multiple sulfur isotopes monitor fluid evolution of an archean orogenic gold deposit abstract the evolution of a gold bearing hydrothermal fluid from its source to the locus of gold deposition is complex as it experiences rapid changes in thermochemical conditions during ascent through the crust although it is well established that orogenic gold deposits are generated during time periods of abundant crustal growth and or reworking the source of fluid and the thermochemical processes that control gold precipitation remain poorly understood in situ analyses of multiple sulfur isotopes offer a new window into the relationship between source reservoirs of au bearing fluids and the thermochemical processes that occur along their pathway to the final site of mineralisation whereas 34s is able to track changes in the evolution of the thermodynamic conditions of ore forming fluids 33s is virtually indelible and can uniquely fingerprint an archean sedimentary reservoir that has undergone mass independent fractionation of sulfur mif s we combine these two tracers 34s and 33s to characterise a gold bearing laminated quartz breccia ore zone and its sulfide bearing alteration halo in the 6 moz au structurally controlled archean waroonga deposit located in the eastern goldfields superterrane of the yilgarn craton western australia over 250 analyses of gold associated sulfides yield a 34s shift from what is interpreted as an early pre mineralisation phase with chalcopyrite pyrrhotite 34s 0 7 to 2 9 and arsenopyrite cores 34s 0 5 to a syn mineralisation stage reflected in au bearing arsenopyrite rims 34s 7 6 to 1 5 this shift coincides with an unchanging 33s value 33s 0 3 both temporally throughout the au hosting hydrothermal sulfide paragenesis and spatially across the au ore zone these results indicate that sulfur is at least partially recycled from mif s bearing archean sediments further the invariant nature of the observed mif s signature demonstrates that sulfur is derived from a homogeneous mif s bearing fluid reservoir at depth rather than being locally sourced at the site of au precipitation finally by constraining the mif s bearing sulfur source to a fixed reservoir we are able to display the thermochemical evolution of the ore fluid in 34s space and capture the abrupt change in oxidation state that causes au precipitation our results highlight the importance of constraining multiple sulfur isotopes in space and time in order to elucidate the source and evolution of any given au bearing fluid

the changing geomorphology of the atchafalaya river louisiana a historical perspective abstract the atchafalaya river in south central louisiana is of geomorphic interest because of the large volumes of flow and sediment transported and its role as a major floodway for the lower mississippi river notwithstanding its potential to become an avulsion pathway at old river since its origin in the sixteenth century it has undergone many transformations and currently receives flow from the red river and partial flow of the mississippi river through multiple pathways flow increased progressively beginning in the mid 1800s and continued until concerns of diversion spurred placement of engineering structures to regulate flows at the juncture between the mississippi and atchafalaya rivers in 1963 prior studies that looked at reaches or six to seven rangelines or cross sections on hydrographic surveys document areal width and depth increases in the uppermost 80 km 50 mi of the atchafalaya river from 1880 to 1974 this study assesses how channel geometry of the atchafalaya river has changed with flow variations since the 1880s by evaluating instantaneous discharge measurements that document change at superior temporal resolution and differences in thalweg values from coupled modern hydrographic surveys which document change at better spatial resolution than prior works while structures have regulated and limited flow into the atchafalaya since 1963 the highest values of discharge cross sectional area and width at simmesport upstream follow river regulation associated geometry data at simmesport and morgan city downstream document that mean bed and thalweg elevations have risen comparison of thalweg elevations on hydrographic surveys from 1963 and 2006 show pronounced local variability in bed elevation change but also reaches dominated by degradation including the red river and uppermost section of the atchafalaya near the old river diversion aggradation occurs in the lower atchafalaya a zone where along channel lakes are filling flow and sediment quantities are changing in part caused by the constructed wax lake outlet and deltas have been growing in atchafalaya bay findings are relevant to river engineering and flood management especially navigation and help in better understanding the evolution of large distributaries and the role of human impacts

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

oxidized and reduced mineral assemblages in greenstone belt rocks of the st ives gold camp western australia vectors to high grade ore bodies in archaean gold deposits hydrothermal sulfide oxide gold mineral assemblages in gold deposits in the archaean st ives gold camp in western australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time reduced alteration assemblages pyrrhotite pyrite occur in deposits in the southwest of the camp e g argo junction deposits and moderately to strongly oxidized assemblages magnetite pyrite hematite pyrite occur in deposits in the central corridor in the northeast e g north orchin revenge deposits reduced mineral assemblages flank the central corridor of oxidized deposits and locally cut across it along e w trending faults oxidized mineral assemblages in the central corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1 000 m below present surface hydrothermal magnetite predates and is synchronous with early phases of gold associated albite carbonate pyrite biotite chlorite hydrothermal alteration later stage gold associated pyrite is in equilibrium with hematite the spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions the 34s values for pyrite for the st ives gold camp range between 8 4 and 5 1 with the negative values occurring in oxidized magnetite rich domains and slightly negative or positive values occurring in reduced pyrrhotitic domains preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the st ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite pyrite to relatively oxidized magnetite pyrite and hematite pyrite both in space and time gold deposition is inferred to have occurred where fluids of contrasting redox state mixed

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

formation and deformation of pyrite and implications for gold mineralization in the el callao district venezuela the el callao mining district is the most important gold producing region in venezuela it is hosted in the paleoproterozoic guasipati el callao greenstone belt which forms part of the guayana craton the venezuelan extension of the guiana shield of south america it consists of volcanic and volcanosedimentary sequences that were affected by several deformation events particularly localized shear zones the colombia mine the largest active mine in the district produces 4 tonnes t 128 600 oz of gold annually with reserves estimated at 740 t 24 moz and grades of up to 60 g t gold mineralization is concentrated within a vein network in the colombia corridor a shear fracture hosted mesh of interconnected quartz ankerite albite veins enclosing fragments of altered metabasaltic host rocks gold occurs mostly in the metabasaltic fragments and is spatially associated with pyrite in which it occurs as invisible gold micron sized native gold inclusions and filling fractures based on optical and scanning electron microscopy backscattered electron observations two types of pyrite are recognized a simple zoned pyrite and a less common oscillatory zoned pyrite both types consist of a mineral inclusion rich core and a clearer rim however in oscillatory zoned pyrite the latter is composed of complex rhythmic overgrowths of alternating as rich and as poor bands laser ablation inductively coupled plasma mass spectrometry la icp ms analysis and elemental mapping reveal the presence of invisible gold in all generations of pyrite the highest concentrations 5 23 ppm au are found in oscillatory zoned pyrite rims which correlate with the highest as concentrations 16 000 23 000 ppm in as poor bands au up to 1 5 ppm and as 300 6 000 ppm concentrations decrease by about an order of magnitude copper bi te sb pb and ag always occur with invisible gold particularly in pyrite cores suggesting that at least part of the gold occurs in sulfosalt nanoparticles of these metals and metalloids visible native gold grains occur as small inclusions throughout core and rim of both pyrite types as well as in fractures within it in both occurrences chalcopyrite sphalerite tellurobismuthite ankerite albite and chlorite accompany native gold and gold fineness ranges between 900 and 930 at an early stage of vein mesh formation pyrite formed in the metabasaltic fragments at the expense of ankerite which in turn resulted from alteration of fe ti oxides gold together with other chalcophile elements was incorporated within the structure of pyrite most likely by destabilization of metal sulfide complexes during ankerite replacement subsequent cyclic reactivations of the shear zone caused development of pressure shadows around pyrite generating local and repeated decreases in pressure which triggered local boiling of the hydrothermal fluid as evidenced by the presence of primary fluid inclusions containing immiscible liquid rich and vapor rich aqueous carbonic fluids this process was responsible for a number of physical chemical changes in the liquid all of which contributed to the formation of the as and au rich overgrowths in pyrite 1 removal of h 2 o into the vapor phase inducing saturation of dissolved metals in the remaining liquid 2 an increase in ph due to partition of h 2 s and co 2 into the vapor thus decreasing the solubility of sulfide minerals and 3 an adiabatic decrease in temperature lowering the solubility of as and au in the liquid waning of this process restored precipitation of as poor pyrite until the onset of a new cycle because pressure drops are more significant adjacent to open spaces oscillatory zoned pyrite probably crystallized near newly formed veins whereas simple zoned pyrite formed away from them previously formed pyrite underwent fracturing during reactivation of the deformation especially through the brittle deformation events that postdated shearing resulting in local pulverization of pyrite this newly created porosity facilitated fluid circulation and remobilization of structurally bound gold as well as of other chalcophile elements ag cu bi te pb and sb which reprecipitated together with pyrite in the form of native gold sulfides and tellurides either as small inclusions or as larger grains within fractures this remobilization process facilitates the exceptionally high gold tenor found in the deposit where the colombia corridor is intersected by the santa maria fault

gold remobilisation and formation of high grade ore shoots driven by dissolution reprecipitation replacement and ni substitution into auriferous arsenopyrite abstract both gold rich sulphides and ultra high grade native gold oreshoots are common but poorly understood phenomenon in orogenic type mineral systems partly because fluids in these systems are considered to have relatively low gold solubilities and are unlikely to generate high gold concentrations the world class obuasi gold deposit ghana has gold rich arsenopyrite spatially associated with quartz veins which have extremely high localised concentrations of native gold contained in microcrack networks within the quartz veins where they are folded here we examine selected samples from obuasi using a novel combination of quantitative electron backscatter diffraction analysis ion microprobe imaging synchrotron xfm mapping and geochemical modelling to investigate the origin of the unusually high gold concentrations the auriferous arsenopyrites are shown to have undergone partial replacement 15 by au poor nickeliferous arsenopyrite during localised crystal plastic deformation intragranular microfracture and metamorphism 340 460 c 2 kbars our results show the dominant replacement mechanism was pseudomorphic dissolution reprecipitation driven by small volumes of an infiltrating fluid that had relatively low s 2 and carried aqueous nicl 2 we find that arsenopyrite replacement produced strong chemical gradients at crystal fluid interfaces due to an increase in s 2 during reaction which enabled efficient removal of gold to the fluid phase and development of anomalously gold rich fluid potentially 10 ppm or more depending on sulphur concentration this process was facilitated by precipitation of ankerite which removed co 2 from the fluid increasing the relative proportion of sulphur for gold complexation and inhibited additional quartz precipitation gold re precipitation occurred over distances of 10 m to several tens of metres and was likely a result of sulphur activity reduction through precipitation of pyrite and other sulphides we suggest this late remobilisation process may be relatively common in orogenic belts containing abundant mafic ultramafic rocks which act as a source of ni and co scavenged by chloride bearing fluids both the preference of the arsenopyrite crystal structure for ni and co rather than gold and the release of sulphur during reaction can drive gold remobilisation in many deposits across broad regions

in situ multiple sulfur isotope analysis by sims of pyrite chalcopyrite pyrrhotite and pentlandite to refine magmatic ore genetic models with growing interest in the application of in situ multiple sulfur isotope analysis to a variety of mineral systems we report here the development of a suite of sulfur isotope standards for distribution relevant to magmatic magmatic hydrothermal and hydrothermal ore systems these materials include sierra pyrite fes2 nifty b chalcopyrite cufes2 alexo pyrrhotite fe 1 x s and vmso pentlandite fe ni 9s8 that have been chemically characterized by electron microprobe analysis isotopically characterized for 33s 34s and 36s by fluorination gas source mass spectrometry and tested for homogeneity at the micro scale by secondary ion mass spectrometry beam sample interaction as a function of crystallographic orientation is determined to have no effect on 34s and 33s isotopic measurements of pentlandite these new findings provided the basis for a case study on the genesis of the long victor nickel sulfide deposit located in the world class kambalda nickel camp in the southern kalgoorlie terrane of western australia results demonstrate that precise multiple sulfur isotope analyses from magmatic pentlandite pyrrhotite and chalcopyrite can better constrain genetic models related to ore forming processes data indicate that pentlandite pyrrhotite and chalcopyrite are in isotopic equilibrium and display similar 33s values 0 2 this isotopic equilibrium unequivocally fingerprints the isotopic signature of the magmatic assemblage the three sulfide phases show slightly variable 34s values 34schalcopyrite 2 9 0 3 34spentlandite 3 1 0 2 and 34spyrrhotite 3 9 0 5 which are indicative of natural fractionation careful in situ multiple sulfur isotope analysis of multiple sulfide phases is able to capture the subtle isotopic variability of the magmatic sulfide assemblage which may help resolve the nature of the ore forming process hence this sims based approach discriminates the magmatic sulfur isotope signature from that recorded in metamorphic and alteration related sulfides which may not be resolved during bulk rock fluorination analysis the results indicate that unlike the giant dunite hosted komatiite systems that thermo mechanically assimilated volcanogenic massive sulfides proximal to vents and display negative 33s values the kambalda ores formed in relatively distal environments assimilating abyssal sulfidic shales

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

trace element and sulfur isotopic evidence for redox changes during formation of the wallaby gold deposit western australia abstract wallaby is a major gold deposit of the st ives gold field of western australia with an estimated resource of 8 million ounces of gold it has a well established paragenesis across five vein sets that displays macroscopic evidence of changing redox through time from hematite to magnetite micro analysis of pyrite from each vein generation shows a progressive and gradual change in redox conditions the sulfur isotope composition has a 34 s range of 7 7 to 9 8 using 3 m spots on the sensitive high resolution iron micro probe stable isotope shrimp si negative values indicative of an oxidized sulfur signature are found in the earliest generation of pyrite which coexists with hematite that also contains high concentrations of as ni zn ag sb cu and pb conversely positive values representative of reduced sulfur are found in later generations of pyrite with lower concentrations of as ni sb cu zn and pb these later pyrite crystals display higher as ni as sb and as bi and lower cu te these geochemical trends are the result of redox controlled transport and partitioning into pyrite of minor and trace elements now within the pyrite structure previous studies suggested a single orogenic event formed the wallaby deposit this is not supported by the present study trace element ratios such as as ni clearly delineate the high au generations and could be used as an exploration tool it is suggested that pyrite from the wallaby gold deposit formed via desulfidation and a gradual change in redox conditions within an evolving fluid and did not result from mixing of two separate fluids as previously advocated utilizing pyrite to link the entire fluid history of wallaby demonstrates the general use of pyrite as a valuable mineral tracer in gold bearing fluid systems

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

stability and abundance of the trisulfur radical ion s 3 in hydrothermal fluids the interpretation of sulfur behavior in geological fluids and melts is based on a long standing paradigm that sulfate sulfide and sulfur dioxide are the major sulfur compounds this paradigm was recently challenged by the discovery of the trisulfur ion s 3 in aqueous s bearing fluids from laboratory experiments at elevated temperatures however the stability and abundance of this potentially important sulfur species remain insufficiently quantified at hydrothermal conditions here we used in situ raman spectroscopy on model thiosulfate sulfide and sulfate aqueous solutions across a wide range of sulfur concentration 0 5 10 0 wt acidity ph 3 8 temperature 200 500 degrees c and pressure 15 1500 bar to identify the different sulfur species and determine their concentrations results show that in the low density 0 2 g cm 3 vapor phase h2s is the only detectable sulfur form by contrast in the denser liquid and supercritical fluid phase together with sulfide and sulfate the trisulfur radical ion s 3 is a ubiquitous and thermodynamically stable species from 200 degrees c to at least 500 degrees c in addition the disulfur radical ion s is detected at 450 500 degrees c in most solutions and polymeric molecular sulfur with a maximum abundance around 300 degrees c in s rich solutions these results combined with revised literature data allow the thermodynamic properties of s 3 to be constrained enabling quantitative predictions of its abundance over a wide temperature and pressure range of crustal fluids these predictions suggest that s 3 may account for up to 10 of total dissolved sulfur s tot at 300 500 degrees c in fluids from arc related magmatic hydrothermal systems and more than 50 s tot at 600 700 degrees c in s rich fluids produced via prograde metamorphism of pyrite bearing rocks the trisulfur ion may favor the mobility of sulfur itself and associated metals au cu pt mo in geological fluids over a large range of depth and provide the source of these elements for orogenic au and porphyry epithermal cu au mo deposits furthermore the ubiquity of s 3 in aqueous sulfate sulfide systems offers new interpretations of the kinetics and mechanisms of sulfur redox reactions at elevated temperatures and associated mass dependent and mass independent fractionation of sulfur isotopes

sulphur isotope evidence for an oxic archaean atmosphere the presence of mass independently fractionated sulphur isotopes mif s in many sedimentary rocks older than approximately 2 4 billion years gyr and the absence of mif s in younger rocks has been considered the best evidence for a dramatic change from an anoxic to oxic atmosphere around 2 4 gyr ago this is because the only mechanism known to produce mif s has been ultraviolet photolysis of volcanic sulphur dioxide gas in an oxygen poor atmosphere here we report the absence of mif s throughout approximately 100 m sections of 2 76 gyr old lake sediments and 2 92 gyr old marine shales in the pilbara craton western australia we propose three possible interpretations of the mif s geologic record 1 the level of atmospheric oxygen fluctuated greatly during the archaean era 2 the atmosphere has remained oxic since approximately 3 8 gyr ago and mif s in sedimentary rocks represents times and regions of violent volcanic eruptions that ejected large volumes of sulphur dioxide into the stratosphere or 3 mif s in rocks was mostly created by non photochemical reactions during sediment diagenesis and thus is not linked to atmospheric chemistry

ruthenium chromium variation a new lithogeochemical tool in the exploration for komatiite hosted ni cu pge deposits we present a new lithogeochemical method to target prospective komatiites that may host ni cu pge deposits the new methodology is based on the geochemical properties of ruthenium ru and chromium cr elements that are immobile under most conditions it relies on a restricted number of carefully selected representative samples and is applicable in highly altered terrains ruthenium is a platinum group element pge that exhibits contrasting geochemical behavior in sulfide saturated and sulfide undersaturated komatiites similarly to other pges ru shows highly chalcophile behavior in magmas that equilibrate with an immiscible sulfide phase however ru is also compatible in chromite in sulfide undersaturated systems if we consider cr concentration as an index of chromite abundance in chromite saturated komatiites we observe that ru increases or decreases systematically with increasing cr according to the sulfide saturation state of the magmatic system in rocks that crystallized from sulfide saturated melts ru contents decrease with increasing cr conversely in rocks that crystallized from sulfide undersaturated melts ru contents increase with increasing cr as a result on the basis of the ru cr variation it is possible to discriminate whether a komatiite melt equilibrated with a sulfide liquid during crystallization the strength of this method compared to previous pge based lithogeochemical techniques derives from combining the traditional use of the geochemical properties of a highly immobile and chalcophile element that records the ore forming process ruthenium with the occurrence of a mechanically and chemically resistant mineral phase chromite which is generally preserved in highly altered komatiites

multiple sulfur isotope analyses support a magmatic model for the volcanogenic massive sulfide deposits of the teutonic bore volcanic complex yilgarn craton western australia we report sensitive high mass resolution ion microprobe stable isotopes shrimp si multiple sulfur isotope analyses 32s 33s 34s to constrain the sources of sulfur in three archean vms deposits teutonic bore bentley and jaguar from the teutonic bore volcanic complex of the yilgarn craton western australia together with sedimentary pyrites from associated black shales and interpillow pyrites the pyrites from vms mineralization are dominated by mantle sulfur but include a small amount of slightly negative mass independent fractionation mif anomalies whereas sulfur from the pyrites in the sedimentary rocks has pronounced positive mif with 33s values that lie between 0 19 and 6 20 with one outlier at 1 62 the wall rocks to the mineralization include sedimentary rocks that have contributed no detectable positive mif sulfur to the vms deposits which is difficult to reconcile with the leaching model for the formation of these deposits the sulfur isotope data are best explained by mixing between sulfur derived from a magmatic hydrothermal fluid and seawater sulfur as represented by the interpillow pyrites the massive sulfide lens pyrites have a weighted mean 33s value of 0 27 0 05 mswd 1 6 nearly identical with 0 31 0 08 mswd 2 4 for pyrites from the stringer zone which requires mixing to have occurred below the sea floor we employed a two component mixing model to estimate the contribution of seawater sulfur to the total sulfur budget of the two teutonic bore volcanic complex vms deposits the results are 15 to 18 for both teutonic bore and bentley much higher than the 3 obtained by jamieson et al 2013 for the giant kidd creek deposit similar calculations carried out for other neoarchean vms deposits give value between 2 and 30 which are similar to modern hydrothermal vms deposits we suggest that multiple sulfur isotope analyses may be used to predict the size of archean vms deposits and to provide a vector to ore deposit but further studies are needed to test these suggestions

a hydrothermal ni as pge geochemical halo around the miitel komatiite hosted nickel sulfide deposit yilgarn craton western australia the remobilization of metals during postdeposition hydrothermal alteration of magmatic sulfide ores has the potential to result in large haloes the recognition of which could potentially enlarge the detectable footprint of this ore type the miitel komatiite hosted nickel sulfide deposit in western australia was used as a case study to investigate the nature and 3 d geometry of the geochemical halo created by the remobilization of base metals gold and platinum group elements pge into the rocks surrounding the mineralization at miitel anomalous metal enrichment is found in the country rocks surrounding the massive sulfides up to 250 m away from the ore this enrichment detected using portable x ray fluorescence pxrf and backed up by laboratory analyses occurs in the mount edwards footwall basalt within decimeters of the contact with the overlying widgiemooltha komatiites it is associated with the presence of nickel arsenides gersdorffite and minor nickeline are concentrated within small quartz and carbonate veinlets and are interpreted to form during the circulation of arsenic rich hydrothermal fluids results of lead fire assay analyses and in situ laser ablation inductively coupled plasma mass spectrometer la icp ms analyses indicate high pge concentrations pd and pt and minor gold associated with the observed nickel and arsenic enrichment results from a larger regional scale study combined with this pge enrichment suggest that the massive nickel sulfides from the miitel ore are the source of the remobilized nickel in the country rocks the presence of pd and pt enriched trace arsenide phases in country rocks and shear zones may be a generally applicable proximity indicator for nickel sulfides in hydrothermally altered terranes

origin of strong winds in an explosive mediterranean extratropical cyclone abstractduring 2 3 december 2012 the black sea and east coast of romania were affected by a rapidly deepening mediterranean cyclone the cyclone developed a bent back front along which short lived

multiple sulfur isotopes and the evolution of earth s surface sulfur cycle abstract the distribution of sulfur isotopes in geological materials reveals information about earth history and biogeochemical processes research during the last several decades has used sulfur isotope geochemistry as a tool to better understand microbial processes harrison and thode 1958 kaplan 1975 monster et al 1979 peck 1959 peck 1962 rees 1973 and sediment diagenesis berner 1969 berner 1982 canfield et al 1993b earth historians also realized this potential as there exists a rich record of environmental change within the sedimentary records canfield and teske 1996 claypool et al 1980 goodwin et al 1976 habicht et al 2002 kah et al 2004 monster et al 1979 shen et al 2001 strauss 1993 thode and goodwin 1983 these applications have championed the use of the two most abundant sulfur isotopes 32s and 34s and provide a rich introduction to what the sulfur isotope record has to offer see canfield 2004 canfield and raiswell 1999 within the last decade this information has been supplemented by new data derived from the less abundant isotopes 33s and 36s the measurement of all four stable sulfur isotopes multiple sulfur isotope geochemistry has expanded our understanding of biological evolution and activity atmospheric chemistry and transport crustal recycling and many more fields related to earth surface processes see farquhar and wing 2003 here i present a review of recent works in multiple sulfur isotope geochemistry with a focus on results that inform our understanding of biogeochemical processes and earth surface evolution

a test of the viability of fluid wall rock interaction mechanisms for changes in opaque phase assemblage in metasedimentary rocks in the kambalda st ives goldfield western australia transitions from pyrrhotite magnetite to pyrite magnetite and pyrite hematite bearing assemblages in metasedimentary rocks in the kambalda st ives goldfield have been shown to be spatially associated with economic gold grades fluid mixing fluid rock interaction and phase separation have been proposed previously as causes for this association textural mineralogical and isotopic evidence is reviewed and thermodynamic calculations are used to investigate the mineralogical consequences of progressive fluid rock interaction in interflow metasediments fluid rock interactions in response to fluid infiltration and or bulk composition variation are plausible mechanisms for production of the observed features

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

bimodal distribution of gold in pyrite and arsenopyrite examples from the archean boorara and bardoc shear systems yilgarn craton western australia this study investigates the microstructures geochemistry and hydrothermal evolution of gold bearing pyrite and arsenopyrite from six orogenic gold deposits in the archean eastern goldfields province western australia scanning electron microscope sem electron microprobe emp and laser ablation inductively coupled plasma mass spectroscopy la icp ms analyses show that the gold bearing minerals possess a number of similar textural features including the occurrence of invisible gold within initial phases of growth and later stage visible gold associated with alteration rims the alteration rims are characterized by a higher than average atomic mass mainly owing to arsenic enrichment and are preferentially located along fractures and grain boundaries in the pyrite and arsenopyrite these observations suggest that visible gold formation is associated with hydrothermal alteration of preexisting pyrite and arsenopyrite textural observations and la icp ms data suggest that some invisible gold was remobilized from early formed pyrite and arsenopyrite to form visible gold during development of these alteration rims gold may also have been added by hydrothermal fluids during a later stage of mineralization in situ geochemistry and phase relationships of alteration rims are used to further constrain the hydrothermal process responsible for formation of alteration rims and visible gold in fractures based on sulfide stability relations our data indicate that development of arsenopyrite alteration rims associated with late stage visible gold formation was related to an increase in temperature maximum increase from 310 to 415 c and up to of sic orders of magnitude increase in sulfur fugacity whereas changes in oxygen fugacity were less important la icp ms analyses show that the relative and absolute variations in selected trace element au ag sb bi ba te pb co and mo concentrations can also be used to distinguish between unaltered and altered pyrite and arsenopyrite in general trace elements within pyrite and arsenopyrite have a relatively uniform distribution whereas later stage alteration rims have more variable trace element distributions although the observed textures are typical of prograde metamorphic coronae we suggest that they are the consequence of variations in fluid conditions and chemistry and that mineralization occurred in response to syn and or postpeak metamorphic fluid infiltration

linking high grade gold mineralization to earthquake induced fault valve processes in the porgera gold deposit papua new guinea processes responsible for high grade gold enrichment are of significant economic interest and have long been elusive structural and geophysical models postulate that earthquake induced fault valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems although a comprehensive geochemical record of these processes has been lacking the porgera gold deposit papua new guinea provides an opportunity to examine the complex processes surrounding high grade au mineralization in structurally controlled hydrothermal systems new high resolution trace element and sulfur isotope data obtained in situ by laser ablation inductively coupled plasma mass spectrometry and sensitive high resolution ion microprobe stable isotope analysis of pyrite across a well constrained paragenetic sequence reveal a temporal evolution in fluid chemistry the data uncover a stratigraphy of repeated high au negative 34 s and low au positive 34 s zones within individual pyrite crystals present in the highest grade gold event stage ii results provide the first clear geochemical documentation of rapid complete switching in ore forming processes preserved in individual pyrite crystals throughout an evolving hydrothermal system it is postulated that geochemical variations are attributable to rapid au deposition during pressure release due to fault failure followed by switching to more reduced rock buffered metal poor fluids from adjacent sediments as the rupture seals the cyclicity of this process is reflected in the pyrite zonation seen at the porgera deposit this study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways revealing that deposition of exceptionally high grade ore at the porgera deposit was the result of multiple events in a fluctuating ore forming environment

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

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

trace element content of pyrite from the kapai slate st ives gold district western australia the kapai slate is a continuous pyrite rich carbonaceous shale horizon within the st ives au district that is spatially related to high grade au mineralization in situ laser ablation inductively coupled mass spectrometry la icpms trace element analyses in situ sensitive high resolution ion microprobe stable isotope shrimp si s isotope analyses and optical microscopy pyrite texture analyses were used to examine the different pyrite types in the kapai slate and au deposits these data were also used to confirm that the trace element signature of sedimentary pyrite can be preserved in rocks that underwent upper to mid greenschist facies metamorphism and significant hydrothermal overprint the data were further utilized to gain a more detailed understanding of the ocean conditions during deposition of the kapai slate and determine whether some of the au and s in the st ives district could have been sourced from the kapai slate seven different types of pyrite were identified fine grained sedimentary pyrite py 1 nodular sedimentary pyrite py 2 remobilized sedimentary pyrite py 3 coarse grained inclusions poor late pyrite py 4 inclusion rich magnetite series pyrite py 5 ore stage pyrite py 6 and pyrite associated with the mafic units py 7 each type of pyrite was found to have distinctive trace element compositions and s isotope signatures the results of the la icpms analyses provide evidence for early trace element enrichment in the kapai slate sedimentary pyrite median values of 158 ppm ni 387 ppm co 82 ppm cu 727 ppm as 1 91 ppm mo 13 ppm se 0 25 ppm au 7 72 ppm te and 3 36 ppm ag for py 1 and 223 ppm ni 158 ppm co 99 ppm cu 856 ppm as 1 27 ppm mo 10 2 ppm se 0 57 ppm au 10 09 ppm te and 6 62 ppm ag for py 2 concentrations of ni and co are low relative to other late archean sedimentary pyrite median of 813 and 465 ppm respectively and mo levels are near that of the euxinic shales of the similar aged jeerinah formation in the hamersley basin western australia these data suggest that the kapai slate was deposited in an anoxic to euxinic basin with relatively low biological productivity the 33 s and 34 s signatures of the sedimentary pyrite suggest two different sources of s positive 34 s and negative 33 s signatures indicate bacterial reduction of so 4 2 from seawater whereas positive 34 s and positive 33 s signatures indicate an elemental s 8 source indicating the pyrite formed later during diagenesis this s isotope signature is consistent with a transition between a near sediment environment to a more distal environment source analyses of the ore phase pyrite yield weakly positive 33 s values this suggests there was a minor contribution of sedimentary s to the more significant oxidized ore forming fluids which is consistent with a small contribution of au from a sedimentary source approximations of the degree of sedimentary pyrite destruction in the pyrrhotite pyrite dominated zones and pyrrhotite magnetite pyrite zones of the northern part of the st ives district were used to calculate the amount of au released from the early sedimentary pyrite the calculation suggests that a minor though possibly locally significant amount of au could have been sourced from the kapai slate

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

anomalous sulfur isotopes trace volatile pathways in magmatic arcs the cycle of sulfur an important volatile in earth9s crust is the driver of many significant processes such as biological evolution climate change and the formation of ore deposits this study investigates the ancient cycle of volatiles by tracing the indelible signal of anomalous sulfur isotopes expressed as 33 s 0 to illuminate the pathway of sulfur recycling through magmatic arcs we selected the ca 2 0 ga glenburgh gold deposit in the glenburgh magmatic arc of western australia as a natural laboratory for this study high precision multiple sulfur isotope analyses of samples from the glenburgh gold deposit and surrounding granitoid rocks yield the largest known sulfur isotope anomalies 33 s up to 0 82 in rocks 2 33 ga globally these data indicate that sulfur and possibly gold originated from multiple geochemical reservoirs in sedimentary rocks subducted beneath the magmatic arc one of which is 2 33 ga multiple sulfur isotope data are able to clarify a process that is cryptic to most other currently available data sets showing that the cycling of volatiles and metals in arc settings occurs on very large scales from the atmosphere hydrosphere through to the lithosphere during crustal generation

multiple sulfur isotopes and the evolution of the atmosphere interest in multiple sulfur isotope analyses has been fueled by recent reports of mass independent sulfur isotope signatures in the geologic record a non zero multiple isotopic signature of sulfur 33s and 36s is produced primarily through photochemical reactions and it is an almost perfect tracer of the source of sulfur once the signature is passed on to a given sulfur reservoir it will be preserved unless there is addition of sulfur with a different 33s or 36s in the geological record this signature has been used to study the evolution of the earth s atmosphere and to trace movement of sulfur through geological systems early in earth s history recently small but significant mass independent signatures have been reported for some younger samples raising the possibility of additional applications of multiple isotope studies the purpose of this review is to introduce and discuss the implications of multiple isotope studies and to focus attention on these anomalous but not uncommon isotopic signatures

gilt by association origins of pyritic gold ores in the victory mesothermal gold deposit western australia sulfur isotope data and calculated fluid rock reaction paths constrain the origin of pyritic gold ores in the victory mesothermal gold deposit of western australia pyrite 34s values range from 4 4 to 5 1 and are negatively correlated with gold tenor in mafic host rocks indicating a significant increase in fluid oxidation state accompanied mineralization geologic evidence argues strongly that mineralization occurred with wall rock sulfidation but this process alone cannot produce the degree of fluid oxidation necessary to explain the sulfur isotope range in contrast either carbonation of ferric iron bearing minerals or separation of immiscible fluid phases both of which are consistent with the available geologic data can drive fluid oxidation to the point of aqueous sulfate dominance coupled decreases in gold solubility and sulfide sulfur isotope values that are predicted by such a scenario provide an explanation for the trend between 34s and gold tenor observed at victory because both wall rock carbonation and fluid phase separation involve loss of aqueous sulfide but do not require precipitation of iron sulfides these processes may explain the poor correlation between gold tenor and pyrite abundance in many mesothermal deposits and the occurrence of some gold ores in iron poor rocks

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

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

mass dependent fractionation of quadruple stable sulfur isotope system as a new tracer of sulfur biogeochemical cycles abstract sulfur isotope studies of post archean terrestrial materials have focused on the ratio 34s 32s because additional isotopes 33s and 36s were thought to carry little information beyond the well known mass dependent relationship among multiple isotope ratios we report high precision analyses of 33s and 36s values defined as deviations of 33s and 36s from ideal mass dependent relationships for international reference materials and sedimentary sulfides of phanerozoic age by using a fluorination technique with a dual inlet isotope ratio mass spectrometer measured variations in 33s and 36s are explained as resulting from processes involve branching reactions two or more reservoirs formed or mixing irreversible processes in closed systems rayleigh distillation amplify the isotope effect we outline how this new isotope proxy can be used to gain new insights into fundamental aspects of the sulfur biogeochemical cycle including additional constraints on seawater sulfate budget and processes in sedimentary sulfide formation the isotope systematics discussed here cannot explain the much larger variation of 33s and 36s observed in archean rock records furthermore phanerozoic samples we have studied show a characteristic 33s and 36s relationship that differs from those measured in archean rocks and laboratory photolysis experiments thus high precision analysis of 33s and 36s can be used to distinguish small non zero 33s and 36s produced by mass dependent processes from those produced by mass independent processes in archean rocks and extraterrestrial materials

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