URANIUM GEOCHEMISTRY OF SELECTED ROCK UNITS FROM THE MARYSVALE VOLCANIC FIELD, PIUTE COUNTY, UTAH

The Marysvale Volcanic Field is an area rich in uranium. This study was undertaken to determine if the uranium deposits might be of volcanogenic origin. The leaching, migration, and redeposition of uranium from silicic volcanic rocks are the primary processes responsible for concentrating uranium in volcanogenic systems. This geochemical study consisted of determining the major, minor and trace element concentrations of the major volcanic units, and the relationships of the rock chemistry to uranium mineralization. The units in the Marysvale Volcanic Field, used in this study, consist of ash-flow tuffs, intermediate lava flows, and associated intrusives of the Bullion Canyon volcanics and ash-flow tuffs, volcaniclastic deposits, domes and stocks of the Mount Belknap volcanics. When compared to overlaying welded tuff or rhyolitic units, results of selected trace element analyses indicate that the vitrophyric samples from the Mount Belknap volcanic units, are all enriched in F, Cs, and U, and that 50% of the vitrophyres are enriched in Cr, Cu, Mo, Ni, Zr, Pb, Sr, V, and Zn. The presence of anamalous Cs in all of the vitrophyres may indicate that an important trace element indicator for uranium exploration has been overlooked. This indicates that the overlying units have been devitrified and have released U as well as other trace elements into the volcanogenic system. This study has reevaluated the Marysvale Central Mining District and has proposed another theory as to the origin of the uranium deposits in that area. This hypothesis places a previously unidentified caldera around the area, and this author has named it the Marysvale caldera. Evidence for this caldera includes: arcurate faults which surround the region; alteration patterns which appear to form a circular pattern along the boundary of the proposed caldera; the presence of small monzonite intrusive bodies appear to ring the caldera; the presence of ash-flow tuffs which thicken appreciably along the northeast boundary of the caldera; and the central intrusive which may represent a resurgent phase of the proposed caldera. The rhyolite flow domes and small stocks associated with the Mount Belknap Volcanics, and the small Red Hills caldera may represent a recycling of caldera evolution with these later volcanics being emplaced along the other caldera structure. This seems to be a viable alternative to the magmatic hydrothermal origin for the uranium deposits presently proposed for the Central Mining District