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Mn oxides as efficient traps for metal pollutants in a polyphase low-temperature Pliocene environment: A case study in the Tamra iron mine, Nefza mining district, Tunisia

Abstract : The Tamra mine, located in the Nefza mining district (NW Tunisia), exploits a 50 m-thick layer of Mio-Pliocene sediments that are heavily mineralized with Fe and other metals (Mn, Pb, Zn), especially in its eastern part, which is highly mineralized in Mn and known as the ''manganiferous zone". The textural and geochemical studies of manganiferous minerals in the Tamra mine have allowed the determination of four main paragenetic stages. Stages 1 and 2 relate to the main pedogenetic event that gave rise to the currently exploited Fe ore deposit. The last two stages relate to mineralizing events closely connected with hydrothermal circulation and leaching of underlying mineralization of the Sidi Driss Pb­Zn sedex deposit, with subsequent crystallisation in the supergene environment. Stage 3 is characterized by the formation of massive romanechite, hollandite and Sr-cryptomelane, while stage 4 results in the formation of coronadite and chalcophanite. 39Ar­40Ar analyses performed on hollandite (stage 3) and coronadite (stage 4) samples yielded ages of 4.7 ± 0.1 Ma and 3.35 ± 0.07 Ma, respectively. Tentative 39Ar­40Ar analyses on chalcophanite provided aberrant results, due to the poor argon retention in this layer-structure mineral. The youngest age corresponds to the late phase of the late Alpine extension event in northern Tunisia, evidenced through an increased regional thermal gradient as well as by a N­S set of normal faults and fractures. The Tamra mine is obviously a polyphase mineral deposit, recording several distinct metal inputs, part of them originating from the underlying Sidi Driss Pb­Zn deposit, while another part is provided by hydrothermal circulations forced by the high thermal gradient. Three springs flowing from the Tamra ore series are regular sources for drinking water used by the local population. Although the Alpine thermal gradient could have facilitated extensive mixing between subsurface oxidizing meteoric fluids and deep reducing hydrothermal tepid fluids, the springs flowing out in the Tamra mine are not significantly polluted with metallic elements (Fe, Pb, Zn, Mn), and can be regarded as drinking waters according to the World Health Organization (WHO) standards. 39Ar­40Ar ages suggest that complex Mn oxides formed early in the deposit history. These oxides incorporated metal ions in their mineral structure and/or acted as high-surface-area substrates favouring heavy metal adsorption (e.g. Pb on Mn oxides). Similar adsorption/co-precipitation processes are also exhibited by iron oxides, which form the major part of the Tamra sediments. These efficient trapping processes most probably account for a restricted migration of ''pollutants" through the neighbouring sediments and the circulating fluids.
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Submitted on : Wednesday, June 15, 2011 - 10:35:28 AM
Last modification on : Wednesday, October 14, 2020 - 4:09:54 AM

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Sophie Decree, Gilles Ruffet, Thierry Putter De, Jean-Marc Baele, Philippe Recourt, et al.. Mn oxides as efficient traps for metal pollutants in a polyphase low-temperature Pliocene environment: A case study in the Tamra iron mine, Nefza mining district, Tunisia. Journal of African Earth Sciences, Elsevier, 2010, 57 (3), pp.249-261. ⟨10.1016/j.jafrearsci.2009.08⟩. ⟨insu-00600568⟩

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