Environmental geochemistry of rare earth elements in Cu-porphyry mine tailings in the semiarid climate conditions of Sarcheshmeh mine in southeastern Iran

Sarcheshmeh mine is one of the largest Oligo-Miocene porphyry Cu deposits in the world. It is located in the Kerman Cenozoic magmatic arc in southeastern Iran. This paper discusses the geo-environmental behavior of rare earth elements (REEs) in the Sarcheshmeh dammed tailing pond, situated in semiarid climate. For this purpose, solid and water samples were collected to consider: (1) weathering/oxidation reactions and upward migration of soluble elements; (2) super-saturation and formation/dissolution of the secondary evaporative phases; (3) hydro-geochemical changes of the surface and groundwater resources; (4) flotation of the Cu-sulfide ore and tailing management strategies. The mean values of the ∑ REEs for fresh and weathered tailings and the evaporative phases were 113.0, 103.7 and 74.6 mg/kg, respectively. When compared with the fresh and weathered tailings, the evaporative phases showed significant enrichment of HREEs and a well-depleted pattern of LREEs (except for that of Eu). This geochemical pattern suggests a higher solubility and mobility potential of HREEs and Eu compared with LREEs, resulting in good fractionation and enrichment of these elements in the surface evaporative layer formed on top of the weathered tailings. The water samples showed a range of hydro-geochemical features under acidic (pH of 2.5 to 4.7), near neutral (pH of 6.4 to 7.9), alkaline (pH of 8 to 10) and highly alkaline (pH > 10) conditions. The maximum concentration of REEs was observed in the acidic waters produced by the weathered tailings after rainfall (∑ REEs of 489 to 1903 μg/L). Although ∑ REEs decreased as the pH value increased, the general trend was compatible with the highest concentration of HREEs rather than LREEs. The speciation calculations indicate that LnSO4+, Ln(SO4)2− and Ln3 + are the primary forms of dissolved REEs at acidic and near neutral pH and the dominant species was LnSO4+. LnCO3+ (40.78%), Ln(CO3)2− (31.68%) and LnSO4+ (31.07%) were the dominant species under alkaline conditions. At high alkaline pH, the speciation pattern changed to LnOH2 + (53.15%), Ln(CO3)2− (44.88%) and LnCO3+ (2.61%), where Ln denotes lanthanide. Based on the results and considering previous works, it appears that ionic potential plays an important role in the geo-environmental behavior of REEs of adsorption tendency, mobility potential and ionic substitution.