Summary:
Deposit Types
The Edikan gold deposits are structurally controlled, orogenic style gold deposits.
Gold occurs both in classic Ashanti-style sediment hosted shear zones, and within granitic plugs and sills or dykes situated along several regional shear structures.
Most of the known gold Mineral Resources at the Edikan Gold Mine are hosted by the granite plugs and sills or dykes, which occur along the same structural corridors that contain the sediment shear hosted gold occurrences. Gold mineralisation within the granites occurs in two to three generations of quartz veins and stockworks with individual veins millimetres to centimetres in thickness and rarely more than a metre thick. Gold is associated with pyrite, lesser arsenopyrite, and traces of sphalerite, chalcopyrite, galena and rutile. Mineralised quartz veining is nearly pervasive throughout the granite host bodies, rarely extending into the adjacent sediments.
The sediment-hosted shear zone deposits consist either of pinch and swell quartz reefs in relatively tight shears or quartz +/- carbonate stockwork veining in broader shear zones. Coarser grained wacke to sandstone sediments are often preferentially mineralised due to their more competent and brittle nature. Pervasive iron carbonate and more localised sericite and silica alteration has affected the host sediments, and fine-grained pyrite with lesser arsenopyrite occurs as disseminations in the host sediments and to a lesser degree in the quartz veins. Most of the gold occurs in veins as disseminations and as free gold along sulphide grain boundaries.
Mineralisation
Gold occurs at Edikan in two main deposit styles:
• Altered granite intrusive bodies (“granitoid”) – occur in at least three clusters and are emplaced in structurally favourable locations within more extensive ductile shear zones.
• Typical Ashanti-style shear zones within metasedimentary rocks (“shear zone”) - occur throughout the licence area. Mineralisation occurs within structurally favourable locations and is typically associated with or is proximal to quartz veining and widths are typically narrow (2 – 10 m).
AF Gap
Gold mineralisation was identified in a single granitoid intrusive over 2 km strike between Abnabna and Fobinso. Most of the gold mineralisation is contained within five zones ranging from 30 – 140 m in width and plunging moderately to steeply NNE. The AF Gap deposit has been mined almost continuously in a series of cut-backs since commencement of milling operations by PMGL and is being mined at the date of this report. The pit is presently about 200 m deep and designed to extend to about 240 m.
Esuajah North
The Esuajah North deposit, located 2 km north of Fetish, occupied a ridge comprising a 400 m long granitoid intrusive which averages about 100 m in width. An open pit to approximately 210 m depth, completed in May 2020, has exploited mineralisation to what was deemed at the time to be the economic limit of open pit mining. The deposit remains open at depth and a cut-back of the open pit may be viable at sufficiently high gold prices.
Esuajah South
The Esuajah South gold deposit is located 1 km southwest of the village of Ayanfuri and is hosted by a 250 m long discrete granitic plug averaging 40 m in width and with a small surface expression. Esuajah South is the highest grade of the granitoid deposits at Edikan with a zone of higher-grade mineralisation (>2 g/t Au) at a depth of 150 m to 250 m below surface. Oxide mineralisation has been completely mined out by AGC via an open pit.
The entire granitic intrusion is altered and mineralised, with minor sediment hosted mineralisation occurring along the margins of the granite and also in thin (1 – 3 m) felsic dykes and in fault zones in the metasediments. The intrusion strikes north-northeast, is sub-vertical and dips steeply to the west. The granite-hosted mineralisation remains open at depth.
No major geological differences are observed between the hanging wall and footwall sediments.
Mineralisation within the granite is associated with two to three generations of quartz veins and stockworks with individual veins millimetres to centimetres in thickness and rarely more than a metre thick. The granite is pervasively sericitized, with alteration intensity the highest immediately adjacent to the quartz veins.
Gold is generally associated with < 3% pyrite, lesser arsenopyrite and traces of sphalerite, chalcopyrite, galena and rutile. Native gold also occurs as very fine grains, commonly along sulphide grain boundaries and in fractures in sulphides, usually at or near vein margins. Coarse visible gold is occasionally observed in the quartz veins. Higher grade gold intersections often tend to be associated with very coarse arsenopyrite +/- sphalerite, chalcopyrite and galena.
Fetish
The Fetish granitoid gold deposit, located east of the high grade Bokitsi North shear deposit, was discovered by sampling a cross-cut from the old Bokitiso Mine in the 1930’s. An intercept of 28.34 m at 3.69 g/t Au was recorded from primary granite at a depth of 91 m.
Fetish comprises two pervasively auriferous intrusive granite bodies and associated host sediment gold mineralisation within an area 550 m long and 300 m wide. The large, low grade eastern granite intrusive is up to 170 m wide, while the western granitoid and host sediment mineralisation is approximately 40 m wide with grades averaging 1.6 g/t Au. The western portion of the open pit takes in the shear zone hosted Bokitsi North lode.
Bokitsi South
The relatively high grade Bokitsi South shear zone hosted deposit was partly mined by AGC and ore was hauled to Obuasi for treatment through the Sansu CIL plant. Based on processing history, the metallurgical recovery of the sulphide mineralisation at Bokitsi was lower than from the granite mineralisation.
Chirawewa
Gold mineralisation at Chirawewa, located 1 km south of Fetish, is contained within, or is associated with the margins of a granitoid intrusive. The main Chirawewa deposit extends for 300 m and ranges from 20 – 100 m in width. Oxide and sulphide mineralisation has been completely mined out at Chirawewa and the pits backfilled.
Mampong
Mampong is located 2 km south of the Abnabna deposit and forms a series of narrow zones hosted in a mix of granite dykes and shear hosted sediments. The majority of the Mampong deposit is low grade with only one zone having consistent grades greater than 1 g/t Au. There is no current MRE for this deposit, and it is unlikely to be mined in the future.
Nkosuo
Gold mineralisation at Nkosuo is hosted by a NNE-trending granitic plug extending at least 600 m in strike and ranging up to 200 m wide. The granite body dips at around 70° to the west in the northern part of the intrusion, steepening to near vertical towards the south. Drilling has defined its northern limit but it remains open to the south, though narrowing to around 100 m. Mineralisation within the granite consists of variably intense quartz stockwork veining with associated quartz-carbonatesericite alteration. The veining and alteration are accompanied by 1-2% disseminated and selvage pyrite and arsenopyrite, with better gold grades generally associated with higher concentrations of arsenopyrite. The enclosing Birimian metasediments are devoid of mineralisation.