metals such as gold and iron. Understanding the location of metallic ores and how to mine them is one of the most ancient fields of geoscientific endeavor. Research involving mineral deposits is similar in some ways to research in oil and gas exploration. Both are strongly affected by economic fluctuations and international politics. When the industries thrive, the larger companies manage extensive research programs that foster close links between exploration projects and scientists from government and universities. At the same time, smaller enterprises can justify ventures that would be considered too risky in a sluggish market. Advances in geophysics, geochemistry, and data-gathering and data-handling capabilities have revolutionized mineral exploration and development, as they have the petroleum industry. Both industries are especially stimulated by applications of new theoretical concepts.

Mineral resources are generally concentrations, sometimes exceptionally high concentrations, of one or more of the materials that constitute the solid-earth. Ninety-nine percent of the crust is made up of only nine chemical elements: oxygen, silicon, aluminum, magnesium, iron, calcium, sodium, potassium, and titanium. It is the other 1 percent of the crust that captures the interest of the mineral exploration geoscientist who wants to know how, when, and especially where concentrations of minerals occur. Interest centers on understanding the processes that form the mineral deposits, the environments in which those processes operate, and the distribution of deposits through space and time. From this understanding, new deposits can be predicted, discovered, and developed, and existing ones can be exploited efficiently.

Mineral deposits form through a wide range of geological processes that may operate at uncommon levels of effectiveness or in unusual associations. In some situations, rocks such as sandstone and limestone qualify as mineral deposits because they are of economic value in a particular place. Sands and gravel deposited in the bed of a mountain stream provide a simple example of such a situation. Where a stream erodes crustal rocks that contain gold, the downstream gravels may contain anomalously high concentrations of the precious metal. This is because the weathered gold particles are denser than the gravels and sands. The stream deposits the gold particles when it loses the capacity to carry particles of that density and size, so the gold accumulates within a particular reach of the stream. These are the placer deposits that have been the sites of gold mining throughout history, from the mythical deposits of Colchis where Jason found the Golden Fleece to those of Sutter's Mill in California that inspired the forty-niners of a century and a half ago.

Less than one part in 10,000 of the metals present in the upper kilometer of the continental crust is concentrated in known mineral deposits. The remainder is widely dispersed at low concentration and, for that reason, is unsuited to economical recovery. Ore deposits are rare indeed! A simple illustrative plot for the metal lead shows how concentration of a typical element varies in the crust (Figure 4.7). The range of concentrations is so large that it is necessary to represent it logarithmically. Common crustal rocks contain about one part in 100,000 of lead by weight; this is represented by the crest of the bell-shaped curve. There are virtually no rocks in which the amount of lead is too small to be measured. The smooth curve represents the gradual increase in lead abundance, with the maximum representing the most probable value for a randomly selected sample. The fall in abundance from this value is not symmetrical. Economically workable lead concentrations—rocks with a lead context of more than a few percent—occupy a place at the far right end of the curve.

Deposits with lower lead content are expected to become economically profitable at some time in the future when all larger deposits are exhausted. A threshold identified as the mineralogical limit separates a deposit in which rocks contain lead-dominant minerals from a deposit of lead so evenly distributed through the rock that it does not form discrete mineral concentrations. Although by definition all lead ores fall within the first category, there is a far greater total amount of lead distributed thinly throughout crustal rocks.

Lead is just one example of crustal element occurrence that can generate a bell-shaped distribution curve. Such distribution patterns indicate the challenge of mineral exploration, which is to discover the small amounts of concentrated materials constituting ore bodies. These concentrations not only are rare but can be extraordinarily localized. For example, seven gold fields covering an area of about 5,000 km 2—no larger than the county of Los Angeles—within the Witwatersrand basin of South Africa have produced more gold than has been discovered over the remainder of the surface. The mercury deposit at Almaden, Spain, has yielded more mercury than any other source and still retains the bulk of the world's reserves. The Bushveld intrusion in South Africa contains 98 percent of the world's chromium reserves, most of them in a single layer.

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