Neoproterozoic shearing and auriferous hydrothermalism along the lithospheric N–S East In Ouzzal shear zone (Western Hoggar, Algeria, North Africa)

Abstract

The East In Ouzzal shear zone (EIOSZ) is a dextral N–S lithospheric lineament that contains a 100 km×5 km gold vein-type district including the two worldclass Amesmessa and Tirek goldfields. The 40Ar/ 39Ar laser-probe dating of ferrotschermakite and biotite from both goldfields confirms the Neoproterozoic age (627–613 Ma) of major syn- to late collisional ductile deformation. Gold-rich (>20 ppm) quartz veins at Amesmessa and at Tirek are oriented N–S and mostly confined to the longitudinal shear corridors. Later veins were diachronously emplaced under a ductile/brittle shear regime. Hydrothermal muscovite from the altered country rocks of a N–S gold-quartz-bearing vein is older at Amesmessa (611 Ma) than at Tirek (575 Ma). At Amesmessa, a second set of E–W-trending, undeformed and gold-poor (<5 ppm) quartz veins lies in the eastern mylonitic part of the EIOSZ. Dating hydrothermal muscovite from the altered country rocks of an E–W undeformed quartz vein suggests that ductile deformation had at least ended by 577 Ma at Amesmessa, whereas shearing was still active at Tirek. Primary gold deposition in the N–S quartz veins occurred during the earliest hydrothermal event. This primary gold was bound to the crystal lattice of hydrothermal pyrites which contain up to 24 ppm Au. However, the economic gold ores are younger than 611–575 Ma which is the timing for deposition of the hydrothermal muscovites from the N–S gold-rich quartz veins at Amesmessa and Tirek. Gold concentration could be related to a late Neoproterozoic reactivation of the shear zone.