NEW U-PB ZIRCON SHRIMP GEOCHRONOLOGY OF VOLCANIC ROCKS OF THE THOMAS RANGE AND THE WORLD CLASS BE-U-F DEPOSIT AT SPOR MOUNTAIN, UTAH: A RECORD OF REGIONAL MAGMATISM FROM 37 TO 5 MA

We report new U-Pb zircon SHRIMP age and trace element data for volcanic rocks in the Thomas Range, including regionally distributed rhyolites of the Spor Mountain Formation [SMF]. The SMF, which hosts the largest productive deposit of beryllium in the world, occurs in an area of extensive alkalic rhyolitic ash flow tuffs and Eocene to Oligocene calderas underlain by Paleozoic and older limestone, dolomite, shale, and quartzite. Recent U-Pb zircon SHRIMP dating (Ayuso et al., 2020 J. Geochem. Expl. 209, 1-22) of fluorite- and bertrandite-rich tuff and topaz-rich rhyolite at Spor Mountain yielded overlapping crystallization ages ca. 26 Ma; a younger zircon population ca. 21 Ma is ascribed to effects of heating and hydrothermal alteration. New U-Pb age determinations for other volcanic rocks in the region include: 1) ca. 37 Ma for the Keg Springs andesite, 2) ca. 35 Ma for the Dell Tuff, 3) ca. 34 Ma for the Joy Tuff, 4) ca. 24 Ma for rhyolite from a prominent vent structure located north of the uplifted Paleozoic carbonate block of Spor Mountain, 5) 22 to 21 Ma for glassy rhyolite lavas in The Dell east of Spor Mountain and adjacent to the ring fault of the Thomas Caldera, 6) ca. 20 Ma for a rhyolite vent in carbonate rocks of Spor Mountain, and 7) ca. 5 Ma for rhyolite of the Topaz Mountain Formation. The rhyolite lava ages suggest that the SMF is part of a protracted period of volcanism. The 26 Ma beryllium tuff is the only unit known to contain phreatomagmatic base-surge deposits containing carbonate debris. Interplay of periodic infusions of Be-enriched magma, ash, and gas with shallow-circulating heated fluids led to large-scale remobilization and concentration of high-grade Be ore, mainly by replacement of carbonate in the tuff. New U-Pb opal SHRIMP data place constraints on timescales of bertrandite-fluorite-opal precipitation. Be-rich opal precipitation spanned ca. 16 million years, from 26 Ma to about 10 Ma. The new age data for regional volcanic rocks refine the volcanic sequence proposed by Lindsey (1979, USGS MI Series I-1179) and shed light on the magnitude of the metallogenic processes and magmatic-to-hydrothermal system required to form a super-large volcanic-hosted beryllium deposit.