2013

Tawlah Specialized Alkaline Granite Prospect, Midyan

Region, Arabian Shield, Kingdom of Saudi Arabia:

Petrology. Structural Implications and REEs-RM

Characterization

Mohamed Th.S. Heikal* and Maher A. Ammawy**

*Geology Department, Faculty of Science, Tanta University, Tanta 31527, Egypt.

E-mail: Mohamed.heikal2010@yahoo.com

** Geology Department, Faculty of Science, Benha University, Egypt

The Tawlah albite granites-quartz synites is one of promising sites of REE- Ta-Nb- bearing alkaline granites in the Midyan Suite, NW Arabian Shield. The granite-syenite association is a highly lecucratic, albite-rich rocks with accessory columbite-tantalite, thorite, monazite, allanite, xenotime, zircon and cassiterite. Ages of 577± 4 Ma (Hedge, 1985) were obtained from isotopic ratios of Sr and Nd by 143Nd/144Nd and 87Sr/86Sr method. The precision is sufficient to indicate that the albite granite is post-orogenic with respect to Najd orogeny. The Tawlah granite is divided into abite-granite (in major) and quartz-syenite (in part). The albite granite is more highly mineralized, has higher modal albite contents and higher Nb/Ta ratios, both in whole surface rocks and in drill hole samples. On the structural point of view, the relationship between fault system and/or shear zones reported and mineral potential provides excellent and interesting evidences of its occurrence. Bearing in mind the Najd fault zone provides interesting evidences concerning the relationship between such faults and REEs-Nb-Ta-Th mineralization. Therefore, the distribution of mineralizations are closely related to the main trends; WNW-ESE, NW-SE and ENE. On the other hand, reactivation of NW Najd fault system has been affected by Teriary rift tectonics, leading to the development of mineralization in Tawlah alkaline granites. On the basis of petrographic, mineralogical signature and geochemical interpretations, it is suggested that the albite granite-quartz syenite of Jabal Tawlah has a magmatic origin indicating post-orogenic overprinting of A-type granites. By analogy with other Nb-Ta-REEs –bearing granites, the sodic bulk composition of Tawlah granite can be explained by fluorine enrichment in the magma, but much of the magmatic fluorine was lost upon crystallization because the low CaO, P2O5 and Al2O3 contents of melt precluded fixation of F in crystalline phases. In fact, the Tawlah alkaline granite prospect is considered to be a promising metallogenic site for REEs and Nb-Ta-Th, where its geologic environment, structural framework and tectonic setting are reasonable for achievement.

Keywords: Tawlah A-type granite, Arabian Shield, REEs-RM- F enrichment, Najd fault zone, metallogenic site.