Abstract
Rare earth elements (REEs), especially heavy rare earth elements (HREEs), are in demand for their current and emerging applications in advanced technologies. Here we perform computer-driven micro-mapping at the millimeter scale of the minerals that comprise Round Top Mountain, in west Texas, USA. This large rhyolite deposit is enriched in HREEs and such other critical elements as Li, Be, and U. Electron probe microanalysis of 2 × 2 mm areas of thin sections of the rhyolite produced individual maps of 16 elements. These were superimposed to generate a 16-element composition at each pixel. Principal components analysis of elements at each pixel identified the specific mineral at that site. The pixels were then relabeled as the appropriate minerals, thereby producing a single mineral map. The overall mineral composition of the 7 studied samples compared favorably with prior analyses of the Round Top deposit available in the literature. Likewise the range of porosity in the maps was consistent with that of previous direct measurements by water saturation. This new statistical and GIS-based technique provides a robust and unbiased approach to electron microprobe mapping. The study further showed that the high-value yttrofluorite grains exhibited little tendency to cluster with other late-stage trace minerals and that the samples extended the previously documented overall homogeneity of the deposit at field scale to this microscopic scale.
Highlights
Round Top Mountain, a Tertiary rhyolite laccolith in Hudspeth County, west Texas, USA is a potentially economically valuable deposit of heavy rare earth elements (HREEs) and other critical elements [1]-[10]
Mineralization is homogenous throughout the laccolith with the exception of the rhyolite margins and synchrotron-based X-ray absorption spectroscopy shows that yttrofluorite hosts almost all of the yttrium and the desirable heavy rare earths (YHREEs) [3]
The computer-generated mineral compositions based on the low pass filtered principal component analysis (PCA) data fall within the range of mineral percentages found in prior research indicating our approach works well
Summary
Round Top Mountain, a Tertiary rhyolite laccolith in Hudspeth County, west Texas, USA is a potentially economically valuable deposit of heavy rare earth elements (HREEs) and other critical elements [1]-[10]. The purpose of this study is to create detailed mineralogical maps from thin sections of Round Top Mountain samples using a new approach that combines multivariate statistical analysis and geospatial analysis through the use of ArcGISTM [11] 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. 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] These analyses confirm the presence of major, minor, and trace elements of potential value within those mineral grains
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