Granites as indicators of uranium provinces

The origins of U provinces can be attributed to acid magmatic sources with juvenile characteristics which transferred radioelements from the lower to the upper crust and which were formed by partial melting of a F- and LIL-enriched lower crustal-upper mantle source region. The granites were emplaced as dry melts which were further enriched at a high level by fractional crystallization and pervasive metasomatism. The granites and their precursor source regions thus represent a primary and fundamental control on the subsequent development of high level U ore-forming processes. A long period of evolution and enrichment in the lower crust-upper mantle region may thus be required prior to the transferrence of these U enriched magmas from the lower to upper crustal levels and Precambrian crust is often reported from areas where this process is thought to have occurred. Indeed it was emphasized by Bowie (1977) that the occurrence of uranium provinces in Colorado, southern Ontario, South Africa and elsewhere is closely associated with Precambrian crust. Major tectonism and crustal instability during the ore-forming processes are also an essential requirement. On the other hand a model involving the anatexis and high level emplacement of sedimentary rocks in continental collision zones does present many difficulties in accounting for the genesis of U provinces: Firstly, uraniferous granites in orogenic belts are shown to have characteristics similar to those of uraniferous granites from anorogenic regions and secondly, black shales which have high U contents also have high levels of Co, Cr, Cu, Ni and V; these elements are however notably lacking in the uraniferous granites we have investigated. Studies of mineralized and metalliferous granites from several U provinces show however that it is usually possible to distinguish mineralized from nonmineralized metalliferous granites using selected indicator elements such as F and B. These factors are illustrated by examples ranging from the Archaean to the Phanerozoic from China, South Africa and Europe. 24 refs, 1 fig