The 72 km² property (Dec/01) is located in the Brong Ahafo region of west-central Ghana, about 30 kilometres south of Sunyani and 290 km northwest of the capital city of Accra. This listing covers the Subika pit and an underground operation below the pit.
GOLD open pit mining operation in shear zone.
* Property summary (Dec/01) on document Pa097122.PDF, p. 5.
* Property report, location and prospect maps (Jan/00) on document Pa036617.PDF p. 17.
* Property review (Aug/98) on document Pa008835.PDF p. 44.
The property has 17 pits with reserves with three (Awonsu and Amoma) in operation as of Dec31/08. The process plant consists of a conventional mill and carbon-in-leach circuit.
The license is located in the Yamfo-Sefwi gold belt, a well-defined zone of many gold occurrences that trends 20 - 30° east of north and stretches northwards from the Ghanaian border with Cote d’Ivoire for a distance of about 200 km. Intruded along this trend are a series of hornblende granodiorites which preferentially exploit a major regional break separating Lower Birimian sediments from Upper Birimian volcanics. The property covers 3 segments of the granodiorite - metasediment contact.
This major break has been mapped for a strike length of 6.4 km in the northern part of the license and for an additional 6.4 km in the southern part and marks the contact between Birimian metasediments to the west and a complex of multiphase granodiorite intrusions to the east. This structure hosts the Yamfo deposit as well as significant mineralization at Kenyase and at Subenso. Thirteen km of the prospective structure occur within the license boundary.
In general, gold mineralization is associated with a brecciated and hydrothermally altered shear-zone of granodiorite lying in thrust/wrench contact with underlying graphitic phyllites of the Birimian Supergroup. The phyllities also contain scattered pods of gold mineralization, but these appear to be limited to a zone lying no more than 20 m from the granodiorite. The thrust contact always dips east at angles varying from 30° to 65°.
Gold mineralization is also associated with a shear zone about 1.2 km east of the granodiorite - metasediment contact that was expressed by a low-level soil geochemical target. The granodiorite hosted shear zone is subparallel to the main trend and dipping from 60° east to sub-vertical.
Geochemical soil sampling identified 8 major gold anomalies, 7 of which are named in alphabetical order Zone A through G, and an eighth zone named Sika Aminaso.
Three zones, A, B, and C in the northern part of the license area have been investigated.
Zone A gold mineralization (locally bonanza grade) is confined mainly to a highly altered and brecciated granodiorite shot through with a dense stockwork of quartz-sericite-chlorite- carbonate-albite veinlets containing varying amounts of dusty pyrite and occasional clots of coarse, visible gold. This style of mineralization is best developed along a contact zone between the granodiorite and graphitic phyllites but is not limited to this horizon. The granodiorite is interpreted to be lying in thrust/wrench fault contact with underlying graphitic phyllites. The thrust contact always dips east at angles varying from 30°E to 65°E. The zone has a strike length of 1.5 km and represents the NE continuation of Normandy LaSource's Kenyase East deposit. The zone contains scattered bonanza gold grades of up to 80 g/t Au.
Zone B partial results indicate that gold mineralization is more patchy than in Zone A. Bonanza grades are present. Zone B has a strike length of 1.6 km. Gold mineralization is localized along the metasedimentary – granodiorite contact, which dips at 30° to 60° to the SE. The highest-grade intervals, however, were intersected 400 m east of the contact, entirely within the granodiorite.
Zone C is located in the northern part of the property and abuts Normandy LaSource’s Bosumkese deposit extending north of the licence boundary. Zone C has a strike length of 2 km. Most of the potentially economic mineralization occurs in the northernmost 400 m of the zone. The main mineralization in this zone occurs some 150 to 200 m east of the metasediment - granodiorite contact entirely within the intrusive. Drilling on 100 m spaced fences and 50 metre hole spacing has delineated several parallel zones of mineralization associated with sheared granodiorite and hydrothermally altered xenoliths. Weathering extends to a depth of 35 m.
Zone E is located approximately 4 km south of Zone A and is located approximately 1,200 m east of the main structure. The mineralized zone was associated with a subtle soil anomaly (+100 ppb and ranging up to 200 ppb). It is a 2.2 km long geochemical anomaly lying entirely within the dominant granodiorite lithology and to the east of the contact zone associated with other mineralized zones. Continuity of mineralization is good. Diamond drilling has returned intersection of up to 82 m grading 4.12 g/t Au.
Subika Expansion (2010)
The Subika expansion project is exploring the opportunity to develop a layback to the Subika pit and an underground operation below the pit to expand Ahafo's production and life of mine. If successful, the project would be Newmont's first underground mine in Ghana.
For the underground expansion, the evaluation is determining the continuity of economic grade and mining costs (driven by geotechnical and hydrology conditions), which requires underground access. The study objectives for 2010 are to advance the decline to access trial stopes, perform underground exploration drilling, prepare for permitting, update haulage evaluation and evaluate optimum interface between potential underground and pit options.
The team began blasting/drilling an underground portal in January 2010 and has advanced more than a quarter mile (425 m) into the exploration decline with nearly 2 miles (3,000 m) of drifting planned for 2010. A Gate 2 decision is expected in 2011.