Ore Body

the general name for an accumulation of ore in any shape. An ore body may correspond to an ore deposit, but more often the deposit includes several ore bodies. The boundary between an ore body and the enclosing rocks may be distinct and discernible to the eye. On the other hand, it may be indistinct, with a gradual transition from the ore body through a zone of impregnated low-grade ores and weakly mineralized rocks to the enclosing rocks. When indistinct, the boundary of the ore body is established during the sampling process, based on the minimum allowable content of metal or mineral in the ore.

Three groups of ore bodies are distinguished by shape: isometric, flat, and elongated in one direction. Isometric ore bodies are accumulations of mineral substance that are approximately equal in all measurements. They include stocks, stockworks, and pockets, relatively small accumulations of ore that are isometric in shape and usually not more than 1–3 m in cross section.

Flat ore bodies—sheets, veins, and lenses—have two long dimensions and one short dimension. The sheet, the most common shape in which sedimentary deposits occur, is a tabular body separated from other rocks by bedding planes. A distinction is made between simple sheets and complex sheets, which have rock interlayers. Sheetlike deposits differ from sheets in their smaller dimensions, discontinuity, and lesser stability of thickness. They are typical of weathering deposits.

Veins are ore bodies formed when a mineral substance fills fracture cavities or when there is metasomatic substitution of mineral substances for rocks along cracks. The plane of contact between the vein and the enclosing rocks is called the selvage. The zones of mineralized lateral rocks of veins create a contact metamorphic aureole that sometimes contains industrial concentrations of valuable components. Where the minerals that fill the veins are unevenly distributed, there is an alternation of sections rich and poor in valuable components; the rich sections in the body of the vein are called ore shoots. Ore shoots may be morphological or concentrated. The former are formed by bulges in the vein, whereas the latter are zones having an increased concentration of valuable components unrelated to change in the morphology of the ore body but rather caused by local alterations of the physicochemical parameters of ore deposition. The latter are sometimes related to the ability of the ore-enclosing rocks to react chemically with solutions. Sometimes they result from a sharp change in the temperature and pressure of solutions, the change leading to a large-scale accumulation of ore minerals.

A lens is a lenticular geological body that tapers out markedly in all directions; its thickness is slight compared to its length. In terms of morphology, lenses and lenticular beds are transitional formations between isometric and flat ore bodies.

Ore bodies elongated in one direction are called ore pipes or pipes. Ore pipes are oval in cross section. They form when an ore substance from magmatic melts or hydrothermal solutions is concentrated; the melts or solutions penetrate from the abyssal parts of the earth’s crust along the line where tectonic fractures intersect or along fractures that intersect easily penetrated rock strata. Sometimes, when melts or hot vapors and gases break through a bed of rock, diatremes are formed; examples are the diamond-bearing kimberlite pipes of Siberia and South Africa. There are ore pipes composed of copper, lead-zinc, and tin; they are up to several kilometers long, and their width in cross section varies from a few meters to several hundred.