Miocene karsts and associated Fe-Zn-rich minerals in Ain Khamouda (Central Tunisia)

Abstract : The karstic system in Ain Khamouda (Central Tunisia) has been briefly exploited for its Pb-Zn mineralization in the first 20 years of the 20th century. Since then, little work has been dedicated to this system, apart from some detailed mineralogical studies. Here we place the Ain Khamouda karsts in their regional geological framework, which is marked by the presence of large basinal-scale MVT Pb-Zn deposit (e.g. at the Djebel Chambi, similar to 10 km to the south of the study area). The karsts are deepening along faults in Turonian-Coniacian limestone (the Douleb Fm), under a Tortonian sandy cover (the Beglia sand) that is withdrawn within the karst infilling. This makes a Tortonian or post-Tortonian age for the karstification likely, as for other comparable karsts located to the northwest of the study area, in the Algerian Constantinois. In Ain Khamouda, the mineralization sequence is a variant of the classical gypsum -> iron -> oxide silicates sequence, with some specific characteristics inherited from the regional Pb-Zn background: a 7 angstrom Zn-clay (berthierine) has been identified, mixed with halloysite, and calamine minerals (hemimorphite, smithsonite) are found in voids and fractures within the karstic system. Undersaturated meteoric fluids flowing downward meet pre-existing or recently formed base metals sulphides, in the subsurface. In the Ain Khamouda area, the downward deepening of karsts along major regional faults brings meteoric fluids in contact with base metal sulphides. The oxidation of base metals sulphides results in the release of sulphuric acid, which also accounts for an increased karstification. Late processes occurring in the Beglia sand appear to be complex, though somewhat anecdotic in terms of volumes. They essentially consist in recurring cycles of reduction (among which biochemical reduction of galena) and oxidation (including the formation of jarosite). The study area offers the opportunity to investigate (1) a karstic system where deepening caves meet pre-existing base metal sulphides, and (2) the consequences this specific setting has, in terms of minerals found within the karst: presence of Zn-bearing clays (berthierine), formation of calamine minerals, minor cycling of lead in the late phases. (C) 2009 Elsevier Ltd. All rights reserved.
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Delphine Bruyère, Thierry de Putter, Sophie Decree, Christian Dupuis, Yves Fuchs, et al.. Miocene karsts and associated Fe-Zn-rich minerals in Ain Khamouda (Central Tunisia). Journal of African Earth Sciences, Elsevier, 2010, 57 (1-2), pp.70--78. ⟨10.1016/j.jafrearsci.2009.07.011⟩. ⟨hal-00693450⟩

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