Timing of the telescoped sediment-hosted Cu-(Ni) and Pb-Zn-Ba mineralization at the Oumjrane vein system (Eastern Anti-Atlas, Morocco) and its relevance to Pre-Cretaceous Central Atlantic and Maghrebian Tethys geodynamics

Abstract

• Apatite U-Pb dating shows 175.6 Ma for copper and 151.3 Ma for Pb-Zn-Ba mineralization. • Mineralizing events align with Central Atlantic drifting and Maghrebian Tethys formation. • Apatite chemistry shows a rock-buffered system as the dominant ore-forming process. The structurally controlled vein-type Oumjrane Cu-(Ni-Pb-Zn-Ba) district (5.7 Mt at 1.5 % Cu) consists of a 25 x 25 km-wide system of steeply dipping ENE- to E-W-trending quartz-sulfide fault veins hosted in a succession of Ordovician low-grade phosphorus-bearing siliciclastic metasedimentary rocks of the first (Upper Darriwilian-Sandbian) and second (Hirnantian) Bani Groups. Based on textural relationships and mineral assemblages, two major sulfide fault-vein infill events are recognized. The early event, which is copper-rich and economically the most productive, consists of chalcopyrite with subordinate gersdorffite (NiAsS) and magnetite, closely intergrown with quartz and apatite-1. The later Pb-Zn-Ba event comprises galena and sphalerite as the principal sulfides embedded in a barite-dominated and apatite-2 matrix. Major and trace element chemistry and LA-ICP-MS U-Pb dating of ore-related apatite from both mineralization events are used to determine the absolute age(s) of mineralization and related geodynamic setting(s). The obtained ages are 175.6 ± 1.1 Ma (MSWD = 1.3) for the first copper-rich event, and 151.3 ± 3.1 Ma (MSWD = 0.46) for the later lead-zinc-barite event. The trace element composition of apatite-1from the first copper event reflects the involvement of regional circulation of predominantly Cl − and OH – , and SO 4 2− hydrothermal basinal brines in a dominantly rock-buffered system. Conversely, apatite-2 chemistry from the late Pb-Zn-Ba event is consistent with a magmatic-hydrothermal origin. High fluid-rock ratios would have resulted in mobilization of metals and their transport into the hydrothermal system through the ENE- to E-W-trending faults which acted as pathways for the upward flow of the ore-forming fluids. From a geodynamic perspective, the new U-Pb ages correlate with the successive peak of the hyper-extension of Maghrebian Tethys and initial stages of Central Atlantic passive margin formation. Remarkably, these post-Variscan radiometric dates offer new insights challenging the previously established timeline for the Anti-Atlas copper belt genetic models, situating it within a broader, rich metallogenic period in North Africa and Europe.