Many uranium and other lithopile element deposits are

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within or adjacent to small fluorine-rich rhyolitic dome complexes or tuff sheets. Examples studied include Spor Mountain, Utah (Be-U-Li-F), the Honeycomb Hills, Utah (Be-U-Li-F), Wah Wah Mountains, Utah (U-F), and the Black Range, New Mexico (Sn-Be-F). The formation of these and similar deposits begins with the emplacement of a rhyolitic magma, fractionated in lithophile metals and complexing fluorine, that rises to a shallow crustal level, where its roof zone may become further enriched in volatiles and the ore elements. During initial explosive volcanic activity, aprons of lithic-rich tuffs and surge deposits are erupted around the vents. These early pyroclastic deposits commonly host the mineralization, owing to their initial enrichment in the lithophile elements, their permeabili y, and the reactivity of their foreign lithic inclusions (particularly carbonates). The breccias are capped by thick topaz rhyolite lavas or welded tuffs that can serve as a source of heat and of additional quantities of ore elements. Devitrification, vapor-phase crystallization, fumarolic alteration, or the formation of lithophysae may free the ore elements from the glass matrix and place them in a form that is readily leached by percolating meteoric waters. Heat from the rhyolitic sheets drives such waters through the system, generally into and up the conduit and out through the early tuffs. Secondary alteration zones (K-feldspar, sericite, silica, clays, fluorite, carbonate, and zeolites) and economic mineral concentrations may form in response to this low temperature (less than 200 76;C) circulation. After cooling, meteoric water continues to migrate through the system, modifying the distribution and concentration of the ore elements (especially uranium). In this model, the ore elements are derived essentially from the volcanic vent complex itself, although contributions from the underlying magma chamber are not excluded. Plutons and country rocks beneath such vent complexes may themselves contain disseminated, vein, or replacement deposits of U, Th, Be, Sn, Mo, W, Nb, Ta, or associated elements.

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