The Hoggar Gold and Rare Metals Metallogenic Province of the Pan-African Tuareg Shield (Central Sahara, South Algeria): An Early Cambrian Echo of the Late Ediacaran Murzukian Event?

Abstract

The Hoggar massif, a part of the Tuareg shield in the Trans-Saharan Pan-African orogen, is endowed in gold and rare metals (Sn, W, Ta, Be). Most of these metals are present at low levels compared to other Precambrian or collisional belts worldwide, but tantalum is concentrated in a series of evolved granitic cupolas, making the Hoggar a promising tantalum province. The Tuareg shield was built in three stages during the convergence of two cratonic masses, the West African Craton (WAC) to the west and the East Saharan craton to the east. The first stage (730–630 Ma; Cryogenian) involved the accretion of island and continental arcs to several cratons. The second stage (630–580 Ma, Ediacaran) was a collisional event, involving (1) northward escape of the Tuareg terranes between the two main cratons, along N-S mega-shear zones with up to 1000 km of lateral displacement; (2) emplacement of large high-K calc-alkaline (HKCA) linear batholiths resulting from mantle-crust interaction through linear lithospheric delamination along the mega-shear zones; and (3) concomitant high temperature-low pressure metamorphism. As a result, the small Eburnean cratons included in the Tuareg shield (In Ouzzal; Laouini-Azrou-n-Fad-Tefedest-Egere-Aleksod, or LATEA) were more or less reworked (metacratonization). The third stage (575–540 Ma, late Ediacaran) was limited to the eastern Hoggar province, and involved the intracratonic collision of terranes within the margin of the East Saharan metacraton (Murzukian event, Fezaa et al. 2010). Immediately following the Murzukian event (540–520 Ma, terminal Ediacaran-early Cambrian) was simultaneous reactivation of late (transtensional) mega-shear zones, intrusion of high-level granite plutons related to rare-metal mineralization, and inception of crustal-scale hydrothermal systems and gold mineralization. The granite plutons comprise a series of A-type granites that evolved towards F-rich (topaz-bearing) alaskites, and true peraluminous, F–Na–Li-rich rare-metal granites (RMGs), with evidence of mixing between the two lineages. However, only the RMG suites are associated with Sn–W quartz veins, whereas Ta-rich cupolas may be found either in the Taourirt lineage (Tim Mersoi and highly fractionated Rechla cupolas) or in the RMG suites (Ebelekan)—the rare-metal enrichment being, in any case, of magmatic origin. Emplacement of these late granites was controlled either by the mega-shear zones, as in the Iskel island arc terrane, or by secondary shears of various orientation (N10° E, N50° E, N140° E) that dissect the terranes, as in the LATEA metacraton. The quartz-gold deposits are of the “orogenic gold” class and display contrasting relationships with the shear-zone systems, from close spatial associations with mega-shear zones as in the shear-zone-hosted In Ouzzal deposits (Tirek-Amesmessa) and the Iskel showings, to the very distal association with the Raghane shear zone as in the Tiririne deposits, through the connexion with secondary N10° E shear zones of the In Abeggui deposits in the LATEA. In all of the deposits, the hosting quartz veins were plastically deformed prior to gold deposition, which was uniformly a very late event occurring under brittle conditions, typically during extension. Both gold lodes and RMGs were emplaced into evolving stress-strain fields, involving a rotation of the shortening direction (σ1) from ca. N100° E to N30–40° E and finally close to N-S. In addition to rare-metal and gold mineralization, minor fracturing, magma emplacement, and hydrothermal activity are associated with these orientations in the entire Tuareg shield, far to the west of the locus of the Murzukian collision. We therefore conclude that the terminal Ediacaran-early Cambrian Hoggar gold and rare-metal province was controlled, at all scales, by the latest transtensional reactivation of Pan-African mega- and second-order shear zones, following and prolonging the late Ediacaran Murzukian event. Both magmatism and hydrothermal circulation were triggered by the heat flux associated with a renewal of linear lithospheric delamination processes that accompanied this reactivation. All of the known gold and rare metal deposits are hosted by metacratonic terranes with an indisputable Eburnean basement, whereas the juvenile terranes are either very poorly endowed (Iskel) or apparently barren. Although the connection with the gold mineralization seems likely, the reasons remain obscure. It may, however, be suggested that gold endowment is ultimately linked to an influx of mantle-derived CO₂ and the formation of ultra-high temperature granulites in the Eburnean lower crust.