ORE

aggregate of economically important minerals that is sufficiently rich to enable separation of the minerals at a profit. Although more than 2,800 mineral species have been identified, only about 100 are considered ore minerals. Among these are hematite, magnetite, limonite, and siderite, which are the principal sources of iron; chalcopyrite, bornite, and chalcocite, the principal sources of copper; and sphalerite and galena, the principal sources, respectively, of zinc and lead. Copper, molybdenum, and gold are commonly found in disseminated depositsi.e., scattered more or less uniformly through a large volume of rock. Copper, lead, and zinc are frequently found in massive sulfide deposits. Many such deposits are believed to have been formed by precipitation from volcanic exhalations on the seafloor or by metasomatic replacement (a process of simultaneous solution and deposition). The term ore was originally applied only to metallic minerals, but its definition has been broadened to include such nonmetallic substances as sulfur, calcium fluoride (fluorite), and barium sulfate (barite). No ore deposit consists entirely of a single ore mineral. The ore is always mixed with unwanted or valueless rocks and minerals that are collectively known as gangue. The ore and the gangue are mined togetheri.e., taken out of the host rock in a mass, by either mechanical or manual means. Then the ore is separated from the gangue by various operations known collectively as mineral processing (q.v.), or ore dressing. The desired metallic element is then extracted from the ore by various smelting, roasting, or leaching processes. After this, the metal may be still further refined (purified) or alloyed with other metals, as in a copper refinery or steel mill. Mining, processing, and refining are thus successive steps in the utilization of an ore deposit to yield a metal.