Reconnaissance geology of the Precambrian rocks in the Wadi Mahraghah quadrangle, Kingdom of Saudi Arabia

The Wadi Mahraghah quadrangle covers an area of 2863 sq km at the eastern edge of the Precambrian Shield in Saudi Arabia. Precambrian rocks are exposed in the western quarter of the quadrangle. To the east the Precambrian rocks are overlain by limestone of Permian age and by Quaternary sand and gravel. The Precambrian rocks consist of granite intruded by a swarm of andesite dikes. These igneous rocks are overlain by sedimentary formations of the Precambrian Murdama Group comprising conglomerate of the Z'reiba Formation which grades upward into graywacke of the Abt Formation. Rhyolite dikes intrude the Abt. Possibly owing to these rhyolite dikes, or possibly as a result of weak mineralization along west-northwest- and east-trending faults related to the Najd fault zone, the area underlain by graywacke and conglomerate is characterized by threshold or anomalous amounts of beryllium in samples of wadi sand. Samples of sand from granitic areas contain threshold amounts of barium and lanthanum, but none is anomalous, no ancient mine nor other megascoplc evidence of mineralization was found; however,