Round Top

Region West, Southwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area Broad area of exposed and potential concealed REE in highly fractionated Tertiary rhyolites. Area extends from New Mexico to south of the Quitman Mountains, including the Sierra Blanca Complex, Round Top deposit, and nearby similar prospects (intrusive rhyolites with high whole rock REE); includes the Cave Peak deposit. Western boundary is the approximate edge of the Rio Grande Rift. See McLemore (2018a) for summary of the deposits in the Texas-New Mexico alkaline belt.
Identified resources Identified REE resource.
Production None.
Status Current and recent exploration at Round Top as a HREE, Sn, U, Be prospect, drilled and analyzed, but never mined.
Estimated resources A resource model for Round Top suggests the deposit contains an estimated Measured and Indicated resource of 364 million metric tons of mineralized rhyolite, with additional Inferred resources of 735 million tons, with a high ratio of HREE to LREE (Hulse and others, 2019, Table 14-10).
Geologic maps Barnes, Shell Oil Co. and others (1983), scale 1:250,000; Elliott (2014), scale 1:24,000.
Geophysical data Recent Rank 1 aeromagnetic and aeroradiometric coverage over part of the region (Bultman, 2021).
Favorable rocks and structures Rhyolite domes or laccoliths.
Deposits Round Top prospects (MRDS dep_id: 10085039; USMIN Site_ID: TX00001).
Evidence from mineral occurrences MRDS; USMIN; Hulse and others (2019).
Geochemical evidence Chemical analyses (Elliott, 2018). Round Top: REE-bearing minerals: bastnaesite-Ce, Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Ca-Th-Pb fluoride, and possibly ancylite-La and cerianite-Ce. The Tertiary rhyolite intrusion at Round Top is enriched in both heavy and light rare earth elements (REE) and other incompatible elements such as Li, Be, F, U, Th, Nb, Ta and Hf (Hulse and others, 2019).
Geophysical evidence Recent Rank 1 aeromagnetic and aeroradiometric data over part of the region (Bultman, 2021). Round Top has a fairly large associated aeromagnetic anomaly (Hulse and others, 2019).
Evidence from other sources McLemore (2018a).
Comments High priority to locate similar deposits in the Trans-Pecos Igneous Province/Belt. The Sierra Blanca Mountains (which includes Round Top) and the southern edge of the Southern Granite and Rhyolite basement terrane based on Parker and others (2017) provided thick continental crust that allowed for fractionation and differentiation of mantle-derived magmas generated as the Farallon plate began to founder that resulted in the highly evolved intrusive rhyolites responsible for these deposits. It is possible that the Trans-Pecos south of the Southern Granite and Rhyolite basement terrane (the Grenville Front) may host both Round Top-type deposits and carbonatites (as seen to the west in Mexico), but this area has abundant flood rhyolites which may indicate a thinner crust.
Cover thickness and description Round Top laccolith is exposed at the surface. Concealed REE deposits in highly fractionated rhyolite deposits (like Round Top) and, probably, REE vein related deposits may be present at depth in mapped sedimentary rocks or concealed by Cenozoic basin fill.
Authors Brent A. Elliott, Joshua M. Rosera.
New data needs Date fluorite throughout belt, focused mapping in certain areas, chemistry from all the laccoliths and Tertiary igneous rocks that have not been sampled and analyzed with any modern geochemistry.
Geologic mapping and modeling needs Some mapping done in the area, but incomplete. 1:250,000 scale maps on the Geologic Atlas of Texas regional maps, and some isolated quadrangles, such as, Elliott (2014), scale 1:24,000.
Geophysical survey and modeling needs New Rank 1 aeromagnetic and aeroradiometric coverage available (Bultman, 2021).
Digital elevation data needs Lidar complete.