Ore and mineral deposits 2

Ore and mineral deposits

Sequence of deposition

Studies of the relationships of minerals in time and space have shown that a fairly constant sequence of deposition, or paragenesis, is characteristic of many mineral deposits. This sequence has been established largely by microscopic observations of the boundary relationships of the minerals in scores of deposits. Subsequent experimental studies of mineral phases have contributed to the knowledge of paragenesis. In magmatic and contact metasomatic ores, silicates form first, followed by oxides and then sulfides. W. Lindgren presented the paragenesis for hypogene mineral associations, and others have discussed the problems involved. The sequence of common minerals starts with quartz, followed by iron sulfide or arsenide, chalcopyrite, sphalerite, bornite, tetrahedrite, galena, and complex lead and silver sulfo salts. It indicates the existence of some fundamental control but attempts to explain the variations in it have been largely unsuccessful, or are applicable to only part of the series or to specific mineralized areas. Local variations are to be expected since many factors such as replacement, unmixing, superimposed periods of mineralization, structural and stratigraphic factors, and telescoping of minerals may complicate the order of deposition.

Paragenesis is generally thought to be the result of decreasing solubility of minerals with decreasing temperature and pressure. It has also been explained in terms of relative solubilities, pH of the solutions, metal volatilities, decreasing order of potentials of elements, free energies, and changing crystal structure of the minerals as they are deposited. R. L. Stanton has reevaluated paragenetic criteria as applied to certain stratiform sulfide ores in sedimentary and metamorphic rocks. He proposes that the textures of such ores do not represent sequences of deposition but are the result of surface energy requirements during grain growth, or annealing of deformed minerals. To explain mineral paragenesis more satisfactorily, many additional experiments must be made to determine phase relations at different temperatures and pressures. See also: Depositional systems and environments; Mineral

Mineralogenetic provinces and epochs

Mineral deposits are not uniformly distributed in the Earth's crust nor did they all form at the same time. In certain regions conditions were favorable for the concentration of useful minerals. These regions are termed mineralogenetic provinces and they contain broadly similar types of deposits, or deposits with different mineral assemblages that appear to be genetically related. The time during which these deposits formed constitutes a mineralogenetic epoch; such epochs differ in duration, but in general they cover a long time interval that is not sharply defined. Certain provinces contain mineral deposits of more than one epoch.

During diastrophic periods in the Earth's history mountain formation was accompanied by plutonic and volcanic activity and by mineralization of magmatic, pegmatitic, hydrothermal and metamorphic types. During the quieter periods, and in regions where diastrophism was milder, deposits formed by processes of sedimentation, weathering, evaporation, supergene enrichment, and mechanical action.

During the 1960s numerous studies were made of the regional distribution of mineral deposits associated with long subsiding belts of sediments, or geosynclines, and with platform areas of relatively thin sediments adjoining the thick geosynclinal wedge. Geosynclinal areas commonly suffer folding and later uplift and become the sites of complex mountain ranges. It has been proposed that the outer troughs and bordering deep-seated faults contain ore deposits of subcrustal origin, the inner uplifts contain deposits of crustal origin, and the platforms contain ores derived from subcrustal and nonmagmatic platform mantle rocks. V. I. Smirnov and others have summarized available information on types of mineral deposits characteristic of the processes most active during the evolutionary stages of geosynclinal and platform regions. In the early prefolding stage of subsidence, subcrustal juvenile basaltic sources of ore fluids prevail, and the characteristic metals are Cr, titanomagnetite, Pt metals, skarn Fe and Cu, and deposits of pyritic Cu and Fe and Mn. In the folding episode, rocks of the geosyncline are melted to produce magma from which ore components are extracted or leached by postmagmatic fluids. The most typical ores of this stage are Sn, W, Be, Ni, Ta, and various polymetallic deposits. The late stage is characterized by ore deposits associated with igneous rocks and other deposits with no apparent relationship to igneous rocks. Smirnov believes these ores originated by the combined effect of subcrustal, crustal, and nonmagmatic sources of ore material. Typical metals of this stage include Pb, Zn, Cu, Mo, Sn, Bi, Au, Ag, Sb, and Hg. In the tectonically activated platform areas, deposits of Cu-Ni sulfides, diamonds, various magmatic and pegmatitic deposits, and hydrothermal ores of nonferrous, precious, and rare metals are found. In addition, there are nonmagmatic deposits of Pb and Zn. Some ore material is believed to be both subcrustal and nonmagmatic in origin. Relative proportions of types of mineralization differ from one region to another.

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