ree mobility in groundwater proximate to the natural fission reactor at bangombe gabon the natural nuclear fission reactor at bangombe is located at a depth of approximately 12 m and has undergone supergene weathering and chemical exchange with groundwater under moderately oxidizing conditions this reactor has been studied as an analogue for the migration of radionuclides in a geologic repository five water samples were taken from a drill hole in the reactor zone and from drill holes that are situated along the direction of the groundwater flow dissolution of phosphates in the weathering profile provided an important mechanism for the mobilization of phosphorous and ree this phosphorous allowed the formation of secondary minerals e g phosphatian coffinites and fe uranyl phosphate hydroxide hydrates the filtered groundwaters dissolved phase obtained directly from the reactor zone had anomalous 143nd 146nd 145nd 146nd and 149sm 147sm isotope ratios of 0 7235 0 4933 and 0 843 respectively confirming mixing between a fissiogenic and normal ree component in this dissoved phase a small fraction of the nd 2 3 at is of fissiogenic origin the suspended loads filtered particulates of the same groundwater from within the reactor show similar isotopic anomalies with a fissiogenic nd contribution of 3 2 at similar fissiogenic nd contributions are observable for groundwater samples from outside the reactor zone 3 m distance both dissolved and suspended load samples from the more distant well 25 m have normal isotopic ratios thus the prevailing physico chemical conditions within this groundwater system allow the migration of the fissiogenic ree over at least 3 m but less than 25 m although ree phases are abundant in the reactor zone the ree concentrations of the water in contact with the sediments of this zone are very low 17 ppt the most important lanthanide and actinide bearing phases are uraninite coffinites and fe uranyl phosphate hydroxide hydrate the presence of all of these phases retards the mobility of the fissiogenic lanthanides and actinides

transport and selective uptake of radium into natural clay minerals abstract understanding of the environmental behavior of ra is important from the viewpoint of the long termed repository safety of radioactive waste but investigation of ra behavior in natural environment is difficult to detect we found isotopic evidence of ra transportation and its selective uptake into clay minerals from pb isotopic analyses illite grains found in calcite veins included in sandstone near the oklo uranium deposit republic of gabon show extremely low 207 pb 206 pb 0 0158 isotopic ratios although the pb isotopic ratios of calcite and quartz coexisting with illite indicate the formation age of each component those of illite do not in addition illite grains having low 207 pb 206 pb isotopic ratios contain a strongly large amount of ba 1230 to 6010 ppm in contrast with low contents of ba in calcite and quartz 207 pb 206 pb isotopic data suggest an excess of 206 pb due to selective adsorption of 226 ra and also ba into illite grains this is a very rare example to show evidence of the selective adsorption behavior of ra from the isotopic excesses of 206 pb although the adsorption ability of ra itself in nature was largely reported

petrology of basaltic sills from ocean drilling program sites 794 and 797 in the yamato basin of the japan sea the basement rocks drilled at ocean drilling program sites 794 and 797 in the yamato basin of the japan sea are composed dominantly of aphyric plagioclase phyric and olivine and plagioclase phyric tholeiitic sills intruded into wet sediments the rocks can be divided into an incompatible element depleted type and an enriched type based on their mineral and bulk rock compositions site 794 drilled an upper suite of dominating aphyric enriched basaltic sills the upper part of which contains relatively calcic plagioclase phenocrysts an85 90 and the lower part of which contains occasionally olivine phenocrysts a lower suite at site 794 is composed of dominating aphyric depleted sills with plagioclase and olivine microphenocrysts occasionally in chilled margins site 797 recovered an upper suite of sparsely plagioclase and olivine phyric to aphyric depleted sills and a lower suite of sparsely plagioclase phyric and aphyric enriched sills the groundmass phases of all suites suggest a systematic enrichment in feo tio2 na2o and k2o during solidification in addition there is evidence that olivine ceases to crystallize approximately when augite appears during groundmass crystallization irrespective of the enriched versus the depleted magma type the rock compositions reveal two contrasting evolution trends which can be related to fractionation processes in crustal magma chambers a tholeiitic evolution trend with increasing feo and tio2 and decreasing al2o3 can be modelled by fractional crystallization of 40 to 50 plagioclase olivine and augite a mildly calc alkalic evolution trend with decreasing feo increasing al2o3 and nearly constant tio2 can be modelled by 8 to 12 olivine fractionation one atmosphere anhydrous melting experiments on a basalt from hole 797c produced liquid compositions very similar to the basalts that exhibit the feo enrichment trend there is mineralogical evidence that the contrasting trends may be related to the effect of small amounts of water during crystallization relatively high water activity will suppress plagioclase crystallization and thus produce the iron depletion trend while nearly anhydrous crystallization involves the early appearance of plagioclase which is responsible for the iron enrichment trend the tholeiitic suites occur in lower parts of the drill cores while the calcalkalic suites occur in the upper parts this suggests a magmatic and tectonic evolution of the japan sea perhaps reflecting a transition between calc alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back arc spreading furthermore any model must be able to account for the range in parental magmas from depleted to enriched types throughout the magmatic history of the yamato basin

isotopic evidence for trapped fissiogenic ree and nucleogenic pu in apatite and pb evolution at the oklo natural reactor abstract a part of the boundary layer of reactor zone 10 at the oklo natural reactor shows a unique petrologic texture which contains high grade uraninite and massive apatite concretions in order to study distribution behavior of fission products around the boundary between the reactor zone and the wall rock and to clarify the relation of migration mechanisms of fission products with geochemical factors in situ isotopic analyses of nd sm gd pb and u in uraninite and apatite from the sample were performed by sensitive high resolution ion microprobe shrimp sm and gd isotopic ratios of uraninite and apatite show evidence of neutron irradiation with fluence between 4 4 6 8 10 19 n cm 2 judging from the isotopic anomalies of nd and u the apatite coexisting with the uraninite plays an important role in trapping fissiogenic lree and nucleogenic 239 pu into the structure systematic pb isotopic data from apatite uraninite galena and minium suggest the following chronological interpretations 1 the apatite formed 1 92 0 01 ga ago and trapped fissiogenic light ree and nucleogenic 239 pu that migrated from the reactor during the criticality 2 the uraninite around the boundary between reactor and sandstone dissolved once 1 1 1 2 ga ago 3 galena grains were formed by u pb mobilization in association with the intrusion of dolerite dyke 0 45 0 83 ga ago 4 minium was derived from recent dissolution of galena under locally oxidizing conditions

sm nd isotopic dating of proterozoic clay material an example from the francevillian sedimentary series gabon clay fractions of 0 4 m and 0 2 m and acid leachates of these clay fractions were separated from two different kerogen rich black shale samples from the francevillian sedimentary series in gabon they yielded identical sm nd isochron ages of2099 115 ma and2036 79 ma with initial nd values of 6 5 2 0 and 9 3 1 5 respectively these ages define multi episode illitization during early diagenesis as suggested by the differentsm nd ratios of the clay fractions the sm nd isotope data of the kerogen source rock up to 13 organic carbon plot between the two isochrons but close to the clay fractions suggesting that this organic matter reached nd isotopic equilibrium with respect to the clay minerals and the diagenetic fluids analyses of fracture filling bitumen showed that it had very elevated143nd 144nd and147sm 144nd ratios these values are higher than the most elevated values of the leachates but plot on the extension of the two isochrons this indicates that the bitumen reached nd isotopic equilibrium with the kerogen source rock from which it was expelled and the authigenic clay minerals secondary migration and chemical evolution of the bitumen appear to have caused the fractionation between sm and nd with an attendant increase in thesm nd ratio the results suggest that the sm nd isotope method has potential for both dating diagenetically related illitization using leachate residue pairs of small size clay fractions and reconstructing geochemical processes during diagenesis in oil producing sedimentary basins

delayed fractionation of basaltic lavas the phenocryst cores of the basaltic lavas from jan mayen and hawaii display a range in compositions the textural features of the phenocrysts also vary both euhedral and skeletal phenocrysts are present in the same thin section apparently the basaltic magmas underwent crystallization within a temperature interval of 50 200 c before they became fractionated the fractionates of basaltic lavas are therefore average compositions of the phenocryst assemblages rather than liquidus compositions this type of fractionation is called delayed fractionation it is considered that most tholeiitic and alkalic basaltic lavas undergo delayed fractionation

alteration of monazite and zircon and lead migration as geochemical tracers of fluid paleocirculations around the oklo okelobondo and bangombe natural nuclear reaction zones franceville basin gabon abstract large scale light rare earth element lree uranium lead and phosphorus migration has been evidenced in the fa lower proterozoic sandstones of the franceville basin gabon hosting oklo natural nuclear reaction zones rz in relation with extensive accessory mineral alteration by highly saline diagenetic brines 28 7 wt nacl eq to 30 wt cacl2 eq at about 140 c and 1 kbar monazite is the most severely altered accessory mineral in the coarse grained sandstones of the basal fa formation detrital monazite crystals are altered to th oh silicate microcrystalline phase with very low concentrations of u and lree the th la ratio increase from non altered th la 0 27 to altered sandstones th la 1 14 shows that about 76 of the lree was leached this corresponds to the leaching of 2 01 109 metric tons at the scale of the fa formation in the franceville basin similarly the th u increase from monazite th u 18 6 to the th silicate phase th u 88 7 is interpreted as a result of an alteration by oxidizing brines with leaching of u together with lree and p it corresponds to the leaching of 9 06 106 metric tons of uranium this amount of uranium largely exceeds the known uranium reserves from the franceville basin in zircon crystals the cores are generally homogeneous weakly fractured and well preserved as attested by the archean ages 2867 24 and 2865 51 ma obtained by ionic microprobe analysis on zircon of the fa formation respectively from the marginal and central parts of the basin their composition corresponds to the pure end member zr hf sio4 poor in th and u th u 1 at the contrary their rims which present several growth zones with cracks fillings are enriched in ree p th and u with higher th u ratios 5 10 both altered monazite and altered zircon contain galena as numerous inclusions in the outer growth zones and as crack fillings for example in zircon the pb of galena crystals 3 23 wt largely exceeds the amount of pb maximum 0 1 wt that would have been produced in situ by radioactive decay in this mineral nearly all the lead were introduced into altered zones of accessories dissolution of accessory minerals occurred at 2000 ma producing a porous and distorted crystal structure which has allowed a later incorporation of pb galena inclusions in altered zircons located in the vicinity of reactor zones have radiogenic lead compositions altered zircon rims and galena inclusions in altered zircon located far from reactor zones have non radiogenic pb isotopic compositions confirming the external origin of lead pb isotopic evolution models indicate a crystallization age sometime after 1000 ma both for galena located close to and far from u mineralizations and reactor zones which may be synchronous with a regional extension event contemporaneous with intrusion of dolerite dyke swarms between 1000 and 750 ma at the scale of the franceville basin the present study also illustrates the different retention capacities of accessory mineral for elements representing analogs of the radiotoxic nuclides in the relatively extreme natural conditions created by the circulation of moderately hot and chloride rich fluids during the diagenesis of a sedimentary basin

francoisite ce a new mineral species from la creusaz uranium deposit valais switzerland and from radium ridge flinders ranges south australia description and genesis the new mineral francoisite ce ce nd ca uo 2 3 o oh po 4 2 6h 2 o is the ce analog of francoisite nd it has been discovered simultaneously at the la creusaz uranium deposit near les marecottes in valais switzerland and at the number 2 uranium workings radium ridge near mt painter arkaroola area northern flinders ranges in south australia francoisite ce is a uranyl bearing supergene mineral that results from the alteration under oxidative conditions of ree and u 4 bearing hypogene minerals allanite ce monazite ce uraninite at les marecottes monazite ce ishikawaite samarskite and an unknown primary u mineral at radium ridge the ree composition of francoisite ce results from a short aqueous transport of ree leached out of primary minerals most likely monazite ce at radium ridge and allanite ce at la creusaz with fractionation among ree resulting mainly from aqueous transport with only limited ce loss due to oxidation to ce 4 during transport

an example of lower proterozoic sediments the francevillian in gabon abstract new mineralogical and isotopic data have been obtained on diagenetic fine fractions of the francevillian sediments together with the rb sr age of a syenitic intrusive and already published isotopic ages determined by means of uranium ore and uranium fission products they allow us to present a more complete geological history of the francevillian the main steps are deposition of the fa formation before 2140 ma deposition of the fb fe formations between 2140 and 2050 ma siliceous diagenesis followed by nuclear reactions between 2050 and 1930 ma carbonate diagenesis at nearly 1870 ma end of cooling at 1700 ma and dolerite intrusion at ca 970 ma the francevillian sediments are to date the oldest true sediments known they are diagenetic lower proterozoic sediments

mobilization and mechanisms of retardation in the oklo natural reactor zone 2 gabon inferences from u ree zr mo and se isotopes abstract mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides to address these issues in situ isotopic analyses by shrimp and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products exchangeable and non exchangeable and their association with mineral phases the distribution patterns of u ree zr and mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality the results emphasize the mobility of zr at oklo this element being often considered as immobile during water rock interactions the main output is the demonstration of the net effects of sorption and coprecipitation processes chlorite and to a lesser extent illite were found to have adsorbed significant amounts of u ree zr and probably th and less sorbing elements such as mo coprecipitation of secondary uo2 and p rich coffinite within the alteration halo is also an important means of retardation the concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour no retention effect was found for se in the immediate vicinity of the reactor and this element may have moved farther from its source of production these results have interesting implications for the evaluation of long term containment of radionuclides they provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary u minerals this study also demonstrates the robustness of these retention processes over extremely long periods of time

isotopic evidence for the retention of90sr inferred from excess90zr in the oklo natural fission reactors implication for geochemical behaviour of fissiogenic rb sr cs and ba in order to investigate the mobility of fissiogenic90sr in the geological environment the zr isotopic compositions of seven samples from one of the newly formed oklo natural reactor zones i e reactor core and adjacent rocks 10 sf84 in the republic of gabon were determined with an inductively coupled plasma mass spectrometer icp ms zr isotopes in uraninite grains from different reactor zones were also measured by secondary ion mass spectrometry sims fissiogenic zr isotopic abundances of three samples from the reactor core have excess90zr which has never before been formed in previous oklo samples in this paper the geochemical behaviour of90zr is discussed by making use of the relative retentivity inferred from the isotopic abundance of sr the excess in90zr suggests dependence on the degree of retention migration of90sr the precursor of90zr in the fission chain in the aqueous phase chemical fractionation between sr and zr could have occurred before radioactive90sr decayed considering the halflife of90sr t1 2 29 1y considerable amounts of the latter have been produced during criticality sr and zr including90zr could have been redistributed between the reactor core and its vicinity the retentivity of fissiogenic90zr in reactor core 10 is not homogeneous in addition the distributions of rb cs and ba is also heterogeneous

redistribution of ree pb and u by supergene weathering studied from in situ isotopic analyses of the bangombe natural reactor gabon abstract the isotopic analyses of rare earth elements ree pb and u in several kinds of minerals from the clay and black shale layers above the bangombe natural reactor gabon were performed using a sensitive high resolution microprobe shrimp to investigate the migration and retardation processes of fission products released from the reactor ree isotopic data of the secondary minerals found in clays and black shales show that most of fission products were effectively trapped in the clays and not distributed into the black shales over the clays which reveals that the clays play an important role in preventing fission products from spreading zircon crystals in the clays heterogeneously contain high u regions up to 28 3 wt with normal 235 u 238 u ratios 0 00725 and significant amounts of fissiogenic ree which suggest the occurrence of significant chemical fractionation between ree and u during the dissolution of reactor uraninite and the recrystallization of secondary u minerals the pb data suggest that galena grains in the clays were also formed by the mixing of the two components during a recent alteration event and that a significant amount of pb was derived from 2 05 ga old original uraninite rather than reactor uraninite the u pb systematics of zircon provide chronological information on the old igneous activity associated with the basement rock formation at 2 8 ga and geochemical evidence of the incomplete mixing of independent pb and u sources this result is consistent with previous chronological results in this area

endogreisen brecciation and fluid activity at the mount bischoff sn deposit north west tasmania australia endogreisens which replace k feld spar quartz dykes in a devonian 360 ma tin deposit at mt bischoff north west tasmania formed from the interaction of unusual solutions probably derived from an underlying leucogranite pluton porphyry dykes and limited quantities of local dolomitic country rock components the intensity of greisenization and ph of the solutions increase inward to the greisenized dykes cores and downward the following types of greisen assemblages indicate increasing degrees of greisenization sericite muscovite quartz tourmaline fluorite topaz quartz tourmaline fluorite weberite prosopite ralstonite ca ralstonite and quartz topaz fluorite where the solutions interacted with dolomite exogreisens consisting of topaz or tourmaline bearing assemblages were formed the greisens were subsequently overprinted to varying degrees by siderite sulphides and hydrous silicates talc serpentine chlorite micas the temperature during greisenization ranged from 180 to 414 c based on fluid inclusions in topaz quartz fluorite sellaite and cassiterite the main greisen forming event occurred at temperatures of 360 20 c the fluids boiled intermittently their salinities ranged from 31 5 to 38 9 wt total dissolved salts consisting of ca k na fe cl hydrocarbon species fluid inclusion data indicate that only 0 5 1 5 km of cover were present above this deposit at the time of formation the greisenized dykes were intruded by and intrude different stages of breccias the breccias consist mainly of country rock and greisenized dyke fragments with rock flour and later tourmaline alteration the mt bischoff greisen system is possibly part of a porphyry tin style deposit formed at near surface conditions 0 5 1 0 km

methodology for bounding calculations of nuclear criticality of fissile material accumulations external to a waste container at yucca mountain nevada abstract the possibility of nuclear criticality however remote in the vicinity of the proposed repository at yucca mountain nevada generates justified concerns and may impact the performance of the repository a heuristic approach is presented here for determining the amount spatial distribution and other characteristics of fissile material accumulation in the rock beneath a waste package that could contribute to such an event this study is concerned primarily with waste packages containing special spent fuel from the department of energy and high level nuclear waste glass mixing with less alkaline waters and the subsequent drop in ph is the mechanism that is most efficient for precipitating fissile material from the waste package internal leachate in contrast to natural deposits in which redox changes are the main precipitation driver external accumulation size is determined by 1 computing the chemical composition of the leachate leaving a package as its internal materials degrade with the batch geochemical code eq3 6 2 determining precipitation of fissile material into mineral phases using the 1d geochemical code phreeqc as the effluent mixes with percolation water and 3 heuristically scaling results to a 3d volume and computing the criticality coefficient using the code mcnp loci for accumulation are the multiple lithophysal cavities and the fracture system a bounding conservative approach is used by necessity in step 3 nuclear criticality is sensitive to small variations in the distribution of fissile material and parameters of natural systems vary by orders of magnitude because the most likely combinations of parameters are not conducive to nuclear criticality this study focuses on extreme values of parameter probabilistic distributions such as limited flow into the package associated with a large percolation rate combinations of material degradation rates favoring actinide release and very high host rock porosity values by considering these combinations most favorable to criticality but unlikely it was concluded that external nuclear criticality is not a concern at the proposed repository

petrology of lavas from the puu oo eruption of kilauea volcano iii the kupaianaha episode 1986 1992 the puu oo eruption has been remarkable in the historical record of kilauea volcano for its duration over 13 years volume 1 km3 and compositional variation 5 7 10 wt mgo during the summer of 1986 the main vent for lava production moved 3 km down the east rift zone and the eruption style changed from episodic geyser like fountaining at puu oo to virtually continuous relatively quiescent effusion at the kupaianaha vent this paper examines this next chapter in the puu oo eruption episodes 48 and 49 and presents new icp ms trace element and pb sr and nd isotope data for the entire eruption 1983 1994 nearly aphyric to weakly olivine phyric lavas were erupted during episodes 48 and 49 the variation in mgo content of kupaianaha lavas erupted before 1990 correlates with changes in tilt at the summit of kilauea both of which probably were controlled by variations in kilauea s magma supply rate these lavas contain euhedral olivines which generally are in equilibrium with whole rock compositions although some of the more mafic lavas which erupted during 1990 a period of frequent pauses in the eruption accumulated 2 4 vol olivine the highest forsterite content of olivines 85 in kupaianaha lavas indicates that the parental magmas for these lavas had mgo contents of 10 wt which equals the highest observed value for lavas during this eruption the composition of the puu oo lavas has progressively changed during the eruption since early 1985 episode 30 when mixing between an evolved rift zone magma and a more mafic summit reservoir derived magma ended the normalized to 10 wt mgo abundances of highly incompatible elements and cao have systematically decreased with time whereas ratios of these trace elements and pb sr and nd isotopes and the abundances of y and yb have remained relatively unchanged these results indicate that the hawaiian plume source for puu oo magmas must be relatively homogeneous on a scale of 10 20 km3 assuming 5 10 partial melting and that localized melting within the plume has apparently progressively depleted its incompatible elements and clinopyroxene component as the eruption continued the rate of variation of highly incompatible elements in puu oo lavas is much greater than that observed for kilauea historical summit lavas e g ba y 0 09 a 1 vs 0 03 a 1 this rapid change indicates that puu oo magmas did not mix thoroughly with magma in the summit reservoir thus except for variable amounts of olivine fractionation the geochemical variation in these lavas is predominantly controlled by mantle processes

evidence of fissiogenic cs estimated from ba isotopic deviations in an oklo natural reactor zone abstract isotopic studies of many elements from the uranium ore natural nuclear reactors at oklo provide useful information on the migration of radioactive nuclides the fissiogenic isotopic composition of ba is particularly interesting as it is an important indication in the search for fissiogenic cs in this report we detail the detection of remarkable isotopic deviations of ba in the oklo samples and estimate the geochemical behaviour of fissiogenic cs from excess ba isotopes six samples systematically collected from borehole sf84 zone 10 at the oklo uranium mine have been analyzed isotopic deviations of ba indicate the existence of fissiogenic cs and ba a good correlation between the elemental abundance of cs and isotopic abundances of excess 135 ba and 137 ba suggests that fissiogenic 135 ba and 137 ba behaved as cs rather than ba

role of organic matter in the proterozoic oklo natural fission reactors gabon africa of the sixteen known oklo and the bangombe natural fission reactors hydrothermally altered elastic sedimentary rocks that contain abundant uraninite and authigenic clay minerals reactors 1 to 6 at oklo contain only traces of organic matter but the others are rich in organic substances reactors 7 to 9 are the subjects of this study these organic rich reactors may serve as time tested analogues for anthropogenic nuclear waste containment strategies organic matter helped to concentrate quantities of uranium sufficient to initiate the nuclear chain reactions liquid bitumen was generated from organic matter by hydrothermal reactions during nuclear criticality the bitumen soon became a solid consisting of polycyclic aromatic hydrocarbons and an intimate mixture of cryptocrystalline graphite which enclosed and immobilized uraninite and the fission generated isotopes entrapped in uraninite this mechanism prevented major loss of uranium and fission products from the natural nuclear reactors for 1 2 b y 24 refs 4 figs

the geology and genesis of the cleveland tin deposit western tasmania fluid inclusion and stable isotope studies the cleveland tin copper deposit comprises several lenses of pyrrhotite cassiterite stannite chalcopyrite mineralization represented by a series of interlayered chert shale beds and mineralized units occurring within an early cambrian succession of spilitic basalt mafic pyro clastic argillite chert and limestone overlain by a sequence of micaceous turbiditic graywacke and argillite south and east of the deposit is the late devonian meredith granite ore paragenesis is divided into four stages with a superimposed mineralogical zonation i quartz fluorite wolframite molybdenite cassiterite as stockwork veins in the footwall rocks ii quartz tourmaline fluorite chlorite pyrrhotite chalcopyrite cassiterite stannite as stratabound lenses parallel to the host rocks with sphalerite and arsenopyrite common at the extremities of the sulfide lenses and quartz veins with sphalerite arsenopyrite chalcopyrite and galena surrounding the deposit iii quartz arsenopyrite stannite as veins and iv fluorite carbonate quartz filling veins and vugs an extensive alteration halo in the hanging wall of the sulfide lenses is defined by increased replacement of pyroxene by actinolite nearer to the ore accompanied by an increase in k rb sn and zn and a decrease in na but is not reflected by the whole rock delta 18 o values fluid inclusion and stable isotope data reveal a systematic trend in depositional temperatures and a marked consistency in composition of the hydrothermal fluid fluids of stages i and ii were low to moderately saline 8 14 nacl but while stage i fluids were of low co 2 content the co 2 content of stage ii fluids was locally high the co 2 in stage ii fluids was probably derived from dissolution of limestone the salinity of the fluid decreased slightly to 7 to 11 percent nacl and was very low in co 2 during stage iv fluid temperatures are estimated at about 480 degrees c for stage i and 500 degrees c for stage ii and 250 degrees to 300 degrees c during stage iv the calculated delta 18 o composition of the water in the hydrothermal fluid was about 8 7 per mil for stage i 10 2 per mil for stage ii and 9 5 per mil for stage iv the delta 34 s values of the sulfides were near to 0 per mil 0 2 to 5 5 per mil throughout the mineralization the consistent salinity and the isotopic compositions indicate a magmatic source for the mineralizing fluid and a lack of mixing with meteoric water the cleveland deposit originated from the metasomatic replacement of limestone beds by tin bearing low to moderately saline fluids of initially low co 2 content derived from a granitic source related to the meredith granite

textural and paleo fluid flow control on diagenesis in the paleoproterozoic franceville basin south eastern gabon abstract the paleoproterozoic 2 15 ga franceville basin south eastern gabon is a continental sedimentary basin that host unmetamorphosed sediments this study involve detailed mineralogy sedimentology and petrography of the basal sedimentary units of fa and lower fb fb1 formations from the basin margin to centre in relation to mineral paragenesis and fluid flow the fa formation conglomerate sandstone and mudstone consists lithofacies of mixed fluvial and fluvio deltaic transitional origin while the overlying fb1 formation includes alternating organic rich black shale and sandstone of marine deposit the medium to coarse grained fluvio deltaic quartz arenite in upper part of fa formation is characterized by pervasive authigenic quartz cementation that reduced the porosity and permeability in the early stage of burial history this provides a resistant framework for subsequent diagenetic modification and also inhibits fluid flow during burial diagenesis in contrast the clay and unstable detrital grains rich fluvial arkosic to sub arkosic sandstones that escaped early quartz cementation show considerable pressure solution at grain contacts these arkosics were less porous and permeable when deposited but transformed to diagenetic aquifers as a result of dissolution of detrital grains during diagenesis and subsequent precipitation of authigenic mineral cements in the resulting secondary porosities from the proximal to distal basin and within sample suites there is no considerable chemical variation in the petrographic distinct generations of the precipitated illite and chlorite suggesting their precipitation from a near equilibrium homogenous pore fluid the predominance of 1mt illite polytype in most lithologies reflects precipitation of the clay minerals and probably other cements in an environment with high fluid rock ratio dolomite anhydrite barite and fe oxides are the main crystallized authigenic minerals aside illite and fe rich chlorite clay minerals the mineralogical assemblages and textural occurrences of the rocks suggest that diagenesis and fluid flow in the fa formation in the franceville basin are mainly controlled by depositional facies

natural fission reactors of oklo fourteen natural fission reactors are known to date in three different ore deposits located in the franceville basin the most famous and the first discovered are those in the oklo uranium ore deposit petrographic geochemical and oxygen isotope studies have been performed in an attempt to answer these questions 1 what are the main characteristics of the uranium ores in which fission reactions took place 2 what are the processes that created such exceptionally high grade ores and 3 what are the effects of such fission reactions on the host rocks the ore which hosts the fission reactions at oklo is characterized by a gangue of clay minerals whereas the normal ore of oklo consists of sandstones these clay minerals are the result of hydrothermal alteration of sandstone hydrothermal circulation was induced by the heat produced by fission reactions the reactor ores are further characterized by very high uranium contents 20 60 and high uranium accumulation by means of silica depletion and by uranium enrichment of the normal ore without uranium depletion hydrothermal convective circulation induced by the fission reactions created very high grade ore next to the reactors hydrothermal processes are thus critical to the propagation of the fission reactions

metallogenesis of the french part of the variscan orogen part ii time space relationships between u au and snw ore deposition and geodynamic events mineralogical and upb data abstract mineralogical geochemical structural and isotopic u pb studies were carried out on representative uranium and gold deposits from the french part of the variscan orogen results are compared with complementary data from the literature in order to propose a reconstruction of the metallogenesis in the variscan terranes with a particular emphasis on u deposits the data show that the early metal concentrations or pre enrichments in the terranes are affected by successive metallogenic stages which caused significant metal transport and deposition thus the significant geodynamic events which affected the variscan terranes were responsible for multistage metal deposition or alteration of the early concentrations and for the superimposed features of the metal deposits u pb geochronological results complemented by a compilation of geochronological data on u and pb zn ba f deposits show that three major stages of u deposition took place in western europe in the periods 325 300 ma 290 260 ma and 190 170 ma in addition small uranium mineralization events have been dated at 425 ma 90 110 ma and as tertiary about 40 ma the permo stephanian stage is associated with late brittle deformation associated with major shear zone movements and constitutes the major stage of u concentration the jurassic phase has a prominent part in the hydrothermal remobilization of uranium and in f ba pb zn deposition throughout western europe especially in the eastern part of the massif central other stages correspond to subordinate uranium migration within variscan terranes during earlier or later geodynamic phases the silurian metamorphism in vendee and the mesozoic to tertiary tectonic movements related to the pyrenean or alpine orogenies in the south eastern part of the massif central and in the alps data available on au and sn w as sb deposits have been considered for comparison with the u metallogenesis stages related to the deposition of such metals display significant similarities with those related to uranium deposition they occurred mostly during visean to permian events related to the intrusion of the late visean sn w and late carboniferous sn w li f granitoids and to the late brittle deformation stages which affected the hercynian basement in the vicinity of major shear zones au as sb however the significant discrepancies between the physical chemical conditions of solubilization transport and deposition of the metal led to distinct deposit location independently of the metal source location within the terranes

the oxygen isotopic composition of uranium minerals a review abstract uranium ore is an essential material in the preparation of nuclear fuel for civilian as well as military uses uranium is first extracted from uranium bearing minerals using a variety of reagents and is precipitated from solutions as yellow cake prior to isotope enrichment processes the disintegration of the former eastern bloc in the 1990s and frequent unrest in the middle east have underscored the need for better characterizing source uranium ores for forensic and attribution purposes the world s major deposits of u occur in several distinctly different geological environments fourteen principal types of u deposits and rocks with elevated uranium contents are recognized with more than 40 subtypes combining our own analysis and literature data we have amassed over 250 oxygen isotope data from 13 major u producing countries which vary widely from 32 to 11 however interpreting the oxygen isotopic composition of uraninite in terms of the composition of the fluid from which it precipitated or interacted with requires knowledge of the fractionation factor and temperature of interactions which are not always available since each deposit type occurs within a unique geologic setting and is generally formed from chemically distinct fluids the chemical compositions of the uranium ores are also distinct uranium deposits that form in igneous rocks have higher trace element compositions relative to sandstone hosted deposits our data shows that one of the most useful techniques for distinguishing between uranium ore is to use a combination of 18 o values and rare earth elements e g la yb ratios these methods may allow investigators to trace uranium ore back to the source

the generation and shape of feeder dykes from mantle sources the criterion for fracture propagation suggests that the primary magma generated in the mantle ascends in a stepwise fashion initially a pulse of magma forms a dyke above the mantle source and ascends for some distance whereafter it stops a subsequent pulse of magma joins the first one and the ascent of the accumulating body of magma continues for some distance each time a new pulse of magma reaches the body of magma situated in the top of the dyke in this manner a dyke is formed that may extend either partly or the whole way from the mantle source to the surface this model may explain the large pressures up to 2000 bars that prevail during basaltic eruptions the calculated shape of the ascending pulses of magma suggests that their heights and widths are similar

acidity salinity diagrams application to greisen and porphyry deposits to a first approximation alteration minerals in greisen and related fluorine rich porphyry deposits lie in the system k 2 o al 2 o 3 sio 2 h 2 o f 2 o sub 1 equilibria in this model system can be depicted on quartz and fluid saturated chemical potential diagrams whose axes are mu hf acidity where mu hf mu hcl mu sub fcl sub 1 and mu kf salinity where mu kf mu nacl mu sub kna sub 1 mu sub fcl sub 1 advantages of this approach are that it reflects the fact that the anion f unlike cl is incorporated in greisen minerals mainly topaz fluorite and micas and that mineral stabilities can be correlated with possible ore forming processes the most important of these processes is probably vapor phase separation from an aqueous brine or magma first or second boiling fluid inclusion studies have shown that boiling occurs in all porphyry and many but not all greisen deposits vapor phase separation from a magma pneumatolysis must similarly be responsible for the formation of greisenlike mineral assemblages in miarolitic topaz rhyolite lava flows and is presumably also responsible for the formation of gem pockets in more deeply seated pegmatites and granites the result of boiling is an increase in mu hf and a decrease in mu kf in the vapor phase and the opposite in the residual brine or magma in deep greisen veins the muscovite stability field is encountered between the k feldspar and topaz fields whereas in f rich relatively shallow porphyry deposits such as henderson colorado boiling causes aqueous fluids to pass directly from the k feldspar ore zone to the overlying topaz magnetite zone muscovite or f bearing mn fe garnet is only encountered farther out as the fluids are condensed neutralized and diluted by cooler meteoric waters when ca fe mn be w o 2 s 2 and co 2 are added to the greisen model system scheelite is shown to transform to wolframite with increasing mu hf and beryl to phenakite with increasing mu hf or mu kf biotite transforms to magnetite hematite siderite pyrite chlorite or fayalite beyond the k feldspar field its stability is enhanced by high mu hf and chlorite cordierite or mn fe garnet transform to an mn fe carbonate oxide or sulfide or biotite beyond the al silicate field the ca content of plagioclase coexisting with fluorite decreases with an increase in either mu hf or mu kf and thus provides an indicator of total fluorine in both acid and neutral species

oil and its biomarkers associated with the palaeoproterozoic oklo natural fission reactors gabon abstract the 2 1 ga fa formation sandstone of the franceville basin in gabon that hosts the oklo natural fission reactors has been discovered to contain abundant palaeoproterozoic oil bearing fluid inclusions inside which syngenetic biomarkers are preserved the oil occurs within h2o and co2 dominated inclusions trapped in syntaxial quartz overgrowths and intragranular and transgranular microfractures in detrital quartz homogeneous fluid inclusion assemblages with oil inside a brine and carbonic fluid indicate that there was intermixing of brines and co2 with liquid hydrocarbons prior to trapping textural relationships and microthermometry suggest that entrapment of oil occurred first during diagenesis of the sandstone at ca 115 to 200 c soon after deposition of the sequence at ca 2 1 ga and secondly during a hydrothermal event at 200 to 285 c linked to the operation of the reactors ca 1 98 ga a series of sequential outside rinse blanks of framework grains analysed by gas chromatography mass spectrometry gc ms prior to fluid inclusion crushing shows a first solvent rinse dominated by hydrocarbons derived from co occurring solid bitumens which have a different geochemical distribution compared to the fluid inclusion oil the crush leach extract containing oil from fluid inclusions is markedly richer than the final outside rinse blank displays significant geochemical differences in compound distribution and is interpreted to include only a very small contribution from solid bitumen the fluid inclusion oil has a high maturity is non biodegraded and lacks clear effects of radiolysis it is therefore likely that the oil that migrated at temperatures higher than those of diagenesis was generated by hydrothermal alteration caused by reactor heated fluids rather than by the criticality event itself biomarkers include hopanes 2 methylhopanes terpanes and steranes indicative of bacteria including cyanobacteria and of eukaryotes the relatively high concentration of regular steranes suggests that the source rock had a substantial eukaryotic contribution trace amounts of c30n propylcholestanes indicate a contribution from marine algae the oil was likely derived from conformably overlying marine black shales that were deposited during the great oxidation event during which the earth s atmosphere became oxygenated the biomarker content of the oil is remarkably similar to that of other oils formed in sedimentary basins between 2 7 and 1 4 ga it is thus suggested that hydrocarbons were derived from a uniform range of organisms in this time period despite apparent fluctuations of atmospheric o2

constraining provenance for the uraniferous paleoproterozoic francevillian group sediments gabon with detrital zircon geochronology and geochemistry abstract the paleoproterozoic sandstones of the ca 2 1 ga francevillian group southeast gabon host the oldest known redox controlled high grade uranium u deposits uranium bearing detrital minerals e g monazite in the francevillian group sediments were derived by erosion of archean basement rocks from the east gabonian block of the northwestern congo craton surrounding the francevillian basin they are viewed as primary sources for u but the nature of these sources remains poorly constrained here u pb ages and hf isotope data for detrital zircon grains are combined with literature geochemical data for igneous rocks from the archean basement to characterize the nature age and origin of the provenance for the francevillian basin and the ultimate source of u for the high grade uranium deposits detrital zircon ages range from ca 3 1 to 2 6 ga with a dominant population between ca 2 90 and 2 80 ga suggesting a major contribution from ca 2 88 to 2 81 ga high k granitoids with a limited contribution from ca 3 1 to 2 84 ga ttgs and 2 75 to 2 7 ga granitoids of the east gabonian block most detrital zircon grains share similar depleted mantle hf model ages between ca 3 08 and 3 30 ga indicating significant recycling of a paleoarchean to mesoarchean crust we suggest that ca 2 9 to 2 8 ga high k u and th granitoids which reflect hybrid melts of both mantle and crustal parentage were the major detrital sources of u to the francevillian basin and ultimately for the u deposits we propose that ca 2 9 to 2 8 ga detrital zircons with hf tdm ages between ca 3 1 and 3 3 ga may thus allow for vectoring u deposits in sedimentary basins across the congo craton

shrimp in situ isotopic analyses of ree pb and u in micro minerals bearing fission products in the oklo and bangombe natural reactors a review of a natural analogue study for the migration of fission products abstract in the precambrian parts of the oklo okelobondo and bangombe uranium deposits of the republic of gabon central africa functioned as natural fission reactors many elements in the oklo and bangombe uranium deposits show variations in isotopic composition caused by a combination of nuclear fission neutron capture and radioactive decay isotopic studies provide useful information to understand the behavior of radionuclides in geological media in our recent work in situ ree pb and u isotopic analyses of individual tiny minerals in and around reactor zones have been performed using a shrimp sensitive high resolution ion microprobe the isotopic results of the shrimp analyses on micro minerals found in and around the oklo and bangombe natural reactors are reviewed in this paper the data suggest the selective uptake behavior of 1 ra into illite and 2 pu into apatite 3 the formation process of secondary minerals bearing fissiogenic ree and depleted u 4 evidence of nuggets particles bearing fissiogenic platinum group elements pge and 5 from the u pb systematics of highly altered zircons the redistribution of u and pb

uraninite recrystallization and pb loss in the oklo and bangombe natural fission reactors gabon abstract the oklo and bangombe natural fossil fission reactors formed ca 2 ga ago in the franceville basin gabon the response of uraninite in the natural reactors to different geological conditions has implications for the disposal of the uo 2 in spent nuclear fuel uraninite and galena from two reactor zones rz16 at oklo and rzb at bangombe were studied to clarify the chronology and effect of alteration events on the reactor zones in addition ion microprobe u pb analysis of zircons from a dolerite dyke in the oklo deposit were completed to better constrain the age of the dyke and thereby testing the link between the dyke and an important alteration event in the reactor zones the analyzed uraninite from rz16 and rzb contains ca 6 wt pbo indicating a substantial loss of radiogenic pb transmission electron microscopy showed that microscopic uraninite grains in the reactor zones consist of mainly defect free nanocrystalline to microcrystalline aggregates however the nanocrystalline regions have elevated si contents and lower pb contents than coarser uraninite crystallites single stage model ages of large millimeter sized galena grains at both rz16 and rzb correlate well with the age of the oklo dolerite dyke 860 39 ma 2 thus the first major pb loss from uraninite occurred at both oklo and bangombe during regional extension and the intrusion of a dyke swarm in the franceville basin 860 890 ma ago uraninite pb isotopes from rz16 and rzb give lower ages of ca 500 ma these ages agree with the chemical ages of the uraninite and show that an ancient pb loss occurred after the intrusion of the dolerite dykes the presence of nanocrystallites in the reactor uraninite indicates internal recrystallization which may have occurred around 500 ma resulting in the 6wt pbo uraninite it is suggested that leaching by fluid interaction triggered by the pan african orogeny was important during this second pb loss event thus there are indications that uraninite at both the oklo and bangombe natural reactors has experienced at least two ancient episodes of pb loss associated with internal recrystallization these recrystallization events have occurred without significantly depleting the 2 ga fission products compatible with the uraninite structure

geochemical fixation of rare earth elements into secondary minerals in sandstones beneath a natural fission reactor at bangombe gabon to study geochemical processes for migration and fixation of fissiogenic rare earth elements ree in association with uranium dissolution in situ isotopic analyses using an ion microprobe were performed on u and ree bearing secondary minerals such as coffinite francoisite uraniferous goethite and uraninite found in a sandstone layer 30 to 110 cm beneath a natural fission reactor at bangombe gabon phosphate minerals such as phosphatian coffinite and francoisite with depleted 235u 235u 238u 0 00609 to 0 00638 contained large amount of fissiogenic light ree while micro sized uraninite grains in a solid bitumen aggregate have normal u isotopic values 235u 238u 0 00725 and small amount of fissiogenic ree components the proportions of fissiogenic and non fissiogenic ree components in four samples from the core of bax03 vary in depth ranging from 30 cm to 130 cm beneath the reactor which suggests mixing between fissiogenic isotopes from the reactor and non fissiogenic isotopes from original minerals in the sandstone significant chemical fractionation was observed between ce and the other ree in the secondary minerals which shows evidence of an oxidizing atmosphere during their formation pb isotopic analyses of individual minerals do not directly provide chronological information because of the disturbance of u pb decay system due to recent geologic alteration however systematic pb isotopic results from all of the minerals reveal the mobilization of fissiogenic isotopes pb and u from the reactor in association with dolerite dyke intrusion 0 798 ga ago and the formation of the secondary minerals by mixing event between 2 05 ga old original minerals and reactor materials due to recent alteration

carbonaceous substances associated with the paleoproterozoic natural nuclear fission reactors of oklo gabon paragenesis thermal maturation and carbon isotopic and trace element compositions field relationships textural and reflectance characteristics and carbon isotopic compositions indicate that at least eight different categories of carbonaceous substances cs occur in the black shales uranium ores and fossil nuclear fission reactors of oklo the categories include kerogens of the mikouloungou coal in the fa formation the c1 pelites of the fa formation the black shale of the fb formation and bitumens present in veinlets in the basal black shale of the fb formation in the fa formation in normal ore and in hydraulic fractures in and near the fossil reactors and in late dolerite dikes bitumen migration occurred at oklo during at least three intervals 1 ca 2 0 ga ago when petroleum liquids derivatives of which are preserved as solid bitumen were generated by thermal maturation of black shale kerogen 2 ca 1 968 50 ma ago when 15 or more pockets of uranium ore achieved criticality and 3 977 to 981 27 ma ago when dolerite dikes intruded radionuclide migration occurred during intervals 2 and 3 bitumenization trends established for organic matter which has undergone thermal maturation fail to discriminate between heat affected and normally matured cs at oklo in and near oklo reactors where several types of solid bitumen commonly occur in close association carbon of the youngest low reflectance bitumen is isotopically heavier than that of older high reflectance bitumen due to thermal cracking this process together with enhanced radiolysis of bitumen in and near the reactors have combined to increase its 13c avg 25 8 compared to that of bitumen samples in normal ore avg 35 5 distant from the reactors graphitized bitumen inhibited migration of 235u and some fission products in uraninite of the natural reactors however alkaline earth elements such as ba sr and mo and alkaline elements rb and cs not compatible in the uo2 crystalline structure have migrated in hydrothermal solutions and in the once fluid oklo bitumens in situ analysis of solid bitumens by laser ablation icp ms detects differences in trace element composition developed as a result of their varying geological histories and evolutionary trends

mesozoic east north american alkaline magmatism part 1 evolution of monteregian lamprophyres quebec canada abstract the cretaceous monteregian alkaline igneous province is composed of shallowly emplaced plutonic complexes and cogenetic dykes and sills the mafic dykes are principally camptonites monchiquites and alnoites and sample the magmas that fed the plutons the three principal dyke suites have different incompatible trace element ratios y ce zr nb and are not interrelated by closed system fractional crystallization zonation of pyroxene grains imply a polybaric crystallization history indicating that magmas ascended as mafic crystal charged mushes intrasuite chemical variations among monchiquites are broadly consistent with segregation of the suspended phenocrysts from interstitial syenitic melts partial resorption of suspended amphibole megacrysts assimilation of syenitic material 5 8 and exsolution of carbonatitic melt are required to explain the variations of ba rb k 2 o th la and tio 2 chemical variations among mafic camptonites are acceptably modelled by fractionation of pyroxene and olivine only implying that their amphibole rich character is the result of postfractionation crystallization under volatile rich conditions high zr nb camptonites and rb rich ac monchiquites could not unequivocably be linked to the main lineages and may represent distinct sources or have been affected by crustal contaminants

hydrothermal and supergene clays of the oklo natural reactors conditions of radionuclide release migration and retention abstract mineralogical chemical and o isotopic analyses were performed on the clay matrices surrounding two natural fission reactors of the oklo deposit in the deeply preserved reactor 10 450 m clays are composed of hydrothermal mg chlorites and sudoites al chlorites the wide range of their o isotopic compositions from 0 0 to 15 1 smow is consistent with crystallisation during conditions of criticality when the reactors created a thermal gradient of 100 c m the temperature in the core of reactor is calculated to have been approximately 400 c in contrast in the shallow reactor 9 120 m deep vermiculitized al chlorites are the products of the weathering of the hydrothermal chlorites these vermiculitized al chlorites are 18 o enriched 18 o 18 smow and underwent interaction with surface fluids 2 to 5 149 sm 147 sm isotopic ratios of the clays of reactor 9 show a wide dispersion 2 3 m of the fissiogenic sm around the reactor the migration of the fissiogenic sm which is usually retained in the deep reactors is here related to the circulation of meteoric water under oxidising conditions

population density and zoning of olivine phenocrysts in tholeiites from kauai hawaii the population density of olivine phenocrysts of the tholeiites display an exponential variation which is typical of igneous as well as contact metamorphic rocks the exponential variation is explained by a new growth probability model which is consistent with experimental work the forsterite content of the olivine phenocrysts decreases with decreasing size various phenocryst features suggest that the tholeiites first crystallized slowly in a magma chamber after which they underwent crystallization for a short period of time in a feeder dyke before eruption took place

olivine phenocrysts of hawaiian olivine tholeiite and oceanite olivine phenocrysts of hawaiian tholeiites display a variety of textures and a variation in composition which is due to supercooled crystallization the highest forsterite content measured is 91 and the highest magnesia content estimated for a tholeiitic liquid is 17 using the olivine geothermometer the liquid of this composition may be either a primary magma or a primitive magma it is not yet possible to specify any particular composition range for the primary hawaiian tholeiites

redistribution of fissiogenic and non fissiogenic ree th and u in and around natural fission reactors at oklo and bangombe gabon abstract elemental abundances and isotopic compositions of rare earth elements ree th and u of twenty eight samples from four kinds of natural fission reactors at the oklo and bangombe uranium ores were determined by mass spectrometry fissiogenic and non fissiogenic ree components of each sample were calculated from the isotopic compositions the geochemical distribution of both fissiogenic and non fissiogenic ree differs among the four reactors each reactor shows the following features possibly associated with the local geochemical conditions 1 reactor 9 at the shallow depth of the oklo deposit contains higher amounts of non fissiogenic ree than the other reactors the distribution of non fissiogenic ree does not reflect the primordial ree abundance patterns and might have been disturbed in and around the reactor due to supergene alteration our results suggest that significant amounts of fissiogenic lighter ree lree such as la ce and pr were mobile in the reactor 2 reactors 10 and 13 located deep underground have much lower contents of non fissiogenic ree than reactor 9 all fissiogenic ree show high retention in the reactors non fissiogenic ree of reactor 10 inform of the primordial ore formation without any geochemical disturbance little redistribution of both fissiogenic and non fissiogenic ree in the reactor 10 was observed and 3 the bangombe reactor located at shallow depth has apparently well preserved fissiogenic ree non fissiogenic ree in the bangombe reactor might have been disturbed possibly by hydrothermal processes the distribution behavior of fissiogenic ree is considered to be related with physico chemical reactor conditions such as nuclear reaction temperature and chemical compositions of each reactor but clear relationship between nuclear parameters and the ree distribution could not be obtained in this study

magma solidification processes beneath kilauea volcano hawaii a quantitative textural and geochemical study of the 1969 1974 mauna ulu lavas sents a late magma component formed in the summit magma storage region our results are consistent with the hypothesis that the components of the mu magmas followed two different routes the deformed olivine bearing magma moved along the deep basal

hydrothermal clay mineral formation in the uraniferous paleoproterozoic fa formation francevillian basin gabon abstract the spatial distribution of neoformed clay minerals was investigated in the paleoproterozoic fa formation of the francevillian basin south east gabon which hosts high grade u ore deposits associated with the only known occurrence of natural nuclear reactors illite appears as the main clay phase in the lower fluvial unit in the mineralized upper fluvio deltaic tidal unit the clay assemblage is more diversified and commonly characterized by illite chamosite berthierine and chamosite berthierine mixed layers usually in association with alteration products of u bearing minerals the clay mineral assemblage thus potentially constitutes a mineralogical marker for the regional distribution of u ore deposits polytype species of the clay phases indicate mineral transformations in an environment characterized by a high fluid rock ratio according to crystalline structure mineral chemistry thermodynamic modeling and geochronology clay phases seem to be mainly hygrometer rather than exclusively a thermometer and their formation as well as associated dissolution precipitation of u bearing minerals took place between ca 2040 and 2010 ma ago at temperatures of about 240 30 c using previous burial estimates this suggests the operation of hydrothermal processes controlled by an external heat source likely associated with volcanism during deposition of the fd and fe formations hydrothermal activity involving oxidized fluids would have driven u remobilization and ultimately formation of high grade u ore deposits the thermal history proposed here allows for a better understanding of the conditions during burial of the sedimentary succession and the origin of its rich u endowment

uranium and sm isotope studies of the supergiant olympic dam cu au u ag deposit south australia the olympic dam cu u au ag deposit in the archean proterozoic gawler craton south australia is a type example of the iron oxide copper gold iocg spectrum of deposits and one of the largest cu u au resources known mineralization is hosted in a lithologically and texturally diverse hematite rich breccia complex developed within a granite of the 1 59 ga gawler silicic province emerging evidence indicates that both the breccia complex and its metal content developed over 1000 ma responding to major tectonic events e g at 1300 1100 825 and 500 ma however metal sources and exact mechanism s of ore formation remain poorly known new high precision 238u 235u data for a set of 40 whole rock samples representing all major lithological facies of the breccia complex show a narrow range 238ucrm112a 0 56 to 0 04 at the scale of sampling there is no correlation of 238u with lithology degree of alteration or u mineralogy although ores with u 5 wt have subtly higher 238u values 0 20 to 0 00 than the majority of samples 0 7 wt u 0 56 to 0 23 the new u isotope data are consistent with published data for uraninites from olympic dam and with published results from high temperature u deposits they overlap completely with the range of 238u values in granitoids including the host granite 0 18 to 0 32 and with estimates of the upper continental crust in general this similarity suggests that olympic dam 238u values reflects the crustal sources of u which probably include felsic volcanic rocks and granitoids the isotopic homogeneity suggests depositional mechanisms that involve minimal isotopic fractionation of u alternatively primary fractionation signatures may have been erased during the long history of the u mineralization high grade u ores may record isotopic neutron capture effects related to fissionogenic neutrons high precision sm isotope data for five high u 5 wt u u sm 500 olympic dam ores define a neutron capture line with correlated depletions in 149sm up to 2e units and excesses in 150sm up to 4e units but fission fragment contributions to sm are below detection these observations provide evidence for small scale neutron capture effects with calculated neutron fluences of 1015 to 1016 n cm 2 similar to those observed in several proterozoic and phanerozoic u deposits the apparent lack of fission fragment contributions in olympic dam high grade ores can be explained with an age of u deposition or re deposition that is substantially younger than the initial 1 59 ga age of the oldest iocg style mineralization the results presented here thus i suggest uranium sources in common likely igneous upper crustal lithologies ii support geochronological evidence for gradual addition of u in several stages over 1000 ma at elevated temperatures of mineralization and iii do not show the high 238u signatures expected from low temperature reworking of older low 238u ores

natural fission reactors in the franceville basin gabon a review of the conditions and results of a critical event in a geologic system abstract natural nuclear fission reactors are only known in two uranium deposits in the world the oklo and bangombedeposits of the franceville basin gabon since 1982 five new reactor zones have been discovered in these deposits and studied since 1989 in a cooperative european program new geological mineralogical and geochemical studies have been carried out in order to understand the behavior of the actinides and fission products which have been stored in a geological environment for more than 2 0 ga years the franceville basin and the uranium deposits remained geologically stable over a long period of time therefore the sites of oklo and bangombeare well preserved for the reactors two main periods of actinide and radionuclides migration have been observed during the criticality under p t conditions of 300 bars and 400 500 c respectively and during a distention event which affected the franceville basin 800 to 900 ma ago and which was responsible for the intrusion of dolerite dikes close to the reactors new isotopic analyses on uranium dioxides clays and phosphates allow us to determine their respective importance for the retention of fission products the uo 2 matrix appears to be efficient at retaining most actinides and fission products such as rees y and zr but not the volatile fission products cd cs xe and kr nor rb sr and ba some fissiogenic elements such as mo tc ru rh pd and te could have formed metallic and oxide inclusion in the uo 2 matrix which are similar to those observed in artificial spent fuel clays and phosphate minerals also appear to have played a role in the retention of fissiogenic rees and also of pu

fate of trace elements during alteration of uraninite in a hydrothermal vein type u deposit from marshall pass colorado usa alteration of uraninite from a hydrothermal vein type u deposit in marshall pass colorado has been examined by electron microprobe analysis in order to investigate the release and migration of trace elements w as mo zr pb ba ce y ca ti p th fe si al during alteration under both reducing and oxidizing conditions the release of trace elements from uraninite is used to establish constraints on the release of fission product elements from the uo2 in spent nuclear fuels uraninite occurs with two different textures 1 colloform uraninite and 2 fine grained uraninite the colloform uraninite contains 1 04 1 75 wt of wo3 0 16 1 70 wt of as2o3 0 06 0 88 wt of moo3 whereas the fine grained uraninite retains 2 25 4 93 wt of wo3 up to 5 76 wt of moo3 and 0 26 0 60 wt of as2o3 the near constant concentration of incompatible w in the colloform uraninite suggests w incorporation into the uraninite structure or homogeneous distribution of w rich nano domains incorporation of w and mo into the uraninite and subsequent precipitation of uranyl phases bearing these elements are critically important to understanding the release and migration of cs during the corrosion of spent nuclear fuel as there is a strong affinity of cs with w and mo zoning in the colloform texture is attributed to variation in the amount of impurities in uraninite for unaltered zones the calculated amount of oxygen ranges from 2 08 to 2 32 apfu atom per formula unit and defines the stoichiometry as uo2 x and u4o9 whereas for the altered zones of the colloform texture the oxygen content is 2 37 2 48 apfu which is probably due to the inclusion of secondary uranyl phases mainly schoepite the supergene alteration resulted in precipitation of secondary uranyl minerals at the expense of uraninite four stages of colloform uraninite alteration are proposed i formation of an oxidized layer at the rim ii corrosion of the oxidized layer iii precipitation of u6 phases with well defined cleavage and iv fracture of the uraninite surface along the cleavage planes of the u6 phases

a petrologic perspective of kīlauea volcano s summit magma reservoir two hundred years of magmatic history are documented by the lavas and tephra sampled from k olauea s historical summit eruptions this paper presents detailed petrographic and geochemical data for a comprehensive suite of samples erupted within or near k olauea s summit caldera since the 17th century our results elucidate the range of magmatic processes that operate within the volcano s summit magma reservoir and document two compositional trends that span nearly the entire known range for the volcano prior to the 1924 summit crater collapse a trend of increasing incompatible element and cao and decreasing sio2 abundances at a constant mgo prevailed thereafter the trend reversed direction and has persisted for the rest of the 20th century including during the current puau a oa o eruption the rapid and systematic nature of these temporal geochemical variations indicates that the summit reservoir is a single relatively small body rather than a plexus of dikes and sills olivine fractionation is the dominant petrologic process within this reservoir petrographic observations and olivine and whole rock geochemical data suggest that the summit reservoir has a crown of aphyric more evolved low density magma differentiation within this crown involving clinopyroxene and plagioclase is more extensive than previously recognized in k olauea summit lavas the effects of crystal fractionation are superimposed upon an evolving hybrid magma composition produced by mixing new mantle derived magmas with more fractionated reservoir magma frequent eruptions of these hybrid reservoir magmas document the rapid variation in parental magma composition these compositional variations correlate with magma supply rate both are thought to be influenced by the degree of melting of small scale source heterogeneities within the hawaiian plume however k olauea s source compositions and partial melting processes have varied only within a narrow range over the past 350 kyr

ore related breccias in volcanoplutonic arcs an overview of breccias related to a variety of base metal precious metal and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation 1 release of magmatic hydrothermal fluids from high level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and or ascent single or multiple intrusion related breccia pipes and pre and intermineral breccias in porphyry copper deposits provide widespread examples 2 magmatic heating and expansion of meteoric pore fluids may lead to brecciation commonly of late or postmineral age and including pebble dikes in porphyry type and related deposits magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of kuroko type massive sulfide deposits many of the resultant breccias underwent limited sedimentary transport overpressuring of heated fluids beneath semipermeable partly self sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings magmatic heating or tectonic disturbance may have triggered brecciation 3 interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious or base metal deposits were generated in this manner these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters 4 magmatic hydrothermal brecciation may lead to disruption of rocks through to the paleosurface decompression and fragmentation and eruption of the top part of an underlying magma chamber pre and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry type and other base and precious metal systems they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits 5 breccias may result from mechanical disruption of wall rocks during subsurface movement of magma any intrusion related deposit may include such intrusion breccias 6 tectonic breccias resulting from fault displacement may accompany any type of ore deposit a continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction

florencite la with fissiogenic rees from a natural fission reactor at bangombe gabon florecite la la ce 1 09 with fissiogenic rees and florecite ce la ce 0 62 have been identified in illite from the clay mantle surrounding a natural 2 ga fission reactor at bangombre and in sandstone beneath the reactor zone respectively florencite ce is apparently unrelated to nuclear processes and occurs with monazite ce apatite tio sub 2 probably anatase zircon and illite grains of florencite ce contain inclusions of thorite chalcopyrite and galena florencite la was found 5 cm from the open quotes core close quotes of the reactor and contains inclusions of galena and u ti bearing phases secondary uraninite and coffinite have precipitated on some of the florencite grains the chemical composition of florencite la as determined by electron microprobe analysis is la sub 0 38 ce sub 0 35 nd sub 0 06 sm sub 0 01 ca sub 0 03 sr sub 0 17 al sub 2 98 fe sub 0 02 sup 3 po sub 4 po sub 3 80 oh sub 0 20 oh sub 6 secondary ion mass spectrometry revealed that between 27 and 30 of nd and 67 and 71 of sm in florencite la is fissiogenic the presence of fissiogenic rees in open quotes florencite close quotes from the reactor zone in bangombe and their preferential concentration in florencite relative to the bulk sample of clay demonstrate that aluminous phosphates may have played a more significant role in the fixation of fissiogenic rees released from uraninite after the sustained fission reactions than sorption ontomore clays 30 refs 3 figs 2 tabs less

on some fundamentals of igneous petrology the age old process of crystal fractionation leading to the diversity of the igneous rocks and earth itself is an exceedingly well understood chemical process in magmatism and physical chemistry but the broader physical aspects of this and related processes have proven elusive on many fronts especially in its relation to the spatial variations in rock composition texture and macroscopic features like layering magmatic systems be they volcanic dikes sills or plutons are generally analyzed with a problem at hand and an end result in mind the processes invoked to solve these problems which are most often purely chemical are often unique to each problem with few if any general principles emerging that are central to understanding the wider perspective of magmatic processes and problems an attempt is made at the outset to provide a list of inviolate magmatic first principles that are relevant to analyzing most magmatic problems these involve initial conditions critical crystallinity solidification fronts transport and emplacement fluxes phenocrysts xenocrysts primocrysts crystal size layering and crystal sorting thermal convection magmatic processes are physical along with these principles two reference magmatic systems are suggested where the initial conditions and outcome are unequivocal the sudbury impact melt sheet and the hawaiian lava lakes sudbury formed in 5 min by superheated magma crystallized to a near uniform sequence while the tiny lava lakes formed of crystal laden slurries form a highly differentiated layered sequence the major difference is in the initial conditions of formation especially the nature of the input materials the challenge is to construct and analyze magmatic systems i e magma chambers sills dikes and lavas using these reference end members and the suggested principles the hawaiian 500 000 year volcanic record exhibits what can be expected as input materials namely a highly varied output of magma of an overall composition reflecting the abundance of entrained olivine primocrysts the provenance of these crystals is varied and within any single sample the population may be highly heterogeneous in composition from crystal to crystal yet the overall pattern of chemical fractionation is exceedingly regular and well defined if similar inputs go to form large intrusions these systems will undoubtedly be dominated by crystal rich slurries which provide a vast set of physical processes promoting exotic layering and at the same time given the effects of annealing and continued crystal growth a final chemical record adhering to all the time honored effects of crystal fractionation the long assumed initial condition of instantaneously emplaced crystal free magmas cannot reasonably produce the observed rock records

evolution of alkalic lavas at haleakala volcano east maui hawaii the postshield and posterosional stages of haleakala volcano contain intercalated alkalic basalt and evolved alkalic lavas isotopic and incompatible element abundance ratios in the haleakala postshield basalts changed systematically with time providing evidence for significant temporal changes in the mantle components contributing to the magmatic sources specifically a depleted i e low87sr 86sr and high143nd 144nd mantle component is more abundant in younger lavas however as magma production rates decreased during the postshield and posterosional stages basaltic melts in magma reservoirs cooled and fractionated leading to evolved residual melts such as hawaiite because primary basalt compositions changed with time the evolved haleakala lavas formed from a range of parental compositions however basalts and evolved lavas of similar age and isotopic ratios sr and nd have major and trace element contents that are consistent with a crystal fractionation model although alkalic basalt and hawaiite are the dominant lavas of the postshield stages of both haleakala and mauna kea volcanoes there are important differences between their lavas for example compositional differences between the hawaiite suites at haleakala and mauna kea indicate that on average the evolved lavas at haleakala formed at lower pressures also at haleakala basalts are intercalated with hawaiites whereas at mauna kea basalts and hawaiites are separated by a sharp boundary these differences probably reflect a higher magma supply rate to the haleakala volcano

characterization of organic matter associated with uranium deposits in the francevillian formation of gabon lower proterozoic abstract elemental analysis organic petrography and high resolution transmission electron microscopy were used to study organic matter in lower proterozoic rocks of the francevillian series in africa results show a convincing relationship between solid bitumens derived from thermal alteration of crude oil and deposition of uraninite ores evidence is presented that suggests the presence of migration paths for crude oil in associated sandstones moreover the solid bitumens appear to have been further altered by radiation damage as a consequence of oxidation and uranium mineralization

the neodymium isotope system as a tool for petroleum exploration abstract mineralogical petrological geochemical and neodymium isotopic investigations strongly suggest isotopic and chemical exchange between authigenic silicate phases and petroleum alteration of organic matter sulfur reduction generation of aqueous acidic solutions with the resultant dissolution of k feldspar leads to in situ crystallization of illite and chlorite as well as neodymium isotopic equilibration between the silicate phases and petroleum these findings are of significance not only to understand the mechanisms leading to neodymium isotopic equilibration between authigenic clay minerals in detrital sediments but also to source rock petroleum in reservoir correlations in particularly when biomarker molecules i e isoprenoid hydrocarbons have been obliterated by prolonged severe diagenesis

oxygen isotopic composition of nano scale uraninite at the oklo okelobondo natural fission reactors gabon high spatial resolution 10 30 m in situ oxygen isotopic analyses by secondary ion mass spectrometry sims coupled with high resolution transmission electron microscopy hrtem were used to show that uraninite from the oklo okelobondo natural fission reactors that occur in near surface environments have low 1 8 o values and nanotextures that are consistent with interaction with ground water these low 1 8 o values 14 4 to 8 5 suggest that the minerals exchanged with meteoric groundwater in contrast reactor zones that occur at depth have largely retained their original o isotopic composition 10 2 to 5 6 and uraninites are well crystallized and essentially defect free these observations clearly demonstrate that by combining both hrtem and in situ o isotopic analyses by sims it is possible to characterize the nano scale porosity and post depositional alteration of u bearing phases

the corrosion of basaltic dykes in evaporites ar sr nd isotope and rare earth elements evidence abstract the corrosion of basaltic dykes from central germany which intruded during the tertiary into rock and potash salts of upper permian age zechstein has been studied the corrosion behavior of these glassy basalts can serve as a natural analogue for the long term corrosion expected for nuclear waste glasses in a salt repository our data demonstrate mobility and fractionation of the rare earth elements ree during a postintrusive circulation of salt brines the processes controlling this behavior of the ree were dissolution and reprecipitation of phosphates the k ar data document the synintrusive assimilation of a salt phase followed by a postintrusive fluid circulation in the peripheral parts this circulation removed the assimilated salt and imported highly radiogenic salt ar which was integrated into newly formed sheet silicates the central part of the dyke was not affected by this circulation and could retain a mantle gas phase the sr isotope data document a synintrusive assimilation of salt sr and a strong exchange with salt fluids during the postintrusive corrosion which was strongest in a zone next to the chilled dyke margins this locally stronger alteration is probably due to different cooling and shrinking rates of the basalt at the margins and in the core of the dyke which led to the formation of a highly permeable fractured zone