Study of heavy metal concentrations in shale deposits of Irankouh Mine (Southwest of Isfahan)

Irankuh Pb-Zn mine is located 20 km Southwest of Isfahan in the Zayandehrud River drainage basin. Shale and carbonate are the host rocks of the ore deposits formed from high concentrations of Pb and Zn. Weathering and erosion of the host rocks of ore deposits, waste water and waste materials arising from mining have a potential for heavy metal pollution of running water, ground water and agricultural soils. The main objectives of this investigation were to study the total concentration of Zn, Pb, Ni, Cu, Co, Ag, and Cd in the shale deposits of the mine, and the physicochemical factors on their distribution and pollution. For measuring heavy metal values, twenty samples were taken from the shale deposits. For distinguishing the shale-forming minerals, thin sections were prepared and studied with a polarizing microscope. The rock forming clay minerals, after preliminary treatments (heating at 550oc, ethylene glycol solvation), were detected by XRD and the organic carbon, calcium carbonate and Eh/pH of the shales were then measured. Heavy metal values in the sample were measured by AAS after mineralization. The correlation coefficient of organic carbon and manganese with metals and also metals-metals were calculated. For comparison of the heavy metal concentration relative to their natural amount (in shale), the enrichment factor was applied. The dark grey to olive-coloured shales in the studied area consisted of clay to fine silt size. Quartz, biotite, muscovite, feldspar and clay minerals comprise the non-opaque minerals of the shales while the clay minerals are illit (60-70%), kaolinite (9-27%) and chlorite (7-22%), respectively. Sphalerite, pyrite and galena are the main opaque minerals in the mine. The mean OC content of the shale is about 2% (0.35-5.5%) with less than 1% calcium carbonate. The pH of the samples is nearly neutral (6.8-7.8) and they have an oxidizing to nearly reducing Eh (18 to -100 v). The average concentration of heavy metals in the shales were Zn=128.05>Pb=42.55> Ni=35.24> Cu=24.18> Co=7.4> Ag=3.33> Cd=0.79 in ppm, respectively. A highly positive correlation between Ag-Pb-Cd, Cd-Zn-Pb and Co-Ni-Mn and also a positive correlation between Zn with organic carbon demonstrate geochemical convenient conditions for concentration of the metals. The calculated enrichment factor has revealed extremely high enrichment for Ag and average for Cd. Other elements are depleted in the sediments. Due to the relative high concentration of the metals and extent of the shales in the study area, activation of the heavy metals under suitable conditions can be a potential source for environmental contamination in the groundwater and runoff water of the basin.