geological setting and petrochemistry of the narracoota volcanics capricorn orogen western australia abstract the narracoota volcanics part of the early proterozoic glengarry group on the northern margin of the yilgarn block were deformed and metamorphosed between 1 7 and 1 8 ga ago and then covered by sediments of the padbury group 1 6 1 7 ga although most of the glengarry group is in place on granitoid basement pre padbury group deformation produced large scale south vergent recumbent folds in parts of the region so that much of the glengarry group is at least parautochthonous it was probably deposited on substantially thinned continental crust the narracoota volcanics in the southern autochthonous part of the region are dominated by thick 4 km chemically uniform successions of tholeiites differing from morb only in their slightly low ti fe and zr fe further north autochthonous and parautochthonous narracoota volcanics are dominated by ultramafic schists of probably pyroclastic origin mafic volcanic rocks associated with these schists range up to 58 sio2 and are very poor in ti and zr they resemble some boninites of the western pacific the southern volcanic rocks are interpreted to reflect open system fractionation of mantle derived magmas in large subcrustal magma chambers similar to those postulated at oceanic ridges the high si values and low ti and zr values in more northerly mafic rocks may reflect hydrous melting and second stage melting respectively the geochemical similarities of some of the narracoota volcanics to boninites suggest an arc subduction related origin for these rocks but there is little evidence from the regional geological setting to support this origin in a continental rifting environment in which the sub continental mantle was locally h2o rich is the favoured interpretation the thick sequences of morb like volcanic rocks their geochemical differences from continental tholeiites and the thin skinned deformation suggest that continental rifting was of major dimensions in the region

gold metallogeny and the copper gold association of the australian proterozoic australian proterozoic gold producing deposits emplaced mainly at 1 55 2 00 ga are divided into the following categories 1 iron oxide dominated brecciahosted cu u au replacement deposits spatially associated with felsic intrusions 273t au 2 stratabound au cu bearing iron formations 152 4t au 3 unconformity style u cu pgm au deposits 53t au 4 iron oxide dominated au cu mineralisation hosted within elements of ductile deformation 146 7t au 5 broken hill and volcanic hosted massive sulphides 150t au 6 iron sulphide dominated veins and replacement zones spatially related to felsic intrusions 150 7t au and 7 iron sulphide dominated veins and replacement zones spatially related to elements of regional deformation 159 9t au categories 1 to 4 are mainly confined to proterozoic rocks constituting an association in which au and cu are commonly present together with variable amounts of u bi co w se te and ree most examples in categories 1 4 fall into either of two groups cu aumagnetite hematite types formed at relatively high temperature 300 450 c and cu u au hematite types formed at 150 300 c we postulate that these ores formed from a common high salinity 15 35 wt nacl equiv low total sulphur a s 10 3 to 10 2 high fo2 fluid type in which metal transport was dominated by chloride complexing the most effective method of metal deposition was fluid mixing achieving a synchronous decrease in fo2 and temperature this unusual oxidised fluid association was favoured in high heat flow extensional settings containing oxidised and or oxidised evaporitic sedimentary sequences the intrusion of oxidised fractionated granites which are commonly temporally associated with metal emplacement acted in some places to heat and focus basinal fluids and in others was the ultimate source of metals

gravity anomalies crustal structure and isostasy associated with the proterozoic capricorn orogen western australia abstract present knowledge of the proterozoic capricorn orogen in western australia is largely based on surface geological studies geophysical studies of the crust of the orogen are limited to a few regional seismic refraction profiles and regional potential field interpretations the aim of this paper is to present a simple gravity constrained crustal model for the capricorn orogen anomalous lows immediately north of the yilgarn craton are prominent features of the capricorn orogen gravity field and geoid these anomalous lows do not correlate with surface geology or elevated topography their character suggests that they reflect thickened crust in the core of the capricorn orogen preserved in a state of mechanical equilibrium for about two billion years the location of the anomalous lows and associated crustal thickening south of the inferred location of the pilbara yilgarn suture suggest that the pilbara craton was subducted southwards beneath the previously combined yilgarn craton and southern gascoyne complex in a simple two dimensional crustal model features such as the bangemall bryah and yerrida basins are assumed to be dominantly supported by the rigid lithosphere of this region effective elastic thickness 50 100 km the yerrida basin and adjacent marymia inlier are modelled as north dipping features the bangemall basin as a southward deepening basin and the lower boundary of the gascoyne complex is best modelled as a north dipping contact at the northern margin of the orogen the sylvania inlier appears to lie at shallow depths 1 km beneath the eastern ashburton basin the rocks of the south eastern hamersley province reach a maximum thickness of about 7 km and thin toward the north a notable feature of the model is that it does not maintain isostatic equilibrium the prolonged erosion of this region and the prominent upper crustal density anomalies mean that it is difficult to develop gravity models that maintain isostatic balance a simple isostatic analysis suggests that the present day topography of the capricorn orogen is over compensated this over compensation can be explained by resistance of the rigid lithosphere of western australia to rebound in response to mesozoic or cainozoic erosion mantle flow acting to pull the crust downward from its equilibrium position could also lead to the apparent lack of isostatic equilibrium

the influence of closed brine pockets and permeable brine channels on the thermo elastic properties of saline ice a model of the thermo elastic behaviour of saline ice is formulated and model solutions describing thermo elastic waves tew propagating into a half space of the ice are investigated the model is based on a proposal that saline ice is a matrix which encompasses both closed brine pockets and permeable channels filled with brine experiments on the thermal expansion of saline ice samples and on tew in saline ice have been performed in the cold laboratories of the university centre in svalbard and in university college london the experimental data are compared with theoretical conclusions the experimental data support our hypothesis that the brine in saline ice is divided between closed pockets and open permeable channels this article is part of the themed issue microdynamics of ice

stratigraphy of the late palaeoproterozoic 2 03 ga wooly dolomite ashburton province western australia a carbonate platform developed in a failed rift basin abstract this paper describes the sequence stratigraphy of the 2 03 ga wooly dolomite the uppermost stratigraphic unit of the horseshoe basin strata of which unconformably overlie the 2772 2410 ma hamersley province unconformably truncate folded 2210 ma dolerite sills and post date the 2195 2145 ma ophthalmia orogeny the horseshoe basin was an intracontinental rift that initiated at 2 05 ga with deposition of 360 m of fluvial and shallow marine siliciclastic sandstones beasley river quartzite followed by eruption of 2 7 km of flood basalt cheela springs basalt and culminated with deposition of 325 m of platformal carbonates and associated volcaniclastic and siliciclastic sediments wooly dolomite the three formations of the horseshoe basin stack conformably with tectonically driven unconformities developed only at a late stage within the wooly dolomite depositional systems of the wooly dolomite appear to have been compartmentalised but connected to an ocean five depositional sequences are identified depositional sequence 1 which developed conformably on the cheela springs basalt records the establishment of a carbonate platform coeval with volcaniclastic sedimentation but it was developed only in the southeastern most sector of the basin resedimented tuffs volcaniclastic siltstones are present in all of the depositional sequences but are most abundant in ds1 depositional sequence 2 cuts deeply into the cheela springs basalt and has a lower section of siliciclastic sedimentary rocks that is overlain by carbonate platform deposits the wyloo dome at an interpreted rift triple junction was an uplifted area until the late stage of ds2 implying that accommodation generation was also fault compartmentalised depositional sequences 3 4 and 5 have predominantly carbonate platform deposits and are also unconformity bound whereas ds4 and ds5 preserve a shelf slope transition subsidence in the horseshoe basin ended with basin inversion during the d1 panhandle deformation event which predated emplacement of an 2008 ma dolerite dyke swarm a subsequent rifting event recorded by the mcgrath basin led to a rift drift transition that finally initiated a west facing atlantic type continental margin

isotopic dating of the migration of a low grade metamorphic front during orogenesis the southern margin of the pilbara craton in northwestern australia underwent regional heating folding and thrusting as well as extensive fluid flow during collision in the paleoproterozoic however the precise timing of this event is uncertain previous geochronologic studies of archean basement rocks have yielded a wide range of younger radiometric dates from ca 2 4 ga to ca 2 0 ga interpreted to record multiple hydrothermal alteration events in situ u pb geochronology of authigenic monazite and xenotime in very low grade archean metasedimentary rocks from across the craton suggests that the pilbara was affected by at least two cryptic thermotectonic events the first between ca 2430 ma and ca 2400 ma and the second between ca 2215 ma and ca 2145 ma the older event is restricted to three localities in the west and its cause is unknown the younger event affected most of the craton 100 000 km 2 spanning a 70 m y period from ca 2215 ma nearest the collisional margin in the south to ca 2145 ma toward the craton interior in the north the widespread geographic and stratigraphic distribution of ca 2 2 ga phosphates suggests that fluid flow was pervasive this event was probably driven by the northward advancing ophthalmian fold and thrust belt that developed during protracted collision the associated low grade metamorphic front migrated over 350 km at an average rate of 5 0 mm yr placing important constraints on the rate and duration of deformation and metamorphism in orogenic settings

deformation induced silica redistribution in banded iron formation hamersley province australia abstract the formation of banded iron formations bif remains controversial despite their potential to provide key information on precambrian atmospheres and hydrospheres it is widely agreed that bif are chemical sedimentary rocks comprising alternating layers of iron oxides and chert formed from poorly known precursor phases many models address the chemical transformation of such precursor iron oxide phases into bif during compaction and diagenesis however the formation of chert and the influence of physical forces in this process have received less attention microstructural analysis of bif from the hamersley province western australia reveals that significant amounts of silica were redistributed by dissolution precipitation creep during both diagenesis and regional scale deformation this physicochemical process led to silica remobilisation and volume loss by stress induced dissolution of microcrystalline quartz in an aqueous fluid the dissolved solid phase was transported by diffusion and fluid flow along grain boundaries or within available porosity and then reprecipitated in low pressure zones leading to local volume increase these processes were further enhanced by rheological contrasts between different minerals resulting in significant variations of chert band thickness microstructural observations combined with quantitative microfabric analysis reveal domains of crystallographic preferred orientations cpo in quartz grains within chert layers the cpo fabrics record strain regimes e g pure and simple shear extension and shortening that modified quartz aggregates by dissolution precipitation creep providing new insights into the metamorphic and deformation history of bif we document microstructures that indicate that non coaxial deformation was active during diagenesis and subsequent deformation of the hamersley province bif further relatively undeformed chert layers may have been similarly affected by significant amounts of dissolution precipitation creep providing a possible explanation for the destruction of primary features we show that different generations of chert alternate on a micro scale suggesting that the role of deformation has been underestimated in many aspects of bif research the discovery of dissolution precipitation creep in low grade metamorphic bif provides a potential link between the physical and chemical processes that occur within these rocks after deposition in the hamersley province dissolution precipitation creep was active from the compaction or diagenesis stage and probably operated during all major extensional and compressional deformation events we also suggest that chert layers without cpo record the least overprint by diagenesis and as the most pristine form of bif should be targeted for future paleo environmental research

tectono metamorphic events in the north atlantic region in the palaeoproterozoic from the view point of high grade metamorphic rocks in the lewisian complex south harris nw scotland abstract the tectono thermal history of the lewisian complex in south harris south harris complex was inferred from its geologic and metamorphic characteristics the lithological assemblages and geochemical features of the complex suggest that its precursory rocks were composed of the subduction related accretionary complex formed in the palaeo convergent margin the complex has suffered the ultra high temperature uht metamorphism that was contemporaneous with the igneous activity to make the south harris igneous complex shic and the subsequent continent continent collisional activity a similar complex recording the geological processes of the subduction the uht metamorphism and the collision has been recognized in the lapland kola belt and new quebec in the palaeoproterozoic this suggests an assembly of micro continents to form the palaeoproterozoic supercontinent in the north atlantic region

continental rifting and the archean proterozoic transition in the western australian shield the norseman wiluna belt dominated by lowermost sequences of deformed and metamorphosed basalts and komatiites and the fortescue group with successions of gently dipping little metamorphosed basalts and rare komatiites are typical examples of archean style greenstones and proterozoic style platform sequences respectively isotopic evidence however indicates that they are broadly coeval and both archean ca 2 7 0 05 ga and geologic evidence suggests that both relate to continental rifting their contrasting volcanism and subsequent deformation are inferred to result from their positions relative to rift axes and the state of the basement the norseman wiluna belt representing a failed rift in stretched relatively hot crust and the fortescue group representing a continental basalt succession on thick relatively cold crust the fortescue group immediately follows a major period of crustal reworking and stabilization whereas eruption of the greenstones of the norseman wiluna belt was closely followed by such an event thus the diachronous change in tectonic style from greenstones to platforms the archean proterozoic transition in this case appears to relate to a finite period of stabilization of continental crust rather than to a fundamental change in tectonic style or process with time

proterozoic tectonic framework and metal deposits of southwestern australia abstract the proterozoic tectonic evolution and metal deposits of southwestern australia reflect a dynamic history of repeated aggregation and dispersal of continental fragments this contrasts with previous interpretations that much of australia was part of a proterozoic supercontinent in which all orogenic activity and metallogenesis was ensialic southwestern australia contains a wide range of sedimentary and intrusive rocks that span the whole of proterozoic time they occur in four proterozoic orogens and as cover sequences on one of the orogens and on an adjacent archaean craton each proterozoic orogen formed during one of three episodes of continental collision which occurred in the early middle and late proterozoic the oldest deposits hamersley group occur on the archaean pilbara craton and are dominated by banded iron formations containing major supergene enriched hematite goethite ores following early proterozoic rifting the archaean pilbara and yilgarn cratons collided and amalgamated forming the intervening capricorn orogen between 2 0 and 1 6 ga granite with associated minor mo w bi pb and u mineralisation is abundant in the vicinity of the suture sedimentary rocks from passive continental margins and foreland basins together with basalt gabbro and serpentinite from oceanic or arc environments were folded and thrust outwards onto the margins of the adjacent cratons and locally contain prominent au cu and pb mineralisation middle proterozoic sedimentary rocks bangemall group that unconformably overlie the capricorn orogen contain significant deposits of pb cu ag and mn the middle proterozoic albany fraser orogen is dominated by huge thrust sheets of high grade gneiss and granite reflecting another episode of continental collision along the southern margin of western australia exposed proterozoic rocks of the pinjarra orogen are high grade gneiss and granite that appear to have been involved in late proterozoic continental collision the paterson orogen comprises early middle and late proterozoic components that were also involved in late proterozoic continental collision it contains major deposits of au cu cu pb zn and u

mineral systems approach applied to gis based 2d prospectivity modelling of geological regions insights from western australia abstract gis based 2d prospectivity modelling of three greenfield geological regions of western australia namely the west arunta orogen west musgrave orogen and gascoyne province was implemented for a range of deposit types including orogenic and intrusion related gold volcanic sediment hosted base metal sulfides magmatic nickel copper and magmatic platinum group element sulfides iron oxide copper gold tin tungsten igneous and metamorphic related rare earth elements surficial uranium and unconformity related uranium conceptual mineral systems models were generated to identify the targeting criteria the inputs to the models were the spatial proxies derived from 1 100 000 to 1 500 000 scale public domain data the results showed similar prospectivity patterns for all of the targeted deposit types except sediment hosted uranium and surficial uranium deposit types once a favourable geodynamic architecture is established it can sustain different mineral systems and produce diverse deposit types depending on the nature of ligands in the source regions and physical chemical environment in the trap regions through repeated reactivation in the subsequent geological history a model is proposed to explain the formation of different deposit types at different stages of tectonic evolution of a province the implication for gis based 2d prospectivity modelling at the scale of geological region is that the prospectivity model may not be deposit type specific further prospectivity modelling should be carried out sequentially at progressively finer scales regional to district to camp scale using only the targeting criteria that are relevant at the specific scale to delineate targets for specific deposit types

exploiting seismic signal and noise in an intracratonic environment to constrain crustal structure and source parameters of infrequent earthquakes in many regions of the world characterized by a relatively low rate of seismicity the determination of local and regional seismic source parameters is often restricted to an analysis of the first onsets of p waves or first motion analysis due to incomplete information about earth structure and the small size of the events when rare large earthquakes occur in these regions their waveforms can be used to model earth structure this however makes the nature of the earthquake source determination problem circular as source information is mapped as structure presented here is one possible remedy to this situation where through a two step approach we first constrain earth structure using data independent of the earthquake of interest in this study we focus on a region in western australia with low seismicity and minimal instrument coverage and use the capra lp temporary deployment to demonstrate that reliable structural models of the upper lithosphere can be obtained from an independent collection of teleseismic and ambient noise datasets apart from teleseismic receiver functions rfs we obtain group velocities from the cross correlation of ambient noise and phase velocities from the traditional two station method using carefully selected teleseismic earthquakes and station pairs crustal models are then developed through the joint inversion of dispersion data and rfs and structural green s functions are computed from a layered composite model in the second step of this comprehensive approach we apply full waveform inversion three component body and surface waves to the 2007 ml 5 3 shark bay western australia earthquake to estimate its source parameters seismic moment focal mechanism and depth we conclude that the full waveform inversion analysis provides constraints on the orientation of fault planes superior to a first motion interpretation

structural evolution of the mount wall region in the hamersley province western australia and its control on hydrothermal alteration and formation of high grade iron deposits abstract the discovery of two relatively small but high grade iron ore deposits near mt wall an intensely faulted part of the southwestern hamersley province provides unique insights into the structural control on ore formation in this region the deposits have many geological features typical of the high grade microplaty hematite group which also contains the much larger mt tom price paraburdoo and mt whaleback deposits the deposits are structurally controlled along early normal faults and contain abundant microplaty hematite and martite and are largely confined to the dales gorge member of the brockman iron formation in addition to the microplaty hematite martite ore there are martite goethite ores and rare magnetite goethite or magnetite hematite ores below the modern weathering surface hydrothermally altered zones in wallrock bif from the lower dales gorge member contain magnetite hematite and carbonate talc bearing mineral assemblages a staged ore genesis model involving early extension and fluid circulation along normal faults hypogene silica leaching and carbonate alteration followed by deep meteoric oxidation with microplaty hematite formation and finally weathering can explain most features of the mt wall deposits the role of deformation was to provide pathways for mineralising fluids and initiate the seed points for the mineralised systems high grade iron in the wellthandalthaluna deposit is situated between the nw to nnw trending boolgeeda creek fault and a synthetic joining splay the northern fault both are high angle normal faults and formed during early extension in this part of the province faults are characterised by localised small scale deformation and brecciation deep carbonate alteration and oxidation recent weathering has penetrated deeply into the fault zones converting the carbonate rich assemblages into goethite mineralisation in the arochar deposit is situated in the overlap or relay zone between two segments of the mt wall fault zone a moderately to steeply southerly dipping normal fault system which at arochar is intruded by dolerite dykes at both locations the ore controlling faults are offset by later nw trending dextral and normal faults fault relay zones or fault splay zones were likely zones of increased permeability and fluid flow during fault development or reactivation and may also have been important in initiating mineralisation in larger deposits such as mt tom price and mt whaleback however structural controls on the largest iron ore deposits are often obscured due to the intensity and scale of ore development whereas they are better preserved in the smaller deposits recognition that carbonate bearing protores at mt wall survived for nearly two billion years until intense recent weathering converted them to martite goethite or magnetite goethite ores may imply that more of the giant hematite goethite deposits of the hamersley province had hydrothermal precursors and were not formed by supergene processes alone

genesis of high grade hematite orebodies of the hamersley province western australia a reply sir we are very thankful for the constructive comments provided by kneeshaw and kepert unfortunately the competitive ownership structure of the iron ore mining industry in the hamersley province limits our personal knowledge of the mount whaleback deposit to occasional mine visits and to the available literature published and unpublished we do however have access to the data on the smaller giles mini and lido deposits 50 and 10 km west of mount whaleback respectively and to extensive exploration mapping in the ophthalmia ranges to the north direct proof for rotation of the orebody as it is preserved at paraburdoo in the approximately 50 dip of the overlying proterozoic hematite conglomerates of the mount mcgrath formation is not preserved at mount whaleback however we believe that there is sufficient information available in the literature to think that a rotational model for a large part of the deposit is perfectly plausible first and above all as stated in taylor et al 2001 f2 folds are more upright with axial planes dipping about 50 s in the southern and western part of the deposit away from the whaleback fault e g as shown on section no 315120e fig 4 in powell et al 1999 interestingly the ore controlling faults are also more upright in this part of the deposit which suggests an intimate link between the dip of these fault planes and dip of the axial planes of f2 folds secondly

did global tectonics drive early biosphere evolution carbon isotope record from 2 6 to 1 9 ga carbonates of western australian basins the 13 ccarb record of well preserved carbonates in outcrop and core is here examined from the 2 6 to 1 9 ga old basins of western australia these data which are constrained by a well defined stratigraphic and tectonic framework and by u pb zircon ages provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere in the latest archaean ca 2 6 ga the secular 13 ccarb curve is flat much like that seen in the later palaeoproterozoic basins of northern australia 1 8 ga this implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable in the early palaeoproterozoic beginning after 2 5 ga and continuing until at least 1 9 ga the 13 ccarb the curve is much more dynamic with significant positive and negative excursions including a major positive excursion 9 pdb close to 2 2 ga these excursions can be correlated with the lomagundi event identified in africa europe and north america previously published studies of the overlying meso to palaeoproterozoic bangemall basin and of 1 8 1 5 ga old basins in northern australia suggest that the 13 ccarb curve became relatively monotonic again after ca 1 8 ga and remained so for most of the following mesoproterozoic comparisons with data from other ancient cratons especially africa suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the neoproterozoic when the western australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins the data are consistent with an earth in which the carbon mass balance was in equilibrium the 13 ccarb curve began to oscillate following the onset of glaciation as the pilbara and yilgarn cratons began to converge during the capricorn orogeny suggesting periods of rapid carbon burial during continental dispersal however the major positive excursion is preserved in carbonates from back arc basins formed as the ocean closed and subduction began since similar tectonic processes can be recognised not only in northern australia but also on other early cratons it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development we explain this coincidence of carbon isotopic excursions

isotope and rare earth element evidence for a late archaean terrane boundary in the southeastern pilbara craton western australia abstract rb sr sm nd and ree data have been obtained from granitic rocks exposed in archaean granite greenstone terrain in the southeastern part of the pilbara craton samples were collected from the kurrana batholith the cooninia billinooka and sylvania inliers and from the bottom of a borehole drilled through the hamersley basin succession in the fortescue valley almost all the samples have 87 rb 86 sr 144 nd and sm nd model ages tdm in the range 3 1 to 3 3 ga these characteristics together with ree patterns differ from published data for granitic rocks of the northern pilbara granite greenstone terrain that have similar geological histories rb sr isotopic systematics have been variably disturbed with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the hamersley basin succession 2 76 ga this is attributed to pervasive burial metamorphism beneath the hamersley basin and to later hydrothermal events some rock units appear to be younger than their northern pilbara equivalents a shear zone separating the mosquito creek synclinorium from the kurrana batholith has the characteristics of a suture zone and is postulated to be the boundary between two distinct terranes that joined to form the pilbara craton between 3 0 and 2 76 ga

the northern margin of the capricorn orogen western australia an example of an early proterozoic collision zone abstract the early proterozoic 2200 1600 ma capricorn orogen is a major zone of deformation metamorphism and granite emplacement between the archaean yilgarn and pilbara cratons of northwest australia capricorn orogen structures are recognized along the southern margin of the pilbara craton deforming the archaean granite greenstone terrain of the sylvania inlier together with rocks of the hamersley ashburton and blair basins in the sylvania inlier deformation is characterized by large scale shear zones interpreted as nne directed thrusts in the hamersley basin orogenic deformation forms the ophthalmia fold belt and can be divided into two groups of structures in the southwest large scale dome and basin folds pre ashburton basin are present in the southeast folds and faults are part of a younger syn ashburton basin n directed foreland fold and thrust belt directly linked to deformation in the sylvania inlier the deformed ashburton basin and adjacent parts of the hamersley basin form the ashburton fold belt two periods of deformation are recognized early recumbent folding d1a post ashburton basin pre blair basin and later dextral wrench faulting and associated folding d2a syn or post blair basin capricorn orogen structures are interpreted as the result of an oblique continent continent collision between the pilbara and yilgarn cratons along the southern pilbara margin collision first occurred in the southeast and migrated northwest with the ashburton basin evolving as a foreland basin late stage dextral wrench faulting was the result of westward extrusion of material caught between the opposing craton margins

rare earth elements in hamersley bif minerals abstract minerals from the hamersley banded iron formation western australia were analyzed for y and rare earth elements yrees by laser ablation icp ms to investigate diagenetic pathways from precursor phases to bif minerals one group of apatites carries the seawater ree signature giving evidence that p and rees thoroughly scavenged from the water column by si ferrihydrite particles were released upon microbial fe 3 reductive dissolution of si ferrihydrite in pore water and finally sequestered mainly in authigenic apatite the absence of fractionation between apatite and seawater suggests that ree were first incorporated into an amorphous calcium phosphate as fully hydrated cations i e as outer sphere complexes the iron oxides and carbonates carry only a small fraction of the whole rock ree budget their ree patterns are distinctly enriched in yb and show some m type tetrad effect consistent with experimental k d ree between solid and saline solution with low carbonate ion concentrations it is deduced that hematite formed at an incipient stage of fe 2 catalyzed dissolution of si ferrihydrite via a dissolution reprecipitation pathway the ree pattern of greenalite found as sub micron particles in quartz in a chert siderite sample is consistent with its authigenic origin by precipitation in pore water after dissolution of a small amount of si ferrihydrite magnetite carries very low yrees ppb level has an homogeneous pattern distinctly enriched in the mid rees compared to hematite and includes a late population depleted in light rees ba and as magnetite forming aggregates and massive laminae is tentatively interpreted as reflecting some fluid aided hematite magnetite re equilibration or transformation at low grade metamorphic temperatures

smnd geochronology of greenstone belts in the yilgarn block western australia abstract sm nd geochronology has been used to date 3 widely spaced metavolcanic greenstone belts in the yilgarn block the measured isochron ages are 2 78 0 03 ga initial nd 2 5 0 3 for greenstones at kanowna in the eastern goldfields province 3 05 0 10 ga initial nd 0 9 0 7 for diemals marda in the southern cross province and 2 98 0 12 ga initial nd 0 7 1 2 for the warriedar fold belt in the murchison province these data show that substantial crustal components of age 3 0 ga exist in the southern cross and murchison provinces and support published evidence that the eastern goldfields province contains no crustal material 2 8 ga these ages are all significantly younger than ages commonly observed in the western gneiss terrain and so support the suggestion of a progressive crustal age trend across the yilgarn block the measured age values depend strongly on the inclusion of data for felsic components of the greenstone belts data for mafic and ultramafic units allow that eruption in the 3 localities could have been contemporaneous at 2 8 ga the age range of metavolcanic rocks within each province is possibly comparable to the maximum possible age difference between study areas 0 2 ga so the documented lithological variations between the greenstone successions of the 3 provinces cannot be attributed to depositional age differences leaching experiments on altered warriedar samples indicate that both alteration and isotopic homogenisation occurred early in the evolution of the system probably prior to the 2 6 ga regional metamorphic event the initial nd values do not conform to previously suggested mantle depletion curves when considered with published data for greenstone belts they suggest complex mantle heterogeneity with end diverging over time to both positive and negative values

episodic ages of greenstones a key to mantle dynamics two greenstone age minima at 2 5 2 2 ga and 1 65 1 35 ga may correlate with ages of supercontinents when a greater proportion of greenstones were recycled into the mantle by subduction from the age distributions of greenstones and juvenile continental crust earth history can be divided into three stages i 2 8 ga when greenstones and microcontinents formed and collided continuously although probably not forming a supercontinent until about 3 ga ii 2 8 1 3 ga where a clear episodisity is apparent in greenstone eruption and collision ages and where two greenstone age minima coincide in age with supercontinents and iii 1 3 ga when greenstones and continents appear to have formed and collided continuously perhaps in response to significant overlap in the times of dispersal and assembly of supercontinents greenstone age peaks 2 7 1 9 and 1 3 ga may reflect catastrophic overturn of the mantle initiated by episodes of sinking of cold lithospheric slabs through the 660 km discontinuity to the d layer initiating an episode of major mantle plume activity continuous production and preservation of greenstones in the last 1 3 gy is consistent with seismic tomography data which suggest a steady sinking of slabs into the lower mantle

the nature of proterozoic reworking of early archaean gneisses mukalo creek area southern gascoyne province western australia abstract the gascoyne province a key segment of the early proterozoic capricorn orogen which separates the archaean pilbara and yilgarn cratons of western australia may be divided into northern central and southern zones the southern zone comprises predominantly archaean granulite facies quartzo feldspathic gneisses and passes southwards into the western gneiss terrain of the yilgarn block this area was part of the lower crust of a stabilized continental landmass at the end of the archaean development of the capricorn orogen 2 ga ago resulted in discontinuous deformation of the northern part of this landmass deformation was concentrated in ductile shear zones leaving blocks of relatively undeformed gneiss between them the shear zones trend between ene wsw and ese wnw and dip to the south at 50 60 strong down dip stretching lineations and asymmetric feldspar porphyroclasts indicate that the shear zones are reverse faults however metamorphic assemblages in early proterozoic dolerite dykes which post date the shear zones show that the area was still in the lower crust following the earliest proterozoic deformation isothermal uplift of the southern gascoyne province northern yilgarn block by 10 km or more occurred 1 7 1 5 ga ago and was accompanied by granitoid intrusion deformation within e w trending shear zones continued after uplift and was accompanied by patchy greenschist facies retrogression of both static and dynamic style on the local scale the effects of early proterozoic deformation on the early archaean gneisses are generally confined to shear zones and their immediate surroundings they comprise 1 realignment of gneissic trends from n s to e w 2 development of quartz ribbon mylonites by elongation of quartz grains into lenses and ribbons and reduction of feldspar grainsize by crystal plastic processes 3 growth of porphyroblastic and corona garnet in felsic and mafic gneisses the later proterozoic deformation occurred at lower temperatures and resulted in 1 further realignment of gneissic trends from northerly to easterly 2 reduction of quartz grainsize by crystal plastic processes and reduction of feldspar grainsize largely by brittle fracture 3 greenschist facies retrogression that the earliest deformation in the southern gascoyne province resulted in crustal shortening and was followed by significant uplift and granitoid intrusion indicates that proterozoic reworking of this area resulted from regional n s compression this favours development of the capricorn orogen in a collision zone between the pilbara and yilgarn cratons rather than in an ensialic mobile zone

precambrian tectonic history of the west australian craton and adjacent orogens this review is a synthesis of the precambrian geology of the west aus tralian craton and its fringing orogens it describes the evidence that has recently been reinvestigated and reinterpreted to reveal a long dynamic history of repeated aggregation and dispersal of crustal fragments cratons are large composite geological units of continental crust that have survived intact for significant periods of geological time they are generally tabular bodies with great areal extent millions of square kilo meters and lithospheric thickness 200 400 km orogens are narrow steep zones of tectonic and metamorphic activity associated with the deposition of sediments and the intrusion of plutonic rocks they occur within or along the margins of continents and are generally a few hundred kilometers wide thousands of kilometers long and extend to the base of the litho sphere most orogens are initially associated with compressive tectonics crustal thickening and the transient formation of mountain belts cratons and orogens may survive for very long periods of time but as part of a thin crust on a dynamic mantle they are inherently mobile and travel as rafts for great distances around the surface of the earth they may be torn apart dispersed and reassembled with other continental fragments the boundaries of these crustal fragments once established remain zones of crustal weakness that may be reactivated as intra continental mobile belts or lines along which cratons are assembled and dismembered cf hoffman 1 988

gold metallogeny of the northern capricorn orogen the relationship between crustal architecture fault reactivation and hydrothermal fluid flow abstract the timing and distribution of gold mineralization in proterozoic orogens is influenced by crustal architecture which is often established long before gold mineralization occurs gold occurrences in such settings are commonly associated with crustal scale faults formed at cratonic margins once established these faults provide critical pathways for hydrothermal and mineralizing fluids which during repeated fault reactivations can result in remobilization or introduction of new auriferous fluids resulting in overprinting gold events recently published geochronological data for the northern part of the proterozoic capricorn orogen in western australia show it has experienced at least three episodes of gold mineralization occurring at c 2400 1770 and 1680 ma this information combined with new data directly linking the timing of hydrothermal activity gold mineralization and fault reactivation show that the crustal scale nanjilgardy and baring downs faults and their subsidiary structures were re activated during discrete orogenic events many of the gold deposits are associated with intracratonic reworking during the 1820 1770 ma capricorn orogeny and 1680 1620 ma mangaroon orogeny intracratonic settings are not normally considered prospective for gold mineralization due to a lack of input of juvenile material however it appears that repeated hydrothermal fluid flow during intracratonic events has the potential to upgrade gold mineralization or increase gold endowment throughout the orogen either through gold remobilization or through introduction of new gold increasing the potential for economic gold deposits

corona textures between kyanite garnet and gedrite in gneisses from errabiddy western australia corona textures which developed in alternating layers in rocks in a supracrustal belt at errabiddy western australia involved a the production of staurolite cordierite and quartz or sapphirine between kyanite and or sillimanite and gedrite and b the production of cordierite between garnet and gedrite these textures are inconsistent with development along the same pressure temperature path in the system feo mgo al2o3 sio2 h2o but can be accounted for if cao mainly in garnet is taken into account the sapphirine bearing kyanite gedrite textures are explained by lower a sio2 during their development the assemblages indicate a consistent pressure temperature p t trajectory involving substantial uplift with only a slight decrease in temperature the history of these rocks includes reheating of originally high grade rocks that had cooled to a stable conductive geotherm followed by substantial essentially isothermal uplift the tectonic environment for this was presumably the one responsible for emplacement of the high grade terrain in the upper crust

revisiting the orogenic energy balance in the western taiwan orogen with weak faults orogens at convergent margins must meet the energetic requirements necessary to lift rocks against gravity allow for frictional sliding along basal detachments and accommodate internal deformation processes the combination of critical taper and kinematic wedge theories predicts the partitioning between these energy sinks as a function of both fault and crustal strengths integrating contemporary estimates of both crustal pore fluid pressures and the coefficient of friction on major faults we find that work associated with internal deformation processes is the dominant energy sink in the western taiwan orogenic wedge these processes consume 54 of the total work budget while the dissipation rates associated with frictional sliding on the basal detachment and lifting rocks against gravity are lower requiring only 11 and 35 respectively these results suggest a mechanical dichotomy in orogenic wedges where the faulting dominates the kinematic deformation budget but internal distributed deformation processes dominate the energy budget

using in situ shrimp u pb monazite and xenotime geochronology to determine the age of orogenic gold mineralization an example from the paulsens mine southern pilbara craton paulsens is a mesothermal orogenic gold deposit located in the wyloo inlier on the southern margin of the pilbara craton of western australia gold occurs in quartz sulfide veins hosted within a folded and faulted gabbro dike from which baddeleyite yields a u pb crystallization age of 2701 11 ma monazite and xenotime in the veins and from hydrothermally altered country rocks yield three distinct u pb dates of ca 2400 1730 and 1680 ma textural relationships between euhedral xenotime and pyrite with rounded native gold inclusions from within the quartz sulfide veins show that the primary gold mineralization was synchronous with xenotime crystallization at 2403 5 ma and coeval with pervasive alteration of the host rocks which yield monazite ages of 2398 37 and 2403 38 ma regional scale hydrothermal events at ca 1730 and 1680 ma are linked to the growth of monazite within phyllitic rocks at 1730 28 and 1721 32 ma carbonate veining at 1655 37 ma and gold remobilization or introduction of new gold at 1680 9 ma the ca 2400 ma age for mineralization and hydrothermal alteration does not correspond with any known deformation event in the region indicating a significantly different and more complicated low temperature tectonothermal evolution for the southern pilbara region than previously recognized the in situ secondary ion mass spectrometry dating of monazite and xenotime employed here will lead to better targeting of orogenic gold deposits in the northern capricorn orogen and these techniques can be utilized for orogenic gold exploration worldwide

pressure temperature time paths of regional metamorphism i heat transfer during the evolution of regions of thickened continental crust the development of regional metamorphism in areas of thickened continental crust is investigated in terms of the major controls on regional scale thermal regimes these are the total radiogenic heat supply within the thickened crust the supply of heat from the mantle the thermal conductivity of the medium and the length and time scales of erosion of the continental crust the orogenic episode is regarded as consisting of a relatively rapid phase of crustal thickening during which little temperature change occurs in individual rocks followed by a lengthier phase of erosion at the end of which the crust is at its original thickness the principal features of pressure temperature time ptt paths followed by rocks in this environment are a period of thermal relaxation during which the temperature rises towards the higher geotherm that would be supported by the thickened crust followed by a period of cooling as the rock approaches the cold land surface the temperature increase that occurs is governed by the degree of thickening of the crust its conductivity and the time that elapses before the rock is exhumed sufficiently to be affected by the proximity of the cold upper boundary for much of the parameter range considered the heating phase encompasses a considerable portion of the exhumation decompression part of the ptt path in addition to the detailed calculation of ptt paths we present an idealized model of the thickening and exhumation process which may be used to make simple calculations of the amount of heating to be expected during a given thickening and exhumation episode and of the depth at which a rock will start to cool on its ascent path an important feature of these ptt paths is that most of them lie within 50 c of the maximum temperature attained for one third or more of the total duration of their burial and uplift and for a geologically plausible range of erosion rates the rocks do not begin to cool until they have completed 20 to 40 per cent of the total uplift they experience considerable melting of the continental crust is a likely consequence of thickening of crust with an average continental geotherm a companion paper discusses these results in the context of attempts to use metamorphic petrology data to give information on tectonic processes 1984 oxford university press

neighbouring orogenic gold deposits may be the products of unrelated mineralizing events abstract models for exploration targeting are often developed by assessing known gold deposits in a region and targeting similar geological features such as geochemical anomalies favourable host rocks or structural settings with the assumption that they represented the footprint of the mineralizing event belvedere and paulsens are gold deposits with similar characteristics located in the wyloo inlier in the north capricorn orogen western australia gold at both deposits is hosted in quartz carbonate sulfide veins within mafic intrusive rock prior to this study they were thought to have formed during the same hydrothermal mineralizing event at belvedere in situ baddeleyite geochronology yields a weighted mean 207pb 206pb crystallization age of 2082 30 ma for the belvedere dolerite which hosts the ore body xenotime intergrown with ore stage alteration minerals and encased in arsenopyrite yielded a 207pb 206pb date of 1681 9 ma interpreted to represent the timing of hydrothermal activity related to gold mineralization at the belvedere deposit despite the similarities between the two deposits our results indicate that they underwent different geological histories with primary gold mineralization at paulsens occurring at c 2400 ma pre dating both the belvedere dolerite and mineralization within the dolerite furthermore field relationships show that the suite of dolerite dykes to which the belvedere dolerite belongs crosscut gold mineralization at paulsens the in situ u pb geochronology techniques employed here combined with field relationships have led to a better understanding on age constraints of gold deposition in the wyloo inlier and challenges the assumption that orogenic deposits in a given region probably formed during a single mineralizing event

early permian sediment provenance and paleogeographic reconstructions in southeastern gondwana using detrital zircon geochronology northern perth basin western australia abstract the perth basin western australia forms part of a north trending belt of intracratonic rift basins that initiated in southeastern gondwana in the late paleozoic existing provenance studies have highlighted the dominance of proterozoic zircons derived from southerly terranes such as the albany fraser orogen leeuwin complex and more distant provinces in east antarctica with limited input from the adjacent archean western australian craton new u pb geochronology data were acquired using a sensitive high resolution ion microprobe shrimp ii on detrital zircons from lower permian formations exposed in the northeastern perth basin in contrast to previous studies archean zircons showing fine oscillatory zoning dominate the age spectra with up to 90 grains ranging between 3 0 and 2 6 ga indicating that granitic parent rocks of the adjacent craton were a major sediment source during the early permian a minor paleoproterozoic age population from the gascoyne complex of the capricorn orogen was likely transported southward and dispersed in the basin by a combination of longshore drift and tidal currents meso and neoproterozoic detritus became prominent only from the middle artinskian and may relate to the establishment of large northward flowing river systems draining the albany fraser province the prydz leeuwin belt and or the wilkes land in east antarctica this study shows that integrating depositional processes and stratigraphic evolution of sedimentary formations with provenance analysis can enhance reconstruction of dispersal pathways to and within basins