South-Central Section - 47th Annual Meeting (4-5 April 2013)

Paper No. 30-6
Presentation Time: 8:00 AM-12:00 PM


O'NEILL, L. Christine, Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, 1 University Station, C1100, Austin, TX 78712, KYLE, J. Richard, Department of Geological Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712 and ELLIOTT, Brent, The Bureau of Economic Geology, The University of Texas at Austin, University Station Box X, Austin, TX 78713,

The Trans-Pecos region of Texas is the site of extensive Paleogene magmatic activity linked to subduction-related processes off southwest North America, including caldera-sourced felsic pyroclastics. This magmatic province also includes numerous plutons, most of which are shallowly emplaced rhyolitic laccoliths. More than 100 of these laccoliths are present in Trans-Pecos Texas and contiguous regions, but only a few have recognized mineral occurrences.

The Sierra Blanca peaks are comprised of five peraluminous laccoliths dated at 36 Ma northwest of the town of Sierra Blanca, Texas. The Round Top laccolith has been the focus of major exploration programs for beryllium in the 1980’s and for REE from 2010-12 that has provided a large sample and information base. Prior studies of the Round Top laccolith indicate enrichment in Li, Be, Zn, Rb, Y, Zr, Nb, Sn, REE, and Th, and a composition of 48-52% potassium feldspar, 28-30% quartz, 8-14% plagioclase feldspar, 4-5% biotite, and 2-3% opaques, dominantly magnetite-hematite. Prior studies indicate the common presence of accessory minerals including secondary zircons, and numerous REE minerals including columbite, yttrofluorite, yttrocerite, cerfluorite, and bastnaesite.

The rhyolite-limestone contact and northwest-trending faults are complexly brecciated with cementation by clay, fluorite, chert, and/or calcite. Unusual element enrichment, including beryllium and uranium, is notably highest at the Round Top intrusion-limestone wall rock contact and within brecciated rhyolite along faults.

Current studies of the Round Top laccolith indicate the presence of five unique rhyolites varying significantly by color, mineralogical composition, and texture, but believed to be of cogenetic origin. Portions of the rhyolites have mottled, crosshatched, or striped textures of unknown origin but are possibly linked to initial magma heterogeneity, and/or subsequent selective hydrothermal alteration. Petrographic, microbeam, and geochemical studies of the Round Top rhyolites are underway in order to determine the geochemical variations and possible REE enrichment differences among the rhyolite types and to characterize the magma chemistry, crystallization history, and hydrothermal processes instrumental in the enrichment of unusual elements.