Mineral exploration is the primary means to define new mineral resources. Following the end of World War II, there was a global economic boom which required the identification and mining of vast numbers of new deposits in order to provide the needed raw materials to sustain the demand. By and large, shallow easy-to-define orebodies were recognized first and developed. In the past 20 years, the discovery performance across virtually all mineral sectors has fallen, resulting in growing concern that if unchecked, there could be shortfalls in a number of commodities within the next 20 years. The collective sense is that there are more deposits to be found, but these are expected to be at greater depths than those that have been typical targets in the past.
To operate in this environment, new approaches for identifying deposits are required and the concept of a mineral systems approach, first suggested 20 years ago, has emerged as a powerful means going forward to build strategies and capabilities. In terms of geophysical exploration, the major change that will be required is a shift from a focus almost entirely on direct targeting with geophysical surveys of deposits, to a staged process where geophysical approaches are used initially to help define the pathways in the earth that carried the mineralizing solutions, which formed the target deposit. These pathways would provide a much larger target and if detected and mapped, should allow explorers to follow the pathway to the location of potential deposits.
This task is different from most geophysical studies, where the focus has typically been on improving the direct targeting capabilities and not the larger scale mapping problem that a mineral systems approach requires. A review of the current state-of-play for a number of major deposit styles shows how geophysical data are being used at present to explore for the larger scale mapping problem. The assessment overall is encouraging but major challenges remain outside of the technical issues of defining a mineral systems strategy that relate primarily to human resources and the commercial environment. With regard to the human resources issue, are there going to be a sufficient number of the right people to develop and implement the required programs? Universities play a critical role in producing new geoscientists but the industry then must take responsibility to train and mentor these people to become functioning professionals. In the commercial environment, at present there is little interest for long-term, strategic programs, either in terms of the needed fiscal support or commitment to undertake the implementation of outcomes. Although governments likely have a greater sense of urgency with regard to this problem, it may be difficult to unilaterally and successfully deal with this complex issue.
Figures & Tables
Earth’s near-surface mineralogy has diversified over more than 4.5 b.y. from no more than a dozen preplanetary refractory mineral species (what have been referred to as “ur-minerals” by Hazen et al., 2008) to ~5,000 species (based on the list of minerals approved by the International Mineralogical Association; http://rruff.info/ima). This dramatic diversification is a consequence of three principal physical, chemical, and biological processes: (1) element selection and concentration (primarily through planetary differentiation and fluidrock interactions); (2) an expanded range of mineral-forming environments (including temperature, pressure, redox, and activities of volatile species); and (3) the influence of the biosphere. Earth’s history can be divided into three eras and ten stages of “mineral evolution” (Table 1; Hazen et al., 2008), each of which has seen significant changes in the planet’s near-surface mineralogy, including increases in the number of mineral species; shifts in the distribution of those species; systematic changes in major, minor, and trace element and isotopic compositions of minerals; and the appearance of new mineral grain sizes, textures, and/or morphologies. Initial treatments of mineral evolution, first in Russia (e.g., Zhabin, 1979; Yushkin, 1982) and subsequently in greater detail by our group (Hazen et al., 2008, 2009, 2011, 2013a, b; Hazen and Ferry, 2010; Hazen, 2013), focused on key events in Earth history. The 10 stages we suggested are Earth’s accretion and differentiation (stages 1, 2, and 3), petrologic innovations (e.g., the stage 4 initiation of granite magmatism), modes of tectonism (stage 5 and the commencement of plate tectonics), biological transitions (origins of life, oxygenic photosynthesis, and the terrestrial biosphere in stages 6, 7, and 10, respectively), and associated environmental changes in oceans and atmosphere (stage 8 “intermediate ocean” and stage 9 “snowball/hothouse Earth” episodes). These 10 stages of mineral evolution provide a useful conceptual framework for considering Earth’s changing mineralogy through time, and episodes of metallization are often associated with specific stages of mineral evolution (Table 1). For example, the formation of complex pegmatites with Be, Li, Cs, and Sn mineralization could not have occurred prior to stage 4 granitization. Similarly, the appearance of large-scale volcanogenic sulfide deposits may postdate the initiation of modern-style subduction (stage 5). The origins and evolution of life also played central roles; for example, redox-mediated ore deposits of elements such as U, Mo, and Cu occurred only after the Great Oxidation Event (stage 7), and major Hg deposition is associated with the rise of the terrestrial biosphere (stage 10; Hazen et al., 2012).