Mineralogical and geochemical investigation of clay-rich mine tailings from a closed phosphate mine, Bartow Florida, USA

Clay-rich mine tailings from phosphate mine operations in Florida are a major environmental and economic problem. Options for reclamation and restoration for these tailings are very limited and are fundamentally restricted by poor physical properties such as low mechanical strength, low hydraulic conductivity, and heavy metal content. The major control on these bulk physical properties is the mineralogy of the materials. Eight continuous push borings were obtained to investigate stratigraphy, mineralogy, aspects of geochemistry, and bulk properties of a deposit of clay-rich mine tailings from a phosphate mine near Bartow, Florida that ceased operations in the early 1970s. Stratigraphy is dominated by laminated clay-rich sediment with minor units of silt and sand. An intact kaolinite liner occurs near the impoundment walls and the impoundment floor has approximately 4 m of relief. Moisture content varies from 4.35 to 57.40 wt% and organic content varies from 0.41 to 9.53 wt%. Bulk XRF investigation indicates that the P2O5 concentrations vary from approximately 4 to 21 wt%. A very strong correlation (r 2 = 0.92) between CaO and P2O5 indicates that apatite is a major control on the phosphate. The strong correlation (r 2 = 0.77) of Al2O3 and TiO2 suggests that the source materials for this deposit are comparatively uniform. A number of heavy metal elements and trace elements occur. Cr, V, Ni, Cu are interpreted to be in phosphate minerals, largely apatite. Sr and Pb are interpreted to be in both phyllosilicates and phosphate minerals. Two populations of apatite were observed in the clay-sized fraction, one that was Fe and Si- bearing and another that was only Si-bearing. Fe-bearing apatite had Fe2O3 contents that varied from 0.38 to 5.32 wt% and SiO2 contents that varied from 0.90 to 3.32 wt%. The other apatite population had a wider range of SiO2 contents that varied from 0.77 to 8.80 wt%. TEM imaging shows that apatite grains are dominantly single crystals with lesser amounts of aggregates. Wavellite commonly occurs as individual or clusters of lath-like crystals and the chemical composition differs from the pure aluminium phosphate end member with average concentrations of components being that of CaO (1.57 wt%), Fe2O3 (1.98 wt%), SiO2 (5.94 wt%). In the clay-sized phosphate minerals investigated no fluorine was found above detection limit (approximately 0.15 wt%), nor was any uranium, radium, heavy metal, or REE element detected. The phyllosilicate mineralogy of the deposit is dominated by smectite (montmorillonite with lesser amounts of nontronite), palygorskite, illite and kaolinite. No systematic variation in the relative proportions of phyllosilicates was observed in the clay deposit. Energy dispersive spectroscopy EDS analysis indicates that chemical compositions of phyllosilicates are somewhat typical but overall are enriched with respect to Fe compared to theoretical end members. The relative enrichment of Fe is interpreted to be a primary sedimentary feature. Ca content in smectite minerals is high and may inhibit stabilization using lime or similar methods. The high percentages of montmorillonite and palygorskite explain the high bulk water contents observed. This investigation provides fundamentally new details regarding clay tailing deposits from closed phosphate mines in central Florida which can be used in restoration and reclamation efforts.