The Oued Belif Hematite-Rich Breccia: A Miocene Iron Oxide Cu-Au-(U-REE) Deposit in the Nefza Mining District, Tunisia

摘要

The upper Miocene Oued Belif ring-shaped breccia is located in the Nefza mining district of northern Tunisia, within the internal zone of the collisional Alpine Maghrebide belt. It encloses chaotic Triassic material (evaporites, altered siltites, and pelites) within an extrusive diapiric structure reactivated in a late Miocene nappe emplacement episode. This deep-rooted structure drove the hydrothermal activity that caused the brec-ciation of the Oued Belif breccia and the emplacement of two generations of shallow felsic magmatism, which, most notably, include a Serravallian granodiorite stock and a Tortonian rhyodacite dome. The inverted cone-shaped matrix-supported Oued Belif breccia comprises; clasts of Triassic material and others deriving from the regional substrate, as well as a very small amount of volcanic-related material that suggests a possible phreatomagmatic origin. Fine-grained, low-Ti, Si-Al-rich hematite (>20 vol %) is the main matrix mineral, along with rare earth element (REE) minerals (bastnaesite, parisite, and subordinate monazite) and U minerals (uraninite). The Oued Belif breccia is not an economic Cu-Au deposit, although it does possess a distinct geochemical anomaly in mafic (Cu-Co-Au) elements, with local Cu enrichment at depth (from drill core material), as well as in felsic (Bi-W-Sn-U) elements. A multiphase alteration episode (K-Fe-(Mg) metasomatism) responsible for the formation of K-feldspar, Fe phlogopite, Fe muscovite, and tourmaline, with fluorite and barite, predates the Fe-light REE (LREE)-U mineralization. The major brecciation event occurred slightly after this alteration and is coeval with the main iron oxide-LREE-U mineralization event and the emplacement of the Oued Belif rhyodacite intrusion. Hydrothermal mineralizing fluids were hot (>540°C), saline and F-CO_2-rich brines, possibly with a mixed, basinal (salt-related) and magmatic (alkaline-related) origin.