Geochemistry of the Spor Mountain rhyolite, western Utah, as revealed by laser ablation ICP-MS, cathodoluminescence, and electron microprobe analysis

Abstract:

The Miocene topaz rhyolite at Spor Mountain in western Utah hosts one of the largest beryllium deposits in the world and was responsible for producing 85% of the beryllium mined worldwide in 2010 (Boland, 2012). The Spor Mountain rhyolite is composed primarily of Ca-poor plagioclase (An8), sodic sanidine (Or40), Fe-rich biotite (Fe/(Fe+Mg)>0.95; Al 1.2-1.4 apfu), and Ti-poor quartz, along with several trace-element rich accessory phases including zircon, monazite, thorite, columbite, and allanite. Cathodoluminescence (CL) studies of quartz show oscillatory zoning, with 80% of the examined crystals displaying euhedral edges and slightly darker rims. CL images were used to guide laser ablation (LA) ICP-MS analysis of quartz, along with analyses of plagioclase, sanidine, biotite, and glass. Ti concentrations in quartz are 20±6 ppm; there is no quantifiable variation of Ti from core to rim within the diameter of the laser spot (53 microns). Temperatures, calculated using Ti in quartz (at 2 kb, aTiO2=0.34), vary between 529±10 C (Thomas et al., 2011), 669±13 C (Huang and Audetat, 2012), and 691±13 C (Wark and Watson, 2006). Two feldspar thermometry yield temperatures of 686±33 C (Elkins and Grove, 1990) and 670±41 C (Benisek et al., 2010). Zr saturation temperatures (Watson and Harrison, 1983) average 711±28 C. Analysis of the glass reveal the Spor Mountain rhyolite is greatly enriched in rare elements (i.e. Li, Be, F, Ga, Rb, Nb, Mo, Sn, and Ta) compared to average continental crust (Rudnick and Gao, 2003). Be in the glass can have as much as 100 ppm, nearly 50 times the concentration in continental crust. REE partition coefficients for sanidine are 2 to 3 times higher in the Spor Mountain rhyolite when compared to other silicic magmas (Nash and Crecraft, 1985; Mahood and Hildreth, 1983), although plagioclase tends to have lower partition coefficients; biotite has lower partition coefficients for LREE and higher partition coefficients for HREE. The patterns of trace element enrichment and depletion are similar to those of the measured partition coefficients, consistent with a major role for extensive fractional crystallization in the origin of the Be enriched magma.