عنوان مقاله :
بررسي تمركز عناصر سنگين Mn, Pb, Cu, Ag, Zn ,Sr, Co, Cd و Ni در پهنه‌هاي ماسه‌اي پلاياي گاوخوني (جنوب شرق اصفهان)
عنوان فرعي :
Study on the distributions of Cd, Co, Sr, Zn, Ag, Cu, Pb, Mn and Ni in the eolian sands of the Gavkuni playa (southeast of Isfahan)
پديد آورندگان :
پاكزاد، حميدرضا نويسنده استاديار گروه زمين‌شناسي دانشگاه اصفهان , , پسندي، مهرداد نويسنده استاديار گروه زمين‌شناسي دانشگاه اصفهان , , علي‌نيايي، زينب نويسنده دانشجوي كارشناسي ارشد زمين‌شناسي دانشگاه اصفهان ,
چكيده لاتين :
Introduction
Heavy metals are continually introduced into hydrosphere, atmosphere and biosphere. These potentially toxic elements can concentrate in sediments in aqueous ecosystems and they can act as pollution sources in particular conditions. Heavy metals mainly concentrate in fine sand particles because of more concentration of heavy minerals in this grain size.
The Gavkhuni playa lake with an approximate area of 550 km2 is located at southeast of Isfahan, Iran. The Zayandehrud permanent river and several seasonal rivers flow into this playa. During quaternary, massive volume of sediments in sand and gravel sizes were carried into this area and deposited. These sediments are mainly deposited in the delta of Zayandehrud river (northwest of Gavkhuni playa) and northwest of the sand dunes in margin of the Zayandehrud river particularly from Varzaneh to the playa. The megafans surrounding the playa can be partly origin of these sediments. The Gavkhuni playa lake is composed of three major flats namely sand, mud and salt flats. The salt flat forms the major part of the playa. Igneous, metamorphic and sedimentary (mainly carbonates) rocks outcrop in the drainage basin of the Gavkhuni playa lake. Schist, gneiss, limestone, shale and andesite are the dominant lithology in this basin.
The main objective of this research is to determine the concentrations of Cd, Co, Sr, Zn, Ag, Cu, Pb, Mn and Ni in the eolian sand deposits, the factors influencing the distributions of these elements and also relationship between the heavy metals and the minerals (particularly heavy minerals) containing these elements.
Material & Methods
The eolian sands (sand dunes and sand flats) of the Gavkhuni playa were sampled in 15 sampling points from north to south of the playa. Grain size distribution of the sediments were determined through dry sieving method. Heavy minerals were separated and studied through microscopy of the prepared thin and polish sections. The concentration of heavy metals was determined by Atomic adsorption Spectroscopy (AAS) after preparation of the samples.
Discussion of Results & Conclusion
The eolian sands located at west of the playa includes sand dunes and sand flats. About 85% of the eolian sands are in fine sand size range. These sediments are composed of igneous, sedimentary and metamorphic rock fragments and also quartz, feldspar and different heavy minerals.
The igneous and metamorphic rock fragments decrease in the eolian sands from north to south of the playa and conversely sedimentary rock fragments increase in the sediments. The silicate heavy minerals of the sediments are zircon, amphibole, pyroxene, epidote, clinozoisite, spinel, idocrase, wollastonite, teremolite-actinolite and apatite in their order of abundances. The non-silicate heavy minerals include hematite, limonite, magnetite, pyrite and sphalerite. The metamorphic rocks outcropping in north of the study area (Feshark Skarn) are the origin of spinel, idocrase, wollastonite and tremolite-actinolite. The volcanic rocks occurring in north and northeast of the basin are the provenance of zircon, amphibole, pyroxene, epidote, clinozoisite, magnetite and apatite.
Cd exists in the carbonates hosting sulfides of Zn, Pb, Cd and Ag complex occurring in the drainage basin of the Gavkhuni playa. This element is more concentrated in the samples containing basic igneous rock fragments and minerals such as plagioclase, biotite and amphibole.
Sr shows positive significant correlation with the calcium carbonate content and its concentration is higher in the samples containing more carbonate grains. Cd and Sr also show high significant positive correlation and their concentration increase with increase in the amount of sedimentary (carbonate) rock fragments.
The Co concentration decreases in the eolian sands from north to south of the playa. The amount of this element increases in the fine-grained samples containing more heavy minerals particularly hematite, goethite and sulfides such as pyrite and chalcopyrite.
The concentration of Ni generally decreases in the eolian sands from north to south of the playa. The Ni concentration indicates negative significant correlation with the calcium carbonate content. The concentration of this element increases in the eolian sands located at the northern part of the playa where the sediments contain more amphibole, pyroxene, biotite, plagioclase and magnetite because Ni exists in structure of these minerals.
The concentration of Ag is minimum in the delta of Zayandehrud River. The Ag concentration is higher in the samples containing more sulfide minerals such as pyrite and chalcopyrite.
The Pb concentration shows significant positive correlation with the calcium carbonate content and the amount of them both increase from north to south of the eolian sands. The concentration of this element increases in the samples containing more calcium carbonate. The Ag concentration is also higher in the samples containing amphibole, augite, biotite, magnetite and pyrite due to substitution of this element in structure of these minerals.
The Cu concentration decreases in the eolian sands towards south of the playa due to the decrease in the amount of sulfide minerals such as pyrite and calcopyite.
The Mn concentration increase in the samples containing more Fe-bearing minerals and sedimentary rock fragments (mostly carbonate).
The concentration of Zn decreases in the eolian sands from north to south of the playa. This element is more concentrated in the fine-grained samples containing more heavy minerals. The higher concentration of Zn in the eolian sand samples taken from northern part of the playa is due to the higher amounts of amphibole, pyroxene, augite, biotite and magnetite in these samples because these minerals substitute Zn in their structures.
According to the calculated Enrichment Factors (EF), Cd and Ag are very highly enriched in the studied sediments, Sr and Co are highly enriched, Ni and Pb are mildly enriched and Mn, Cu and Zn show slightly enrichment.
