Summary:
The style gold mineralisation at Sukari is classified as orogenic gold and comprises a broadly mineralised granodiorite dislocated by major shear/vein hosted higher grade mineralised zones. Gold mineralisation is hosted mainly by granodiorite and diorite at Little Sukari, with some mineralisation extending into the surrounding metasediments.
The Sukari granodiorite strikes north-northeast and typically dips between 50° and 75° to the east. The granodiorite has a strike length of approximately 2.3km, and ranges in thickness from approximately 100m in the south to 600m in the north. Gold mineralisation within is not homogenous and its deposition has been influenced by major longlived structures that experienced continuous reactivation.
Deposit descriptions
Geometry
The granodiorite host for the mineralisation has a strike length of approximately 2.3km, and ranges in thickness from 100m in the south to approximately 600m in the north. Gold mineralisation is not continuous. Gold deposition was influenced by major long-lived structures, the most important of which are tabular sheets of crackle breccia found on the east and west contacts of the granodiorite hosting >1g/t Au. The high-grade Main Reef and Hapi Reef (Amun Zone) are the major areas of brecciation. The lower grade material (<1g/t Au) found predominantly within the open pit is associated with disseminated sulphides throughout the southern, narrower portion of the granodiorite. The Cleopatra Zone to the north is made up of two extension quartz vein zones (30cm quartz veinlets hosted by granodiorite; 5-20m wide; 350-400m long) dipping shallowly (32°) to the northwest (dip direction: 316°) grading 1-1.5g/t Au with limited migration of gold into the country rock.
Mineralisation controls
Gold mineralisation is structurally controlled, with the southern end of the Sukari granodiorite containing the highest grades. The first-order structural control is steep shear zones found mainly on the contacts of the granodiorite. The second-order control is a shallow angle short shear, parallel to bedding. The third-order control is early east/west trending, north- and south-dipping transverse faults. Gold mineralisation is late and post-dates these structures. The structures were subsequently filled by andesitic dykes or altered to kaolinite.
Gold mineralisation is found within quartz veins, breccia, and shears, and hosted within disseminated sulphides and sulphide veinlets within stacked extensional veins.
Vein geometry
Quartz veins and veinlets are commonly found intruding the granodiorite and the metavolcano-sedimentary association and form a fissure-filling system. The quartz veinlet thicknesses vary between few millimetres up to 10- 20m. Quartz veins are grouped into three sets of orientations:
• East-west (older).
• Northwest-southeast (younger).
• Northeast-southwest.
The Main Reef vein strikes 20–30° northeast and dips 25–50° southeast. It attains a thickness of 2.5m at the upper level, and is composed of massive, milky-white quartz with sulphides. In the northeast-southwest directions, the mineralised zones are located along shear fractures paralleling the contact between the metavolcano-sedimentary country rocks and granodiorite. It consists of the main northeast auriferous quartz veins, accompanied by a series of subparallel contiguous veinlets and offshoots forming a vein system.
The Sukari Main Reef and Hapi Reef are the most significant mineralised features in the high-grade Amun Zone. The Sukari Main Reef is a 0.2-2m thick quartz reef with massive, laminated, and breccia habit, while the Hapi Reef comprises a zone of stockwork quartz veins and stylolitic quartz, sulphide and sericite veins, as well as throughgoing laminated and massive quartz reefs.
The most conspicuous feature of the mineralised granodiorite is the intensive hydrothermal alteration of the country rocks on both sides of the mineralised veins. Brecciated veins consist of brecciated vein quartz and granodiorite rock fragments or granodiorite fragments in a matrix of vein quartz ±sulphides ±hematite. Shear veins appear to be rare, whilst extensional veins are distinguished by their short strike lengths and normally form stacked arrays between thin linking shears.
The orientation of the shear zones, not the extensional veins, indicates the large-scale direction of continuity of a stacked vein array that is commonly arranged en-echelon.
Sulphides
Gold mineralisation is intimately related with sulphides; pyrite is the most abundant sulphide, followed by arsenopyrite. Higher gold grades are associated with increased arsenopyrite concentration. The sulphides, occur as fine grained, subhedral disseminations in altered granodiorite and as blebby sub- to euhedral crystals and finer disseminations in quartz veins, fractures and breccias. Pyrite is found in all the mineralised zones.
Arsenopyrite is most common in the zones of higher-grade gold mineralisation, notably in the Main and Hapi Reefs, and breccias. Arsenopyrite is less abundant in the stacked extensional zones and minor quartz veins.
Pyrite and arsenopyrite exhibit deformation and even brecciation textures, whilst younger, native gold fills stringers and tiny holes in this deformed pyrite and arsenopyrite. Other sulphides such as galena, chalcopyrite, sphalerite, pyrrhotite have been noted. Sphalerite is sometimes a significant sulphide mineral. Abundant exsolved chalcopyrite bodies are randomly distributed in the sphalerite host. The sphalerite-chalcopyrite association seems to be filling and replacing the older pre-existing pyrite.
Gold
Visible gold occurs as anhedral grains in milky-white extensional and breccia quartz veins and as intergrowths with pyrite and arsenopyrite, commonly in narrow shear veins at quartz vein margins and margins to clasts in hydraulic quartz vein breccias.
High-purity gold commonly occurs free in quartz and anhydrite veining, on the margins of pyrite and arsenopyrite crystals, and as microfracture fillings. Gold is fine grained and ranges from 1µm to 40µm.
Alteration
The intrusion-hosting intermediate andesitic volcano-sedimentary rocks have generally been altered to a carbonate (ankerite, calcite)-silica-sericite-chlorite assemblage.
The granodiorite itself has undergone varying degrees of alteration, including silicification, sericitisation, carbonatisation, albitisation and more advanced kaolinisation. Sericite and silica are the most prevalent alteration products, closely associated with shears and stockworks. The extent of granodiorite alteration corresponds to the intensity of the extensional veins and their proximity to major shear structures This often manifests as a zonal alteration halo encircling breccia-quartz vein-shears, characterised by a central zone of intense kaolin-sulphidesericite alteration, transitioning to a sericite-silica ± albite intermediate zone, and further outward to a weaker sericite-silica-carbonate environment.
Silica, sericite, and carbonate alterations are pre- to syn-mineralisation, with gold mineralisation spatially associated with phases of silica, kaolin, sericite, and sulphides.
Sericite occurs in all granodiorites as well as in shears, as vein selvedge, veins, and blebby masses. Kaolinite alteration occurs along shear and fracture zones such as the Main Reef, but its occurrence is not consistent along these structures. The alteration is distinctly white, clayey to sandy (from resistant quartz grains in clay matrix), hosted in bleached rock, and is associated with strong fine-grained pyrite and elevated gold grades.