Tectono-magmatic events and genesis of Fe-Pb-Zn resources in Nefza area, NW Tunisia

Abstract

Fe-ore and Pb-Zn (Ag) resources of Nefza region occur within hydrothermally altered sericite-schists and breccias, with Neogene molasse and lacustrine sediments and with a sequence of tertiary granodiorite-rhyodacite and basalts. Intrusions of intermediate igneous rocks, whose correlate with the successive Alpine tectonic events, started at 15 Ma and continued to near 5 Ma. Four main stages of mineralization are identified: (1) the first stage started during or soon after the maximum alpine compression (Langhian-Lower Serravallian) and magmatism. Silicification and carbonatization prevailed and sericite-bearing sediments formed by K-sericite alteration. (2) During the second stage, a mixture of marine and meteoric ore-forming fluids (H2O, H2CO3, F and Cl; temperature range, 315–500 °C) has deposited mineralizations. The sericite schist assemblages and breccias were extremely altered and four types of minerals were formed: (i) silica, feldspar, Mg-micas, chlorite-vermiculite, Mg-clays, and kaolinite-halloysite; (ii) fluorite and sorbed U; (iii) Fe-(Mn) oxyhydroxides; (iv) Zn, Pb, Mn, and Fe, deposited in the carbonates, in coronadite and in minor sulfides. (3) The third stage was related to the extensional tectonic phase (Upper Serravallian-Lower Tortonian) and leads to the subsidence of Miocene grabens and deposition of molasse with interbeded pyroclastites. The strata-bound iron ores were reworked from pre-existing Fe-mineralization hosted in folded structures. (4) During the fourth stage (8 to 5 Ma), magmatic activity changed from calc-alkaline granitoid to alkaline basalt. This correlates with a moderate Atlasic compression, thrust faulting, and Miocene graben folding. During this stage, Pb (Ag)-Zn sulfides were deposited in veins that cut across the predating iron deposits, with neoformed Mg-rich clays and carbonates (calcite, siderite, ankerite) with noticeable Zn and Mn enrichments.