The Round Top Project is owned by Texas Mineral Resources (TMRC), (formerly TRER), and is subject to a joint-venture and option agreement between USA Rare Earth LLC and TMRC, with USA Rare Earth LLC as the operating partner.
In November 2018, USA Rare Earth LLC entered into an option and development agreement with TMRC to acquire up to 80% interest in the Round Top project, subject to certain minimum expenditures, project milestones, and conditions.
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Summary:
The rhyolite itself comprises the REE mineralized body. Magmas with a peralkaline composition are known to have high concentrations of incompatible elements such as U, REE, Th, and Zr.
The rhyolite magma that developed Round Top Peak probably cooled too quickly to develop a coarse-grained texture or to develop zones with high REE concentrations. A quick cooling rate would cause a fine-grained texture of the rhyolite and even distribution of the REE minerals. The rhyolite magma was saturated in fluorine, which is reflected in the high percentage of fluorine accessory minerals that are distributed throughout the rhyolite mass. As the magma cooled, fluorine saturated fluids exsolved from the crystallizing magma. These fluorine rich fluids accumulated in interstices and vugs between the earlier crystallized minerals and deposited REE minerals and other accessory minerals in the interstices. The REE deposit at Round Top Peak can be classified as quartz saturated peralkaline (A-1) granite with a rhyolitic texture and a composition similar to certain pegmatites.
REE mineralization is hosted by the Round Top Peak laccolith. The rhyolite is fine grained with a microporphyritic texture. The porphyry phenocrysts consist of alkali-feldspar with albite cores, clear quartz grains, and minor brown to clear Li-mica. Within the quartz grains or crystals, inclusions along planes of crystallization have been observed. The groundmass is aphanitic and consists of quartz, feldspar, and mica with vugs or vesicles. The vugs may be lined with quartz, feldspar, fluorite, cryolite, and li-mica crystals. Some vugs are filled with kaolinite or fluorite and are surrounded by coarsely crystalized minerals. The vugs occur in bands and can be locally clustered in isolated locations. Late-stage fractionation of volatile components, such as F, CO 2 or H 2 O, from the crystallizing rhyolite probably formed these vugs.
Round Top Peak displays some pegmatitic characteristics, including an abundance of cryolite, lithium rich micas, rutilated quartz and vapor rich fluid inclusions (Price et al., 1987). Peralkaline rhyolites and pegmatites can contain an abundance of incompatible elements including REEs. The Round Top Peak rhyolite is enriched in incompatible elements including Li, F, Rb, Y, Zr, Nb, Sn, Ta, Pb, REE, Th, and U.
Isolated zones of brown rhyolite are present and are often related to fault structures or near the contact between the rhyolite and sedimentary rocks. In these brown zones, the iron minerals are replaced by goethite and limonite giving the rhyolite a brown color. Tan rhyolite is found along the contact between the rhyolite and sedimentary rocks. Tan rhyolite can also occur as mottling in the red and pink rhyolites located near mineralized faults and the contact between the intrusive and sedimentary rocks. The tan rhyolites were probably altered by vapor phase or hydrothermal fluids and consist of kaolinite clay and residual quartz phenocrysts. Magnetite and hematite are absent or present in only trace amounts. Degree of alteration varies and can be represented by a complete replacement of the feldspars by kaolinite to a partial replacement. Multiple colored fluorites often occur as fracture fillings and replacements in the tan rhyolites that contact the sedimentary rocks.
REE distribution and grades were not affected by the hematitic alteration of the rhyolite. However, the vapor phase or hydrothermal alteration of the tan rhyolite had an impact on the REE grade. The more intensely altered tan rhyolite zones can have a lower REE grade than the other four rhyolite phases.