Ma’aden Gold and Base Metals company (MGBM) is a wholly owned subsidiary of Maaden Saudi Arabian Mining Co. (Ma’aden), operates five gold mines, i.e. Mahd Ad-Dahab, Al-Amar, Bulghah, As-Suq and Ad-Duwayhi and a processing plant at Sukhaybarat.
- subscription is required.
Summary:
Geology.
The Bulghah deposit lies within Pre-Cambrian rocks of the Arabian Shield, located in the western part of the Arabian Peninsula. Gold mineralisation is predominantly hosted by mesothermal gold veins, often associated with a 680Ma to 640Ma orogeny that amalgamated various terranes into the Arabian Shield. Gold is also associated with a 615Ma phase of diorite-granodiorite magmatism in the north-eastern Shield, and post 640Ma strike-slip faulting in the Najd fault system.
Gold mineralisation at Bulghah is hosted by a tonalite/diorite intrusion, which strikes roughly north-south. The tonalite/diorite intrusion is fault/shear bounded on the east and west sides, with the contacts dipping roughly 60° towards the east. The intrusion ranges from 200m to 500m wide, narrowing towards the south, and extends to the north and south for approximately 1,000m with a depth of about 450m. The intrusion is bounded to the east by meta-volcanics, primarily andesite, and by meta- sediments to the west. The meta-sediments include a distinct marble unit. The intrusion grades from tonalite to diorite in the southeast and northern parts of the project area. The diorite has a coarser texture, with hornblende and plagioclase phenocrysts in a light coloured matrix. The intrusion is cross- cut by numerous aplite dykes, which are sub- horizontal, and frequently have irregular geometries. The aplite dykes also occur on or near the contacts between the country rock and the intrusion.
The area is also structurally complex, with three phases of deformation identified in the area. The first phase was east-west extension, resulting in north-south trending, steep to moderate cataclastic faults, which are thought to have acted as the conduits for mineralising fluids. This was followed by an east-west compressional tectonic event, resulting in north-south trending thrust faults that resulted in displacement of mineralisation by shallow dipping north-south striking thrust faults. The final phase was the development of east-west striking faults, which, cross-cut the gold mineralisation and displace it laterally. The intrusion is thought to have been emplaced during the first stage of deformation. Gold mineralisation occurs along the contact faults, and into the footwall and hangingwall rocks. The thrust faults have been mapped in the pit and appear to offset the mineralisation.
Gold mineralisation is structurally complex, associated with fault zones, micro-fractures, quartz (± carbonate) veinlets and with haematite alteration. In general, mineralisation occurs predominately as an intrusive body along quartz filled fractures zones, shears, and joints and is associated with north-south triking and steeply dipping faults. Sulphide minerals associated with the gold mineralisation include arsenopyrite, pyrite (± minor pyrhotite), chalcopyrite, sphalerite and other trace sulphides. There is a close association between gold and arsenic, with visible gold often occurring with arsenopyrite. Fine disseminated pyrite occurs throughout the area, and is not necessarily associated with gold mineralisation. There is some quartz veining in the area, but these are considered to be of limited continuity, and are rarely associated with the gold mineralisation.
Mineralisation is subdivided into oxide and sulphide ore. Supergene mineralisation extends from surface to 30m to 35m below surface. Mineralisation is associated with iron oxides, primarily limonite and minor hematite. Sulphide mineralisation occurs just below the “redox” boundary at depths of 24m or greater. A transition zone of about 5m occurs between the base of oxide mineralisation and sulphide mineralisation. Microprobe and microscopic studies indicate that gold is associated with borders and fractures within arsenopyrite grains. Gold mineralisation is associated to a lesser extent with quartz and calcite and rarely with pyrite. In the supergene zone gold is associated with hematite after sulphides. Quartz veinlets and hematite staining are generally indicators of high grade mineralisation and the presence of pyrhotite is also noted in some portions of the pit.
Hydrothermal alteration of the intrusive follows structural preparation related to the first phase of north-south striking cataclastic zones, and related fractures and veins. The unfractured tonalite is generally unaltered. The fractured tonalite grades from weak propyllitic to phyllic alteration destroying the rock fabric. Silicification and feldspathisation are associated with veining and correlate with sulphide intensity. Calcite veinlets and stringers are associated with low and high grade mineralisation.
Mafic dykes less than 1m in width intrude the mine area. These dykes are unmineralised and are not believed to affect the tenor of mineralisation. Larger (aplitic) dykes consisting of andesite porphyry and quartz porphyry have been noted with maximum widths or 4m.
High grade gold mineralisation is associated with structures with a strike of 025° and 030° with dips of 75° southeast and 45° to 75° southwest.
Mineralisation.
The gold mineralization occurs in quartz filled veins associated with steep north-south faults. Gold mineralization is associated with arsenopyrite, pyrite (± minor pyrrhotite), chalcopyrite, sphalerite and other trace sulphides. Mineralization is subdivided into oxide, transitional and sulphide ore. Oxide mineralization extends from surface to about 35 meters depth and is underlain by sulphide mineralization with a transitional zone of about 5 meters between the oxide and sulphide mineralization zones.