The craton7 is resistant to geologic change, and its history therefore is characterized by a lack of events and by stability. On this basis, once a craton is established, no mineral deposits should form in it. However, the outward simplicity is misleading. Most shields are composed of stabilized roots of multiple orogenes welded around a central core, as in the Canadian shield. Ore deposits formed in the orogenes when they were active and mobile. Therefore, all the aspects of deep, high-temperature uranium mineralization apply in the shields. Where the shields have remained high-standing, as in Canada and South Africa, the orogene roots should contain high-temperature mineral deposits. Late, low-temperature, shallow uranium veins could well exist also; but, if the craton lacks a sedimentary veneer, a postcraton age may be difficult to demonstrate, and the young age must be inferred from textures and mineralogy.
Many cratons or portions thereof have subsided moderately to accumulate thin sedimentary covers, and thus are platforms. The sedimentary covers are flat and featureless (except near mobile belts, where isolated uplifts occur, or along taphrogenic fault zones) and are notoriously barren of epigenetic mineral deposits. The craton has merely been lowered a notch and given a concealing cover, and general stability continues.
Crustal orogenic and igneous processes which could mobilize crustal metals and stimulate mineralization are foreign to such an environment of stability, and ore deposits so formed are scarce. However, the shields and platforms have not escaped tectonic activity (Fig. 41). They are relatively rigid and have been broken into a mosaic of individual blocks by taphrogenic faulting. The pattern of faults and their relation to the marginal orogenic belts (Gabelman, 1973) suggest that craton fragmentation was related to the differential encroachment of ocean floor, the differential translation of orogenic-belt segments into the craton, and perhaps the extension of transform faults beneath the continent. Entire continents seem to have been affected (North America is a good example), and the effect is what might be expected from the differential passage of active mantle beneath a passive continent. Areas of rupturing in the craton could overlie transform faults in deep crust or mantle. Boundaries between blocks are either zones of short multiple faults, as along the 38th Parallel lineament, or lineaments of local structural or volcanic features which only suggest faulting. This variation in boundary style would indicate that deep faults die out upward and that transcurrent movement is absorbed in local arches and basins (a situation comparable to the passive crumpling of a stationary rug on a moving floor).
Figures & Tables
Migration of Uranium and Thorium-Exploration Significance
The uranium resource industry since the late 1960s has presented a paradox to those concerned with the growing energy shortage and the relative ability of uranium resources to respond to the need on a timely basis. This publication reviews the possible ways that uranium in the earth might be concentrated into economic deposits, and considers what industry should be able to expect from an exploration effort. Some of the chapters in this volume include: Fundamental sources of uranium and thorium; Mechanisms of uranium and thorium transfer to the crust; Shallow uranium mobilization processes; Geochemical distinction of uranium moneralization processes; and Oceanic migration history of uranium and thorium.