Greisen and pseudo-greisen in the Tamanrasset area (Central Hoggar, Algeria): Petrography, geochemistry and insight on the fluids origin from mica chemistry

Abstract

In the Tamanrasset area of the Pan-African Tuareg Shield, some of the late Pan-African intrusions collectively known as the Taourirt granites are associated with tin and tungsten prospects of both the quartz-vein and greisen types, with a variety of greisen styles. The mineralized intrusions are albite-topaz-protolithionite granites and microgranites. In the present work, we combine new observations and mica analyses with a reinterpretation of existing wall-rock geochemical data in order to assess the possible role of both magmatic-hydrothermal and external fluids in the greisenizing process at Tamanrasset.Greisens at Tamanrasset appear to result from two process: the first is acid leaching forming quartz-topaz and quartz-rich greisens, followed by an episyenitization which is characterized by quartz dissolution and replacements by Li-rich micas, leading to mica-rich greisens (that could better be named “pseudo-greisens”). Distinct trends recorded by the mica compositions in the granitic rocks and the greisens allow to disentangle the involvement of magmatic-hydrothermal fluids, on the one hand, and of mainly metamorphic external fluids, on the other hand, in the greisenizing process. The latter fluids were CO2-rich, and could have contributed to the source of the acidity.