calcified microbial reefs in cambrian series 2 north china platform implications for the evolution of cambrian calcified microbes abstract this study focuses on the microbial reefs of the zhushadong formation cambrian series 2 in shandong province china in order to understand the evolution of calcified microbes in the north china platform during the cambrian series 2 and 3 the microbial reefs occur in a thin unit ca 3 m thick over an area of 1 km 2 they consist of three types of thrombolite based on their mesostructures rimmed grainstone patch and dendritic the thrombolites mainly occur in various coarse grained carbonate facies including crudely stratified oolitic grainstone stromatolitic grainstone and disorganized limestone conglomerate calcified microbes in the thrombolites include epiphyton kordephyton a tubiform microbe bija tarthinia renalcis amgaina and razumovskia the zhushadong thrombolites were formed within a grainstone shoal and experienced repeated burial and exposure the rimmed thrombolite and grainstone patch thrombolite experienced abundant input of carbonate grains forming grainstone patches in contrast the dendritic thrombolite formed solely by calcification of microbes that mainly include epiphyton tarthinia and the tubiform microbe the outer crusts of the rimmed thrombolite were formed by amgaina under high energy conditions the diverse calcified microbes of the zhushadong formation form the earliest assemblage of their type in the north china platform their descendants mostly epiphyton subsequently thrived forming a ca 180 m thick microbialite oolite dominated succession during the cambrian series 3 zhangxia formation although the reefs in the zhushadong formation are much smaller than those of the overlying zhangxia formation their calcified microbes are more diverse this most likely reflects changes in depositional environments e g abundant siliciclastic input and tidal effects vs those of a stable carbonate platform and or global changes within reef environments e g end cambrian series 2 extinction of archaeocyaths and calcified microbes a decrease in diversity of calcified microbes in the north china platform where archaeocyaths were absent may help to account for evolutionary trends in calcified microbes that occurred independently of archaeocyath influence

modern lacustrine microbialites towards a synthesis of aqueous and carbonate geochemistry and mineralogy abstract lacustrine microbialites record evidence of their depositional environments in their morphology mineralogy and geochemistry this synthesis reviews geochemical data both for microbialites and lake water for 21 modern lacustrine microbialite occurrences younger than 15 000 years although these data are limited several trends and associations are identified that provide useful criteria to aid interpretation of the environmental settings of ancient analogues these microbialites are either thrombolites or thrombolitic stromatolites they form in diverse settings including karst volcanic coastal and inland athalassic lakes surveyed lakes are mostly closed basins subject to evaporative processes lakes vary from stratified to totally mixed examples the major hydrochemical types include na cl ca so4 ca hco3 and soda lakes the salinity and alkalinity of the lakes correlates as expected with the mode of mineralization which in turn is reflected in the microstructure of the microbialites a correlation between the 13ccarbonate and the ca2 alkalinity ratio ca alk is observed generally lakes with ca alk 1 are subject to carbonate precipitation driven by either uptake of co2 that results in a broad range of positive and negative 13ccarbonate or mineralization mediated by sulphate reducing bacteria in saline to hypersaline environments that results in negative 13ccarbonate ca alk 39 are associated with hydromagnesite dolomite high mg calcite hmc and monohydrocalcite are associated with the total aqueous concentration of mg cmg occurring only in lakes with cmg 75 meq l evaporites e g gypsum are related to high lake water salinities the absolute concentration of aqueous si commonly associated with dissolution of diatom tests influences precipitation of crystalline and or amorphous silicates silica to form microbialites with si concentrations above 0 54 mmol l either as amorphous phases or as minerals such as stevensite and kerolite combined the trends observed in this survey demonstrate that modern lacustrine microbialites can preserve information symptomatic of the environments in which they formed however in a limited number of examples this is not the case as subsequent diagenesis may alter the original mineralogy to a point where the geochemical evidence conserved from the depositional environment is lost additionally some geochemical information may be the product of isolated microenvironments within living biofilms and thus not necessarily directly related to the chemistry of ambient lake water further misleading conclusions would result if the hydrochemistry at the time of study had changed from those prevailing when the microbialites were mineralized thus all the correlations developed in this work represent hypotheses to be tested rather than interpretations to be accepted this compilation demonstrates how microbialites respond to different lacustrine hydrochemical environments and is intended to provide a guide for future field studies and laboratory simulations this review also highlights the general scarcity of trace element data for lake waters and microbialites however some recent studies suggest that lacustrine microbialites may fractionate rare earth elements ree during uptake from water whereas marine microbialites appear to record the ree distribution from seawater without modification this possible difference in ree behaviour could have implications for the interpretation of palaeoenvironmental proxies

polybessurus like fossils as key contributors to permian triassic boundary microbialites in south china abstract massive occurrences of microbialites near the permian triassic boundary have been interpreted as being due to benthic microbial proliferation in unusual marine conditions after the end permian mass extinction here we present large populations of polybessurus like microfossils in the microbialites of the south china craton to reveal their morphological structures growth patterns and ecological implications these distinctive microfossils are composed of layered stalks and spherical endpoints the spheroids 20 40 m in diameter comprising external thin micritic walls and internal sparry parts are interpreted as coccoid cyanobacteria the stalks consist of multiple stacked cup micritic envelopes consisting of micron sized magnesian calcite dolomite grains and are catalogued as calcified remains of coccoid cyanobacterial extracellular polymeric substances eps early microbially mediated calcification of eps increased the preservation potential of microfossils the microfossils stand upright as coccoid unicells supported by the highly unidirectional secretion of eps revealing their benthic behaviours the successively secreted eps lift the cells away from the substrate expanding their ecological niches to gain more sunlight for photosynthesis in the barren marine floor the distinctive polybessurus like microfossils can be viewed both in laminae of stromatolites and in spotted digitate clots of thrombolites suggesting their critical roles in microbe dominated reefs in the aftermath of the end permian mass extinction the morphological features and growth patterns of the polybessurus like fossils resemble their proterozoic counterpart polybessurus bipartitus fairchild ex green et al 1987 the widespread polybessurus like fossils in the permian triassic boundary microbialites of the south china craton suggest pervasive retrogressive ecosystems on carbonate platforms after the largest extinction event during the earth s history

holocene loess in illinois revealed by osl dating implications for stratigraphy and geoarchaeology of the midwest united states abstract most uplands in illinois are blanketed by late wisconsinan peoria loess and the conventional wisdom is that little or no dust accumulation occurred during the holocene 11 7 ka to present in a recent effort to investigate if illinois holocene loess was deposited and preserved we applied optically stimulated luminescence osl dating to seven locations where loess is thick these include four sites along the mississippi river in westernmost illinois and three loess bluffs south of the illinois river in central illinois our results suggest that holocene aged loess is preserved along the mississippi river in the state of illinois in contrast no holocene loess was found south of the illinois river in the central portion of the state in addition to its spatially limited distribution holocene loess thicknesses range only up to 1 m nevertheless loess stratigraphy and osl dating results provide the first holocene loess reported east of the missouri river our new finding also resolves the long time archeological puzzle that a number of artifacts of several thousand years old were buried by upland loess along the mississippi river

the multiple sulfur isotope architecture of the golden mile and mount charlotte deposits western australia the golden mile and mount charlotte deposits in the kalgoorlie terrane western australia display three main mineralization styles fimiston comprised of interconnected shear zones associated with ankerite pyrite hematite magnetite gold telluride alteration oroya made up of breccia bodies with v muscovite ankerite pyrite pyrrhotite gold telluride alteration and mount charlotte which consists of vein arrays with symmetrical ankerite sericite albite pyrite pyrrhotite gold alteration pyrite is in equilibrium with gold in all three mineralization styles and has been selected as a proxy to record the sulfur source of the mineralizing fluids as well as the nature of the hydrothermal processes the 34s 33s and 36s analyses on pyrite grains from the different mineralization styles including oxidized and reduced sulfide oxide assemblages reveal 1 a large variation in 34s from 12 6 to 23 5 and 2 a previously unrecognized occurrence of anomalous 33s and 36s signatures from 1 0 to 1 1 and from 2 3 to 0 9 respectively it is argued that the mineralizing fluids that formed the golden mile and mount charlotte deposits record mixing among three components mantle sulfur oxidized seawater sulfur e g so4 and reduced elemental sulfur e g s8 petrographic evidence in conjunction with 33s and 36s data suggest that mif s was acquired during the deposition of shales and basalts present in the kalgoorlie terrane and later mixed with mantle derived sulfur during the mineralization events the negative 34s values that predominate in fimiston style mineralization are consistent with a prevalence of oxidized fluids during the ore forming process as reflected by the presence of hematite pyrite magnetite gold assemblages conversely the positive 34s values that dominate in the mt charlotte and oroya mineralization styles reflect a reducing environment as reflected by the presence of pyrite pyrrhotite gold assemblages

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

geochemical variations in peoria loess of western iowa indicate paleowinds of midcontinental north america during last glaciation peoria loess deposited in western iowa during the last glacial maximum lgm shows distinct geochemical and particle size variations as a function of both depth and distance east of the missouri river geochemical and particle size data indicate that peoria loess in western iowa probably had two sources the missouri river valley and a source that lay to the west of the missouri river both sources indicate that lgm paleowinds in western iowa had a strong westerly component similar to interpretations of previous workers a compilation of loess studies in iowa and elsewhere indicates that westerly winds were dominant during loess transport over much of the midcontinent south of the laurentide ice sheet which is not in agreement with paleowinds simulated by atmospheric general circulation models agcms agcms consistently generate a glacial anticyclone with easterly or northeasterly winds over the laurentide ice sheet and the area to the south of it loess deposition in the midcontinent during the lgm may be a function of infrequent northwesterly winds that were unrelated to the presence of the glacial anticyclone

geological framework of the archean and paleoproterozoic tanami region northern territory the tanami region a poorly exposed mostly paleoproterozoic province within the north australian craton hosts a number of significant gold deposits in diverse settings rare exposures of 2 520 2 500 ma amphibolite facies archean gneiss and metasedimentary rocks form basement to the thick overlying metasedimentary succession of the 1 880 1 830 ma tanami group the basal unit of the tanami group is the dead bullock formation a fining upward deep water succession dominated by siltstone carbonaceous siltstone iron rich siltstone and mafic sills carbonaceous and iron rich lithologies in the upper dead bullock formation represent important hosts for gold mineralization the conformably overlying killi killi formation represents turbiditic sedimentary rocks that are correlated with the widespread lander rock beds of the arunta region sedimentation of the tanami group was terminated by regional deformation and greenschist to amphibolite facies metamorphism during the tanami event d1 m1 at around 1 830 ma the tanami group is unconformably overlain by rhyolite siliciclastic sedimentary rocks and felsic ignimbrite of the ware group that were deposited at about 1 825 1 810 ma subsequent ese wnw to se nw directed shortening d2 followed by ne sw to e w directed shortening d3 has resulted in open ne f2 and nw f3 trending folds in both the tanami and ware groups voluminous granitoids dominated by i type biotite granodiorite and monzogranite were intruded in the interval 1 825 1 790 ma and have been subdivided using geochemical criteria into the birthday frederick and grimwade suites basalt and immature sedimentary rocks of the mount charles formation are restricted in extent to the tanami mine corridor and are interpreted to reflect a continental rift succession that was deposited around 1 800 ma with an early archean sedimentary provenance steep s to se dipping f4 fold structures of tanami and ware group metasedimentary rocks many spatially associated with 1 825 1 790 ma granitoid intrusions indicate a period of sse directed regional shortening d4 syn to post the regional granitoid intrusive phase a network of n to nw striking faults several of which are interpreted as oblique thrusts with a component of left lateral movement indicates a period of d5 convergence during wsw ene to e w directed shortening the tanami mine corridor fault system comprises a network of n ne to ene striking d5 faults that merge with n to nw striking faults and probably accommodated movement between granite core domains d5 faulting is associated with the main phase of gold mineralization in suitable structural lithological traps the paleoproterozoic basement of the tanami region is unconformably overlain by quartz sandstone lithic arenite and conglomerate of the pargee sandstone pargee sandstone may represent syn tectonic sedimentation related to the 1 730 ma strangways orogeny and is unconformably overlain by the late paleoproterozoic platform cover succession of the birrindudu group the paleoproterozoic basement and cover sequences have subsequently undergone several episodes of faulting collectively termed d6 the paleoproterozoic evolution of the tanami region is interpreted to have occurred in an intracratonic setting but was fundamentally influenced by tectonic events in the adjacent halls creek orogen 1 835 1 805 ma halls creek orogeny and arunta region 1 815 1 800 ma stafford event the boundaries between the tanami region and kimberley region to the northwest and the arunta region to the southeast are transitional and are largely defined by the presence or absence of identifiable dead bullock formation

contemporaneous formation of vein hosted and stratabound gold mineralization at the world class dead bullock soak mining camp australia we investigate the relative and absolute timing associated with the formation of the world class dead bullock soak dbs mining camp the study area is situated in the paleoproterozoic granites tanami gold province in the northern territory australia and consists of five orogenic gold deposits including the callie deposit with a resource of 14 2 moz au at dbs the ore is hosted in paleoproterozoic metasedimentary rocks and consists of two contrasting styles of mineralization vein hosted and stratabound the vein hosted mineralization consists of visible gold in quartz chlorite carbonate veins preferentially hosted in finely laminated decarbonized siltstones in contrast the stratabound mineralization consists of free gold associated with arsenopyrite concentrated in bedding parallel sulfide rich layers hosted in iron rich siltstones this study combines structural and petrographic analysis with detailed u pb geochronological data on hydrothermal and metamorphic xenotime and monazite to constrain the relative and isotopic timing of ore formation in the dbs mining camp the results of this study show that both mineralization styles developed within a series of ne trending and se dipping vein corridors or structural pathways associated with the development of axial planar fabric related to small scale f2 folds u pb geochronology shows that both mineralization styles formed at c 1805 ma approximately 35 m y after peak metamorphism at c 1840 ma a post mineralization hydrothermal event is locally remobilizing gold and yields an age of c 1790 ma which provides a minimum age for gold deposition

was phanerozoic reef history controlled by the distribution of non enzymatically secreted reef carbonates microbial carbonate and biologically induced cement throughout most of the phanerozoic reef rigidity resulted as much or more from early lithification by microbial carbonates and biologically induced cements non enzymatic carbonates than from biological encrustation of or by large enzymatically secreted metazoan skeletons reef framework is divided into four categories 1 skeletal metazoan 2 non skeletal microbialite stromatolite and thrombolite 3 calcimicrobe and 4 biocementstone in which small or delicate organisms serve as scaffolds for rigid cement crusts the last three categories are dominated by non enzymatic carbonates skeletal framework and non skeletal microbialite framework were the most abundant framework types through the phanerozoic the composition and abundance of skeletal framework was controlled largely by mass extinction events but most reefs consisted of both microbialite and skeletal organisms in a mutually beneficial relationship microbialite framework was abundant throughout the palaeozoic and early mesozoic but declined after the jurassic calcimicrobe framework was important during the cambrian early ordovician and devonian and biocementstone framework was important from the late mississippian to the late triassic the phanerozoic history of reefs does not correlate well with the stratigraphic distribution of large skeletal reef builders or with a variety of physicochemical parameters including sea level history wilson cycle or global climate cycles because non enzymatic carbonates result from induction by non obligate calcifiers and not enzymatic precipitation by obligate calcifiers the distribution of these carbonates was controlled to a larger extent by temporal changes in physicochemical parameters affecting the saturation state of sea water with respect to carbonate minerals changes in pco ca mg ratios cation concentrations and temperature may have affected the abundance of non enzymatic carbonates and hence reefs independently from the effects of these same parameters and mass extinction events on skeletal reef biota the decline in abundance of reefal microbialite and absence of calcimicrobe and biocementstone reef framework after the jurassic may be a result of relatively low saturation states of sea water owing to increased removal and sequestration of finite marine carbonate resources by calcareous plankton since the jurassic reef history is difficult to correlate with temporal changes in specific global parameters because these parameters affect skeletal biota and biologically induced carbonate precipitation independently hence reef history was regulated not just by skeletal reef biota but by parameters governing non enzymatic carbonates

the importance of early crustal architecture for subsequent basin forming magmatic and fluid flow events the granites tanami orogen example abstract the paleoproterozoic granites tanami orogen gto consists of tightly folded predominantly greenschist facies rocks the orogen a component of the north australian craton includes a ca 1864 ma succession of turbiditic and pelagic sedimentary rocks interbedded with volcanic rocks and abundant dolerite sills in a back arc basin setting which is intruded by collision related ca 1800 ma granitic rocks integrated 4d interpretation of deep seismic profiles gravity and magnetic field observations associated with available surface geology and igneous isotope geochemistry suggest that the gto forms an imbricated crust developed on a partially inverted s dipping rifted archean basement the rifting took place in the paleoproterozoic around 1864 ma and was followed by e w orientated compressional deformation ca 1850 ma with the development of northerly trending isoclinal folds and associated faults orthogonal collisional tectonics followed at ca 1800 ma which is associated with s verging kilometre scale folds faults and orogenic lode gold deposits in the region recognition of the preserved early rift architecture in the gto has major implications for the subsequent evolution of the orogen the early architecture has influenced all subsequent deposition deformation magmatic and fluid events including spatial control of the ca 1864 ma rock succession to the location of intrusive bodies and fluid transfer during formation of ca 1800 ma orogenic gold mineralization

tectono thermal evolution of the palaeoproterozoic granites tanami orogen north australian craton implications from hornblende and biotite 40ar 39ar geochronology abstract the palaeoproterozoic granites tanami orogen gto hosts a number of gold deposits located in the southern margin of the north australian craton the major stratigraphic succession is the palaeoproterozoic tanami group which is subdivided into the dead bullock formation and conformably overlying killi killi formation new geochemical data for the ca 1864 coora and groundrush dolerite sills in the dead bullock formation suggests that they have the same characteristics with the enriched back arc basin basaltic rocks from the former stubbins formation such as tholeiitic affinity high tio 2 contents 0 94 to 1 24 wt and low mg 41 45 slightly enriched lile rb th u and k weakly depleted hfse nb ta and relatively flat ree patterns their magma was generated by high degree decompressional melting 5 15 of the asthenosphere source with an input of 3 4 subduction related material the petrological and geochemical similarities of igneous rocks provide new evidence for the assignment of the ca 1864 ma former stubbins formation and the mount charles formations in the dead bullock formation of the tanami group these conclusions confirmed that the extensive palaeoproterozoic tanami group was deposited in a back arc basin environment hornblende and biotite 40 ar 39 ar geochronological study identified three major tectono thermal events in the gto since the deposition of the dead bullock formation the ca 1840 ma 40 ar 39 ar cooling age of metamorphic hornblende from the coora and groundrush dolerite sills in the dead bullock formation provided precise age constraint for the first palaeoproterozoic tectono thermal event during the evolution of the granites tanami back arc basin this age is highly consistent with the ca 1850 1840 ma subduction and peak metamorphism events in the north australian craton nac associated with the halls creek orogeny in the halls creek orogen and the tennant orogeny in the tennant creek inlier the 40 ar 39 ar age of 1753 8 ma for the early biotite bi b from the dead bullock formation is a cooling age record of the peak greenschist facies metamorphism during the ca 1795 ma tanami orogeny which is interpreted as the continental continental collisional event between the gto and the arunta orogen in the central australian craton to the south the approximately 40 million year slow cooling is a combined result of exhumation heat released from the upwelling asthenospheric mantle regional magmatism and long term crustal residence this tectono thermal event was associated with gold mineralisation in the gto and the proterozoic nac was finally cratonised during this period the late biotite bi g is a recrystallization product of hydrothermal alteration its 40 ar 39 ar age of 1718 8 ma revealed the last tectono thermal overprint to the gto and arunta orogen this age reflected the distal thermal influence by the ca 1723 ma strangways event in the arunta orogen

relative and absolute timing of gold mineralization along the meliadine trend nunavut canada evidence for paleoproterozoic gold hosted in an archean greenstone belt the meliadine area of the neoarchean rankin inlet greenstone belt is located on the western shore of hudson bay canada and is host to several important orogenic or mesothermal gold deposits supracrustal rocks comprise alternating southeast striking panels of mafic volcanic and clastic sedimentary rocks both of which are host to banded iron formation the dominant structural element is the pyke break a several kilometer wide high strain zone characterized by multiple foliations and regionally important shear zones gold mineralization is spatially associated with these shear zones the maximum age of mineralization is constrained by the observation that auriferous veins crosscut ca 2450 ma mafic dikes and a minimum age is constrained by undeformed postore lamprophyre dikes emplaced at ca 1830 ma highly concordant shrimp determined ages on hydrothermal monazite indicate auriferous veining occurred at ca 1850 ma these relative and absolute timing constraints suggest gold was concentrated in neoarchean supracrustal rocks during paleoproterozoic tectonothermal activity that was widespread in the western churchill province at ca 1850 ma these observations do not preclude the presence of archean gold occurrences in the rankin inlet greenstone belt but no early deformed ores can be linked to auriferous veins introduced at ca 1850 ma suggesting remobilization from earlier formed deposits was not a factor in concentrating gold at meliadine

protracted and polyphased gold mineralisation in the agnew district yilgarn craton western australia abstract the archean orogenic gold deposits exhibits common characteristics including coeval emplacement age often proposed to be associated with a single late tectonic gold mineralisation event although valid for a large majority of deposits in the yilgarn craton this model does not account for certain atypical gold deposits in the world class agnew district yilgarn craton western australia structural and paragenetic relationships combined with a compilation of geochronology and multiple sulfur isotopes indicate that mineralisation developed during a two stage process involving different fluid sources the initial event dated at c 2660 ma is related to the onset of folding during kalgoorlie orogeny and presents characteristics compatible with magmatic intrusion related mineralisation a second mineralisation event dated at c 2630 ma presents a mineral and metal inventory typical to archean orogenic like gold mineralisation and developed at a later stage of the orogenesis measurements of 34s and 33s from spatially constrained samples from the two temporally distinct mineralising events further ascertain contrasting fluid compositions and or reservoirs combined with geochronological data obtained on the poly magmatic lawlers granitic complex we argue for a protracted ore forming process the first stage of mineralisation has a magmatic affinity and is followed by a second stage of mineralisation whereby au bearing hydrothermal solution incorporated an archean sedimentary sulfur reservoir we propose that the change in source reservoir for sulfur in hydrothermal fluids may reflect a regional change in tectonic regime from convergent to transcurrent motion allowing fluids to be first sourced from mantle derived magmas followed later by metamorphic fluids

isotopic composition of organic and inorganic carbon from the mesoproterozoic jixian group north china implications for biological and oceanic evolution a b s t r a c t analyses of marine carbon isotope profiles have provided much of our current understanding of the evolution of earth surface environments particularly in the latter portion of the proterozoic eon earlier mesoproterozoic successions however have received comparatively little attention due to the relatively subdued nature of carbon isotope variation in this study we present high resolution isotopic profiles from three sections in the yanshan basin north china craton that combined comprise the entirety of the early mesoproterozoic 1600 1400 ma calymmian period jixian group high resolution profiles of both carbonate and organic carbon provide critical data for global comparison and permit us to better con strain both the pattern and origin of isotopic variation in the mesoproterozoic marine carbonate rocks of the jixian group show generally muted isotopic variation with average values near 0 consistent with previous observations from the early mesoproterozoic data furthermore record an increase in isotopic variation through the succession that is interpreted to reflect a long term decrease in pco2 and conse quently in the isotopic buffering capacity of marine dissolved inorganic carbon dic by contrast the isotopic composition of marine organic matter suggests facies dependent differences in carbon cycling organic carbon compositions suggest a dominance of autotrophic carbon fixation and aerobic decom position in shallow water environments and increased remineralization by anaerobic heterotrophs in deeper water environments correlation between organic carbon composition and depositional environ ment are interpreted to reflect differences in carbon cycling within benthic microbial mats under low oxygen conditions and dynamically maintained stratification of marine waters

episodic deposition of illinois valley peoria silt in association with lake michigan lobe fluctuations during the last glacial maximum the chronology and cause of millennial depositional oscillations within last glacial loess of the central lowlands of the united states are uncertain here we present a new age model that indicates the peoria silt along the illinois river valley accumulated episodically from 28 500 to 16 000 cal yr bp as the lake michigan lobe margin fluctuated within northeastern illinois the age model indicates accelerated loess deposition coincident with regional glacial advances during the local last glacial maximum a weakly developed paleosol the jules geosol represents a period of significantly slower deposition from 23 700 to 22 000 cal yr bp a gastropod assemblage based reconstruction of mean july temperature shows temperatures 6 10 c cooler than modern during peoria silt deposition stable oxygen and carbon isotope values 18 o and 13 c of gastropod carbonate do not vary significantly across the pedostratigraphic boundary of the jules geosol suggesting slower loess accumulation was a result of reduced glacial sediment supply rather than direct climatic factors however a decrease in 18 o values occurred between 26 000 and 24 000 cal yr bp synchronous with the lake michigan lobe s southernmost advance this 18 o decrease suggests a coupling of regional summer hydroclimate and ice lobe position during the late glacial period

neoproterozoic loess and limits to snowball earth abstract neoproterozoic tillites overlain by limestones and dolostones cap carbonates have been interpreted as evidence of abrupt climate change from glaciers to tropical seas on the assumption that cap carbonates were marine this assumption is here challenged by evidence for loess deposition of the nuccaleena formation of south australia from granulometry angular randomly oriented silt modal diameter 7 and sedimentary structures climbing translatent cross stratification linear dunes in addition a variety of palaeosols in the nuccaleena formation have red clayey horizons replacive micritization expansion cracks and thufur mounds as well as low carbon and oxygen isotope values high strontium isotope ratios and geochemical evidence of leaching and clay formation this cap carbonate has no definitively marine features and is here interpreted as periglacial loess overlying the fluvioglacial intertidal elatina formation so it is not a record of abrupt global warming from snowball earth other cap carbonates around the world differ in various ways and may have formed in different depositional environments but could profitably be reconsidered from the new perspective of the loess depositional model proposed here supplementary material tabular comparison of marine and non marine interpretations of cap carbonates petrographic textures and composition and new isotopic and chemical analyses of nuccaleena formation are available at http www geolsoc org uk sup18443

variations in loess accumulation rates in the mid continent united states as reflected by magnetic susceptibility detailed magnetic susceptibility records of late wisconsin loess sequences in the southwest indiana region show several distinct changes in magnitude that reflect variations in loess accumulation rate the comparison of the magnetic susceptibility signal and sedimentary features within the loess reveal the following relations 1 low magnetic susceptibility values correspond to incipient paleosols that record periods of low loess accumulation rates 2 highly variable values within intermediate to high magnetic susceptibility zones correspond to stratified loess that represents periods of high loess accumulation rates and 3 moderate to high magnetic susceptibility values correspond to massive loess that may reflect periods of moderate to high loess accumulation rates we suspect that soil moisture controls the loess accumulation rate and if so our work shows that soil moisture alternated about 24 times between 25 and 14 ka in the middle of north america

late quaternary loess in northeastern colorado part ii pb isotopic evidence for the variability of loess sources a new application of the pb isotopic tracer technique has been used to determine the relative importance of different silt sources for late wisconsin loess in the central great plains of eastern colorado samples of the peoria loess collected throughout the study area contain k feldspar derived from two isotopically and genetically distinct sources 1 glaciogenic material from early and middle proterozoic crystalline rocks of the colorado province and 2 volcaniclastic material from the tertiary white river group exposed on the northern great plains pb isotopic compositions of k feldspar in loess from two dated vertical sections at beecher island and last chance colorado vary systematically implying climatic control of source availability we propose a model whereby relatively cold conditions promoted the advance of front range valley glaciers discharging relatively little glaciogenic silt but strong winds caused eolian erosion of white river group silt due to a decrease in vegetation cover during warmer periods valley glaciers receded and discharged abundant glaciogenic silt while surfaces underlain by the white river group were stabilized by vegetation isotopic data from eastern colorado loess sections record two warm cold warm cycles during late wisconsin time between about 21 000 and 11 000 radiocarbon yr b p similar to results from other studies in the united states and greenland

sequence organization and growth patterns of late mesoproterozoic stromatolite reefs an example from the burovaya formation turukhansk uplift siberia abstract the latest mesoproterozoic burovaya formation is a 600 1000 m thick stromatolite mud dominated carbonate unit demarcated by an erosional unconformity into two subformations containing stromatolite reefs of variable size and geometry the lower subformation consists of two distinct stratigraphic sequences the lower sequence records shelf to ramp evolution and includes small scale stromatolite reefs developed within an upper subtidal shelf settings at the transgressive systems tract and overlying mud dominated highstand deposits accumulated in lower subtidal to peritidal environments the second sequence of muddy and fine grained sediments makes up a lowstand systems tract and documents a progressive structural and environmental differentiation of the burovaya platform the upper subformation records a third stratigraphic sequence containing transgressive and highstand systems tracts represented by reef bearing and reef cap facies complexes the reef bearing complex characteristic of the transgressive and the bulk of the highstand systems tracts contains a giant 20 25 km wide 10 km long up to 550 m thick stromatolite reef and surrounding deep water carbonate mudstones containing sporadic slump block deposits the reef was located on the distally steepened part of the ramp and contains distinct internal facies organization defined by particular stromatolite microstructure and morphology and prevailing environments late highstand development of the burovaya platform was marked by the termination of reef accretion infilling of basinal relief by shallow water carbonate and shale sediments of the cap complex and subsequent subaerial exposure the history of the burovaya reef is indicative of the interplay between reef growth potential and platform geometry defined by regional tectonic and sea level dynamics

gold mineralisation throughout about 45 ma of archaean orogenesis protracted flux of gold in the golden mile yilgarn craton western australia the golden mile deposit was discovered in 1893 and represents today the largest archaean orogenic lode gold system in the world 50 m oz produced gold the golden mile deposit comprises three major styles of gold mineralisation fimiston oroya and charlotte styles fimiston style lodes formed at 250 to 350 c and 100 to 200 mpa and are controlled by brittle ductile fault zones their subsidiary fault zone and vein networks including breccias and open cavity infill textures and hydrothermally altered wall rock fimiston lodes were formed late d1 prior to d2 regional upright folding hydrothermal alteration haloes comprise a progression toward the lode of diminishing chlorite an increase in sericite and in fe content of carbonates lodes contain siderite pyrite native gold 17 different telluride minerals au ag tellurides contain 25 of total gold tourmaline haematite sericite and v rich muscovite oroya style lodes formed at similar p t conditions as the fimiston lodes and are controlled by brittle ductile shear zones associated dilational jogs that are particularly well developed at the contact between paringa basalt and black shale interflow sedimentary rocks and altered wall rock the orebodies are characterised by micro breccias and zones of intense shear zone foliation very high gold grades up to 100 000 g t au and the common association of tellurides and vanadian mica green leader oroya lodes crosscut fimiston lodes and are interpreted to have formed slightly later than fimiston lodes as part of one evolving hydrothermal system spanning d1 and d2 deformation ca 2 675 2 660 ma charlotte style lodes exemplified by the mt charlotte deposit are controlled by a sheeted vein stockwork complex of north dipping quartz veins and hydrothermally altered wall rock the mt charlotte orebody formed at 120 to 440 c and 150 to 250 mpa during movement along closely spaced d4 2 625 ma and reactivated d2 faults with the quartz granophyre in the golden mile dolerite exerting a strong lithological control on gold mineralisation veins consist of quartz carbonate minor scheelite and wall rock alteration comprises chlorite destruction and growth of ferroan carbonate sericite pyrite native gold pyrite pyrrhotite is zoned on the scale of vein haloes and of the entire mine giving a vertical temperature gradient of 50 100 c over 1 000 vertical metres the structural hydrothermal model proposed consists of four major stages 1 d1 thrusting and formation of fimiston style lodes 2 d2 reverse faulting and formation of oroya style lodes 3 d3 faulting and dissecting of fimiston and oroya style lodes and 4 d4 faulting and formation of mt charlotte style sheeted quartz vein system the giant accumulation of gold in the golden mile deposit was formed due to protracted gold mineralisation throughout episodes of an archaean orogeny that spanned about 45 ma fluid conduits formed early in the tectonic history and persisted throughout orogenesis with the plumbing system showing a rare high degree of focussing efficiency and duration in addition to the long lasting fluid plumbing system the wide variety of transient structural and geochemical traps multiple fluid sources and precipitation mechanism contributed towards the richest golden mile in the world

tephrochronology and paleoclimatology of the last interglacial glacial cycle recorded in alaskan loess deposits abstract alaskan loess deposits contain long paleoclimatic records analogous to those found in marine and lacustrine sediments and in ice sheets the sedimentology and geophysical properties of alaskan loess were influenced by climatically forced changes in wind intensity storminess temperature and precipitation loess deposits also contain evidence of episodic permafrost and paleosol formation explosive volcanic eruptions paleoecologic changes in high latitude regions quaternary fossils and early human sites and artifacts the old crow tephra ca 140 000 10 000 bp dates the transition between full glacial conditions and the last interglaciation in central alaska climatic amelioration in alaska apparently began prior to astronomically forced increases in insolation a finding consistent with the well dated devil s hole record however time series analysis of loess proxy climate data also demonstrates the effects of orbital precession and obliquity on alaskan climate change numerous short term climate events are recorded in alaskan loess deposits including a climate reversal corresponding to the younger dryas

evidence for two stages of mineralization in west africa s largest gold deposit obuasi ghana the supergiant obuasi gold deposit is the largest deposit in the paleoproterozoic birimian terranes of west africa with 62 moz of gold past production resources the deposit is hosted in the paleoproterozoic kumasi group sedimentary rocks composed of carbonaceous phyllites slates psammites and volcaniclastic rocks intruded by different generations of felsic dikes and granites a three stage deformation history is defined for the district the d1 ob stage is weakly recorded in the sedimentary rocks as a layer parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at obuasi the d2 ob is the main deformation stage affecting the obuasi district and corresponds to a nw se shortening tight to isoclinal folding as well as intense subhorizontal stretching occurred during d2 ob parallel with the plane of a pervasive ne striking subvertical foliation s2 ob finally a n s shortening event d3 ob refolded previously formed structures and formed a distinct ene striking variably dipping s3 ob cleavage that is domainal in nature throughout the deposit two economic styles of mineralization occur at obuasi and contribute equally to the gold budget these are 1 gold bearing sulfides dominantly arsenopyrite mainly disseminated in metasedimentary rocks and 2 native gold hosted in quartz veins that are as much as 25 m wide microstructural evidence such as strain shadows surrounding gold bearing arsenopyrite parallel with s2 ob but folded by s3 ob indicates that the sulfides were formed during d2 ob concentrations of as much as 700 ppm au are present in the epitaxial growth zones of the arsenopyrite grains although the large mineralized quartz veins are boudinaged and refolded indicating their formation during d2 ob field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by d3 ob where the s3 ob cleavage crosscuts the quartz veins in the main ore zones thus these observations constitute the first evidence for multiple stages of gold deposition at the obuasi deposit futhermore three dimensional modeling of stratigraphy structure and gold orebodies highlights three major controls on oreshoot location which are 1 contacts between volcaniclastic units and pre d 1 felsic dikes 2 fault intersections and 3 f3 ob fold hinges the maximum age for the older disseminated gold event is given by the age of the granites at 2105 2 ma which is within error of hydrothermal rutile in the granites of 2098 7 ma the absolute age of the younger gold event is not known

the timing of gold mineralization across the eastern yilgarn craton using u pb geochronology of hydrothermal phosphate minerals the highly mineralized eastern goldfields of the eastern yilgarn craton is an amalgamation of dominantly neoarchaean granitoid greenstone terranes and domains that record a history of early rifting followed by westward directed collision with initial arc formation collision and clastic basin formation and final accretion to the western yilgarn proto craton between 2 66 and 2 60 billion years ago the gold deposits that define this region as a world class gold province are the product of orogenic processes that operated during accretion late in the tectonic history after initial compressional deformation d1 d2 and the majority of granitoid magmatism minor gold was also deposited throughout the entire tectonic history in magmatic hydrothermal related systems however such mineralization mostly 0 3 g t gold is nowhere economic unless it overprints or is overprinted by much higher grade orogenic gold lodes robust shrimp u pb geochronology of gold related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events d3 d4 shortly before cratonization however westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m y between c 2 65 ma in the east including laverton district kurnalpi terrane to c 2 63 ma in the west including kalgoorlie terrane across the eastern part of the craton the robust geochronology refutes previous suggestions that significant gold mineralization events extended from de to d4 in the evolution of the orogen and that the kalgoorlie gold deposits formed over a period of 45 m y the crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous but must be modified to account for diachroneity of orogenic events and gold mineralization across the eastern goldfields

a diverse calcified cyanobacteria assemblage in the latest ediacaran abstract a three dimensionally preserved phosphatized cyanobacterial fossil assemblage tentatively identified as a distinct calcified cyanobacterial combination of oscillatorialeans and rivulariacean is recognized in the latest ediacaran in the southern shaanxi province china based on the morphological and preservation modes oscillatorialean cyanobacteria herein contain 3 genera which are girvanella obruchevella and subtifloria with thin walled tubes tangled coiled or aligned a kind of fan shaped fossils composed of filaments with tapered trichomes and pseudo branches cambricodium is clarified as calcified rivulariacean cyanobacteria prosperous calcified cyanobacteria co occurring with the earliest mineralized skeletal tubular fossils cloudina and sinotubulites in addition to contributing new morphological and preservation information the discovery of these cyanobacteria perhaps strengthens the case for prosperity in cyanobacterial calcification in the terminal neoproterozoic

a pronounced fall in the caco3 saturation state and the total alkalinity of the surface ocean during the mid mesozoic abstract geochemical models suggest that the calcium carbonate saturation state caco3 and total alkalinity ta of the ocean fluctuated considerably in the geologic past under the influence of the continental weathering hco3 source and the biological caco3 sink the mid mesozoic is a key period in this respect because the advent of planktic calcifying organisms introduced the calcium carbonate compensation mechanism buffering oceanic caco3 and ta with respect to perturbations of the carbonate cycle as a result caco3 and ta are thought to have fallen drastically in the mid mesozoic and excluding short lived perturbations of the carbon cycle to have remained similar to present values ever since this important idea put forward using geochemical models is tested here using the geological record of marine calcified cyanobacteria it is shown that under the most conservative of assumptions oceanic caco3 must have been 6 10 and ta 3 mm in the permo triassic prior to the advent of planktic calcifying organisms it is likely though that oceanic caco3 10 16 and ta 4 6 mm were greater than these minimum estimates during most of this time interval in the late cretaceous once the pelagic caco3 sink was well in place oceanic caco3 4 and ta 2 mm had fallen to values compatible with those of the modern ocean this analysis implies a large fall in oceanic caco3 and ta between 180 and 90 myr a time interval that encompasses the advent of planktic calcifying organisms and the introduction of the carbonate compensation mechanism this cyanobacteria based reconstruction strengthens the claim that pelagic calcification revolutionised the carbonate cycle it also highlights without implying a cause effect link that biocalcifying organisms evolved in an ocean with an elevated hco3 h ratio a condition known to favour biological calcification in laboratory experiments

discussion the timing of gold mineralization across the eastern yilgarn craton using u pb geochronology of hydrothermal phosphate minerals the presentation of recent geochronological work on orogenic gold deposits in the eastern goldfields of the yilgarn craton western australia claims to prove that there is a single broad event of gold mineralization and that structural work demonstrating that there are a number of discrete gold mineralization events is wrong this new data demonstrates no such thing as this data no doubt the best that can currently be produced shows a very wide and inconsistent range in ages geochronology is not yet able to reliably separate these events which appear to be spread over an interval of perhaps 30 ma up to 2635 ma

rock magnetism and pedogenetic processes in luvisol profiles examples from central russia and central mexico abstract despite a vast literature concerning magnetic properties of loess sequences we still do not fully understand how magnetic components and properties are related to particular soil forming processes that can vary with each type of genetic horizon in order to establish the role of lithogenic factors in the link between magnetic properties and soil forming processes we carried out a study of two complete profiles of buried interglacial luvisols one formed in loess in russia alexandrovsky quarry and the other in volcaniclastics in mexico barranca tlalpan in both profiles soil genetic horizons have contrasting differences of their magnetic properties in the alexandrovsky profile the magnetic susceptibility is enhanced in the paleosol compared to parent material in the barranca tlalpan sequence enhancement is absent in the soil profile increase of fine grained magnetic components in the soil is attributed to neoformed minerals however this process cannot compensate for the loss of lithogenic magnetic minerals in any of the genetic horizons and the resulting trend is depletion in the whole soil profile the pedogenic environment of eluvial horizons in both luvisols is destructive to all magnetic components both primary and secondary higher concentrations of antiferromagnetic components hematite and goethite found in e horizons are related to redoximorphic processes

paleoproterozoic gold deposits in the bald hill and coyote areas western tanami western australia significant gold deposits in the western tanami region of western australia include deposits in the bald hill and coyote areas the ca 1 864 ma bald hill sequence of turbiditic and mafic volcanic rocks hosts the kookaburra and sandpiper deposits and a number of smaller prospects the ca 1 835 ma turbiditic killi killi formation hosts the coyote deposit and several nearby prospects the kookaburra deposit forms as a saddle reef within a syncline and the sandpiper deposit is localized within graphitic metasedimentary rocks along a limb of an anticline gold in these deposits is hosted by anastomosing quartz pyrite arsenopyrite veins within quartz sericite schist with disseminated arsenopyrite pyrite and marcasite after pyrrhotite based on relative timing relationships with structural elements the auriferous veins are interpreted to have been emplaced before or during the ca 1 835 1 825 ma tanami orogeny regional d1 gold deposition is thought to have been caused by pressure drops associated with saddle reef formation kookaburra and chemical reactions with graphitic rocks sandpiper the coyote deposit the largest in the western tanami region consists of a number of ore lenses localized along the limbs of the coyote anticline which formed during the tanami orogeny the largest lenses are associated with the gonzalez fault which is located along the steeply dipping southern limb of this fold gold was introduced at ca 1 790 ma into dilatant zones that formed in local perturbations along this fault during later reactivation regional d5 towards the end of a period of granite emplacement gold is associated with quartz chlorite pyrite arsenopyrite galena sphalerite veins with narrow 5 mm chloritic selvages a quartz muscovite biotite k feldspar tourmaline actinolite arsenopyrite assemblage which is interpreted to relate to granite emplacement overprints the regional greenschist facies metamorphic assemblage the mineralogical similarity between this overprinting assemblage and the vein assemblage suggests that the auriferous veins at the coyote deposit are associated with the granite related metamorphic metasomatic assemblage gold deposition is thought to have been caused by pressure drops within dilatant zones

multi stage enrichment processes for large gold bearing ore deposits abstract a review of previous studies of the world s large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation these observations suggest that multistage processes may even be a requirement for the formation of large deposits in some deposits e g witwatersrand boddington cadia sukhoi log or carlin the different enrichment processes occur millions of years apart in others such as many large porphyry deposits the different stages are much closer in time in many deposits particularly sedimentary hosted deposits early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes e g sukhoi log kalgoorlie the presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps this model has important implications as the presence or absence of multiple gold events could be used to discriminate at the exploration and feasibility stages between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment

neoproterozoic reef microstructures from the little dal group northwestern canada basinal facies in the neoproterozoic little dal group contain large reefs up to 300 m in height and 8 km in diameter in contrast to the stromatolitic composition typical of earlier proterozoic buildups little dal reefs consist mainly of three different calcareous microfossils a tubule thread microfossil analogous to the calcimicrobe girvanella in paleozoic reefs formed laminar reticulate structures like those in ordovician bioherms a clotted to saccate microfossil comparable morphologically to paleozoic renalcis constructed botryoidal to encrusting masses an enigmatic component of multicellular appearance that has attributes of an early calcified metaphyte exhibits the encrusting sheetlike habit characteristic of middle to late paleozoic algae these reefs represent an intermediate stage between proterozoic stromatolitic buildups and paleozoic metazoan calcimicrobial reefs and biogenic mounds

mesoproterozoic carbon dioxide levels inferred from calcified cyanobacteria filamentous and shrub like carbonate fabrics produced by in vivo cyanobacterial sheath calcification in stromatolites of the ca 1200 ma society cliffs formation baffin and bylot islands arctic canada are 400 m y older than previously reported examples in vivo sheath calcification is promoted by carbon dioxide concentrating mechanisms ccms and is a direct ecophysiological link to atmospheric co 2 concentration ccms are induced in present day cyanobacteria under experimental conditions when pco 2 is below 0 36 10 times present atmospheric level pal society cliffs calcified cyanobacteria consequently imply pco 2 levels of 2 of 7 10 pal in the late mesoproterozoic combined petro graphic experimental and modeling results therefore suggest that mesoproterozoic pco 2 concentrations were not substantially different from phanerozoic values and were significantly less than previous estimates of up to 200 pal assuming 10 lower solar luminosity in the late mesoproterozoic pco 2 levels of 10 pal or less require the presence of additional greenhouse gases for maintenance of an ice free earth at 10 pal pco 2 methane concentrations of 100 200 ppm would have been sufficient to sustain warm earth surface conditions the low atmospheric oxygen and limited marine sulfate concentrations required to sustain atmospheric methane provide additional support for sulfur isotope models that suggest protracted oxygenation of earth9s proterozoic biosphere

late palaeoproterozoic 1800 1600ma stromatolites cuddapah basin southern india cyanobacterial or other bacterial microfabrics abstract late palaeoproterozoic stromatolites from the vempalle formation of the cuddapah supergroup south eastern india contain well preserved microfabrics these exhibit a range of complex clotted to bush like micritic fabrics set in microsparite and sparite cements no calcified microfossils confidently referable to calcified cyanobacteria have been recognized although micritic masses crudely resembling the phanerozoic calcified cyanobacterium angusticellularia are present it is suggested that these vempalle stromatolitic microfabrics compare more closely with modern calcified non cyanobacterial biofilms this interpretation questions the widespread assumption that cyanobacteria were important components of stromatolite building microbiotas during the proterozoic and instead emphasizes the possibility that examples similar to those studied were dominated by non cyanobacterial bacteria it is generally thought likely that cyanobacteria were important components of stromatolite building microbiotas during the proterozoic and that early lithification by carbonate precipitation was widespread and intense if these assumptions are correct then the apparent absence of diverse calcified cyanobacteria suggests that these organisms which are conspicuous in the early palaeozoic underwent evolutionary radiation close to the neoproterozoic cambrian boundary

proterozoic microbial reef complexes and associated hydrothermal mineralizations in the banfora cliffs burkina faso abstract the proterozoic guena souroukoundinga formation of the mopti arm gourma aulacogen southerm taoudeni basin consists of a shale dominated succession up to 200 m thick with scattered microbial reef complexes quarry exposures of the tiara reef complex allow reconstruction of a transect across back reef peritidal laminites reef margin and peri reef ooidal shoals and fore reef slope strata microbial carbonate productivity nucleated on isolated palaeohighs during transgression whereas its end was controlled by two tectonically induced drowning pulses that led to the successive record of onlapping kerogenous limestones and pelagic shales reef carbonates are crosscut by fractures and fissures occluded by hydrothermal mineralizations which are related to the rifting activity of the gourma aulacogen the tiara reef complex is similar to other proterozoic reefs in being composed nearly entirely of stromatolites although calcimicrobial filamentous and thromboid textures are locally abundant which contrast with their scarcity or absence in coeval stable platform microbial reefs of the northern taoudeni basin

the timmins porcupine gold camp ontario anatomy of an archean greenstone belt and ontogeny of gold mineralization new structural and geochronological data and interpretations from the timmins porcupine gold camp abitibi subprovince more precisely define the stratigraphy and the generation timing and effects of deformation tisdale basalts and porcupine volcaniclastic and turbiditic sedimentary rocks assemblage rocks exhibit three major foliations s2 s3 s4 plus a late constrictional fabric of which s3 and s4 are also present in younger timiskaming assemblage sedimentary rocks pre timiskaming folding has long been recognized in tisdale assemblage rocks but no foliation had previously been attributed to such an event d1 regional uplift and extension produced a low angle unconformity between the tisdale and porcupine assemblages with partial excision of upper tisdale stratigraphy and perhaps of the blake river assemblage d2 involved imbrication of a set of south over north thrust sheets rooted in the porcupine destor deformation zone d3 en echelon folds along the northern flank of the porcupine destor deformation zone are a result of left lateral strike slip along this zone the timiskaming basin lying along the curvilinear trace of the porcupine destor deformation zone formed as a d3 transtensional half graben and the dome fault formed its faulted margin s4 foliation formed during subsequent right lateral strike slip movement and folding of s3 foliation intense d5 constrictional strain deformed earlier lineations as the previously dilatational jog that hosts timiskaming assemblage rocks acted as a compressional jog during right lateral movement along the porcupine destor deformation zone these d2 to d5 phases represent an overall thrusting strike slip transpressional regime there were several phases of gold mineralization in the timmins porcupine gold camp clasts of ankerite veins occur in timiskaming assemblage conglomerate at the dome mine and d3 foliation and folds crosscut early ankerite veining and alteration and some quartz veining thus low grade mineralization and possibly the quartz fuchsite veins predate timiskaming sedimentation however the bulk of gold postdates timiskaming sedimentation quartz carbonate tourmaline gold veins hollinger mcintyre hoyle pond dome aunor delnite mines formed within timiskaming assemblage and tisdale assemblage rocks these gold deposits developed largely as oblique slip and extensional vein arrays formed during north south shortening and local strike slip faulting younger deposits pamour are associated with d4 dip slip faulting and were weakly deformed during late d4 thus the largest known archean lode gold camp formed throughout protracted orogenesis consisting of a broadly transpressional regime with unusually well developed early thrusting

biosedimentology of a silurian thrombolite reef with meter scale growth framework cavities a completely dolomitized silurian reef at maumee ohio u s a is uncommon among early paleozoic reefs in that its framework consists of about 95 thrombolite boundstone and 5 corals the reef lithified syndepositonally as evidenced by centimeter to meter scale growth framework cavities within it that are completely filled with internal sediment the largest cavities 2 3 5 m in diameter are prob ably the largest ever recorded in a thrombolite reef four main types of components are distinguished in the reef frame work 1 dense bushy mesoclots 2 dense rounded to subrounded mesoclots 3 saccate microstructures and 4 crusts mostly struc tureless but also containing dense bushy mesoclots the dense bushy components are abundant rounded to sub rounded mesoclots less so and the remaining components are rare the dense bushy and rounded to subrounded mesoclots are interpreted to represent the calcified re mains of what were initially either aggregates of the probable filamen tous cyanobacterum angulocellularia or aggregates of renalcid calci microbes or both and the saccate mesoclots as the calcimicrobe ren alcis sensu stricto it is speculated that syndepositonal lithification of the reef framework was accomplished by impregnation of unlithified protomesoclots by micrite to create mesoclots along with the formation of micritic cement around and between mesoclots that served to fuse together successive generations of mesoclots the direction of branching of bushy mesoclots in the reef indicates that unlithified protomesoclots were able to grow upward laterally and downward this ability along with syndepositional lithification to create mesoclots and to cement them together enabled the protome soclots and mesoclots to create reef knobs and eventually to roof over spaces that were centimeters to three or more meters in width and height to create growth framework cavities features indicative of rap id deposition including graded bedding poor sorting of bioclasts and textural inversion are common in internal sediment in meter size cav ities these features suggest that storms possibly hurricanes were at least as important and probably more so than everyday wave en ergy in transporting sediment into all cavities in the reef regardless of size

paleocurrent analysis for the late pleistocene holocene incised valley fill of the yangtze delta china by using anisotropy of magnetic susceptibility data abstract anisotropy of magnetic susceptibility ams analysis has been conducted on samples from borehole core cm 97 from the yangtze river changjiang incised valley fill china to determine the paleocurrent directions to help in reconstructing sedimentary paleoenvironments borehole cm 97 consists of fluvial unit 1 estuarine units 2 6 and deltaic units 7 9 sediments after the last glacial maximum in ascending order the ams results show that the paleocurrent directions for the tide dominated estuarine and deltaic sediments were westerly or northwesterly directed flood tide dominated but give no definite trend for the fluvial sediments comparison between the paleocurrent directions inferred from primary sedimentary structures and in situ ams data shows that they are in good agreement confirming the applicability of ams as a good paleocurrent indicator for sediments deposited in coastal tide dominated environments considering that these sediments were strongly tide influenced and the tidal pattern since 12 kyr bp has not changed significantly we think that the westerly or northwesterly current direction most probably resulted from flood tidal currents and the sedimentary paleoenvironment was a flood tide dominated estuary or delta additionally it has been found that downhole changes of some ams parameters including the mean magnetic susceptibility k the corrected anisotropy degree p j and the magnetic foliation f clearly mark the defined stratigraphic boundaries in the borehole this further extends the validity of ams as a good stratigraphic marker in addition to its long known credibility as a sensible paleocurrent recorder

source and origin of roxana silt and middle wisconsinan midcontinent glacial activity abstract thick roxana silt middle wisconsinan in central and southwestern illinois traditionally has been interpreted as loess derived from valley train deposits in the ancient mississippi river valley winters et al h a winters j j alford and r l rieck quaternary research 29 25 35 1988 recently suggested that the roxana was not directly related to glacial activity but was derived from sediment produced by increased shoreline and spillway erosion associated with a fluctuating ancestral lake michigan because 1 paleoenvironmental and paleohydrologic conditions inferred in the hypothesis are unlikely for a loess depositional system and 2 loess did not accumulate during late wisconsinan deglaciation under conditions similar to those hypothesized we suggest the hypothesis should be rejected roxana distribution suggests the major source was drainage from the upper mississippi river valley and variations in loess thickness in illinois can be explained by consideration of valley width depth orientation and postdepositional erosion tills in the headwaters region of the ancient mississippi drainage system in minnesota and wisconsin occur in the appropriate stratigraphic position and have colors and mineralogic compositions that suggest they could be the parent till of the roxana we believe a valley train source for thick roxana is most probable and urge continued consideration of middle wisconsinan glaciation in the upper great lakes area

paleoclimatic forcing of magnetic susceptibility variations in alaskan loess during the late quaternary visual matches and statistical tests suggest correlations between marine isotope curves retrodictive solar insolation at lat 65 n and magnetic susceptibility profiles through late quaternary age alaskan loess sections the susceptibility changes largely appear to reflect variability in magnetite content due to climatically controlled changes in wind intensity and competence magnetic susceptibility profiles through massive loess can provide stratigraphic context for intercalated paleosols and tephras a prominent paleosol correlated with marine isotope stage 5 occurs several metres above the old crow ash in loess sections indicating that this important tephra is older than suggested by thermoluminescence dates and may have been deposited ca 215 25 ka

sims u pb dating of vein hosted hydrothermal rutile and carbon isotope of fluids in the wulong lode gold deposit ne china linking gold mineralization with craton destruction abstract wulong is the largest lode gold deposit in the liaodong peninsula northeastern north china craton ncc gold mineralization mainly occurs in the quartz sulfide veins which are hosted in jurassic monzogranite and early cretaceous diorite dikes here rutile grains are firstly identified in the auriferous quartz pyrite vein the vein hosted rutiles have textural associations with quartz chlorite pyrite and bi sulfosalts and host fluid inclusions indicating that they are unequivocally of hydrothermal origin and co precipitate with gold the rutile grains are characterized by high w and v concentrations and u shaped rare earth elements pattern with the greatest depletion in mree due to excess zr incorporation during fast rutile crystal growth process from the f rich ore forming fluids zr in rutile thermometers provide unrealistic crystallization temperatures 550 c which are much higher than the previous homogenization temperatures data of fluid inclusions yu et al 2018 all these geochemical signatures also support that rutiles formed from the hydrothermal fluids the 13c values of co2 gases in low salinity aqueous carbonic fluid inclusions from the mineralized quartz range from 9 6 to 1 7 average 5 7 indicating mantle magmatic sources for the ore forming fluids sims u pb dating yields a more precise and robust age of 122 3 1 1 ma n 23 mswd 0 82 for the gold mineralization at wulong this new date is roughly coeval with the ore hosted diorite dikes and the sanguliu granitic pluton most in 125 120 ma such close temporal association provides additional weights of evidence for a magmatic affinity to the gold formation regionally the rutile u pb age indicates that the gold mineralization occurs in an extensional tectonic regime caused by the thinning and destruction of the lithosphere beneath the ncc meanwhile the timing of gold mineralization is the same as the peak age of ncc destruction therefore we propose that the wulong deposit has a genetic link with this craton destruction this is similar to numerous other early cretaceous gold deposits across the ncc such as those in the xiaoqinling and jiaodong districts our study indicates that rutile is a robust u pb chronometer for the lode gold deposit but may be not a good geothermometer in the case of f rich hydrothermal fluid system

climate change in southern illinois usa based on the age and δ13c of organic matter in cave sediments abstract matrix supported diamicton and uniform to laminated silty fine grained sediment deposited from about 42 500 to 27 600 cal yr b p under slackwater conditions nearly filled two caves in southwestern illinois at some point most of the sediment was flushed from the caves and from about 22 700 to 4000 cal yr b p floods deposited a drape of sandy and silty sediment on remnant slackwater successions cobbly alluvium and bedrock especially from 7700 to 4000 cal yr b p clay mineral analyses of the slackwater cave sediment reveal a provenance of chiefly petersburg silt a smectite and illite rich proglacial lacustrine unit present in the overlying illinois episode glacial succession today remnants of the ancient subterranean slackwater deposits nearly fill several secondary passages and in at least two locations cover a cobble mantled strath terrace 1 3 to 1 5 m above active stream channels slumping and sinkhole formation appear to have been important mechanisms for deposition of the ancient subterranean deposits slumping of these surficial deposits and associated vegetation can occur along the flanks of sinkholes in addition to sinkhole formation and enter caves however the finer organics some of them comminuted during transport into the caves become part of the cave alluvium this finer organic fraction is the modern analog of the humified organic matter disseminated in slackwater sediment dated in this investigation by radiocarbon methods twenty four 14c ages on humified organic matter provide chronologic control the 13c values of the organic matter reflect the proportion of c4 type to c3 type vegetation growing in and around swallets and sinkholes at the time of redeposition drought tolerant c4 type vegetation was more prevalent relative to c3 type vegetation from 42 500 to 31 200 cal yr b p compared to conditions from 28 800 cal yr b p to the present the 13c values are consistent with the results from other investigations of speleothems and organic matter from loessial paleosols

digital reconstruction and stratigraphic evolution of a microbial dominated isolated carbonate platform terminal proterozoic nama group namibia abstract a terminal proterozoic ca 550 ma isolated carbonate platform developed downdip on a carbonate ramp of the nama group of southern namibia the stratigraphic evolution of the platform was reconstructed from an extensive dataset that was compiled by using digital surveying technologies and traditional outcrop observations digital field geology techniques enabled rapid and accurate mapping over the 10 km outcrop belt with spatial and geological information captured at scales ranging from centimeters to kilometers furthermore these digital geology techniques made the reconstruction and interpretation of the development of the carbonate platform considerably more efficient and quantitative the carbonate platform deposits encompass three accommodation cycles in which each subsequent cycle experienced progressively greater influence of a long term accommodation increase aggradation and progradation during the first cycle produced a flat topped sheet like platform deposits of the first cycle show a coarsening upward motif grading from dominantly columnar stromatolitic thrombolites and interbedded mudstone grainstone at the base to dominantly massive and cross bedded dolostones toward the top the second cycle features aggradation and is typified by the formation of a distinct margin and development of a bucket shaped geometry the margin contained thrombolite mounds and domes whereas columnar stromatolitic thrombolites dominated the platform interior the third and last cycle records a gradual deepening trend with initial aggradation and formation of well bedded thin deposits in the interior and microbial mounds at the margins while the interior drowned the platform margins kept up with rising sea level and complex pinnacle reefs formed which contained fused and coalesced thrombolite mounds flanked by bioclastic grainstones and collapse breccias the top of the unit consists of a set of isolated large thrombolite mounds flanked by shale and is interpreted to indicate the final stage of the carbonate platform before drowning the overall geometric evolution and spatial scale of this platform from flat topped to bucket shaped with elevated margins is recorded in many proterozoic and phanerozoic isolated carbonate platforms this study clearly illustrates that accommodation changes are a fundamental control on the development of carbonate platforms regardless of whether they are formed from microbial or metazoan components or are of precambrian or phanerozoic age significantly the architectural elements and spatial dimensions are comparable throughout earth history

magnetic susceptibility and mineral zonations controlled by provenance in loess along the illinois and central mississippi river valleys abstract magnetic susceptibility ms patterns have proven useful for regional stratigraphic correlations of zones within thick oxidized peoria and roxana silts along the illinois and central mississippi river valleys for more than 350 km variations in ms of c horizon loess are controlled by silt sized magnetite content and are interpreted to reflect changes in sediment provenance due to fluctuations of the superior and lake michigan glacier lobes and the diversion of the mississippi river to its present course grain size distributions and scanning electron microscopic observations indicate that stratigraphic changes in ms are not significantly influenced by eolian sorting or diagenetic dissolution respectively three compositional zones lower middle and upper are delineated within peoria silt which usually can be traced in the field by ms the occurrence of clay beds interstadial soils and or subtle color changes these zones can be correlated with but are generally of more practical use than previously studied dolomite zones mckay 1977 or clay mineral zones frye et al 1968 however mineralogical analyses can help to substantiate zone boundaries when in question ms and compositional zones may indirectly record a climatic signal primarily through the effect that global cooling has had on ice lobe fluctuations in the upper mississippi drainage basin

the geochemistry of loess asian and north american deposits compared abstract loess is widely distributed over asia and north america and constitutes one of the most important surficial deposits that serve as terrestrial records of the quaternary the oldest pleistocene loess in china is likely 2 6 ma thus spanning much or all of the pleistocene in north america most loess is no older than the penultimate glacial period with the exception of alaska where the record may go back to 3 0 ma on both continents loess deposits date primarily to glacial periods and interglacial or interstadial periods are represented by paleosols both glacial and non glacial sources of silts that comprise the bulk of loess deposits are found on both continents although loess has been considered to be representative of the average upper continental crust there are regionally distinctive compositions of loess in both asia and north america loess deposits in asia from yakutia tajikistan and china have compositionally distinct major element compositions due to varying abundances of silicate minerals carbonate minerals and clay minerals in north america loess in the mississippi river valley the great plains and alaska are also distinguishable with regard to major element composition that reflects highly diverse source sediments trace element geochemistry sc th zr and the rare earth elements also shows regional diversity of loess bodies in both asia and north america on both continents most loess bodies show significant contributions from later cycle altered sedimentary rocks as opposed to direct derivation from igneous rocks further some loess bodies have detectable contributions from mafic igneous rocks as well as major contributions from average upper crustal felsic rocks intercalated paleosols in loess sections show geochemical compositions that differ significantly from the underlying loess parent materials ratios of soluble to insoluble elements show depletions in paleosols due to chemical weathering losses of calcite dolomite plagioclase mica apatite and smectite in asia and north america the last interglacial paleosol is more weathered than equivalent modern soils which could be due either to a climate that was warmer and more humid a longer period of pedogenesis or both in asia early pleistocene loess and paleosols are both more weathered than those from the middle and late pleistocene forming prior to a mid pleistocene aridification of asia from uplift of the tibetan plateau understanding the geochemistry of loess and paleosols can tell us much about past atmospheric circulation past temperature and moisture regimes and even tectonic processes

a late quaternary loess paleosol record at kurtak southern siberia a high resolution paleoclimatic record for the last two glacial interglacial cycles has been documented at kurtak in southern siberia a unique 34 m high late quaternary loess section section 29 with a series of 32 variably developed paleosol horizons provides evidence of a strongly fluctuating but patterned climatic change mineralogical composition and grain morphology of the undifferentiated aeolian and colluviated loess indicate a local provenance and a short transport of the silt derived by glacial erosion of the granitic and metamorphic bedrock in the kuznetskiy alatau and eastern sayan mountains and subsequently subaerially redeposited from the alluvial plain in the yenisey river valley the succession of chernozemic brunisolic and gleyed regosolic soils alternating with nonweathered or weakly weathered loess events matched by parallel changes in magnetic susceptibility can be correlated with marine isotope stages 1 7 the magnetic susceptibility pattern is completely opposite to

mesoproterozoic deep water reefs from borden peninsula arctic canada a superbly exposed stromatolite reef complex occurs in the victor bay formation near strathcona river on northern baffin island individual reefs are up to 130 m thick and nearly 1 km in length and their development was clearly related to their position in the facies spectrum and to sea level dynamics in the first sea level cycle metre scale reefs grew amongst mid ramp calcarenites and outer ramp shales during slow sea level rise a 25 m thick oblate reef tract separating mid ramp and outer ramp facies formed during the highstand the greatest period of reef growth was during the second sea level cycle pinnacle reefs nucleated on the karsted upper surface of the oblate reef tract and aggraded rapidly in response to rising sea level producing structures with more than 75 m of depositional relief a gradual symmetrical succession of stromatolite growth forms from stratiform to cylindrical columns to conical columns and then back through cylindrical columns to stratiform is mirrored by evidence in offreef deposits for deepening to a maximum flooding surface and then shallowing the tops of these high standing reefs were karsted during the following regression while dolomite cryptodomes grew as sheets on their submerged flanks and as progradational tongues extending basinward of the reefs continued sea level fall resulted in subaerial exposure of the entire reef complex and the extensive formation of surface and subsurface karst these proterozoic slope buildups are similar to phanerozoic deep water reefs in size shape prevalence of synsedimentary lithification presence of neptunian dykes and in their well developed vertical zonation of reefbuilders however they differ in being constructed exclusively by stromatolites rather than being mud mounds with small skeletal elements and in lacking halos of perireefal sand and gravel sized calcareous debris their responses to changes in sea level were strikingly similar to those shown by their younger counterparts and suggest that sequence stratigraphic concepts derived from studies of phanerozoic reefs can also be applied to the proterozoic

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

origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry muhs d r bettis iii e a aleinikoff j n mcgeehin j p beann j skipp g marshall b d roberts h m johnson w c benton r 2008 origin and paleoclimatic significance of late quaternary loess in nebraska evidence from stratigraphy chronology sedimentology and geochemistry gsa bulletin 120 11 12 1378 1407

loess studies in central united states evolution of concepts abstract few words in the realm of earth science have caused more debate than loess it is a common term that was first used as a name of a silt deposit before it was defined in a scientific sense because this loose deposit is easily distinguished from other more coherent deposits it was recognized as a matter of practical concern and later became the object of much scientific scrutiny loess was first recognized along the rhine valley in germany in the 1830s and was first noted in the united states in 1846 along the lower mississippi river where it later became the center of attention the use of the name eventually spread around the world but its use has not been consistently applied over the years some interpretations and stratigraphy correlations have been validated but others have been hotly contested on conceptual grounds and semantic issues the concept of loess evolved into a complex issue as loess and loess like deposits were discovered in different parts of the us the evolution of concepts in the central us developed in four indefinite stages the eras of 1 discovery and development of hypotheses 2 conditional acceptance of the eolian origin of loess 3 bandwagon popularity of loess research and 4 analytical inquiry on the nature of loess toward the end of the first era around 1900 the popular opinion on the meaning of the term loess shifted from a lithological sense of loose silt to a lithogenetic sense of eolian silt however the dual use of the term fostered a lingering skepticism during the second era that ended in 1944 with an explosion of interest that lasted for more than a decade in 1944 r j russell proposed and h n fisk defended a new non eolian property based concept of loess the eolian advocates reacted with surprise and enthusiasm each side used constrained arguments to show their view of the problem but did not examine the fundamental problem which was not in the proofs of their hypothesis but in the definition of the term between 1944 and about 1950 the debates about loess reached a maximum level of complexity the main semantic problem was submersed in peripheral arguments about physical properties and genetic interpretations the scholarly treatment of the subject by fisk and russell stimulated quality responses from a diversity of earth scientists interested in academic and applied studies particularly geo history pedology soil mechanics and stratigraphy the long lasting popularity of loess studies during the bandwagon era lasted to about 1970 by that time the analytical and technical interests had attracted the mainstream into the fourth era with a focus beyond the old arguments although fisk and russell found themselves defending an unpopular theory they stimulated a scientific interest in the late quaternary history of the mississippi valley that may never be exceeded

questioning the microbial origin of automicrite in ordovician calathid demosponge carbonate mounds calathid demosponge carbonate mounds are a feature of early to middle ordovician shallow marine carbonate depositional environments of tropical to subtropical palaeolatitudes these mounds contain an important amount of autochthonous non skeletal microcrystalline calcium carbonate automicrite conventionally considered microbial in origin here the automicrite of calathid demosponge carbonate mounds tarim basin north west china is broken down into five distinct fabrics an in situ peloidal spiculiferous fabric am 1 an in situ peloidal fabric am 2 an aphanitic microtubular fabric am 3 a minipeloidal fabric am 4 and a laminoid cerebroid fabric am 5 type am 1 occurs with am 2 being succeeded by an assemblage of am 3 and am 4 types am 4 and am 5 are separated by an erosional disconformity a good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite probably via the permineralization of non living organic substrates adsorbing dissolved metal humate complexes using a spreadsheet with six parameters and 17 characters am 1 to am 4 turn out to be non microbial in origin instead these automicrites represent relics of calcified metazoan tissues such as siliceous sponges non spiculate sponges or the basal attachment structures of stalked invertebrates fabric am 5 is a microbial carbonate but is post mound in origin forming a drape within a reefal framework established by am 4 the five automicritic fabrics individually or as an assemblage are a common element of ordovician calathid demosponge carbonate mounds in general the reassessment of the origins of these automicritic fabrics holds consequences for understanding of the great ordovician biodiversification event in terms of community structure reef ecology and reef evolution episodically these fabrics are also present in other carbonate build ups stretching from the neoproterozoic over the entire phanerozoic eon the massive calcification of metazoan soft tissue am 1 to am 4 characterizes episodes and conditions of enhanced marine calcification and might be of value to refine secular trends of pco2 ca concentration and mg ca ratio at the scale of individual sedimentary basins

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt

age dating and the orbital theory of the ice ages development of a high resolution 0 to 300 000 year chronostratigraphy using the concept of orbital tuning a continuous high resolution deep sea chronostratigraphy has been developed spanning the last 300 000 yr the chronology is developed using a stacked oxygen isotope stratigraphy and four different orbital tuning approaches each of which is based upon a different assumption concerning the response of the orbital signal recorded in the data each approach yields a separate chronology the error measured by the standard deviation about the average of these four results which represents the best chronology has an average magnitude of only 2500 yr this small value indicates that the chronology produced is insensitive to the specific orbital tuning technique used excellent convergence between chronologies developed using each of five different paleoclimatological indicators from a single core is also obtained the resultant chronology is also insensitive to the specific indicator used the error associated with each tuning approach is estimated independently and propagated through to the average result the resulting error estimate is independent of that associated with the degree of convergence and has an average magnitude of 3500 yr in excellent agreement with the 2500 yr estimate transfer of the final chronology to the stacked record leads to an estimated error of 1500 yr thus the final chronology has an average error of 5000 yr

composition and history of giant carbonate seep mounds mesoproterozoic borden basin arctic canada abstract the mesoproterozoic ikpiarjuk formation borden basin nunavut consists of a series of very large 200 m thick kms diameter dolostone mounds that accumulated on the floor of a restricted basin the mounds are isolated from contemporaneous shallow water carbonate rocks accumulated at the same time as black shale in deep water regions of the basin and are geographically limited to the vicinity of basin scale fault zones the mounds contain extensive deposits of benthic thrombolites a microbial texture that is not common in mesoproterozoic carbonate rocks field relationships of individual mounds indicate that mound growth was initiated during regional deposition of organic rich black shale mounds developed significant relief above the basin floor and some mound tops were eventually exposed and karsted during differential uplift the mounds accumulated in a deep water sub photic zone setting as a result of fluid venting along basin scale faults interpreted basin depths indicate that the thrombolitic mound texture is probably not related to cyanobacteria chemosynthetic microbes exploited the chemical gradient in the vicinity of the vents and built a mound framework of thrombolites framework voids contain both carbonate mud and isopachous linings of fibrous cement indicating that the mixing of vent fluid with basin water favoured production of abiogenic carbonate minerals in addition to providing a chemical gradient for microbes to exploit proterozoic deep water carbonates and thrombolites are rare and the ikpiarjuk formation mounds represent microbial buildups that are fundamentally different from other examples in the precambrian rock record

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

signature of magnetic enhancement in a loessic soil in nebraska united states of america our study of a loessic soil profile from east central nebraska shows that the a horizons of the modern soil are characterized by higher concentrations of fine grained b0 1 am magnetic minerals this pedogenic magnetic component leads to higher values of concentration dependent parameters such as magnetic susceptibility v isothermal remanent magnetization irm and anhysteretic remanent magnetization arm combined with increases in frequency dependent susceptibility vfd and arm irm ratios hysteresis properties are relatively insensitive towards the presence of this pedogenic magnetic component the magnetic properties of the soil profile are dominated by ferrimagnetic magnetite or maghemite analyses of bsoftq irm sirm and bhardq irm hirm however do show that approximately 80 90 of the remanence carrying magnetic component exists in the form of hematite or goethite and that the magnetically enhanced horizons are enriched in both ferri and antiferromagnetic minerals the pedogenic magnetic component is most likely caused by the conversion of paramagnetic iron bearing minerals to ferriand antiferromagnetic minerals soil compaction lessivage or decalcification cannot explain the observed magnetic soil properties magnetic analyses of loess paleosol sequences from the midwestern united states may yield valuable information about regional variability of paleoclimate based on the fine grained nature of the pedogenic magnetic component we expect grain size dependent proxies arm arm irm vfd to yield better paleoclimatic information than low field magnetic susceptibility used in previous analyses d 2004 elsevier b v all rights reserved

microbial carbonates the geological record of the calcified bacterial algal mats and biofilms summary deposits produced by microbial growth and metabolism have been important components of carbonate sediments since the archaean geologically best known in seas and lakes microbial carbonates are also important at the present day in fluviatile spring cave and soil environments the principal organisms involved are bacteria particularly cyanobacteria small algae and fungi that participate in the growth of microbial biofilms and mats grain trapping is locally important but the key process is precipitation producing reefal accumulations of calcified microbes and enhancing mat accretion and preservation various metabolic processes such as photosynthetic uptake of co2 and or hco3 by cyanobacteria and ammonification denitrification and sulphate reduction by other bacteria can increase alkalinity and stimulate carbonate precipitation extracellular polymeric substances widely produced by microbes for attachment and protection are important in providing nucleation sites and facilitating sediment trapping microbial carbonate microfabrics are heterogeneous they commonly incorporate trapped particles and in situ algae and invertebrates and crystals form around bacterial cells but the main component is dense clotted or peloidal micrite resulting from calcification of bacterial cells sheaths and biofilm and from phytoplankton stimulated whiting nucleation interpretation of these texturally convergent and often inscrutable fabrics is a challenge conspicuous accumulations are large domes and columns with laminated stromatolite clotted thrombolite and other macrofabrics which may be either agglutinated or mainly composed of calcified or spar encrusted microbes stromatolite lamination appears to be primary but clotted thrombolite fabrics can be primary or secondary microbial precipitation also contributes to hot spring travertine cold spring mound calcrete cave crust and coated grain deposits as well as influencing carbonate cementation recrystallization and replacement microbial carbonate is biologically stimulated but also requires favourable saturation state in ambient water and thus relies uniquely on a combination of biotic and abiotic factors this overriding environmental control is seen at the present day by the localization of microbial carbonates in calcareous streams and springs and in shallow tropical seas and in the past by temporal variation in abundance of marine microbial carbonates patterns of cyanobacterial calcification and microbial dome formation through time appear to reflect fluctuations in seawater chemistry stromatolites appeared at 3450 ma and were generally diverse and abundant from 2800 to 1000 ma inception of a proterozoic decline variously identified at 2000 1000 and 675 ma has been attributed to eukaryote competition and or reduced lithification thrombolites and dendrolites mainly formed by calcified cyanobacteria became important early in the palaeozoic and reappeared in the late devonian microbial carbonates retained importance through much of the mesozoic became scarcer in marine environments in the cenozoic but locally re emerged as large agglutinated domes possibly reflecting increased algal involvement and thick micritic reef crusts in the late neogene famous modern examples at shark bay and lee stocking island are composite coarse agglutinated domes and columns with complex bacterial algal mats occurring in environments that are both stressed and current swept products of mat evolution ecological refugia sites of enhanced early lithification or all three

modern sangamon and yarmouth soil development in loess of unglaciated southwestern illinois abstract the thebes section in unglaciated southwestern illinois contains a well preserved 500 kyr loess paleosol sequence with four loesses and three interglacial soils various magnetic mineralogical and elemental properties were analyzed and compared over the thickness of soil sola these proxies for soil development intensity have the following trend yarmouth geosol sangamon geosol modern soil quartz plagioclase zr sr and tio 2 na 2 o ratios were most sensitive to weathering frequency dependent magnetic susceptibility and anhysteretic remanent magnetization greatest in a horizons also correspond well with soil development intensity neoformed mixed layered kaolinite expandables suggestive of a warm humid climate were detected in the sangamon and yarmouth soil sola clay illuviation in soils was among the least sensitive indicators of soil development differences in properties among interglacial soils are interpreted to primarily reflect soil development duration with climatic effects being secondary assuming logarithmic decreases in weathering rates the observed weathering in the sangamon geosol is consistent with 50 kyr of interglacial weathering oxygen isotope stage 5 compared to 10 kyr for the modern soil oxygen isotope stage 1 we propose that the yarmouth geosol in the central midwest formed over 180 kyr of interglacial weathering including oxygen isotope stages 7 9 and 11

carbonate platform development in a paleoproterozoic extensional basin vempalle formation cuddapah basin india abstract sedimentological investigation of the late paleoproterozoic orosirian vempalle formation of the cuddapah basin dharwar craton india reveals three facies association that range from supratidal to deep subtidal sedimentary rocks of this succession are dominated by heterolithic carbonate mudstone intraformational carbonate conglomerate and a variety of columnar domal and stratiform microbialite facies deposition occurred in an extensional regime during development of a low gradient ramp where the distribution of microbialite facies is distinctly depth partitioned a gradual increase in synoptic relief of columnar stromatolites through the section and the upward transition from stratiform to columnar microbialites record a prolonged marine transgression with little or no influx of terrigenous detritus siliciclastic influx along the northeastern side of the shelf reflects the redistribution of topographic highs concomitant with large scale volcanic activity redistribution of topographic highs eventually led to progradation of peritidal facies and shutting down of the carbonate factory earthquake induced ground shaking and voluminous volcanism experienced by this platform point to the reactivation of a deep seated mantle plume that resulted in thermal doming of the dharwar crust prior to the onset of cuddapah deposition isotopic and elemental chemistry of a selection of vempalle formation carbonate rocks record elevated mn2 and fe2 concentrations and depleted carbon isotope values in inner ramp lagoonal facies relative to more open marine stromatolitic facies patterns of isotopic and elemental variation suggest the presence of geochemically distinct water masses either within the water column or within substrate pore fluids that resulted from a combination of globally low marine oxygenation and restricted oceanographic circulation in inner ramp environments these data suggest that even in the aftermath of early paleoproterozoic oxygenation that ocean chemistry was heterogeneous and strongly affected by local basin conditions

calcified rivulariaceans from the ordovician of the tarim basin northwest china phanerozoic lagoonal examples and possible controlling factors abstract a distinctive association of rivulariacean like calcified microfossils is recognized in back reef lagoon facies on the bachu tazhong platform in the lianglitag formation katian stage upper ordovician of the tarim basin based on investigation of 4500 thin sections from 35 well drill cores the genera include hedstroemia ortonella zonotrichites cayeuxia and garwoodia most of which have features comparable with present day calcified cyanobacteria such as rivularia calothrix and dichothrix rivulariaceae nostocales a similar association is present in lagoonal and other restricted nearshore shallow marine carbonate environments during much of the paleozoic and mesozoic this suggests the sustained presence of a rivulariacean dominated cyanobacterial association characteristic of back reef lagoonal environments at the present day uncalcified rivularia calothrix and dichothrix remain common in back reef lagoon mangrove swamp rocky shore salt marsh and saline lake environments the ability of these cyanobacteria to grow in environments low in inorganic nitrate and phosphate could help to explain this distribution cenozoic decline in marine calcified rivulariaceans is attributed to global reduction of seawater carbonate saturation state the phanerozoic record of calcified rivulariacean cyanobacteria appears to sensitively reflect long term variations in the carbonate and nutrient chemistry of marine environments

microbial metazoan reefs of the terminal proterozoic nama group c 550 543 ma namibia thrombolite and stromatolite reefs occur at several stratigraphic levels within the terminal proterozoic nama group c 550 543 ma of central and southern namibia the reefs form integral parts of several carbonate platforms within the nama group including the kuibis platform of the northern nama basin zaris subbasin and huns platform witputs subbasin of the southern nama basin the reefs are composed of both thrombolites and stromatolites that form laterally continuous biostromes isolated patch reefs and isolated pinnacle reefs ranging in scale from a metre to several kilometres in width in the majority of cases the reefs occur stratigraphically as an integral facies within the transgressive systems tracts of sequences making up the kuibis and huns platforms this suggests that a regime of increasing accommodation was required to form well developed reefs though reefs also occur sporadically in highstand systems tract settings within a given transgressive systems tract a regime of increasing accommodation through time favours the transition from sheet like biostromal geometries to more isolated patch and pinnacle biohermal geometries similarly increasing accommodation in space such as a transect down depositional dip shows a similar transition from more sheet like geometries in updip positions to more isolated geometries in downdip positions reefal facies consist of thrombolitic domes columns and mounds with well developed internal clotted textures in addition to stromatolitic domes columns and mounds with crudely to moderately well developed internal lamination stromatolites are better developed in conditions of relatively low accommodation and updip locations under conditions of higher current velocities and greater sediment influx thrombolites are better developed in conditions of relatively high accommodation and low sediment influx both types of microbialites are intimately associated with the first calcifying metazoan organisms which may have attached themselves to the sediment surface or otherwise lived within sheltered depressions within the rough topography created by ecologically complex mats the appearance of thrombolitic textures during terminal proterozoic time is consistent with colonization of cyanobacterial mats by higher algae and metazoans which would have been an important process in generating clotted textures fabrics in the nama thrombolites are well preserved and show evidence of thrombolitic mesoclots being overgrown by fibrous marine carbonate interpreted as former aragonite this was followed by emplacement of geopetal micrite fills and precipitation of dolomite as an isopachous rim cement followed by occlusion of remaining porosity by blocky calcite spar

enigmatic chambered structures in cryogenian reefs the oldest sponge grade organisms abstract previously undescribed chambered structures are common and widespread in the cryogenian post sturtian glacial carbonates of the oodnaminta reef complex adelaide geosyncline south australia the rasthof berg aukas formation namibia and the gauss formation namibia these carbonate structures have millimetre to centimetre scale chambers separated by well defined and generally thin micritic walls chamber walls now consist of dolomite but were probably originally aragonitic the chambers may have a lobate polygonal or dendritic morphology and are often further divided into smaller chambers chambered structures occur as reefal growth frameworks as cavity fillings in neptunean dykes and growth cavities and as intercolumnar material within stromatolite frameworks in the oodnaminta reef complex these structures are only present in the sub photic deep water framework these structures probably represent the calcified remains of an organism or community of organisms that was globally distributed and widespread for a significant time period following the sturtian glaciation no precisely analogous structures have been previously described from modern or ancient settings but the complexity and degree of organization suggests a significant evolutionary advance over older proterozoic fossils the closest morphological analogues for the structures are a some types of reef dwelling sponges and b some complex microbialites from archaean and paleoproterozoic carbonates the structures lack spicules and ostia found in sponges ruling out a true poriferan origin however it is plausible that they are proto sponges sponge grade organisms or complex microbial precursors to sponge grade organisms whatever their affinity we suggest these structures record a significant evolutionary event on the path towards organic complexity

characterisation of soils by mineral magnetic measurements processes of iron mineral authigenesis diagenesis and dissolution in interaction with primary inputs of iron minerals act to produce vertical differentiation of soil magnetic properties thus magnetic iron oxides in common with other iron forms may both respond to and reflect soil forming processes investigation of the degree of detectable and persistent magnetic differentiation within different soil materials can provide insights into the nature and direction of distinct sets of soil processes evidence of clear association between magnetic variation and discrete types of soil environment also has applied significance in several areas of environmental study for example the observed magnetic variation can be used to discriminate between individual soils and soil horizons for the purposes of soil surveying sediment tagging and tracing and empirical modelling of source sediment linkages this paper reports the use of mineral magnetic techniques to characterise the iron oxide assemblages within soils of varying type and provenance the effect of different pedogenic regimes have been investigated through measurement of a range of magnetic parameters including magnetic susceptibility frequency dependent susceptibility and anhysteretic and saturation remanences contrary to current pedological thought but in accord with other magnetically based studies e g mullins the presence of magnetite maghemite typically of superparamagnetic single domain grain size within soils has been found to be an extremely widespread phenomenon its contemporary neoformation within the soil environment is inferred however the input of artificially generated ferrimagnetic material is indicated for one of the magnetically enhanced soils examined here and the possibility of extraneous sources of magnetite in topsoils should be considered before pedogenic processes of magnetic enhancement are inferred from the data presented discrimination of industrially derived magnetic particles appears possible on the basis of their very low levels of frequency dependent susceptibility fd low anhysteretic remanence arm and dissociation from the finer clay sized not the formation but the active dissolution of ferrimagnetic minerals is indicated for soils affected by the processes of gleying and podsolisation eluvial horizons of podsolised profiles demonstrate almost total loss of detectable magnetic content variations in the form of the iron reprecipitated in the underlying illuvial layers may reflect differences in the pedological characteristics of these horizons within gleyed waterlogged soils processes of magnetite dissolution appear to be grain size specific preferential reduction of those grains of ultrafine superparamagnetic and fine single domain magnetic grain size is indicated where soil forms a major contributor to catchment sediment loads this has significance in the area of source sediment modelling soil derived from permanently wet or highly leached catchment areas is unlikely to contribute to peaks in the magnetic content of deposited sediments further only those erosional processes that operate at a faster rate than those of magnetic depletion will have the capacity to produce peaks in sediment ferrimagnetic concentrations such as have been observed within the historical magnetic record

broad synchroneity of three gold mineralization styles in the kalgoorlie gold field shrimp u pb and 40ar 39ar geochronological evidence there has been a long standing controversy regarding the timing and number of gold mineralization events at kalgoorlie uranium pb dating of zircons and hydrothermal monazite and xenotime as well as ar 40 ar 39 analysis of metasomatic fuchsite and white mica are used to date pre to synore dikes alteration and orebodies in order to resolve this issue the majority of gold mineralization at kalgoorlie including ductile brittle fimiston brittle ductile oroya and brittle mount charlotte style gold are different expressions of a complex mineralizing system that was active at broadly the same time at ca 2 64 ga gold mineralization was thus deposited in both ductile and brittle structures at approximately the same crustal level at broadly the same time limier similar pt conditions this giant ore system formed after ea 2 69 ga basic magmatism intrusion of the golden mile dolerite sill at 2680 9 ma and intrusion or calc alkaline feldspar quartz porphyry dikes at 2670 5 ma gold mineralization was broadly coeval with lamprophyre dike intrusion at 2642 6 ma and overlapped the waning stages of hornblende and albite bearing porphyry dike emplacement at 2650 6 ma and regional metamorphism subsequent brittle deformation in the kalgoorlie gold held way accompanied by hydrothermal activity that may have led to some late gold mineralization or remobilization in extensional quartz vein arrays in the golden mile between about 2 61 and 2 60 ca this late hydrothermal activity and associated brittle deformation marks the last event to significantly affect the rocks it kalgoorlie and may be related to uplift lift and final cooling of the terrane despite this late event the geometry of the kalgoorlie gold field and its contained lode systems has remained essentially the same since the time of gold mineralization

shrimp u pb analyses of detrital zircon a window to understanding the paleoproterozoic development of the tanami region northern australia poorly exposed paleoproterozoic turbiditic to shallow marine sedimentary rocks of the tanami basin nt australia are largely the erosional products of either the 1 87 1 85 hooper orogeny and or magmatism associated with the 1 87 ga nimbuwah event dating of detrital zircon from six of the principal sedimentary units shows that deposition spanned at least the period 1 84 1 77 ga collectively the detrital zircon ages reveal a progression in provenance that is a record of the development of the orogen the basal dead bullock formation contains only zircon derived from archean basement and no contemporaneous products of orogeny its deposition age is inferred to be 1 87 1 84 ga orogenic 1 86 ga zircon appears in the overlying killi killi formation deposited between 1 84 1 82 ga and persists probably due to recycling in all overlying units except one the mount charles formation the accepted stratigraphic position of this unit might be incorrect

the role of decarbonization and structure in the callie gold deposit tanami region of northern australia the callie deposit is the largest 6 0 moz au of several gold deposits in the dead bullock soak goldfield of the northern territory s tanami region 550 km northwest of alice springs the callie ore lies within corridors up to 180 m wide of sheeted en echelon quartz veins where they intersect the 500 m wide hinge of an ese plunging f1 anticlinorium the host rocks are the blake beds of the paleoproterozoic dead bullock formation which consist of a 350 m thick sequence of lower greenschist facies graphitic turbidites and mudstones overlying in excess of 100 m of thickly bedded siltstones and fine sandstones the rocks are fe rich and dominated by assemblages of chlorite and biotite both of which are of hydrothermal and metamorphic origin a fundamental characteristic of the hydrothermal alteration is the removal of graphite a process which is associated with bleaching and the development of bedding parallel bands of coarse biotite augen gold is found only in quartz veins and only where they cut decarbonized chloritic rock with abundant biotite augen and no sulfide minerals auriferous quartz veins differ from barren quartz veins by the presence of ilmenite apatite xenotime and gold and the absence of sulfide minerals the assemblage of gold ilmenite apatite xenotime indicates a linked genesis and mobility of ti p and y in the mineralizing fluids geochemical analysis of samples throughout the deposit shows that gold only occurs in sedimentary rocks with high feo feo fe2o3 and low c c co2 ratios 0 8 and 0 2 respectively this association can be explained by reactions that convert c from reduced graphitic host rocks into co2 and reduce ferric iron in the host rocks to ferrous iron in biotite and chlorite these reactions would increase the co2 content of the fluid facilitating the transport of ti p and y from the host rocks into the veins both co2 and ch4 produced by reaction of h2o with graphite effervesced under the lower confining pressures in the veins this would have partitioned h2s into the vapor phase destabilizing au bisulfide complexes the loss of co2 and h2s from the aqueous phase caused precipitation of gold ilmenite apatite and xenotime it is proposed that this process was the main control on gold precipitation oxidization of iron in the very reduced wall rocks resulting in reduction of the fluid provided a second mechanism of gold precipitation in previously decarbonized rocks contributing to the high grades in some samples although sulfide minerals especially arsenopyrite did form during the hydrothermal event host rock sulfidation reactions did not play a role in gold precipitation because gold is absent near rocks or veins containing sulfide minerals sulfide minerals likely formed by different mechanisms from those associated with gold deposition both the fold architecture and subsequent spatially coincident sinistral semibrittle shearing ensured that the ore fluids were strongly focused into the hinges of the anticlines within the anticlines a reactive cap of fine grained graphitic reduced fe rich turbidites above more permeable siltstones and fine sandstones impeded fluid flow ensuring efficient removal of graphite and the associated effervescence of co2 from the fluid caused the precipitation of gold exploration for similar deposits should focus on the intersection of east west shear zones with folds and fe rich graphitic host rocks

chambered structures from the ediacaran dengying formation yunnan china comparison with the cryogenian analogues and their microbial interpretation enigmatic chambered structures have been reported forming reef frames in cryogenian interglacial carbonates prior to the commonly acknowledged microbial metazoan reefs at the terminal ediacaran and interpreted as fossils of possible sponge grade organisms a better constraint on the affinity of these structures is partly hindered by few analogues in other time periods this study describes similar structures from peritidal dolostones of the ediacaran denying formation from yunnan china samples were investigated using optical microscopy and three dimensional 3 d reconstruction based on grinding tomography the dengying chambered structures are comparable with cryogenian structures in basic construction but are not frame building and show variations in overall shape and inhabiting facies two dimensional 2 d cross sections show that thin homogeneous micritic laminae are the basic building blocks of the chamber walls thick walls represent parallel accretion of these laminae and thin walls developed from the angular growth of a single lamina or merging of multiple laminae in 3 d space the laminae primarily correspond to continuous surfaces which sometimes contain sub circular holes while a few represent filamentous elements connected to the surfaces the morphological features and growth pattern of the dengying chambered structures indicate that they are likely to be calcified microbial constructions rather than skeletal remains of basic metazoans however aside from the cryogenian and dengying examples comparable chambered constructions with laminae based architecture are yet unknown from other fossil or extant microbialites further work investigating related structures is needed to determine the microbial consortia and controlling environmental factors that produced these chambered structures

origin of last glacial loess in the western yukon tanana upland central alaska usa loess is widespread over alaska and its accumulation has traditionally been associated with glacial periods surprisingly loess deposits securely dated to the last glacial period are rare in alaska and paleowind reconstructions for this time period are limited to inferences from dune orientations we report a rare occurrence of loess deposits dating to the last glacial period 19 ka to 12 ka in the yukon tanana upland loess in this area is very coarse grained abundant coarse silt with decreases in particle size moving south of the yukon river implying that the drainage basin of this river was the main source geochemical data show however that the tanana river valley to the south is also a likely distal source the occurrence of last glacial loess with sources to both the south and north is explained by both regional synoptic scale winds from the northeast and opposing katabatic winds that could have developed from expanded glaciers in both the brooks range to the north and the alaska range to the south based on a comparison with recent climate modeling for the last glacial period seasonality of dust transport may also have played a role in bringing about contributions from both northern and southern sources

palaeoecology and evolution of marine hard substrate communities abstract marine organisms have occupied hard substrates since the archaean shells rocks wood and sedimentary hardgrounds offer relatively stable habitats compared to unconsolidated sediments but the plants and animals which inhabit them must develop means to gain and defend this premium attachment space hard substrate communities are formed by organisms with a variety of strategies for adhering to and or excavating the substrates they inhabit while mobile grazers organically attached and even soft bodied organisms may leave evidence of their former presence in ancient hard substrate communities a superior fossil record is left by sessile encrusters with mineralised skeletons and by borers which leave trace fossils furthermore encrusters and borers are preserved in situ retaining their spatial relationships to one another and to the substrate spatial competition ecological succession oriented growth and differential utilisation of exposed vs hidden substrate surfaces can all be observed or inferred hard substrate communities are thus excellent systems with which to study community evolution over hundreds of millions of years here we review the research on modern and ancient hard substrate communities and point to some changes that have affected them over geological time scales such changes include a general increase in bioerosion of hard substrates particularly carbonate surfaces through the phanerozoic this is at least in part analogous to the infaunalisation trends seen in soft substrate communities encrusting forms show an increase in skeletalisation from the palaeozoic into the mesozoic and cenozoic which may be a response to increasing levels of predation hard substrate communities considering borers and encrusters together show a rough increase in tiering through the phanerozoic which again parallels trends seen in soft substrate communities this extensive review of the literature on living and fossil hard substrate organisms shows that many opportunities remain for large scale studies of trends through time at the community and clade levels palaeontologists will especially benefit by closer integration of their work with that of neontologists particularly in aspects of ecology such as larval recruitment competition and succession

last glacial loess in the conterminous usa the conterminous united states contains an extensive and generally well studied record of last glacial loess the loess occurs in diverse physiographic provinces and under a wide range of climatic and ecological conditions both glacial and non glacial loess sources are present and many properties of the loess vary systematically with distance from loess sources united states midcontinent last glacial loess is probably the thickest in the world and our calculated mass accumulation rates mars are as high as 17 500 g m 2 yr at the bignell hill locality in nebraska and many near source localities have mars greater than 1500 g m 2 yr these mars are high relative to rates calculated in other loess provinces around the world recent models of last glacial dust sources fail to predict the extent and magnitude of dust flux from the mid continent of the united states a better understanding of linkages between climate ice sheet behaviour routing of glacial meltwater land surface processes beyond the ice margin and vegetation is needed to improve the predictive capabilities of models simulating dust flux from this region r 2003 elsevier ltd all rights reserved

petrology of lower cretaceous carbonate mud mounds albian n spain insights into organomineralic deposits of the geological record the process of organomineralization is increasingly well understood with respect to modern carbonate sediments accumulating adjacent to tropical reef atolls and reef caves mineralization related to non living organic substrates results in autochthonous micrite production automicrites automicrites are the main constructive element of lower cretaceous albian carbonate mud mounds in northern spain these slope mud mounds occur within transgressive and early highstand system tracts encompassing several macrobenthic ecological zones they are clearly separated from the biocalcifying carbonate factory urgonian carbonate platforms in both space and time within these build ups most automicrites were initially indurated and accreted to form a medium relief growth framework automicrites have a uniform presumably high mg calcite precursor mineralogy they show an inorganic stable isotope signature 13c around 3 3 within the range of early marine cements and skeletal compounds lacking major vital effects epifluorescence microscopy shows that they have facies specific fluorescence which is similar to skeletal compounds of acanthochaetetes but clearly different from allomicritic sediment and cements which are mostly non fluorescent the edta soluble intracrystalline organic fraction siof of albian automicrites shows an amino acid spectrum that is similar to shallow subsurface samples from their modern counterparts gel electrophoresis of the siof demonstrates an exclusively acidic character and a mean molecular size range between 20 and 30 kda experiments in vitro inhibition tests indicate that the siof has a significant ca2 binding capacity fluorescence and chemical characteristics of siof point to a main substance class such as humic and fulvic acids compounds that form from pristine organic matter during early diagenesis biomarker analyses provide evidence for the crucial role of biodegradation by heterotrophic microorganisms but no biomarker for cyanobacteria has been found primary sources of organic material should have been manifold including major contributions by metazoans such as sponges it is concluded that many carbonate mud mounds are essentially organomineralic in origin and that the resulting fabric of polygenetic muds polymuds may represent ancestral metazoan reef ecosystems which possibly originated during the neoproterozoic

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

evidence for the dissolution of magnetite in recent scottish peats abstract magnetic hydrological and geochemical data for 11 scottish peat cores confirm previous reports that anthropogenic pollutants constitute the principal source of ferrimagnets to such environments but indicate that postdepositional fe3o4 persistence may be more limited than formerly envisaged in all cores the lower stratigraphic limit of magnetic enrichment coincides with the water table below which negative mv values and enhanced ratios of fe mn and cu zn depict an abrupt transition to a strongly anoxic regime dissolution of fe3o4 in the reducing inundated zone is also signified by the more rapid down core loss of magnetic properties in cores from topographically depressed sphagnum lawns than in those from more elevated calluna hummocks as a consequence the peak and cumulative sirm values for cores from the former settings appear disproportionately low and the magnetostratigraphic records from adjacent but topographically variant sites are frequently asynchronous in contrast to most marine and lacustrine sediments dissolution of fe3o4 in peat induces a refinement of magnetic mineral assemblages highlighted by declining sirm arm ratios with depth and a hardening of remanence near the base of the magnetically enhanced strata sem micrographs of magnetic extracts from one scottish core indicate that this refinement results from preferential dissolution along crystallographic weaknesses within multidomain phases ultimately causing their disintegration and the adoption of single domain type magnetic behavior rates of dissolution are considerably greater than those reported for marine sediments probably due to the unusually high dissolved sulfide concentrations in mire waters

roxana silt of the upper mississippi valley lithology source and paleoenvironment significant lithological differences exist between the middle wisconsinan roxana silt loess of the upper mississippi valley the valley north of 41 5 and that of the type area near st louis missouri primary differences are that the upper mississippi valley roxana silt is much thinner finer textured and more weathered than that of the type area extrapolation of radiocarbon dates brackets sedimentation of upper mississippi valley roxana silt between 55 and 27 ka which is in close agreement with dates from the type area radiocarbon dates and paleobotanical data indicate that the upper mississippi valley roxana silt accumulated at a maximum long term average sedimentation rate of about 7 cm 1 000 yr within a boreal forest environment whereas the type area roxana silt accumulated at a maximum long term average sedimentation rate of about 60 cm 1 000 yr within a prairie like environment multiple regression analysis shows that more than 75 of the variance in thickness and texture between the upper mississippi valley roxana silt and that of the type area is explained by parameters of source valley width source valley azimuth and distance of the depositional site from the source valley this indicates that although the lithologic characteristics and sedimentation rates between the two regions are quite different they are consistent with a unified loess sedimentation sys tem linked to the ancient mississippi river valley a proglacial source for the roxana silt and a middle wisconsinan advance of the lauren tide ice sheet is a topic of scientific debate because recent findings show a lack of roxana correlative till deposits in the midwest alternative hypotheses include a glaciolacustrine source a regional slope erosion source and a cold climate desert dust source tests of these hypotheses support a proglacial origin for the roxana silt multiple regression analysis of loess thickness and texture along with paleobotanical evidence indicates that the roxana silt was blown from the ancient mississippi valley rather than from a cold climate desert source or from flood plains linked to a glaciolacustrine source geochemical and mineralogical composition of the upper mississippi valley roxana silt compared to out wash derived peoria loess indicates that detritus from regional hillslope erosion is not a unique component of the roxana silt combined data indicate that the midcontinental roxana silt was derived as loess blown from flood plains of middle wisconsinan proglacial rivers between 55 and 27 ka

sedimentary geochemistry geochronology and sequence stratigraphy as tools to typecast stratigraphic units and constrain basin evolution in the gold mineralised palaeoproterozoic tanami region northern australia abstract rare earth elements rees high field strength elements and detrital zircon populations typecast and establish the provenance of siliciclastic sedimentary rocks of the palaeoproterozoic tanami group this group is the main host of gold in the tanami region an important proterozoic gold province in northern australia these data also define chronostratigraphic surfaces which provide a framework for constructing sequence stratigraphic models for the tanami basin this basin model provides insight into the likely spatial distribution of deep water facies which are the best trap rocks for epigenetic gold deposition the largest gold deposits are hosted in the well bedded carbonaceous siltstone of the dead bullock formation which is the basal unit of the tanami group at the callie deposit which is the largest deposit in the region the host dead bullock formation is characterised by 1 enrichments of mgo tio2 cr sc 2 light ree depletion relative to paas 3 a dominant population of detrital zircons with an age of 2500 ma in contrast turbidites of the overlying killi killi formation which is less strongly mineralised are characterised by 1 enrichments in zr la sc la sm 2 relatively flat ree pattern and 3 the presence of a major population of detrital zircons with ages of 1880 1840 ma these characteristics are independent of grain size and are interpreted to indicate that the dead bullock formation was derived from a provenance dominated by archean mafic and felsic rocks whereas the killi killi formation was derived from a provenance dominated by felsic rocks of palaeoproterozoic age these characteristics which are best shown using zr sc versus th sc diagrams can be used as an exploration tool to differentiate the more favourable dead bullock formation from the less prospective killi killi formation these data combined with regional geologic data provide a first order understanding of the make up of the tanami basin fill with implications for the distribution of siltstone dominated facies that are the best host rocks for gold

geochemical characterisation of xenotime formation environments using u th abstract xenotime ypo4 is a trace component in many metasedimentary and some igneous rocks altered rocks and many hydrothermal ore assemblages where it forms in response to a range of different processes from igneous crystallisation to low temperature early diagenesis due to its wide range of formation temperatures its suitability and isotopic robustness for reliable u pb geochronology dissolution precipitation characteristics during overprint events and widespread occurrence it is one of the most valuable minerals for u pb geochronology leading to four dimensional 4d studies of many terranes the formation environment of xenotime can be deduced from careful petrography and reliable u pb in situ geochronology within a 4d framework further xenotime is a physically robust mineral during sediment transport and may carry distinctive geochemical fingerprints including age to secondary environments from published works we review the u th contents of well characterised examples of xenotime formation to provide chemical fingerprints which assist in identifying the xenotime formation environment although the u th characteristics of xenotime from all formation environments show considerable overlap we make the following observations which may be distinctive of some ore formation environments i hydrothermal xenotime formed from low salinity ore fluids e g iron ore and orogenic gold deposits trend to have the lowest u contents 10 iii diagenetic xenotime has the most variable and highest u th whereas iv xenotime from precambrian orogenic gold deposits has the least variable and lowest u th petrogenetic inferences from these observations for ore deposit research and exploration include i support for the homogenising effect of the crustal scale of the fluid system in precambrian orogenic gold deposits and ii the potential for distinctive u th geochemical fingerprints for u ores and some au ores these characteristics may be reflected in detrital xenotime grains recovered from routine exploration sampling programs

evidence at lomax illinois for mid wisconsin 40 000 yr b p position of the des moines lobe and for diversion of the mississippi river by the lake michigan lobe 20 350 yr b p abstract an abrupt change in environment from a wetland to a deeper slackwater lake at 20 345 85 yr b p occurred in a second order valley tributary to the mississippi river near lomax illinois the age of this shift is associated with the overflow of glacial lake milan and diversion of the mississippi river from the princeton valley rock island to peoria to its present course rock island to st louis the diversion occurred due to blockage of the princeton valley segment of the ancient mississippi river by the lake michigan lobe impoundment of glacial lake milan and eventual overflow of the lake southwest of rock island across a former drainage divide near andalusia illinois fossil ostracode assemblages in the slackwater sediment at lomax indicate changes in the post diversion full glacial paleohydrology and based on multivariate analysis hint at the area s paleoclimate an older part of the succession at lomax is consistent with glaciation in the upper iowa river basin about 40 000 yr b p aggradation of sediment rich in coarse silt is attributed to a response of the ancient iowa river basin associated with deposition of the glacigenic sheldon creek formation by the des moines lobe