INVESTIGATION OF RESIDUAL MINERAL CONTENT OF BAUXITE STOCKPILES, SALINE MINING DISTRICT, ARKANSAS, U.S.A.
Abstract
The former mining town of Bauxite is in the Saline County Mining District, in central Arkansas. Bauxite is located approximately 40 km (25 mi) southwest of Little Rock just east of Benton, Arkansas. This area contains extensive deposits of bauxite ore and nepheline syenite previously mined by Aluminum Company of America using open pit and subsurface mining techniques. This study incorporates five bauxite stockpiles (SP1, Clay, SP2, SP3, SP4) with varying amounts of alumina content situated on approximately 10 acres of land within an area of the former mine recognized as Section 20. Over the past several years, there has been renewed economic interest in minerals that can be derived from bauxite and related residues due to the rapid evolution in modern technologies including batteries, alloys, lasers, and much more. There have been investigations on bauxite ore and residue around the world, but the minerals within each setting where bauxite forms may vary depending on parent material, dissolution patterns, and time, potentially yielding elements that may have economic value.
One-hundred twenty-eight (128) rock samples were collected from five stockpiles and analyzed using X-ray fluorescence (XRF) and Laser Ablation – Inductively Coupled Plasma – Mass Spectrometry (LA-ICP-MS). Several samples had elements were determined to be enriched using XRF and/or LA-ICP-MS with respect to background concentrations in the Earth’s upper continental crust and/or a NIST-698 sample (Jamaican bauxite standard), including of which are on the 2022 List of Critical Minerals. Comparing XRF and LA-ICP-MS results, Si, Ca, Rb, Sr, Zr, Nb, and Mo had very strong correlations. A table showing correlations among elements tested in XRF and LA-ICP-MS compared to Fe, Ti, and Al was created to assess for elemental relationships.
In addition to element information, volume analyses of each stockpile were completed using both open source (QGIS) and commercial (ArcGIS Pro) GIS software for processing. A programmed flight with a DJI Mavic Pro 2 drone was used to complete the survey and obtain images to create maps of the study area. For all stockpiles, QGIS and ArcGIS had a statistically significant difference in mean volume calculations. The largest percent difference between each software for the averaged stockpile volumes was 4.2% Mean volumes were consistently higher in ArcGIS for each stockpile. Maps depicting the spatial distribution of elements (Sr, Zr, Nb, Si, Ca, and Mo) were generated using ArcGIS Pro to assess mobility by evaluating changes in element concentrations of the five stockpiles. Higher values of Sr, Zr, Nb, and Ca were noted in stockpiles with higher alumina (Al2O3) content. The hydrolysates (Zr and Nb) with the exception of Mo, mobilized around the base and slope of SP2, the tallest stockpile among the five in this study.
The chemical characteristics of each stockpile may provide understanding to elemental patterns and metal affinities within bauxite ores. The relationship between bauxite stockpile aluminum concentrations and the presence of elements associated with critical minerals could potentially lead to a future economic supply of important resources.
