3

DEPLETION OF MINERAL RESOURCES

At first glance, sustainability and mineral resource development appear to be in conflict. Mining depletes finite resources and in a strict sense, therefore, is inherently unsustainable. For instance, there is only a finite amount of copper in the earth’s crust, and each unit of copper extracted increases the fraction of the total copper resource base that is in use. Thus, it can be argued that if we continue to mine we will eventually exhaust the available supply of minerals.

This perspective, however, ignores the dynamics of mineral supplies. In practice the non-renewable character of minerals may be less constraining than it might seem. Five factors make the benefits from mining much more sustainable than they initially appear to be. First, through the process of exploration and development, mining companies continually reinvigorate, augment, or “sustain” their reserves.2 Current reserves represent only a small portion of the mineral resources remaining in the earth’s crust. Exploration and development lead to the discovery and proving up of previously unknown mineral deposits and—perhaps just as important—additional reserves at existing mines and known deposits. Technological improvements in exploration increase the discovery rate of mineral deposits and at the same time reduce discovery costs. Predictive models for massive sulfide deposits, for example, allow targeting of completely buried deposits by using the combination of structural projections and favorable stratigraphic horizons in volcanic rocks.

2  

Reserves are minerals that are known to exist and can be produced at a profit with current technology under prevailing economic conditions.



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MINERAL RESOURCES AND SUSTAINABILITY: CHALLENGES FOR EARTH SCIENTISTS 3 DEPLETION OF MINERAL RESOURCES At first glance, sustainability and mineral resource development appear to be in conflict. Mining depletes finite resources and in a strict sense, therefore, is inherently unsustainable. For instance, there is only a finite amount of copper in the earth’s crust, and each unit of copper extracted increases the fraction of the total copper resource base that is in use. Thus, it can be argued that if we continue to mine we will eventually exhaust the available supply of minerals. This perspective, however, ignores the dynamics of mineral supplies. In practice the non-renewable character of minerals may be less constraining than it might seem. Five factors make the benefits from mining much more sustainable than they initially appear to be. First, through the process of exploration and development, mining companies continually reinvigorate, augment, or “sustain” their reserves.2 Current reserves represent only a small portion of the mineral resources remaining in the earth’s crust. Exploration and development lead to the discovery and proving up of previously unknown mineral deposits and—perhaps just as important—additional reserves at existing mines and known deposits. Technological improvements in exploration increase the discovery rate of mineral deposits and at the same time reduce discovery costs. Predictive models for massive sulfide deposits, for example, allow targeting of completely buried deposits by using the combination of structural projections and favorable stratigraphic horizons in volcanic rocks. 2   Reserves are minerals that are known to exist and can be produced at a profit with current technology under prevailing economic conditions.

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MINERAL RESOURCES AND SUSTAINABILITY: CHALLENGES FOR EARTH SCIENTISTS Discovery of dozens of gold, copper, and uranium deposits over the last several decades amply demonstrate the rewards of technically sound exploration. Mineral exploration and mine development, in effect, create wealth (reserves) even as resources are extracted. Undiscovered mineral resources have little value until they are discovered and developed. But this optimistic picture must be tempered by the fact that there is increasing competition for the area available for exploration and reserve augmentation at least within the borders of the United States. Second, technological change sustains mineral production. Technological improvements in mining and mineral processing reduce costs and permit profitable mineral production from previously uneconomic, mineralized rock. Minerals can be extracted from rock that otherwise would either remain in the ground or in waste piles and tailings. For example, the “micron” gold deposits in Nevada and Utah, which currently dominate U.S. gold production, rely on recovery technology that had not been perfected twenty-five years ago. The available evidence suggests that additions to reserves through discovery and technologic change have more than offset reductions through depletion of existing mines over the last several decades (Table 1). Third, even when a mine is physically depleted, recycling can be thought of as an extension of primary mining. Recycling is, in fact, an important source of many metals (Table 2). Fourth, a scarce resource can sometimes be replaced by a more abundant substance. For example, aluminum can replace copper in long distance transmission power lines.

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MINERAL RESOURCES AND SUSTAINABILITY: CHALLENGES FOR EARTH SCIENTISTS Table 1. Estimated World Reserves of Selected Minerals and Metals, 1940s-1980s (million metric tons of contained metal, end of decade)1   1940s 1950s 1960s 1970s 1980s Aluminum2 1,605 3,224 11,600 22,700 23,200 Copper 91 124 280 543 566 Lead 31-45 45-54 86 157 120 Zinc 54-70 77-86 106 240 295 Source: Crowson 1994, who compiled data from Minerals Handbook 1994-95. Notes: 1. These data are crude estimates and they are subject to many uncertainties. Nevertheless they provide a picture of the effects of exploration and technological change on mineral reserves. 2. Gross weight of bauxite. Table 2. Old Scrap Recovery1 as a Percent of U.S. Apparent Consumption of Selected Metals, 1960-1990 (percent)2   1960 1970 1980 1990 Aluminum 5 4 11 22 Copper 27 25 28 25 Iron and Steel ? ? 15-20 22 Lead 40 37 54 69 Zinc 6 5 6 9 Source: Combined from U.S. Bureau of Mines Minerals Yearbooks 1962-94. Notes: 1. Old scrap recovery represents metal recovered from discarded and obsolete products that have reached the end of their useful lives. 2. Apparent consumption = U.S. primary metal production + secondary production from old scrap + imports - exports.

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MINERAL RESOURCES AND SUSTAINABILITY: CHALLENGES FOR EARTH SCIENTISTS Fifth, and more broadly, even when a mine is physically depleted, its value may persist if an appropriate portion of the proceeds are invested in human capital (such as education or health care) or man-made physical capital (such as infrastructure or technology). Elaboration of this contentious point is beyond the scope of this report. Challenges Concerning Depletion Challenge #1: To develop a better scientific basis for discussions of the adequacy of mineral resources. An important need is to develop a better methodology for characterizing submarginal mineral resources—i.e., those that are not exploitable under current economic conditions with known technologies, but which could become economically feasible under different conditions. Another need is to understand the “mineralogical barrier”, the fact that mineral and metal resources occur not only in several distinct ranges of enrichment or concentration, but also in different chemical states. Recovery becomes dramatically more difficult (e.g., more energy intensive) as we move from one mineralogical type (e.g., sulfide minerals) to another (e.g., silicate minerals). Details on these concentration ranges are very incomplete, and hence we know little about this aspect of our mineral-resource inventory (see Skinner, 1976; Gorgon, et al., 1987). Challenge #2: To develop better data on factors involved in mineral supply, that should be used in public-policy analysis and decision making. An important need is better data on annual changes in mineral reserves, both additions and reductions. A related need is better data on how the discovery and development of mineral resources occurs over time —for example, historical discovery rates and the factors that influence discovery rates, and information on where additional

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MINERAL RESOURCES AND SUSTAINABILITY: CHALLENGES FOR EARTH SCIENTISTS reserves come from (i.e., the relative importance of discovery of previously unknown deposits, development of previously known but undeveloped deposits, extensions of existing mines, and adoption of better techniques of mining and mineral processing). These types of data are necessary inputs to ongoing efforts to incorporate resource depletion into the national income and product accounts (see National Research Council, 1994). By incorporating resource depletion and reserve additions into these accounts, we will have better measures of real economic growth and national wealth. Although we are a long way from having useful mineral accounts, the study by Carson (1994) and her colleagues at the U.S. Department of Commerce is an important initial step in establishing this type of approach to mineral resources. Challenge #3: To better communicate to policymakers and the public the dynamic nature of mineral supply, thus putting the prospect of “running out ” in the proper context. Better science and data alone are not enough. The sustainability debate challenges earth scientists to communicate more clearly with policymakers and the public. Mineral reserves represent only a small portion of the mineral wealth of the earth’s crust. Reserves are like inventory—they are continually replenished from previously uneconomic or undiscovered mineralization. What should be of concern is not the limited extent of reserves, but rather the process by which reserves are replenished. Challenge #4: To incorporate recycling and reuse into the concept of sustainability . Ultimately, recycling must play a major role in our use of metals, but we have much to learn about the problems and practices of recycling. This area needs research, ideas, and education.

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