THE WITWATERSRAND DEEP MICROBIOLOGY PROJECT

 

Mining Map of Carletonville Mining Region

 

Surface geology map of the mines near Carletonville.  Kloof mine shaft 7 and 4 and Driefontein consolidated mines 9 and 5 and Mpening mine are demarcated in white.  The pale blue unit is the 2.5 Ga Transvaal Dolomitic Aquifer.  Where it is exposed the aquifer in unconfined and an open system.  Strata dip to the south so that the dolomite plunges beneath the 2.4 Ga Pretoria group, BIF, volcanics, diabase sills and shale cutting of the dolomite from surface recharge.  The NNE dikes are the 1.45 Ga Pilansberg alkalic dykes that compartmentalize ground water flow.  Prior to extensive mine dewatering the ground water flowed from east to west following the topographic gradient, welling up at the springs (eyes) at the eastern border of the dykes.  Today pumping has dropped the water table a few hundred meters and groundwater flows from the north.  A major fault that also cuts all strata up to the mid-Pretoria group is the Bank fault.

Simplified geological map of the Carletonville mining district, west Wits line, showing the relative positions of Mpeneng Mine (Western Deep Levels) (sampled in 1996), West Driefontein, East Driefontein (now Driefontein Consolidated), and Kloof mines. East Driefontein, Shaft #5 is indicated in pink. Pink line is strike of crossection plotted in following figure. The geological contact is between the 2.2 Ga Transvaal Dolomite (brick gray) and the overlying Pretoria Group (tan) terrigenous sediments with the contact dipping 7 degrees to the south. The Transvaal Dolomite is the second major source of water for the mines. It is partitioned into "compartments" by 1.4 Ga syenite dikes radiating from the Pilansberg Complex (within the Bushveld Complex) to the north. Water from the dolomite enters into the underlying Witwatersrand beds primarily through fissures arranged en echelon to these dike aquicludes. The Witwatersrand units beneath vary from being "wet" mines, like West Driefontein, to "dry" mines like Doornfontein (Wolmarans, 1986). This is not controlled by the natural recharge rate into the dolomite, which is uniformly quite high. The Oberholzer Compartment averages 56 megaliters/day.

The degree of "wetness" appears to be controlled by the type of "fissures". At West Driefontein and Venterspost Mine the fissures are "dirty" or highly porous and permeable; whereas, at Doornfontein mine the fissures tend to be mylonitic and much less permeable. The Venterspost mine pumps upwards to 50 megaliters/day from their subsurface operations. The vertical hydraulic conductivity may also be related to the relative orientations of the regional principal stress (arrow in figure) and the en echelon fissures (Gay and Jager, 1986).

The "wet" mines also appear to be "hot" or express a higher geothermal gradient than "dry" mines. The lowest geothermal gradient measured, 9oC/km, was at East Driefontein Mine. The highest geothermal gradient, 15oC/km, was measured at Kloof mine. The correlation of high geotherms with wet conditions, however, implies that a component of deeper, hotter, fissure water may be flowing upwards through the Witwatersrand units.

Since mining operations began in this area the water table has been lowered by several hundred meters in the Bank compartment. Originally the water table dipped to the west, but today it is flat with cones of depression centered around "pump chambers" at shaft 4-W and 4-E. The water originally flowed through the cavernous portions of the Transvaal dolomite, but today the water table is well below the cavernous zone into a lower porosity zone.
 
 


 

 

To date, samples collected from Driefontein consolidate occur near the deeper, southwestern portion of the mine property where very little active mining has occurred.  The rusty stipple pattern represents mined carbon leader zones and the purple stipple represents overlying mined VCR (Ventersdorp Contact Reef) zones.  5 Shaft is old East Driefontein 5 shaft.  5 shaft at the time it was sampled in 1998 was just starting production from the carbon leader and is now in full production mode.  9 shaft used to be a West Driefontein shaft and is not in operation, but in development.

References:

Gay, N.C. and Jager, A.J. (1986) The influence of geological features on problems of rock mechanics in Witwatersrand
    Mines. In Mineral Deposits of Southern Africa (Anhaeusser, C.R. and Maske, S, eds.)
    Africa vols. 1&II, Geol. Soc. S. Afr., Johannesburg, 753-772.

Wolmarans, J.F. (1986) Some engineering-geological and hydrological aspects of mining on the West Wits line.
    In Mineral Deposits of Southern Africa (Anhaeusser, C.R. and Maske, S, eds)
    vols. 1&II, Geol. Soc. S. Afr., Johannesburg, 791-796.
 
 

Cross Section of East Driefontein Mining Region


 



Simplified geological cross section from West Driefontein to East Driefontein. Service water is chilled at the surface to 4¡C and treated with chlorine and bromine disinfectants before it descends shaft #5 to mining levels (blue arrows). From the shaft the service water is pumped 1-2 kilometers to the stopes where the "carbon" leader is being mined. It is used at the stopes to cool the circulating air, control dust levels and cool drilling equipment. From the stopes the now hot water flows back towards the base of the elevator shaft where it is pump to the surface in two stages (red arrows). Dolomite water is drawn from the IPC pump chamber at shaft #4 to augment water supply at shaft #5. The hot water is chilled in cooling towers before being returned to the subsurface. Cool air descends into the elevator shafts and is drawn through the access tunnels to the stopes. From the stopes the air usually ascends until it reaches a 2 kilometer deep fan drift shaft where the now hot air is sucked to the surface at hurricane velocities to exit the fan drift.

E5-46-borehole is located on level 46 in the volcanics of the Ventersdorf Formation. Fissure water seeps at E5-46 and on level 48 are associated with the contact between a Ventersdorf dike and the volcanics. E5 sump is a pond of water beneath that borehole. All carbon leader samples from E5 come from the stope connecting levels 46 and 48. E5-48-FW is a ceiling drip at what was then the end of the level 48 access tunnel in the Boonton Shale. These fissure waters may represent a mixture of dolomite water descending from the Transvaal dolomite above through the fractures and formation waters ascending from the Witwatersrand formation.
 

 

Cross Section of West Driefontein Mining Region-Home of the Dream Borehole!


 





Simplified cross section from 9-W shaft to 8-W shaft.  38 level connects 9-W with Tertiary 6-W.  Dream borehole is located approximately 1 km west of 9-W and penetrates 780 meters at 60o.  The carbon leader lies just above the base of the borehole.


 

Core log for the "Dream Borehole", location of water/gas/bio samples collected with bailer (red dots) and the maximum temperatures recorded by the temperature strips.  Temperature maximum is centered on the faulted contact between the Ventersdorp volcanics and Witwatersrand quartzites (VCR) from which hot water may be emanating.
 

 

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