Cryolite-bearing and rare metal-enriched rhyolite, Sierra Blanca Peaks, Hudspeth County, Texas.
This report describes the round top intrusion, one of five shallow rhyolite laccoliths in the Tertiary trans-Pecos Texas magmatic province near Sierra Blanca, Texas, which was chemically modified by pegmatitic vapor-phase crystallization. Evidence of pegmatitic crystallization includes the presence of cryolite (Na3AlF6), Li-rich trioctahedral micas with variable Fe contents, rutilated quartz, and vapor-rich fluid inclusions defining quartz overgrowths on magmatic grains. The laccoliths are enriched in Li, Be, F, Zn, Rb, Y, Zr, Nb, Sn, Rees, and Th and are depleted in Mg, Ca, and Ti relative to typical calc-alkaline rhyolites. They are peraluminous, making them unusual among trans-pecos silicic igneous rocks, which are typically peralkaline or metaluminous. The presence of cryolite and the strong hree enrichment, which may also be the result of vapor-phase crystallization, sets the Sierra Blanca rhyolites apart from otherwise chemically similar topaz-bearing rhyolites. Accessory biotite and lirich micas are enriched in f, mn, and zn and are depleted in Mg and Ti. Rare metals are contained in part by bastnaesite(?), cassiterite, columbite, priorite(?), Nb-rich rutile, tantalite, thorite, hree-rich xenotime, yttrocerite and yttrofluorite, and zircon. The rhyolites are the sources of f and be in beryllium deposits in fluoritized limestones along the contacts with the laccoliths.