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Figure 8.1 Decline in (a) average copper ore grade in the United States (Ayres et al. 2001); (b) average lead ore grade in Australia (Mudd 2007), and (c) average gold ore grade in Australia (Mudd 2007).

of hundreds or even thousands (Skinner 1976). This has been referred to as the "mineralogical barrier," depicted in Figure 8.2, where dashed and solid lines refer to geochemically scarce and abundant metals, respectively. Steen and Borg (2002) have estimated the cost of producing metal concentrate from the Earth's crust and found it to be very high compared to current costs of concentrates produced from metallic ores. In this chapter, I address primarily the effect of ore grade on energy consumption at grades above this mineralogical barrier (see Figure 8.2).

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