Geochemistry, Petrogenesis and Alteration of Rare-Metal-Bearing Granitoids and Mineralized Silexite of the Al-Ghurayyah Stock, Arabian Shield, Saudi Arabia

Abstract

New data are presented for the rare-metal bearing A-type granitoids of the Al-Ghurayyah stock in the northwestern segment of the Arabian Shield, a composite pluton intruding metamorphosed volcano-sedimentary successions of the Silasia Formation. Metals in the granitoids are variably enriched, with up to 1 990 µg/g Zn, 7 680 µg/g Zr, 2 316 µg/g Nb, 232 µg/g Ta, 485 µg/g Hf, 670 µg/g Th, 137 µg/g U and 1 647 µg/g total rare earth elements (REE). The silexite is highly mineralized and yields higher maximum concentrations of several metals than the granitoids, including up to 1 860 µg/g Y, 9 400 µg/g Zr, 878 µg/g Hf, 1 000 µg/g Th, and 2 029 µg/g total REE. The Al-Ghurayyah stock has been assigned to an intraplate setting. Lithospheric delamination led to generation of mantle melts that supplied heat to melt the juvenile crust of the ANS. The fluorine and rare-metal enriched parental magma evolved by fractional crystallization. The quartz-rich silexite, distinct in character from ordinary hydrothermal vein quartz, is inferred to be co-genetic with the granitoids on the basis of their similar REE patterns; it is interpreted as a small volume of residual magma enriched in SiO₂, volatiles, and trace metals. Mineralization took place both at the magmatic stage and later during a hydrothermal stage that concentrated these elements to economic grades.

Keywords

Arabian Shield / Al-Ghurayyah / rare metals / silexite / peralkaline granite / hydrothermal metasomatism

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