novel retrieval of volcanic so2 abundance from ultraviolet spectra projects i firb miur simulazione dei flussi lavici con gli automi cellulari n rbau01rmz4 002 ii sviluppo di sistemi di monitoraggio funded by dipartimento di protezione civile della regione sicilia ingv istituto nazionale di geofisica e vulcanologia sezione di catania italy iii eu funded sixth framework programme project 18354 novac iv leverhulme trust fellowship

hydrothermal alteration and the formation of aluminous haloes around sulfide deposits the sulfide deposits of chizeuil saone et loire occur in a late devonian volcano sedimentary sequence within an acid volcanic unit and close to a carboniferous granite batholith sulfide bodies mainly pyrite are enclosed in andalusite bearing siliceous rocks lacking primary textures and recrystallized by the granitic thermal metamorphism

silicification hydrothermal alteration in an archean geothermal system within the amulet rhyolite formation noranda quebec silicified andesitic flows divided into two alteration facies mottled epidote quartz altered andesite and silicified andesite silicification was contemporaneous with volcanism silicified andesites of the amulet rhyolite are products of extensive regional alteration and present a larger exploration target for massive sulfide deposits than does proximal chloritic and sericitic alteration beneath orebodies modified journal abstract

geochemistry and origin of the ore chimney formation a transported paleoregolith in the grenville province of southeastern ontario canada the ore chimney formation is a mid proterozoic metapelitic rock unit that separates mafic to intermediate metavolcanic rocks of the grenville supergroup from an overlying clastic metasedimentary succession the flinton group it has been interpreted alternatively as a paleoregolith or a product of hydrothermal alteration of the volcanic rocks in a major shear zone the formation is thin 10 m massive to poorly layered crops out over 40 km along strike and is typically composed of alternating layers of biotite schist muscovite biotite schist hornblende biotite schist and hornblende schist it typically contains coarse porphyroblasts of garnet and less commonly of staurolite kyanite epidote and or hornblende thirty five samples of the ore chimney formation fifty samples of the underlying metavolcanics and fourteen samples of the overlying clastic metasediments were analysed for major and twenty eight trace elements including ree immobile trace element ratios ti zr zr y nb y la yb indicate that the ore chimney formation is locally derived from the underlying metavolcanics bulk chemistry field relations and petrographic observations strongly suggest that the ore chimney formation is a locally reworked weathering product of the metavolcanic rocks

geology genesis and exploration implications of the footwall and hanging wall alteration associated with the hellyer volcanic hosted massive sulfide deposit tasmania australia hellyer is a large 16 2 million metric tons high grade 13 9 zn 7 1 pb 0 4 cu 168 g t ag 2 5 g t au sea floor mound style polymetallic volcanic hosted massive sulfide vhms deposit located in the mount read volcanics of western tasmania the deposit is hosted by the que hellyer volcanics a sequence of late middle cambrian mafic to felsic coherent volcanics and polymict volcaniclastics hydrothermal alteration occurs in the regional footwall immediate footwall and hanging wall alteration in the regional footwall is confined to patchy quartz albite and chlorite with minor sericite epidote and hematite underlying hellyer is a zoned alteration pipe with a central siliceous core quartz sericite which passes into zones of chlorite chlorite carbonate sericite chlorite and finally sericite quartz stringer envelope zone on the margin overlying the central part of the deposit within the hanging wall basalt is a distinctive and zoned alteration plume five alteration zones have been identified fuchsite chlorite carbonate quartzalbite and sericite fuchsite dominated alteration occupies the central portion of the hanging wall alteration plume chlorite and carbonate alteration surrounds the fuchsite zone with carbonate zones forming near to the ore deposit and chlorite zones extending above and lateral to the carbonate outward is quartz ablite alteration which extends laterally into distal sericite alteration mass change calculations for the footwall and hanging wall indicate that in general the footwall alteration zones are depleted in cao na2o la sr ni cr and v but have enrichments of fe2o3 mno mgo k2o s and most metals compared to the host basalt the hanging wall alteration has gained cao k2o na2o co2 s in rb ba ag as mo sb cs and tl while fe2o3 mno mgo p2o5 la sr pb zn th u cd and nd are depleted cao na2o cr v and ni are depleted in the footwall andesite but enriched in the hanging wall alteration plume this relationship suggests that these elements were sourced from the breakdown of feldspars pyroxenes and andesitic groundmass of the footwall lithologies and transported in the hydrothermal fluid into the overlying basalt and precipitated as albite calcite and white micas in the hanging wall alteration the development of alteration associated with the hellyer vhms deposit occurred in three stages stage 1 regional footwall alteration was formed by unfocused hydrothermal convection of seawater down into the recently deposited volcanic pile at temperatures between approximately 250 degrees celsius and 200 degrees celsius and at low to moderate water rock ratios stage 2 alteration formed by structurally controlled fluid flow from a deep intensifying hydrothermal convection system and created the footwall alteration pipe decreasing water rock ratios and temperatures over a range of 350 degrees celsius to less than 200 degrees celsius led to the development of the concentric alteration pipe mineral zones based on modeling of whole rock d18ofr values geochemical modeling and mineral assemblages the siliceous core is interpreted to have formed at temperatures near 350 degrees celsius the chlorite rich alteration zone at 300 degrees celsius to 250 degrees celsius and the outer sericite rich alteration at temperatures of 250 degrees celsius to 150 degrees celsius the final synmineralization fluid flow event stage 3 created the hanging wall alteration plume after rapid burial of the deposit by basalt continuation of upward hydrothermal fluid flow created the zoned hanging wall alteration distribution of hanging wall alteration assemblages suggests a temperature gradient from approximately 250 degrees celsius for the fuchsite zone to lower temperatures 150 degrees celsius for the distal quartz albite and sericite alteration zones alteration mineralogy mineral chemistry lithogeochemistry and stable isotope characteristics of the footwall and hanging wall alteration have been combined into a comprehensive set of vectors which can be used in exploration for vhms deposits in similar geologic settings

rare earth element mobility at constant inter ree ratios in the alteration zone at the phelps dodge massive sulphide deposit matagami quebec rare earth elements ree from la to ho were mobile and enriched in hydrothermally altered rocks below the archean phelps dodge cu zn volcanogenic massive sulphide deposit in northwestern quebec largest net enrichment was in the moderately altered quartz chlorite zone where la concentration increased six fold and lanybn steepened from 1 9 to 13 0 the intensely altered chloritite zone had both minor net ree enrichment and depletion yb and lu were immobile throughout both zones the mobile ree were added and subtracted in constant chondrite normalized inter ree proportions 1 0 la 0 79 ce 0 57 nd 0 49 sm 0 01 eu 0 10 tb and 0 02 hb small additions of eu relative to generally larger additions of lree and tb produced enhanced negative eu anomalies the ree were mobilized at the hot 300 c core of the alteration system and deposited at the cooler periphery other sites of intense alteration and water rock interaction display similar ree changes indicating that selective ree enrichment at constant inter ree ratios is a widespread phenomenon

evolution of photoautotrophy and early atmospheric oxygen levels abstract current photochemical models suggest that oxygen levels in the prebiological atmosphere were extremely low most probably remaining in the range 10 8 10 14 pal present atmospheric level it is therefore reasonable to assume that only life processes were able to overwhelm these minor o2 pressures with free oxygen resulting from the reduction of carbon dioxide to the carbohydrate level during photoautotrophic carbon fixation using water as an electron donor 2 h 2 o co 2 rarr h ch 2 o h 2 o o 2 it is by now well established that reduced organic carbon is a common constituent of sedimentary rocks from the very start of the geological record 3 8 ga ago both direct assays and inferences derived from a carbon isotope mass balance suggest that the corg content of archaean sediments was not basically different from that of geologically younger rocks this poses the problem of the existence 3 5 ga ago of an oxidation equivalent of such a formidable ancient corg reservoir which depending on the model adopted for the growth of the sedimentary mass through time might have amounted to between 20 and 100 of the present one low atmospheric oxygen pressures in the early precambrian that are inferred from retarded oxidation reactions notably in the ancient continental weathering cycle are likely therefore to indicate extremely rapid processes of oxygen consumption in other parts of the system e g hydrosphere rather than the general absence of photosynthetic oxidation equivalents during this time

mobilization and mechanisms of retardation in the oklo natural reactor zone 2 gabon inferences from u ree zr mo and se isotopes abstract mineralogical and isotopic studies were carried out on the natural nuclear reaction zone 2 from the oklo deposit to evaluate the mobility of several nuclear reaction products in response to the alteration of the reaction zone and to identify the mechanisms which could retard the transport of released radionuclides to address these issues in situ isotopic analyses by shrimp and a selective extraction procedure were performed to constrain the structural location of nuclear reaction products exchangeable and non exchangeable and their association with mineral phases the distribution patterns of u ree zr and mo isotopes reveal that substantial amounts were released from the core and migrated through the hydrothermal alteration halo over metric distances owing to uraninite dissolution and advective transport by hydrothermal solutions during and soon after criticality the results emphasize the mobility of zr at oklo this element being often considered as immobile during water rock interactions the main output is the demonstration of the net effects of sorption and coprecipitation processes chlorite and to a lesser extent illite were found to have adsorbed significant amounts of u ree zr and probably th and less sorbing elements such as mo coprecipitation of secondary uo2 and p rich coffinite within the alteration halo is also an important means of retardation the concentration of radionuclides released from the reactor were probably high and they display solubility limited transport behaviour no retention effect was found for se in the immediate vicinity of the reactor and this element may have moved farther from its source of production these results have interesting implications for the evaluation of long term containment of radionuclides they provide a simple illustration of the performance of a clay barrier in the uptake of radionuclides by sorption onto clays and reincorporation in secondary u minerals this study also demonstrates the robustness of these retention processes over extremely long periods of time

rare earth element mobility around the volcanogenic polymentallic massive sulfide deposit at que river tasmania australia abstract the que river volcanogenic base and precious metal deposit is located in the cambrian mt read volcanics of western tasmania the host andesites and dacites adjacent to mineralization have been hydrothermally altered during mineralization and the entire sequence has undergone devonian regional prehnite pumpellyite facies metamorphism rocks petrographically recognizable as andesites exhibit chondrite normalized ree patterns similar to modern orogenic high k andesitic rocks with ce yb n ratios ranging from 6 4 to 8 6 dacites are more fractionated with ce yb n ranging from 10 6 to 16 0 like the andesites the dacites are characterized by negative eu anomalies eu eu 0 73 0 85 except for a more extreme eu anomaly in one sample eu eu 0 57 polymict volcaniclastic rocks have patterns very similar to those of the andesites the chondrite normalized ree patterns for the hydrothermally altered volcanics fall into three groups that can also be distinguished petrographically a single chloriterich sample has a substantial eu anomaly and shows less relative depletion in the middle ree than do either the andesites or the dacites highly silicified samples are all characterized by large negative eu anomalies eu eu 0 37 0 46 and many are relatively unfractionated with ce yb n ratios ranging from 3 5 to 7 7 a third group of samples characterized by the development of phengitic mica and carbonate show ree patterns similar to the andesites although their eu anomalies are generally smaller a sample of massive zn 1bpb ore has very low ree abundances showing light ree enrichment and a small negative eu anomaly eu eu 0 82 massive pyrite has a similar pattern except for higher overall abundances and a small positive eu anomaly eu eu 1 10 except for the eu anomalies the andesites and dacites show no evidence for ree mobility eu anomalies in the andesites are probably due to alteration of phenocrystic plagioclase and subsequent removal of divalent eu substantial ree mobility has occurred in the hydrothermally altered volcanic rocks the degree of mobility correlating with alteration style and intensity eu has been leached from both former plagioclase phenocrysts and the groundmass some of the eu leached from zones of the most intense alteration may have been redeposited in lower temperature more distal alteration zones

filamentous fossil bacteria from the archean of western australia abstract four morphotypes of structurally preserved filamentous fossil bacteria have been discovered in petrographic thin sections of laminated carbonaceous cherts from the 3500 ma old warrawoona group of northwestern australia these tubular and septate microfossils are interpreted here as being syngenetic with warrawoona sedimentation as such they are apparently the oldest such fossils now known in the geological record the diversity of this assemblage and the evident complexity of its individual components suggest that the beginnings of life on earth may have appreciably pre dated the deposition of the warrawoona sediments

carboniferous volcanic rocks of the magdalen islands gulf of st lawrence volcanic rocks of mississippian age occur on the magdalen islands as cap rocks and within collapse breccias above salt diapirs that have formed the islands they consist of coarse volcaniclastic deposits and basaltic flows intruded by minor mafic dykes and plugs petrologic studies of the basaltic rocks show that they are extensively altered original plagioclase clinopyroxene olivine and interstitial glass are partially to entirely replaced by mixtures of chlorite sericite smectite sphene carbonate epidote albite potassium feldspar and iron oxides and the samples display a relatively wide range in chemical compositions especially mobile were k na and ca and most samples are classified as potash spilites poenites using standard discriminant diagrams for mafic igneous rocks it can be seen that the basalts appear to range from continental tholeiitic to continental alkalic however relict clinopyroxene compositions and the presence of kaersutitic amphibole and titaniferous biotite in so

hydrothermal alteration in volcanic rocks eastern part of the lukavice group železne hory mountains czech republic many rocks mapped as felsic metavolcanics in the eastern part of the lukavice group are shown to be altered mafic metavolcanics similar to those in the noranda and flin flon snow lake mining districts canada the relatively fresh rocks of the lukavice group are rhyolite dacite andesite and andesite basalt of calcalka line character assuming no substantial volume change during alteration ti p la ce yb lu th partly sc and v contents remained unchanged altered rocks are enriched in fe mg k and si and depleted in na ca and zr some elements show both increased and decreased contents in altered rocks mg ba sm eu tb and hf although hydrothermal alteration in the lukavice group is of large extent it is of the proximal kuroko style and not of regional amulet rhyolite style implications for a large hydrothermal system within a volcanic pile are discussed in relation to the ordovician lukavice group and its mineral deposits and to some other parts of the bohemian massif with volcanosedimentary sequences of the same age

geology and wall rock alteration at the hercynian draa sfar zn pb cu massive sulphide deposit morocco abstract draa sfar is a siliciclastic felsic volcanogenic massive sulphide vms zn pb cu deposit located 15 km north of marrakesh within the jebilet massif of the western moroccan meseta the draa sfar deposit occurs within the sarhlef series a volcano sedimentary succession that hosts other massive sulphide deposits e g hajar kettara within the dominantly siliciclastic sedimentary succession of the lower central jebilet at draa sfar the footwall lithofacies are dominated by grey to black argillite carbonaceous argillite and intercalated siltstone with localized rhyodacitic flows and domes associated in situ and transported autoclastic deposits and lesser dykes of aphanitic basalt and gabbro thin to thick bedded black carbonaceous argillite minor intercalated siltstone and a large gabbro sill dominate the hanging wall lithofacies the main lithologies strike nne ssw parallel to a pronounced s1 foliation and have a low grade chlorite muscovite quartz albite oligoclase metamorphic assemblage the draa sfar deposit consists of two stratabound sulphide orebodies tazakourt to the south and sidi m barek to the north both orebodies are hosted by argillite in the upper part of the lower volcano sedimentary unit the tazakourt and sidi m barek orebodies are highly deformed sheet like bodies of massive pyrrhotite up to 95 pyrrhotite with lesser sphalerite galena chalcopyrite and pyrite the draa sfar deposit formed within a restricted sediment starved fault controlled anoxic volcano sedimentary rift basin the deposit formed at and below the seafloor within anoxic pelagic muds the argillaceous sedimentary rocks that surround the draa sfar orebodies are characterized by a pronounced zonation of alteration assemblages and geochemical patterns in the more proximal volcanic area to the south the abundance of medium to dark green chlorite progressively increases within the argillite toward the base of the tazakourt orebody chlorite alteration is manifested by the replacement of feldspar and a decrease in muscovite abundance related to a net addition of fe and mg and a loss of k and na in the volcanically distal and northern sidi m barek orebody alteration within the footwall argillite is characterized by a modal increase of sericite relative to chlorite a calcite quartz muscovite assemblage and a pronounced decrease in chlorite characterize argillite within the immediate hanging wall to the entire draa sfar deposit the sympathetic lateral change from predominantly sericite to chlorite alteration within the footwall argillite with increasing volcanic proximity suggests that the higher temperature part of the hydrothermal system is coincident with a volcanic vent defined by localized rhyodacitic flow domes within the footwall succession

the behaviour of so called immobile elements in hydrothermally altered rocks associated with volcanogenic submarine exhalative ore deposits evidence is available that some elements notably zr tio2 y sc ce and nb are largely immobile during the alteration of volcanic rocks owing to metamorphism hydrothermal events and weathering e g floyd and winchester 1978 however it is shown by reference to analyses of rocks from the environment of five volcanogenic massive sulphide bodies that while zr tio2 and ce are mostly immobile even during intense hydrothermal alteration y and particularly sc and nb may be extremely mobile when elements are removed by solution in a hydrothermal fluid it seems that reaction rates are such that these elements are almost totally removed from the rock therefore of the so called immobile trace elements only zr and tio2 may be used with any reliability to identify the degree of magmatic differentiation in an hydrothermally altered rock however if an element has been mobile it is usually readily identified as having moved

evidence for the transition to an oxygen rich atmosphere during the evolution of red beds in the lower proterozoic sequences of southern africa abstract strata 2 5 to 1 8 ga in southern africa appear to span the transition from a non oxidizing atmosphere to one characterized by free oxygen red beds in 2 3 to 2 1 ga unconformity bounded sequences have matrices of stained clay minerals and are spatially associated with volcanic rocks ironstones or banded iron formations which evidently were sources of iron for the matrices however red beds in 1 9 ga unconformity bounded sequences not only matrix staining but also hematitic coatings on grains these beds also have far fewer interstratified ferruginous lithologies because red beds with stained clay matrices banded iron formations pyritic witwatersrand type conglomerates and iron depleted paleosols all occur in the 2 2 ga pretoria group formation of these lithologies may not have been time dependent until atmospheric oxygen approached 10 2 atm by 1 9 ga atmospheric oxygen was 10 2 atm causing lateritic weathering and red beds with hematitic coatings on grains

evidence for the transition to an oxygen rich atmosphere in the rooiberg group south africa a note abstract the rooiberg felsite group is a rare example of a conformable succession that apparently spans the period of evolution of an oxygen rich atmosphere this conclusion is reached indirectly from a review of the proterozoic stratigraphy of south africa and directly from a consideration of the rooiberg itself the atmospheric transition coincides with the stratigraphic interval between the dark silicic lavas and associated grey volcaniclastics of the damwal formation and the overlying red rhyodacites and intercalated red bed sedimentary rocks of the selonsrivier formation radiometric age measurements indicate that this transition occurred after 2224 21 ma but before 2090 40 ma

ifit a simple method for measuring volcanic so2 without a measured fraunhofer reference spectrum abstract accurate quantification of the emission rate of sulphur dioxide so2 from volcanoes provides both insights into magmatic processes and a powerful monitoring tool for hazard mitigation the primary method for measuring magmatic so2 is differential optical absorption spectroscopy doas of uv scattered sunlight spectra in which a reference spectrum taken outside the plume is used to quantify the so2 slant column density inside the plume this can lead to problems if the reference spectrum is contaminated with so2 as this will result in a systematic underestimation of the retrieved so2 slant column density and therefore emission rate we present a new analysis method named ifit which retrieves the so2 slant column density from uv spectra by directly fitting the measured intensity spectrum at high spectral resolution 0 01 nm using a literature solar reference spectrum and measured instrument characteristics this eliminates the requirement for a measured reference spectrum providing a point and shoot method for quantifying so2 slant column densities we show that ifit retrieves correct so2 slant column densities in a series of test cases finding agreement with existing methods we propose that ifit is suitable for both traverse measurements and permanent scanning stations and could be integrated into volcano monitoring networks at observatories finally we provide an open source software implementation of ifit with a user friendly graphical interface to allow users to easily utilise ifit

on the use of different spectral windows in doas evaluations effects on the estimation of so2 emission rate and mixing ratios during strong emission of popocatepetl volcano abstract reported so2 emission values at strongly degassing volcanoes might be underestimated scanning spectroscopic instruments are widely used at volcanoes to quantify so2 emission rates and occasionally molar ratios between so2 and other gases such as bro frequently differential optical absorption spectroscopy is applied to evaluate spectra in the ultraviolet range if radiative transfer effects are not considered in the evaluations the retrieved column densities may vary depending on which spectral window is used we compare the use of different windows for data collected at popocatepetl volcano during strong degassing periods when high column densities lead to especially high dependence on the applied evaluation window we propose the use of three different windows 310 322 nm 314 7 326 7 nm and 322 334 nm and a simple algorithm to choose the most accurate column density from them the so2 evaluation using proper spectral windows would allow a more realistic estimation of the so2 emission rates

alteration hfse mineralisation and hydrocarbon formation in peralkaline igneous systems insights from the strange lake pluton canada abstract two characteristics of peralkaline igneous rocks that are poorly understood are the extreme enrichment in hfse notably zr nb y and ree and the occurrence of fluid inclusions dominated by methane and higher hydrocarbons although much of the hfse enrichment can be explained by magmatic processes the common intense alteration of the parts of the peralkaline intrusions most enriched in these elements suggests that hydrothermal processes also play an important role in hfse enrichment likewise although the origin of the higher order hydrocarbons that occur as inclusions in these rocks is still debated there is strong evidence that at least in some cases their formation involved hydrothermal processes the issues of hfse enrichment and hydrocarbon formation in peralkaline intrusions are examined using data from the strange lake pluton a small middle proterozoic intrusion of peralkaline granite in northeast canada this pluton contains some of the highest concentrations of zr ree and y ever reported in an igneous body and is characterised by abundant hydrocarbon dominated fluid inclusions in rocks that have been hydrothermally altered including those that form a potential hfse ore zone we show that hfse at strange lake were partly concentrated to near exploitable levels as a result of their transport in a high salinity magmatic aqueous liquid and that this fluid coexisted immiscibly with a carbonic phase which reacted with hydrogen and iron oxides generated during the associated hydrothermal alteration to produce hydrocarbons via a fischer tropsch synthesis as a result hydrocarbons and hfse mineralization are intimately associated we then go on to show that hydrothermal alteration hfse mineralisation and hydrocarbons are also spatially associated in other peralkaline complexes and present a model to explain this association which we believe may be applicable to any peralkaline intrusion where hfse enrichment was accompanied by calcium metasomatism hematisation and hydrothermal fluorite we also suggest that even where these criteria are not satisfied hydrothermally enriched hfse and hydrocarbons will be intimately associated simply because they are products of the same initial magmatic fluid finally we speculate that the association of hfse and hydrocarbons may in some cases actually be genetic if as seems possible unmixing or effervescence of a reduced carbonic fluid from the original magmatic fluid caused changes in temperature ph f o 2 or the activity of volatile ligands sufficient to induce the deposition of hfse minerals

geochemistry of two metavolcanic arc suites from the central metasedimentary belt of the grenville province southeastern ontario canada samples of two subalkaline metavolcanic suites the tudor formation ca 1 28 ga and the overlying kashwakamak formation have been analysed for major elements and 27 trace elements including rare earth elements the tudor formation is tholeiitic and contains mainly basaltic flows whereas the kashwakamak formation is calc alkaline and contains mainly andesitic rocks with minor felsic rocks the succession has been regionally metamorphosed to upper greenschist lower amphibolite facies trace element abundances and ratios indicate that rocks of the tudor and kashwakamak formations are island arc type geochemical modelling using rare earth elements zr ti and y indicates that the tudor volcanic rocks are not derived from a single parental magma through simple fractional crystallization equilibrium partial melting of a heterogeneous proterozoic upper mantle can explain the trace element abundances and ratios of tudor formation volcanic rocks the intermediate to felsic rocks of the kashwakamak formation appear to have been derived from a separate partial melting event the data are consistent with an origin of the arc either on oceanic crust or on thinned continental crust and with accretion of the arc to a continental margin between the time of extrusion of tudor volcanic rocks and that of kashwakamak volcanic rocks

the time bound character of uranium mineralising processes with special reference to the proterozoic of gondwana abstract it is more useful to consider the time bound character of uranium mineralising processes than the time bound character of uranium deposits this is because present uranium deposits are the result of a series of mineralising steps which occurred over hundreds of millions of years we have identified five periods which are characterised by moderately distinct mineralising processes 1 1 before 2800 ma uranium was fractionated into the upper crust during late archaean early proterozoic cratonisation by magmatic and metamorphic processes the granitic terranes were an important source of uranium for subsequent mineralising events 2 2 2800 2200 ma although the atmosphere was probably not reducing at this time the level of oxygen was sufficiently low for uraninite to be transported and concentrated in sedimentary basins cratonisation was a prerequisite for the formation and preservation of long lived basins rapid burial and reducing conditions during diagenesis were important for preserving the uranitite the uranium was mobilised during diagenesis and metamorphism 3 3 2200 1200 ma after 2000 ma when the oxygen level in the atmosphere increased markedly the movement of uranium in solution as uranyl ions became more important the first marine shelf sequences containing chemical and organic sediments to be deposited on the cratons formed excellent sites for the syn sedimentary diagenetic or metamorphic concentration of uranium in some cases this mineralisation was economic in other cases it may have been just a step in a succession of mineralising events continental clastic sediments which overlie these shelf sediments were excellent hosts for large hydrothermal circulation systems which could remobilise the uranium from either granites or sediments the lack of organic and argillaceous material in these continental sequences resulted in the precipitation of uranium near the basal unconformity where there were reducing sediments 4 4 1200 400 ma with the start of modern plate tectonics there was greater potential for the thermally activated remobilisation of uranium the period 1200 900 ma marked the final mineralisation in some important deposits the pan african and brazilian events 700 400 ma mobilised uranium of uncertain origin and concentrated it in previously deposited marginal marine sediments 5 5 400 0 ma the incorporation of organic material into continental clastic sediments allowed uranyl ions to be precipitated before they reached the sea to form the important sandstone type of deposits any time bound mineralising process may be either the final or an intermediate step in forming an economic uranium deposit which of these it is depends on the subsequent history of the area on whether the uranium is preserved dispersed or reconcentrated that many major uranium deposits occur on late archaean early proterozoic cratons emphasises the importance of the long term preservation of sedimentary sequences even though the uranium may be remobilised several times within the sediments

pinwarian 1 50 ga volcanism and hydrothermal activity at the eastern margin of the wakeham group grenville province quebec volcanic belts developed along the southeastern continental margin of laurentia between 1 70 and 1 30 ga and subsequently metamorphosed at high grade are today largely concealed among gneiss complexes of the grenville province at the eastern end of the wakeham group and in the la romaine supracrustal belt to the east four 1 50 ga volcanic centres were found among gneissic synvolcanic intrusions typical of the 1 52 1 46 ga pinwarian continental magmatic arc upper amphibolite to granulite facies rhyolitic to dacitic lavas and coarse lapillistone overlie or are intimately associated with arenites typical of the wakeham group garnetite ironstone carbonate rock calc silicate rock and sillimanite bearing nodules veins and gneiss locally preserving lapilli are also present the distribution paragenesis and modes of most of these latter units differ from those of normal metasediments but are diagnostic of metamorphosed exhalites and hydrothermal alteration zones in the la romaine supracrustal belt

peralkaline and calc alkaline hosted volcanogenic massive sulfide deposits of the bonnifield district east central alaska volcanogenic massive sulfide vms zn pb cu ag au deposits of the bonnifield mining district formed during late devonian early mississippian magmatism along the western edge of laurentia the largest deposits dry creek and wtf have a combined resource of 5 7 million tonnes at 10 zn 4 pb 0 3 cu 300 grams per tonne g t ag and 1 6 g t au these polymetallic deposits are hosted in high field strength element hfse and rare earth element ree rich peralkaline pantelleritic metarhyolite and interlayered pyritic argillite and mudstone of the mystic creek member of the totatlanika schist formation mystic creek metarhyolite and alkali basalt chute creek member constitute a bimodal pair that formed in an extensional environment a synvolcanic peralkaline quartz porphyry containing veins of fluorite sphalerite pyrite and quartz intrudes the central footwall at dry creek the anderson mountain deposit located 32 km to the southwest occurs within calc alkaline felsic to intermediate composition metavolcanic rocks and associated graphitic argillite of the wood river assemblage felsic metavolcanic rocks there have only slightly elevated hfses and rees the association of abundant graphitic and siliceous argillite with the felsic volcanic rocks together with low cu contents in the bonnifield deposits suggests classification as a siliciclastic felsic type of vms deposit bonnifield massive sulfides and host rocks were metamorphosed and deformed under greenschist facies conditions in the mesozoic primary depositional textures generally uncommon consist of framboids framboidal aggregates and spongy masses of pyrite sphalerite the predominant base metal sulfide encloses early pyrite framboids galena and chalcopyrite accompanied early pyrite formation but primarily formed late in the paragenetic sequence silver rich tetrahedrite is a minor late phase at the dry creek deposit gold and ag are present in low to moderate amounts in pyrite from all of the deposits electrum inclusions occur in dry creek sphalerite contents and ratios of trace elements in graphitic argillite that serve as proxies for the redox state of the bottom waters in the basin indicate that dry creek mineralization took place in suboxic to periodically anoxic bottom waters trace element data show higher contents of tl mn as in pyrite from the anderson mountain deposit compared to the dry creek or wtf deposits and thus suggest that anderson mountain may have formed at lower temperatures or under slightly more oxidizing conditions no exact modern analogue for the tectonic setting of the bonnifield vms deposits is known although the back arc regions of the okinawa trough and woodlark basin satisfy the requirement for a submarine extensional setting adjacent to a continental margin limited occurrences of peralkaline volcanic rocks occur in these two potential analogues but the peralkalinity of those rocks is much less than that of the mystic creek member metarhyolites in the bonnifield district the highly elevated trace element e g zr nb contents of mystic creek metarhyolites suggest that a better analogue may be a submarine rifted continental margin the calc alkaline composition of the host rocks to the anderson mountain deposit suggests that mineralization there formed in a continental margin arc outboard of the extended continental margin setting of the peralkaline hosted dry creek and wtf deposits

variations in the abundance of photosynthetic oxygen through precambrian and paleozoic time in relation to biotic evolution and mass extinctions evidence from mn fe ratios abstract this paper reports new information about variations in the abundance of photosynthetic oxygen through precambrian and paleozoic time non detrital marine sediments cherts limestones and dolomite were analysed for nh2oh hcl acetic acid extractable mn and fe and the mn fe ratio a proxy for the oxidation reduction potential of the sediment at the time of deposition was plotted against geologic age the method has never before been applied to ancient sediments but previously published data produced independently by other methods confirmed its applicability and underlying assumptions the mn fe ratio was unexpectedly high ca 3 416 ga implying localised oxidation due to oxygen production by cyanobacteria but fell dramatically over the interval 3 416 3 298 ga suggesting mass mortality or mass extinction of early archaean cyanobacteria owing to asteroid impacts however the ratio increased continuously though at episodically varying rates from a minimum at 1 8783 ga to a maximum at 0 680 ga signifying accumulation of oxygen in the atmosphere and hydrosphere the rate of increase was relatively high at first but dropped abruptly at some point during the interval 1 8783 1 6 ga possibly signalling the appearance of eukaryotic herbivores the ratio increased exponentially from 1 6 to 0 8 ga and then rose more rapidly from 0 8 to 0 680 ga indicating a late proterozoic upsurge of oxygen production whereupon it fell catastrophically to a minimum in the cambrian reflecting widespread anoxia due to mass extinction of ediacaran organisms the crisis at the precambrian cambrian boundary was followed by a logarithmic increase from the cambrian to the permian indicating a resurgence of photosynthetic activity

radiative transfer corrections for accurate spectroscopic measurements of volcanic gas emissions there is widespread use of passive remote sensing techniques to quantify trace gas column densities in volcanic plumes utilizing scattered sunlight as a light source examples include passive doas cospec and the so2 camera in order to calculate trace gas concentrations or volcanic emission fluxes knowledge about the optical path through the plume is necessary in the past a straight photon path through the plume has always been assumed although it was known that this is not always true here we present the results of model studies conducted specifically to quantify the effects of realistic radiative transfer in and around volcanic plumes on ground based remote sensing measurements of so2 the results show that measurements conducted without additional information on average photon paths can be inaccurate under certain conditions with possible errors spanning more than an order of magnitude both over and underestimation of the true column density can occur actual errors depend on parameters such as distance between instrument and plume plume so2 concentration plume aerosol load as well as aerosol conditions in the ambient atmosphere as an example a measurement conducted with an so2 camera is discussed the results of which can only be correctly interpreted if realistic radiative transfer is considered finally a method is presented which for the first time allows the retrieval of actual average photon paths in spectroscopic i e doas measurements of adequate resolution by allowing for a wavelength dependent column density during the evaluation of doas measurements we show how radiative transfer effects can be corrected using information inherently available in the measured spectra thus greatly enhancing the accuracy of doas measurements of volcanic emissions

the origin of chlorite tremolite carbonate rocks associated with the thalanga volcanic hosted massive sulfide deposit north queensland australia the thalanga polymetallic massive sulfide deposit is hosted by felsic volcanics of the cambro ordovician mount windsor subprovince in north queensland the sheetlike sulfide deposit consists of several semiconnected lenses totaling 6 6 million metric tons mt which lie at a single stratigraphic level between underlying subaqueously emplaced rhyolite and overlying dacitic to andesitic lavas sills and volcaniclastic rocks the ores and host rocks have been deformed and metamorphosed to upper greenschist facies associated with the west thalanga orebody are strata bound lenses composed of chlorite tremolite dolomite and calcite that previously have been interpreted as exhalites they extend up to a few hundred meters along the mineralized horizon and locally also exist in the stratigraphic footwall around the fringes of the deposit rhyolitic volcanics beneath the deposit are extensively altered a pervasive sericite chlorite pyrite zone that extends greater than 200 m into the footwall envelops semistratiform proximal quartz pyrite chlorite stringer zones major and trace element geochemical data indicate that zr ti and al remained essentially immobile in the footwall rocks despite intense hydrothermal alteration involving significant mass transfers of mobile elements the chlorite tremolite carbonate assemblages associated with ore lenses have immobile element ratios identical to those of the altered rhyolites carbonate rich assemblages retain some relict spheroidal and rhombic textures in dolomite which suggest formation at an early hydrothermal stage in a matrix of chlorite or in open spaces the chlorite tremolite carbonate rocks probably represent metamorphic equivalents of hydrothermal chlorite quartz dolomite calcite or mg smectite quartz dolomite calcite assemblages the formation of tremolite by metamorphic reaction of quartz and dolomite is consistent with regional metamorphic conditions with allowance for metamorphic decarbonation carbon and oxygen isotope data from dolomite and calcite are consistent with precipitation of primary dolomite at 170 to 250 c from seawater dominated fluid the chlorite tremolite carbonate rocks at thalanga are consequently reinterpreted as altered rhyolitic volcaniclastics containing variable proportions of hydrothermal carbonate they probably formed in a zone of mixing of hydrothermal fluid and seawater in a permeable substrate close to the paleosea floor siliceous ironstones that are present in distal parts of some thalanga ore lenses have very low immobile element contents and are probably true exhalites

are the 3 800 myr old isua objects microfossils limonite stained fluid inclusions or neither bridgwater et al 1 issued a cautionary note concerning several reports published by pflug and co workers2 5 describing objects called yeast like microfossils isuasphaera isua pflug from a metamorphosed quartzite of the 3 800 myr old isua supracrustal belt of south west greenland bridgwater et al believe that the objects described by pflug et al 2 5 are indistinguishable from limonite stained fluid inclusions and hence are non biogenic i show here that the objects are neither limonite stained fluid inclusions nor microfossils but are limonite stained cavities from the otherwise complete dissolution by weathering of ferruginous dolomite grains in these rocks several supporting arguments presented by both sides are believed to be invalid and others are ambiguous in view of the extensive research on the earliest life forms and then significance to evolution to early geochemical cycles and to the origin of the atmosphere and some ore deposits the exact nature of the isua objects and particularly the validity of the evidence either for or against a biological origin are of considerable importance a careful evaluation of the evidence from isua is particularly pertinent as bona fide precambrian fossils are also found in chemically similar but much younger silica rich environments

emplacement crystallization and alteration of spinifex textured komatiitic basalt flows in the archaean nondweni greenstone belt southern kaapvaal craton south africa a well exposed succession of spinifex textured komatiite flows is reported from the archaean nondweni greenstone belt located near the southern margin of the kaapvaal craton the flows are relatively thin 1 5 m compared to similar occurrences in other greenstone belts they are characterised by well developed cone structures of highly elongate amphibole crystals after augite which fan downwards from the tops of the flows extreme development of coned spinifex has not been reported from other greenstone belts and points to specific thermal conditions prevailing in the nondweni environment the zones of bladed spinifex are contained between layers of random spinifex and overlie a lower cumulus layer originally of augite orthopyroxene and minor olivine the observed major and trace element distributions through a 1 7 m thick spinifex textured flow are consistent with a model involving concentration of phenocryst phases resulting in significant fractionation upwards in the flow approximately 40 of the spinifex textured phenocrysts grew in situ after the lithological units were established collapse and displacement of the coned crystal networks originally attached to the top of the flow are shown to have influenced the distribution of liquid within the flow and accentuated the fractionation associated with the spinifex textured units are massive aphyric and brecciated flows which show distinct chemical cycles through the succession the brecciated zones have compositions with 18 mgo and are characterised by ovoid bodies that are not pillows and may represent magmatic reworking and movement of a partly congealed flow post solidus alteration is considered to have caused early hydration of the original mineralogy and also introduced sio2 and na2o into the upper part of the flow by way of microfractures the observed alteration is different to that of mid ocean ridge basalts and a subaerial shallow water environment is suggested

the key tuffite matagami camp abitibi greenstone belt canada petrogenesis and implications for vms formation and exploration the key tuffite is a stratigraphic marker unit for most of the zinc rich volcanogenic massive sulfide deposits of the matagami camp in the abitibi greenstone belt this 2 to 6 m thick unit was previously interpreted as a mixture of ash fall andesitic to rhyolitic tuffaceous components and volcanogenic massive sulfide vms related chemical seafloor precipitate exhalative component previous attempts to develop geochemical exploration vectoring tools using metal content within the key tuffite were mostly inconclusive due to the complex nature of the key tuffite unit and a poor understanding of its composition origin and relationship with the vms forming hydrothermal systems detailed mapping and thorough lithogeochemistry of the key tuffite in the vicinity of the perseverance and bracemac mcleod deposits indicate that the key tuffite is a homogeneous calc alkaline andesitic tuff that was deposited before the vms deposits were formed the unit is mostly devoid of exhalative component but it is strongly hydrothermally altered close to orebodies this is characterized by a strong proximal chloritization and a distal sericitization which grades laterally into the unaltered key tuffite neither the key tuffite nor the ore was formed by seafloor exhalative processes for the two studied deposits this probably explains why previously proposed exploration models based on metal scavenging proved unsuccessful and suggests that a re evaluation of the exhalative model should be done at the scale of the mining camp however as shown in this study hydrothermal alteration can be used to vector towards ore along the key tuffite

paleomagnetic and geological investigation into southern sinai volcanic rocks and the rifting of the gulf of suez two volcanic occurrences from southern sinai were sampled for paleomagnetic petrographic and geochemical investigations 29 oriented samples were taken from six sites of the middle jurassic basaltic intrusion at wadi budra southwest sinai 28 55aen 33 20aee after alternating field af demagnetization and tilt correction all samples exhibit normal magnetization with a mean paleomagnetic direction d 63 3 i 8 0 and a 95 10 2 this yields a paleopole position at lat 25 n long 133 e and a 95 8 6 also six oriented blocks 34 core samples were taken from the sant catherine doleritic dike at wadi sahl el raha south sinai 28 33aen 33 55aee after af demagnetization all sites showed normal polarity with a mean direction of magnetization d 3 i 39 and a 95 15 3 and a paleopole position at lat 83 e long 179 e and a 95 16 5 the paleomagnetic pole of w budra is found to be incompatible with poles of similar age from africa the cause of this cannot be uniquely determined but most probably could be referred to local tectonic rotations and or geomagnetic secular variations avecting the paleomagnetic directions of these samples in contrast the tertiary pole of wadi sahl el raha is consistent with the part of the apparent polar wander path apwp of africa for the time period 20 ma oligo miocene age petrographical and geochemical investigations revealed that the volcanic rocks of both localities are continental within plate rift related alkali olivine basalt w budra and olivine dolerite sahl el raha the primary magnetite is the main opaque mineral present in the samples which represents the main carrier of magnetization in these rocks 2000 elsevier science b v all rights reserved

magmatic composition and tectonic setting of altered volcanic rocks of the fennell formation british columbia the upper paleozoic fennell formation in south central british columbia consists of basalts and associated marine sediments metamorphosed at low greenschist facies conditions although the microphe

evidence for hfse and ree mobility during calc silicate metasomatism mesoarchean 3075ma ivisaartoq greenstone belt southern west greenland abstract the mesoarchean ca 3075 ma ivisaartoq greenstone belt consists dominantly of variably deformed and metamorphosed mafic to ultramafic flows basalts to picrites in addition there are minor serpentinites gabbros clinopyroxene rich cumulates and volcaniclastic and siliciclastic sedimentary rocks primary volcanic structures such as pillows and volcanic breccia are well preserved in low strained zones the belt underwent intense metasomatic hydrothermal alteration between 3075 and 2961 ma an early stage of calc silicate metasomatism stage i resulted in the formation of epidosites in pillow cores pillow interstitials and intrusive margins of gabbroic dykes this alteration appears to have resulted from sea floor hydrothermal alteration a second stage of calc silicate metasomatism stage ii formed centimeter to meter scale layers concordant to discordant relative to the regional foliation these layers consist of diopside garnet epidote plagioclase quartz titanite vesuvianite calcite actinolite scheelite the second stage calc silicate rocks are associated with shear zones and their origin is attributed to interaction of reactive fluids with mafic and ultramafic rocks during regional metamorphism both stages of calc silicate metasomatism caused significant mobilization of major elements si mg ca fe na k and mn lile rb cs sr ba pb and lree heavy rare earth elements hree and hfse th nb ta zr ti remained relatively immobile during stage i metasomatic alteration but were variably disturbed during the stage ii metasomatism accordingly the stage ii calc silicate rocks exhibit evidence for significant mobility of th nb and ta on a local scale 1 4 m whereas zr and ti were not significantly mobilized this resulted in calc silicates with complementary trace element patterns displaying extreme negative to positive nb anomalies nb nb 0 19 7 28 transition metals ni v co cr and sc were relatively immobile during both metasomatic events amphibolites spatially related to the second stage calc silicate rocks preserve mineralogical and major element characteristics consistent with a mafic to ultramafic composition however their near primary magmatic trace element compositions were variably modified due to strong ree mobility during the stage ii metasomatic event this gave rise to various ree characteristics including lree enriched lree depleted and u shape ree patterns some of these element patterns resemble those shown by modern boninites and mid ocean ridge basalts n morb in contrast the least altered amphibolites exhibit similar trace element patterns to those displayed by the least deformed non metasomatized mafic and ultramafic pillow flows they possess relatively flat trace element patterns with negative nb ta anomalies indicating a subduction zone geochemical signature

mass change during extreme acid sulphate hydrothermal alteration of a tertiary latite styria austria abstract the tertiary latitic rock from gossendorf in the gleichenberg volcanic area styria austria has in places been completely altered to various associations of the secondary minerals opal c ct alunite kaolinite and montmorillonite the associations occur in a zonal arrangement and correspond to the characteristics of the silicic advanced argillic and argillic alteration terminology all elements investigated were mobilized during the hydrothermal acid sulphate alteration process except ti which was selected as the immobile monitor element to calculate mass changes drastic differences in gains and losses of certain elements which refer to the different alteration conditions were found to exist not only between different alteration zones but also within such zones the gains depend on the newly formed minerals the losses however on the increase of the porosity in the altered rocks for one altered rock characterizing theoretically the alteration processes in gossendorf mass grains were calculated for so 3 h 2 o v and sr and mass losses for sio 2 al 2 o 3 fe 2 o 3 mno mg0 cao na 2 o k 2 o p 2 o 5 cr co ni cu zn rb y zr nb and ba with respect to an unaltered latite the net mass balance calculation i e the difference of elemental gains and losses shows a net mass loss of about 14 60 g the mass exchange was accompanied by a moderate average decrease in volume of 6 6 the chemical species added to the rock during alteration so 3 v 2 o 3 and sro probably originated from a volcanic source h 2 o could have been of exogenous and or endogenous origin the results of the mass change calculations of major minor and trace elements suggest that the secondary mineral associations and their zonal arrangement at gossendorf have been formed by a strongly acidic hydrothermal solution rich in so 4 2 which underwent chemical variations by reaction with the latite the calculations indicate that for the formation of the altered rocks and their zonal distribution the elements of the latitic precursor rock a volcanic source for s v and sr and probably exogenic water were necessarily involved based on these results a general chemical characteristic for alteration assemblages in epithermal acid sulphate districts is presented al 2 o 3 fe 2 o 3 mg0 cao so 3 and h 2 o by weight proves to be suitable to characterize silicic advanced argillic and argillic alteration

hydrothermal alteration and physical volcanology of archean rocks in the vicinity of the headway coulee massive sulfide occurrence onaman area northwestern ontario the headway coulee zn pb cu ag prospect occurs as a series of 13 lenses of vein and disseminated type sulfides which are associated with hydrothermally altered archcan mafic lavas and felsic hydroclastic rocks mafic lavas form a 4 to 6 km wide sequence that is locally interrupted by a narrow lens shaped horizon 30 200 m x 2 km of thin to thickly bedded ash rich tuffs the felsic tuffs are subdivided into two units which represent two periods of explosive activity the felsic tuffs and many of the mafic lavas have undergone hydrothermal alteration during the waning stages of explosive activity alteration zones and their metamorphosed equivalents both crosscut and are conformable to stratigraphy they have been divided into six mineralogically and chemically distinct types least altered calcite sericite iron chlorite chloritoid and kyanite least altered rocks underlie approximately 50 percent of the area are all mafic in composition and have been affected primarily by greenschist facies metamorphism calcite type alteration is the earliest hydrothermal event recognized in the volcanic rocks and forms a 600 by 1 600 m zone which crosscuts stratigraphy calcite rich rocks grade laterally into least altered or sericite rich rocks they were formed by the addition of co 2 and the minor loss of na fe and mg whereas ca remained relatively immobile sericite type alteration postdates calcite alteration and occurs primarily as elongate lenses up to 1 000 m within felsic tuffs it also occurs in anomalous amounts within iron chlorite and chloritoid rich mafic lavas prior to formation of chlorite and chloritoid the sericite alteration zone was 3 000 by 100 to 1 200 m in size and crosscut the stratigraphy sericite is thought to have formed from a low temperature evolved seawater fluid which circulated only in the very upper portions of the hydrothermal system at low water rock ratios this fluid became enriched in k and si and was heated enough to rise buoyantly along fractures and more permeable lithological units water rock interactions formed sericite and quartz at the expense of plagioclase feldspar iron chlorite type alteration and its metamorphosed equivalent chloritoid type alteration crosscut both calcite and sericite alteration and chlorite replaces sericite within both mafic and felsic rocks within felsic tuffs this alteration is characterized by the addition of fe and mn and the loss of na k ca si and co 2 volume may decrease by as much as 30 percent within mafic lavas however the alteration is characterized by addition of fe with minor ca and loss of k and co 2 there was negligible volume change iron chlorite is believed to have formed from an evolved high temperature seawater fluid which circulated at depth interaction with mafic rocks at low water rock ratios enriched the fluid in fe mn h super while depleting it in ca and na this acidic and base metal rich fluid as it neared the surface interacted with sericite and calcite rich rocks to form iron chlorite iron carbonate and a hydrous aluminum silicate phase pyrophyllite within the discharge channel close to the water rock interface the solution extensively leached the ash tuffs to produce a rock composed dominantly of sio 2 and hydrous aluminum silicate phases this rock represents a leached cap to the hydrothermal system and a local discharge site later greenschist facies metamorphism converted the quartz hydrous aluminum silicate assemblage to kyanite quartz and the iron chlorite plus hydrous aluminum silicate assemblage to chloritoid base metals were precipitated below and or immediately above the discharge site

archean chemical weathering at three localities on the canadian shield abstract sapropelite developed on a granodiorite basement below the archean steeprock group in the western superior structural province of canada weathered materials from a 3 15 ga granitic basement to the archean yellowknife supergroup in the slave structural province and weathered detritus from contemporaneous komatiites and gneissic basement of the archean prince albert group in northern churchill structural province have been analysed the deviation of the weakly metamorphosed weathered material from the chemical composition of their precursors is comparable to that seen in more recent weathered profiles from similar source materials this includes the enhancement of ferric to ferrous iron ratios and potash to soda lime and magnesia ratios there is also an increase in lighter ree and a depletion of heavier ree quartz rich sediments immediately above the unconformity derived from the basement granitoids may be enriched in aluminum and iron and yield chloritoid under proper metamorphic conditions sediments formed from mixed sources such as weathered gneissic basement and contemporaneous komatiitic volcanics yield anomalous resistate sedimentary mixtures such as chromiferous quartzites based on the data given here there is no need to postulate that different weathering and or ground water systems were operative during the archean compared to more recent times on the other hand the complicated weathering and diagenetic history that altered material undergoes below a subsiding unconformity relies as much or more on the buffering capacity of the enclosed rock mass as on the atmospheric conditions prevalent at the time of weathering hence weathering profiles old and new are more a response to local environmental conditions within the rock than the composition of the atmosphere

early atmospheric oxygen levels constraints from archaean photoautotrophy various independent lines of evidence suggest that photoautotrophic carbon fixation is a very ancient process that had attained control of the terrestrial carbon cycle from at least 3 5 ga if not 3 8 ga since it is generally accepted that only biological processes could substantially increase the negligible oxygen pressures sustained on the early earth by inorganic photochemistry probably between 10 8 and 10 14 present atmospheric level pal the advent of photoautotrophy would provide a limiting condition for the oxygenation of terrestrial near surface environments however as the size of the free oxygen reservoir is determined by the kinetics of the geochemically relevant oxygen consuming reactions we may assume that the start of photosynthetic oxygen production did not result in an instantaneous buildup of modern p o 2 levels low atmospheric oxygen pressures generally inferred for the archaean and lower proterozoic earth would therefore indicate extremely rapid processes of oxygen consumption in certain compartments of the ancient exogenic system notably the hydrosphere rather than the non existence of photosynthetic oxygen production at this time

reply to discussion on geochemistry and geodynamic setting of volcanic and plutonic rocks associated with early archaean volcanogenic massive sulfide mineralisation pilbara craton vearncombe s e kerrich r 1999 precambrian research 98 243 270 by c w brauhart p morant

microfossil like objects from the archaean of greenland a cautionary note recent reports1 4 have described yeast like microfossils isuasphaera isua pflug2 in 3 800 myr old5 metaquartzites from the isua supracrustal belt of south west greenland a biogenic interpretation of these objects is inconsistent with the tectonic history of the isua region6 8 with the petrology of the metaquartzites and with the morphology of the microstructures themselves the putative microfossils are indistinguishable from limonite stained fluid inclusions microstructures which are demonstrably inorganic and post depositional in origin as such we contend here that these objects should not be regarded as evidence of early archaean life forms

trace sulfate in mid proterozoic carbonates and the sulfur isotope record of biospheric evolution concentrations of oceanic and atmospheric oxygen have varied over geologic time as a function of sulfur and carbon cycling at or near the earth s surface this balance is expressed in the sulfur isotope composition of seawater sulfate given the near absence of gypsum in pre phanerozoic sediments trace amounts of carbonate associated sulfate cas within limestones or dolostones provide the best available constraints on the isotopic composition of sulfate in precambrian seawater although absolute cas concen trations which range from those below detection to 120 ppm sulfate in this study may be compromised by diagenesis the sulfur isotope compositions can be buffered sufficiently to retain primary values stratigraphically controlled 34 s measurements for cas from three mid proterozoic carbonate successions 1 2 ga mescal limestone apache group arizona usa 1 45 1 47 ga helena and newland formations belt supergroup montana usa and 1 65 ga paradise creek formation mcnamara group nw queens land australia show large isotopic variability 9 1 to 18 9 1 1 to 27 3 and 14 1 to 37 3 respectively over stratigraphic intervals of 50 to 450 m this rapid variability ranging from scattered to highly systematic and overall low cas abundances can be linked to sulfate concentrations in the mid proterozoic ocean that were substantially lower than those of the phanerozoic but higher than values inferred for the archean results from the belt supergroup specifically corroborate previous arguments for seawater contributions to the basin limited sulfate availability that tracks the oxygenation history of the early atmosphere is also consistent with the possibility of extensive deep ocean sulfate reduction the scarcity of bedded gypsum and the stratigraphic 34 s trends and 34 s enrichments commonly observed for iron sulfides of mid proterozoic age copyright 2005 elsevier ltd

hostrocks to the stratabound iron formation hosted starra gold copper deposit australia this paper documents the timing geochemistry and possible origin of sodium enrichment associated with proterozoic iron formation hosted copper gold ore lenses at the starra deposit australia the ore lenses are immediately underlain by variably sheared albitemagnetite hematite pyrite bearing rocks and overlain by mainly unaltered less sheared meta sediments evidence indicating that albite was abundant prior to deformation and also prior to the main hydrothermal addition of fe s cu and au includes 1 regional albite development at this stratigraphic level 2 alteration of albite by ore related sericite and 3 inclusion of unfoliated inclusions of albite and hematite in ore related pyrite nevertheless albite also developed around dolerites and within hangingwall shears during metamorphism and deformation least altered albite footwall rocks have sio2 67 31 73 13 wt al2o3 9 89 12 51 wt total alkali elements 6 40 9 43 wt and hfs elements e g zr 179 275 ppm ti zr 7 6 14 1 comparable with felsic volcanics but have high relative na2o contents 5 08 5 81 wt and variable to high na2o k2o ratios 0 05 68 37 an episode of alkali alteration by high na fluids prior to the ore related alteration is postulated to account for sodic compositions after chemical consideration of the alternatives 1 arkosic sediments 2 trondhjemitic volcaniclastics and 3 na metasomatism of staveley formation sediments during the most intense ore alteration isocon analysis indicates a density increase of 23 compared to the least altered albitised hostrocks cu fe au sn w zn k ba and rb were enriched during the mineralising process whereas mn ca si nb and zr were lost and al ti and y were not changed and were used to define the isocon the loss of some high field strength elements during this alteration indicates that igneous rock classification diagrams based on immobile element ratios should be used with caution for albitemuscovite magnetite pyrite assemblages associated with iron formation hosted copper gold deposits zircon was partially soluble whereas titanium oxides were probably stable in the saline high temperature 260 380 c acid ph 3 9 6 0 fluids envisaged for ore related alteration

modern weathering in outcrop samples versus ancient paleoredox information in drill core samples from a mesoarchaean marine oxygen oasis in pongola supergroup south africa there is growing evidence from mo and fe stable isotope tracers that shallow water sediments of the 2 95 ga mozaan group pongola supergroup in south africa were deposited under oxidizing conditions while mineralogical elemental and s isotopic evidence favors the contemporaneous atmosphere to have remained reducing in contrast the first reported cr stable isotope data for white mfolozi riverbed outcrop samples of the ijzermijn iron formation if mozaan group and drill core samples of the underlying paleosol of the nsuze group were interpreted to require oxidative weathering conditions during the time of deposition more recent cr isotope data of at least the ijzermijn if samples of the same white mfolozi riverbed outcrop however suggested that modern weathering effects can superimpose oxidative signatures on paleo redox proxies this study compares 234u 238u activity ratios along with stable mo and fe isotope data of corresponding drill core and white mfolozi riverbed outcrop samples in order i to test 234u 238u disequilibria as a potential tool to identify samples affected by modern weathering and ii to further investigate weathering effects on outcrop samples particularly with regard to the stable mo and fe isotope systematics all drill core samples from the ijzermijn if have 234u 238u activity ratios that are within uncertainties of secular equilibrium i e a value of 1 and thus were not significantly affected by modern weathering related u gain or loss or other sub recent rock water interaction by contrast the outcrop if and shale samples from the white mfolozi riverbed show very strong deviations from the secular equilibrium 234u 238u activity ratio furthermore these deviations are accompanied by the observations that i outcrop samples have on average heavier 56 54fe values 0 96 than drill cores 1 59 and ii positive correlations in drill core samples between log fe mn and 98 95mo or 56 54fe are disrupted by inclusion of the outcrop samples as are negative correlations between log mno and 98 95mo however 56 54fe and 98 95mo of our outcrop samples do not correlate with the extent of 234u 238u disequilibria as such 234u 238u activity ratios fail to act as a direct identification tool for modern weathering effects on stable fe and mo isotopes in outcrop samples overall this study confirms the adsorption of isotopically light mo onto mno2 particles in oxic surface water conditions in the epicontinental pongola sea reduction of the mn oxides together with microbial fe reduction in the reduced sediment pile of the pongola basin led to the formation of mn and fe carbonates during early diagenesis storing the observed stable fe and mo isotopic signatures we thus extend the so called mno2 shuttle first proposed for the neoarchean 2 46 ga old koegas if to late mesoarchean shallow marine depositional settings

rare earth element mobility in alteration pipes below massive cuzn sulfide deposits abstract a convincing demonstration of trace element mobility should satisfy two criteria 1 the range of trace element concentrations in the rock before and after alteration must be known with confidence and 2 the absolute abundances and the ratios of the mobile trace elements should vary between altered and unaltered rocks these criteria have been applied to the host felsic volcanics and alteration pipes associated with several canadian massive cu zn sulfide deposits although there is no evidence of rare earth element ree mobility in the pervasive low grade alteration envelopes adjacent to the deposits the light and middle ree lree and mree plus y have been systematically mobilized relative to the heavy ree hree and other trace elements in the alteration pipes beneath the ore zones there is evidence of minor lree and mree depletion in the most altered rocks at the south bay mine uchi lake and of significant lree depletion in the alteration pipe beneath the mattabi mine sturgeon lake however the strongest mobility is found at the kidd creek mine where the lree were leached from the rocks in the footwall stringer zone and redeposited in a metasomatic chlorite zone directly beneath the massive ore horizon and the mree and y were preferentially lost from the system producing distinctive chondrite normalized ree patterns with tb yb n ratios less than unity eu2 was found to be more mobile than the trivalent ree especially at kidd creek where there is a marked increase in the magnitude of the eu anomaly in the most altered samples high field strength elements such as u th hf and zr have not decoupled and apparently remained relatively immobile during all but the most intense alteration i e kidd creek preliminary data suggest that the degree of ree mobility increases with the size of the deposit thus ree mobility is a potentially useful method of distinguishing between small and large massive sulfide deposits at an early stage in exploration however this hypothesis requires further testing

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

trace element geochemistry of metabasaltic rocks from the yukon tanana upland and implications for the origin of tectonic assemblages in east central alaska we present major and trace element geochemical data for 27 amphibolites and six greenstones from three structural packages in the yukon tanana upland of east central alaska the lake george assem

stromatolites at 3 500 myr and a greenstone granite unconformity in the zimbabwean archaean two controversial areas of geological endeavour are the establishment of the antiquity of life and the tectonic setting of greenstone sequences we record here the recent discoveries in the fort victoria greenstone belt of stromatolites in limestones assigned to 3 500 myr minimum age sebakwian group rocks of the rhodesian archaean craton within zimbabwe and a nearby outcrop of a thin sedimentary formation basal to a thick 2 700 myr volcanic pile resting with definite unconformity on 3 500 myr mushandike granite