Abstract
The Mississippi Valley-type deposits of the Touissit-Bou Beker district are hosted by a 25 m thick sequence of diagenetically and hydrothermally dolomitized carbonate platform rocks of Aalenian-Bajocian age. The sulfide mineralization consists principally of galena and sphalerite and occurs as open-space fillings of voids and moderate to massive replacement of the medium- to coarse-grained host dolostone. Five types of dolomite have been distinguished, two of which (D 1 and D 2 ) are of replacement origin, whereas HD 1 , HD 2 , HD 3 occurring as open-space filling are of hydrothermal affiliation. Main ore controls include stratigraphy and lithology, carbonate dissolution, paleogeography, faults or faulted rocks, and availability of organic matter. Fluid inclusion data, along with Na-Cl-Br leachate, indicate that the ore-forming fluids correspond to evolved NaCl-CaCl 2 -KCl-MgCl 2 basin-derived hot (100° ± 20°C) saline brines (>20 wt % NaCl equiv) that acquired their high salinities and Ca/Na ratios through evaporation of seawater, and subsequent dolomitization and fluid-rock interactions. Stable isotope data for replacement and hydrothermal dolomites are tightly clustered and overlapping, with δ 18 O and δ 13 C values from 20.5 to 21.2 and 0.2 to 0.7‰, respectively. Similarly, sulfides yield δ 34 S values between 11.2 and 1.9‰, whereas those corresponding to the nearby Triassic gypsum cluster yield around 14‰. Altogether, these isotopic compositions are consistent with a basinal-type fluid with reduced sulfur very likely being derived through thermochemical reduction of dissolved sulfate, resulting in metal precipitation, and carbon of mainly marine Aalenian-Bajocian carbonate origin with a minor biogenic component. 87 Sr/ 86 Sr values of replacement dolostone are similar to those of ore-related hydrothermal dolomites, ranging from 0.70746 to 0.70833 and from 0.70769 to 0.70828, respectively, and are different from those of the Visean rhyodacite (0.71849–0.72167). Lead isotope ratios ( 206 Pb/ 204 Pb = 18.319–18.390; 207 Pb/ 204 Pb = 15.620–15.680; 208 Pb/ 204 Pb = 38.452–38.650) of sulfides are consistent with Pb being derived from the Visean rhyodacite and associated volcaniclastic rocks. The intimate link between faults and mineralization suggests the strong possibility of brine flow along both ENE-trending regional-scale faults and NW-SW-trending local-scale faults. The data suggest that MVT mineralization was emplaced during the late Neogene-Quaternary (i.e., ca. 15–0 Ma), possibly as a result of subsurface gravity-driven fluid flow in response to the collision between the African and Eurasian plates.
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