Geochemistry of gold-bearing metamorphic rocks of the Natitingou area, Atacora structural unit, Northwestern Bénin (West Africa): Implications for Au genesis

Abstract

The Natitingou area within the Atacora Structural Unit (ASU) of the Pan-African Dahomeyide orogenic belt, northwestern Bénin, is characterized by the monocyclic metasedimentary formations, amphibolites and orthogneisses which belong to the Internal zone of the belt. Representative samples of these different gold-bearing rocks have been analyzed geochemically to determine their petrogenesis, mode of emplacement, tectonic setting and role in the genesis of gold mineralization in the area. The orthogneisses are subalkaline, peraluminous and emplaced in a syn-collisional arc setting while the amphibolites are metaluminous, tholeiitic with an E-MORB affinity and emplaced in a rifted lithospheric setting. The metasedimentary rocks, comprising quartzites and mica schists are cross-cut by auriferous quartz veins. The metasedimentary rocks are recycled sediments varying from wackes to sublitharenite in composition and deposited in a passive margin setting. The quartz veins similar to the metasedimentary rocks are enriched in LREE, depleted in HREE and showed a negative Eu-anomaly (0.31–0.72). Their chondrite-normalised REE patterns deciphered them as Upper Continental Crustal materials which were probably derived from the reworking of their host metasedimentary rocks. Gold concentration in quartzites, amphibolites, mica schists, quartz veins and orthogneiss were 0.004–9.587, 0.26–1.76, 0.0005–0.464, 0.023–0.147 and 0.0−0.0005 ppm, respectively. Devolatilization process of the country-rocks, is the most probable mechanism for the ore-forming fluid, which could have been generated during the Pan-African Dahomeyide event. Remobilisation of gold and associated metal (As, W, Cu, Mo, Sb, and Sn) after the Pan-African Dahomeyide orogeny is suggested for enrichment of gold along shear and fault zones of the Natitingou area.