The age of the hydrothermal Co-Ni-As-(Au) mineralization in the Bou Azzer-El Graara inlier was determined using different isotopic systems on ore and mineralization-related gangue minerals, namely Re-Os on molybdenite, Sm-Nd on carbonates, and U-Pb on brannerite. Re-Os dating of molybdenite was difficult owing to the intimate intergrowth of molybdenite with Co arsenides and the fact that sampling by drilling complex sulfide mixtures resulted in poor concentration of molybdenite. Accordingly, Re-Os dating of the impure molybdenite-sulfide mixtures yielded inconsistent ages in the range 400 to 350 Ma. This scatter is attributed to (1) inadequate concentration of pure molybdenite, (2) complex Re-Os systematics, possibly involving excess radiogenic Os derived from older generation(s) of sulfide, and/or (3) mixture of at least two Re-Os systems consisting of a component with pre-Hercynian history and later disturbance by Hercynian metamorphism. Carbonates and brannerite that coexist with molybdenite gave more consistent ages of 308 ± 31 Ma (Sm-Nd) and 310 ± 5 Ma (U-Pb), respectively. Because both ages agree within their errors, we regard the brannerite U-Pb age of 310 ± 5 (2σ) Ma as the best and most precise estimate for the age of the Co-Ni-As-(Au) mineralization. Although an earlier onset of mineralization cannot be ignored totally, the new age data underline that the principal Co-Ni-As-(Au) mineralization at Bou Azzer occupying the main ore-bearing structures was driven by and formed during the end of the Hercynian orogeny.