Round Top Mountain rhyolite (Texas, USA), a massive, unique Y-bearing-fluorite-hosted heavy rare earth element (HREE) deposit

Round Top Mountain in Hudspeth County, west Texas, USA is a surface-exposed rhyolite intrusion enriched in Y and heavy rare earth elements (HREEs), as well as Nb, Ta, Be, Li, F, Sn, Rb, Th, and U. The massive tonnage, estimated at well over 1 billion tons, of the deposit makes it a target for recovery of valuable yttrium and HREEs (YHREEs), and possibly other scarce elements. Because of the extremely fine grain size of the mineralized rhyolite matrix, it has not been clear which minerals host the YHREEs and in what proportions. REE-bearing minerals reported in the deposit included bastnäsite-Ce, Y-bearing fluorite, xenotime-Y, zircon, aeschynite-Ce, a Ca-Th-Pb fluoride, and possibly ancylite-La and cerianite-Ce. Extended X-ray absorption fine structure (EXAFS) indicated that virtually all of the yttrium, a proxy for the HREEs, resided in a coordination in the fluorite-type crystal structure, rather than those in the structures of bastnäsite-Ce and xenotime-Y. The YREE grade of the Round Top deposit was just over 0.05%, with 72% of this consisting of YHREEs. This grade was in the range of the South China ionic clay deposits that supply essentially all of the world's YHREEs. Because the host Y-bearing fluorite is soluble in dilute sulfuric acid at room temperature, a heap leaching of the deposit appeared feasible, aided by the fact that 90%–95% of the rock consists of unreactive and insoluble feldspars and quartz. The absence of overburden, remarkable consistency of mineralization grade throughout the massive rhyolite, proximity (a few km) to a US interstate highway, major rail systems and gas and electricity, temperate climate, and stable political location in the world's largest economy all enhanced the potential economic appeal of Round Top.

Peaks of rhyolite rock at Sierra Blanca, Texas, USA. Seen at far left, 375-m-tall Round Top Mountain contains a possible 475,000 tons of YHREOs. Examination of this rhyolite at the atomic level by X-ray absorption spectroscopy points to Y-bearing fluorite as the host mineral of essentially all of the Y, and by proxy all of the HREEs, in the massive deposit. Because the Y-bearing fluorite is soluble in dilute sulfuric acid, extraction by heap leach with minimal environmental impact should be possible. Uniform YHREO grade throughout the Round Top rhyolite and its fine grain size also are favorable to heap leach operation. YHREO grade of the deposit is similar to that of the south China ion-adsorption clays that provide nearly all the global supply of these valuable materialsFigure optionsDownload as PowerPoint slide