Where once there was great optimism about the economic exploitation of very low-grade uranium deposits (0.002 to 0.010% U₃O₈) such as occur in the Chattanooga Shale and the Conway Granite, there now is almost equivalent pessimism. A 1978 National Research Council report "rules out" shales and granites as future uranium sources, because of the "enormous" mining and processing costs required and because the environmental impacts of the "many huge operations" that would be needed are not likely to be acceptable. Such a conclusion appears premature. Gold is won at an after-tax profit from deep-mined refractory ores in which it is found in concentrations as low as 3 ppm (0.0003%), much lower than the low end of the range cited for uranium in shales and gr nites; the price-grade relations for gold suggest that 0.006% U₃O₈ rock could become ore at a price well below $150/lb. The energy balance appears positive even for once-through LWR burning; with breeders, it would be strongly positive. To meet the projected 1980 United States annual requirement of 19,000 short tons of U₃O₈ from a single mine in rock containing 0.006% U₃O₈ at 70% recovery would require moving a minimum of 1.24 million short tons of rock a day--a large amount, but easily within the ability of present technology (5 or 6 large copper pits would be the equivalent), and possibly not unacceptable in a time of energy scarcity. The major constraint on potential uranium supply remains political, rather than geologi al or technological.

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