Rand Group Research Articles

A new interpretation of soft-sediment deformation features (SSDs) in the Parktown Formation (lower West Rand Group); the physical processes of deformation during deposition

Abstract In the Parktown Formation of the lower West Rand Group (Witwatersrand Supergroup), remarkably well-preserved soft-sedimentary deformation features (SSDs) occur in a number of locations in three stratigraphic horizons across the Vredefort Mountainland and in key sites in Johannesburg. The SSDs were the inevitable products of static liquefaction during early deposition in the Witwatersrand Basin. The trigger to static liquefaction is interpreted to be autogenic and related to cyclic fluctuations in pore fluid pressure(s) during capped dewatering. Allogenic triggers are not indicated. We suggest that static liquefaction was an integral physical process to sediment deposition on the Parktown Formation offshore platform, but a process not previously recognised as important to the modification of the West Rand Group stratigraphy across the Witwatersrand Basin.

Stratigraphy and Basin Analysis of the West Rand Group, Witwatersrand Supergroup

Abstract The Mesoarchaean West Rand Group displays a layer-cake stratigraphy with lithostratigraphic units correlatable on a basin-wide scale. The ~5 km-thick succession consists of fluvial braidplain and shelf deposits, which range from shallow inner shelf marine orthoquartzites, outer shelf argillites to starved shelf iron-formations. Minor diamictites are of debris flow origin and are possibly related to glacial activity. Three major sequences are present: Sequence I (Hospital Hill Subgroup) is bounded by an angular unconformity at the base of the Orange Grove Formation and a low-angle unconformity at the base of the Promise diamictite. Sequence II (Government Subgroup) extends from the base of the Promise diamictite to a well-defined low-angle unconformity at the base of the Koedoeslaagte Formation. Sequence III (Jeppestown Subgroup) comprises the succession between the Koedoeslaagte Formation and the Maraisburg Formation, up to the low-angle unconformity at the base of the Main Reef. Sequence I was deposited during a period of highstand of sea-level, Sequence II during a period of relative lowstand, and Sequence III during a period of relative highstand coupled with high rates of sediment supply. Isopach, depofacies and palaeocurrent analyses indicate that strata in the western to northwestern parts of the basin were deposited under more proximal sedimentary conditions compared to those in the central or southeastern parts of the basin. There is little relationship between the present outline of the basin and the distribution of depofacies or isopachs of sequences, and it is therefore concluded that the original sedimentary basin was significantly larger in areal extent. Depofacies and thickness distribution, as well as synsedimentary deformation of strata, indicate that the basin was most probably of flexural tectonic origin. These findings strongly support deposition in a wide, shallow, and rather stationary foreland basin, with an axial zone towards the west/northwest and a low amplitude peripheral bulge to the east/southeast. Such shallow foreland basins, with abundant sediment bypassing, are thought to be associated with windward-facing orogenic fronts. High rates of erosion along such fold-thrust belts lead to ineffective loading and advancement of the orogenic front, as well as an oversupply of sediment.

Uranium distribution in the promise reefs of the Mesoarchean Westrand Group, Witwatersrand Supergroup, South Africa

The Mesoarchean Westrand Group (2932 – 2923 Ma) constitutes the lower portion of the Witwatersrand Supergroup. Compared to the overlying Central Rand Group which is well known for its world-class conglomerate-hosted gold-uranium deposits, the economic potential of palaeoplacers in the Westrand Group has not been fully explored. This study aims to assess the uranium potential of the Promise Reefs in the Westrand Group. Field exposures of the Promise Reefs and boreholes drilled down-dip from the surface outcrops were investigated in the Westrand area. Three reef zones were recognized consisting of well-mineralized, matrix-to clast-supported, small pebble conglomerates with associated medium- to coarse-grained quartzite.Radioactive minerals found in the conglomerates comprise leucoxene, brannerite, carbon nodules, monazite, allanite, thorite, and uraninite.Analyses of U and Th from borehole samples reported mean values of 120 ppm for U and 60 ppm for Th. Maximum U and Th concentrations are 630 ppm and 125 ppm, respectively. The highest U/Th ratio was 17.4 with a mean value of 2. The conglomerates from surface outcrops have maximum U and Th concentrations of 111 ppm and 140 ppm, respectively, with a mean U value of 19.5 ppm and a mean Th value of 26.6 ppm, and a mean U/Th ratio of 0.6. Comparison of the data shows that surface outcrops generally report lower U concentrations than their unweathered equivalents.Also, U assays by γ-spectrometry and INAA of the same samples from surface outcrops yielded inconsistent results. As U exploration by field γ-spectrometry relies on the existence of 234U/238U equilibrium and because of the U depletion in the surface samples, further investigations using α-spectrometry and Pb-isotope analyses were employed.The results obtained from α-spectrometry point to complex U redistribution processes in the surface conglomerate with several stages of uranium loss and/or gain. The bulk of the U was removed during an early stage of surface exposure and supergene alteration, probably more than 1.4 Ma ago.The estimation of the original pre-weathering U concentration in the surface conglomerates is based on the measured Pb-isotope ratios 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb and the Pb and Th concentrations. The data were inserted into U-Th-Pb age equations which were reformulated and solved for the U concentrations. To test the method, for a suite of borehole samples the same treatment was applied. The calculated fresh-rock U tenor indicates the occurrence of supergene uranium losses which are of the same magnitude as the U losses caused by experimental leaching of fresh conglomerate samples. The high amount of radiogenic Pb found in the surface samples points to severe uranium losses earlier than 100 Ma ago.The data show that U redistribution in weathered Mesoarchean conglomerates is not governed by systematic processes which obscure the original pre-weathering U concentration. Application of the Pb isotope technique proves to be the only method yielding usable estimates of the fresh-rock uranium tenor in deeply weathered terranes.

Linking Archaean climate change with gold metallogeny

First large-scale concentration of gold to ore grade in Earth’s crust took place at around 2.9 Ga in quartz-pebble conglomerates, well before endogenous deposits, such as porphyry and epithermal systems or orogenic-type deposits, started to play a significant role from ca. 2.75 Ga onwards. The conglomerate-hosted gold placers, typified by those in the Mesoarchaeaen Witwatersrand Basin in South Africa, are thought to have been sourced from the leaching of background levels of gold in the Archaean continental crust, promoted by deep chemical weathering under a reducing acidic atmosphere. Gold dissolved in meteoric waters was trapped by possibly acidophile microbes, fossil remnants of which are preserved as kerogen layers in the 2.9 Ga lower Central Rand Group of the Witwatersrand Supergroup. Mechanical reworking of the delicate microbial mat-bound gold led to rich gold placers from 2.9 Ga onwards. Well-endowed gold placers older than 2.9 Ga are conspicuously missing, although suitable siliciclastic host rocks, even containing relics of former microbes, are known from continental sediment sequences as old as 3.22 Ga. A review of climate-sensitive rock types and geochemical data on the extent of chemical weathering reveals that the period from 2.96–2.91 Ga was cold, with repeated glaciations as evidenced by glaciogenic diamicite, whereas warmer and (or) wetter climates reigned from 2.90 to 2.78 Ga. In concert with the temporal distribution of placer gold accumulation, the conclusion is reached that chemical weathering rate, dictated by climate, was a key determining factor in the exogenous binding of Au into sedimentary deposits.

A positive syn-fold test from the Neoarchaean Klipriviersberg Group of South Africa: Quo vadis Vaalbara?

Abstract The existence of Vaalbara, the combined Neoarchaean to Palaeoproterozoic Kaapvaal-Pilbara supercraton, is questionable during the early Neoarchaean when scrutinised through the lens of recent Australian and South African palaeomagnetic data. Remarkably similar ~2.7 to 2.5 Ga geological successions (with near bed-for-bed correlatability) support a coherent Vaalbara at the end of the Neoarchaean. Here we report palaeomagnetic and rock magnetic results from the Klipriviersberg Group of South Africa, which is the oldest rock sequences used to define Vaalbara originally. A positive syn-fold test illustrated a high-temperature remanence component acquired during the formation of the Witwatersrand syncline. This fold structure predates the Vredefort Impact Structure and its formation is synchronous with the deposition of the Mesoarchaean Central Rand Group and extrusion of the Klipriviersberg Group. The studied rocks of the Klipriviersberg Group are not directly dated, but most are likley younger than 2 780 to 2 789 Ma, based on detrital zircon ages from the lowermost Ventersdorp Supergroup and U-Pb baddeleyite ages for mafic sills that intrude the Witwatersrand Supergroup that are regarded as feeders of the Kliprivierberg Group lavas, but older than the overlying 2 720 to 2 750 Ma Platberg Group. The Klipriviersberg Group pole is at 27.7°S, 32.7°E with an A95 of 11°. A comparison of Meso- to Neoarchaean palaeopoles from the Kaapvaal and Pilbara cratons suggests their shared drift path traversing the polar circle and thus supports the existence of Vaalbara across the 2.78 to 2.70 Ga interval.

Polyphase Archean to Paleoproterozoic deformation along the northwest margin of the Johannesburg Dome, Kaapvaal Craton

Despite extensive gold exploration, the reasons for the dome shape of the Paleoproterozoic Johannesburg Dome (JD) remain elusive. We investigated the structural record of an outlier of Archean to Paleoproterozoic supracrustal rocks (the Zwartkops Hills Outlier, ZHO) overlying the JD to investigate its tectonic evolution. Steep gneissic fabrics in the basement pre-date the intrusion of ∼ 3225 Ma trondhjemite (new U–Pb zircon age). Two Archean deformation events affected West Rand Group rocks and pre-date Platberg Group deposition. To the south of the ZHO, a newly recognized, large-scale, recumbent fold formed during top-to-the-south shearing. In the central part of the ZHO, steeply W-dipping stratification results from E-W shortening. Two more deformation events are Paleoproterozoic in age and affected all units. A shallowly S-SW-dipping cleavage formed in the biotite zone of the greenschist facies is associated with N–S shortening and top-to-the-north shearing. The cleavage transects the large recumbent fold and is observed down to the upper 30 m of the basement. Finally, a weak N–S shortening event is inferred from kink folds and north-dipping axial planar cleavage. These data are consistent with the existing regional models of (i) south- and east-directed fold-and-thrust belts formed during sedimentation of the Archean Central Rand Group, (ii) north-verging schist belt formed during, or after the Transvaal Supergroup deposition, and (iii) impingement of the Bushveld Complex onto the JD during N–S shortening. These findings are integrated with previous regional sedimentological, seismic, and structural studies to suggest that the JD granitoid basement formed a paleo-high during the deposition of the Central Rand and Platberg groups. Once buried in the upper crust, the paleo-high acted as a rigid granitoid promontory that indented weak surrounding cover rocks during Paleoproterozoic N–S contraction events, creating the dome shape of the Johannesburg Dome.

U-Pb-Hf isotopes and shape parameters of zircon from the Mozaan Group (South Africa) with implications for depositional ages, provenance and Witwatersrand – Pongola Supergroup correlations

The Mozaan Group represents the youngest unit of the c. 2.9 Ga Pongola Supergroup located along the south-eastern margin of the Kaapvaal Craton. It comprises a ca. 4800 m thick succession of clastic sedimentary rocks intercalated by minor chemical and volcano-sedimentary rocks deposited in shallow marine to fluvial environments, and is stratigraphically correlated with the auriferous Witwatersrand Supergroup. This correlation, however, is speculative, in particular as systematic information about depositional ages and sediment provenances are absent. To address these problems, we present new combined sets of U-Pb ages, Hf isotopes, and shape parameters (width, length, aspect ratios and roundness) of >700 detrital zircon grains from seven samples of the Mozaan type profile in the Hartland area. These data reveal a switch in provenance between the lower and upper Mozaan Group. Zircons in sandstones of the lower Mozaan Group (Sinqeni to Ntombe formations) were supplied from surrounding proto-Kaapvaal Craton, and those in upper Mozaan Group rocks (Delfkom to Ntanyana formations) predominately from a juvenile hinterland, comprising sources as far as the Pietersburg and/or Kimberley blocks, which became amalgamated to the proto-Kaapvaal Craton at 2.97–2.87 Ga. Significant overlap of zircon age spectra, Hf isotope data, and maximum depositional ages (2908 ± 8 Ma to 2866 ± 7 Ma) suggest similar sources for upper Mozaan Group and Central Rand Group sediments of the Witwatersrand Basin. In contrast, sedimentary rocks of the West Rand Group have no counterparts in the Pongola Basin, except for the Orange Grove Formation, which shows good agreement with the Sinqeni Formation. The provenance switch indicated by the age-Hf isotope data is not identified by zircon shape parameters. These rather reflect differences in depositional environment (littoral, fluvial, volcanogenic), related to the duration and energy of sediment transport and reworking, as indicated by specific patterns in grain size vs. roundness diagrams.

Source-to-Sink Analysis of the Gold-Hosting Mesoarchean Main and Mondeor Formations (Central Rand Group) in the Area South of Johannesburg, Witwatersrand Basin, Kaapvaal Province, South Africa

AbstractTo date, all source-to-sink research has focused on the Phanerozoic and Proterozoic stratigraphic record. This study attempts to apply source-to-sink principles to the 2.8–3.0 Ga placer gol...

A geochemical study of the Crown Formation and Bird Member lavas of the Mesoarchaean Witwatersrand Supergroup, South Africa

Abstract The ca. 2.97 to 2.80 Ga Witwatersrand Supergroup, South Africa, represents the oldest intracontinental sedimentary basin of the Kaapvaal craton. Two volcanic units occur in this supergroup: the widespread Crown Formation lavas in the marine shale-dominated West Rand Group and the more geographically restricted Bird Member lavas, intercalated with fluvial to fluvio-deltaic sandstone and conglomerate of the Central Rand Group. These units remain poorly studied as they are rarely exposed and generally deeply weathered when cropping out. We report whole-rock major and trace elements, Hf and Nd-isotope whole-rock analyses of the lavas from core samples drilled in the south of the Witwatersrand basin and underground samples from the Evander Goldfield in the northeast. In the studied areas, both the Crown Formation and Bird Member are composed of two units of lava separated by sandstone. Whereas all the Crown Formation samples show a similar geochemical composition, the upper and lower volcanic units of the Bird Member present clear differences. However, the primitive mantle-normalized incompatible trace element concentrations of all Crown Formation and Bird Member samples show variously enriched patterns and marked negative Nb and Ta anomalies relative to Th and La. Despite the convergent geodynamic setting of the Witwatersrand Supergroup suggested by the literature, the Crown Formation and Bird Member are probably not related to subduction-related magmatism but more to decompression melting. Overall, the combined trace element and Sm-Nd isotopic data indicate melts from slightly to moderately depleted sources that were variably contaminated with crustal material. Greater contamination, followed by differentiation in different magma chambers, can explain the difference between the two signatures of the Bird Member. Finally, despite previous proposals for stratigraphically correlating the Witwatersrand Supergroup to the Mozaan Group of the Pongola Supergroup, their volcanic units are overall geochemically distinct.

Quality of life post-reconstruction of the bile tract

Presenter: Norberto Portillo Lopez MD | Universidad de la Frontera Background: Bile duct reconstruction secondary to bile duct injury associated to cholecystectomy is a complex surgery and an important aspect is quality of life before the procedure. Methods: Quality of life study conducted in a concurrent cohort for convenience. The cohort is composed of 32 patients undergoing bile duct reconstruction for bile duct injury associated to cholecystectomy. To have a comparison group, a cohort of patients undergoing cholecystectomy without incident was chosen. These cohorts were matched 1: 1 by age (± 1 year), gender and type of surgery. The SF-36 questionnaire was applied with the score proposed by the RAND group personally or by telephone. Results: Results: The bile duct reconstruction patient cohort consists of 32 patients, with an average age of 47 ± 17.68 years; predominantly female gender (78%). The average number of hospi-talization days was 20 ± 11.85. The average follow-up was 7 ± 5 years. The average score obtained by patients undergoing bile duct reconstruction vs cholecystectomy without com-plications was: Physical Function: 77,5 ± 21.81 vs 78,91 ± 25,42; Physical Role: 60,94 ± 37,53 vs 65,63 ± 36,89; Body pain: 47,86 ± 21,95 vs 43,2 ± 23,49; General Health: 57,84 ± 10,45 vs 56,13 ± 17,26; Vitality: 42,97 ± 11,56 vs 47,34 ± 16,26; Social Function: 52,73 ± 20,84 vs 49,61 ± 16,03; Emotional Role: 66,41 ± 31,8 vs 71,72 ± 36,01; Mental Health: 42,38 ± 10,94 vs 44 ± 6,82; finding no statistically significant difference between the 2 cohorts. Conclusion: The present study found no significant difference in quality of life between patients with bile duct injury associated to cholecystectomy and bile duct reconstruction in comparison with cholecystectomized patients without complications

Age of the Archaean Murchison Belt and mineralisation, South Africa

Abstract The east-northeast-trending Murchison-Thabazimbi Lineament in northern South Africa is one of the world’s most important structures for its control on world-class mineral deposits, Proterozoic sedimentary basins and giant igneous intrusions. The deepest exposed Archaean parts of the lineament are the Murchison Belt. Bounded by granitoids, the belt comprises greenschist to amphibolite facies volcano-sedimentary strata with isoclinal folds and the 7 km thick meta-igneous Rooiwater Complex. The Rooiwater Complex is intruded by a northern regional granitoid dated at 2 929 ± 7 Ma by SHRIMP U-Pb on zircons. Using field relationships, published isotopic age data and new SHRIMP zircon dates we confirm the age Rooiwater Complex at 2 965 Ma, showing it to be contemporaneous with the Archaean volcanic and sedimentary formations, the meta-igneous Complex being the lower sequence in a ~2 980 to 2 960 Ma island arc. Despite being implicated as a source of gold for the world’s largest natural accumulation of gold in the Witwatersrand Basin, the absolute age of Sb-Au mineralisation in the Murchison Belt is poorly constrained. We have utilised SHRIMP U-Pb geochronology to date monazites from a Sb-Au ore sample from the granitoid-hosted Malati Pump orebody and determine ages for two different generations of monazite, both associated with ore minerals. The older age of 2 832 ± 23 Ma is from a minority of grains and is interpreted to date the primary Sb-Au mineralisation, about 120 Ma after belt formation. This age predates, or is possibly synchronous with, sedimentation of the upper-Witwatersrand Central Rand Group. The younger age of 1 968 ± 17 Ma from a majority of monazite grains is unrelated in time to known events and interpreted here as a cryptic hydrothermal reworking of the Sb-Au ores in this deposit.

The Significance of Erosion Channels on Gold Metallogeny in the Witwatersrand Basin (South Africa): Evidence from the Carbon Leader Reef in the Carletonville Gold Field

Abstract Within the eastern portion of the Carletonville gold field, the gold- and uranium-rich Carbon Leader reef of the Central Rand Group (Witwatersrand Supergroup) is truncated by an erosion channel. This channel is asymmetrical and lenticular in shape, measuring 150 to 180 m in width and up to 100 m in depth. High-resolution seismic data show that the erosion channel cuts from the Carbon Leader reef into all older units of the Central Rand Group down to the Roodepoort Formation of the underlying West Rand Group. A total of seven boreholes were drilled into the channel, revealing that it is composed of quartzite at its base (9 m thick), overlain by deformed (lower) and laminated (upper) chloritoid-bearing shale (21 m thick) and quartzite (18 m thick). The Carbon Leader reef is highly enriched in gold (5–40 g/t Au), whereas the gold tenor of the erosion channel fill is in general much lower (<1 g/t Au), although locally grades of as much as 3.8 g/t Au are reached. Detailed seismic, sedimentological, and petrographic analyses revealed that the channel was filled with locally sourced sediments from the Main Formation. A closed-system mass balance further demonstrates that gold in the erosion channel could have been entirely sourced from the Carbon Leader reef. Sediment load played a crucial role in the distribution of gold in the channel, thus supporting a stratigraphically controlled modified placer model for the origin of gold in the Carbon Leader reef.

High-resolution Integrated Geophysical Investigation at the Lancaster Gold Mine, Krugersdorp, South Africa

An integrated geophysical approach using seismics and geoelectrical techniques was employed to investigate the architecture of historic narrow-reef workings and a proposed open-pit mine at Lancaster Gold Mine, near Krugersdorp, South Africa. The mining activities in the area were mainly carried out within the Kimberley Reef Package in the upper Central Rand Group of the Witwatersrand Supergroup, which hosts several gold-bearing conglomerates (locally known as reefs). The reefs are generally thin (≤ 2 m thick) and dip between 28° and 32° south. The low-velocity weathered layer introduces significant static shifts in the reflection seismic data. Moreover, environmental noise from drilling and trucking, and the prominent bedrock-overburden contact that produces various wave conversions (P-S conversion), caused undesirable noise that contaminates the shot gathers as high-amplitude, source-generated and monochromatic noise. The noise was removed from the shot gathers using frequency and velocity filtering techniques. The final depth-migrated sections are characterised by high-resolution images of the subsurface from ~10 to ~150 m depth, which are constrained by the borehole information. The reflection seismic data delineate the interfaces between different rock layers and the stopes. The refraction and resistivity tomograms, on the other hand, provide more detailed images of the top 20–50 m of the subsurface and depict the approximate shallow geometry of fluid migration paths, mined-out areas, and bedrock-overburden boundaries. The integrated results indicate that the study area is characterised by a weathered surface layer with variable low P-wave velocity (400–1200 m/s) and resistivity (150–800 Ωm). The deeper layer reveals an increase in resistivity and velocity, and it’s characterized by discontinuities, weak zones, cavities or water-bearing zones due to the mining activities. The combined borehole and geophysical data provide valuable information regarding the physical characteristics of the subsurface and can be helpful for future risk management decisions, environmental and engineering studies in the area.

An Introduction to the Proceedings of the AAOP’s State-of-the-Science Conference on the Economic Science of Lower-Limb Prosthetic Rehabilitation

Recent years have witnessed an increased interest in the economic science associated with prosthetic rehabilitation. This has been evidenced in a series of economic publications, reports, and analyses. Recognizing the increasing relevance of economics in health care policy and decision making, the American Academy of Orthotists and Prosthetists held a State-of-the-Science Conference on November 2–3, 2017, on this important topic. Several of the presenters shared materials created through recent collaborations with the American Orthotic and Prosthetic Association. The participation of both organizations in this event allowed for a comprehensive consideration of all available economic publications in a single setting. The following proceedings include reprints of relevant economic publications, multidisciplinary perspective pieces, and an introduction to economic science for those working in rehabilitation of individuals with amputation.Figure 1: Phillip M. Stevens, MEd, CPO, FAAOP.Figure 2: M. Jason Highsmith, PhD, DPT, CP, FAAOP.The proceedings begin with two systematic reviews encompassing the existing body of economic science in prosthetic science through November 2015. The first comprises a scoping review of the small number of available evaluations germane to transtibial prosthetic care, whereas the latter is a narrative review of descriptive economic literature through the same period. These reviews are followed by an introduction to health care economics for those working in prosthetic rehabilitation in which prosthetic publications and applications are described in economic terms and principles. This includes introductions to the concepts of value and value assessment, economic evaluation strategies of benefit, consequence, effectiveness and utility, quality-adjusted life years, incremental cost-effectiveness ratios, and willingness-to-pay thresholds. This is followed by a perspective piece on the value of prosthesis-enabled mobility inclusive of the impact of mobility on quality of life, its role in the avoidance of negative outcomes associated with prolonged immobilization, the burden of disease associated with limb loss as characterized by disability-adjusted life years, and the health benefits associated with physical activity with a prosthesis. These broad introductory pieces are followed by more focused perspective pieces from the consumer, the provider, and the manufacturer with respect to economic considerations in prosthetic rehabilitation. A supplemental perspective piece representing international economic approaches and considerations is also considered. These perspective pieces are followed by reprints of three of the most recent and influential economic analyses, all commissioned by the American Orthotic and Prosthetic Association. These include retrospective analyses of Medicare claims data by Dobson et al. and an economic modeling exercise on the impact of microprocessor knees on individuals with transfemoral amputation performed by the RAND group. Collectively, this material represents a current, comprehensive consideration of available economic principles and data as they pertain to lower-limb prosthetic rehabilitation. The hope is that this document will usher in an increased awareness of the relevance of economic considerations in future clinical trials and coverage policies.

U-Pb and Hf isotopes of detrital zircons from the Pongola Supergroup: Constraints on deposition ages, provenance and Archean evolution of the Kaapvaal craton

The c.3.0–2.9 Ga Pongola Supergroup located on the south-eastern margin of the Kaapvaal Craton preserves some of the most pristine Archean successions but the timing of volcanic and sedimentary processes, and sediment sources remain relatively poorly known. Combined zircon U-Pb ages and Hf isotope data of sandstones from the Nsuze and Mozaan Groups are presented from key locations of the Northern, Central and Southern Domains of the 270 km long belt. These data indicate important lateral and temporal variations and therefore changes in source areas. The basal Pongola sedimentary rocks (Mantonga Formation) in the Northern Domain were derived from the c.3.4 Ga basement, and also most likely from reworked Moodies Group sedimentary rocks of the Barberton greenstone belt. Zircon detritus in Nsuze and lower Mozaan Groups sandstones shows similar age-Hf isotope spectra, and a remarkable overlap with data from magmatic and (meta)sedimentary rocks of the southeastern Kaapvaal craton. The data point to a proximal source for Nsuze sandstones in the Northern Domain and a different provenance for the Southern Domain sandstones. A possible signature of the extensive 3.10–3.07 Ga Mpuluzi granite is seen only in the Nsuze sandstones and basal Mozaan of the Central Domain. The data indicate for the first time that Cr-rich sandstones of the Ndikwe Thrust Segment of the Southern Domain were deposited at <2.92 Ma, and therefore belong to the uppermost Mozaan Group. Furthermore, they reveal a significant switch in provenance from the lower to upper Mozaan Group from a reworked Archean basement towards a more juvenile hinterland with εHft = +0.9 to +6.3 between 3.07 and 2.92 Ga; similar to the source previously reported for the Central Rand Group of the Witwatersrand Basin. The juvenile zircon detritus in the upper Mozaan Group requires sources as far as the Pietersburg and/or Kimberley Blocks, which became amalgamated to the central block of the Kaapvaal Craton at 2.97–2.87 Ga, but perhaps also a source outside of the present day Kaapvaal Craton, and possibly as far as the Pilbara Craton.

The Use of the Delphi and Other Consensus Group Methods in Medical Education Research: A Review.

Consensus group methods, such as the Delphi method and nominal group technique (NGT), are used to synthesize expert opinions when evidence is lacking. Despite their extensive use, these methods are inconsistently applied. Their use in medical education research has not been well studied. The authors set out to describe the use of consensus methods in medical education research and to assess the reporting quality of these methods and results. Using scoping review methods, the authors searched the Medline, Embase, PsycInfo, PubMed, Scopus, and ERIC databases for 2009-2016. Full-text articles that focused on medical education and the keywords Delphi, RAND, NGT, or other consensus group methods were included. A standardized extraction form was used to collect article demographic data and features reflecting methodological rigor. Of the articles reviewed, 257 met the inclusion criteria. The Modified Delphi (105/257; 40.8%), Delphi (91/257; 35.4%), and NGT (23/257; 8.9%) methods were most often used. The most common study purpose was curriculum development or reform (68/257; 26.5%), assessment tool development (55/257; 21.4%), and defining competencies (43/257; 16.7%). The reporting quality varied, with 70.0% (180/257) of articles reporting a literature review, 27.2% (70/257) reporting what background information was provided to participants, 66.1% (170/257) describing the number of participants, 40.1% (103/257) reporting if private decisions were collected, 37.7% (97/257) reporting if formal feedback of group ratings was shared, and 43.2% (111/257) defining consensus a priori. Consensus methods are poorly standardized and inconsistently used in medical education research. Improved criteria for reporting are needed.

Uranium in South Africa: Exploration and Supply Capacity

The major uranium deposits types are quartz-pebble conglomerate of the Central Rand Group (Witwatersrand Supergroup) and the Dominion Group, sandstone of the Karoo Uranium Province, surficial deposits of the Namaqualand region, as well as coal of the Springbok Flats Basin. Recent exploration activities (both greenfields and near-mine) have increased the resource potentials of these deposits. South Africa's Council for Geoscience has conducted high resolution magnetic and radiometric surveys in the Namaqualand region. Witwatersrand Basin is the only deposit where uranium mining is active at present in South Africa. Total uranium production in 2015 was 448 t. However, with production expected to increase in the Witwatersrand Basin, production is expected to reach 700 t U by the end of 2018. Furthermore, with uranium production envisaged to begin in the Karoo Uranium Province, uranium production may exceed 1000 t U by the year 2020.

A Review of the Witwatersrand Basin - The World’s Greatest Goldfield

The world's greatest deposit of gold is hosted by the Archaean Witwatersrand sedimentary basin, situated in the central portion of the Kaapvaal Craton of South Africa. The geological setting of this remarkable clastic sedimentary deposit, which has yielded more than one third of all the gold ever produced on the planet, is discussed. The stratigraphy and structure of the Witwatersrand Supergroup is reviewed together with the sedimentology, mineralogy and geochronology of the more important auriferous conglomerate (reef) horizons. Geophysical methods of exploration are discussed briefly. The Witwatersrand Basin was formed in response to a series of crustal plate movements from the north and west in a foreland basin setting. Deposition commenced in a shallow marine environment with quartzite and shale, including prominent iron rich shale horizons forming the West Rand Group (WRG). The overlying dominantly arenaceous Central Rand Group (CRG) was deposited in a basin which regressed as it filled. Important gold deposits formed on braided fluvial plains which sometimes terminated in a shoreline. Gold reefs at the base of the CRG show evidence of later marine reworking. Higher in the sequence, fluvial deposits become dominant. The focus of this paper is the Central Rand Group where most of the gold mineralisation occurs. Detrital gold (largely within sulphides) and uranium (as uraninite) were originally concentrated along with placer minerals such as chromite and zircon, at the base of fluvial cycles. In addition to the placer mineralisation, prokaryotic bacteria flourished and provided carbon-rich material that was highly effective in the concentration of non-detrital gold. Although most of the lithologies, including reef horizons, have great continuity and can be correlated across much of the Witwatersrand Basin, discrete entry points of reef material, with distributary channels and associated ore shoots, can generally be clearly defined. Three major events reshaped and metamorphosed the Witwatersrand. Lateral crustal plate movements caused severe faulting and folding, especially along the western and northern sides of the Basin. The second event was the catastrophic Vredefort meteorite impact which struck the centre of the basin. The resulting fluid movement which pervaded the Witwatersrand strata caused widespread metasomatic alteration. Localised alteration and remobilisation of the gold mineralisation is thought to have been in partial response to this event. The third major event was the emplacement of the Bushveld Complex, which is the largest layered igneous intrusion of its kind on Earth. The heat generated by this intrusion certainly exceeded that of the Vredefort event, also making it an important agent of mineral alteration and remobilisation, especially along the northern flank. The Great Debate on the origin of the gold has been the subject of heated discussions for decades. The three main points of view are a placer source, a hydrothermal source and a combination of the two called the modified placer theory. Recognition of the problem was realised soon after the discovery of the gold and continues to this day. Evidence has been presented in favour of all arguments, lending more credence for the modified placer process. Although the Witwatersrand is a mature goldfield with declining production, it is estimated that it still contains six times more gold than the world's second largest goldfield. Much of the remaining resource occurs at considerable depths; however, there are still opportunities for extracting some of this resource, as well as generally somewhat lower grade mineralisation, at moderate to shallow depth. The Basin thus remains a major exploration target. Gold mining in the Witwatersrand Basin has been responsible for the creation of the metropolis of Johannesburg and many thriving mining towns and has had a major influence in the shaping of South Africa. It still plays an important role in the country's economy.

The reliability of ∼2.9 Ga old Witwatersrand banded iron formations (South Africa) as archives for Mesoarchean seawater: Evidence from REE and Nd isotope systematics

Pure marine chemical sediments, such as (Banded) Iron Formations, (B)IFs, are archives of geochemical proxies for the composition of Precambrian seawater and may provide information about the ancient hydrosphere-atmosphere system. We here present rare earths and yttrium (REY) and high precision Sm–Nd isotope data of ∼2.90 Ga old Superior-type BIFs from the Witwatersrand Supergroup, South Africa, and compare those with data for near-contemporaneous BIFs from the correlative Pongola Supergroup (Superior-type BIF) and from the Pietersburg Greenstone Belt (Algoma-type IF), respectively. All Witwatersrand samples studied display the typical general REY distribution of Archean seawater, but their REY anomalies are less pronounced and their immobile element concentrations are higher than those of other pure (B)IFs. These observations indicate the presence of significant amounts of detrital aluminosilicates in the Witwatersrand BIFs and question the reliability of the Contorted Bed and Water Tower BIFs (Parktown Formation, West Rand Group) as archives of Mesoarchean seawater. Significant post-depositional alteration of the REY budget and the Sm–Nd isotope system is not observed. The Nd isotopic compositions of the purest BIF samples, i.e. the most reliable archives for Witwatersrand seawater, show initial εNd values between −3.95 and −2.25. This range is more negative than what is observed in ambient shales, indicating a decoupling of suspended and dissolved loads in the “near-shore” Witwatersrand Basin seawater. However, εNd range overlaps with that of the correlative Pongola BIF (Alexander et al., 2008). The deeper-water Algoma-type Pietersburg BIF shows more positive (i.e. more mantle-like) εNd2.9Ga values, supporting the hypothesis that a significant amount of its REY inventory was derived from black smoker-style, high-temperature hydrothermal fluids that had altered seafloor basalts. In marked contrast, the dissolved REY budgets (including the Nd isotopic compositions) of the Witwatersrand and Pongola seawater, however, were dominated by similar terrigenous REY sources from the Kaapvaal Craton.

Evidence for an Intrabasinal Source and Multiple Concentration Processes in the Formation of the Carbon Leader Reef, Witwatersrand Supergroup, South Africa

Laser ablation-inductively coupled plasma-mass spectrometer (LA-ICP-MS) trace element maps of pyrite and gold from the Carbon Leader Reef in the Witwatersrand basin provide new evidence that a significant proportion of the pyrite and gold was intrabasinal, derived from the West Rand Group or equivalent shales stratigraphically below this reef. Rounded detrital pyrite grains in the Carbon Leader Reef vary from compact to porous to sooty, with similar textures and composition to diagenetic pyrites reported from sedimentary rocks in the West Rand Group. Detrital porous and sooty pyrites contain 0.4 to 11.3 ppm solid-solution gold, sulfur isotope δ34S values from -17 to +16%, and high levels of As and Te, plus a wide range of other trace elements (in decreasing order: Ni, Co, Sb, Cr, Cu, U, Pb, Bi, Mo, Zn and Ag). The sooty pyrite is the most Au, Te, and Mo rich and is intergrown with alumino-silicates and organic matter. The detrital pyrite textures, S-isotopes and geochemistry resemble diagenetic pyrite developed under suboxic to anoxic bottom water conditions. The LAICP-MS maps also show that the detrital pyrite grains have euhedral hydrothermal pyrite overgrowths containing micro-inclusions of gold, brannerite, and Ni-As sulfides. The pyrite overgrowths are also enriched in solid-solution gold, tellurium, and other trace elements including, in decreasing order, As, Co, Ni, Cr, Cu, Mn, Pb, Bi, and Ag. Fractured and brecciated pyrite associated with brittle bedding-parallel fracture zones in the Carbon Leader Reef are also enriched in these elements due to alteration of pyrite surrounding the fractures. Laser ICP-MS Pb isotope determinations on the cores of detrital pyrite indicate an age between 2750 and 2950 Ma with hydrothermal overgrowths originating between 2100 and 2020 Ma incorporating highly radiogenic Pb. This study demonstrates that both of the two competing theories for the origin of the Witwatersrand gold reefs are likely to be correct. We suggest that the hydrothermal event was widespread (kilometer scale) and involved basinal fluids that scavenged gold, tellurium, arsenic, and trace elements (Co, Ni, Cr, Cu, Mn, Pb, Bi, Ag) from gold-bearing sedimentary units in the Central Rand Group. © 2013 Society of Economic Geologists, Inc.