a combined in situ oxygen silicon isotopic and fluid inclusion study of a chert sample from onverwacht group 3 35ga south africa new constraints on fluid circulation abstract cherts have been extensively used as a proxy of precambrian environmental conditions of the earth cherts from the onverwacht group 3 5 3 3 ga south africa are considered as well preserved and have been used as a paleo oceanic thermometer especially by using the oxygen isotope composition of the chert nevertheless there are several lines of evidence size and number of the quartz veins that fluids circulated through these cherts after their deposition these cherts are composed mainly of microquartz with large quartz veins 100 m and iron carbonates as ankerites a combined study of oxygen silicon and fluid inclusions study have been performed on one chert sample from the onverwacht group in order to assess the effect of these fluid circulations on the chert oxygen and silicon isotopic compositions in situ oxygen and silicon isotopic compositions on microquartz and vein quartz were measured at high spatial resolution using a multicollector cameca 1270 ion probe crpg cnrs nancy moreover fluid inclusions present in the quartz veins are considered to determine the origin and conditions of the fluid circulation three types of primary fluid inclusions carbonic lc aqueous carbonic fluids lc w and aqueous fluid lw have been observed the temperatures of this fluid circulation were determined by microthermometry and using quartz carbonate pair thermometry at between 200 c and 300 c fluid pressures range from 100 mpa to 30 mpa which correspond to depths ranging from 4 to 1 km this evolution could be related to an exhumation associated with a dilution and cooling process this fluid circulation could be associated to the gold bearing fluid responsible of the sheba fairview ore system dated at 3074 6 ma the results of the isotopic study show that these fluid circulations homogenised the oxygen isotope composition but not the entire silicon isotope composition of the microquartz the 18 o value calculated for the fluid show that it equilibrated with carbonate rocks and is similar to the composition of the gold bearing fluid the silicon isotope composition of the microquartz allows determination of the initial 30 si of the microquartz precursor this precursor has been precipitated from a hydrothermal fluid hence a combined 18 o 30 si and fluid inclusion study allows the conditions of the fluid circulation to be constrained and the origins of chert to be determined

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

continental growth and convergence related arc plutonism in the mesoarchaean evidence from the barberton granitoid greenstone terrain south africa u pb zircon ages from trondhjemitic gneisses of the badplaas domain located in the southwestern parts of the mesoarchaean barberton granitoid greenstone terrain of south africa document a hitherto largely unrecognized plutonic event the geochronological results indicate the addition of juvenile felsic crust over a period of 60 myr between ca 3290 and 3230ma and prior to the main collisional event in the granitoid greenstone terrain at 3230 ma the timing and duration of plutonism together with the structural and compositional heterogeneity of the badplaas domain suggest that the badplaas domain represents part of a convergence related magmatic arc on a regional scale the spatial and temporal relationships between plutonism metamorphism and deformation are interpreted to preserve an almost complete inventory of a mesoarchean arc trench system this includes 1 the 3290 3230ma convergence related magmatic arc 2 a largely coeval deposition of the back arc type volcano sedimentary 3260 3225ma fig tree group of the barberton greenstone belt 3 relics of the underplated crust preserved in high p low t rocks to the immediate east of the magmatic arc juxtaposed against 4 high t medium p rocks of the overlying plate along the major crustal structure of the inyoni shear zone these findings corroborate the notion that thermal and rheological conditions on the early earth supported at least locally crustal growth and the extraction of buoyant felsic crust along convergence related magmatic arcs

the norian plateosaurus bonebeds of central europe and their taphonomy abstract plateosaurus is probably the single most common dinosaur in the late triassic known from over 40 localities in the norian knollenmergel mudstones of central europe in addition it is also one of the best known triassic dinosaurs due to the discovery of many complete skeletons these complete skeletons as well as the majority of all other material are actually derived from only a handful of localities three of which are compared here to one another in detail frick northern switzerland trossingen southwestern germany and halberstadt central germany are recognized as the result of the same taphonomic processes and can all be described with the term plateosaurus bonebed this type of bonebed is characterized by its large size measured in thousands of square meters the large number of incomplete and complete plateosaurus skeletons and the lack of other vertebrate fossils with the exception of the primitive turtle proganochelys the uniformly upright carcass position precludes post mortem transport and strongly argues for in situ death shallow depressions in the thick floodplain muds of the knollenmergel must have formed efficient traps for the heaviest animal at that time plateosaurus the animals became mired and were scavenged upon by small theropods whose shed teeth are always present in plateosaurus bondebeds miring explains the deep position of the feet and hindquarters and in combination with scavenging and weathering the bias towoards bones of the posterior body region the lack of juvenile individuals is due to their lower foot pressure which was below the threshold for becoming mired

mesoarchean assembly and stabilization of the eastern kaapvaal craton a structural thermochronological perspective 1 understanding the construction and stabilization of archean continental lithosphere has important implications for models of tectonic and thermal regimes in the early earth as well as for the subsequent evolution of continents we provide new constraints for the amalgamation and stabilization of the eastern kaapvaal craton in the vicinity of the barberton greenstone belt circa 3 3 3 1 ga isotope dilution thermal ionization mass spectrometry u pb geochronology and thermochronology are combined with mapping and structural analysis around the margins of this belt in order to constrain movement on major shear zones that separate high grade orthogneiss terranes from low grade supracrustal rocks and to relate this movement to the timing of differential exhumation new and existing data are consistent with accretion at 3 23 ga within an oblique subduction plate boundary manifested in the bgb as an asymmetric flower like structural geometry for 100 ma following terrane assembly transform boundary tectonics manifested partially as transtension on reactivated faults led to large scale juxtaposition of middle to lower crustal basement orthogneiss complexes against upper crustal greenstone sequences and episodic emplacement of granitic batholiths stabilization of this portion of archean lithosphere involved at least a three stage process 1 creation of a thick and rigid mantle lithosphere during crustal growth subduction and terrane assembly from circa 3 30 to 3 23 ga 2 generation of a rigid crust during strike slip and transtensional tectonics coupled with migration of granites and heat producing elements into the upper crust between circa 3 2 and 3 1 ga and 3 erosion and removal of the uppermost crust during regional peneplanation that lasted until circa 2 9 ga

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

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

first report of cisuralian early permian charcoal layers within a coal bed from baode north china with reference to global wildfire distribution abstract fossil charcoal is reported for the first time from a cisuralian coal bed of the shanxi formation in the qiaotou section baode shanxi north china based on anatomical characteristics these charcoal fragments consist of coniferous or cordaitalean xylem unidentified primary xylem and cordaitalean and possible fern leaves these charcoal fragments represent the evidence of palaeowildfire taking place in tropical peat swamps during the cisuralian in cathaysia the palaeowildfire is most likely to be a surface fire and burning litter and shrubby vegetation fire frequency for this early permian peat swamp might have been on the order of 176 294 588 1429 years close to modern values compared with modern analogues the north china block during the cisuralian was probably wet in general but could be occasionally seasonally dry for short time intervals previous charcoal and inertinite records moreover indicate that palaeowildfires were globally common during the cisuralian overall more wildfire evidence was found in the artinskian kungurian than the asselian sakmarian except in the euramerican realm probably due to more suitable regional climate vegetation to fuel fires and taphonomic circumstances

spatial and temporal distribution of the orogenic gold deposits in the late palaeozoic variscides and southern tianshan how orogenic are they abstract a principal uncertainty in models of orogenic ore deposits concerns their ages relative to orogenic processes the yardstick of the relation has resided loosely in the peak of metamorphism age estimates in the variscides and tianshan indicate that most orogenic ore deposits were formed in the course of the late carboniferous to middle triassic with a peak between 305 and 280 ma their locations settings and ages suggest an association with coeval lithosphere scale strike slip deformation which played a role in the destruction of the orogenic system following crustal shortening this destruction tends to invalidate the orogenic association of the orogenic ore deposits observation of a mantle signature in some of these ore deposits and in coeval magmatic rocks suggests that the strike slip belts tapped into sub crustal levels here lithological composition had been modified by subduction during the stage of construction consequently any orogenic element in the ore deposit models is apart from location only an element of inheritance of subduction modified lithosphere features acquired during construction of the orogen these features may however well have been crucial for the formation of the ore deposits in view of the translithospheric extent of the strike slip belts fluids which contributed to the ore deposits may have come from at least the entire in part metasomatized lithosphere column the ore deposits could probably not have been formed without these deep reaching strike slip systems a model is proposed for the geological setting of the orogenic gold deposits in the variscides and the southern tianshan it consists of plutono metamorphic elements in the mantle in the lower and in the middle crust within a transpressional to transtensional tectonic framework of translithospheric faults and associated domes and pull apart basins with shallow marine to terrestrial sediments and interlayered felsic and mafic volcanics the thermal engines of these systems were probably in localized upwellings of the asthenosphere prompted and controlled by the lithosphere scale deformation at the time at an even larger scale oroclinal bending of the old orogenic backbone may have played yet another role in the localization of the ore deposits the thesis of specific localized tectono thermal engines is at odds with the original proposals of a relation between orogenic ore deposits and regional metamorphism it suggests that the late palaeozoic gold antimony mercury ore deposits in the variscides and the southern tianshan are more akin to the intrusion related ore systems

sources of sulphur for the proterozoic kiggavik uranium deposit nunavut canada the thelon basin is temporally and spatially related to the athabasca basin in saskatchewan canada which hosts the highest grade unconformity related uranium deposits in the world several uraniu

district to camp controls on the genesis of komatiite hosted nickel sulfide deposits agnew wiluna greenstone belt western australia insights from the multiple sulfur isotopes the agnew wiluna greenstone belt of western australia is the largest komatiite hosted nickel sulfide belt in the world and contains two world class ni cu pge deposits and a host of smaller deposits this study focuses on the broader scale geology of this greenstone belt in order to understand the key controls on the genesis of the komatiite hosted ni cu pge deposits with specific focus on camp to district controls we apply multiple sulfur isotopes to this geologic framework and conclude not only that the addition of crustal sulfur is a prerequisite for ore genesis in komatiite systems but above all that the sulfur required to generate world class deposits is most likely derived from barren volcanic massive sulfide lenses which are spatially and genetically associated with felsic volcanic and volcaniclastic sequences that were emplaced coevally with large komatiitic sills and channelized lava flows multiple sulfur isotope data can be utilized in exploration at the deposit to district scales at the deposit scale the spatial pattern of mass independent s isotope values 33 s provides crucial insight into the identification of proximal high grade and high tenor ores in mineralized systems in fact sulfur data reflect the assimilation process that occurred upon komatiite emplacement whereby hot turbulent magma thermomechanically eroded and assimilated exhalative sulfides spatially located close to vent with negative to near zero 33 s values whereas less turbulent flows interacted with distal sulfidic shales having 33 s values above 0 per mil accordingly the spatial variation of multiple sulfur isotope values in magmatic sulfides and associated host rocks may be utilized as a vector towards high grade ores of poorly known systems at the district scale rather than ascertaining what controls the distribution of komatiite hosted ni cu pge deposits the appropriate question to ask is what controls the distribution of country rock sulfides considering that exhalative sulfides may be crucial to ore genesis in komatiite systems we propose that felsic lava domes unambiguously mark their vents and can be directly mapped or inferred from gravity data this work provides the first step in identifying district scale control on komatiite hosted ni cu pge deposits this is the scale that has high impact on exploration for new komatiite hosted nickel sulfide belts globally

prolonged magmatism and time constraints for sediment deposition in the early archean barberton greenstone belt evidence from the upper onverwacht and fig tree groups the single zircon evaporation shrimp ion microprobe and conventional dissolution techniques were used to determine 207pb 206pb and u pb ages on samples from the upper onverwacht and fig tree groups of the early archean barberton greenstone belt south africa zircons from dacitic rocks of the upper hooggenoeg formation yield ages of 3445 3452 ma a tuff in the basal kromberg formation has a mean age of 3416 5 ma a tuffaceous band 5 cm thick in the uppermost kromberg formation contains igneous zircons with a mean age of 3334 3 ma the 1700 m section of kromberg formation between these two samples is composed of basaltic lavas minor komatiites and cherty metasediments the overlying mendon formation is composed of interbedded komatiitic lavas and metasediments with a minimum thickness of 600 m a cherty stromatolitic metasediment 300 m above the base contains several thin ash layers with a mean zircon age of 3298 3 ma the basal fig tree group has units as old as 3259 3 ma and upper units in the fig tree are as young as 3225 3 ma xenocrystic zircons in the upper onverwacht and overlying fig tree group samples suggest that successive igneous units inherited zircons from underlying units and that over several hundred million years episodes of intermediate to felsic igneous activity took place at 20 40 ma intervals structural repetition by isoclinal folding and thrust faulting are important components of late greenstone belt evolution but should not obscure the importance of the prolonged interval of magmatic evolution represented by the thick pile of volcanic rocks observed in the barberton greenstone belt

a biogeochemical influence on the secular distribution of orogenic gold orogenic gold deposits are unevenly distributed in geologic time the richest period of orogenic gold deposit formation was the phanerozoic which followed a 1 b y period of relatively meager deposit generation during the proterozoic this proterozoic low gold period developed despite continent building episodes which should have been appropriate for orogenic gold genesis here a brief review of the changes in global geochemistry brought on by evolving interactions between plate tectonics and the biosphere is conducted before the consequences for gold uptake into sedimentary pyrite are discussed it is suggested that the extensive oxygenation of the deep oceans during the second great oxidation event over the period 635 to 510 ma produced conditions where gold was soluble in the deep oceans from that point on regions where bacterial sulfate reduction drove formation of sedimentary pyrite also destabilized this soluble gold promoting its uptake into pyrite and allowing formation of metallogenically enhanced sedimentary sequences these sediments would have become ideal source rocks for gold deposits during subsequent collisional tectonics and metamorphism this biogenic influence on source region enhancement may explain the association during the phanerozoic eon between some of the world s most gold productive regions and sedimentary sequences that contain pyritic carbonaceous rocks

the record of triassic charcoal and other evidence for palaeo wildfires signal for atmospheric oxygen levels taphonomic biases or lack of fuel abstract as wildfires are today important sources of disturbance in many terrestrial ecosystems it is of great interest to understand how different environmental parameters and fire activity interacted during past periods of the earth history fossil charcoal inertinites and pyrogenic polycyclic aromatic hydrocarbons pahs represent the only direct evidence for the occurrence of such palaeo wildfires in the present study a review of published data together with new data on the occurrence of fossil charcoal for the permian and the triassic is presented for a long time it has been speculated that an assumed lack of evidence for palaeo wildfires during the triassic should be explained by a large drop in atmospheric oxygen concentration following or during the end permian mass extinction event preventing the occurrence of wildfires however evidence for palaeo wildfires is relatively common in many middle and late triassic strata whereas such evidence is almost totally lacking from early triassic sediments the interpretation of this charcoal gap or depression is difficult as many factors e g atmospheric oxygen concentration taphonomical biases lack of sediments suitable for the preservation of macroscopic charcoal lack of fuel and ignorance of scientists may have influenced not only the production but also the preservation and recovery of evidence for palaeo wildfires during this period thus it is not clear whether this early triassic charcoal gap can also be seen as evidence for an assumed wildfire gap or not without any doubt further investigations on the early triassic record of charcoal and other evidence for palaeo wildfires will be necessary before this problem can be solved in fact it can be expected that the number of published records of early triassic evidence for palaeo wildfires will increase in the future as more and more scientist working on sediments of this age may become aware of the interest in fires from this time this will certainly make it possible to give a much better picture of the temporal and regional distribution of wildfires during this period in the future

variations in atmospheric sulphur chemistry on early earth linked to volcanic activity the nature of the atmospheric sulphur cycle on the early earth has been difficult to reconstruct an analysis of sulphur isotopes from 3 2 billion year old volcanic rocks suggests that episodic volcanism released pulses of sulphur dioxide that was then broken down by ultraviolet photodissociation

an overview of the relationship between granitoid intrusions and gold mineralisation in the archaean murchison province western australia in the archaean murchison province of western australia granitoid batholiths and plutons that intruded into the ca 2 7 2 8 ga and ca 3 0 ga greenstone belts can be divided into three major suites suite i is a ca 2 69 ga monzogranite granodiorite suite which was derived from anatexis of old continental crust and occurs as syn tectonic composite batholiths over the entire province suite ii is a trondhjemite tonalite suite termed i type derived from partial melting of subducted basaltic crust which intruded as syn to late tectonic plutons into the greenstone belts in the northeastern part of the province where most of the major gold deposits are situated one of the suite ii trondhjemite plutons has a pb pb isochron age of 2641 36 ma and one of the structurally youngest tonalite plutons has a minimum pb pb isochron age of 2630 1 4 3 ma suite iii is a ca 2 65 2 62 ga a type monzogranite syenogranite suite which is most abundant in the largely unmineralised southwestern part of the province gold deposits in the province are mostly hosted in brittle ductile shear zones and were formed at a late stage in the history of metamorphism deformation and granitoid emplacement at one locality mineralisation has been dated at 2636 8 4 2 ma through a pyritetitanite pb pb isochron lead and sr isotope studies of granitoids and gold deposits indicate that although most gold deposits have initial pb isotope compositions most closely similar to those of suite ii intrusions both suite i and suite ii intrusions or their source regions could have contributed solutes to the ore fluids these preliminary data suggest that gold mineralisation in the murchison province was temporally and spatially associated with suite ii i type granitoids in the northeastern part of the province this association is consistent with the concept that archaean gold mineralisation was related to convergent style tectonic settings as generation of both suite ii i type granitoids and hydrothermal ore fluids could have been linked to the dehydration and partial fusion of subducted oceanic crust and old sialic crust or its anatectic products may also contribute solutes to the ore fluids integration of data from this study with other geological and radiogenic isotope constraints in the yilgarn block argue against direct derivation of gold ore fluids from specific i type granitoid plutons but favour a broad association with convergent tectonics and granitoid magmatism in the late archaean

paleoarchean sulfur cycle and biogeochemical surface conditions on the early earth barberton south africa abstract this study presents the first multiple sulfur isotope dataset on sulfides from the ca 3 5 3 2 ga onverwacht group in the paleoarchean barberton greenstone belt bgb of south africa in situ 34 scdt and 33 s values of pyrite n 568 are reported from a wide range of hydrothermal volcanic and sedimentary environments and are used to explore mid archean biogeochemical sulfur cycling samples are from fresh drill core collected by the barberton scientific drilling project that intercepted cherts metabasalts and sheared ultramafics of the 3 3 3 35 ga kromberg formation the sedimentary units of the 3 432 ga noisy formation and the unconformably underlying metabasaltic pillow lavas of the 3 472 ga hooggenoeg formation pyrite in quartz carbonate veins in the lower diamictite of the noisy sequence records the largest range and most negative 34 scdt values so far reported from an archean terrain 34 s cdt 55 04 to 27 46 the noisy sediments also contain detrital and diagenetic pyrites with a significant variation in 33 s of between 0 62 to 1 4 and 34 scdt of between 7 00 and 12 6 interpreted to reflect tectonic exposure of these early sediments to atmospheric shallow marine conditions early marine pyrites from the kromberg cherts also display strong positive 33 s values up to 2 50 with narrow range in 34 scdt values 6 00 to 1 50 whereas hydrothermal veins in the basal ultramafic shear zones preserve magmatic values 0 this study reveals a potential proto tectonic control on atmospheric geodynamic and hydrothermal environments available for early sulfate reducing and or methanogenic microbes in the paleoarchean no evidence for microbial sulfate reduction or disproportionation was identified in the kromberg cherts despite previous morphological claims for microbial life highly variable and negative 34 scdt values were found in the noisy turbidites and hooggenoeg pillow lava breccia supporting the presence of microbial sulfate reduction in early tectono sedimentary basins and in the paleoarchean sub seafloor respectively in light of current controversies surrounding sulfur isotope studies in similar aged rocks of the pilbara craton west australia we argue that microbial elemental sulfur disproportionation was not a preferred metabolic pathway on the paleoarchean earth

the hydrothermal fluid of archaean lode gold deposits at different metamorphic grades compositional constraints from ore and wallrock alteration assemblages a low salinity mixed aqueous carbonic fluid is common to all archaean lode gold deposits throughout the range of mineralising conditions from sub greenschist to lower granulite facies temperatures alteration assemblages and fluid inclusion data give constraints on the fluid composition fluid xco2 is 0 1 0 3 in typical greenschist facies mesothermal deposits at higher temperatures the assemblages are consistent with formation from a fluid of similar composition but slightly higher or lower xco2 cannot be ruled out and fluid inclusion data indicate that ch4 may be an important component in ore fluids at these temperatures fluid ph is neutral or weakly alkaline at all conditions a range of relative oxidation states of four orders of magnitude fo2 is indicated at any temperature with deposits more oxidising relative to qfm at lower temperature sulphur contents of the fluids vary from 10 to 10 3 5m s with a trend towards lower sulphur contents at lower temperatures the relative concentrations of major cations in solution are similar at all conditions with na k ca although ca may be less abundant at low temperatures the broad similarities in ore fluid composition at all temperatures give support to crustal continuum models in which archaean lode gold mineralisation involved either a single fluid moving through the middle and upper crust or derivation of ore fluids by similar processes at different crustal levels many of the compositional differences between high and lowtemperature ore fluids may be attributed to evolution of deep sourced hydrothermal solutions as they rise along structurally controlled conduits the constancy of major ore fluid component concentration e g co2 cl k suggests fluid buffering and high fluid rock ratios along fluid pathways fluid buffered conditions can also explain the ore fluid fo2 temperature relations with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state in contrast the concentrations of components present in lesser abundance in archaean gold ore fluids e g s ca h were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature or were rock buffered through fluid rock reactions extrapolation to high temperatures of the k na and ca contents of the gold bearing fluids indicates that their composition is consistent with derivation from or final equilibration with rocks of intermediate granitic composition thus giving support to isotopic and geological arguments for ore fluid source regions external to the greenstone belts the fluid oxidation states are characteristic of a wide range of potential source rocks including mantle derived igneous rocks calc alkaline granitoids and magmas and seaflooraltered metabasalts strongly oxidised magmatic sources or unusually oxidising source processes e g co2 streaming during granulitisation of the lower crust are therefore not required in the genesis of arch an lode gold deposits

metasomatic diamond growth a multi isotope study 13c 15n 33s 34s of sulphide inclusions and their host diamonds from jwaneng botswana abstract the formation of diamond through metasomatic events from volatiles enriched fluids melts brought into pre existing mantle rocks has been suggested from a series of independent studies however the link between these hypothetical volatile rich fluids and deep seated mineral inclusions mostly silicate and sulphides entrapped in diamonds remains unclear yet the relationship between these two species may provide the key to our understanding of diamond crystallization in order to address the relationship between the origin and formation of diamonds and their mineral inclusions we carried out the first coupled stable isotopic study 13 c 15 n and multiple s isotopes as 34 s 33 s 33 s of diamonds containing sulphide inclusions using samples from the jwaneng kimberlite as our model system sulphides extracted from the present collection of 55 diamonds belong to either eclogitic e type 52 diamonds or peridotitic p type 3 diamonds paragenetic suites as attested by their cr and ni content cr 0 18 wt and ni 14 wt for p types sulphur isotopic compositions have been measured in situ by multicollector secondary ion mass spectrometry and reveal that peridotitic sulphide inclusions in diamonds n 4 lie on the terrestrial fractionation line whereas mass independent sulphur isotope fractionations mif are preserved inside eclogitic sulphides inclusions 0 5 33 s n 33 such large mif values extended here to the negative values recorded in archean sediments are thought to be produced through photochemical reactions in the archean atmosphere which implies that sulphides inclusions contain an archean sedimentary sulphur component that was transferred to the earth s mantle in contrast to this isotopic dichotomy between e type and p type sulphides the geochemical characteristics of their host diamonds 13 c 15 n n content n speciation are largely homogeneous throughout the population and differ from silicate bearing diamonds previously studied in the same locality sulphide bearing diamonds appear to define a distinct population which might reflect a different crystallization process characterized by carbon and nitrogen isotopic compositions falling within the range of unfractionated mantle values the lack of a distinct isotopic signature of recycled sedimentary nitrogen and carbon in sulphide bearing diamonds leads us to infer that sulphide bearing jwaneng diamonds were not derived from the same chemical reservoir as their inclusions one possible formation mechanism that is compatible with our observations is diamond crystallization from a mantle derived carbon bearing fluid associated with pre existing sedimentary sulphide minerals this model has important implications for the interpretation of diamond ages obtained from sulphide inclusions using re os systematics here we suggest that the incorporation of a significant amount of mantle related sulphur during a diamond growth event into the original sulphide could lead to a re equilibration of the re os isotope composition in any inclusion and accordingly account for some scatter on re os isochrons

stress analysis post orogenic extension and 3 01 ga gold mineralisation in the barberton greenstone belt south africa gold deposits in the barberton greenstone belt bgb have formed along a common network of brittle ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at 3015 ma at a deposit scale mineralised shears occur in en echelon arrays distributed as riedel p and anti riedel shears with envelopes that connect groups of mine workings along nnw ene ne and ese trends the dip of the mineralised shear envelopes is generally moderate to steep 50 80 the existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with fry analysis stress inversion for mineralised shear zones from workings near 28 mines using a bingham axial distribution and an optimized right dihedron approach provide near identical results with 1 vertical and 3 horizontal nw the stress regime is generally radial extensional or pure extensional at the scale of the greenstone belt the mineralised shear envelopes comprise two sets of near orthogonal trends positioned symmetrically with respect the measured far field stress 1 a nw and ne trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike slip and 2 a nnw and ese trending pair accommodating dextral transtensional and sinistral transtensional movements respectively the fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage pb pb ages of 3017 1 17 8 ma and 3008 8 16 ma from a syn mineralisation porphyry dyke near golden quarry indicates gold deposition at 3015 ma i e 85 ma after the craton had stabilised older porphyry dykes intruded near sheba and consort mines have been dated at 3100 ma and 3260 ma respectively and provide a minimum and maximum age constraint on accretionary deformation in the area we propose that the main phase of gold enrichment in the bgb occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane mineralised brittle ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension mine camps display a cluster spacing of 12 km which suggests a regular distribution of controlling shears possibly as a function of the elastic rigidity of the lithosphere rather than linking mineralisation to orogenic processes during greenstone belt accretion we link gold mineralisation to the opening of fault bounded intracratonic basins including the pongola and witwatersrand basins and concomitant igneous activity

formation of gold deposits review and evaluation of the continuum model abstract the continuum model has been one of the more widely advocated genetic models for gold only deposits such as those found in archaean greenstone belts it postulates that hydrothermal gold deposits were formed throughout a 20 25 km vertical crustal profile from temperatures above 700 c to below 180 c and that the deposition occurred synchronous with the peak of metamorphism the continuum model is reviewed at this stage because we believe that it does not successfully account for many aspects of gold deposit formation the most obvious shortcoming is in considering ore deposits found in rocks of uppermost amphibolite and granulite facies domains where temperatures were appropriate for partial melting the main chemical condition that favours melting is access to h2o from either aqueous fluid or the breakdown of hydrous minerals a major gold forming hydrothermal event at the peak of high grade metamorphism as implied by the continuum model is incompatible with partial melting of wallrocks instead of forming a hydrothermal gold deposit an aqueous fluid introduced during partial melting would be consumed to produce further melt five gold deposits are documented from high metamorphic grade domains within four separate archaean cratons one is the type example used in the continuum model the other four have been significant producers partial melting has been recorded in the wallrocks adjacent to gold mineralisation at big bell hemlo challenger and renco gold deposits in the sulphide rich ore itself at challenger and hemlo deposits and as cross cutting dykes and migmatites at griffins find for these five deposits and indeed in general the continuum model does not easily account for aspects of the fluid source fluid composition gold transport and metal deposition the evidence that has been used to support deposit formation at the peak of high grade metamorphism as required by the continuum model is generally compatible with one or more alternative genetic models textural and structural siting of the gold high variance mineral assemblages proportion of various gold host rocks and relative element enrichments at high metamorphic grade can be accounted for by gold formation at greenschist facies conditions followed by progress of the deposit to high metamorphic grade metamorphic model moreover the type examples in the continuum model of deposits in sub greenschist facies domains appear to be incorrectly ascribed and are likely to be in greenschist facies domains the continuum model does not adequately explain archaean gold deposits

sulfur isotope and metal variations in sulfides in the bif hosted orogenic cuiaba gold deposit brazil implications for the hydrothermal fluid evolution abstract textural chemical and multiple sulfur isotope analyses of sulfides from the archean world class cuiaba orogenic au deposit quadrilatero ferrifero district brazil were conducted to track distinct trace element signatures from syngenetic to epigenetic hydrothermal pyrite types in different host units and to deduce the nature and source of the mineralizing fluids gold mineralization in cuiaba is hosted in greenschist facies volcanic and sedimentary host rocks belonging to the archean rio das velhas supergroup especially in association with quartz veins and sulfide minerals the bif hosted fonte grande sul orebody is controlled by quartz veins and disseminated gold bearing pyrite pyrrhotite and arsenopyrite in the wall rocks five pyrite types are classified based on textural relations as spongy syngenetic py1 porous early py2 smooth main py3 and smooth isolated and overgrown late stage py4 and py5 types variably present in metamorphosed carbonaceous pelites bif and andesite besides pyrite pyrrhotite and arsenopyrite are also defined according to textural and paragenetic differences and they display significant trace element contents trace element abundance maps and la icp ms analyses display trace element incorporations in py1 present only in carbonaceous pelite py2 and py3 in all host rocks and py4 and py5 only in bif py1 yields high as co ni pb and ag concentrations whereas py2 formed by agglomeration of py1 maintains high trace element concentrations with slightly less of those elements trace element incorporation in py3 is similar to py1 in carbonaceous pelite early stage py2 and main stage py3 in bif and andesite have increased co and ni but have less au and as late stage py4 is characterized by lower trace element concentrations whereas py5 is further enriched in as bi co ni and pb the hydrothermal alteration is divided into early main and late stage which correlate to the sulfide hydrothermal evolution the study shows that carbonaceous pelite is pre enriched in co ni and pb whereas certain elements like ag au bi and as are only hydrothermally concentrated either during the early main or late stage in bif and andesite this geochemical pattern supports a syngenetic versus hydrothermal origin of distinct elements including some trace elements now in the ore zones multiple sulfur isotope data suggest that mineralizing fluids at the cuiaba gold deposit reflect a complex mixing of sulfur evolved from three possible sources seawater mantle and reduced elemental sulfur spongy syngenetic py1 yields 33s values ranging from 2 28 to 0 25 separated into two ranges the first from 2 28 to 1 97 and the second from 0 96 to 0 25 the first range suggests py1 in the carbonaceous pelite was deposited in a seawater environment these pyrites probably mixed with later mantle sulfur carrying a near zero 33s signature which is indicated by the second 33s range or could have mixed with fluids sourced from sedimentary rocks at depth carrying a positive 33s signature the second scenario is more common in sedimentary and diagenetic pyrites an unequivocal contrast is shown by early main and late stage pyrite types py2 py3 py4 and py5 in carbonaceous pelite bif and andesite which present a continuous process of crustal assimilation values towards increasingly positive 34s and 33s values this confirms evidence for a sedimentary derived sulfur source as also indicated by fluid inclusion data of the andesite hosted orebody at the cuiaba deposit

field geochemical and u pb isotopic constraints from hypabyssal felsic intrusions within the barberton greenstone belt south africa implications for tectonics and the timing of gold mineralization abstract field mapping geochemical and u pb zircon isotopic data are reported for two undeformed feldspar quartz porphyries from within the central region of the barberton greenstone belt 3450 ma and from a porphyry at the deepest level of one of the largest gold mines in the barberton area fairview mine one porphyry the mudpools porphyry has an age of circa 3230 ma and provides a minimum age on unusual bif ferruginous shale hosted mudpool structures field mapping constraints and similar geochemical signatures between the mudpools porphyry and a second porphyry the ironstone pod porphyry suggest that 3230 ma can also be considered the minimum age of the equally unusual ironstone pods the two porphyries cross cut fig tree sediments which are regionally deformed d 2 and intrude up a thrust fault s suggesting pre 3230 ma ages for d 2 deformation and thrusting geochemical data trace elements ree indicate that these two porphyries are high level co magmatic equivalents of the nearby 3227 ma kaap valley tonalite which in part intrudes the lower sequences of the greenstone bell since the edge of the kaap valley pluton is also deformed by the d 2 deformational event either d 2 is diachronous or d 2 in the center of the belt is not equal to d 2 at the edge of the greenstone belt zircon from the fairview mine porphyry indicates a maximum age of 3126 21 ma 2 a gold mineralized shear which cross cuts the fairview porphyryl is responsible for hydrothermal alteration of the porphyry u pb analysis of hydrothermal rutile associated with gold from the porphyry gives an age of 3084 18 ma 2 and may represent the age of gold mineralization associated with this late shearing it appears that 3230 ma is a important age in early barberton greenstone belt history signifying intrusion of the kaap valley tonalite adjacent to the greenstone belt together with the emplacement of high level co magmatic equivalents porphyries of the kaap valley tonalite along regional thrust faults shear zones within the greenstone belt gold bearing hydrothermal fluids have utilized similar shear zones 150 ma after emplacement of both the porphyries in the central region of the greenstone belt and the kaap valley tonalite

atmospheric sulfur in the orogenic gold deposits of the archean yilgarn craton australia the origin of sulfur and gold in archean orogenic gold systems should provide significant insights into the dynamics of fluid movement in the crust of the early earth but is poorly constrained and highly debated our natural laboratory to address this knowledge gap is the metal endowed yilgarn craton western australia where we measured the multiple sulfur isotope signatures of representative sulfide bearing auriferous samples from 24 archean orogenic gold deposits varying in size and geological setting utilizing chemically conservative mass independent fractionated sulfur mif s isotope signatures we fingerprinted a major source of sulfur in these deposits contrary to previous studies our data show that they display mif s isotope anomalies with 33 s values ranging from 1 18 to 2 04 most of the studied deposits show a sulfur signature that is consistent with a crustally derived source unlike smaller deposits which may form with sulfur derived from a single sedimentary sulfur source therefore providing a coherent archean atmospheric signal in their 33 s 36 s slope 0 9 1 5 the formation of giant deposits may require sourcing from a wider range of sulfur reservoirs and using different processes as reflected in their apparently random 33 s 36 s slopes

sulfur isotope evidence for the origin of 3 4 to 3 1 ga pyrite at the princeton gold mine barberton greenstone belt south africa abstract the princeton mine is one of several mesothermal gold deposits found in the barberton greenstone belt the host rocks of the ore bodies are carbonaceous and pyritic shales of the fig tree group 3 4 to 3 2 ga in age the methothermal gold mineralization has occurred during the latest thrusting and deformation events at 3 1 ga micro analyses of sulfur isotopes of pyrite were performed on four representative samples collected from the princeton mine the 34 s values were determined in situ on individual pyrite crystals or aggregates of fine grained pyrite by the laser microprobe method 58 analyses and also on aggregates of separated pyrite crystals by the conventional cu 2 o combustion method 11 analyses trace element concentrations of pyrite were also determined using an electron microprobe based on their grain sizes and chemical compositions the pyrite samples examined are divided into two groups 1 fine grained pyrite and 2 coarse grained pyrite the fine grained group includes disseminated pyrite and pyrite laminae concordant to sedimentary bedding their textures suggest that the fine grained pyrite was formed during sedimentation of the carbonaceous shale at 3 4 to 3 2 ga the 34 s values of the fine grained pyrite range from 0 8 to 4 4 within a micro scale area 1 mm 2 mm 29 laser microprobe analyses suggesting that the microbial reduction of seawater sulfate was responsible for the formation of fine grained pyrite this further suggests that the 3 4 to 3 2 ga ocean already contained an appreciable amount of sulfate which is in contrast to the previously popular theory that the archean ocean was h 2 s rich and pyrite in archean sedimentary rocks was formed by an inorganic process coarse grained pyrite is associated with auriferous quartz veins and often contains gold grains trace element concentrations of coarse grained pyrite such as as concentrations are different from these of fine grained pyrite the petrographic features suggest that the most coarse grained pyrite was directly precipitated from high temperature hydrothermal fluids coarse grained pyrite has homogeneous 34 s values ranging from 1 1 to 3 6 29 laser microprobe analyses suggesting that source of sulfur for the coarse grained pyrite was reduced sulfur species in hydrothermal fluids

multiple sulfur isotope evidence for surface derived sulfur in the bushveld complex abstract the rustenberg layered suite of the bushveld complex originated in the mantle but is characterized by variably radiogenic strontium neodymium lead and osmium initial isotope ratios and more elevated 18 o values than expected from a partial melt of primitive mantle it is not clear whether bushveld magmas assimilated material from the crust or whether they were instead generated from an eclogitic mantle source that contained crustal material here we report the 33 s of fifteen whole rock and sulfide separate samples from both the main and critical zones of the bushveld including the ug2 chromitite and the merensky reef all samples have positive 33 s values 0 11 0 02 1 and 34 s values 2 2 0 6 1 non zero 33 s is fundamentally a surface process since high temperature processes are not known to produce significant change in 33 s the positive 33 s indicates that the bushveld sulfur contains a sedimentary or surface derived component the homogeneity of 33 s values for this suite of samples contrasts with the heterogeneity displayed by strontium neodymium lead and osmium initial isotope ratios of the bushveld and requires that the sulfur contamination occurred prior to the emplacement of the bushveld in the upper crust possible sources of contamination include lower crust and sub continental lithospheric mantle limited multiple sulfur isotopic measurements of these reservoirs exist more work needs to be done to constrain whether they contain surface derived sulfur with non zero 33 s and if they do whether the concentrations and isotopic compositions of these reservoirs can explain the measured 33 s of the bushveld

coupled fe and s isotope variations in pyrite nodules from archean shale ron and sulfur isotope compositions recorded in ancient rocks and minerals such as pyrite fes2 have been widely used as a proxy for early microbial metabolisms and redox evolution of the oceans however most previous studies focused on only one of these isotopic systems herein we illustrate the importance of in situ and coupled study of fe and s isotopes on two pyrite nodules in a c 2 7 ga shale from the bubi greenstone belt zimbabwe fe and s isotope compositions were measured both by bulk sample mass spectrometry techniques and by ion microprobe in situ methods secondary ion mass spectrometry sims spatially resolved analysis across the nodules shows a large range of variations at micrometer scale for both fe and s isotope compositions with 56 fe and 34 s values from 2 1 to 0 7 and from 0 5 to 8 2 respectively and 33 s values from 1 6 to 2 9 the fe and s isotope variations in these nodules cannot be explained by tandem operation of dissimilatory iron reduction dir and bacterial sulfate reduction bsr as was previously proposed but rather they reflect the contributions of different fe and s sources during a complex diagenetic history pyrite formed from two different mineral precursors 1 mackinawite precipitated in the water column and 2 greigite formed in the sediment during early diagenesis the in situ analytical approach reveals a complex history of the pyrite nodule growth and allows us to better constrain environmental conditions during the archean

origin of mineralizing fluids of the sediment hosted navachab gold mine namibia constraints from stable o h c s isotopes the navachab gold mine in the damara belt of central namibia is characterized by a polymetallic au bi as cu ag ore assemblage including pyrrhotite chalcopyrite sphalerite arsenopyrite bismuth gold bismuthinite and bismuth tellurides gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near vertical sequence of shelf type metasedimentary rocks including marble calcsilicate rock and biotite schist the sequence has been intruded by abundant syntectonic lamprophyre aplite and pegmatite dikes documenting widespread igneous activity coeval with mineralization the majority of quartz from the veins has 18o values of 14 to 15 per mil v smow the total variations in 18o values of the biotite schist and calcsilicate rock are relatively small 12 14 whereas the marble records steep gradients in 18o values 17 21 the lowest values being recorded at the vein margins despite this there is no correlation between 18o and 13c values and the carbonate content of the rocks indicating that fluid rock interaction alone cannot explain the isotopic gradients in addition the marble records increased 13c values at the contact to the veins possibly related to a change in the physicochemical conditions during fluid rock interaction gold is interpreted to have precipitated in equilibrium with metamorphic fluid 18o 12 14 d 40 to 60 at peak metamorphic conditions of ca 550 c and 2 kbars consistent with isotopic fractionations between coexisting calcite garnet and clinopyroxene in the alteration halos the most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the damaran metapelites that underwent prograde metamorphism at amphibolite to granulite facies grades although there is no isotopic evidence for the contribution of magmatic fluids they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients ca 80 c km 1 in the mine area

biotite and apatite as tools for tracking pathways of oxidized fluids in the archean east repulse gold deposit australia fluid pathways in hydrothermal ore deposits can potentially be traced through alteration zoning patterns and the chemistry of mineral phases i e major trace elements and isotopic composition which provide insight into the physico chemical gradients within and adjacent to pathways world class neoarchean gold deposits of the eastern yilgarn can be zoned with respect to alteration assemblages but in many cases fluid pathways are poorly understood which makes it difficult to identify the nature and source of fluids this study conducted detailed core logging sem mineral mapping of alteration assemblages and microprobe analysis of biotite and apatite in order to identify chemical gradients across the east repulse gold deposit of the st ives gold camp eastern yilgarn western australia results show that the east repulse deposit is vertically zoned with relatively oxidized sulfate rich alteration in the footwall of the deposit anhydrite celestine and or barite sulfide rich alteration in the core of the deposit pyrite magnetite with minor barite and celestine and relatively reduced sulfide only assemblages pyrite pyrrhotite with minor pentlandite millerite and cobaltite in the hanging wall of the deposit gold was identified as inclusions in pyrite and associated biotite implying a link between biotite pyrite alteration and gold mineralization the abundance of f in biotite and apatite progressively decreases from the footwall of the deposit into the mineralized core this trend of decreasing f in biotite and apatite with decreasing depth is consistent with an hypothesis that oxidized f and k bearing fluids were focused along subvertical pathways beneath the deposit within or subjacent to granitoid dikes the biotite apatite geothermometer shows that these fluids were relatively hot 480 60 c at the time of alteration these relatively high temperature oxidized f and k bearing fluids may have been derived from a proximal magmatic source

different carbon reservoirs of auriferous fluids in african archean and proterozoic gold deposits constraints from stable carbon isotopic compositions of quartz hosted co 2 rich fluid inclusions stable carbon and when present nitrogen isotope ratios of fluid inclusions in quartz from selected gold deposits in ghana and zimbabwe have been analyzed using a crushing device interfaced to an isotopic ratio mass spectrometer irms in order to constrain possible sources of the auriferous fluids the study revealed a striking difference in stable carbon isotopic compositions of co2 in quartz hosted fluid inclusions from archean and paleoproterozoic orogenic gold deposits and points to diverse sources of co2 in the studied deposits whether this finding can be generalized for other archean and proterozoic orogenic gold deposits worldwide remains open however a significant co2 contribution by mantle degassing can be ruled out for every deposit studied devolatilization of greenstone belt rocks is the most likely source for co2 in some archean au deposits in zimbabwe whereas co2 in proterozoic vein type au deposits in the west african craton is most likely derived from corg bearing metasedimentary rocks the 13cco2 values of high density co2 rich water poor inclusions hosted in quartz pebbles from the world class au bearing conglomerate deposits at tarkwa ghana differ considerably from the 13cco2 values of similar high density co2 rich inclusions in vein quartz from the giant ashanti deposit ghana and disprove the idea of derivation of the tarkwaian quartz and gold from an older equivalent to the ashanti vein type gold deposit

sandstone body and shale body dimensions in a braided fluvial system salt wash sandstone member morrison formation garfield county utah excellent three dimensional exposures of the upper jurassic salt wash sandstone member of the morrison formation in the henry mountains area of southern utah allow measurement of the thickness and width of fluvial sandstone and shale bodies from extensive photomosaics the salt wash sandstone member is composed of fluvial channel fill abandoned channel fill and overbank flood plain strata that were deposited on a broad alluvial plain of low sinuosity sandy braided streams flowing northeast a hierarchy of sandstone and shale bodies in the salt wash sandstone member includes in ascending order trough cross bedding fining upward units mudstone intraclast conglomerates single story sandstone bodies basal conglomerate abandoned channel fill multistory sandstone bodies and overbank flood plain heterolithic strata trough cross beds have an average width thickness ratio w t of 8 5 1 in the lower interval of the salt wash sandstone member and 10 4 1 in the upper interval fining upward units are 0 5 3 0 m thick and 3 11 m wide single story sandstone bodies in the upper interval are wider and thicker than their counterparts in the lower interval based on average w t linear regression analysis and cumulative relative frequency graphs multistory sandstone bodies are composed of two to eight stories range up to 30 m thick and over 1500 m wide w t 50 1 and are also larger in the upper interval heterolithic units between sandstone bodies include abandoned channel fill w t 33 1 and overbank flood plain deposits w t 70 1 understanding w t ratios from the component parts of an ancient sandy braided stream deposit can be applied in several ways to similar strata in other basins for example to 1 determine the width of a unit when only the thickness is known 2 create correlation guidelines and maximum correlation lengths 3 aid in interpreting the controls on fluvial architecture and 4 place additional constraints on input variables to stratigraphic and fluid flow modeling the usefulness of these types of data demonstrates the need to develop more data sets from other depositional environments

preservation of exceptional vertebrate assemblages in middle permian fluviolacustrine mudstones of kotel nich russia stratigraphy sedimentology and taphonomy the kotel nich locality in european russia has long been a rich source of high quality tetrapod fossils including pareiasaurs dicynodonts gorgonopsians and theriodonts the age of the kotel nich locality has been debated but it corresponds to early severodvinian in the russian stratigraphic scheme equivalent to the late capitanian late middle permian on the international time scale remarkably the majority of specimens are complete quite unlike those from most russian permo triassic red bed localities commonest of all are 1 2 metre long pareiasaur skeletons of the genus deltavjatia preserved in hollows on top of a consolidated palaeosol horizon previous taphonomic scenarios in the russian literature have included suggestions that the animals were overwhelmed beneath sand dunes mired in soft fluviatile sediments caught at the bottom of a deep lake trapped in burrows or dumped in fluviatile scours it is probable that the pareiasaurs were searching for water in a time of catastrophic aridification and died weakened in shallow hollows in this case we also emphasise the importance of floodplain microtopography in creating the sedimentary conditions necessary for the preservation of exceptional vertebrate assemblages in a slowly aggrading fluviolacustrine setting

numerical modeling of fault controlled fluid flow in the genesis of tin deposits of the malage ore field gejiu mining district china a distinct element finite difference model based on geologic data from the tin polymetallic hydrothermal deposits of malage is constructed to simulate the geologic processes of fault controlled fluid migration during hydrothermal ore genesis the history of hydrothermal ore genesis is strongly affected by the interaction between fault deformation and fluid flow an injected fluid induces fault slip by lowering the effective stress across a fault the subsequent movement on the fault enhances the fault aperture which provides a channelway that both facilitates and focuses fluid flow the variation of fluid injection under certain stress conditions causes cyclic movement on faults this cyclic failure gives rise to pulsating fluid flow along the faults which may be responsible in part for the precipitation zoning of mineral deposits as an important structural condition for ore formation in any fault related mineralized region like malage the linking up of fracture networks not only allows appropriate long distance channels for fluid migration but also provides the appropriate conditions for ore deposition at particular structural sites this study indicates that different fluid flow patterns i e changes in fluid flow rate or velocity with time and space can reflect the variation of structural environment for instance the flow velocity is increased gradually over time on the relatively connected faults such as the northwest southeast trending fault and the western half of the east west trending yuanlao fault in the center of the malage ore field these are the main pathways for the ore forming fluids in the smaller secondary faults the flow velocity fluctuates more rapidly with time and space in some instances causing conditions that are favorable to mineral deposition the main ore deposition sites occur on the smaller secondary faults the detailed variation of fluid and aperture conditions modeled in this complex fault array can be related to the formation and location of mineral deposits both predictively and conceptually the modeling also demonstrates that the fault deformation and fluid flow are sensitive to the principal stress orientation if the orientation of sigma 1 is changed by more than 15 degrees there are very different results for the calculated fault motion pattern hydraulic aperture and fluid flow

vertebrate preservation in fluvial channels abstract two taphonomic modes for attritional vertebrate assemblages in channels are proposed based on the sedimentary context of the vertebrate remains and taphonomic features of the bones themselves the channel lag mode includes bones that are buried with coarse lithologies near the bases of active channels the channel fill mode occurs in fine grained to mixed fills of abandoned channels the extreme for a channel lag assemblage would be a cluster of allochthonous abraded unidentifiable fragments and the extreme for a channel fill assemblage would be a cluster of autochthonous unbraded complete skeletons between these extremes there is a broad spectrum of possible taphonomic histories for bones in channels but distinct channel lag vs channel fill modes can be recognized in fluvial deposits in different tectonic and climatic settings throughout the phanerozoic physical and biological processes that affect the different modes produce different samples of vertebrate paleocommunities with bones in the channel lag mode representing transported remains from a variety of habitats whereas channel fill assemblages are more autochthonous and habitat specific channel facies channel pattern and alluvial architecture are used to develop hypotheses concerning how the taphonomic modes relate to different scales of fluvial processes fluvial systems with numerous abandoned channels provide more sites for preservation of relatively complete fossil vertebrates in channel fills while systems that continually rework sediments by lateral migration preserve more vertebrate remains as channel lags large scale physical controls on channel pattern and fluvial architecture probably have had significant effects on the quality and quantity of the vertebrate record throughout the history of land vertebrates taphonomic modes provide a basis for comparing faunas with similar preservational histories throughout the geologic record and they can help to minimize biases in important paleobiological parameters such as diversity estimates and the timing of appearance and extinction events

an atmospheric source of s in mesoarchaean structurally controlled gold mineralisation of the barberton greenstone belt abstract the barberton greenstone belt of southern africa hosts several mesoarchaean gold deposits the ores were mostly formed in greenschist facies conditions and occur as hydrothermal alteration zones around extensional faults that truncate and post date the main compressional structures of the greenstone belt ore deposition was accompanied by the intrusion of porphyries which has led to the hypothesis that gold may have been sourced from magmas because the transport of au in the hydrothermal fluids is widely believed to have involved s complexes tracing the origin of s may place strong constraints on the origin of au we measured multiple s isotopes in sulfide ore from sheba and fairview mines of the barberton greenstone belt to distinguish deep s sources e g magmas from surface s sources i e rocks of the volcano sedimentary succession that contain s processed in the atmosphere preserved as sulfide and sulfate minerals ion probe sims analyses of pyrite from ore zones indicate mass independent fractionation of s isotopes 33s 0 6 to 1 0 and the distribution of the analyses in the 33s 34s space matches the distribution peak of previously published analyses of pyrite from the entire volcano sedimentary succession notwithstanding that the h2o co2 components of the fluids may have been introduced from a deep source external to the greenstone belt rocks the fact that s bears an atmospheric signature suggests the hypothesis that the source of au should also be identified in the supracrustal succession of the greenstone belt our findings differ from conclusions of previous studies of other archaean shear hosted au deposits based on mineralogical and isotopic evidence which suggested a magmatic or mantle source for au and imply that there is no single model that can be applied to this type of mineralisation in the archaean

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

the plant debris beds of the early cretaceous barremian wessex formation of the isle of wight southern england their genesis and palaeontological significance abstract the early cretaceous barremian wessex formation of the isle of wight primarily represents high sinuosity fluvial but also lacustrine and terrestrial deposition on a low relief floodplain occupying half grabens with active normal faults immediately to the north plant debris beds form a very small proportion of the succession but are the main source of vertebrate fossils including dinosaurs these conspicuous but thin grey units are distributed randomly throughout the wessex formation and are of limited lateral extent the lower part of most plant debris beds comprises a poorly sorted matrix supported conglomerate the upper part of most consists of structureless mudstone which usually grades up into colour mottled but predominantly red and purple floodplain mudstones the plant debris beds contain an unusual mixture of freshwater molluscs diverse aquatic and terrestrial vertebrates and mainly terrestrial plants with a variable quality of preservation we conclude that each plant debris bed represents a locally generated sheetflood which was then transformed on the floodplain into a debris flow by the acquisition of surface material the debris flows surged into and were deposited in depressions on the alluvial plain including abandoned chute and cut off channels oxbows and ponds these units are therefore intrabasinal and interfluvial and the fossils that they contain are locally derived similar deposits of palaeontological significance have not been reported elsewhere in the literature in view of this their origin and sedimentology their palaeontological significance and the taphonomy of the flora and fauna they contain are discussed

synmetamorphic lode gold deposits in high grade archean settings lode gold deposits that formed under conditions of the middle to upper amphibolite and granulite facies have been identified in the yilgarn block of western australia these deposits which form a continuum with lower metamorphic grade mesothermal gold depos its show that at least some of the fluids responsible for mesothermal gold deposits originated in or below the middle crust studies of 18 o suggest that fluid compositions were controlled by the wall rocks at least for some elements these more proximal gold deposits provide more information about deep fluid sources than do investigations of higher level mineralization deposited from fluids strongly modified by wall rock interaction during the early part of their ascent

early permian depositional environments and pond bonebeds in central archer county texas abstract the lower permian redbeds of north texas contain the oldest abundant non aquatic vertebrate fauna in the past collecting of fossils and study of their paleozoology has been emphasized with the exception of the pioneering work of olson 1958 1977 little attention was paid to sedimentological paleoecological and taphonomical aspects of the fauna this study focuses on the sedimentary framework paleoenvironment fossil content and taphonomy of the most common type of fossil occurrence the pond bonebed five sedimentary facies are recognized meanderbelt sandstones large straigt channel fill sandstones small straight channel fill sandstone interbedded facies and floodplain mudstone these are the components of small meandering river systems with local tributaries and crevasse splays pond bonebeds occur as gray claystone lenses in red floodplain mudstones deposited in oxbow lakes or swamps the fauna is largely aquatic orthacanthus ectosteorhachis paleoniscoids sagenodus or semiaquatic eryops trimerorachis archeria edaphosaurus with subordinate terrestrial elements diadectes dimetrodon fossil plants found as impressions petrifactions and charcoal are indicative of swamps psaronius and well drained soils conifers all of the organic debris accumulated gradually by biological and physical processes occasional isolated but complete vertebrate skeletons are found in the floodplain mudstones

trace element zoning of sulfides and quartz at sheba and fairview gold mines clues to mesoarchean mineralisation in the barberton greenstone belt south africa abstract the fairview and sheba mines are two of the major gold mines in the paleoarchean barberton greenstone belt of southern africa at these mines gold is associated with quartz carbonate rutile veins and occurs both as invisible gold finely dispersed in sulfides primarily pyrite and arsenopyrite and as visible electrum grains hosted in pyrite up to approximately 1000 ppm au are contained in pyrite and up to approximately 1700 ppm in arsenopyrite mapping of trace element distribution in sulfide minerals using electron microprobe and proton probe techniques revealed multiple events of ore formation and au mineralisation at fairview mine three stages of pyrite formation were identified the last of which is associated with arsenopyrite electrum and other sulfide minerals sphalerite chalcopyrite galena gersdorffite and sb sulfides at sheba mine pyrite was deposited in two stages and electrum is associated with the second stage at both mines the last stage of sulfide formation is the main stage of au deposition and is associated with mobilisation of au as sb cu zn and ni the host rock composition seems to have affected the composition of pyrite since higher ni and co concentrations up to 1 4 and 1 6 wt respectively have been measured in meta ultra mafic host rocks in comparison with chert and metagreywacke arsenopyrite is chemically zoned and has sb and s rich cores and as and ni rich rims this zoning indicates variations in fluid compositions decreasing sb and increasing ni and crystallisation conditions increasing as content for increasing temperature geothermometric estimates based on the as content of arsenopyrite as 32 at indicate temperatures up to 420 c for the crystal rims petrographic and cathodoluminescence observations of quartz associated with gold mineralisation show only local brittle deformation and no plastic deformation this supports the notion that the ore transporting veins were emplaced late in the deformation history variations of cathodoluminescence of quartz are correlated with changing al contents al 0 16 wt and can be related to fluctuations in the ph of the mineralising fluids

taphonomy of the lower permian geraldine bonebed in archer county texas abstract lower permian admiral formation redbeds in north texas are famous for their vertebrate fauna this paper provides a detailed taphonomic analysis of the geraldine bonebed in central archer county texas one of the most important fossil vertebrate occurrences in the region the geraldine bonebed is situated in the floodbasin facies red and gray mudstones with abundant psaronius roots of a small meandering river system such basins were covered with a dense swamp forest with a high diversity of vertebrates the geraldine bonebed has yielded from an area of 30 m 2 44 partly articulated skeletons of four genera of tetrapods and the remains of another eight vertebrate taxa the bones lie on a layer of fern and conifer foliage and wood intermingled with fossil charcoal the sediment the occurrence of wasserleichen and the skeleton orientation indicate post mortem transportation and deposition in water a single mass death event which could have been a forest fire drove the animals into a pond where they perished carcasses floating in water together with plant debris were concentrated by wind induced drift into this exceptional fossil deposit a recent example of this process of bonebed formation after mass death was described by weigelt 1927 from the texas gulf coast

minimal insect herbivory for the lower permian coprolite bone bed site of north central texas usa and comparison to other late paleozoic floras abstract an examination of 598 leaves from the lower permian late sakmarian coprolite bone bed cbb site in north central texas usa reveals low herbivory levels based on analyses of foliar surface area removed 0 25 and frequency of herbivorized leaves 15 6 these values contrast with a similar study from a somewhat younger mid artinskian nearby site where analogous values are 2 55 and 31 8 respectively as at the younger site when compared to all other co occurring plant taxa cbb pteridosperms were overwhelmingly herbivorized particularly the peltasperm autunia cf conferta and the medullosan odontopteris cf lingulata the latter preserving an exceptionally high herbivory level of 3 31 these two host plant taxa accounted for 96 8 of all herbivorized leaves and 96 4 of all leaf surface area removed by insects for the bulk flora four subgroups of feeding were documented margin feeding 70 1 of all occurrences hole feeding 24 1 and minor skeletonization earliest occurrence and galling distinctive gall types one on the midveinal region of a cf conferta and the other on a branchlet of walchia piniformis also occurring in the european early permian add to an existing but depauperate insect gall record for the pennsylvanian and permian emerging evidence indicates that herbivory intensity was spatiotemporally heterogeneous across fluvially associated landscapes during the early permian in both western euramerica texas usa and western gondwana parana basin brazil differing by an order of magnitude within each of these areas localities with the highest percentages of bulk leaf surface area removed by herbivores in both realms ranged from 2 39 to 2 97 this range approximates about a third of modern surface area removal values by insect herbivores in both subtropical to tropical floras

does gold in orogenic deposits come from pyrite in deeply buried carbon rich sediments insight from volatiles in fluid inclusions the origin of volatiles in fluid inclusions was reviewed for testing the involvement at depth of carbonaceous pyritic sedimentary rocks as the source for orogenic gold mineralization fluid inclusions from selected deposits were analyzed by solid probe mass spectrometry fluids are mostly aqueous carbonic with variable amounts of n 2 ch 4 c 2 h 6 ar h 2 s h 2 and he for fluids with ch 4 and c 2 h 6 their ratios c 1 c 2 range from 2 6 to 25 5 indicating that c 2 h 6 is sourced from thermally degraded organic matter proportions of co 2 ch 4 c 2 h 6 and h 2 are highly variable and can be explained by hydrothermal reactions where c 2 h 6 is degraded to co 2 by water consumption such reactions may account for the problematic co 2 rich h 2 o poor fluids associated with some of the richest gold districts conditions needed for c 2 h 6 degradation are also fundamental for forming gold deposits such as hs enriched fluids for carrying gold and local weakly oxidizing conditions for promoting gold precipitation the c 2 h 6 content is recorded in fluids from mesoarchean to cretaceous gold deposits providing support for a general model where fluids and gold were sourced from deeply buried carbon rich and pyrite gold bearing sedimentary rocks

no mass independent sulfur isotope fractionation in auriferous fluids supports a magmatic origin for archean gold deposits the source of the auriferous fluids that formed archean greenstone gold deposits remains controversial metamorphic fluids fluids derived from felsic magmas mantle fluids and meteoric water have all been suggested we report the results of ion microprobe sensitive high resolution ion microprobe stable isotopes multiple sulfur isotope analyses 32 s 33 s 34 s conducted to constrain the source of sulfur in four archean gold deposits the analyses are of pyrite samples that are genetically related to the gold mineralization from the bellerophon victory and wallaby deposits in the eastern goldfield superterrane of the yilgarn craton western australia together with sulfide from black shale from the golden mile deposit and st ives gold camp ore associated pyrite from the beattie gold deposit of the abitibi greenstone belt of the superior craton in canada was measured to test whether the results obtained from the yilgarn craton apply to other archean gold deposits sulfur analyses from sulfide from shale show mass independent fractionation mif with 33 s as high as 1 8 whereas those from the 4 archean gold deposits are dominated by mass dependent fractionation with only a minor contribution of mif sulfur in this respect archean gold deposits contrast with archean nis deposits which are dominated by mif sulfur the absence of mif in the pyrite from the four archean gold deposits analyzed in this study is consistent with the auriferous fluid originating from a felsic magma or mantle source but inconsistent with the metamorphic and meteoritic fluid hypotheses

reassessment of archean crustal development in the barberton mountain land south africa based on u pb dating this paper presents u pb ages for 23 samples from the barberton mountain land an archean granite greenstone terrain in the kaapvaal craton of south africa a reexamination of the geology of this area in the light of previous u pb dating and the new ages indicates that much of its deformation can probably be accounted for by a single short lived compressional event within the time span 3230 3215 ma which was synchronous with deposition of the moodies group and the upper part of the fig tree group sediments this event may have been responsible for thrusting of much older sequences in the onverwacht group including slivers of 3 54 ga old gneiss circa 3 47 ga old komatiitic and tholeiitic sequences and circa 3 45 ga old felsic sequences key constraints on the timing of deformation are previously determined 3227 3225 ma ages for detrital zircons in thrusted units of the fig tree group a 3229 4 3 ma age for a deformed porphyry which is pre or synthrusting a previously determined 3227 3 ma age for an undeformed porphyry in an inferred thrust fault 3227 1 ma ages for both the kaap valley pluton and an ignimbrite in the stolzburg syncline both of which predate regional upright folding and a 3216 2 ma age for the dalmein pluton which appears to cut regional upright ne sw trending folds in the kromberg syncline part of the final phase of deformation associated with regional compression areally extensive granitoid sheets and syenogranitic plutons were subsequently emplaced into the barberton region at about 3105 ma the linear configuration of the syenogranitic plutons which are also aligned with the gabbroic usushwana complex as well as the geometry of the granitoid sheets suggests that the circa 3105 ma magmatism was caused by incipient rifting around a nw se trending crustal fracture this episode was probably coeval with a late regional transtensional phase of deformation recorded in the greenstone belt some of the above interpretations are in conflict with previous models for early development of the barberton greenstone belt a 3352 6 ma age from a metagabbro which is comagmatic with a proposed ultramafic sheeted dike complex is over 100 m y younger than the probable age of volcanic rocks which host the dikes this conflicts with the suggestion that much of the onverwacht group comprises an ophiolitic section obducted in an intraoceanic setting at circa 3 45 ga that partially melted to form trondhjemitic magmas which intruded along active thrust faults present data suggest that despite its antiquity and the diversity of ages and lithologies found within it the barberton greenstone belt can generally be understood in terms of actualistic tectonic processes at convergent margins

paleoarchean sulfur cycling multiple sulfur isotope constraints from the barberton greenstone belt south africa mass dependent and mass independent sulfur isotope fractionation archived in volcanic and sedimentary rocks from the barberton greenstone belt 3550 3215 ma south africa provide constraints for sulfur cycling on the early earth four different sample suites were studied komatiites and tholeiites barite massive and disseminated sulfide ores and non mineralized black shales variable but generally slightly positive 34s values between 0 7 and 5 2 negative 33s values between 0 50 and 0 09 and a negative correlation between 34s and 33s as well as between 33s and 36s for komatiites and tholeiites from the komati formation and from the weltevreden formation are outside the expected range of unfractionated juvenile sulfur instead results suggest alteration of oceanic crustal rock sulfur through interactions with fluids that most likely derived their sulfur from seawater barite from the mapepe formation displays positive 34s values between 3 1 and 8 1 and negative 33s values between 0 77 and 0 34 the mass independent sulfur isotope fractionation indicates an atmospheric sulfur source notably photolytic sulfate whereas the positive 34s values suggest bacterial sulfate reduction of the marine sulfate reservoir non mineralized black shale samples from the presumed stratigraphic equivalent of the mapepe formation show positive 34s values between 0 0 and 1 3 and positive 33s values between 0 59 and 2 45 these results are interpreted to result from the reduction of photolytic elemental sulfur carrying a positive 33s signature positive 34s values ranging from 0 7 to 3 5 and slightly negative 33s values between 0 17 and 0 12 characterize massive and disseminated sulfides from the bien venue prospect results suggest unfractionated juvenile magmatic sulfur source as the primary sulfur source but a contribution from recycled seawater sulfate which would be indicative of submarine hydrothermal activity cannot be ruled out massive and disseminated sulfides from the m hlati prospect are distinctly different from massive and disseminated sulfide from the bien venue prospect they show negative 34s values between 1 2 and 0 1 and positive 33s values between 2 66 and 3 17 thus displaying a sizeable mass independent sulfur isotopic fractionation again these samples clearly exhibit the incorporation of an atmospheric mif s signal the source of sulfur for these samples has positive 33s values suggesting a connection with photolytic elemental sulfur in conclusion the sulfur isotope signatures in paleoarchean rocks from the barberton greenstone belt are diverse and indicate the incorporation of different sources of sulfur for komatiites and tholeiites barite and massive and possibly also disseminated sulfides from bien venue multiple sulfur isotopes are related to ambient seawater sulfate and its photolytic origin while massive and disseminated sulfides from m hlati and non mineralized black shales are related to a second photolytic elemental sulfur end member

new u pb and 40ar 39ar ages from the northern margin of the barberton greenstone belt south africa implications for the formation of mesoarchaean gold deposits gold mineralization in the mesoarchaean new consort gold mine in the barberton greenstone belt south africa is situated in the immediate hanging wall of an exhumed composite granitoid gneiss dome known as the stentor pluton the granitoid greenstone contact is characterized by a contact parallel high strain zone that separates the mid crustal gneisses from the greenschist facies greenstone belt in contrast to the lower greenschist facies deposits in the central parts of the greenstone belt the mineralized shear zones at new consort are hosted by distinctly higher grade metamorphic rocks ranging from upper greenschist to upper amphibolite facies conditions a distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the stentor gneiss dome against the overlying greenstone belt examination of the structural and petrologic evolution of the mineralized high strain zone combined with new u pb and ar ar data and previously published ages indicates a multistage tectono metamorphic evolution early high t medium p metamorphism ca 600 700 c and 5 1 kbar in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen parallel ne sw extension and associated detachment shearing during the main d2 accretionary event in the centre of the greenstone belt between ca 3260 and 3225 ma recent work has shown that these early high t fabrics were reactivated at ca 3105ma during the subsequent d3 tectonism and associated emplacement of large potassic granite batholiths the gold mineralization was coeval with ne sw extensional exhumation and solid state doming of the stentor gneiss dome during the subsequent d4 deformation this late stage deformation and the gold mineralization postdate the emplacement of the d3 related potassic granites by more than 50 million years as indicated by 1 a u pb age of 3046 27ma for a retrograde titanite generation in a d2 related amphibolite sample 113 and 2 a 3027 7 5ma age for hydrothermal titanite from the so called footwall lens mineralization which temperature has been estimated at 590 40 c this high t event is also recorded by a gcorrected h 3040 4ma hornblende ar ar age from the same sample suggesting that cooling of the stentor gneiss dome shortly followed the exhumation a considerably younger ar ar age 2966 4 ma has been obtained for an amphibolite from the contact parallel high strain zone pointing to non uniform cooling following the mineralization the gold mineralization was largely contemporaneous with the emplacement of nnw trending syn kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca 3030 3040 ma the mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the barberton greenstone belt which has previously been dated at ca 3080 ma in conjunction with previously published ages the data indicate the au mineralization in the barberton greenstone belt to have occurred during at least two mineralization episodes at ca 3080 and 3040 ma the main phase of au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid greenstone terrain that may have lasted some 200 ma

raman characterization of carbonaceous material in the macraes orogenic gold deposit and metasedimentary host rocks new zealand abstract raman spectroscopic and petrographic analyses were performed on samples collected from zones distal and proximal to the macraes gold deposit in the otago schist of new zealand to characterize the features and possible origins of carbonaceous material cm and to assess the potential role of cm in the formation of gold deposits cm is a common component in meta sedimentary orogenic gold deposits and it has been proposed that cm contributes to gold mineralization processes but the details of the mechanisms responsible are not fully understood documentation of the origins of the otago schist cm will improve our understanding of the role of cm in gold deposits this work has identified four types of cm of varying thermal maturity and origins from prehnite pumpellyite grade to lower greenschist grade samples in prehnite pumpellyite and pumpellyite actinolite grade rocks low maturity cm 1 coexists with framboidal pyrite indicating an in situ sedimentary origin with a potential association with the source of gold low crystallinity cm 2 is also found in low grade samples and is likely to have been deposited from fluids unrelated to gold mobilization cm 3 is the highest maturity cm recognized cm 3 is found in samples from the highest metamorphic grades studied lower greenschist facies where bands of cm 3 cross cut the foliation cm 3 is therefore thought to have been transported by fluids though possibly only at short length scales cm 4 is less mature than cm 3 and is found in mineralized rocks in association with sulfide minerals and gold cm 4 is likely to have a depositional origin but its precise role with respect to gold mineralization has not been identified

mapping mine wastes and analyzing areas affected by selenium rich water runoff in southeast idaho using aviris imagery and digital elevation data abstract remotely sensed hyperspectral and digital elevation data from southeastern idaho are combined in a new method to assess mine waste contamination waste rock from phosphorite mining in the area contains selenium cadmium vanadium and other metals toxic concentrations of selenium have been found in plants and soils near some mine waste dumps eighteen mine waste dumps and five vegetation cover types in the southeast idaho phosphate district were mapped by using airborne visible infrared imaging spectrometer aviris imagery and field data the interaction of surface water runoff with mine waste was assessed by registering the aviris results to digital elevation data enabling determinations of 1 mine dump morphologies 2 catchment watershed areas above each mine dump 3 flow directions from the dumps 4 stream gradients and 5 the extent of downstream wetlands available for selenium absorption watersheds with the most severe selenium contamination such as the south maybe canyon watershed are associated with mine dumps that have large catchment watershed areas high stream gradients a paucity of downstream wetlands and dump forms that tend to obstruct stream flow watersheds associated with low concentrations of dissolved selenium such as angus creek have mine dumps with small catchment watershed areas low stream gradients abundant wetlands vegetation and less obstructing dump morphologies

fluid sources and the role of abiogenic ch4 in archean gold mineralization constraints from noble gases and halogens abstract the st ives goldfield of western australia hosts high grade gold mineralization in a variety of lithologies within 2 7 ga greenstone belts the low salinity co 2 bearing fluids responsible for gold mineralization could have been generated either by metamorphic processes within the greenstones or from external magmatic sources and independently sourced abiogenic ch 4 has been suggested as an important control on gold mineralization in order to better constrain possible fluid sources we applied a combination of he ne ar kr xe isotope and cl br i analyses to quartz and carbonate veins containing h 2 o co 2 fluid inclusions pyrite magnetite hematite and quartz veins containing ch 4 rich fluid inclusions pyrrhotite pyrite samples containing h 2 o co 2 fluid inclusions have higher br cl and i cl values and less radiogenic noble gas isotope signatures than suggested for ch 4 rich fluid inclusions the samples containing ch 4 fluid inclusions have maximum 40 ar 36 ar of 50 000 20 ne 22 ne of 9 8 and maximum 21 ne 22 ne of 0 56 in contrast h 2 o co 2 fluid inclusions have maximum 40 ar 36 ar of only 20 000 and maximum 21 ne 22 ne of 0 14 the use of 40 ar 39 ar methodology to measure k simultaneously with 40 ar 36 ar enables us to show the highest 40 ar 36 ar ratios are representative of the fluids trapped at 2 65 ga and have not been significantly modified by post entrapment 40 ar r production 40 ar r radiogenic 40 ar furthermore the 40 ar 36 ar ratio is correlated with the 21 ne 22 ne and 136 xe 130 xe ratios demonstrating that other heavy noble gas isotope ratios are also close to their original composition the highly radiogenic noble gas isotope signatures are unlikely to have been generated within the greenstone host rocks because seawater altered volcanic rocks have high abundances of non radiogenic atmospheric noble gas isotopes in contrast the strongly radiogenic noble gas isotope signatures are likely to have been generated in ancient basement rocks underlying the greenstones at 2 65 ga the combined noble gas and halogen data are consistent with models in which redox reactions triggered by interaction of independently and distally sourced h 2 o co 2 and ch 4 localized high grade gold mineralization we suggest that the fluids were intimately associated with crustal magmatism in the st ives goldfield and that abiogenic ch 4 could be an important moderator of redox processes in other crustal environments

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

source of pb in orogenic lode gold mineralisation pb isotope constraints from deep crustal rocks from the southwestern archaean yilgarn craton australia southern cross was one of the earliest gold mining centres in western australia over 142 tonnes of gold have been produced from the district and on a gold per hectare basis the southern cross greenstone belt in the southwestern yilgarn craton is the most productive of western australia s archaean greenstone belts the sw yilgarn craton is characterised by high grade amphibolite to granulite facies metamorphism extensive granitoid magmatism and older greenstone volcanism ages compared to the well known greenschist facies metamorphism and younger 2 7 ga eruption ages which dominate in the eastern goldfields province the pb isotope compositions of deep seated granitoids in the sw archaean yilgarn craton which were emplaced coeval with a craton wide major orogenic lode gold mineralization event at about 2 64 2 63 ga have been determined for 96 whole rock and 24 k feldspar samples the pb isotope data of the granitoids are consistent with a crustal origin for their genesis probably by reworking partial melting of older continental crust the pb isotope composition of greenstones which are the main host rocks for gold mineralisation and pyrites from the komatiite hosted syngenetic ni deposits in the amphibolite facies forrestania greenstone belt have also been determined with initial pb isotope ratios higher than that for the eastern goldfields province the pb isotopic character of the orogenic lode gold deposits in the region is intermediate between coeval granitoid and greenstone pb indicating that the ore fluids contained metals from both reservoirs the pb in the ore fluid of the most deeply formed deposit griffin s find overlaps the isotopic composition of coeval granitoids indicating the deep seated granitoid magmatism was the primary source for pb in the ore fluids

tectonic history of the barberton greenstone belt south africa 490 million years of archean crustal evolution rocks of the 3570 3080 ma barberton greenstone belt record a geological history of at least 490 million years m y and were affected by regional thermal perturbations for over 1500 m y small tectonic wedges of 3570 3511 ma schistose tonalite are the oldest rocks yet recognized but little is known about their tectonothermal history the earliest well recognized tectonothermal events 3490 3450 ma represent mid ocean ridge like processes these were followed by two periods of arc related and trench related processes separated by 160 m y the first 3445 3416 ma recording an intraoceanic suprasubduction like environment and the second 3260 3225 ma intra arc and interarc like processes culminating in arc amalgamation accretion like convergent processes dominated between 3230 ma and 3080 ma overlapping with the assembly of the kaapvaal craton at 3080 ma the barberton greenstone belt experienced a sudden shift from dominantly convergent to dominantly transtensional tectonism and was mineralized with mesothermal gold along a regional shear system soon afterward transtension became distributed across the entire kaapvaal craton heralding the onset of formation of the witwatersrand basin by 3070 ma

pyrite and pyrrhotite textures and composition in sediments laminated quartz veins and reefs at bendigo gold mine australia insights for ore genesis the various types textures and compositional zoning of pyrite in the gold bearing saddle reefs quartz veins and surrounding sedimentary rocks provide new information on the potential source and timing of gold and arsenic and related fluid processes responsible for mineralization at bendigo nodular diagenetic pyrite in the black shale tops to sandstone turbidites is enriched in invisible gold and arsenic with mean values of 0 61 ppm au 1 300 ppm as and au ag 1 based on la icpms analyses other elements enriched in the diagenetic pyrite within the organic rich shales are mn zn mo cu v ba ag cd tl co ni bi pb and te in contrast euhedral and growth zoned hydrothermal pyrite in the turbidites and bedding parallel laminated quartz veins contains lower contents of most trace elements but has higher contents of invisible au and as especially on the outermost rim of the pyrite the gold rich pyrite rims generally become thicker a few to hundreds of microns in proximity to the gold bearing saddle reefs pyrite in the reef commonly has the highest levels of invisible au and as and the lowest levels of other trace elements it is characterized by au ag 1 and au pb 0 01 in the deepest stratigraphic levels below known productive gold reefs diagenetic pyrite in the most carbonaceous shales has been replaced by pyrrhotite during metamorphism la icpms analyses reveal that the disseminated pyrrhotite contains similar levels of ni and co to the diagenetic pyrite but is strongly depleted in as and au the spatial relationships between organic rich shales folded bedding parallel laminated quartz veins and gold arsenic bearing saddle reefs combined with the consistent trends in the trace element composition of pyrite hosted by these three geological elements is interpreted to indicate that the black shales were an initial source of au and as and the laminated quartz veins acted as the initial pathways for hydrothermal fluid flow carrying au and as from the source shales to the saddle reefs maximum gold and arsenic input into the reefs principally as free gold plus arsenopyrite occurred late during deformation toward the end of the hydrothermal cycle and is expressed by the au as rich rims to hydrothermal pyrite in the sedimentary host rocks laminated quartz veins and reefs this corresponds with final fold lockup and the development of through going fault arrays linking adjacent anticlines the source of au and as for this final and most economically important fluid flow event is considered to be from carbonaceous shales deeper in the basin where original gold bearing diagenetic arsenian pyrite reacts with organic matter and is converted to pyrrhotite with release of au as and s to the metamorphic fluid

archean gold relation to granulite formation and redox zoning in the crust the archean was the principal period for gold mineralization these rocks compose 12 of the exposed crust but have produced more than half of the world9s gold most major lode deposits formed at 2 7 0 2 ga this corresponds to the principal time of crustal thickening and stabilization an essential part of which was formation of granulite the anhydrous refractory base for the crust upward streaming of mantle co 2 along shear zones in the lower crust is one means for dehydrating amphibolite to form granulite and is accompanied by removal of large ion lithophile lil elements to higher crustal levels archean lode gold deposits were formed along major shear zones at intermediate crustal depths they are enclosed by zones of co 2 metasomatism many of them kilometres in extent 13 c data indicate that the co 2 is juvenile some major archean deposits have been shown to have formed from relatively oxidized hydrothermal fluids co 2 streaming with lil element depletion could only have occurred under relatively oxidized conditions that permitted a free co 2 vapor to exist in the lower crust such conditions would also have favored dissolution of gold and associated sulfide and their transfer to the mid crust in a co 2 h 2 o fluid the h 2 o coming from dehydration of amphibolite

pyrite mine waste and water mapping using hymap and hyperion hyperspectral data hyperspectral low spatial resolution hyperion data are used to map mine waste from massive sulphide ore deposits mostly abandoned on the iberian pyrite belt hymap high spatial resolution data are used for crossed interpretation mine dams mill tailings and mine dumps in a variable state of pyrite oxidation are recognizable ponds of acid water in the mine sites are spectrally outstanding and with simple image processing procedures mappable in addition acid water with different chemical composition is mappable with hyperspectral data whether of high or low spatial resolution a sequence of hyperspectral image processing algorithms used to produce the maps is suggested therefore hyperspectral data are invaluable in giving quick hints on the quality of the rapidly changing state of the contamination generated by sulphide mine waste enabling the authorities to activate mitigation procedures

pyrite physical and chemical textures pyrite may crystallize initially in forms as diverse as framboids or cubes depending upon the temperatures and pressures fluid rich diagenesis or low grade metamorphism clearly results in thorough recrystallization and the common formation of cubes once these have formed the pyrite becomes much more refractory and retains many characteristics even in deposits which have undergone penetrative deformation this is in strong contrast to the behavior of most of the accompanying sulfides which often undergo ductile deformation solid state or chemical remobilization and annealing pyrite deforms sparingly until there is brittle failure however there may be significant pyrite corrosion and regrowth during metamorphism as the result of sulfur exchange with other minerals especially pyrrhotite pyrite fabrics may also be significantly modified by pressure solution or coble creep optical microscopic examination and electron microprobe chemical mapping of pyrites from a variety of mineral deposits including several high grade metamorphic ones reveals that the pyrites frequently contain both physical and chemical textures that may be interpreted in terms of the depositional and the post depositional history of the deposits inclusions of sulfides or other minerals reveal information on the timing of the crystallization or recrystallization of the pyrite chemical mapping of elements such as ni co and as reveals information on the relative time of transport of these elements in the ore fluids

hyperspectral remote sensing monitoring of pyrite mine wastes a record of climate variability pyrite belt spain monitoring of mine waste on sulphide deposits through hyperspectral remote sensing data contributes to predicting surface water quality quantitatively estimating acid drainage and metal contamination on a yearly basis the mineralogy of surface crusts loaded with highly soluble salts is a record of available humidity and temperature along the year a temporal monitoring of salt efflorescence on mine wastes at a mine site in the iberian pyrite belt spain has been mapped in this work using hyperspectral airborne hymap data climate change estimations are made based on oxidation stages derived from well known sequences of minerals tracing sulphides oxidation intensity using archive spectral libraries therefore mine waste weathering products of sulphide mapped from airborne hyperspectral remote sensing data can be used as a short term record of climate change providing a useful tool for assessing environmental geoindicators in semi arid areas

stratigraphic framework of upper pennsylvanian and lower permian marine to continental transition wichita falls lawton and sherman quadrangles north central texas approximately 1 900 ft 580 m of continental rocks of latest virgilian wolfcampian and early leonardian age are exposed in an area of about 3 800 mi2 9 900 km2 between the brazos and red rivers of north central texas the stratigraphic complexity of these strata has impeded internal correlation and mapping since the rocks were first described by w f cummins in the late 19th century precise correlation of this discontinuously stratified fluvial sequence with well defined limestone bounded fluvial deltaic and marine formations to the south has been hampered because of 1 the pronounced change in lithology that accompanies this marked facies transition 2 a shift in strike of approximately 65 that coincides with a change in facies t act thus amplifying the stratigraphic complexity of the region and 3 generally poor exposure of the gently inclined strata the continental sequence is composed of approximately 25 major and numerous minor upward fining principally fluviogenic cycles sandstone units 5 60 ft or 1 5 18 m thick mark the bases of these cycles and occur as regionally persistent zones with multistory and multilateral geometry these generally resistant cuesta forming units interfinger with limestone bearing strata or are separated from them along strike by intervening zones of red mudstone the fine grained upper portion of the cycles 10 to 100 ft or 3 to 30 m thick is predominantly concretionary red mudstone although gray and variegated claystone lenses thin siltstone and sandstone beds and lenticular and channel fill conglomerates are characteristic precise mapping of sandstone units and correlation with prominent limestone pinch outs have permitted a stratigraphic tie with the marine section of the colorado and brazos river valleys continental rocks are divided into the bowie and wichita groups equivalent marine strata are divided into the cisco and albany groups formations have been defined in each group to allow a maximum degree of intergroup correlation end of article last page 145

reworking of atmospheric sulfur in a paleoarchean hydrothermal system at londozi barberton greenstone belt swaziland anomalous fractionation of the minor isotopes of sulfur 33s 36s in archean pyrite is thought to reflect photochemical reactions in an anoxic atmosphere with most samples falling along a reference array with 36s 33s 1 small deviations from this array record microbial sulfate reduction or changes in atmospheric source reactions here we argue that reworking of atmospheric sulfur with distinct minor sulfur isotope ratios 36s 33s 1 produced additional variability in sulfide 33s and 36s values in a 3 52 ga hydrothermal barite deposit at londozi barberton greenstone belt swaziland in situ measurement of the four stable sulfur isotopes in pyrite revealed 36s 33s relationships and a 36s 33s trend 3 2 0 4 which is significantly different from the co variation between 36s and 33s in the co existing barite that reflects ambient paleoarchean seawater sulfate this argues against biological or thermochemical sulfate reduction at the time of barite deposition and requires incorporation of sulfide generated in a chemically distinct atmosphere before 3 52 ga we propose a model that combines reworking of this sulfur by hydrothermal leaching deep mixing with juvenile sulfur and surface mixing with biogenic sulfide to explain the observed variation in 34s 33s and 36s these interactions between abiotic and biological processes in the londozi hydrothermal system complicate the interpretation of biosignatures based on deviations in 33s and 36s from the archean reference array

mineralogical and geochemical investigation of clay rich mine tailings from a closed phosphate mine bartow florida usa clay rich mine tailings from phosphate mine operations in florida are a major environmental and economic problem options for reclamation and restoration for these tailings are very limited and are fundamentally restricted by poor physical properties such as low mechanical strength low hydraulic conductivity and heavy metal content the major control on these bulk physical properties is the mineralogy of the materials eight continuous push borings were obtained to investigate stratigraphy mineralogy aspects of geochemistry and bulk properties of a deposit of clay rich mine tailings from a phosphate mine near bartow florida that ceased operations in the early 1970s stratigraphy is dominated by laminated clay rich sediment with minor units of silt and sand an intact kaolinite liner occurs near the impoundment walls and the impoundment floor has approximately 4 m of relief moisture content varies from 4 35 to 57 40 wt and organic content varies from 0 41 to 9 53 wt bulk xrf investigation indicates that the p2o5 concentrations vary from approximately 4 to 21 wt a very strong correlation r2 0 92 between cao and p2o5 indicates that apatite is a major control on the phosphate the strong correlation r2 0 77 of al2o3 and tio2 suggests that the source materials for this deposit are comparatively uniform a number of heavy metal elements and trace elements occur cr v ni cu are interpreted to be in phosphate minerals largely apatite sr and pb are interpreted to be in both phyllosilicates and phosphate minerals two populations of apatite were observed in the clay sized fraction one that was fe and si bearing and another that was only si bearing fe bearing apatite had fe2o3 contents that varied from 0 38 to 5 32 wt and sio2 contents that varied from 0 90 to 3 32 wt the other apatite population had a wider range of sio2 contents that varied from 0 77 to 8 80 wt tem imaging shows that apatite grains are dominantly single crystals with lesser amounts of aggregates wavellite commonly occurs as individual or clusters of lath like crystals and the chemical composition differs from the pure aluminium phosphate end member with average concentrations of components being that of cao 1 57 wt fe2o3 1 98 wt sio2 5 94 wt in the clay sized phosphate minerals investigated no fluorine was found above detection limit approximately 0 15 wt nor was any uranium radium heavy metal or ree element detected the phyllosilicate mineralogy of the deposit is dominated by smectite montmorillonite with lesser amounts of nontronite palygorskite illite and kaolinite no systematic variation in the relative proportions of phyllosilicates was observed in the clay deposit energy dispersive spectroscopy eds analysis indicates that chemical compositions of phyllosilicates are somewhat typical but overall are enriched with respect to fe compared to theoretical end members the relative enrichment of fe is interpreted to be a primary sedimentary feature ca content in smectite minerals is high and may inhibit stabilization using lime or similar methods the high percentages of montmorillonite and palygorskite explain the high bulk water contents observed this investigation provides fundamentally new details regarding clay tailing deposits from closed phosphate mines in central florida which can be used in restoration and reclamation efforts

plume related oceanic plateaus as a potential source of gold mineralization we propose that the formation of anomalously gold enriched provinces on opposite sides of the pacific basin at ca 120 ma can be traced back to the formation of a mantle plume related oceanic plateau in the permian fertile fragments of this oceanic plateau travelled across the developing pacific basin and arrived at the accretionary margin of the north china and american cratons respectively some time earlier than 120 ma formation of the oceanic plateau proximal to a midoceanic rise or triple junction implies a long cooling history and reduced buoyancy by the time it arrived at an active subduction zone consequently in this model at least a part of the plateau material could have subducted like normal oceanic crust metamorphic devolatilization caused the release of gold into the overlying crust thereby fertilizing the advective hydrothermal system which gave rise to among others gold provinces in the sierra nevada foothills california and jiaodong peninsula china this process could be a mechanism for the complete temporal and spatial decoupling of the initial mantle plume activity formation of a fertile primitive oceanic source rock and the establishment of a highly endowed gold province at an accretionary margin

factors in the formation of the giant kalgoorlie gold deposit abstract kalgoorlie golden mile plus mt charlotte accounts for more than half the gold won from the 2000 deposits that comprise the widespread gold mineralization of the archaean yilgarn block of western australia it is also one of the largest gold deposits globally 1800 t au production and reserves an analysis of the various stages of formation of the kalgoorlie deposits from source through solution of gold in hydrothermal fluids and their infiltration along channelways through to gold precipitation at depositional sites reveals several features that make kalgoorlie favourably disposed as a site for a major gold deposit however none alone account for its anomalously large size it is concluded that the giant kalgoorlie gold deposit owes its size and unique position in western australian gold production history to favourable host rock chemical composition favourable host rock thickness and mechanical properties and to the favourable geometry of its host units and hosting greenstone belt at favourable pt conditions within the greenschist facies collectively these factors together with proximity to the regional scale boulder lefroy fault allowed highly focussed fluid flow on both a deposit and district scale in this regard it is significant that the structural setting of gold mineralization and the greenstone architecture at the largest canadian archaean deposit timmins are remarkably similar to kalgoorlie orebody size distribution histograms for both the yilgarn block and the abitibi belt canada are highly skewed with extremely few giant deposits but numerous deposits of 10 tonnes gold existing this topology can be accounted for in terms of system efficiency the probability of all six factors in the ore forming system being optimized high heat flow large fluid reservoir proximity to major structure efficient fluid focussing and large and compositionally favourable host lithology is extremely small in contrast the probability of two processes coupling to give low efficiency is very large resulting in numerous small deposits kalgoorlie appears to have formed by the same basic processes that formed the plethora of smaller yilgarn gold deposits however it represents a mineralized domain in which there was a conjunction of a number of favourable parameters both on a regional and deposit scale that produced high system efficiency and allowed the effective fluid focussing and gold deposition that formed this giant deposit

chronology of early archaean granite greenstone evolution in the barberton mountain land south africa based on precise dating by single zircon evaporation abstract we report precise 207 pb 206 pb single zircon evaporation ages for low grade felsic metavolcanic rocks within the onverwacht and fig tree groups of the barberton greenstone belt bgb south africa and from granitoid plutons bordering the belt dacitic tuffs of the hooggenoeg formation in the upper part of the onverwacht group yield ages between 3445 3 and 3416 5 ma and contain older crustal components represented by a 3504 4 ma old zircon xenocryst fig tree dacitic tuffs and agglomerates have euhedral zircons between 3259 5 and 3225 3 ma in age which we interpret to reflect the time of crystallization a surprisingly complex xenocryst population in one sample documents ages from 3323 4 to 3522 4 ma we suspect that these xenocrysts were inherited during the passage of the felsic melts to the surface from various sources such as greenstones and granitoid rocks now exposed in the form of tonalite trondhjemite plutons along the southern and western margins of the bgb and units predating any of the exposed greenstone or intrusive rocks several of the granitoids along the southern margin of the belt have zircon populations with ages between 3490 and 3440 ma coeval with or slightly older than onverwacht felsic volcanism while the kaap valley pluton along the northwestern margin of the belt is coeval with fig tree dacitic volcanism these results emphasize the comagmatic relationships between greenstone felsic volcanic units and the surrounding plutonic suites some of the volcanic and plutonic units contain zircon xenocrysts older than any exposed rocks these indicate the existence of still older units possibly stratigraphically lower and older portions of the greenstone sequence itself older granitoid intrusive rocks or bodies of older unrelated crustal material our data show that the onverwacht and fig tree felsic units have distinctly different ages and therefore do not represent a single tectonically repeated unit as proposed by others unlike the late archaean abitibi greenstone belt in canada which formed over about 30 ma exposed rocks in the bgb formed over a period of at least 220 ma the complex zircon populations encountered in this study imply that conventional multigrain zircon dating may not accurately identify the time of felsic volcanic activity in ancient greenstones a surprising similarity in rock types tectonic evolution and ages of the bgb in the kaapvaal craton of southern africa and greenstones in the pilbara block of western australia suggests that these two terrains may have been part of a larger crustal unit in early archaean times

archean lode gold deposits fluid flow and chemical evolution in vertically extensive hydrothermal systems abstract the range of conditions of formation of lode gold deposits from the sub greenschist to the lower granulite facies in archean greenstone belts and the generally steeply plunging vertically continuous pipe like or tabular geometries of individual deposits indicate long distance hydrothermal fluid advection along well defined channelways in the upper and middle crust from presently available gold solubility data destabilisation of gold bisulphide complexes through h 2 s loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole this inference is supported by the s au ratios of ores sulphur and au precipitation in the hydrothermal system is estimated to be relatively inefficient with only 10 50 of s or au contained in the fluid precipitated over any kilometre length of fluid channelway the relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile

archean gold mineralization and oxidized hydrothermal fluids sulfate minerals hematite and isotopically fractionated sulfides are relatively rare in rocks of archean age several archean au deposits however contain one or more of these features indicative of oxidized hydrothermal fluids they include hemlo mcintyre hollinger macassa lake shore and ross which are all in ontario canada and kalgoorlie in australia examples of lesser deposits are in hislop township and in the matachewan area ontario canada with the exception of hemlo these are quartz carbonate vein deposits spatially and temporally associated with felsic plutonism because metamorphic fluids and archean surface waters were mostly reducing the most feasible general source for the fluids was felsic magmas parent magmas were relatively oxidized hydrous and co 2 rich the oxidized character may have favored the incorporation of au into the partial melt and its partitioning into magmatic aqueous fluids high contents of co 2 are conducive to the separation of an aqueous fluid from magma at greater depth than otherwise fluid inclusion data support the formation of major archean au vein deposits at deeper crustal levels than more recent magmatic hydrothermal mineralization such as porphyry cu deposits co 2 rich fluids can unmix into two phases at relatively high temperatures and pressures a saline aqueous liquid and a low salinity h 2 o co 2 liquid this can cause partitioning of cl complexed base metals from other metals including au sb and as the extensive carbonatization around most deposits has consistent delta 13 c values in the range of juvenile magmatic co 2

comparing orthomagmatic and hydrothermal mineralization models for komatiite hosted nickel deposits in zimbabwe using multiple sulfur iron and nickel isotope data trojan and shangani mines are low grade 0 8 ni komatiite hosted nickel sulfide deposits associated with ca 2 7 ga volcano sedimentary sequences of the zimbabwe craton at both mines nickel sulfide mineralization is present in strongly deformed serpentinite bodies that are enveloped by a complex network of highly sheared silicified and sulfide bearing metasedimentary rocks strong polyphase structural metamorphic metasomatic overprints in both the trojan and shangani deposits make it difficult to ascertain if sulfide mineralization was derived from orthomagmatic or hydrothermal processes or by a combination of both multiple s fe and ni isotope analyses were applied to test these competing models massive ores at shangani mine show mass dependent fractionation of sulfur isotopes consistent with a mantle sulfur source whereas s isotope systematics of net textured ore and disseminated ore in talcose serpentinite indicates mixing of magmatic and sedimentary sulfur sources potentially via post magmatic hydrothermal processes a restricted range of strongly mass independent 33s values in ore samples from trojan mine likely reflects high temperature assimilation of sulfur from supracrustal rocks and later superimposed low temperature hydrothermal remobilization iron isotope values for most ni bearing sulfides show a narrow range suggesting that in contrast to sulfur nearly all of iron was derived from an igneous source negative ni isotope values also agree with derivation of ni from ultramafic melt and a significant high temperature fractionation of ni isotopes fe isotope values of some samples from shangani mine are more fractionated than expected to occur in high temperature magmatic systems further suggesting that hydrothermal processes were involved in either low grade ore formation liberation of ni from olivine by sulfur bearing hydrothermal fluids or remobilization of existing sulfides potentially inducing secondary ni sulfide mineralization

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

ore genesis of the bake gold deposit southeastern guizhou province china constraints from mineralogy in situ trace element and sulfur isotope analysis of pyrite abstract the bake gold deposit is exceptional for its high native gold abundance relative to the other orogenic gold deposits in the southeastern guizhou district the grades of gold can be up to 2 to 3 wt in bonanza ore and native gold preferentially occurs within voids and fractures in pyrite arsenopyrite and quartz gold content can be up to 6 9 ppm in hydrothermal pyrite however the majority of the pyrite analyses have au contents less than 1 ppm mean 0 32 ppm based on la icpms analyses meanwhile empa reveals that arsenopyrite has a maximal au content of up to 0 14 wt but most analyses are below detection limit 0 01 wt moreover native gold grains in cataclastic pyrite and arsenopyrite are commonly high purity with fineness ranging from 949 to 977 given that paragenetically early hydrothermal pyrite and arsenopyrite contain variable but relatively high gold concentrations than later generations of sulfide it is suggested that early invisible gold may have been remobilized to form at least part of the microscopic visible gold post mineralization shear zones which cut the vein bearing anticline may have led to the deformation of the lode quartz vein system on the macro level and microscopically sulfide fragmentation and elemental remobilization although diagenetic pyrite from unaltered host rocks is enriched in invisible gold with mean values at 0 83 ppm evidence supporting the release of au from them is weak corresponding evidence comes from the distinct sulfur isotope signatures in different pyrite generations diagenetic pyrite has 34s values in the narrow range of 9 27 to 10 75 whereas hydrothermal pyrite shows a broad range 6 81 to 17 42 based on la mc icpms analyses the mesoproterozoic lengjiaxi group underlying the xiajiang group is most likely the source region on the basis of geological background and assuming the metamorphic devolatilization model

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

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

oxygen and strontium isotopes from fossil shark teeth environmental and ecological implications for late palaeozoic european basins article i nfo fossil shark remains occur in both marine and nonmarine late palaeozoic deposits therefore their palaeo ecology is controversial the oxygen and strontium isotopic composition of biogenic fluorapatite in 179 teeth scales and spines predominantly of hybodontid lissodus and xenacanthiform orthacanthus xenacanthus bohemiacanthus triodus sharks from various late carboniferous moscovian to early permian artinskian basins of europe are used as ecological tracers to decipher diadromous or obligate freshwater lifestyle of the investigated taxa the 18 op values of the different shark teeth range from 11 7 to 20 2 within the different basins with mean values of 16 9 0 5 for the bohemian massif 16 2 0 8 for eastern germany 18 2 1 0 for southwestern germany 18 5 0 7 for southern central spain 17 6 0 4 for