isotopic dating of the migration of a low grade metamorphic front during orogenesis the southern margin of the pilbara craton in northwestern australia underwent regional heating folding and thrusting as well as extensive fluid flow during collision in the paleoproterozoic however the precise timing of this event is uncertain previous geochronologic studies of archean basement rocks have yielded a wide range of younger radiometric dates from ca 2 4 ga to ca 2 0 ga interpreted to record multiple hydrothermal alteration events in situ u pb geochronology of authigenic monazite and xenotime in very low grade archean metasedimentary rocks from across the craton suggests that the pilbara was affected by at least two cryptic thermotectonic events the first between ca 2430 ma and ca 2400 ma and the second between ca 2215 ma and ca 2145 ma the older event is restricted to three localities in the west and its cause is unknown the younger event affected most of the craton 100 000 km 2 spanning a 70 m y period from ca 2215 ma nearest the collisional margin in the south to ca 2145 ma toward the craton interior in the north the widespread geographic and stratigraphic distribution of ca 2 2 ga phosphates suggests that fluid flow was pervasive this event was probably driven by the northward advancing ophthalmian fold and thrust belt that developed during protracted collision the associated low grade metamorphic front migrated over 350 km at an average rate of 5 0 mm yr placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings

deformation induced silica redistribution in banded iron formation hamersley province australia abstract the formation of banded iron formations bif remains controversial despite their potential to provide key information on precambrian atmospheres and hydrospheres it is widely agreed that bif are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases many models address the chemical transformation of such precursor iron oxide phases into bif during compaction and diagenesis however the formation of chert and the influence of physical forces in this process have received less attention microstructural analysis of bif from the hamersley province western australia reveals that significant amounts of silica were redistributed by dissolution precipitation creep during both diagenesis and regional scale deformation this physicochemical process led to silica remobilisation and volume loss by stress induced dissolution of microcrystalline quartz in an aqueous fluid the dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low pressure zones leading to local volume increase these processes were further enhanced by rheological contrasts between different minerals resulting in significant variations of chert band thickness microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations cpo in quartz grains within chert layers the cpo fabrics record strain regimes e g pure and simple shear extension and shortening that modified quartz aggregates by dissolution precipitation creep providing new insights into the metamorphic and deformation history of bif we document microstructures that indicate that non coaxial deformation was active during diagenesis and subsequent deformation of the hamersley province bif further relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution precipitation creep providing a possible explanation for the destruction of primary features we show that different generations of chert alternate on a micro scale suggesting that the role of deformation has been underestimated in many aspects of bif research the discovery of dissolution precipitation creep in low grade metamorphic bif provides a potential link between the physical and chemical processes that occur within these rocks after deposition in the hamersley province dissolution precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events we also suggest that chert layers without cpo record the least overprint by diagenesis and as the most pristine form of bif should be targeted for future paleo environmental research

bottom current depositional model and characterization of the paraburdoo member surrounding the paraburdoo spherule layer hamersley basin western australia abstract the stratigraphy within the wittenoom formation s paraburdoo member of the hamersley basin in western australia has lacked a significant and identifiable marker bed until the recent discovery of the paraburdoo spherule layer psl in this paper we correlate this stratigraphy residing at the three known exposed locations of the psl within the hamersley basin and use these correlations in conjunction with detailed observations and analyses to assess the type of sedimentation recorded by these alternating sand and mud beds we also characterize in detail these same several meters of stratigraphy using the psl as a distinctive marker bed and add to the existing depositional model of the hamersley basin we find that 1 the strata surrounding the psl was not deposited by sediment gravity flows but rather long acting deep marine bottom current s 2 the rhythmicity recorded as alternating sand and mud beds is the result of increasing and decreasing current velocities 3 grain size decreases while mud content increases to the east within the basin 4 bed thinning occurs to the west within the basin 5 microbial activity may be recorded in laminations in the finer grained mud beds present within these strata and 6 the diagenetic history changes across the hamersley basin

a tale of two basins stratigraphy and detrital zircon provenance of the paleoproterozoic turee creek and horseshoe basins of western australia discussion

evidence for microbial life in synsedimentary cavities from 2 75 ga terrestrial environments fluviolacustrine sediments of the 2 75 ga hardey formation fortescue group australia preserve pendant columnar structures with stromatolitic lamination within synsedimentary cavities the millimeter sized finger like columns strongly resemble microbialites from modern basaltic caves and indicate the likely presence of microbial biofilms the ancient microbial columns are preserved by chert and locally occur as reworked clasts indicating a near depositional age for the structures sulfur isotopic analysis of pyrite in the columns and adjacent carbonaceous matrix yields 34scdt cdt canyon diablo troilite values between 8 5 and 19 showing significant fractionation characteristic of biological cycling of sulfur organic matter in cavity ceilings and shale matrix has 13cpdb pdb peedee belemnite values between 55 and 43 suggesting the presence of methanotrophs our results suggest that 2 75 ga terrestrial environments supported a microbial ecosystem including microbes that inhabited synsedimentary hollows extending the fossil record of coelobionts by 1 5 b y subsurface cavities represent a new habitable microenvironment for early life on earth and an analogue for ancient life on mars

dust to dust evidence for the formation of primary hematite dust in banded iron formations via oxidation of iron silicate nanoparticles abstract conventional models for the deposition of banded iron formations bifs envisage the oxidation of upwelled ferrous iron and the precipitation of ferric oxide hydroxide particles in surface waters that settled to form laterally extensive layers of iron rich sediment a fundamental tenet of this model is that fine grained hematite so called dusty hematite in least altered bifs represents the dehydration product of original oxide hydroxide precipitates however this premise has never been proven we have investigated the origin of the earliest formed iron oxides in chert in well preserved bifs of the 2 63 2 45 billion year old hamersley group australia we find that laminated chert in bifs show progressive stages of in situ alteration from gray green chert containing iron silicate nanoparticles to red chert with abundant hematite dust analysis of textures by transmission electron microscopy of samples from the transition zone between gray green and red chert reveals that dusty hematite formed after partial dissolution of iron silicate nanoparticles by the precipitation of iron oxides in resulting cavities these observations suggest that hematite dust is not a relict of an original seawater precipitate but the end product of post depositional oxidation our observations are consistent with paleomagnetic results from the hamersley group which record two major phases of magnetic remanence carried by hematite that post date deposition by more than 200 million years our results may provide an alternative explanation for the origin of jasper in bifs deposited before the start of the great oxidation event about 2 4 billion years ago if correct it follows that hematite dust is not a reliable proxy for paleoenvironmental conditions or biological processes in early precambrian seawater furthermore our results suggest that the primary iron precipitate in bifs was iron silicate mud that was silicified at or just below the sediment water interface a hypothesis that requires neither dissolved oxygen nor photosynthetic life but was an inorganic chemical process reflecting anoxic oceans enriched in iron and silica

biogenic versus metamorphic origins of diverse microtubes in 2 7 gyr old volcanic ashes multi scale investigations abstract titanite bearing microtubes in metabasalts have been interpreted as evidence for early microbial life thriving in the oceanic crust up to 3 5 ga ago based on similarities with microtubular pitting occurring in modern volcanic glass an alternative abiogenic origin as ambient inclusions trails aits has been debated but textural evidence such as the absence of diagnostic terminal inclusions was used to discard this possibility here we describe a variety of microtubes occurring in pyroclastic tuff of the 2 72 ga tumbiana formation western australia some of which resemble titanite filled microtubes reported in metamorphosed archean pillow basalts to assess the origin of the tumbiana microtubes we investigated their structure and chemistry and their root zone down to the sub micrometer scale using a combination of raman microspectroscopy scanning electron microscopy sem focused ion beam fib milling and scanning transmission electron microscopy stem coupled with energy dispersive x ray spectrometry edxs we report four types of microtubes type 1 comprises calcite microtubes type 2 comprises aits showing terminal pyrite and trails with negative crystal shapes type 3 comprises titanite microtubes in chlorite displaying morphologies similar to those reported in 3 5 ga old archean lavas at the sub micrometer scale they display a dendritic structure depending on their localization type 3 microtubes have been divided in three main sub types i type 3a extends in pores filled by chloritized cement ii type 3b occurs in pyroclasts and iii type 3c overgrows and thus post dates type 2 aits type 4 comprises titanite microtubes confined to quartz and calcite rims types 3 and 4 microtubes are terminated by sub micrometric carbonaceous and or mineral inclusions altogether these features suggest that the tumbiana titanite microtubes have formed as dendritic aits associated with displacement of mineral and or organic inclusions in pyroclasts and in surrounding void filling cements during recrystallization they are therefore unlikely to represent relics of the activity of early microbial life rather abiotic features formed in the presence of organic matter during burial metamorphism in that sense they may represent some sort of indirect evidence for life

the ridgeway gold copper deposit a high grade alkalic porphyry deposit in the lachlan fold belt new south wales australia a discussion sir wilson et al 2003 in their important paper on the ridgeway au cu deposit describe hydrothermal alteration about the ridgeway porphyry ore system however their findings are substantially more valuable if taken into account with the published works on regional burial metamorphism of the district smith 1968 1969 these works described burial metamorphism of the ordovician volcanic sequences including the hosts to the ridgeway deposit based on district and regional zonation of secondary mineral assemblages this discussion briefly presents this context and focuses on the key issue of discriminating the porphyry related hydrothermal alteration from the effects of burial metamorphism because porphyry related hydrothermal minerals and later metamorphic mineral assemblages can be similar their discrimination is crucial in mapping alteration styles as vectors in exploration for concealed ore

tempe fossae mars a planetary analogon to a terrestrial continental rift tempe terra the northeastern part of the tharsis region on mars is characterized by several extensional structures differing in style and age the tempe fossae a particularly well developed system of graben striking n45 e have been studied for the first time in detail on the basis of viking orbiter imagery and mars observer laser altimeter mola topographic data the graben system appears to be unique in the whole region because of its extent the remarkable width and depth of the graben the varying graben pattern and the associated volcanism single graben can be up to 35 km wide and 120 km long while their depth can reach nearly 3000 m they typically show an asymmetric architecture often in halfgraben or step fault style the graben system widens and changes its style along strike from ne to sw from a single very deep and narrow graben to a complex set of several shallower sinuous graben and halfgraben crustal extension was measured in the northeastern part from the observable throw and amounts to 2 5 3 1 km volcanic structures can be found at several locations along the graben in the sw the graben system seems to be connected with the tempe terra volcanic province marked by flood basalts and plains volcanism craters were counted on volcanic units unaffected by extensional tectonics crater statistics indicate an absolute crater model age for the upper end of the extensional deformation of 3 5 ga the dimensions and the characteristics of the mapped graben system resemble those of terrestrial continental rifts and a comparison between them and the kenya rift reveals striking similarities therefore the tempe fossae are interpreted as a martian analog to a continental rift associated with an underlying mantle plume this hypothesis seems to be supported by recent geophysical models based on topography and gravity data from mars global surveyor which indicate regional uplift for tempe terra

methylhopane biomarker hydrocarbons in hamersley province sediments provide evidence for neoarchean aerobiosis abstract bacteriohopanepolyols with a ring methyl substituents are bacterial lipid biomarkers that are presently known to originate from just a few specific taxa when preserved in ancient rocks the fossilized hydrocarbon skeletons of these molecules have the potential to provide biogeochemical records of those taxa and their respective physiologies considering both their occurrences in modern organisms and their proterozoic and phanerozoic sedimentary distributions hopanes carrying 2 methyl or 3 methyl substituents are proposed to be derived from cyanobacteria and oxygen respiring methanotrophs respectively here we report the distribution of methylhopanes in 2 72 2 56 billion year old neoarchean rocks from the hamersley province on the pilbara craton the relative abundance of c 31 3 methylhopane but not that of c 31 2 methylhopane shows a strong correlation to the carbon isotopic composition of co occurring kerogen insoluble particulate organic matter the unanticipated nature of this relationship provides evidence for a shallow water locus of carbon cycling through aerobic oxidation of ch 4 and coincidentally a means to demonstrate biomarker syngenicity 2 methylhopanes are most abundant in both shale and carbonate from shallow water sediments and a positive correlation to carbonate abundance contrasts to variable but generally lower 2 methylhopane abundances of deeper water facies these observations are consistent with their origin from cyanobacteria which were likely the cornerstone of microbial communities in shallow water ecosystems providing molecular oxygen fixed carbon and possibly fixed nitrogen

sedimentology of the paleoproterozoic kungarra formation turee creek group western australia a conformable record of the transition from early to modern earth abstract this paper presents the first detailed sedimentological analysis of the paleoproterozoic kungarra formation the lowermost of three formations comprising the turee creek group in western australia which was deposited across the rise in atmospheric oxygen the great oxidation event or goe and the transition from early to modern earth the data show that the kungarra formation has a gradational conformable lower contact with underlying banded iron formation of the hamersley group and predominantly comprises an upward shallowing succession from deepwater shales and siltstones through rippled fine grained sandstones and stromatolitic carbonates to tidal flat deposits that immediately underlie coastal fluvial deposits of the overlying koolbye formation at the base of the kungarra formation is a gradual transition from alternating units of magnetic green shale and thin units of banded iron formation that pass upsection to units of non magnetic shale and ferruginous chert and grey chert reflecting a gradual loss of iron from the world s oceans accompanying the rise of atmospheric oxygen a falling stage systems tract is recognised above this transition in the hardey syncline area capped by stromatolitic carbonates and a period of exposure marked by an erosional unconformity and carbonate beachrock two glacio eustatic cycles are recognised within the middle to upper parts of the kungarra formation each of which is marked by the rapid onset of falling systems tracts and characterised by falling systems tracts during and following diamictite deposition stratigraphic data are used to infer a depobasin filled by a sediment wedge prograding from southeast to northwest in contrast to previous models of a north northeastward deepening foreland basin the lack of seismites or internal unconformities within the formation precludes a foredeep setting rather deposition is interpreted as having occurred within an intracratonic basin with detritus sourced from erosion of uplifted bedrock to the southeast

actinolite forming reaction at low pressure and the role of fe 2 mg substitution the 6km thick karmutsen metabasites exposed over much of vancouver island were thermally metamorphosed by intrusions of jurassic granodiorite and granite observation of about 800 thin sections shows that the metabasites provide a complete succession of mineral assemblages ranging from the zeolite to pyroxene hornfels facies around the intrusion the reaction leading to the appearance of actinolite which is the facies boundary between prehnite pumpellyite and prehnite actinolite facies was examined using calcite free karmutsen metabasites collected from the route along the elk river in the prehnite pumpellyite facies xfe3 fe3 fe3 al in prehnite pumpellyite and epidote buffered by the four phase assemblage prehnite pumpellyite epidote chlorite systematically decreases with increasing metamorphic grade such a trend is the reverse of that proposed by cho et al 1986 this may be related to the higher f o 2 in the mt menzies area the actinolite forming reaction depends on the value of xfe3 in pumpellyite if using a low value of fe3 3 89 pr 0 06 0 48 ep 0 26 0 60 chl h2o 2 10 pm 0 08 0 17 act 0 88 qz is delineated the number in parentheses stands for the xfe3 value in ca al silicates on the other hand replacing the xfe3 of 0 08 in pumpellyite with a higher xfe3 value 0 24 changes the reaction to 0 41 pm 0 02 chl 0 42 qz 0 11 pr 0 62 ep 0 10 act h2o the first hydration reaction forms pumpellyite and actinolite on the high temperature side whereas the second dehydration reaction consumes pumpellyite to form prehnite epidote and actinolite the former reaction seems to explain the textural relationship of ca al silicates in the study area however actinolite forming reaction changes to a different reaction depending on the compositions of the participating minerals although in the other area even physical conditions may be similar to those in the study area chemographic analysis of phase relations in the pra facies indicates that the appearance of prehnite depends strongly on the bulk feo mgo ratio this may explain the rarity of prehnite in common metabasites in spite of the expected dominant occurrence in the conventional pseudo quaternary ca al fe3 fm system an increasing feo mgo ratio stabilizes the pr act assemblage and reduces the stability of the pm act one therefore the definition of pumpellyite actinolite facies should include not only pm act but also the absence of pr act assemblages in addition to the possible role of high x co 2 cho and liou 1987 and or high f o 2 to mask the appearance of prehnite the effect of the feo mgo ratio is emphasized

geological history of the early proterozoic paraburdoo hinge zone western australia abstract palinspastic profiles across the paraburdoo hinge zone which forms the southwestern boundary to the hamersley shelf indicate the existence of a fore trough adjacent to the ashburton trough on the edge of the hamersley shelf the fore trough existed during turee creek group and lower wyloo group times of deposition and is characterized by unconformities or shallow water facies on its margins the distribution of mature iron ores in the hamersley group suggests a relationship of this type of ore body to the fore trough margins continual uplift or a gradual migration northeastwards of a geanticline is suggested for the ashburton trough region from post hamersley group times to pre ashburton formation times of deposition

ubiquitous occurrence of basaltic derived paleosols in the late archean fortescue group western australia abstract the 2 76 ga old mount roe basalt paleosols mr 1 and mr 2 recognized near the base of the 2 76 2 69 ga fortescue group in western australia represent some of the oldest definite examples of archean paleoweathering profiles the loss of fe and the absence of pedogenic carbonates in these reference paleosols have been considered as strong evidence for low oxygen and moderate carbon dioxide in the late archean atmosphere respectively however the robustness of such interpretations suffers both from the scarcity of paleosol exposures and the superposition of post weathering alteration over primary soil profiles here we report new exposures of the mr 1 paleosol as well as a number of new basalt derived paleosol occurrences distributed in the mount roe basalt and the 2 73 ga old kylena formations of the fortescue group we show that all these paleosols including mr 1 and mr 2 were strongly affected by post weathering reductive alteration nevertheless we discovered early lithologic units locally preserved within the hydrothermally altered paleosols which feature distinct chemical and mineralogic compositions these include i 13c depleted carbonaceous rich diaspore pyrophyllite boudins likely inherited from the hydrolysis and bauxitization of the parent basalt and ii green hard core material mostly composed of fe sericite associated with authigenic sphene and containing early relics of sulfate bearing iron rich smectite or berthierine we argue that smectite relics may constitute a part of the primary pedogenic mineral assemblage while berthierine and sphene formed during diagenesis through the circulation of reductive fluids because the depletion of iron observed in the fortescue paleosols is not solely due to pedogenesis but also to post weathering alteration particular care has to be taken when bulk chemical profiles of iron in such paleosols are used as atmospheric paleobarometer recognition of the regional scale syn depositional alteration of the fortescue subaerial basalts over the north pilbara suggests the onset of intense continental weathering associated with the uplift and emergence of the pilbara craton during continental break up and rifting such a regional onset of continental weathering may have provided large amounts of nutrients to nearby marine and or lacustrine systems favoring the development of microbial life in shallow waters

making magnetite late again evidence for widespread magnetite growth by thermal decomposition of siderite in hamersley banded iron formations abstract banded iron formations bifs are marine chemical sedimentary rocks that were widely deposited before the start of the great oxidation event 2 45 2 32 ga they represent an important archive into the chemistry of the precambrian oceans and atmosphere and the evolution of the marine biosphere it is believed that bifs were derived from ferric oxides hydroxides that were deposited by iron oxidizing bacteria or by reaction with oxygen released by cyanobacteria hematite is traditionally interpreted to represent the dehydrated product of the ferric oxide hydroxide precipitate whereas magnetite is thought to have formed from ferric oxides hydroxides via bacterial iron reduction and oxidation of microbial biomass however studies are equivocal about when and how magnetite formed in bifs we present the results of a detailed petrographic study of magnetite in bifs of the hamersley group western australia we find no evidence that magnetite was precipitated from seawater or was a product of bacterial iron reduction instead most of the magnetite formed by replacement of siderite and other ferrous rich minerals after burial our observations suggest that magnetite growth was linked to the breakdown of siderite during metamorphism published experimental data indicate that in the presence of water and under the metamorphic conditions experienced in the southern pilbara craton 200 350 c the thermal decomposition of siderite could account for much of the magnetite in the hamersley bifs 3feco3 h2o fe3o4 3co2 h2 the relationship between magnetite and the primary precipitates which are preserved as greenalite nanoparticles and microgranules in chert bands suggests that much of the magnetite formed in siderite rich iron silicate bands that developed by compaction of non silicified iron silicate muds these bands acted as reactive pathways for fluid flow and the formation of magnetite if most of the magnetite formed after burial then it is necessary to reassess assumptions about the chemistry of the original precipitates and the role of microbial life in the redox cycling of iron our results are consistent with models for the deposition of bifs that center around a precursor sediment comprising greenalite precipitates and diagenetic siderite that was partially replaced by magnetite and hematite after burial

ree chemistry and sm nd systematics of late archean weathering profiles in the fortescue group western australia two weathering profiles each consisting of an upper sericite rich zone and a lower chlorite rich zone are preserved between flows of the mt roe basalt in the fortescue group hamersley basin western australia ree concentrations in samples from these two profiles which originally developed ca 2 760 ma show large variations depending on stratigraphic position lree abundances and la yb n are greatest at depths of 3 6 m below the paleosurface of the mt roe 1 profile and are somewhat lower in samples above this level the lrees reach concentrations 6 9 times greater than in the underlying basalt and thus appear to have been mobilized downward in the paleosol and concentrated in its middle part lree concentrations in the 2 profile show a similar distribution but with a sharp increase in all ree concentrations within 50 cm of the paleosurface the distinction between the ree profiles in the two paleosols may be related to the difference in the overlying material the 1 paleosol is overlain by a few meters of sediments and then by basalt whereas the 2 paleosol is directly overlain by basalt the lrees appear to have been mobilized both during chemical weathering of the parental basalt and during later lower greenschist facies metamorphism and metasomatism of the paleosols remobilization of the rees during the regional metamorphism of the fortescue group is confirmed by a whole rock sm nd reference isochron of mt roe 1 samples with an age of 2 151 360 ma variable initial 143nd 144nd values of unweathered basalt samples which may represent the paleosol protolith prevents a confident determination of the magnitude of lree mobility both the initial mobilization of the rees during weathering and the metasomatic remobilization appear to have taken place under redox conditions where ce was present dominantly as ce3 because ce anomalies are not developed within the sericite zone samples regardless of concentration europium anomalies in the paleoweathering profile are somewhat variable and were probably modified by mobilization of eu2 at metamorphic conditions in all samples the hrees appear to have been relatively immobile and correlate with al ti cr v zr and nb sm nd systematics and ree patterns of four unweathered basalt samples indicate derivation of the mt roe basalts from a heterogeneous and enriched source having epsilon nd between 4 0 and 7 4 initial 143nd 144nd values of these basalts are even lower than those reported by nelson et al 1992 for fortescue group basalts and indicate a substantial crustal component in the generation of mt roe basalts

the joffre banded iron formation hamersley group western australia assessing the palaeoenvironment through detailed petrology and chemostratigraphy abstract the joffre member of the brockman iron formation is by volume the largest single known banded iron formation bif in the world here we present detailed petrology and chemostratigraphy through the entire 355 m core section of this 2 45 billion year old unit oxide bif and silicate carbonate oxide bif dominate the lithology with minor amounts of interbedded stilpnomelane mudrock stilpnomelane rich tuffaceous mudrock and calcareous mudrock besides chert and magnetite the prominent mineralogy is riebeckite ankerite hematite stilpnomelane and crocidolite the bif is characterized by an average of 50 wt sio 2 and 44 5 wt fe 2 o 3 and an overall low abundance of al 2 o 3 2 sn of 0 24 the rey patterns also show a positive la sn anomaly no ce sn anomaly and a weakly developed positive y sn anomaly iron isotopes 56 fe with positive 56 fe values of 0 04 to 1 21 suggest that a large part of the hydrothermal iron was partly oxidized in the upper water column and subsequently precipitated as ferric oxyhydroxides no epiclastic grains have been found rather submarine hydrothermal fluids and fine grained volcanogenic detritus controlled bif chemistry the former source is reflected through a constant positive eu sn anomaly throughout the core average eu sn anomaly of 1 6 with a peak of 2 1 between 100 and 155 m depth while the latter source is best reflected through the stilpnomelane rich tuffaceous mudrock consisting of volcanic ash fall tuff with relict shards set in a stilpnomelane matrix the mudrock is overlain by well preserved wavy laminae and laminae sets of stilpnomelane microgranules that likely originated from re worked volcanic ash formed either on the seafloor or in the water column prior to deposition an enriched hree to lree pattern a high iron content 30 wt and a 56 fe value of 0 59 collectively imply that the mudrock facies interacted with the fe rich seawater prior to deposition the tio 2 zr ratio of the bif and the associated mudrocks suggest a felsic only source related to the same style of volcanics as the slightly younger woongarra rhyolites given the observation that the dominant control on the seawater chemistry was associated with felsic volcanics we speculate that the fine grained pelagic ash particles may have sourced bio available nutrients to the surface water this would have facilitated enhanced biological productivity including bacterial fe ii oxidation which is now recorded as the positively fractionated 56 fe iron oxide minerals in the joffre bif alongside submarine hydrothermal input to the basin the dominant control on the ocean chemistry seems to have been through volcanic and pyroclastic pathways thereby making the joffre bif poorly suited as a chemical proxy for the study of atmospheric oxygen and its weathering impact on local landmasses

oxygen and carbon isotope and cation geochemistry of metasomatic carbonates and fluids bergell aureole northern italy abstract oxygen and carbon isotope geochemistry and ca mg geothermometry are used to investigate temperatures fluid compositions and mechanisms of vein formation in dolomitic marble roof pendants of the bergell intrusive complex northern italy ca mg and oxygen isotope geothermometry suggests that the mineralogically zoned clinohumite chondrodite calcite forsterite calcite diopside calcite and tremolite calcite veins formed over a range of temperature of approximately 520 to 380 c most veins apparently formed near 400 c from h2o rich c o h fluids xco2 0 1 0 25 abrupt 5 14 isotopic discontinuities over several millimeters across the vein front are coincident with the mineralogically indicated vein fronts suggesting that mass transport was controlled by an infiltration mechanism decarbonation did not play a significant role in determining the isotopic composition of the vein calcite the isotopic data suggest a metasomatic fluid of rather constant composition and fluid rock ratios as high as 100 such ratios are minimum estimates but not inconsistent with fluid rock ratios calculated for complete reaction of the dolomitic wall rock with a fluid which provides the required silica

the antiquity of oxygenic photosynthesis evidence from stromatolites in sulphate deficient archaean lakes the tumbiana formation about 2700 million years old was largely deposited in ephemeral saline lakes as judged by the unusual evaporite paragenesis of carbonate and halite with no sulfate stromatolites of diverse morphology occur in the lacustrine sediments some with palimpsest fabrics after erect filaments these stromatolites were probably accreted by phototropic microbes that from their habitat in shallow isolated basins with negligible sulfate concentrations almost certainly metabolized by ozygenic photosynthesis

gold metallogeny of the northern capricorn orogen the relationship between crustal architecture fault reactivation and hydrothermal fluid flow abstract the timing and distribution of gold mineralization in proterozoic orogens is influenced by crustal architecture which is often established long before gold mineralization occurs gold occurrences in such settings are commonly associated with crustal scale faults formed at cratonic margins once established these faults provide critical pathways for hydrothermal and mineralizing fluids which during repeated fault reactivations can result in remobilization or introduction of new auriferous fluids resulting in overprinting gold events recently published geochronological data for the northern part of the proterozoic capricorn orogen in western australia show it has experienced at least three episodes of gold mineralization occurring at c 2400 1770 and 1680 ma this information combined with new data directly linking the timing of hydrothermal activity gold mineralization and fault reactivation show that the crustal scale nanjilgardy and baring downs faults and their subsidiary structures were re activated during discrete orogenic events many of the gold deposits are associated with intracratonic reworking during the 1820 1770 ma capricorn orogeny and 1680 1620 ma mangaroon orogeny intracratonic settings are not normally considered prospective for gold mineralization due to a lack of input of juvenile material however it appears that repeated hydrothermal fluid flow during intracratonic events has the potential to upgrade gold mineralization or increase gold endowment throughout the orogen either through gold remobilization or through introduction of new gold increasing the potential for economic gold deposits

geology and major and trace element chemistry of late archean weathering profiles in the fortescue group western australia implications for atmospheric po2 abstract we have identified two thick weathering profiles between flows of the mt roe basalt unit of the fortescue group in the pilbara area of western australia the age of weathering is constrained by u pb zircon dating to lie between 2765 and 2715 ma the profiles herein referred to as the mt roe 1 and 2 paleosols were mapped in detail for 1500 m and 760 m along strike respectively samples were collected along traverses across both profiles where they were well developed and well exposed the profiles which may be structurally offset parts of the same weathering horizon have well developed upper sericite zones 1 15 m thick these grade downward into chlorite rich zones which are chemically and physically heterogeneous the chlorite zones grade downward into unweathered basalts the mt roe 1 profile is overlain by 2 6 m of bedded probably lacustrine sediments these in turn are covered by succeeding lava flows evidence of a surficial origin for the paleosols includes mud cracks ripple marks and rip ups of the paleosol material in the post paleosol sediments soft sediment deformation and probable dewatering structures are present along the paleosol sediment contact the mt roe 2 profile is overlain directly by the succeeding lava flow with little or no intervening sediments some brecciation and pillowing is observed along the base of the overlying basalt flow in both cases probably due to emplacement of lava into a shallow lake or onto wet soil in the sericite zones of both profiles nearly all of the fe mn mg and zn and a substantial fraction of the ca pb na ni and si have been leached the chlorite zones have elevated fe pb ni and zn contents and appear to be repositories of elements lost from the sericite zone al ti zr and th were immobile in the mt roe 1 paleosol and are concentrated by factors of 2 5 2 6 over the unweathered basalt v u and cr were also nearly immobile major element and trace element data indicate that weathering took place under conditions of low atmospheric oxygen k is strongly enriched toward the upper contact of the sericite zone and was introduced during post weathering metasomatism together with rb and sr na and ca are also enriched near the top of each profile and small amounts of each may have been introduced during regional low grade metamorphism of the hamersley basin at 2168 10 ma

regional scale metasomatism in the fortescue group volcanics hamersley basin western australia implications for hydrothermal ore systems mafic to intermediate volcanic rocks of the fortescue group form the lowermost stratigraphic unit of the 100 000 km2 hamersley basin on the southern margin of the archaean pilbara craton western australia these represent one of the oldest 2 8 2 7 ga known examples of a continental flood basalt sequence a regional burial metamorphic gradient extends across the basin from prehnite pumpellyite facies in the north to epidote actinolite greenschist facies in the south and west superimposed on this metamorphic gradient regional scale metasomatism has affected extensive areas of the fortescue group metasomatized mafic lavas are characterized by well developed assemblages dominated by pumpellyite quartz or epidote quartz associations the mineral associations of metasomatic domain types broadly match the distribution of metamorphic isograd indicator minerals with a southward and westward increase in the proportion of epidote a continuum exists between least altered rocks that preserve the regional metamorphic signature and the most intensely altered metasomatized rocks metasomatism is essentially continuous over a stratigraphic strike length of 100 km and across a strike width of 20 km regionally metasomatically altered volcanic rocks occur widely across the hamersley basin and its outliers over an area of some 450 km by 200 km metasomatic alteration is most conspicuous in the lower grade metamorphic zones because pumpellyite and epidote rich rocks are green to yellow green in outcrop whole rock geochemical data indicate that metasomatism is associated with strong depletions in alkalis na k li rb alkali earths mg sr be ba and heavy first transition series metals mn fe co ni cu zn with a significant enrichment in si calcium shows more variable mobility such geochemical trends particularly depletions in fe mn and base metals co ni cu zn together with the metasomatic mineral assemblages are comparable with those associated with the sub sea floor circulation of seawater particularly in relation to the metal depleted root zones of base metal deposits petrographic features indicate that the development of metasomatic mineral associations post dates the formation of regional metamorphic assemblages consequently it is interpreted that the hydrothermal fluid flowed through the buried pile after sufficient time to allow for the metamorphic mineral assemblages to approach equilibrium with ambient p t conditions the hydrothermal depletion in fe observed across the fortescue group mafic lavas is intriguing given the abundance of iron ore in the overlying hamersley group and a possible connection cannot be ignored furthermore the scale of fluid flow observed in the fortescue group which occurred through zones of inherent permeability such as vesicular and brecciated lava flow tops without the aid of major cross cutting structures has significant implications for the size of metasomatic systems in other mafic volcanic terranes with potential consequences for the exploration for hydrothermal mineral deposits

neighbouring orogenic gold deposits may be the products of unrelated mineralizing events abstract models for exploration targeting are often developed by assessing known gold deposits in a region and targeting similar geological features such as geochemical anomalies favourable host rocks or structural settings with the assumption that they represented the footprint of the mineralizing event belvedere and paulsens are gold deposits with similar characteristics located in the wyloo inlier in the north capricorn orogen western australia gold at both deposits is hosted in quartz carbonate sulfide veins within mafic intrusive rock prior to this study they were thought to have formed during the same hydrothermal mineralizing event at belvedere in situ baddeleyite geochronology yields a weighted mean 207pb 206pb crystallization age of 2082 30 ma for the belvedere dolerite which hosts the ore body xenotime intergrown with ore stage alteration minerals and encased in arsenopyrite yielded a 207pb 206pb date of 1681 9 ma interpreted to represent the timing of hydrothermal activity related to gold mineralization at the belvedere deposit despite the similarities between the two deposits our results indicate that they underwent different geological histories with primary gold mineralization at paulsens occurring at c 2400 ma pre dating both the belvedere dolerite and mineralization within the dolerite furthermore field relationships show that the suite of dolerite dykes to which the belvedere dolerite belongs crosscut gold mineralization at paulsens the in situ u pb geochronology techniques employed here combined with field relationships have led to a better understanding on age constraints of gold deposition in the wyloo inlier and challenges the assumption that orogenic deposits in a given region probably formed during a single mineralizing event

detecting the source of heinrich layers an organic geochemical study 1 there are controversies regarding the origin of heinrich layer 3 h3 the massive ice rafting and meltwater event in the north atlantic during the last glacial cycle spanning a time window between 29 and 30 kyr b p some argue in favor of a laurentide ice sheet source similar to other heinrich layers while a contending view argues for the european ice sheet source existing geochemical proxies such as 40ar 39ar 206pb 204pb or nd etc could not be used to distinguish among various sources of ice rafted debris in h3 because of their low abundances suggesting a background glacial sediment signal in order to circumvent this problem a biomarker based approach is used to characterize the provenance of h layers 2 3 and 4 and other non heinrich layers the presence of hopanes and steranes and their aromatic counterparts in the h layers is incompatible with recent sediments and is attributed to the transportation of organic matter because of the glacial erosion of source rocks the most diagnostic and useful signatures of this ancient organic matter in the h layers are the dominance of c34 hopanoids over c33 and the occurrence of isorenieratane along with palaerenieratane biomarkers signatures in h layers 2 and 3 of the labrador sea suggest no difference in their source hydrocarbon distributions suggest that these sediments were derived from the middle to late ordovician and silurian source rocks of the hudson bay of eastern canada biomarker data of the h layer 4 from the northwest atlantic reveal that the sediments of this layer have a similar source to the h layers in the labrador sea

new insights into archean sulfur cycle from mass independent sulfur isotope records from the hamersley basin australia we have measured multiple sulfur isotope ratios 34 s 33 s 32 s for sulfide sulfur in shale and carbonate lithofacies from the hamersley basin western australia the v 33 s values v 33 swn 33 s30 515un 34 s shift from 31 9 to 6 9x over a 22 m core section of the lower mount mcrae shale v2 5 ga likewise sulfide sulfur analyses of the jeerinah formation v2 7 ga yield v 33 s values of 30 1 to 8 1x over a 50 m section of core despite wide variations in v 33 s and n 34 s these two shale units yield a similar positive correlation between v 33 s and n 34 s in contrast pyrite sulfur analyses of the carawine dolomite v2 6 ga yield a broad range in n 34 s 3 2 to 16 2x but a relatively small variation and negative values in v 33 s 32 5 to 31 1x the stratigraphic distribution of n 33 s n 34 s and v 33 s in western australia allows us to speculate on the sulfur isotopic composition of archean sulfur reservoirs and to trace pathways in the archean sulfur cycle our data are explained by a combination of massindependent fractionation mif in the atmosphere and biological mass dependent fractionation in the ocean in the archean volcanic sulfur bearing gas species were photolysed by solar ultraviolet uv radiation in an oxygen free atmosphere resulting in mif of sulfur isotopes aerosols of s8 with v 33 ss 0 and sulfuric acid with v 33 s6 0 formed from the products of uv photolysis and carried mass independently fractionated sulfur into the hydrosphere the signatures of atmospheric photolysis were preserved by precipitation of pyrite in sediments pyrite precipitation was mediated by microbial enzymatic catalysis that superimposed mass dependent fractionation on mass independent atmospheric effects multiple sulfur isotope analyses provide new insights into the early evolution of the atmosphere and the evolution and distribution of early sulfur metabolizing organisms a 2003 elsevier science b v all rights reserved

microbially influenced formation of 2 724 million year old stromatolites nanoscale evidence suggests that the tumbiana formation stromatolites in australia were influenced by microbial activity in the stromatolites clusters of organic globules are closely associated with 2 724 million year old aragonite crystals