Hamama polymetallic Au prospect in the Egyptian part of the Arabian Nubian Shield: How a single occurrence can improve the knowledge of a gigantic metallogenic province.

The Arabian Nubian Shield is a massive juvenile accretionary orogenic belt formed due to the assembly of Gondwana. It is exposed along the margins of the Red Sea and hosts hundreds of gold prospects as well as a limited number of operating gold mines. Until recently, the widespread occurrence of gold across the shield was thought to be exclusively the result of Neoproterozoic Gondwanan tectonics and related ore-forming processes. This proposal suggests that the shield contains gold mainly in older volcanogenic massive sulfide (VMS) deposits and younger orogenic gold deposits. This widely accepted proposal complies with the geological history of the shield, but it remains unproven, given that most prospects are still understudied, with only a handful having ore-stage geochronological constraints.

The Hamama polymetallic gold prospect in Egypt hosts a Zn-Pb rich stratabound ore with inferred gold and silver resources of 230 koz and 7836 koz, respectively. The prospect area is mainly covered by submarine metavolcanics-metavolcaniclastics assemblage, and the ore has been long classified as a syngenetic Au-bearing VMS based on the general geologic setting and old literature data. Interestingly, modern exploration activities confirmed the absence of massive sulfide lenses at depth and elucidated that the ore in Hamama is hosted exclusively in an oxidized horizon composed essentially of  carbonates, silica and barite. The oxidized gossan cap extends between the metavolcanics-metavolcaniclastics for 3.2 km in a NE-ENE direction. Boreholes penetrate the oxidized cap usually to ~30 m depth, beneath which the unoxidized host rock occurs. The mineralized host, as whole, is intersected in the boreholes at an average depth of 120 m. It is intensely brecciated and fractured, and has been previously described as an exhalite or a carbonatized felsic volcanic rock.

Our detailed petrographic study on deep drill cores retrieved from two representative diamond drillholes supported by frequent field campaigns reveals that the mineralized horizon is a dolomitic formation representing the base of the sedimentary cover in the region, which is reported for the first time in this study. Based on its fossil content and local paleogeography, we reinterpret this ore-bearing formation as a part of the Late Cenomanian-Early Turonian carbonate platform of NE Africa deposited in a paleovalley between the Neoproterozoic basement. The structural complexity of the shield, the thick oxidized cap, and the diagenetic and hydrothermal processes played a major role in the previous misinterpretation of the host rock. This finding refutes the Neoproterozoic VMS deposit model in Hamama area and introduces a new sedimentary formation of economic potential to the Egyptian stratigraphic sequence, which we call Abu Garida dolostone. As a starting point, this study provides the first evidence for the presence of significant Late Phanerozoic gold inside the shield. Finally, this interesting case study elucidates that the history of gold precipitation across the shield is more complex than previously thought.