lode gold mineralization in the tanami region northern australia the tanami region of northern australia has emerged over the last two decades as the largest gold producing region in the northern territory gold is hosted by epigenetic quartz veins in sedimentary and mafic rocks and by sulfide rich replacement zones within iron formation although limited geochronological data suggest that most mineralization occurred at about 1 805 1 790 ma during a period of extensive granite intrusion although structural relationships suggest that some deposits predate this period there are three main goldfields in the tanami region the dead bullock soak goldfield which hosts the world class callie deposit the granites goldfield and the tanami goldfield in the dead bullock soak goldfield deposits are hosted by carbonaceous siltstone and iron formation where a late d5 structural corridor intersects an early f1 anticlinorium in the granites goldfield deposits are hosted by highly sheared iron formation and are interpreted to predate d5 the tanami goldfield consists of a large number of small mostly basalt hosted deposits that probably formed at a high structural level during d5 the d5 structures that host most deposits formed in a convergent structural regime with 1 oriented between e w and ene wsw structures active during d5 include ne trending oblique thrust dextral faults and ese trending sinistral faults that curve into n to nnw trending reverse faults localized in supracrustal belts between and around granite complexes granite intrusions also locally perturbed the stress field possibly localizing structures and deposits forward modeling and preliminary interpretations of reflection seismic data indicate that all faults extend into the mid crust in areas characterized by the n to nw trending faults orebodies also tend to be n to nw trending localized in dilational jogs or in fractured competent rock units in areas characterized by ese trending faults the orebodies and veins tend to strike broadly east at an angle consistent with tensional fractures opened during e w to ene wsw directed transpression many of these deposits are hosted by reactive rock units such as carbonaceous siltstone and iron formation ore deposition occurred at depths ranging from 1 5 to 11 km from generally low to moderate salinity carbonic fluids with temperatures from 200 to 430 c similar to lode gold fluids elsewhere in the world these fluids are interpreted as the product of metamorphic dewatering caused by enhanced heat flow although it is also possible that the fluids were derived from coeval granites lead isotope data suggest that lead in the ore fluids had multiple sources hydrogen and oxygen isotope data are consistent with both metamorphic and magmatic origins for ore fluids gold deposition is interpreted to be caused by fluid unmixing and sulfidation of host rocks fluid unmixing is caused by three different processes 1 co2 unmixing caused by interaction of ore fluids with carbonaceous siltstone 2 depressurization caused by pressure cycling in shear zones and 3 boiling as ore fluids move to shallow levels deposits in the tanami region may illustrate the continuum model of lode gold deposition suggested by groves mineralium deposita 28 366 374 1993 for archean districts

a flexible approach to assessing synchroneity of past events using bayesian reconstructions of sedimentation history the dating of depths in two or more cores is frequently followed by a study of the synchroneity or otherwise of events reflected in the cores the difficulties most frequently encountered are a determining precisely the depths associated with the events and b determining the ages associated with the depths there has been much progress in recent years in developing tools for the study of uncertainties in establishing chronologies this has not yet been matched by similar progress in modelling event depth relationships this paper proposes a simple and flexible approach showing how uncertain events can be married to uncertain chronologies difficulties in studying event depth age relationships typically involve a confounding of two different problems first what exactly do we mean by an event a point in history a single depth in the core corresponding to a single time or a depth time range sometimes event is in fact a shorthand for a space time process do the data reflect more than one type of event process this can reflect vagueness in definition second what are the sources and implications of the uncertainties here we illustrate the issues involved by examination of several features seen in north european holocene pollen records the alnus rise is regarded as a diachronous early holocene event in contrast the ulmus decline is widely seen as a near synchronous event in the mid holocene the third feature we examine is the interval between the ulmus decline and the first occurrence of cerealia type pollen the evidence for these events lies in cores of lake sediment from which are determined a the proportions of pollen at many depths and b radiocarbon age estimates from usually fewer depths for this illustration we focus on six sites we draw attention to a new and flexible method implemented in the free r software package bchron haslett j parnell a 2008 a simple monotone process with application to radiocarbon dated depth chronologies journal of the royal statistical society series c 57 4 399 418 for the establishment of the uncertainties surrounding the dating of samples in such cores we illustrate its flexibility by assessing the synchroneity of past events

geologic evidence for chaotic behavior of the planets and its constraints on the third order eustatic sequences at the end of the late paleozoic ice age abstract high resolution 948 samples measurements of anhysteretic remanent magnetization arm were performed on the 200 m thick early middle permian maokou formation of the shangsi section south china the arm variations are quasi periodic and the wavelengths of significant cycles collectively present a ratio of 20 5 2 1 throughout the formation corresponding to long orbital eccentricity short orbital eccentricity obliquity and precession cycles a strong obliquity 44 and 33 kyr signal suggests that waning and waxing of the ice sheet in eastern australia at the end of the late paleozoic ice age lpia exerted a significant influence on global climate and sea level a floating astronomical time scale ats is developed using the 405 kyr orbital eccentricity cycle as an astronomical calibration target this results in estimation of the roadian and wordian stages duration as 3 7 0 4 myr and 2 9 0 4 myr respectively prominent 2 myr cycles likely originated from earth and mars secular frequencies g4 g3 and 1 myr cycles from s4 s3 these periodicities are shorter than those observed in the cenozoic era which may be due to the chaotic behavior of the planets but still reflecting 2 1 secular resonance between earth and mars third order eustatic sequences are linked to the s4 s3 obliquity term which suggests a glacioeustatic controlling mechanism during this transitional stage from paleozoic icehouse to mesozoic greenhouse

migration of brines in the basement rocks of the athabasca basin through microfracture networks p patch u deposit canada abstract the unconformity type uranium deposits of the athabasca basin saskatchewan canada are hosted near the unconformity between a middle proterozoic intracratonic sedimentary basin and an archean to paleo proterozoic metamorphic and plutonic basement these deposits which are considered to be the richest u deposits in the world are the result of massive basinal fluid migrations in the basement rocks this study shows that basinal brines have strongly penetrated into the basement not only through faults and major pathways but also by way of dense networks of microfractures which favoured the percolation of fluids down to considerable depths hundred metres below the unconformity and their chemical modification salinity increase by interaction with basement lithologies these processes are one of the major causes of uranium mobility within the basement rocks and the formation of unconformity type mineralization microfracture networks which opened during the basinal brine stage ca 1600 1400 ma are interpreted as sets of mode i cracks corresponding to a specific stage of deformation and occur as fluid inclusion planes after healing the stress field at that stage 1 n130 150 e subvertical partly reopened the earlier microcrack networks 1 n80 110 e and n130 150 e subvertical issued from the trans hudson orogeny late retrograde metamorphic stage ca 1795 1720 ma the circulation of the two types of fluids carbonic and brines occurs thus at two distinct events trans hudson orogeny late retrograde metamorphism for carbonic fluids and maximal burial diagenesis for brines but the same main microfissure geometry was used by the fluids this demonstrates the existence of a similar stress field direction acting before and after the basin formation moreover the brine circulations in the basement acted in a wider volume than the clay rich alteration halo surrounding the u ores generally considered as the main envelope of fluid percolation outside the fault systems the data on the chemistry of the fluids and on the geometry of their migration at various scales emphasise the fundamental role of the basement in the chemical evolution of highly saline brines linked to unconformity related uranium mineralization in the athabasca basin

measurement of u pb ages of uraninite and davidite by laser ablation hr icp ms laser ablation high resolution inductively coupled plasma mass spectrometry la hr icp ms is a rapid accurate and inexpensive technique for making in situ u pb isotopic measurements of uraninite and davidite the advantages of this method include 1 mineral separation and chemical digestion are not required 2 measurements on complex samples are feasible because significant isobaric interferences can be resolved and 3 accurate u pb and 207pb 206pb dates on 10 25 m spots can be obtained rapidly the la hr icp ms method is applied to u oxide minerals from four deposits and prospects from northern australia and the new dates are compared to previously published conventional thermal ionization mass spectroscopy tims dates and to known ages of geologically important events these comparisons permit us to assess elemental fractionation of u and pb for uraninite and davidite of the new method relative to zircon as well as its geochronological accuracy and precision u pb apparent ages measured previously agree well with our measurements for el sherena palette and mt isa additionally the upper intercept 207pb 206pb dates for adelaide river 701 190 ma 1 and palette 841 94 ma 1 uraninite measured here are similar to those previously obtained for palette 730 ma nabarlek 920 ma and koongarra 870 ma and the upper intercept date for el sherena uraninite 1573 160 ma is within error of that previously determined for ranger 1550 15 and 1472 40 ma such apparent age agreement for uraninite and similarly for davidite indicates that u and pb fractionations are within error of that for zircon whereas the inherent imprecision of our dates and their associated mswd values greater than 2 5 probably indicate that multiple resetting events affected our samples analytically these results demonstrate that the la hr icp ms technique provides excellent spatial resolution while also removing argide phosphide sulfide and halide interferences that can otherwise lead to erroneous data when using quadrupole icp ms geologically the individual 207pb 206pb and upper intercept u pb dates of uraninite from adelaide river and palette are ca 800 ma possibly reflecting recrystallization of uraninite during the break up of rodinia

a shear hosted au cu bi metallogenic event at 1660 ma in the tennant creek goldfield northern australia defined by in situ monazite u pb th dating abstract high grade au cu and bi ores in the tennant creek goldfield have been mined from hydrothermal magnetite and or hematite rich ironstone bodies less well known is a style of au cu bi mineralisation hosted by quartz vein systems within shear zones outside ironstones we report ion probe shrimp u pb th ages of 1659 13 ma and 1659 15 ma for hydrothermal monazite associated with this mineralisation style at the orlando east au cu bi deposit and navigator 6 au prospect respectively the dated monazite at orlando east is intergrown with cu and bi sulfides and au in quartz chlorite veins within a shear zone at navigator 6 the monazite occurs both in late stage chalcopyrite bearing quartz hematite veins and within earlier formed sheared chloritic breccia that also contains anomalous au although the full significance of the 1660 ma monazite ages has yet to be determined the results indicate the introduction of au cu and bi in quartz vein systems at 1660 ma or remobilisation of earlier 1850 ma main stage au cu bi from mineralised ironstones and into quartz veins 10 s 100 s of metres from the initial mineralisation in either case hydrothermal fluids capable of transporting significant concentrations of ore metals were present in the tennant creek goldfield at 1660 ma synchronous with deformation a key implication is that rock sequences up to 200 million years younger than the ironstone hosted au cu bi deposits may now be considered prospective for a second stage of au cu bi mineralisation the results also validate non ironstone shear hosted deposits in the tennant creek region as an alternative exploration target to the well known high grade but low tonnage tennant creek ironstone hosted deposits

an oceanic model for lithological and faunal changes tested on the silurian record a model for oceanic cycles is presented the cycles are expressed as changes between more humid low latitude and cooler high latitude climates p episodes and dryer low latitude and warmer high latitude climates s episodes the cyclicity may be self regulating through changes in the co 2 storage capacity of the deep ocean due to temperature changes and through changes in the oceanic input and output of dissolved carbonate the model permits over 30 predictions regarding observable changes in both deep water and shelf lithologies and variations in the abundance diversity and extinction rate of planktic and benthic faunas sedimentary changes involve fluctuations in the rate of clay deposition and carbonate production the advance and retreat of carbonate deposition on the shelf edge the timing of oolite reef and black shale formation and changes between oxic and anoxic deep water faunal effects include both iterative evolution and intermittently widespread taxa lazarus taxa these predictions agree well with changes observed in well known sequence of wenlock and ludlow age as well as changes seen in selected older and younger sequences

refining the early devonian time scale using milankovitch cyclicity in lochkovian pragian sediments prague synform czech republic abstract the early devonian geological time scale base of the devonian at 418 8 2 9 myr becker et al 2012 suffers from poor age control with associated large uncertainties between 2 5 and 4 2 myr on the stage boundaries identifying orbital cycles from sedimentary successions can serve as a very powerful chronometer to test and where appropriate improve age models here we focus on the lochkovian and pragian the two lowermost devonian stages high resolution magnetic susceptibility in 5 to 10 cm sampling interval and gamma ray spectrometry grs 25 to 50 cm sampling interval records were gathered from two main limestone sections po ar cs 118 m spanning the lochkov and praha formations and pod barrandovem 174 m praha formation both in the czech republic an additional section bran ovy 65 m praha formation was sampled for grs every 50 cm the in and grs records are very similar so in variations are driven by variations in the samples paramagnetic clay mineral content reflecting changes in detrital input therefore climatic variations are very likely captured in our records multiple spectral analysis and statistical techniques such as continuous wavelet transform evolutive harmonic analysis multi taper method and average spectral misfit were used in concert to reach an optimal astronomical interpretation the po ar cs section shows distinctly varying sediment accumulation rates the lochkovian essentially equivalent to the lochkov formation fm is interpreted to include a total of nineteen 405 kyr eccentricity cycles constraining its duration to 7 7 2 8 myr the praha fm includes fourteen 405 kyr eccentricity cycles in the three sampled sections while the pragian stage only includes about four 405 kyr eccentricity cycles thus exhibiting durations of 5 7 0 6 myr and 1 7 0 7 myr respectively because the lochkov fm contains an interval with very low sediment accumulation rate and because the praha fm was cross validated in three different sections the uncertainty in the duration of the lochkov fm and the lochkovian is larger than that of the praha fm and pragian the new floating time scales for the lochkovian and pragian stages have an unprecedented precision with reduction in the uncertainty by a factor of 1 7 for the lochkovian and of 6 for the pragian furthermore longer orbital modulation cycles are also identified with periodicities of 1000 kyr and 2000 2500 kyr

basin related uranium mineral systems in australia a review of critical features abstract this paper reviews critical features of basin related uranium mineral systems in australia these mineral systems include proterozoic unconformity related uranium systems formed predominantly from diagenetic fluids expelled from sandstones overlying the unconformity sandstone hosted uranium systems formed from the influx of oxidised groundwaters through sandstone aquifers and calcrete uranium systems formed from oxidised groundwaters flowing through palaeochannel aquifers sand and calcrete the review uses the so called source pathway trap paradigm to summarise critical features of fertile mineral systems however the scheme is expanded to include information on the geological setting age and relative timing of mineralisation and preservation of mineral systems the critical features are also summarised in three separate tables these features can provide the basis to conduct mineral potential and prospectivity analysis in an area such analysis requires identification of mappable signatures of above mentioned critical features in geological geophysical and geochemical datasets the review of fertile basin related systems shows that these systems require the presence of at least four ingredients a source of leachable uranium and vanadium and potassium for calcrete uranium deposits suitable hydrological architecture enabling connection between the source and the sink site of accumulation physical and chemical sinks or traps and a post mineralisation setting favourable for preservation the review also discusses factors that may control the efficiency of mineral systems assuming that world class deposits result from more efficient mineral systems the review presents a brief discussion of factors which may have controlled the formation of large deposits in the lake frome region in south australia the chu sarysu and syrdarya basins in kazakhstan and calcrete uranium deposits in the yilgarn region western australia

early silurian glaciation along the western margin of gondwana peru bolivia and northern argentina palaeogeographic and geodynamic setting the record of the ordovician silurian transition within the southern peru bolivia palaeozoic basin extending across peru bolivia and northern argentina is characterized by a diamictite bearing unit which overlies several different ordovician units and underlies mid silurian shales we review the lithostratigraphy sedimentology and biostratigraphy of this unit revising stratigraphic relationships and several ideas and interpretations previously proposed analysis of facies and lithofacies associations allow for a reinterpretation of sedimentary processes and environments involved during its deposition turbidite and shale interbeds within the diamictites indicate a deep marine environment with interbedded mud flows debris flows slumps and large displaced slabs providing evidence for sediment instability and resedimentation chitinozoan biostratigraphy indicates a llandovery age in contrast with previous proposals of a hirnantian age and suggests the need for more detailed sedimentologic and biostratigraphic studies and a reassessment of this diamictite unit in relation to adjacent regions in south america the resedimented character of the deposit explains some of the ordovician fauna previously described which must be considered as recycled from underlying units in situ fauna needs to be reassessed as it may be endemic diachronic and or indicative of migrations within gondwana glacially faceted and striated clasts as well as large granitoid boulders within the resedimented materials provide evidence for glaciation of the source area and are interpreted as recycled from former glacigenic deposits the evidence found in the central andes indicates a glaciomarine origin for the diamictites and corroborates glaciation of a western source area prior to the late llandovery telychian the precise age of glaciation in this part of gondwana cannot be confirmed do to a lack of true tillites tectonic deformation and the resulting relief are respectively identified as the origin for sediment instability and for local glaciation along the active margin of western gondwana during the late ordovician and early silurian the new data imply that the ordovician silurian boundary was not preserved in some areas of the basin and that an erosional hiatus disconformity with variable regional significance is present between different ordovician units and the diamictite unit late ordovician late caradoc early ashgill euxinic black shales locally present in the central altiplano and eastern cordillera of bolivia may represent the youngest ordovician unit in the region and more research should be focused on them paleogeographic reconstruction of the early silurian peru bolivia basin depicts the extension of the diamictite unit southwards into northern argentina and western paraguay connecting with the parana basin through the asuncion arch to the north it continued into northern peru and ecuador connecting with the silurian record in colombia and venezuela and to the northeast into brazil solimoes and amazonas basins the sedimentary record of glaciation in the peru bolivia basin was due to the development of local ice fields to the west of the basin related with tectonism and resulting reliefs along the active margin of gondwana and probably did not coincide with the development of the north african late ashgillian ice cap

long period orbital climate forcing in the early palaeozoic facies indicators and geochemical proxies of early palaeozoic global climate cooling suggest episodes of fluctuating glacio eustasy and severe cold or glaciation from the mid late cambrian to silurian c 85 myr with a mean frequency of 2 6 myr long period orbital time series predicted through the phanerozoic are used to generate sine waves to test against these data the null hypothesis of no orbital influence is rejected with a high confidence level cooling episodes appear most frequent through the late ordovician leading up to the hirnantian glacial maximum but even greenhouse intervals of the early mid ordovician and early silurian provide evidence for periodic cooling episodes

axial obliquity control on the greenhouse carbon budget through middle to high latitude reservoirs carbon sources and sinks are key components of the climate feedback system yet their response to external forcing remains poorly constrained particularly for past greenhouse climates carbon isotope data indicate systematic million year scale transfers of carbon between surface reservoirs during and immediately after the late cretaceous thermal maximum peaking in the cenomanian turonian circa 97 91 million years myr ago here we calibrate albian to campanian 108 72 myr ago high resolution carbon isotope records with a refined chronology and demonstrate how net transfers between reservoirs are plausibly controlled by 1 myr changes in the amplitude of axial obliquity the amplitude modulating terms are absent from the frequency domain representation of insolation series and require a nonlinear cumulative mechanism to become expressed in power spectra of isotope time series mass balance modeling suggests that the residence time of carbon in the ocean atmosphere system is by itself insufficient to explain the myr scale variability it is proposed that the astronomical control was imparted by a transient storage of organic matter or methane in quasi stable reservoirs wetlands soils marginal zones of marine euxinic strata and potentially permafrost that responded nonlinearly to obliquity driven changes in high latitude insolation and or meridional insolation gradients while these reservoirs are probably underrepresented in the geological record due to their quasi stable character they might have provided an important control on the dynamics and stability of the greenhouse climate

the heartbeat of the oligocene climate system a 13 million year continuous record of oligocene climate from the equatorial pacific reveals a pronounced heartbeat in the global carbon cycle and periodicity of glaciations this heartbeat consists of 405 000 127 000 and 96 000 year eccentricity cycles and 1 2 million year obliquity cycles in periodically recurring glacial and carbon cycle events that climate system response to intricate orbital variations suggests a fundamental interaction of the carbon cycle solar forcing and glacial events box modeling shows that the interaction of the carbon cycle and solar forcing modulates deep ocean acidity as well as the production and burial of global biomass the pronounced 405 000 year eccentricity cycle is amplified by the long residence time of carbon in the oceans

pacing of the latest ordovician and silurian carbon cycle by a 4 5 myr orbital cycle abstract the cyclic eccentricity precession and obliquity parameters of the earth s orbit and axial motion have been demonstrated to have a large impact on climate on short 104 to 105 yr timescales during the cenozoic and mesozoic amplitude modulation am of these parameters on longer 106 yr timescales induces grand orbital cycles that have also been detected for large parts of the cenozoic and mesozoic however the response of paleoclimatic variables to these cycles is unknown for the paleozoic due to past chaotic transitions in the orbital motion of the inner planets the late ordovician to early devonian remains an enigmatic period of geological time during the hirnantian extensive southern hemisphere glaciation was associated with the second largest mass extinction in earth s history the subsequent silurian was met with several high amplitude 4 carbon isotope 13c excursions associated with equally erratic climatic biotic and eustatic fluctuations here i conduct spectral analysis of carbonate 13c and conodont apatite oxygen isotope 18o compilations for the katian 448 ma to mid lochkovian 415 ma the resulting spectra document a prominent cyclicity of 4 2 3 7 to 4 8 myr in the carbon and climate cycles similar to dominant periods found in cenozoic astronomical solutions related to the long term am of the 100 kyr eccentricity band 400 kyr eccentricity band and axial obliquity variations in the 4 5 myr eccentricity and axial obliquity cycles respectively enhanced organic carbon burial or reservoir stability leading to fluctuations in the paleozoic 13c record this process led to a decline in atmospheric co2 to threshold levels after which astronomical forcing triggered the onset of glaciation and extensive global cooling associated with positive 18o excursions during the hirnantian the early and late llandovery the telychian sheinwoodian boundary the homerian the late ludfordian and the late pridoli early lochkovian

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

uranium and gold deposits in the pine creek orogen north australian craton a link at 1 8 ga abstract the pine creek orogen pco in the north australian craton nac hosts world class unconformity related u deposits and numerous orogenic or intrusion related au deposits whose timing and genesis are the subject of ongoing controversy this study reports new geochronology results for an unconformity related u deposit ranger 1 number 3 orebody to constrain the timing of processes leading to alteration development and u mineralisation within a regional geological and metallogenic context it also examines why some u deposits in the eastern pco contain elevated au concentrations unlike their unconformity related u counterparts in canada and elsewhere key results are as follows i the ranger 1 deposit and the pco au deposits share several characteristics of hydrothermal mineralogy including chlorite white mica monazite xenotime bearing alteration assemblages some sulfides pyrite chalcopyrite silica quartz veining or silicification and au ii the primary host structure at the ranger 1 deposit is a shear zone which was reactivated at low grade metamorphic conditions as during formation of many of the syn tectonic au deposits in the pco iii ion probe u pb dating of low th monazite in an assemblage of hydrothermal white mica and fe rich chlorite from an outer alteration zone at the ranger 1 no 3 orebody yielded an age of 1800 9 ma this age is 60 million years earlier than the oldest previously reported age of unconformity related u mineralisation in the pco and is within error of several u pb ages reported for monazite and xenotime associated with au mineralisation in the pco iv iron rich chlorite intimately associated with the dated monazite is similar compositionally and texturally to chlorites in alteration zones at other u deposits in the pco two scenarios for the formation of the ranger 1 deposit are consistent with the available evidence we prefer a two stage model where an fe chlorite white mica monazite hydrothermal alteration assemblage with probable gold was introduced replacing medium grade metamorphic assemblages within a reactivated shear zone at ca 1800 ma contemporaneous with the formation of some au deposits within the pco subsequently u ore was formed within this environment at 1740 ma and or later via the reaction of oxidised u and mg bearing basin derived fluids with chemical reductants in the basement the new u pb geochronology result for monazite points to an important hydrothermal event at the site of the ranger 1 deposit during regional tectonothermal activity of the shoobridge event at 1 8 ga in the pco and suggests that this pre ore alteration and tectonism was a key factor in localising later unconformity related u mineralisation the pre ore alteration at 1800 ma at the ranger 1 deposit therefore provides a link between the genesis of the unconformity related u deposits and the orogenic or intrusion related au deposits that formed in the pco during the shoobridge event at the broader scale the newly identified 1800 ma hydrothermal activity at the ranger 1 deposit may be part of a craton wide metallogenic and tectonothermal event at 1 8 ga that includes au mineralisation in the pco and in the granites tanami orogen as proposed in other studies

stone properties and weathering induced by salt crystallization of maltese globigerina limestone most monuments and buildings in the maltese islands are constructed of the local globigerina limestone today this globigerina limestone shows considerable damage in many buildings particularly through alveolar weathering which is frequently very intense owing to malta s marine environment salt crystallization in the stone s pore spaces has been recognized as the main weathering process responsible for the deterioration of the country s monuments in order to obtain more information on the fabric dependent weathering processes of globigerina limestone detailed analyses were carried out globigerina limestone samples obtained from stone types with two different known qualities were characterized according to petrographical geochemical and physical properties these included porosity pore radii distribution and tensile strength as well as water and humidity transport properties investigations by means of salt crystallization tests on quarry samples of both stone types reinforced the idea that the extent of salt weathering depends on salt type and concentration and pore space properties visible weathering damage was recorded and evaluated for a representative monument the church of santa marija ta cwerra in siggiewi by means of a monument mapping method which was carried out twice over a period of 9 years 1995 and 2004 the identified weathering forms were also correlated with a previously developed weathering model for globigerina limestone according to the results of the mapping salt analyses carried out on samples from the church and salt loading tests on quarry samples there exists a significant correlation between visible damage and salt load the zoning of weathering damage is obviously related to different salt concentrations the zone with severe weathering damage is characterized by a high concentration of halite consequently salt weathering represents the main damage process for the globigerina limestone of malta the maltese archipelago of malta comino and gozo lies in the central mediterranean sea approximately 90 km south of sicily during the geological epochs of the oligocene and miocene 30 5 ma ago extensive sedimentation took place in this area this sedimentation led to the laying down of beds of limeand mudstones pedley et al 2002 the maltese globigerina limestone formation is one of five main formations and varies in thickness from 20 to over 200 m the material used for building is located stratigraphically in the lower part of the globigerina limestone formation called the lower globigerina limestone during the deposition of the sediments that eventually formed this stone far away from the continent and below the action of waves only minor but variable amounts of clay in suspension were carried from a distant land source the globigerina limestone also contains numerous shells algae and planktonic fossils mainly the planktonic foraminifera globigerina pedley et al 2002 as can be widely seen in the maltese islands the local limestone has always been used as the predominant building material the maltese prehistoric temples which were constructed approximately 6000 years ago bear testimony to this cassar vannucci 2001 between 1530 and 1798 the order of the knights of st john built kilometres of fortifications to protect the island from the expanding ottoman empire fortifications impressive churches fig 1 and palaces were built of the local building stone during this period the capital city of valletta is included in the unesco world heritage list as are the prehistoric temples even today the local building stone is still much in demand many modern buildings are constructed using the globigerina limestone fig 2a and many local quarries are still active fig 2b from p ikryl r smith b j eds building stone decay from diagnosis to conservation geological society london special publications 271 189 198 0305 8719 07 15 00 the geological society of london 2007

early silurian sea level changes global sea level fluctuated markedly during the early silurian probably as a result of the waxing and waning of ice sheets in the south american portion of gondwana the highest sea levels of the silurian are recorded by the telychian upper crispus lower griestoniensis and spiralis lower lapworthi biozones other highstands occurred in the early aeronian during the convolutus zone mid aeronian guerichi zone and late turriculatus zone early telychian and early sheinwoodian low sea levels characterized much of the argenteus and sedgwickii zones aeronian the utilis subzone late guerichi early turriculatus zones early telychian the late telychian commencing in the mid lapworthi zone and after a period of apparently only small amplitude sea level fluctuations in the late sheinwoodian and earliest homerian the mid late homerian in particular the early nassa zone facies and faunal changes in the lower silurian do not support the p and s model of jeppsson and others but are consistent with the sea level changes proposed herein mid telychian marine red beds appear to have been deposited during a minor sea level fall immediately after a period of very high sea levels rather than during a transgressive episode as previously suggested comparison of the sea level curve presented herein with those constructed in the past is hampered by the lack of precision currently possible in the correlation of early silurian deep water graptolitic and shallow water shelly sequences improving the precision of this correlation should be a priority for future research

the k ar and 40ar 39ar methods revisited for dating fine grained k bearing clay minerals abstract the strengths and weaknesses of the two ar isotopic methods k ar and 40ar 39ar were evaluated on the basis of respective recent applications mainly on low temperature k bearing illite type clay minerals this review includes a presentation of basic analytical and technical aspects for both methods as well as a discussion of varied claims on the two methods and of requests about sample preparation and characterization whenever possible the advantages and weaknesses of each method were compared on coeval results obtained by both methods on the same mineral separates the comparative review examines stratigraphic dating of glauconites indirect dating of low temperature ore deposits dating of burial related illitization and dating of polyphased tectono thermal activity more specifically of fault gouges some pending questions such as the necessary encapsulation due to 39ar recoil and its restoration into step heating patterns are also raised together with the new potential of ar dating of nanometric illite crystals weakness of the k ar method is in its pioneering status that makes many believe that it is no longer accurate because of its traditional analytical aspects and of the k determinations leading to somewhat large uncertainties however precise evaluation of varied applications points to a k ar method having probably larger applicability in sedimentary to diagenetic environments than the 40ar 39ar method the drawbacks become less important if the method is applied to nanometer sized clay minerals in diagenetic to low grade metamorphic environments in this instance the extracted size fractions are generally homogeneous and the relative uncertainty given by the age calculations if mathematically justified can be reduced by duplicate analyses weakness of the 40ar 39ar method is in its basics such as the 39ar recoil the necessary encapsulation the reintegration of the 39ar into the step heating patterns and the meaning of the step heating patterns that are more suggestive of variable 39ar reservoirs created among the clay particles by irradiation than of meaningful geologic ages if the k ar method is the preferred method for dating diagenetic clay processes such as glauconitization illite crystal nucleation and growth or low temperature hydrothermal activities then the 40ar 39ar method has more potential in dating low temperature tectono thermal activities and in detailing mixtures of multi generation illite

tectonic evolution of the pine creek inlier northern territory the pine creek inlier c 66 000 km2 contains early proterozoic sediments and volcanics deposited 1900 ma ago in a basin formed by crustal extension of c 2500 ma granitic basement about 10 km of supracrustals accumulated in 20 ma in shallow marine to continental environments during sagging related to post extensional subsidence deposition in this pine creek geosyncline was succeeded by an orogenic period lasting for 180 ma 1870 1690 ma the top end orogeny felsic and mafic intrusives were emplaced before and after the main deformation which ranged in style from open to tight upright folds in low grade areas in the centre of the region to reclined multiple isoclinal folds in areas of medium to high grade metamorphism in the northeast and west during late orogenesis rift related felsic volcanics and continental sediments were deposited on the metamorphosed sequence to form the katherine volcanic sequence representing a second period of crustal extension the lower part of this sequence was itself mildly deformed by the last significant perturbation of the top end orogeny the hiatus between these early proterozoic events and middle proterozoic deposition is marked by a saprolitic weathered profile indicating a stable subaerial period perhaps as long as 150 ma the cover rocks are the basal part of the mcarthur basin sequence which extends eastward into queensland in the pine creek region they are plateau forming sandstone generally 600 m thick thickening to 2000 m in basins near katherine formed by further movement of the late early proterozoic rift systems the region has remained stable since proterozoic time with only minor reactivation of rift structures to allow accumulation of locally thicker mesozoic and eocene sequences and intrusion of minor mafic to intermediate dykes in the late proterozoic and palaeozoic later eustatic movement led to incursion of shallow palaeozoic and mesozoic seas

major orogenic gold episode associated with cordilleran style tectonics related to the assembly of paleoproterozoic australia new in situ sensitive high resolution ion microprobe shrimp u pb analyses of hydrothermal phosphates associated with orogenic gold mineralization in the paleoproterozoic ashburton and pine creek gold provinces of northern australia provide ages of ca 1740 and ca 1730 ma respectively argon argon analyses of gold related hydrothermal mica from the tanami gold province of northern australia provide ages ca 1730 ma it is important to note that late orogenic events across the western half of australia coincide with gold metallogenesis across this time interval in several widely separated provinces thus this orogenic gold episode is interpreted to relate to tectonic events during the amalgamation of various continental blocks to form paleoproterozoic australia it is potentially earth s best preserved record of orogenic gold formation during a major early precambrian continental assembly event

limestones under salt decay tests assessment of pore network dependent durability predictors three portuguese limestones two grainstones semi rijo and moca creme commercial types and a travertine were subjected to salt crystallisation test en 12370 1999 grainstones specimens showed higher weight loss than the travertine ones results are discussed using parametric and nonparametric statistics in relation to pore space characteristics that have been considered durability predictors evaluated at the macroscopic water imbibition and microscopic mercury injection porosimetry levels several of the pore space parameters indicate differences between grainstones and travertine but these differences are much lower than the differences in weight loss results of the durability dimensional estimator dde seem to present a discrepancy with higher values in the rock type having lower weight loss travertine additionally weathering patterns of travertine samples show marked irregularities a feature that is not assessed by weight loss these irregularities are attributed to heterogeneous spatial distribution of detritic components a feature that is not assessed by pore space parameters comparing the grainstones moca creme showed higher values of mass loss and several durability predictors suggested noteworthy differences in relation to semi rijo however these differences are much lower than differences in weight loss calculated crystallisation pressures are even higher for semi rijo it is proposed that petrographical aspects of moca creme namely heterogeneity related to the presence of bioclasts and veinlets could contribute to explain the higher weathering susceptibility of this rock type these results highlight the need to include petrographical features in assessments of durability and the usefulness of visual description on the characterization of salt weathering

rapid cenozoic glaciation of antarctica induced by declining atmospheric co2 the sudden widespread glaciation of antarctica and the associated shift towards colder temperatures at the eocene oligocene boundary 34 million years ago refs 1 4 is one of the most fundamental reorganizations of global climate known in the geologic record the glaciation of antarctica has hitherto been thought to result from the tectonic opening of southern ocean gateways which enabled the formation of the antarctic circumpolar current and the subsequent thermal isolation of the antarctic continent5 here we simulate the glacial inception and early growth of the east antarctic ice sheet using a general circulation model with coupled components for atmosphere ocean ice sheet and sediment and which incorporates palaeogeography greenhouse gas changing orbital parameters and varying ocean heat transport in our model declining cenozoic co2 first leads to the formation of small highly dynamic ice caps on high antarctic plateaux at a later time a co2 threshold is crossed initiating ice sheet height mass balance feedbacks that cause the ice caps to expand rapidly with large orbital variations eventually coalescing into a continental scale east antarctic ice sheet according to our simulation the opening of southern ocean gateways plays a secondary role in this transition relative to co2 concentration

sequestration of 12c in the deep ocean during the early wenlock silurian positive carbon isotope excursion abstract marine carbonates from three biostratigraphically well dated silurian sections from the mid continent of north america were analyzed for carbon isotopic 13 c carb study the early wenlock positive carbon isotope excursion ireviken excursion is recorded in three sections from tennessee iowa and ohio with values of 4 5 this global shift in the carbon isotopic ratio of marine waters began during a protracted extinction event ireviken event that spanned the llandovery wenlock boundary although several oceanographic models have been developed to account for the coincident changes in lithology biology and carbon isotope stratigraphy during this interval of the silurian their proposed causal connections among glaciation oceanography primary productivity and the global carbon cycle remain a matter of debate this investigation provides carbon isotopic and lithologic evidence that the ireviken excursion occurred during a time of relatively oligotrophic conditions a third order transgressive regressive cycle and increased carbonate production throughout the mid continent of north america the ireviken excursion is recorded in relatively pure carbonate sequences and is immediately preceded by a significant unconformity in each of the measured sections it is proposed that the ireviken excursion was the product of sequestration and burial of 12 c in deep water as a result of altered deep ocean circulation

did cooling oceans trigger ordovician biodiversification evidence from conodont thermometry the ordovician period long considered a supergreenhouse state saw one of the greatest radiations of life in earth9s history previous temperature estimates of up to 70 c have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time we present a very different global climate record determined by ion microprobe oxygen isotope analyses of early ordovician silurian conodonts this record shows a steady cooling trend through the early ordovician reaching modern equatorial temperatures that were sustained throughout the middle and late ordovician this favorable climate regime implies not only that the oxygen isotopic composition of ordovician seawater was similar to that of today but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period

strontium and carbon isotope stratigraphy of the llandovery early silurian implications for tectonics and weathering abstract a high resolution 87sr 86sr curve and paired 13c carbonate organic data set is generated for the llandovery series from the ikla drill core in estonia a 13c carbonate curve is also presented from the pancake range in nevada observed 87sr 86sr values in the ikla drill core are at a minimum in the early llandovery rhuddanian stage 0 7079 to 0 7080 and then trend to more radiogenic ratios in the basal part of the telychian stage an 87sr 86sr high near 0 7084 is observed in the telychian at the top of the studied section the range of values is in general agreement with the data from previous sample sets of brachiopods and conodonts recovered from localities in north america and europe that record a rising trend in the 87sr 86sr ratio throughout the llandovery from approximately 0 7080 to 0 7084 the major increase in the 87sr 86sr ratio during the late llandovery may be due to weathering of radiogenic source rocks that were uplifted during early silurian continent continent collisions the sr rise potentially coincides with the occurrence of an unusually thick sequence of k bentonite beds representing large magnitude ash falls in the early telychian a previously documented negative 13c excursion in marine carbonates in the lower telychian interval of the ikla core is quasi synchronous with the increase in 87sr 86sr our new organic matter 13c data from the ikla core confirm that this negative 13c carbonate excursion is not a result of diagenesis furthermore a negative 13c excursion in carbonates from the early telychian portion of the pancake range section in nevada seems to confirm the global scope of this carbon cycle perturbation

high resolution late silurian correlations between gotland sweden and the broken river region ne australia lithologies conodonts and isotopes high resolution correlations partly with a precision better than 10 ka are achieved between late ludfordian sequences on gotland sweden on baltica and a section cog through the coral gardens formation along the broken river northeastern australia on gondwana despite these sections having been on different palaeocontinents facing different oceans the interval is characterised by rapid very large faunal isotopic and lithologic changes lithologies are remarkably similar in the two areas consisting of in order from below marls with thin limestone beds flaggy limestones oncoidal crinoidal limestone oncoidal marls terrigenous clastics silty clay mudstone sandstone oolite and cliff gorge forming limestones further independent correlation dates several of the lithological changes as coeval the 813 c excursion in whole rock carbonates is one of the three largest ones known during the phanerozoic the increase in delta c 13 is very similar in the two areas from below 1 parts per thousand to c 9 parts per thousand vpdb though two gotland samples yielded 9 71 parts per thousand and 10 54 parts per thousand the corresponding cog interval included fewer carbonate layers than needed to definitely exclude that this difference is due to a lack of suitable rocks for sampling compared with the lithologies and the delta c 13 curves the conodont faunas display some divergence but key taxa permit precise correlations for much of the studied interval the conodont assemblages change stepwise from very diverse in the polygnathoides siluricus zone to a low diversity fauna dominated markedly by a single taxon upper icriodontid subzone returning abruptly to a comparatively diverse ozarkodina snajdri zone fauna a method resembling graphic correlation in some respects is used to propose a similarly high resolution correlation through a longer interval for future testing c 2006 elsevier b v all rights reserved less

testing the limits of paleozoic chronostratigraphic correlation via high resolution 500 k y integrated conodont graptolite and carbon isotope δ13ccarb biochemostratigraphy across the llandovery wenlock silurian boundary is a unified phanerozoic time scale achievable the resolution and fidelity of global chronostratigraphic correlation are direct functions of the time period under consideration by virtue of deep ocean cores and astrochronology the cenozoic and mesozoic time scales carry error bars of a few thousand years k y to a few hundred k y in contrast most of the paleozoic time scale carries error bars of plus or minus a few million years m y and chronostratigraphic control better than 1 m y is considered high resolution the general lack of paleozoic abyssal sediments and paucity of orbitally tuned paleozoic data series combined with the relative incompleteness of the paleozoic stratigraphic record have proven historically to be such an obstacle to intercontinental chronostratigraphic correlation that resolving the paleozoic time scale to the level achieved during the mesozoic and cenozoic was viewed as impractical impossible or both here we utilize integrated graptolite conodont and carbonate carbon isotope delta c 13 carb data from three paleocontinents baltica avalonia and laurentia to demonstrate chronostratigraphic control for upper llandovery through middle wenlock telychiansheinwoodian similar to 436 426 ma strata with a resolution of a few hundred k y the interval surrounding the base of the wenlock series can now be correlated globally with precision approaching 100 k y but some intervals e g uppermost telychian and upper shein woodian are either yet to be studied in sufficient detail or do not show sufficient biologic speciation and or extinction or carbon isotopic features to delineate such small time slices although producing such resolution during the paleozoic presents an array of challenges unique to the era we have begun to demonstrate that erecting a paleozoic time scale comparable to that of younger eras is achievable less

late cretaceous orbitally paced carbon isotope stratigraphy from the bottaccione gorge italy abstract a refined astronomical tuning of the upper albian lower campanian is proposed from the bottaccione reference section gubbio central italy long term eccentricity cycles filtered from a high resolution 13c record were tuned to the highly stable 405 kyr cycles of the new la2010 astronomical solution for the earth s orbital elements the achieved orbital tuning provides a new precise and accurate age model for dating biostratigraphic magnetostratigraphic and carbon isotope events through a 23 myr long record cycles of 8 0 4 7 3 4 and 2 4 myr modulate the entire 13c record thus extending their detection from the cenozoic to 100 ma and represent primary and stable long term oscillation modes of earth s climate ocean system although an ultimate driver of these long term periodicities is lacking we speculate that specifically the periodicity at 4 7 myr represents a homologue of the present eccentricity grand cycles evolved by the chaotic behaviour of solar system planets during the mesozoic the long term periodicities potentially reflect an unexplored expression of the low frequency response of the carbon cycle to global biogeochemical dynamics of major nutrients particularly phosphorus associated with modulation of inputs to the ocean in turn triggered by high order marine transgressions and formation of highly productive shelf seas this very long term eccentricity control modulated by periodic low energy cycles is suggested to play a crucial role in carbon cycling controlling a chain of climate sensitive global biogeochemical processes on the earth finally these grand cycles provide a potential tool for geological correlation and provide a robust constraint for accurate calculation of the orbital evolution of the solar system

astronomical cycles of middle permian maokou formation in south china and their implications for sequence stratigraphy and paleoclimate abstract high resolution 930 samples magnetic susceptibility ms and anhysteretic remanent magnetization arm analyses were conducted on the 49 m thick middle permian maokou formation at dukou south china spectral analysis of covariant ms and arm series reveal milankovitch cycles with eccentricity obliquity and precession a 5 7 myr floating 405 kyr eccentricity cycle calibrated astronomical time scale was constructed and used as a geochronometer for estimating the durations of the conodont zones the results indicate that the durations of the jinogondolella postserrata j altudaensis j prexuanhanensis and j xuanhanensis conodont zones were 490 kyr 560 kyr 820 kyr and 750 kyr respectively the 405 kyr calibrated ms and arm series also show prominent 1 myr cyclicity which may correspond to the secular frequencies of earth and mars s 4 s 3 in the middle permian the early capitanian time interval was synchronous with the kamura cooling event and the accentuated obliquity cycling may signal ice sheet expansion the modulations of the obliquity have a close relationship with third order eustatic sequences of the wordian and capitanian stages suggesting a glacioeustatic origin during the middle permian

clay alteration associated with proterozoic unconformity type uranium deposits in the east alligator rivers uranium field northern territory australia clay minerals associated with the unconformity type uranium deposits of the east alligator rivers uranium field have been used to determine the spatial distribution of hydrothermal alteration and the conditions of uranium deposition within the altered basement underlying the kombolgie unconformity alteration develops a similar pattern at regional and smaller scales with a close relationship between the degree of alteration and the amount of structural discontinuities observed on both sides of the unconformity e g faults fractures breccias these discontinuities were mechanically active during the hydrothermal activity the hydrothermal clay minerals partly to totally replace diagenetic dickite and illite in the kombolgie sandstones and metamorphic phengite and chlorite in the basement rocks the time space alteration sequence established in this study can be summarized as follows 1 early crystallization of illite illite smectite i s mixed layer minerals lree sr bearing aluminum phosphate sulfates aps in sandstones above the unconformity illite i s mixed layer minerals sudoite aps on both sides of the unconformity and in fault gouges and illite i s mixed layer minerals chlorite apatite deeper in the basement and 2 late precipitation of chlorite in the open fracture network mainly centered below the unconformity the main stage of uranium deposition was associated with chloritization in the basement rocks smaller particle size lath shaped or flaky morphology common interstratification with expandable layers lower crystallinity and mg rich chemical compositions can be used to distinguish the hydrothermal clays from the other diagenetic and metamorphic illite and or chlorite hydrothermal illite can be distinguished from diagenetic and metamorphic illite phengite by its poor crystallinity along the c axis interstratification with minor amounts of smectite layers and a 1m polytype in which trans vacant octahedral sites predominate 1 mt chemically the hydrothermal illite differs from diagenetic illite by a lower interlayer charge and a higher mg content the hydrothermal chlorite is distinguished from the metamorphic chlorite by its smaller particle size spherulitic habit poor crystallinity along the c axis and common association with corrensite and or chlorite corrensite mixed layers most of the hydrothermal chlorite is mg rich and has a structural formula close to that of clinochlore xfe 0 20 near the uranium deposits the spherules of hydrothermal chlorite are chemically zoned from xfe 0 10 to xfe 0 40 which is interpreted as the result of a change in the oxidation state of the hydrothermal solution during the chlorite crystallization the space time alteration sequence established for clay minerals and the observed transition of aps to apatite are interpreted as the result of an increasing degree of interaction of the basement rocks with infiltrating acidic and oxidizing sub basinal brines in turn these fluids tend to be neutralized and reduced by their ongoing interaction with ferrous iron bearing phyllosilicates fe chlorite and biotite and iron sulfides of metamorphic origin the ore deposits are surrounded by zones in which the degree of fluid rock interaction was very high and chloritization is most abundant the octahedral mg content of the associated hydrothermal illite from the sandstone can be used as a guide to indicate the possible presence of uranium orebodies in the basement rocks tens to hundreds of meters below the crystal structures of the hydrothermal illite and chlorite suggest that these minerals crystallized at temperatures approximatively 30 to 50 c lower than those expected at the peak diagenesis of the kombolgie sandstones near the unconformity

shelters over the megalithic temples of malta debate design and implementation the maltese megalithic temples constructed between the mid fourth and mid third millennia bc are unique and are amongst the oldest stone buildings of such complexity in the world they are of great local and international significance embodying symbolic educational and recreational values these temples are currently suffering from a series of severe problems associated with the deterioration of materials as well as structural problems seen in a number of serious collapses in recent years in 2000 it was decided that these vulnerable prehistoric structures needed to be protected from the direct impact of environmental factors by means of a temporary open sided shelter conceived as a large parasol designed to be as light as possible in visual as well as in physical terms the erection of two of these shelters took place during 2008 2009 the performance of the shelters is currently being assessed by environmental monitoring which already indicates an improvement in conditions beneath the shelters when compared to conditions on site before sheltering

graptoloid evolutionary rates track ordovician silurian global climate change graptoloid evolutionary dynamics show a marked contrast from the ordovician to the silurian subdued extinction and origination rates during the ordovician give way during the late katian to rates that were highly volatile and of higher mean value through the silurian reflecting the significantly shorter lifespan of silurian species these patterns are revealed in high resolution rate curves derived from the conop constrained optimization scaled and calibrated global composite sequence of 2094 graptoloid species the end ordovician mass depletion was driven primarily by an elevated extinction rate which lasted for c 1 2 ma with two main spikes during the hirnantian the early silurian recovery although initiated by a peak in origination rate was maintained by a complex interplay of origination and extinction rates with both rates rising and falling sharply the global 13 c curve echoes the graptoloid evolutionary rates pattern the prominent and well known positive isotope excursions during the late ordovician and silurian lie on or close to times of sharp decline in graptoloid species richness commonly associated with extinction rate spikes the graptoloid and isotope data point to a relatively steady marine environment in the ordovician with mainly background extinction rates changing during the katian to a more volatile climatic regime that prevailed through the silurian with several sharp extinction episodes triggered by environmental crises the correlation of graptoloid species diversity with isotopic ratios was positive in the ordovician and negative in the silurian suggesting different causal linkages throughout the history of the graptoloid clade all major depletions in species richness except for one were caused by elevated extinction rate rather than decreased origination rate

cyclostratigraphy and its revolutionizing applications in the earth and planetary sciences over the past 25 yr the science of stratigraphy has evolved to include time correlative data from vastly disparate components of the earth system not least of these is the global signal afforded by cyclostratigraphy which has recorded the evolution of earth s astronomical milankovitch forcing of insolation and the paleoclimate system fossil astronomical signals are collected from cyclic sedimentary sequences by detailed sampling and study of facies geochemistry mineralogy rock magnetism color etc in step with the documentation of astronomically forced paleoclimate from ever older older geologic times innovations in computational science have provided ever longer high accuracy astronomical model targets that can be used for time scale calibration the earth s orbit is affected by motions of other planets notably the orbital perihelia of venus and jupiter which impose a dominant 405 k y eccentricity cycle on earth s orbital evolution the large mass of jupiter stabilizes this cycle over hundreds of millions of years the cyclostratigraphic record of 405 k y cycles is therefore often used to correct chronologies affected by variable sedimentation earth s shape and rotation rate are influenced by tidal dissipation and climate friction these effects affect earth s precession rate through time thus a record of earth moon evolution is also embedded in cyclostratigraphy the geochronologic value of cyclostratigraphy has been affirmed through intercalibration with high precision radioisotope dating which today has the potential to define the ages of stratigraphic horizons with 2 uncertainties at the scale of a precession cycle precession index phasing relative to that of the obliquity elucidates the seasonal nature of astronomical forcing of the paleoclimate system cyclostratigraphy contributes to our knowledge of planetary dynamics for times prior to the current ca 50 ma limit of accurate astronomical solutions and it will guide our future understanding of solar system evolution and the evidence for chaotic diffusion astronomical modeling is undergoing its own revolution with development of new numerical integrators to extend accuracy further back in time finally space exploration has revealed prominent sedimentary bedding and ice stratigraphy on the surface of mars with patterns suggestive of astronomical forcing analogous to earth

orbital forcing of the paleocene and eocene carbon cycle multimillion year proxy records across the paleocene and eocene show prominent variations on orbital time scales the cycles which have been identified at various sites across the globe preferentially concentrate spectral power at eccentricity and precessional frequencies it is evident that these cycles are an expression of changes in global climate and carbon cycling paced by astronomical forcing however little is currently known about the link between orbital forcing and the carbon cycle climate system and the amplitude of associated atmospheric co2 variations here we use simple and complex carbon cycle models to explore the basic effect of different orbital forcing schemes and noise on the carbon cycle our primary modeling target is the high resolution 7 7 myr long benthic isotope record at ocean drilling program site 1262 in the south atlantic for direct insolation forcing as opposed to artificial eccentricity tilt precession one major challenge is understanding how the system transfers spectral power from high to low frequencies we discuss feasible solutions including insolation transformations analogous to electronic ac dc conversion dc ing regarding mechanisms we focus on tropical insolation and a long term carbon imbalance in terrestrial organic burial oxidation but do not rule out other scenarios our analysis shows that high latitude mechanisms are unlikely drivers of orbitally paced changes in the late paleocene early eocene lpee earth system furthermore we provide constraints on the origin and isotopic composition of a possible lpee cyclic carbon imbalance source responding to astronomical forcing our simulations also reveal a mechanism for the large 13c eccentricity lag at the 400 kyr period observed in paleocene oligocene and miocene sections we present the first estimates of orbital scale variations in atmospheric co2 during the late paleocene and early eocene

a textural and geochemical guide to the identification of hydrothermal monazite criteria for selection of samples for dating epigenetic hydrothermal ore deposits hydrothermal monazite is one of the most reliable geochronometers for u pb dating of epigenetic or hydrothermal ore deposits because its blocking temperature at ca 700 c is higher than that of most metamorphic conditions although igneous monazite is routinely used for age dating igneous rocks only a few ore deposits have been dated by hydrothermal monazite this is due to the similar morphology of hydrothermal and igneous monazite which prevents the reliable distinction of hydrothermal monazite in particular we demonstrate that hydrothermal monazite can be distinguished from igneous monazite by its unique geochemical signature its local abundance and to common association with hydrothermal mineral assemblages our data and the data of others reported in the literature suggest that the low tho 2 content of hydrothermal monazite 0 1 wt is distinct from that of igneous monazite 3 to 5 wt and may be used to determine their genesis

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

seeing red in cyclic stratigraphy spectral noise estimation for astrochronology 1 fundamental to the development of astronomical time scales is the recognition of oscillatory variability within stratigraphic data and its evaluation relative to a null noise hypothesis in this study monte carlo simulations are used to investigate the suitability of two commonly used noise hypotheses the conventional and robust ar1 approaches and the results highlight important limitations in both for cyclostratigraphic application perhaps most problematic the robust ar1 method can result in inflated confidence level estimates and excessive clumping of false positives within the low frequency portion of the spectrum especially when the underlying noise process has a high lag 1 autocorrelation given typical cyclostratigraphic records this technique will often impose significant eccentricity band variability even in the case of pure ar1 noise an alternative spectral noise estimation method is proposed to overcome these problems which simultaneously allows for departures from the ar1 assumption and obtains high statistical power that is the ability to accurately identify astronomical signals when they are present in the data we apply the method to un tuned 18o data from miocene sediments of the ceara rise indicating statistically significant spectral power at frequencies that are consistent with the published orbital interpretation of weedon et al 1997 furthermore evaluation of the frequency arrangement of the significant spatial bedding periods using the average spectral misfit method for astrochronologic testing reveals that the null hypothesis of no orbital influence can be rejected with a high degree of confidence the 99 8 confidence level

visions of ice sheets in a greenhouse world abstract backstripped eustatic estimates from new jersey and the russian platform show large 25 m and rapid 18 o record highlights the enigma this was the warmest interval of the past 200 my yet it was bracketed by two inferred eustatic falls of 25 m that were associated with two large 0 75 deep sea 18 o increases 92 93 ma mid turonian and 96 ma mid cenomanian we reconcile records of warm high latitudes with glacioeustasy by proposing that late cretaceous early eocene ice sheets generally reached maximum volumes of 8 12 10 6 km 3 20 30 m glacioeustatic equivalent but did not reach the antarctic coast hence coastal antarctica hence deep water remained relatively warm even though there were significant changes in sea level as the result of glaciation unlike the oligocene and younger icehouse world these ice sheets only existed during short intervals of peak milankovitch insolation leaving antarctica ice free during much of the greenhouse late cretaceous to middle eocene these results highlight the need to re evaluate the paradigm that polar ice sheets did not exist during times of warm high latitude climates

early silurian glaciations in brazil abstract the opening of the brazilian paleozoic basins during the late ordovician was followed by a tectonic stabilization in the silurian at the time of the early silurian glaciations deposition of continental sandstones prevailed in the basins offshore environments were present in the amazonas basin and to a smaller extent in the parana basin the ice sheets advanced over fluviatile and littoral sediments except for the south margin of the amazonas basin where the glacial diamictites immediately overlie marine facies the ice produced much meltwater even when advancing and this was probably climatically controlled the ice movements were from the northwest at the northern margin of the amazonas basin and towards the northeast at the southern margin in the parnaiba basin and its outliers the jatoba basin and cariri valley the ice came from southeast and probably also from east further to the south in the parana basin it came from the north and northeast four glaciations have been dated to the early silurian the oldest is dated as latest ashgill and or earliest llandovery in the parana basin as evidenced from tillites in the upper rio ivai and lower vila maria formations including the former iapo formation it is contemporaneous with a glaciation in the cape basin south africa the next glaciation is dated as early middle llandovery gregarius graptolite zone and is present in the amazonas basin and the andean belt and probably also in the parnaiba basin and its outliers the two youngest tillites were widespread in the northern brazilian basins amazonas parnaiba and jatoba basins and cariri valley as well as in the andean belt and are dated as early late llandovery possibly starting already in the late middle llandovery and latest llandovery to earliest wenlock the latter tillite is the most widespread one assuming these glaciations are related to polar positions the apparent migration of the south pole ice cap must have been quite rapid from central north africa in late ashgill towards central brazil in the middle late llandovery the glaciation in the parana basin took place up to 45 from the late ashgillian south pole the cool climate continued during the silurian but no new glaciations are known before latest devonian

paradox of late paleozoic glacioeustasy models of euramerican cyclothem deposition invoke orbitally driven glacioeustasy to explain widespread cyclic marine and nonmarine late paleozoic sedimentary sequences base level fluctuations of 100 m have been estimated for the deposition of mid continent north american subpycnoclinal black shales subaerial exposure relief of algal bioherms in the sacramento mountains and russian platform carbonates similar to the pleistocene these glacioeustatic fluctuations are thought to be driven by variations in orbital parameters to evaluate this hypothesis a coupled general circulation model ice sheet model was used to simulate the effects of both transient orbital changes and variable atmospheric p co 2 concentrations on late paleozoic continental ice sheets in our model large continental ice sheet inception is simulated at and below p co 2 levels of 280 ppm model results predict that while changing orbital parameters results in dynamic ice sheet behavior the maximum orbitally induced sea level fluctuation is 25 m the model also demonstrates that the complete ablation of ice sheets formed at 280 ppm 7 9 10 7 km 3 sea level change 135 m requires an increase in atmospheric p co 2 to levels 2240 ppm these results present a potential paradox while our model is able to simulate widespread gondwanan glaciation it is unable to reproduce significant orbitally driven glacioeustatic fluctuations without very large magnitude carbon cycle perturbations we discuss possible solutions to this paradox

a chronology of paleozoic sea level changes sea levels have been determined for most of the paleozoic era 542 to 251 million years ago but an integrated history of sea levels has remained unrealized we reconstructed a history of sea level fluctuations for the entire paleozoic by using stratigraphic sections from pericratonic and cratonic basins evaluation of the timing and amplitude of individual sea level events reveals that the magnitude of change is the most problematic to estimate accurately the long term sea level shows a gradual rise through the cambrian reaching a zenith in the late ordovician then a short lived but prominent withdrawal in response to hirnantian glaciation subsequent but decreasingly substantial eustatic highs occurred in the mid silurian near the middle late devonian boundary and in the latest carboniferous eustatic lows are recorded in the early devonian near the mississippian pennsylvanian boundary and in the late permian one hundred and seventy two eustatic events are documented for the paleozoic varying in magnitude from a few tens of meters to 125 meters

astronomical cycle origin of bedded chert a middle triassic bedded chert sequence inuyama japan abstract astronomical forcing is one of the main drivers of climate change and astronomical cyclicity recorded in sediments provides a clue to understand the dynamics of the global climate system bedded cherts consist of rhythmic alternations of chert and shale beds although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles e g fischer 1976 hori et al 1993 conclusive proof remains elusive to explore this possibility we established a continuous high resolution lithostratigraphy of middle triassic bedded chert in central japan the average duration of each chert shale couplet is 20 kyr similar to that of the precession cycle spectral analysis of a bed number series of thickness variations in chert beds was performed assuming that each chert shale couplet represents a 20 kyr precession cycle the results reveal cycles involving approximately 200 20 5 and 2 3 beds corresponding to periodicities of approximately 4000 400 100 and 40 60 kyr respectively by further assuming that the 20 bed cycle represents a 405 kyr eccentricity cycle of constant and stable periodicity we converted the bed number series to a time series spectral analysis of the time series revealed distinct periodicities of 3600 117 97 and 38 kyr in addition to 405 kyr besides 3600 kyr these periodicities agree well with the 120 95 and 37 kyr periodicities for eccentricity cycles and the obliquity cycle during the triassic moreover we detected amplitude modulation of the approximately 100 kyr cycle of thickness variations in chert beds with a 405 kyr periodicity which may correspond to amplitude modulation of 100 kyr climatic precession cycle with the 405 kyr periodicity the approximately 3600 kyr periodicity described above and 1800 kyr periodicity manifested as the amplitude modulation of the 405 kyr cycle are correlated to present day long term eccentricity cycles of 2400 and 4800 kyr evolved by chaotic behavior of solar planets collectively these similarities in the periodicities of dominant cycles their hierarchy and the nature of amplitude modulation of cycles in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles thereby indicating the potential of bedded chert as a template for a mesozoic cyclostratigraphy

paleoenvironmental changes in the silurian indicated by stable isotopes in brachiopod shells from gotland sweden ratios of stable carbon and oxygen isotopes in brachiopod shells more than 370 specimens esp atrypa reticularis from the silurian of gotland sweden have been analysed preservation of biological skeletal ultrastructures observed in sem micrographs and cathodoluminescence analyses indicate that usually no diagenetical alteration occurs the silurian of gotland consists of 440 m carbonate deposits spanning the late llandovery to late ludlow epochs 431 411 m y repeatedly uniform sequences of micritic limestones and marks are interrupted by complex structured reefs and adjacent platform sediments previously the alteration of facies is interpreted as the result of sea level fluctuations caused by a gradual regression with superimposed minor transgressive pulses the silurian sequence of gotland exhibits principally parallel carbon and oxygen isotope records corresponding closely to the topostratigraphic units lower values occur in periods dominated by deposition of marly sequences higher values are observed in periods dominated by reefs and extended carbonate platforms the isotope ratios are influenced by local as well as global factors the oxygen isotope ratios are interpreted to reflect paleosalinity changes due to varying freshwater input rather than changes in paleotemperature consequently the facies distribution of gotland is interpreted as resulting from changes in terrigenous input caused by different rates of continental weathering and freshwater runoff rather than by sea level fluctuations periods of arid climate and therefore anti estuarine downwelling of oxygenated surface water appear as short episodes of reef growing 1 5 m y in an epoche characterized by a tropic humid climate which causes an estuarine circulation and the upwelling of co2 rich deep water carbon isotope ratios are obviously connected to these changes in circulation by the advection of 13c rich surface water arid episodes or upwelling of 13c depleted deep water humid episodes of a silurian ocean which itself reveals generally euxinic deep water conditions due to the burial of organic carbon in black shales

did changes in atmospheric co2 coincide with latest ordovician glacial interglacial cycles article i nfo the late ordovician hirnantian stage 444 million years ago was one of three time periods during the past half billion years in which large continental glaciers formed over earth s polar regions the effects of this glaciation were far reaching and coincided with one of the largest marine mass extinction events in earth history the cause of this ice age is uncertain and a paradoxical association with evidence for high atmospheric co2 levels has been debated precise linkages between sea level ice volume and carbon isotope 13 ccarb and 13 corg proxy records of pco2 have been poorly understood due in part to uncertainties in stratigraphic correlation and the interpretation of globally important sections although correlation difficulties remain recent hirnantian biostratigraphic studies now allow for improved correlations here we show that consistent trends in both 13 ccarb and 13 corg from two well dated stratigraphic sequences in estonia and anticosti island canada coincide with changes in late ordovician hirnantian climate as inferred from sea level and the extent of ice sheets the integrated datasets are consistent with increasing pco2 levels in response to ice sheet expansion that reduced silicate weathering ultimately the time period of elevated pco2 levels is followed by geologic evidence of deglaciation

hydrothermal alteration of monazite in the precambrian crystalline basement of the athabasca basin saskatchewan canada implications for the formation of unconformity related uranium deposits microanalytical studies of basement rocks below the athabasca sandstone basin indicate that monazite is the dominant uranium bearing mineral in the study area drill core samples of hydrothermally altered basement show that monazite is commonly altered to a th silicate phase and uranium has been significantly mobilized on average 75 of the uranium bound to monazite is leached out during monazite alteration in contrast no substantial mobilization of uranium from detrital minerals e g zircon has yet been observed in the athabasca sandstones it is suggested that hydrothermal alteration of granitic rocks especially potassic pegmatoids and potassic orthogneisses of the sub athabasca basement represents the most important uranium source for the unconformity type deposits

carbon and oxygen isotopic composition of silurian brachiopods gotland sweden palaeoceanographic implications abstract one hundred and seventy two non luminescent brachiopod shells from the silurian sequence of gotland were investigated for their carbon and oxygen isotopic composition diagenetic alteration of the shell calcite was excluded by means of petrographic trace element and cathodoluminescence analysis oxygen and carbon isotopic signals vary considerably within the uppermost llandovery to the ludlow average 18o and 13c values of well defined stratigraphic units range from 2 4 to 5 9 and 1 to 8 respectively we observed parallel trends in the carbon and oxygen isotope curves with the most prominent shifts occurring in the lower wenlock upper wenlock and middle ludlow average 18o values 2 4 to 2 9 recorded in the middle ludlow are among the highest 18o values reported for the early palaeozoic the recorded 18o variations cannot solely be explained by changes in sea surface temperature and salinity the variations in the isotopic signals correlate negatively with changes in sea level heavy carbon and oxygen signals coincide with sea level lowstands whereas light signals are recorded during sea level highstands sea level highstands were probably characterized by the formation of warm saline waters on subtropical epicontinental shelves inducing oxygen deficiency in the basins alumshale facies the low 18o values are attributed to 18o depletion in surface waters owing to the storage of 18o enriched saline waters in the basins low 13c values are explained by a low surface water productivity since nutrients were not supplied to the photic zone but rather stored in the deeper ocean during sea level lowstands colder waters that formed in higher latitudes may have represented the main intermediate water source a better ventilation of the basins is documented by the deposition of well aerated sediments whereas the high 18o values are explained by the change in oceanic circulation the high 13c values are attributed to an enhanced primary productivity and possibly to an enhanced burial of corg the model discussed here may provide an attractive scenario for accounting for the recorded short term variations in the isotopic signals without the necessity of introducing an isotopic correction factor for silurian sea water

a 70 million year astronomical time scale for the deep sea bedded chert sequence inuyama japan implications for triassic jurassic geochronology abstract the astronomical time scale ats has provided high resolution geochronology however the early mesozoic ats is still under construction partly due to the lack of continuous pelagic sequences of the early mesozoic here we present ca 70 myr long ats constructed from the early mesozoic deep sea bedded chert sequence exposed in the inuyama area central japan the sedimentary rhythms of bedded chert display a full range of climatic precession related cycles 20 kyr cycle as a chert shale couplet and 100 405 2000 to 4000 and 10 000 kyr cycles as chert bed thickness variation the newly established ats inuyama ats is tuned by 405 kyr eccentricity cycle and is anchored at the end triassic radiolarian extinction level as 201 4 0 2 ma this inuyama ats gives ages consistent with the radiometric ages projected to the inuyama deep sea sequence using biostratigraphy and carbon isotope stratigraphy the inuyama ats provides the age constraints for the triassic and jurassic stage boundaries which support the long norian option of muttoni et al 2004 because the deep sea bedded chert sequence covers a long time interval before the cretaceous the ats for the bedded chert will serve as a template for the astrochronology of mesozoic and older ages

testing ore deposit models using in situ u pb geochronology of hydrothermal monazite paleoproterozoic gold mineralization in northern australia the inability to establish absolute ages for gold deposition in the pine creek orogen of northern australia has led to conflicting ore deposit models ranging from intrusion related which predict that gold mineralization was synchronous with granite magmatism ca 1835 1820 ma to orogenic which place ore deposition nearly 100 m y later here we present ion microprobe u pb geochronology for a mineralized quartz reef from tom s gully mine mount bundey northern territory australia and nearby granitic rocks and associated contact aureoles isotopic dating of zircon and monazite indicates that intrusion and contact metamorphism occurred ca 1825 ma whereas hydrothermal monazite from the auriferous quartz reef gives a mean 207pb 206pb age of 1780 10 ma interpreted as the time of gold mineralization mineralization therefore postdated intrusion by 45 m y and preceded a postulated ca 1740 1730 ma cratonwide orogenic gold event by 50 m y hence neither the intrusion related model nor the recently proposed orogenic model is applicable combined with a reevaluation of age data from the nearby goodall gold deposit our data suggest that mineralization coincides with and may be related to an episode of regional low grade metamorphism deformation and fluid circulation shoobridge event our results demonstrate the importance of high precision in situ geochronology and detailed petrography for deciphering age relationships in ore deposits and of testing the veracity of models for ore formation

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

carbon isotope stratigraphy of lochkovian to eifelian limestones from the devonian of central and southern europe lower to middle devonian carbonates of the prague syncline the carnic alps the montagne noire and the cantabrian mountains were investigated for 13ccarb and 13corg these values were measured on bulk rocks selected components and cements many carbonates exhibit primary marine values but some are altered by diagenesis a 13c curve can be presented for the latest pridolian to emsian time interval several sharp or broad positive excursions are obvious in the woschmidti postwoschmidti sulcatus kitabicus late serotinus and kockelianus conodont zones the excursion at the silurian devonian boundary is known worldwide and therefore considered global in nature some of the others are described for the first time from central and southern europe and their global nature has to be verified by further investigations in other regions most excursions relate to and or started during major regressions whereas sea level highstands correspond to minimal 13c values similar relationships between sea level changes and 13c have been observed from other early palaeozoic intervals the transgressive chotec and kacak events are marked by positive isotope excursions this type of combination is usually observed in late palaeozoic to cenozoic black shale events

age and paragenesis of mineralisation at coronation hill uranium deposit northern territory australia coronation hill is a u au platinum group elements deposit in the south alligator valley sav field in northern australia south of the better known unconformity style u east alligator rivers ear field the sav field differs from the ear by having a more complex basin basement architecture a volcanically active fault trough jawoyn sub basin developed on older basement and then was disrupted by renewed faulting before being buried beneath regional mcarthur basin sandstones that are also the main hanging wall to the ear deposits primary mineralisation at coronation hill formed at 1607 26 ma rather than 600 900 ma as previously thought and so it is likely that the sav was part of a single west mcarthur basin dilational event most ore is hosted in sub vertical faults and breccias in the competent volcanic cover sequence this favoured fluid mixing acid buffering forming illite and oxidation of fe2 and reduced c rich assemblages as important uranium depositional mechanisms however reduction of u in fractured older pyrite pb model age of 1833 67 ma is an important trap in diorite some primary ore was remobilised at 675 21 ma to form coarse uraninite ni co pyrite networks containing radiogenic pb coronation hill is polymetallic and in this respect resembles the egress style u deposits in the athabascan basin canada however these are all cover hosted a hypothesis for further testing is that coronation hill is also egress style with ores formed by fluids rising through basement hosted fault networks u reduction by diorite pyrite and carbonaceous shale and into veins and breccias in the overlying jawoyn sub basin volcano sedimentary succession

ordovician and silurian sea water chemistry sea level and climate a synopsis following the cambrian explosion and the appearance in the fossil record of most animal phyla associated with a range of new body plans the ordovician and silurian periods witnessed three subsequent major biotic events the great ordovician biodiversification event the end ordovician extinction the first animal extinction and second largest of the five mass extinctions of the phanerozoic and the early silurian postextinction recovery there are currently no simple explanations for these three major events combined extrinsic geological and intrinsic biological factors probably drove the biodiversifications and radiations and the appearance and disappearance of marine habitats have to be analysed in the frame of changing palaeogeography palaeoclimate and sea water chemistry the present paper reviews the relationships of the three biotic events to chemical and physical processes occurring in the ocean and atmosphere during the ordovician and silurian including sea level changes geochemical proxies 13c 18o 87sr 86sr of the ocean waters and the evolution of the atmosphere oxygen and carbon dioxide content less

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

a 9 myr cycle in cenozoic δ13c record and long term orbital eccentricity modulation is there a link abstract the 65 myr long cenozoic carbon isotope record 13 c of zachos et al 2001 2008 documents a strong long term cycle with a mean pseudoperiodicity close to 9 myr this cyclicity modulates the 2 4 myr eccentricity cycle amplitude hinting at a possible link between long term astronomical and geological variations some phase shifts between 9 myr 13 c and astronomical cycles suggest that additional processes e g tectonics contribute to these long term carbon cycle variations the strong response of 13 c to long term eccentricity periods 9 myr 2 4 myr 400 kyr supports the hypothesis that the long time residence of carbon in the oceans amplifies lower frequency or dampens higher frequency orbital variations additionally the strong expression of low amplitude 9 myr eccentricity cycle in the 13 c record could be explained by energy transfer process from higher to lower frequency cycles and all eccentricity components modulate the carrier climatic precession cycles finally the paleocene eocene thermal maximum petm 55 9 ma event which corresponds to a pronounced 13 c negative excursion is situated within a strong decrease in the most prominent 9 myr 13 c cycle hinting at a link between accelerated rates in 13 c variations and the petm this specific 9 myr 13 c cycle seems to be amplified by non orbital mechanisms in atmosphere continent ocean system such as previously suggested methane release from gas hydrate and volcanism

glacial onset predated late ordovician climate cooling the ordovician glaciation represents the acme of one of only three major icehouse periods in earth s phanerozoic history and is notorious for setting the scene for one of the big five mass extinction events nevertheless the mechanisms that drove ice sheet growth remain poorly understood and the final extent of the ice sheet crudely constrained here using an earth system model with an innovative coupling method between ocean atmosphere and land ice accounting for climate and ice sheet feedback processes we report simulations portraying for the first time the detailed evolution of the ordovician ice sheet we show that the emergence of the ice sheet happened in two discrete phases in a counterintuitive sequence of events the continental ice sheet appeared suddenly in a warm climate only during the second act and set against a background of decreasing atmospheric co2 followed steeply dropping temperatures and extending sea ice the comparison with abundant sedimentological geochemical and micropaleontological data suggests that glacial onset may have occurred as early as the middle ordovician darriwilian in agreement with recent studies reporting third order glacioeustatic cycles during the same period the second step in ice sheet growth typified by a sudden drop in tropical sea surface temperatures by approximate to 8 degrees c and the further extension of a single continental scale ice sheet over gondwana marked the onset of the hirnantian glacial maximum by suggesting the presence of an ice sheet over gondwana throughout most of the middle and late ordovician our models embrace the emerging paradigm of an early paleozoic ice age

the ireviken event in the lower silurian of gotland sweden relation to similar palaeozoic and proterozoic events abstract for a long time the silurian was thought to represent a time of stable environmental conditions in the greenhouse period that followed the late ordovician glaciation during the past decade knowledge about the silurian has increased markedly and today it is known that the conditions in the silurian were much more variable than previously assumed detailed isotopic investigations have revealed several distinct positive excursions in both carbon and oxygen isotope values in low latitudes these periods of high c and o isotope values are in many cases characterised by the growth of reefs and the formation of extended carbonate platforms the sediments deposited during these excursions contain impoverished fossil assemblages especially with respect to conodonts graptolites and trilobites a conspicuous isotope excursion coincident with facies changes and a marked mass extinction is observed near the llandovery wenlock boundary this event is called the ireviken event after its type locality on the island of gotland sweden jeppsson l 1987 in palaeobiology of conodonts ellis horwood ltd chichester pp 129 145 here isotope data from nine sections at the nw coast of gotland are presented that cover the time interval of the ireviken event the 13c mean values rise from 1 4 to 4 5 and the 18o values increase from 5 6 to 5 0 the relative timing of stable isotope development extinctions and facies development is discussed it is shown that first extinctions precede the isotope excursion this indicates that extinction events and stable isotope development are only indirectly connected but might reflect the same causes other events characterised by similar relationships between positive isotope excursions mass extinctions and facies development are found in younger parts of the silurian late wenlock and ludlow in the late ordovician the late cambrian and with some reservations in the proterozoic the similarities between these events indicate analogous controlling mechanisms for the silurian climatic changes between humid and arid conditions in low latitudes were postulated in an earlier study of the authors a palaeoceanographic climatic model was postulated which is consistent with most of the sedimentological palaeontological and geochemical data here we demonstrate that this model may be applicable also for the older events