re os dating of pyrite confirms an early diagenetic onset and extended duration of mineralization in the irish zn pb ore field the irish midlands region contains one of the world s largest hydrothermal zn pb ore districts but uncertainty exists in the timing of mineralization relative to host rock ages consequently genetic models for ore formation are poorly constrained and remain controversial here we use re os geochronology to show that ore stage pyrite from the lisheen deposit formed at 346 6 3 0 ma shortly after host rock deposition pyrite from the silvermines deposit returns an age of 334 0 6 1 ma indicating that at least some mineralization occurred during later burial these age determinations show that the much younger paleomagnetic ages reported for the irish zn pb deposits reflect remagnetization during the variscan orogeny a process that we suggest affects paleomagnetic dating more widely the re os ages overlap with the ages of lower carboniferous volcanic rocks in the midlands which are the product of magmatism that has been invoked as the driving force for hydrothermal activity the relatively low initial os ratios for both lisheen 0 253 0 045 and silvermines 0 453 0 006 are compatible with derivation of os from these magmas or from the caledonian basement that underlies the ore deposits

tectonic setting of synorogenic gold deposits of the pacific rim abstract more than 420 million oz of gold were concentrated in circum pacific synorogenic quartz lodes mainly during two periods of continental growth one along the gondwanan margin in the palaeozoic and the other in the northern pacific basin between 170 and 50 ma these ores have many features in common and can be grouped into a single type of lode gold deposit widespread throughout clastic sedimentary rock dominant terranes the auriferous veins contain only a few percent sulphide minerals have gold silver ratios typically greater than 1 1 show a distinct association with medium grade metamorphic rocks and may be associated with large scale fault zones ore fluids are consistently of low salinity and are co2 rich in the early and middle palaeozoic in the southern pacific basin a single immense turbidite sequence was added to the eastern margin of gondwanaland deformation of these rocks in southeastern australia was accompanied by deposition of at least 80 million oz of gold in the victorian sector of the lachlan fold belt mainly during the middle and late devonian lesser devonian gold accumulations characterized the more northerly parts of the gondwanan margin within the hodgkinson broken river and thomson fold belts additional lodes were emplaced in this flyschoid sequence in devonian or earlier palaeozoic times in what is now the buller terrane westland new zealand minor post devonian growth of gondwanaland included terrane collision and formation of gold bearing veins in the permian in australia s new england fold belt and in the jurassic early cretaceous in new zealand s otago schists collision and accretion of dozens of terranes for a 100 m y long period against the western margin of north america and eastern margin of eurasia led to widespread latest jurassic to eocene gold veining in the northern pacific basin in the former location late jurassic and early cretaceous veins and related placer deposits along the western margin of the sierra nevada batholith have yielded more than 100 million oz of gold additional significant ore forming events during the development of north america s cordilleran orogen included those in the klamath mountains region california in the late jurassic and early cretaceous the klondike district yukon by the early cretaceous the nome and fairbanks districts alaska and the bridge river district british columbia in the middle cretaceous and the juneau gold belt alaska in the eocene gold bearing veins deposited during the late jurassic and early cretaceous terrane collision that formed the present day russian far east have been the source for more than 130 million oz of placer gold the abundance of gold bearing quartz carbonate veins throughout the gondwanan north american and eurasian continental margins suggests the migration and concentration of large fluid volumes during continental growth such volumes could be released during orogenic heating of hydrous silicate mineral phases within accreted marine strata the common temporal association between gold veining and magmatism around the pacific rim reflects these thermal episodes melting of the lower thickened crust during arc formation slab rollback and extensional tectonism and subduction of a slab window beneath the seaward part of the forearc region can all provide the required heat for initiation of the ore forming processes

textural and stable isotopic evidence for the genesis of the tynagh base metal deposit ireland at the tynagh mine county galway four major stages of mineralization can be recognized that span the diagenetic and postlithification history of the host waulsortian limestones 1 growth of colloform and granular pyrite clots during the early diagenesis of the micritic host 2 rapid geopetal precipitation of microcrystalline sphalerite galena barite and dolomite within a dilatant fracture system that developed as a consequence of the forceful injection of metalliferous fluids into the coherent waulsortian carbonate mudbanks adjacent to the tynagh fault 3 veining and replacement of the host and previously precipitated phases by tennantite chalcopyrite bornite arsenopyrite medium to coarsely crystalline galena and coarse sparry barite and 4 finally precipitation of calcite within veinlets and cavities and dolomitization of large bodies of previously unmineralized waulsortian limestones sulfur isotope ratios indicate that contemporaneous lower carboniferous seawater acted as the source of sulfate for precipitation of barite delta 34 s 21 1 to 17 4ppm stage 1 delta 34 s 3 1 to 15 5ppm and stage 2 delta 34 s 4 1 to 26 0ppm sulfides were precipitated by reduced sulfur generated during open system bacterial reduction of this sea water sulfate source isotopically heavier delta 34 s or 0ppm reduced sulfur of deep seated origin was introduced from the fault zone during stage 3 this mixed with bacteriogenic sulfur migrating into the waulsortian bank limestones to precipitate stage 3 sulfides and sulfosalts delta 34 s 11 1 to 23 0ppm mineralized limestones delta 13 c 2 1 to 4 0ppm delta 18 o 17 6 to 27 1ppm ore stage carbonates delta 13 c 2 8 to 3 4ppm delta 18 o 17 1 to 26 9ppm and postore carbonates delta 13 c 4 0 to 2 6ppm delta 18 o 15 9 to 25 8ppm are depleted in the heavier isotopes of carbon and oxygen with respect to unmineralized limestones delta 13 c 3 1 to 4 1ppm delta 18 o 22 2 to 28 1ppm the depletion in 13 c is attributed to the mixing of host derived carbon with bacterially oxidized carbon and during stage 4 deep seated carbon the 18 o depletion of the mineralized host is a consequence of isotopic exchange with the ore fluids an oxygen isotope fractionation temperature of 200 degrees c and a delta 18 o smow value of 10 5 per mil for the coexisting water is obtained from a single stage 3 quartz albite pair collected from the fault zone if these values are accepted the delta 18 o values of stage 3 carbonates can only be interpreted in terms of mixing of this isotopically heavy water introduced from the fault with isotopically lighter water already present or migrating into the bank limestones

zn pb mineralisation in the silvermines district ireland a product of burial diagenesis carbonate sulphide cement stratigraphic relationships in the host rock and ore have been used to constrain the age of mineralisation at the silvermines zinc lead barium deposit the base metal sulphides post date planar dolomite and replace stylolites furthermore the pre mineralisation planar dolomites also replace stylolites these and other diagenetic observations indicate that the base metal sulphides formed at burial depths greater than 800 m but probably predate the variscan deformation since pressure shadows overgrow base metal sulphides this indicates that the sulphides are of epigenetic origin constraining the age of mineralisation to between the late chadian 347 ma and the late westphalian 307 ma however the most likely age for mineralisation based on widespread macro stylolite development is asbian 339 ma or younger no evidence of synsedimentary sulphides in the form of hydrothermal chimneys vent faunas or sulphides intergrown with marine cements was observed at silvermines mineralised breccias black matrix breccias late stage internal sediments and dissolution zones within the carbonate cements all appear to be produced by hydrothermal karsting that occurred during the mineralisation process fluid inclusion homogenisation temperatures for ore stage calcites up to 300 c approach the peak temperature estimates derived from regional maturation parameters 270 to 310 c from conodont alteration indices and vitrinite reflectance this suggests that homogenisation temperatures represent maximum heating temperatures probably during variscan time rather than mineralisation temperatures

magnetostratigraphical dating of loess deposits in china the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene the new palaeomagnetic measurements reported here on a bore hole section near lochuan shaanxi province suggest a clearly defined magnetic polarity zonation which extends below the olduvai subchron a late pliocene age of 2 4 myr is assigned to the oldest loess sediments measured the intensity variations of natural remanent magnetization nrm and low field magnetic susceptibility are indicative of climatic changes during loess deposition

lewis ponds a hybrid carbonate and volcanic hosted polymetallic massive sulphide deposit new south wales australia the lewis ponds zn pb cu ag au deposit located in the eastern lachlan fold belt central western new south wales exhibits the characteristics of both volcanic hosted massive sulphide and carbonate hosted replacement deposits two stratabound massive to disseminated sulphide zones main and toms occur in a tightly folded upper silurian sequence of marine felsic volcanic and sedimentary rocks they have a combined indicated resource of 5 7 mt grading 3 5 zn 2 0 pb 0 19 cu 97 g t ag and 1 9 g t au main zone is hosted by a thick unit of poorly sorted mixed provenance breccia limestone clast breccia and quartz crystal rich sandstone whereas toms zone occurs in the overlying siltstone pretectonic carbonate chalcopyrite pyrite and quartz pyrite stringer veins occur in the footwall porphyritic dacite south of toms zone strongly sheared dolomite chalcopyrite pyrrhotite veins directly underlie the toms massive sulphide lens the mineralized zones consist predominantly of pyrite sphalerite and galena paragenetically early framboidal dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host main zone but are rarely preserved in the annealed massive sulphide in toms zone main and toms zones are associated with a semi conformable hydrothermal alteration envelope characterized by texturally destructive chlorite dolomite and quartz rich assemblages dolomite chlorite quartz calcite and sulphides have selectively replaced breccia and sandstone beds in the main zone host sequence whereas the underlying porphyritic dacite is weakly sericite altered vuggy and botryoidal textures resulted from partial dissolution of the dolomite altered sedimentary rocks and unimpeded growth of base metal sulphides carbonate and quartz into open cavities the intense chlorite rich alteration assemblage underlying toms zone grades outward into a weak pervasive sericite quartz assemblage with distance from the massive sulphide lens limestone clasts and hydrothermal dolomite at lewis ponds are enriched in light carbon and oxygen isotopes the dolomite yielded 13cvpdb values of 11 to 1 and 18ovsmow values of 6 to 16 liquid vapour fluid inclusions in the dolomite have low salinities 1 4 7 7 equiv wt nacl and homogenization temperatures 166 232 c for 1 000 m water depth dolomitization probably involved fluid mixing or fluid rock interactions between evolved heated seawater and the limestone bearing facies prior to and during mineralization 34svcdt values range from 2 0 to 5 0 in the massive sulphide and 3 9 to 7 4 in the footwall carbonate chalcopyrite pyrite stringer veins indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater the sulphide mineral assemblages at lewis ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization low temperature dolomitization of limestone bearing facies in the main zone host sequence created secondary porosity and provided a reactive host for fluid rock interactions main zone formed by lateral fluid flow and sub seafloor replacement of the poorly sorted breccia and sandstone beds base metal sulphide deposition probably resulted from dissolution of dolomite fluid mixing and increased fluid ph pyrite sphalerite and galena precipitated from a relatively low temperature 150 250 c hydrothermal fluid in contrast toms zone was emplaced into fine grained sediment at or near the seafloor above a zone of focused up flowing hydrothermal fluids copper rich assemblages were deposited in the toms zone footwall and massive sulphide lenses in main and toms zones as the hydrothermal system intensified during the d1 deformation fracture controlled fluids within the lewis ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins lewis ponds is a rare example of a synvolcanic sub seafloor hydrothermal system developed within fossiliferous limestone bearing facies the close spatial association between limestone hydrothermal dolomite massive sulphide and dacite provides a basis for new exploration targets elsewhere in new south wales

the physics of wind blown loess implications for grain size proxy interpretations in quaternary paleoclimate studies abstract loess deposits are recorders of aeolian activity during past glaciations since the size distribution of loess deposits depends on distance to the dust source and environmental conditions at the source during transport and at deposition loess particle size distributions and derived statistical measures are widely used proxies in quaternary paleoenvironmental studies however the interpretation of these proxies often only considers dust transport processes to move beyond such overly simplistic proxy interpretations and toward proxy interpretations that consider the range of environmental processes that determine loess particle size distribution variations we provide a comprehensive review on the physics of dust particle mobilization and deposition furthermore using high resolution bulk loess and quartz grain size datasets from a last glacial interglacial sequence we show that because grain size distributions are affected by multiple often stochastic processes changes in these distributions over time allow multiple interpretations for the driving processes consequently simplistic interpretations of proxy variations in terms of only one factor e g wind speed are likely to be inaccurate nonetheless using loess proxies to understand temporal changes in the dust cycle and environmental parameters requires i a careful site selection to minimize the effects of topography and source distance and ii the joint use of bulk and quartz grain size proxies together with high resolution mass accumulation rate calculations if possible

the maryport fault the post caledonian tectonic history of southern britain in microcosm the maryport fault forms part of the faulted southern margin of the northumberland solway basin seismic reflection data reveal a complex history of repeated fault reactivation large oblique normal displacements on the fault controlled early carboniferous extensional basin development subsequently variscan basin inversion was accompanied by partial reversal of the fault in permian and mesozoic times renewed oblique normal displacements strongly influenced basin subsidence further partial reverse displacement occurred as the solway basin was again inverted probably in mid tertiary times in response to alpine compression the maryport fault is most unusual therefore in providing direct evidence of the principal post caledonian tectonic events in southern britain

on diagenesis dolomitisation and mineralisation in the irish zn pb orefield marine calcite cementation and lithification of carboniferous carbonate sediments hosting zn pb mineralisation in the irish orefield occurred at or near the seafloor a relatively early fine grained grey replacive dolomite preferentially developed in micrite is widely developed in the waulsortian limestone formation the main host to mineralisation and is pervasive in the southeastern midlands in proximity to the leinster massif this dolomite formed after the first four main stages of calcite cementation but probably also developed within tens of metres of the seafloor as evidenced by incorporation of clasts of dolomite in intraformational sedimentary breccias later coarse grained white dolomite preferentially replaced coarser components of the waulsortian limestone and infilled residual vuggy porosity whilst some of this coarse dolomite may be related to the fine replacive dolomite event a common spatial association with fault zones coupled with primary fluid inclusion data suggest that a significant proportion of this phase precipitated during the onset of fault controlled subsidence and widespread hydrothermal circulation within the irish midlands area fluids up to 250 c and 10 15 wt nacl equivalent sourced from a lower palaeozoic basement equilibrated fluid reservoir infiltrated the carbonate sequence via faults and fractures the more localised development of dolomite cemented breccias white matrix breccias that are frequently associated spatially with mineralisation was a consequence of the increased focusing of these hydrothermal fluids ore formation was broadly synchronous with development of the white dolomite breccias but only happened where mixing occurred between the hydrothermal ore fluids and localised near surface reservoirs of low temperature h2s rich brine in the waulsortian this process led to the precipitation of a distinctive black dolomite that forms a broad halo to massive sulphides although ore stage sulphides postdate significant diagenesis of the host rocks and often display epigenetic textures the fact that much of the cementation occurred soon after carbonate deposition means that mineralisation does not have to have formed after significant burial in fact the occurrence of clasts of hydrothermal dolomite and sulphides in intraformational debris flow breccias is only consistent with mineralising processes occurring in the near seafloor environment relatively soon after host rock deposition the regional development of a distinctive pink dolomite associated with faults and fractures was a post ore event and is considered to mark a regional brine migration linked to the onset of the variscan orogeny the development of this new tectonic and flow regime may have been responsible for the cessation of economic mineralisation in ireland

magnetism of loess palaeosol sequences recent developments abstract investigation of the global loess record primarily as a palaeoclimatic archive continues unabated one important aspect of this research effort is the determination of the magnetic properties of the loess itself and of the palaeosols developed within it in earlier work these have provided a record of major long term past climatic changes however recent studies are now revealing sub milankovitch signals with teleconnections from the chinese loess plateau clp to the greenland and antarctic ice caps as well as to climate related events in sediments recovered from the north atlantic these high resolution profiles cover to the last 130 000 years however progress is also being made in extending the record down below the loess sensu stricto of the clp into the so called red clay so that the aeolian record can now be carried back beyond 7 ma bp several authors have attempted to obtain quantitative estimates of palaeoprecipitation by comparing magnetic susceptibility of palaeosols with values for modern soils although the broad picture is qualitatively clear wetter interglacials dryer glacials the actual quantitative values provided by the various techniques differ significantly in the classic chinese sites palaeosols are always magnetically enhanced due to in situ pedogenic processes however evidence is now emerging to indicate that this is not the only magnetoclimatological model in cases such as siberia where wind vigour dominates over pedogenesis the opposite pattern is found with magnetic susceptibility minima in palaeosols this also occurs in situations where the sediments are water logged for significant amounts of time leading to gleying and concomitant loss of magnetic minerals the use of magnetic remanence as a geomagnetic polarity clock requires caution early interpretations were acceptable for establishing broad chronological control however recent scrutiny suggests that remanence lock in may be delayed by as much as 20 000 years furthermore synthetic magnetostratigraphies modelled on the basis of realistic sediment characteristics indicate that there are limits to the amount of geomagnetic detail likely to be resolved this seems to be the case for the blake event which manifests itself in a very variable way at different sites in the clp

quantitative estimates of pedogenic ferromagnetic mineral formation in chinese loess and palaeoclimatic implications abstract the quasi continuously deposited loess sediments of the central chinese loess plateau constitute one of the most complete and long lasting records of quaternary palaeoclimate on land much of this record has been uncovered by measurement and interpretation of the magnetic properties of the loess the polarity of natural remanent magnetization has provided absolute age dates by correlation with the geomagnetic polarity time scale magnetic low field susceptibility has been used to reconstruct the cyclic changes of pleistocene palaeoclimates although the origin of the signal is still poorly understood from comparison with the 10 be concentration throughout the sequence we are able to determine the contributions to susceptibility from inherited and authigenic sources for the last 130 ky since the production of authigenic magnetite is directly related to regional precipitation annual palaeoprecipitation rates can be calculated palaeoprecipitation rates similar to present day rates are derived for oxygen isotope stage 5 including the last interglacial in contrast cold periods corresponding to oxygen isotope stages 2 and 4 experienced negligible precipitation the beginning and end of the time interval represented by oxygen isotope stage 3 however was characterized by short episodes when precipitation exceeded the holocene average

partly pedogenic origin of magnetic variations in chinese loess quaternary sequences of interbedded aeolian silts loess and buried soils palaeosols potentially provide one of the best terrestrial records of past climates1 5 magnetic susceptibility variations in loess and palaeosol sequences from china are strongly correlated with climate induced fluctuations of oxygen isotope ratios in deep sea sediments2 4 6 8 as an explanation of this correlation it has been suggested that the recorded variations in magnetic susceptibility depend primarily on the degree of dilution by non magnetic bulk loess constituents of a uniform magnetic assemblage derived from remote but unknown sources6 8 it has also been suggested that such a model involving changes in the accumulation rate of loess superimposed on a constant flux of magnetic particles provides a relative geological timescale through the control of loess deposition by astronomically modulated climate fluctuations7 here we report a preliminary test of this model using rock magnetic properties especially sensitive to variations in magnetic grain size our results show that palaeosols are characterized by much finer magnetic grain size assemblages than are the intervening loess units this suggests that a simple model based on constant magnetic influx and dilution by variable amounts of non magnetic loess is inadequate our magnetic measurements establish the close comparability of the fine grained magnetic minerals in the palaeosol samples to those in contemporary soils and thus point to a partially pedogenic origin for the magnetic mineral assemblages in the palaeosols

morphological characters and multi element isotopic signatures of carbonates from chinese loess paleosol sequences abstract morphological characters and multi element isotopic compositions of carbonates from the loess paleosol sequences in northwestern china are examined to explore the origin of the minerals samples are collected from various sections ranging from holocene to 0 9 ma within the sequences and fractions with grain sizes 45 and 45 m exhibit an almost perfect spherical shape while those 13 c and 18 o values between minerals and biologically originated samples indicates that the 206 pb 204 pb 207 pb 204 pb 208 pb 204 pb and 87 sr 86 sr these results suggest the utility of fine carbonate particles in the soil profile in reconstructing a potentially higher resolution 13 c and 18 o time series to elucidate the paleoclimatic fluctuation in the chinese loess plateau during pleistocene the discovery of the nano rod calcite in chinese loess together with previous findings of the similar mineral form in asian dust strongly suggests the possibility that these highly reactive caco 3 form may alter the aerosol properties during transport

magnetic susceptibility of chinese loess and its bearing on paleoclimate magnetic susceptibility measurements are reported from 468 samples spanning a 160 m thick section of loess near the city of baoji shaanxi province an independent absolute chronology is provided by geomagnetic polarity reversals which indicate that the sediments represent the geological interval 0 to 2 5 million years ago fourier analysis of the susceptibility time series reveals strong spectral peaks at quasi periods of 100 000 and 40 000 years in agreement with the astronomical theory of climatic forcing the longer period is due to the eccentricity of the earth s orbit the shorter to the obliquity of the ecliptic the expected peak near 20 000 years due to precession appears only weakly however we do find strong evidence for a 400 000 year quasi periodicity particularly prior to 1 32 million years ago this is firmly predicted by astronomical calculations but has rarely been recognized in geological data

paleorainfall reconstructions from pedogenic magnetic susceptibility variations in the chinese loess and paleosols the rock magnetic properties of the chinese loess and paleosols constitute a unique and sensitive record of east asian paleoclimate through the quaternary period systematic variations in the concentration and grain size of the magnetic minerals in these sediments have produced systematic variations in the magnetic susceptibility signal which can be easily and rapidly measured at many sites across the loess plateau variations in many other rock magnetic properties can be used to identify the key shifts in ferrimagnetic grain size but magnetic susceptibility alone is sufficiently sensitive to record stadial and interstadial climate stages as well as glaciations and interglaciations past changes in rainfall and monsoon activity for this region are reconstructed from the susceptibility variations the susceptibility record is calibrated using the modern relationship between rainfall and pedogenic susceptibility on the loess plateau our rainfall reconstructions identify enhanced summer monsoonal activity in the chinese loess plateau region in the early holocene and the last interglaciation in the presently semiarid western area of the plateau annual precipitation in interglacial times was up to 80 higher than at present in the more humid southern and eastern areas values were up to 20 higher than today s levels during the last glaciation precipitation decreased across the entire plateau typically by 25 the relationship between pedogenic susceptibility climate and weathering age was examined over the northern hemisphere temperate zone and the observed positive correlation between rainfall and susceptibility indicates that climate rather than soil age is the predominant factor that controls pedogenic susceptibility enhancement in loess soils

late pleistocene continental climate and oceanic variability recorded in northwest pacific sediments core v21 146 provides a continuous record of northwest pacific pelagic sedimentation spanning the past 530 000 years downcore variations of 18o from benthic foraminiferal calcite were correlated to the specmap record to provide an age model for late pleistocene paleoclimatic and paleoceanographic variation fluxes of caco3 and eolian material have been determined by combining information about linear sedimentation rates bulk densities and component percentages the mass accumulation rate mar of caco3 varies from 58 to 1435 mg cm kyr 1 and exhibits the pacific ocean pattern of higher accumulation associated with glacial periods for the younger portion of the record however the pattern of caco3 accumulation appears to shift during the mid brunhes so that prior to 350 000 years ago it exhibits an opposite pattern of greater accumulation during interglacial periods the flux of eolian dust is a proxy measure of the source area aridity and correlates well to the loess soil stratigraphy in china dust fluxes range from 43 to 718 mg cm kyr 1 and are greater by a factor of about 4 during glacial times cross spectral analysis of the eolian flux and oxygen isotope records shows an inphase and coherent relationship at each of the major orbital periodicities of 100 41 and 19 kyr eolian grain sizes generally used as a proxy record of wind intensity range from 8 4 2 9 m to 6 2 13 6 m a change in grain size variability occurs near about 300 ka such that the older portion is characterized by lower frequency and higher amplitude fluctuations than the younger portion power spectra for the eolian grain size record show dominant peaks at 100 and 50 kyr and a broad 25 35 kyr peak but coherency to the oxygen isotope curve at only the 100 and 33 kyr periodicities the phase relationship for eccentricity between the eolian grain size and 18o records suggests that at the 100 kyr frequency the smallest grain sizes are associated with glacial conditions

regional dolomitization of the waulsortian limestone in southeastern ireland evidence of large scale fluid flow driven by the hercynian orogeny the lower carboniferous waulsortian limestone has been affected by late diagenetic regional burial dolomitization over an area of 7000 km 2 extending 170 km northeastward from the hercynian front the dolostone is composed of two components 1 a very fine crystalline replacive component and 2 a coarse crystalline baroque component that fills vuggy porosity developed within the replacive dolostone fluid inclusions within the coarse crystalline component of the regional dolostone indicate that the dolomitizing solutions were warm 100 c and moderately saline 10 13 wt nacl equivalent the replacive dolomite displays systematic decreases in crystal size and degree of xenotopic texture northward both components of the regional dolostone display a regular northward increase in mean 18 o values the replacive dolomite has 87 sr 86 sr values that decrease northward these laterally variable characteristics suggest that regional dolomitization resulted from large scale north directed fluid flow of heated brines developed in response to topographic uplift associated with collapse of the continental margin south of ireland during the hercynian variscan orogeny

loess stratigraphy in central china abstract loess sequences in central china provide a detailed record of climate changes during the last 2 5 million years at least 44 major shifts from glacial to interglacial conditions occurred during this time in china prolonged loess deposition lasting more than 40 millennia occurred eleven times with only four of these intervals in brunhes chron the climate oscillations took place with increased frequency between 2 4 and 0 5 ma compared to the last half million years major shifts toward cooler and drier climates occurred at about 2 4 ma 1 2 ma and 0 5 ma while less pronounced shifts are dated to 1 65 ma 0 8 ma and 0 2 ma three of the last four reversals of the earth s magnetic field whose lithostratigraphic position has been detail are recorded in a glacial deposit the low field magnetic susceptibility of loess and soils which reflects the concentration of ultrafine magnetite is related to the degree of pedogenesis and serves as a proxy measure of climate the accumulation rate of the magnetite was fairly constant throughout the last 1 2 million years and can serve as a measure of time susceptibility records from localities near xifeng and luochuan located 160 km apart are compared and used to define informal lithostratigraphic units in the loess plateau of china

paleoclimatic significance of the mineral magnetic record of the chinese loess and paleosols the origins of the magnetic susceptibility variations of the chinese loess and paleosols are explored by scanning and transmission electron microscopy of magnetic extracts and by magnetic modeling of magnetic hysteresis data to provide quantified estimates of the major magnetic components microscopy identifies several distinct size and shape characteristics in the magnetic carriers lithogenic magnetites intact and abraded dominate the coarse grained magnetic fraction the smallest of the coarse grains is 2 i m the remaining magnetic materal is ultrafine in size with two types of magnetite particles present type a particles strongly resemble soil magnetites produced by inorganic precipitation type b particles which occur rarely are probably bacterial in origin quantitative modeling of these magnetic assemblages shows that over 90 of the susceptibility variations is accounted for by the superparamagnetic magnetite component compared to the loess units the paleosols are richer in magnetite particularly of superparamagnetic size and have a threefold higher ratio of magnetite to hematite we identify pedogenic formation of magnetite as the major contributor to the loess magnetic record matching this record against other paleoclimatic records we find an extremely high correlation with the standard 18o record the chinese loess sequences record a very high resolution magnetic stratigraphy directly related to changing climate

the chinese loess in xifeng i the primary study on magnetostratigraphy of a loess profile in xifeng area gansu province summary 1848 specimens were collected systematically from about 900 horizons in the loess profile of xifeng gansu province china the magnetostratigraphy is established on the basis of the palaeomagnetic results after thermal cleaning usually at 300 c this result has been found to be generally consistent with previous work in luochuan the susceptibility varies regularly with the appearance of loess and palaeosol in the profile and thus could be used as a criterion to classify the loess and palaeosol sequence

the different climatic response of pedogenic hematite and ferrimagnetic minerals evidence from particle sized modern soils over the chinese loess plateau abstract in recent years increasing interest in loess studies has focused on qualitative and quantitative paleoclimatic reconstruction using the imperfect antiferromagnetic mineral hematite however the linkage between the hematite formation and climatic variables remains controversial here we present the results of a comprehensive investigation of the magnetic properties and statistical analysis of a suite of clay and silt fractions of modern soil samples from 179 sites across the chinese loess plateau clp and adjacent regions our objective was to clarify the relationships between modern climatic variables and pedogenic hematite as well as pedogenic ferrimagnetic minerals first order reversal curve measurements were also conducted on representative particle sized subsamples from a n s transect to understand the differences in magnetic mineralogy between the two fractions our results show that pipette extraction separates the fine grained superparamagnetic sp and most of the single domain sd magnetic grains into the clay fraction and that the remaining silt fraction displays the magnetic properties of coarse pseudo single domain psd or a mixture of multidomain md psd and a few sd particles only the pedogenic clay fraction shows a strong correlation with climatic variables the application of redundancy analysis helps to distinguish the climate variables controlling the formation of ferrimagnetic minerals and hematite during pedogenesis on the clp pedogenic ferrimagnetic minerals are sensitive to mean annual precipitation while pedogenic hematite formation is preferentially dependent on mean annual temperature the confirmation of the temperature dependent nature of hematite on the clp provides a new possibility for quantitatively reconstructing the paleotemperature history of chinese loess paleosol sequences

geology s pb isotopes and 40 ar 39 ar geochronology of the zhaxikang sb pb zn ag deposit in southern tibet implications for multiple mineralization events at zhaxikang several au sb sb au pb zn and sb pb zn ag deposits are present throughout the north himalaya in southern tibet china the largest sb pb zn ag deposit is zhaxikang 18 mt at 0 6 wt sb 2 0 wt pb 3 5 wt zn and 78 g t ag zhaxikang veins are hosted within n s trending faults which crosscut the early middle jurassic ridang formation consisting of shale interbedded with sandstone and limestone deposited on a passive continental margin ore paragenesis indicates that zhaxikang mineralization occurred in two main phases composed of six total stages the initial phase was characterized by assemblages of fine grained mn fe carbonate arsenopyrite pyrite sphalerite stage 1 followed by relatively coarse grained mn fe carbonate fe rich sphalerite galena pyrite stage 2 the second phase was marked by assemblages of quartz pyrite fe poor sphalerite and ag rich galena tetrahedrite sericite stage 3 quartz sb pb sulfosalt minerals mainly composed of boulangerite and jamesonite stage 4 quartz stibnite cinnabar stage 5 and quartz calcite stage 6 sulfides of stage 2 have 34sv cdt of 8 4 12 0 206pb 204pb ratios of 19 648 to 19 659 207pb 204pb ratios of 15 788 to 15 812 and 208pb 204pb ratios of 40 035 to 40 153 sulfides of stage 3 have similar 34sv cdt of 6 1 11 2 and relatively more radiogenic lead isotopes 206pb 204pb 19 683 19 792 stage 4 sb pb sulfosalt minerals have 34sv cdt of 5 0 7 2 and even more radiogenic lead 206pb 204pb 19 811 19 981 by contrast stibnite of stage 5 has 34sv cdt of 4 5 7 8 and less radiogenic lead 206pb 204pb 18 880 18 974 taken together with the geological observations that the pb zn bearing mn fe carbonate veins were crosscut by various types of quartz veins sphalerite and galena of stage 2 underwent dissolution and remobilization and that sb pb fe sulfosalts formed at the expense of pb from stage 2 galena and of fe from stage 2 sphalerite we argue that the early pb zn veins were overprinted by later sb rich fluids stage 2 fluids were likely acidic and oxidized and leached lead from high grade metamorphic rocks of the greater himalayan crystalline complex ghc and sulfur from reduced rocks such as slate of the ridang formation along n s trending faults leading to precipitation of pb zn sulfides and mn fe carbonate and formation of solution collapse breccias later sb rich fluids leached pb from the ghc and the pre existing sulfides and deposited fe poor sphalerite ag rich galena tetrahedrite sb pb sulfosalts and stibnite in quartz veins that cut pre existing pb zn bearing mn fe carbonate veins the sb rich fluids also likely leached pb from early cretaceous gabbro and formed stibnite at shallow levels where early pb zn bearing mn fe carbonate veins are absent a sericite 40ar 39ar plateau age of 17 9 0 5 ma from stage 3 veins represents the timing of the onset of stage 3 mineralization

source of the climate signal recorded by magnetic susceptibility variations in indian ocean sediments the origins of magnetic susceptibility variations in deep sea sediments from the indian ocean ocean drilling program odp leg 117 have been examined to identify the specific connections between climate shifts and sediment magnetic properties in a previous study the magnetic susceptibility variations in odp hole 722b were identified as an outstanding proxy paleoceanographic record despite evidence of postdepositional loss of magnetic minerals through reductive diagenesis this paradox is addressed by assessing the contributions of ferrimagnetic detrital iron oxides bacterial magnetosomes and paramagnetic detrital fe silicate minerals to the magnetic susceptibility signal in addition to detailed magnetic analyses mineralogical morphological and grain size data have been obtained from representative magnetic extracts for hole 722b we find that magnetic responses to climate change result from 1 fluctuations in the volume magnetic susceptibility which is primarily controlled by carbonate dilution 2 a ferrimagnetic signal which is restricted to the upper 7 meters below sea floor mbsf and which largely reflects source area aridity and 3 a paramagnetic susceptibility record below 7 mbsf which is coincident in frequency with variations in lithogenic grain size

magnetic susceptibility of loess in china abstract the low field susceptibility of the loess deposits in the chinese loess plateau is due to both paramagnetic and ferromagnetic minerals paramagnetism can be seen in the less to unweathered loess horizons where superparamagnetic contributions which arise from ultrafine magnetite particles are not as strongly developed as in the heavily weathered palaeosols frequency dependent susceptibility and minimum unblocking temperatures of low temperature thermoremanent magnetization generally indicate finer grain sizes in the palaeosols as compared to the loess beds this grain size difference and the fact that susceptibilities of the same stratigraphic palaeosol layer vary by an order of magnitude between the western and central regions of the plateau are considered as strong evidence that the ultrafine magnetite is not of detrital aeolian origin but was formedin situ the pedogenetic processes are clearly related to palaeoclimatic changes therefore the various susceptibility properties may serve for a detailed reconstruction of the continental palaeoclimatic conditions for the last 2 5 ma since the palaeomagnetically dated beginning of loess sedimentation in china

pleistocene climates in china dated by magnetic susceptibility the magnetic susceptibility of loess and interbedded soils in central china varies with the degree of pedogenesis and serves as a proxy measure of climate the concentration of magnetic minerals in the sediment is inversely proportional to sedimentation rates throughout the brunhes epoch susceptibility measurements combined with the reversal stratigraphy provide a time scale that is independent of astronomic chronology on this scale the susceptibility record closely parallels the oxygen isotope fluctuations in deep sea sediments suggesting a close interdependence of the chinese dust falls the volume of land based ice and global climate

magnetism of chinese loess deposits summary the age of chinese loess deposits has long been disputed biostratigraphical and earlier magnetostratigraphical investigations placed the entire loess formation within the pleistocene and ascertained a maximum loess age of about 1 2 myr a new collection of nearly 500 samples from a natural outcrop and a borehole section near lochuan lat 35 8 n long 109 2 e shaanxi province has been dated by magnetic stratigraphy thermal cleaning of the natural remanent magnetization nrm removes a strong secondary component of viscous origin along the present geomagnetic field which resides largely in magnetite the characteristic nrm component is due to haematite which is thought to be of chemical origin rhythmical intensity variations of nrm and initial susceptibility depend on the loess lithology and may reflect climatic changes during loess deposition the palaeomagnetic results are consistent between the two sections and yield a clearly defined magnetic polarity zonation the brunhes matuyama boundary and the jaramillo subchron have been positively identified in both outcrops at exactly the same stratigraphic level the olduvai subchron has been found in the borehole section which records the entire loess sequence most probably the formation of chinese loess began shortly after the matuyama gauss polarity transition therefore a late pliocene age of about 2 4 myr is assigned to the oldest loess sediments measured

outcrop analog for cyclic shelf reservoirs san andres formation of permian basin stratigraphic framework permeability distribution geostatistics and fluid flow modeling vertical and horizontal transects were sampled from core and outcrop of the san andres formation at lawyer canyon guadalupe mountains new mexico to assess permeability variation in a geologic framework of upward shallowing carbonate cycles and to show the potential effect these variations have on viscous dominated flow behavior in analogous reservoirs these cycles occur in a ramp crest facies tract are 3 13 m 10 45 ft thick and contain both vertical and lateral variation of lithofacies thicker cycles consist of a basal dolomudstone which is overlain by burrowed dolopackstone and capped by bar flank ooid peloid dolograinstone and bar crest ooid dolograinstones in vertical transects permeability is extremely variable about the mean yet upward increasing trends coinciding with the succession of lithofacies typify a given cycle semi variance analysis shows permeability to be uncorrelated vertically at distances greater than 5 5 m 18 ft which is the average cycle thickness suggesting that the cycles may equate to a fluid flow unit in a reservoir semi variance analysis of measurements collected along a horizontal transect within bar crest dolograinstones of a single cycle shows permeability is uncorrelated at distances greater than 3 6 m 12 ft this correlation distance appears to be controlled by alternating porous and tightly cemented zones that formed during dolomitization vertical and lateral variogram models were fit to the spatial parameters to generate a variety of conditionally simulated permeability fields fluid flow simulations show viscous dominated flow behavior is compartmentalized by both the individual cycles and groups of cycles the basal dolomudstones are potential baffles to flow crossover between cycles but poorly developed cycles i e those that are mud rich and lack well developed bar flank and bar crest facies result in the greatest compartmentalization of fluid flow within a succession of cycles

structural and paleotopographic controls of lead zinc mineralization in the silvermines orebodies republic of ireland occurrence in lower carboniferous and devonian strata in northern tipperary the deposits comprise lower discordant zones deposited epigenetically along the channelways of the ascending ore solutions and upper late tournaisian stratiform syngenetic syndiagenetic zones of sedimentary exhalative origin mineralization occurs in a variety of sediments whose composition reflects the nature of the exhaled fluids and the variable physiochemical conditions in their depositional sites as a result of periodic tectonism on the fault zone these mineralized sediments were subjected to varying degrees of brecciation in a very unstable depositional environment the stratiform deposits of silvermines are synsedimentary products of the exhalation of mineralizing fluids from northwest trending feeder zones on to a late tournaisian sea floor of highly irregular paleotopography modified journal abstract

routine staining of drill core to determine carbonate mineralogy and distinguish carbonate alteration textures carbonate staining has long been utilized with thin sections similar techniques are also extremely useful for distinguishing the mineralogy and texture of different carbonate minerals in drill cores the simplest method for routine staining of whole drill core utilizes a dilute hydrochloric acid solution containing both alizarin red s and potassium ferricyanide this solution is painted on to the drill core to allow for the rapid identification of calcite ferroan calcite ferroan dolomite ankerite and rhodochrosite production scale staining of whole drill core rapidly provides a wealth of information on carbonate mineralogy and textural relationships in a number of deposit types examples from the irish zn pb district the mcarthur river zn pb ag district the ruby creek cu co deposit and the kansanshi copper deposit are presented to illustrate the type of information that can be gained through use of this inexpensive and rapid technique

environmental magnetism principles and applications 1 in environmental magnetism rock and mineral magnetic techniques are used to investigate the formation transportation deposition and postdepositional alterations of magnetic minerals under the influences of a wide range of environmental processes all materials respond in some way to an applied magnetic field and iron bearing minerals are sensitive to a range of environmental processes which makes magnetic measurements extremely useful for detecting signals associated with environmental processes environmental magnetism has grown considerably since the mid 1970s and now contributes to research in the geosciences and in branches of physics chemistry and biology and environmental science including research on climate change pollution iron biomineralization and depositional and diagenetic processes in sediments to name a few applications magnetic parameters are used to routinely scan sediments but interpretation is often difficult and requires understanding of the underlying physics and chemistry thorough examination of magnetic properties and of the environmental processes that give rise to the measured magnetic signal is needed to avoid ambiguities complexities and limitations to interpretations in this review we evaluate environmental magnetic parameters based on theory and empirical results we describe how ambiguities can be resolved by use of combined techniques and demonstrate the power of environmental magnetism in enabling quantitative environmental interpretations we also review recent developments that demonstrate the mutual benefit of environmental magnetism from close collaborations with biology chemistry and physics finally we discuss directions in which environmental magnetism is likely to develop in the future

3 d modeling of the lisheen and silvermines deposits county tipperary ireland insights into structural controls on the formation of irish zn pb deposits

protolith relations of the gravina belt and yukon tanana terrane in central southeastern alaska metamorphic rocks west of the coast mountains batholith in central southeastern alaska are divided into the gravina belt taku terrane and newly defined ruth assemblage the ruth assemblage comprises metapelite quartzose metaclastic strata quartzite marble felsic metatuff mafic metavolcanic rocks and orthogneiss depositional and emplacement ages of 367 pm 10 ma and 345 pm 13 ma inferred from discordant u pb zircon analyses on felsic metatuff and granodioritic orthogneiss respectively require that at least portions of the ruth assemblage be late devonian and early mississippian in age the assemblage is similar in age and protolith to and thus correlated with the yukon tanana terrane the gravina belt is characterized by upper jurassic and lower cretaceous mafic volcanic rocks and tuffaceous turbiditic clastic strata that unconformably overlie the alexander terrane metamorphic rocks that structurally underlie the taku terrane and ruth assemblage are included in this assemblage trace

rainfall dependent transformations of iron oxides in a tropical saprolite transect of hainan island south china spectral and magnetic measurements 1 the iron oxide content of soils and sediments controlled by weathering and pedogenesis is generally considered a reasonable indicator of climate previous studies in temperate zones have established a positive correlation in aerobic soils between ferrimagnets and low to moderate rainfall the correlation seems to be reversed under extreme climates with high rainfall here we present a transect of saprolitic soils from hainan island south china with high rainfall 1440 2020 mm yr little temperature variation 23 24 c and extreme weathering along this transect we observed that both hematite concentration and magnetic susceptibility decrease with increasing rainfall whereas goethite concentration displays a large increase however there is no systematic trend in the total amount of iron oxides related to chemical weathering intensity along the transect goethite is the favored mineral phase of iron oxide with increasing rainfall and accumulates at the expense of hematite and maghemite through the dominance of rainfall driven processes these pedogenic processes coincide with the fundamentals of previous nonmonotonic models of hematite magnetic susceptibility and rainfall control of an inflection point this study also verifies a common genetic relationship between hematite and pedogenic ferrimagnets across a wide climate range a conceptual model considering both rainfall and temperature is proposed to help interpret the mechanism of ferrimagnet formation and changes in the rainfall inflection point

10be and magnetic susceptibility in chinese loess 10be and magnetic susceptibility have been measured in the top 12 m of a loess profile from luochuan central china comparison of the 10be concentration record with the specmap 18o profile of the last 130 ka imbrie et al 1984 results in a new time scale which is in good agreement with that of kukla et al 1990 the calculated loess accumulation rates can be compared with existing records of dust falls in pacific deep sea sediments and antarctic ice cores a simple model for the 10be dust flux demonstrates that a significant part of the magnetic susceptibility signal in palaeosol horizons is due to in situ pedogenic production during times of high loess accumulation cold palaeoclimate this contribution is negligible

magnetic mineralogy of chinese loess and its significance summary the topmost 16 metres of the loess section at xifeng central chinese loess plateau which cover the last 128000 years consist of two palaeosol beds interlayered by a loess horizon hysteresis parameters frequency dependent field and temperaturedependent susceptibility show very similar and stable magnetic mineral constituents in both loess and palaeosol samples the concentration and grain size of magnetite maghaemite and haematite control the magnetic properties of chinese loess susceptibility and loess thickness are closely related to geographic position and present climate across the loess plateau the loess and palaeosol magnetic properties are largely controlled by the fine grained magnetite formed in various quantities and grain sizes under the influence of pedogenesis during variable palaeoclimatic conditions therefore regional and global palaeoclimatic development during the last 2 5 myr can be reconstructed from susceptibility records in the loess plateau

thermobarometric constraints on the structural evolution of the coast mountains batholith central southeastern alaska thermobarometric data from amphibolite facies metamorphic rocks west of the coast mountains batholith provide important constraints on the structural evolution of the mid cretaceous sumdum fanshaw

modelling of bed contours and cross sections adjacent to planar normal faults the geometry of beds in the volume surrounding a single blind planar fault depends on i the axial ratio of the fault surface ellipse ii the ratio between the maximum displacement and the maximum dimension of the fault surface iii the displacement distribution on the fault surface from the point of maximum displacement to zero displacement at the tip line loop fault surface displacement profile and iv the rate at which the displacement decreases to zero along a normal to the fault surface reverse drag displacement profile these relationships which are either known or can be estimated are incorporated in a computer model which generates structural contours on horizons intersected by a planar fault structural contour patterns also vary with the pre faulting dip and strike of beds the dip and strike of the fault and the level at which a bed intersects the fault surface when the boundary conditions are relaxed to include synsedimentary faults the ratio of hangingwall to footwall displacement varies systematically with fault dip a synthetic structural contour map is matched with that of an actual fault model isopachs based on simple depositional models are generated for synsedimentary units cross sections are generated for synsedimentary faults by sequential fault growth and sedimentary deposition and show features similar to actual cross sections the soft domino model is applicable primarily to dip slip tectonic faults with maximum displacements of up to about 5 km listric faults are not considered

chemical mass transfer during hydrothermal alteration of carbonates controls of seafloor subsidence sedimentation and zn pb mineralization in the irish carboniferous abstract petrographic and geochemical data were acquired to evaluate models for the formation of hydrothermal dolomite breccias that host zinc lead orebodies in the rathdowney trend of the irish orefield and assess element mobility during hydrothermal alteration of carbonates macroscopic and microscopic textures indicate that the breccias largely formed by a pseudobrecciation process involving dissolution and replacement of precursor waulsortian dolomite of diagenetic origin during subseafloor hydrothermal fluid flow geochemical analyses show that compared with the waulsortian dolomite precursor breccia clasts and matrix contain respectively higher concentrations of al ti and light rare earth elements which are linearly correlated indicative of mass loss during hydrothermal interaction bulk rock mass loss estimates average 33 indicating extensive dissolution of carbonate although petrographic evidence and limited clast rotation or translation rule out the development of significant open space this supports a mechanism of alteration whereby hydrothermal fluids migrating along grain boundaries microcracks and solution seams drove dissolution and replacement of the surrounding host rock with partial collapse of the transient microporosity produced progressive interaction led to the gradual isolation of domains clasts surrounded by replacement dolomite matrix massive sulfide mineralization formed subsequently by stratabound replacement of the dolomite pseudobreccia by hydrothermal fluids preferentially focused within the more permeable and possibly more reactive lithology this process involved further volume loss 30 of the replaced rock quantitative mass transfer analysis indicates that during hydrothermal interaction net loss of na p mn sr and sometimes cu and net gain in li k fe as well as the ore associated elements typically occurred marked fractionation of the ree is observed eu was added implicating elevated temperature acidic and reduced hydrothermal fluids ce was also added possibly derived from remobilization of pre sulfide oxide mineralization the lree were typically immobile or partially removed and the hree were preferentially mobilized and transported beyond the scale of sampling thus the hree together with na p mn sr and possibly cu may form a distal chemical anomaly at the base of the waulsortian limestone formation the net addition of li k and be in dolomite breccia matrix samples combined with their increase in concentration due to dolomite dissolution produced a broad anomaly of these elements extending for at least 1 5 km from the center of the lisheen deposit these various geochemical anomalies have potential utility in mineral exploration a total volume loss of up to 68 at lisheen is consistent with a marked thinning of the waulsortian biomicrite facies above massive sulfide the observed thickening of upper and supra waulsortian facies in the trough developed above this zone is interpreted to reflect infill of a seafloor depression up to 90 m deep developed in response to underlying hydrothermal dissolution this model implies mineralization initiated during the latter stages of waulsortian deposition in the late courceyan early chadian at a depth of approximately 150 m below the seafloor

the carbonate hosted lisheen zn pb ag deposit county tipperary ireland the lisheen zn pb ag deposit located in county tipperary in southeast central ireland consists of a group of strata bound pyritic sulfide lenses containing 22 mt of ore with an overall grade of 11 5 percent zn 1 9 percent pb 26 g t ag and 16 percent fe hosted in lower carboniferous early mississippian carbonate rocks sulfide bodies at lisheen are restricted to the waulsortian mudbank complex and the lisduff oolite unit of the ballysteen limestone which compose the stratigraphically lowest nonargillaceous carbonate horizons in the local lower carboniferous succession the deposit is located within a major right stepping relay zone of a segmented normal fault system sulfide bodies occur adjacent to the normal faults thickest development of sulfides generally occurs at points of maximum throw metal zoning is well developed relative to these structures the waulsortian mudbank complex and portions of the overlying succession were subjected to an early premineralization regional scale dolomitizing event that increased permeability the regional dolostone produced by this event is cut and replaced by hydrothermal dolomite referred to as black matrix breccia and white matrix breccia associated with mineralization hydrothermal dolomitization was most intense in the waulsortian mudbank complex though it locally affected rocks from the silurian basement to more than 200 m above the level of major sulfide precipitation the majority of the lisheen deposit consists of fe zn pb ag sulfides hosted in the black matrix breccia regional dolomite and to a much lesser extent undolomitized limestone of the waulsortian complex sulfide bodies at lisheen consist of pyrite marcasite sphalerite and galena with subsidiary sulfosalt minerals arsenopyrite and copper sulfides sulfides replace carbonate minerals and infill dissolution cavities gangue minerals intergrown with sulfides include dolomite calcite barite and quartz mineral textures indicate a regular paragenetic sequence of sulfide deposition from early precipitation of colloform iron sulfide containing minor sphalerite and galena through deposition of a complex assemblage of sphalerite galena iron sulfides sulfosalt minerals and copper sulfides distribution of alteration and sulfide zones indicates that the lisheen deposit formed as a prograding hydrothermal system textural and mineralogic evidence suggests that the hydrothermal fluids were relatively acidic and that reaction with the carbonate host rocks was a primary reason for sulfide precipitation on the basis of the probable age of faulting and crosscutting relationships of the regional and hydrothermal dolomite the maximum age of mineralization at lisheen was at least 10 m y after host rock deposition

dependence of near surface magnetic susceptibility on dust accumulation rate and precipitation on the chinese loess plateau magnetic susceptibility ms of surface sediment varies systematically across the loess plateau in central china decreasing exponentially from 200 10 8 m3 kg at the northern margin of the qinling shan to 30 10 8 m3 kg near the southern margin of the mu us desert ms correlates highly with loess median grain size r2 0 79 which decreases south southeastward across the plateau it also correlates with mean annual temperature mat and mean annual precipitation map r2 0 58 and 0 60 respectively and with their product mat map r2 0 83 which is considered a measure of potential pedogenic activity because regional isopleths depicting grain size and the primary meteorological parameters are nearly parallel it is difficult to determine their relative influence on ms a simple ms model based on the observed spatial variation in loess thickness permits quantitative assessment of the effect of the dust accumulation rate on the ms signal of surface sediment and isolates the likely role of climate in the production of magnetic minerals the model suggests that 84 of the loess ms variance is dictated by the diluting effect of dust and 10 11 is associated with meteorological factors primarily precipitation the observed and modeled relationships support hypotheses that attribute variations in ms in the loess paleosol succession to varying rates of dust deposition and in situ production of magnetic minerals in the accretionary soils both of which are controlled by monsoon climate

paleomagnetic evidence for an early permian age of the lisheen zn pb deposit ireland the lisheen zn pb deposit is one of the major lower carboniferous carbonate hosted base metal deposits of the irish midlands the timing of ore mineralization and therefore its genesis remains much debated because of a lack of absolute age determinations paleomagnetic analysis of 432 specimens from 12 sites in ore mineralization and 10 sites in host rocks at lisheen isolated a well defined stable characteristic remanent magnetization chrm direction using af step demagnetization except in one site of massive sulfide mineralization and two sites of postore pink dolomite thermal step demagnetization and saturation remanence tests show that the chrm is carried dominantly by single domain magnetite a paleomagnetic fold test is negative indicating that the chrm postdates burial lithification and deformation the chrm directions from eight host rock and 12 zn pb mineralized sites are indistinguishable at 95 percent confidence the combined chrm directions give a mean paleopole at 41 6 s 18 8 w semiaxes of the oval of 95 confidence along and perpendicular to the site pole great circle dp 1 7 dm 3 3 that when compared to the robust apparent polar wander path for laurentia rotated into european coordinates gives an age of 277 7 2 ma for the chrm this 277 7 ma age is close to the paleomagnetic ages of 290 9 2 ma for zn pb mineralization at galmoy and 269 4 2 ma for the ba mineralization at silvermines which are 10 km northeast and 30 km west of lisheen respectively this early permian magnetization postdates peak variscan orogenic heating that is observed in all carboniferous strata in ireland our preferred interpretation is that the paleomagnetic data in conjunction with other thermal data support an entirely epigenetic origin in contrast to the pre variscan syn genetic or syndiagenetic models favored by previous authors we propose a topographically driven fluid flow either from the south of ireland or from closer to the deposit which was modified by localized convection during the variscan orogeny

basaltic maar diatreme volcanism in the lower carboniferous of the limerick basin sw ireland lead zinc exploration drilling within the limerick basin sw ireland has revealed the deep internal architecture and extra crater deposits of five alkali basaltic maar diatremes these were emplaced as part of a regional north east south west tectonomagmatic trend during the lower carboniferous period field relationships and textural observations suggest that the diatremes erupted into a shallow submarine environment limerick trace element data indicates a genetic relationship between the diatremes and extra crater successions of the knockroe formation which records multiple diatreme filling and emptying cycles deposition was controlled largely by bathymetry defined by the surrounding waulsortian carbonate mounds an initial non diatreme forming eruption stage occurred at the water sediment interface with magma water interaction prevented by high magma ascent rates this was followed by seawater incursion and the onset of phreatomagmatic activity magma water interaction generated poorly vesicular blocky clasts although the co occurrence of plastically deformed and highly vesicular clasts indicate that phreatomagmatic and magmatic processes were not mutually exclusive at a later stage the diatreme filled with a slurry of juvenile lapilli and country rock lithic clasts homogenised by the action of debris jets the resulting extra crater deposits eventually emerged above sea level so that water ingress significantly declined and late stage magmatic processes became dominant these deposits largely confined to the deep vents incorporate high concentrations of partially sintered globular and large raggy lapilli showing evidence for heat retention our study provides new insights into the dynamics and evolution of basaltic diatremes erupting into a shallow water 20 120 m submarine environment

the origin of a mississippi valley type deposit in the viburnum trend southeast missouri petrologic and isotopic studies of the base metal sulfide mineralization in the buick mine have established the following paragenesis an early stage of fine grained pyrite and marcasite replaced the dolostone host rock the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation delta 34 s sp 16 5 to 19 0ppm following the sphalerite two main stages of galena were precipitated a cuboctahedral stage delta 34 s gn 21 3 to 10 0ppm during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite and a cubic stage delta 34 s gn 10 5 to 0 9ppm the two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite delta 34 s mc 2 3 to 4 1ppm was precipitated and followed by a period of mild etching of the cubic galena minor amounts of pale sphalerite delta 34 s sp 14 0 to 16 5ppm were then precipitated white and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite the cubic galena and the pale sphalerite after the pale sphalerite was precipitated chalcopyrite quartz and siegenite were precipitated followed by calcite and finally kaolinite the paragenetic and textural data for both galena and sphalerite in the buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation similar textural evidence for galena reported by other workers for different mines in the viburnum trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire viburnum trend fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the viburnum trend demonstrating that the ore forming solutions in the viburnum trend were probably at temperatures in the range 105 degrees to 141 degrees c assuming a maximum pressure correction of about 11 degrees c and were extremely saline containing about 4 0 to 5 0 molal chloride consideration of typical chemical analyses of fluid inclusions from other mississippi valley type deposits suggests that the maximum likely concentration of dissolved co 2 in the ore forming solutions was about 0 05 molal at these concentrations the ore forming solutions that travelled to the site of deposition could have had ph9s about two units less than neutral and still have been in equilibrium with dolomite or calcite the ranges of sulfur and lead isotopic compositions of galena from the buick mine are very similar to those established by earlier studies of the old lead belt and adjacent subdistricts however the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the viburnum trend suggesting that galena was precipitated from many solutions each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions the textural paragenetic and isotopic data for the buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition instead the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur as h 2 s in the same solutions to the sites of deposition the reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite which demonstrate that low but economically significant concentrations of lead or zinc could have been transported as chloride complexes together with equivalent concentrations of reduced sulfur at temperatures of 100 degrees to 150 degrees c and ph9s of one to two units less than neutral and could have been precipitated by ph increases cooling or dilution at the sites of deposition

structural geological setting of the elura zn pb ag massive sulphide deposit australia abstract the elura deposit is located in the cobar basin one of several intracratonic basins developed during silurian devonian time in the central province of the lachlan orogen eastern australia the deposit contained a total pre mining resource of 45 mt at 8 5 zn 5 3 pb and 69 g t ag the deposit is hosted by an early devonian sedimentary sequence which transits from a shallow water shelf facies into a deep water turbidite facies the host rocks comprise open platform reef limestone back reef calcareous mudstone open platform clastic sediments and distal turbiditic mudstone and siltstone interbedded with fine grained sandstone csa siltstone the structural setting of the deposit is at the north northeastern margin of the half inverted cobar basin adjacent to the north northwest trending elura growth fault and north northeast transform transfer buckwaroon fault these major structures controlled lithofacies distribution during basin formation flow and focus of metal bearing fluids during mineralisation as well as strain distribution during basin inversion the deposit comprises two mineralised systems massive sulphide mineralisation hosted in fine grained turbidites and semi massive sulphides hosted in limestone the dominant mineral assemblage in the turbidite hosted mineralisation is pyrite sphalerite pyrrhotite galena with minor chalcopyrite whilst mineralisation in the limestone comprises pyrite sphalerite and minor galena and pyrrhotite the elura deposit is interpreted as a syn diagenetic replacement massive sulphide deposit formed by mixing of metamorphic fluids derived from basement and basinal brines discharged along the growth fault in comparison with other base metal deposits the elura deposit is similar to the irish type base metals deposits during basin inversion the deposit underwent high strain and greenschist metamorphic conditions which caused modification and distortion

the last interglacial glacial cycle in fennoscandia abstract the climate and glacier fluctuations during the last interglacial glacial cycle in fennoscandia seem to correspond roughly in time with the climate fluctuations recorded in france germany holland however the climate in fennoscandia was considerably colder and the coldest phases are recorded as periods with glacier fluctuations admittedly the exact age and amplitudes of some fluctuations are still much debated

u pb geochronology of late cretaceous and early tertiary plutons in the northern coast mountains batholith u pb geochronologic studies demonstrate that steeply dipping sheetlike tonalitic plutons along the western margin of the northern coast mountains batholith were emplaced between 83 and 57 perhaps 55 ma less elongate tonalitic granodioritic bodies in central portions of the batholith yield ages of 59 58 ma coeval with younger phases of the tonalitic sheets large granite granodiorite bodies in central and eastern portions of the batholith were emplaced at 51 48 ma trends in ages suggest that the tonalitic bodies generally become younger southeastward and that at the latitude of juneau plutonism migrated northeastward across the batholith at 0 9 km ma variations in the age shape location and degree of fabric development among the various plutons indicate that late cretaceous paleocene tonalitic bodies were emplaced into a steeply dipping dip slip shear zone that was active along the western margin of the batholith postkinematic eocene plutons were emplaced at shallow crustal levels inhe

structural and geochronologic relations along the western flank of the coast mountains batholith stikine river to cape fanshaw central southeastern alaska abstract geologic and u pb geochronologic studies in central southeastern alaska provide constraints on the mid cretaceous to mid tertiary deformation west of the coast mountains batholith the ne dipping sumdum and fanshaw faults record the w directed emplacement of the ruth assemblage yukon tanana terrane and taku terrane over the subjacent gravina belt and alexander terrane ductile fabrics of the sumdum fanshaw fault system truncate early formed foliation and thrust faults in albian and older gravina belt strata u pb age data from syn and post tectonic plutons suggest that deformation was ongoing at 92 9 3 0 ma but had mostly ceased by 90 ma the sumdum fanshaw fault system marks the tectonic boundary between the alexander wrangellia terrane and inboard yukon tanana and stikine terranes and lies within a thrust belt that extends from southern alaska to northern washington mid cretaceous structures are truncated to the east by ne dipping ductile fabrics of the coast shear zone sheet like paleocene tonalites were emplaced into and deformed within the shear zone undeformed mid eocene dikes cross cut the tonalites kinematic relations in the coast shear zone suggest a complex displacement history that includes both east side up reverse and west side up normal shear the shear zone probably accommodated the collapse of overthickened crust developed during mid cretaceous shortening

geology of the western flank of the coast mountains between cape fanshaw and taku inlet southeastern alaska the western flank of the coast mountains batholith between cape fanshaw and taku inlet is underlain primarily by jura cretaceous strata of the gravina belt pre permian permian and triassic strata of the taku terrane and mid proterozoic to upper paleozoic rocks of continental margin affinity the continental margin rocks include mid proterozoic to lower paleozoic quartzite and marble of the tracy arm assemblage mid paleozoic metavolcanic and subordinate metasedimentary rocks of the endicott arm assemblage and upper paleozoic quartz rich metaturbidites and metaconglomerate of the port houghton assemblage we suggest that these three assemblages are correlative with components of the yukon tanana terrane which underlies a large region of yukon and eastern alaska rocks of the gravina belt taku terrane and yukon tanana terrane are juxtaposed along west vergent thrust faults of mid cretaceous age and are internally deformed and disrupted along latest cretaceous to early eocene dip slip and possibly strike slip shear zones these rocks and structures together with mid cretaceous to eocene plutons of the coast mountains batholith separate rocks of the alexander wrangellia terrane to the west from rocks of the stikine terrane to the east mid cretaceous thrust faults in the area belong to a system of thrusts that form the inboard margin of the alexander wrangellia terrane from central alaska to northwestern washington the continental margin rocks in the northern coast mountains may be part of the in situ north american margin that has been overthrust by the stikine and adjacent terranes alternatively these rocks may have 1 rifted from and then returned to the north american margin 2 moved 800 km along left lateral or right lateral faults from elsewhere along the north american margin or 3 originated near a continent other than north america

on the growth of colloform textures a case study of sphalerite from the galmoy ore body ireland abstract colloform textures have been described from many of the world9s sulphide ore deposits and involve formation of distinct microcrystalline layers these textures provide information related to sequential stages of ore formation yet their mechanism of growth is poorly understood this study has analysed a series of colloform samples from the galmoy zn pb mine ireland results indicate that the growth sequence is not always intuitive and layers that appear stratigraphic upon cursory observation may not be the crystal preferred orientation cpo of discrete colloform layers abruptly switches between different orientation axes examination of the same layers using in situ laser sulphur isotope analysis reveals equally striking changes in 34 s signature between end member bacteriogenic 25 and hydrothermal 10 sources although these results combined with petrographic observation allow the determination of a probable growth history there is no correlation between shifting cpo and 34 s signature trace element analysis reveals changes in trace element sequestration between colloform layers cadmium and chlorine rich layers correspond to a hydrothermal sulphur source whereas iron rich layers correspond to a bacteriogenic sulphur source in the absence of alternative mechanisms we suggest that the most likely factors influencing cpo changes are temperature and degree of supersaturation during two fluid mixing

spatial and temporal reconstructions of changes in the asian palaeomonsoon a new mineral magnetic approach abstract a new quantitative approach is proposed for estimating palaeoprecipitation across the chinese loess plateau at present there is a strong rainfall gradient across the plateau from 300 mm yr in the near desert conditions in the northwest to over 750 mm yr in the southeast just 700 km distant we find that the concentration of ferrimagnetic iron oxide minerals in nine modern soil types represented by 37 individual soil profiles is strongly correlated with this contemporary rainfall gradient the ferrimagnetic concentration rises along this gradient from 0 01 in the northwest to over 0 2 in the southeast the nine modern soil types have been used in the construction of a rainfall vs magnetic susceptibility least squares regression climofunction past variations of loess soil iron oxide content are easily established through magnetic susceptibility measurements and so can be used to reconstruct the rainfall of former interglacial and glacial periods the physical and pedological basis of the rainfall vs susceptibility relationship is discussed and potential limitations of our rainfall reconstruction method are explored our palaeoclimate reconstructions indicate dramatic changes in rainfall due to variations in the structure of the asian monsoon the rainfall variations are about four times greater than has been suggested for this region by atmospheric general circulation modelling our data indicate increased rainfall throughout central china both in interglacial periods and in the early holocene the increases in monsoonal rain were particularly pronounced at our westernmost sites adjacent to the northeastern edge of the tibetan plateau in contrast for glacial periods a reduction in rainfall is found across the whole loess area with the greatest decreases in the southeast

displacement gradients on fault surfaces abstract the maximum displacements and the dimensions of single faults are systematically related and the variation in displacement from a maximum at the centre of a fault to zero at an elliptical tip line loop is described by a simple theoretically derived expression these relationships are used to examine the theoretical ranges and distributions of rates of change of displacement on fault surfaces the dimensions and maximum displacement of a fault can be estimated from a limited number of displacement gradient measurements estimates are further constrained by a knowledge of the effective shear modulus of the rocks containing a fault an understanding of displacement gradients can be applied to problems commonly encountered in mining operations and in the interpretation and use of seismic reflection data in addition to field problems a comparison of theoretical displacement gradients with measured displacement gradients from british coalfield faults shows that i fault surface ellipticities are between 1 5 and 2 5 and ii gross displacement gradients expressed as the ratio of maximum displacement long axis radius of the fault ellipse range from 0 02 to 0 002 the measured displacement gradients indicate that the shear moduli of the faulted rocks range from 3 to 11 gpa which is within the known range for coal measure rocks displacement gradients along an array of fault segments are similar to those of single faults for analysis of displacement gradients it is necessary to sum the discontinuous and continuous components of displacement in extreme cases all the displacement may be accommodated by continuous deformation

environmental magnetism past present and future environmental magnetism involves the application of rock and mineral magnetic techniques to situations in which the transport deposition or transformation of magnetic grains is influenced by environmental processes in the atmosphere hydrosphere and lithosphere the first explicit description of environmental magnetism as a distinct field was in 1980 since that time environmental magnetism has become a broad field that is finding application in an ever increasing array of scientific disciplines in this review of the present state of environmental magnetic studies we divide the field into three broad but arbitrary categories the first involves the use of mineral magnetic assemblages in the geological record to study physical processes in depositional environments this category includes the correlation of sediment cores using magnetic susceptibility measurements studies of geomagnetic field behavior the analysis of depositional and postdepositional mechanical processes that affect sediments and the examination of magnetic parameters that might represent proxies for paleoclimatic variation the second category encompasses studies of the processes responsible for variations in the magnetic minerals brought into a sedimentary environment these provenance investigations include studies of changes in catchment derived sediment in lakes fluctuations in contributions from terrigenous aeolian and glaciogenic components in deep sea sediments and the origin of atmospheric particulates the final category addresses in situ changes and transformations of magnetic minerals in sedimentary environments including pedogenesis authigenetic diagenetic formation of ferrimagnetic phases dissolution of magnetic minerals due to reductive diagenesis and contributions of biomagnetism to sedimentary magnetism because environmental magnetism can address problems in so many disciplines and because many of these problems may be inaccessible to other techniques it is likely that the scope of environmental magnetism will continue to expand rapidly environmental magnetism is capable of providing important data for studies of global environmental change climatic processes and the impact of humans on the environment all of which are major research initiatives in the international scientific community these factors suggest that environmental magnetism has a bright and diverse future

10be in chinese loess we report a first series of10be measurements on chinese loess covering the last 750 000 years the10be concentration shows a tendency to decrease with increasing grain size leaching experiments with acid solutions show that10be is strongly bound to particles and therefore mobility due to dissolution can be excluded the10be record in loess shows features similar to those observed in 180 records from deep sea sediments which strongly reflect climatic variations this allows one to correlate the10be loess records with 180 deep sea records and to establish a time scale based on this time scale accumulation rates of loess and10be fluxes can be calculated these reveal generally higher values for cold arid and windy periods than for warm humid and calmer intervals

geology isotope geochemistry and fluid inclusion investigation of the robat zn pb ba deposit malayer esfahan metallogenic belt southwestern iran abstract the robat zn pb ba deposit in the arak mining district in the malayer esfahan metallogenic belt memb has an estimated reserve of 5 million tonnes mt ore grading 2 1 wt zn and 1 4 wt pb and 4 mt barite ore this deposit hosted by early cretaceous carbonate rocks formed in an extensional back arc environment between the sanandaj sirjan zone and central iranian microcontinent sulfide and barite ores display a strong lithological control and form stratabound stacked ore lenses which are hosted in two horizons by silicified and dolomitized limestone they are generally best developed below marl layers within intercalated shale marl and limestone the textural and mineralogical data indicate two main stages of sulfide and barite mineralization at the robat deposit the first stage comprised deposition of volumetrically minor early fine grained disseminated sulfides and barite near the seafloor in unconsolidated sediments subsequently sub seafloor replacement of barite carbonate and early sulfide laminae bands occurred by precipitation of late sulfides and other hydrothermal minerals within the early cretaceous limestone dominant sequence the minerals in this later stage were deposited in a close relationship with quartz veins electron microprobe analyses revealed that sphalerite has a very high fe content averaging about 1 23 wt and is relatively enriched in as ga ge cd ag and bi which is similar to irish type carbonate hosted deposits in addition the ag content of galena is comparatively low although concentrations of ga ge cd zn and sb are fairly high fluid inclusion microthermometric study of hydrothermal quartz revealed two types of primary two phase l v fluid inclusions in hydrothermal quartz type i fluid inclusions with irregular shapes were recorded along crystal growth zones whereas type ii inclusions are observed as isolated inclusions with relatively regular shapes that are less common than type i type i and ii inclusions exhibit homogenization temperatures of 140 239 c and 233 290 c with salinities of 2 2 11 8 and 5 5 11 5 wt nacl equiv respectively fluid inclusions extracted from quartz displayed dsmow between 67 and 56 and measurements of hydrothermal quartz yielded 18ofluid smow of 12 9 to 23 5 the 206pb 204pb 207pb 204pb and 208pb 204pb ratios of galena samples fell in the ranges of 18 412 18 427 15 642 15 647 and 38 556 38 580 respectively the different ratios for the pb isotopes suggest a dominant continental crust origin for pb in the ores geological textural mineralogical and isotopic data may be cumulatively interpreted to indicate that the robat carbonate hosted deposit formed from sub seafloor replacement like commonly observed for irish type base metal deposits

the sr and nd isotopic variations of the chinese loess plateau during the past 7 ma implications for the east asian winter monsoon and source areas of loess abstract 143 nd 144 nd and 87 sr 86 sr ratios of the acid insoluble residues of the red clay and overlying loess paleosols from the lingtai profile of the loess plateau china were investigated the results show that variations of the 87 sr 86 sr ratios of the lingtai profile can be divided into two stages from 7 ma b p to 2 5 ma b p the acid insoluble residues in the red clay rc formation are characterized by higher 87 sr 86 sr ratios with an average of 0 7230 from 2 5 ma b p to the present the acid insoluble residues in the wuchen loess wl4 ws1 lishi loess l15 s1 malan loess l1 and holocene loess s0 have relatively lower 87 sr 86 sr ratios and display an overall descending trend from 0 7223 at 2 5 ma b p to 0 7182 at the present among three possible interpretations for the variations of 87 sr 86 sr ratios of acid insoluble residues of the loess provenance change chemical weathering change or grain size distribution change only grain size distribution change provides a satisfactory interpretation this implies that the east asian winter monsoon strength was weak and relatively stable from 7 ma b p to 2 5 ma b p but became continuously enhanced from 2 5 ma b p to the present all the red clay and the overlying loess paleosols in the lingtai profile have generally identical e nd 0 implying that the source regions of these eolian deposits of the chinese loess plateau may be relatively stable and has not changed since 7 ma b p

mineral magnetic record of the chinese loess and paleosols variations in the magnetic susceptiblity of the chinese loess and its interbedded paleosols correlate strikingly well with the deep sea oxygen isotope record low susceptibility values are found for the loess layers and high values are found for the soil horizons two interpretative models have been advanced to account for these magnetic variations both of these models discount any significant in situ formation of magnetite during soil forming periods instead they infer relative concentration of detrital or atmospheric magnetite at these times we critically examine the assumptions made in these models and on the basis of some new mineral magnetic data and fromalized magnetic flux calculations offer an alternative explanation of the loess magnetic record our model identifies pedogenic formation of magnetite as the major contributor to the high magnetic susceptibility of the paleosols the formation and preservation of this pedogenic magnetite is dependent on soil froming conditions and hence reflects the regional climate

visible and near infrared spectra of carbonate rocks reflectance variations related to petrographic texture and impurities grain size statistics derived from thin section point measurements were used to classify carbonate rock textures and to identify related spectral characteristics in the visible and near infrared 0 4 2 5 m wavelength range although the statistics did not permit all the carbonate textural varieties generally recognized by petrologists to be distinguished three broad textural classes of pure carbonate material were found that produced distinctive spectra further refinement of the statistical technique may permit other textural classes to be identified however internal scattering in carbonate rocks is a complex phenomenon involving the grain size distribution as well as the nature and abundance of inclusions and internal grain defects to yield an optimum textural classification for spectroscopic studies all such scattering contributors would have to be quantified the spectral effects of iron oxide clay silica and organic matter impurities were examined by comparing texturally similar pure and impure carbonate rock samples of these impurities organic matter had the greatest spectral influence and amounts as low as 0 01 wt commonly caused marked reductions in sample brightness and absorption band intensity in contrast the spectral effects of rock texture were subtle and could only be identified in samples that were nearly free of organic matter impurities the pronounced organic matter spectral effects imply that the organic matter in most carbonates is very finely disseminated samples collected from different regions usually showed different degrees of organic spectral quenching however no correlations were evident between sample reflectance and contained kerogen h c ratios or kerogen amounts additional analytical work is needed to examine organic matter compositional variation and associated spectral quenching effects in detail from the present study it appears likely that contained organic matter will be a dominant factor in the visible and near infrared spectral expression for most carbonate rocks and perhaps for other sedimentary lithologies spectral quenching by organic matter will likely complicate remote sensing and laboratory spectral determinations of carbonate composition on the other hand variations in quenching may prove to be useful for distinguishing rocks that have experienced unusual diagenetic thermal or other conditions affecting the organic matter contents

two phase evolution of accretionary margins examples from the north american cordillera abstract the coast mountains orogen southeastern alaska and the salmon river suture zone western idaho record the accretion of oceanic crustal fragments to the north american cordilleran margin structural elements associated with these boundaries include a western accretion related thrust belt and steeply dipping post accretionary shear zone the shear zones the coast shear zone southeastern alaska and western idaho shear zone idaho are steeply dipping crustal scale structures characterized by steep isotopic and geophysical gradients the presence of syndeformational tabular calc alkaline tonalitic plutons concordant with shear zone fabrics locally preserved dextral strike slip indicators and pervasive down dip mineral and elongation lineations are consistent with an intra arc transpressional origin for both structures in addition to strike slip translation the structures accommodated exhumation of high pressure moderate temperature metamorphic rocks within the thrust belt to the west and moderate pressure high temperature batholithic rocks to the east similarities between the structures in alaska and idaho suggest that the observed two stage history of underthrusting of accreting crustal fragments followed by modification by intra arc transpressional shear zones is a common tectonic process our analysis implies that recognition of processes active during the accretion of crustal fragments to continental or cratonal margins is commonly inhibited by post accretion deformation and provides insight into lower crustal processes presently operative under modern strike slip accretionary boundaries in addition the intra arc transpressional shear zones e g the coast and western idaho shear zones are structures that could have accommodated paleomagnetically determined large magnitude transcurrent displacements

zinc isotope variation in hydrothermal systems preliminary evidence from the irish midlands ore field little is known about the range or controls on the zinc isotope composition of terrestrial materials and no systematic studies have been carried out on ore forming systems we have obtained zinc isotope data from 19 sphalerite samples formed over a range of well constrained precipitation conditions from the irish zn pb ore field the results reveal variation in 66zn where 66zn 66zn 64zn sample 66zn 64zn standard 1 1000 from 0 17 to 1 33 per mil relative to the lyon jmc 3 0749l zinc standard this variation is significant compared to the external reproducibility 0 12 2 and the data show very good mass dependency with 67zn and 68zn values thus natural variations in the zinc isotope composition of these ore minerals can be resolved our results span the entire range of 66zn values measured on terrestrial geologic samples to date the data suggest that variations in the primary source rock composition or precipitation temperature are unlikely to be important controls on the zinc isotope composition of sphalerite in the ore field we suggest that the variation is most likely due to a kinetic fractionation involving the preferential incorporation of light zinc isotopes in sphalerite precipitated rapidly under disequilibrium conditions however we cannot rule out the possibility of mixing of zinc derived from two isotopically distinct sources the significant variation in zinc isotope compositions we have observed in the irish ore field confirms that such fractionations can provide new insights into mineralizing processes in the earth s crust

sediment magnetic signature of climate in modern loessic soils from the great plains our magnetic analysis of 40 modern loessic soil profiles from the midwestern united states shows that the upper soil horizons of all sites are characterized by an increase in the concentration of fine grained do0 1mm ferrimagnetic minerals magnetite and or maghemite our sites were selected from stable upland positions along a precipitation gradient extending from sw nebraska mean ann precip o500 mm yr to central missouri mean ann precip 41000 mm yr changes in magnetic remanence parameters irm arm and magnetic susceptibility w between soil and parent material vary systematically along the transect and are likely to reflect changes in source area and transport distance magnetic enhancement is calculated for several magnetic parameters by taking the ratio menhanced mparent material where m stands for either arm irm or w our analyses show that magnetic enhancement of modern soils based on irm and w correlates with changes in mean annual precipitation the best correlation however is observed when arm is used as a proxy of magnetic enhancement which might reflect the bias of arm towards small single domain grains of likely pedogenic origin our study shows that magnetic soil properties in combination with non magnetic proxies such as solum depth b horizon color depth to carbonates or other weathering indices have the potential to yield reliable reconstructions of paleoclimate in parts of the great plains where well preserved loessic paleosols exist

two stage exhumation of midcrustal arc rocks coast mountains british columbia 1 new geologic data from the central gneiss complex along douglas channel help delineate the burial and exhumation of the late cretaceous to eocene coast mountains magmatic arc arc plutonism was on going between 90 and 60 ma replaced by formation of dikes to 52 ma supracrustal rocks were buried to midcrustal levels by 90 ma and garnet and kyanite grew from 90 through 70 ma subsequent exhumation was nearly isothermal and took place in two stages the first occurred from 70 to 59 ma as the arc contracted obliquely and sillimanite replaced kyanite exhumation was slow 0 5 mm yr probably accomplished by erosion aided by coaxial crustal thinning exhumation rates about doubled in the second stage of exhumation after 59 ma and before 52 ma penetrative deformation ended prior to intrusion of the quottoon pluton at 59 ma and 6 5 kbar production of cordierite rims on garnet at 4 5 kbar and 700 c signaled the end of near isothermal decompression prior to 52 ma rapid cooling 100 c 106 years of the central gneiss complex followed cordierite growth temperatures dropped to 250 by 48 ma the increase in exhumation rate and the subsequent rapid cooling are attributed to excision of 6 km of crust on a detachment system on the northeastern side of the central gneiss complex comparison to other parts of the coast mountains arc the sierra nevada and fiordland new zealand shows that the amount and tempo of exhumation vary greatly within and between arcs suggesting that the processes accommodating exhumation vary significantly

palaeoclimatic and sedimentary history from magnetic susceptibility of loess in china the magnetostratigraphic age of the oldest loess sediments in the central chinese loess plateau investigated so far has recently been placed at ca 2 4 million years b p thus glaciations in central asia which are believed to provide the detritus for the formation of chinese loess commenced at the same time as the first ice rafted debris sank into the ocean basins of the northern hemisphere shackleton et al 1984 lithologic palaeontologic geochemical and especially rock magnetic variations throughout the loess column are indicative of palaeoclimatic fluctuations during loess deposition the variations of magnetic low field susceptibility in loess closely resemble those of the marine oxygen isotope record shackleton and opdyke 1976 and establish for the first time a continuous record of the pleistocene glacial and interglacial stages in a continental section spectral analysis of the rock magnetic time series indicates slow apparent sedimentation during interglacial periods which are characterized mainly by calcium dissolution and related compaction of the soils variable aeolic sediment influx as well as erosion and or polygenetic soil formation may also strongly influence the sedimentary history

the magnetic properties of particle sized samples from the luo chuan loess section evidence for pedogenesis abstract measurements of magnetic susceptibility and of laboratory remanences have been carried out on particle size fractions of loess and palaeosol samples from luo chuan china they show that the sub micron clay fraction of the palaeosol is greatly enriched in fine grained

the role of intrusions in the formation of irish type mineralisation the stonepark prospect is located in county limerick south central ireland multiple zones of zn pb mineralisation have been identified at stonepark and these are approximately 5 km west of the pallas green prospect at stonepark the sulphide bodies are hosted within the waulsortian limestone and closely resemble other irish type deposits the mineralisation is composed of pyrite marcasite sphalerite and galena with gangue fe dolomite and calcite cements a key difference at stonepark is the presence of chadian aged volcanic rocks knockroe volcanics that intrude into and overlie the waulsortian limestone subsequent hydrothermal brecciation of the waulsortian limestone and knockroe intrusions resulted in the formation of tabular polymict breccia bodies containing mixed carbonate and clasts of intrusive rocks these have then been overprinted by massive sulphide mineralisation further syn mineralisation brecciation has overprinted the earlier breccias drilling has demonstrated a spatial relationship between the volume of intrusive rocks dykes and polymict breccias and zn pb mineralisation this association suggests that the intrusive rocks provided a mechanism for the introduction of the mineralising fluids into the breccia bodies this is significant as to date no large controlling fault has been identified as is seen at other irish type deposits further work is required to understand the alteration process of the intrusive rocks and how this may relate to the mineralising process

single grain heavy mineral provenance of garnet and amphibole in the surveyor fan and precursor sediments on the gulf of alaska abyssal plain implications for climate tectonic interactions in the st elias orogen abstract the st elias orogen formed as a result of the northwestward drift of the yakutat terrane along and final collision with the alaska margin in the miocene its exhumation coincided with changing glacial conditions that are considered to have strongly interacted with mountain building processes a significant part of the record of these tectonic climatic interactions is stored in sediments on the gulf of alaska abyssal plain including the surveyor fan our study examines temporal provenance changes of miocene through pleistocene sediments of the surveyor fan gulf of alaska to constrain the dynamics of exhumation and mass transfer from the evolving st elias orogen to the adjacent surveyor deep sea fan we present single grain geochemical data of amphibole and garnet and 40 ar 39 ar cooling ages of biotite and amphibole together with point counting data of heavy minerals from sands and silts from two sites in the distal and proximal fan drilled by iodp expedition 341 in 2013 sites u1417 and u1418 respectively a shift in heavy mineral composition during the miocene predating the onset of glaciation points to a tectonically induced change in erosion centers and sediment transport probably caused by the rise of the st elias mountains garnet and amphibole data suggest the chugach metamorphic complex is the main sediment source implying input to the surveyor fan from sources relatively far in the north during the miocene changing provenance signals from the miocene to pliocene suggest rising input from the lower grade metamorphic areas at the southern flanks of the orogen indicating the advance of glaciers to the tidewater line providing material from this flanking region higher input from the chugach metamorphic complex in all pleistocene sediments suggests the northern hemisphere glaciation at the plio pleistocene boundary caused erosion and sediment yield from the interior of the orogen climatic changes at the mid pleistocene transition did not cause significant changes in the provenance signal

regional fluid flow and the genesis of irish carboniferous base metal deposits the irish carbonate hosted base metal deposits have long been an enigmatic subclass of deposit some of the irish deposits harberton bridge allenwood are clearly epigenetic hosted in breccia pipes and have close affinities to mississippi valley type deposits others are characterised by stratabound and sometimes stratiform mineralisation abbeytown navan while a third group is associated closely with concordant dolomitic breccias and cavity fill mineralisation at the base of carbonate mud mounds lisheen ballinalack when the stratigraphic and textural evidence is reviewed it is apparent that all of the base metal mineralisation occurred in already compacted sediments hence mineralisation probably occurred at depths of at least several 100 m depth below the sea bed the mineralisation occurred either during transtensional lower carboniferous basin development or subsequently during the onset of variscan shortening fluids may have been derived from dewatering of variscan driven deformation to the south of the carbonate platform with fluid flow through major fracture zones and basal clastic aquifers alternatively high heat flow produced by lower carboniferous extension may have driven the mineralising system syn genetic models are extremely difficult to sustain

understanding the microscale spatial distribution and mineralogical residency of re in pyrite examples from carbonate hosted zn pb ores and implications for pyrite re os geochronology abstract accurate and precise geochronology using the re os isotopic system in pyrite is an invaluable tool for developing and confirming genetic models of ore systems however as a bulk method the results produced by pyrite re os geochronology are commonly complex and many imprecise isochrons exist in the literature using la icpms methods it is now possible to map and quantify re distribution at the ppb level allowing an unprecedented look into the re os systematics of pyrite bearing ore two samples from the lisheen zn pb ore deposit in ireland showing disparate re os isotopic behavior were investigated in situ sulfur isotope measurements using sims an analytical technique not previously attempted on the irish deposits was used to supplement the re os dataset a massive pyrite sample from the main zone produced a precise low scatter isochron 346 6 3 0 ma mswd 1 6 the re distribution in this sample is relatively homogeneous with the re budget dominated by pyrite containing 1 5 ppb re but the 34s varies significantly from 45 2 to 8 2 a second more paragenetically complex sample from the derryville zone produced a younger age with high scatter 322 11 ma mswd 206 and this also displays a large variation in 34s 53 to 4 the cores of grains of main stage iron sulfide are depleted in trace elements and show low re abundances a second re os dataset from zn pb mineralization at hawker creek nunavut canada was produced from massive pyrite that displays low re concentrations in general therefore the complexities of pyrite re os geochronology can result from impurities in mineral separates attempts to eliminate impurities through different mineral separation techniques e g crushing heavy liquid separation magnetic separation acid leaching are frequently only partially successful and therefore full characterization of any resulting mineral separates is extremely important we conclude that la icpms mapping of re and mo distributions is essential for the identification of such impurities although other trace element la icpms maps in situ sulfur isotope measurements and petrographic evidence were of limited use in assessing the re budget of a sample they are invaluable in linking the documented re distribution obtained through la icpms to re os geochronological results

ore depositional process in the navan zn pb deposit ireland the navan zn pb deposit in ireland is hosted by a lower carboniferous carbonate dominated sedimentary sequence in excess of 97 percent of the ore is hosted by a courceyan shallow water succession termed the pale beds the ore occurs as complex strata bound tabular lenses often dislocated by faulting and truncated by a submarine erosional slide structure above this surface unconformity lies the economically minor but genetically important conglomerate group ore hosted by a complex debris flow termed the boulder conglomerate timing constraints on the mineralization can be defined by the relationship to the erosion surface and style of mineralization suggesting a late chadian or early arundian age approximately 345 ma the bulk of the ore formed below the erosion surface in the semilithified to lithified pale beds ore the mineralogy of the economic ores is relatively simple being dominated by sphalerite and galena in a 5 1 ratio pyrite and marcasite are present in subordinate amounts in the majority of the pale beds ore but dominate the conglomerate group ore and the pale beds ore immediately below the erosion surface particularly lens 2 1 gangue minerals are dominated by calcite dolomite and barite the morphology of the mineralization reflects subsea floor emplacement the orebody occurs as numerous strata bound horizons ranging from intense high grade massive sulfides to lower grade sulfide layers separated by barren limestone in all cases there is strong evidence for tectonic disruption during ore deposition sulfides were dominantly deposited as open space growth and replacement of host rock open space textures include dendritic skeletal stalactitic internal sediment geopetal and coarse bladed forms the general absence of coarse euhedral cavity lining textures or substantial collapse brecciation suggests that large cavities were not present prior to mineralization replacement textures range from delicate pseudomorphs of biodasts to more destructive granular styles sulfide sulfur isotope analyses exhibit two dominant groupings of delta 34 s from around 23 to 5 and 0 to 15 per mil respectively and a third minor grouping around 32 to 28 per mil at least two sources of sulfur are invoked values around 23 to 15 per mil are considered to be the result of bacteriogenic reduction of lower carboniferous seawater sulfate approximately 20ppm with a characteristic fractionation around 35 to 45 per mil away from sulfate this is the most common range of delta 34 s in the deposit values around 8 to 15 per mil are interpreted to be hydrothermal sulfide transported with the metal bearing fluid mixing of these two components is inferred from the isotopic data and trends in the mineral paragenesis barite has a mean of delta 34 s 21 or 2 per mil which falls within the range generally recognized for lower carboniferous seawater sulfate we consider that contemporaneous seawater sulfate was the dominant source of sulfur in the barite at navan implying that seawater accessed the pale beds at the time of mineralization

micromorphology and genesis of paleosols in the luochuan loess section china pedostratigraphic and environmental implications the loess paleosol sequence in the well known potou section near luochuan covers the entire pleistocene back to 2 5 ma in a much more complete sequence than any known from europe or north america to deduce a record of the pleistocene climatic history an understanding of the genesis of the paleosols is important main diagnostic features normally observable in surface soils have disappeared in the case of fossil soils therefore micromorphology is of outstanding importance because it enables the reconstruction of original properties and polygenetic soil development especially in pedocomplexes the upper part of the loess paleosol sequence can well be compared with the 18o record shown in the specmap curve of the brunhes epoch although the sequence of paleosols shows partly an even more detailed record of cold and warm stages and allows more specific informations regarding the paleoclimate of the interglacial periods cf fig 2 the last four paleosols represented by the s1 pedocomplex the upper part of the s2 and the s3 are genetically well comparable with the natural holocene soil indicating a similar climate with a tall grass steppe vegetation the s6 is a steppe soil also whereas the five soils of the s4 the s5 complex and the s7 are forest soils indicating more moist and partly warmer interglaciations than the holocene the loess paleosol sequence of the matuyama epoch seems to give more paleoclimatic informations than the deep sea cores known so far the lishi loess below the b m boundary down to 84 m in the section contains two more soils s11 and s13 which are developed under steppe vegetation in interglaciations similar to the present one and four forest soils the s8 complex the upper soil of the s9 and the s10 indicating again more moist interglaciations than the holocene most of the numerous soils in the wusheng loess are probably formed synsedimentarily as alternation of soil formation and addition of loess the red clay at the basis consists of four intensively developed but truncated paleosols the parent material of at least the lower part must be regarded as pliocene loess