Geochemistry of Trace Metals and Rare Earth Elements in Stream Water, Stream Sediments and Acid Mine Drainage from Darrehzar Copper Mine, Kerman, Iran

摘要

Darrehzar porphyry copper mine is located 10km south of the famous Sarcheshmeh porphyry copper mine in Kerman province, Iran. Water and sediment samples in the Darrehzar porphyry copper mine were collected and analyzed in order to characterize the environmental effects of regional geology and mining activity. Stream affected by acid mine drainage (AMD) are Ca-SO_4~(2-) in type, while unaffected streams are Ca-HCO_3~- type. Acid drainage, with pH values as low as 2.5, is quite rich in dissolved copper (mean 306.53 mg/L) > iron (77.83 mg/L) > aluminium (48.62 mg/L) > manganese (6.90 mg/L) > zinc (2810.42 μg/L) > cobalt (831.55 μg/L) > nickel (298.47 μg/L). The high sulfate concentration makes sulfate the major controlling factor of electrical conductivity. The strong correlation among the majority of the elements indicates either similar geochemical behavior or a common origin, (mostly sulfides). Calculation of Acid Mine Drainage Index (AMDI) indicates that Darrehzar mine is classified as dangerous in view of AMD hazards. In Darrehzar porphyry copper mine, total rare earth element content (∑REE) in AMD (5.54-417.76 μg L~(-1)) is higher than natural water (REE in upstream and downstream water is 0.87-1.40 and 0.83- 1.71 μg L~(-1) respectively). The North American Shale Composite (NASC) normalized REE patterns of AMDs revealed strong enrichment of middle REE (MREE), and heavy REE (HREE) compared with light REE (LREE). Speciation modeling indicates the predominance of aqueous species i.e., LnSO_4~+, Ln(SO_4)_2~-, Ln~(3+), LnCO_3~+, LnHCO_3~(2+) and Ln(CO_3)_2~- (Ln refers to any lanthanide element). In upstream, where pH is 7.2 ≤ pH, REEs primarily exist as LnCO_3~+, and HREE bicarbonate complexes (Ln (CO_3)_2~-) exceed those of LREEs. Based on mean geoaccumulation indices (I_(geo)), Darrehzar river sediments at mine site display the following trend:Cu > Mo > Pb > Sb > Cd. Calculated potential ecological risk index (RI) indicates higher risk at mine site. The mean potential ecological risk factors (E_i) for sediment metals display the following trend:Mo > Cd > Cu > As > hbox Pb > Ni > Cr > Zn.