Saeid Baghban, PhD
C.Y. Dong, D.D. Gregory , Q.F. Wang
The Ayanfuri deposit is the earliest notable occurrence of gold-bearing granitoid in the Birimian of Ghana, West Africa. The deposit hosts > 9 Moz total aggregate gold with auriferous intrusions mainly of granodiorite composition contributing about 80% of total endowment.
Gold mineralization within steep shear zones of the metasedimentary country rock also contribute to the gold endowment in the region. Litho-structural mapping has become necessary at lower mining levels in bedrock to ascertain structural fabrics and deformation history established within saprolite material. Detailed mapping was carried out by measuring and recording all planar and linear elements observable on the steep pit walls with a scale of 1:2000.
Pit mapping displays a plethora of structures resulting from the superposition of three generations of structures (D₁, D₂, D₃).
- D₁ is expressed by a bedding-parallel foliation (S₁) involving a thin-skinned tectonic event.
- D₂ records a NW-SE shortening and resulted in regional upright folding and development of a penetrative regional S₂ foliation and steep brittle-ductile faults recording evidence of reverse and strike slip displacements within the metasedimentary country rocks.
- D₃ deformation resulted in the formation of conjugate EW- and NW-trending steep crenulation cleavages and minor folds of the primary layering, S₁ and S₂.
Within the intrusives, series of NE trending faults developed in response to the D₂ deformation; they are sub-parallel or at a low angle to the NE-trending narrow shear zones of the metasedimentary. These faults recorded reverse, reverse-oblique and strike slip movements as the results of multiple tectonic reactivations.
Third order structures are NW, WSW-, EW- and WNW-trending, low angle, south-verging thrust faults that crosscut obliquely the second order structures and contain very significant gold mineralization. Veins within the deposit are controlled by the geometry and orientation of structures hosting them.
- V₁ veins are bedding parallel quartz carbonates veins that are deformed and folded and clearly postdate mineralization.
- V₂ are quartz-carbonates-sulphide veins sub-parallel to S₂ foliation and the steep NE shear zones.
- V₃ veins are predominant within the intrusive body. They trend NW and crosscut earlier planar structures and veins. Structural mapping at lower mining level (i.e. 1020mRL) resonates with earlier findings of the Ayanfuri deposit.