A Unique Yttrofluorite-Hosted Giant Heavy Rare Earth Deposit: Round Top Mountain, Hudspeth County, Texas, USA

Round Top Mountain is a surface-exposed peraluminous rhyolite laccolith, enriched in heavy rare earth elements, as well as niobium-tantalum, beryllium, lithium, fluorine, tin, rubidium, thorium, and uranium. The extreme extent of the deposit (diameter one mile) makes it a target for recovery of valuable yttrium and HREEs, and possibly other scarce elements. The Texas Bureau of Economic Geology estimated the laccolith mass as at least 1.6 billion tons. A Preliminary Economic Assessment for Texas Rare Earth Resources listed an inferred mineral resource of 430,598,000 kg REOs (rare earth oxides), with over 70% Y+HREEs (YHREE). Put in global perspective, China is thought to produce ~25,000 tons YHREE per year, and exports but a small fraction of that. Because of the extremely fine grain size of the late-phase fluorine-carried critical fluid mineralization, it has not been clear which minerals host the YHREEs. X-ray Absorption Spectroscopy experiments at the Stanford Synchrotron Radiation Lightsource revealed that virtually all of the YHREE content resides in yttrofluorite, rather than in the other reported REE minerals in the deposit, bastnaesite and xenotime. The extended x-ray absorption fine structure (XAFS) spectra of the sample suite were all quite similar, and proved a close match to known model compound specimens of yttrofluorite from two locations, in Sweden and New Mexico. Small spectral variation between the two model compounds and among the samples is attributable to the variable elemental composition and altervalent substitutional nature of yttrofluorite (Ca [1-x] Y,REE [x])F[2+x]. We found no other reported deposit in the world in which yttrofluorite is the exclusive, or even more than a minor, YHREE host mineral. Leaching experiments show that the YHREEs are easily liberated by dissolution with dilute sulfuric acid, due to the solubility of yttrofluorite. Flotation separation of the yttrofluorite had been demonstrated, but was rendered inefficient by the micron-scale grain size of the yttrofluorite. Our laboratory leaching experiments with different acid strengths, grain sizes, and exposure times showed up to 90% recovery of the YHREEs. As expected, similar recoveries were obtained from longer exposure times at lower sulfuric acid concentration. Optimal grain size is in the 2-10 mm range. Thus a heap leach of the deposit is likely feasible, aided by the fact that 90-95% of the rock comprises insoluble and unreactive quartz and feldspars. The absence of overburden, proximity (a few km) to an interstate highway and major rail systems, temperate climate, and favorable political location enhance the potential and appeal for development of a heap leach operation. The grade of the deposit is just over 0.05% total rare earth elements plus yttrium. Although some might consider this sub-economic, it is in the range of the South China ionic clay deposits that supply essentially all of the world's YHREEs. Further, the grade is remarkably consistent through 1657 samples from 64 reverse-circulation drill holes with a total sampled interval of 30,353 feet. This consistency of grade permits accurate economic assessment and prediction, an unchanging ore grade and mine feedstock over life of mine, and a single REE separation chemistry to be developed. Thus mine and separation procedures need only be developed and optimized once.