Ore and mineral deposits

Ore deposits are naturally occurring geologic bodies that may be worked for one or more metals. The metals may be present as native elements, or, more commonly, as oxides, sulfides, sulfates, silicates, or other compounds. The term ore is often used loosely to include such nonmetallic minerals as fluorite and gypsum. The broader term, mineral deposits, includes, in addition to metalliferous minerals, any other useful minerals or rocks. Minerals of little or no value which occur with ore minerals are called gangue. Some gangue minerals may not be worthless in that they are used as by-products; for instance, limestone for fertilizer or flux, pyrite for making sulfuric acid, and rock for road material.

Mineral deposits that are essentially as originally formed are called primary or hypogene. The term hypogene also indicates formation by upward movement of material. Deposits that have been altered by weathering or other superficial processes are secondary or supergene deposits. Mineral deposits that formed at the same time as the enclosing rock are called syngenetic, and those that were introduced into preexisting rocks are called epigenetic.

The distinction between metallic and nonmetallic deposits is at times an arbitrary one since some substances classified as nonmetals, such as lepidolite, spodumene, beryl, and rhodochrosite, are the source of metals. The principal reasons for distinguishing nonmetallic from metallic deposits are practical ones, and include such economic factors as recovery methods and uses.

Most mineral deposits are natural enrichments and concentrations of original material produced by different geologic processes. Economic considerations, such as the amount and concentration of metal, the cost of mining and refining, and the market value of the metal, determine whether the ore is of commercial grade. To be of commercial grade, for example, the following metals must be concentrated in the amounts indicated: aluminum, about 30%; copper, 0.7–10%; lead, 2–4%; zinc, 3–8%; and gold, silver, and uranium, only a small fraction of a percent of metal. See also Geochemical prospecting; Mineral.