Metallogeny

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Metallogeny

 

the branch of the theory of minerals that investigates regional uniformities in the formation and location of ore deposits. Metallogeny provides the scientific basis for predicting the distribution of different groups of ore deposits. The founders of metallogeny in the USSR were V. A. Obruchev, S. S. Smirnov, and lu. A. Bilibin; abroad the French geologist L. de Launay was a founder of this branch. Metallogeny proceeds from the idea that during successive stages of the development of the earth’s crust particular assemblages of ore deposits form in the large structural subdivisions of the crust. These subdivisions have their own characteristic processes of sediment accumulation, tectonic activity, and magmatism. The formation of groups of ore deposits occurs differently in geosynclines and on platforms.

The transformation of geosynclines into folded regions is accompanied by the appearance of three series of igneous rocks and associated ore deposits. During the early stage (the bowing of the floor of the geosyncline and accumulation of a thick layer of basaltoid volcanogenic-sedimentary rocks) four types of igneous rock develop: spilite-keratophyre with pyrite deposits of copper, zinc, and sometimes lead; peridotite with magmatic chromite deposits; gabbro-pyroxenite-dunite with magmatic deposits of titanium-magnetite ores; and plagiogranite-plagiosyenite with skarn deposits of iron and copper. In the middle stage of geosynclinal development, during the main phases of folding, two granitoid igneous rock formations develop: granodiorite formations with skarn and hydrothermal deposits of tungsten (scheelite), gold, copper, molybdenum, lead, and zinc and granite formations with pegmatite, albitite, and greisen deposits of tin, tungsten (wolframite), tantalum, lithium, and beryllium. In the late stage, which is transitional from a geosynclinal to a platform regime, two formations of igneous rocks are intruded: small hypabyssal intrusions with compositions ranging from dioriteporphyries to granite-porphyries and syenite-porphyries with various plutonic hydrothermal ore deposits of nonferrous, rare, precious, and radioactive metals; and andesite-dacites with equally varied volcanogenic hydrothermal ore deposits.

The above schema of metallogeny for geosynclines is generalized and usually does not manifest itself in full. In actual folded regions that have arisen on the site of geosynclines, either ore deposits characteristic of the early and middle stages of geosynclinal development form or else deposits of the middle and late stages predominate. Accordingly, there are two profiles of geosynclinal metallogeny. In the basaltoid profile typical of eugeosynclines, ore deposits of the first two stages predominate—for example, in the Urals. In the granitoid profile, which is characteristic for miogeosynclines, deposits of the last two stages are developed—for example, in the Verkhoiansk region.

The formations of igneous rocks and ore deposits associated with them are distributed in a regular manner within geosynclines, creating an ordered metallogenic zonation in folded regions. Spilite-keratophyre and plagiogranite-plagiosyenite formations of the early stage with their characteristic deposits, primarily iron and copper ores, are located in eugeosynclines. In eugeosynclinal troughs the section of the earth’s crust is thinner and has no granitic layer, as a result of which the metallogeny of eugeosynclinal troughs is exclusively basaltoid. In the internal zones of miogeosynclines and the medial uplifts that form on their site, there develop chains of middle-stage granite-formation massifs; the sites have associated zones of pegmatite, albitite, and greisen deposits of rare elements. The internal zones of miogeosynclines are characterized by complete sections of the earth’s crust with a well-developed granitic layer; granitoid metallogeny is typical in these cases. The zones between eugeosynclinal troughs and the peripheral zones of miogeosynclines are where granodiorite middle-stage formations and associated ore deposits are found. The abyssal fractures that delineate the major structural-formational zones of geosynclines control the injection of peridotites and gabbro-pyroxenites of the early stage. In so doing, they determine the position of the belts of magmatic deposits of chromites and titanium-magnetites. They also, however, determine the position of the hypabyssal plutonic and volcanic formations of igneous rocks of the late stage. These formations mark the position of the belts of plutonic and volcanogenic hydrothermal deposits of nonferrous, rare, precious, and radioactive metals associated with the igneous rocks.

The metallogeny of the platforms is determined by the three stages of development of their internal geological structures: formation of the folded base, creation of the sedimentary cover, and tectonic-magmatic activation.

During the stage in which the folded base is formed, there develop deposits of the folded zones that correspond to the characteristics of geosynclinal metallogeny. During the formation of the sedimentary cover of the platforms stratified sedimentary deposits of ore, non-ore, and combustible minerals form. The completeness of development and the composition of deposits that form in the course of tectonic-magmatic activation of the platforms depend on the intensity of the activation.

On weakly activated platforms there are no marked tectonic deformations and no igneous rocks associated with a given stage in the development of platforms. There may, however, be telethermal or stratiform deposits of copper, lead, zinc, fluorite, and barite ores, which some investigators consider to be derivatives of deep-seated igneous rocks. The stratiform deposits of lead and zinc ores in the Paleozoic cover of the North American platform are examples of such deposits.

Activated platforms are characterized by the formation of gently folded deformations and occasional fractures and by the injection of unique igneous rocks during the platform period of geological history. For example, at the end of the Paleozoic and beginning of the Mesozoic the Siberian Platform was curved into broad, gentle folds that formed uplifts and depressions divided by fractures. In the depressions traps formed with accompanying magmatic deposits of sulfide copper-nickel ores; intrusive alkali rocks accompanied by gold-ore formation developed in the uplifts. Diamond-bearing kimberlites and ultrabasic alkali rocks accompanied by carbonatite deposits of apatite and rare elements were injected along the fractures.

Intensively activated platforms are characterized by the injection of hypabyssal granitic rocks and hydrothermal deposits of gold, tin, molybdenum, zinc, lead, and other metals.

The recurrence of similar processes of ore deposit formation during the earth’s geological history has made it possible to identify a number of successive metallogenic epochs, and the formation of analogous groups of ore deposits under similar geological conditions has made it possible to identify metallogenic provinces of the geosynclinal and platform types.

REFERENCES

Bilibin, III. A. Metallogenicheskie provintsii i metallogenicheskie epokhi. Moscow, 1955.
Magak’ian, I. G. Osnovy metallogenii materikov. Yerevan, 1959.
Smirnov, V. I. Ocherki metallogenii. Moscow, 1963.
Smirnov, S. S. Ocherki metallogenii Vostochnogo Zabaikal’ia. Moscow-Leningrad, 1944.
Shcheglov, A. D. Metallogeniia oblastei avtonomnoi aktivizatsii. Lenin-grad, 1968.

V. I. SMIRNOV

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