source requirements for the golden mile kalgoorlie significance to the metamorphic replacement model for archean gold deposits the golden mile at kalgoorlie represents a giant archean hydrothermal gold system localized by ductile shear zones and hosted mainly by a differentiated tholeiitic sill chlorite carbonate and pyrite alteration zones cover the whole mineralized area 1 km 3 km and calculations suggest that for the golden mile production around 1200 t au the amounts of components added to these alteration zones are 340 mt co2 20 mt k and 5 mt s if one adopts a metamorphic replacement model for gold mineralization in which all ore components derive from devolatilization of greenstones at amphibolite facies or above these data suggest that a source area involving a 5 km thick greenstone slab of area 8 km 8 km could produce the necessary co2 k s h2o and au this is considered a reasonable volume of greenstone belt and under such a model the minimum spacing of large gold deposits would be approximately 20 km along strike neither special au enriched source rocks nor unreasonably large volumes of greenstone b

paleoarchean detrital zircon ages from the earliest tectonic basin in the barberton greenstone belt kaapvaal craton south africa abstract detrital zircon grains in coarse grained sedimentary rocks of the noisy formation traditionally considered as the h6 unit of the uppermost hooggenoeg formation in the paleoarchean barberton greenstone belt bgb south africa are investigated by laser ablation la icp ms to constrain their u pb and 207pb 206pb ages for depositional age and provenance determination drill core and field observations indicate that these rocks are clastic polymictitic diamictites and sandstones a wide range in 207pb 206pb ages between ca 3600 and 3430 ma is reported the youngest detrital zircon grain identified has an age of 3432 10 ma constraining the maximum depositional age of the sedimentary sequence and the formation of a major underlying erosional unconformity the youngest grains correspond in age to the ca 3460 3430 ma tonalite trondjemite granodiorite ttg gneisses of the stoltzburg terrane and felsic intrusions of the uppermost hooggenoeg formation that constitute most of the songimvelo block of the bgb on the kaapvaal craton the 207pb 206pb ages of the oldest grains identified corresponds to plutonic sources identified in older proto continental blocks of the kaapvaal craton namely the ca 3509 ma steynsdorp block and ca 3667 3223 ma ancient gneiss complex agc this new data indicates provenancial linkage between the songimvelo block and older proto continental blocks exposed on the kaapvaal craton at ca 3432 ma it is argued here that the noisy formation is a remnant of the earliest tectonic basin in the bgb that developed during major tectonic accretion and uplift at ca 3432 ma that lead to formation of the erosional unconformity in conjunction with new drill core and field observations the geochronological constraints presented here support plate tectonic processes operating in the proto kaapvaal craton that resulted in its formation as a stable continental landmass as early as ca 3432 ma

late archean convergent margin tectonics and gold mineralization a new look at the norseman wiluna belt western australia on a world scale the most important hydrothermal gold deposits occur either in late archean ca 2700 ma greenstone belts or at late paleozoic to quaternary convergent plate boundaries in the latter environments epithermal and porphyry hosted deposits form during subduction related magmatism in volcanic arcs and deep level meso thermal deposits form during deformation in continental margin orogenic belts the mesothermal deposits are similar to archean gold deposits in greenstone belts such as the norseman wiluna belt in western australia we here suggest that this similarity exists because the norseman wiluna belt is an orogenic belt with a tectonic history similar to that of younger mineralized convergent margins such as the north american cordillera it is most likely that the norseman wiluna belt resulted from the interaction of lithospheric plates during the major period of continental growth and stabilization that occurred in the late archean the distribution of gold mineralization being controlled by convergent margin tectonics

gneiss domes and extensional deformation in the highly mineralised archaean eastern goldfields province western australia abstract the recognition of large areas of gneiss within granitic complexes of the northeastern part of the eastern goldfields province of western australia based on analysis of regional aeromagnetic data has resulted in a re evaluation of the structural data for that region five magnetic domains have been established in the granitic complexes which are marginal to the main greenstone belts at least two of these domains have a distinctive domal form and also have internal domeshaped regions the ballard and laverton domains have extensive areas of layered gneissic and migmatitic rocks which flank the internal domes boundary relationships at the laverton and ballard domains show that the domes are concordant sheets and the metamorphic conditions of the boundary rocks imply that the sheets were intruded at a middle crustal level strain conditions in the boundary zones indicate that emplacement of the gneiss domains to the crustal level of the surrounding lower grade greenstones was initially along relatively gently dipping shear zones which were active during uplift structural relationships around both the domal granitic complexes and the later granite batholiths however show that regional shortening deformation events were not caused directly by the emplacement of any of the granitic rocks the tectonic hypothesis presented for the eastern goldfields is similar to that proposed for the formation of cordilleran metamorphic core complexes the uplift event responsible for the crustal extension required to allow the emplacement of the gneiss may have been driven by the same thermal input responsible for the generation of the voluminous granites intrusive into both the gneiss complexes and the surrounding greenstone belts

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

the effect of heterogeneous stress and strain on metamorphic fluid flow mary kathleen australia and a model for large scale fluid circulation in the proterozoic mary kathleen fold belt northern australia infiltration of large volumes of externally derived fluid occurred synchronously with regional amphibolite facies metamorphism and deformation this paper develops a model of structurally controlled fluid migration by comparing the distribution of fossil fluid pathways with the inferred stress and strain patterns during the deformation intense fluid flow was localized within strong relatively brittle meta intrusive bodies and in discrete veined brecciated and altered zones around their margins in metasediments folded in a ductile manner outside these areas fluid infiltration was negligible the direct correlation between structural styles and the magnitude of veining and metasomatism suggests control of permeability enhancement and hence fluid flow by deformation finite difference modelling of a strong body in a weaker matrix has been used to evaluate the variation of stresses during the deformation from which it is clear that stress and strain heterogeneities have systematically influenced the development and maintenance of metamorphic fluid pathways particular regions in which mean stress may be significantly lower than the average lithostatic pressures include the strain shadow zones adjacent to the strong bodies other dilatant zones around the bodies and the bodies themselves this geometry is favourable not only for localized brittle deformation under amphilobite facies conditions but also for focused fluid flow in the low mean stress regions as evidenced by the abundance of veins fluid access through these metamorphic aquifers occurred during tensile failure episodes with particularly large dilations and decimetre scale veining in areas of strain incompatibility it appears likely that fluid circulated many times through the fold belt with flow concentrated in the metamorphic aquifers a model is developed that explains both the structurally focused fluid flow and the postulated multi pass recirculation by dilatancy pumping the pump engines comprising the low mean stress zones

the granny smith gold deposit the role of heterogeneous stress distribution at an irregular granitoid contact in a greenschist facies terrane the granny smith gold deposits formed late in the structural history of the yilgarn block at a high crustal level in a largely brittle structural regime gold mineralisation is located along a n s striking fault which wraps around the contact of a small granitoid intrusion in different sections of the fault mineralisation may be developed in the granitoid in the adjacent sedimentary sequence and or along the contact between them in the granitoid gold mineralisation is in conjugate networks of thin carbonate quartz veins and their alteration halos small displacements along veins are common in contrast veins and faults in the sedimentary rocks are subparallel to bedding spatial variations in the conjugate vein orientations indicate that the local stress field was heterogeneous and controlled by the shape of the granitoid contact the greatest variations in vein and implied stress orientations occur in zones where the contact is most irregular these are also the areas of richest mineralisation fluid flow was thus focused in a regional scale low mean stress region created by the geometry of the granitoid intrusion its irregular contact caused deposit scale variations in fluid flow and resulted in heterogeneous gold grades along the contact zone

fluid wall rock interaction in an archean hydrothermal gold deposit a thermodynamic model for the hunt mine kambalda gold mineralization at the hunt mine kambalda is situated in a 20 m thick steeply dipping shear zone within a sequence of rather uniform metabasalts zonal alteration of the host rocks occurred around fractures which acted as conduits during gold mineralization limited major element mobility ensured assemblages of low variance over most of the alteration zone providing a suitable environment for the application of thermodynamics in conjunction with detailed petrographic geochemical electron microprobe analysis of mineral compositions and fluid inclusion studies microthermometry equilibrium equations involving fluid components and alteration minerals are used to determine conditions during the alteration event and in particular activity gradients of components profiles across the schist zone show increasing f sub h 2 s around auriferous quartz veins log f sub h 2 s 0 6 but constant f sub o 2 log f sub o 2 29 7 these results are near the magnetite pyrrhotite pyrite triple point for the inferred temperature of 350 degrees c and explain the pyrite pyrrhotite magnetite zonation sequence outward from veins the profile of x sub co 2 is very close to the value of 0 25 determined independently from fluid inclusions the k super h super profile shows increases adjacent to auriferous veins corresponding to the zone of biotite stability using a value for fluid salinity of 2 equiv wt percent nacl ph was calculated to be around 6 9 the solubility of gold in solution was estimated for the calculated conditions for the complex au hs super 2 the maximum solubility is around 0 5 mg kg super 1 i e 0 5 ppm for au cl super 2 it is close to 10 super 6 mg kg super 1 indicating the efficacy of sulfide transport of gold compared to chloride transport under these conditions the calculated decrease in f sub h 2 s away from the auriferous quartz veins results in an 80 percent or greater decrease in gold solubility suggesting hat the sulfidation of fe rich wall rocks is a geologically reasonable and very efficient mechanism for precipitating gold from solution at elevated temperatures

strontium isotope systematics of hydrothermal minerals from epigenetic archean gold deposits in the yilgarn block western australia strontium isotope compositions are presented for samples of hydrothermal scheelite and tourmaline from one granitoid hosted and 14 greenstone hosted archean gold deposits in the yilgarn block western australia the direct measurement of the 87 sr 86 sr ratios of rb poor minerals such as scheelite and tourmaline which show an intimate paragenetic relationship to gold allows an accurate estimate of the local strontium isotope composition of the archean gold bearing hydrothermal fluids the population of greenstone hosted gold deposits is sub divided according to gangue mineralogy into gold skarns with garnet pyroxene biotite and amphibole biotite calcite wall rock alteration which formed in the lower parts of the green stone belts at depths of 10 to 15 km and medium temperature gold deposits with biotite or sericite ankerite albite wall rock alteration which formed in the upper parts of the belts at depths of 3 to 7 km the 87 sr 86 sr ratios of the fluids vary from 0 7020 to 0 7035 in skarns with garnet pyroxene biotite alteration from 0 7020 to 0 7043 in skarn quartz orebodies with amphibole biotite calcite alteration and from 0 7014 to 0 7028 in the medium temperature gold deposits the variations in the strontium isotope composition of scheelite from the same mining district are interpreted to reflect mixing between radiogenic strontium introduced by the hydrothermal fluid and nonradiogenic strontium released from the wall rocks during interaction with this fluid the greenstone host rocks of all deposits mainly metamorphosed ultramafic or mafic volcanic rocks and their intrusive equivalents are characterized by the low initial 87 sr 86 sr ratio 0 7007 0 7012 and low rb sr ratio of the archean mantle on the other hand the initial 87 sr 86 sr ratios of granodiorite and granite plutons in the batholiths which underlie and border the greenstone belts vary from 0 7023 to 0 7044 the strontium isotope data indicate that the deep seated gold skarns formed predominantly from fluids which evolved from or interacted with anatectic magmas in the granitoid batholiths the data do not as clearly constrain the origin of the fluids in the higher level medium temperature hydrothermal systems they are however inconsistent with models involving predominantly metamorphic fluids generated by the devolatilization of the lower parts of the greenstone belts

a comparison of progressive hydrothermal carbonate alteration in archean metabasalts and metaperidotites because of major differences in both bulk chemical composition and silicate mineralogy between metabasalts and metaperidotites valid comparison of the degree or intensity of carbonate alteration cannot be made in terms of weight per cent co2 molar co2 cao is preferred as an index of the intensity of carbonate alteration in metabasalts molar co2 cao in carbonatized metabasalts is independent of cao mgo and only mildly sensitive to bulk composition and to the proportions of tremolite and clinozoisite molar co2 cao reflect the proportions of calcite and dolomite in metabasalts and the proportions of dolomite and magnesite in metaperidotites however neither molar co2 cao nor the proportions of dolomite and magnesite are reliable measures of carbonate alteration in metaperidotites of variable composition because both are strongly dependent on mgo cao in the whole rock the preferred alteration index in metaperidotites is m co2 m cao mgo feo which represents the proportion of total relevant cations that exist in carbonate form an empirical equation relating molar co2 cao in metabasalts x and mco2 m cao mgo feo in metaperidotites y is y 0 16 0 30 x

geobarometry differential block movements and crustal structure of the southwestern abitibi greenstone belt canada the southern volcanic zone of the archean abitibi greenstone belt comprises several fault bounded tectonic blocks with distinct volcanic sedimentary sequences pressure temperature time p t t histories granite compositions and metallogenic provinces pressure distributions have been mapped out from an extensive new database derived from al in hornblende and other geobarometers in the composite crustal structure of the southern volcanic zone determined from the pressure time data the lacorne block and pontiac subprovince represent deeper crustal levels overlain by rocks exposed at the surface in the round lake or lac abitibi type blocks which are in turn capped by those exposed at the surface in a blake river rouyn type block exposure of the deeper crustal levels and juxtaposition of these distinct tectonic blocks are interpreted to have occurred by differential block uplift caused by strike slip faulting which was related to oblique collisional tectonics during late tectonic development of the superior province in late archean time

surface water influx in shallow level archean lode gold deposits in western australia archean shallow level lode gold deposits in western australia are located in rocks of prehnite pumpellyite to lower greenschist facies and are characterized by 1 predominantly brittle shear zones that formed well after volcanism and late in the structural history 2 breccia hosted lodes 3 low temperature ore and gangue minerals such as silver sulfosalts stibnite and precursor chalcedony and 4 vertical metal zonation of the orebodies ore fluid temperatures ranged from 150 to 325 c and a depth of em placement of 5 km is inferred isotopic analyses of alteration minerals such as quartz sericite and chlorite indicate higher dfluid 6 9 relative to smow 1 n 19 and lower 18ofluid 4 4 2 3 relative to smow 1 n 167 values compared to such values for hydrothermal fluids that deposited au in other archean lode gold deposits with similar structural timing the fluid inclusion and stable isotope data and the presence of chalcedony in this deposit style are best explained by the influx of surface water at shallow levels and interaction of that water with advecting metamorphic magmatic fluids

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

crustal scale shear zones and their significance to archaean gold mineralization in western australia many large archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts although in detail they are sited in subsidiary brittle ductile fault structures fluids originating from a deep source follow a complex path and re equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels temperature and pressure conditions at or below the amphibolite greenschist metamorphic boundary where most gold deposits are located favour the establishment of brittle ductile and brittle subsidiary structures the preferred structural setting of gold deposits physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient strongly localized fluid flow into the latter where lower temperatures and suitable host rocks with high fe fe mg ratios favour gold deposition the multi source origin and continuous re equilibration of the fluid with crustal rock which includes granitoid and greenstone belt lithologies of different ages is reflected in the diverse isotopic and geochemical signature of the gold deposits

the relations between mean rock stress and fluid flow in the crust with reference to vein and lode style gold deposits abstract fluid movement in the crust at the typical depths of formation of vein and lode style gold deposits will be dominated by upward flow focussing of upward flow into discrete channelways as required to form gold deposits is due to lateral gradients in fluid pressure fluid channelways should have lower fluid pressures than surrounding rock if lateral gradients are induced by variations in permeability this is however at variance with inferences from quartz vein structures of high relative fluid pressures during the formation of gold deposits focussing into zones of low mean rock stress will in contrast be associated with high relative fluid pressures in the zone of focussed fluid flow variations in mean rock stress are a direct consequence of a regional deviatoric stress acting on an inhomogeneous rock sequence analysis of stress fields shows a wide variety of potential sites of low mean stress dependent on the geometry of rock units and on patterns of faults fractures or shear zones a model of fluid focussing in the crust due to variations in mean stress is thus consistent with the large variety of structural setting of vein and lode gold deposits observed in nature use of the model through the technology of stress mapping has the potential to generate viable exploration targets

the coolgardie goldfied western australia district scale controls on an archaean gold camp in an amphibolite facies terrane archaean lode gold deposits in the coolgardie goldfield western australia occur in mafic and ultramafic rocks which have been metamorphosed to the amphibolite facies mineralisation was broadly synchronous with peak metamorphism the main phase of granitoid emplacement and regional deformation several different structural styles are represented by the deposits of the coolgardie goldfield mineralisation occurs along sheared felsic porphyry ultramafic rock contacts in gabbro hosted quartz vein sets in fault bounded quartzvein sets and in laminated quartz reefs sited in brittleductile shear zones the structures hosting mineralisation formed in response to a progressive deformation event related to granitoid emplacement in an east west compressional far field stress regime but with local heterogeneous stress orientations this occurred after an earlier period of thrust stacking with probable north south directed tectonic transport two contrasting styles of goldrelated wallrock alteration are associated with the auriferous lodes of the coolgardie goldfield a high temperature assemblage formation temperature 500 c characterised by the proximal alteration assemblage garnet hornblende plagioclase pyrrhotite contrasts with a medium temperature assemblage formation temperature 500 c consisting of calcic amphibole biotite plagioclase calcite arsenopyrite pyrrhotite the distribution of the two styles of gold related alteration is controlled by distance from voluminous syntectonic granitoids located to the west of the coolgardie goldfield with the high temperature style of alteration more proximal to the granitoid greenstone contact than the medium temperature style the occurrence of gold deposits that formed under amphibolite facies conditions throughout the coolgardie goldfield supports a crustal continuum model for archaean lode gold deposits which proposes that gold is deposited in metamorphic environments that range from the sub greenschist to granulite facies in addition the data from coolgardie suggest that syntectonic synmineralisation granitic plutons may play a significant role in controlling the style of gold associated wallrock alteration at deep crustal levels

craton scale distribution of archean greenstone gold deposits predictive capacity of the metamorphic model genetic models should not only attempt to explain features of archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts the application of genetic models developed with such a philosophy can have predictive capacity in gold exploration the metamorphic model one of a number of current genetic models for archean gold deposits involves generation of auriferous h 2 o co 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion crustal scale fault shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites declining temperature below the amphibolite greenschist transition combined with sulfidation of fe rich host rocks is a particularly efficient au depositing mechanism the model suggests that the major controlling factors in declining order of importance are 1 high temperature low pressure generally low strain metamorphic regimes 2 crustal scale fault systems 3 unaltered fe rich rocks tholeiites banded iron formation at a suitable stratigraphic metamorphic level and 4 abundant au enriched komatiites and s rich sediments in source regions the low pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most if not all generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in archean terranes all four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2 7 or 0 1 b y old rift phase greenstone belts interpreted to have formed under conditions of high crustal extension and extensive crustal thinning i e with predicted high geothermal gradients high fault density limited synvolcanic alteration and abundant komatiites and sulfidic sediment horizons more widespread commonly older platform phase greenstone belts which formed under conditions of lesser crustal extension and thinning are less mineralized as predicted by the metamorphic model the ability of the model to explain the regional distribution of known archean gold deposits lends credence to its predictive capacity for example 1 greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential 2 the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated 3 areas with the highest potential can be defined in rift phase greenstone belts by the coincidence of critical parameters such as specific host rock sequences and metamorphic facies 4 prospective structural sites for gold mineralization can be postulated in platform phase greenstone belts affected by diapiric tectonics and 5 the possible spacing of large deposits can be estimated further the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny including alteration patterns metamorphic p t paths and regional scale structural regimes

metamorphism in archaean greenstone belts calculated fluid compositions and implications for gold mineralization an inescapable consequence of the metamorphism of greenstone belt sequences is the release of a large volume of metamorphic fluid of low salinity with chemical characteristics controlled by the mineral assemblages involved in the devolatilization reactions for mafic and ultramafic sequences the composition of fluids released at upper greenschist to lower amphibolite facies conditions for the necessary relatively hot geotherm corresponds to those inferred for greenstone gold deposits xco2 0 2 0 3 this result follows from the calculation of mineral equilibria in the model system cao mgo feo al2o3 sio2 h2o co2 using a new expanded internally consistent dataset greenstone metamorphism cannot have involved much crustal over thickening because very shallow levels of greenstone belts are preserved such orogeny can be accounted for if compressive deformation of the crust is accompanied by thinning of the mantle lithosphere in this case the observed metamorphism which was contemporaneous with deformation is of the low p high t type for this type of metamorphism the metamorphic peak should have occurred earlier at deeper levels in the crust i e the piezothermal array should be of the deeper earlier type however at shallow crustal levels the piezothermal array is likely to have been of deeper later type as a consequence of erosion thus while the lower crust reached maximum temperatures and partially melted to produce the observed granites mid crustal levels were releasing fluids prograde into shallow crustal levels that were already retrograde we propose that these fluids are responsible for the gold mineralization thus the contemporaneity of igneous activity and gold mineralization is a natural consequence of the thermal evolution and does not mean that the mineralization has to be a consequence of igneous processes upward migration of metamorphic fluid via appropriate structurally controlled pathways will bring the fluid into contact with mineral assemblages that have equilibrated with a fluid with significantly lower xco2 these assemblages are therefore grossly out of equilibrium with the fluid in the case of infiltrated metabasic rocks intense carbonation and sulphidation is predicted if as seems reasonable gold is mobilized by the fluid generated by devolatilization then the combination of processes proposed most of which are an inevitable consequence of the metamorphism leads to the formation of greenstone gold deposits predominantly from metamorphic fluids

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

lead isotope constraints on the age and source of gold mineralization in the archean yilgarn block western australia model mineralization ages based on whole earth lead isotope growth curves are used to date sulfides mainly galena with minor fe sulfides and pb tellurides related on one hand to stratiform base metal and ni cu mineralization and on the other to predominantly lode type gold mineralization in greenstone belts of the archean yilgarn block western australia galenas related to undoubted syngenetic volcanogenic base metal mineralization and fe sulfides to komatiite associated ni cu deposits yield pb model ages that are broadly consistent with sm nd pb pb and u pb zircon greenstone belt geochronology such deposits from the murchison province give model ages of ca 3 0 b y or greater whereas those of the eastern goldfields province give model ages of ca 2 85 to 2 80 b y for the northeastern part of the province and ca 2 75 to 2 70 b y for the highly mineralized norseman wiluna belt the majority of galenas related to gold mineralization record model ages of ca 2 85 to 2 75 b y for the murchison province and ca 2 75 to 2 70 b y for the southern cross province with most deposits from the norseman wiluna belt having model ages around or below ca 2 7 b y a few examples of gold mineralization all but one economically insignificant have galenas with more complex multistage lead isotope histories with mineralization ages lying in the proterozoic lead model ages of galenas from gold deposits in the murchison province are consistently younger then those related to syngenetic mineralization those from gold deposits in the southern cross province broadly coincide with the best estimates of the age of peak metamorphism and those from the norseman wiluna belt are indistinguishable or slightly younger than the ages of syngenetic base metal and ni cu mineralization in the same belt the lead isotope compositions of most gold related galenas yield 238 u 204 pb mu values that are consistent with a previously proposed epigenetic model for economically important gold mineralization in this model lead and by inference gold was derived from basalts and komatiites by major hydrothermal systems which developed due to devolatilization of the greenstone pile during regional metamorphism from between 250 m y to less than 50 m y after eruption the small deposits that give younger mineralization ages may be related to or affected by minor thermal events which occurred later in the history of the yilgarn block and which were also responsible for resetting rock and mineral rb sr systems the contrast in isotopic histories between galenas and possibly fe sulfides from major and minor gold deposits may have application as an exploration tool significantly higher 238 u 204 pb values for sulfides from the norseman gold deposits provide support for the suggestion that crust older than 3 0 b y existed prior to formation of the greenstone belts of the yilgarn block

rapid dewatering of the crust deduced from ages of mesothermal gold deposits the large scale migration of fluids through the continental crust has been well documented but there is no consensus regarding the timing of fluid migration relative to orogenic episodes or rates of crustal dewatering1 here we present40ar 39ar dates for muscovites from quartz veins along a major shear zone in southeast alaska which show that the veins were emplaced in the early eocene during the late stages of orogenic deformation hydrother mal activity took place for only about 1 myr and along a distance of at least 200 km the fluids were generated by metamorphic reactions in subducted crust along the north american plate margin and were apparently trapped in the crust by the low permeabilities accompanying a convergent tectonic regime until 56 myr ago the rapid dewatering event coincided with a change in plate motion at 56 55 myr which caused a shift from convergent to partly transcurrent tectonics we suggest that this change in tectonic regime led to increased crustal permeabilities and hence the possibility of large scale fluid migration

archaean au ag mineralisation at racetrack near kalgoorlie western australia a high crustal level expression of the archaean composite lode gold system the racetrack au ag deposit in the archaean yilgarn block western australia is hosted by a porphyritic basalt in a low greenschist facies setting and is associated with a brittle strike slip fault system three distinct and successive stages of hydrothermal activity and late quartz carbonate veining resulted in multiple veining and or brecciation stages i and ii are au bearing whereas stage iii and late veins are barren the ore shows features of both classic epithermal and mesothermal deposits alteration assemblages typified by sericitization carbonization silicification and chloritization are similar to those of mesothermal gold deposits wheras the quartz vein textures including comb rosette plumose and banded ore mineralogyof arsenopyrite pyrite chalcopyrite sphalerite galena freibergite tetrahedrite tennantite fahlore electrum and gold and metal associations cu as ag sn sb w au and pb are more characteristics of epithermal deposits fluid inclusions related to stage ii are two phase and aqueous with 1 8 average 4 wt nacl equiv and co2 content of 0 85 molal pressure corrected homogenisation temperatures range from 190 c to 260 c mineral assemblages indicate that ore fluid ph ranged between 4 2 and 5 3 fo 2 between 10 38 8 and 10 39 6 bars and m s between 10 3 2 and 10 3 6 calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the mineralisation but evidence for deep crustal pb indicates that deeply sourced fluids were also involved the deposit is interpreted to have formed in a shallow environment via mixing of deeply sourced fluids from at least as deep as the base of the greenstone belt with surface waters it therefore represents the upper crustal end member of the crustal depth spectrum of archaean lode gold mineralisation

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

the source of lead in archaean lode gold deposits of the menzies kalgoorlie kambalda region yilgarn block western australia the archaean greenstone terrane between menzies and kambalda exhibits a coherent although deformed stratigraphic sequence intruded by granitoids and bounded by major nnw trending shear and or fault zones the greenstone terrane hosts a large number of lode gold prospects and deposits including the giant kalgoorlie deposits the initial pb isotope compositions of lode gold deposits as determined from ore related galena and pyrite vary systematically in a linear trend on a207pb 204pb versus206pb 204pb diagram which reflects crustal heterogeneity at the time of mineralisation deposits hosted within a 90 km section of the menzies boorara shear zone have a uniform radiogenic initial pb isotope composition irrespective of temperature of mineralisation and proximity to granitoid gneiss in plan view the pb in these deposits is considered to be derived largely from older felsic crust underlying the greenstone belt and was accessed via this major shear zone system deposits in a transect unrelated to a major shear zone show a systematic correlation between initial pb isotope compositions and proximity to granitoid gneiss and or to mineralisation temperature these compositions are less radiogenic than those within the menzies boorara shear zone but trend on a207pb 204pb versus606pb 204pb diagram between this isotope signature and the uniform pb isotope signature which characterises the 100 km greenstone transect from the mt pleasant area through kalgoorlie to kambalda these data are interpreted to reflect pb derivation from discrete crustal segments within and below the greenstones and require that mineralisation was related to crustal scale hydrothermal systems that accessed both sialic mid to lower crust and the greenstone succession

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

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

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

the archean lode gold deposits at wiluna western australia high level brittle style mineralization in a strike slip regime the wiluna lode hosted gold deposits comprise a group of individual archean lode hosted gold deposits located in the northern part of the norseman wiluna belt western australia collectively they are the fifth largest gold producer in the yilgarn block the deposits are hosted in a series of very low grade metamorphosed prehnite pumpellyite facies mafic and ultramafic lava flows with abundant primary igneous textures preserved lode hosted gold mineralization is controlled by the wiluna strike slip fault system which consists of north and northeast trending and west and east dipping sets of principal displacement faults and associated subsidiary structures the movement on all faults was dextral with minor oblique reverse or oblique normal motion and maximum apparent displacement of 1 350 m the lode deposits are centered on structural inhomogeneities in the principal displacement faults e g divergent bends and dilational jogs including horsetail splays and or subsidiary structures e g overstepping and intersecting faults the plunge of the ore shoots is generally moderately to steeply to the south and is controlled by the intersections between 1 lithologic layering and fault planes 2 two or more fault planes 3 a fault plane and an earlier formed barren or weakly mineralized quartz reef that is semiconcordant to the regional stratigraphy and 4 a fault plane and a felsic intrusion the shape and orientation of ore shoots becomes more complex downdip where they are controlled by additional features such as drag folds and link structures fault hosted ore breccias including implosion and net veined breccias and brecciated veins are significant loci of gold mineralization in all lode deposits they contain mainly quartz chalcedony carbonates fuchsite and arsenopyrite pyrite and or stibnite mineralization occurs also in shear veins and extensional veins and finely disseminated along foliation planes of the mineral fabric formed oblique to the principal displacement faults textures of implosion breccias at wiluna indicate a history of incremental extensional opening across dilational sites and multiple episodes of brecciation and hydrothermal recementation quartz textures such as vugs cavities comb textures and cockade textured quartz are ubiquitous indicating the preservation of open space due to low effective confining pressure geologic relations indicate that the mineralized strike slip fault system was generated late in the tectonic history of the greenstone belt and that high grade ore zones formed late in the strike slip deformation event the very low grade metamorphic facies and the brittle style of deformation in the host rocks combined with the style of mineralization indicate that the fault and mineralizing system formed in a shallow near surface crustal environment textural evidence combined with the tectonic setting of the lode deposits favors the suction pump mechanism as a fluid infiltration mechanism for the lode hosted gold deposits within the wiluna strike slip fault system