REE-BE-U-ENRICHED FLUORITE MINERALIZATION OF THE ROUND TOP AND LITTLE ROUND TOP LACCOLITH-CRETACEOUS LIMESTONE CONTACT ZONE AND RHYOLITE BRECCIAS, SIERRA BLANCA PEAKS, TRANS-PECOS TEXAS

The Trans-Pecos region of Texas is the site of extensive Paleogene magmatic activity linked to subduction-related processes off southwestern 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. Typically the diverse metallic and industrial mineral concentrations form as replacement deposits near the laccolith contacts with reactive carbonate rocks of Permian to Cretaceous ages.

The Sierra Blanca peaks comprise of five peraluminous laccoliths dated at 36 Ma northwest of the town of Sierra Blanca in Hudspeth County. Round Top laccolith was the focus of a major exploration program for beryllium in the 1980’s that provided a large sample and information base; Round Top and Little Round Top are currently being evaluated for their REE resource potential. Prior studies of the Sierra Blanca laccoliths have indicated that they are enriched in Li, Be, Zn, Rb, Y, Zr, Nb, Sn, REE, and Th. The Round Top rhyolite typically consists of 48-52% potassium feldspar, 28-30% quartz, 8-14% plagioclase feldspar, 4-5% biotite, and 2-3% opaques, dominantly magnetite-hematite. Preliminary microbeam studies indicate the common presence of subhedral zircons, with anomalous concentrations of U, Th, and Hf. Nb-Pb-rich grains probably are columbite-tantalite.

The rhyolite-limestone contact in addition to northwest trending fault structures, are complexly brecciated with cementation by clay, fluorite, silica, and/or calcite. Unusual element enrichment, including beryllium and uranium, is notably highest at the Round Top intrusion-limestone wall rock contact and within the rhyolite breccias of the fault structures. Petrographic and geochemical studies of the Round Top and Little Round Top rhyolite-wall rock and fluorite cementation alteration-mineralization relationships are in progress to evaluate the role of magma chemistry, crystallization history, and hydrothermal processes in the enrichment of unusual elements.