Laurence Robb - Introduction to Ore-Forming Processes

1 Chrysocolla – (Cu,Al)2H2Si2O5(OH)4·nH2O

The development of a geological time scale has been the subject of a considerable amount of thought and research over the past few decades and continues to occupy the minds and activities of stratigraphers and geochronologists around the world. The definition of a framework within which to describe the secular evolution of rocks, and hence the Earth, has been, and continues to be, a contentious exercise. The International Commission on Stratigraphy (ICS is a working group of the International Union of Geological Sciences: IUGS) has assumed the official role of developing the geological time scale, a task that is continuously being modified and improved upon. The work of the ICS is periodically published as a book, such as Harland's (1989) seminal A Geologic Time Scale – which has now been superseded a number of times by works such as Gradstein et al. (2004 and 2012) and Ogg et al. (2016). In these books reference is made to the timing of various events and processes and the provision of a time scale to which readers can refer. Figure 5is a time scale based on the 2018 version of the International Stratigraphic Chart, published and sanctioned by the ICS and IUGS ( http://www.stratigraphy.org). In this diagram global chronostratigraphic terms are presented in terms of eons, eras, periods, and epochs, and defined by absolute ages in millions of years before present (Ma). Also shown are the approximate positions on the time scale of many of the ore deposits and metallogenic provinces referred to in the text.

Figure 5Geological time scale after the International Commission on Stratigraphy ( http://www.stratigraphy.org/index.php/ics-chart-timescale). Also shown are the ages of the various deposits and metallogenic provinces mentioned in the book.

One of the major issues that characterized social and economic development toward the end of the twentieth century revolved around the widespread acceptance that the Earth's natural resources are finite, and that their exploitation should be carried out in a manner that will not detrimentally affect future generations. The concept of “sustainable development” in terms of the exploitation of mineral occurrences implies that current social and economic practice should endeavor not to deplete natural resources to the point where the needs of the future cannot be met. This would seem to be an impossible goal given the unprecedented population growth over the past century and the fact that many commodities may become depleted within the next 100 years. The challenge for commodity supply over the next century is a multifaceted one and will require a better understanding of the Earth system, improved incentives to promote more efficient recycling of existing resources, and the means to find alternative sources for commodities that are in danger of depletion.

There has been a dramatic rise in global population over the past 150 years. The number of humans on Earth has risen from one billion in 1830 to over seven billion at the start of the twenty‐first century. Most predictions suggest that the populations of most countries will start to level off over the next 30 years and that global numbers will stabilize at around eleven billion people by the end of the twenty‐first century. Societies in the next 100 years are, nevertheless, facing a scenario in which the demand for, and utilization of, natural resources continues to increase, and certain commodities might well become depleted in this interval. Production trends for commodities such as oil, bauxite, copper, and gold ( Figure 6) confirm that demand for resources reflects population growth and is likely to continue to do so over the next few decades. World oil production increased precipitously until the late 1970s, but since then a variety of political and economic factors have contributed to moderating demand ( Figure 6a), thereby ensuring a longer‐term reserve base. A similar leveling of production is evident for bauxite ( Figure 6b) but such a trend is not yet evident for the precious metals such as gold or platinum. For some commodities, such as copper ( Figure 6c), the world reserve base is also leveling off, a feature that in part also reflects fewer new and large discoveries. Critical shortages of most natural commodities are not likely to present a problem during the early part of the twenty‐first century (Einaudi 2000), but this situation will deteriorate unless strategies for sustainability are put into place immediately. Another area of concern is related to those strategically important metals for which the security of supply has become an issue. Many metals, including the rare earth elements, tungsten, the platinum group elements, tantalum and niobium, have been cited as “critical metals” because their supply has been affected, not by naturally diminishing resource bases, but by socio‐economic and political factors.

Figure 6Global 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 2illustrates 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.