hot aqueous solutions that may have had a magmatic or a metamorphic or a surface or another source. Think of metals being carried in solution and then precipitating out. It’s sometimes useful to think of an Oil process. You need to have a source, a mechanism that drives the Oil out from the source rock into a potential reservoir. You need that potential reservoir, and then you need a trap to contain the Oil in your reservoir. That is when you get the accumulation. In some ways it’s very similar when you’ve got a hydrothermal geological process, a hot aqueous solution, you need to have a source, the material needs to be leached or brought into solution in 1 area, perhaps in the magma, which is being fractionated and is being continually enriched as non-mineral bearing crystals crystallise out, or you can have the magma as a heat engine which is driving fluids through a big rock mass. Essentially you need some kind of source of the metals, you need a heat engine, and it could be tectonics, it could be pure collisional processes, it could be burial, it could be force, it could be intrusion related heat thing but you need to get the metal together, get it into the solution, and move it through a rock mass up to a point where it’s going to want to preferentially precipitate out and it’s going to want to stay there and get concentrated over time. Pulse after pulse after pulse of fluid coming through, it’s going to want to drop out the metal at that spot there. That is how you get your concentration of 80 times for Copper, or 250 times for Gold because you found a geological setting that is favourable for precipitation.

These are porphyry Copper deposits of the Andes, of PNG, of Bingham, of Arizona. These are Golden vein deposits in the Greenstone. It can be to ore, or hydrothermal, it can be lateral secretion, diffusion of ore-forming material from the country rocks, but it’s essentially carried by hot minerals. They can be driven by metamorphic processes. It’s hot fluids finding the catalysts for precipitation. Precipitation is controlled by many factors but in principle, you need to look at a change in pressure, a change in temperature, and a change in the chemistry. If you’ve got a solution that is rich in metals, that solution may be able to carry the metals at a certain pressure, at a certain temperature, at a certain chemistry, but if you changed those 3 things, or changed 1 of those 3 things, or 2 of those things, or all 3, the solution may not be able to hold the metal in solution form and therefore it will precipitate out.

Many of these hydrothermal fluids are weakly acidic so you get this wall rock interaction. As the hydrothermal fluid comes through the rock, it interacts with the wall rocks which changes the chemistry. For example, this is where you get Skarn deposits. You have an intrusion and an acidic, magmatic fluid circulating in place next to a Limestone deposit or Carbonate deposit, which is typically alkaline. The 2 meets, it drops the acidity of the fluid, the solution, the metals are no longer stable in the solution and therefore they precipitate out and as the heat from the intrusion keeps chucking out or driving acidic hydrothermal fluids from the intrusion, and it keeps getting buffered on the Carbonates, therefore you keep getting this precipitation in your Skarn deposit. That is where you get your concentration factor.

The interaction of your fluid will depend on the grain size of the rock that it’s coming into contact with in terms of the physical state of the rock that it comes into contact with. Is it highly sheared or unsheared? Is it deformed or under formed? Does it have good permeability through fracturing or is it sealed together? Is it plastic or is it brittle? Are there cracks? Is it opening up? I’ve just mentioned the buffering of solutions. This enrichment process depends on so many factors.

When you are exploring for mineral deposits, you have to really hope that you’ve got a big mineralising system. You want something that feels pumped and juicy, but it can be the case that it’s almost too flushed. The mineral fluids have come through, there’s a lot of silica, it’s a big hydrothermal alteration system but actually, there’s been no concentration system that’s kept the metals, the value material, tightly concentrated. It’s been flushed through. What you’re really looking for is the large hydrothermal system and the cubic kilometres of altered rock, but you absolutely equally need a localising factor and a focus point.

Quite often that is dilation of a space opening. This is where the understanding of your structure is so critical. You need to have your structural models well understood. You need to know this is where the force is and therefore if you point two fingers together, that point there, you’ve got compression and that’s unlikely to encourage fluid flow and yet forces in geology are never entirely orthogonal or straight on, there’s always at some angle, therefore if you’ve got something that’s not quite straight, there’s always a space opening, there’s always some dilation.

You need to understand your stress and your strain, compensation in rocks and in particular your pressure shadows, and where the fluid might flow. You may have seen quite a lot of presentations with people talking about the primary structures or first-order structures, and secondary structures, and the intersection between the second-order structures and the third-order structures that you get the space creation and that’s for the preferential localisation of fluid flows. That needs to be understood. It can happen on a regional scale over tens of kilometres, and it can happen on the deposit scale over tens of metres, or if you’re in a complex geological area, it can happen down on the metre scale and even down to the centimetric.

I’ve got a little exercise for you now. With your elbows by your side, put your hands together in a flat-handed prayer position, but don't point your fingers straight up, lower your finger tips to about 45o, or half way between your nose and horizontal / parallel to the floor.  Open your palms a little bit, like reading a book without letting your neighbours see (keep your hands nice and flat).  What you should be able to see is that even though your 2 hands represent two steepling tilting planes that are facing in towards each other (where they meet along little fingers), your little fingers have a line of intersection (where the two hands meet) that is just a line and not a plane, and it is in a different orientation  - it should in fact be dipping towards your belly button. That is the intersection line between 2 planes. Frequently there are structures where fluid flow comes up one or both of the planes , but it will preferentially congregate on that line where the 2 planes meet. There is a concentration of fluid flow along the intersection of the 2 planes. You have 2 planar features and where they intersect is a linear feature. Quite often in a mineral deposit, particularly for Gold deposits, there is preferential mineralisation down a plunging chute and that is what we’re talking about. It’s on a more complex version of the intersection between these 2 planes. You can put your hands down now.

The whole thing is further complicated when that has a life, the Gold that’s deposited there and the rocks solidify, and then they get buried again and the whole thing gets deformed and folded and sometimes sheared. It gets heated up again and you might get Gold moving around, being brought Ito the solution again and re-precipitated. It can be fearfully complicated. Remember that you’re looking for things that are going to assist nature to concentrate mineralisation in 1 spot. You’re looking for space creation, favourable chemistry, the right kind of fracturing, or the right kind of capping. For example, in some Uranium deposits, it might be that you have got some impermeable layer, some clay over the top that enables your roll front Uranium to pool underneath and just concentrate and concentrate. It’s getting to know the specifics of the kind of geology that you’re looking for.

I was going through the table of the simple classification of theories of mineral deposits, but I got completely sucked into the hydrothermal fluid flow. There are many other ways to make your mineral deposits. You can have mineral deposits formed at surface through mechanical accumulation. You can have a concentration of heavy or durable minerals into placer deposits such Rutile-Zircon Ilmenite sands. You can have Gold placers, Diamond placers; that’s the mechanical accumulation. You can have weathering features as Kaolin, which is the ingredient that goes into clays, which is weathered from Granite. Or you can have the leaching of silicates to get Bauxites.

At surface, you can have sedimentary precipitates, precipitation of particular elements in particular sedimentary environments, with or without the intervention of biological organisms at various points in the earth’s history. For example, between 1,800M and 3,000M-years ago, 2.5Bn-years ago, you had the Banded Iron formations of the Precambrian Shield. You had Quartz and Iron layers and layers and layers in a reducing environment, a climate without oxygen, and you get the big, Banded Iron formation deposits being laid down such as in the Hammersley Basin in Brazil, and also in South Africa. You can also have Manganese deposits like Groote Eylandt, just east of Darwin in Australia where the Manganese was being precipitated out into a Manganese-rich sea. You’ve got Zechstein Evaporites across Europe.

Then you can have residual processes where you have the leaching of rocks at the surface, so the soluble elements get leached away leaving concentrations of insoluble elements such as Nickel Laterites, the Bauxites of Jamaica, etc. Then there’s this secondary or supergene enrichment which is re-precipitation and/or rainwater circulation that leaches elements from the upper parts of mineral deposits and then re-precipitates them at a lower level in the deposit so you can get an enriched cap. Sometimes you can just have it as an erosional feature, sometimes you can have it where it actually lifts, enriches, at the upper levels as well. You might get an enriched oxide layer. The most famous one typically is the Chalcocite Blanket, which is a supergene enrichment of the Copper in the rock where the Copper in the upper area gets leached, re-precipitated down as Chalcocite, a very Copper-rich mineral, as a black blanket over the top of your buried porphyry. Then ideally, you’ll have a degree of erosion that takes off the leached upper portion so that it doesn’t take away the Chalcocite Blanket.

It’s very hard to find an economic deposit; most people find very few in their lifetime. A few people find more than 1. Most projects that you see on an exploration basis are not going to make it. You can take yourself to the right area by understanding metallogenic provinces. You can roughly work out where these minerals are going to be found but because we’ve got centuries, thousands of years, millennia of work in these mineral deposits, we roughly know where they’re going to be found and we roughly know what age groups you’re going to find them in.

For example, your Archean Greenstone belts, you’re going to find in the Archean, which is 3,800M-years ago to 2,500M-years ago. 3.8Bn to 2.5Bn-years ago. If you’re looking for Gold or Silver, that kind of stuff, you want to be in the Archean - you don’t have to be, but it certainly doesn’t harm to be in the Archean Greenstone belt. Similarly, the Iron ore deposits have almost all got their roots in the Archean Banded Iron formations. Just making a few observations of things that have stuck with me over my interactions with various geologies and mineral deposits over time.

There’s no rhyme or reason to this, no structure, just a few observations. If you’re looking for Gold, or when geologists are exploring for Gold, in a sense Gold is where it is. If you’re panning for it or exploring in hard rock, it’s important not to over-interpret too early. Just stick to the assay results, finding it where it is. If you’re panning for it, which is worth doing if you’ve got an interest in that kind of stuff, it doesn’t travel far. It’s so much denser than anything else; it’s got a density of 19.6g/cm cube so it’s almost twice as dense as Lead, it’s 7 times denser than most rock. If you’re looking for it, it just falls and so you don’t need to go very far. It hasn’t travelled far. That is also a useful exploration tool. If you’re panning as an exploration tool, it typically hasn’t travelled far and if you’re looking for it in rock, just because in the early days it’s always been associated with a particular mineral adularia like a Pyrite, or an Arsenopyrite. It doesn’t mean that it will continue to do so. I’ve seen it many times where the Gold actually in certain areas just does its own thing so stick to the assays for Gold.

The second thing, understanding structural controls in an orebody can take a lot of time. It’s hard to know where the Gold is going. If you’re an observer of results coming from an exploration team, don’t expect too much too early. Quite often it can take you 40 drill holes to understand exactly what the controlling structures on Gold are and even then, you won’t have it thoroughly nailed down.

Sometimes it’s easier but Gold and structure can be hard to understand. You need to have a really good geologist and a lot of drilling before you can start cracking the code of the orebody. You don’t want to be too rigorous. The code might work for a year or 2, you might be able to get a feel for the deposit, and then the further you go, the deeper or perhaps on strike, it might not behave in the same way. You always have to keep an open mind.

This is where you get separation of all minerals by fractional crystallisation and related processes during magmatic differentiation. What this means is that in your magma mix, you might have an elevation of PGNs of Nickel, and what happens is the first minerals that precipitate out as this thing cools down or changes pressure or temperature may not be rich in Nickel, for example, of PGNs, but as more and more crystals crystallise, precipitate out, the remaining melt is richer and richer and richer in Nickel until finally when the Coper and the Nickel do precipitate out, they form a significant portion at that stage of the fractionated crystal part.

You can get Copper-Nickel orebodies in Sudbury, Canada, in the USSR. You can get Titanium deposits such as in Allard Lake in Quebec in Canada. These can be very extensive. Think of the Bushveld or the Norilsk-Talnakh orebodies of central northern Russia. Those are valuable and key producers of Nickel, Copper, and PGNs.

Hydrothermal Orebody

These are the depositions from hot aqueous solutions that may have had a magmatic or a metamorphic or a surface or another source. Think of metals being carried in solution and then precipitating out. It’s sometimes useful to think of an Oil process. You need to have a source, a mechanism that drives the Oil out from the source rock into a potential reservoir. You need that potential reservoir, and then you need a trap to contain the Oil in your reservoir. That is when you get the accumulation. In some ways it’s very similar when you’ve got a hydrothermal geological process, a hot aqueous solution, you need to have a source, the material needs to be leached or brought into solution in 1 area, perhaps in the magma, which is being fractionated and is being continually enriched as non-mineral bearing crystals crystallise out, or you can have the magma as a heat engine which is driving fluids through a big rock mass. Essentially you need some kind of source of the metals, you need a heat engine, and it could be tectonics, it could be pure collisional processes, it could be burial, it could be force, it could be intrusion related heat thing but you need to get the metal together, get it into the solution, and move it through a rock mass up to a point where it’s going to want to preferentially precipitate out and it’s going to want to stay there and get concentrated over time. Pulse after pulse after pulse of fluid coming through, it’s going to want to drop out the metal at that spot there. That is how you get your concentration of 80 times for Copper, or 250 times for Gold because you found a geological setting that is favourable for precipitation.

These are porphyry Copper deposits of the Andes, of PNG, of Bingham, of Arizona. These are Golden vein deposits in the Greenstone. It can be to ore, or hydrothermal, it can be lateral secretion, diffusion of ore-forming material from the country rocks, but it’s essentially carried by hot minerals. They can be driven by metamorphic processes. It’s hot fluids finding the catalysts for precipitation. Precipitation is controlled by many factors but in principle, you need to look at a change in pressure, a change in temperature, and a change in the chemistry. If you’ve got a solution that is rich in metals, that solution may be able to carry the metals at a certain pressure, at a certain temperature, at a certain chemistry, but if you changed those 3 things, or changed 1 of those 3 things, or 2 of those things, or all 3, the solution may not be able to hold the metal in solution form and therefore it will precipitate out.

Many of these hydrothermal fluids are weakly acidic so you get this wall rock interaction. As the hydrothermal fluid comes through the rock, it interacts with the wall rocks which changes the chemistry. For example, this is where you get Skarn deposits. You have an intrusion and an acidic, magmatic fluid circulating in place next to a Limestone deposit or Carbonate deposit, which is typically alkaline. The 2 meets, it drops the acidity of the fluid, the solution, the metals are no longer stable in the solution and therefore they precipitate out and as the heat from the intrusion keeps chucking out or driving acidic hydrothermal fluids from the intrusion, and it keeps getting buffered on the Carbonates, therefore you keep getting this precipitation in your Skarn deposit. That is where you get your concentration factor.

The interaction of your fluid will depend on the grain size of the rock that it’s coming into contact with in terms of the physical state of the rock that it comes into contact with. Is it highly sheared or unsheared? Is it deformed or under formed? Does it have good permeability through fracturing or is it sealed together? Is it plastic or is it brittle? Are there cracks? Is it opening up? I’ve just mentioned the buffering of solutions. This enrichment process depends on so many factors.

When you are exploring for mineral deposits, you have to really hope that you’ve got a big mineralising system. You want something that feels pumped and juicy, but it can be the case that it’s almost too flushed. The mineral fluids have come through, there’s a lot of silica, it’s a big hydrothermal alteration system but actually, there’s been no concentration system that’s kept the metals, the value material, tightly concentrated. It’s been flushed through. What you’re really looking for is the large hydrothermal system and the cubic kilometres of altered rock, but you absolutely equally need a localising factor and a focus point.

Quite often that is dilation of a space opening. This is where the understanding of your structure is so critical. You need to have your structural models well understood. You need to know this is where the force is and therefore if you point two fingers together, that point there, you’ve got compression and that’s unlikely to encourage fluid flow and yet forces in geology are never entirely orthogonal or straight on, there’s always at some angle, therefore if you’ve got something that’s not quite straight, there’s always a space opening, there’s always some dilation.

You need to understand your stress and your strain, compensation in rocks and in particular your pressure shadows, and where the fluid might flow. You may have seen quite a lot of presentations with people talking about the primary structures or first-order structures, and secondary structures, and the intersection between the second-order structures and the third-order structures that you get the space creation and that’s for the preferential localisation of fluid flows. That needs to be understood. It can happen on a regional scale over tens of kilometres, and it can happen on the deposit scale over tens of metres, or if you’re in a complex geological area, it can happen down on the metre scale and even down to the centimetric.

I’ve got a little exercise for you now. With your elbows by your side, put your hands together in a flat-handed prayer position, but don't point your fingers straight up, lower your finger tips to about 45o, or half way between your nose and horizontal / parallel to the floor.  Open your palms a little bit, like reading a book without letting your neighbours see (keep your hands nice and flat).  What you should be able to see is that even though your 2 hands represent two steepling tilting planes that are facing in towards each other (where they meet along little fingers), your little fingers have a line of intersection (where the two hands meet) that is just a line and not a plane, and it is in a different orientation  - it should in fact be dipping towards your belly button. That is the intersection line between 2 planes. Frequently there are structures where fluid flow comes up one or both of the planes , but it will preferentially congregate on that line where the 2 planes meet. There is a concentration of fluid flow along the intersection of the 2 planes. You have 2 planar features and where they intersect is a linear feature. Quite often in a mineral deposit, particularly for Gold deposits, there is preferential mineralisation down a plunging chute and that is what we’re talking about. It’s on a more complex version of the intersection between these 2 planes. You can put your hands down now.

The whole thing is further complicated when that has a life, the Gold that’s deposited there and the rocks solidify, and then they get buried again and the whole thing gets deformed and folded and sometimes sheared. It gets heated up again and you might get Gold moving around, being brought Ito the solution again and re-precipitated. It can be fearfully complicated. Remember that you’re looking for things that are going to assist nature to concentrate mineralisation in 1 spot. You’re looking for space creation, favourable chemistry, the right kind of fracturing, or the right kind of capping. For example, in some Uranium deposits, it might be that you have got some impermeable layer, some clay over the top that enables your roll front Uranium to pool underneath and just concentrate and concentrate. It’s getting to know the specifics of the kind of geology that you’re looking for.

I was going through the table of the simple classification of theories of mineral deposits, but I got completely sucked into the hydrothermal fluid flow. There are many other ways to make your mineral deposits. You can have mineral deposits formed at surface through mechanical accumulation. You can have a concentration of heavy or durable minerals into placer deposits such Rutile-Zircon Ilmenite sands. You can have Gold placers, Diamond placers; that’s the mechanical accumulation. You can have weathering features as Kaolin, which is the ingredient that goes into clays, which is weathered from Granite. Or you can have the leaching of silicates to get Bauxites.

At surface, you can have sedimentary precipitates, precipitation of particular elements in particular sedimentary environments, with or without the intervention of biological organisms at various points in the earth’s history. For example, between 1,800M and 3,000M-years ago, 2.5Bn-years ago, you had the Banded Iron formations of the Precambrian Shield. You had Quartz and Iron layers and layers and layers in a reducing environment, a climate without oxygen, and you get the big, Banded Iron formation deposits being laid down such as in the Hammersley Basin in Brazil, and also in South Africa. You can also have Manganese deposits like Groote Eylandt, just east of Darwin in Australia where the Manganese was being precipitated out into a Manganese-rich sea. You’ve got Zechstein Evaporites across Europe.

Then you can have residual processes where you have the leaching of rocks at the surface, so the soluble elements get leached away leaving concentrations of insoluble elements such as Nickel Laterites, the Bauxites of Jamaica, etc. Then there’s this secondary or supergene enrichment which is re-precipitation and/or rainwater circulation that leaches elements from the upper parts of mineral deposits and then re-precipitates them at a lower level in the deposit so you can get an enriched cap. Sometimes you can just have it as an erosional feature, sometimes you can have it where it actually lifts, enriches, at the upper levels as well. You might get an enriched oxide layer. The most famous one typically is the Chalcocite Blanket, which is a supergene enrichment of the Copper in the rock where the Copper in the upper area gets leached, re-precipitated down as Chalcocite, a very Copper-rich mineral, as a black blanket over the top of your buried porphyry. Then ideally, you’ll have a degree of erosion that takes off the leached upper portion so that it doesn’t take away the Chalcocite Blanket.

It’s very hard to find an economic deposit; most people find very few in their lifetime. A few people find more than 1. Most projects that you see on an exploration basis are not going to make it. You can take yourself to the right area by understanding metallogenic provinces. You can roughly work out where these minerals are going to be found but because we’ve got centuries, thousands of years, millennia of work in these mineral deposits, we roughly know where they’re going to be found and we roughly know what age groups you’re going to find them in.

For example, your Archean Greenstone belts, you’re going to find in the Archean, which is 3,800M-years ago to 2,500M-years ago. 3.8Bn to 2.5Bn-years ago. If you’re looking for Gold or Silver, that kind of stuff, you want to be in the Archean - you don’t have to be, but it certainly doesn’t harm to be in the Archean Greenstone belt. Similarly, the Iron ore deposits have almost all got their roots in the Archean Banded Iron formations. Just making a few observations of things that have stuck with me over my interactions with various geologies and mineral deposits over time.

There’s no rhyme or reason to this, no structure, just a few observations. If you’re looking for Gold, or when geologists are exploring for Gold, in a sense Gold is where it is. If you’re panning for it or exploring in hard rock, it’s important not to over-interpret too early. Just stick to the assay results, finding it where it is. If you’re panning for it, which is worth doing if you’ve got an interest in that kind of stuff, it doesn’t travel far. It’s so much denser than anything else; it’s got a density of 19.6g/cm cube so it’s almost twice as dense as Lead, it’s 7 times denser than most rock. If you’re looking for it, it just falls and so you don’t need to go very far. It hasn’t travelled far. That is also a useful exploration tool. If you’re panning as an exploration tool, it typically hasn’t travelled far and if you’re looking for it in rock, just because in the early days it’s always been associated with a particular mineral adularia like a Pyrite, or an Arsenopyrite. It doesn’t mean that it will continue to do so. I’ve seen it many times where the Gold actually in certain areas just does its own thing so stick to the assays for Gold.

The second thing, understanding structural controls in an orebody can take a lot of time. It’s hard to know where the Gold is going. If you’re an observer of results coming from an exploration team, don’t expect too much too early. Quite often it can take you 40 drill holes to understand exactly what the controlling structures on Gold are and even then, you won’t have it thoroughly nailed down.

Sometimes it’s easier but Gold and structure can be hard to understand. You need to have a really good geologist and a lot of drilling before you can start cracking the code of the orebody. You don’t want to be too rigorous. The code might work for a year or 2, you might be able to get a feel for the deposit, and then the further you go, the deeper or perhaps on strike, it might not behave in the same way. You always have to keep an open mind.

Magnetic Separation

today I will start by introducing the concept of Ore within the context of mineral resources. And if and when complexities arise from a discussion of such a simple three-letter word as “ore”, I will branch out and follow a down a few side alleys that

Pierre Gy began his career in French Equatorial Africa (Congo) working on the small M’Fouati lead mine as the Mineral Process Engineer in 1946, where he was in charge of the processing plant and associ-ated laboratories. In 1947 the Paris-based head office asked Pierre to estimate the grade of a 200,000 t, apparently low-grade stockpile that had been dormant since 1940. He soon recognised i) that fragments on the stockpile varied from several tonnes to fine dust, ii) he knew nothing about sam-pling, iii) there was no meaningful literature available and iv) that he would have to improvise. This request planted the seed of life-long interest in his mind.On his return to Paris in 1949, his work in a mineral-processing laboratory also constantly brought issues of “sampling” to his attention, in particular the question of “the minimum sample weight necessary to achieve a certain degree of reliability”.1 In his search through the available literature, such as there was, Gy found that Brun-ton2 claimed that the minimum sample weight was proportional to the cube of the top particle size, while Richards3 sug-gested that the square of the particle size was important. Brunton2 based his ideas on the “constant proportionality factor”, mean-ing that for samples with different fragment top sizes, the same number of fragments was required, but Gy1 was concerned that variations in grade or density had not been properly incorporated.It was the magnitude of financial transac-tions in the coal trade based on assays for ash and sulphur in “coal samples” that pro-moted much of the early research into sam-pling. Gy tells about UK- and USA-based researchers that “realised that sampling actually generated errors that could have a financial impact”, and so began the interest in investigating coal properties in regard to particle top size, sample mass and sample variance.

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