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Subtle evidence of several types suggests that diatreme feeding of mantle volatiles into receptive host formations at tectonically weak loci (centers) may be a more important factor in forming productive uranium districts than has been recognized. The known highest abundance of uranium and thorium in alkalic hypabyssal and volcanic rocks differentiated in the mantle, as well as the recognition that this alkalinity is the product of a previous enrichment in volatiles occasioned by that differentiation, illustrates the concentration of radioelements in the volatile fraction. These observations support the theoretical inference made in Chapter 4 that mantle degassing is a principal means of radioelement transfer to the crust. Another pertinent inference is the formation of a further differentiation product of concentrated volatiles and fluid without magma which should contain an even greater concentration of radioelements than the alkalic rocks or certain carbonatites. Such a gas could easily be the preferred diatreme drilling agent.

Juvenile gases are usually evidenced at the surface as fumaroles or solfataras, and these are known mostly in volcanic regions. Uranium has been noted in gases from such regions-by Stoiber and Rose (1968, 1972) and Rose et al (1 970) in the basaltic Central American volcanoes, and by Mittempergher (1970) in the alkalic volcanic material of Italy. Stoiber et al (1971) also recognized hydrocarbon compounds in volcanic gas; thus, UCI4 could form by reaction with hydrocarbon chlorides. The occurrence of uranium and thorium in volcanic regions, particularly in the alkalic volcanic rocks, is common. However, large uranium districts do not occur in the most prolific volcanic regions, and volcanic rocks are only subtly and questionably associated with the best uranium districts. Therefore, if uraniferous gas feeding is to be a significant transfer mechanism, gas feeding would have to be partly or largely independent of lava feeding. This implies the further differentiation beyond the uraniferous alkaline magmas. Some fumaroles are known in nonvolcanic areas—such as the CO2 geysers of the San Rafael swell in Utah, a uraniferous region—but distinction between juvenile and sedimentary origin is difficult.

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