selective alteration of sargassum lipids in anoxic sediments of the orca basin abstract the orca basin represents a unique depositional environment where sediments are overlain by several hundred meters of anoxic brine layers of the seaweed sargassum are found as discrete layers within the sediments and appear intact by visual examination to examine lipid diagenesis in this environment major lipid classes sterols fatty acids and alcohols in plants collected in the water column were compared to those of plants found in sediments and dated at 380 years old by 14 c measurements plant fronds stipes and air bladders were analyzed separately to compare the preservation of lipids among major sargassum tissues all lipids showed significant changes in distribution and abundance in preserved plants with a general loss of unsaturated components the concentration of several saturated fatty acids however remained unchanged or increased in older plant tissues suggesting that thet were more resistant to decomposition and that reduction of unsaturated acids to saturated forms may have occurred over time sterols showed contrasting results with nuclear saturated sterols stanols decreasing in abundance by an average of 62 in preserved tissues while nuclear unsaturated sterols stenols showed only a mean decrease of 41 two major sterols cholest 5 en 3 ol and cholesta 5 24 dien 3 ol were reduced on average by 92 2 and 93 5 respectively while the major sterol in all sargassum tissues 24 ethylcholesta 5 24 28 e dien 3 ol showed no significant change in concentration over time results of lipid analysis for preserved and fresh sargassum suggests that selective degradation may occur in anoxic marine environments counter to pathways typically seen in other systems

archaean oil migration in the witwatersrand basin of south africa abstract the late archaean witwatersrand supergroup of south africa hosts the largest known gold uranium pyrite ore deposits oil preserved in fluid inclusions in quartz grains in siliciclastic sedimentary rocks of that supergroup implies that hydrocarbon generation and migration occurred during the archaean and may have been involved in mineralization processes through reference to phanerozoic analogues oil bearing fluid inclusions entrapped in healed microfractures in detrital quartz grains and in early syntaxial quartz overgrowths imply that the onset of oil migration coincided with early to intermediate stages of burial while intra granular porosity was still preserved multiple generations of oil migration are indicated by i oil inclusions within early diagenetic cements at different levels in the stratigraphic succession ii more than one type of oil in entrapment sites iii oil entrapment in multiple stages of the quartz paragenetic sequence oil generation and migration are considered to have occurred throughout and for some considerable time after development of the witwatersrand basin consistent with progressive burial and kerogen maturation in more than one tectonic regime oil bearing fluid inclusions within detrital sandstone fragments suggest that oil migration also occurred in a sedimentary succession on the kaapvaal craton prior to 2 9 ga oil in the witwatersrand supergroup was most likely derived from multiple source areas with the principal source probably being shales within the lower witwatersrand supergroup the hydrocarbon migration history of the basin has important implications for understanding the textural relationship between gold bituminized oil and uraninite in the giant gold uranium pyrite ore deposits

mineral chemical study of u bearing minerals from the dominion reefs south africa the neo archean dominion reefs 3 06 ga are thin meta conglomerate layers with concentrations of u and th bearing heavy minerals higher than in the overlying witwatersrand reefs ore samples from uranium one africa s rietkuil and dominion exploration areas near klerksdorp south africa were investigated for their mineral paragenesis texture and mineral chemical composition the ore and heavy mineral assemblages consist of uraninite other uraniferous minerals fe sulphides ni co sulfarsenides garnet pyrite pyrrhotite monazite zircon chromite magnetite and minor gold sub rounded uraninite grains occur associated with the primary detrital heavy mineral paragenesis u ti u th minerals pitchblende colloform uraninite and coffinite are of secondary re mobilised origin as evidenced by crystal shape and texture most of the uranium mineralisation is represented by detrital uraninite with up to 70 2 wt uo2 and up to 9 3 wt tho2 re crystallised phases such as secondary pitchblende without th coffinite u ti and u th phases are related to hydrothermal overprint during low grade metamorphism and are of minor abundance

phanerozoic analogues for carbonaceous matter in witwatersrand ore deposits thoriferous bitumen nodules in quartz pebble conglomerates and pebbly sandstones of phanerozoic age in the british isles are analogues for the uraniferous bitumen nodules flyspeck in the archean witwatersrand gold uranium deposits they are comparable in morphology relationship to bedding planes replacive behavior toward quartz mineralogy fragmentation and displacement of inclusions in each case the quartz rich sedimentary host rocks represent regionally important transgressive deposits the contrast in dominant radioelement thorium or uranium probably reflects differences in mobility of the elements as a function of variable eh conditions the phanerozoic thoriferous nodules occur only in pale low eh beds which lack uranium deposits but in the archean witwatersrand deposits fractionation of uranium and thorium may not have been as marked in the phanerozoic examples there is good evidence for migration of hydrocarbons through the host rocks and the bitumen nodules can confidently be attributed a hydrocarbon origin by analogy this supports proponents of a hydrocarbon origin for the witwatersrand carbon rather than a microbial origin

occurrence and origin of ultralaminar structures in amorphous kerogens of various source rocks and oil shales abstract forty low maturity type i ii kerogens from source rocks and oil shales ranging in age from infra cambrian to miocene were examined by transmission electron microscopy tem when previously observed by light microscopy and or uv fluorescence microscopy the above samples like a very large number of kerogens seemed homogeneous and amorphous in fact tem revealed the presence of ultralaminar structures in 22 of these amorphous kerogens the abundance of ultralaminae depends on the considered sample and some of the tested kerogens are nearly exclusively composed of ultralamina accumulations based on thickness three main groups of ultralaminae can be identified parallel studies on extant algae indicated that some of the above ultralaminae likely derived from the resistant outer walls of microalgae the fossilization of such outer walls should occur via the selective preservation pathway and result in the formation of highly aliphatic ultralaminar kerogens the lack of ultralaminae in infra cambrian kerogens is consistent with a cyanobacterial origin

the lipid chemistry of an interfacial sediment from the peru continental shelf fatty acids alcohols aliphatic ketones and hydrocarbons a sample of the sediment water column interface which lies on the continental shelf under the peru upwelling regime has been examined for fatty acids fatty alcohols ketones and hydrocarbons fatty acids were the most abundant compound class ranging from c12 c24 with 16 0 as the major component 765 5 g g dry sediment the alcohols were dominated by 3 7 11 15 tetramethylhexadeca 2 en ol phytol with even chain n alcohols in the range c14 c20 the ketones consisted of c37 c39 di and tri unsaturated alken 2 ones and alken 3 ones both alkanes and alkenes were present in the hydrocarbon fraction the alkanes ranging from c13 c20 and comprising both straight chain and isoprenoid compounds the alkenes consisting of isomeric pairs of c25 branched trienes and tetraenes the data indicate that the organic content has been contributed very largely from marine sources probably mainly from phytoplankton and bacteria showing little terrigenous influence the presence of labile compounds such as polyunsaturated fatty acids with two to six double bonds implies that the sediment has undergone very little diagenetic alteration and the lipids are probably largely unchanged from the state in which they actually reached the sediment they may therefore serve as a useful baseline in assessing diagenesis in older sediments where diagenetic transformations are more advanced

metamorphism of the witwatersrand gold fields a review abstract there is widespread evidence for greenschist facies metamorphism in different rock types within all the witwatersrand gold mines the silicate mineral assemblages suggest near uniform peak metamorphic conditions equivalent to the chlorite zone in all the major witwatersrand gold fields assemblages of coexisting pyrophyllite chloritoid have been recorded in most of the gold mines and the critical assemblage of pyrophyllite chloritoid muscovite chlorite quartz tourmaline rutile pyrite is found in most gold fields kaolinite paragonite biotite pyrrhotite and arsenopyrite have more local distribution pelites shales quartzites meta conglomerates metabasalts and some dykes record these metamorphic assemblages albeit in different mineral proportions and pyrophyllite and chloritoid are recorded from many gold producing reefs andalusite kyanite garnet and or cordierite bearing assemblages indicate higher metamorphic grades in three discrete areas along the nw margin of the witwatersrand basin to the ne of the evander gold field and around the collar of the vredefort dome concentric metamorphic facies around the vredefort dome culminate in granulite facies assemblages near the centre it is important to note that there are no definitive published data constraining the temporal relationships between the granulites and amphibolites of the vredefort dome greenschist facies assemblages in the vredefort collar rocks thermal metamorphism related to intrusion of the bushveld complex and the regional greenschist facies metamorphism documented in the gold fields metamorphic conditions of 350 50 c and up to 3 kbars are inferred for the gold fields with temperatures increasing to around 700 c in granulites of the vredefort dome other geothermometric methods including illite crystallinity fluid inclusions carbon elemental ratios and vitrinite reflectance are compatible with the above estimates but are each associated with considerable temperature uncertainties fluids in equilibrium with pyrophyllite muscovite quartz pyrite paragonite are inferred during metamorphism suggesting slightly acid conditions and elevated sulphur activities regional greenschist facies metamorphism documented in the witwatersrand gold mines was accompanied by widespread deformation large fluxes of aqueous fluid and considerable element mobility the low intermediate pressure of this metamorphic terrain and the lack of large scale tectonic thickening favours underplating by mafic magmas as the likely cause of the metamorphic pattern no model for gold genesis in the witwatersrand basin is complete without accommodating post burial processes including metamorphism deformation and alteration any placer gold would be overprinted by these processes and any epigenetic gold could potentially be introduced during these periods of thermal and fluid activity it is also important to note at least two quite unusual features of the witwatersrand metamorphic assemblages the virtual absence of feldspar from the 3 km thick central rand group over 300 km and the narrow range of metamorphic grade inferred for the gold mines over this same distance

morphology of witwatersrand gold grains from the basal reef evidence for their detrital origin thousands of rounded particles of fine grained gold released from a crossbedded gold specimen of the basal reef placer in the welkom gold field after acid digestion of the host quartz arenite have revealed morphological features and abrasion textures that are characteristic of detrital gold the size range which is between 0 038 and 0 473 mm has a lognormal distribution with a mean of 2 88 phi units 0 136 mm and a standard deviation of 0 48 this is a characteristic of well sorted sediment the shapes are predominantly disclike with toroidal forms being most common a corey shape factor of 0 08 indicates extreme flattening the preponderance of these shapes is interpreted to be the result of selective sorting by entrainment of particular shapes during extensive transport a knobby surface texture similar to placer grains recovered from both recent and devonian placers and produced during laboratory controlled tumbling with sand and cobbles in water is considered distinctive of sediment abrasion secondary gold comprising 25 percent of the total gold content is finer grained has an interstitial jagged form and displays unabraded smooth to crystalline surfaces study of this remarkable specimen has produced unequivocal evidence of the detrital origin of gold in the extensive basal reef placer and led to research concerning its metamorphic overprints

relationship between detrital zircon age spectra and the tectonic evolution of the late archaean witwatersrand basin south africa abstract shrimp u pb detrital zircon age spectra for 14 samples from the west rand and central rand groups of the witwatersrand basin indicate that felsic source rocks mostly formed at 3090 3060 and 3000 2870 ma with a smaller proportion formed prior to 3100 ma provenance evaluation indicates that source rocks have largely vanished through erosion such that greenstone terrains extending from the amalia kraaipan belt in the southwest to the murchison belt in the northeast are models of but not the actual source areas analysis of detrital zircon age modes reveals that the west rand and central rand groups record different basins the west rand group is characterised by age subpopulations with a much smaller range and less complex distribution than those in the central rand group furthermore detrital zircon age spectra for the west rand group generally decrease in complexity up section whereas they become increasingly more complex in the central rand group it is concluded that the source area for the west rand group is consistent with a passive margin setting with the decrease in complexity up section of zircon age spectra being the consequence of increasing maturity of the hinterland conversely the source area of the central rand group was far more dynamic with an increasing age range of source rocks being exposed through time pulsed tectonic rejuvenation of the source area is implied contemporaneous emplacement uplift and erosion of granitoids supports a magmatic fold thrust belt as the source and a complementary retroarc basin setting detrital xenotime grains record the ages of high u granitoids emplaced in the fold thrust belt during deposition of the central rand group and are much closer to depositional ages than are detrital zircon grains change from a passive margin to a retroarc basin across the west rand central rand boundary is emphasised by change from simple to increasingly complex detrital zircon age spectra this study establishes that detrital zircon geochronology can overcome the problem of overly quartzose compositions in archaean sandstones due to severe chemical weathering and authigenic alteration thereby enabling distinction between passive margin and retroarc basin stages through systematic changes in the age complexity of the source area

concentric eyewall asymmetries in hurricane gonzalo 2014 observed by airborne radar abstracttwo eyewall replacement cycles were observed in hurricane gonzalo by the noaa p3 tail ta radar and the recently developed nasa high altitude imaging wind and rain airborne profiler hiwrap radar these observations captured detailed precipitation and kinematic features of gonzalo s concentric eyewalls both before and after the outer eyewall s winds became the vortex maximum winds the data were analyzed relative to the deep layer environmental wind shear vector during the beginning eyewall replacement cycle stages the inner and outer eyewalls exhibited different asymmetries the inner eyewall asymmetry exhibited significant low level inflow updrafts and positive tangential acceleration in the downshear quadrants consistent with observational and theoretical studies the outer eyewall asymmetry exhibited these features in the left of shear quadrants further downwind from those of the inner eyewall it is suggested that the low level inflow occurring at the outer but not at the inner eyewal

stable carbon isotope ratio variations of organic matter in orca basin sediments abstract orca basin is a highly reducing basin on the slope of the gulf of mexico stable carbon isotope ratios and total organic carbon percentages were determined for two cores within the basin and one control core outside the basin the results show that the organic carbon content of the basin cores is consistently 2 3 times greater than that of the control core the pleistocene holocene boundary indicated by a break in the 13 c depth profile occurs at a greater sediment depth in the basin cores than in the control core a small sampling interval has made it possible to detect an unexplained fine structure in the 13 c profile not previously observed

origin and diagenetic transformations of c25 and c30 highly branched isoprenoid sulphur compounds further evidence for the formation of organically bound sulphur during early diagenesis abstract a number of c25 and c30 highly branched isoprenoid hbi sulphur compounds e g thiolanes 1 oxo thiolanes thiophenes and benzo b thiophenes with 2 6 10 14 tetramethyl 7 3 methylpentyl pentadecane and 2 6 10 14 18 pentamethyl 7 3 methylpentyl nonadecane carbon skeletons were identified in sediments ranging from holocene to upper cretaceous these identifications are based on mass spectral characterisation desulphurisation and in some cases by comparison of mass spectral and relative retention time data with those of authentic standards the presence of unsaturated c25 and c30 hbi thiolanes in a recent sediment from the black sea age 3 6 10 3 a strongly supports their formation during early diagenesis the co occurrence of hbi polyenes c25 and c30 and unsaturated hbi thiolanes c25 and c30 possessing two double bonds less than the corresponding hbi polyenes in this recent sediment testifies to the formation of unsaturated hbi thiolanes by a reaction of inorganic sulphur species with double bonds of the hbi polyenes furthermore a diagenetic scheme for hbi sulphur compounds is proposed based on the identification of hbi sulphur compounds in sediment samples with different maturity levels

the growth and concentration of uranium and titanium minerals in hydrocarbons of the carbon leader reef witwatersrand supergroup south africa abstract uranium and gold bearing pyrobitumen from the carbon leader reef in the witwatersrand basin south africa was investigated by high resolution transmission electron microscopy this study provides evidence for the in situ growth of uraninite and anatase nanocrystals in the pyrobitumen implying mobilization and concentration of uranium and titanium by formerly mobile liquid hydrocarbons individual nanocrystals of uraninite and anatase are pervasively distributed and locally isolated within the pyrobitumen matrix crystallization of uraninite and anatase led to the formation of complex nanocrystal aggregates by oriented attachment in which anatase generally provided nuclei for the growth of uraninite single nanocrystals of curite occur locally in channel ways within masses of uraninite nanocrystals consistent with later auto oxidation of uraninite and limited release of water during hydrocarbon maturation on the basis of evidence for the migration of liquid hydrocarbons in the witwatersrand basin and the presence of abundant uraninite and anatase nanoparticles in pyrobitumen a new model is proposed for the transport and concentration of uranium and titanium in the carbon leader reef according to this model liquid hydrocarbons that were circulating in the witwatersrand basin dissolved detrital u ti bearing minerals and transported the uranium and titanium until thermal degradation immobilized the hydrocarbons by solidifying them as pyrobitumen the latter process involved the release of volatiles and the destruction of bonds that may have held the uranium and titanium in solution thereby inducing the growth of individual uraninite and anatase nanocrystals and the formation of complex nanocrystal aggregates within the pyrobitumen

nature and origin of carbon in the archaean witwatersrand basin south africa the nature and origin of the carbonaceous matter carbon in the gold bearing reefs of the archaean witwatersrand basin of south africa has long been a subject of controversy and debate the importance of the carbon results from its direct association with the gold grade the studies reported here were directed at resolving the nature and origin of carbon techniques used include stope scale macroscopic mapping mesoscopic hand specimen examination and microscopic studies incorporating the use of optical reflectance measurement macroscopic mapping has demonstrated the braided habit of carbon bodies over tens of square metres with a close parallel to sedimentology mesoscopic observations however show the vein like carbon seams are often cross cutting sedimentary features with the carbon occupying an anastomosing set of bed parallel fractures microscopic observations show a globular form for the carbon and inclusions of uraninite particles in seam carbon the globules are generally modified into elongated spindles oriented perpendicular to bedding optical properties are dominated by generally high reflectance and anisotropy with a swirling liquid crystal structure classifying seam carbon as carbonaceous mesophase it is this fine mesophase texture that has previously been wrongly attributed to plant algal morphology highly significantly the optical anisotropy is bed perpendicular rather than bed parallel as in normal sedimentary burial indicating mesophase formation during fracture opening both reflectance and bireflectance of the seam mesophase show a range of values at all scales no stratigraphic or depth trends were apparent though geographic variation points to localization of the maximum temperature event s given high anisotropies maximum reflectance values in the 4 8 r o range and minima in the 1 2 r o range give arithmetic mean reflectance values of about 4 0 r o kinetic modelling equates these values with short term 50 000 years temperature events of c 400 c or burial temperatures of c 280 c lower reflectance values of c 2 0 r o obtained from the shales suggests thermal detachment of permeable and impermeable beds a characteristic of pulsed hydrothermal flux the included uraninite grains influence the orientation of the surrounding anisotropic mesophase demonstrating that uraninite predated the mesophase formation event regional palaeo stress frozen into the mesophase anisotropy indicates a nw se deviatoric stress the combination of hydrocarbon radiation and hydrothermal fluid flow marks witwatersrand basin pyrobitumens out as being different from conventional oil putative source rocks for the oil are identified in the intra basinal witwatersrand shale units lithological control in carbon seams is explained by the poro perm and mechanical properties of the unconformity sequence of footwall reef conglomerates and hanging wall quartzites favouring fracture along the unconformity surfaces an overpressured oxidizing fluid created the fracture network that hosted the oil and also introduced uranium as uranyl ion on meeting a reducing agent oil uraninite precipitated stabilizing the oil by polymerization further hotter hydrothermal flux then coked the bitumen to often fibrous spindle shaped mesophase continued flow deposited gold in the available pore spaces most notably those formed by lateral contraction between mesophase spindles the speculative role of methane in the reductive precipitation of gold is indicated by the presence of gold in any co eval porosity largely independent of the mineral matrix

biogenic hydrocarbon gases and sulfate reduction in the orca basin brine the composition of light hydrocarbon gases in the orca basin an anoxic hypersaline intraslope depression on the continental slope of the northern gulf of mexico indicates that both methane and ethane are biogenic in nature with a c1 c2 c3 ratio of 730 and a 13c of methane of 73 relative to the pdb standard the concentrations of methane 750 mm and ethane 1300 mm in the orca basin brine are higher than any other marine anoxic basin these high levels result not from high rates of productivity but from the long residence time of the brine in the basin due to its high stability toward mixing with overlying seawater 1 z 3 2m both methane and ethane show well mixed distributions in the brine these distributions probably result from convective mixing of the isohaline brine pool due to normal heat flow from the basin sediments methane and ethane maxima above the pycnocline at the brine seawater interface reflect in situ production and or consumption in the aerobic water column concurrent maxima in suspended particulate material distributions in this region suggest methane may be produced there in anaerobic microenvironments associated with the suspended matter reduced rates of anaerobic decomposition including sulfate reduction in the brine sediments are inferred from preserved sargassum fronds in the sediments vertical sulfate profiles in most cores and the sediment organic carbon content which is two to three times higher in sediments below the high salinity brine than in the normal gulf sediments nearby

detrital origin of hydrothermal witwatersrand gold a review the witwatersrand basin is the largest known gold province in the world the gold deposits have been worked for moren than 100 years but there is still controversy about the ore forming process detailed petrographic studies often reveal that the gold is late in the paragenetic sequence which has led many researchers to propose a hydrothermal origin for the gold however observations such as the occurrence of rounded disc like gold particles next to irregularly shaped or idiomorphic secondary gold particles in the same sample suggest an initial detrital gold source within the witwatersrand strata mineral chemical and isotopic data together with sem cathodoluminescence imaging and fluid inclusion studies provide evidence for small scale variations in the fluid chemistry a requirement for the short range mobilization of the gold the existing data and observations on the witwatersrand rocks support a model of hydrothermally altered metamorphosed placer deposits with at least two subsequent gold mobilization events hydrothermal infiltration in early transvaal time 2 6 2 5 ga and during the 2 020 ga vredefort impact event

chemical evidence of kerogen formation in source rocks and oil shales via selective preservation of thin resistant outer walls of microalgae origin of ultralaminae abstract new structures termed ultralaminae were recently observed by transmission electron microscopy usually in high amounts in a number of kerogens from oil shales and source rocks morphological similarities were noted between ultralaminae and the thin ca 15 nm resistant outer walls composed of non hydrolyzable macromolecules algaenans commonly occurring in extant chlorophyceae especially in the cosmopolitan genus scenedesmus identification of the pyrolysis products of s quadricauda algaenan showed i a highly aliphatic structure based on a macromolecular network of long up to c32 polymethylenic chains probably cross linked by ether bridges and ii a close correlation based on the formation of n alkylnitriles between this algaenan and two ultralaminar kerogens the rundle oil shale mainly composed of ultralamina accumulations and the green river shale ultralaminae dispersed within an amorphous matrix these fossil ultralaminae therefore likely originated from the selective preservation of the thin algaenan containing outer walls of scenedesmus and or of other chlorophyceae containing outer walls of a similar morphology and composition relative distributions of n alkylnitriles and of n alkanoic acids in the pyrolysates of s quadricauda algaenan and of the rundle oil shale indicated that nitriles are not derived from secondary reactions of carboxylic acids but originate from preexisting nitrogen functions likely amides previous evidence of kerogen formation via selective preservation of algaenans was restricted to rather uncommon kerogens the present results added to ultralamina common occurrence and abundance point to a wide involvement and to a large contribution of the selective preservation of algaenan containing thin outer walls of chlorophyceae in the formation of kerogens in a number of lacustrine source rocks and oil shales all the available information suggest that the three way correlation selective preservation of algaenans from thin resistant chlorophycean outer walls formation of fossil ultralaminae presence of n alkylnitriles with a typical distribution in the pyrolysates of ultralaminar kerogens observed in this study might be of a general character

zircon ion microprobe studies bearing on the age and evolution of the witwatersrand triad abstract the ventersdorp and witwatersrand supergroups and the dominion group collectively referred to as the witwatersrand triad represent some of the oldest and economically most important volcano sedimentary basins within the kaapvaal craton of southern africa precise age constraints which are vital to models aimed at understanding the formation mineralisation and evolution of these basins have been hampered by analytical and geological problems these problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins felsic and mafic volcanics from the ventersdorp supergroup yield ages of 2709 4 ma and 2714 8 ma respectively constraining the timing of the closure of the witwatersrand basin a maximum age for the witwatersrand basin is indicated by the age of 3074 6 ma for volcanics from the underlying dominion group thus a period of c 350 ma was available for the accumulation of the full witwatersrand basin succession unfortunately dating within the witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin these new data show that previous age determinations on the ventersdorp witwatersrand and dominion basins were inaccurate and that the economically important witwatersrand basin must now be recognised as archaean

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 al2o3 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

basin internal derivation of hydrocarbons in the witwatersrand basin south africa evidence from bulk and molecular δ13c data abstract gold in the quartz pebble conglomerates of the late archean witwatersrand basin south africa is often intimately associated with carbonaceous matter of organic biogenic origin which occurs in the form of stratiform carbon seams and paragenetically late bitumen nodules both carbon forms are believed to be formed by solidification of migrating hydrocarbons this paper presents bulk and molecular chemical and stable carbon isotope data for the carbonaceous matter all of which are used to provide a clue to the source of the hydrocarbons these data are compared with those from intra basinal shales and overlying dolostone of the transvaal supergroup the 13c values of the extracts from the witwatersrand carbonaceous material show small differences up to 2 4 compared to the associated insoluble organic matter this suggests that the auriferous rocks were stained by mobile hydrocarbons produced by thermal and oxidative alteration of indigenous bitumens a contribution from hydrocarbons derived from intra basinal witwatersrand shales cannot be excluded individual aliphatic hydrocarbons of the various carbonaceous materials were subjected to compound specific isotope analysis using on line gas chromatography combustion stable isotope ratio mass spectrometry gc c irms the limited variability of the molecular parameters and uniform 13c values of individual n alkanes 31 1 1 7 and isoprenoids 30 7 1 1 in the witwatersrand samples exclude the mixing of oils from different sources carbonaceous matter in the dolostones shows distinctly different bulk and molecular isotope characteristics and thus cannot have been the source of the hydrocarbons in the witwatersrand deposits all the various forms of witwatersrand carbon appear indigenous to the witwatersrand basin and the differences between them are explained by variable in general probably short centimeter to meter scale hydrocarbon migration during diagenesis and subsequent hydrothermal infiltration

origin of uranium deposits revealed by their rare earth element signature uranium deposits form in a wide range of geological settings including deep magmatic to surficial conditions and range in age from archaean to recent these temporal and spatial variations have given rise to an extreme diversity of ore deposits however understanding their conditions of formation has remained challenging this article reports rare earth element ree abundances measured by microbeam methods in uranium oxides for a series of worldwide uranium occurrences the ree patterns are very specific to each deposit type and directly reflect the conditions of their genesis we propose an evaluation of the first order parameters controlling the ree behaviour in each mineralised system this study demonstrates that the ree pattern is the most efficient tool for constraining the geological models of uranium deposits and for genetically discriminating new uranium discoveries this approach may form the starting point for a new procedure in the fight against nuclear trafficking

a major archean gold and crust forming event in the kaapvaal craton south africa the 2 89 to 2 76 gigayear old conglomerates of the central rand group of south africa host an immense concentration of gold the gold and rounded pyrites from the conglomerates yield a rhenium osmium isochron age of 3 03 0 02 gigayears and an initial 187 os 188 os ratio of 0 1079 0 0001 this age is older than that of the conglomerates thus the gold is detrital and was not deposited by later hydrothermal fluids this middle archean gold mineralization event corresponds to a period of rapid crustal growth in which much of the kaapvaal craton was formed and is evidence for a significant noble metal flux from the mantle

diphytanyl glycerol ether distributions in sediments of the orca basin abstract archaebacterially produced diphytanyl glycerol ether dpge was examined in core sediments from the orca basin an anoxic hypersaline basin in the northwestern gulf of mexico to observe its spatial variability and potential origin a differential extraction protocol was employed to quantify the isopranyl glycerol ethers associated with unbound intermediate bound and kerogen bound lipid fractions archaebacterial lipids were evident at all depths for the unbound and intermediate bound fractions concentrations of dpge ranged from 0 51 to 2 91 g g dry sediment at the surface and showed secondary maxima deeper in basin sediments intermediate bound dpge concentrations exhibited an inverse relationship to unbound dpge concentrations kerogen bound dpge concentrations were normally below detection limits earlier studies describing the general homogeneity of lipid components within the overlying brine and at the brine seawater interface suggest that the large scale sedimentary dpge variations observed in this study result from spatial and temporal variations in in situ production by methanogenic or extremely halophilic archaebacteria

the origin of uraninite bitumen nodules and carbon seams in witwatersrand gold uranium pyrite ore deposits based on a permo triassic analogue permo triassic dongara sandstones from the perth basin of western australia contain solitary bitumen nodules that are analogous to uraniferous bitumen nodules or fly speck carbon in late archean witwatersrand gold uranium pyrite ore deposits nodules in both contain cores of radioactive minerals and occupy depositional sites where heavy minerals were concentrated solitary bitumen nodules in the dongara sandstone formed during burial by radiation induced polymerization of liquid hydrocarbons migrating through primary porosity around detrital monazite grains similar bitumen nodules in witwatersrand ore deposits are likewise interpreted to have formed during burial although the radiation emitting detrital grains were uraninite carbon seams in witwatersrand ore deposits are stratiform bodies of coalesced bitumen nodules by analogy with solitary nodules individual nodules in carbon seams formed during burial as liquid hydrocarbons migrated through primary porosity past detrital uraninite grains but they differ from solitary nodules in that many have been stretched perpendicular to bedding to form an ellipsoidal or columnar structure although a columnar structure is consistent with deformation by vertical extension that deformation clearly postdated nodule formation in some seams elongated nodules are associated spatially with framework hosted bedding parallel microfractures which are commonly filled with fibrous secondary minerals again bedding parallel microfracturing clearly postdated nodule formation columnar textured bitumen and bedding parallel fractures are interpreted to be postdiagenetic structural fabrics whereas carbon seams appear to have been a microfocus of a compressive deformation neither columnar textured bitumen nor bedding parallel fractures are present in the dongara sandstone and they are not present in every witwatersrand carbon seam either supporting the contention that columnar textured bitumen and bedding parallel fractures are focused structural fabrics that postdate nodule formation recognition that solitary bitumen nodules and carbon seam hosted bitumen nodules of witwatersrand ore deposits are analogous in origin to bitumen nodules in the dongara sandstone supports historical mineralogical observations that the witwatersrand basin contained uraninite placers indeed carbon seams are most likely the former sites of heavy mineral uraninite strands given the chemical instability of uraninite under oxidizing conditions detrital uraninite in witwatersrand strata supports the long held contention that the archean atmosphere contained significantly less oxygen than the modern atmosphere

hydrothermal gold mineralization in the witwatersrand basin the origin of the gold mineralization in the witwaterstrand basin of south africa the largest known gold province has been controversial for decades with arguments favouring detrital1 2 placer modified placer3 4 and hydrothermal5 6 origins here we present the results of an extensive geological study of witwatersrand rocks which show that the gold and associated uranium mineralization is hydrothermal in origin and postdates a regional high temperature alteration event alteration processes identified on a small scale can be mapped out regionally as roughly strata bound zones of acid metasomatism extending far into the basin the fluid flow responsible for this alteration was concentrated in small scale structures localized along lithological boundaries we find that the gold precipitated as a consequence of interactions of the fluid with shale derived hydrocarbons present within the basin

thiophenic biomarkers for palaeoenvironmental assessment and molecular stratigraphy reconstruction of ancient depositional environments is an important objective in the earth sciences the complex assemblages of organic molecules in sediments include compounds that can be related to known biochemicals produced by specific biota from well defined habitats1 3 and these chemical fossils are applied frequently to reconstruct palaeoenvironments especially in studies of organic and petroleum geochemistry and in chemical oceanography most of the compounds used in this way are hydro carbons2 4 although oxygenated for example alcohols acids and ketones and nitrogen containing for example porphyrins compounds have also been applied successfully1 2 here we report on the use of organic sulphur compounds as geochemical tools for palaeoenvironmental and stratigraphic assessment depth profiles of specific sulphur compounds related to certain organisms show significant variations reflecting changes both in sources of organic matter and in physical conditions such as salinity of the depositional environment in a number of cases these variations are not otherwise revealed by changes in lithology geochemical bulk data and or absolute and relative amounts of hydrocarbon biological markers moreover several organic sulphur compounds reveal the presence of unprecedented biochemicals in these palaeoenvironments

trace element distribution in uraninite from mesoarchaean witwatersrand conglomerates south africa supports placer model and magmatogenic source the first systematic analyses of the trace and rare earth element ree distribution in uraninite from various gold bearing conglomerates of the mesoarchaean central rand group in south africa s witwatersrand basin by in situ laser ablation inductively coupled plasma mass spectrometry confirms a placer origin for the uraninite and a magmatogenic provenance thereof the chemistry of commonly rounded to sub rounded uraninite is highly variable from grain to grain but generally marked by elevated th w bi mo ta y ree contents and unusually high au concentrations especially the high th contents and the chondrite normalised ree patterns are incompatible with post sedimentary hydrothermal genetic models for the u mineralisation and point to derivation of the detrital uraninite from a high temperature magmatogenic presumably granitic to pegmatitic source the elevated au concentrations of as much as 67 ppm in this uraninite are unique to the witwatersrand and hint at a granitic hinterland that was enriched in both u and au thus presenting a potential source domain for some of the detrital gold in the witwatersrand conglomerates minute fracture fills of brannerite in close proximity to the larger rounded uraninite grains are devoid of detectable bi mo ree and au and have only very low concentrations of th w ta and y this is explicable by crystallisation from a low temperature hydrothermal fluid thus witwatersrand u phases show analogous to many other ore constituents such as pyrite and gold clear evidence of partial short range mobilisation of originally detrital particles by post sedimentary fluids