REE-BE-U-ENRICHED FLUORITE MINERALIZATION OF THE ROUND TOP LACCOLITH-CRETACEOUS LIMESTONE CONTACT ZONE, 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 are replacement deposits near the laccolith contacts with reactive carbonate rocks of Permian to Cretaceous ages.

The Sierra Blanca peaks comprise five peraluminous laccoliths dated at 36 Ma northwest of the town of Sierra Blanca in Hudspeth County. The Round Top laccolith was the focus of major exploration program for beryllium in the 1980’s that provided a large sample and information base; Round Top currently is being evaluated for its 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, and 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 is complexly brecciated with the breccias cemented with clay and/or fluorite. Unusual element enrichment, including beryllium, is notably highest at the Round Top intrusion-limestone wall rock contact. Additional petrographic and geochemical studies of the Round Top rhyolite-wallrock alteration-mineralization relationships are in progress to evaluate the role that magma chemistry, crystallization history, and hydrothermal processes have played in the enrichment of unusual elements.