Geological and geophysical investigations to delineate the subsurface extension and the geological setting of Al Ji'lani layered intrusion and its mineralization potentiality, Ad Dawadimi District, Kingdom of Saudi Arabia

In the present study, Al Ji'lani layered intrusion was subjected to integrated field, petrographic, processing of ASTER data, and geophysical investigations to delineate its subsurface extension and to determine the chronological order of the exposed rocks. The intrusion is surrounded by foliated granodiorite and both were intruded by younger granite. Processing of ASTER data revealed that the intrusion incorporated foliated granodiorite masses along its NE corner indicating its younger age (postorogenic) setting contrary to what have been proposed by previous authors. Also, this is further confirmed by the presence of an offshoot from the intrusion in the South-East corner as well as freshness and undeformed nature of the gabbroic rocks. Petrographically, the gabbroic rocks are characterized by the presence of kelyphytic coronas around olivine in contact with plagioclase, magnetite-orthopyroxene symplectites after olivine, and symplectites between plagioclase and magnetite/ilmenite. These textures are explained in terms of interaction with late deuteric magmatic fluids and not to metamorphism as believed before. The extensive geophysical analyses of the Al Ji'lani prospect using aeromagnetic data suggest complicated combination of magnetic bodies composed mainly of gabbroic rocks intruding the foliated granodiorite with variable magnetic susceptibilities. Gradient analysis, tilt angle and edge detection techniques extracted the shallow subsurface magnetic boundaries and a probable multiple bodies in the subsurface are detected. The 3-dimensional constraint inversion using parametrized trust region algorithm revealed the deep subsurface distribution of magnetic susceptibilities of the bodies. Two resolved bodies are clear, a northern more shallow body, and a southern, deeper and laterally extend to the south and southwest. The calculated volume from the inverted model representing the Al Ji'lani layered intrusion is approximately 518.7 km(3) as calculated to 6.0 km depth. The body could be extended to a deeper depth if a different proposed model geometry is adjusted. The surface area of the exposed body is only 42.39 km(2). Several magnetic anomalies are defined within the intrusion and are considered potential sites of mineralization. The south east corner of the gabbroic intrusion is traversed by a shear zone trending ENE-WSW which hosts sulfide-bearing quartz veins with high silver content (Samrah Prospect) associated with an offshoot from the layered gabbro. The shear zone should be followed to the west where the intrusion extends for a distance of about 10 km in the subsurface to the southwest of the exposed part of the intrusion.