Petrology, geochemistry and REE mineralization of sodalite syenites in the Tijirit gold district, Archean shield of the Reguibat ridge (Mauritania)

Abstract

The blue sodalite syenites of Tijirit are unusual magmatic rocks, located in the Archean part of the Reguibat ridge within the Ahmeyim greenstone belt in a NS to NNE-SSW trending shear corridor. They form two distinct intrusive bodies, one light-colored and the other dark-oxidized. In view of the lack of previous work, the aim of this project is to specify the main petrological and geochemical characteristics of these particular rocks, and to define their relationship with gold mineralization, with a view to proposing regional prospecting guides. A detailed mineralogical investigation of the syenite silicate assemblages and their mineralizations was undertaken using scanning electron microscopy (SEM), and their major and trace element compositions were analyzed by ICP- AES and ICP- MS. These syenites consist of K-feldspar, albite and sodalite with a secondary assemblage of natrolite and cancrinite. Accessory biotite (annite) is replaced by aegirine and fluorite. Zircon, pyrochlore, Nb-Ta-U-bearing phases and monazite form the mineralized aggregates with fluorite, rare-earth silicates and fluoro-carbonates. The light syenite displays zoned pegmatoid pockets, whereas the dark syenite is characterized by abundant phases of iron and titanium oxides (hematite, ilmenite, Nb-rutile) and carbonates (calcite and Sr-REE carbonates). Tijirit syenites are miaskitic, close to agpaitic, with an average (Na + K)/Al mol.% of 0.96. Our samples reveal high levels of mineralization in REE, Nb and F, with most REE contained in zircon, pyrochlore (Ce-pyrochlore), fluorocarbonates, bastnaesite and monazite, which indicate their affinity to NYF-type magmatism. Geochemical analysis shows OIB-type affinities from a mantle source that generated a Na-enriched residual liquid under fluorine- and chlorine-enriched active fluid conditions for the light-colored syenite and a less evolved term for the oxidized syenite. The blue syenites were emplaced at a deep lineament that allowed the circulation of mixtures of possible mantle fluids responsible for the gold mineralization in the shear corridor of the Ahmeyim greenstone belt.