Tourmaline associated with orogenic gold occurrences in the Silet terrane, South Algeria: Evolution of composition and redox state during hydrothermal crystallization

In the Silet terrane (western Hoggar, southern Algeria), gold occurs in association with tourmaline in quartz veins. In the Assouf Mellen, Seldrar, and Idreksi occurrences, gold-bearing veins cut diorite and granodiorite complexes of the pre-orogenic Tonalite-Trondhjemite-Granodiorite group (868–840 Ma). These veins trend dominantly in the N-S, NE-SW, and NW-SE directions, and are preferentially developed at the intersection zones of secondary NE-SW and NW-SE fault splays off the main 4°30′ fault zone (Iskel Shear Zone). Free gold is present as inclusions disseminated in quartz and tourmaline, and as fissure-fillings in deformed tourmaline crystals. Gold was also found as inclusions in chalcopyrite and in iron oxides.

Based on the mineral chemistry, the tourmaline belongs to the alkali group and represents the schorl–dravite and foitite–oxy-foitite series and shows evolution trends to bosiite/povondraite; tourmaline components from the calcic group (lucchesiite, magnesio-lucchesiite) are minor only. Tourmaline in Silet was likely formed as Al-deficient Fe³⁺-enriched schorl (Tur I), later recrystallized (Tur II) and enriched in Mg and Al with higher X-site vacancy (Mg-rich schorl to Fe-rich dravite). In the Seldrar occurrences (middle part of Silet terrane), gold is associated with tourmaline with high initial Fe³⁺-contents (Tur I) while the assemblage of the later Tur II is barren. The evolution towards Fe³⁺-rich schorl may be explained as a result of more oxidizing conditions in the late stage of formation of the mineralized quartz veins. Part of gold was remobilized by low-temperature weathering processes.