lead isotope geochemistry of paleoproterozoic layered intrusions in the eastern baltic shield inferences about magma sources and u th pb fractionation in the crust mantle system abstract u pb data for plagioclase and sulfide are reported from the burakovka and olanga layered mafic ultramafic complexes in karelia these 2 44 2 45 ga complexes differ in the age of their host rocks and in post emplacement metamorphic history acid leaching of optically discrete plagioclase populations revealed three components of pb present in the olanga plagioclases initial radiogenic and alteration related leaching in hcl and hno3 was found to remove most of u and radiogenic pb from plagioclase and to fractionate u from pb the alteration component abundant in the olanga plagioclase is only partially removed by leaching the olanga sulfides also contain a significant radiogenic component plagioclase from the unmetamorphosed burakovka complex yields more reliable initial pb ratios the pb isotopic compositions of plagioclase from the three intrusions of the olanga complex form a single steep linear trend which shows excess scatter due to residual radiogenic and alteration pb calculated single stage 1 of about 7 9 in the kivakka intrusion is close to the archean depleted mantle value but combined pb sr and nd isotopic systematics suggest the presence of an enriched component from either lithospheric or slightly contaminated plume source internal variations in initial pb and sr isotopic ratios and pb isotopic systematics of sulfides are explained by late magmatic fluid exchange with 2 7 ga country rocks the burakovka complex emplaced into the pre 3 1 ga crust shows large correlated variations in 207pb 204pb nd t and 87sr 86sr t in plagioclase initial pb nd and sr isotopic variations in the burakovka complex are interpreted as a result of a multistage contamination process including contamination en route and during emplacement and local assimilation of enclosing gneiss in addition the burakovka mantle source was probably modified with addition of sediment derived pb by a subduction zone fluid time integrated th u 4 3 in mantle sources and crustal contaminants of the olanga and burakovka complexes are uniform and are similar to the archean mantle value this implies that u pb fractionation in the archean paleoproterozoic crust mantle system probably occurred by hydrothermal pb transfer that fractionated pb from u but not th from u

a time scale for granitoid emplacement in the archean murchison province western australia by single zircon geochronology abstract in the late archean murchison province of western australia four major granitoid suites intruded into two stratigraphically distinct supracrustal sequences the luke creek and mount farmer groups single zircon u pb ages from five granitoid samples representing three of the four suites were measured by ion microprobe the earliest of the suites the pegmatite banded gneiss intruded luke creek group greenstones at 2919 12 ma 1 the luke creek group and the pegmatite banded gneiss protolith were subsequently deformed and then overlain by mount farmer group greenstones large volumes of monzogranite then intruded greenstones throughout the province two samples from this suite gave crystallization ages of 2704 51 and 2681 6 ma this package of greenstones gneiss and monzogranite was folded and metamorphosed and then intruded by two suites of post folding granitoids two samples from the more southern of these suites gave ages of 2641 5 and 2602 14 ma two additional samples were collected from a tonalite pluton previously assigned to the northern post folding granitoid suite they gave crystallization ages of 2760 8 and 2784 22 ma 1 this tonalite is now interpreted as a subvolcanic pluton associated with the andesite and dacite volcanism which produced the nearby mount farmer group greenstones no xenocrystic zircons with ages significantly in excess of 3 0 ga were found in any of the seven samples implying that little or no mid archean or older rocks are present in the region lower concordia intercept ages and 207 pb 206 pb ages from individual analyses indicate the presence of mid to late proterozoic activity throughout the murchison province concordia intercept ages were calculated using a least squares fit of a plane in three dimensions this approach obviates both the need to assign a common lead isotopic composition and the assumption that no pre recent events have disturbed the population s u pb system a further refinement of the data assessment process involved the use of the internal scan by scan scatter for defining the precision of the individual pb pb isotopic ratios compared with the observed scatter ion counting error estimates were found to frequently underestimate the uncertainties in the 207 pb 206 pb and 204 pb 206 pb ratios

the scale of ore depositional systems an important restraint on epigenetic vs remobilized syngenetic origins for archaean mesothermal gold deposits there is general consensus that the majority of gold deposits in the important group hosted by volcanic or intrusive host rocks within archaean granitoid greenstone terrains have an epigenetic origin however there is controversy surrounding those stratabound deposits hosted by fe rich sedimentary rocks such as banded iron formation bif for which epigenetic syngenetic and remobilized syngenetic lateral secretion origins have all been proposed in western australia such controversial gold deposits in fe rich sedimentary units have similar ore mineralogies ore element ratios and in places alteration assemblages and on the craton scale show very similar structural controls to the other deposits equivalent relationships are recorded for similar deposits in other cratons all western australian deposits occur along kilometre scale shear or fault zones linked to trans craton obliqueto strike slip shear zones that were a focus for i carbonation with a mantle like isotopic signature and ii emplacement of high level a and i type granitoids felsic porphyries and or calc alkaline lamprophyres the scale of the mineralizing systems and the broad contemporaneity of mineralization as indicated by radiogenic isotope studies is much larger than that envisaged in conceptual secretion models which involve volumetrically small locally enriched source rocks instead archaean gold mineralization in western australia at least is probably the result of high fluid flux during deformation hightemperature metamorphism and magmatism which may be related to tectonism at convergent plate margins as in modern examples there is strong evidence for similar tectonic and mineralizing processes from the canadian shield

tectonic controls on the endowment of neoarchean cratons in volcanic hosted massive sulfide deposits evidence from lead and neodymium isotopes review and analysis of initial lead isotope ratios from archean volcanic hosted massive sulfide vhms and lode gold deposits and of neodymium isotopes from igneous rocks from the geologic provinces that host these deposits identifies systematic spatial and temporal patterns both within and between the provinces the abitibi wawa subprovince of the superior province is characterized by highly juvenile lead and neodymium most other archean provinces however are characterized by more evolved isotopes although domains within them can be characterized by juvenile isotope ratios the analysis indicates that the endowment measured as the quantity of metal contained in geologic resources per unit surface area of vhms and komatiite associated nickel sulfide deposits is related to the isotopic character and therefore the tectonic history of provinces that host these deposits provinces with extensive juvenile crust have significantly higher endowment of vhms deposits possibly as a consequence of higher heat flow and extension related faults provinces with evolved crust have higher endowment of komatiite associated nickel sulfide deposits possibly because such crust provided either a source of sulfur or a stable substrate for komatiite emplacement in any case initial radiogenic isotope ratios can be useful in predicting the endowment of archean terranes for vhms and komatiite associated nickel sulfide deposits limited data suggest similar relationships may hold in younger terranes lead isotope data may provide constraints on supercraton reconstructions in the archean as lead isotope systematics appear to be highly provincial similar isotope systematics between the superior province and the eastern goldfields superterrane may indicate a linkage during the neoarchean consistent with the similar geologic and metallogenic histories of these provinces

the timing of gold mineralization across the eastern yilgarn craton using u pb geochronology of hydrothermal phosphate minerals the highly mineralized eastern goldfields of the eastern yilgarn craton is an amalgamation of dominantly neoarchaean granitoid greenstone terranes and domains that record a history of early rifting followed by westward directed collision with initial arc formation collision and clastic basin formation and final accretion to the western yilgarn proto craton between 2 66 and 2 60 billion years ago the gold deposits that define this region as a world class gold province are the product of orogenic processes that operated during accretion late in the tectonic history after initial compressional deformation d1 d2 and the majority of granitoid magmatism minor gold was also deposited throughout the entire tectonic history in magmatic hydrothermal related systems however such mineralization mostly 0 3 g t gold is nowhere economic unless it overprints or is overprinted by much higher grade orogenic gold lodes robust shrimp u pb geochronology of gold related hydrothermal xenotime and monazite supports structural studies that gold mineralization occurred during late transpressional events d3 d4 shortly before cratonization however westward migration of collision and accretion produced a complementary diachroneity in the timing of gold mineralization of 5 to 20 m y between c 2 65 ma in the east including laverton district kurnalpi terrane to c 2 63 ma in the west including kalgoorlie terrane across the eastern part of the craton the robust geochronology refutes previous suggestions that significant gold mineralization events extended from de to d4 in the evolution of the orogen and that the kalgoorlie gold deposits formed over a period of 45 m y the crustal continuum model is applicable within terranes where orogenic gold depositional events were penecontemporaneous but must be modified to account for diachroneity of orogenic events and gold mineralization across the eastern goldfields

petrogenesis of amphibole biotite calcite plagioclase alteration and laminated gold silver quartz veins in four archean shear zones of the norseman district western australia the norseman mining district in the archean yilgarn block western australia has produced 140 t of gold and about 90 t of silver from 11 24 106 t of ore the district is located within a metamorphic terrane of mafic and minor ultramafic greenstones intruded by granite cupolas and swarms of porphyry dykes the orebodies consist of laminated quartz veins controlled by narrow 0 5 5 m reverse shear zones that in general follow the contacts of metapyroxenite or porphyry dykes petrological studies of four shear zones exposed on the regent shaft 14 level ajax shaft 10 level and in the stope above the north royal shaft 5 level show that the host rocks were metamorphosed to hornblende plagioclase amphibolites and actinolite chlorite rocks at temperatures of 500 550 c prior to mineralization at the localities studied intense wall rock replacement and low grade 0 5 g t gold mineralization are confined to ductile or brittle ductile shear structures alteration is similar in both ultramafic and mafic

the source of ore fluids and solutes in archaean lode gold deposits of western australia abstract the ore fluids which produced archaean lode gold deposits in sub amphibolite facies rocks of the yilgarn block western australia are characteristically low to moderate salinity h 2 o co 2 ch 4 fluids wall rock alteration and ore deposition were from a generally reducing near neutral fluid at 200 350 c and 1 3 kbar although more oxidising fluids produced some assemblages in the giant golden mile deposit gold was transported as reduced sulphur complexes the ore fluid composition and stable and radiogenic isotope data indicate multiple sources for the fluid and solute deep plumbing systems are invoked with the fluids being either metamorphic fluids that derived solutes from the lower and upper crusts or fluids exsolved from deep seated granitoids formed by anatexis of such crust

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

the savage lode magnesian skarn in the marvel loch gold silver mine southern cross greenstone belt western australia part 2 pressure temperature estimates and constraints on fluid sources the savage lode gold skarn orebody in the marvel loch mine southern cross greenstone belt western australia replaces foliated metakomatiites in a subvertical ductile shear zone and is located within the broad metamorphic aureole of the ghooli dome granodiorite granite batholith pressure estimates based on metamorphic as well as alteration mineral parageneses indicate that skarn formation took place at p 4 1 kbar 1 kbar 100 mpa corresponding to a burial depth of about 13 km metamorphism reached temperatures of 550 630 c in the mine area and preceded skarn formation pressure temperature calculations constrain the maximum temperature of the hydrothermal fluid in the savage lode to 640 20 c during the formation of early olivine calcite and diopside amphibole rocks and to 500 600 c during the formation of phlogopite chlorite calcite schists and quartz diopside veins lower fluid temperatures 500 to 400 c are recorded by retrograde serpentine after olivine talc after tremolite a

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

structural geology across a proposed archaean terrane boundary in the eastern yilgarn craton western australia abstract the eastern goldfields province of the archaean yilgarn craton western australia can be divided into several nw se trending terranes with different stratigraphic signature at least two main phases of penetrative ductile deformation affect the terranes d1 is a phase of nappe type transport mainly to the north along gently dipping ductile shear zones it is interpreted as a phase of regional extension contemporaneous with intrusion of massive granitoid plutons and possibly contemporaneous with deposition of felsic volcanics and metasediments in the greenstone belts terrane boundaries were probably formed during d1 d2 gave rise to steep nw se to n s trending foliations and upright folds d2 is strongly partitioned over the area for strain and vorticity the pattern of partitioning seems to be dependent on the position of granitoid plutons which give rise to development of d2 strain shadows d1 structures have been preserved mainly in these strain shadows terrane boundaries are transected by boundaries of d2 structural domains minor intrusion of granitoids continued during and after d2 d1 is possibly associated with arrival of a mantle plume below the yilgarn craton and d2 may be a phase of regional shortening in a thinned lithosphere weakened by the thermal event

deciphering the tectonic evolution of archaean greenstone belts the importance of contrasting histories to the distribution of mineralization in the yilgarn craton western australia abstract the distribution of archaean metallic mineral deposits reflects tectonic controls operating during the evolution of granitoid greenstone terranes tectonic analysis of these terranes not only allows recognition of parameters that may be important in interpreting the tectonic histories of greenstone belts but also the resultant tectonic control on mineral deposit distribution in the yilgarn craton of western australia the late archaean 2 7 ga norseman wiluna belt is most intensely mineralized with both komatiite associated ni cu deposits and epigenetic gold deposits this belt is dominated by 2 7 ga supracrustal successions and its tectonic history is that of an orogenic belt at an obliquely convergent craton or proto craton margin tectonostratigraphic associations preserved resemble younger volcanic arc and back arc basin crust and many represent the remains of a volcanic arc and back arc basin complex developed in response to westward dipping subduction these sequences were deformed metamorphosed and intruded by granitoids relatively soon 30 ma after the major period of volcanism with gold mineralization occurring late in the orogenic event possibly during uplift in contrast greenstone belts in the murchison and southern cross provinces are dominated by older supra crustal successions that contain 2 9 ga volcanosedimentary associations characterized by extensive oxide facies banded iron formations bifs bifs appear to have been deposited in shelf or platform environments in intra oceanic or passive craton margin settings komatiite associated ni cu mineralization and volcanogenic cu zn mineralization are locally hosted by favourable tectonostratigraphic associations the main period of deformation metamorphism and gold mineralization in these provinces was broadly synchronous with that in the norseman wiluna belt as was regional granitoid emplacement during this event strain was probably transmitted from the eastern margin of the craton via a yilgarn wide system of shear zones faults and fold belts gold mineralization is developed best in greenstone belts such as the meekathara mt magnet belt that contain large scale fault systems and internal granitoids similar tectonic controls on the distribution of mineral deposits can also be recognized in other archaean cratons and the heterogeneous distribution of archaean mineral deposits probably reflects a range of tectonic settings as well as different stages in the evolution of individual cratons

diverse metal sources of archaean gold deposits evidence from in situ lead isotope analysis of individual grains of galena and altaite in the ross and kirkland lake deposits abitibi greenstone belt canada lead isotope compositions for individual grains of galena and altaite pbte were determined in situ using a secondary ion mass spectrometer sims galena was collected from the ross deposit and altaite from the kirkland lake kl deposits in the southern abitibi greenstone belt superior province of canada the samples from kl are more radiogenic than those from the ross deposit isotopic compositions vary significantly between different grains in each deposit and form broad linear arrays in 207pb 204pb 206pb 204pb and 208pb 204pb 206pb 204pb diagrams the linear arrays of pb isotope data are attributed to mixing of pb from different sources at least two sources are required for individual deposits one with low u pb and th pb ratios and the other with high ratios lead minerals occurring with au are less radiogenic than those that are not obviously associated with au suggesting that au was supplied from low u pb sources such as sulphides or older ultramafic mafic rocks while most data are consistent with the derivation from local rocks highly radiogenic pb with relatively low 207pb 206pb ratios recorded at kl require post archaean mineralization or derivation of the pb from an unusual crustal source with low the latter interpretation is favored because of the lack of textural evidence and because it is difficult to dissolve and precipitate altaite at low temperatures the presence of a pb reservoir with low is also inferred from the data of archaean banded iron formations and volcanogenic massive sulphide deposits different isotopic patterns of the two deposits suggest different sources of metals in the two deposits while this conclusion does not reject the derivation of fluids from the lower crust or mantle the data are not in accord with conceptual models invoking a common source reservoir for metals the study suggests that fluids which may have a common origin leached metals and other constituents from the upper crustal rocks during their ascent the proposed model different origins for different constituents explains much of the conflicting evidence presented by archaean au deposits including provinciality of mineralogy and relatively uniform fluid inclusion and c isotope data from many au deposits

the distribution of 3 0 ga and 2 7 ga volcanic episodes in the yilgarn craton of western australia abstract the results of a conventional zircon u pb investigation of the ages of felsic volcanic rocks in a number of greenstone belts in the yilgarn craton of western australia indicate that volcanism took place during two distinct episodes at ca 3 0 and ca 2 7 ga each episode spans a time period of about 75 ma and the two ages of volcanism are spatially separated 3 0 ga volcanic rocks are found in greenstone belts in the northern part of the western gneiss terrain and in the murchison province whereas 2 7 ga volcanic rocks are present in the southern part of the western gneiss terrain and in the southern cross and eastern goldfields provinces there is no evidence of inheritance in the zircon u pb systems which would support an older crustal origin for these rocks present results together with previous studies of the ages of granites and gneisses suggest that the volcanic episodes were accompanied by broadly contemporaneous granite plutonism it is suggested that the evolution of the yilgarn craton took place in three main stages the first was the development of a stable 3 4 ga granitic nucleus which incorporated earlier generations of granitoid a 3 7 ga layered complex and possibly 4 2 ga primitive crust this was followed by the two separate granite greenstone episodes at ca 3 0 and ca 2 7 ga involving the addition of significant amounts of new crust each episode was characterised by the accretion of extensive granite greenstone terranes at the margins of the stable craton the intrusion of granite plutons into the evolving craton and the reworking of older cratonic rocks to form granites and gneisses with inherited isotopic systems

a holistic model for the origin of orogenic gold deposits and its implications for exploration the term orogenic gold deposits has been widely accepted but there has been continuing debate on their genesis early syn sedimentary or syn volcanic models and hydrothermal meteoric fluid models are now invalid magmatic hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships the most plausible models involve metamorphic fluids but the source of these fluids is equivocal intra basin sources within deeper segments of the hosting supracrustal successions the underlying continental crust subducted oceanic lithosphere with its overlying sediment wedge and metasomatized lithosphere are all potential sources several features of precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic fluid source these include the presence of hypozonal deposits in amphibolite facies domains their anomalous multiple sulfur isotopic compositions and problems of derivation of gold related elements from devolatilization of dominant basalts in the sequences the phanerozoic deposits are largely described as hosted in greenschist facies domains consistent with supracrustal devolatilization models a notable exception is the jiaodong gold deposits of china where ca 120 ma gold deposits are hosted in precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization other deposits in china are comparable to those in the massif central and elsewhere in france in that they are hosted in amphibolite facies domains or clearly post date regional metamorphic events imposed on hosting supracrustal sequences if all orogenic gold deposits have a common genesis the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold bearing sulfide rich sedimentary package or the associated metasomatized mantle wedge with co2 released during decarbonation and s and ore related elements released from transformation of pyrite to pyrrhotite at about 500 c although this model satisfies all geological geochronological isotopic and geochemical constraints and is consistent with limited computer based modeling of fluid release from subduction zones the precise mechanisms of fluid flux are model driven and remain uncertain from an exploration viewpoint the model re emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction related orogenic belts and importance of continental scale lithosphere tapping fault and shear zones to focus large volumes of auriferous fluid it confirms the importance of the consistent spacing between world class deposits broadly equivalent to the depth of the moho as derived from empirical observations

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

diagenesis and very low grade metamorphism of volcaniclastic sandstones from contrasting geodynamic environments north island new zealand the murihiku and waipapa terranes the mesozoic murihiku and waipapa terranes are two accretionary wedges of linked forearc and trench sediments respectively that were juxtaposed in the early cretaceous late triassic to late jurassic murihiku terrane volcaniclastic sediments are folded into a regional syncline and have been diagenetically altered there is a general relationship between zeolite occurrence clay mineralogy vitrinite reflectance and stratigraphic position youngest jurassic sediments contain heulandite analcime and stilbite whereas late triassic to mid jurassic sediments have laumontite and heulandite in detail the zeolite distribution is complicated tuffaceous horizons on the eastern limb of the syncline are calcitized rather than zeolitized post diagenetic fractures associated with uplift are laumontite filled the inferred geothermal gradient is c 15 c km 1 the waipapa terrane is an accretionary complex dominated by imbricated terrigenous sediments of triassic and jurassic age with enclosed permian to jurassic pelagic sediments and basalts late jurassic sediments are massive volaniclastic sandstones the sediments are non foliated and metamorphic minerals in the massive sandstones have crystallized in specific domains the observed metamorphic succession of prehnite pumpellyite and pumpellyite actinolite facies assemblages was overprinted in the imbricated rocks during a thermal event that was late in the deformation sequence and broadly coincident with hydraulic fracturing and veining the metamorphic successions in the two terranes and their relationships to structural features are in excellent accord with accretionary complex models

chronology of the mount magnet granite greenstone terrain yilgarn craton western australia implications for field based predictions of the relative timing of granitoid emplacement abstract in this paper we report ion probe u pb ages for zircons from thirteen granitoids and five supracrustal rocks from the mount magnet area of the archaean murchison province of western australia the oldest granitoids in the area are two porphyry dykes and three small apparrently underformed bodies that intrude the greenstones the porphyries have ages of 2715 and 2752 ma and the undeformed granitoids range in age between 2696 and 2716 ma the external monzogranites that surround the greenstone belt are larger than the internal granitoids and show variable evidence of deformation from a foliated texture to recrystallization at grain boundaries with the exception of a gneissic sample which has an age of 2710 ma they are younger than the undeformed granites with ages between 2640 and 2694 ma a late leucocratic vein in one sample is 20 ma younger than the mesocratic foliated rock it cuts the young age for the external monzogranite is unexpected and it implies that the supracrustal rocks away from the margins of the mount magnet greenstone belt and the small granitoids that intrude them were cold and rigid at the time of emplacement of the monzogranites so that they behaved as a coherent block we suggest that the stress associated with the deformation even that produced foliation gneissic textures and recrystallization in the external granitoids was dissipated at the margins of the greenstone belts so that the cores of these belts were little affected by this event the age difference of 20 ma between the melanocratic and leucocratic phases in the same rock suggest slow cooling and crystallization in the lower or middle crust followed by tectonic emplacement into the upper crust the mount magnet granitoids contain abundant xenocrystic zircons that show three pronounced peaks in their age distribution a broad peak between 2910 and 2950 ma and sharp peaks at 2800 ma and 2730 ma these ages correlate with eruption of volcanic rocks in the murchison we suggest that the thermal events that produced these periods of volcanism were also recorded in the lower crust which later melted to produce the mount magnet granitoids two felsic volcanics from near the top of the stratigraphy yield ages of 2704 and 2727 ma and a chert the warramboo galtee moore chert of the windaning formation an age of 2798 ma not 3 0 ga as had previously been thought the 2727 ma felsic volcanic unit unconformably overlies the warramboo galtee moore chert and is in turn overlain by the wattle creek basalt of the mount farmer group this suggests that the mount farmer group has an age of 2 7 ga and that 2 7 ga mafic volcanism is present in the murchison the younger external monzogranites which make up the bulk of the granitoids in the mount magnet region may have derived their heat from the mantle plume head that is believed to be responsible for basaltic and komatiitic volcanism of the norseman wiluna greenstone belt however the smaller internal granitoids are older than the plume related volcanic activity of the eastern yilgarn and require an additional heat source to have affected the northern part of the yilgarn craton

round hill shear zone hosted gold deposit macraes flat otago new zealand evidence of a magmatic ore fluid the 0 8 moz round hill shear zone hosted gold deposit is one of several located on or near the hyde macraes shear zone macraes flat new zealand field relationships show stockwork veins are the oldest part of the hydrothermal system followed by flat and ramp veins hanging wall shear veins formed early but have been modified by late deformation fluid inclusion microthermometric results for dominant type i inclusions show that flat and ramp veins have the same average wt percent nacl equiv values 2 2 0 6 and were precipitated from the same fluid salinity data for these veins overlap with those from hanging wall veins average 1 3 0 3 and stockwork veins 1 6 0 6 for vein margins 0 9 0 3 for vein centers although both of the latter trend toward lower overall wt percent nacl equiv values suggesting mixing between relatively dilute and more saline end members all of the above veins have similar temperatures of homogenization t h ranging between 110 and 188 c fluid inclusion trapping temperatures are considered to be 300 c equating to a 10 km depth of formation carbon dioxide dissolved in the hydrothermal fluid is responsible for up to one third of fluid inclusion apparent salinities stockwork and hanging wall veins thus have true salinities of 1 wt percent nacl equiv 0 17 molal and flat and ramp veins have true salinities of fluid inclusion volatiles from round hill and other macraes deposits are dominated by h 2 o 99 02 99 69 mol with lesser co 2 0 14 0 76 mol n 2 0 03 0 32 mol and ch 4 0 06 0 17 mol noticeable concentrations of hydrocarbons occur in many of the samples fluid inclusion cation and anion data show few components of relatively low concentration are dissolved in the hydrothermal fluid consistent with co 2 corrected microthermometric measurements a range in fluid inclusion d h 2 o values obtained for the round hill deposit is coincident with the different vein types and also indicates mixing between two fluids one fluid is considered to be o isotope shifted meteoric water as trapped by the early stockwork veins and has the lowest d h 2 o values of 80 per mil the other fluid is represented by flat and or ramp and hanging wall veins that have d h 2 o values up to 40 per mil the macraes d h 2 o data show a trend toward the magmatic water end member for geothermal systems associated with convergent plate boundaries and are consistent with a 25 to 50 percent magmatic component in the macraes hydrothermal system

2 7 ga plume associated vhms mineralization in the eastern goldfields superterrane yilgarn craton insights from the low temperature and shallow water ag zn au nimbus deposit economic volcanic hosted massive sulfide vhms deposits of the archaean yilgarn craton of western australia are restricted to zones of juvenile crust as revealed through regional nd pb and hf isotopic variations and the geochemistry of felsic volcanic rocks interpreted as archaean paleo rift zones one of these runs n s through the eastern goldfields superterrane broadly coincident with the kurnalpi terrane and is associated with the high grade ca 2690 ma teutonic bore jaguar and bentley deposits plus sub economic vhms mineralization further south to date only small historic cu occurrences e g anaconda and barren pyritic lenses have been recognized in the older 2 7 ga plume dominated lower stratigraphy of the eastern goldfields superterrane the nimbus ag zn au deposit 12 1 mt at 52 g t ag 0 9 zn and 0 2 g t au is located approximately 10 km east of kalgoorlie near the margin of the kurnalpi terrane its origin has been contentious for a number of years with previous models favouring seafloor sub seafloor vhms mineralization or a high sulfidation fault hosted system we present a detailed account of the deposit its host stratigraphy and associated hydrothermal alteration plus two new shrimp u pb zircon ages pb isotope galena and o isotope zircon constraints compared to other vhms occurrences in the yilgarn craton the nimbus deposit is unusual in terms of its tectono stratigraphic position the geochemistry of its host sequence i e fi affinity felsic volcanic rocks ocean plateau like low th basalts mineralogy e g abundance of ag sb pb as bearing sulfosalts high hg low cu and quartz carbonate sericite dominated alteration assemblages classification of nimbus as a shallow water and low temperature vhms deposit with epithermal characteristics i e a hybrid bimodal felsic deposit is consistent with its position near the margin of the kurnalpi paleo rift zone and radiogenic 238u 204pb values the recognition that the nimbus deposit is associated with c 2705 ma plume magmatism opens new areas for vhms exploration in the eastern goldfields superterrane over a strike length exceeding 500 km

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

the age of the giant golden mile deposit kalgoorlie western australia ion microprobe zircon and monazite u pb geochronology of a synmineralization lamprophyre dike open pit and underground mines in the golden mile at kalgoorlie western australia have produced more than 1 475 metric tons t of gold since 1893 despite the economic importance of the deposit the age of the mineralized shear zone array and its temporal relationship to other structures and to porphyry intrusions in the host greenstone terrane are poorly constrained in this study the shrimp ion microprobe has been used to date zircons and monazites recovered from a chlorite carbonate altered synmineralization lamprophyre dike intruded into sericite ankerite altered paringa basalt below the high grade oroya hanging wall shear zone analyses of magmatic hydrothermal zircons define a weighted mean 207 pb 206 pb age of 2642 6 ma n 37 mswd 0 88 analyses of cogenetic hydrothermal monazites yield a concordant 207 pb 206 pb age of 2637 20 ma n 9 mswd 1 8 the two alteration and or mineralization ages determined independently in two different minerals are inconsistent with recent structural models relating mineralization to the early compressional events of the d 1 to d 4 deformation sequence for the terrane instead they confirm earlier structural models relating mineralization to the postfolding wrench tectonic regime which generated regional sinistral faults d 3 coeval with mantle derived lamprophyric and initial felsic magmatism in the middle to lower archean crust

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

age and origin of granitic rocks in the kalgoorlie norseman region of western australia implications for the origin of archaean crust three granites from the norseman area at stennet rocks buldania rocks and east of lake johnston have u pb zircon ages of 2691 8 95 c l 2689 22 and 2661 8 ma respectively one stennet rocks contains 3100 3200 ma cores providing support for oversby s 1975 argument that all three plutons were derived by anatexis of much older basement common leads within the zircons show evidence of having been derived from within the rock and represent mixtures of initial rock leads and radiogenic leads the unusually 208pb rich compositions of the common leads suggest the breakdown of a th rich phase perhaps uranothorite as a major contributor to the radiogenic component available zircon geochemical and isotopic data are used to develop a two stage model for archaean crustal development in this region with formation of primitive crust of broadly andesitic composition from depleted mantle or basalt beginning at 3400 ma followed by extensive thermal reworking and possibly continued growth over the period 3000 3300 ma in the second stage at 2700 ma anatexis resulted in large scale intra crustal differentiation giving a thick upper granitic layer and a lower crust residual from anatexis it is this second major thermal event that is recorded by most geochronometers implicit in the andesitic crustal growth model is that because the sm nd and rb sr ratios of newly formed archaean crust are close to mantle values this process is transparent to the sm nd and rb sr isotope systems until recycling of incompatible element rich sediment becomes important following the late archaean production of a wide spread differentiated upper crust

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

the porphyry gold association in the norseman wiluna belt of western australia implications for models of archaean gold metallogeny abstract porphyritic minor intrusions felsic intermediate porphyries and lamprophyres occur in the vicinity of many gold deposits in the archaean norseman wiluna belt of western australia but are not restricted to this environment unlike the abitibi belt of canada there is no correlation between the presence of such intrusions and the economic status of a deposit the timing of intrusive activity with respect to spatially associated mineralization is variable with mainly instances of pre sometimes 10 ma earlier and some examples of post ore emplacement being recorded in many cases the porphyry gold association reflects the importance of structural control to the localization of mineralization intrusions may exploit the same zones of crustal weakness as the ore fluids although such a relationship need not imply contemporaneity or may act as structural traps for mineralization owing to rheological contrast between intrusion and country rock the composition and petrogenesis of the minor intrusions are both variable and there is no evidence of a specific suite of intrusions preferentially associated with mineralization spatial temporal and petrogenetic considerations preclude direct evolution of the gold bearing fluids from the minor intrusions and militate against magmatic theories of ore genesis rather both the igneous activity and the mineralization are viewed as expressions of a deep seated tectono thermal event that affected the yilgarn block in the late archaean

the relationship between archaean gold mineralization and spatially associated minor intrusions at the kambalda and norseman gold camps western australia lead isotope evidence many archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits a comparison of the pb isotopic signature of gold related galenas from kambalda and norseman with that of spatially associated minor intrusions at the time of mineralization indicates that the ore fluid pb cannot have been derived solely from the intrusions or their source regions for both study areas the galena pb isotopic compositions are bracketed by those of local volcanic mafic and intrusive largely felsic rock types this is consistent with the ore fluid having derived metallic components from the crust or crustally derived granitic rocks and the mantle or mantle derived rocks of the greenstone succession via metamorphic dewatering or mantle crustal degassing interaction of granite derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of pb isotopic signatures the norseman data as a whole are more radiogenic than the kambalda data for broadly synchronous mineralization reflecting the greater abundance of older granitic rocks with respect to mafic ultramafic rocks in the norseman district the provinciality exhibited by the pb isotopic composition of the ore fluid indicates that the gold mineralizing process formed galena whose pb isotopic composition was very sensitive to local variations in crustal pb isotopic composition either within the source region of the fluid or along fluid conduits

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

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

signatures of multiple mineralization processes in the archean orogenic gold deposit of the pampalo mine hattu schist belt eastern finland the hattu schist belt is an emerging gold producing ore district in the western part of the archean karelian province of the fennoscandian shield the belt consists of 2 76 to 2 70 ga tonalite granodiorite and leucogranite intruded into a mafic felsic epiclastic volcanic sequence of an only slightly older age complex and successive folding shearing and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism 550 50 c 3 5 kbar at ca 2 70 ga orogenic gold deposits are hosted by the highly strained zones that developed during the archean deformation of the belt however previous k ar and rb sr geochronological studies indicated that a second tectonothermal overprint affected the hattu schist belt between 1 7 and 1 8 ga during the svecofennian orogeny results of our field mineralogical textural and pb isotope studies suggest that ore deposition at pampalo was initiated by hydrothermal processes at the time of the emplacement of feldspar porphyry and tonalite intrusions and dikes at around 2 72 ga albitization and quartz tourmaline biotite muscovite veining characterize this hydrothermal activity the major stage of gold ore deposition can be confined to the subsequent development of high strain zones in an intermediate felsic tuffaceous unit mafic schist characterized by biotite carbonate pyrite alteration ar ar studies revealed the complete resetting of the ar ar system in muscovite and biotite due to the svecofennian orogeny with closure of the isotopic system at 1 81 ga results of pb isotope studies of hydrothermal k feldspar galena and altaite by laser ablation inductively coupled plasma mass spectrometry indicate that k feldspar alteration and remobilization of metals also took place during the svecofennian reactivation of high strain zones the petrographic manifestation of this process is the replacement of albite by hydrothermal k feldspar fluid inclusion data from hydrothermal k feldspar suggest that carbonic aqueous low salinity fluids interacted with the archean ore at 350 to 400 c and 1 8 to 2 4 kbar during the svecofennian overprint results of mass transfer calculations indicate that potassium gain correlates with increase of gold concentrations in the mafic schist and feldspar porphyry units therefore the svecofennian overprint locally also enhanced the grade of gold mineralization mostly along competency differences of the mafic schist and feldspar porphyry blocks dikes later percolation of relatively low temperature 300 c saline basin fluids in some fractures of the crystalline basement also left their pb isotope and fluid inclusion signatures on the mineralization while further modifying the composition of the ore occurrences of hydrothermal proto ores as well as fluid flow events along structures reactivated by overprinting orogenic processes are not exceptional features in archean orogenic gold provinces this study shows that the combination of in situ pb isotope studies of u poor alteration minerals and pb rich ore minerals together with evaluation of relationships between gold enrichment and hydrothermal alteration and fluid inclusion studies can be very useful in determining the significance and conditions of overprinting processes as well as their potential implications for genetic and exploration models

criteria for determining initial lead isotopic compositions of pyrite in archaean lode gold deposits a case study at victory kambalda western australia abstract the initial pb isotopic compositions of ore associated sulphides are important in isotopic tracer studies studies of archaean lode gold deposits from western australia indicate that the pb isotopic data distribution for ore associated galenas and pyrites is commonly linear and determination of the initial pb ratio relies on selecting the least radiogenic composition this detailed pb isotopic study of sulphides from the 32 ore zone which transects a variety of rocks in the victory mine mineralisation dated at 2627 ma has resolved the origin of the pb isotopic heterogeneities which yield linear arrays it has led to the development of criteria for sample selection which more efficiently and effectively target pyrite samples which best preserve the initial pb ratio of the sulphide and hence the ore fluid the most important controls on the displacement of the measured pb ratio of pyrite from the initial pb ratio are in order of importance 1 pb content of pyrite 2 pyrite abundance 3 proximity of pyrite to ore fluid channelway i e vein and 4 host rock the host rock is important because the th u and pb contents and presence of minor u th bearing phases influence the amount and nature of any post formation pb represented in the pyrite data the first three parameters control the magnitude of that influence a high pb content high pyrite abundance and setting in a vein or at a vein margin produce a pb isotopic composition which is more likely to be dominated or buffered by fluid pb i e more accurately reflects the initial pb isotopic ratio conversely pyrite with a low pb content low abundance and siting distal from veining is likely to be more susceptible to post formation modification by contributions from the host rock resulting in a more radiogenic pb isotopic composition

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