Characteristics of Conventional Mineral Deposits

Although there is wide agreement that mineral resources are nonrenewable, agreement is lacking on just how fixed our mineral stocks really are. At one end of the debate are fixed-stock geologists, environmentalists, and other neo-Malthusians who maintain that Earth has a finite number of mineral deposits. At the other end are opportunity-cost economists and other optimists who maintain that Earth's stock of deposits will expand as increasing prices allow us to exploit lower grade deposits (Tilton 1996). The continuing debate between these camps was revisited recently when Gordon et al. (2006) indicated that the amount of copper that the world will need by 2100 (if population reaches 10 billion and world copper usage equals that of the U.S.) will exceed the estimated copper resources of the planet. Tilton and Lagos (2007) responded that the real cost of copper to society (relative to other costs) has not increased significantly over the last 130 years and that this trend could continue through 2100, thus supplying demand.

Regardless of which of these camps is correct, all participants agree that, in the absence of abundant cheap energy, mineral resources to supply society's needs must come from mineral deposits where geological processes have concentrated the commodity of interest to a level that is higher than its average abundance in Earth's crust. Mineral deposits provide society with a tremendous head start; de Wit (2005) has estimated that the total energy expended by Earth to form a copper deposit is about 10 to 20 times the current market value of copper. By taking advantage of these concentrations, we avoid the need to expend this energy in our quest for copper and other mineral resources.

Formation of mineral deposits requires special geological processes that are localized in time and space. With the exception of a few commodities that are recovered from seawater (e.g., boron and magnesium) and from the atmosphere and other gases (e.g., nitrogen, helium, and sulfur), most mineral resources must come from the solid Earth or lithosphere. This reflects the fact that the solid Earth has a more complex composition than the atmosphere and oceans, and undergoes a wider range of processes that concentrate these elements into mineral deposits. Mineral resources can be present in the lithosphere as elements, minerals, or rocks, and a better understanding of this distinction is critical to any analysis of sustainability. According to simple definitions, elements are any of the more than 100 known substances (of which 92 occur naturally) that cannot be separated into simpler substances and that singly or in combination constitute all matter; minerals are naturally occurring, homogeneous, inorganic solids with a definite chemical composition and crystalline structure; and rocks are aggregates of minerals and other solid materials. A few elements, including gold, silver, copper, and sulfur, are found in the native (elemental) state in nature and are therefore minerals in their own right. Most elements, however, combine (chemically) with other elements in nature to make minerals, and minerals are then combined (physically) to make rocks.

Recovering mineral commodities from mineral deposits requires both mining and processing. Mining involves removal of ore from Earth. (Ore is the general term for any combination of elements, minerals, and rocks that contains a high enough concentration of the desired element, mineral, or rock to be produced economically.) In almost all cases, the desired material is mixed in its ore with other material (usually minerals) of less or no interest, and this requires that the ore be processed to separate the desirable material from waste. In almost all cases, processing requires more energy than mining. Table 7.1 divides the common nonfuel mineral resources into two groups based on whether they are used principally in mineral/rock form or elemental form. This grouping reflects the energy that must be used to recover the commodity of interest.

Mineral resources used in rock and mineral form require less energy to produce because they are processed largely by physical rather than chemical methods. In this group are large-volume construction materials including aggregate, sand and gravel, and crushed stone and smaller-volume materials such as diatomite and pumice, which usually require only washing, sizing, and possibly crushing to be useable. Rocks that require chemical processing before use include limestone and perlite, which are heated to drive off CO2 and H2O, respectively. Minerals that are used in their original or minimally processed state include halite (NaCl) for de-icing roads, diamond (C) for jewelry, garnet and feldspar for abrasives, barite in drilling muds, clays in ceramics and paper, and wollastonite in spark plugs. Other minerals that require additional, usually chemical, processing include halite for production of chemical compounds containing Na and Cl as well as alimentary salt, gypsum (CaSO4 ■ 2H2O) which is heated to drive off some H2O to make plaster, and calcite (CaCO3), as mentioned above, which is heated to drive off CO2 to make lime (CaO) and cement.

Energy consumption is much higher for mineral resources that are used in elemental form, ranging from antimony to zirconium, because they must be released from their host minerals by chemical processes. Processing to release

Table 7.1 Nonfuel mineral resources: (a) those used in rock or mineral form and (b) those used in elemental form. Data showing world mine production, reserve base, and their ratio along with level of recycling (from USGS Mineral Commodity Surveys for 2007). Amounts are given in metric tons except diamond, which is given in carats. Amount of recycling is shown as L (large, <50% of mine production), M (medium, 50-100%), S (small, <10-1%), I (insignificant, <1% to none), ND (no data).

Table 7.1 Nonfuel mineral resources: (a) those used in rock or mineral form and (b) those used in elemental form. Data showing world mine production, reserve base, and their ratio along with level of recycling (from USGS Mineral Commodity Surveys for 2007). Amounts are given in metric tons except diamond, which is given in carats. Amount of recycling is shown as L (large, <50% of mine production), M (medium, 50-100%), S (small, <10-1%), I (insignificant, <1% to none), ND (no data).

(a) Mineral or rock form

Production

Reserve base

Years

Recycling

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