Metallogeny and Genesis of Fault-Filling Barite-Sulfide Veins (Ougnat, Morocco): Petrography, Fluid Inclusion, and Sr-S Isotopic Constraints

Abstract

The Ougnat Massif of the eastern Anti-Atlas (Morocco) hosts barite and sulfide vein-type deposits of vital economic importance. With over 150 mineralized structures reported in the Ougnat Massif, the ore-bearing ones are predominantly composed of barite, quartz, calcite, and minor portions of sulfides. The mineralized veins are driven by NW-SE and NE-SW to E-W oblique-slip opening faults that cross both the Precambrian basement and its Paleozoic cover. The mineralized structures occur as lenses and sigmoidal veins that follow stepped tension fracture sets oblique to the fault planes. These geometries and kinematic indicators of these structures point to a predominantly normal-sinistral opening in a brittle-ductile tectonic setting. The S isotopic compositions of barite from the Ougnat Massif (+10.8 to +19.5‰) fall mostly within the range of δ34S values of Late Triassic to Jurassic seawater, thus suggesting that some of the SO2− in barite comes from seawater sulfate. This range of δ34S values also corresponds approximately to the hydrothermal barite context. The 87Sr/86Sr ratios of barite, which range from 0.710772 to 0.710816, lie between the radiogenic strontium isotopic compositions of deposition by hydrothermal solutions, and also coincide with the non-radiogenic isotopic signature of Triassic to Jurassic seawater. Based on a fluid inclusions study, the ore-forming fluids were a mixture of two or more fluids. A deep hot fluid with an average temperature of 368 °C leached the granodiorites and volcanic-sedimentary complex of the Ouarzazate Group. This fluid provided the hydrothermal system with most of the Ba, radiogenic Sr, and some of the dissolved S. A second, shallow fluid with an average temperature of 242 °C was derived from Late Triassic to Jurassic seawater. The barite mineralization of the Ougnat Massif constitutes a typical example of vein-type mineralization that occurred along the northern margin of the West African Craton and regionally tied to the central Atlantic opening.