8/14/2019 Fluorescein Dye Penetration in Round Top Rhyolite
(Hudspeth County, Texas, USA) to Reveal Micro-Permeability a
1/2
CONTROL ID: 1820700
TITLE: Fluorescein Dye Penetration in Round Top Rhyolite
(Hudspeth County, Texas, USA) to Reveal
Micro-permeability and Optimize Grain Size for Heavy REE Heap
Leach
AUTHORS (FIRST NAME, LAST NAME): Lorraine Marie Negron1, Juan W
Clague1, Dan
Gorski3, Maria A Amaya2, Nicholas E Pingitore1, 2
INSTITUTIONS (ALL): 1. Geological Sciences, The University of
Texas at El Paso, El Paso, TX,
United States.
2. School of Nursing, The University of Texas at El Paso, El
Paso, TX, United States.
3. Texas Rare Earth Resources, Sierra Blanca, TX, United
States.
ABSTRACT BODY: Millimeter- and micrometer-scale permeability of
fine-grained igneous rocks has
generated limited research interest. Nonetheless, the scale and
distribution of such micro-permeability
determines fluid penetration and pathways, parameters that
define both the ability to heap leach a rock
and the optimal grain size for such an operation. Texas Rare
Earth Resources is evaluating the possibility
of heap leaching of yttrium and heavy rare earth elements
(YHREE) from the peraluminous rhyolitelaccolith that forms
one-mile-diameter Round Top Mountain. The YHREEs in this immense,
surface-
exposed deposit (minimum 1.6 billion tons, Texas Bureau Economic
Geology) are dilute and diffuse,
suggesting leaching as the best option for recovery. The REE
grade is 0.05% and YHREEs comprise
more than 70% of the total REE content.
The YHREEs are hosted exclusively in micron-scale yttrofluorite
grains, which proved soluble in dilute
sulfuric acid. Laboratory experiments showed YHREE recoveries of
up to 90%. Within limits, recoveries
decrease with larger grain sizes, and increase with acid
strength and exposure time. Our research
question centers on dissolution effectiveness: Is YHREE
recovery, relative to grain size, limited by (1)
diffusion time of acid into, and dissolved solids, including
YHREEs, out of the micro-permeability paths
inherent in the rock particles; (2) the effective lengths of the
natural micro-permeability paths in the rock;
or (3) the putative role of the acid in dissolving new
micro-paths into the grains? The maximum grain size
should not exceed twice the typical path length (unless acid
creates new paths), lest YHREEs in the core
of a larger grain than that not be reached by acid. If instead
diffusion time is limiting, longer leach time
may prove effective.
Rather than perform an extensive and expensive series of
laboratory leaching experimentssome of
which would be several months in durationto determine optimal
grain size, we developed a technique
to efficiently determine the limits of penetration of water into
the rhyolite.
We cut parallel-sided slabs of Round Top rhyolite at staged
thickness up to 10 mm. We then wet one
side and view the opposite side over time under UV light to
detect breakthrough of the fluorescein dye.
Because of its extremely low visual detection limits, well below
the ppm level, the dye has been widely
used in biochemical research, as a tracer in surface and ground
water studies, in delineating invisible
cracks in such structural material as motor blocks, and in
detecting corneal abrasions.
We have been successful in detecting breakthrough at different
rhyolite thicknesses. Continuing studies
focus on mapping of the 2-dimensional distribution of the
permeability via hand lens and low-power
