The Oued Belif Hematite-Rich Breccia: A Miocene Iron Oxide Cu-Au-(U-REE)Deposit in the Nefza Mining District, Tunisia
De Putter, T.
Earth and related Environmental sciences
Geodynamics and mineral resources
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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 drive the hydrothermal activity that caused the brecciation of the Oued Belif Breccia (OBB) 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 OBB 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 phreato-(magmatic) origin. Fine-grained, low-Ti, Si-Al-rich hematite (≥ 20 vol. %) is the main matrix mineral, along with REE-minerals (bastnaesite, parisite and subordinate monazite) and U-minerals (uraninite). The OBB is not an economic Cu-Au deposit, although it does possess a distinct geochemical anomaly in mafic (Cu-Co-Au) elements in which local Cu enrichment are at depth (from drill core material) as well as in felsic (Bi-W-Sn-U) elements. A multi-phase 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-LREE-U mineralization. The major brecciation event occurred slightly after this alteration and is coeval with: (i) the main iron-oxide-LREE-U mineralization event and (ii) the emplacement of the Oued Belif rhyodacite intrusion. Hydrothermal mineralizing fluids were hot (≥ 540°C) saline and F-CO2-rich brines, possibly with a mixed, basinal (salt-related) and a magmatic (alkaline-related) origin. Except for its lack of economic level Cu and Au content, the OBB shares most typical attributes of the hematite-group of IOCG deposits. One may therefore classify it as either an Iron-Oxide associated-Alkali Altered (IOAA) (Porter, 2010) or as an Iron-Oxide Uranium (IOU) (Skirrow, 2010) mineral system. K-Ar dating of K-feldspar sets the OBB formation at 9.2 ± 0.25 Ma, which could make it the youngest presently known representative of the IOAA-IOCG class of deposits. From a geodynamic viewpoint, the Oued Belif structure fits within a still active collisional belt even though the OBB was formed in a post-collisional stage. In this respect, Oued Belif differs from most other IOCGs, especially from older Proterozoic ones. It nevertheless shares two essential characteristics considered necessary to yield IOCGs (Skirrow, 2010), namely a metasomatised subcontinental lithospheric mantle (SCLM) and a lithospheric delamination process. Identifying the OBB as a member of the IOAA-IOCG family opens new frontiers within the study of IOCG deposits within the geodynamic environment of collisional belts. Future research in this area should pay particular attention to the circum-Mediterranean segment of the Alpine belt.
CitationDecrée, S.; Marignac, C.; De Putter, T.; Yans, J.; Clauer, N.; Dermech, M.; Aloui, K.; Baele, J.M. (2013). The Oued Belif Hematite-Rich Breccia: A Miocene Iron Oxide Cu-Au-(U-REE)Deposit in the Nefza Mining District, Tunisia. , Economic Geology, Vol. 108, 1425-1457,