| Abstrakt: |
The deposition of the Witwatersrand Supergroup commenced with a major marine transgression across the granite/greenstone terrain of the Kaapvaal Craton, depositing the predominantly marine West Rand Group. Possibly as a consequence of continental convergence the craton became subject to regional transpression, which began to fragment the widely distributed West Rand Group into a number of separate sub-basins, flanked by uplifts. The sedimentation style changed from shallow marine to fluvial, initiating the deposition of the Central Rand Group. The sub-basins in which sedimentation took place were separated by faults, anticlines or monoclines. In the Far East Rand these sub-basins included the Evander, Delmas, South Rand and East Rand Basins. Complete separation of these sub-basins only occurred after deposition of the Kimberley Reef. Left lateral transpression caused syn-depositional periclinal folding of Witwatersrand strata trending northwestsoutheast. Important structures that developed at this stage include the Springs Monocline, which formed a ledge separating the East Rand from the main Witwatersrand Basin, and the Fold-Fault, which formed a ridge dividing the Evander Basin into two. Contrary to previous interpretations, the Fold-Fault is not a basin margin structure because the Evander Basin continues to its northeast, and hence it is an intra-basinal fold. During the early phases of Central Rand Group deposition in the East Rand Basin, sediments were derived from greenstone poor granitic source rocks in the north and northwest. In contrast, in the adjoining Evander area, pre-Kimberley sediment was derived from greenstone-dominated terrain to the northeast. Underground mapping of the immediate footwall strata of the Kimberley Reef at Evander has provided unambiguous evidence for glacial activity at that time. It is unclear whether glaciation was involved in the formation of the Kimberley Reef itself. However, glacial activity was responsible for the deposition of extensive glaciogenic sediments across the Far East Rand in the form of diamictites (termed puddingstones in the older literature) and associated channel deposits (MK1, eskers?). A period of erosion followed this glacial episode and gave rise to an extensive gravel lag across the erosion surface, which constitutes the Kimberley Reef at Evander. The erosion surface on which the reef lies is thus a large pediment. The palaeocurrents responsible for this degradation flowed from the west and southwest and thus indicate a reversal of the local palaeoslope relative to pre-Kimberley Reef times. Palaeocurrent measurements in the Kimberley Reef and its hangingwall are consistent in their azimuths suggesting that flow was dispersed across the pediment rather than originating from a local point source. The sources of gold which became concentrated in the gravel lag appear to have been predominantly the LK1 and MK2 conglomerates and possibly gold present in the diamictites. The footwall strata had been derived from a greenstone-rich source in the east. Their greenstone signature was carried over into the Kimberley Reef, notably the presence of banded iron formation clasts and high concentrations of osmiridium and chromite. Northwest-southeast folding continued during Kimberley Reef formation. A synclinal trough bounded along its northeastern flank by an anticline, the Fold-Fault, developed across the pre-Kimberley surface. This structure created accommodation space, which caused local thickening of the Kimberley Reef package. Thickening of the reef in the Fold-Fault area is therefore not a consequence of fluvial channelisation but a result of thickening due to synsedimentary warping. Palaeocurrent directions in this trough continued in a northeasterly direction, perpendicular to the warp axis. The synclinal feature associated with the Fold-Fault remained active after the formation of the Kimberley Reef. As a consequence, isopachs of hangingwall strata also thicken towards the trough. The gold grade in the Kimberley Reef is inversely related to reef thickness: where the reef is thick, the gold grade is generally low. Most of the gold was scavenged from the footwall. The scavanged material was winnowed on the erosion surface, producing the high grades characteristic of the thin Kimberley Reef present at the southwestern and northwestern margins of the basin. In the trough which borders the western flank of the Fold-Fault, grade is lower due to an increase in quartzite partings. The Fold-Fault is a syn- to post depositional anticline and it is expected that the reef will thin onto the structure. Palaeocurrents are perpendicular to the anticline and therefore gold grades are expected to increase as the reef thins. Because of the local derivation of much of the Kimberley Reef, the proximal to distal indicators such as changes in pebble size, U/Au ratio and gold fineness are weakly developed to absent. Carbon is often associated with gold in the Witwatersrand Basin but in the case of the Evander Goldfield, carbon does not appear to have played a significant role in gold enrichment on a regional scale. However, there is anecdotal evidence of gold being associated with carbon on a local scale. We suggest that the burial of the Kimberley Reef occurred as a result of an increase in sediment supply to the depository. Distal deposits which characteristically contain a high proportion of quartzite partings (HW1 and HW2), transgressed across the palaeo-plain and buried the Kimberley pediment surface. [ABSTRACT FROM AUTHOR] |
