Pb and Sr Isotopic Study of Mineralization at Mahd Adh Dhahab and Nuqrah, Arabian Shield

Abstract

Lead extracted from 5 samples of galena from the Au-Agbase metal vein deposit of Mahd adh Dhahab shows a tightly clustered isotopic composition with an average two-stage Pb-Pb model age of 685 Ma. Vse of the Tatsumoto group of constants yields an average age of 766 Ma. A minimum age of769::!: 5 had previously been established by V-Pb dating of zircon separated from the Ramram granite intruding the Mahd Group, which hosts the mineralization. The obtained Pb-Pb dates are, however, older than a Rb-Sr isochron date of 645 ::!: 19 Ma obtained on a porphyritic rhyolite plug which also intrudes the Mahd Group. This relationship ascertains the noninvolvement of the rhyolite in the ore formation. With the exception of the Mahd adh Dhahab deposit, all dated vein-type deposits in the Shield have Pb-Pb dates that range between 550 and 450 Ma. Lead extracted from 3 samples of the stratiform massive sulfide deposit of An Nuqrah prospect also has a uniform isotopic composition similar to that of Mahd adh Dhahab. It yields an average Pb-Pb model date of 685 Ma, with a possibility of being too young by 30-50 Ma. A Rb/Sr isochron age on Asfar granite batholith emplaced into Hulayfah Group rocks hosting the mineralization also gave a 685 Ma date. The deformation effects of the batholith on the ore minerals implies that the granite intrusion occurred shortly after the deposition of the Hulayfah Group volcanosedimentary rocks, and their contemporaneous massive sulfides. Common lead ages obtained are similar to ages obtained previously on other massive sulfide deposits of the Arabian Shield. Application of the plumbotectonics model indicates evolution of mineralization in both areas within an island arc environment. However, the similarity of Pb isotope compositions between An-Nuqrah and Mahd adh Dhahab does suggest that the source regions of lead in the two areas have remarkably similar isotopic composition.