Introduction to Ore-Forming Processes. Laurence Robb

Figure 6 Global production trends for oil (a), bauxite (b), copper (c), and gold (d) over the twentieth century.

      Source: After compilations in Craig et al. (1996).

The depletion of commodities in the Earth's crust is particularly serious for those metals that are already scarce in terms of crustal abundances and for which high degrees of enrichment are required in order to make viable ore deposits. Figure 2 illustrates the point by referring to the production of iron as a baseline measure against which extraction of other metals can be compared (Skinner 1976; Einaudi 2000). Those elements which fall above the Fe production line (notably Au, Ag, Bi, Sb, Sn, Cu, Pb, and Zn) are being extracted or depleted at faster rates, relative to their crustal abundances, than Fe. It is these metals that are in most danger of depletion in the next 50 years or so unless production is ameliorated or the reserve base is replaced. Conversely, those metals that plot beneath the Fe production line (such as Ti, Mg, and Al) are being extracted at slower rates than Fe and are in less danger of serious depletion during this century.

One of the ways in which metallogeny can assist in the creation of a sustainable pattern of resource utilization is to better understand the processes by which ores are concentrated in the Earth's crust. The replacement of the global commodity reserve base is dependent on exploration success and the ability to find new ore deposits that can replace those that are being depleted. It is, of course, increasingly difficult to find new and large deposits of conventional ores, since most of the accessible parts of the globe have been extensively surveyed and assessed for their mineral potential. The search for deeper deposits is an option but this is dependent to a large extent on the availability of technologies that will enable mining to take place safely and profitably at depths in excess of 4000 m (currently the deepest level of mining in South African gold mines). Another option is to extract material from inaccessible parts of the globe, such as the ocean floor, a proposal that has received serious consideration with respect to metals such as Mn and Cu. Again, there are technological barriers to such processes at present, but these can be overcome, as demonstrated by the now widespread exploration for, and extraction of, oil and gas from the sea floor. Environmental barriers to sea floor exploitation are more serious and difficult to overcome, as evident from catastrophic oil spillages in many parts of the globe. A third option to improve the sustainability of resource exploitation is to extract useful commodities from rocks that traditionally have not been thought of as viable ores. Such a development can only be achieved if the so‐called “mineralogical barrier” (Skinner 1976) is overcome. This concept can be described in terms of the amount of energy (or cost) required to extract a commodity from its ore. It is, for example, considerably cheaper to extract Fe from a banded iron‐ formation than it is from olivine or orthopyroxene in an igneous rock, even though both rock types might contain significant amounts of the metal. The economics of mining and the widespread availability of banded iron‐formations dictate that extraction of Fe from silicate minerals is essentially not feasible. The same is not true of nickel. Although it is cheaper and easier to extract Ni from sulfide ore minerals (such as pentlandite) there is now widespread extraction of the metal from nickeliferous silicate minerals (garnierite) that form during the lateritic weathering of ultramafic rocks. Even though Ni is more difficult and expensive to extract from laterite than from sulfide ores, the high tonnages and grades, as well as the widespread development and ease of access of the former, mean that they represent viable mining propositions despite the extractive difficulties. Ultimately, it may also become desirable to consider mining iron laterites, but this would only happen if conventional banded iron‐formation hosted deposits were depleted, or if the economics of the whole operation favored laterites over iron‐formations. This is not likely to happen in the short term, but, if planned for, the scenario does offer hope for sustainability in the long term. In short, sustainable production of mineral resources requires a thorough understanding of ore‐forming processes and the means to apply these to the discovery of new mineral occurrences. It also requires the timely development of technologies, both in the earth sciences and in related fields of mining and extractive metallurgy, that will enable alternative supplies of mineral resources to be economically exploited in the future.

      Mining and Environmental Responsibility

      A global population of possibly eleven billion people by the end of the century presents a major challenge in terms of the supply of most of the world's natural resources. What is even more serious, though, is the enormous strain it will place on the Earth's fragile environment arising from the justifiable expectation that future societies will provide an adequate standard of living, in terms of food, water, housing, technology, recreation, and material benefits, to all their people. In addition to commodity supply problems, the twenty‐first century will also be characterized by unprecedented depletion of even more critical resources in the form of soil, water, and clean air (Fyfe 2000). Legislation that is aimed at dealing with issues such as atmospheric pollution and greenhouse gas emissions, erosion, factory waste and acid drainage, de‐forestation, the protection of endangered species, overgrazing, and crop fertilization, is highly desirable but far from globally achievable because it is perceived as a luxury that only the developed world can afford.

      The study of ore‐forming processes is occasionally viewed as an undesirable topic that ultimately contributes to the exploitation of the world's precious natural resources. Nothing could be further from the truth. An understanding of the processes by which metals are concentrated in the Earth's crust is essential knowledge for anyone concerned with the preservation and remediation of the environment. The principles that underpin the natural concentration of ores in the crust are the same as those that can be utilized to tackle issues such as the control of acid mine drainage, and soil and erosion management. Mining operations around the world are required to assume responsibility for reclamation of the landscape once the resource has been depleted. The industry now encompasses a range of activities extending from geological exploration and evaluation, through mining and beneficiation, and eventually to remediation and environmental reclamation. This is the mining cycle and its effective management in the future will be a multidisciplinary exercise carried out by highly skilled scientists and engineers. Earth systems science, and in particular the geological processes that gave rise to the formation of mineral deposits, will be central to the future custodianship of the Earth's natural resources.

Summary

      The discipline of “economic geology” and in particular the field of metallogeny (the study of the genesis of ore deposits) remains critical to the teaching of earth systems science. A holistic approach involving the integration of knowledge relevant to the atmosphere, biosphere, and lithosphere is now regarded as essential to understanding the complexities of the Earth system. The development of environmentally responsible policies for the sustainable production of all natural resources will demand a thorough knowledge of the nature and workings of the Earth system. Central to this is an understanding of metallogeny and the nature and origin of the entire spectrum of mineral resources, including the fossil fuels. The classification and description of ore forming processes can most effectively be achieved in terms of host rock associations, namely igneous, hydrothermal,