Paleoweathering, Provenance and tectonic setting of gold-bearing neoproterozoic rocks from the Atacora Structural Unit, Pan-African Dahomeyide belt, northwestern B´enin

Abstract

The Atacora Structural Unit (ASU) is part of the Pan-African belt located in Northwestern B´enin Republic. It consists of metasedimentary rocks of greenschist facies and composed of quartzites, mica schists and chloritesericite- quartz schists. A petrochemical study of these rock types was undertaken, geared at inferring their paleoweathering, provenance and tectonic setting. Texturally, the rocks are, medium to coarse-grained with angular to rounded clasts. Detrital component statistics classified them as sublitharenite/quartzarenite. Geochemically, they can be classified as arkose/subarkose and sublitharenite rocks. The average values for PIA, CIA, and CIW for the quartzites, mica schists and schists range from 75% to 98%, reflecting an intensive degree of chemical weathering. The LREE enrichment (LaN/YbN = 9.56), negative europium anomaly (Eu/Eu* = 0.77), and fractionated HREE patterns of chondrite-normalized, are comparable to melts generated from crustal continental materials. The geochemical studies show that the studied rocks were sourced from ancient quartzose sedimentary rocks. Trace element ratios Cr/Th, Zr/Sc, Th/Sc, Th/Co and La/Th and rare earth elements of the source components indicate that these mature materials were chiefly derived from felsic rocks, though the involvement of basic rocks (basalts) cannot be ruled out. Chondrite-normalized REE patterns indicate that the metasedimentary rocks are sourced from pre-existing sedimentary rocks in the ASU, which rocks were mainly derived from the Dahomeyide basement and probably from the Amazonia craton. The major and trace elements discriminant diagrams suggest these metasediments were deposited in a passive-margin setting, though continental island arc setting signature was also identified.