Abstract

Ore-forming processes are discussed in relation to the hypothesis that the earth originally had a basaltic crust, and that the continents have been derived from this crust and the mantle. The transformation was accomplished by a combination of disintegration, decomposition and redistribution of rock materials at the earth's surface and reorganization of geosynclinal accumulations during orogenesis. Limited concentrations of metals may have formed during the solidification of the outer part of the earth, but these could not become available as ores except through drastic later disturbances of the outer crust. Ore-forming processes are considered in 4 groups: 1) Concentration by surface processes; 2) Concentration during the filling of geosynclines; 3) Concentration during orogenesis; 4) Concentration during stable platform modifications. Surface processes produce several types of ore deposits and help to shape the "charges" of geosynclines. During the accumulation stage in geosynclines, deposits enriched in metals may form 1) from the differentiation of basic intrusives as magmatic sulfide or oxide deposits, or as cavity filling, replacement or hotspring deposits formed by late stage hydrothermal extracts from these magmas, 2) through the outward migration of quantities of water originally included in the sedimentary fill. This water, with many substances dissolved, including metals, ranges in temperature from cool to hot, depending on the depths reached. In the late stages of accumulation, water in the deepest parts should be above the critical temperature and metamorphic changes in the sediments may have set in on an appreciable scale. All deposits formed before major deformation sets in, must be raised high above their level of formation to become available as ores. This is accomplished during the deformation stage and such ore deposits are normally deformed with the rest of the rocks. During the deformation stage in geosynclines, water solutions are moved about; metamorphic changes result in driving off of more water, both free and combined, and these solutions may form additional ores. Differential melting with solution of sedimentary and volcanic materials in deep parts gives rise to magmas with more or less metals, depending on the make-up of the material melted. Injection of magmas to higher levels results in driving off more water from the surrounding rocks and the magma itself may, at a late stage in its crystallization, yield solutions capable of forming metal-rich cavity filling and replacement deposits. Such injections and ore forming processes continue during a prolonged period following the climax of deformation. Those deposits brought close to the surface by subsequent erosion are available as ores, if rich enough in metals. Stable platform adjustments may result in the formation of ores related to layered basic intrusives, or to volcanic centers; also ores of Zn, Pb, Cu, V, and U in cover rocks. To improve understanding in discussions of deposits formed by hot water solutions, it is desirable that the word hydrothermal, used in reference to ore deposits or solutions, be stripped of all implications that the deposits or the solutions were derived from magmas.

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