Reconnaissance geology of the Bi'r Al Badriyah quadrangle, Kingdom of Saudi Arabia

Abstract

The Bi'r al Badriyah quadrangle covers an area of 2843 sq km in the extreme eastern part of the Precambrian Shield in central Saudi Arabia. The Precambrian rocks in the southeastern part of the area are unconformably overlain by limestone of Permian age, which occupies only a small part of the quadrangle. Three great sequences of Precambrian rocks are recognized and called, from oldest to youngest, the Halaban Group, the Bi'r Khountina Group, and the Murdama Group. From evidence within the quadrangle itself the three groups are seen to be separated by erosional unconformities, and the Halaban and Bi'r Khountina Groups are intruded by granitic and gabbroic plutonic rocks and a wide variety of dikes. Exposures north of the quadrangle show that the Murdama Group is also intruded by granitic rocks. The Halaban Group, consists of three formations called, from oldest to youngest, the Umm Mushraha Formation, the Jebal al Egfool Formation, and the Wadi al Jifr Formation. These rocks are variably metamorphosed, but characteristically they are at the epidote-albite amphibolite facies of regional metamorphism, are polymetamorphic, and rest unconformably on ancient granite gneiss. The Umm Mushraha Formation consists of amphibolite, schistose andesite, and greenstone associated with minor meta-agglomerate, meta-graywacke, and marble. The Jebal al Egfool Formation consists of sheared and metamorphosed volcanic rocks of intermediate composition, and the Wadi al Jifr Formation is made up of metamorphosed felsic volcanic rocks. Rocks of the Bi'r Khountina Group are rather similar to the Halaban Group in original composition, but they are separated from the Halaban by an angular unconformity and have distinctive formations of conglomerate (the Idsas Formation), marble (the Fawara Formation), and graywacke (the Abu Sawarir Formation) at the base. Most of the Bi'r Khountina Group consists of andesitic volcanic rocks in the Badriyah Formation. Mostly, the Bi'r Khountina Group is unmetamorphosed or feebly metamorphosed to the greenschist facies. Locally, as in the aureole of intrusive rocks, the Bi'r Khountina Group reaches higher metamorphic grades. The rocks of the Murdama Group unconformably overlie the Bi'r Khountina Group, and basal conglomerates of the Murdama contain fragments of distinctive rocks intrusive into the Bi'r Khountina. The Murdama Group is composed of the Z'relba Formation, Consisting of conglomerate and schistose conglomerate and the Abt Formation, a thick sequence of laminated, mainly fine-grained, sedimentary rocks. Metamorphism in .the Murdama increases toward the north, and along the north edge of the area the Murdama Group is represented by chlorite-sericite schists. Three major episodes of folding are recorded in the Precambrian rocks. The earliest episode folded the rocks of the Halaban Group along north-trending axes which are now truncated by the unconformity at the base of the Bi'r Khountina Group. After the rocks of the Bi'r Khountina and Murdama Groups were deposited, the three groups were folded along northwest-trending axes of regional scale. Later, possibly at a time of major faulting along a northwesterly direction, the rocks were again folded on north-trending axes. Samples of rock materials collected at 105 localities in the quadrangle were analyzed for 27 elements. Only one anomalous value was found: a low positive anomaly for lead at the Umm Mushraha gold mine. Threshold amounts of Ag, B, Cr, Cu, La, Mn, Mo, NI, Sc, Sn, Ti, V, and Zr were detected. The distribution of copper and molybdenum suggests that two plutons of biotite-hornblende granite in the central part of the quadrangle, and the rocks between them, should be examined for possible disseminated copper and molybdenum of the porphyry type. The distribution of scandium and detrital scheelite suggests that a zone adjacent to phyllonite in the northeastern quarter of the quadrangle be examined for tungsten associated with amphibolite.