W.L. Pohl:
Economic Geology, Principles and Practice: Metals, Minerals, Coal and Hydrocarbons
— an Introduction to Formation and Sustainable Exploitation of Mineral Deposits

(published April 2011)

Introduction

Human societies need sufficient water, productive soil, energy in different forms, organic, and mineral raw materials as a base for their physical existence. An additional important requirement is a healthy natural and socio-economic environment.

Economic Geology is a subdiscipline of the geosciences; according to Lindgren (1933) it is “the application of geology”. Today, we might call it the scientific study of the Earth’s sources of mineral raw materials, and the practical application of the acquired knowledge. Considering the life cycle of a mine, economic geology leads in the search for new mineral deposits and in their detailed investigation. It contributes to economic and technical evaluations, which confirm the feasibility of a project and result in the physical establishment of a new mine. While mining goes on, economic geology provides many services that assist rational exploitation, foremost by continuously renewing mineable reserves, and by reducing effects on the mine’s environment to a minimum. Possible negative impacts of mining include surface subsidence, lowering the water table, various emissions, and unstable or environmentally doubtful waste rock dumps. In the phase of mine closure, economic geology helps to avoid insufficient or outright wrong measures of physical and chemical stabilisation, recultivation and renaturalisation.

In recent years, the economic evolution of industrial and of rapidly developing countries caused incisive changes in supply and consumption of mineral raw materials. China rather than Europe or North America provides world markets with essential metals and minerals, although at the same time importing large quantities of needed feedstock for expanding industry and improving the people's quality of life. The future supply with petroleum appears to be unreliable, but its role as the main source of liquid fuels for transport is hardly dented by biofuels and other developments. Wind and geothermal energy are increasingly contributing to electricity production, yet without coal, nuclear power and natural gas, industrial economies would soon break down and developing nations would be locked in poverty. Ours is a time of transition but we cannot yet discern the outcome. Whatever it will be, metals, minerals and energy raw materials are certain to remain a precondition of human welfare.

What are ore deposits?

Ore deposits or mineral deposits are natural concentrations of useful minerals or rocks, which can be economically exploited. Concentrations that are too small or too low-grade for mining are called occurrences or mineralisations. It is very important to understand the economic implications of the difference between these terms. Unfortunately, their wrong application is common and leads to fundamentally misleading deductions. Therefore, the denomination economic ore deposit may be used when a clear attribution to this class is to be emphasised. Note that not all ores are strictly natural – it is very common that waste of a former miners’ generation is today’s profitable ore, such as tailings of earlier gold or diamond mining.

Mineral deposits are basically just valuable rocks, so that their formation is compared with processes that have produced ordinary rocks, and is investigated with petrological methods (Robb 2005). They can also be thought of as a geochemical enrichment of elements or compounds in the earth’s crust that is determined by their chemical properties (Railsback 2003). The ratio between the content of a valued element in an ore deposit and its crustal average (Clarke values, Wedepohl 1995) is called the concentration factor. Formation of an iron ore deposit with the typical grade of 60% Fe relative to an average crustal iron concentration of about 5% requires 12-fold concentration. A copper deposit that has 1% Cu compared to the crustal average of 0.007% Cu in the crust exhibits a 140-fold enrichment. Gold ore with 10 gram/tonne "distilled" from ordinary rocks with 0.002 g/t Au attests to a 5000-fold concentration.

Manifold are the processes and factors leading to the concentration of elements and minerals, including the formation of mineral deposits (Robb 2005). Final causes are the dynamic interactions between the Earth’s core, mantle and crust, and of the hydro-, bio- and atmosphere. Cooling and devolatilization of the Earth and unmixing of the system in the geological-geochemical cycle and during the transfer of elements have important roles (Lehmann et al. 2000b). With reference to the origin, endogenous and exogenous process systems are distinguished. The first are those resulting from the dynamics of the Earth’s interior, that are ultimately driven by the Earth’s heat flow. At present the total heat flow at the Earth's surface — 46±3 terawatts (1012 J/s) — is the result of heat entering the mantle from the core, of mantle cooling, radiogenic heating of the mantle from the decay of radioactive elements and of various minor processes (Lay et al. 2008). Exogenous processes take place at the Earth’s surface and are mainly due to the flow of energy from the sun. In rare cases, extraterrestrial processes have contributed to the formation of mineral deposits by impact of meteorites and asteroids.

The origin of mineral deposits is often due to a complex combination of several processes, boundary conditions and modifying factors, collectively making up metallogenetic, or minerogenetic systems. Evidence for such systems that operated in the geological past is always fragmentary. Some questions can possibly be answered by studying presently active ore-forming systems (eg the black smokers in the deep oceans), but this method (“actualism”) has limitations. Because of the unknown factors there is often room for different interpretations (hypotheses) of the scientific facts. Economic geology strives to improve continuously the genetic models of ore formation, ie complete schemes of these systems. This effort is assisted by progress in many other sciences (from biology to physics), but the reverse is also true. Economic geology provides a fascinating insight into geological systems that are extremely rare and can only be illuminated by studying mineral deposits. The practical mission of economic geology is the provision of metals and minerals that society requires. Of course, this implies cooperation with other scientific, technical and financial professionals.

Mining in the stress field between society and environment

Cum semper fuerit inter homines de metallis dissensio, quod alii eis praeconium tribuerent, alii ea graviter vituperarent (the original text in Latin by Georgius Agricola 1556). In English: “People were always divided in their opinion about mining, as some praised it highly while others condemned it fiercely.”

Agricola reports that enemies of mining in his time deplored not only harmful effects on the immediate environs but even moral aspects: they accused mining of advancing greed. Today, this remains one motive of opposition to the industry, but fundamental rejection of any extraction of minerals is more common. The main reasons given are that mining visibly uses the land and often leaves a profound and enduring change.

Undoubtedly, mining adds to the pressure exerted on natural systems by growing human populations. Yet, well managed and responsible mining provides a net-positive, long-term contribution to human society and to ecosystem wellbeing (ICMM 2012). Its overall balance of benefits, costs and risks is positive. True, there often are sound arguments against mining at a specific location. Compromises should be sought, however, because mineral deposits cannot be installed at arbitrary places. Their locations are predetermined by nature. An example are sand and gravel deposits in river plains. Today, these raw materials are so scarce in many regions that they have to be protected against other claims (eg housing developments). Yet, everyone consumes minerals and mineral-derived products for homes, heating, transport, computers, and numerous articles of daily life. Mining provides these minerals. Recycling replaces only part of primary production.

Land use by mining is very small (0.3% of the global ice-free area: Hooke et al. 2012) and only locally visible. Biofuel agriculture, sun and wind energy plants require much more land. Indeed, they create additional demand for minerals (e.g. fertiliser, metals for machines and processing plants, transport). Toxic elements such as arsenic and cadmium are essential for sustainable energy production, for example in photovoltaics. In many cases, even low foot-print technologies like geothermal power plants have serious problems with waste such as brines, salt, toxic and heavy metals (most notably arsenic, mercury and radionuclides). This demonstrates that there are no simple solutions for a sustainable economy without mining. On the contrary, it is undeniable that conservation of our quality of life, and development for the major part of humans who still lack the most basic necessities for a life in dignity require both, mineral raw materials and an intact environment.

Mining without an impact on the environment is impossible (Fig. 1.1), but the industry strives to minimize negative effects (Fig. 1.2) and to improve the welfare of affected communities (“green mining”). Green mining operations create an enriched landscape of re-constructed ecosystems, which provide humans with a variety of services (e.g. food, flood and erosion control, areas for recreation and aesthetics, and clean water; Kareiva et al. 2011). Examples include lignite and clay pits which bequeath beautiful new lakes. Hard rock mines and quarries may grow into rare islands of nature in a sea of human occupation. Many of these sites support rare and threatened species from archaea and bacteria to plants and animals, helping to preserve biodiversity (Batty 2005).

Reversing mineral extraction, mines also have an extremely important role as deep disposal sites for the safe storage of society’s unavoidable toxic and radioactive waste. Chemically dangerous waste is usually stored in worked sections of suitable underground mines. For highly toxic and radioactive waste, the construction of dedicated underground disposal mines is the best solution for protecting the biosphere. Underground disposal takes lessons from nature that has preserved high concentrations of hazardous solid or gaseous substances in the form of mineral deposits over many millions of years (e.g. sulfide metal ore, natural gas, uranium, and even the remains of natural nuclear reactors).

The World Commission on Environment and Development ("Brundtland-Report", Brundtland 1987) extended the concept of sustainable development to non-renewable resources. Clearly, few mineral resources lend themselves to the concept of sustainability as it was formulated about 300 years ago for the management of forests, “that the amount of wood cut should not exceed the growth rate” (Carlowitz 1713). Such exceptions may be salt or potassium harvested from sea water. Most metals and minerals are non-renewable and their use should be managed according to the following rules: i) consume as little as possible; ii) optimise the recycling rate; and iii) increase the efficiency of using natural resources, especially of energy. The original concept of sustainability considered mainly the interests of later generations. In the Rio Declaration (UN Conference on Environment and Development 1992) the concept of intrageneration fairness was added, to allow for the interests of the living generation of mankind.

In fact, the rapidly growing world population and demands for a better life enforce a continuing growth of raw materials production. Yet, every individual extractive operation must have the acceptance of public opinion. To reach that aim, all stakeholders must profit, and the mine’s social as well as the natural environment need to be improved. The radical call that sustainability requires immediate termination of any extraction of minerals is, of course, social and economic nonsense (Gilpin 2000). Let us use needed resources in the interest of living humans, and let us trust in technical and economic inventiveness and ingenuity to provide for later generations.

The mineral resources conundrum

But is there a sufficient mass of minerals for an ever-increasing mine production? Because of the limited size of our planet it is true that geological resources are principally finite, although very large indeed. The search for most minerals has hardly gone deeper than a few hundred metres below the surface, and only land, shallow seas and the margins of the vast oceans are fully explored for petroleum and gas deposits. Even in the well-known Gulf of Mexico, the new giant Tiber oilfield was only recently (2009) discovered. Metal mining on the sea floor has a large future potential. Giant unconventional oil and gas resources opened up in America are fundamentally altering geopolitics of global energy supply.

In contrast to resources, reserves that can be exploited at present economic and technological conditions are a small part only of the total geological endowment, because searching and defining reserves is a capital investment that must be paid back with interests. Due to the rules of depreciation of a future income, reserves are typically defined only for the next 10-30 years. The result is that at any time, a division of total reserves by the yearly consumption (the R/C ratio or “life-index”) will predict that in ten (or twenty, or thirty) years time “the world will run out of” the respective mineral. This fundamental error was famously made by the Club of Rome when it predicted this dire fate for the years 1990-2000 (Meadows et al. 1974). Because predictions of impending catastrophes are always popular this gave the Club of Rome’s hypothesis a sweeping impact. Actually, the imminent end of important minerals was announced many times in the past but never arrived. The term “life-index” is misleading, and the figure is rather an indication of specific conditions, which dictate production and marketing of individual metals and minerals. With few exceptions, individual R/C ratios change little over time-scales of several decades.

In the future, just as in the past, science and technology will continue to provide the mineral raw materials needed by society, both by finding new deposits, by recycling and by providing natural or synthetic functional replacement (Wellmer & Dalheimer 2012). The recycling rate of metals is increasing. End-of-life recycling of metals such as iron, copper and zinc has reached >50% and nearly 90% for toxic lead whereas many high-technology metals (lithium, indium, rare earth elements) are hardly ever recycled, mainly because of unfavourable economics. In the case of a number of complex alloys, separation is virtually impossible (Reck & Graedel 2012). In most cases, temporary scarceness of certain critical raw materials is caused by political constraints that distort markets (e.g. European Commission 2010). Furthermore, exploiting lower-grade ores, producing functional replacements for certain minerals and metals, and recycling of materials all need energy. Accordingly, energy is the most important natural resource of all.

In an interesting thought experiment, MacKay (2009) arbitrarily defined sustainable coal consumption by streching resources for 1000 years. As expected, the calculation demonstrates that the present burn-rate is far too high, and that the annual growth of consumption reduces the availability to less than 100 years. For radicals, this calls for immediate termination of using coal. Accounting for the time needed to profoundly change a large sector of energy supply, moderates suggest to expand use of renewable energy sources including the nuclear option (Lovelock 2006).

It is undeniable that there are physical limits to the availability of certain quality classes of raw materials. Severe problems arising from this fact are not to be expected as long as the unlimited resource of human creativity is given the freedom and incentives to search for solutions. The continuously expanding reserve base for practically all minerals, roughly in parallel to increasing consumption, is the best proof of this principle in the mining industry.


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