Central Rand Group Research Articles

Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa

ABSTRACTA characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found.The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup.In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation.Palaeocurrents of the Middelvlei placer are towards the south‐southeast in the western parts of the study area and towards the south‐east in the east. Maximum pebble size, however, decreases towards the west‐southwest, that is in a direction roughly perpendicular to the palaeocurrents.A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike.The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward‐thinning wedge of underlying strata.The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.

Sedimentology and gravel bar morphology in an Archaean braided river sequence: the Witpan Conglomerate Member (Witwatersrand Supergroup) in the Welkom Goldfield, South Africa

Abstract The Witpan conglomerate member is the lower, thicker, more channelized of the two auriferous conglomerates that comprise the Archaean ‘A’ Reefs (Aandenk Formation, Central Rand Group, Witwatersrand Supergroup) mined in the southern part of the Welkom Goldfield, Republic of South Africa. It is interpreted as the deposit of a degrading, gravelly, braided river system with well developed topographical differentiation. Three morphological elements have been identified in the Witpan Conglomerate member, namely Major Channels, Minor Channels and Terrace areas which are comparable to Levels 1, 2 and 3 of the modern Donjek River. Nine facies are present in the Witpan Conglomerate member combined into eight facies assemblages which show differing three dimensional architecture and relationship to channels. The four conglomeratic bar facies assemblages (FA1 to FA4) are interpreted as longitudinal, diagonal, transverse and side bars respectively and are combined to form compound bars. The three quartzitic channel fill facies assemblages (FA5 to FA7) represent active, slough and abandoned channel fills. The eighth facies assemblage (FA8) forms the interchannel terrace areas. The distribution of gold mineralization in the Witpan Conglomerate member is related to sedimentological factors including the presence of massive gravels, the accessibility of these gravels to flood water and the formation of areas of confined, convergent flow. Hence the bar facies assemblages are the richest followed by channel fill facies assemblages with the interchannel facies assemblage having generally low gold concentrations. Of the bars, diagonal bars are the most favourable for gold mineralization followed by longitudinal bars then side bars with transverse bars being the lowest.

Placer formation during progradational fluvial degradation; the late Archean Middelvlei gold placer, Witwatersrand, South Africa

Previous workers have established experimentally that geomorphologic conditions of degradation are favorable for the transport and concentration of heavy minerals and should therefore be conducive to the formation of placers. This principle may apply generally to the late Archean gold-uranium placers of South Africa, most of which rest on erosion surfaces.The Middelvlei gold placer, a quartz-pebble conglomerate which occurs near the base of the Central Rand Group of the Witwatersrand Supergroup, is mined selectively in a gold field known as the West Wits Line situated west of Johannesburg. The placer is considered part of a depositional system which overlies a regional mudstone marker, the Green Bar Member, and comprises three members, the Footwall Member, the Middelvlei placer, and the Hanging-wall Member.The Footwall Member consists predominantly of trough crossbedded quartz wackes, with lag conglomerates and massive mudstone as subordinate lithofacies. Thickness of this member varies from about 35 to more than 100 m and is generally smaller in the eastern parts of the gold field, where it is overlain by a large-pebble-type Middelvlei placer. Paleocurrent distributions of the Footwall Member are unimodal in all parts of the gold field with mean directions toward the southeast in the eastern regions and toward the south-southeast in the western parts. Directional variances are typically smaller than 1,600.The Middelvlei placer overlies the Footwall Member unconformably in most parts of the gold field, but in some southern parts of the western region no sedimentary break between these two members is evident. The placer comprises interbeds of quartz-pebble conglomerate, mostly massive, cross- and horizontally bedded quartz wacke, and thin massive mudstone.Maximum pebble size is about -6.0 phi and sizes are generally greater in the eastern parts of the gold field. Paleocurrent distributions are also unimodal, and in any particular area the means are not significantly different from those of the underlying Footwall Member. Directional variances are typically smaller than 2,000.The Hanging-wall Member consists mainly of trough crossbedded quartz wackes and subordinate lag conglomerates. For the western regions of the gold field information on paleo-currents is sparse, but in the eastern region the distribution is unimodal, with a mean not significantly different from that of the underlying Middelvlei placer.The Middelvlei depositional system is thought to be fluvial in origin and was probably deposited in braided streams. The Footwall Member and the Middelvlei placer constitute a coarsening-upward megacycle and are thought to represent a prograding braid plain. In most areas the transport of coarse-grained gravel during progradation was accompanied by erosion of the underlying Footwall Member.Gold concentrations are generally higher in areas of severe degradation of the Footwall Member than in regions where little or no basal erosion by the placer has taken place and where a conformable relationship between the two members exists. Statistical analyses revealed that the gold-rich facies of the placer is a single, relatively thin bed of massive, clast-supported conglomerate with large pebbles.

Changing tectono-sedimentary scenarios relevant to the development of the Late Archaean Witwatersrand Basin

Recent studies of the Witwatersrand Basin indicate that contrary to earlier proposals the shape and size of the basin and style of sedimentation were very different early and late in basin history. This reflects a geotectonic evolution which may involve an overall Wilson cycle. Progressive stages prior to, during and after basin development are described in order to map this evolution. Stage 1 saw the structural framework of the Kaapvaal Craton which was to accommodate future basin development. Fault zones representing lineaments and sutures associated with the structural evolution of various greenstone belts of the Kaapvaal Craton were rejuvenated (especially during late stages of Witwatersrand Supergroup deposition) to act as primary synsedimentary fault zones. Stage 2 involved the development of the Dominion Basin which has been interpreted as an extensional basin associated with NNE trending faults, although volcanics show calc-alkaline affinities. Stage 3 saw widespread deposition over the Kaapvaal Craton of a variety of subtidal sediments of the West Rand Group and its correlatives including the Pongola Sequence. This epicontinental style of sedimentation was either associated with a thermal cooling phase related to the Dominion Group extension or with the initiation of a foreland basin. The conformable or disconformable relationship between the West Rand Group and the underlying Dominion Group would point to the former. During stage 4 major changes in sedimentary patterns occurred. Upper Central Rand Group sediments were deposited and draped over compressive basement block-faults, which interacted with the sedimentation causing reverse faults/ monoclines and locally provided sources for coarse gravel. The basin was shrinking considerably in size and fluvial sediments derived from developing pediment areas and uplifted basement interacted with a reduced marine water body. The basin was controlled by NE-SW directed compression which caused a relative tectonic escape of the central part of the Kaapvaal Craton to the SE. The reason for these changes was the initial orogenesis associated with the subduction history preceding and the ultimate A-subduction history during the collision of the Zimbabwe Craton with the Kaapvaal Craton. Stage 5 saw the outpouring of the Klipriviersberg Group flood basalts into the Witwatersrand Basin. Early in this stage the tectonism was similar to that for stage 4, but late in the extrusive history a change in feeder dyke patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the patterns probably heralds the major extensional tectonics developed during stage 6, involving the deposition of the Platberg Group. The latter developed in an extensional rift basin caused by the tectonic inversion of faults from their previous reverse state to a normal movement. This has been interpreted as the result of impactogenal rifting. In the aftermath the collapse of the orogen allowed the deposition of the Pniel sequence and Wolkberg Group in extensional rift basins and the associated thermal cooling led to the development of the Chuniespoort Group chemical sedimentation. At no time did the Vredefort Dome affect this sedimentary history, in fact this was a post-Bushveld event which was followed by the emplacement of the Johannesburg Dome. The Vredefort Dome did, however, have a major structural impact on the Witwatersrand Basin and may have played a major role in its long term preservation up to the present day.

The nature of the Witwatersrand hinterland; conjectures on the source area problem

Although the Au and U deposits of the Witwatersrand basin are generally regarded as modified placer in origin, the nature of the source area and the ultimate origin of Au and U still remain a major enigma. A number of domal outcrops of the Archean basement are exposed in the Witwatersrand hinterland, revealing the presence of a diverse suite of granitoids and a minor (<20%) greenstone belt component. In addition, samples from over 130 boreholes drilled adjacent to the basin have revealed the character of subsurface Archean basement beneath early Proterozoic sedimentary and volcanic cover. The granitic subsurface basement is commonly characterized by pervasive hydrothermal alteration evident as propylitization, sericitization, carbonation, and greisenization, as well as a variety of vein-related alterations comprising quartz + or - K-feldspar + or - chlorite + or - CO 3 + or - sulfide + or - fluorite parageneses. These hydrothermally altered granites appear to have been preserved beneath early Proterozoic cover but have largely been eroded from surface outcrops. The hydrothermally altered granites, which are believed to be < or = 3,050 Ma, are significantly enriched in Au and U with respect to both surface granitoids from the Witwatersrand hinterland and the entire range of granites from the Barberton Mountain Land.The presence of enriched hydrothermally altered granites in the Witwatersrand hinterland lends support to the view that mineralization processes in the basin were dominated in the first instance by placer processes. The remarkable concentrations of Au and U that are unique to the Central Rand Group of the Witwatersrand basin indicate that the sediments in the upper portions of the sequence tapped a fertile source not present in the hinterland during earlier cycles of sedimentation. The erosional remnants of this source area are represented by the hydrothermally altered granites which are considered to have been a suite of high-level granitoids, the upper portions and roof zones of which were mineralized in Au and U and easily accessible to erosion. The emplacement of these granites, and the development of extensive zones of attendant, high-level, hydrothermal alteration, was synchronous with Witwatersrand sedimentation. The hydrothermally altered granites not only contributed to the Au- and U-rich detritus in the Witwatersrand, but their emplacement may have been linked causally to the deposition of the upper part of the sedimentary sequence.

The interaction between tectonics and mudflow deposits within the main conglomerate formation in the 2.8−2.7 Ga Witwatersrand Basin

The Black Bar is an erosively based unit comprising diamictites overlain by mudstones and interbedded siltstones and sandstones. It is situated within the fluvially deposited Main Conglomerate Formation, an economically important component of the Witwatersrand Supergroup. The unit is well developed on East Rand Proprietary Mines Ltd. (E.R.P.M.), situated ∼ 25 km east of Johannesburg. Conglomerate reefs (Angelo Leader, Main Reef, Main Reef Leader, South Reef) were deposited on erosional surfaces which bound tectono-stratigraphic packages. The Black Bar also represents a tectono-stratigraphic package in the middle of the Main Conglomerate Formation. The Main Conglomerate Formation was deposited during a phase of tectonism which has been associated with nearby marginal faults in previous papers. This tectonism manifests itself in the study area as a rising anticline which acted as a syndepositional topographic high over which successive packages were deposited on erosion surfaces. Regionally, the Main Conglomerate Formation forms the base of the Central Rand Group which directly overlies the Jeppestown Shale Subgroup, a sequence of fine-grained sediments deposited in a large marine or lacustrine water body. It therefore marks a rapid transition to subaerial fluvial conditions. The Black Bar extends into the East Rand Goldfield and is an important stratigraphic marker in the stratigraphy of the Main Conglomerate Formation, because of its correlation with the Green Bar in the Carletonville Goldfield and the ‘Cab’ in the West Rand. The Black Bar facies distribution on E.R.P.M. has been used in this paper to explain regional variations between the Carletonville and East Rand Goldfields. Deposition of the Black Bar diamictites is attributed to a mudflow mechanism. Texturally, clasts and sand grains are similar to those found in underlying conglomerates and associated sandstones. Geochemically the Black Bar has a similar major element composition to the underlying Jeppestown Shale. It is therefore likely that the conglomerates, sandstones and shale all acted as source components to the Black Bar. It is thought that tectonic activity and gravitational instability were the important factors in the development of the Black Bar. Movements on marginal faults would have increased depositional slopes and triggered slumps and mudflows. Tectonic uplift on the margin was complemented by basin subsidence. This led to a transgression, represented by the deposition of turbidites and laminated mudstones and siltstones which form the upper portion of the Black Bar package. This implies that the diamictites, like the mudstones, were largely deposited subaqueously. Further uplift reintroduced fluvial conditions with the erosively based Main Reef Leader and South Reef packages.

Pebble Morphology of an Ancient Conglomerate: The Middelvlei Gold Placer, Witwatersrand, South Africa

ABSTRACT A highly weathered outcrop afforded an opportunity to obtain pebbles for a morphometric clast study of a normally well indurated Early Proterozoic conglomerate, the Middelvlei gold placer. This quartz-pebble conglomerate occurs towards the base of the Central Rand Group of the Witwatersrand Supergroup, South Africa, and is a principal gold orebody mined in a goldfield known as the West Wits Line. The placer has been regarded as fluvial mainly because of a unimodal paleocurrent distribution, repeated fining-upward conglomerate-sandstone sequences, and the lenticular nature of its beds. More than 1,000 quartz pebbles from five separate conglomerate beds were measured. Pebble form was defined by maximum projection sphericity, coefficient of flatness and oblate-prolate index, all calculated from long, intermediate, and short pebble axes. A normal distribution was obtained for the first two form indices. For all five conglomerate beds, the means of all three indices are significantly greater than empirical lower limits determined by previous workers to distinguish fluvial from beach pebbles in modern gravels, confirming the fluvial origin of the beds. Although the range of pebble sizes in this study is relatively small, a significant decrease in sphericity with increasing size was found. Pebble roundness was determined for 444 undamaged pebbles by visual comparison with chart images of Krumbein (1941). Comparison of the overall mean of 0.59 with figures obtained in studies of modern rivers suggests relatively long transport distances, of the order of 160 km, for the placer.

Geochemistry of Archean shales from the Witwatersrand Supergroup, South Africa: Source-area weathering and provenance

With a few exceptions, shales from the Archean Witwatersrand Supergroup (~2800 Ma) in South Africa are depleted in Na, Ca, LILE, REE and HFSE compared to Phanerozoic shales. Cr, Co and Ni are enriched in all Witwatersrand shales and Fe and Mg are high in shales from the West Rand Groups (WRG) and lower Central Rand Group (CRG). Shales from the CRG and uppermost WRG are enriched in Na, Al, LILE, REE, HFSE and transition metals relative to shales from the lower WRG. Chondrite-normalized REE patterns for all Witwatersrand shales are enriched in light-REE and exhibit small to moderate negative Eu anomalies. A positive correlation of REE and Al 2O 3 contents in the shales suggests that REE are contained principally in clay minerals. Relative to shales from the CRG, shales from the WRG exhibit depletions of Na, Ca and Sr, a feature probably reflecting intense chemical weathering of their source rocks. CIA indices in Witwatersrand shales are variable (chiefly 70–98), even within the same shale unit. Such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism. Compared to present-day upper continental crust, all but the Orange Grove, Roodepoort, and K8 shales appear to have been derived from continental sources depleted in LILE, REE, and HFSE and enriched in transition metals. Computer mixing models based on six relatively immobile elements (Th, Hf, Yb, La, Sc, Co) and four source rocks indicate that the relative proportions of granite, basalt and komatiite increased with time in sediment source areas at the expense of tonalite. The contributions of basalt and komatiite appear to reach a maximum during deposition of the Booysens shale, and granite during deposition of the K8 shales and possibly during deposition of the Orange Grove shales.