Hydrogeology of the Venetia Diamond Mine, South Africa

Venetia Mine is located in the northern part of the Limpopo Province. The mean annual rainfall for the area is 344mm and the mean annual evaporation is in the order of 2650mm/annum. Geologically, Venetia Mine is situated in the Central Zone of the Limpopo Belt where a large variety of rock types are developed. These include quartzites, dolomitic marbles, magnetite rich quartzites, amphibolites, gneisses and schist's. During the hydrogeological investigation a number of boreholes were drilled and were subjected to numerous conventional and unconventional aquifer tests. The country rock of the mine is hydraulically tight but has a large storage capacity for groundwater. Faults and fractures within the country rock enhance the hydraulic conductivity in places and serve as drainage lines into the open pit. The kimberlite may be divided into two significant horizontal hydrogeological units, the so-called "overshot zone" created by drilling and blasting and the undisturbed kimberlite. The "overshot zone" has a high density of artificial fractures and is between 2m and 5m thick. Test pumping of the in-situ kimberlite of the Venetia Diamond Mine has revealed unusually high hydraulic values. Post closure, evaporation might exceed the inflow of groundwater into the pit. it will, therefore, either be dry except for runoff from precipitation or develop into a brine lake where accumulation of salts will take place in the pit. The drawdown of the groundwater table will continue indefinitely because evaporation exceeds recharge. A pit lake model is planned to simulate post closure conditions.