Jabal Hamra REE-mineralized silexite, Hijaz region, Kingdom of Saudi Arabia

The Jabal Hamra silexite, a crescent-shaped stock 300 m long by 100 m wide, averages over 6000 ppm combined REE and is the Kingdom's highest-grade resource of these elements. It is anomalously radioactive (total-count gamma radiation 1000–3000 cps), has high average contents of U (75 ppm) and Th (263 ppm) and is also enriched in Nb, Zr, Y, Sn and Ta.

The silexite crystallized as a pressure-quenched rock resembling aplite, and was subsequently pervasively cataclased. It was derived by differentiation of a quartz alkali-feldspar syenite magma. Petrologic continuity can be demonstrated from quartz alkali-feldspar syenite through leucocratic and amphibole-bearing alkali-feldspar granite to silexite. Although the geochemical signature of the mineralization resembles that of mineralized Arabian alkali granites, the nature of the associated rocks and therefore the genesis of the deposit are significantly different.