To delineate the extent and grade of the deposit, in 2011 Texas Rare Earth Resources Corporation (TRER), now with a new name, Texas Mineral Resources Corporation (TMRC), drilled a series of 5.25-inch (13-cm) diameter reverse circulation (RC) holes into the mountain for a total length of 26,915 feet (8204 m) [24] . They cored two more holes, drilled parallel to 2 RC holes to confirm composition. The RC cuttings were collected from 5-foot (1.5 m) intervals and stored in sample bags sealed in plastic buckets.

Borehole locations are seen in Figure 1. The drill holes south of the blue line on the map were not included in this study. These were drilled on the southern flanks of the mountain, which can have an extensive erosional cover. That material no longer maintains the same elemental or mineralogical composition as the bulk rhyolite due to physical and chemical weathering and selective removal of minerals by erosion. Picking the boundary between the eroded material and the bulk rhyolite with certainty was not possible from the borehole log records, so we chose to censor the list of holes and eliminate these holes from our analysis. A total of 1671 samples from 66 drill holes was then available for study.

A 1-kg sample was created from each 5-foot (1.5-m) interval of each drill hole for laboratory determination of 38 minor and trace elements. As results were received by TMRC that indicated overall uniform elemental concentrations, the 5-foot (1.5-m) sample intervals were composited into 10- to 30-foot (3- to 6.1-m) intervals, based on the lithologies determined from the borehole logs.

Multi-element analyses were performed by ICP-MS (inductively coupled plasma mass spectrometry) at ALS Chemex, a certified analytical laboratory in Vancouver, British Columbia, Canada. Duplicates, blanks, and standards were included in the sample stream at appropriate intervals. Additional details of the entire drilling and analysis programs can be found in the TMRC Preliminary Economic Assessment document [24] .

Figure 2, top, presents yttrium content in ppm, x-axis, versus elevation (MSL, i.e., relative to mean sea level), y-axis, of drill cuttings from the 66 selected drill holes. A total of 1671 samples are plotted, each color-coded to its drill hole, labelled on right side of the figure. Bottom left diagram, labelled “−1”, eliminates the highest and lowest cutting from each drill hole. Those cuttings typically were contaminated by contact with the overlying and underlying strata during emplacement of the Round Top laccolith. The hot magma melts portions of the surrounding containing rock, yielding a mixed contact zone that no longer reflects the overall composition of the laccolith. The third graph, at bottom right, shows elimination of the uppermost and lowermost two drill cutting samples from each hole. Low concentration outliers are seen to be removed progressively

by this process. Thus, the plot labeled −2 best characterizes the elemental variation of the bulk of the deposit. If Round Top were to be open “pit” mined (or more properly stated, cut down from the top), only the minor upper contact zone would be lower grade material that might be discarded rather than processed. Later excavation would not continue into the lower contact zone. Note that after elimination of the 4 bounding samples from each well, a total of 1408 samples were plotted, and two very short drill holes from which only a total of 7 samples had been recovered were eliminated. The rest of the plots, seen in Figure 3 and Figure 4, are all of the “−2” type, with 1408 samples.

Figure 3 presents graphs of concentrations of the HREEs Dy, Ho, Tm, and Yb and of the incompatible elements Nb and U versus depth. Figure 4 shows similar graphs for LREEs Ce, Pr, Nd, Eu, and HREEs Gd and Tb.

A scan of all the graphs reveals a limited range of variation for every element, with a limited number of positive and negative outliers. Despite the censoring of the uppermost and lowermost samples from each drill hole, there still are more negative than positive outliers. This suggests that contact zone contamination, with mixing in of the country rock and dilution of the laccolith, may have extended even farther into the rhyolite. It also is likely that weathering of the rhyolite and selective dissolution of soluble minerals removes some of these elements along the fractures and joints found throughout the bulk rhyolite.

To further emphasize the consistency of REE grades, note that for Y (Figure 2) nearly all the samples fall between 200 and 270 ppm. Thus, they fall within +/− 35 ppm of the visual range average of 235 ppm. That is a variation of +/− 15%, more remarkable still if one considers that the reproducibility for a single sample likely is at least +/− 5% due to sample preparation and instrument analysis. The element Y was chosen for this analysis because it by far is the most abundant REE in Round Top, 4 times more than Yb, the next most abundant. It therefore was likely to have the least sample preparation and analytical error and thus its distribution would best demonstrate natural REE variation in the deposit.

A number of the graphs appear to show a very slight enrichment in REEs with depth in Round Top Mountain. There is a barely perceptible negative slope in the central, dense set of data points, ignoring the peripheral outliers. From a mining standpoint the possible enrichment is not an economic or technical consideration; it falls well within the small range of variation seen at any depth.

Figure 5 confirms the slight enrichment in some REEs with depth at Round Top. The box symbol, average concentration of the element in the lower half of the laccolith, plots slightly above the diamond symbol, upper half average. The figure also demonstrates that there is little difference in concentrations of most elements between the upper and lower sections of the deposit.

These graphs (Figures 2-4) attempt to portray the variability of the REEs through a 3-dimensional object, Round Top Mountain. The plots, however, only reveal variability in a single dimension, the vertical location or depth in the mountain of each sample. For the other two dimensions, two additional sets of graphs could present each sample in its east-west location (on the vertical graph axis) and then in its north-south location. But as can be seen from the graphs, there is so little variation of values of each element that significant geographic variation is not possible. Further, if, say, the values in a specific drill hole were higher than in others we would see a vertically oriented set of circles of a single color (each drill hole is

color-coded in the graph) to the right of the main cluster of points. Such a distribution of the sample points is not found in any of the graphs.

Thus the depth plots serve to confirm that the REEs in the deposit are homogenously distributed throughout the sampled portion of Round Top Mountain.

Uranium, a potential economic target, exhibits a much larger set of positive outliers than the REEs. This suggests deposition of secondary U-containing minerals at some time after emplacement of the laccolith, presumably in cracks and fractures in the rhyolite. Uranium enrichment also has been observed in surface outcrops of the surrounding rock units proximal to the rhyolite.

Niobium, another potential economic target, exhibits a narrow range of variation, as seen in Figure 4. Niobium at Round Top occurs chiefly as the mineral columbite, now regarded as columbite-Fe and columbite-Mn, (Fe, Mn) Nb₂O₆ (orthorhombic) [30] . Columbite is not a major host for REEs at Round Top but exhibits the same homogeneous distribution as the REEs throughout the sampled portion of the deposit.

The significance of the even distributions of U and Nb, and other minor and trace elements not included here, is that they confirm the well-mixed nature of the rhyolite laccolith [20] [21] [31] [32] [33] . Thus an even grade is not just an oddity of the REEs, but reflects the well-mixed nature of the deposit itself.

This point is confirmed in Figure 5, although comparison of elemental averages in just upper and lower sections of the rhyolite is less strong evidence than the comparison of well over a thousand samples in Figures 2-4. Note that the suite of 38 elements includes incompatible elements, some of which are incorporated in the major mineralogy of the deposit (e.g., Rb, a large-ion lithophile element (LILE), resides chiefly in the potassium feldspar substituting for the K cations) and those that are concentrated in the late-stage minor minerals such as the high field strength elements (HFSE), including the REEs, Nb, Ta, U, Zr, etc. Because Figure 5 includes elements incorporated in both early- and late-stage crystallization of the laccolith, it supports a model of thorough mixing of the magma at all stages of cooling.

We have no explanation for the one noticeable anomaly, seen for Ba; most likely it is present as a post-laccolith vein or fracture deposit. In any event it is not a target economic element.

The figures demonstrate, with high reliability, the even grade of the target YHREEs as well as the LREEs through the majority of the deposit. Given the geologic setting and the nature and cooling―crystallization processes of a laccolith intrusion it is reasonable to assume that the even grade characterizes REE distribution in the portions of the deposit not included in the extensive drilled and sampled region.

Previous study has shown that: 1) yttrofluorite hosts nearly all of the REEs at Round Top; and 2) the yttrofluorite grains are very fine, few larger than 10 µm in dimension, with most only a few µm or less in size [20] [21] [31] [32] [33] [34] . This fine grain size makes extraction via conventional froth flotation prohibitively expensive due to the extreme grinding and milling required to release the minuscule yttrofluorite grains from their enclosing gangue matrix. Because of the porosity of the rhyolite and its largely insoluble composition (90% - 95% feldspar and quartz) heap leaching of 3- to 4-cm chunks releases up to 90% of YHREEs in laboratory experiments. The small grain size of the yttrofluorite and its pervasive distribution are related to the posited deposition from a fluorine-rich carrier fluid, the last remnant of the parent magma [20] [21] [31] [32] [33] . This carrier, likely a supercritical fluid, permeated the entire laccolith, and cooled to yield the remarkably even distribution of yttrofluorite and other minor minerals rich in incompatible elements. The data in Figure 5 are consistent with this model of thorough magma mixing and pervasive final phase homogeneous deposition of incompatible elements.

From a mining standpoint, this consistent grade is a very positive feature of the deposit. As discussed earlier, estimation of the total amount of HREEs available for potential mining is greatly simplified, compared to, say, a mineralized vein deposit. The feedstock for the leach pad would have a near-constant HREE content for the life of the mine. All processing could be optimized during the earliest operations, yielding great savings in potential operating costs over time. From an investment point of view, the constant grade is a very positive feature of the Round Top deposit.

From an economic standpoint there is already enough material whose composition is well constrained by past drilling at Round Top to accommodate many decades of mining. Nonetheless, since the drilling programs concentrated their efforts on one side of the mountain, it would be interesting from a scientific standpoint to increase this coverage. That would assist in additional geological and geochemical studies to develop better models of the nature, timing, and sequence of mineralization. Such models would help in exploration for other examples of this currently unique and potentially quite valuable type of deposit [34] .