Summary:
The Aggeneys copper-lead-zinc-silver deposits occur in the Precambrian metavolcanic metasedimentary Bushmanland Group which forms part of the Namaqualand Metamorphic Complex. The Bushmanland Basin occupies an area measuring around 18,000km in the western half of the Namaqualand-Natal Mobile Belt.
Ore at the Black Mountain Mine is more copper-rich, in contrast to the other deposits to the east which are all more zinc-rich. This deposit comprises two superposed massive sulphide bodies namely the thicker Upper Ore Body (UOB) and a thinner Lower Ore Body (LOB). Both ore bodies, which also carry disseminated sulphides, are hosted in the banded iron formation. The iron formation horizons are both separated by and enveloped in northwest-dipping schist, which is overlain by a thick quartzite formation.
The UOB is comprised of three types of iron formation: magnetite quartzite, magnetite- amphibolite and barite-magnetite. Garnet-quartzite forms a halo around the UOB; it is locally enriched in copper (up to 3% copper (Cu)).
The LOB consists of baritic to quartzitic schist with disseminated sulphides which grades into magnetiteamphibolite. The footwall to the massive sulphide lenses is characterized by abundant sillimanite.
The dip is to the north at 55° (fifty-five degrees) near surface and varies from almost sub horizontal to 40° in the lower western portion of the ore bodies. The contacts of the massive sulphide ore with the host rock are sharp. The ore bodies extend over a strike length of 1,600m from a surface outcrop in the west to about 800m in the east. The stratigraphy consists primarily of footwall schists, which contain little or no water. An unconsolidated weak zone 3m thick, consisting of graphite and mica rich ground, occurs in the footwall. The ore bodies and the hanging wall quartzite's contain water which is associated with fissures and cracks.
On a regional scale the area has been subjected to several phases of faulting and folding which has resulted in fracture zones. The surface rocks are invariably jointed and in some areas open partings are present along the east-west striking bedding planes. Much of the jointing and fracturing however extends to depths of less than 200m below surface and it appears that deep open fracturing of geohydrological significance only occurs on quartzite gneiss contacts where late stage folding and fracturing has occurred. The fractured contact zones may act as preferential flow paths for ground water.
Black Mountain contains significant lead and copper mineralisation with zinc and silver, while Broken Hill, which is presently being mined, contains the highest grades of lead, zinc and silver, with lesser, although still economically important, copper.
Four major sediment-hosted lead-zinc-copper-silver deposits: Broken Hill, Swartberg, Big Syncline and Gamsberg, occur in the Aggeneys area.
The Deep ore body's western extremity is approximately 390 m east of, and 240 m below, the current deepest level of the mine (800 m below surface). It has a known down plunge extent of 1 100 m and is open at depth. The deepest position of the ore body is 1 680 m below surface. The Deep ore body is sub-divided into five geologically distinct zones each comprising of iron formation and massive sulphide. Lead-zinc copper-silver mineralisation occurs as fine to coarse disseminations or interbanded in the iron formations. Mineralisation in the massive sulphide is fine-grained and often brecciated. Economic ore occurs in all of the five ore body zones and is predominantly situated at or close to the footwall of each zone. The Deep ore body is contained in a synformal structure with a steep (63°-70°) and extensive southern limb.
The dip is to the north at 55° (fifty-five degrees) near surface and varies from almost sub horizontal to 40° in the lower western portion of the ore bodies. The contacts of the massive sulphide ore with the host rock are sharp. The ore bodies extend over a strike length of 1,600m from a surface outcrop in the west to about 800m in the east. The stratigraphy consists primarily of footwall schists, which contain little or no water. An unconsolidated weak zone 3m thick, consisting of graphite and mica rich ground, occurs in the footwall. The ore bodies and the hanging wall quartzite's contain water which is associated with fissures and cracks.