alteration mineralogy and stable isotope geochemistry of paleoproterozoic basement hosted unconformity type uranium deposits in the athabasca basin canada unconformity type uranium deposits are characterized by mineralization developed along the contact between younger sandstone cover and underlying crystalline basement rocks mineralization may extend up to 400 m into the underlying basement rocks whereas sandstone hosted unconformity type deposits have been well studied deposits hosted primarily in the basement have not this study examines the deposits at rabbit lake dawn lake and mcarthur river in the athabasca basin of canada which are hosted by the metamorphic archean and early paleoproterozoic rocks forming the basement to younger late paleoproterozoic sandstones alteration is similar in the three deposits and is characterized by three distinct paragenetic stages 1 preore alteration involving illitization of plagioclase and amphibole followed by chloritization of biotite and illite which formed at ca 230 c 2 ore stage alteration characterized by uraninite and coarse grained illite which formed at ca 240 c 3 postore alteration comprising spherulitic dravite vein chlorite quartz calcite and fe cu co and pb sulfides which formed at ca 135 c fluid circulation associated with emplacement of later mackenzie dikes initiated partial recrystallization of uraninite a later stage of alteration includes kaolinite and iron hydroxide precipitation formed at much lower temperatures of ca 50 c stable isotope compositions of the alteration minerals in conjunction with their paragenesis indicate that oxidized basinal fluids d 43 to 21 18o 3 8 were derived primarily from evolved seawater and leached uranium from the overlying sandstones of the athabasca formation and transported it into the basement via infiltration along fracture zones associated with reverse faults graphitic units in the basement and preore alteration served as both physical fractured zones and chemical reductants traps for the uranium mineralization the basinal fluids were responsible for the preore illite chlorite synore uraninite illite and the early postore alteration events this differs from many other sandstone hosted deposits where both oxidized basinal and reduced basement derived fluids were responsible for uranium precipitation

jurassic subsidence history in the hebrides n w scotland abstract geohistory analyses have been applied to the generally well dated triassic jurassic successions of three areas in the hebrides basin north west scotland and the importance of making corrections for decompaction palaeobathymetry and eustatic changes of sea level are assessed the thermo tectonic subsidence patterns in the skye areas north suggest a three phase history of crustal stretching though only the first phase is well developed in the mull area south episodes of relatively rapid subsidence are 1 1 late triassic c210 ma until earliest toarcian 186 ma 2 2 latest toarcian 180 ma until late bathonian c161 ma 3 3 early oxfordian 154 ma until at least early kimmeridgian incomplete these are separated by toarcian 186 180 ma and latest bathonian callovian c161 154 ma episodes of either little subsidence or of uplift but at slower rates than eustatic rise of sea level the intervals between phases at 30 ma and 26 ma are remarkably similar but the wider significance of this remains to be established

the nolans bore rare earth element phosphorus uranium mineral system geology origin and post depositional modifications nolans bore is a rare earth element ree u p fluorapatite vein deposit hosted mostly by the 1805 ma boothby orthogneiss in the aileron province northern territory australia the fluorapatite veins are complex with two stages 1 massive to granular fluorapatite with inclusions of ree silicates phosphates and fluoro carbonates and 2 calcite allanite with accessory ree bearing phosphate and fluoro carbonate minerals that vein and brecciate the earlier stage the veins are locally accompanied by narrow skarn like garnet diopside amphibole wall rock alteration zones shrimp th pb analyses of allanite yielded an age of 1525 18 ma interpreted as the minimum age of mineralisation the maximum age is provided by a 1550 ma shrimp u pb age for a pegmatite that predates the fluorapatite veins other isotopic systems yielded ages from 1443 to 345 ma implying significant post depositional isotopic disturbance calculation of initial end and 87sr 86sr at 1525 ma and stable isotope data are consistent with an enriched mantle or lower crust source although post depositional disturbance is likely processes leading to formation of nolans bore began with north dipping subduction along the south margin of the aileron province at 1820 1750 ma producing a metasomatised volatile rich lithospheric mantle wedge about 200 million years later near the end of the chewings orogeny this reservoir and or the lower crust sourced alkaline low degree partial melts which passed into the mid and upper crust fluids derived from these melts which may have included phosphatic melts eventually deposited the nolans bore fluorapatite veins due to fluid rock interaction cooling depressurisation and or fluid mixing owing to its size and high concentration of th 2500 ppm in situ radiogenic heating caused significant recrystallisation and isotopic resetting the system finally cooled below 300 c at 370 ma possibly in response to unroofing during the alice springs orogeny surface exposure and weathering of fluorapatite produced acidic fluids and intense near surface kaolinitised zones that include high grade supergene enriched cheralite rich ores

basin evolution and unconformity related uranium mineralization the camie river u prospect paleoproterozoic otish basin quebec the paleoproterozoic otish basin quebec hosts several uranium prospects that until recently remained underexplored and poorly understood in this study the camie river u prospect which shows similar charac teristics to high grade unconformity related u deposits is the focus of an integrated basin analysis in the western otish basin conglomerate and sandstone of the indicator formation which were deposited in at least six depositional sequences were affected by insignificant early diagenetic compaction and cementation this allowed the for mation of regional peak diagenetic aquifers which became muscovite altered due to interaction with fluids having 18 o and 2h values similar to those of seawater influenced basinal brines at 250 c u mineralization at camie river occurred at 1721 20 ma based on a 207 pb 206 pb date obtained by laser ablation of uraninite which coincides with a phase of the otish gabbro intrusion that has been dated at ca 1730 ma the intrusive event promoted circulation of u bearing basinal brines triggering u mineralization at several locations in the western otish basin interaction of basinal brines with the otish gabbro produced coarse grained chlorite tourmaline and phengitic muscovite which decreased the fluid conducting capabilities of diagenetic aquifers and resulted in fault zone and fracture dominated fluid flow subsequent fluid alteration events produced limited u remobilization sulfides sudoite and siderite between ca 1670 and 1410 ma based on mineral paragenesis and 40 ar 39 ar dates of muscovite metamorphic fluids having high 18 o values and temperatures around 300 c accompanied 1 2 to 1 0 ga grenville orogene sis and subgreenschist grade metamorphism in the otish basin but were present at low water rock ratios at camie river and therefore produced little alteration post grenville uplift of the otish basin likely produced late low temperature alteration minerals that have been influenced by recent meteoric water suggesting that the fault zones and fractures the minerals occupy remain as preferential fluid flow pathways to the present day radiogenic pb and the characteristic trace elements mo w nb have also preferentially dispersed from the mineralization along fault zones fractures and depositional sequence boundaries and can be used to explore for camie river style u mineralization

significance of alteration assemblages for the origin and evolution of the proterozoic nabarlek unconformity related uranium deposit northern territory australia the proterozoic nabarlek unconformity related uranium deposit in the alligator rivers uranium field is hosted by paleoproterozoic amphibolite grade metamorphosed semipelitic sedimentary rocks and amphibolite schist high grade ore is confined to the nabarlek fault a reverse fault shear zone that crosscuts a series of interbedded muscovite quartz biotite schists and amphibolite petrographic studies on polished thin sections combined with electron microprobe analyses x ray diffraction fluid inclusion data o h and u pb isotope values as well as 40 ar 39 ar dating have identified up to eight significant fluid events beginning with the precipitation of early quartz veins during uplift of the myra falls metamorphics at 1830 ma and ending with limited uraninite recrystallization during reactivation of the nabarlek fault between 1380 and 750 ma quartz veins that likely formed toward the end of the top end orogeny represent the earliest recorded fluid event fluid inclusion data and 18 o and d values indicate that these veins formed from basement derived fluids that may have been heated by a cooling nabarlek granite during circulation through reverse faults shear zones the next fluid event represented by fine grained sericite and chlorite occurred when fluids passed into these faults and altered the metamorphic minerals following the exhumation of the basement and deposition of the kombolgie subgroup at ca 1760 ma the intrusion of the oenpelli dolerite at ca 1720 ma resulted in the local remobilization of silica and the precipitation of quartz associated with minor pyrite and dolomite around the reverse faults uranium mineralization is associated with an inner and outer alteration halo that extends as far as 1 km from the nabarlek fault alteration in the outer halo began as early as 1700 ma and is dominated by chlorite and sericite which formed when a 200 c fluid flowed into the nabarlek fault from the overlying kombolgie subgroup u pb and 207 pb 206 pb dating reveals that massive uraninite precipitated at ca 1640 ma and formed together with illite and hematite at ca 200 c chlorite was not coeval with uraninite precipitation stable isotope values indicate that the pre and synore alteration assemblage formed from basinal brines with 18 o fluid and d fluid values of 2 2 and 25 10 per mil respectively reactivation of the nabarlek fault at ca 1360 1100 and 900 ma is indicated by u pb and 207 pb 206 pb dating of uraninite these ages correlate with the intrusion of the maningkorrirr phonolitic dikes and the derim derim dolerite at ca 1316 40 and 1324 4 ma respectively the amalgamation of australia and laurentia during the grenville orogen at ca 1140 ma and the breakup of rodinia between 1000 and 750 ma fluid incursions associated with these events precipitated much of the chlorite that has previously been related to uraninite precipitation drusy quartz veins that host high salinity fluid inclusions and sulfides particularly galena also formed after the initial uraninite forming event finally erosion of the kombolgie subgroup and subsequent weathering of the deposit resulted in the recent formation of kaolinite and numerous secondary uranium minerals these data constrain individual events more precisely than previous studies and thus advance the current genetic model to a level that takes into account the multiple stages of fluid overprinting that occurred over a period of at least 800 m y

geochemistry and geochronology of the kiggavik uranium deposit nunavut canada the thelon basin nunavut canada is similar in age size and geology to the u producing athabasca basin in saskatchewan the kiggavik project area located immediately south of the aberdeen sub basin of the thelon basin contains u deposits and showings along a 30 km long ne sw structural trend the basement hosted kiggavik deposit comprising three separate mineralized zones main centre and east is located at the northern end of this trend multiple styles of u mineralization and alteration were identified at the kiggavik deposit disseminated euhedral uraninite u1 1284 53 ma has elevated pb contents up to 14 wt pbo and is associated with illite and hematite alteration vein style uraninite u2 284 19 ma forms along fractures contains lower pb contents 3 9 wt pbo and is partly altered to ca si rich 4 5 wt cao and 3 3 wt sio2 uraninite highly altered u2 is associated with galena late u minerals u3 have biogenic textures and are pb free indicating that they formed recently 1 ma based on sims in situ u pb geochronology the minimum age of uraninite at the kiggavik deposit is 1284 53 ma however this is likely a reset age associated with the mackenzie dyke event 1267 ma oxygen and hydrogen stable isotope compositions of illite and muscovite suggest that the kiggavik deposit has been overprinted by high latitude surficial fluids derived from snowmelt

sedimentary basin inversion caused by igneous underplating northwest european continental shelf a considerable body of evidence indicates that many of the extensional sedimentary basins in the vicinity of the british isles underwent permanent exhumation during the tertiary the most dramatic indicator of this process is the present day absence of as much as 4 km of anticipated postrift thermal subsidence in basins just north and west of scotland any explanation of this observation must take into account the fact that the entire region has very small long wavelength free air gravity anomalies this important constraint implies either that the crust has been thickened or that low density material has been added to or formed from the lithosphere and rules out models that invoke flexural effects arising from the opening of the north atlantic tertiary epeirogeny is often attributed to compression that is assumed to be related in a general sense to alpine mountain building however to remove 3 km of sedi mentary rock from a basin 100 km wide requires 15 km of shortening minor tertiary compression is observed all over the continental shelf but nowhere is it sufficient to account for the required amount of uplift and erosion in addition exhumation dramatically increases from south to north whereas the observed compression decreases markedly in the same direction at the beginning of the tertiary rifting associated with the initiation of the iceland plume generated substantial volumes of melt inversion of rare earth element concentrations of mgorich igneous rocks suggests that a minimum of 5 km of melt was produced beneath at least part of the continental shelf we infer that much of this melt remains trapped within the lithosphere presumably close to the moho which acted as a density filter such underplating will generate rapid uplift

low temperature oxygen isotopic fractionation in the uraninite uo3 co2 h2o system oxygen isotope fractionation factors for uraninite water and uo3 water were determined from a series of uraninite co2 and uo3 co2 exchange experiments between 100 and 300 c the measured fractionation factors are similar to the theoretical fractionation factors of hattori and halas 1982 at high temperatures and zheng 1991 at low temperatures and are nonlinear over the temperature range examined regression through these curve gives 1000 ln uo2 h2o 16 58 106 t2 77 52 103 t 77 48 1000 ln uo3 h2o 2 21 106 t2 25 06 103 t 45 90 with 2 errors of 2 5 diffusion coefficients of oxygen with temperature for uraninite and uo3 can be represented by d uraninite 3 1 10 10 exp 63981 rt and d uo3 1 3 10 15 exp 21 197 rt where the activation energies for uraninite and uo3 are 64 0 0 2 kcal mole and 21 2 0 2 kcal mol respectively exceptionally low 18o values of natural uraninites i e 32 to 19 5 from unconformity type uranium deposits in saskatchewan in conjunction with theoretical and experimental uraninite water and uo3 water fractionation factors suggest that primary uranium mineralization is not in oxygen isotopic equilibrium with coeval clay and silicate minerals the low 18o values have been interpreted as having resulted from the low temperature recrystallization of primary uranium mineralization in the presence of relatively modern meteoric fluids having 18o values of ca 18 despite petrographic and u pb isotope data that indicate limited alteration therefore it appears that uranium minerals can exchange oxygen isotopes with later fluids with only limited actinide remobilization and minor disturbances to their original chemical compositions and textures

jurassic sequence stratigraphy in the hebrides basin nw scotland abstract stratigraphical analysis of the triassic jurassic succession outcropping in the hebrides in terms of genetic sequences has identified seven major sequences and seven minor sequences the major sequence boundaries are defined at abrupt facies changes which usually represent deepening events often associated with a hiatus they are dated as 1 late triassic 2 mid sinemurian birchi subzone turneri zone diachronous to oxynotum zone 3 early toarcian exaratum subzone falciferum zone 4 latest toarcian aalensis subzone levesquei zone 5 late bajocian dichotoma subzone garantiana zone 6 late bathonian discus zone 7 early callovian medea subzone jason zone integrating stratigraphical with subsidence history analyses enables identification of basin tectonics as the causal mechanism for sequence boundaries 1 2 4 and 5 eustatic rises of sea level during toarcian and callovian phases of relative tectonic quiessence were the causes of sequence boundaries 3 6 and 7 late jurassic tectonic events are not reflected in the sequence stratigraphy on the basis of available data

mineralogy geochronology and genesis of the andrew lake uranium deposit thelon basin nunavut canada the thelon basin located in nunavut canada shares many similarities with the u producing athabasca basin in saskatchewan the kiggavik project area located near the northeastern edge of the thelon basin contains u deposits and showings along the 30 km long ne sw kiggavik andrew lake structural trend the andrew lake deposit is near the southern end of this trend pre mineralization is characterized by quartz carbonate veins that occupy fault systems later reactivated as conduits for u mineralizing fluids a four phase genetic model is proposed for the andrew lake deposit phase 1 comprises vein style uraninite u1 1031 23 ma that is associated with illite and hematite and contains variable pbo contents 0 2 9 5 wt phase 2 is characterized by altered uraninite u2 530 ma that is associated with coffinite altered uraninite u3 1 ma characterizes phase 3 and occurs as centimetre scale roll fronts in phase 4 all three uraninite stages and coffinite are altered to boltwoodite al

the structure of the rockall trough from seismic refraction and wide angle reflection measurements there are few detailed descriptions of the deep crustal structure of failed cratonic rifts and passive margins despite their import ance in constraining the variety of structural models invoked to account for their common structural symmetry or gross half graben form1 3 such deep crustal structure profiles also have considerable value in defining the continent ocean boundary con straining plate reconstructions and determining the overall setting of sedimentary basin development the rockall trough in the north atlantic ocean is a sinuous elongate bathymetric depression 200 km wide and more than 2 000 m deep lying between rockall plateau and the british isles and the structure nature and age of its underlying crust are disputed in the absence of deep crustal structure profiles the presence of oceanic crust has been assumed because the trough has to be accommodated in reconstructions of the north atlantic although stretched continental crust is equally plausible the nature of the crust is important for the precise reconstruction of the late jurassic early cretaceous plate history and setting of sedimentary basins of the north atlantic4 8 the nature of the crust beneath the trough as well as the structure thickness and composition of the sedimentary cover is also relevant to oil exploration here we report the first observations of deep crustal structure across the rockall trough and its margins from seismic refraction and wide angle reflection profiles our results show that the northern rockall trough is underlain by highly stretched continental crust and not by oceanic crust as previously postulated

detrital zircon age constraints on basement history on the margins of the northern rockall basin detrital zircon dating has proven to be an effective way to constrain ages of submerged basement terranes on the margins of the northern rockall basin a region where direct evidence of crustal affinities is scarce or absent zircons have been dated from sandstones of palaeocene oligocene age known to have been derived from the east hebridean platform and west rockall and george bligh highs the results show that the hebridean platform is a westward extension of the lewisian complex with archaean and palaeoproterozoic ages that can be directly correlated with events identified in the outer hebrides and northwest scotland the detrital zircons derived from the hebridean platform also provide evidence for a mesoproterozoic thermal event and two phases of intrusions in the palaeozoic the rockall high consists of a palaeoproterozoic terrane dated as c 1760 1800 ma similar to ages previously determined from both basement samples and detrital sediment the data also provide evidence for the subsequent intrusion of alkaline igneous rocks in the palaeocene eocene the george bligh high represents an archaean terrane heavily affected by palaeoproterozoic tectonothermal events and was also the site of intrusion of alkaline igneous rocks in the palaeocene

characteristics and origin of the oak dam east breccia hosted iron oxide cu u au deposit olympic dam region gawler craton south australia oak dam east is a large 560 mt of 41 56 wt fe cu au u bearing massive fe oxide deposit in the northeastern gawler craton south australia it is one of several unmined deposits in the same province as the giant olympic dam deposit and it lies beneath more than 500 m of younger cover rocks the oak dam east breccia complex formed in a near surface environment and was characterized by an early history of skarnlike albite calc silicate magnetite formation from hypersaline 400 500 c fluids large scale brecciation was an early feature and the resulting complex was progressively filled in and replaced by 1 hematite and goethite hematite stage i 2 magnetite apatite quartz actinolite magnetite stage 3 hematite goethite quartz chalcedony pyrite chlorite monazite hematite stage ii and 4 chalcopyrite pitchblende illite hydromus covite florencite carrolite cu u au mineralization stage hematite stage ii assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent nacl equiv that were boiling at 170 to 190 c and had 18o values more negative than 3 7 to 1 7 per mil interpreted as meteoric water the upper 200 m of the breccia complex contains spectacular colloform hematite void fillings interpreted to have replaced hydrothermal goethite the cu u mineralized zones resemble olympic dam ore in terms of geometries being grossly subhorizontal 10 to 70 m thick and crosscutting with respect to the host breccias the mineralization is zoned vertically from pyrite 34s 6 to 0 at depth to shallower chalcopyrite 34s 14 to 7 high grade pitchblende occurs centrally within the chalcopyrite zone e g 10 m of 0 46 cu and 3 700 ppm u3o8 straddling a boundary between k mica illite alteration and underlying fe mg chlorite alteration the deposit is assigned to the 1570 to 1595 ma iron oxide cu au u event in the eastern gawler craton on the basis of its similarity to other cu au u deposits in the province an la icpms pb isotope age for monazite in hematite stage ii was calculated to be 1455 20 ma and possibly as young as 1370 54 ma to explain all isotopic data this is an age of isotopic resetting our preferred model is that cu u mineralization developed within a stratified hydrothermal brine complex analogous to the salton sea geothermal system copper fe and s in the deep brine was sourced from an evolving deep reservoir with local compositional controls on sulfur isotopes including cooling boiling and replacement of previous pyrite chalcopyrite u veins and disseminations formed as a result of mixing with overlying steam heated u rich saline ground water mixing induced cooling and dilution only occurred episodically following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex by contrast high grade uo2 formation occurred as deep upflow waned via reaction of the upper brine with hematite stage ii fe mg chlorite along an interface that descended with time this resulted in an increase of u grades through time caused by leaching and reprecipitation

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

review and update of the applications of organic petrology part 1 geological applications abstract organic petrology developed as coal petrology at the beginning of the 20th century dedicated mainly to the study of coals because of their utilization in industry coal petrology was then considered a branch of coal science later with the development of specialized nomenclature classification of coal components and the standardization and improvement of analytical microscopical methods this discipline expanded in interests and name becoming organic petrology organic petrology carries a broader context being as well a tool applied in the study of dispersed organic matter in sedimentary rocks due to its importance in exploration for fossil fuel resources at present organic petrology is a discipline widely recognized for its role in fundamental and applied research with respect to both coal utilization and in geosciences throughout the 20th century several important monographs have been published on the discipline of organic petrology including stach s textbook of coal petrology 1st edition 1935 2nd 1975 3rd 1982 updated as the more general organic petrology by taylor et al 1998 more recently the text applied coal petrology the role of petrology in coal utilization was published by suarez ruiz and crelling 2008 this review is the first in a two part review series that describes and updates the role of organic petrology in geosciences a second part complementing this one and focused on the applications of organic petrology to other scientific fields will follow

peraluminous igneous rocks as an indicator of thermogenic methane release from the north atlantic volcanic province at the time of the paleocene eocene thermal maximum petm unusual cordierite bearing peraluminous dacites produced by melting of organic rich sediments by intrusion of basaltic magma are found within the north atlantic volcanic province navp calculations suggest that formation of the dacites radiometric dated at 55 9 0 3 ma and possibly widespread could have released an average of 4 500 gt range from 3 000 to 6 000 gt of carbon as methane with a 13c of about 35 published model results suggest that such a methane release could explain the negative 13c excursion in the oceans and atmosphere the extreme global warming and the marked dissolution of carbonates in the deep oceans that accompanied the concurrent paleocene eocene thermal maximum petm outgassing from melting of sediments and formation of dacites possibly in conjunction with methane produced in contact metamorphic aureoles and by methane hydrate release provides a novel way of explaining the petm and its timing

hydrogen isotope systematics of submarine basalts the d h ratios and water contents in fresh submarine basalts from the mid atlantic ridge the east pacific rise and hawaii indicate that the primary d h ratios of many submarine lavas have been altered by processes including 1 outgassing 2 addition of seawater at magmatic temperature and 3 low temperature hydration of glass decreases in d and h2o from exteriors to interiors of pillows are explained by outgassing of water whereas inverse relations between d and h2o in basalts from the galapagos rise and the famous area are attributed to outgassing of ch4 and h2 a good correlation between d values and h2o is observed in a suite of submarine tholeiites dredged from the kilauea east rift zone where seawater added directly to the magma affected only the isotopic compositions of hydrogen and argon analyses of some glassy rims indicate that the outer millimeter of the glass can undergo lowtemperature hydration by hydroxyl groups having d values as low as 100 d values vary with h2o contents of subaerial transitional basalts from molokai hawaii and subaerial alkali basalts from the society islands indicating that the primary d values were similar to those of submarine lavas extrapolations to possible unaltered d values and h2o contents indicate that the primary d values of most thoteiite and alkali basalts are near 80 5 the weight percentages of water are variable 0 15 0 35 for mor tholeiites about 0 25 for hawaiian tholeiites and up to 1 1 for alkali basalts the primary d values of 80 for most basalts are comparable to those measured for deep seated phlogopites these results indicate that hydrogen in marked contrast to other elements such as sr nd pb and o has a uniform isotopic composition in the mantle this uniformity is best explained by the presence of a homogeneous reservoir of hydrogen that has existed in the mantle since the very early history of the earth

an 40 ar 39 ar investigation of the contact effects of a dyke intrusion kapuskasing structural zone ontario an 40ar 39ar study on biotite and hornblende from baked tonalite gneiss adjacent to a 14 m wide kapuskasing dyke in the chapleau block of the kapuskasing structural zone employed resistance furnace step heating of bulk mineral separates single grain laser step heating and laser spot dating of individual grains in thick section all three methods yielded concordant results for the biotite indicating that it has been uniformly overprinted at ca 2 05 ga this date is probably a close estimate of the age of the dyke intrusion the argon data from the hornblende are more difficult to interpret all three methods suggest that the hornblende was partially overprinted and then affected by a later influx of excess argon from the external environment laser spot dates record an apparent age gradient extending radially outward from 2 25 ga in the grain cores to 2 4 2 7 ga in the grain rims corresponding 37ar 39ar values show no systematic variation across the hornblende although 38ar 39ar ratios display a similar pattern to the age distribution with lower values in the cores than in the rims the bulk separate and single grain step heating spectra mimic a volume diffusion ar loss profile that would appear to be inconsistent with the laser spot dates three possible interpretations to explain this apparent discrepancy may involve either experimental difficulties an effective radius of diffusion which is much smaller than the actual grain radius or an effective radius of diffusion that is of the order of the grain radius it is clear that for the 40ar 39ar dating of samples that have experienced complex geologic histories it is important to utilize both resistance furnace and laser techniques

stable isotope geochemistry of alteration associated with the key lake uranium deposit canada the key lake uranium deposit occurs within a shear zone that intersects the unconformity between early proterozoic basement gneiss and overlying middle proterozoic athabasca group sandstone uranium is concentrated near the unconformity and is associated with intense illite chlorite and kaolinite alteration the delta d and delta 18 o values of the clay minerals suggest that several different fluids have affected the ores and surrounding host rocks alteration of the basement gneisses associated with the uranium mineralization consists mainly of mg chlorite that equilibrated about 200 degrees c with a fluid having a delta d value near 20 per mil and a delta 18 o value near 5 per mil an illite rich sandstone alteration halo preserved only locally above the deposit equilibrated about 200 degrees c with a basinal brine having a delta d value near 65 per mil and a delta 18 o value near 5 per mil the uranium was concentrated near the unconformity by the mixing of these two distinct fluids the delta 18 o values of coexisting quartz and illite sampled from the shear zone hosting the uranium mineralization yield equilibration temperatures of 190 degrees to 230 degrees c which is postulated to be the temperature of the ore forming event fracturing in the sandstone above the deposit controlled late low temperature 50 degrees c kaolinitization of preexisting hydrothermal illite and chlorite by meteoric waters having delta d values of 50 or 30 per mil and delta 18 o values of 7 or 3 per mil the youngest event to affect the key lake deposit is related to the influx of modern meteoric waters that have lowered the delta d values but not greatly affected the delta 18 o values of some of the clay minerals associated with the shear and breccia zones this latest event has recrystallized much of the hydrothermal mg chlorite associated with the uranium to produce an fe mg chlorite with delta d values as low as 170 per mil

o and pb isotopic analyses of uranium minerals by ion microprobe and u pb ages from the cigar lake deposit we apply a rapid and accurate in situ technique to make u pb isotopic measurements of complexly intergrown uranium minerals and oxygen isotopic analyes of uraninite from the unconformity type cigar lake uranium deposit secondary uranium minerals intergrown with uraninite such as coffinite usio4 nh2o and calciouranoite cau2o7 5h2o were identified by highresolution transmission electron microscopy hrtem in situ u pb results from three stages of uraninite and coffinite define well correlated arrays on concordia with upper intercepts of 1461f47 1176f9 and 876f14 ma f1r these ages are interpreted as the minimum ages of mineralization correlate with the timing of clay mineral alteration f1477 ma associated with these unconformity type uranium deposits the ages of magnetization events at 1600 1450 and 900 ma from the athabasca basin and the grenvillian orogeny at f1100 ma in situ u pb isotopic analyses of uraninite and coffinite can document the pb u heterogeneities that occur on the scale of 15 30 am thus providing relatively accurate information regarding the timing of fluid interactions associated with the evolution of these deposits the high spatial resolution and precision of the ion microprobe allow us to measure d 18 o values of 20 100 am unaltered portions of uraninites from cigar lake the range of d 18 o values 33 9 to 20 4x are among the lowest reported for unconformity type deposits confirming that conventional fluorination analyses of material sampled at the mm scale are insufficient to avoid contamination from isotopically heavier coffinite and calciouranoite d 2002 elsevier science b v all rights reserved

pre ore hydrothermal alteration in an unconformity type uranium deposit hydrothermal alteration associated with the formation of unconformity type uranium deposits is manifested mainly by intense chloritic replacement of metamorphic mineral assemblages at the koongarra deposit northern australia three stages of pre ore alteration were identified in chloritized metamorphic muscovite using optical microscopy the electron microprobe and transmission and analytical electron microscopy tem and aem during the first stage metamorphic muscovite was altered to a mica with very low interlayer occupancy k content as low as 0 35 compared to 1 for ideal muscovite high octahedral cation content up to 2 3 whereas ideal muscovite has 2 and high si content high resolution tem images show that these changes in composition were accompanied by frequent layer terminations in contrast to the metamorphic muscovite whereas selected area electron diffraction patterns indicate that the altered muscovite remained a two layer polytype the alteration of muscovite appears more extensive in the outer alteration halo and has been almost completely eradicated in the ore zone where chloritization is more complete during the second stage of the pre ore alteration the altered muscovite was replaced by mg rich serpentine lizardite and clinochlore high resolution tem images show two 10 a mica layers coalescing to one 14 a chlorite layer this type of coalescence appears to involve transformation of one dioctahedral layer of the mica structure to one trioctahedral brucite layer in the chlorite structure through dissolution of the tetrahedral sheets and cation influx in the octahedral site the final stage of the pre ore alteration resulted in replacement of lizardite and clinochlore by fe rich chlorite comparison of the alteration observed in metamorphic muscovite with the alteration observed in metamorphic biotite and garnet shows clearly that chloritization of these phases was also a pre ore alteration event pre ore alteration was principally characterized by the evolution of chlorite from a mg rich phase to an ferich one

the time bound character of uranium mineralising processes with special reference to the proterozoic of gondwana abstract it is more useful to consider the time bound character of uranium mineralising processes than the time bound character of uranium deposits this is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years we have identified five periods which are characterised by moderately distinct mineralising processes 1 1 before 2800 ma uranium was fractionated into the upper crust during late archaean early proterozoic cratonisation by magmatic and metamorphic processes the granitic terranes were an important source of uranium for subsequent mineralising events 2 2 2800 2200 ma although the atmosphere was probably not reducing at this time the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins cratonisation was a prerequisite for the formation and preservation of long lived basins rapid burial and reducing conditions during diagenesis were important for preserving the uranitite the uranium was mobilised during diagenesis and metamorphism 3 3 2200 1200 ma after 2000 ma when the oxygen level in the atmosphere increased markedly the movement of uranium in solution as uranyl ions became more important the first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn sedimentary diagenetic or metamorphic concentration of uranium in some cases this mineralisation was economic in other cases it may have been just a step in a succession of mineralising events continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments the lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity where there were reducing sediments 4 4 1200 400 ma with the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium the period 1200 900 ma marked the final mineralisation in some important deposits the pan african and brazilian events 700 400 ma mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments 5 5 400 0 ma the incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea to form the important sandstone type of deposits any time bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit which of these it is depends on the subsequent history of the area on whether the uranium is preserved dispersed or reconcentrated that many major uranium deposits occur on late archaean early proterozoic cratons emphasises the importance of the long term preservation of sedimentary sequences even though the uranium may be remobilised several times within the sediments

radiometric correction of c band imagery for topographic effects in regions of moderate relief the combined effects of topography slope look angle and aspect on c band synthetic aperture radar sar data on the radiometric quality of sar images in a region of moderate relief are examined a correction method was used to attenuate the change of illumination across the swath due to the antenna pattern ground data were integrated into the analysis using a digital terrain model dtm correction functions based on the cosine of the incidence angle were applied to the thematic classes and to the grouped classes in order to reduce the effects related to topography it was found that it is possible to eliminate part of the radiometric variations created by moderate topographic relief after the corrections were applied a reduction was noted of the variance in the radiometric values of the spectral signatures of the cover types which ranged between 3 03 and 9 47 depending on the correction function used

the structure and subsidence of rockall trough from two ship seismic experiments coincident multichannel seismic reflection and refraction profiles acquired northwest of britain during a two ship seismic experiment in the southern rockall trough provide evidence for a thin 6 km syn rift crust below rockall trough the variation of seismic velocity with depth is consistent with either oceanic crust similar to that found in other parts of the north atlantic or with thinned continental crust heavily intruded by syn rift igneous rocks the syn rift crust beneath rockall trough is now buried by nearly 5 km of sediments that are intruded by tertiary volcanics varying in extent from isolated sills to a large sill complex extending 100 km laterally the basement imaged on seismic profiles exhibits a band of subhorizontal reflectors that extend laterally over distances of up to 40 km which they interpret as sediments intercalated with submarine lava flow generated during the opening of the trough estimates of stretching from crustal thinning and from subsidence indicate that rockall trough has undergone extensive rifting beta 6 upwelling asthenosphere beneath the thinned lithosphere generated at least 1 3 km thickness of melt as it decompressed the molten rock rose upward from the mantle until it was in part extruded as lava flows and in partmore intruded into the crust less

age of uranium mineralization at the jabiluka and ranger deposits northern territory australia new u pb isotope evidence the ranger and jabiluka uranium deposits are the largest in the alligator rivers uranium field which contains at least 20 percent of the world s low cost uranium reserves ore occurs in early proterozoic metasediments below an unconformity with sandstones of the 1 65 b y old kombolgie formation this study has used u pb isotope data from a large number of whole rock drill core samples with a variety of mineral assemblages and textures ranger samples indicate a well defined age of 1 737 or 20 m y consistent with earlier suggestions of the antiquity of this deposit this age is distinctly pre kombolgie so the ranger deposit cannot have been formed by processes requiring its presence the age is consistent however with mineralization related to heating associated with either the emplacement of early postmetamorphic granites or possibly with intrusion of the nearby oenpelli dolerite in contrast data for the least altered jabiluka ores yield a concordia intercept age of 1 437 or 40 m y significantly younger than the ranger age and also younger than the kombolgie formation this age may correspond to a regional thermal event as indicated both by mafic dikes of roughly this age and a zircon lower intercept age from a nearby granite gneiss thus our data indicate that there must be at least three distinct periods of major u mineralization in the alligator rivers uranium field including the previously determined approximately 900 m y age of the nabarlek deposit despite the fact that all of the deposits share strong similarities in their host rock mineralogy alteration and geologic setting both ranger and jabiluka reflect a common profound isotopic disturbance at about 400 to 600 m y this disturbance which was especially pronounced at jabiluka may correspond to the development of basins and associated basalt flows to the west and southwest as suggested by crick et al 1980

mesozoic rocks from the hebrides shelf and implications for hydrocarbon prospectivity in the northern rockall trough abstract shallow boreholes drilled in the west lewis and west flannan basins outer hebrides shelf have proved rocks of probable latest bajocian bathonian to barremian age where previously a mainly permo triassic infill has been assumed the full geological succession of the basins remains unproved although the permo triassic history of the basins may be similar the rocks encountered suggest that the basins may have dissimilar geological histories during the jurassic and early cretaceous lithologies are mainly fine grained poorly sorted sandstones or organic rich mudstones some correlation can be made with the isle of skye and the west shetland area the sandstones are poor hydrocarbon reservoirs but the middle jurassic and ryazanian mudstones although immature where drilled have excellent oil source rock potential and may have generated large amounts of hydrocarbons if they are present at depth in the adjacent northern rockall trough

changes in the hoar isotope composition of clays during retrograde alteration abstract k ar ages of illite alteration associated with middle proterozoic athabasca unconformity type u deposits in saskatchewan range from 414 to 1493 ma the k ar ages correlate with water contents and d values such that illites with young k ar ages have d values as low as 169 and water contents as high as 7 7 wt whereas illites with older ages have d values near 70 and water contents near 4 wt water extracted at 400 c from illites with low d values and high water contents has low d and 18 o values similar to those of modern meteoric water suggesting that some of the illites associated with the original deposition of the ore underwent varying degrees of retrograde alteration the alteration is initiated by hydration of sites in the interlayer region of the illite which results in the partial resetting of the k ar ages and introduction of excess structural water in the form of interlamellar water the interlamellar water is enriched in 18 o by about 7 per mil relative to the water that physically surrounded the clay particle further alteration decreases the d value and increases the 18 o value of the illite by isotopic exchange between the mineral and the interlamellar water although the chemical compositions and xrd patterns of the altered illites indicate that no detectable smectite component is present in the samples the isotopic results suggest that the altered illites may be an early precursor in the formation of mixed layer illite smectite by retrograde alteration of pure illite the wide variation of d values of chlorite and kaolinite from these u deposits is analogous to that of the illite suggesting that retrograde alteration of clays by meteoric water can be substantial the general association of altered clays with areas containing the highest concentrations of u is probably related to localized permeability within the ore zone

geochemical modeling of the formation of an unconformity type uranium deposit petrologic studies of alteration associated with uranium mineralization in unconformity type uranium deposits have established a detailed paragenetic sequence of the preore and synore alteration in addition fluid inclusion studies have determined the presence of two distinct fluids and thermodynamic analysis of the alteration assemblages has defined their basic characteristics these fluids were probably basinal brines derived from the overlying sandstones and have been distinguished by their different salinities fluids responsible for alteration and ore formation were na ca cl brines speciation calculations suggest that the hydrothermal fluids responsible for the formation of unconformity type uranium deposits were distinguished by their high oxygen fugacity and relatively low ph uranium was transported mainly as uranyl chloride complexes reconstruction of the reactions associated with the formation of the deposits employing chemical mass transfer calculations helps unravel the geochemical processes operating during their genesis reaction path calculations indicate that fe present in aluminosilicates in the host rocks is the principal reductant of oxidized uranium present in the ore forming fluids the use of variable host rock lithologies results in the reproduction of mineralogical features unique to each deposit along with the characteristics common to all unconformity related uranium deposits the calculations show that the observed paragenesis is a function not only of the fluid chemistry but also of the host rock lithologies the modeling indicates that although an orebody can be produced with or without graphite as a reactant its presence leads to a better agreement with the observed alteration paragenesis

geochemistry of organic rich cretaceous and jurassic mudstones in the west lewis and west flannan basins offshore north west scotland implications for source rock presence in the north east rockall trough the west lewis and west flannan basins serve as important calibration sites for understanding the source rock distribution and quality in the north eastern part of the rockall trough in the west lewis basin two shallow boreholes penetrated organic rich oil prone bathonian mudstones which were deposited in a fresh to brackish water environment these mudstones will generate oils rich in paraffins upon maturation organic rich oil prone ryazanian mudstones were discovered in both the west lewis and west flannan basins they are offshore marine mudstones with a predominance of algal organic matter akin to the kimmeridge clay in the north sea and basins west of shetland the bathonian mudstones will generate their hydrocarbon products at higher temperatures than the ryazanian mudstones the presence of lower cretaceous and middle jurassic oil prone mudstones in basins on the hebrides shelf is intriguing and suggests that similar source rocks may be present along the north eastern margin of the rockall trough

unusual triterpane distributions in lacustrine oils abstract a series of diahopanes and a second unknown early eluting series of triterpanes are reported in lacustrine oil samples from three different basins we suggest that these compounds together with 18 h 30 norneohopane show distributions which vary with the salinity of the depositional environment

control of hydrocarbon seepage intensity on level of biodegradation in sea bottom sediments offshore surface geochemical surveys which target the surface expression of potential migration pathways for sampling such as fault scarps or diapiric features have become a commonly applied approach in the petroleum industry results of such surveys help to reduce risk on key exploration play elements and are used to evaluate prospects and to predict hydrocarbon phase and expected properties based on geochemical surveys conducted by exxonmobil in many basins worldwide there is an interrelation of the seep intensity concentration and level of biodegradation results from offshore west africa where many active macroseeps show moderate to severe biodegradation and a frontier basin offshore united kingdom rockall trough where active microseeps show no evidence of biodegradation are compared the specific biochemical controls on the difference in biodegradation proneness are not known although it appears that a certain threshold of oil concentration is needed to sustain an active bacterial community or to exceed clay adsorption capacities that may protect microseeps from biodegradation it is notable that the 25 norhopane series often considered an indication of severe biodegradation in reservoir oils has not been recognized in even ultra severely biodegraded seeps this suggests that different biodegradation pathways may be followed in marine surface seeps versus those in subsurface hydrocarbon accumulations a likely scenario in light of the fact that physiologically diverse bacterial communities are prevalent under different physiochemical conditions

the nabarlek uranium deposit northern territory australia some petrologic and geochemical constraints on genesis the nabarlek uranium deposit northern territory australia is confined to a shear zone in contorted and metasomatized early proterozoic schists and occurs near an unconformity with overlying middle proterozoic sandstone massive chlorite or sericite or hematite rocks breccias and intensely altered schists are characteristic of the ore zone and at least three generations of chlorite have been recognized and described the primary ore mineral assemblage is dominated by uraninite intergrown with chlorite the orebody was sericitized at or after 920 m y ago resulting in the widespread replacement and breakdown of chlorite the formation of hematite and the solution and redeposition of uraninite redox reactions involving iron and uranium are evident in sericitized rocks containing hematite and residual uraninite high u th ratios in the ore zone suggest that uranium was transported to the site of deposition as a uranyl complex although no carbonaceous material occurs in the ore zone isotopic data on minor carbonate associated with uraninite suggest that organic material was originally associated with the mineralization and indicate that these carbonates have evolved from the interaction of hydrothermal fluids with this material erosion of the middle proterozoic cover rocks in the recent past has exposed themore deposit to the effects of weathering less

unconformity related rare earth element deposits a regional scale hydrothermal mineralization type of northern australia rare earth element ree orebodies are typically associated with alkaline igneous rocks or develop as placer or laterite deposits here we describe an economically important heavy h ree mineralization type that is entirely hydrothermal in origin with no demonstrable links to magmatism the mineralization occurs as numerous xenotime rich vein and breccia orebodies across a large area of northern australia but particularly close to a regional unconformity between archean metasedimentary rocks of the browns range metamorphics and overlying proterozoic sandstones of the birrindudu group the deposits formed at 1 65 to 1 61 ga along steeply dipping faults there is no known local igneous activity at this time depletion of hrees in the browns range metamorphics together with the similar nonradiogenic nd isotope composition of the orebodies and the browns range metamorphics indicates that ore metals were leached directly from the browns range metasedimentary rocks we propose an ore genesis model that involves fluid leaching hrees from the browns range metamorphics and subsequently mixing with p bearing acidic fluid from the overlying sandstones in fault zones near the unconformity the union of p and hrees via fluid mixing in a low ca environment triggered extensive xenotime precipitation this mineralization is unlike that of any other class of ree ore deposit but has a similar setting to unconformity related u deposits of australia and canada so we assign it the label unconformity related ree further discoveries of this ree mineralization type are expected near regional unconformities within proterozoic intracontinental sedimentary basins across the globe

multipolarization sar data for surface feature delineation and forest vegetation characterization this paper presents the utility of multipolarization synthetic aperture radar sar data for surface feature delineation and forest vegetation characterization three channels of ratioed data vv hh vh hh and vh vv are generated from the hh vv and vh polarization data v vertical h horizontal the ratioed data are linearly stretched to yield a digital number within a range of 0 to 255 the techniques for reducing sar speckle noise and for measuring the degree of separation are discussed for surface feature delineation the results indicate that cross polarization as well as cross polarization ratioed data better delineate those surface features that are difficult to separate by like polarization data the results suggest using a median value filtering technique to reduce within plot data fluctuation to increase the separability measure for forest vegetation characterization the results indicate that multipolarization sar data may be used to estimate forest properties such as total tree biomass basal area and tree height

variations in the uranium isotopic compositions of uranium ores from different types of uranium deposits abstract variations in 238u 235u and 234u 238u ratios were measured in uranium minerals from a spectrum of uranium deposit types as well as diagenetic phosphates in uranium rich basins and peraluminous rhyolites and associated autunite mineralisation from macusani meseta peru mean 238u values of uranium minerals relative to nbl crm 112 a are 0 02 for metasomatic deposits 0 16 for intrusive 0 18 for calcrete 0 18 for volcanic 0 29 for quartz pebble conglomerate 0 29 for sandstone hosted 0 44 for unconformity type and 0 56 for vein with a total range in 238u values from 0 30 to 1 52 uranium mineralisation associated with igneous systems including low temperature calcretes that are sourced from u rich minerals in igneous systems have low 238u values of ca 0 1 near those of their igneous sources whereas uranium minerals in basin hosted deposits have higher and more variable values high grade unconformity related deposits have 238u values around 0 2 whereas lower grade unconformity type deposits in the athabasca kombolgie and otish basins have higher 238u values the 234u values for most samples are around 0 in secular equilibrium but some samples have 234u values much lower or higher than 0 associated with addition or removal of 234u during the past 2 5 ma these 238u and 234u values suggest that there are at least two different mechanisms responsible for 238u 235u and 234u 238u variations the 234u 238u disequilibria ratios indicate recent fluid interaction with the uranium minerals and preferential migration of 234u fractionation between 235u and 238u is a result of nuclear field effects with enrichment of 238u in the reduced insoluble species mostly uo2 and 235u in oxidised mobile species as uranyl ion uo22 and its complexes therefore isotopic fractionation effects should be reflected in 238u 235u ratios in uranium ore minerals formed either by reduction of uranium to uo2 or chemical precipitation in the form of u6 minerals the 238u values of uranium ore minerals from a variety of deposits are controlled by the isotopic signature of the uranium source the efficiency of uranium reduction in the case of uo2 systems and the degree to which uranium was previously removed from the fluid with less influence from temperature of ore formation and later alteration of the ore uranium isotopes are potentially superb tracers of redox in natural systems

the geology of the uk hatton rockall margin abstract seismic potential field sea bed sample and shallow drilling data have been used to investigate the geology of the uk part of the rockall plateau most of the rockall plateau is underlain by early proterozoic rockall bank islay terrane metamorphic basement although george bligh bank and the north eastern end of hatton bank may be underlain at depth by lewisian rocks the basement crops out on rockall bank but occurs at a much deeper level beneath hatton bank and hatton basin the widespread palaeogene lavas of rockall plateau have been sourced from large central igneous complexes rockall swithin sandastre mammal lyonesse and sandarro local volcanoes and fissures some of the lavas may also have been derived from the incipient rift axis west of hatton bank at the time of continental break up differences in seismic facies allow the extent of the lavas from the large centres to be mapped where the lavas are absent beneath parts of hatton bank either due to non emplacement or subsequent erosion during the cenozoic seismic data image the underlying mesozoic and older rocks here bgs boreholes 99 1 and 99 2a have proved mid cretaceous albian shales and sandstones respectively beneath a thin cenozoic cover the mesozoic and older rocks appear to be quite extensive and are folded and faulted mesozoic strata are not observed on rockall bank in uk waters the post lava cenozoic succession of rockall plateau can be sub divided into three megasequences separated by two unconformities which by analogy with rockall basin have been designated c30 intra late eocene and c10 intra early pliocene the ages can be corroborated by dsdp odp boreholes in the hatton basin the cenozoic succession contains eocene prograding fan sequences contourite deposits on the flanks of the banks and shallow faulting some of which affects the present day sea bed structurally rockall high appears mainly as a large basement block whereas hatton high is more complex exhibiting half grabens inversion structures and reverse fault geometries due to cenozoic tectonism the northern end of hatton high is a large anticline with its axis aligned ene wsw the structural differences between the highs may be due to their locations with rockall high exhibiting fewer tectonic effects due to its more landward intraplate setting further away from plate margin deformation

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

a new look at the rockall region offshore ireland abstract a 700 km wide angle reflection refraction profile carried out in the central north atlantic west of ireland crossed the erris trough rockall trough and rockall bank and terminated in the western hatton rockall basin the results reveal the presence of a number of sedimentary basins separated by basement highs the rockall trough with a sedimentary pile up to 5 km thick is underlain by thinned continental crust 8 10 km thick some major fault block structures are identified especially on the eastern margin of the rockall trough and in the adjacent erris trough the hatton rockall basin is underlain by westward thinning continental crust 22 10 km thick sedimentary strata are up to 5 km thick the strata in the rockall trough and hatton rockall basin probably range in age from late palaeozoic to cenozoic however the basins have different sedimentation histories and differ in structural style the geometry of the crust and sediments suggests that the rockall trough originated by pure shear crustal stretching associated with rift deposits and cenozoic thermal sag strata in contrast the development of the erris trough located on unthinned continental crust was facilitated by shallow brittle extension with little deep crustal attenuation a two layered crust occurs throughout the region the lower crustal velocity in the hatton rockall basin is higher than that in the rockall trough the velocity structure shows no indication of crustal underplating by upper mantle material in the region

geology of the nabarlek uranium deposit northern territory australia the nabarlek deposit is an unusually high grade uranium orebody 1 84 u 3 o 8 12 000 metric tons hosted by amphibolite and semipelitic schist of lower proterozoic age it is unusual in the alligator rivers uranium field in being hosted by amphibolite the disposition of the ore is controlled by a prominent reverse fault which is part of a larger zone of reverse faulting the fault zone is nearly perpendicular to prominent metamorphic s 2 and s 3 fabrics a geometry which might enhance the fluid rock ratio a striking feature of the deposit is the extent and intensity of hydrothermal alteration which extends more than 1 km from the ore alteration remote from the ore resulted in pseudomorphous replacement of metamorphic biotite amphibole and feldspar under conditions of a low fluid rock ratio and mass transfer ore is associated with a small desilicification zone within and superimposed on this larger envelope the fluid rock ratio and mass transfer resulting mainly in silica loss were high within this interior zone and produced intense pervasive alteration leading to an assemblage of phengitic white mica magnesian chlorite and hematite large volumes of fluid were required to effect the massive desilicification and deposit such a volume of uranium given the current knowledge of the solubility of quartz and uraninite ore deposition predominantly as uraninite postdated intense phengitic white mica alteration chlorite however is the main gangue mineral possibly reflecting a change in redox conditions accompanying ore deposition a redox control on ore deposition is supported by the sparse ocurrence of host rock graphite which evidently participated in ch 4 forming reactions abundant hematite in the alteration halo suggests the presence of an oxidized fluid log f sub o 2 approx 35 at 200 degrees c which was thus suitable for transporting ore forming quantities of uranium probably as chloride complexes primary mineralization underwent incongruent dissolution to coffinite in equilibrium with a vanadian illite kaolinite and secondary hematite assemblage residual ore weathering of primary and residual ore produced yellow and green secondary uranium minerals both pseudomorphous after earlier ore types and as crusting of open goethite lined fractures

age dating of individual grains of uraninite in rocks from electron microprobe analyses it is shown that an electron microprobe analysis of the u th and pb content of a single uraninite uo2 grain may be used to calculate the age of formation of that grain this is known as a chemical age examples are shown for a range of natural samples with a wide range of ages comparison with ages derived from isotopic measurements shows the results to be valid approximate formulae which have been proposed for the calculation of the chemical age are not sufficiently reliable especially where the th content is high and for older ages a method for calculating the age by iteration is presented this uses the full and exact formula and is shown to produce a reliable age for the uraninite compared to isotopic measurements the method is particularly useful for samples where the minerals and mineral textures record information on more than one event two examples of samples recording information on dual ages are discussed to show how these ages can be related to events in the formation of the host rocks in one example an alluvial heavy mineral concentrate is shown to contain uraninites from two distinct sources with significantly different ages in the other example two phases of u mineralisation are distinguished in the same rock sample one phase relates to the genesis of the sediments from which the rock was formed the second phase records remobilisation of the u during later tectonic movement

formation of u rich mineralizing fluids through basinal brine migration within basement hosted shear zones a large scale study of the fluid chemistry around the unconformity related cigar lake u deposit saskatchewan canada abstract the evolution of basinal brines through percolation into basement rocks is studied in the paleo to mesoproterozoic athabasca basin and its underlying basement this specific geological context includes the evolution of a nacl rich basinal brine to a u mineralizing cacl2 rich brine that is of particular importance for the understanding of the formation of giant u deposits the objective of this study is to determine the physico chemical parameters p t x of the basinal brines and their changes both within the athabasca group sandstones and the underlying basement lithologies on the scale of several km around the world class unconformity related cigar lake u deposit particularly this study focuses on the basement hosted fault systems that are considered to be the main fluid flow pathways a comprehensive study including microthermometry raman spectroscopy and la icp ms analyses was carried out on fluid inclusions as was a semi quantitative estimation of the microfracture paleoporosity based on the surface density of fluid inclusion planes initial nacl rich brines were trapped during early diagenesis c a 1 7 1 65 ga in dickite rich sandstones under a three to seven kilometers thick sedimentary cover at around 90 to 150 c under hydrostatic pressure or at 120 210 c assuming lithostatic conditions at the time of u deposits formation ca 1 46 1 29 ga these brines percolated through the entire basement but preferentially within the damage zones of some graphite rich faults formed during the ca 1 8 ga trans hudson orogeny and active during the subsequent retrograde metamorphism and exhumation ca 1 8 1 72 ga the circulation of the basinal brines into these structures was enhanced and structurally controlled by the microfracture network inherited from earlier brittle reactivation during basement uplift the resulting high microfracture paleoporosity in the vicinity of some but not all of these structures favored intense interaction between the brines and basement rocks that induced the formation of illite sudoite alteration haloes in the basement the percolating brines scavenged mn k ba sr and especially u and evolved from the original nacl dominant composition toward a higher salinity cacl2 rich composition the resulting u bearing brine mixed with the unmodified diagenetic nacl rich brine in above and all along the reactivated faults either during thermal convection in isobaric settings and higher temperatures 110 200 c or through episodic tectonic reactivation in isothermal settings through episodic pressure fluctuation in direct vicinity of the u deposits the interactions between the brines and the basement rocks were the strongest which is marked by the highest intensity of alteration of the host rocks thus the formation of ore forming fluids resulted from the interaction between nacl rich basinal brines and highly microfractured basement rocks this study reinforces the hypothesis that interaction with basement rocks enhanced by strong fluid flow in cataclastic zones can significantly modify the physico chemical characteristics of basin derived brines leading to the formation of mineralizing fluids and the genesis of giant ore deposits such as the unconformity related u deposits

significance of alteration assemblages for the origin and evolution of the proterozoic nabarlek unconformity related uranium deposit northern territory australia a discussion sir in their recent paper on the nabarlek uranium deposit polito et al 2004 appear to have misunderstood wilde and wall s 1987 description of outer and inner halo alteration wilde and wall 1987 defined the outer alteration as pseudomorphous replacement of amphibole by chlorite biotite by chlorite and rutile or anatase and feldspar by finegrained white mica sericite and not as chlorite hematite schist chlorite muscovite hematite schist and chloritesericite schist up to 1 km from the nabarlek fault polito et al p 117 contrary to the assertion by polito et al 2004 p 117 that quartz is relatively rare in the outer alteration zone metamorphic rocks of the outer halo have been pervasively silicified beneath and along the margins of the oenpelli dolerite and quartz veining is common along pre ore faults beyond the ore zone the suggestion of polito et al 2004 that pervasive silicification was coincident with intrusion of the oenpelli dolerite lacks credible supporting evidence an equally plausible interpretation is that the lithologic boundary between the competent and homogeneous oenpelli dolerite and its strongly foliated and inhomogeneous host rocks focused fluid flow during post intrusion deformation the inner halo as defined by wilde and wall 1987 extends for no more than 50 m from ore and is manifested as nearcomplete pervasive replacement of metamorphic quartz by fine grained clay sized chlorite and white mica involving extreme desilicification of the host rocks fig 1a b in many samples from nabarlek and other uranium deposits from the east alligator rivers uranium field there is evidence of preservation of metamorphic and outer halo phases notably coarse metamorphic muscovite and biotite often with interlayered chlorite in pervasive fine grained alteration products fig 1a d fig 1 a photomicrograph plane polarized light of altered schist from inner alteration zone at nabarlek dh96 60m ch

post kombolgie metasomatism at jabiluka northern territory australia and its significance in the formation of high grade uranium mineralization in lower proterozoic rocks diagenetic lithification of kombolgie sandstones followed by a metasomatic event which produced veining and replacement metasomatism was controlled by brecciation and bedding within the sandstones the high grade uranium ores are interpreted to be the product of a low temperature 100 degrees 200 degrees c hydrothermal system of ground water which was driven by a regional heating event also manifested by phonolite and or diabase intrusions the kombolgie sandstones were an essential part of the hydrologic system which formed the high grade orebodies after lithification of the lower part of the thick clastic section but possibly contemporaneous with diagenetic events in the upper part the processes operating in the alligator rivers uranium field were essentially similar to those in the athabasca district of northern saskatchewan the primary difference probably being that no organic material was remobilized into the overlying sandstone to act as a reductant in the australian case modified journal abstract

fluid inclusion evidence on the origin of some australian unconformity related uranium deposits fluid inclusions in altered and unaltered host rocks and hydrothermal quartz calcite and dolomite from the unconformity related uranium deposits of koongarra nabarlek and jabiluka northern territory australia were examined using standard microthermometry laser raman microprobe and scanning electron microscopy decrepitated gases were analyzed in a mass spectrometer the relationship of fluid inclusions to alteration and mineralization was established by occurrence restricted to the alteration zones around the deposits and the presence in fluid inclusions of solid phases which also occur as megascopic phases in the alteration secondary halite saturated fluid inclusions in altered host rock quartz and in early hydrothermal quartz veins are believed to relate to hydrothermal alteration associated with ore deposition these inclusions are identical to primary inclusions in silicified and boron metasomatized kombolgie formation sandstone above the jabiluka 2 deposit durak et al 1983 and hence sample post kombolgie fluids the trapped fluid is saline and calcium rich ca 23 equiv wt cacl 2 and sulfate poor so super 2 4 0 001 m no gases were detected in the vapor phase of such inclusions and the median vapor disappearance temperature is 140 degrees c primary inclusions in postore dolomite veins and secondary inclusions in host rock and hydrothermal quartz sample postore hydrothermal fluid this fluid is hypersaline and also calcium rich typically 35 equiv wt cacl 2 and is saturated with as many as 11 solid phases including dolomite and iron rich chlorite vapor bubbles in these inclusions contain methane and minor n 2 and the median vapor disappearance temperature is 127 degrees c aromatic hydrocarbons detected by mass spectrometry are possibly related to this population the calcic brines are thought to have been terrestrial in origin and derived from the oxidized red bed sediments overlying the deposits macarthur basin ore deposition was a response either to reduction of the ore bearing fluid by direct interaction with graphite and ferrous iron in the wall rocks or by mixing with a methane and higher hydrocarbon bearing brine represented by postore fluid inclusions ore deposition and related hydrothermal alteration occurred between 200 degrees and 300 degrees c under a depth of cover of ca 2 km this temperature is consistent with independent temperature constraints and suggests an anomalous thermal gradient during ore formation

nd sr isotope constraints on the age and origin of unconformity type uranium deposits in the alligator rivers uranium field northern territory australia the results of an sm nd and rb sr isotope study on the large and rich unconformity type uranium gold platinum group elements deposits in northern australia indicate identical sm nd ages for primary uranium mineralization at nabarlek 1 616 or 50 m y and jabiluka ii 1 614 or 132 m y in excellent agreement with new and previously published rb sr data on u ores and altered host rocks in contrast the sm nd ages are older than published u pb ages which range from 900 to 1 437 m y sm nd data on uraninites from koongarra are strongly scattered but the geologic setting and nd model ages for the least disturbed samples suggest an age between 1 600 to 1 650 m y identical to the age of primary mineralization at nabarlek and jabiluka ii the narrow range of sm nd ages for these three deposits 1 600 1 650 m y indicates that mineralization took place soon after the deposition of unconformably overlying sandstones and volcanics of the 1 650 m y old kombolgie formation relatively high initial epsilon nd values and low 87 sr 86 sr ratios in the ores indicate that ore fluids derived their metal content from two main sources a preunconformity source comprising the local wall rocks lower proterozoic metasediments and amphibolites and a postunconformity source including the volcano sedimentary rocks of the kombolgie formation which form part of the evaporite bearing mcarthur group at nabarlek the kombolgie formation isotopic signature is obscured by the intensive wall rock alteration involving isotopically similar amphibolites and dolerites but mass balance calculations indicate major contributions of u and heavy rare earth elements from sources outside the alteration zone the most likely external metal sources are again the postunconformity volcanics and sandstones in contrast isotopically more evolved sources such as u rich late archean basement gneisses and post tectonic high level granites are less probable metal sources on the basis of their much lower epsilon nd and higher 87 sr 86 sr values suggesting that the ore forming hydrothermal activity did not penetrate the metamorphic basement except in the mineralized shear zones the new data lend support to models of ore deposition that involve reaction of hot oxidized u bearing formation meteoric waters whose source is the postunconformity rocks e g the kombolgie formation with reducing metasediments of the lower proterozoic basement highly altered volcanics within the kombolgie formation are favored as major u sources the origin of the mineralization at ranger may have involved different processes recent u pb data indicate a 1 737 m y age for mineralization at this deposit significantly earlier than that of the other deposits and most importantly earlier than that of deposition of the kombolgie formation the sm nd data for ranger uraninites do not form an isochron but indicate a possible age range from 1 600 to asymp 1 750 m y data resolution is insufficient to allow reliable distinction between a pre or post kombolgie age however nd sr initial ratios for ranger point to unreasonably high epsilon nd and high 87 sr 86 sr value sources when calculated for any age younger than 1 650 m y the age of kombolgie sedimentation and basin formation in contrast uraninite nd sr initial ratios at 1 737 m y are easily explained as mixtures involving nd and sr leached from various lower proterozoic and late archean sources the data are thus more compatible with a pre kombolgie origin for uranium mineralization at ranger

assessment of tropical forest stand characteristics with multipolarization sar data acquired over a mountainous region in costa rica a digital terrain elevation data set was coregistered with radar data for assessing tropical forest stands characteristics both raw and topographically corrected l band polarimetric radar data ac quired over the tropical forests of costa rica were analyzed and cor related with field collected tree parameter data to study the stand char acteristics the results of analyses using 18 out of 81 plots for sites a and b indicated that per plot bole volume and tree volume are related to synthetic aperture radar sar data particularly at site a the topographically corrected sar data appear to produce the same find ings as those of uncorrected data