zircon shrimp dating confirms a palaeoarchaean supracrustal terrain in the southeastern kaapvaal craton southern africa abstract we report shrimp zircon ages for felsic volcanic rocks of the early archaean nondweni greenstone belt ngb located in northern kwazulu natal province south africa the ngb is part of a chain of greenstone belt remnants that occurs south of the well known barberton greenstone belt bgb two samples of felsic volcanic rocks of the basal toggekry formation yielded zircon ages of 3 53 ga whereas significantly younger ages of 3 41 ga were obtained for two samples of felsic rocks from near the top of the witkop formation the latter date now constrains the age of stromatolite like structures in cherts associated with the felsic rocks which probably represent some of the oldest preserved evidence of life on earth the zircon ages also indicate that the ngb correlates well chronologically with the bgb and is one of the oldest volcano sedimentary successions in the southeastern kaapvaal craton geochemical data for the felsic ngb rocks suggest formation by partial melting or crystal fractionation of a mafic source that was contaminated by older crust possibly in a back arc environment initial e nd values for ngb rocks range from 1 2 to 0 3 and suggest involvement of pre greenstone continental crust that was possibly extensive in the southeastern part of the kaapvaal craton during palaeoarchaean times

gold speciation and transport in geological fluids insights from experiments and physical chemical modelling this contribution provides an overview of available experimental thermodynamic and molecular data on au aqueous speciation solubility and partitioning in major types of geological fluids in the earth s crust from low temperature aqueous solution to supercritical hydrothermal magmatic fluids vapours and silicate melts critical revisions of these data allow generation of a set of thermodynamic properties of the auoh aucl 2 auhs and au hs 2 complexes dominant in aqueous hydrothermal solutions however other complexes involving different sulphur forms chloride and alkali metals may operate in high temperature sulphur rich fluids vapours and melts the large affinity of au for reduced sulphur is responsible for au enrichment in s rich vapours and sulphide melts which are important gold sources for hydrothermal deposits thermo dynamic speciation and partitioning data and their comparison with au and s contents in natural fluid inclusions from magmatic hydrothermal gold deposits provide new constraints on the major physical chemical parameters temperature pressure salinity acidity redox and ubiquitous fluid components sulphur carbon dioxide arsenic affecting au concentration transport precipitation and fractionation from other metals in the crust the availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations

salinity history of the earth s early ocean it is commonly thought that the oceans are becoming saltier with time as sodium and chlorine are weathered out of continental rocks and transported to the sea here we argue that the salinity of the early ocean was 1 5 to 2 times the modern value and that it did not decline significantly until surprisingly late in the earth s history if correct this theory could help explain why the evolution of higher life took so long

gold in the oceans through time during sedimentation and diagenesis of carbonaceous shales in marine continental margin settings au is adsorbed from seawater and organic matter and becomes incorporated into sedimentary pyrite la icpms analysis of over 4000 sedimentary pyrite grains in 308 samples from 33 locations around the world grouped over 123 determined ages has enabled us to track in a first order sense the au content of the ocean over the last 3 5 billion years gold was enriched in the meso and neoarchean oceans several times above present values then dropped by an order of magnitude from the first great oxidation event goe1 through the paleoproterozoic to reach a minimum value around 1600 ma gold content of the oceans then rose with perturbations through the meso and neoproterozoic showing a steady rise at the end of the proterozoic 800 to 520 ma which most likely represents the effects of the second great oxidation event goe2 gold in the oceans was at a maximum at 520 ma when oxygen in the oceans rose to match current maximum values in the archean and proterozoic the au content of seawater correlates with the time distribution of high mg greenstone belts black shales and banded iron formations suggesting that increases in atmospheric oxygen and marine bio productivity combined with the higher background of au in komatiitic and mg rich basalts were the first order causes of the pattern of au enrichment in seawater we suggest the lack of major au deposits from 1800 to 800 ma is explained by the low levels of au in the oceans during this period

geochemistry and timing of the marginal basin and arc magmatism in the philippine sea available data enable the recognition of three periods of back arc crustal generation and three pulses of volcanic activity along the associated island arcs of the philippine sea the geochemistry of the basalts from the back arc basins of different ages indicates that in most cases they are identical to mid ocean ridge basalts and therefore should have similar sources and origins in contrast the island arc rock composition is variable in space and time reflecting the different source and more complex nature of corresponding magmatism geomagnetic studies and recent deep sea drilling project results suggest an alternate sequence of back arc and arc magmatic cycles both geochemical and geological observations provide important constraints on models of magmatism and extensions tectonics at convergent plate boundaries

the gold content of some archaean rocks and their possible relationship to epigenetic gold quartz vein deposits gold mineralization in archaean granite greenstone environments especially gold quartz veins contributes considerably to the world s gold production the formation of epigenetic gold mineralization in greenstone belts is generally explained by the metamorphic secretion theory this theory is based on the assumption that the source of the gold may be komatiitic or tholeiitic lavas pyritic chemical or clastic sediments and even granitic rocks from which as a result of regional metamorphic overprinting gold was extracted and concentrated in suitable structures it has been shown that in proposed potential source rocks gold is predominantly associated with sulfide minerals and thus relatively easily accessible to secretion and reconstitution processes a large number of various rock types originating from granite greenstone terranes of the kaapvaal and the rhodesian cratons were geochemically investigated and the following ranges for gold determined volcanic rocks komatiitic and tholeiitic 0 1 372 ppb granitic rocks of the basement 0 3 7 8 ppb iron rich chemical sediments 1 0 667 ppb statistical treatment of the data reveals that volcanic rocks as well as iron rich chemical sediments are favorable sources for epigenetic gold mineralization formed by metamorphic secretion while the granitic rocks make less suitable primary gold sources this finding explains the close spatial relationship which is common between gold quartz veins and greenstone belts the conspicuous abundance of epigenetic gold mineralization in the archaean however is attributed to the unique geologic and metamorphic history of the granite greenstone terranes

evidence for a 3 45 billion year old magnetic remanence hints of an ancient geodynamo from conglomerates of south africa 1 paleomagnetic and rock magnetic analyses of 3445 million year old dacite conglomerate clasts and parent body rocks from the barberton greenstone belt south africa define two dominant components of magnetization one component unblocked at low temperature is an overprint acquired 180 million years ago the other component is unblocked at high temperatures and passes a conglomerate test indicating that this component is older than the depositional age of the conglomerate 3416 ma the high unblocking temperature component shows scatter in the parent body rocks that can be explained by the effects of modern lightning strikes archean overprinting and the presence of multidomain magnetic grains that are conducive to carrying secondary magnetizations alternatively this scatter can be explained by exotic magnetization scenarios in the absence of a dynamo including magnetization by an external field related to solar wind interaction with the atmosphere such exotic mechanisms can be tested with the acquisition of paleointensity data while more scattered than paleomagnetic data recording the more recent geomagnetic field the high unblocking temperature component in the dacite parent body shows some consistency and the simplest explanation of the data is that they reflect a geodynamo 3445 million years ago

gold distribution in supracrustal rocks from archean greenstone belts of southern africa and from paleozoic ultramafic complexes of the european alps metallogenic and geochemical implications many archean gold veins are thought to have formed as a result of metamorphic secretion processes with the source of gold being gold enriched supracrustal rocks however in many gold provinces no elevated gold concentrations can be detected in the proposed source beds in this study 98 samples of mafic and ultramafic rocks and 32 samples of ferruginous chemical sediments from south african greenstone belts and from the belingwe greenstone belt in zimbabwe were analyzed for their mg al cr fe co ni and au contents the analyses were carried out by means of xrf aas and neutron activation analysis and the data evaluated by various statistical methods the gold values of the volcanics range from 0 1 to 372 ppb with the mean content at 10 8 ppb and the gold values of the sediments vary from 0 5 to 667 ppb with the mean content being 129 9 ppb as a comparative population 56 samples from paleozoic european ultramafic complexes were investigated for the same suite of elements the gold values range from 0 1 to 25 ppb and possess a mean content of 2 0 ppb for the samples from southern africa statistical analyses indicate a complete lack of relationship between gold and rock forming minerals significant differences in gold abundance were observed in samples originating from geologically and geochemically similar greenstone terranes these differences may be a result of heterogeneous gold abundance in the upper mantle differentiation trends of gold in the ultramafic to mafic range were not detected in this study ferruginous chemical sediments of the algoma type were found to contain more gold than younger ones of the superior type for volcanic rocks the analytical data suggest that the principal gold carriers are accessory sulfides and to a lesser extent intergranular gold in particulate form rock forming minerals appear to contribute less than 0 5 ppb to the gold in these rocks most supracrustal rocks contain sulfides and thus contain gold readily accessible to leaching metamorphic hydrothermal solutions which may deposit gold in nearby dilatant zones it is proposed that in greenstone terranes metamorphic overprint constitutes the key factor determining the formation of gold veins the conspicuous abundance of such gold mineralizations in archean greenstone terranes is considered to be a result of the unique evolution of these terranes

silver isotopes as a source and transport tracer for gold a reconnaissance study at the sheba and new consort gold mines in the barberton greenstone belt kaapvaal craton south africa

the gold content of volcanogenic massive sulfide deposits volcanogenic massive sulfide deposits contain variable amounts of gold both in terms of average grade and total gold content with some vms deposits hosting world class gold mines with more than 100 t au previous studies have identified gold rich vms as having an average gold grade expressed in g t exceeding the total abundance of base metals expressed in wt however statistically meaningful criteria for the identification of truly anomalous deposits have not been established this paper presents a more extensive analysis of gold grades and tonnages of 513 vms deposits worldwide revealing a number of important features in the distribution of the data a large proportion of deposits are characterized by a relatively low gold grade 2 g t with a gradual decrease in frequency towards maximum gold grades defining a log normal distribution in the analysis presented in this paper the geometric mean and geometric standard deviation appear to be the simplest metric for identifying subclasses of vms deposits based on gold grade especially when comparing deposits within individual belts and districts the geometric mean gold grade of 513 vms deposits worldwide is 0 76 g t the geometric standard deviation is 2 70 g t au in this analysis deposits with more than 3 46 g t au geometric mean plus one geometric standard deviation are considered auriferous the geometric mean gold content is 4 7 t au with a geometric standard deviation of 26 3 t au deposits containing 31 t au or more geometric mean plus one geometric standard deviation are also considered to be anomalous in terms of gold content irrespective of the gold grade deposits with more than 3 46 g t au and 31 t au are considered gold rich vms a large proportion of the total gold hosted in vms worldwide is found in a relatively small number of such deposits the identification of these truly anomalous systems helps shed light on the geological parameters that control unusual enrichment of gold in vms at the district scale the gold rich deposits occupy a stratigraphic position and volcanic setting that commonly differs from other deposits of the district possibly due to a step change in the geodynamic and magmatic evolution of local volcanic complexes the gold rich vms are commonly associated with transitional to calc alkaline intermediate to felsic volcanic rocks which may reflect a particularly fertile geodynamic setting and or timing e g early arc rifting or rifting front at the deposit scale uncommon alteration assemblages e g advanced argillic aluminous strongly siliceous or potassium feldspar alteration and trace element signatures may be recognized e g au ag as sb bi hg te suggesting a direct magmatic input in some systems

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

geochemistry of archean carbonaceous cherts deposited at immature island arc setting in the pilbara block western australia abstract archean carbonaceous meta cherts upper greenschist to amphibolite facies from the nickol well unit the cleaverville group 3 2ga in the roebourne district in west pilbara are characterized by variable al2o3 tio2 zr tio2 and cr th values and by high concentrations of carbon and some trace metals distribution patterns of the cherts on a newly proposed diagram al2o3 tio2 element al2o3 indicate that concentrations of cr v and ni can be explained by contributions of clastic components whereas concentrations of cu zn and pb are much higher than those expected solely from clastic contributions detrital material in the cherts was derived from a wide variety of volcanic sources mafic ultramafic felsic sources consistent with the inferred geological setting of the cleaverville group immature island arc setting excess cu zn and pb are probably from hydrothermal solutions as suggested by positive eu anomalies and enrichment in heavy rare earth elements ree in chondrite normalized patterns high concentrations and light isotopic values of c 17 to 24 per mil in the cherts indicate that the precursory sediments were enriched in organic materials although it is unclear whether the organic material was derived from organic particles that settled from the ocean surface or benthic microbes in either case it is possible that this material played an important role in the accumulation of heavy metals cu zn and pb in the sediments

isovolumetric silicification of early archean komatiites geochemical mass balances and constraints on origin stratiform quartz phyllosilicate carbonate alteration zones are a common feature of archean greenstone terranes and determining their origin is an important aspect of understanding the nature of shallow crustal alteration during the early history of the earth komatiites and komatiitic basalts of the uppermost onverwacht group ca 3 4 3 5 ga barberton greenstone belt south africa were regionally metasomatized prior to major tectonic deformation to the assemblage quartz dioctahedral mica chlorite pm dolomite subsequent metamorphism of these rocks has been restricted to greenschist facies and they thus record processes that may be obscured in analogous sequences from more highly metamorphosed terranes the isovolumetric nature of much of the alteration and the relative immobility of elements such as al zr and y permit a quantitative assessment of elemental gains and losses during metasomatism for each m 3 of komatiite altered approximately 10 3 kg one ton of mgo were removed

mobility of au and related elements during the hydrothermal alteration of the oceanic crust implications for the sources of metals in vms deposits volcanogenic massive sulphide vms deposits are commonly enriched in cu zn and pb and can also be variably enriched in au as sb se and te the behaviour of these elements during hydrothermal alteration of the oceanic crust is not well known ocean drilling program odp hole 1256d penetrates a complete in situ section of the upper oceanic crust providing a unique sample suite to investigate the behaviour of metals during hydrothermal alteration a representative suite of samples was analysed for au as sb se and te using low detection limit methods and a mass balance of metal mobility has been carried out through comparison with a fresh mid oceanic ridge basalt morb glass database the mass balance shows that au as se sb s cu zn and pb are depleted in the sheeted dyke and plutonic complexes by 46 12 27 5 2 5 0 5 27 6 8 4 0 7 9 6 1 6 7 9 0 5 and 44 6 respectively arsenic and sb are enriched in the volcanic section due to seawater derived fluid circulation calculations suggest that large quantities of metal are mobilised from the oceanic crust but only a small proportion is eventually trapped as vms mineralisation the quantity of au mobilised and the ratio of au to base metals are similar to those of mafic vms and ten times enrichment of au would be needed to form a au rich vms the cu rich affinity of mafic vms deposits could be explained by base metal fractionation both in the upper sheeted dykes and during vms deposit formation

the variscan post collisional volcanism in late carboniferous permian sequences of ligurian alps southern alps and sardinia italy a synthesis abstract in the considered wide sector of the west mediterranean southern europe the collisional phase of the variscan orogeny during late carboniferous and permian times was followed by magmatic intrusive and effusive activity and sedimentation into intracontinental alluvial to lacustrine basins originated by wrench to normal fault systems the first volcanic cycle generally late carboniferous early permian in age is represented by early calc alkaline andesites and rhyolites in variable amounts and by following large volume of rhyolites and by dacites both andesites and rhyolites show k normal and high k calc alkaline character the origin of the liquids of the first cycle is ascribed to partial melting processes at the mantle crust interface telescoped within a thickened crust the melting is considered as the consequence of thermal re equilibration following isostatic disequilibrium and the subsequent collapse of the orogenic belt the ascent of liquids occurred in a trans tensional regime the second magmatic cycle is represented by alkaline magmatism and exhibits typical anorogenic features consistent with a rifting regime this event was no more related with the collapse of the variscan belt but rather to the post variscan global re organization of plates that evolved during late triassic times to the neo tethyan rifting in both cycles important differences in timing areal distribution and outpoured volumes arise

silica alteration zones in the barberton greenstone belt a window into subseafloor processes 3 5 3 3 ga ago abstract silicification of volcanic rocks ranging from komatiitic to dacitic in composition is a common phenomenon of the 3 5 to 3 2 ga old volcano sedimentary succession of the barberton greenstone belt silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons sio2 contents increase upsection from the original igneous value up to 92 silicification is associated with a depletion of most elements commonly mobile during water rock interaction while k2o rb and ba are enriched in these zones some transition metals sc v and cr were immobile whereas ni and co and some other metals cu zn were mobile and are depleted the 18o values of volcanic rocks show a positive linear relationship with silica content and range from 9 0 to 17 3 the element depletion enrichment patterns and oxygen isotope data indicate low temperature c 100 150 c hydrothermal processes for the origin of the alteration zones the loss of most elements coupled with the addition of sio2 as well as ree systematics indicate high water rock ratios and a fluid ree composition typical of archaean seawater that had a 18o value of 0 hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau like setting and resulted in the establishment of shallow subseafloor convection cells diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3 5 ga ago

gold in seawater abstract a suite of seawater samples has been analyzed for au by inductively coupled plasma quadrupole mass spectroscopy icpms the recently developed method involves preconcentration by anion exchange followed by flow injection sample introduction into a vg plasmaquad icpms profiles in the atlantic gulf stream northeast pacific and mediterranean are presented in addition the first measurements of au in hydrothermal fluids are reported the measured concentrations of au in the atlantic and northeast pacific 50 150 fmol l are within a factor of 2 3 of recently reported values for pacific waters 1 and nearly three orders of magnitude less than reported in the literature prior to 1988 indicating contamination problems with the earlier data in general au concentrations for atlantic and pacific samples reported here are the same 50 fmol l however differences of up to a factor of 2 between the oceans cannot be ruled out due to large uncertainties associated with the pacific data mediterranean deep waters contain higher concentrations of au 100 150 fmol l this is attributed to proximal aeolian dust and or riverine sources gold is enriched in hydrothermal fluids 10 100 s of pmol l with respect to ambient seawater although the fluxes remain poorly constrained preliminary budget considerations indicate that rivers and hydrothermal venting are the most significant sources of au to the oceans

organic geochemistry and mineralogy i characterisation of organic matter associated with metal deposits abstract as geochemical appraisals of mineral regions of commercial prospectivity evolve the organic matter associated with metal rich ores has attracted greater attention petroleum basin and modern seafloor hydrothermal vent studies have suggested that organic matter can have a significant influence on the behaviour of mineralising fluids there have been many isolated reports of certain organic compositional or morphological e g pyrobitumen features showing an apparent relationships with hydrothermal fluids or minerals raising expectations that organic based parameters might be useful to mineral exploration however the understanding of organic inorganic relationships in earth systems is far from complete for example the detailed mechanics of the interaction of organics with hydrothermal fluids over geological time remain largely undefined organic geochemistry studies have traditionally involved the measurement and interpretation of the hydrocarbon composition of sedimentary rocks here we review the types of aliphatic hydrocarbons aromatic hydrocarbons and metalloporphyrins often detected from organic geochemical investigations in mineral rich regions such molecular data can be particularly diagnostic of biochemical sources and the palaeo environments at the time mineral associated organic matter was deposited sub surface trends of hydrocarbon alteration may also reflect major biogeochemical processes such as thermal maturity and biodegradation organic geochemistry data can also occasionally provide information about the nature e g origin composition temperatures and migration pathways of hydrothermal fluids and can make a contribution to holistic ore genesis models the well preserved organic matter associated with the economic here s your chance pb zn ag mine paleoproterozoic barney creek formation mcarthur basin australia and the transition metal rich early permian kupferschiefer formation germany poland have attracted significant attention a more detailed summary of the organic character of these deposits is provided to highlight the contribution organic geochemistry can make to understanding mineralisation processes most organic geochemical studies of highly mineralised regions however have not adequately addressed the significance of organic matter to mineralisation a slightly different analytical focus than traditionally used for exploration appraisal of petroleum hydrocarbons may be required to properly evaluate the significance of organic species to the mobilisation transport and deposition of ore metals the characterisation and subsequent thermodynamic modeling of organic substances and complexes within metalliferous hydrothermal systems will contribute to a better understanding of the nature and role of organic inorganic fluids or other affiliated organics in ore systems

the geochemistry of black shales from the chuniespoort group transvaal sequence eastern transvaal south africa geochemical and mineralogical data have been obtained from black carbonaceous shales interbedded with dolomite in the chuniespoort group of the late archean early proterozoic transvaal sequence the black shales are fine grained and generally argillaceous and contain on average 1 2 percent total c organic 0 7 percent s 7 5 ppb au 60 ppm cu 330 ppm cr and 125 ppm ni these metal concentrations are similar to or slightly higher than the values given for the u s geological survey black shale standard sdo 1 all other heavy and transition metals however are depleted in the chuniespoort shales relative to sdo 1 it is well known that high concentrations of cr and ni occur in archcan shales the enhanced cr and ni values found in the chuniespoort shales are a reflection of their antiquity and provenance and are not linked to preferential uptake of these metals during sedimentation and or diagenesis the chuniespoort shales are characterized by excess fe with respect to that required to form pyrite and have extremely variable c s ratios as well this together with the wide ranging s isotope ratios is taken as an indication that biogenically produced sulfide was augmented by sulfide introduced from an extraneous source whole rock s isotopes further indicate that s could have been derived from biogenic and inorganic reduction of sulfate as well as from hydrothermal fluids interelement correlations suggest that organic carbon does not play a role in hosting significant concentrations of base and transition metals rather element associations indicate that v and cr are preferentially contained in illite hydromuscovite and that ni co zn cu fe pb and mo probably reside in pyrite and or chalcopyrite gold is only weakly correlated with other elements which may indicate that its distribution is not solely determined by the presence of sulfide minerals the fact that au cu and s c ratios show an increase in samples with delta 34 s values around 0 per mil suggests that these elements are likely to have an epigenetic hydrothermal origin the limited syngenetic metal accumulation can be explained by the stable cratonic environment in which the sediments were deposited and the absence of submarine hydrothermal springs venting metalliferous fluids into the basin other factors such as the low metal potential of the provenance the rate of sedimentation and the degree of oxygenation of the bottom waters of the basin may have been additional factors limiting the concentration in and extraction of metals from seawater the fact that the chuniespoort shales host significant epigenetic gold occurrences particularly in the sabie pilgrim9s rest gold field is largely related to their chemical characteristics which promoted permeability and high absorption capacities for metals in hydrothermal fluids because of their low metal concentrations elsewhere in the basin the shales cannot however be considered the source rocks for gold mineralization

estimations of clarkes for carbonaceous biolithes world averages for trace element contents in black shales and coals abstract black shale and coal clarke values are the average trace element contents in the world black shales and coals these calculations are made in russian geochemistry but up to now are poorly known in the west modern tables of black shale and coal clarkes are presented based on comprehensive calculations using very large amount of information thousands analyses of black shales coals and coal ashes for trace elements in black shale geochemistry three figures were calculated for each main lithologies terrigenous tuff chert and carbonate two clarke estimations are presented named lithological k1 and lithostratigraphical k2 in coal geochemistry seven figures were calculated for each trace element average content in hard coals and their ashes average content in brown coals and their ashes average content in all coals and their ashes and coal affinity index or coalphile index average content in all ashes clarke values of sedimentary rocks the black shale and coal clarkes presented here provide an important scientific base for many geochemical comparisons and issues

protoliths of the 3 8 3 7 ga isua greenstone belt west greenland abstract the isua greenstone belt fig 1 contains the oldest known relatively well preserved metavolcanic and metasedimentary rocks on earth the rocks are all deformed and many were substantially altered by metasomatism but both the deformation and metasomatism were heterogeneous transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated likewise different kinds and different episodes of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths new geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents by this means the primary nature of the schists that make up most of the isua greenstone belt was reinterpreted and a new map that better portrays the primary nature of the rocks has been produced the previously mapped stratigraphy was found to be of little value in understanding the geology stratigraphic units were defined by different and diverse criteria such as current composition structure metamorphic texture and inferred protoliths much of this stratigraphy represents a misinterpretation of the primary nature of the rocks the new work indicates that most of the isua greenstone belt consists of fault bounded rock packages mainly derived from basaltic and high mg basaltic pillow lava and pillow lava breccia chert bif and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks a previously mapped extensive unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite

gold mobility during palaeoarchaean submarine alteration abstract seafloor alteration provides large amounts of solutes to the hydrosphere in order to investigate gold mobility during water rock interaction prior to 3 billion years ago low detection limit analysis of au concentrations was carried out on rocks from marine alteration zones stratiform zones recording low temperature 150 c seafloor alteration are a characteristic feature of greenstone belts older than 3 0 ga hydrothermal processes were operating on and immediately below the seafloor giving rise to extensive silicification of sub seafloor volcanic rocks and silicification of seafloor sediments in order to investigate gold mobility during silicification unaltered and variably silicified volcanic rocks and associated cherts from palaeoarchaean greenstone successions c 3 4 ga of south africa were analyzed results show mobility of gold during silicification of mafic ultramafic rocks and transfer to the archaean ocean some gold was incorporated into carbonaceous marine sediments overlying the alteration zones a combination of pervasive silicification rarity of black shales and low gold content in komatiites can explain the low mineralization potential of palaeoarchaean greenstone belts for orogenic gold deposits

coupled silicon oxygen isotope fractionation traces archaean silicification silica alteration zones and cherts are a conspicuous feature of archaean greenstone belts worldwide and provide evidence of extensive mobilisation of silica in the marine environment of the early earth in order to understand the process es of silicification we measured the silicon and oxygen isotope composition of sections of variably silicified basalts and overlying bedded cherts from the theespruit hooggenoeg and kromberg formations of the barberton greenstone belt south africa the 30si and 18o values of bulk rock increase with increasing amount of silicification from unsilicified basalts 0 64 30si 0 01 and 8 6 18o 11 9 to silicified basalts 30si and 18o values as high as 0 81 and 15 6 respectively cherts generally have positive isotope ratios 0 21 30si 1 05 and 10 9 18o 17 1 except two cherts which have negative 30si values but high 18o up to 19 5 the pronounced positive correlations between 30si 18o and sio2 imply that the isotope variation is driven by the silicification process which coevally introduced both 18o and 30si into the basalts the oxygen isotope variation in the basalts from about 8 6 to 15 6 is likely to represent temperature dependent isotope fractionation during alteration our proposed model for the observed silicon isotope variation relies on a temperature controlled basalt dissolution vs silica deposition process