Geochemical fractionation pattern and environmental behaviour of rare earth elements (REEs) in mine wastes and mining contaminated sediments; Sarcheshmeh mine, SE of Iran

Abstract

The geochemical fractionation pattern and environmental behaviour of rare earth elements (REEs) were investigated in the mining/processing wastes and contaminated sediments associated with the Sarcheshmeh mine, one of the largest Oligo-Miocene Cu-porphyry deposits in the world. For this purpose, the total concentration and the fractionation pattern of REEs were determined using microwave-assisted multi-acid digestion (HCl + HNO3 + HF) and BCR (Community Bureau of Reference, European Commission) sequential extraction methods, respectively. The normalizing patterns of the total concentration of REEs based on the world average shale (WAS) and average chondrite (AC) for the samples composed of primary lithogenic minerals have corresponded with the higher enrichment of LREEs than HREEs, a pattern that was similar to the Sarcheshmeh porphyry rocks. The BCR fractionation pattern of REEs in theses samples was mainly dominated by the residual fraction. By contrast, the normalizing patterns of the total concentration of REEs in the samples composed of secondary minerals or amorphous solid phases showed higher enrichment of HREEs than the LREEs. Surprisingly, depending on the mineralogy and environmental conditions, the BCR fractionation pattern of REEs in these samples was mainly associated with the both readily acid-soluble and reducible fractions. For example, the BCR fractionation pattern of REEs in the Sarcheshmeh mine pit sediments, which were enriched in amorphous Fe oxides, was mainly controlled by the reducible fraction (>46%), while these elements were mainly related to the acid-soluble fraction (>80%) in the sediment sample collected from the surface evaporative layer formed on top of the old weathered tailings. On the other hand, the BCR fractionation pattern of REEs was controlled by the both acid-soluble (>32%) and reducible fractions (>43%) in the samples collected from sediments and efflorescence salts formed along the waste rock drainages. These results showed that statement any general geochemical pattern and environmental behaviour for the REEs without considering the key environmental factors may lead to miss-understanding or even contrasting interpretations. This study emphasizes on the importance of the mineralogy, the conditions of weathering and oxidation reactions, and even the climate and topography on the BCR fractionation pattern and geochemical behaviour of REEs in the environment.