ore, metal-bearing mineral mass that can be profitably mined. Nearly all rock deposits contain some metallic minerals, but in many cases the concentration of metal is too low to justify mining the ore. Ores are usually found concentrated in deposits with a definite gradation of metal concentration from the ore to the surrounding rock. The ore often occurs in veins, which are ore-filled fissures in the rock. The veins vary in thickness from only fractions of an inch to several hundred feet. Minerals with no commercial value, called gangue minerals, are usually found mixed with the ore in the vein. Some veins are buried deep within the ground, but others lie close to the surface. Veins of ore exposed to weathering are often eroded and redeposited in placers and alluvial deposits, e.g., ore-bearing stream and lake gravels or beach sands. Some ores are simple chemical compounds, while others are chemically complex minerals. Important ores of aluminum, iron, manganese, and tin are oxides; important ores of antimony, copper, lead, mercury, nickel, silver, and zinc are sulfides. Some metals, called native metals, occur uncombined in nature, e.g., copper, gold, platinum, and silver. The recovery of metals from their ores is one area of the field of metallurgy.

bog iron ore: see limonite.

Ore Mountains: see Erzgebirge.

or ore dressing

Mechanical treatment of crude ores to separate the valuable minerals. Mineral processing was at first applied only to ores of precious metals but later came to be used to recover other metals and nonmetallic minerals. It is also used during coal preparation to enrich the value of raw coal. The primary operations are comminution and concentration. Comminution is carried out by large jaw crushers and by smaller cylindrical grinding mills. Common methods of concentration are gravity separation and flotation separation. Gravity methods include jigging (ground ore is fed into a pulsating body of water so that the heavier mineral fractions settle out, leaving lighter wastes at the top) or washing the ore down inclined planes, spirals, or shaking tables so that mineral and waste fractions settle in different areas. Seealso beneficiation; mining.

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Aggregate of economically important minerals that is sufficiently rich to separate for a profit. Although more than 3,500 mineral species are known, only about 100 are considered ore minerals. The term originally applied only to metallic minerals (see native element) but now includes such nonmetallic substances as sulfur, calcium fluoride (fluorite), and barium sulfate (barite). Ore is always mixed with unwanted rocks and minerals, known collectively as gangue. The ore and the gangue are mined together and then separated. The desired element is then extracted from the ore. The metal may be still further refined (purified) or alloyed with other metals.

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