Abstract

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 brecciation 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-CO2–rich brines, possibly with a mixed, basinal (salt-related) and magmatic (alkaline-related) origin.

Except for its lack of economic Cu and Au content, the Oued Belif breccia shares most typical attributes of the hematite group of iron oxide copper-gold (IOCG) deposits. One may therefore classify it either as an “iron oxide-associated–alkali-altered” (Porter, 2010) or as an “iron oxide uranium” (Skirrow, 2010) mineral system. K-Ar dating of K-feldspar sets the Oued Belif breccia formation at 9.2 ± 0.25 Ma, which could make it the youngest presently known representative of the iron oxide-associated–alkali-altered–IOCG class of deposits.

From a geodynamic viewpoint, the Oued Belif structure fits within a still active collisional belt, even though the Oued Belif breccia was formed in a postcollisional stage. In this respect, Oued Belif differs from most other IOCG deposits, especially from older Proterozoic ones. It nevertheless shares two essential characteristics considered necessary to yield IOCG deposits (Skirrow, 2010), namely a metasomatized subcontinental lithospheric mantle and a lithospheric delamination process.

Identifying the Oued Belif breccia as a member of the iron oxide-associated–alkali-altered–IOCG family opens new frontiers within the study of IOCG deposits within the geodynamic environment of collisional belts. Future research in this area should focus particularly on the circum-Mediterranean segment of the Alpine belt.

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