Evaluation of mineralogical changes of sediments affected by Dar-e-Allo copper mine, south of Kerman: application for environmental studies

Abstract

Abstract The present study has been performed to evaluate the prospective effects of activity mining on the mineralogy of sediments in the Dar-e-Allo copper mine, south of Kerman. Sediments affected by Dar-e-Allo mine are divided into six sedimentary groups including: natural background sediments, sediments of waterways leading to the mine, sediments of the Sarmashk River, sediments under the waste rock dump, sediments containing secondary phases, Fe-Mg oxy-hydroxide sediments, and evaporative sediments. Mineralogical studies of sediments as an indicator to evaluate the environmental effects of mining are classified into five main groups including primary and unaltered, carbonate, clay, sulfate and oxide minerals. Sediments in the operational area of the Dar-e-Allo copper mine are associated with extreme mineralogical diversity. The sulphide minerals are the most important source of acid mine drainage and secondary minerals such as gypsum, starkeyite, copiapite, magnesiocopiapite and natrojarosite are the most important temporary reserves of potentially toxic elements (PTEs) and H+ ions. The dissolution of the mentioned minerals, especially during the initial flushing events in the wet season, cause a sharp increase in the acidity and concentration of PTEs in the surface runoff. Keywords: Sediments, Mineralogy, Environment, Daralo Copper Mine Introduction Mining operation and extraction of sulphide ore cause the oxidation of a group of sulphide minerals (pyrite and chalcopyrite) and the production of acid mine drainage (Woo and Choi 2001; Milu et al. 2002; Sinclair 2007). Sediments play an important role in physical, chemical and biological processes resulting from acidic runoff. This research attempt to evaluate the impact of mining activities on the natural sediments of the area by studying the mineralogy of surface sediments. Determining the role of these sediments in the absorption and release of PTEs from the sediments and entry into the water as a threat has great importance. The Dar-e-Allo Cu mine is one of the largest copper mines in the southeastern part of the Urumieh-Dokhtar Magmatic Belt (UDMB), about 120km south of Kerman, Iran. The oldest lithologic unit of this region is Eocene in age. The petrology of the area is predominantly composed of igneous and volcanic rocks. The host of Cu ores in the Dar-e-Allo mine is a massive granodiorite (Alimohamadi et al. 2015). The goal of this study is to evaluate the impact of mining activities on the mineralogy of sediments. Materials and Methods Thirty-one sediment samples from five sedimentary systems were collected at the end of the dry season (September 2019), when evaporative phases are formed due to intense evaporation and the supersaturation process. The sampling locations included waste rock drainages, sediments along the natural streams, evaporative deposits, sediments containing ferrous compounds and natural background sediment. After drying, these samples were sieved. XRD (X-Ray Diffraction) analysis were performed on the samples for identification of minerals by the qualitative method at the Zar-Azma laboratory in Tehran. Discussion of Results & Conclusions Mineralogical results confirmed the presence of 19 minerals in the composition of sediments. The identified minerals are classified into five main groups including primary and unaltered, carbonate, clay, sulphide and oxide minerals. Quartz, albite, orthoclase and hornblende are the most important primary minerals of lithogenic origin in sediment samples. Calcite and siderite are the only carbonate minerals identified in the sediment samples. The six minerals include chlorite, muscovite-illite, kaolinite, illite, montmorillonite and clinoptilolite are the important minerals resulting from alteration identified in the sediment samples. Moreover, five minerals including gypsum, copiapite, magnesiocopiapite, starkeyite and natrojarosite are the minerals that have stored sulfate ions in their composition. Oxide minerals include hematite and goethite. Mineralogical results show that each of the primary and secondary mineralogical compositions will show different environmental effects in the short and long term on the surrounding vital ecosystems. Albite and orthoclase as the major minerals through homogeneous or heterogeneous weathering (consumption of H+ or the production of HCO3-) can play an important role in reducing the acidity of weathering solutions and increasing the absorption of PTEs (Lottermoser 2003). Carbonate minerals neutralize the acid by forming HCO3- or H2CO3 (Skousen et al. 2000; García-Valero et al. 2020). Clay minerals can remove PTEs from Contaminated drains through cation exchange or surface adsorption (Ren et al. 2023). The consumption of H+ ions and the acidity of mine drainage decreases as a result of weathering of clay minerals (Elghali et al. 2021). As a result of the evaporation of sulfated waters, evaporate and secondary minerals with different compositions are deposited (Hammarstrom et al. 2003; Hammarstrom et al. 2005). The presence of copiapite minerals is proof of the acidic conditions of the sedimentation environment (Carbone et al. 2013). Gypsum is another important evaporative mineral in acidic drainage environments (Carbone et al. 2013). The formation of the starkeyite indicates intense evaporative conditions and the presence of Fe sulfide compounds (Sracek et al. 2004). The abundance of Na+ in the water of the mine area has provided suitable conditions for the natrojarosite formation (Bavi 2021). The Na+ required for the formation of natrojarosite is released from the weathering of albite, which is a common mineral in intermediate and acidic rocks (Desborough et al. 2010). The active presence of gypsum as a high degree of dissolution mineral in sulphide sediments (S23-S25) and very high to a dangerous degree of pollution (Bavi et al. 2023) are proof the temporary storage of H+ and PTEs in the mineralogical structure of gypsum. While sulfate salts containing Fe2+, Mn2+, Fe3+, and Al3+ (for example, starkeyite, copiapite, magnesiocopiapite, and natrojarosite) are insoluble (Lottermoser 2003) and are not easily to release H+ and PTEs to the aquatic system. The presence of these evaporite minerals with high dissolution intensity in acidic conditions (S17, S18, S23, S25) corresponds with a very high degree of contamination (Bavi et al. 2023). Hematite and goethite are stable iron oxides that have an active absorption surface and are capable of absorbing cations and anions from the surrounding environment (Carbone et al. 2013). Therefore, these minerals have a potential application in protecting the environment and absorbing PTEs from water and reducing their concentration in the solution phase. Sulphide minerals are the most important source of acid mine drainage, which depending on the composition of the host rock or sediments can cause acid production and the release of PTEs over a continuous time. In the studied area, the evaporate sediments have the highest amount of secondary minerals. The formation of these sediments only temporarily causes the storage of PTEs and H+ ions. Therefore, as a natural cleaning process, they play an important role in preventing the movement and transfer of PTEs into the environment. With the beginning of the wet season, especially during the first flood, water pollution will increase sharply; But with repeated rainfall, the intensity of pollution will decrease. Such a cycle of pollution transfer in the water environment and sediment will be repeated every year.

Keywords

• sediments
• mineralogy
• environment
• daralo copper mine