constrained 3d modeling of the mesoproterozoic benagerie volcanics australia abstract the benagerie volcanics form part of a hotspot track that affected large sections of eastern australia during the mesoproterozoic understanding the location and controls on eruption centres of the benagerie volcanics and coeval plutons is difficult because they do not outcrop and are known only from limited drillhole data we have applied various combinations of forward and inversion modeling of magnetic and gravity to well constrained petrophysics and drillholes areas to characterize the 3d geometry and extent of the volcanic province and its crustal architecture thereby adding to a better understanding of the emplacement mechanisms involved we interpret the sw limit of the benagerie shear zone corridor to be a major terrane boundary separating relatively magnetic crust to the sw from crust with low magnetic susceptibility to the ne 2d and 3d models show a pronounced thickening of felsic material coincident with the shear zone that may be interpreted as a plutonic root thereby corroborating interpretations that this shear zone acted as a structural conduit for the ascent and emplacement of magma

investigation into the use of 2d elastic waveform inversion from look ahead walk away vsp surveys pore pressure in sediments beneath salt in the gulf of mexico varies widely creating a potentially dangerous and difficult drilling challenge estimating elastic parameters of sediments beneath salt is key to the prediction of pore pressure and reducing the drilling risk in exiting the base of the salt in this paper we investigate the ability of 2d waveform inversion to recover the elastic parameters in the sedimentary layer beneath the salt from a walk away vsp vertical seismic profile carried out with the receivers in the salt with the objective of estimating pore pressure at the base of the salt to be estimated using traditional methods we propose an effective method for performing the inversion and apply this method to a blind test of a large and realistic synthetic dataset to facilitate the design of a vsp survey suitable to this type of inversion we also present an analysis into the effects of the receiver array receiver spacing it is shown that the resulting velocity estimates are sufficiently accurate to predict the pore pressure within the limits required for drilling

reverse time migration a prospect of seismic imaging methodology abstract reverse time migration rtm is a seismic imaging method to map the subsurface reflectivity using recorded seismic waveforms the practice in exploration seismology has long established a two fold approach of seismic imaging using velocity modeling building to establish the long wavelength reference velocity models and using seismic migration to map the short wavelength reflectivity structures among various seismic migration methods for different situations rtm is the only method that is capable to use all seismic wave types that can be computed numerically being initiated in early 1980 s rtm seeks an image of the subsurface reflectivity as the best match in an image space between the extrapolation of time reversed waveform data and the prediction based on estimated velocity model and source parameters judging the image quality in the same space of forming the images is more advantageous than the approaches of modeling and inversion which seek the solution in the model space but judge its fitness in data space considering that most seismic migration applications today still use primary reflection as the only signal the capability of rtm to use all computable wave types is unique and helpful reducing the imaging artifacts due to mistaking non primary waves as primary reflections hence we refer to those rtm algorithms using only primary reflections as the first generation rtm methods and the rtm algorithms making a full use of primary reflections multiple reflections and other non primary waveform data as the second generation rtm methods this paper reviews the development history of the rtm along with its major challenges current solutions and future perspectives

geochronology of the sequence hosting the broken hill pb zn ag orebody australia robust stratigraphic control and constraints on the timing of events have been established for the paleoproterozoic metasedimentary and metavolcanic succession that hosts the broken hill pb zn ag orebody in western new south wales geologically consistent depositional intrusive and metamorphic ages are elucidated by means of shrimp u pb analyses of zircon and integrated field studies comparisons of the new shrimp results with those of previous studies clarify some previously untenable age interpretations a range of zircon provenance ages is found in albitic metasedimentary rocks from one of the oldest recognized parts of the willyama supergroup thackaringa group the zircons indicate maximum depositional ages of 1720 to 1730 ma and a minimum stratigraphic age for the lower thackaringa group is constrained by the 1704 3 ma intrusive alma gneiss the latter is the same age as gneissic granitoid rocks in the redan zone southern broken hill domain 1703 3 1705 3 ma felsic magmatism of this age has long been known in the contiguous olary domain south australia but has not been recognized previously at broken hill depositional ages in the broken hill group the upper part of which hosts the pb zn ag orebody indicate that the entire package was deposited within a period of ca 10 m y an age of 1685 3 ma for felsic metavolcaniclastic rocks in the hores gneiss spatially and stratigraphically associated with the ore horizon corroborates previous work this age is within error of that determined for the hanging wall and footwall unit rasp ridge gneiss of the orebody 1683 3 ma it suggests that felsic magmatism in the rasp ridge gneiss was coeval with that of the hores gneiss previous interpretations which suggested that the hores gneiss is not a single stratigraphic entity but comprises younger intrusions having ages from 1690 to1640 ma are not supported by our data and conclusions the stratigraphic integrity of the broken hill group excluding some mafic intrusions is further supported by results from an older formation near its base where tuffaceous metasedimentary rocks from the ettlewood calc silicate member allendale metasediments define an age of 1693 4 ma this is in good agreement with the age of 1693 5 ma for the immediately overlying parnell formation provenance ages obtained from detrital zircons in broken hill group psammites indicate a remarkable uniformity of paleoproterozoic and late archean source s generally matching provenance ages are found in the overlying sundown group psammopelites suggesting a stable flux of sediment fill as the basin deepened and matured similar provenance is indicated for upper paragon group sedimentary rocks and inherited zircons in most of the younger granitoids have age spectra similar to those of the detrital zircons the perpetuation of these age patterns in both clastic detritus and magmatic xenocrysts thus broadly fingerprints exposed and buried older crustal elements that provided material to produce the willyama supergroup basin fill tuffaceous metasiltstones in the middle paragon group bijerkerno metasediments are interpreted to have been derived partly from air fall volcaniclastic material and give consistent depositional ages of 1655 4 and 1657 4 ma in the northern broken hill and euriowie blocks respectively younger tuffaceous metasiltstones in the upper paragon group dalnit bore metasediments provide a stratigraphically consistent maximum depositional age of 1642 5 ma zircon ages from granitoid intrusions that bracket the deformation events suggest that the d 2 and d 3 deformations are inseparable within the interval 1597 3 to 1591 5 ma low th u zircon overgrowths are ubiquitous and irrespective of stratigraphic position record metamorphic ages of 1600 ma metamorphic rutile has the same age confirming that high grade metamorphic event s and d 2 and d 3 structural events were approximately synchronous we find no isotopic support for any pre 1600 ma high grade metamorphic events using newly acquired local age control for the galena pb isotope growth curve the new data support a stratigraphic syngenetic or diagenetic model of primary ore formation for the broken hill orebody at ca 1690 ma although the present distribution of the lodes likely reflects displacement as a result of later structural remobilization

1600 1500 ma hotspot track in eastern australia implications for mesoproterozoic continental reconstructions mesoproterozoic a type magmatic rocks in the gawler craton curnamona province and eastern mount isa inlier form a palaeo curvilinear belt for reconstructed plate orientations the oldest igneous rocks in the gawler craton are the hiltaba granite suite c 1600 1575 ma the youngest in the mount isa inlier are the williams naraku batholiths c 1545 1500 ma the belt is interpreted as a segment of a hotspot track that evolved between c 1600 and 1500 ma this hotspot track may define a quasilinear part of australia s motion between 1636 and 1500 ma and suggests that australia drifted to high latitudes an implication of this interpretation is that australia and laurentia may not have been fellow travellers leading to the formation of rodinia a hotspot model for a type magmatism in australia differs from geodynamic models for this style of magmatism on other continents this suggests that multiple geologic processes may be responsible for the genesis of proterozoic a type magmas

mitigation of defocusing by statics and near surface velocity errors by interferometric least squares migration with a reference datum abstractimaging seismic data with an erroneous migration velocity can lead to defocused migration images to mitigate this problem we first choose a reference reflector whose topography is well known from the well logs for example reflections from this reference layer are correlated with the traces associated with reflections from deeper interfaces to get crosscorrelograms interferometric least squares migration ilsm is then used to get the migration image that maximizes the crosscorrelation between the observed and the predicted crosscorrelograms deeper reference reflectors are used to image deeper parts of the subsurface with a greater accuracy results on synthetic and field data show that defocusing caused by velocity errors is largely suppressed by ilsm we have also determined that ilsm can be used for 4d surveys in which environmental conditions and acquisition parameters are significantly different from one survey to the next the limitations of ilsm are that it requires prior knowledge of

a structural metamorphic study of the broken hill block nsw australia the prograde pressure temperature p t path for the complexly polydeformed proterozoic broken hill block australia has been reconstructed through detailed structural analysis in conjunction with calculation of compositionally specific p t pseudosections of pelitic rock units within a high temperature shear zone that formed early in the tectonic evolution of the terrane whilst the overall p t path for the broken hill block has been interpreted to be anticlockwise the prograde portion of this path has been unresolved our results have constrained part of this prograde path showing an early heating event m1 at p t conditions of at least c 600 c and c 2 8 4 2 kbar associated with an elevated geothermal gradient c 41 61 c km 1 this event is interpreted to be the result of rifting at c 1 69 1 67 ga or at c 1 64 1 61 in the broken hill block early rifting was followed by an episode of lithospheric thermal relaxation and burial during which time sag phase sediments of the upper broken hill stratigraphy paragon group were deposited following sedimentation a second tectonothermal event m2 d2 occurred this event is associated with peak low pressure granulite facies metamorphism c 1 6 ga and attained conditions of at least 740 c at c 5 kbar a regionally pervasive high temperature fabric s2 developed during the m2 d2 event and deformation was accommodated along lithology parallel high temperature shear zones the larger scale deformation regime extensional or shortening of this event remains unresolved the m2 d2 event was terminated by intense crustal shortening during the olarian orogeny during which time the first mappable folds within the broken hill block developed

ion microprobe dating of paleozoic granitoids devonian magmatism in new zealand and correlations with australia and antarctica abstract precise ion microprobe u pb zircon ages have been obtained from a representative set of paleozoic igneous rocks from the western province of the south island new zealand granitoid rocks forming the karamea batholith and related plutons in the buller terrane all yield crystallisation ages that are indistinguishable within a 5 ma uncertainty at 375 ma middle late devonian previous workers have suggested that the batholith was emplaced over a long time interval and comprises rocks ranging in age from 370 to 310 ma with the bulk in the early carboniferous granitoid rocks with carboniferous ages 330 ma do occur further to the west and younger cretaceous granitoids occur within the karamea batholith but these are not volumetrically significant a sample from the ultramafic mafic riwaka complex in the adjacent takaka terrane gave a crystallisation age of 376 9 5 6 ma 2 indicating emplacement coeval with the karamea batholith the paleozoic granitoids contain a large amount of inherited zircon with distinct 390 500 600 and 1000 ma components the 390 ma age corresponds to widespread plutonism in the lachlan fold belt in se australia the 500 600 ma ross delamerian age and 1000 ma grenville age age components have also been observed in granitoids from the lachlan fold belt and in ordovician metasedimentary rocks from se australia and new zealand the inherited zircons in the karamea batholith could be derived from continental basement at depth from the incorporation of upper crustal material in the granitoid magmas or both the devonian granitoids in new zealand can now be correlated with rocks of similar age in northern victoria land admiralty intrusives and marie byrd land ford granodiorite in west antarctica in ne tasmania and in the central part of the lachlan fold belt in se australia on a reconstruction of the sw pacific margin of gondwana prior to later break up it is possible to trace out a semi continuous magmatic belt in excess of 2000 km in length

the 1800 1610 ma stratigraphic and magmatic history of the eastern succession mount isa inlier and correlations with adjacent paleoproterozoic terranes the chronostratigraphic and magmatic framework for the proterozoic mount isa inlier northwest queensland has been substantially revised to match insights provided by isotopic age constraints accumulated over recent years this material and existing chronometric constraints is used to construct a new time space plot for the inlier and to compare eastern succession volcanosedimentary accumulations with temporally equivalent units in the western fold belt the accumulation of volcanosedimentary sequences in the eastern succession occurred in four main periods namely 1890 1850 ma 1800 1750 ma 1710 1690 ma and 1680 1610 ma temporally equivalent units can be identified in the western fold belt periods of sedimentary accumulation were immediately preceded by bimodal igneous activity across the inlier the locus of bimodal volcanism migrates from west to east with time intracontinental rifting is the most likely tectonic setting for the sedimentary basins the majority of the soldiers cap group was deposited between ca 1675 and 1650 ma with the exception of units in the boomarra horst now known to belong to older sequences deposited ca 1775 1750 ma and the llewellyn creek formation at snake creek which may be as old as ca 1690 ma upper amphibolite facies rock units in the southwest e g the gandry dam gneiss are likely to be higher metamorphic grade temporal equivalents of the soldiers cap group five rock units previously assigned to the cover sequence 2 mary kathleen group namely the answer slate kuridala formation staveley formation marimo slate and agate downs siltstone are now known to have been deposited between ca 1675 and 1610 ma these units bar the kuridala formation have been removed from the cover sequence 2 mary kathleen group and given the informal name young australia group the kuridala formation has been included in the soldiers cap group the rock units within the young australia group are likely to be lateral facies equivalents of the younger units within the soldiers cap group rock units within the mount albert group although undated are considered likely to be temporal equivalents of the soldiers cap group and the young australia group the roxmere quartzite is included here in the young australia group that it tectonically overlays the ballara quartzite and mitakoodi quartzite represent a single regionally extensive sequence deposited at ca 1755 ma the double crossing metamorphics previously mapped as basement are likely to correlate with the argylla formation and mitakoodi quartzite most of the eastern succession to the west of the pilgrim fault is likely to have been a topographic high between ca 1780 and 1755 ma prior to the ballara quartzite unconformably overlying the ca 1780 ma argylla formation detrital zircons of argylla and wonga ages indicate that the mary kathleen belt is likely to have been a topographic high at least periodically during accumulation of the soldiers cap and young australia groups soldiers cap group units in the southeast contain numerous detrital zircons giving ages between ca 1730 and 1710 indicating a possible siliciclastic source to the east the host rocks to the pb zn ag deposits are restricted to cover sequence 3 units deposited between ca 1680 and 1590 ma and with the exception of century young from the southeast towards the northwest cu au deposits and prospects are hosted in units from all the major periods of volcanosedimentary accumulation however many of the most significant deposits are found within the younger sequences

superwide angle one way wave propagator and its application in imaging steep salt flanks one way wave propagators have been used extensively in forward and inverse problems the advantages of one way wave propagators are the fast speed of computation and the large savings in computer resources however regular one way wave propagators have inherent angle limitation to within 90 of the preferred direction e g z direction we propose to overcome the limitation by a superwide angle one way method which combines and interpolates two orthogonally propagated one way wavefields in the 2d case a weight function determined by the propagation angle is employed to combine the two one way wavefields in the current implementation we use the wavefield gradient to estimate the propagation angle numerical experiments show that the proposed method has accurate superwide angle 90 propagation and can model turning waves well by applying the superwide angle one way wave propagator on several synthetic model data sets we can demonstrate the validity of the method in imaging steep and overhanging

a basin system and fluid flow analysis of the zn pb ag mount isa type deposits of northern australia identifying metal source basinal brine reservoirs times of fluid expulsion and organic matter reactions paleoproterozoic rocks of northern australia host one of the world s largest base metal repositories and are the world s most important zinc repository the mcarthur mount isa cloncurry mineral belt contains several world class zn pb ag u cu and cu au deposits ewers and fergusson 1980 williams 1998 betts et al 2003 large et al 2005 fig 1 the province has the potential to host additional base metal and uranium reserves advances in exploration techniques in the 1980s led to the discovery of several major new zn and cu au deposits including cannington century ernest henry and osborne however recent exploration results have been disappointing and new exploration strategies are required if the region is to further its growth potential and if new resources are to be realized beneath shallow cover fig 1 map of northeastern australia showing the principal tectonostratigraphic subdivisions for the region the map highlights the areas of study covered by the thematic papers in this volume and the location of the principal zn pb ag cu and cu au deposits the locations of profiles p1 and p2 the comalco seismic grid and deep boreholes including desert creek 1 are shown in the inset map bottom left between 1975 to 1995 geoscientists from geoscience australia the geological survey of queensland and the northern territory geological survey mapped the paleoproterozoic outcrop belt of northern australia at a scale of 1 100 000 subsequently researchers at monash university undertook detailed studies of the deformation history of the mount isa block placing the ore deposits within a tectonic context e g o dea et al 1997 betts et al 1998 2003 betts and lister 2002 researchers at james cook university carried out structural metamorphic and mineralization studies across the mount isa block with their principal focus concentrating on the deposits and their immediate environs bell et al 1988

reinterpretation of the tectonic context of high temperature metamorphism in the broken hill block nsw and implications on the palaeo to meso proterozoic evolution abstract the origin of high temperatures during regional low pressure granulite facies metamorphism within the proterozoic broken hill block australia has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca 1 71 1 67 ga and then maintained during a ca 1 62 ga short lived mid crustal extensional event mid crustal extension at ca 1 62 ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients and occurred prior to activity along d2 high temperature shear zones peak low pressure granulite facies metamorphism ca 1 60 ga and crustal shortening during the olarian orogeny ca 1 60 1 59 ga this reinterpretation places the broken hill block within an environment in which multiple episodes of transient extension ca 1 71 1 62 ga were followed by a switch to shortening at ca 1 60 ga the interpreted tectonic environment is a continental back arc setting located in the over riding plate of a subduction zone along the southern margin of palaeoproterozoic australia

stratigraphic and structural constraints on the proterozoic tectonic history of the olary block south australia abstract the lower proterozoic of the olary block is divided into five rock suites defining a conformable stratigraphic sequence correlated with the willyama supergroup of the broken hill block which has a probable depositional age no greater than 1800 ma the sequence was deformed and metamorphosed during the olarian orogeny at 1580 ma definition of the stratigraphy permits regional structural analysis and the resolution of two folding events an initial recumbent fold terrain redeformed by upright folds with a near vertical axial surface the olarian orogeny comprises a single regional compression axis with both folding events indicating a se direction of transport the described tectonic fabrics are consistent with simple shear deformation generating the early recumbent terrain

textural evidence for extensive melting of the broken hill orebody galena pyrrhotite and chalcopyrite in samples of zn rich ore from the broken hill orebody show textures similar to those in partially melted metamorphic rocks including strings of low melting phases separating lithons of minerals likely to be restite low dihedral angles between low melting temperature minerals and restitic minerals and enlargement of these dihedral angles due to partial annealing analysis of existing experimental work from the system fe pb zn s indicates that at 1 bar galena and sphalerite coexisting with pyrrhotite of the compositions found at broken hill would have been partially melted at temperatures of about 750 c at 5 kbars this melting would occur at 780 c this temperature is well within the range of metamorphic temperatures reported from the area from these data we conclude that the broken hill orebody was partially molten at peak metamorphism with much of the pb cu ag sb and as in the melt and fe and zn residing mostly in the restite differentiation during cooling enriched the residual melt in sb as ag and other low melting temperature chalcophile elements the main sulfide melt froze after the last penetrative deformation to affect the orebody at temperatures below 720 c and possibly as low as 650 c sulfosalts crystallized out of the remaining melt at temperatures of 600 to 450 c and the final melt crystallized native bismuth at ca 250 c

imaging of offset vsp data acquired in complex areas with modified reverse time migration a modified reverse time migration algorithm for offset vertical seismic profiling data is proposed this algorithm performs depth imaging of target areas in the borehole vicinity without taking into account the overburden originally recorded seismograms are used reliable results can be obtained using only the velocity profile obtained along the well the downgoing wavefield emitted from a surface source is approximated in the target area using the transmitted p wave recorded by the receivers deployed in the well this is achieved through a reverse time extrapolation of the direct transmitted p wave into the target area after its separation in offset vertical seismic profiling seismograms generated using a finite difference scheme for the solution of the scalar wave equation the proposed approach produces kinematically reliable images from reflected pp and ps waves and furthermore can be applied as a salt proximity tool for salt body flank imaging based on the transmitted ps waves our experiments on synthetic data demonstrate that the modified reverse time migration provides reliable depth images based on offset vertical seismic profiling data even if only the velocity profile obtained along the borehole is used

local vertical seismic profiling vsp elastic reverse time migration and migration resolution salt flank imaging with transmitted p to s waves to avoid the defocusing effects of propagating waves through salt and overburden with an inaccurate overburden velocity model we introduce a vertical seismic profiling vsp local elastic reverse time migration rtm method for salt flank imaging by transmitted p to s waves this method back projects the transmitted ps waves using a local velocity model around the well until they are in phase with the back projected pp waves at the salt boundaries the merits of this method are that it does not require the complex overburden and salt body velocities and it automatically accounts for source side statics in addition the method accounts for kinematic and dynamic effects including anisotropy absorption and all other unknown rock effects outside of this lo cal subsalt velocity model numerical tests on an elastic salt model and offset 2d vsp data in the gulf of mexico using a finite difference time domain staggered grid rtm scheme partly demonstrate the effectiveness of this method over interferometry

delamerian aged metamorphism in the southern curnamona province australia implications for the evolution of the mesoproterozoic olarian orogeny monazite electron microprobe u th pb and garnet sm nd isotopic data from metapelitic assemblages in the willyama supergroup in the southern curnamona province south central australia indicate that the terrain underwent regional greenschist to amphibolite grade metamorphism during the c 500 ma delamerian orogeny the delamerian aged mineral assemblages include prograde garnet staurolite and kyanite bearing associations that overprint andalusite and sillimanite bearing assemblages that are interpreted to have developed during the c 1600 ma olarian orogeny importantly the development of secondary kyanite bearing assemblages in the southern curnamona province has been used previously to suggest that the olarian orogeny followed an anticlockwise p t evolution if such assemblages are the product of c 500 ma metamorphism then the anticlockwise p t path is an apparent path due to the overprint of a distinct metamorphic cycle c 1100 ma later making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny this study highlights the utility of in situ geochronology linking age data to petrologically important phases and assemblages

vsp beyond time to depth vsp or vertical seismic profile was originally designed and is currently primarily used to give us time to depth for seismic well ties beyond time to depth a number of possibilities exist recently there has been considerable interest in vsp imaging ray et al 2003 paulsson et al 2004 hornby et al 2004 hornby et al 2005a with extensive surveys being acquired both on land and offshore modeling studies using full waveform finite difference method fdm payne et al 1994 van gestel et al 2003 show us what we can image for a particular acquisition geometry and geology with best image results seen with 3d vsp surveys incorporating a large vsp array in the well and a 2d source pattern acquired using a surface seismic shooting vessel traditionally vsp imaging has been implemented using surface seismic processing algorithms however the vsp geometry poses its own challenges and unique opportunities in this article we explore some imaging methods to attempt to take advantage of the vsp g

imaging above an extended reach horizontal well using converted shear waves and a rig source abstractwe evaluated a method for using 3c vertical seismic profile data to image acoustic interfaces located between the surface source and a downhole receiver array the approach was based on simple concepts adapted from whole earth seismology in which observed compressional and shear wavefields are traced back to a common origin however unlike whole earth and passive seismology in which physical sources are imaged we used the observed compressional and shear wavefields to image secondary sources scatterers situated between the surface source and the downhole receiver array the algorithm consisted of the following steps first estimating the receiver compressional wavefield second using polarization to estimating the shear wavefield third deconvolving the shear wavefield using estimates of the source wavelet obtained from the direct compressional wave fourth the compressional and shear wavefields were back projected into the volume between the source and receivers where finally an imag

geochronology of early to middle proterozoic fold belts in northern australia a review abstract this geochronological review integrates geologically relevant isotopic data from the several early to middle proterozoic fold belts of northern australia a number of firm chronological benchmarks largely established by means of u pb zircon studies now provide a time framework within which to better quantify the tectonostratigraphic history of many of these terranes the apparent worldwide dearth of crustal events in the earliest proterozoic between 2500 and 2000 ma was broken in northern australia with the advent of a major crust forming episode between 1880 and 1850 ma ago these rocks may have evolved from an initial mantle or lower crustal melting and fractionation process about 400 300 ma earlier that is evidenced from sm nd model ages the extent involvement and or degree of assimilation of any pre existing archaean crust is small but whether or not it is negligible or zero is not satisfactorily known the 1880 1850 ma event the barramundi orogeny is well represented in northern australia both lithologically and analytically and the rocks that pre and post date it have a similar tectonic setting over very wide areas of the continent two consistent and widespread elements of this tectonism are regional metamorphism and deformation of the first cycle of supracrustals barramundi orogeny and crystallization of i type k rich felsic magmas closely following and in some cases overlapping with deformational processes in the halls creek inlier the ages of this rapid deep crustal to supracrustal tectonic transition cannot be distinguished within the range 1860 1850 ma this major yet short lived tectonism is mirrored in other fold belts such as pine creek tennant creek and mount isa in which u pb zircon ages give slightly older constraints of 1885 1870 ma 1920 1870 ma and 1885 10 ma respectively the ensuing felsic magmatism in these latter terranes continued for a few tens of millions of years with the crystallization and emplacement of comagmatic volcanic plutonic pairs throughout in the interval 1870 1860 ma in the georgetown and arunta inliers direct evidence of this otherwise ubiquitous event if present is masked by younger high grade metamorphism in the latter however metamorphic crustal components having approximate protolith ages of this order can be inferred from their evolved isotopic signatures the geochronological and geochemical coherency of the extensive early proterozoic 1870 ma magmatism contrasts with that of post 1800 ma magmatism which is less extensive generally anorogenic commonly bimodal in character and varies in age from one terrane to another several major sedimentary basins were being filled at this time major igneous events at 1790 1740 ma 1670 ma and 1500 ma are recognized elongate sublinear volcanic and plutonic belts 1790 1740 ma old are present at mount isa in high grade granitic gneisses of the arunta inlier a few u pb zircon ages and extrapolated rb sr protolith ages are coincident at about 1760 1750 ma indicating that this event may be important in the development of the division 2 sequences of that fold belt the platformal hart dolerite and oenpelli dolerite two of the largest mafic intrusions in the world were emplaced at 1762 25 ma and 1688 13 ma younger felsic plutonism and volcanism gave rise to rapakivi type granites in the mount isa tennant creek and the granites tanami inliers at 1670 ma a time close to that of the aileron deformational metamorphic event in central australia further at 1650 ma there is growing evidence of an unexpectedly coherent hydrothermal episode that reset or disturbed many rb sr ages gave rise to pegmatites and was possibly associated with mineralization in the davenport and eastern arunta inliers discrete middle proterozoic deformational events between 1610 and 1470 ma punctuate the tectonostratigraphic history in the arunta mount isa and georgetown fold belts where they are best documented

subsalt 3d vsp imaging at deimos field in the deepwater gulf of mexico at deimos field in the deepwater gulf of mexico surface seismic imaging cannot fully image sedimentary structure near the steeply dipping base of salt 3d vertical seismic profile vsp imaging was introduced to the deepwater gom to improve seismic imaging of complex structural and stratigraphic features ray et al 2003 hornby et al 2006 and recently has been used for imaging blind spots in surface seismic coverage hornby et al 2005 hornby et al 2007 with this method 3d surveys are conducted using a surface source vessel and downhole geophones here we investigate the use of 3d vsp imaging to complement the surface results with additional coverage beyond that of the surface seismic adjacent to the complex salt body

characterization of a proposed palaeoproterozoic suture in the crust beneath the curnamona province australia abstract we use constrained forward and inverse modeling of bouguer gravity data to investigate a regional positive gravity anomaly that defines but is not confined to the eastern margin of the curnamona province best fit results imply that the positive gravity values are sourced from a relatively dense terrane 0 04 mg m 3 with respect to surrounding crust located at middle crustal levels 16 27 km depth which is likely dismembered and offset into discrete slivers by later tectonism consideration of pre 1500 ma plate reconstructions of the australian continent correlates this terrane with an interpreted buried suture in the mount isa inlier of northern australia where allochthonous crust was amalgamated to the australian margin at ca 2 20 1 85 ga bouguer gravity forward and inverse modeling also delineates a shallow west dipping crustal scale structure that possibly represents a primary boundary between precambrian basement of australia and the eastern fold belts accreted to the precambrian margin during the palaeozoic

a sedimentary overlap assemblage links australia to northwestern laurentia at 1 6 ga abstract the columbia nuna supercontinent existed from approximately 1 9 ga to 1 3 ga laurentia was part of columbia and the western edge of laurentia current coordinates was likely proximal to a large landmass during parts of this interval here we present detrital zircon ages of a paleoproterozoic sedimentary succession in northern yukon canada that bear on the evolution of columbia the sedimentary succession is preserved as clasts within 1 60 ga hydrothermal megabreccias analyses of detrital zircon reveal abundant 1 78 1 68 ga zircon with evolved hf isotope values 16 1

did the giant broken hill australia zn pb ag deposit melt abstract recent published research has proposed that some metamorphosed massive sulfide deposits including the broken hill deposit australia have partially melted here we discuss the evidence for and against this process the broken hill deposit is located in the curnamona craton within a rift in an apparent upward coarsening sequence of clastic metasediments into which paleoproterozoic mafic and felsic melts intruded the deposit was metamorphosed to granulite facies conditions 750 800 c and 5 6 kbar and was subjected to at least five periods of deformation at peak p t conditions a melt phase was produced in silicate rocks especially granitoids and psammopelites and in ore characterized by the systems cu as s ag pb s ag sb as s cu sb pb s sb as s cu sb s and fe as s however the proportion of minerals in these s bearing systems is insignificant 2 feo mno cao al 2 o 3 p 2 o 5 co 2 zns pbs fes feass feas 2 which is the most relevant system to broken hill ores p t conditions were never high enough to produce a melt phase in this system authors arguing for sulfide rock melting suggest that mn rich lithologies garnetite and quartz garnetite which are intimately associated with the pb rich ores at the broken hill deposit were produced by a reaction between mn rich sphalerite and aluminous wall rocks however to produce such rocks would require mn contents of sphalerite compositions that are unrealistically high and yet to be found in nature moreover the implication of the melt model is that wherever mn rich quartz garnetite and garnetite are found sphalerite should be located next to them this is clearly not the case the presence of polyphase sulfide inclusions within garnet in garnetite has also been considered by some in the literature to be sulfide melt inclusions with the implication that the enclosing garnet was a product of melting however we consider these sulfides to be products of hydrothermal processes during retrograde metamorphism it is impossible to form garnetite and quartz garnetite in rocks within the curnamona craton that formed at upper greenschist lower amphibolite facies conditions by partial melting p t conditions were too low we consider that the fe and mn component of garnetite and quartz garnetite are products of exhalation and inhalation at or near the sea floor and that these rocks are meta exhalites or meta inhalites some garnet rich rocks also formed by metasomatic processes throughout the protracted metamorphic history that affected the curnamona craton

sedimentary model for the giant broken hill pb zn deposit australia during the past 15 years there has been increasing acceptance of a volcanic exhalative origin for the giant 300 mt broken hill pb zn orebody supposedly silicic pyroclastic rock types have been identified in the host rock succession and because of an association with basic volcanics amphibolites comparison is made with phanerozoic continental rifts the orebody has also been described as distal and deep water we report that the so called silicic volcanic rocks are all normal clastic sediments the metasedimentary succession more than 5 km thick is interpreted as a series of major continental fluvio deltaic progradational wedges interfingering with marine transgressive cycles deposited in a gradually deepening basin pb zn mineralization is hosted by shallow marine sands that later became reservoirs for the metal bearing brines basin formation stratigraphic architecture and pb zn mineralization can all be interpreted in terms of concepts understood from studies of the development of younger sedimentary basins the inference is that pb zn mineralization was generated by compactive expulsion of metal bearing brines during accumulation of the sedimentary pile

the tam ky phuoc son shear zone in central vietnam tectonic and metallogenic implications abstract the tam ky phuoc son shear zone tkpssz is one of the most pronounced crustal structures in central vietnam and juxtaposes high grade rocks of the kontum massif against lower grade rocks of the truong son fold belt an internal zone comprises highly strained rocks including ultramafic mafic tectonites bounded by mylonitic zones that straddle tens to hundreds of kilometers an external zone comprises mainly high grade metamorphic complexes southern flank and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone northern flank detailed structural analysis reveals that the shear zone is a multi deformed terrane d1 produced a regional high grade schistosity and mylonitization d2 generated regional northwest southeast trending fold arrays and brittle ductile shear zones d3 locally produced northeast trending folds brittle faulting occurred during d4 d5 u pb dating of zircon and monazite suggests that d1 involved metamorphism and felsic magmatism at ca 430 ma recording part of a regional collisional orogeny monazite and titanite growth at ca 250 240 ma in basement rocks is synchronous with widespread syn to post d2 magmatism between ca 260 and 245 ma suggesting a second major collisional event during the indosinian orogeny d3 may have occurred as part of or soon after this permo triassic event deformation during post collisional stages perhaps in response to extrusion wedging and oroclinal rotation of terranes led to post d3 structures d4 d5 the e w trending tkpssz is here shown to be a continuation of the n s trending po ko shear zone pksz most of the significant lode gold occurrences in central vietnam occur along this tkpssz pksz structure and are associated with and controlled by d1 ductile to ductile brittle high strain zones mineralized later stage structures support remobilization and reconcentration of ore during subsequent events at ca 240 ma re os molybdenite age the tkpssz pksz d1 structure likely represents part of a paleosuture zone marking the closure of an ancient ocean basin through terrane assembly in the early paleozoic

age constraints on the hydrothermal history of the prominent hill iron oxide copper gold deposit south australia the mesoproterozoic prominent hill iron oxide copper gold deposit lies on the fault bound southern edge of the mt woods domain gawler craton south australia chalcocite bornite chalcopyrite ores occur in a hematitic breccia complex that has similarities to the olympic dam deposit but were emplaced in a shallow water clastic carbonate package overlying a thick andesite dacite pile the sequence has been overturned against the major steep east west hangingwall fault beyond which lies the clastic to potentially evaporitic blue duck metasediments immediately north of the deposit these metasediments have been intruded by dacite porphyry and granitoid and metasomatised to form magnetite calc silicate skarn pyrite chalcopyrite the hematitic breccia complex is strongly sericitised and silicified has a large sericite chlorite halo and was intruded by dykes during and after sericitisation this paper evaluates the age of sericite formation in the mineralised breccias and provides constraints on the timing of granitoid intrusion and skarn formation in the terrain adjoining the mineralisation the breccia complex contains fragments of granitoid and porphyry that are found here to be part of the gawler range volcanics hiltaba suite magmatic event at 1600 1570 ma this indicates that some breccia formation post dated granitoid intrusion monazite and apatite in fe p ree albite metasomatised granitoid paragenetically linked with magnetite skarn formation north of the hangingwall fault grew soon after granitoid intrusion although the apatite experienced u pb lree loss during later fluid mineral interaction this accounts for its calculated age of 1544 39 ma to the south of the fault within the breccia 40ar 39ar ages yield a minimum age of sericitisation cu fe ree of dykes and volcanics of 1575 ma firmly placing prominent hill ore formation as part of the gawler range volcanics hiltaba suite magmatic event within the olympic cu au province of the gawler craton

water activity changes across an amphibolite granulite facies transition broken hill australia phase compositions in pelitic and mafic gneisses place tight constraints on pressure ranging from 3 up to 6 kb and to a lesser extent on temperature 500 up to 800 c during prograde regional metamorphism of the willyama complex broken hill se australia these limits allow an evaluation of water activity across the terrain using various equilibria in pelitic and mafic gneisses the stability of cummingtonite and biotite over much of the terrain places upper limits on temperature and the presence of syn metamorphic partial melts in the metasediments places lower limits on a h2o garnet biotitesillimanite k feldspar quartz relations combined with the partial melting data suggest a decrease in water activity from near 1 0 in the lower grade zones to 0 5 0 2 in the broken hill little broken hill part of the two pyroxene zone this result is compatible with less precise hornblende orthopyroxene clinopyroxene quartz relations

meta evaporite sequence exhalative chemical sediments and associated rocks in the proterozoic willyama supergroup south australia implications for metallogenesis abstract the willyama supergroup comprises proterozoic meta sedimentary and meta igneous rocks in western new south wales and eastern south australia and contains the giant broken hill pb zn ag deposit in addition to numerous small stratiform stratabound and vein mineral deposits new lithological geochemical and fluid inclusion data from the olary block west of broken hill suggest the presence of former evaporite rocks now metamorphosed to a variety of chemically and mineralogically unusual lithologies these evaporite rocks overlie relatively oxidised strata and are in turn overlain by reduced graphite bearing pelites rocks of sodic composition abound represented by albite quartz rich rocks calcite actinolite epidote garnet clinopyroxene inferred to represent metamorphosed analcime rich strata of felsic volcaniclastic provenance with variable proportions of carbonates for example calcite dolomite trona and shortite these rocks are interspersed with potassic rocks possibly also with an evaporitic origin in addition to normal clastic meta sediments the presence of halogen rich minerals in a distinctive mineral assemblage hypersaline fluid inclusions pseudomorphic voids and minerals after gypsum and carbonate in addition to tourmalines with isotopically light boron isotopes is interpreted as evidence for a non marine evaporite parent rock for portions of the olary block types of mineral occurrences in the olary block form a continuum between chemical sediments formed by the exhalations of hydrothermal fluids to intrasediment deposits inferred to have been formed due to the chemical changes imposed on these and or diagenetic fluids during their passage through the sedimentary pile disseminated stratabound pyrite including cobaltian types and chalcopyrite extend along strike for many kilometres and are interpreted to have formed by a diagenetic mechanism the presence of massive sulphide bearing strata iron formations barite tourmalinite and manganiferous rocks are typical of chemical sediments derived from hot spring sources the olary block meta evaporites have similar whole rock compositions to tertiary analcime rich rocks of the green river formation wyoming these new data imply that portions of the adjacent broken hill block may also comprise meta evaporites for example the quartz albite rich strata of the thackaringa group and the calcsilicates of the lower broken hill group and that the disseminated cobaltian pyrite mineralisation in the thackaringa group may not be of exhalative origin

felsic metavolcanic rocks related to the broken hill pb zn ag orebody australia geology depositional age and timing of high grade metamorphism two key issues related to the origin of the massive broken hill pb zn ag orebody are the genesis of its host rocks and their depositional age these granulite facies layered gneisses have been ascribed alternately to volcanic and nonvolcanic regimes determination of the depositional age of the sequence has never been seriously addressed north of broken hill amphibolite facies precursors to the granulite facies gneisses show compelling evidence of a volcanic origin in their texture chemistry and zircon morphology these felsic metavolcanic rocks are in the upper part of the broken hill group stratigraphically equivalent to the broken hill orebody the present work traces textural zircon morphological and isotopic changes across the amphibolite to granulite facies transition in order to determine the primary depositional age of this supracrustal sequence and the timing of highgrade regional metamorphism ion microprobe shrimp u pb data on zircons from the lower grade amphibolite facies felsic metavolcanic rocks define a major magmatic population at 1690 or 5 ma this is considered to be the age of zircon crystallization and eruption of felsic volcanics in this part of the broken hill group these shrimp data clarify inheritance and discordance problems that had plagued conventional multigrain u pb analyses zircon results from granulite facies stratigraphic equivalents of these rocks near southern cross and at broken hill have patterns similar to those of the amphibolite facies rocks including igneous relics that provide the same primary age additionally zircon in the highest grade granulite facies rocks approximately 800 degrees c forms metamorphic overgrowths surrounding the magmatic relics these younger zircon selvedges have a pooled age of 1600 or 8 ma which determines the timing of this granulite facies event consistent u pb monazite results yield this same age 1599 or 9 ma suggesting that the monazite blocking temperature is close to that of zircon or that cooling to 725 degrees c was achieved relatively rapidly this is some 60 m y younger than the previously accepted age for high grade metamorphism at broken hill irrespective of metamorphic grade in the gneisses all biotite rb sr ages have been reset to approximately 480 ma a reflection of early paleozoic delamerian orogenesis inherited zircon grains interpreted as xenocrysts and in some cases mantied by 1690 ma igneous rims have older proterozoic to late archcan ages 1780 2700 ma the presence of xenocrystic zircon components points to an ensialic tectonic setting for the broken hill block and reflects accidental incorporation in the felsic volcanic protoliths of lithic fragments and or complex igneous sources such as found in crust exposed in the gawler craton to the west the 1690 or 5 ma depositional age determined here provides a firm chronologic tie between the early proterozoic sequences enclosing major stratiform pb zn ag orebodies at broken hill mount isa 1670 or 19 ma and mcarthur river 1690 or 27 ma in northern australia in light of the uncertainties over timing and mode of origin of these deposits the similar ages of their host sequences have important implications for early proterozoic tectonics and metallogeny

refined proterozoic evolution of the gawler craton south australia through u pb zircon geochronology abstract through the application of both conventional u pb zircon analyses and small sample u pb isotopic analyses the nature and timing of tectonic events leading to the formation of the gawler craton have been defined more precisely constraints on deposition of early proterozoic iron formation bearing sediments have been narrowed down to the period 1960 1847 ma deformed acid volcanics including the economically important moonta porphyry have zircon ages of 1790 and 1740 ma the voluminous acid gawler range volcanics and correlatives to the east were erupted over a short interval at 1592 2 ma and were intruded by anorogenic granites at 1575 ma small sample zircon analyses proved to be an extremely valuable adjunct to conventional analyses generally yielding more concordant data which forced a curved discordia through an upper intercept slightly younger than from a conventional straight line discordia

excess40ar in metamorphic rocks from broken hill new south wales implications for40ar 39ar age spectra and the thermal history of the region 40ar 39ar age spectrum analyses of samples from broken hill new south wales indicate that the region has experienced a complex thermal history following high grade metamorphism 1660 ma ago the terrain cooled slowly 3 c ma 1 until about 1570 ma ago when the temperature fell below about 500 c following granitoid emplacement 1500 ma ago the region remained relatively cold until affected by a thermal pulse 520 40ma ago causing temperatures to rise to 350 c in some places during this event accumulated40ar was released from minerals causing a significant ar partial pressure to develop laboratory ar solubility data combined with the40ar 39ar age spectra gives a local estimate of this partial pressure of 10 4atm the region finally cooled below 100 c about 280 ma ago 40ar 39ar age spectrum analyses of hornblende plagioclase and clinopyroxene containing excess40ar are characterized by saddle shaped age spectra detailed analysis of plagioclase samples reveals a complex diffusion behaviour which is controlled by exsolution structures this effect in conjunction with the presumed different lattice occupancy of excess40ar with respect to radiogenic40ar appears to be responsible for the saddle shaped age spectra

subseafloor origin for broken hill pb zn ag mineralization new south wales australia pb isotope analyses combined with sensitive high resolution ion microprobe u pb zircon dating suggest that the giant broken hill orebody formed over an 6 m y period coincident with a number of small broken hill type deposits lower in the stratigraphic sequence the formation of the smaller but significant pinnacles deposit was also episodic over 7 m y and up to 10 m y prior to the formation of broken hill using relative ages and stratigraphic position together with lithologic and isotopic variation a subsea floor origin for the deposits including the broken hill pb zn ag orebody is proposed mn p rich banded iron formation stratigraphically above the orebody may have been derived from spent ore forming fluids exhaled onto the seafloor extensive magnetite disseminations in metasedimentary rocks at the same level as the banded iron formation probably represent distal signatures of the same hot springs

geochronological constraints on orogenic events in the arunta inlier a review early workers subdivided the arunta inlier central australia into three major tectonic provinces classified the principal tectonostratigraphic units into three divisions and separated the tectonomagmatic history into five discrete events ranging in age from 1800 ma to 300 ma a review of the evidence for tectonic boundaries within the arunta inlier suggests that no major structural discontinuity exists between the northern and central tectonic provinces but that the redbank thrust zone is at least in part a province boundary the tectonostratigraphic division concept also requires revision as division 3 rocks of the northern and southern arunta cannot be time equivalents division 1 rocks grade into division 2 in the northern region and division 3 of the northern region may be equivalent to division 1 in the central region rocks originally assigned to divisions are better grouped into a larger number of lithological assemblages until better stratigraphic and isotopic correlations can be made compilation of rb sr and 40ar39ar age determinations obtained before 1984 indicate an almost complete age spectrum from 1800 ma to 1000 ma but age clusters exist at 1400 ma 1100 ma and 400 300 ma which have geological significance subdivision of older ages into discrete tectonic events was not justified from this data alone but more recent u pb isotopic analysis of zircons in rocks designated to particular structural metamorphic events has revealed a complex tectonic history in the 1880 1600 ma interval the northern and central provinces underwent major tectonism between 1780 1730 ma but much of the southern province was not deformed until 1600 ma the revised tectonic history nomenclature is proposed for the arunta inlier following deposition of a widespread turbiditic unit at or before 1880 ma possibly on thinned archaean crust the northern and central provinces underwent localized tectonism magmatism at 1880 ma yuendumu tectonic event and 1820 ma stafford tectonic event prior to the widespread strangways orogeny which is subdivided into an early phase at 1780 1770 ma and late phase at 1745 1730 ma it is not clear whether the strangways orogeny is a single arunta wide event or a series of separate short lived smaller scale events although the major deformation in all areas appears to have involved w to sw directed thrusting in the southern province the major events occurred at 1680 ma argilke tectonic event and 1600 ma chewings orogeny the latter involving n directed thrusting that was penecontemporaneous with post tectonic pegmatite intrusion in the northern province suggesting linkage of both regions at this stage the younger history of the inlier is characterised by several stages of uplift and isolated thermal magmatic events major mesoproterozoic crustal exhumation anmatjira uplift phase has been interpreted from widespread 1500 1400 ma ages including regional scale closure of 40ar 39ar systems in hornblende from mafic granulite outcrop scale rb sr total rock isotopic systems in mylonitized granitoids and sm nd garnet hornblende pairs in mylonitized amphibolites a 1150 ma thermal event in the southern part of the inlier teapot magmatic event is recorded by intrusion of granite pegmatites and the alkaline mordor igneous complex and by isotopic resetting of rb sr mineral systems dolerite dyke injection at 1050 ma reflects limited extensional tectonism that may have related to formation of the amadeus basin the second major uplift event was the alice springs orogeny at 400 300 ma an unusual intracratonic thick and thin skinned thrust event that involved limited retrograde amphibolite facies but extensive greenschist facies metamorphism and restricted pegmatite intrusion

cooling and exhumation history of the northeastern gawler craton south australia abstract we present biotite 40ar 39ar ages and from the palaeo to mesoproterozoic mount woods inlier in the northeastern gawler craton south australia in combination with garnet biotite fe mg exchange modelling using the thermal history program these data are used to constrain a post peak metamorphic p t path cooling rates and a temperature time path for the inlier consistently well defined plateaus at 1 53 ga of biotite 40ar 39ar age spectra indicate that the rocks cooled through 300 c at this time cooling rates within the inlier are 3 9 c ma from peak high temperature low pressure metamorphism at 1 59 ga to 1 53 ga cooling rates subsequently decreased to 4 c ma cooling is suggested to be a function of cessation of voluminous magmatism and mantle heat input in conjunction with exhumation initiated along the southwestern margin of the inlier ca 1 592 1 582 ga the temperature time path of the inlier has a concave shape the peak metamorphic and ensuing cooling history of the mount woods inlier are akin to the nearby curnamona province coober pedy ridge and mabel creek ridge within the south australian craton deformation during high grade metamorphism within these terranes was accommodated at mid crustal levels and large scale exhumation did not occur until the terranes cooled and were able to be exhumed as coherent rigid blocks along discrete shear zones and faults

influence of preexisting low metamorphic grade sedimentary successions on the distribution of iron oxide copper gold mineralization in the olympic cu au province gawler craton the primary driver for mineralization in the olympic cu au province of the eastern gawler craton south australia was a major early mesoproterozoic thermal event partly manifest as extensive a type i type and mafic magmatism magmatism of this age is present across the gawler craton and the curnamona province to the east yet hematite rich iron oxide copper gold iocg deposits are apparently restricted to a north south belt along the eastern margin of the gawler craton whereas geochemical differences between the early mesoproterozoic granites in the eastern versus western gawler craton appear to influence the type of mineralization in these regions there are also stark differences in the nature of the rocks into which these magmas intruded metamorphic data from the gawler craton and curnamona province show that most of the western southern and northern gawler craton underwent amphibolite to granulite facies metamorphism either during a ca 2465 to 2410 ma event sleafordian orogeny a ca 1730 to 1690 ma event kimban orogeny or both in contrast across the eastern gawler craton and curnamona province potentially evaporate bearing sedimentary successions including the ca 1750 ma wallaroo group and ca 1710 to 1640 ma willyama supergroup were largely unmetamorphosed prior to the early mesoproterozoic magmatic and metallogenic event thus a clear spatial partitioning of paleoproterozoic orogenesis exists in the western southern and northern gawler craton including a broad spatial correlation between rocks of low metamorphic grade and the region of iocg mineralization multiple fluid sources e g sedimentary metamorphic and magmatic were important in generating the chemistry suitable to precipitate cu au u ree bearing minerals in the olympic cu au province therefore we suggest that the preservation of rocks at low metamorphic grade containing formation waters and porosity provided a fertile geochemical environment into which high temperature felsic and mafic magmas and their associated hydrothermal cells were emplaced and developed this example from the eastern gawler craton and curnamona province suggests that prior orogenic events can operate as a first order control on the location of large mineral provinces

tectonic framework and evolution of the gawler craton southern australia the gawler craton is the major crustal province in the southern australian proterozoic and is pivotal in models seeking to describe the evolution of proterozoic australia the craton is host to the olympic dam iron oxide cu au u ree deposit as well as a number of other iron oxide copper gold iocg au and iron ore deposits the evolution of the gawler craton is dominated by two major phases of tectonic activity late archean and late paleoproterozoic early mesoproterozoic in total spanning ca 1 billion years the late archean 2560 2500 ma basin development was coeval with arclike felsic magmatism and mafic ultramafic magmas including komatiites collisional deformation between 2480 and 2420 ma led to ca 400 m y of tectonic quiescence conceivably within an early paleoproterozoic continental interior the second major phase of tectonic activity was in the middle to late paleoproterozoic and early mesoproterozoic 2000 1450 ma paleoproterozoic tectonism was initially dominated by rift related events which produced a series of basins over the interval ca 1900 to 1730 ma some of which contain significant iron ore reserves transient contractional deformation at ca 1850 ma led to batholith scale granitic magmatism in part derived from melting of the late archean basement major basin development was finally terminated by the 1730 to 1690 ma kimban orogeny the effects of which are widespread and include the formation of crustal scale shear systems granitic magmatism and low to high grade metamorphism the kimban orogeny was followed by a renewed period of extension between ca 1680 and 1640 ma leading to local magmatism and sedimentation which appears in part to have been coeval with the high grade ca 1650 ma ooldean event in the western gawler craton at ca 1620 to 1615 ma the generation of the arc related st peter suite in the southern gawler craton implies that craton components were located at an active plate margin the arc environment was superseded by a transition to a continental interior regime which coincided with the voluminous gawler range volcanics 1595 1590 ma and hiltaba suite granitoids 1595 1575 ma the hiltaba suite is dominated by high t fractionated felsic rocks with coeval mafic magmatism confirming a mantle involvement emplacement of the hiltaba suite coincided with the major mineralizing interval in the gawler craton regionally two distinct mineral systems have been recognized the olympic iocg province in the eastern part of the craton and the gold dominated systems within the central gawler gold province the spatial distribution of iocg versus au dominated mineral systems appears to reflect regional variations in crustal composition and hiltaba suite petrogeneses hiltaba aged granites in the olympic iocg province are isotopically more evolved richer in u and th and oxidized compared to similar aged granites in the au dominated province modern day heat flow in the iocg province is significantly higher 90 10 mwm 2 compared to the au dominated province 54 5 mwm 2 suggesting there are important lithospheric compositional differences between the two metallogenic provinces which may reflect an older phase of craton assembly widespread northwest southeast contractional deformation coeval with the emplacement of the hiltaba suite is expressed by the formation and or reactivation of shear zones ranging up to crustal scale within this regime northwest trending structures such as those in the vicinity of the olympic dam deposit are likely to have accommodated dilation associated with strike slip movements intersection between these structures and northeast trending contractional faults may have formed suitable structural traps for 1590 to 1580 ma mineralization the timing of syn hiltaba deformation overlaps with transpression and medium to high grade metamorphism associated with the ca 1570 to 1540 ma kararan orogeny which dominates the geophysical architecture of the central northern part of the craton the youngest phase of deformation in the craton is expressed by reactivation of shear zones between ca 1470 and 1450 ma and regional cooling

evidence and timing of crustal extension versus shortening in the early tectonothermal evolution of a proterozoic continental rift sequence at broken hill australia 1 low pressure p high temperature t metamorphism previously attributed to crustal thickening accompanying nappe emplacement in the paleoproterozoic willyama supergroup at broken hill is reinterpreted as a response to crustal extension d1 and bimodal magmatism commencing circa 1700 ma d1 deformation occurred under amphibolite granulite facies conditions producing layer parallel fabrics s1 and metapelitic mineral assemblages in which andalusite dominated over sillimanite crustal extension ceased soon after 1640 ma to be followed by crustal thickening higher pressure metamorphism and northeast vergent recumbent folding d2 attendant on inversion of the original d1 extensional structures d2 recumbent folding has previously gone unrecognized but probably peaked around 1600 ma before further amphibolite facies metamorphism between 1600 and 1590 ma accompanying upright folding and northwest directed thrust faulting d3 in the light of this revised tectonothermal history geodynamic models erected to explain elevated thermal gradients and low p high t granulite facies metamorphism in tectonically thickened crust may not be as relevant to broken hill as once thought above average crustal temperatures and low p high t metamorphism in the willyama supergroup more likely occurred in response to voluminous magmatic intrusion accompanying crustal thinning mantle upwelling and emplacement and unroofing of midcrustal rocks along a major detachment surface a second major structural and or stratigraphic break in the willyama supergroup is indicated by the absence of d1 fabrics from the uppermost part of the sequence

strain localisation and high grade metamorphism at broken hill australia a view from the southern cross area abstract progressive localisation of strain within proterozoic high grade rocks in the southern cross area of the willyama supergroup broken hill has resulted in the formation of shear zones of highly strained d2 fabrics separated by less strained d2 zones that preserve pre tectonic relationships complex fold interference patterns and abundant pegmatites related to a d1 m1 deformation metamorphic event the metamorphic peak m1 is characterised by biotite sillimanite quartz alkali feldspar plagioclase with garnet or cordierite in metapelites formed at about 750 c and 400 500 mpa presumably relatively late in m1 as garnet has overgrown the s1 biotite sillimanite fabric the d2 strain is concentrated in high strain zones that form sub planar and anastomosing networks of apparently simple s l tectonite fabrics associated with subhorizontal shearing and are locally overprinted by d2 leucosomes in the more aluminous metasediments m2 involved identical metamorphic conditions as m1 late d2 muscovite bearing pegmatites intruded the area a further proterozoic d3 m3 event localised in the sites of d2 high strain is characterised by the growth of staurolite and garnet across a muscovite fabric at about 600 c at 400 600 mpa resulting from the addition of h2o to the original upper amphibolite facies assemblages younger phases of retrogression generally involving thrusting are usually localised in sites of pre existing high strain they are characterised by chlorite muscovite bearing assemblages it is proposed that the events d1 m1 d2 m2 and d3 m3 occur within separate heating events separated by periods of repose at the ambient geotherm

redatuming through a salt canopy and target oriented salt flank imaging we describe a new shortcut strategy for imaging the sediments and salt edge around a salt flank through an overburden salt canopy we tested its performance and capabilities on 2d synthetic acoustic seismic data from a gulf of mexico style model we first redatumed surface shots using seismic interferometry from a walkaway vertical seismic profile survey as if the source and receiver pairs had been located in the borehole at the positions of the receivers this process creates effective downhole shot gathers by completely moving surface shots through the salt canopy without any knowledge of overburden velocity structure after redatuming we can apply multiple passes of prestack migration from the reference datum of the bore hole in our example first pass migration using only a simple vertical velocity gradient model reveals the outline of the salt edge a second pass of reverse time prestack depth migration using full two way wave equation was performed with an updated velocity model that consisted of the velocity gradient and salt dome the second pass migration brings out dipping sediments abutting the salt flank because these reflectors were illuminated by energy that bounced off the salt flank forming prismatic reflections in this target oriented strategy the computationally fast redatuming process eliminates the need for the traditional complex process of velocity estimation model building and iterative depth migration to remove effects of the salt canopy and surrounding overburden this might allow this strategy to be used in the field in near real time