Ore and granitization

Abstract

The process of granitization may be regarded as an approximate reestablishment of the geochemical equilibrium attained when the earth first cooled, but which was later upset by the large quantities of predominantly basic volcanic rocks brought into the sial by physical, as distinct from chemical, forces.On the basis of the granitization hypothesis, an attempt is made to explain the variation in composition of granitic rocks and of associated ore deposits during geological time by relating them to preceding volcanic activity. This leads to a possible explanation of the origin of some metalliferous provinces--e.g., the Western Australian gold province.Evidence is offered in support of the hypothesis that, in the process of the formation of a granitic rock, the valuable elements of ore deposits are concentrated in inverse ratio to the extent to which they are incorporated by isomorphous substitution in the common rock-forming minerals. This is held to explain, to a large extent, the association of specific ores with specific types of granitic rocks.Attention is drawn to the economic importance of distinguishing between synchronous and subsequent batholiths and suggestions are made for prospecting in areas in which synchronous batholiths occur.It is considered important to distinguish between ore deposits formed during a period of granitization and those of volcanic--or abyssal--origin: e.g., in the former case, given favorable conditions, some sort of zonal arrangement of the mineralization is to be expected; in the latter case, fluctuations in the depth of geoisotherms might easily cause ores, theoretically belonging to various depth-zones, to occur at the same vertical horizon.