even common elements on the Moon are "worth more than their weight in gold." This is misleading. As discussed at length below, these high transport costs also apply to any capital and maintenance equipment required to put those elements in a useful form. Hence the high cost cut both ways, so that a large concentration of the desired element in potential lunar raw materials remains extremely important. Intermediate cases exist, of course; but these examples nevertheless indicate that the sources for high-volume commodities must be as pure as possible. Such commodities, of course, are the ones required for space development. Contrast Not only must the desired element be sufficiently concentrated, it must be amenable to extraction. The nature of the chemical and physical state of the potential ore material versus the host is another fundamental determinant of separation costs. For example, many base metals, although relatively rare, occur as sulfides. Commonly these sulfides are dispersed - "disseminated" - in a much larger volume of silicate rock. Examples of disseminated sulfide deposits arc "porphyry copper" bodies, the source of much of the world's Cu. Such a deposit consists of a granitic igneous body in which copper and other sulfides are dispersed, mostly in the upper part. In the main part of the orebody, copper typically amounts to ~0.4% [e.g., Gordon et al., 1987, p. 38], To mine such a deposit, the rock is mined in bulk and crushed; then after crushing, the sulfide grains are easily separated from the silicate gangue by froth flotation. That is, the contrast in physical properties between sulfide and silicate permits their efficient separation. Were the Cu dispersed instead in a separate sulfide mineral, it probably could not be extracted profitably because the physical properties would be too similar for efficient beneficiation. Chrysocolla (a copper silicate) ores, for example, must be leached. Another example is the "Carlin-type" gold deposits of Nevada. Typical "sulfide ore" in such deposits consists of disseminated sulfides and minor arsenides, antimonides, etc., in which a trace of gold is present. Parts of such orebodies typically contain "oxidized ore" zones, however, and these are more valuable because they are easier to work with (Percival et al., 1988], In these zones, the sulfides have been oxidized by subsequent geologic events. As gold has low affinity for oxygen, the oxidation has caused the gold to segregate into microscopic grains of native metal. The concentration of gold is the same, but it is now easier to process. (Even so, new technology still had to be developed, as the native gold grains are far too small to be recovered with conventional extraction processes). The costs of winning an element from its ore are, of course, ultimately constrained by its oxidation potential, which determines the minimum energy that must be expended. For certain metals such as Al this is indeed a major part of the cost. Indeed, of the common metals in the crust, only Fe has been known from antiquity, because it has by far the lowest oxidation potential; hence it can be
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