Geological and geochemical studies in the Wadi Bidah District, Kingdom of Saudi Arabia

Abstract

Geological and geochemical followup studies of airborne electromagnetic anomalies in the Wadi Bidah district, southwestern Saudi Arabia, did not reveal metals of economic grade. Investigation of an anomaly enclosing the Rabathan ancient mine disclosed tightly folded and sheared Proterozoic tuffaceous rocks interlayered mostly with chert, dolomite, carbonaceous rocks, and volcanic wacke including cherty iron-manganese formations slightly anomalous in copper and zinc. Three drill holes placed to test anomalies within these formations yielded negative results. Studies of a long, narrow anomaly north of the Rabathan area indicated a similar geological environment. This northern area also contains limited zones that are highly anomalous in copper and zinc and extensive zones that are slightly anomalous in those metals. Drilling was not undertaken in this area. The Bilajimah airborne electromagnetic anomaly west of Wadi Bidah coincides with a broad synclinorium of layered felsic turfs and gossans. Geochemical studies indicated slightly anomalous copper, zinc, and silver values in gossans within the anomaly area. Two drill holes intersected carbonaceous rock that contained approximately 15 percent pyrrhotite and traces of sphalerite and chalcopyrite. Two geophysically anomalous areas west of Wadi Bidah surround ancient mines at Mahawiyah and Khayal al Masna'ah. Results of geochemical sampling at these workings were positive. An airborne electromagnetic anomaly located in the Assifar area in the southwestern corner of the Wadi Bidah district is underlain principally by metasedimentary rocks that include large linear zones of cherty iron-manganese formation and a few gossans .containing secondary base metal minerals. Detailed mapping and sampling of the Mulhal ancient mine, located west of Wadi Bidah, revealed two types of polymetallic gossans : (1) stratiform deposits interlayered with ignimbrites and mafic volcanic rocks and (2) barite-bearing gossanous material in shear zones that grade into hydrothermally altered shear zones and extend beyond the mine area. The gossans and gossanous shear zones contain anomalous amounts of gold, silver, lead, copper, zinc, barium, and selenium. Two gossans west of Wadi Bidah were mapped and sampled in detail; both gossans are interlayered, with siliceous volcanic rocks. Although the gossan at Jabal Mohr covers a large area, it contains low amounts of precious and base metals. The gossan at Mulhal No. 2 contains moderate to high amounts of gold, silver, copper, lead, and zinc.