SHRIMP U-PB GEOCHRONOLOGY OF THE SPOR MOUNTAIN FORMATION AND OF BE-U-RICH OPAL, UTAH: CONTINUOUS OPAL FORMATION, EPISODIC BE-U MINERALIZATION, AND REMOBILIZATION EVENTS

The Spor Mountain Formation (SMF) hosts the largest deposit of volcanogenic-epithermal beryllium in the world. New U-Pb ages (± 2s errors) obtained by SHRIMP indicate that zircon from the tuff member of the SMF, which hosts the Be-U mineralization, yield a mean crystallization age of 24.1 ± 0.5 Ma. Zircon from overlying altered rhyolite and unaltered rhyolite of SMF yield mean ages of 25.2 ± 1.1 and 25.2 ± 0.7 Ma, which overlap the Be tuff age. Beryllium ore (in tuff breccia) is found mainly in cm-to-m-wide, irregularly layered nodules (calcite, chalcedony, opal, fluorite, and bertrandite). SHRIMP U-Pb age analyses of carbonate-free opal in the analyzed nodules have a range of ²⁰⁶Pb/²³⁸U ages from ~55 Ma to ~2 Ma. Ages older than ~25 Ma are reliable if the opals have not gained or lost U or its daughters. Older opals are found in volumetrically minor relict lithic fragments in nodules and overlap ages of older Oligocene and Eocene volcanics in the region. Younger ages are found in successive opal layers. A previous study of bulk opal samples from a nodule identified ages of ~21.8 Ma, 13-16 Ma, 8-9 Ma in SMF and ~3.5 Ma in opal veins in the 6 Ma Topaz Mountain Formation (Ludwig et al., 1980). SHRIMP multi-element trace element analyses collected concurrently with opal ages show that high values of Be/Si in opal (~5,000-20,000), Be/F, Be/P, and Be/U are generally correlated with the oldest ages (>25 Ma), which are found in opal fragments in cores of some nodules; outward, younger opal layers (~25 to ~7 Ma) decline in Be/Si values but include spikes of >5,000. A prominent U/Si spike (>300) occurs in opal between ~6 Ma and ~4 Ma, which may reflect the age of nearby U mineralization at the Yellow Chief U deposit. The occurrence of Be-rich opal fragments older than 25 Ma within the mineralized tuff suggests that some opal-Be mineralization is related to older volcanic events. Younger opal (<25 Ma), which represents the main stage of Be-mineralization, is thought to have formed by hydrothermal fluid interacting with Be-rich glass in tuff. Beryllium in late nodular opal (<6 Ma) may reflect remobilization of Be by hydrothermal fluids related to the Topaz Mountain Formation. The world-class deposits at Spor Mountain likely formed by prolonged magmatic-hydrothermal processes that included multiple Be mineralization and remobilization events.