Conglomerate Horizons Research Articles

A mineralogical perspective on the recovery of uranium from brannerite-rich ore at Cooke Section, West Rand Goldfield, South Africa

Abstract The recovery of uranium from quartz-pebble conglomerates of the Witwatersrand Basin is accomplished through sulphuric acid dissolution under oxidising conditions. At Cooke Section on the West Rand Goldfield, the extraction process has been plagued by low to moderate yields on the order of 40 to 75%, as opposed to a target recovery of 80%. This has been ascribed to the high abundance of brannerite in the ore, which has traditionally been more problematic to leach. In addition to brannerite, poor metallurgical recoveries may also be associated with processing inefficiencies related to comminution, residence time, acid dosage and leach temperature. In view of this, a range of ore samples (channel samples) were collected from four uranium-bearing conglomerate horizons at Cooke Section (the A1, A5, E9EC and UE1A reefs) for detailed mineralogical and metallurgical characterisation, involving automated mineralogical analysis, and laboratory-scale leach testwork. The mineralogical results show that the major uranium-bearing minerals of uraninite, coffinite and brannerite are fine-grained (~80% passing 32 micron) and exhibit high degrees of mineral exposure to the lixiviant (~99%). Despite these favourable attributes, the elemental deportment data indicate that brannerite accounts for approximately 43% of the combined uranium budget. Further inspection shows that brannerite can be subdivided into three compositional subtypes: uraniferous brannerite (~13% U deportment), brannerite (~25% U deportment) and titaniferous brannerite (~5% U deportment). Baseline laboratory leach tests, which replicated plant leach conditions of 30 kg/ton acid, 4 kg/t oxidant, 24 hour residence time and 60°C leach temperature, yielded elevated dissolutions between ~77% and ~96%, with a combined overall uranium recovery of ~94%. These results are not consistent with the low yields obtained at the processing plant, and suggest that the high level of uranium recovery can be attributed to the effective leaching of brannerite (most likely uraniferous brannerite and brannerite). In view of prevailing market conditions, variability tests were carried out on a representative bulk composite sample to investigate the potential to achieve similar yields under more cost-effective leaching conditions. In these tests, a single parameter was varied (e.g. acid dosage), while the remaining parameters remained at baseline conditions. The results demonstrate that uranium recoveries of ~80% can be achieved on Cooke Section ores at low acid dosages and high temperatures (18 kg/t, 60°C) or at moderate acid dosages and low temperatures (23 kg/t, 30°C). The associated reduction of input costs would represent a significant cost-saving for the Ezulwini gold and uranium recovery plant. It is concluded that the poor uranium yields encountered during commercial processing of the ore are most likely related to undiagnosed inefficiencies in the treatment plant, such as excessive acid consumption related to elevated temperatures/oxidant addition and/or insufficient leach residence times, especially when recirculating, continuous flow-through leaching systems are in use. The broader implication of this study is that uranium processing operations beyond Cooke Section may be able to optimise their process designs and reduce input costs by quantifying the different types of brannerite within their ores through automated mineralogical analyses. The present study thus demonstrates the value of a geometallurgical approach in enhancing the understanding of uranium recovery through acid leaching.

An insight to the initiation of Cretaceous sedimentation in Northeast Indian Craton

The time transgressive Cretaceous sedimentation in northeast Indian craton is represented by the deposition of the Upper Cretaceous Mahadek sediments in Meghalaya. The Lower unit of these sediments is comprised of alternate sandstone and conglomerate horizons, and the Upper unit is composed of arkosic to sub-arkosic sandstones. These sediments are overlain by the calcareous litho-units of Langpar Formation of Cretaceous–Paleocene age. Lithofacies identified from various exposures and studied lithologs reflect a variation from marginal marine to marine depositional set-up in the Mahadek sediments. The Lower Mahadek facies is dominated by large-scale trough beds of medium to coarse-grained sediments which initiates with a conglomerate horizon succeeded by thin beds of shale and fine-grained sandstone with parallel laminations, which are followed by successive deposition of gritty sand and pebbly beds with coarsening upward sequence, suggesting product of delta plain in a marginal marine setting. Moreover, the bioturbated coaly fine-grained sandstone is further representative of tidal flat-tidal channel swamp formed in reducing condition and the trace fossil assemblages depict the channel fills. The Upper Mahadek facies is dominated by glauconitic-gritty fossiliferous sandstone showing fining upward sequence and hummocks, typical of marine environment. This fining upward sequence along with the vertical and lateral facies variation implies superimposition of retrograding shorelines on an earlier progradational alluvial fan sand complex. These different-scale cycles are interpreted as the sedimentary response to movements of the basin margin faults at variable frequencies, related to the contemporary faults. This depositional model of the Cretaceous sediments may further help in paleogeographic reconstruction of the northeast Indian craton during the Cretaceous period.

Ice‐margin fluctuation sequences and grounding zone wedges: The record of the Late Palaeozoic Ice Age in the eastern Karoo Basin (Dwyka Group, South Africa)

AbstractIn the eastern part of the Karoo Basin of South Africa, the sedimentary record of the Late Palaeozoic Ice Age, the Dwyka Group, consists of an up to 200 m thick accumulation of massive to crudely stratified diamictite occasionally interstratified with siltstone, sandstone and conglomerate horizons. Three distinct sedimentary units, separated by intervening glacial erosion surfaces, are viewed as ice‐margin fluctuation sequences. The lowermost one, resting on highly uneven, glacially abraded Archaean basement, has been interpreted as a grounding zone wedge deposited after the retreat and stabilization of the ice margin after the inundation of the Karoo Basin. The grounding zone wedge interpretation is based on its thickness (up to 100 m), the dominance of diamictite, and its facies assemblage and inferred depositional processes (rain‐out of debris, dropstone dumping, mass and debris flow, till). Overlying the grounding zone wedge, deposits are sedimentary units interpreted as glaciofluvial or ice‐contact delta and grounding zone wedges, respectively. By analogy with Quaternary sedimentary sequences, deposition of the Dwyka Group in the study area might have been very rapid (tens to hundreds of thousand years) and may hence correlate with the ultimate deglacial sequence of the Western Karoo Basin, as both successions are covered by the postglacial Ecca Group. Although commonly observed and imaged on modern, high‐latitude continental shelves, grounding zone wedges have never been interpreted in the ancient geological record. This paper therefore outlines a model defining criteria necessary for their identification.

3D seismic attributes for structural mapping and enhancement of deep gold mining: a case study from the West Wits Line goldfields, South Africa

Volumetric and horizon-based seismic attributes were carried out on the 3D reflection seismic data acquired on the deep crystalline rock environments of the Witwatersrand Basin (South Africa). This study aimed at improving the delineation of minor faults that cross-cut the gold-bearing quartz pebble conglomerate horizons (termed reefs) in the complex geology of the West Wits Line (WWL) goldfield using different seismic attributes. The 3D seismic data show delineation of faults that cross-cut and displace the target horizons by tens to hundreds of metres, leading to the delineation of the ore resources in faulted areas of the mines. In particular, the ant-tracking technique has provided better stratigraphic and structural imaging in complex faulted areas, relative to conventional interpretation methods. Other improvements include more accurate mapping of the depths, dip and strike of the key seismic horizon (Roodepoort shale) using the edge detection attributes, yielding a better understanding of the interrelationship between fault activity, reef distribution and the relative chronology of tectonic events. The integration of the 3D reflection seismic data, seismic attributes and information from borehole logs and underground mapping has provided better imaging and modelling of important fault systems that might have a direct effect on mining. This information could be used for future mining planning and designs to: (1) assess and mitigate the risks posed by mining activities, and (2) improve the resource evaluation of the gold-bearing reefs in the WWL goldfield.

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.

Lithofacies and sequence stratigraphic analysis of the Upper Jurassic siliciclastics in the eastern Kopet-Dagh Basin, NE Iran

The Upper Jurassic (Oxfordian-Kimmeridgian) Mozduran Formation is the most important gas reservoirs of the northeast Iran. Siliciclastic facies of this formation in eastern most parts of the basin have not been studied yet. Therefore, four stratigraphic sections of Mozduran Formation have been selected in the Kole-Malekabad, Kale-Karab, Deraz-Ab and Karizak to interpret depositional history and analyze depositional sequences. Based on texture and sedimentary structures, 14 slilciclastic lithofacies were identified and classified into four categories, including conglomerate (Gms, Gp, Gt), sandstone (Sh, Sp, St, Sr, Sl, Sm, Se), mud rock (Fl) and intermediate sandstone-mud rock (Sr (Fl), Sr/Fl, Fl (Sr)). Identified lithofacies formed four architectural elements CH, SB, LA and FF. Lithofacies characteristics and architectural elements with mostly bimodal pattern of paleocurrents show that the majority of Mozduran lithofacies deposited in the coastal environment (tidal influence). Sequence stratigraphic analysis shows that the Kole-Malekabad section consists of two depositional sequences while other sections are characterized by three depositional sequences. The lower and upper sequence boundaries of the Mozduran Formation in all stratigraphic sections are SB1 that are distinguished by paleosol and sometime conglomerate horizons. Most of depositional sequences in studied sections are composed only of TST and HST. The TST deposits consist mostly of quartzarenite and litharenite petrofacies that have been deposited in the tidal zone. HST packages are mostly including mud rocks with interdeds of sandstone lithofacies that are deposited in supratidal setting. The LST facies is recognized only in the DS3 (equivalent to the second depositional sequences of the Kole-Malekabad), which consist of conglomerate facies. Instead, the Kole-Malekabad section is often composed of supratidal gypsiferrous shales, indicating sea level fall in the study area.

Pyrite Zoning as a Record of Mineralization in the Ventersdorp Contact Reef, Witwatersrand Basin, South Africa

The Archean metasedimentary succession of the Witwatersrand basin, South Africa, hosts the largest Au deposit in the world. Gold mineralization is mostly concentrated in conglomerate horizons, or “reefs,” and is tightly associated with pyrite. Trace element zoning of pyrite from the Ventersdorp Contact Reef, studied by X-ray elemental (As, Ni, Co, and Pb) maps, electron microprobe analysis, and laser ablation-inductively coupled plasma-mass spectrometry, indicates successive stages of pyrite formation, each characterized by different textures and trace element composition (As ≤2.2 wt %, Ni ≤1.37 wt %, Co ≤1.98 wt %). Four generations have been distinguished: generation 1 is detrital and includes compact (nonporous), porous, and laminated pyrite; generations 2 to 4 are postsedimentary/authigenic. Generation 4 pyrite formed at near-peak metamorphic conditions (T = 270°–350°C, chlorite geothermometry). Porous and concentrically laminated pyrite grains (generation 1) are particularly enriched in Au (average 6.4 ppm, maximum 70 ppm), in addition to Sb, Tl, Pb, Mn, Mo, Cu, and Ag, in comparison with compact pyrite types of all generations. In these grains, Au, occurring as “invisible gold,” and other trace elements might be finely dispersed with the phyllosilicates filling the pyrite pores. Trace element composition of porous and concentrically laminated pyrite is reminiscent of pyrite known to form in suboxic to anoxic environments (black shales). The presence of Au in detrital pyrite indicates an early introduction of Au in the Ventersdorp Contact Reef. Gold is also present as secondary inclusions of electrum associated with the last pyrite generation (generation 4), together with sphalerite, chalcopyrite, galena, and pyrrhotite.

Preservation of terranes during the assembly of the Annieopsquotch Accretionary Tract: Inferences from the provenance of a Middle Ordovician ophiolite to arc transition, central Newfoundland Appalachians1Geological Survey of Canada Contribution 20100463.2This article is one of a series of papers published in this CJES Special Issue: In honour of Ward Neale on the theme of Appalachian and Grenvillian geology.

The Newfoundland Appalachians are a classic area for studying the record of arc development and terrane accretion processes with excellent exposure to different crustal levels that are minimally deformed and metamorphosed. This area also provides a link between the once continuous Appalachian and Caledonian orogens. The Annieopsquotch Accretionary Tract lies along the Red Indian Line, within the peri-Laurentian realm of the central Newfoundland Appalachians. The Darriwilian (468–461 Ma) tectonostratigraphic units of the Annieopsquotch Accretionary Tract are commonly characterized by polymictic volcanogenic conglomerate horizons. A conglomerate horizon at the interface between a suprasubduction zone ophiolite and its calc-alkaline volcanic arc cover sequence is herein investigated for zircon and geochemical provenance. Geochronology revealed a maximum age of deposition of 467 ± 4 Ma with zircon inheritance ranging from ca. 500 to 2800 Ma, consistent with a peri-Laurentian continental basement source. Four types of volcanogenic conglomerate clasts are noted on the basis of lithogeochemistry: arc andesite; calc-alkaline basalt; tholeiitic basalt; and non-arc rhyodacite. Tholeiitic basalt clasts are likely locally derived, perhaps from the underlying Skidder Formation. Other volcanic clasts do not have any known geochemical equivalents in the Annieopsquotch Accretionary Tract and hence appear to be exotic. The dominant zircon population suggests that the exotic clasts were derived from a ca. 467 Ma peri-Laurentian andesitic volcanic arc that once formed part of the Annieopsquotch Accretionary Tract.

Characterization of the Geology of Subsurface Shallow Conglomerate using 2D Electrical Resistivity Imaging at Baragadi, Panna District, Madhya Pradesh, India

The 2D electrical resistivity imaging study is the rising tool used for characterization of the geology of subsurface diamondiferous shallow conglomerate and geological condition at Baragadi, Panna District,Madhya Pradesh, India. In the present study, the 2D electrical resistivity imaging of subsurface shallow conglomerate has been generated using through Computerized Resistivity Meter (CRM -500) and the Wenner electrodeconfiguration has been used for 2D electrical resistivity imaging studies. The measured apparent resistivity values have been subjected to inversion so as called a resistivity pseudosection by using RES2DINV 3.56 software. Resistivity contrast technique has been adopted to distinguish the rock properties in the form of resistivity distribution displayed in the imaging sections and to delineate the distribution of diamondiferous conglomerate horizons withinclay and kankar deposits in the Baragadi shallow congromerte lower part of Upper Vindhayan Formation.

O- AND H-ISOTOPE STUDY OF THE CARBON LEADER REEF AT THE TAU TONA AND SAVUKA MINES (WESTERN DEEP LEVELS), SOUTH AFRICA: IMPLICATIONS FOR THE ORIGIN AND EVOLUTION OF WITWATERSRAND BASIN FLUIDS

The Carbon Leader Reef is a ~1 m thick conglomeratic unit with a thin bituminous base, and is one of the major gold-bearing conglomerate horizons in the Central Rand Group of the Witwatersrand Basin. It consists of alternating conglomerate and quartzite layers and was metamorphosed under greenschist facies conditions. Bulk rock δ18O and δD values of the Carbon Leader Reef range from 7.2‰ to 10.8‰ (mean = 10.6‰) and −27 to −65 (mean = −41‰), respectively. The narrow range in δ18O values, together with the lack of correlation between the δ18O value and the modal % matrix minerals, suggests that the original detrital minerals and the authigenic matrix minerals have similar oxygen isotope composition. Calculated δD values of the fluid, assuming that it was in isotope equilibrium with the bulk rock at the estimated peak metamorphic temperature of 350°C, range from −1‰ to −40‰. This suggests that the ultimate origin of the fluid was a mixture of meteoric and metamorphic water. These data are not consistent with the ingress of large quantities of externally derived fluid post burial metamorphism. Gold-rich sections of the Carbon Leader Reef do not have significantly different δ18O values than the adjacent gold-poor section, which suggests that gold mineralization is not related to interaction with significant amounts of externally derived hydrothermal fluids.

Late Archaean to Palaeoproterozoic geotherms in the Kaapvaal craton, South Africa: constraints on the thermal evolution of the Witwatersrand Basin

ABSTRACTThe c. 2.97–2.71 Ga Witwatersrand Basin located in the Kaapvaal craton of South Africa represents a remnant of a large Late Archaean sedimentary basin that hosts the world's premier gold deposit within a series of conglomerate horizons. Evidence of postdepositional gold mobility within these conglomerates associated with hydrothermal–metamorphic activity has led to speculation about the Late Archaean to Palaeoproterozoic geothermal gradients in the basin. We use surface heat flow and heat production data from rocks in the basin and its environs in order to calculate detailed temperature profiles for the central Kaapvaal craton that show that the steady state crustal geotherm during the Late Archaean and Palaeoproterozoic was relatively cool at 15–20 K km−1. The geotherm in the upper crustal strata is also largely unaffected by substantial increases in the heat flow into the base of the crust. Consequently, regional greenschist facies metamorphism of the basin sediments could only have been achieved during a transient thermal event that advected heat into the upper crust. The most likely candidate for this is the Bushveld magmatic event at 2.06 Ga.

Nagthat Formation: An example of a progradational, tide-dominated Proterozoic succession in Kumaun Lesser Himalaya, India

The Proterozoic Nagthat Formation of the Krol-belt succession, in the Nainital area, is composed mainly of fine- to coarse-grained quartzarenite with a subordinate amount of purple to grey sandstone, siltstone-shale and conglomerate horizons. The association with spilitic lava flows, variable palaeocurrent trends and the restricted lateral extent of the Nagthat Formation within the Krol-belt succession imply an active role for tectonism in the basin of deposition. In the upward coarsening succession of the Nagthat Formation, six major lithofacies have been identified: medium- to coarse-grained gravelly quartzarenite (Lithofacies A), planar cross-bedded, medium-grained quartzarenite (Lithofacies B), horizontally laminated, fine-grained quartzarenite (Lithofacies D), interbedded sandstone-shale (Lithofacies E) and matrix-supported conglomerate (Lithofacies F). The constituent lithofacies are repetitive in nature, forming upward fining unit cycles and interpreted to reflect deposition as upper shore-face, shoals and bars, barrier-beachface, tidal channels (inlets), intertidal–sandflat–mixedflat environments and, occasionally, in the form of gravity flows in subtidal channels. The general upward coarsening succession of the Nagthat Formation represents deposition in a progradational (regressive) barrier island system. The palaeocurrent pattern in the Nagthat Formation is distinctly polymodal and indicates sediment distribution across the roughly NW–SE trending shoreline, in response to a dominating flood tidal current system. The palaeocurrent pattern shows higher variability in the upper shore-face deposits than in the tidalflat domain. A recycled metasedimentary terrain served as the source for the Nagthat Formation, probably supplying the sediments from E, NE and S directions.

Rn-generated206Pb in hydrothermal sulphide minerals and bitumen from the Ventersdorp Contact Reef, South Africa

The auriferous conglomerate horizons (reefs) in the Witwatersrand Basin of South Africa are in many places cut by hydrothermal quartz veins that frequently contain sulphide, bitumen, and, less commonly, free gold. New Pb isotopic results for the Ventersdorp Contact Reef which has experienced particularly intense hydrothermal alteration, reaffirm the radiogenically enriched nature of the Pb in this reef and provide additional insight into its origin. This study focuses on analyses of galena, chalcopyrite, pyrrhotite, and bitumen from quartz veins, which presumably formed during the 2.020 Ga Vredefort meteorite impact event. The radiogenic, mainly uranogenic, component of the Pb appears to have been derived almost entirely from uraninite in the surrounding reef rock. Assigning a 2.02 Ga age of mineralization and constructing secondary isochrons for paragenetically early galena and chalcopyrite, ages of the source uraninite are calculated as 2.6–2.4 Ga. No special significance is ascribed to these source ages, which likely reflect extensive radiogenic Pb loss from originally somewhat older detrital uraninite during transport, sedimentation, and post-burial alteration. Analyses of detrital(?), syngenetic, and epigenetic pyrite from a reef conglomerate define a subsidiary linear array with a considerably shallower slope. Interpreted as a secondary isochron, the array gives an implausibly young mineralization and/or source age indicative of a superimposed isotopic disturbance.

Gold mobilizing fluids in the Witwatersrand Basin: composition and possible sources

Crush-leach data were obtained, using High Performance Gradient Ion-Chromatography and Capillary Electrophoresis, on individual generations of aqueous fluid inclusions in hydrothermal quartz from three different auriferous conglomerate horizons (reefs) in the late Archaean Witwatersrand Basin, South Africa. These data, supplemented by oxygen isotope analyses of hydrothermal quartz and in combination with microthermometric analyses, help to constrain the chemical composition, pH, temperature of formation and the possible source of the mineralizing fluid which, in places, was capable of mobilizing some of the primarily detrital gold in the fluvial Witwatersrand sediments. The dominant cations in the aqueous fluid inclusions are Na+ and Ca2+, with Cl− or HCO 3 − being the dominant anion, whereas K+, Mg2+, and SO 4 2− are subordinate. Most fluid inclusions have elevated NH 4 + concentrations which are directly correlated with those of N03. In a number of samples small amounts of organic acids (formate, propionate, and acetate) were also detected. A largely meteoric source is inferred for the gold-mobilizing fluids in the Witwatersrand reefs because of a lack of Br in the fluid, a composition distinctly different from that of seawater, the presence of organic acids, and δ18Ofluid values around O%o. The fluids are ascribed to hydrothermal infiltration triggered by the 2020 Ma Vredefort impact which also created a secondary permeability in the form of a dense network of micro-fractures preferentially in the conglomerate beds of the already metamorphosed Witwatersrand rock sequence. This fluid differs from the regional metamorphic fluid in the basin by having a considerably higher pH (5.7–7.2). The difference in pH might explain why the older, fairly acidic metamorphic fluid was apparently less capable of mobilizing the gold as gold solubility reaches its peak at the pH calculated for the fluid ascribed to the impact.

Transpressional development of the Australia‐Pacific boundary through southern South Island, New Zealand: Constraints from Miocene‐Pliocene sediments, Waiho‐1 borehole, South Westland

Abstract Clast compositions of Waiho‐1 conglomerate horizons record dextral movement of the borehole site past different basement terranes on the opposite side of the Alpine Fault. Metavolcanic and volcanogenic metagreywacke clasts in sediments aged c. 4.5 Ma indicate deposition adjacent to exposures of Caples, Dun Mountain—Maitai, or Brook Street basement terranes. Sediments aged c. 4 Ma contain Torlesse Terrane and Haast Schist clasts. Relocation of the Waiho‐1 borehole site using published finite rotations, combined with the observed change in provenance at c. 4.5 Ma, requires that the source rocks had relationships similar to those observed at the present day, that is, that most curvature of basement terranes in southern South Island, New Zealand, was already in place at 4.5 Ma. Major increases in sedimentation rate recorded by Waiho‐1 borehole at c. 12 and 5 Ma are linked to changes in the Australia‐Pacific plate vector. Subsidence before 12 Ma may have been related to thermal subsidence of the Emerald Basin, or/and middle Miocene foreland basin development. Clast compositions of middle—late Miocene (c. 12–5 Ma) sediments record reverse movement on the South Westland Fault Zone and uplift southeast of the Alpine Fault in areas where meagreywacke basement rocks were exposed. Fiordland was low lying and contributing little sediment to the basin in middle—late Miocene time. Large volumes of Pliocene—Quaternary (c. 5–0 Ma) sediment derived from southeast of the Alpine Fault reflect widespread uplift of the Southern Alps. Schist clasts in an early Pliocene sample indicate that amphibolite facies Alpine Schist was exposed by c. 4 Ma.

Crustal Evolution in the Source Region of the Witwatersrand Sediments, South Africa — Evidence from Quartz Pebble and Quartzite Geochemistry

Various hypotheses have been proposed for the evolution of the continental crust which incorporate different models of growth through geologic time. The most widely used and tested model is that of extensive growth by intracrustal melting, starting about 3.2 Ga ago (Taylor & McLennan, 1985). Ages of single, detrital zircons from various Witwatersrand stratigraphic horizons (Fig.l) indicate a general trend of decreasing ages of source rocks from about 3.2 to 3.1 Ga for the Dominion Reef to less than 2.8 Ga for the Ventersdorp Contact Reef (VCR). A small number of older zircons with ages between 3.2 and 3.5 Ga occur in most sediments. There also appears to exist a gap in zircon ages at about 3.26 Ga indicating the end of an older tectonic cycle. A major new cycle of crustal evolution appears to have started at about 3.1 to 3.0 Ga, judging from the appearance of large numbers of detrital zircons of that age in Witwatersrand sediments ( Barton, E.S. and Hallbauer, D.K., unpublished data). Evidence for crusta l evolut ion in the Witwatersrand source region is also available from a different avenue, the geochemistry of quartz. On average, quartz is the major component of Witwatersrand sediments and quartz pebbles of the 'vein quartz' type are a prominent component of most conglomerate horizons. More than 200 single pebbles from different reefs and quartz from some primary sources have

Phosphorites, hardgrounds and syndepositional solution subsidence: A palaeoenvironmental model from the miocene of the Maltese Islands

Two major phosphorite conglomerate horizons occur throughout the central Mediterranean Maltese Islands within the Lower-Middle Miocene, Globigerina Limestone Formation. The lower conglomerate lies directly upon a hardground and both conglomerates include lithoclasts of hardground material. Variation in clast size and bed thickness indicates a source area to the west and north of the present-day islands of Malta and Gozo. Particularly thick phosphorite conglomerates occur in solution subsidence structures in western Gozo, being developed as a result of synsedimentary seafloor collapse. These structures indicate that phosphorite clasts were deposited during several discrete pulses of turbulence, punctuated by periods of non-depositional hardground formation or normal marine pelagic sedimentation. Geochemical analysis of the contained phosphorite clasts within the conglomerates shows that P 2O 5 generally is concentrated around the outer margins of the pebbles and contributes up to 17% by weight of some samples. Some 654,580 m 3 and 837,870 m 3 of phosphorite-bearing strata are contained within the Xlendi and Qawra subsidence structures, respectively. The Maltese Islands represent part of a Miocene carbonate shelf, whose margin lay nearby to the west and which was subject to periodic erosion by strong currents transporting phosphorite pebbles eastwards to their present sites. These currents further contributed phosphorus to semi-autochthonous phosphorite developments located within the present Maltese area. The Maltese phosphorites are related to other phosphorite areas in Sicily. A major province of Miocene phosphogenesis is proposed to account for these occurrences. It extends along the western margins of the Malta-Ragusa Rise.

Kemik Sandstones, Arctic National Wildlife Refuge (ANWR), Alaska: ABSTRACT

In the Sadlerochit Mountains area of ANWR, the Kemik Sandstone of Hauterivian-Barremian age ranges to at least 35 m (120 ft) of very well sorted, fine-grained quartzose sandstone with minor pebble conglomerate. It is an elongate body traceable for over 160 km (100 mi) from the eastern Sadlerochit Mountains into the subsurface near the Sagavanirktok River to the west. In the northeast, it crops out in a belt about 16 km (10 mi) wide; to the southwest in the subsurface, it expands to about 65 km (40 mi) wide. It is a potential petroleum reservoir in the subsurface of ANWR, but its distribution north and east of the Sadlerochit Mountains is unknown. The Kemik overlies a regional angular unconformity with increasing depth of truncation northward; it is overlain by and intertongues with an unnamed pebble-shale unit. Data suggest that the Kemik was derived from erosion of Lisburne Group carbonate rocks and a terrain of mature sedimentary rocks north and east of the outcrop belt. Conglomerate clasts consist dominantly of white leached spicular and foraminiferal chert, silicified carbonate, and quartzite. Although the sand has a nearly uniform grain size from east to west, conglomerate clasts are most abundant to the northeast and become smaller and less abundant westward; few are noted west of the Canning River. This distribution is suggestive of longshore drift from the northeast. A shoreface depositional environment, possibly a barrier island along a coast with low tidal flux, seems to be represented in the Kemik. Beds up to 2 m (6 ft) thick with sweeping low-angle cross stratification are interbedded with parallel laminated units and scattered, thin, maroon siltstone-mudstone beds. Vertical burrows up to 0.5 m (20 in.) long and truncated by overlying beds are conspicuous in some areas. A sparse pelecypod fauna is present, usually in the maroon silty horizons. The Kemik and other coeval sandstone and conglomerate horizons in the subsurface, such as the Put River sands and the Point Thomson sands, are probably separate bodies of sediment derived from uplifted blocks along the rifted Arctic Alaska plate margin. End_of_Article - Last_Page 672------------

The geochronology of uraniferous minerals in the Witwatersrand Triad; an interpretation of new and existing U-Pb age data on rocks and minerals from the Dominion Reef, Witwatersrand and Ventersdorp Supergroups

Uranium and lead analyses of rock samples from the Witwatersrand, Ventersdorp, and Transvaal supergroups give mainly discordant ages. Samples from the Upper Witwatersrand of the Orange Free State give 207 Pb/ 206 Pb ages of ca. 3000 Ma. These data when considered together with earlier total conglomerate U -Pb analyses from the Dominion Reef Supergroup lead to the conclusion that the uraniferous minerals of the Dominion Reef, Witwatersrand, Ventersdorp and Transvaal conglomerates are 3050 ± 50 Ma old. In the northern parts of the Witwatersrand Basin the parent uraniferous minerals experienced a major reworking at 2040 ± 100 Ma which brought about the partial or complete resetting of the original 3050 Ma age. Radiogenic lead released during this reworking crystallized as galena in veins and fissures which cut across the uraniferous conglomerate horizons. This reworking appears to have had little effect in the Orange Free State to the south. Its age and geographical extent suggest it was caused by thermal effects which accompanied the emplacement of the Bushveld Igneous Complex at 1950 ± 150 Ma. Samples from the south, which were relatively unaffected by the ca. 2040 Ma reworking generally show the effects of recent uranium loss. In the northern part of the basin discordant age patterns characteristic of lead loss have been imposed on uranium-lead systems which were generally reset (partially or completely) by the ca. 2040 Ma event. The presence of 3050 Ma old minerals in sedimentary sequences which are probably younger than ca. 2740 Ma suggests the simple interpretation that the uraniferous minerals are predominantly detrital.

Statistical application of plagioclase extinction in provenance studies

Estimating the determining composition of plagioclase by a rapid extinction method using albite twin lamellae is successful, when treated statistically and cautiously, in establishing provenance area of a sediment. The method involves recording all symmetrical extinction angles of albite twin lamellae. It differs from the Michel-Levy method which records only the maximum value for each thin section. Recording all symmetrical extinction angles allows the detection of a polymodal population of plagioclase compositions. The apparent anorthite content of plagioclase (An) in two conglomerate horizons and three granite bodies is used: (1) to relate conglomerate horizons separated by a body of water, (2) to relate both conglomerate horizons to a common provenance area (two of the granite bodies), and (3) to show assimilation of the conglomerate into the third granite body, allowing areal extension of the conglomerate palaeo-environment. The usefulness of this approach is limited.