THE DISTRIBUTION OF TIN-ORE DEPOSITS WITHIN FOLDED ZONES

Abstract

The author makes an attempt to establish the general laws governing distribution in time and position of tin-bearing centers and areas in the course of the geological history of the earth; on the basis of methods of regional metallogenic analysis, developed in the U. S. S. R. by a group of scientists under the guidance of G. A. Bilibin. The author has considered the principal geotectonic and structural geological features and factors controlling the regular occurrence during a certain stage of development of a folded zone, of a certain type of tin-bearing intrusions and, hence, of a certain mineralogical-geochemical type of tin deposits. Considerable tin concentration evidently did not take place during the initial and early stages in development of folded zones. The middle stages of development, corresponding to the period of transformation of a geosyncline into a folded belt, were highly productive with regard to tin. Tin accumulations are associated with syntectonic batholithic intrusions of acid and ultra-acid “potassium” granites. Tin-bearing batholiths and their linear belts are controlled by regional ancient deepseated fractures, confined either to the points of anticlinorium contact (central upheavals) with the adjacent inner synclinoria or to the contact points of downwarpings with consolidated geoanticlinal zones. The late stages of development of geosynclinal regions corresponding to the period of consolidation of the folded territory also produced considerable tin concentrations. According to the depth of the controlling intrusions, the showings of tin occurrences belonging to the late stages of development are subdivided into two natural groups: the hypabyssal and the subvolcanic. Tin ore-bearing areas belonging to the late stages of development, are confined to the following two types of geotectonic environments: 1) the mobilized marginal parts of geosynclines in places of their contact with projections of platforms of intermediate massifs (blocks) and consolidated geoanticlinal structures. A number of large tin ore-bearing areas is confined to structures of late superimposed graben-like downwarpings (complicated by fractures) with a powerful development of terrestrial volcanism; 2) the inner portions of geosynclines, where the tinore-bearing areas are confined to structural facial zones of inner synclinoria, more frequently in places of contact of the latter with zones of central uplifts. The distribution of the world tin resources in the course of geological times is as follows: The Precambrian: 3. 3 percent (Nigeria, Rhodesia, Congo, Union of South Africa); The Caledonian folding: 6. 6 percent (Africa and Australia); The Variscian folding: 18.1 percent (Cornwall, Erzgebirge, the Pyrenees, Australia, Tasmania); The Kimmeridgian folding: 63.1 percent (South- Eastern Asia). Simultaneously with the growth of commercial tin concentrations from the Pre-Cambrian to the Mesozoic, complication of the nature of tin mineralization is observed; from tin-bearing pegmatites in the Pre-Cambrian to complex multicomponent deposits in the Mesozoic and the Cenozoic. Author