ADVANCES IN DEPOSIT GENESIS FOR THE WORLD-CLASS VOLCANIC-HOSTED BERYLLIUM DEPOSITS AT SPOR MOUNTAIN, UTAH, USA

Beryllium (Be) ores that form the world-class deposits at Spor Mountain, Utah, are hosted in a sequence consisting of lithophile element-rich, topaz-bearing rhyolite lava flows and lithic-rich, phreato-magmatic base-surge and tuff deposits situated mainly along the ring fracture of an Oligocene-aged caldera. Recent field studies of ore bodies in the district (Monitor, Blue Chalk, Rainbow, Fluoro, and Southwind) provide new insights into the nature of the tuffaceous, lithic-rich layers that host high-grade Be mineralization. Ore zones consist of altered tuff that contains abundant mineralized veinlets, nodules, and geodes. The nodules and geodes are composed mainly of multiple generations of opal (~100-5300 ppm Be), fluorite, ~1-2% bertrandite (Be₄Si₂O₇(OH)₂), Mn-oxides, and calcite. Three types of siliceous mineralization of different ages are recognized: 1) relict lithic fragments of nodular opal-fluorite, 2) layered nodules of opal-fluorite-quartz, and 3) geodes of jasper-opal-fluorite-quartz. New geochemical modeling indicates that the alteration assemblages and opal-fluorite-bertrandite nodules younger than ~25 Ma likely formed by hydrothermal processes under pH-buffered, isothermal to cooling conditions (~250º-100ºC); the F-, Mn-, Li-, Si-, and Be-bearing fluids replaced carbonate clasts and coated relict nodules of opal-fluorite-bertrandite and other lithic fragments caught up in the base-surge deposits. Models for Be transport and accumulation include remobilization of Be from glass in tuff or direct release of Be (plus volatiles) from cooling of hypabyssal rhyolite (or pluton) at depth. The new data highlight the importance of quantifying the timing and flux of Be into the system and the relative roles of magmatic fluids and meteoric water in the precipitation of bertrandite, fluorite, and other minerals over time. The widespread occurrence of geochemically similar volcanic rocks in the southwestern U.S. indicates the potential for additional deposits of volcanic-hosted Be, which can safeguard a long-term, reliable, and stable supply of U.S.-produced Be to domestic and global markets.