New annular carbonatite-derived Fe-REE-Nb ore structure in southern Morocco: Unveiling a large alkaline province in NW Africa

Abstract

Owing to their natural affinity to concentrate critical metals, including rare earth elements (REE), carbonatite rocks and their weathering products constitute economically important exploration targets. Linked to a growing demand driven by new green technologies, cratonic margins have increasingly undergone prospection for such critical metal deposits. This study presents a new carbonatite occurrence in the peri-cratonic terrain of the Reguibat Shield (Morocco). This recently discovered Terghat structure, with its wide and distinctive, ring-shaped outcrops (Ø = 9.5 km), is covered by both extensive regolith and silica breccia rocks. Rare outcropping carbonatites comprise calcite carbonatites with subordinate apatite, Fe-oxides, and phlogopite, with minor carbocernaite, synchysite, and ancylite as principal REE-bearing phases. Together with whole-rock trace element profiles, these features are strikingly similar to those observed in ring-shaped structures of the Twihinate-Lamlaga-Lahjayra (TLL) complex, within the western Oulad Dlim Massif. Such similarities are suggestive of a genetic link between the Terghat carbonatite and Cretaceous alkaline magmas of the TTL complex. Thick Fe-oxide/hydroxides bearing regolith also resemble carbonatite-derived regolith with total rare earth element oxide (TREO) and Nb₂O₅ enrichment (up to ∼ 3.7 % and 3.5 %, respectively), associated with crandallite-group minerals, monazite, and supergene residual pyrochlore. The silica breccia is also crosscut by monazite-rich veins and hydrothermally altered pyrochlore, linked to intense metasomatism affecting pre-existing quartzitic rocks. In spite of their different mineralogy, these lithofacies display similar REE fractionation with high LREE enrichment, inherited from underlying carbonatites. Accordingly, both hydrothermal alteration and subsequent weathering have played an important role in the redistribution of critical elements and metal deposition. As observed, the Terghat structure represents a new opportunity to investigate carbonatite-related critical element enrichment through sub-solidus processes. It also offers a promising link to broader alkaline magmatic activity and underscores the potential for future discoveries of REE and other critical mineral deposits in NW West African Craton margin.