Microprobe Mapping of Rare Earth Element Distribution in Round Top Yttrofluorite Deposit

Abstract

The electron microprobe maps the spatial distribution of elements in a rock at a sub-mineral-grain scale to provide a basis for understanding mineralization processes and to determine optimal strategies for extraction of valuable target elements. Round Top Mountain (near the town of Sierra Blanca, Hudspeth County, west Texas, USA) is a peraluminous rhyolite laccolith that is homogeneously mineralized at over 500 ppm rare earths, more than 70% of which are yttrium and heavy rare earths (YHREEs). The massive deposit is exposed at the surface as a mountain some 2 km in diameter and 375 m in height. Round Top Mountain also contains Li, Be, U, Th, Nb, Ta, Ga, Rb, Cs, Sn, and F. The valuable YHREEs are hosted in yttrofluorite, which is soluble in dilute sulfuric acid. Texas Mineral Resources Corporation proposes to surface mine, crush, and heap leach the deposit. The distribution of YHREEs, and that of other trace elements, is remarkably homogeneous at outcrop drill hole scale. Here we document that YHREE mineralization appears pervasive through the rhyolite at a millimeter scale. Back scattered electron (BSE) and characteristic X-ray maps reveal the fine grain size and apparently random and dispersed spatial distribution of the yttrofluorite that hosts Round Top's valuable YHREEs. The yttrofluorite grains do not appear to cluster at special mineralized locations, e.g., in pores or along cracks in the rhyolite. The same is apparently true of such other potentially valuable minerals as cassiterite and uranium species. These findings confirm that the distribution of YHREEs in Round Top Mountain rhyolite is homogeneous through different orders of magnitude of scale, i.e., from outcrop (as seen in the companion work in this volume) to sub-thin section. The material thus is ideal for a heap leach operation where homogeneous feedstock is crucial to consistent and economic operation. The findings also confirm and explain why mechanical separation would prove very difficult and expensive due to the astronomical number of yttrofluorite grains in even a golf-ball-size piece of Round Top rhyolite.How to cite this paper: Pingitore

Remarkably Consistent Rare Earth Element Grades at Round Top Yttrofluorite 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.…”

ArcGIS<sup>TM</sup> and Principal Component Analysis of Probe Data to Micro-Map Minerals in Round Top Rare Earth Deposit

“…The previous study [9] defined the mineral deposits and confirmed the potential resource but they did not provide definitive information on how to extract the desired minerals efficiently and economically. For that reason, we conducted further analyses, including more experimental techniques, to better understand a basis for recovering the HREEs at Round Top Mountain economically.…”

“…Polished thin sections underwent an ultrasonic cleaning bath, an ethanol rinse, and carbon coating prior to analysis. Thin section preparation procedure followed [9].…”

“…These mineral maps will improve our understanding of the mineralization process at Round Top Mountain and inform approaches to potential extraction of that mineral wealth. This research is an extension of [9] where electron microprobe mapping was used to outline the microscopic distribution of heavy rare earth elements (HREEs) and better understand the mineralization and potential extraction processes.…”

“…Round Top Mountain underwent chemical alteration by a late-stage fluorine vapor phase that enriched it in HREEs and other incompatible elements [2] [6] [12]. In the search for and delineation of mineral deposits from Round Top Mountain previous studies [3] [8] [9] have included standard methods such as the collection of rock samples and preparation of thin sections for analysis with optical petrography and scanning electron microscopes. Electron probe microanalysis (EPMA) employs an electron beam excitation to determine the elemental composition of individual grains or portions of grains in thin sections [13].…”