Obuasi Mine – Ghana
BLA designs open stoping mine in Ghana
Obuasi Mine – Anglogold Ashanti
Bottom-up Stacks & Crowns Longitudinal Retreat Open Stoping - Conceptual Study of Likely Stress and Deformation Effects
Gold mineralisation within the Obuasi area has a proven continuous lateral extent of about 8 km along strike and continues to an explored depth of over 2000m below surface. The main shear dips steeply to the north-west at 70º and is deformed resulting in an anatomising structural pattern. Characteristic of this type of shear hosted deposit is the development of strong pinch and swell structures.
There are variations in thickness, continuity, dip and strike of mineralised lodes. The result is a complex geological structure with unpredictable geometry and grade distribution. High-grade zones within the ore shears are surrounded by barren or very low-grade shear or host rocks. There are two main mineralisation types. The main historical source of high grade ore is from quartz veins in shear zones associated with free gold. The second type is disseminated sulphides (arsenopyrite predominant) in metavolcanics and metasediments. The ore zones are defined by the presence of mineralised quartz or presence of sulphides.
The narrow secondary shears now becomes the focus of mining and will form the core mining of any future deeper level extraction, below 1500m.
Obuasi Mine is situated along a north easterly striking 300 km wrench fault system known as Ashanti volcanic belt. This belt is the most prominent of the five Birimian Supergroup gold belts found in Ghana and comprises sedimentary and mafic volcanic rocks. The Birimian was deformed, metamorphosed and intruded by syn and post-tectonic granitoids. Folding trends are dominantly north east.
South western major faulting has taken place along the same trends. The contrast in competency at the contact between the metavolcanic rocks to the east and the more argillaceous rocks to the west formed a plane of weakness.
During crustal movement, this plane became a zone of shearing and thrusting. Gold mineralisation occurs within this graphite-chlorite- sericite fault zones. The shears are associated with silica, carbonate and sulphide hydrothermal alteration and occur in tightly folded Lower Birimian schists, phylites and greywackes.
As part of the future deepening and new initiatives at Obuasi Mine to reduce the amount of off reef development the long retreat narrow to wide stoping configurations or better known as Longitudinal open stoping will be utilized.
For this report only the Bottom-up Longitudinal Retreat Open Stoping method was analysed.
Different variations of mining sequences was analysed consisting of a parallel or horizontal slice versus a diagonal or Christmas tree shape. MAP3D-SV is an elastic numerical program, which was used for the numerical analyses done for Obuasi Mine. This numerical modelling program incorporates full three-dimensional simulation capabilities and is ideally suited to massive mining environments. It allows the user to include a host of material properties to simulate the effects of alternate material zones.
Mining was taken as being at a constant depth of 1500m with a reef dip of 75°. The width of the orebody was taken as 5m and 10m respectively with multiple stacks of three and five sublevels being mined. The sublevels were spaced 12m, 16m and 20m apart separated by a solid 12m or 16m reef crown pillar. Panel lengths were taken as 20m for a 10m in width orebody and 30m for a 5m in width orebody. At the different reef widths of 5m and 10m, rib pillars 5m in width was left or the stopes was backfilled with 10% cemented fill. Each panel was backfilled before proceeding to the next mining step.
