minerals Article Porosity and Permeability of Round Top Mountain Rhyolite (Texas, USA) Favor Coarse Crush Size for Rare Earth Element Heap Leach Lorraine Negron 1, Nicholas Pingitore 1,* and Daniel Gorski 2 1 Department of Geological Sciences, The University of Texas at El Paso, El Paso, TX 79968-0155, USA; [email protected] 2 Texas Rare Earth Resources, 539 El Paso St., Sierra Blanca, TX 79851, USA; [email protected] * Correspondence: [email protected]; Tel.: +1-915-747-5754 Academic Editor: Samuel Frimpong Received: 31 December 2015; Accepted: 17 February 2016; Published: 24 February 2016 Abstract: Water-saturation porosity and dye-penetration permeability measurements of Round Top Mountain rhyolite confirm that a ½-inch (13-mm) crush size would permit efficient acid heap leaching of yttrium and heavy rare earth elements (YHREEs) hosted in yttrofluorite, a YHREE-substituted variety of fluorite. Laboratory acid leaching has extracted up to 90% of the YHREEs. The bulk insoluble gangue mineralogy of the rhyolite, 90% to 95% quartz and feldspars, assures low acid consumption. Different crush sizes were weighed, soaked in water, and reweighed over time to determine water-penetration estimated porosity. Typical porosities were 1% to 2% for gray and 3% to 8% for pink varieties of Round Top rhyolite. The same samples were re-tested after soaking in dilute sulfuric to simulate heap leaching effects. Post-leach porosity favorably increased 15% in pink and 50% in gray varieties, due to internal mineral dissolution. Next, drops of water-based writing ink were placed on rhyolite slabs up to ~10 mm thick, and monitored over time for visual dye breakthrough to the lower side. Ink penetration through 0.5 to 2.5-mm-thick slabs was rapid, with breakthrough in minutes to a few hours. Pink rhyolite breakthrough was faster than gray. Thicker slabs, 4 to 10 mm, took hours to three days for breakthrough. Porosity and permeability of the Round Top rhyolite and acid solubility of the yttrofluorite host should permit liberation of YHREEs from the bulk rock by inexpensive heap leaching at a coarse and inexpensive nominal ½-inch (13-mm) crush size. The rate-limiting step in heap leach extraction would be diffusion of acid into, and back-diffusion of dissolution products out of, the crushed particles. The exceptional porosity and permeability that we document at Round Top suggest that there may be other crystalline rock deposits that economically can be exploited by a coarse-crush bulk heap leach approach. Keywords: heavy rare earth elements; yttrofluorite; heap leach; yttrium; dysprosium; YHREE; rhyolite; Round Top 1. Introduction Yttrium and the heavy rare earth elements (YHREEs) are vital components of the materials in the optical, electronic, and mechanical products that define 21st Century technologies [1]. Virtually, all YHREEs currently are sourced from the famed south China ionic clay deposits [2]. Taxation, regulatory policies, and environmental degradation are challenges to the steady supply of those elements. Expanded demand for select YHREEs might exceed supply in the future. Thus, there is a global search for alternative sources beyond China, driven by these factors, as well as by trade policies and economic considerations [3,4]. Minerals 2016, 6, 16; doi:10.3390/min6010016 www.mdpi.com/journal/minerals Minerals 2016, 6, 16 2 of 13 Minerals 2016, 6, 16 2 of 13 RoundRound Top MountainTop Mountain in Hudspeth in Hudspeth County, County, west Texas, west USA,Texas, is aUSA, surface-exposed is a surface-exposed peraluminous (Al2Operaluminous3 > Na2O + K (Al2O2O +3 CaO)> Na2O rhyolite + K2O + laccolith,CaO) rhyolite a mushroom-shaped laccolith, a mushroom-shaped igneous intrusion. igneous Theintrusion. mountain is roughlyThe mountain 2 km in is diameter roughly 2 and km overin diameter 1200 m and high, over with 1200 a m rhyolite high, with mass a rhyolite estimated mass at estimated some 1.5 at billion tonnessome (Figure 1.5 billion1)[ 5 tonnes]. The (Figure rock is 1) enriched [5]. The rock in rareis enriched earth elementsin rare earth (REEs) elements in excess(REEs) in of excess 500 ppm, of of which500 the ppm, economically of which desirablethe economically heavy REEsdesirable and yttriumheavy REEs (YHREEs) and yttrium comprise (YHREEs) approximately comprise 72%. The rhyoliteapproximately intrusion 72%. is LateThe Eocenerhyolite inintrusion age, dated is Late radiometrically Eocene in age, at 36dated Ma radiometrically (million years) at [6 ].36 Although Ma originally(million emplaced years) [6]. at shallowAlthough depth originally beneath emplaced the Earth’s at shallow surface, depth subsequent beneath erosionthe Earth’s ofthe surface, overlying subsequent erosion of the overlying strata has left this rock, more resistant to gradual weathering, as strata has left this rock, more resistant to gradual weathering, as a prominent topographic feature. a prominent topographic feature. Thus, there is little or no overburden that would need to be Thus,removed there is littleto commence or no overburden surface mining that wouldby blasting, need moving, to be removed crushing, to and commence stacking surfaceoperations mining to by blasting,prepare moving, a heap crushing, leach pad. and stacking operations to prepare a heap leach pad. Figure 1. Round Top Mountain yttrium and the heavy rare earth elements (YHREE) deposit, Figure 1. Round Top Mountain yttrium and the heavy rare earth elements (YHREE) deposit, Hudspeth Hudspeth County, west Texas, USA. Virtually the entire mountain, extending a short distance into County, west Texas, USA. Virtually the entire mountain, extending a short distance into the subsurface, the subsurface, is mineralized. Note drill rig on peak for scale. is mineralized. Note drill rig on peak for scale. Round Top is enriched in a suite of incompatible elements, those that are due to their ionic Roundradius and Top charge is enriched are not in aeasily suite incorporated of incompatible into elements,common igneous those that rock-forming are due to theirminerals. ionic At radius and chargeRound areTop, not these easily include incorporated YHREEs, light into REEs common (LREEs), igneous Sn, Be, rock-forming Li, Cs, Rb, U, minerals. Th, Ga, Nb, At and Round Ta. Top, theseAlthough include YHREEs, a number light of REE-bearing REEs (LREEs), minerals Sn, Be, have Li, been Cs, Rb,reported U, Th, in Ga, the Nb, deposit, and Ta.synchrotron-based Although a number X-ray absorption spectroscopy documented that yttrofluorite hosts essentially all of the YHREEs [7]. of REE-bearing minerals have been reported in the deposit, synchrotron-based X-ray absorption Yttrofluorite is a variety of the common mineral fluorite (CaF2, isometric) in which the trivalent (3+) spectroscopy documented that yttrofluorite hosts essentially all of the YHREEs [7]. Yttrofluorite is Y and HREEs substitute for ~5% to 30% of the bivalent Ca2+ cations, yielding the formula a variety(Ca1−x of,Y,HREE the commonx)F2+x [8]. mineral No other fluorite yttrofluorite-hosted (CaF2, isometric) YHREE in whichdeposit thehas trivalent yet been (3+)described; Y and the HREEs 2+ substituteRound for Top ~5% Mountain to 30% deposit of the bivalentmineralogy Ca at presentcations, appears yielding to be the unique. formula (Ca1´x,Y,HREEx)F2+x [8]. No other yttrofluorite-hostedMineralization is pervasive YHREE and deposit homogeneous has yet beenthrough described; the laccolith, the Round with the Top exception Mountain of the deposit mineralogyrhyolite atmargins, present appearswhere mineralizing to be unique. fluids interacted with the enclosing country rock and Mineralizationconcentrated or diluted is pervasive various elements. and homogeneous YHREE mineralization through the occurred laccolith, via pervasive with the deposition exception of the rhyolitefrom late margins, stage (end where of the magma mineralizing cooling fluidsand solidification) interacted fluorine-enriched with the enclosing fluid, country in which rock the and concentratedincompatible or diluted elements various had concentrated elements. [5,9,10]. YHREE mineralization occurred via pervasive deposition The target YHREE-containing yttrofluorite occurs in the fine-grained matrix or groundmass of from late stage (end of the magma cooling and solidification) fluorine-enriched fluid, in which the the rhyolite. Large crystals (up to perhaps 250 μm in diameter) of feldspars and quartz formed incompatibleduring early elements slow cooling, had concentrated and were encased [5,9,10 ].in a fine-grained matrix that formed during rapid Thecooling target when YHREE-containing the magma was emplaced yttrofluorite near the occurs Earth’s in surface. the fine-grained Most of the matrixyttrofluorite or groundmass grains of therange rhyolite. from perhaps Large crystals 10 to 1 μ (upm or to less perhaps in diameter. 250 µ mThe in small diameter) size of ofthe feldspars target yttrofluorite and quartz grains formed duringrenders early such slow conventional cooling, and physical were encased beneficiation in a fine-grained techniques matrixas froth that flotat formedion, gravity during separation, rapid cooling whenmagnetic the magma separation, was emplaced etc. uneconomical, near theEarth’s chiefly, surface. due to Mostthe extremely of the yttrofluorite fine, and grainsperhaps range fromunachievable, perhaps 10 to particle 1 µm orsize less required in diameter. to liberate The the small minuscule size of theyttrofluorite target yttrofluorite crystals by grainssequential renders suchcrushing, conventional grinding, physical