Summary:
Geological Setting
The Kibali deposits are hosted within the Kibali Greenstone Belt (otherwise referred to as Moto granite- greenstone terrane), bounded to the north by the West Nile Gneiss and to the south by plutonic rocks of the Watsa district. The Kibali Greenstone Belt is an elongate west-northwest-east-southeast trending terrane containing Archean aged volcano-sedimentary conglomerate, carbonaceous shales, siltstone, banded iron formations, sub aerial basalts, mafic intermediate intrusions (dykes and sills) and multiple intrusive phases that range from granodiorite to gabbroic in composition. Based on textures and types of lithologies present in the stratigraphy, the rocks within the Kibali permit area are interpreted as having been laid down in an aqueous environment.
The majority of the primary lithologies are clastic (sedimentary) in origin, possibly being developed in a regional extensional environment such as a rift graben or half graben. At Kibali, the gold deposits are largely hosted in siliciclastic rocks, banded iron formations, and cherts that were metamorphosed under greenschist facies conditions, situated along a curvilinear zone 20 kilometers long and up to one kilometer in width, known as the “KZ Trend”. Gold mineralization is concentrated in gently northeast to northnortheast-plunging fold axes whose orientations are generally parallel with a prominent lineation in the mineralized rocks.
The Kibali deposits differ from many orogenic gold deposits as they are hosted within a thrust stack sequence with ductile to brittle-ductile deformational structures and a complex folding history. There are two principal structure sets: northwest-southeast striking, northeast dipping thrust faults and a series of sub-vertical northeast-southwest shear structures both of which, in association with the folding, are considered important mineralizing controls. Unlike many other orogenic gold deposits, mineralization within the Kibali district typically lacks significant phases of quartz-rich veins.
Mineralization
Gold deposits of the Kibali district are classified as Archaean orogenic gold deposits. Ore-forming H2O-CO2-rich fluids migrated along a linked network of gently northeast-dipping shears and north- northeast plunging fold axes that are commonly referred to as the KZ Trend. The auriferous KZ Trend is a complexly deformed fault system specifically developed along the boundary between the younger sedimentary basin in the west of the belt that juxtaposes the older rocks to the east. Mineralisation occurred during the later stages of subsequent regional deformation which resulted in inversion of the basin and the development of reverse faults and folds. Ongoing deformation during hydrothermal activity resulted in the development of lodes in a variety of related structural settings within the KZ Trend.
The mineralized deposits of the Kibali district are associated with halos of quartz, ankerite, and sericite (ACSA-A) alteration that extend into the adjacent rocks.
The Chauffeur and Durba (KCD) deposit is the principal mineralized occurrence along the Sessenge-KCD Trend. It consists of five semi-vertically stacked lodes (3000, 5000, 9000, 11000 and 12000), hosted within the volcanosedimentary units. The location of the individual lodes within the KCD deposit are intimately controlled by the position, shape, and orientation of a series of gently northeast-plunging tight to isoclinal folds. The lodes may be linked genetically by large-scale recumbent folding developed between two bounding northeast trending structures. Higher grade developed in zones of strong to intense alteration that overprinted and texturally-destroyed previous breccia, foliation and lithological textures. These are broadly categorized as the 3000 lodes, 5000 lodes, and the 9000 lodes, all of which plunge towards the northeast at low to moderate angles.
Both the Gorumbwa and Kombokolo deposits occur along a north-east trending mineralized corridor located 800 meters to the west of the main Sessenge-KCD structural zone. Both deposits are considered to be formed from the same mineralizing event, with similar alteration and structural characteristics to the KCD deposit but significantly smaller in size.
The Rhino and Agbarabo deposits located further to the north of the KCD deposit are positioned within smaller and more open folds with a similar north-eastward plunge. These satellite deposits exhibit mineralization primarily along a banded iron formation (“BIF”) horizon's base, suggesting it acted as an aquiclude in confining hydrothermal fluids primarily to the underlying rocks. Multiple mineralized lenses are open down-plunge but characterized by a generally narrow width (30-50 meters).
The Mengu Hill deposit lies on the KZ North structure, to the northwest of Pakaka and to the south of Mofu-Oere. The mineralized lens is cigar-like in shape and plunges shallowly to the north-northeast. Mineralization remains open down plunge.
The Aerodrome-Pakaka-Pamao deposits are located along the KZ North trend, in the gently north-north-east- to east-dipping shear zone. The presence of significant arsenopyrite at Pakaka distinguishes it from other deposits and prospects along the northern half of the KZ Trend. The structures combine to produce a broad northeast plunging open anticlinal structure, with Pamao on the west limb, and Pakaka on the east. The weathering profile at Pakaka is relatively deep up to 70 meters.
The Mengu Village deposit is located near the northwest end of the Pakaka-Mengu Trend. The mineralization is tabular in form, trending northwest and dipping shallowly to the northeast, and is hosted by conglomerates with thin ironstone and carbonaceous shale intercalations.
The Megi-Marakeke-Sayi deposit comprises three individual deposits, Megi, Marakeke, and Sayi, separated by lower grade mineralization but are mined in a single open pit. The Megi-Marakeke-Sayi deposit occurs as multiple tabular lenses that trend northwest and dips gently to the northeast.
The Kalimva/Ikamva and Oere deposits are all located along the major lineament of the KZ North Trend, north of Mengu Hill. These deposits are broadly similar in geology. The mineralized lodes in Kalimva, show a shallowly north-northeast-plunging ore-shoot along a moderate to steeply east-dipping structure locally called the Kalimva Deformation Zone and interpreted as an equivalent of the Ikamva deposit.