Evidence of an Archean-Paleoproterozoic micro-continent remobilized during the Neoproterozoic from the western Hoggar (Tuareg Shield, Algeria): Geodynamic evolution and implication on gold mineralization

The western boundary of the Tirek terrane in the Hoggar contains the two largest gold deposits in Algeria, namely the Amesmessa and Tirek gold fields. New geological mapping of this area and results of a combined petrological, geochemical and geochronological study (U-Pb zircon geochronological (LA-)ICP-MS method) reveals a Paleoproterozoic TTG basement, dated at 1965 ± 18 Ma (inherited zircons from 2771 to 2006 Ma), overlain by a supracrustal unit made up of quartzites and metapelites. Detrital zircons from metapelites yielded ages ranging from Archaean to Paleoproterozoic (2710 Ma–2050 Ma). Furthermore, sills and dykes of sub-alkaline orthogneisses that intrude the previous lithologies have been dated at 1843 ± 19 Ma. On the other hand, the migmatitic granitic-granodioritic batholith, which covers 50 % of the studied area and intrudes all units, has a subduction magmatism affinity and is dated at 663 ± 4 Ma.

The study area was affected by a HT-LP metamorphic overprint during a clockwise P-T evolution with a peak conditions around 800–850 °C at 5 kbar followed by a decompression, then a cooling reaching the P-T conditions of 600 °C and 3 kbar. Uranium-Pb dating of monazite yields an age of 578 ± 5 Ma, which is interpreted to reflect the time of peak metamorphic overprint synchronous with the emplacement of synkinematic granites along shear zones with transcurrent movement. During this event, the western border of the studied area, the East Ouzzalian Shear zone, was the site of magmatic intrusions as well as the circulation of various fluids. This promoted an important fluid/rock interaction event, leading to extraction of gold by fluid percolation through the rocks and its deposition along N-S/NE-SW quartz vein ore shoots. The comparison with the western terranes of the Tuareg Shield (In Ouzzal, Iforas, Tassendjanet, and Kidal terranes) suggests that Tirek and these terranes represent relics of a single Archean-Paleoproterozoic continent. Currently, the Archean lower crust cross out in the central part of this micro-continent. The latter is mainly affected by Paleoproterozoic granulite facies metamorphism, unlike the Paleoproterozoic middle crust, which is exposed on either side and was strongly remobilized during the Pan-African orogeny (850–570 Ma), allowing gold mineralization during the later stages of this event.