an extensive crustally derived 3325 to 3310 ma silicic volcanoplutonic suite in the eastern pilbara craton evidence from the kelly belt mcphee dome and corunna downs batholith abstract new sensitive high resolution ion microprobe zircon u pb dating indicates that the eastern pilbara craton contains an extensive suite of variably deformed 3325 to 3310 ma rhyolites high level quartz feldspar porphyries and granitoids these range in composition from na rich trondhjemite low k rhyolite to k rich monzogranite with silica rich 68 biotite hornblende bearing porphyritic granodiorites and monzogranites most abundant initial volcanism accompanied regional extension at 3325 ma and was followed by intrusion of high level porphyritic stocks and emplacement of massive to foliated batholithic granitoids at 3315 ma the compositions of this volcano plutonic suite suggest that the most likely source for parent magmas is older continental crust with tonalite trondhjemite granodiorite suite compositions crustally derived silicic volcanic rocks and granitoids have traditionally been considered rare in pre 3 0 ga granitoid greenstone terranes evidence from the eastern pilbara indicates that magmatic recycling of continental crust has occurred at least as early as 3 3 ga

new constraints on the evolution of the mallina basin and their bearing on relationships between the contrasting eastern and western granite greenstone terranes of the archaean pilbara craton western australia abstract the northeast trending mallina basin in the central part of the archaean pilbara craton separates western and eastern granite greenstone terranes with significantly different structural histories and age distribution patterns the basin which is up to 70 km wide and has a strike length of more than 150 km is mainly composed of metamorphosed turbidites assigned to the de grey group in the north these rocks are in faulted contact with metamorphosed volcanosedimentary rocks of the whim creek group new shrimp u pb zircon dates show that the turbidite sequence and the whim creek group were deposited within the same period between ca 3010 and 2940 ma and that the basement to the mallina basin is as young as ca 3015 ma geochronological structural and stratigraphic data now mitigate against previous models which suggested that the whim creek group was significantly younger than the turbidite sequence and that the whim creek group evolved in a late ensialic pull apart basin the mallina basin reinterpreted to include at least part of the whim creek group is now believed to be an extensional basin containing an overlap sequence an ensialic setting is preferred in which the basement to the basin consisted of ca 3170 3020 ma granites and greenstones rifted by major northeast trending faults a continental block to the south and east of which the presently exposed east pilbara granite greenstone terrane is a remnant influenced sedimentation in most of the basin previous tectonostratigraphic models that correlated across and beneath the mallina basin and interpreted the entire granite greenstone terrane of the pilbara craton in terms of evolution during two cycles of arc accretion are not compatible with the new geochronological data and stratigraphic interpretations

young ores in old rocks proterozoic iron mineralisation in mesoarchean banded iron formation northern pilbara craton australia abstract the origin of bedded iron ore deposits developed in greenstone belt hosted algoma type banded iron formations of the archean pilbara craton has largely been overlooked during the last three decades two of the key problems in studying these deposits are a lack of information about the structural and stratigraphic setting of the ore bodies and an absence of geochronological data from the ores in this paper we present geological maps for nearly a dozen former mines in the shay gap and goldsworthy belts on the northeastern margin of the craton and the first u pb geochronology for xenotime intergrown with hematite ore iron ore mineralisation in the studied deposits is controlled by a combination of steeply dipping ne and se trending faults and associated dolerite dykes simultaneous dextral oblique slip movement on se trending faults and sinistral normal oblique slip movement on ne trending faults during initial ore formation are probably related to e w extension uranium lead dating of xenotime from the ores using the sensitive high resolution ion microprobe shrimp suggests that iron mineralisation was the cumulative result of several proterozoic hydrothermal events the first at c 2250 ma followed by others at c 2180 ma c 1670 ma and c 1000 ma the cause of the first growth event is not clear but the other age peaks coincide with well documented episodes of orogenic activity at 2210 2145 ma 1680 1620 ma and 1030 950 ma along the southern margin of the pilbara craton and the capricorn orogen farther south these results suggest that high grade hematite deposits are a product of protracted episodic reactivation of a structural architecture that developed during the mesoarchean the development of hematite mineralisation along major structures in mesoarchean bifs after 2250 ma implies that fluid infiltration and oxidative alteration commenced within 100 myr of the start of the great oxidation event at c 2350 ma

archaean tectonic processes a case for horizontal shortening in the north pilbara granite greenstone terrane western australia abstract the archaean pilbara granitoid greenstone terrane ggt has been the focus of numerous studies on archaean geology especially the classic dome and basin area around marble bar in the east pilbara this area has been used as evidence for different tectonic processes i e that vertical tectonics or diapirism was a cause for archaean deformation this paper provides evidence to support regional horizontal plate interaction stresses as being largely responsible for the compressive deformation of the pilbara ggt at least from ca 3 2 ga the relative chronology of meso to macro scale structural elements are presented for a number of selected areas across the pilbara ggt these locally identified events are correlated with a regional pilbara wide structural framework of deformation events that are constrained by geochronological and stratigraphic controls the dome and basin geometry characteristic of the east pilbara was established after 3 2 ga and was successively modified by repeated orthogonal extensional and compressive subhorizontal events the result has been a locally complex development of polyphase structural elements with consistent overprinting relationships that can be correlated across much of the pilbara from 3 2 ga diapirism did not cause these deformation elements although it may have modified them

granite greenstone terranes in the pilbara block australia as coeval volcano plutonic complexes evidence from u pb zircon dating of the mount edgar batholith the mount edgar batholith in the pilbara block western australia consists in part of a gneiss complex that resembles other archean high grade gneiss terranes the complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession the warrawoona group the contact between the greenstone belt and the gneiss complex is tectonic field evidence indicates that the banded gneisses are the oldest part of the batholith zircon u pb ages for three samples are 3429 13ma 3443 10ma and3448 8ma all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession pidgeon 1978 1 two granitoids representing pre and post tectonic phases respectively give zircon ages of 3304 10ma and3314 13ma thereby constraining the major structural events to be contemporary with granite magmatism consistent with a diapiric model for the tectonic evolution of the batholith collins 1989 2 these ages are similar to the zircon age for terminal volcanism in the warrawoona group pidgeon 1984 3 but are 100ma older than the precisely defined rb sr whole rock ages obtained throughout the batholith collins and gray 1989 4 batholith wide resetting or late closure of rb sr isotopic systems is implied possibly associated with late fluid circulation the temporal coincidence between plutonism and volcanism in the mount edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the 3300ma granites and the 3440ma gneisses in the batholith the isotopic evidence combined with the structural data indicates that the granites cannot be derived from the surrounding greenstone belt further there is no compelling evidence for a sialic basement to the greenstone succession

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

fault and stratigraphic controls on volcanogenic massive sulphide deposits in the strelley belt pilbara craton western australia abstract early archaean fe zn cu volcanogenic massive sulphide deposits of the strelley belt pilbara craton occur at the top of a volcanic dominated sequence at the interface of felsic volcanic rocks and siliceous laminites beneath an unconformity overlain by clastic sedimentary rocks the structure of the sulphur springs and kangaroo caves vms deposits is relatively simple with the present morphology reflecting original deposition rather than significant structural modification the rocks have been tilted giving an oblique cross sectional view of discordant high angle deep penetrating faults in the footwall which splay close to the zones of volcanogenic massive sulphide mineralization faults do not extend far into the overlying sedimentary cover indicating their syn volcanic and syn mineralization timing both the sulphur springs and kangaroo caves sulphide deposits are located within elevated grabens in a setting similar to massive sulphide mineralization in modern back are environments mineralization at sulphur springs and kangaroo caves is located at the edge of the grabens at the site of intersecting syn volcanic extensional faults

the age and origin of younger granitic plutons of the shaw batholith in the archaean pilbara block western australia the whole rock pb pb method has been used to date four of the younger mainly adamellite late tectonic plutonic phases within the ca 3 5 ga shaw batholith of the archaean east pilbara block three suites give ages within error of 2966 ma porphyritic granites at 2948 50 ma leuco adamellites at 2943 46 ma and garden creek adamellite at 3007 48 ma the post tectonic cooglegong adamellite gives an age of 2847 34 ma the sm nd model isotopic systematics of all four suites indicate derivation from crust ranging between ca 3200 and 3600 ma in age the sources for these four younger plutonic phases were heterogeneous and although exhibiting some isotopic characteristics of the older 3 5 3 3 ga calc alkali plutonic suites were more depleted in the lil elements rb u and th in addition the garden creek adamellite and the cooglegong adamellite lack the very fractionated and hree depleted ree patterns characteristic of both the older calc alkali plutonic rocks and the porphyritic granites and leuco adamellites the crust underlying the shaw batholith at ca 2950 ma must have been both markedly heterogeneous and variably depleted a conclusion consistent with the complex tectonic and plutonic evolution of this region

tensile and compressive growth structures relationships between sedimentation deformation and granite intrusion in the archaean coppin gap greenstone belt eastern pilbara western australia abstract the coppin gap greenstone belt is composed of an east west striking asymmetric synclinorium between the muccan and mount edgar batholiths along the contact with the muccan batholith the vertical to overturned northern limb is intensely sheared folded and tectonically reduced the southern limb is relatively undisturbed and irregularly intruded by the mount edgar batholith progressive unconformities and sedimentary infilling of growth structures in the gorge creek group record the deformation history the unconformities constrain which of the now steeply dipping shear zones have to be rotated with their cover sequence before they can be interpreted kinematically four mid archaean clastic systems have been recognized the first and third are transgressive littoral quartzose and feldspathic arenites the second and fourth comprise conglomeratic and ferruginous alluvial fan related deposystems east verging thrust tectonics affected the belt after unconformable deposition of the first deposystem on the early archaean warrawoona group in the western extremity of the belt primary tectonic relief was filled with the second deposystem thrust folds were overprinted at right angles by folding sub parallel to the developing belt after truncation the belt was covered by the third and fourth systems in the eastern part of the belt relief created by belt parallel folds affecting the first deposystem was filled by conglomerate overlain by basalts a sequence also assigned to the whim creek group we relate systematic depocentre shifts as observed in the bamboo creek syncline to differential doming of the adjacent batholiths buried thrusts and shear zones may have caused km scale repetition of the underlying sequences the observed relationships place uncertainty on the validity of the existing stratigraphic correlations which now need geochronologic work to resolve part of the thrusting appears to have taken place by inversion on pre existing detachment shear zones in the underlying warrawoona basalts and ultramafic rocks the shear zones descend from a high stratigraphic level in the east towards deeper levels in the west the first motion has been west block down listric normal growth faulting in places accompanied by synsedimentary explosive intrusion of black chert veins mega brecciation and back tilt of fault blocks geometrically these early structures cannot be related to the geometry of batholith doming and are interpreted as the result of either regional tensile stress local caldera collapse or both mechanisms acting synchronously the mid archaean influx of siliciclastic sediments in the coppin gap belt coincides with this change from a tensile to a complex compressive deformation the latter caused by regional thrusting and interbatholith folding late deformation of the belt includes further steepening of the strata and eastward oblique dextral and 060 sinistral strike slip

regional alteration systems associated with volcanogenic massive sulfide mineralization at panorama pilbara western australia outstanding exposure across the panorama district of the archean pilbara block of western australia reveals a cross section through a ca 3 24 ga massive sulfide bearing volcanic pile and an underlying coeval subvolcanic intrusion in an area of low metamorphic grade and very low strain this has provided a rare opportunity to map a complete regional scale hydrothermal alteration system associated with volcanogenic massive sulfide mineralization and to assess the role of a subvolcanic intrusion in driving such a system four major alteration facies can be defined in the panorama district these are feldspar bearing background alteration and feldspar sericite quartz alteration and completely feldspar destructive sericite quartz and chlorite quartz alteration these alteration facies affect all of the volcanic pile and the upper parts of the underlying subvolcanic intrusion regional semiconformable alteration zones which are predominantly feldspar bearing are crosscut by broad corridors of transgressive feldspar destructive alteration a convective hydrothermal model is invoked to explain this distribution of alteration facies where semiconformable alteration zones represent seawater recharge and transgressive feldspar destructive alteration zones mark the path of evolved seawater discharging back to the sea floor the interpreted discharge sites are coincident with base metal sulfide deposits at the top of the volcanic pile geologic relationships and the distribution of alteration zones clearly implicate the strelley granite as the subvolcanic intrusion which drove the hydrothermal alteration systems granophyric textures and miarolitic cavities in the upper parts of the intrusion suggest that a magmatic fluid phase was exsolved early in its crystallization history granite hosted copper zinc tin veins near the top of the intrusion are interpreted to be a product of this magmatic fluid this raises the possibility of a substantial contribution of magmatically derived metal to the overlying volcanogenic massive sulfide hydrothermal alteration systems

origin of radioactive barite sinter flybye springs northwest territories canada cold springs emerging a long the contact between devonian limestone and shale units in the northwestern canadian cordillera are presently depositing a radium enriched barite sinter a geological cross section through the springs area shows that groundwaters could circulate through a mainly limestone aquifer to depths of approximately 2 km some shales and volcanic rocks associated with the aquifer contain barium bound in feldspars barite pyrite and organic matter hosted in shale and radium in feldspars or produced by the radioactive decay of uranium associated with organic matter hosted in shale spring waters are of the type characteristic of water that has equilibrated with clay minerals a subsurface equilibration temperature of 34 c was determined by silica geothermometry and 31 c by magnesium corrected na k ca2 geothermometry emerging waters are partly mixed with surface runoff and therefore these temperatures represent only minimum values assuming a normal geothermal gradient these tem

constraints on the age of the warrawoona group eastern pilbara block western australia abstract zircons from mafic and felsic volcanic rocks in the type area of the warrawoona group the basal archaean greenstone succession of the eastern pilbara block have been dated precisely using the ion microprobe shrimp the results allow two alternative time frames for the duration of the warrawoona group dependent on how the dated zircons are considered to relate to the volcanic rocks our favoured interpretation requires a hiatus of 135 5 ma between the duffer formation at 3 46 ga and the overlying felsic volcanic rocks of the wyman formation and a hydrothermal or later magmatic origin for zircons of age 3 33 ga within one duffer formation sample and the underlying metabasalts the alternative time frame requires a short time for deposition of the entire group less than 15 ma at 3 33 ga and a xenocrystic origin for the 3 46 ga zircons of the duffer formation outside the type area of the warrawoona group the age of an intrusive granodiorite requires that greenstones be older than 3 43 ga and the group formed over an interval of 120 ma visibly different zircons within one of the duffer formation samples were found to be palaeozoic in age and presumably constitute hydrothermal growth of new zircon within the rock at low temperature similar zircons were found within samples from other rock units but with a spread of proterozoic ages

modes of formation of anomalously high radioactivity in oil field brines many oil field brines contain anomalously high radium concentrations a theoretical model proposed here suggests that such concentrations can be a function of the geochemical environments of petroleum reservoirs or can be caused by uranium occurrences associated with oil pools the model suggests that on a triangular diagram with barium radium 228 and radium 226 as the axes brines associated with uranium occurrences will plot relatively far toward the radium 226 corner radium rich brines associated with normal mineral assemblages will fall relatively close to the ba ra 228 side of the diagram geochemical differentiation of the radium source may provide a useful tool in exploration for uranium accumulations in petroliferous areas

the diverse structure of archean lode gold deposits of the southwest mosquito creek belt east pilbara craton western australia the mosquito creek belt has been the largest source of metasediment hosted lode au in the southeast archean pilbara craton most of the deposits lie along the belt parallel structures of the middle creek line au and the blue spec line sb au which strike as a mostly open arc in an approximately east west orientation the metasedimentary rocks were multiply deformed with five deformation phases recognized despite being aligned along these trends in detail the deposits vary significantly in their mineralization style over short distances along strike these styles include a competence controlled strata bound disseminated style golden eagle a competence controlled crosscutting tensional vein fault style otways shearers and a shear zone hosted style barton s hopetoun north controls on mineralization can be attributed to changes in lithology and the partitioning of strain during late stage regional shortening d 4c that was oriented north northwest at around 2900 ma the development of veins and associated faults at a high angle to the main d 4c trend may reflect a later deformation event d 5 which resulted in brittle faulting under mostly north northeast compression the structural chronology of overprinting relationships established for the mosquito creek belt is consistent with that determined in other parts of the pilbara craton e g mallina basin this coherent structural pattern together with consistent mineralization ages suggests that far field forces controlled deformation and raises the prospectivity of the pilbara craton for lode au systems elsewhere e g the mallina basin

mineralization and regional alteration at the mons cupri stratiform cu zn pb deposit pilbara craton western australia the mons cupri cu zn pb deposit is hosted by conglomerate of the ca 2 965 ma cistern formation which forms part of the bookingarra group in the central pilbara tectonic zone of the pilbara craton the deposit has two distinct mineralized zones a 5 to 10 m thick stratiform zn pb rich semi massive sulphide lens which overlies a funnel shaped cu rich stringer zone the deposit is located 5 20 m stratigraphically below the contact of the cistern formation with the overlying rushall slate a high mg basalt the comstock member is located near the base of the rushall slate regional alteration mapping indicates that feldspar destructive alteration facies are restricted to the vicinity of the deposit with intense chloritic alteration restricted to the stringer zone and a narrow cross cutting zone that can be traced for 700 m to the east in the footwall of the deposit feldspar destructive zones are well developed in the comstock member indicating that mineralization occurred after deposition of the rushall slate the mons cupri stratiform semi massive sulphide lens is hosted by a conglomerate and textural evidence indicates that this lens formed by replacement these observations combined with pb isotope data suggest that the mons cupri deposit formed epigenetically at least 30 ma after deposition of its host rock

archean layered mafic ultramafic intrusions in the west pilbara craton western australia a synthesis of some of the oldest orthomagmatic mineralizing systems in the world considerable exploration interest has been generated by the platinum group element pge and ni cu potential of the archean layered mafic ultramafic intrusions in the west pilbara craton western australia the munni munni intrusion contains the largest resource of pge associated with a layered intrusion in australia and the radio hill and mount sholl intrusions host significant resources of ni cu co sulfides titaniferous magnetite layers remobilized sulfides and structurally controlled hydrothermal polymetallic deposits have also been a focus for exploration in recent years the ca 2 9 ga munni munni andover radio hill mount sholl and sherlock layered intrusions are a cogenetic suite of high level 5 kbars bodies that represent some of the oldest mineralizing systems of their type in the world although they display similar field relationships and mineralogical geochemical and isotopic features their contrasting chalcophile metal distribution patterns show that the timing and mechanism of the s saturation event were critical for the development of pge enriched sulfide bearing layers and basal segregations of base metal sulfides the intrusions form thick 5 5 km dikelike bodies or relatively thin 0 5 2 km sheets and sills emplaced at different levels along major lithological discontinuities in the upper crust rhythmically layered ultramafic components are generally thinner than and occur along the northern sides of more massive overlying mafic components the ultramafic zones consist of dunite lherzolite wehrlite olivine websterite clinopyroxenite and websterite inverted pigeonite gabbronorite magnetite gabbro olivine gabbro anorthositic gabbro and anorthosite comprise the mafic sequences olivine and clinopyroxene were generally the first minerals to crystallize except in the andover intrusion where orthopyroxene preceded clinopyroxene and possibly reflects greater contamination of the parent magma by felsic crustal material the crystallization of chromite was inhibited in the ultramafic zones by the partitioning of cr into early crystallizing clinopyroxene thus downgrading the potential for pge chromite associations the pge mineralization in the munni munni intrusion occurs in the upper levels of a porphyritic plagioclase websterite orthocumulate layer directly below the ultramafic gabbroic zone contact mineral compositional trends and nd isotope data indicate that a pd pt au enriched s undersaturated magnesian basaltic magma was frequently injected into a small magma chamber during formation of the ultramafic zone a major influx of more fractionated s saturated tholeiitic gabbroic magma related to the resident magnesian magma rapidly inflated the chamber and induced turbulent magma mixing that resulted in the formation of the pge bearing porphyritic websterite layer in contrast the parent magmas that formed the mount sholl radio hill andover and maitland intrusions were saturated in s before they were emplaced into the magma chambers in these intrusions gravitational and structural controls were important for the concentration of pge poor 5 400 ppb pt pd au massive ni cu co sulfides in depressions and structural embayments along the basal contacts beneath the thickest sequences of mafic ultramafic cumulates the parent magmas to the west pilbara intrusions were siliceous high magnesian basalts of al depleted komatiitic affinity barberton type with 9 to 12 percent mgo 15 to 25 ppm sc 12 to 18 ppm y low al2o3 tio2 ca 11 or half chondrite ratios and light rare earth enrichment chondrite normalized la sm 2 7 la lu 9 0 they were generated with garnet in the residual asthenospheric mantle with probable involvement of a pre 3 0 ga subduction modified lithospheric mantle isotopic and geochemical modeling suggests that the magmas were contaminated by ca 3 0 to 3 3 ga archean tonalitic to granodioritic crust before being emplaced into high level magma chambers

u th pb isotope data indicate phanerozoic age for oxidation of the 3 4 ga apex basalt abstract the occurrence of ferric oxides in archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient earth recognizing that post archean weathering may also produce oxide minerals drill cores have been used to obtain samples beneath the depth of phanerozoic weathering the first core of the archean biosphere drilling project abdp 1 documented hematite as alteration products in 3 4 ga basalts from the marble bar area of the pilbara craton nw australia and this has been used to infer the presence of an o2 bearing atmosphere in the archean it is possible however that despite recovery of samples from 100 m depth oxidation of the basalts occurred much younger than the depositional age in this study the age of oxidation of the apex basalt from the abdp 1 drill core at marble bar is constrained by u th pb geochronology lead and u concentrations of the basalts from the abdp 1 drill core vary greatly between

evidence for diachronous archean lode gold mineralization in the yilgarn craton western australia a shrimp u pb study of intrusive rocks the currently accepted model for the archean lode gold deposits of the yilgarn craton postulates that they represent a coherent group of epigenetic deposits the majority of which formed during a craton scale broadly synchronous hydrothermal event late in the tectonothermal evolution of the granite greenstone terranes at ca 2640 to 2630 ma felsic rocks from the southern eastern goldfields which host or are cut by gold mineralization have shrimp ii u pb zircon ages of 2673 or 3 ma at mount charlotte 2669 or 17 ma at mount percy 2663 or 3 ma at racetrack and 2657 or 8 ma at porphyry all these ages are consistent with gold mineralization at ca 2640 to 2630 ma intermediate to felsic dikes cut typical syn to postmetamorphic lode gold mineralization at the mount mcclure and jundee deposits in the yandal greenstone belt in the north of the kurnalpi terrane the dikes give ages of 2656 or 4 2663 or 4 and 2668 or 10 ma from mount mcclure and 2656 or 7 ma from jundee requiring that mineralization and peak regional metamorphism in the belt occurred prior to ca 2660 ma however both the characteristics of the jundee and mount mcclure deposits and the relative timing of mineralization with respect to the metamorphic and structural history of the belt are similar to that seen for gold deposits elsewhere in the yilgarn craton this implies that mineralization at jundee and mount mcclure was produced prior to 2660 ma by similar processes to those seen elsewhere in the yilgarn at 2640 to 2630 ma peak metamorphism in the western higher metamorphic grade terranes of the yilgarn was not reached until ca 2630 ma some 10 to 30 m y after peak metamorphism in the kalgoorlie terrane and more than 30 m y after metamorphism in the yandal belt in addition almost all of the published robust ages supporting gold mineralization at ca 2640 to 2630 ma are from the west of the craton consideration of the new data from the yandal belt in conjunction with previously published geochronology throws doubt on the hypothesis that lode gold mineralization occurred approximately synchronously across the yilgarn craton rather it suggests that mineralization along with regional metamorphism is earlier by at least 30 m y in the northeastern yilgarn craton

extensional structures during deposition of the 3460 ma warrawoona group in the eastern pilbara craton western australia abstract structures in the granite greenstone terrain of the eastern pilbara were mapped in both the coongan belt greenstones and shaw batholith granitoids important structures that are coeval with an early magmatic event in both the granitoids and the greenstones were identified in the coongan greenstones a brittle extensional fault array is syn kinematic with felsic volcanics of the duffer formation dated at 3452 16 ma pidgeon 1978a in the underlying shaw batholith a major ductile shear zone was mapped at the contact with the coongan belt although the foliation in the shear zone forms part of a domal structure the stretching lineations constantly plunge to the ene the sense of shear as recorded in the shear zone is consistently east up with subsidiary sinistral strike slip movement on the e w trending part and dextral movement on the n s trending part a wedge shaped granitoid intruded into the shear zone during deformation this granitoid is part of the north shaw suite which has been dated at 3467 6 ma mcnaughton et al 1988 the structures in the greenstones and in the batholith have therefore formed during the same event at different crustal levels the structural kinematic magmatic metamorphic and geochronological data are consistent with a model in which these structures represent the downdip part of a dome with a core complex type of geometry in turn this implies that the domal geometry of the shaw batholith was initiated at mid crustal levels at a very early stage namely during deposition of the warrawoona group in the evolution of the pilbara craton as extension occurred during intrusion and extrusion of large volumes of magma the original distribution and thickness of granitoid bodies and greenstones were probably largely determined by the extensional geometry the initial domal shape of the shaw batholith has been enhanced during later deformation phases which caused steepening of n s trending structures

partial convective overturn of archaean crust in the east pilbara craton western australia driving mechanisms and tectonic implications strain varies systematically from weakly developed outward dipping s tectonites in the 3320 3310 10 ma mount edgar batholith to intensely deformed subvertical l tectonites in greenstones of the warrawoona syncline a consistent greenstone down batholith up sense of shear is recorded in batholithic domal margins and adjacent high grade supracrustal rims lineations converge to a central zone of subvertical extension zone of sinking along the synclinal axis at domal margins early kinematic granitoid sheets and intrusive diatexites are subconcordant to a well developed dome parallel schistosity but late to post kinematic intrusives are discordant high level plutons all granitoids are the same age within analytical error these syn doming features conform with structural tests for diapirs and differ from those expected during metamorphic core complex formation and cross folding diapirism is part of a larger process involving partial convective overturn of the crust based on strain patterns and kinematic criteria we argue that deformation was initiated by sinking of greenstones which rapidly subsided the domes then rose passively as accommodation structures ongoing doming d4 partitioned strain to the southern rim of the batholith and uplifted a wedge of the warrawoona syncline keel we consider that crustal overturn occurred in response to mantle plume activity eruption of a 5 10 km thick 3325 ma mafic ultramafic greenstone pile onto an older granite greenstone terrain created a negatively buoyant crust but convective overturn occurred some 5 20 ma later triggered by widespread anatexis thermal softening convective overturn may have been a common archaean process occurring in response to a hotter mantle and represents an end member deformation mechanism that includes thrust accretion of lithotectonic assemblages in other archaean terranes such as the superior province of canada

evidence for multiphase deformation in the archean basal warrawoona group in the marble bar area east pilbara western australia abstract the results of a structural analysis of the marble bar greenstone belt in the mcphee reward area of the eastern pilbara block in western australia indicate that the area has undergone a multiphase deformation on shears which approximately parallel the trace of stratigraphy cf primary igneous layering around the talga talga anticline early extensional movements were associated with the formation of the warrawoona group chiefly the duffer formation and indicate a wnw ese extensional direction similar to the syn warrawoona extension found in the area of the north shaw batholith extension was followed by thrusting with transport to the east the talga talga anticline is a culmination which formed during this event by thrust stacking a major shear in the area the duffada shear was reactivated during granodiorite emplacement with transport to the ne in their present geometry the shears have thrust movements on the south side of the talga talga anticline but normal movements on the north side after the reactivation came a nw se shortening which was accompanied by conjugate shear activity on north trending sinistral and west trending dextral shears and constrained to 2950 ma in the north shaw area the final movements were associated with a n s shortening which resulted in an e w trending thrust which cuts the northern flank of the talga talga anticline and nw and ne trending conjugate faults there is no evidence of radially oriented stretching lineations or slickensides which would be expected if the granitoid bodies which make up the mount edgar batholith were emplaced by a solid state diapiric process the net result of deformation was tectonic disruption of the stratigraphic stacking within the type ara of the talga talga subgroup in the warrawoona group this may offer an additional explanation for anomolously young dates of the talga talga subgroup with respect to the duffer formation and also indicates that the type section for the talga talga subgroup is a lithotectonic sequence

the waning stage of a greenstone belt the mesoarchaean mosquito creek basin of the east pilbara western australia abstract the mosquito creek basin stands apart in the archaean granite greenstone terrane of the east pilbara craton because of the rectangular outcrop but crescent shaped subcrop and its composition of low grade metamorphic mainly turbiditic clastic sedimentary rocks these sediments belong to the c 2 9 ga mosquito creek formation tectonic interpretations of the basin have varied from a simple synclinorium to an accretionary prism an extensional passive margin and more recently into a rift basin separating the kurrana terrane from the east pilbara terrane ept combined sedimentological and structural geological investigations show coarse clastic fan delta marginal facies with stacked unconformities rimming an e w basin centre with predominantly finer grained mass flow dominated sediments a n s traverse therefore contains a complete cross section of the mosquito creek basin shortened to about 50 of its original width by tight folding and thrusting the basin fill shows one major fu sequence from fan delta sandstone to distal turbidites unconformably overlying an older partially preserved clastic sequence of comparable facies a large slump unit occupies the basin axis the unconformities along the margins interfere with folding of the sediments which also influenced basin centre turbiditic palaeocurrents the basin is interpreted as a near symmetrical underfilled compressional intramontane clastic sedimentary basin unconformably superposed on a volcanic greenstone belt rock assemblage with enclosed low angle shear zones compression started during sedimentation under conditions of net subsidence within a rim syncline between adjacent domes this occurred after phases of low angle extensional and compressional deformation of the substrate not related to the present day geometry of the granitoid domes the geological setting of the mosquito creek basin does not differ fundamentally from that of other adjacent e woriented greenstone belts of the ept the basin is inferred to represent the last stratigraphically uppermost and because of subsequent erosion rarely preserved stage of greenstone belt evolution

upb zircon geochronology of archaean felsic units in the marble bar region pilbara craton western australia abstract archaean supracrustal rocks that are well exposed in the marble bar region pilbara craton have been assigned to the warrawoona group and younger sequences the predominantly volcanic warrawoona group previously dated at 3300 to 3500 ma is largely basaltic with locally intercalated thick felsic volcanic units the supracrustal rocks have been folded and are distributed around the margins of large roughly circular to ovoid granitic gneissic batholiths a u pb zircon geochronology study was undertaken to obtain precise age constraints for some of the ore deposits in the area especially the big stubby north pole barite and miralga creek zn pb cu au deposits in support of efforts to improve archaean lead isotope models the results also help significantly in interpreting stratigraphic relationships and crustal evolution in the area the new conventional zircon u pb data indicate that most of the warrawoona group from the duffer formation upwards was deposited over a period of 20 ma from 3470 ma to 3450 ma the age of the duffer formation is closely constrained by results of 3471 5 ma and 3465 3 ma and the age of the upper salgash subgroup appears to be established at 3454 1 ma the panorama formation at the southern margin of the north pole dome is 3458 1 9 ma unless the duffer formation was deposited over a period of at least 10 ma this casts doubt on a previously suggested panorama duffer correlation the age of the lower part of the north pole succession is greater than 3458 ma consistent with the correlation of the basal greenstones at north pole with the talga talga subgroup the north pole chert barite unit which is well known for its preservation of early archean stromatolites microfossils and evaporite sediments is clearly older than 3458 ma the 3325 ma age now established for the wyman formation shows that this unit must be excluded from the warrawoona group the age of the talga talga subgroup remains uncertain a felsic schist in the mount ada basalt which has been dated at 3449 3 ma is interpreted to be a sill probably related to the 3450 ma granitoid bodies an age 3724 ma for a zircon xenocryst in the panorama formation is the oldest obtained so far for zircon from the pilbara granite greenstone terrane and indicates the existence of pre pilbara supergroup sialic basement ages of 3465 3 ma and 3449 2 ma applied to previous pb isotope data for the big stubby and miralga creek deposits respectively in combination with comparable data for other syngenetic archaean sulphide deposits yield a general single stage pb evolution model with the parameters t0 4560 ma a0 9 0818 b0 9 9002 and c0 29 343 this gives model ages in accord with known constraints for many archean deposits a model lead age of 3490 ma for the north pole deposit which is older than 3457 ma may be too great but attempts to directly date this deposit have not yet succeeded there are striking parallels between the warrawoona group ages reported here and those recently obtained for the barberton region kaapvaal craton south africa the talga talga subgroup and lower onverwacht tjakastad subgroup sequences have approximately equivalent constraints and an age for the upper salgash subgroup agrees closely with those for the hoogenoeg formation of the upper onverwacht group geluk subgroup

hyperspectral mapping of mineral assemblages associated with gold mineralization in the central pilbara western australia the indee district located in the central pilbara tectonic zone of the north pilbara terrain in western australia contains archean lode gold deposits and epithermal gold systems hydrothermal alteration assemblages associated with these deposits contain the minerals pyrophyllite white mica chlorite and tourmaline known prospects are surrounded by flat areas of poorly exposed calcretized bedrock calcretes rock float and sandy soils two adjacent lines of airborne hyperspectral data hymap were acquired to produce mineral abundance maps these data were processed to correct for atmospheric and illumination conditions and then calibrated to reflectance data using field spectra this analysis successfully mapped the distribution of pyrophyllite white mica mg chlorite fe chlorite calcite dolomite kaolinite tourmaline hematite and goethite alteration assemblages associated with the camel 2 deposit which is located along the mallina shear zone were found to be characterized by pyrophyllite and al rich white mica the latter having an aloh absorption feature at 2 194 m in contrast alteration assemblages associated with epithermal deposits south of the mallina shear zone are characterized by al poor white mica with a longer wavelength absorption feature at 2 210 m the shift reflecting the change in al chemistry tschermak substitution tourmaline is also present near the epithermal like deposits suggesting that it may be an integral part of these systems fe chlorite is associated with both the mesothermal and epithermal systems but is also pervasive as a regional metamorphic mineral regolith related carbonates are common throughout the area and their compositions are determined by the hyperspectral processing mineral abundance images were verified by pima portable infrared mineral analyser analysis of ground samples and assays of rocks in previously unknown areas of pyrophyllite and white mica confirmed the presence of anomalous gold these results show that mineral maps and compositional information derived from hyperspectral analysis are extremely valuable for exploration even in poorly exposed regolith dominated districts such as indee

early archean hot springs above epithermal veins north pole western australia new insights from fluid inclusion microanalysis the world s most ancient biogenic structures are found in the north pole dome of western australia where 3 47 gyr old algal mats and stromatolites are closely associated with bedding conformable and discordant laminar quartz chalcedony and barite barite rich quartz hydrothermal veins with similar mineralogy occur throughout the stratigraphy below the conformable biogenic structures with the exception of the large volume of barite these bedding conformable and discordant laminar quartz veins exhibit textures and associated hydrothermal alteration quartz chalcedony chlorite illite calcite adularia pyrite typical of epithermal deposits formed from near neutral ph fluids we characterize the physical and chemical conditions of the ancient water responsible for depositing both the discordant and conformable quartz chalcedony barite as it passed through the upper parts of the archean crust field relationships combined with new fluid inclusion data suggest that the best documented stromatolites in the north pole dome occur adjacent to quartz chalcedony bands formed from cool 120 c low salinity 3 wt nacl equiv waters higher temperature up to 300 c more saline up to 10 wt nacl equiv and co2 h2s rich ch4 aqueous fluids occur in deeper level veins rare inclusions that are unusually rich in co2 containing liquid and gaseous co2 and liquid h2o support the existence of multiple batches of hydrothermal fluids with variable densities and gas contents oxygen isotope data 8 7 3 7 suggest that the causative fluids comprised admixtures of deeply circulated surface water with variable input of magmatic components our findings reveal that the earliest life known on earth lived in and around a hydrothermal system with temperatures from 300 c at depth to 120 c near the paleosurface in an environment closely analogous to modern hot springs developed above epithermal veins evidence exists for the introduction of different batches of hydrothermal fluids with variable densities and gas contents during the development of veins these findings support previous studies that demonstrate that the processes that form epithermal deposits have been active throughout geologic time and the present day distribution of epithermal deposits is dominantly a result of preservation not process

melting of archaean sialic crust under high ah2o conditions genesis of 3300 ma na rich granitoids in the mount edgar batholith pilbara block western australia the voluminous 3300 ma old granitoids of the mount edgar batholith pilbara block western australia are typically trondhjemites and silicic tonalites with sio2 ranging from 65 75 wt they have high sr average 425 ppm at 70 6 sio2 and na2o 3 5 5 0 wt contents with k2o rapidly increasing 1 0 4 5 wt with increasing silica so that the most fractionated are granodioritic a lack of restite minerals calcic plagioclase cores and mafic microgranitoid enclaves suggests that the granitoids were produced from chemically evolved sources and that they crystallised from melts approximating the bulk composition of the batholith 70 6 wt sio2 based on experimental data mass balance and trace element modelling these silicic melts are considered to have been produced by high degrees of partial melting involving complete plagioclase dissolution of tonalitic dacitic precursors similar in composition to the dacites of the 3450 ma greenstone succession the data are consistent with field and isotopic evidence from the gneiss complex of the batholith where orthogneisses have formed virtually in situ from older 3450 ma banded tonalitic gneisses that resemble coeval metamorphosed felsic volcanic rocks of the duffer formation or deeper level equivalents a number of granitoids from other batholiths of the pilbara block are geochemically similar to the mount edgar batholith and probably were derived from similar sources the following three stage model of crustal growth is envisaged 1 production of a thick primitive mafic crust at 360 ma remnants of which exist as gabbroic anorthosite xenoliths in some granitoids mcnaughton et al 1988 2 partial melting of the mafic crust at 3500 3450 ma to produce tonalitic dacitic rocks at shallow mantle depths 60 km associated with formation of the major greenstone belts of the pilbara block 3 infracrustal melting of the tonalitic dacitic rocks at 3300 ma to produce the abundant silicic tonalitic trondhjemitic sodic granitoids uniformitarian and non uniformitarian tectonic models involving arc magmatism rifting mantle plumes and delamination are discussed for the three stage crustal growth process but the available geological evidence does not favour any particular model the data indicate that archaean granitoids are similar to those produced in modern subduction related environments but the process of archaean magmatism might have been quite different

radium isotopes in saline seepages south western yilgarn western australia abstract water samples from saline seepages in the south western yilgarn block of western australia contain high activities of the four naturally occurring radium isotopes activities of up to 310 pci l for 226ra and 1720 pci l for 228ra were measured and the 228 ra 226 ra ratio averaged 6 1 activities of the two short lived radium isotopes were also high 223ra activities of up to 94 pci l were found with an average 226 ra 223 ra ratio of 3 3 considerably lower than the natural abundance ratio of 21 4 activities of up to 23 pci l 227ac the long lived t 1 2 22 years grandparent of 223ra were also measured the analysis of surface granite samples the probable source rocks of the radium gave th u activity ratios of around 1 5 the higher 228 ra 226 ra ratios of the waters were attributed to readily leached 228ra in the weathered granites as a result of thorium remaining after weathering leach experiments on u th ore by nacl solutions showed that all four radium isotopes were equally leached sulphate anions reduced the 226ra and 228ra leaching to a greater extent than for 223ra and 224ra suggesting that the latter isotopes were being supported in solution by parent isotopes in particular this suggested 227ac was leached into the sulphate solution but this does not fully account for the amount of 227ac seen in the seepage waters

the source of radium in anomalous accumulations near sandstone escarpments australia abstract the source of ra has been determined in water samples from four areas in australia where anomalously high surface concentrations of 226 ra have accumulated from groundwaters all four anomalies were located adjacent to sandstone formations and the groundwaters which were generally all acidic and low in dissolved salts appeared to be meteoric water with short ground residence times uranium 226 ra and 228 ra concentrations of waters feeding the anomalous areas were comparable to those found in standing waters within the sandstones the 226 ra 228 ra isotopic ratios were distributed about a median of 1 1 which suggests that the waters are in contact with rocks with near normal u th ratios and hence that the ra in the anomalies was derived from within the sandstones the presence of the short lived ra isotopes 223 ra and 224 ra in high concentrations in most spring waters feeding these anomalies suggests that ra enters groundwaters by recoil following alpha decay of a precursor parent radionuclide within mineral grains thus although three of the areas were considered prospective for u the radioactive anomalies studied appear to be due to natural transfer of ra from the sandstones to the surface environment in no case were the anomalies related to nearby known or undiscovered u deposits accordingly a geochemical procedure which includes ra isotopic measurements is recommended for evaluating radioactive anomalies for u exploration this procedure should enable selection of only those anomalies with the highest potential for further exploration by more expensive techniques

stratigraphy and sedimentology of an early archean felsic volcanic sequence eastern pilbara block western australia with special reference to the duffer formation and implications for crustal evolution abstract a sequence of early archean felsic volcanic and volcaniclastic rocks and chert is present in the upper half of the warrawoona group throughout greenstone belts in the eastern pilbara block western australia it includes three formations the duffer formation which makes up the bulk of the sequence consists of up to 5 km of metamorphosed dacitic to rhyolitic volcaniclastic breccia and lava and lesser amounts of tuffaceous rocks and mafic lava it is overlain in several sections by the sandy panorama formation and both are capped by silicified orthochemical and biogenic deposits of the towers formation including both the strelley pool and marble bar cherts these units are identifiable in at least six greenstone belts in the eastern pilbara block the duffer formation is divided into four facies a structureless and graded breccia b complexly organized conglomerate c turbiditic tuff breccia and tuff and d lava these facies represent deposition on predominantly subaqueous volcaniclastic aprons flanking felsic volcanic centers a three stage model of the evolution of these aprons includes 1 an early stage of felsic and mafic eruptions and subaqueous deposition 2 a middle stage during which the bulk of the sequence was deposited involving shallow water to subaerial felsic volcanism and subaqueous deposition and 3 a terminal stage of subaerial felsic eruptions and subaerial deposition post duffer felsic volcanism was marked by explosive pyroclastic activity which resulted in widespread deposition of ash in surrounding shallow water and subaerial systems this phase of activity is represented by the panorama formation after the cessation of volcanism erosion of the emergent vents and regional subsidence led to an interval of orthochemical and biogenic sedimentation represented by the towers formation paleocurrent and facies patterns in the duffer and panorama formations support earlier proposals that the intrusion of granitoid rocks in the eastern pilbara block was coeval with felsic volcanism it seems likely that this volcanism was the surficial expression of plutonism that represented early stages of cratonization in the eastern pilbara the calc alkaline volcanism and depositional styles represented by this sequence are similar to those associated with many phanerozoic arcs the abundance of shallow water facies closely spaced and possibly non linear distribution of felsic centers close association of felsic units with thick sequences of basalt erupted at or near sea level sparse syn volcanic deformation and lack of granitic detritus suggest however that the setting of this archean felsic volcanism was in many details unlike that of most modern arcs

lode gold and epithermal deposits of the mallina basin north pilbara terrain western australia the mallina basin in the central west part of the north pilbara terrain is one of the more mineralized parts of the pilbara craton three styles of au mineralization are present in the region 1 lode au deposits associated with sericite carbonate pyrite alteration assemblages 2 lode au deposits associated with pyrophyllite bearing alteration assemblages and 3 lode sb au deposits gold resources have been identified at the withnell and camel deposits just to the north of the east trending mallina shear zone east trending mineralized zones at the withnell deposit are typical of turbidite hosted lode au deposits in that they are late tectonic syn to late local d3 and associated with extensive carbonate sericite alteration gold mineralization occurs in planar quartz carbonate veins and with pyrite and arsenopyrite in the host rocks to the veins fluid inclusion analyses indicate mainly low salinity carbonic inclusions with moderate homogenization temperatures the lode sb au deposits have a similar structural setting and fluid characteristics at the camel 2 deposit au is associated with syn to late d3 pyrophyllitic shear zones that have overprinted an earlier carbonate alteration assemblage structural relationships and limited pb isotope data suggest that the lode au mineralization in this district formed at about 2900 ma in addition to the lode au and sb au deposits north northwest trending quartz veins have textures typical of low sulfidation epithermal deposits the lithogeochemistry of samples collected from these veins indicates local enrichment of au as sb ag te and hg which is consistent with an epithermal origin based on geologic relationships and pb isotope data these deposits are interpreted to have formed in the mallina basin at ca 2750 ma if they are true epithermal deposits then they are among the oldest known their presence suggests that preservation not archean age is the key control on the temporal distribution of epithermal deposits

review secular tectonic evolution of archean continental crust interplay between horizontal and vertical processes in the formation of the pilbara craton australia the archean pilbara craton contains five geologically distinct terranes the east pilbara karratha sholl regal and kurrana terranes all of which are unconformably overlain by the 3 02 to 2 93 ga de grey superbasin the 3 53 3 17 ga east pilbara terrane ep represents the ancient nucleus of the craton that formed through three distinct mantle plume events at 3 53 3 43 3 35 3 29 and 3 27 3 24 ga each plume event resulted in eruption of thick dominantly basaltic volcanic successions on older crust to 3 72 ga and melting of crust to generate first tonalite trondhjemite granodiorite ttg and then progressively more evolved granitic magmas in each case plume magmatism was accompanied by uplift and crustal extension the combination of conductive heating from below thermal blanketing from above and internal heating of buried granitoids during these events led to episodes of partial convective overturn of upper and middle crust these mantle melting events caused severe depletion of the subcontinental lithospheric mantle making the ep a stable buoyant unsubductable continent by c 3 2 ga extension accompanying the latest event led to rifting of the protocontinent margins at between 3 2 and 3 17 ga after 3 2 ga horizontal tectonic forces dominated over vertical forces as revealed by the geology of the three terranes karratha sholl and regal of the west pilbara superterrane the c 3 12 ga whundo group of the sholl terrane is a fault bounded 10 km thick volcanic succession with geochemical characteristics of modern oceanic arcs including boninites and evidence for flux melting that indicate steep archean subduction at 3 07 ga the 3 12 ga sholl terrane 3 27 ga karratha terrane and c 3 2 ga regal terrane accreted together and onto the ep during the prinsep orogeny this was followed by development of the de grey superbasin an intracontinental sag basin and widespread plutonism 2 99 2 93 ga as a result of orogenic relaxation and slab break off craton wide compressional deformation at 2 95 2 93 ga culminated with 2 91 ga accretion of the 3 18 ga kurrana terrane with the ep this compression caused amplification of the dome and keel structure in the ep final cratonization was effected by emplacement of 2 89 2 83 ga post tectonic granites

ground water geochemistry of 228ra 226ra and 222rn abstract 228ra 226ra and 222rn activities were determined on over 150 ground water samples collected from drilled public water supply wells throughout south carolina a wide range of aquifer lithologies were sampled including the crystalline rocks of the piedmont and sedimentary deposits of the coastal plain a significant linear relationship between log 228ra and log 226ra n 182 r 0 83 was indistinguishable between piedmont and coastal plain ground water median 228 ra 226 ra activity ratios for the piedmont 1 2 and coastal plain 1 3 ground water are close to estimated average crustal 232 th 238 u activity ratios of 1 2 to 1 5 corresponding to th u weight ratios of 3 5 to 4 5 a linear correlation was also found between log 222rn and log 226ra for piedmont n 68 r 0 62 and coastal plain n 89 r 0 64 ground water however the median 222 rn 226 ra activity ratio for piedmont ground water 6100 was much higher than for coastal plain ground water 230 higher excess 222rn activities may be due to greater retention of 226ra by the chemically active piedmont aquifers compared to the more inert sand aquifers sampled in the coastal plain the relationship between log 228ra and log 226ra was used to predict total ra 228 ra 226 ra distributions in appalachian and atlantic and gulf coastal plain ground water predictions estimate that 2 4 of appalachian and 5 3 of atlantic and gulf coastal plain ground water supplies contain total ra activities in excess of the 5 pci l limit established by the u s environmental protection agency these predictions also indicate that 40 50 of these ground water wells may be overlooked using the presently suggested screening activity of 3 0 pci l of 226ra for 228ra analysis

genetic framework for the classification and distribution of archean rare metal pegmatites in the north pilbara craton western australia the pilbara pegmatite province in the archean north pilbara craton contains at least 120 pegmatite deposits in over 27 pegmatite groups and fields including the giant mount cassiterite tantalum orebody in the wodgina pegmatite district generally rare metal pegmatites in the province are hosted by mafic ultramafic volcanic dominated supracrustal sequences of predominantly greenschist facies adjacent to and rarely within domal multiphase granitoid gneiss complexes the most fractionated pegmatites are hosted by tongues or inliers of greenstone belts protruding into or within the larger granitoid complexes most of these are within two of the six tectonostratigraphic domains of the east pilbara granite greenstone terrane of the north pilbara craton they tend to be clustered along and within 5 km at surface of major faults and craton scale lineaments that coincide with or are parallel to domain boundaries all of the major tantalum deposits wodgina mount cassiterite tabba tabba strelley pilgangoora are along a north northeast trending corridor that is within one tectonostratigraphic domain most rare element pegmatites are within 5 km at surface of their apparent parent pluton and all are within 10 km where the pluton has been identified although granitic rocks of the north pilbara craton were emplaced over an 800 m y period 3600 2800 ma rare element class pegmatites can be tied to a number of post tectonic plutons of a younger granite suite that were emplaced into most of the granitoid complexes and adjacent greenstone belts at 2890 to 2830 ma subtle differences in the composition of these granites that have given rise to different petrogenetic pegmatite suites can be identified in processed radiometric data from airborne surveys these parental younger granites are enriched in lithophile and volatile elements and have a highly fractionated character that permitted formation of significant pegmatite mineralization they represent the culmination of the crustal evolution of the north pilbara craton and the onset of cratonization as part of a global appearance of rare metal pegmatite mineralization post 3 ga

age source and stratigraphic implications of pb isotope data for conformable sediment hosted base metal deposits in the proterozoic aravalli delhi orogenic belt northwestern india abstract stratiform sediment hosted base metal deposits in the aravalli delhi orogenic belt of northwestern india are hosted by the sediment dominated bhilwara aravalli and delhi sequences of proterozoic age pb isotope data for 36 samples from five ore districts provide useful stratigraphic and approximate age information model pb ages using a newly developed proterozoic model are 1800 ma for the rampura agucha rajpura dariba and saladipura deposits 1700 ma for the zawar deposits and 1100 ma for the ambaji and deri deposits mineralization in this orogenic belt thus occurred at three stages in middle to upper proterozoic times the oldest of these reflected by deposits in the bhilwara sequence was of broad regional extent the rampura agucha deposit is thus hosted by highly metamorphosed rocks that belong to the bhilwara belt and not to an older supracrustal component of the archean banded gneissic complex the host rocks to the saladipura deposit are bhilwara equivalent and are not part of the delhi supergroup as previously mapped and therefore stratigraphic assignments in the area must be re examined model ages of 1100 ma for the ambaji deri deposits are the only evidence obtained in this study regarding the age of the delhi supergroup and are in apparent conflict with much older ages indicated by previous studies rocks of such late upper proterozoic age may be restricted to the southwestern segment of the orogenic belt or alternatively thrust stacking and or interfolding of lithologically similar sequences of a wide range of ages throughout the belt may not yet be evident because of the limited age and isotopic data presently available all the data can be readily explained by the extraction of pb from sediments derived from an upper crustal basement the isotopic compositions of the zawar deposits provide the clearest evidence for an ancient u enriched upper crustal source the ambaji deri pb data suggest a mature status of the delhi are when these deposits were formed

episodic concentration of gold to ore grade through earth s history abstract concentration of gold to economic ore grades has been highly episodic through earth s history with different sedimentary magmatic and hydrothermal processes having operated as principal driving forces at different times comparison of the various types of gold deposits their secular and spatial distribution and the amounts of gold contained in them combined with thermodynamic considerations reveals that the formation of gold deposits reflects fundamental changes in environmental conditions biological evolution and styles of large scale tectonism over the past four billion years here i suggest that most of the gold in the earth s crust was first concentrated at around 2 9 billion years ga ago by early photosynthesising microbes under an oxygen deficient atmosphere sedimentary reworking of auriferous microbial mats provided the source for the richest known gold palaeoplacers between 2 9 and 2 7 ga and an overall gold enrichment of coeval marine sediments only after modern style plate tectonics had begun to operate sediments enriched in gold had been accreted along active plate margins subducted and subcontinental lithospheric mantle had been correspondingly metasomatised did the formation of larger orogenic porphyry type and epithermal and to a lesser extent also other hydrothermal gold deposits become possible thus the principal secular peak in orogenic gold formation at 2 75 2 55 ga can be explained

chronological correlations between the pilbara and kaapvaal cratons abstract the early geological development of the pilbara and kaapvaal cratons has many features in common attempts have been made to correlate geologically similar features of the two cratons and it has been postulated that they originated as contiguous components of a single continent vaalbara during this time the early geological histories of the pilbara and kaapvaal cratons are here compared in detail and the evidence that they were initially contiguous is assessed these comparisons indicate significant differences in the chronologies of magmatic events within the granite greenstone crusts of the pilbara and kaapvaal cratons in addition igneous correlatives emplaced during ca 2985 and 2782 ma magmatic events on the kaapvaal craton have not been identified on the pilbara craton and a well defined 2760 ma magmatic event manifest as widespread emplacement of granitic rocks into the pilbara granite greenstone basement and eruption of flood basalts of the lower part of the fortescue group is absent from the kaapvaal craton furthermore similarities in first and second order transgression regression cycles within the sedimentary supracrustal sequences may be attributable to global sea level fluctuations and thus may be irrevelant to the question of former contiguity however similarities in some aspects of the geological development of the pilbara and kaapvaal cratons imply that there were periods extending for between 60 and 200 ma of the archaean era during which the style of crust formation intensity of volcanism and subaerial erosion and magnitude of sea level fluctuations may have varied on a global scale such similarities include the overall duration of formation of the granite greenstone crusts from ca 3650 to 3100 ma the onset of craton wide erosion in the interval ca 3125 to 3000 ma the major episodes of flood basaltic volcanism between 2760 and 2680 ma the predominance of chemical carbonate and banded iron formation sedimentation between ca 2630 and 2440 ma and the transition to widespread clastic sedimentation within the interval 2440 to 2200 ma

the timing of mineralization in the archean north pilbara terrain western australia mineralizing events in the north pilbara terrain of western australia occurred between 3490 and 2700 ma and include the oldest examples in the world of many ore deposit types these events were pulsed and associated with major volcano plutonic episodes volcanic hosted massive sulfide porphyry cu sn ta pegmatite mafic ultramafic hosted ni cu pge and epithermal deposits and deformation events lode au deposits in many cases the mineralizing events are associated with extension either in rifts pull apart or back arc basins or as the consequence of doming associated with granitoid diapirism although mineralizing events occurred throughout the evolution of the north pilbara terrain the largest deposits are related to the development of the mallina basin which is located between the older east and west pilbara granite greenstone terranes four volcano plutonic and three deformation events occurred in and around the mallina basin between 2950 and 2840 ma mineralization in the east and west pilbara granite greenstone terranes is less intense and occurred over a much longer period compared to other archean granite greenstone terrains the north pilbara terrain is poorly endowed the only known world class deposit in this region is the wodgina ta sn pegmatite deposit but large iron ore deposits are being mined in the northeastern north pilbara terrain the long history of crustal development and slow rate of crustal growth may be responsible for the diversity of mineral deposits and could account for the apparent poor endowment of the north pilbara terrain

timing of gold mineralization in the mt york district pilgangoora greenstone belt and implications for the tectonic and metamorphic evolution of an area linking the western and eastern pilbara craton abstract hydrothermal gold mineralization occurs in banded iron formation and amphibolite host rocks respectively in the main hill and zakanaka deposits in the northern sector of the pilgangoora greenstone belt of the pilbara craton the supracrustal sequences of the greenstone belt have been folded into n s trending megascale isoclinal f2a folds which have been refolded into open to tight f2b folds d2 to d3 strike slip shear zones developed preferentially along the limbs of f2a isoclinal folds and along the granitoid greenstone belt contact gold mineralization is controlled by f2b folds and shear zones were interpreted to have been activated or reactivated during d2b deformation in both deposits gold associated hydrothermal alteration comprises zoned quartz calc silicate veins and pyrrhotite pyrite at the zakanaka deposit and quartz amphibole biotite diopside and pyrrhotite lollingite arsenopyrite at the main hill deposit thermobarometric and textural studies indicate that gold mineralization occurred synchronous with the peak of amphibolite facies metamorphism in both deposits lead isotope studies on gold associated hydrothermal microcline titanite and pyrrhotite in the zakanaka deposit yield a pb pb isochron with an age of 2888 2 6 1 ma for the alteration and hence gold mineralization this age is confirmed by a concordant pb pb age using shrimp on hydrothermal titanite from the zakanaka deposit of 2904 70 ma the initial isotopic ratio in the ore system estimated using the isotopic composition of hydrothermal microcline is much more primitive than 2888 ma pb from accepted growth curves lead isotope modeling using the possible ages of rock formation and metamorphic events in the eastern pilbara craton shows that the primitive pb isotope signature is consistent with the 2888 ma age of alteration if the pb was derived from old crust which underwent an early high grade depletion event the age of hydrothermal gold associated alteration at zakanaka indirectly constrains the age of the f2b folding event and the peak of amphibolite facies metamorphism in the pilgangoora greenstone belt to ca 2 89 ga the special position of the pilgangoora greenstone belt in the central pilbara permits the interpretation of the age of gold mineralization in the light of the tectonic evolution in both parts of the craton and indicates that mineralization and the associated structural and metamorphic events post dated the main magmatic evolution in both the east and the west pilbara craton but coincided with late deformational events along strike slip shear zones in the west pilbara and late syn to post tectonic granitoids and associated metamorphism in the east pilbara

geochronology and stratigraphic relationships of the sulphur springs group and strelley granite a temporally distinct igneous province in the archaean pilbara craton australia abstract rocks from the sulphur springs group a newly identified stratigraphic succession hosting volcanogenic massive sulphide vms base metal deposits in the soanesville belt pilbara craton northwestern australia have been analyzed by sensitive high resolution ion microprobe shrimp u pb zircon mass spectrometry to determine their age and tectonostratigraphic relationships most volcanism occurred between 3238 and 3235 ma distinctly different from the time interval spanned by the wyman formation type section is 3315 ma other dates 3325 ma with which it was previously correlated outlying volcanic rocks with similar geochemistry at the bernts and obelix prospects also produce dates from 3240 to 3235 ma and are therefore best interpreted as structurally detached fragments of the sulphur springs group the strelley granite an intrusion which shares geochemical characteristics with the overlying volcanic rocks also yields similar ages thus extending the period of temporally linked extrusive intrusive silicic magmatism in the evolution of the pilbara craton model pb pb dating of galena from the zn cu mineralization at the various vms prospects has yielded ages consistent with the zircon u pb ages of the host rocks indicating that the hydrothermal mineralizing systems were contemporaneous with and apparently initiated by igneous activity an epicontinental tectonic setting for the sulphur springs group during vms mineralization is implied by the broad range of old zircon ages obtained from a tuff at the base of the volcanic pile as the igneous episode coincides with the peak regional metamorphic event recorded in other east pilbara supracrustal rocks and with voluminous intrusive activity in nearby granitoid complexes it marks a major event in the crustal evolution of the eastern pilbara craton

geochemistry and geodynamic setting of volcanic and plutonic rocks associated with early archaean volcanogenic massive sulphide mineralization pilbara craton abstract early archaean volcanic rocks and syn volcanic granites at strelley east pilbara craton western australia were formed during an extensional tectonic phase associated with an active convergent continental margin the volcanic sequence comprises a bimodal suite of fractionated sub alkalic tholeiitic basalts andesites and dacites siliceous laminites and volcanogenic massive sulphide vms deposits in the footwall and hangingwall calc alkaline dacites the footwall rocks are characterized by th lapm 1 variably fractionated lree and relatively high y and yb 33 and 3 ppm respectively negative normalized anomalies of nb p ti are consistent with derivation via slab dehydration fusing the mantle wedge the co magmatic and syn volcanic strelley granite is enriched in lile relative to hfse and has distinctively low al2o3 11 14 and la yb n 4 10 suggesting a relatively mature and cool subducted oceanic lithosphere as for the majority of phanerozoic arc granitoids and convergent margin magma compositions the chemical composition of the footwall basaltic to dacitic rocks and the sub volcanic strelley granite display signatures highly prospective for vms mineralization large ve eu anomalies low zr y ratios high level magmatic fractionation comparable with the noranda fiiia type suite the hangingwall dacites have more pronounced hree fractionation than footwall counterparts and lower y and yb the chemistry of the footwall volcanic rocks and granite reflect typical phanerozoic conditions of steep subduction hydrous metasomatism in the mantle wedge and fractionation similar characteristics are also evident in plutonic volcanic suites associated with vms deposits in the late archaean as at noranda canada early extension with nascent back arc development coupled with steep subduction is considered critical to the development of vms deposits

microbial remains and other carbonaceous forms from the 3 24 ga sulphur springs black smoker deposit western australia well preserved carbonaceous materials including filamentous microbial remains have been isolated from an archaean sedimentary package overlying the sulphur springs volcanic hosted massive sulphide vhms deposit in the pilbara of western australia one of the oldest best preserved vhms deposits on earth the isotopically light carbonaceous material 13 c values of 26 8 to 34 0 v pdb occurs as finely striated lenticular to banded organic matter om within fine grained epiclastic sediments and silicified materials that overlie the deposit the predominant banded lenticular carbonaceous material occurs parallel to original sedimentary bedding planes and is distinctly visible in light microscopy transmission electron microscopy tem reveals this om as bundles of filamentous and tubular structure closely resembling both modern day and more ancient microbial forms documented from sea floor hydrothermal environments total organic carbon toc has a range of 1 0 to 2 3 while the thermal maturity of the filamentous microbial bundles indicated by reflectance ro points to maximum temperatures since deposition ca 3 24 ga of around 90 100 c a factor that has enabled the preservation of their morphology these results and the intimate association of sulphides with the om are suggestive of a well developed archaean sediment hosted microbial community situated within a basinal environment associated with an active centre of seafloor hydrothermal activity 2007 elsevier b v all rights reserved

radium content and the 226ra228ra activity ratio in groundwater from bedrock the relative abundance of 226ra and 228ra were determined in the groundwater from 125 drilled wells containing from 0 1 to 51 3 pci l of 226ra the determination of 228ra was carried out with a liquid scintillation counter by measuring only the weakly energetic particles emitted from 228ra thus the interference from the daughter nuclides of 226ra was avoided without specific separation of 228ac the direct measurement of 228ra made the method decisively simpler and faster in terms of the chemistry involved the concentration of 228ra was found to be independent of the amount of 226ra present in the samples the concentrations of 228ra were nearly the same over the whole range of 226ra concentrations and the average sol226ra228ra ratio sharply increased as the 226ra content of water increased the 226ra228ra ratio in the drilled wells varied from 0 3 to 26 abnormally high 226ra228ra ratios were found in areas with known uranium deposits as well as in several drilled wells at other locations the abnormally high 226ra228ra ratios present in groundwater suggest that the radioactivity anomaly is caused by uranium deposits and not by common rocks in samples with a low radioactivity level the average 226ra228ra ratio was slightly below unity corresponding to the typical u th ratio of granite the most common kind of rock in the study area the samples from the rapakivi area proved to be exceptional in that they had a low 226ra228ra ratio independent of the concentration of 226ra

post kombolgie metasomatism at jabiluka northern territory australia and its significance in the formation of high grade uranium mineralization in lower proterozoic rocks diagenetic lithification of kombolgie sandstones followed by a metasomatic event which produced veining and replacement metasomatism was controlled by brecciation and bedding within the sandstones the high grade uranium ores are interpreted to be the product of a low temperature 100 degrees 200 degrees c hydrothermal system of ground water which was driven by a regional heating event also manifested by phonolite and or diabase intrusions the kombolgie sandstones were an essential part of the hydrologic system which formed the high grade orebodies after lithification of the lower part of the thick clastic section but possibly contemporaneous with diagenetic events in the upper part the processes operating in the alligator rivers uranium field were essentially similar to those in the athabasca district of northern saskatchewan the primary difference probably being that no organic material was remobilized into the overlying sandstone to act as a reductant in the australian case modified journal abstract

late archaean crustal extension sedimentary basin formation flood basalt volcanism and continental rifting the nullagine and mount jope supersequences western australia abstract the mount bruce megasequence set formerly the mount bruce supergroup is a succession of volcanic and sedimentary rocks that was deposited on the pilbara craton of western australia in the late archaean to early proterozoic its history can be described and explained in terms of modern style sequence stratigraphy and of phanerozoic style plate tectonics the megasequence set comprises the chichester range megasequence that was deposited on a rifted to divergent continental margin and the overlying hamersley range megasequence that was deposited on a convergent continental margin the chichester range megasequence comprises in order of superposition the nullagine and mount jope supersequences and the marra mamba supersequence package the nullagine and mount jope supersequences are each the rock record of one phase of a protracted two phase late archaean 2770 2700 ma continental break up the nullagine supersequence consists of the mount roe sequence which comprises mostly subaerial basalt and the overlying hardey sequence package which comprises mostly terrigenous sedimentary rocks that were deposited in extensional sedimentary basins the mount jope supersequence consists of in order of superposition the kylena sequence which comprises mostly basalt the tumbiana sequence which comprises mostly tuff and tuffaceous rocks and the maddina sequence package which comprises mostly basalt the nullagine supersequence was deposited during an episode of wnw ese directed crustal extension that possibly led to the development of a rift to the west of the present pilbara craton the mount jope supersequence was deposited during the subsequent development of a wnw ese trending rift the position of which approximates the present southern craton margin regional scale uplift was associated with the development of both supersequences particularly with the nullagine supersequence basement strike slip faults were reactivated as dip slip structures during crustal extension and controlled the locations of some hardey sequence package extensional sedimentary basins nullagine supersequence and mount jope supersequence rifting led to a divergent continental margin bordering an ocean along the south and probably the far west of the craton post rift thermal subsidence was accompanied by the accumulation of mudrock carbonate sedimentary rock and iron formation of the marra mamba supersequence package in a shelf setting the implications of these interpretations are that at 2800 ma the pilbara craton was significantly larger than at present and that it had a sufficiently thick subcontinental lithosphere that divided it in a manner analogous to the break up of phanerozoic continents the nullagine and mount jope supersequences contain three stacked flood basalt successions which were erupted over a time period of between of 55 and 80 m y stacked flood basalt successions covering such a long time period are not known in the phanerozoic and are tangible evidence of a hotter late archaean mantle

the interplay of evolved seawater and magmatic hydrothermal fluids in the 3 24 ga panorama volcanic hosted massive sulfide hydrothermal system north pilbara craton western australia the 3240 ma panorama volcanic hosted massive sulfide vhms district is unusual for its high degree of exposure and low degree of postdepositional modification in addition to typical seafloor vhms deposits this district contains greisen and vein hosted mo cu zn sn mineral occurrences that are contemporaneous with vhms orebodies and are hosted by the strelley granite complex which also drove vhms circulation hence the panorama district is a natural laboratory to investigate the role of magmatic hydrothermal fluids in vhms hydrothermal systems regional and proximal high temperature alteration zones in volcanic rocks underlying the vhms deposits are dominated by chlorite quartz albite assemblages with lesser low temperature sericite quartz k feldspar assemblages these assemblages are typical of vhms hydrothermal systems in contrast the alteration assemblages associated with granite hosted greisens and veins include quartz topaz muscovite fluorite and quartz muscovite sericite chlorite ankerite these vein systems generally do not extend into the overlying volcanic pile fluid inclusion and stable isotope studies suggest that the greisens were produced by high temperature 590 c high salinity 38 56 wt nacl equiv fluids with high densities 1 3 g cm3 and high 18o 9 3 0 6 these fluids are compatible with the measured characteristics of magmatic fluids evolved from the strelley granite complex in contrast fluids in the volcanic pile including the vhms ore forming fluids were of lower temperature 90 270 c lower salinity 5 0 11 2 wt nacl equiv with lower densities 0 88 1 01 g cm3 and lower 18o 0 8 2 6 these fluids are compatible with evolved paleoarchean seawater fluids that formed the quartz chalcopyrite sphalerite cassiterite veins which are present within the granite complex near the contact with the volcanic pile were intermediate in temperature and isotopic composition between the greisen and volcanic pile fluids t 240 315 c 18o 4 3 1 5 and are interpreted to indicate mixing between the two end member fluids evidence of mixing between evolved seawater and magmatic hydrothermal fluid within the granite complex together with the lack of evidence for a magmatic component in fluids from the volcanic pile suggest partitioning of magmatic hydrothermal from evolved seawater hydrothermal systems in the panorama vhms system this separation is interpreted to result from either the swamping of a relatively small magmatic hydro thermal system by evolved seawater or density contrasts precluding movement of magmatic hydrothermal fluids into the volcanic pile variability in the salinity of fluids in the volcanic pile combined with evidence for mixing of low and high salinity fluids in the massive sulfide lens is interpreted to indicate that phase separation occurred within the panorama hydrothermal system although we consider this phase separation to have most likely occurred at depth within the system as has been documented in modern vhms systems the data do not allow the location of the inferred phase separation to be determined

radioactive springs in the watershed of a proposed reservoir in sequoyah county oklahoma origin and environmental effect radioactive radium rich springs discharging in the watershed of a proposed impoundment on lower lee creek in sequoyah county oklahoma were determined not to represent a significant radiological effect the impact of these springs on the proposed lake will be negligible owing to a combination of the following factors extremely low rate of discharge of the springs even following periods of heavy rainfall effective dilution of the spring water by the creek into which the springs discharge dilution factor of about 1 100 and precipitation of radiobarite just below the mouths of the springs the spring water probably rises to the surface along a fault and fracture system present in the study area the presence of daughter products belonging to the sup 238 u as well as the sup 232 th decay chains suggests that these nuclides are not a surface manifestation of buried sedimentary uranium deposits because such deposits do not contain significant concentrations of thorium the geologic setting of the area precludes the presence of vein type uranium thorium deposits the primary factor responsible for the high radium content of the springs is most likely rock to fluid transfer of alpha decaying nuclei in the sup 238 u and sup 232 th series in the deep relatively static rock fluidmore system the fluid is a very sensitive indicator of this phenomenon taking place in a reducing environment in the absence of processes capable of removing radium isotopes from solution less

hematite formation by oxygenated groundwater more than 2 76 billion years ago geoscientific drilling in the marble bar area of the pilbara craton western australia resulted in the discovery of locally abundant hematite in archean basalts 200 m below the present land surface the hematized basalts occurring along a bedding parallel shear zone are cross cut by pyrite veinlets 3 mm in width and contain euhedral pyrite grains 10 500 m in diameter with sharp crystal edges indicating that the hematite formed before the pyrite we have dated the pyrite in the veinlets at 2 763 0 016 ga using the re os method therefore the hematite formed prior to 2 763 ga the basalts containing the hematite belong to the apex basalt of the warrawoona group and were erupted onto the archean seafloor at 3 46 ga due to 2 9 ga orogenic deformation and subsequent deep erosion the apex basalt was exposed at the surface of a continental landmass prior to 2 77 ga sometime in the period between 2 9 ga and 2 77 ga the basalt section we describe was less than 200 m below the late archean land surface and within range of groundwater percolation through the shear zone in the basalts geological mineralogical and geochemical lines of evidence strongly suggest that the infiltration of o2 rich groundwater through the bedding parallel shear in the basalts formed hematite prior to 2 76 ga and hence oxygenated surface environments at least localized and or short lived emerged more than 300 million years before the widely accepted great oxidation event during 2 45 and 2 32 ga

the archaean whim creek belt an ensialic faultbounded basin in the pilbara block australia abstract the whim creek belt in the western pilbara block australia is an elongate structural basin 80 km long and up to 15 km wide it contains a c 2950 ma old sequence of bimodal volcanics volcaniclastics and terrigenous sediments which unconformably overlies granitoids and older deformed and metamorphosed supracrustal sequences the volcano sedimentary sequence was deposited in terrestrial to marine environments in a fault bounded basin with approximately the same configuration as the present whim creek belt alluvial and subaqueous fanglomerate facies were deposited along the northern margin of the basin with submarine fan and basinal facies deposited longitudinally in the centre of the basin the basin margin faults are now part of a regional pattern of strike slip faults which were active both prior to and after basin formation and probably had a significant lateral component during basin evolution the whim creek belt is thus interpreted as an archaean rift or pull apart basin similar basins containing bimodal volcanics and terrigenous sediments occur at the head of the present gulf of california e g salton trough although a direct tectonic analogy cannot be assumed this interpretation implies that the archaean tectonic pattern in the western pilbara involved externally imposed deformation and was not solely the result of gravity driven uprise of granitic batholiths

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

integrated pb and s isotope investigation of sulphide minerals from the early archaean of southwest greenland we present high spatial resolution secondary ion mass spectrometry sims measurements of pb and s isotopes in sulphides from early archaean samples at two localities in southwest greenland secondary pyrite from a 3 71 ga sample of magnetite quartz banded iron formation in the isua greenstone belt which has previously yielded unradiogenic pb consistent with its ancient origin contains sulphur with a mass independently fractionated mif isotope signature delta 33 s 3 3 parts per thousand this reflects the secondary mineralization of remobilized sedimentary s carrying a component modified by photochemical reactions in the early archaean atmosphere it further represents one of the most extreme positive excursions so far known from the early archaean rock record sulphides from a quartz pyroxene rock and an ultramafic boudin from the island of akilia in the godth a over circle bsfjord have heterogeneous and generally radiogenic pb isotopic compositions that we interpret to represent partial re equilibration of pb between the sulphides and whole rocks during tectonothermal events at 3 6 2 7 and 1 6 ga both these samples have delta 33 s 0 within analytical error and therefore show no evidence for mif sulphur these data are consistent with previous interpretations that the rock cannot be proven to have a sedimentary origin our study illustrates that sims s isotope measurements in ancient rocks can be used to elucidate early atmospheric parameters because of the ability to obtain combined s and pb isotope data but caution must be applied when using such data to infer protolith when information from geological context petrography and chronology i e by pb isotopes is combined and fully evaluated delta 33 s signatures from sulphides and their geological significance can be interpreted with a higher degree of confidence c 2005 elsevier b v all rights reserved