Summary:
Sukari is classified as an orogenic gold deposit and comprises a broadly mineralised granodiorite porphyry dislocated by major shear/vein hosted higher grade mineralised zones. Gold mineralisation is hosted mainly by granodiorite, 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 long-lived structures that experienced continuous reactivation.
Mineralised Zones
Gold mineralisation at Sukari is hosted mainly by granodiorite and to some extent in metasediments. Geochemical whole rock analysis confirmed the Sukari intrusion is a granodiorite (A-type granite) high in silica, zirconium, niobium, gallium, yttrium and cerium. The intrusion is a mid-crust melt which has been strongly fractionated to plagioclase and magnetite. Magmas that fractionate magnetite are generally sulphide saturated, and therefore depleted in gold, however the Sukari granodiorite is superoxidised (i.e., sulphate not sulphide). Gold mineralisation sits within the magnetite fractionated portion of the granodiorite. This is supported by the enrichment of thorium, lanthanum, cerium, phosphorous, zirconium, hafnium, depletion of vanadium relative to titanium and decreasing vanadium/scandium ratio with decreasing scandium.
The whole rock geochemical data was combined with ASD mineralogical data to show that gold is spatially associated with muscovite while chlorite and phengite are distal. The geochemical pathfinders defined are sulphur and arsenic to gold, copper and nickel show the extents of the granodiorite, while antimony shows a high on the granodiorite margins, where molybdenum and bismuth are low.
The Sukari host is an oxidized granodiorite, confirmed by its pyrite-hematite-magnetite-anhydrite mineralogy. However, the very high arsenic within the orebody is evidence of a very strong reduction process. The ammonia detected in the ASD work further supports this interpretation. Therefore, gold bearing fluids are considered to have been sourced from the magma and the surrounding carbonaceous sediments.
The Sukari granodiorite represented a favourable host because of its composition relative to mineralising fluids, and its mechanical properties. Evidence includes that granodiorite dykes in the hangingwall of the main granodiorite body show gold mineralisation of essentially the same character as that in the main granodiorite, and wall rocks immediately adjacent to those dykes are barren. The dykes range in thickness from a few centimetres to several metres.
Structural observations indicate that the Sukari granodiorite acted as a rigid body surrounded by weaker rocks. Footwall and hanging wall rocks have taken up strain by development of strong schistosity, likely accompanied by large decreases in volume. The granodiorite has taken up strain by development of predominantly brittle fault structures.
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 within this is not continuous and its deposition has been influenced by major long-lived structures, the most important of which are tabular sheets of crackle breccia, the principal ones being the high-grade Main Reef and Hapi Reef (Amun Zone). Figure 7.6 illustrates the overall shape and size of the granodiorite host and the geometry of the different ore zones.
Vein Geometry
Quartz veins and veinlets are commonly found intruding the granodiorite and the metavolcanosedimentary association constituting a fissure-filling system. The thickness of the quartz veinlets varies between few millimetres up to 10-20m. The quartz veins are grouped into three sets: • E-W (older) • NW-SE (younger) • NE-SW.
The main vein strikes 20–30° NE and dips 25– 50° SE. It attains a thickness of 2.5m at the upper level, and is composed of massive, milky white quartz with sulphides. In NE-SW directions, the mineralised zones are located along shear fractures paralleling the contact between the metavolcano-sedimentary country rocks and granodiorite. It is composed of the main NE auriferous quartz veins, accompanied by a series of subparallel contiguous veinlets and offshoots forming a vein system zone.
The most conspicuous feature of the Sukari 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 shears, not the extensional veins, indicates the large-scale direction of continuity of a stacked vein array that are commonly arranged en-echelon.
Sulphide Development
Gold mineralisation at Sukari is intimately related to sulphides; pyrite is the most abundant sulphide, followed by arsenopyrite. High gold grades are associated with increased arsenopyrite concentration. The sulphides, which are believed to belong to the same paragenesis, 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 Reef and in the Hapi Reef, and breccias.
Arsenopyrite is not 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 in core 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 as electrum is para-genetically first as it is often occluded in pyrite and followed secondly by high purity gold (>900 fine) depleted in silver. Gold is fine grained and ranges from 1µm to 40µm.
As expected, most of the gold is found in the granodiorite but can also be found in the metasediments, serpentinites and volcanoclastics in Amun, Horus and Ptah. This complies with distance analysis indicating that most of the gold sits 40 metres within and 20 metres away from the granodiorite contact.