Environmental mineralogy of Cu-porphyry mine tailings, a case study of semi-arid climate conditions, Sarcheshmeh mine, SE Iran

Abstract

Sarcheshmeh porphyry copper mine is located in semiarid clime conditions, in the Central Iranian Volcano-Plutonic Copper Belt. Hydro-geochemical and climatological conditions produce suites of secondary mineral assemblages in the Sarcheshmeh mine tailings, which are very interesting and important from the environmental point of view. Solid samples were collected from the fresh and weathered tailings and also from the surface evaporative layer formed on top of the old impoundments. Samples were investigated by using X-ray diffraction (XRD), ore microscopy and chemical fractionation methods. The primary mineralogy of fresh tailings, which is controlled by alteration/mineralization in the Sarcheshmeh mine, was mainly composed of quartz, pyrite, muscovite (sericite), albite, orthoclase, ±clinochlore±chalcopyrite and±illite. This mineral assemblage was subjected to the oxidation/weathering reactions in dried impoundments. Acidic conditions of the weathering zone were compatible with the precipitation of jarosite [KFe3+3(SO4)2(OH)6], hydronium jarosite [(H3O)Fe3+3(SO4)2(OH)6], natrojarosite [NaFe3+3(SO4)2(OH)6], alunite [KAl3(SO4)2(OH)6], butlerite [Fe3+(SO4)(OH)2H2O] and gypsum [CaSO4 2H2O]. A great variety of soluble secondary minerals, typical of arid and semiarid environment, were distinguished in the well-developed evaporative layer on top of the old weathered tailings. These minerals, which are formed due to the upward migration of multi-elemental contaminated waters enriched in Al, Ca, Cl, Cu, Fe2+, Fe3+, H, K, Mg, Mn, Na, O, S, and Zn, were mainly from: (1) simple hydrated sulfate salts of the divalent metal cations (gypsum, hexahydrite, kieserite, leonite, poitevinite, and Zinc-melanterite); (2) simple hydrated salts of the trivalent ions (alunogen, kalinite, and tamarugite); (3) mixed divalent–trivalent salts (lishizhenite, römerite, and Zincobotryogen); and (4) chloride containing minerals (eriochalcite, halite, and mitscherlichite). Secondary minerals in the evaporative zone were found in several unique mineral assemblages such as poitevinite–tamarugite–alunogen–hexahydrite. Formation, transformation, and dissolution of evaporative minerals are induced by the wetting/drying cycles. Mineralogical results, which are supported by the hydro-geochemical and sequential extraction data, are useful for the prediction of contamination potential of tailings, the fate of environmental contaminants, and also the possible remediation strategies.