Biogeochemical study of chromite bearing zones in Forumad area, Sabzevar ophiolite, Northeastern Iran
Introduction
Biogeochemical techniques used in mineral prospecting are based on soil and plant relationships (Al-Farraj and Al-Wabel, 2007, Ebong et al., 2007, Ghaderian and Baker, 2007, Ololade et al., 2007, Pratas et al., 2005, Reeves et al., 2007). Soil forms the upper layer of the regolith and supports plant life. Knowledge of soil chemistry is an essential prerequisite for the proper understanding of biogeochemical patterns. Close relationship between the soil and its vegetation is to the extent that it can be considered as an integral part of the biogeochemical technique (Palmer, 1995).
Plant growing on soil is dramatically affected by the host soil composition which leads to the selection of a specific flora. Plants respond to elemental composition of host soil in three ways: exclusion, indication or accumulation. Biogeochemists use soil-indicator plants to prospect ore deposits of valuable metals (Altinözlü et al., 2012, Brooks, 1998, Ghaderian and Baker, 2007, Pratas et al., 2005, Reid and Hill, 2012). The biogeochemical techniques focus on chemical analyses of plants and provide a cost-effective means to assess the composition of near-surface materials in order to detect the presence of ore deposits beneath the earth surface. There is also an increasing interest in the study of plant–soil relationship for phytoremediation, phytoextraction and phytomining (Al-Farraj and Al-Wabel, 2007, Ebong et al., 2007, Ololade et al., 2007, Reid and Hill, 2010).
Since serpentine plants have been well studied and ultramafic rocks are abundant on the earth's crust, plants growing on serpentinized rocks are successfully used in biogeochemical prospecting (Freitas et al., 2004, Sequeira and Pinto da Silva, 1991). The serpentinized ultramafics are rich in Fe and Mg with high concentration of Ni and Cr, thus soils formed on these geological materials contain high levels of magnesium and iron and also have much more nickel and chromium compared to many other soils. For example, high contents of Ni were reported from some serpentine derived soils (up to 2%) (Altinözlü et al., 2012). Serpentine soils tend to be shallow and dry (Proctor and Nagy, 1992), thus plant species growing on these soils and their physiognomy are different from those of non-serpentine areas. The soils are also characterized by low concentrations of some essential nutrients such as N, P, K, and high Mg/Ca ratio. Manganese content of the soils is slightly higher than many other soil types (Ghaderian et al., 2007, Proctor, 1999).
Ultramafic rocks are widely exposed in Iran and most of them belong to Neo-Tethyan ophiolite belt (Rajabzadeh et al., 2013), hence soils derived from serpentinized ultramafics cover substantial areas in many locations of this country (Fig. 1). The metallic deposits have been described by some authors from Iranian section of Neo-Tethyan ophiolite (Rajabzadeh and Moosavinasab, 2013, Rajabzadeh et al., 2013). Although these ophiolites contain important chromite ores exploited in many areas, there is little information on the flora and biogeochemistry of them.
Sabzevar ophiolite massif in northeastern Iran at Forumad area is regarded as one of the important mining areas for chromite ore deposits. In this ophiolite, the podiform chromite deposits intensely surrounded by serpentinized dunite envelopes, generally formed at the basal part of the ophiolite lithological column. As far as we know, despite high potential for mining activities in this area, no study has been conducted with a biogeochemical point of view. The main goal of the current research is to develop the application of biogeochemical methods in Sabzevar ophiolite belt at Forumad area for chromite prospecting. Due to the high resistance of chromite (major Cr-bearing mineral) against weathering and high concentrations of Ni in olivine structure (principal mineral of dunite which is the host rock of chromite) (Naldrett, 2004), in this study, soil and plant relationship was examined in order to present indicator plant species, accumulating Ni with the aim of their utility in biogeochemical exploration as additionally or integrated with other traditionally methods of the chromite ore deposits.
Section snippets
Field description
The Sabzevar ophiolite is part of the northern branch of Neo-Tethyan ophiolite belt in the Middle East that links the Mediterranean ophiolites (e.g. Turkish, Troodos and Greek) to more easterly Asian ophiolites (e.g. Tibetan). The age of ophiolite emplacement into continental margin is estimated to be Late Cretaceous and it may be related to the Lesser Caucasus development (Shojaat et al., 2003). The ophiolite belt in northeastern Iran appears in a region of 150 km long and 10–30 km wide,
Sample collection
The choice of sample spacing should comply with the distribution of desired species and soils. As Forumad area is situated in semi-arid to arid climate, it contains poor distribution of plant communities. By and large, survey design with the spacing of 500 m between sampling sites and 700 m between transects were defined. The representative background samples of soil and plant were collected from soils developed on non-mineralized rocks of the Sabzevar ophiolite at a distance of 2 km far from
soil
As depicted in Table 1, all analyzed soils contain high amounts of total elements (Cr + Ni + Co + Mn + Fe + Mg + Ca). These concentrations are in the range for typical ultramafic soils elsewhere in the world (Alexander et al., 2007, Brooks, 1987). However, the Mg/Ca ratio is lower than many other serpentine sites (Proctor and Woodell, 1975). The exchangeable fraction of Cr, Ni, Co, Mn and, Fe is low, whereas the exchangeable fraction for alkali (Na and K) and alkali earth elements (Mg and Ca) is
Discussion
Podiform chromite deposits at Forumad ophiolite massif are surrounded by intensively serpentinized dunite envelopes that generally occurred at the basal part of the ophiolite lithological column. Iron replacement with Ni in olivine up to 500 μg g− l, which is the main constituting mineral of dunite (Naldrett, 2004), leads soil formed from serpentinized dunite to be exclusively rich in Ni. Field observations and chemical analytical data helped us to classify the studied soils into 2 groups:
Conclusion
Plants growing on serpentinized rocks at Sabzevar ophiolite could be successfully used as additionally methods or integrated with other traditional methods for tracing the chromite ore hosting dunites. A strong relationship between chromium and nickel was observed in soils derived from magmatic dunite, indicating that Ni is a good pathfinder for tracing the chromite ore bearing-dunites. Nickel is more bioavailable than Cr and is an essential constituent of plant enzymes. Determined BCF for Ni
Acknowledgments
Ghaderian SM, (Dept. of Biology, University of Isfahan, Iran) is greatly acknowledged for kindly providing helpful comments during this study. We appreciate Shiraz University Research Council (1391-MSc-Ghasemkhani) that supported this work. The authors are also grateful to Mr. Rahimi, the director general of the Karizshahr Chromite Mines Company for his assistance in the field work. The authors also would like to appreciate Mrs. Ghasemkhani, English instructor at Milajerd PNU University for
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