Dear readers, after the first year and a half of SULTAN blogs, I hope you know quite a lot about us ESRs by now and understand what we are doing within the SULTAN project. But I am sure you may still have some questions, like ‘Why is Work Package 1 (WP1) about geology?’, ‘Don’t geologists just study boring rocks?’, and ‘What on Earth in geometallurgy?’. Perhaps these questions have never crossed your mind, but I am going to try to answer them anyway in this blog!
What do geologists actually do?
The general definition of geology is the science concerned with the solid Earth, the rocks of which it is composed, and the processes by which they change over time [1]. According to Sheldon Cooper of The Big Bang Theory, “Geology is the Kardashians of science”. But where did geologists get such a bad rep? And why to people seem to think geology is boring?
Actually, geological studies cover a wide range of inter-connected and important subjects (Fig. 1), from natural hazards (earthquakes and volcanoes) to environmental studies (water management, pollution), and engineering (rock stability for construction or tunnels, dams etc) to oil and gas. Geological research also focuses on areas such as palaeontology (dinosaurs!) and the study of other planets, such as looking for signs of habitable environments in the geological record of Mars. So one geologist may work on a completely different topic to another geologist. How can such a wide range of studies be boring?!
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Figure 1 The diverse range of different fields of study within geology [2]
Of course, in the case of SULTAN, we geologists from WP1 are interested in the geology of ore deposits and mining. An ore deposit is defined as a concentration of valuable metal-bearing minerals in the Earths’ crust which can be economically extracted [3]. Economic geology is the study of mineral raw materials that can be used for economic and/or industrial purposes [4]. As part of SULTAN, we are interested in metallic mineral deposits and mineral resources. By using common geological techniques such as geochemistry, mineralogy, geophysics and petrology, we can better understand, describe, and exploit an ore deposit, or in this case a mine waste deposit [4].
What is geometallurgy?
That is a good question. Geometallurgy is a branch of geology that we are interested in for the SULTAN project, but if you ask 10 geometallurgists to define it, they will probably all come up with slightly different answers. Basically, geometallurgy combines geology with metallurgy, mining engineering, mineral economics and geoenvironmental parameters when studying ore deposits, to maximise the value of the resource whilst minimising technical, operational and financial risks [5].
Let me try and explain in a more understandable way. Rocks are complex mixtures of different mineral grains, and the properties of a rock depends on the properties of the individual minerals and how they are bound together, as well as other textural properties, such as the size of the grains. The behaviour of the ore throughout the entire mining value chain is affected by geology, from exploration, to mining and processing, to waste disposal and refining of final metal products [6,7].
For instance, let’s say we have found a gold deposit with very high gold content. You may think ‘Great, let’s mine it! We will make a lot of money!’. First, we have to take a closer look at how the gold grains occur. Really, we want coarse gold grains, in-between other grains so that we can crush the rock, easily liberate the gold grains (i.e. the surface of the grains becomes free) and recover them with a simple gravity circuit. However, we may find the exact opposite: the gold occurs in the crystal lattice of pyrite, and the pyrite grains are themselves totally enclosed in quartz grains (see Fig. 2 for an illustration of this). Now, it is not as simple anymore to recover the gold. We would need to crush and mill the ore a lot to liberate the pyrite grains from the quartz, then use froth flotation to recover the pyrite, then leach the pyrite concentrate to recover the gold.
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Figure 2 Example of (A) coarse gold grains occurring on grain boundaries, and (B) gold occurring within the crystal lattice (in solid-solution) of pyrite grains, which are finely disseminated within quartz grains. In (A), the gold would be more easily liberated during crushing, whereas in (B), the pyrite grains would be harder to liberate and would require finer grinding, increasing energy consumption. More processing stages would be required to recover the gold in (B), increasing the costs of gold recovery.
Perhaps there are many, thin veins running through a very hard rock, but the veins are separated by great distances and there would be no easy way to mine the veins. Now imagine that the deposit is found in a very remote area, with no pre-existing infrastructure, and many protected species of animals and plants. All of these kinds of factors combined may mean that it is not economic at all to mine the deposit. This may sound very unlucky, but it is not unusual to encounter such problems! By applying a geometallurgical approach early on in a project, risks can be reduced, and we can determine how we would expect the ore to behave during mineral processing to recover the desired metals from the rock [5-7].
Are geology and geometallurgy important for SULTAN?
Of quartz! WP1 of SULTAN is for ‘Geological mapping and geometallurgy’ of the three tailings case studies (Neves Corvo, Portugal; Plombieres, Belgium; Freiberg, Germany). This includes sampling the tailings and waste materials, characterising them and carrying out geometallurgical and geostatistical modelling. Our aim is to understand the composition of the tailings materials, what metals are there and what minerals they occur in, if the metals can be economically extracted, if the tailings poses a threat to the environment and if the tailings could be used in other applications, such as building materials.
All of this requires a deep understanding of many factors, from the geology of the original ores to the methods used to originally process them and the deposition of the tailings. We will analyse many samples of the tailings to tell us about the mineralogy and geochemistry of the materials, and then use geostatistical techniques to build models of the tailings storage facilities. Using automated mineralogy techniques, we can study the rock textures to assess geometallurgical parameters and determine what processes would be needed to recover any valuable metals in the tailings, or if it is likely that the tailings will produce acid mine drainage from the oxidation of sulphide minerals such as pyrite.
In this way, we are treating the tailings and mine wastes as if they are primary ore deposits which need to be characterised and modelled. A rock is defined as a naturally occurring solid mass or aggregate of minerals, and they are usually grouped into three main types of rock: igneous, sedimentary and metamorphic [8]. So do tailings really fit to this? Well, they may not be naturally occurring, since they are formed by anthropogenic activities, but tailings are still aggregates of minerals. In fact, tailings are often deposited in tailings storage facilities as a slurry with high water content. The tailings particles are then deposited in a way similar to how a river deposits sediments. So perhaps we should view tailings as man-made sedimentary deposits! (see a little joke in Fig. 3). Complicated layering in the tailings is then over-printed by later weathering and metal transport processes, so we could go even further and say that tailings can be altered man-made sedimentary deposits. And what if the tailings have high contents of valuable metal-bearing minerals, and it would economic to recover said metals? Could the tailings then be called a secondary ore deposit?
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Figure 3 Can tailings be called a rock? Well, they are deposited by sedimentary-style processes [9]
After centuries of mining, there are many mine waste and tailings deposits all over the world. Many of these could indeed be exploited again as secondary results, while many instead are hazardous to the environment due to pollution (acidic waters and heavy metals) they are releasing, or issues with dam instability. It is becoming increasingly important that we re-assess such deposits, to minimise damage to the environment, risk of dam collapse, and recover any value that remains. After all, it is much easier to re-mine an old tailings deposit which sits on the surface than a deep and low-grade deposit, where the ore needs to be mined, transported to the surface and crushed!
So, to conclude, you may think that geologists are old men with beards who like to hammer rocks and have an unhealthy interest in boring old rocks. I hope that I have managed to convince you that this is not entirely true (we are not all old men with beards), and that geology is an important and interesting subject for many reasons!
References
[1] https://en.wikipedia.org/wiki/Geology [2] https://www.geolsoc.org.uk/Geology-Career-Pathways/What-is-Geology/Subject-Areas [3] https://en.wikipedia.org/wiki/Ore [4] https://en.wikipedia.org/wiki/Economic_geology [5] Dominy, S.C., O’Connor, L., Parbhakar-Fox, A., Glass, H.J. and Purevgerel, S., 2018. Geometallurgy—A route to more resilient mine operations. Minerals, 8(12), p.560. [6] Hunt, J.A. and Berry, R.F., 2017. Geological contributions to geometallurgy: a review. Geoscience Canada, 44(3), pp.103-118. [7] Lipton, I. 2019. Geometallurgy: The cornerstone of whole-of-mine optimisation. The AusIMM Bulletin, 52. [8] https://en.wikipedia.org/wiki/Rock_(geology) [9] https://memebase.cheezburger.com/puns/tag/sherlock-holmes