Document Type : Original Article
Department of Geology, Faculty of Science, University of Isfahan, Isfahan, Iran
Irankuh Pb-Zn mine is located in 20 km SW of Isfahan in the Zayandehrud river drainage basin. Shale and carbonate are host rocks of the ore deposits. High concentration of Pb and Zn formed ore complex in these rocks. Weathering and erosion of the host rocks of ore deposits, waste water and waste material arising from mining have made potential for pollution of running water, ground water and agriculture soils by the heavy metals. 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 physicochemical factors on their distribution and pollution.
Materials and methods:
For measuring heavy metals values, twenty samples were taken from the shale deposits. For distinguishing the shale forming minerals, thin sections were prepared and studied by polarizing microscope. The rock forming clay minerals, after preliminary treatments (heating at 550oc, ethylene glycol solvation), were detected by XRD. Organic carbon, calcium carbonate and Eh/pH of the shales were measured. Heavy metals values in the sample were measured by AAS after mineralization. Correlation coefficient of organic carbon and Mn with metals and also metals-metals were calculated. For comparison of the heavy metal concentration relative to their natural amount (in shale), enrichment factor was applied.
Results and discussion:
The dark grey to olive colored shales in the studied area consist of clay to fine silt size. Quartz, biotite, muscovite, feldspar and clay minerals are the non-opaque minerals of the shales. The clay minerals are illit (60-70%), kaolinite (9-27%) and chlorite (7-22%) respectively. Sphalerite, pyrite and galena are the main opaque minerals of the mine. The mean OC content of the shale is about 2% (0.35-5.5%) and 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 are Zn=128.05>Pb=42.55> Ni=35.24> Cu=24.18> Co=7.4> Ag=3.33> Cd=0.79 in ppm, respectively. Highly positive correlation between Ag-Pb-Cd, Cd-Zn-Pb and Co-Ni-Mn and also positive correlation between Zn with organic carbon show 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 relative high concentration of the metals and their extent of the shales in the study area, activation of the heavy metals in suitable conditions can be a potential source for environmental contamination in the groundwater and runoff water of the basin.