Environmental geochemistry of As and Pb in a copper low-grade dump, Miduk copper mine, Kerman province, SE Iran

In this study, the geochemistry of As and Pb was investigated in a copper low-grade dump of the Miduk copper mine, Iran. The bulk chemical analyses were combined with complementary mineralogical approaches including X-ray Diffraction (XRD), Scanning Electron Microscope (SEM-EDS), Electron Probe Micro-Analysis (EPMA), and Raman Spectroscopy (RS) to characterise a high pyrite dump. Environmental stability of the newly formed minerals was investigated using leaching experiments and thermodynamic modelling employing PHREEQC code. The distribution of As and Pb with minerals were investigated in addition to the extent of their mobilisation. The bulk concentrations of As (Mean = 27.28, Min = 12.4, and Max = 59.5 mg/kg) and Pb (Mean = 72.05, Min = 19, and Max = 159 mg/kg) were within the normal range of the mine waste rocks. Arsenic and lead were associated with pyrite crystal and the enclosed form of galena in pyrite, respectively. The oxidation of pyrite was confirmed according to the high concentration of SO42− (ranges from 3950 to 286,000 mg/L) in the waste leachate and the prevalence of oxidation evidence around the pyrite particles. Due to the lack of carbonate minerals within the low-grade dump, the pH values of waste leachates were acidic (ranges from 1.47 to 4.23). The precipitation of secondary minerals formed by weathering including coquimbite, plumbojarosite, and brochantite was identified. The association of As and Pb along with hydroxosulphates was confirmed. Consequently, the presence of Pb-As-jarosite-group minerals was one of the main controlling factors on the low concentration of As (ranges from 0.05 to 8.25 mg/L) and Pb (ranges from 0.07 to 1.61 mg/L) in the waste leachates of the dump. The highest solubility of both As and Pb was observed at pH = 2. The oxidation of pyrite occurred during the 96 h leaching test.