LEACHING OF URANIUM FROM FELSIC VOLCANICS AND VOLCANOCLASTICS; MODEL, EXPERIMENTAL STUDIES AND ANALYSIS OF SITES

This work tests the premise that felsic volcanic rocks, particularly ash-flow tuffs, can serve as source rocks for uranium deposits. The applicability of this idea to several geologic environments is investigated. A genetic model is developed dealing with the behavior of uranium during and subsequent to ash-flow tuff deposition. It is based upon previously described investigations, geologic logic, data presented here, and speculation. Chemical changes resulting from early post-depositional processes such as devitrification and hydration are detailed. The influence of later post-depositional faulting, heating, erosion, alteration by ground water and hydrothermal systems and the influence of underlying units is discussed. Uranium may be released during the initial degassing, during hydrothermal alteration, and/or during later diagenesis. Experimental studies have been designed and carried out to simulate natural leaching conditions such as might occur during near surface diagenesis. Synthetic ground waters have been pumped through pulverized uraniferous vitrophyres in experiments at 20, 60 and 80(DEGREES)C (and atmospheric pressure). Na, K, Ca, Mg, Si, CO(,2), Cl, SO(,4) and minor element contents have been determined for the starting and product solutions and solids. The most significant chemical changes take place quickly, within a matter of days. Near equilibrium is attained within 7 days. Several starting and product leachant solution were analyzed fluorimetrically for uranium. They show significant increases in uranium contents, from less than 1 ppb at the start to greater than 10 ppb maximum. Such leachant solutions might be significant transport agents of uranium given geologic time. Leaching at low temperatures appears to involve a thin surface reaction and diffusion layer. Both dissolution and ion exchange influence the leachant composition. Vitrophyre-altered vitrophyre pairs from the Pena Blanca Uranium District were analyzed for major and minor element contents. Significant changes were seen in the content of uranium and other minor elements commonly associated with uranium. The U-K gamma ray spectrometry analyses of these samples show important trends similar to those seen in samples from Virgin Valley, Nevada, and Date Creek, Arizona. Volcanoclastic sediments are also subject to uranium leaching and enrichment during diagenesis. Fresh glass and partially-zeolitized units in Presidio County, Texas, Zeolite deposits at Resse River and Eastgate, Nevada, a uranium-enriched zeolite locality at Virgin Valley, Nevada, and a uranium deposit in zeolitized lacustrine deposits at Date Creek, Arizona, were studied. Thorium, uranium and potassium gamma ray spectrometry analyses of volcanoclastic sediments from a number of different environments demonstrates that definite thorium, uranium and potassium signatures are developed at the different stages in the diagenetic processes. Erionite, clinoptilolite and montmorillonite, three common diagenetic products in volcanoclastic sediments exhibit different U/K ratios regardless of variations in the thorium content. Tri-plots of the thorium, uranium and potassium gamma-ray spectrometry analyses are shown to be an important tool in the exploration for volcanogenic uranium deposits