Highly evolved rare-metal bearing granite overprinted by alkali metasomatism in the Arabian Shield: A case study from the Jabal Tawlah granites

Abstract

Jabal Tawlah granites represent a post-collisional composite stock of late Ediacaran age, exposed in the Midyan district of northwest Arabian Shield. The stock is extended in the northwest and tapers southeastward due to structural control by a NW-SE-striking fault, and a southern master fault exhibits dextral movement by an ENE-WSW fault trend. Two main phases of magmatic activity are recognized in the Jabal Tawlah stock. The early phase (rim) includes albite granite and quartz syenite, which was intruded by a late phase of microgranite. Most of the Tawlah granites are highly mineralized, which can be attributed to the Najd fault system and/or shear zones. The mineralization is represented by small fluorite veins and disseminated minerals such as columbite-tantalite, xenotime, thorite, cassiterite, fergusonite, pyrochlore, allanite, monazite, samarskite, zinnwaldite and bastnaesite. The Tawlah granites show some degree of alteration including silicification and albitization. Textural and geochemical data indicate that the Tawlah granites developed from a magmatic source, and were then overprinted by later sodic metasomatism. Ore minerals crystallized under both magmatic and hydrothermal conditions. The parent magma of Tawlah stock was generated through partial melting of a juvenile continental crust, followed by extensive fractional crystallization. The Tawlah granites should be considered as a potential resource to warrant exploration for REEs and other rare metals.