m Marginal reserves es


Inferred marginal reserves


1 Demonstrated T subeconomic resources i

Inferred subeconomic resources

Other occurrences

Includes nonconventional and low-grade materials

Figure 7.1 System of reserve and resource classification of the USGS.

Figure 7.1 System of reserve and resource classification of the USGS.

considered resources. The lower atmosphere is essentially homogeneous and does not contain enriched zones that might be considered mineral deposits, although gases such as helium and methane are enriched in some gas-rich zones in the porous lithosphere.

In Figure 7.1, resources are divided along the vertical axis into categories based on their economic character, including a wide range of natural and human factors such as amenability to mining and processing, environmental and tax burdens related to production, and current and anticipated markets for the commodity. Along the horizontal axis, resources are divided into geological categories based on whether they have been discovered (identified) or are just thought to be present (undiscovered). Identifi ed resources are divided into demonstrated and inferred resources based on the degree to which they have been sampled, and the demonstrated resources are further divided into measured and indicated categories, reflecting the level of assurance gained from better sampling.

Reserves are that part of the resource that has been clearly delineated physically in terms of both dimension and concentration of the commodity of interest, and for which extraction is economically attractive (Figure 7.1). Material that has been sampled adequately, but is not economically extractable, constitutes marginal reserves. It is worth noting here that large deposits can enter the reserve category only if they are subjected to careful sampling, usually involving tens of thousands of subsurface samples, which are usually obtained by drilling thousands of meters of holes into the suspected deposit at a cost of millions of dollars. Interpretation of the data to obtain a statistically valid measure of the size and degree of enrichment (grade) of the deposit is based on an entire body of statistics known as geostatistics (Goovaerts 1997), which concerns spatially distributed data. Even slight errors in these estimates can make a big difference in profi tability because most mines use ore that is at or near the lower limit for economic recovery (cut-off grade) to maximize overall recovery.

The "reserve base" consists of reserves, marginal reserves, and part of demonstrated subeconomic resources, all of which have been demonstrated (discovered and sampled) rather than simply indicated. Most deposits in the reserve base are conventional deposits that are similar to those being mined today. Subeconomic deposits in the reserve base have lower grades or other marginal economic features that have limited their exploitation. Many of these were discovered during periods of high commodity prices that encouraged exploration in marginal terrains and deposits but that did not last long enough to allow the deposits to be developed and mined.. For example, the Casino porphyry copper deposit in the Yukon of Canada, with 964 million tons of material (though not yet ore) containing 0.22% Cu and minor Au and Mo, was discovered in 1969 but has not yet been developed.

Reserve base (Figure 7.1) is the only widely available quantitative basis that we have for estimating long-term sustainability. Dividing the reserve base by annual production provides a simple indication of sustainability for conventional deposits. Reserve base/production ratios of this type for which 2007 data are available range from a low of 16 years to a high of 4375 years (Table 7.1), and have an exponential frequency distribution with an arithmetic mean of 350 years (Figure 7.2). If we use 60 years as two generations, then slightly more than three-quarters of the commodities are sustainable according to the NRC definition mentioned at the start of this chapter. None of the commodities meet the 200,000-year sustainability requirement of Pickard (2008).

Diamonds (industrial and gem) and strontium have the lowest reserve base relative to present consumption (Figure 7.2). Of these, natural diamond production might be supplemented by synthetic stones and strontium demand is likely to decrease as large cathode-ray tubes are no longer used in televisions. Antimony, arsenic, gold, indium, silver, and tin fall into the next lowest reserve-base category relative to present consumption. Although all of these elements except indium exist in their own deposits in nature, tin is the only dominant element in deposits from which it is mined. Significant amounts of gold and silver, most arsenic and antimony, and all indium are by-products of base metal smelting and refining. Demand over the next decade or so is likely to decrease for arsenic, remain stable for antimony and gold, and increase for indium and possibly tin and silver. Recent changes in demand include decreasing use of arsenic as a wood preservative (Brooks 2007), increasing purchase of gold for exchange-traded funds (George 2006), and decreasing use of silver in its traditional markets of jewelry, photography, and silverware countered by increasing use in coinage, solder, and electronic identification devices (Brooks 2006). The main demand for indium is in indium-tin oxide coatings in flat-panel displays in computers, televisions, and other electronic devices—a market that is expanding (Tolcin 2006). Graedel (2002) has pointed out that

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