SHRIMP U-Pb zircon geochronology of volcanic rocks hosting world class Be-U mineralization at Spor mountain, Utah, U.S.A.

Abstract We report for the first time U Pb data establishing the age of the Spor Mountain Formation, a volcanic sequence that hosts the largest deposit of beryllium in the world. Determining the age of rocks hosting the beryllium deposit is critical for improved recognition of exploration targets containing additional volcanic-hosted beryllium resources. The deposit occurs in an area of extensive alkalic rhyolitic ash flow tuffs and Oligocene and Eocene calderas underlain by Paleozoic and older limestone, dolomite, shale, and quartzite. Data were obtained by analysis of zircon from the Spor Mountain Formation using the Sensitive High-Resolution Ion Microprobe-Reverse Geometry (SHRIMP-RG) technique. We show that lower fluorite and bertrandite-rich tuff and upper topaz-rich rhyolite have overlapping crystallization ages. Zircon from the crystal- and lithic-rich beryllium tuff produced two populations with a predominant group yielding a weighted average 206Pb/238U age of 25.59 + 0.29/−0.45 Ma (± 2σ errors; 41 spots that form a coherent age group) and a separate younger age group at 20.84 + 1.29/−0.64 Ma (16 spots). Spot analyses for each group include cores and rims and show no evidence for older or inherited cores.. The best estimate of the age of crystallization of the beryllium tuff is taken as 25.59 + 0.29/−0.45 Ma. Younger spot analyses that are not part of the coherent group likely reflect Pb loss from thermal events related to renewed influx of hydrothermal F-rich fluids into the tuff. Uranium concentrations and 206Pb/238U spot ages show no statistically significant correlation for the beryllium tuff. Zircon from the overlying, capping rhyolite yields weighted mean 206Pb/238U crystallization ages between ca. 25.1 Ma and 26.2 Ma. The rhyolite that yielded the slightly older age, at ca. 26.2 Ma, shows a strong positive association between U content and 206Pb/238U spot age (correlation coefficient, r = 0.88). We interpret this to be a consequence of the effects of high-U concentrations in the zircon matrix as evidenced by dark zones in crystals. Zircon temperatures from tuff range from ~600 °C to 1200 °C and in rhyolite from ~525 °C to 1000 °C. Beryllium in zircon from tuff and rhyolite overlap (~1 to 150 ppm) and do not correlate with U Pb spot age, Hf, Ti, and other trace element abundances or ratios. REE contents of zircon (chondrite-normalized) are depleted in the light REE, enriched in the heavy REE, and show prominent positive Ce and negative Eu anomalies. V-shaped REE patterns are characteristic of zircons that are not part of the coherent age group. Whole rock ratios of tuff and rhyolite, such as Th/U (>0.1) are typical of igneous zircons, plot in the field of continental settings (e.g., U/Yb ~0.8–6, Hf ~10,000–26,000), and are consistent with an origin from sources relatively enriched in Pb. The apparent uniqueness of the Spor Mountain Formation reflects the interplay of alkaline (A-type) trachytic to rhyolitic magma enriched in Be, U, Li, and F and a protracted history of fractional crystallization. Other beryllium occurrences near Spor Mountain highlight the prospective potential for additional resources of volcanic-hosted beryllium.