Mineralization associated with felsic plutonic rocks in the Arabian Shield

Abstract

Mineralization genetically related to a crystallizing felsic pluton is generally emplaced in its apical or marginal zones, or in satellite dikes, sills and stocks. In the Arabian Shield, as elsewhere, the style and nature of mineralization is primarily controlled by the composition and form of the host intrusion. Five types of rare-element mineralization have been recognized: (1) Deposits of Nb, Zr, Y, rare-earth elements (REE), Sn, Ta, U, Th and F are related to alkali granites and their hydrothermal derivatives. Mineralization is generally disseminated in porphyritic microgranite stocks (e.g. Ghurayyah and Umm al Birak) or concentrated in apical or marginal, layered pegmatite—aplite complexes (e.g. Jabal Sa'id and Jabal Sumr al Ishar). Veins containing fluorite and galena (e.g. Jabal Hadb ad Dayahin) are also known. (2) Deposits of Sn, W, Ta, Pb, Zn, Bi, Ag, Mo, Be and F are related to muscovite-bearing, low-Ca granites and their hydrothermal derivatives. Mineralization generally occurs in quartz veins and stockworks, or in the greisenized apical or marginal portions of stocks, cupolas or sheets. The most important deposits of this type are the Ba'id al Jimalah wolframite-bearing stockwork and the cassiterite deposit at Jabal as Silsilah. (3) Deposits of Mo, W, Sn, Bi, Cu, Pb, Zn, Ag and As are related to biotite-bearing high-Ca granites and granodiorites and their hydrothermal derivatives. Mineralization generally occurs in quartz-vein swarms in the apical and marginal zones of the plutons, as at Bir Tawilah (W Mo Sn), Jabal Thaaban (Mo W Bi) and Wadi Sidarah (Mo Bi Ag). Veins containing Pb, Zn and Ag occur at Samrah. (4) Deposits of Au, Ag, Cu, Pb and Zn, and contact replacement deposits of Fe and Fe Cu, are related to rocks of dioritic, tonalitic or granodioritic composition. Two main types of vein deposit can be distinguished: high-temperature epithermal veins and vein swarms containing combinations of Au, Ag, Te, As, Mo, Cu, Bi (e.g. Ad Duwayh); and low-temperature Au—quartz veins. Magnetite and cupriferous magnetite vein and replacement deposits occur in the contact zones of several diorite—granodiorite complexes (e.g. Methgal and Jabal Idsas). (5) Deposits of Nb, Zr, REE, F (and minor Cu, Pb, Zn, Ag) are associated with low-Ca syenitic rocks. The most important deposits are the Ablah fluorite pipe, a pegmatite—aplite—intrusion breccia complex in a ring-structured quartz syenite to granite pluton, and the Jabal Hamra REE-mineralized silexite stock. Magnetite contact replacement deposits are associated with the Jabal Dhur at Kabir syenite. Twenty-two mineralized districts and provinces within the Arabian Shield are provisionally defined. Within these, mineralization is closely linked to felsic plutonism. All significant Nb Zr—REE mineralization related to alkali granite is confined to the northwestern portion of the Shield, whereas significant Sn (cassiterite) and W (wolframite) mineralization is confined to the east. The eastern Shield is metallogenically zoned in a manner similar in scale and character to that of the Andean Cordillera. Such metal-distribution patterns probably reflect differences in the composition and structure of the underlying crust.