Petrology of oolitic ironstones of Ashumaysi Formation of Wadi Fatima, western Arabian Shield, Saudi Arabia

Abstract

The present study concerns Oligo–Miocene oolitic ironstones of the Ashumaysi Formation between Jeddah and Makkah in the western part of the Arabian Shield, western Saudi Arabia. The Ashumaysi Formation contains oolitic iron ore as 2–3 m thick beds. The formation is exposed along the western margin of Wadi Ashumaysi. It dips toward the east and unconformably overlies Precambrian rock and conformably underlies a Miocene basalt flow of the Sita Formation.The Ashumaysi ironstones are mainly hematitic sandstone and oolitic ironstone. The oolitic ironstones vary in color (reddish, brownish, yellowish, or violet). Petrographic analysis indicated that the ironstones are composed of different shapes of ooids (ooliths), pelloids, and rarely pisoids (pisoliths) within ferruginous cement and matrix, which consists of clay minerals with fine to medium grained quartz XRD with EDX analysis of representative samples indicated that hematite, goethite, chamosite, and quartz are the main components of the oolitic ironstone.The main processes responsible for the formation of these oolitic ironstones are progressive diagenetic dehydration and recrystallization of the amorphous ferrous hydroxides and the formation of oolitic and pisolitic goethite and hematite within different fabrics.