Understanding Critical Mineral Enrichment in the Mineralized Al-Ghurayyah Granite Pluton and Associated Pegmatites, NW Saudi Arabia: Insight on Mineral Exploration in the Arabian Shield

The Arabian-Nubian Shield (ANS) comprises the largest tract of juvenile, mantle-derived continental crust on Earth. ANS granitoids thus record the major development and evolution of this juvenile Neoproterozoic continental crust and extensive distribution of A-type granitoids. Whereas 900 – 630 Ma I-type granitoids dominate the early plutonic evolution of the ANS, within-plate A-type granitoids dominate 630 –570 Ma plutons and provide a record of the final collision between eastern and western Gondwana. Some transitional A/I-type granitoids are highly mineralized in HFSEs and REEs. The Al - Ghurayyah granitoid complex represents a small but highly mineralized pluton exposed in the Midyan terrane of NW Saudi Arabia, the easternmost of a belt of similar bodies in Egypt (Stern et al. in press). Four components of the intrusion are identified: Arfvedsonite granite, amphibole-free porphyritic coarse and medium-grained granite, and microgranite. Peripheral melt-rock interaction generated albitite and marginal units adjacent to host Neoproterozoic metavolcanics. The granitoids are highly fractionated Fe-rich metaluminous to peraluminous A-type, suggesting within-plate affinity. Chondrite and primitive mantle-normalized patterns are similar for granite components. Abundances of HFSE and REE from the different components suggest comagmatic evolution with extreme HREE fractionation (La/YbN=0.034 – 1.752) and negative Eu anomalies (Eu/Eu*=0.046 – 0.375). U-Pb SHRIMP dating of zircons provides an age of 647 ± 9.3 Ma, in the range of 692-628 mineralized A/I-type granites in Egypt. Average ∑REEs from the main intrusion is 1,162 µg/g, with a maximum value of 1,917 µg/g housed within zircons and other accessory phases of the marginal unit. Nb and Ta mineralization (mean of 303 µg/g Nb, 25 µg/g Ta) is strongly correlated (R2=0.74), concentrated in columbite. Tungsten (W) concentration increases towards the marginal unit. Granite REEs tetrad effects (TE1,3) are from 1.25 – 1.67 and display a weak negative correlation with Zr/Hf (R2=0.08) and Y/Ho (R2=0.14), suggesting primary enrichment as a result of extreme fractionation with subsequent fluid–rock interaction. Three types of pegmatites are documented: (1) northern pegmatites (type 1), emplaced into metavolcanics, exhibit complex mineral zonation and highly elevated ∑REEs (436 – 13,115 µg/g) and Li (13–3,010 µg/g), but low concentration of U (2.82 – 94.3 µg/g) and Th (25.3 – 600 µg/g); (2) southwestern pegmatites (type 2) emplaced into microgranite exhibits simple mineral zonation with anomalous abundance of zircon (up to 10% of the mode), high ∑REEs (up to 6,000 µg/g), and extremely high radioactive element abundances  (1,160 µg/g Th and 348 µg/g U). The main HFSEs- and REEs-bearing phases for type 1 and 2 are zircon, pyrochlore, columbite, rutile, xenotime, and cassiterite; (3) southeastern non-zoned pegmatites (type 3) emplaced into the contact of arfvedsonite bearing granite, with low ∑REEs and radioactive elements, but abundances of Li (up to 1,775 µg/g) housed within the mica. We conclude the critical mineral enrichment is primarily magmatic controlled by F- ligands in the melt for the granite. Different types of pegmatite are attributed to different processes and conditions.  Exploration for rare metal mineral deposits should concentrate along and around the reconstructed E-W trending Egypt-Arabia 630-690 Ma mineralized granite belt.