| Abstract:Alkaline metasomatism (AM) is a type of hydrothermal process leading to increase in alkali metals and dipolarization of mean atomic valence of wall rocks. According to the confining nature of reaction substance systems, it may be divided into following three main classes: 1. Closed alkaline metasomatism: This class includes alkaline autometamorphism occurring within granitic intrusives, alkaline rocks, pegmatites, carbonatites, etc. It has four characteristic values which all approach to zero: the interval between the formation of the hydrothermal solutions and that of the wall rocks (value c) Jdistance of silica from alkaline metasomatic rocks (AMR) (value b); distance of orebodies from AMR (value c), distance of soclium metasomatic rocks from potassium metasomatic rocks (value d). It seems to be the most important hydrothermal process forming ore deposits of Nb, Ta, Li, Be, Rb, Cs, TR, P,U, Th inside various magmatic bodies. 2. Open alkaline metasomatism: It is in linear distribution along regionai faults and may appear in any rocks. Value a varies from several tens to hundreds of million years, Values b, c. d are so large (generally over l km) that they go beyond the extent of the deposit. This class is the most important ore-forming process of alkaline hydrothermal uranium deposits, which can be observed in north and south China, the Soviet Union, Canada, India, Brazil, South Africa, Sweden, etc. and is characterized by either pure soclium metasomatism or pure potassium one. 3. Semi-open alkaline metasomatism: In this class, alkaline metasomatic rocks are always connected with the surrounding environment by some fractures .above them. The values of a, b, c, d are between those of the two above-described classes. Although the orebodies, siliceous bodies and AMR appear separately, they remain occurring within the same deposit. Most of the AMR are mixture of K-, Na-metasomatic rocks. This class is the main metasomatic process giving birth to a great variety of deposits in the world, such as quartz-vein-type W, Sn, Au, Ag, Cu, Mo deposits, porphyrite-type Fe deposits, skarn-type Fe, Cu deposits and porphyry-type Cu, Mo, W, Sn, Au deposits. AM has five geochemical characteristics: 1) the incompatibility of K, Na in solid phase products, i. e. sodic minerals are preferentially replaced in the K-bearing hydrothermal solutions, whereas potassic minerals are preferentially replaced in the Na-bearing solutions, 2) desilicification, i. e. quartz in the rocks is dissolved and then moves in the alkaline hydrothermal solutions, 3) oxidation, i. e., Fe2+ is transformed into Fe3+, 4) the participation of large amounts of acidic anion radicals, such as Cl-, F-, S2-, CO32-, SO42-, PO43-in reaction and the introduction of some of them into solid phase products, 5) augmentation and concentration of high valence (>divalence) oxyphile elements, such as Al3+, Fe3+, V3+, ∑TR, Ti4+, Zr4+, Hf4+, Th4+, U4+, C4+, Sn4+, Nb5+, Ta5+, P5+, W6+, U6+, etc. The co-concentration of above mentioned elements might be ascribed to the following three laws: l) law of atomic valence equilibrium of rocks, 2) law of co-migration of complex anions, 3)specialization of concentrated elements: Cu, Mo, Rb, ∑Y etc. in K-metasomatism, Nb, Ta, Cs, Fe, etc. in Na- metasomatism, and U, Th, and Au in both cases. The evolutional line of AM is represented by the alternation of alkaline waves and acid waves. During the late stage of evolution, there frequently appears acid tail composed of quartz, chalcedony, alumstone, gypsum, barite, carbonates and ore-forming polymetallic sulfides; in the alkaline waves the alternation of K-waves and Na-waves also occur many times. As a result, sodic, potassic, siliceous and ore constituents form complex vertical and horizontal zonalities. |
