iron oxide copper gold iocg deposits through earth history implications for origin lithospheric setting and distinction from other epigenetic iron oxide deposits the iron oxide copper gold iocg group of deposits initially defined following discovery of the giant olympic dam cu u au deposit has progressively become too embracing when associated deposits and potential end members or analogs are included the broader group includes several low ti iron oxide associated deposits that include iron oxide p rich iron oxide f and ree rich fe or cu au skarn high grade iron oxide hosted au cu carbonatite hosted cu ree and f rich and iocg sensu stricto deposits consideration of this broad group as a whole obscures the critical features of the iocg sensu stricto deposits such as their temporal distribution and tectonic environment thus leading to difficulties in developing a robust exploration model the iocg sensu stricto deposits are magmatic hydrothermal deposits that contain economic cu and au grades are structurally controlled commonly contain significant volumes of breccia are commonly associated with presulfide sodic or sodic calcic alteration have alteration and or brecciation zones on a large commonly regional scale relative to economic mineralization have abundant low ti iron oxides and or iron silicates intimately associated with but generally paragenetically older than fe cu sulfides have lree enrichment and low s sulfides lack of abundant pyrite lack widespread quartz veins or silicification and show a clear temporal but not close spatial relationship to major magmatic intrusions these intrusions where identified are commonly alkaline to subalkaline mixed mafic even ultramafic to felsic in composition with evidence for mantle derivation of at least the mafic end members of the suite the giant size of many of the deposits and surrounding alteration zones the highly saline ore fluids and the available stable and radiogenic isotope data indicate release of deep volatile rich magmatic fluids through devolatization of causative mantle derived magmas and variable degrees of mixing of these magmatic fluids with other crustal fluids along regional scale fluid flow paths precambrian deposits are the dominant members of the iocg group in terms of both copper and gold resources the 12 iocg deposits with 100 tonnes t resources are located in intracratonic settings within about 100 km of the margins of archean or paleoproterozoic cratons or other lithospheric boundaries and formed 100 to 200 m y after supercontinent assembly their tectonic setting at formation was most likely anorogenic with magmatism and associated hydrothermal activity driven by mantle underplating and or plumes limited amounts of partial melting of volatile rich and possibly metal enriched metasomatized early precambrian subcontinental lithospheric mantle sclm fertilized during earlier subduction probably produced basic to ultrabasic magmas that melted overlying continental crust and mixed with resultant felsic melts with devolatilization and some penecontemporaneous incorporation of other lower to middle crustal fluids to produce the iocg deposits preservation of near surface deposits such as olympic dam is probably due to their formation above buoyant and refractory sclm which resisted delamination and associated uplift most precambrian iron oxide p rich or magnetite apatite kiruna type deposits have a different temporal distribution apparently forming in convergent margin settings prior to or following supercontinent assembly it is only in the phanerozoic that iocg and magnetite apatite deposits are roughly penecontemporaneous in convergent margin settings the phanerozoic iocg deposits such as candelaria chile occur in anomalous extensional to transtensional zones in the coastal cordillera which are also the sites of mantle derived mafic to felsic intrusions that are anomalous in an andean context this implies that special conditions possibly detached slabs of metasomatized sclm may be required in convergent margin settings to generate world class iocg deposits it is likely that formation of giant iocg deposits was mainly a precambrian phenomenon related to the extensive mantle underplating that impacted on buoyant metasomatized sclm generally smaller and rarer phanerozoic iocg deposits formed in tectonic settings where conditions similar to those in the precambrian were replicated

trace elements in magnetite from massive iron oxide apatite deposits indicate a combined formation by igneous and magmatic hydrothermal processes iron oxide apatite ioa deposits are an important source of iron and other elements e g ree p u ag and co vital to modern society however their formation including the namesake kiruna type ioa deposit sweden remains controversial working hypotheses include a purely magmatic origin involving separation of an fe p rich volatile rich oxide melt from a si rich silicate melt and precipitation of magnetite from an aqueous ore fluid which is either of magmatic hydrothermal or non magmatic surface or metamorphic origin in this study we focus on the geochemistry of magnetite from the cretaceous kiruna type los colorados ioa deposit 350 mt fe located in the northern chilean iron belt los colorados has experienced minimal hydrothermal alteration that commonly obscures primary features in ioa deposits laser ablation inductively coupled plasma mass spectroscopy la icp ms transects and electron probe micro analyzer epma wavelength dispersive x ray wdx spectrometry mapping demonstrate distinct chemical zoning in magnetite grains wherein cores are enriched in ti al mn and mg the concentrations of these trace elements in magnetite cores are consistent with igneous magnetite crystallized from a silicate melt whereas magnetite rims show a pronounced depletion in these elements consistent with magnetite grown from an fe rich magmatic hydrothermal aqueous fluid further magnetite grains contain polycrystalline inclusions that re homogenize at magmatic temperatures 850 c smaller inclusions 500 ppm concentrations

rate of sediment accumulation in lakes of udaipur rajasthan india using the210pb method sediment accumulation rates have been determined in udai sagar fateh sagar and pichola lakes which lie in the arid climatic zone using the210pb method the rate of sedimentation estimated in these lakes is 8 9 3 42 and 2 80 mm yr respectively river inflow mining activity and lithology contribute to controling the sediment deposition pattern in udai sagar lake whereas in fateh sagar and pichola lakes anthropogenic activity and erosion of loosely bound terrigenous material through wave action and varying lithology are the major contributors furthermore a contribution from atmospheric fallout has not been ruled out due to occurrence of frequent duststorms owing to strong wind conditions aided by barren hills devoid of vegetation as well as dumping of mining wastes in the area the higher tds and lower sedimentation rate in pichola lake compared to fateh sagar are due to the limestone lithology and better maintenance in the former such as construction of parks graveled roads etc to check the flow of silt into the lake fluctuations in metal content for example v cr co ni sr and sedimentation rates are due to the variation in the sediment input at different time intervals

expandability of anchizonal illite and chlorite significance for crystallinity development in the transition from diagenesis to metamorphism in mixed layer illite smectite containing 80 illite the presence of an expandable component and changes in its abundance are difficult or impossible to detect with the x ray diffraction xrd method using conventional polar interstratification compounds e g solvation with ethylene glycol although high grade diagenetic and very low grade metamorphic vlgm samples do not expand in response to ethylene glycol treatment the illite crystallinity ic improves significantly with increasing diagenetic metamorphic grade improvement of ic in anchizonal to epizonal samples previously was thought to be related primarily to increase in crystallite thickness and decrease in the number of stacking faults the present study reexamines the contribution of expandable components to ic in shale and slate samples from the gaspe peninsula for both the 2 m and 0 1 m size fractions in the high grade diagenetic and vlgm zone as tools we have used xrd patterns and high resolution transmission electron microscopy hrtem lattice fringe images of samples treated with n alkylammonium cations these studies reveal the presence of expandable components including r1 and r3 ordered structures and expandable illite expandable 2 1 clay minerals decrease in abundance with increasing metamorphic grade the improvement of ic with grade is due to the decrease in expandable layers and also to the decrease in the number of lattice defects and an increase in crystallite size stacking faults may be the main source of lattice distortion only when phyllosilicates consist of thick non expandable layer silicates during late diagenesis the chlorites become ordered and are stabilized they are the dominant phyllosilicate minerals present together with illites in the illite chlorite facies of the anchizone lattice fringe images and xrd patterns document the presence of a short range corrensite like r1 ordered phase and thick chlorite packets

gratkorn a benchmark locality for the continental sarmatian s str of the central paratethys this paper presents one of the richest and most complete vertebrate faunas of the late middle miocene 12 ma of central europe up to now sixty two vertebrate taxa comprising all major groups fishes amphibians reptiles birds mammals have been recorded based on sedimentological and palaeobiological evidences this fossillagerstatte is assumed to originate from a floodplain paleosol formed on top of a braided river sequence the fauna points to a highly structured somewhat vegetated landscape with a wide array of habitats e g fluvial channels sporadically moist floodplains short lived ponds savannah like open areas and screes it was preserved due to a rapid drowning and the switch to a freshwater lake environment palaeoclimatological data derived from pedogenic features as well as from biota indicate an overall semi arid subtropical climate with distinct seasonality mean annual precipitation 486 252 mm mean annual temperature 15 c this underlines the late middle early late miocene dry spell in central europe from taphonomical point of view the irregularly distributed but roughly associated larger vertebrate remains refer to an in situ accumulation of the bone bed splintered bones gnawing marks as well as rhizoconcretions and root corrosion structures record some pre and post burial modification of the taphocoenose however the findings of pellet remains argue for a very fast burial and thus to a low degree of time averaging for this reason the fossil fauna reflects the original vertebrate community rather well and is a cornerstone for the understanding of late middle miocene terrestrial ecosystems in this region certainly gratkorn will be one of the key faunas for a high resolution continental biostratigraphy and the comprehension of europe s faunal interchanges near the middle late miocene transition

episodic deposition and 137cs immobility in skan bay sediments a ten year 210pb and 137cs time series abstract a geochronology time series provides a powerful tool for elucidating sedimentary processes such as episodic deposition and diffusive mobility of particle reactive constituents depth distributions of 210 pb and 137 cs from skan bay alaska were determined for sediment cores collected in 1980 1984 1987 and 1990 sediment x radiographs reveal distinct layers indicating that sediments were not continuously mixed by bioturbation however the geochronology time series is inconsistent with an undisturbed steady state sediment column profiles from 1980 1984 and 1990 reveal subsurface regions in which 210 pb activity is relatively constant in addition the depth of the primary 137 cs maximum reflecting the 1963 peak in atmospheric bomb testing does not increase in a regular fashion between 1980 and 1990 the 210 pb and 137 cs geochronologies can be reconciled by removing the effects of an instantaneous depositional event the average 210 pb sedimentation rate corrected for episodic deposition in cores that were collected over a ten year period 0 241 0 006 g cm 2 yr 1 is in excellent agreement with the average 137 cs sedimentation rate 0 258 0 008 g cm 2 yr 1 calculated from three stratigraphic markers peak fallout 1963 first appearance in the sediment record 1952 and the chernobyl accident 1986 the mobility of bomb derived 137 cs under in situ conditions was evaluated by a time dependent numerical model applied to the 137 cs time series the model indicates that bomb derived cesium is immobile in skan bay sediments with a solid liquid distribution coefficient k d of 10 5 ml g 1

210pb geochronology of sediment accumulation rates in mexico city metropolitan zone as recorded at espejo de los lirios lake sediments abstract a sedimentary sequence from the espejo de los lirios lake in the metropolitan zone of mexico city mzmc was analyzed in detail for 210 pb and 137 cs activities allowing to assess the evolution of the accretion rates in the lake during the last 84 4 years the core revealed the deposition of contemporaneously indigenous lake sediments to the site interspersed with eroded bedrock from the catchment evidencing episodic increases in sediment accretion which varied from 0 14 to 0 93 cm year 1 increasing trends in sedimentation rates were found to be closely related to demographic expansion of the metropolitan zone of mexico city and explained on the basis of the erosive processes produced as a result of deforestation created as a consequence of the urban and industrial development of the mzmc

lake levels since about 40 000 years ago at lake chalco near mexico city abstract diatoms magnetic susceptibility organic content and14c ages of sediments from a 26 m core suggest that lake chalco in the southern part of the basin of mexico went through a series of major fluctuations during the late pleistocene and the holocene before ca 39 00014c yr b p the lake was very deep about 8 10 m alkaline and saline it then became shallow

sedimentation rates in lake chapala western mexico possible active tectonic control abstract lake chapala a major natural freshwater reservoir belongs to the watershed of the rio lerma which originates from the mexico city area 450 km eastwards the east west trending lake chapala basin is located on the active tectonic feature citala rift sediments of cores collected in the lake were studied for their 210pb 226ra 137cs and 239 240pu activities in order to evaluate sedimentation rates profiles of magnetic susceptibility allowed stratigraphic correlations to be made since some fine levels display more or less high susceptibility values this allows an average sediment accumulation rate to be calculated the sediment accumulation rate was calculated assuming that the activity at the interface of a defined site is constant in the course of time since any global decrease or increase in the rates could not be observed the direct flux of atmospheric 210pb was evaluated from the inventory in a horizontal soil 0 25 0 3 dpm g 1 except from two cores at the west the fluxes to sediment were higher than the direct atmospheric flux the sedimentation rate s temporal variations along the cores seem to be mainly related to annual precipitation changes with rates lower during the dryness epochs 1945 1957 and 1979 1991 the rates measured in the western part of the lake range approximately from 1 to 2 mm year 1 the lowest rates in the central and eastern areas are higher than 2 4 2 5 mm year 1 and exceed 3 5 mm year 1 0 066 g cm 2 year 1 near the rio lerma prodelta as the lake is not deeper eastward than westward it can be suggested that relative subsidence related to the active tectonic framework compensates the higher sedimentation rate

dating reservoir deposits to reconstruct sediment yields from a small limestone catchment in the yimeng mountain region china abstract in this study we use reservoir deposits to reconstruct sediment yields over the past 50 years from the quananzi catchment a small limestone catchment in the yimeng mountain region china three sediment cores were collected from the reservoir and the chronology of each core was established using 137cs and 210pbex dating the area specific sediment yields ssy for different time periods since dam construction were estimated from each core by referring to the original capacity curves of the reservoir the estimated ssy from the quananzi catchment in each core for each period ranged between 0 9 0 3 and 2 6 0 9 t ha 1 y 1 with a mean value of 1 8 0 6 t ha 1 y 1 which is low compared with reported ssy from catchments worldwide the mean value was only about 1 7 of the equivalent value reported from the jiangzhang catchment in the granite area of the yimeng mountain region this low ssy reflects the shortage of erodible luvisols resistance to erosion and the limited effect of human disturbance in this small limestone catchment the decrease in ssy from 1959 to1972 suggests that the effect of the deforestation that occurred in 1958 on soil erosion decreased with time the continual increase of ssy through the two subsequent periods after 1972 reflects the expansion and intensification of cultivation during this time however the decreased ssy over the latest period of 1998 2012 can be related to the limitations imposed by the karst landscape on cultivation and to the impact of increasing numbers of residents leaving the area since the end of the 1990s to become temporary migrant workers elsewhere resulting in a decrease in human activity in the area overall the trend in ssy reflects the general history of changing human disturbance in this small limestone catchment over the past 50 years the results indicate that reservoir deposits provide a means of estimating the historical sediment yields from a karstic environment

210pb chronology and trace metal geochemistry at los tuxtlas mexico as evidenced by a sedimentary record from the lago verde crater lake lago verde is a fresh water maar found on the lower slopes of san martin volcano at the sierra de los tuxtlas mexico currently the northernmost remnant of the tropical rain forest in america 210 pb and 137 cs analyzed in a sediment core were used to reconstruct the historical fluxes of ag cd cu pb hg and zn to the site during the last 150 yr the 210 pbxs derived sediment accumulation rates the magnetic susceptibility c n ratios and 13 c data evidenced background conditions at the lake until 1960s when enhanced erosion related to the clearing of large forested areas at los tuxtlas promoted higher accumulation rates of a heavier and more magnetic sedimentary material recent sediments from lago verde were found enriched by pb 26 fold natural concentration level ncls and moderately enriched by cd cu zn and hg 6 5 4 and 4 fold corresponding ncls respectively the fluxes of cu hg pb and zn have significantly increased since 1940s with peak ratios of total modern to pre industrial fluxes of 11 11 19 and 49 respectively the lake occupies a relatively pristine non industrialized basin and therefore the increased metal fluxes might be related to long distance aeolian transport of trace metals

a simple model for the interpretation of magnetic records in lacustrine and ocean sediments abstract many sources of magnetic minerals have been implicated as potential influences on the magnetic record of lake and ocean sediments the state of the art of our knowledge of these sources and their properties has been used to define a model which indicates those sedimentary environments where specific magnetic mineral sources may influence the magnetic record authigenically formed magnetic minerals are only likely to be found in estuaries and eutrophic lakes cosmic deposition eolian deposition and magnetic bacteria may be important in low accumulation rate deep ocean sediments whereas catchment derived material is only likely to be the sole source of the magnetic record in high accumulation rate low productivity lakes fly ash is only likely to be a dominant source in very low accumulation rate lake and ocean sediments

sedimentological characterization of palaeo soils in the northern part of the basin of mexico superficial or buried soils are important palaeostratigraphic indicators that have been poorly studied in the basin of mexico this paper summarizes soil genesis and evolution based on the study of 15 soil profiles with palaeosol layers an environmental morphogenetic and pedological sequence was established based on granulometric micromorphological geochemical and field soil studies to identify the properties and characteristics of the pedogenetic environments in which they were formed results indicate two continuous and recurrent phenomena which altered the genesis of the soils they are a torrential erosive processes under tectonic influences and arid conditions and b volcanic deposits that buried and fossilized soils both processes interrupt the evolution and sequence of soil profiles consequently from the upper talus slopes to the lower portions stretching the length of the central lowest plains there are many palaeo soil discontinuities that have not been considered into stratigraphic studies of the lacustrine plains

silicon in magnetite high resolution microanalysis of magnetite ilmenite intergrowths magnetite ilmenite oxidation exsolution intergrowths from an original titanomagnetite microphenocryst from an ash flow tuff unit have been studied using conventional transmission electron microscopy and analytical electron microscopy silicon has been found to be in solid solution in all of the magnetite studied and in some of the coexisting ilmenite the average value in magnetite is 1 2 wt si equivalent to solid solution of 9 mole fe2sio4 silicon is also present in very small silicate inclusions and as unusual si rich domains of uncertain origin in magnetite the inclusions and domains may be irregularly distributed through the magnetite in sizes well below those resolvable with the electron microprobe microprobe analyses for si in magnetite generally reflect these heterogeneities in addition to a component presumably in solid solution the petrologic implications of the data can be assessed only when relevant thermochemical data become available and the distribution of si in magnetite is better understood

clay mineral distribution in surface sediments of the south atlantic sources transport and relation to oceanography abstract surface samples mostly from abyssal sediments of the south atlantic from parts of the equatorial atlantic and of the antarctic ocean were investigated for clay content and clay mineral composition maps of relative clay mineral content were compiled which improve previous maps by showing more details especially at high latitudes large scale relations regarding the origin and transport paths of detrital clay are revealed high smectite concentrations are observed in abyssal regions primarily derived from southernmost south america and from minor sources in southwest africa near submarine volcanoes of the antarctic ocean south sandwich bouvet island smectite contents exhibit distinct maxima which is ascribed to the weathering of altered basalts and volcanic glasses the illite distribution can be subdivided into five major zones including two maxima revealing both south african and antarctic sources a particularly high amount of mg and fe rich illites are observed close to east antarctica they are derived from biotite bearing crystalline rocks and transported to the west by the east antarctic coastal current chiorite and well crystallized dioctaedral illite are typical minerals enriched within the subantarctic and polarfrontal zone but of minor importance off east antarctica kaolinite dominates the clay mineral assemblage at low latitudes where the continental source rocks west africa brazil are mainly affected by intensive chemical weathering surprisingly a slight increase of kaolinite is observed in the enderby basin and near the filchner ronne ice shelf the investigated area can be subdivided into ten large scale clay facies zones with characteristic possible source regions and transport paths clay mineral assemblages of the largest part of the south atlantic especially of the western basins are dominated by chlorite and illite derived from the antarctic peninsula and southernmost south america and supported by advection within the circumantarctic deep water flow in contrast the east antarctic provinces are relatively small assemblages of the eastern basins north of 30 s are strongly influenced by african sources controlled by weathering regimes on land and by a complex interaction of wind river and deep ocean transport the strong gradient in clay mineral composition at the brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins

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

palynological and magnetic susceptibility records of lake chalco central mexico abstract pollen and magnetic susceptibility data from a 11 27 m long core drilled in the central part of lake chalco 99 00 w 19 15 n located at the southeastern part of the basin of mexico are interpreted in terms of paleoenvironmental changes the record of this subtropical highland lake dated by radiocarbon methods spans approximately 24 000 yr b p and documents changes in climate vegetation lake level and volcanic activity seven palynozones were established by using the method of cluster analysis for correlation purposes zones 7 6 and 5 between 20 600 and 18 300 yr b p indicate reduced forests with extensive grasslands relatively high input of sediments by water discharge of surface streams and high lake level the evidences obtained in zone 4 between 18 300 and 17 500 yr b p suggest a dry and warm climate with xerophytic vegetation lower lake level and erosion rates zone 3 between 17 500 and 10 000 yr b p shows a trend to increasing moisture and cooler temperatures volcanic activity during this period is intense and strongly influences the natural environments at around 12 000 yr b p a clear expansion of forests and fulfillment of the lake recorded at the beginning of the holocene an oak forest expansion and a lowering of lake levels are initiated and by the mid holocene zone 2 subhumid temperate conditions are detected although lake level continues to be low the human impact is recorded in the last 4000 yr

mineral magnetic analyses of sediment cores recording recent soil erosion history in central tanzania abstract sediment cores covering the period from ca 1835 to 1988 ad were retrieved from lake haubi located in a severely eroded area in the kondoa district central tanzania the results of mineral magnetic analyses undertaken on the sediment cores reflect two distinctly different depositional environments before ca 1902 ad the basin formed a seasonally inundated swamp which subsequently turned into a lake the swamp sediment is black uniform and extremely clay rich it contains antiferromagnetic minerals e g haematite but lacks ferrimagnetic minerals e g magnetite due to post depositional dissolution the lake sediment is also very clay rich but laminated here ferrimagnetic minerals magnetite dominate the magnetic assemblage the soil erosion history of the catchment has been reconstructed using results based on the mineral magnetic analyses and on the sedimentation rates obtained from 210 pb datings whereby variations in magnetic concentrations and ratios attributed to variations in sediment influx are assumed to reflect soil erosion within the catchment the results from the magnetic analyses are in general agreement with the sedimentation rates high sediment accumulation occurred around the turn of the century and increased generally since 1935 with particularly high rates between ca 1945 and 1950 and from ca 1955 to the present the reconstructed soil erosion history has been compared to both historical records of anthropogenic activity in the kondoa district and to rainfall data from this comparison we infer that effects on soil erosion from variations in rainfall are subordinate to those induced by man

silician magnetite from the dales gorge member of the brockman iron formation hamersley group western australia we report silician magnetite from banded iron formation bif in the dales gorge member of the brockman iron formation hamersley group western australia magnetite mesobands typically consisting of individual 100 m microlaminae are revealed to be composed of silician magnetite overgrowths on magnetite silician magnetite overgrowths contain from 1 to 3 wt sio 2 whereas low si magnetite domains contain less than 1 wt sio 2 silicon solid solution is present in the magnetite crystal lattice as determined by in situ micro x ray diffraction and high resolution transmission electron microscopy three textures are distinguished in magnetite mesobands 1 magnetite sub microlaminae with silician magnetite overgrowths 2 recrystallized magnetite fragments with silician magnetite overgrowths and 3 a complex intergrowth of magnetite and silician magnetite all three textures are found in magnetite mesobands from the bif4 5 and bif12 16 macrobands of the dales gorge type section drill core ddh 47a from wittenoom western australia magnetite domains contain numerous submicrometer to micrometer inclusions of quartz carbonate stilpnomelane and apatite whereas silician magnetite overgrowths are devoid of mineral inclusions the presence of mineral inclusions in magnetite indicates the bif oxide precipitate was not chemically pure iron oxyhydroxide oxide magnetite domains display textures formed during soft sediment deformation that are the earliest and best preserved relict sedimentary structures in this bif silician magnetite is the dominant iron oxide in the dales gorge bif and is present in many other sub greenschist facies bifs worldwide we suggest the former presence of organic matter creates reducing conditions necessary to stabilize silician magnetite thus silician magnetite is a potential biosignature in bifs

geochemistry of magnetite from proterozoic fe cu deposits in the kangdian metallogenic province sw china fe cu deposits in the kangdian fe cu metallogenic province sw china are hosted in paleoproterozoic meta volcanic sedimentary sequences and are spatially associated with coeval mafic intrusions several well known examples are the giant lala dahongshan and yinachang deposits they have a common paragenetic sequence of an early fe oxide stage associated with sodic alteration and a late cu sulfide stage associated with potassic carbonate alteration magnetite dominates the fe oxide stage of these deposits but is also present in the cu sulfide stage of the lala deposit this study uses trace element compositions of magnetite to examine the nature and origin of the ore forming fluids the magnetite has variable concentrations of ti al mg mn si v cr ca co ni sc zn cu mo sn and ga which are thought to have been controlled mainly by fluid compositions and or intensive parameters e g temperature and oxygen fugacity fo2 fluid rock interaction and coprecipitating mineral phases appear to be less important in controlling the magnetite compositions magnetite grains in the fe oxide stage of the lala and dahongshan deposits have comparable trace element compositions and were likely precipitated from chemically similar fluids high ni contents of magnetite in both deposits coupled with previous isotopic data and the fact that the two deposits are spatially associated with coeval mafic intrusions strongly suggest that the ore forming fluids were genetically related to the mafic magmas that formed the intrusions magnetite grains in the fe oxide stage of the yinachang deposit have much lower v and ni but higher sn and mo contents than those of the lala and dahongshan deposits and are thus thought to have precipitated from more oxidized and mo sn rich fluids that may have evolved from relatively felsic magmas magnetite grains from the cu sulfide and fe oxide stages of the lala deposit are broadly similar in composition but those in the cu sulfide stage have slightly higher cu zn and mn and are thought to have crystallized from relatively low temperature and cu zn mn rich fluids evolved from the fluids of the early fe oxide stage our results show that magnetite from the fe cu deposits in the kangdian province banded iron formation fe skarn deposits diabase hosted hydrothermal fe deposits and magmatic deposits has significantly different compositions we propose that covariations of co ni zn sn and co ni mn can be used to effectively discriminate different deposit types

quaternary environments by m a j williams d l dunkerley p de deckker a p kershaw and t j stokes edward arnold new york 1993 329 pp price 16 95 isbn 0 7131 6590 1 paper back

the effect of sediment mixing on pb 210 accumulation rates for the washington continental shelf abstract nine cores from the washington continental shelf were examined by radiochemical techniques in order to evaluate the effect of mixing on the calculation of sediment accumulation rates from pb 210 profiles th 234 profiles indicate mixing coefficients of 140 cm 2 yr 1 for the seabed offshore from the columbia river and 47 cm 2 yr 1 for the seabed of the mid shelf silt deposit 75 km north of the columbia river these large mixing coefficients demonstrate that particles can penetrate to the base of the intensely mixed surface layer 10 cm within one year after emplacement at the seabed surface observed depths of cs 137 penetration within the seabed are compared with depths predicted from the surface mixed layer thickness and the pb 210 accumulation rate the observed and predicted depths agree well at all but one station at this station a combination of th 234 co 60 pb 210 and cs 137 profiles suggests that active mixing occurs below the intensely mixed surface layer the agreement of pb 210 and cs 137 data at the other stations however indicates a general absence of deep mixing on the washington shelf thus the apparent accumulation rates calculated from pb 210 profiles below the intensely mixed surface layer on the washington shelf generally reflect the true rates of sediment accumulation

210pb derived chronology and the fluxes of 210pb and 137cs isotopes into continental shelf sediments east chukchi sea alaskan arctic abstract eleven sediment cores and eight sediment trap samples from the continental shelf of the eastern chukchi sea were analyzed for 210pb cs and 2226ra in selected samples the organic carbon content and stable carbon isotope ratios were also determined the excess 210pb 210pbxs concentrations in the 1 cm core tops are generally lower than those of surficial sediments of subarctic shelves and arctic lakes the sediment inventory derived depositional fluxes of 210ph vary between 0 09 and 0 38 dpm cm 2y 1 with a mean of 0 18 dpm cm 2 y 1 the apparent mass sediment accumulation rates which vary between 31 and 324 mg cm 2 y 1 with a mean value of 133 mg cm 2 y 1 are about an order of magnitude lower than the yearly particle flux obtained from the sediment trap experiments this difference between the relatively long term and short term yearly accumulation rates is likely due to the occasional resuspension of fine particles in the continental shelf areas and their advective transport to sites other than their initial deposition the sediment inventory of 137cs varied between 0 64 and 4 09 dpm cm 2 with a mean of 2 18 dpm cm 2 a comparison of this inventory with the expected fallout inventory of 137cs suggests that there had been no detectable input of 137cs to the study area from the former ussr nuclear dump sites

sediment source tracing using environmental magnetism a new approach with examples from australia environmental magnetism can be used for sediment source tracing in situations such as stream junctions where two catchments deliver magnetically distinguishable well combined average source mixes to a trunk stream consistent mineral magnetic parameter relationships show that spatially and temporally constant magnetic mineral assemblages occur along reaches of natural streams for most of the confluence mixing situations examined it is possible to distinguish between tributaries using magnetic parameter relationships using these relationships relative tributary contributions to the resultant downstream mix can be quantified the method can be used for tracing suspended and bedload sediments so that the principal source catchments can be determined by a sequence of measurements at stream junctions along a drainage network there is considerable potential for using this new and easily applied method for studies of sediment delivery processes and sediment movement in fluvial environments