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

initial hf isotope compositions in magmatic zircon from early proterozoic rocks from the gawler craton australia a test for zircon model ages abstract we evaluate the validity of age information obtained from two stage hf model ages in zircon multiple isotope investigations of minerals from the kiana granite of the archean sleafordian complex from the southern gawler craton australia reveal a complex geologic evolution of the suite rubidium sr mineral ages define a tectono thermal event at 1673 4 4 2 ma associated with the kimban orogeny id tims u pb ages from single zircons yield an upper concordia intercept at 2348 21 ma which marks the time of magmatic emplacement whole rock nd model ages yield crustal residence ages of 2 8 ga for each single zircon grain the lu hf isotope systematic was determined by solution mc icpms two stage hf model ages for the magmatic zircons vary by as much as 500 myrs depending on the choice of lu hf for the source of the host rock putting further emphasis on the need to constrain the source lu hf of a rock used for model age calculations an average lu hf for mafic crust was assumed for the source reservoir of the rocks to calculate its hf isotope evolution through time which yields zircon model ages of 3 5 ga that coincide with the initial growth episode postulated for the gondwana supercontinent it is demonstrated that two stage zircon model ages deduced from initial hf isotopes can provide important age information for the source of igneous rocks and consequently on important crustal formation events even when conventional geochronologic investigations fail e g due to thermo tectonic overprints or mixing

early paleozoic high mg diorite granodiorite in the eastern kunlun orogen western china response to continental collision and slab break off abstract phanerozoic high mg diorites and granodiorites are known to be related to diverse subduction styles at convergent margin settings but have not been linked to collision processes the santonggou complex in the eastern kunlun orogen is composed of a dominant granodiorite intruded by small stocks and dykes of porphyritic granodiorites and diorites which are all characterized by high mgo and ni but low yb and y contents mafic microgranular enclaves mmes occur in the granodiorite and the porphyritic granodiorites la icpms u pb analyses of zircon from the granodiorite porphyritic granodiorite and porphyritic diorite yield similar ages of 432 2 ma 432 2 ma and 427 3 ma respectively plagioclase in the granodiorite exhibits complex zonations with resorbed and patchy cores plagioclase and hornblende phenocrysts in the porphyritic granodiorites and diorites also show textural and compositional zonations the highest e nd t value of 0 6 in mmes suggests asthenosphere derived mafic magma these lines of mineralogical and geochemical evidence together with results from geochemical modeling suggest that the porphyritic diorites were derived from mantle metasomatized slab melts the porphyritic granodiorites and the granodiorite were generated by simultaneous assimilation and fractional crystallization of the magma approximated by the porphyritic diorites plus mixing of asthenosphere derived mafic and evolved granodioritic magmas pressure estimates from the al in hornblende geobarometer suggest that the granodiorite had been uplifted to near the surface before the emplacement of the porphyritic granodiorites partial melting of the slab upwelling of the asthenosphere and rapid orogenic uplift at 430 ma suggest that the cessation of subduction and the initiation of collision as well as slab break off occurred almost simultaneously in the eastern kunlun orogen

the geology and genesis of the tarcoola gold deposits south australia the tarcoola goldfield is located in the gawler craton in northwestern eyre peninsula south australia the gold deposits are hosted in the middle proterozoic tarcoola formation comprised of the fluviatile peela coglomerate member the shallow marine fabian quartzite member and the marine sullivan shale member mineralization in the goldfield consists of north northeast to north northwest trending gold bearing quartz veins with associated hematite pyrite arsenopyrite sphalerite chalcopyrite galena electrum and gold adamellite in contact with the tarcoola formation has previously been included in the middle proterozoic hiltaba suite granitoids on the basis of an apparent intrusive relationship with the tarcoola formation and the gold quartz veins were interpreted as being genetically related to the cooling pluton however detailed field and petrographic studies have demonstrated that the contact between the tarcoola formation and adamellite is a nonconformity hence there is no genetic relationship between the mineralization and the adamellite oxygen isotope data indicate that an oreforming fluid derived from convective circulation of meteoric or seawater or from formation water underwent isotope exchange with sediments of the tarcoola formation a magmatic heat source for the hydrothermal system is suggested by the presence of intrusive igneous rocks including dykes of aplite quartz monzonite and microdiorite sulphur isotope characteristics of the mineralization can be explained by reduction of seawater sulphate or dissolution of disseminated sulphides in the sedimentary sequence metals were probably derived from rocks of the tarcoola formation a complex paragenetic sequence involved deposition of minerals in several stages separated by episodes of fracturing fluid inclusions in quartz and fluorite show that deposition took place over a temperature range of about 340 to 110 c from a low salinity fluid analyses of chlorite coexisting with sulphide minerals suggest deposition of sulphides from about 300 to 170 c gold was transported as au hs 2 and deposition appears to have coincided with a sudden decrease in fo2 at around 260 to 250 c

neodymium isotopic constraints for the origin of mesoproterozoic felsic magmatism gawler craton south australia mesoproterozoic felsic magmatism of the gawler range volcanics and hiltaba suite granites occurred at 1585 1595 ma across much of the gawler craton south australia nd isotopic analysis of this felsic magmatism combined with petrological and geochemical arguments suggest derivation by partial melting of both paleoproterozoic and archean crust the majority of samples analyzed have nd isotopic and geochemical characteristics compatible with the involvement of paleoproterozoic crust stabilized during the 1 85 1 71 ga kimban orogeny as sources for the mesoproterozoic magmatism others require derivation from sources dominated by archean rocks this cycle of paleoproterozoic crustal stabilization followed by involvement of this crust mesoproterozoic felsic magmatism is one previously documented from many parts of mesoproterozoic laurentia on the basis of models proposing east australia antarctica to be the conjugate landmass at the rifted margin of western north america it appears that the voluminous mag

characteristics origin and significance of mesoproterozoic bedded clastic facies at the olympic dam cu u au ag deposit south australia mesoproterozoic bedded clastic facies occur in the olympic dam breccia complex which hosts the olympic dam cu u au ag deposit south australia contacts of the bedded clastic facies with breccia of the olympic dam breccia complex are faulted and or brecciated fragments of the bedded clastic facies occur in the breccia complex the bedded clastic facies comprise five main facies associations four of which are organised into mappable units these associations have distinct textures and components and have not been mixed major sources of detritus were felsic and mafic volcanic units and granitoids of the 1590 ma gawler silicic large igneous province archean and paleoproterozoic zircons in well sorted quartz rich sandstone indicate that older gawler craton basement successions also contributed sediment to the bedded clastic facies accumulation of the bedded clastic facies was at least partly contemporaneous with rhyolitic explosive eruptions from vent s probably tens of km away because bubble wall shards are present in tuffaceous mudstone facies deformation of the bedded clastic facies prior to final lithification produced folds that have diverse shapes and sizes and that lack cleavage the folds have a reasonably consistent gentle plunge to the northeast these soft sediment folds could be related to slumping on a northeast striking paleoslope that dipped either to the northwest or southeast the depocenter in which these facies accumulated may have been bounded by a combination of northeast striking and northwest striking faults because the bedded clastic facies occur at the top of the olympic dam breccia complex and contributed clasts to it the fault controlled sedimentary basin was in place when the breccia complex and the olympic dam ore deposit formed

zircons from the acraman impact melt rock south australia shock metamorphism u pb and 40ar 39ar systematics and implications for the isotopic dating of impact events abstract this study presents the first optical and scanning electron microscopic characterization and u pb shrimp dating results for zircon grains separated from the most likely autochthonous impact melt rock in the central domain of the large 40 90 km eroded ediacaran acraman impact structure in south australia microtextural characteristics define five zircon subtypes corresponding to different levels of progressive shock metamorphism from virtually unshocked monocrystalline zircon grains that exhibit original magmatic zoning in cathodoluminescence images to fully granular zircons that have completely lost their primary zoning pattern and locally contain neocrystallized submicrometer sized spots of zro 2 probably baddeleyite that pseudomorph pre impact zircon the granular zircons correspond to the highest observed level of shock metamorphism and impact induced recrystallization zro 2 bearing granular zircons indicate shock pressures in excess of 65 70 gpa which are considerably higher than previous shock pressure estimates for the acraman impactites u pb systematics of untreated and chemically abraded melt rock zircons indicate that u pb ratios of the acraman zircons were variably reset during impact weakly shocked crystalline grains yield ages on concordia at 1 59 1 60 ga reflecting the magmatic age of the gawler range volcanics only the entirely granular zircon population was apparently impact reset but based on an ediacaran age from stratigraphic constraints on the ejecta layer experienced significant post impact pb loss the microcrystalline nature of granular zircons could have promoted pb diffusion and recoil in post impact time as suggested by grain size dependent diffusion and recoil modeling a positive correlation of u concentration and shock level suggests that granularization might have preferentially occurred in initially u rich probably metamict zircons 40 ar 39 ar dating of a melted yardea dacite clast from the acraman melt rock as well as k feldspar separated from shocked yardea dacite resulted in post impact alteration plateau ages suggestive of hydrothermal events at 500 ma and 450 ma that selectively affected the impactites exposed in the central domain of the acraman impact structure our study demonstrates that the acraman impact is particularly difficult to date in the absence of accurate and precise isotopic ages for acraman the ediacaran ejecta stratigraphic age of 635 541 ma is considered the most reliable age constraint currently available for the timing of the large acraman impact

geochronology of mesoproterozoic gold mineralization in the gawler craton and temporal links with the gawler range volcanics

detrital zircon ages improving interpretation via nd and hf isotopic data abstract the bulk of sedimentary provenance studies use similarities in the detrital zircon age patterns or barcodes in sedimentary systems and potential source regions to make interpretations about palaeogeographic settings while this age only approach is generally considered to be effective it is limiting because the timing of zircon growth events may not be unique to specific terrains and important rock forming events may be associated with little zircon growth to a significant extent these limitations can be overcome by employing additional isotopic data sets such as sm nd and lu hf that provide information on the crustal evolution of the source region and allow comparisons with sedimentary packages as an example the age spectra of detrital zircons in palaeoproterozoic metasedimentary rocks in the eastern gawler craton in southern australia are virtually identical to the dominant zircon growth timelines in adjacent older domains of the gawler craton presenting a prima facie case that it was the source region however whole rock nd isotopic data indicate that the pre existing proto gawler craton was isotopically too crustally evolved e nd 1850 ma 10 to have supplied the bulk of the sediment to the relatively more juvenile metasedimentary units e nd 1850 ma 1 to 5 in addition zircon hf isotopic compositions from ca 2000 ma detrital zircons in the metasedimentary rocks e hf 2000 ma 2 to 5 are significantly more juvenile than 2000 ma rocks in the adjacent gawler craton e hf 2000 ma 2 to 5 the combination of bulk rock nd and hf zircon data suggest that the gawler craton is not a viable source region for the metasedimentary packages despite the striking similarity between detrital zircon ages and zircon crystallisation events within the craton

the 2 7 2 0 ga volcano sedimentary record of africa india and australia evidence for global and local changes in sea level and continental freeboard abstract the 2 7 2 0 ga volcano sedimentary records of the african indian and australian cratons indicate two broadly defined periods of extensive drowning of the emergent continental areas concomitant with lowered freeboard carbonate banded iron formation bif platforms characterised the first such event at ca 2 6 2 4 ga africa and australia to 2 7 ga india these earlier globally enhanced sea levels are ascribed to increased mid ocean ridge activity possibly related to breakup of a postulated late archaean southern supercontinent alternatively a transition from global scale catastrophic mantle overturn events to the onset of plate tectonics may have occurred in the late archaean nelson 1998 earth planet sci lett 158 109 119 both explanations of increased mid ocean ridge activity are compatible with significant early to middle archaean crustal growth armstrong 1981 phil trans r soc london a 301 443 472 with the emergent high freeboard cratons being subjected to aggressive weathering and erosion enhanced continental crustal growth near the archaean proterozoic boundary mclennan and taylor 1982 j geol 90 347 361 related to the development of significant island arc complexes would have resulted in common lowered freeboard enhanced sea level conditions at the passive margins of the southern cratons the diachronous nature of these earlier transgressions in the various cratons may reflect the effect of local tectonic movements and or the thermal state of the cratons from ca 2 4 2 2 ga cratons that make up the present day continents of india africa and australia had relatively high continental freeboard and lowered sea levels glacigenic deposits are preserved on the kaapvaal africa singhbhum india and pilbara australia cratons the second broadly defined drowning event at ca 2 15 ga was probably due to post glacial climatic amelioration freeboard was reduced by the combination of eustatic rise and the reestablishment of aggressive weathering as warmer palaeoclimates returned in india carbonates were more prominent than the siliciclastic sediments including prominent black shales seen in africa and australia

characteristics and origin of the oak dam east breccia hosted iron oxide cu u au deposit olympic dam region gawler craton south australia oak dam east is a large 560 mt of 41 56 wt fe cu au u bearing massive fe oxide deposit in the northeastern gawler craton south australia it is one of several unmined deposits in the same province as the giant olympic dam deposit and it lies beneath more than 500 m of younger cover rocks the oak dam east breccia complex formed in a near surface environment and was characterized by an early history of skarnlike albite calc silicate magnetite formation from hypersaline 400 500 c fluids large scale brecciation was an early feature and the resulting complex was progressively filled in and replaced by 1 hematite and goethite hematite stage i 2 magnetite apatite quartz actinolite magnetite stage 3 hematite goethite quartz chalcedony pyrite chlorite monazite hematite stage ii and 4 chalcopyrite pitchblende illite hydromus covite florencite carrolite cu u au mineralization stage hematite stage ii assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent nacl equiv that were boiling at 170 to 190 c and had 18o values more negative than 3 7 to 1 7 per mil interpreted as meteoric water the upper 200 m of the breccia complex contains spectacular colloform hematite void fillings interpreted to have replaced hydrothermal goethite the cu u mineralized zones resemble olympic dam ore in terms of geometries being grossly subhorizontal 10 to 70 m thick and crosscutting with respect to the host breccias the mineralization is zoned vertically from pyrite 34s 6 to 0 at depth to shallower chalcopyrite 34s 14 to 7 high grade pitchblende occurs centrally within the chalcopyrite zone e g 10 m of 0 46 cu and 3 700 ppm u3o8 straddling a boundary between k mica illite alteration and underlying fe mg chlorite alteration the deposit is assigned to the 1570 to 1595 ma iron oxide cu au u event in the eastern gawler craton on the basis of its similarity to other cu au u deposits in the province an la icpms pb isotope age for monazite in hematite stage ii was calculated to be 1455 20 ma and possibly as young as 1370 54 ma to explain all isotopic data this is an age of isotopic resetting our preferred model is that cu u mineralization developed within a stratified hydrothermal brine complex analogous to the salton sea geothermal system copper fe and s in the deep brine was sourced from an evolving deep reservoir with local compositional controls on sulfur isotopes including cooling boiling and replacement of previous pyrite chalcopyrite u veins and disseminations formed as a result of mixing with overlying steam heated u rich saline ground water mixing induced cooling and dilution only occurred episodically following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex by contrast high grade uo2 formation occurred as deep upflow waned via reaction of the upper brine with hematite stage ii fe mg chlorite along an interface that descended with time this resulted in an increase of u grades through time caused by leaching and reprecipitation

structural controls on localized intraplate deformation and seismicity in southern australia insights from local earthquake tomography of the flinders ranges data from an array of 24 seismometers are used to image the crust beneath the flinders ranges southeast australia with the goal of improving our understanding of crustal structure rheology and the mechanism responsible for the localized intraplate deformation that characterizes this region a subset of p and s wave traveltimes is inverted to jointly recover earthquake hypocenters p wave velocity structure and v p v s anomalies the p wave velocity model reveals a spatial correlation between major negative velocity perturbations and concentrations of seismicity in particular a cluster of seismicity is observed within a distinct low velocity region between the archean mesoproterozoic gawler craton and the palaeo mesoproterozoic curnamona province from 7 to 20km depth we postulate that this may be associated with a pre existing structural weakness in the crust that arises primarily from rifting between the curnamona province and the gawler craton another area characterized by a high level of seismicity overlies a major sequence of n s trending ross delamerian thrust faults which correspond to a band of low v p and particularly v p v s the lack of evidence for elevated heat flows in both of these seismogenic regions suggests that thermally induced weakness is unlikely to play a dominant role instead the dynamic behavior of this intraplate region appears to be caused by a serendipitously oriented regional stress field provided by far field forces that originate from the boundary between the pacific and australian plates which acts upon preexisting structural weaknesses in the lithosphere

magnetic crustal structures in northern fennoscandia abstract cooperation between the nordic countries has produced a set of geophysical and geological maps in the region north of the 66th parallel in northernmost europe the area covers the transition from oceanic crust to the baltic shield a section of the caledonides and a part of the arctic platform an interpretation of the aeromagnetic and gravity maps reveals several large structural units with distinct boundaries and characteristic anomaly patterns in combination with data on rock physical properties some general magnetic crustal features can be outlined 1 1 typical occurrence of magnetically banded ocean crust anomalies 2 2 the occurrence of magnetically banded patterns in the continental crust connected with metasupracrustal formations separated by culminations of high density and high magnetization basement domes and megashear zones thrusts and transcurrent faults with large displacements 3 3 occurrence of magnetic granitoid batholiths 4 4 general low remanence of shield rocks allowing straightforward interpretation of magnetic total field anomalies and 5 5 general low magnetization of caledonian thrusted units allowing mapping of the depth to their magnetic basement merging magnetic gravity and petrophysical data allows constraints to be applied when modelling crustal structures some of the large scale structural features are illustrated and some examples of profiles through the upper crustal structures are shown

earliest paleoproterozoic high grade metamorphism and orogenesis in the gawler craton south australia the southern cousin in the rae family neoarchean earliest paleoproterozoic rocks of the gawler craton preserved within the mulgathing complex south australia record metamorphism and deformation during an earliest paleoproterozoic tectonothermal event known as the sleafordian orogeny we present metamorphic data from pelitic gneisses of the mulgathing complex from three locations challenger mt christie and blackfellow hill in order to constrain the pressure temperature time p t t evolution of this orogeny in situ u pb dating via laser ablation inductively coupled plasma mass spectrometry shows that monazites within garnet porphyroblasts have ages ca 2470 ma providing an estimate of the timing peak p t conditions monazite crystallisation from melt occurred at ca 2460 ma in the presence of peritectic garnet either late on the prograde path during biotite dehydration melting or immediately following peak t lobate and patchy rims in leucosome hosted monazite suggest a dissolution reprecipitation process occurred during cooling across the elevated dry solidus at ca 2440 ma monazite younger than ca 2430 2420 ma that is located at grain boundaries between matrix minerals likely reflects monazite crystallisation during cooling across the dry solidus and or subsolidus mineral breakdown during the waning stages of the thermal event phase diagram modelling of pelitic gneisses suggests peak metamorphic conditions at the three localities overlap at 7 kbar and 850 c with apparent thermal gradients in the order of 125 135 c kbar 35 45 c km metamorphism involved a tight anticlockwise p t t path based on mineral assemblage paragenesis and indicates a slow shallow decompressive cooling path sub parallel to the apparent thermal gradient in the order of few degrees per myr a protracted magmatic history involving bimodal magmatism associated with a thinned and underplated extensional basin in the mulgathing complex was likely a thermal precursor to the high thermal gradient metamorphism and convergent transpressive deformation a significant period of post 2 4 ga quiescence in the gawler craton subsequent to basin inversion suggests that this orogenesis may have occurred interior to a plate margin the similarity in timing and style between the sleafordian orogeny in the gawler craton and the 2 5 2 28 ga arrowsmith orogeny of the rae craton supports the notion that these terranes together with the broader gawler adelie craton of the mawson continent the north australia craton elements of the north china sino korean and dharwar cratons formed part of a family of neoarchean terranes described as the rae family of cratons with a distinctive tectono metamorphic record as compared with other archean cratons we suggest the mawson continent is the southern member of this family dominated as it is by terranes now present in the northern hemisphere

1760 ma magnetite bearing protoliths in the olympic dam deposit south australia implications for ore genesis and regional metallogeny spatial associations between banded iron formation and iron oxide cu au iocg style mineralization are well documented in the gawler craton south australia but the possible genetic relationships between these two distinct types of mineralization are hitherto unclear a texturally conspicuous generation of coarse grained silician magnetite intergrown with carbonates and quartz is observed in drillholes intersecting the outer shell of the olympic dam iocg type deposit this magnetite is characterised by high u content 50 ppm siliceous chemistry and unusual zonal textures with respect to si fe nanoprecipitates direct dating of this magnetite by laser ablation inductively coupled plasma mass spectrometry yields reproducible 207pb 206pb dates 1761 16 ma that are significantly older than the granite hosting the deposit 1593 ma or the mineralized breccias constituting the cu u au ag resource 1592 1589 ma the older fe rich crustal material can be correlated with the 1 76 1 74 ga meta sedimentary wallaroo group host to fe rich horizons across the gawler craton including locations 15 km nw of olympic dam a generation of granitic rocks which intruded bedrock at 1 75 ga are present 30 km ne of olympic dam and likely exsolved hydrothermal fluids that enriched pre existing magnetite bearing protoliths in both u and ree such material was physically and likely chemically incorporated into the outer shell at olympic dam some 150 ma later during granite uplift along faults the coincidence between fe rich horizons bif and 1750 ma granitoids may have provided iocg systems with an additional source of both fe and u that predates the 1 59 ga craton scale metallogenic event the uranium concentrations in some south australian iocg systems represent major global anomalies in the element a combination of the fortuitous geological circumstances outlined here may help explain the highly anomalous accumulation of uranium found at olympic dam

prominent hill a hematite dominated iron oxide copper gold system the discovery of iron oxide copper gold iocg mineralization at prominent hill in the mount woods inlier south australia by minotaur resources ltd in november 2001 was based on an empirical exploration model that focused specifically on the hematite end member of the iocg ore deposit model type copper and gold bearing hematite rich breccias formed by repetitive hydrothermal brecciation milling and explosive venting within a volcanic setting high pressure spalling chemical corrosion of breccia clasts and compositional layering within the breccias are distinctive features iron oxide sericite silica alteration is pervasive within and marginal to the main breccias and is surrounded by a wider zone of less intense alteration iron copper gold barium fluorine uranium cerium and lanthanum are strongly correlated and associated with a relatively low temperature hydrothermal system mineralization was synchronous with volcanism and sedimentation within a narrow east west trending graben that developed at 1585 ma

interpretation of a ca 1600 1580 ma metamorphic core complex in the northern gawler craton australia abstract the mount woods domain in the gawler craton south australia records a complex tectonic evolution spanning the palaeoproterozoic and mesoproterozoic the regional structural architecture is interpreted to represent a partially preserved metamorphic core complex that developed during the 1600 1580 ma hiltaba event making this one of the oldest known core complexes on earth the lower plate is preserved in the central mount woods domain which comprises the mount woods metamorphics these rocks yield a detrital zircon maximum depositional age of 1860 ma and were polydeformed and metamorphosed to upper amphibolite to granulite facies during the 1740 1690 ma kimban orogeny the upper plate comprises a younger succession the skylark metasediments deposited at 1750 ma within the upper plate sedimentary and volcanic successions of the gawler range volcanics were deposited into half graben that evolved during brittle normal faulting the skylark shear zone represents the basal detachment fault separating the upper and lower plate of the core complex the geometry of normal faults in the upper plate is consistent with ne sw extension both the upper and lower plates are intruded by 1795 1575 ma hiltaba suite granitic and mafic plutons the core complex was extensively modified during the 1570 1540 ma kararan orogeny exhumation of the western and eastern mount woods domain is indicated by new 40ar 39ar biotite cooling ages that show that rock packages in the central mount woods domain cooled past 300 c 50 c at 1560 ma which was 20 million years before equivalent cooling in the western and eastern mount woods domain exhumation was associated with activity along major syn kararan orogeny faults

iron oxides constrain bif evolution in terranes with protracted geological histories the iron count prospect middleback ranges south australia abstract when studied at appropriate scales of observation and using complementary methods the intrinsic textural and geochemical complexity of iron oxides can provide unparalleled insights into the evolution of iron ore resources from banded iron formation deposition to ore formation iron count is one of 20 deposits and prospects within the archean middleback ranges iron ore belt south australia in which iron oxides from bifs and ores display variable textural and trace element signatures integrated petrography iron oxide laser ablation mapping and geochemistry and in situ dating of bif derived ores was undertaken to assess whether features observed in samples from this representative prospect might be attributed to localized and superimposed overprint processes the prospect consists of a west and east ridge each characterized by a sequence of overprints expressed as interconversions between iron hydr oxides and accompanying variation in trace element concentrations pseudomorphic replacement of early magnetite by hematite martite followed by replacement by iron hydroxides is recognized in samples from the east ridge further overprints are expressed as veining brecciation and crystallization of micro platy hematite rich in granitophile elements sn mo w u martitization of early magnetite in the west ridge is accompanied by enrichment in zn co and ni in martite subsequent recrystallization of martite iron hydroxides by granoblastic hematite is marked by enrichment in ti ta nb ree and granitophile elements and co crystallization of ree minerals this is followed by crystallization of micro platy hematite similarly rich in these elements enrichment in ti and granitophile elements in granoblastic hematite is interpreted to infer crystallization from highly saline fluids generated by interaction between evaporite bearing sedimentary rocks and granite derived fluids post archean australian shale normalized rey fractionation trends of iron oxides from the east ridge are consistent with precipitation of iron rich minerals from a mixture of anoxic seawater and hydrothermal vent fluids analogous trends from the west ridge lack a seawater signature contrasts in rey fractionation trends between the two ridges is interpreted as a combination of the loss of primary seawater signature due to severe overprinting in the west ridge and to lateral variation in the marine depositional environment results validate the hypothesis that variation in iron oxide trace element signatures seen throughout the middleback ranges iron ore belt results from overprinting due to superimposed tectono magmatic events that have locally affected each deposit despite undergoing different sequences of ore forming stages as well as possibly distinct depositional environments the two ridges share a common overprint dated here at 1790 ma using in situ u pb geochronology of co existing hematite monazite and xenotime this age is considered to represent the timing of interaction between granite derived fluids with ore hosting rock such interpretation is supported by the geochemical signatures of the dated iron oxides showing enrichment in granitophile elements and coincident ages for the wertigo granite and myola volcanics which occur proximal to the prospect and were emplaced erupted during intracontinental rifting the approach undertaken here combining petrography iron oxide geochemistry and in situ dating of bif derived ores is generically applicable to other iron resources within the middleback ranges or analogous terranes elsewhere

timing of iron oxide cu au u hydrothermal activity and nd isotope constraints on metal sources in the gawler craton south australia the eastern gawler craton hosts australia s premier uranium bearing iron oxide copper gold iocg belt the 500 km long olympic cu au u province in addition to the olympic dam cu u au and prominent hill cu au deposits numerous barren and weakly mineralized iocg prospects are present in the province new geochronological data for hydrothermal minerals combined with constraints from host rock ages demonstrate that alteration and associated iocg mineralization formed between 1570 and 1600 ma in three districts of the province this iocg hydrothermal activity temporally overlaps with magmatism of the hiltaba suite and gawler range volcanics titanites in paragenetically early magnetite bearing alteration in the olympic dam and prominent hill districts yield u pb ion probe ages of 1576 5 and 1567 10 ma respectively molybdenite in veins crosscutting magnetite biotite and albitic alteration in the moonta wallaroo district have re os ages of 1574 6 and 1599 6 ma respectively these represent minimum ages of the iocg alteration assemblages in this district a muscovite 40ar 39ar age of 1575 11 ma provides a minimum age of paragenetically later hematitic alteration in the olympic dam district neodymium isotope compositions were determined for 44 whole rock samples from barren and weakly mineralized cu au prospects and host rocks in the olympic dam and prominent hill districts the new geochronological framework enables comparison of the nd isotope data across two iocg districts at the time of formation of the olympic dam deposit ca 1590 ma magnetite rich weakly cu mineralized alteration from five prospects yields a relatively narrow range end 1590 values of 5 8 to 4 1 both hematite and magnetite rich alteration yield generally similar end 1590 values that match values from fresh and weakly altered paleoproterozoic metasedimentary and metagranitic rocks 6 6 to 3 5 as well as from most felsic hiltaba suite intrusions and gawler range volcanics in the eastern gawler craton ca 6 to 4 these data are consistent with crustal sources for ree and by implication for associated copper in the barren and weakly mineralized prospects mineralization and alteration in these minor iocg systems can be geochemically discriminated from the giant olympic dam deposit where greater inputs of mantle derived ree and other ore components are evident 2007 society of economic geologists inc

tectonic evolution of proterozoic australia proterozoic australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic this paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes the assembly was completed between 1300 and 1100 ma when the crustal fragments were combined as an early component of the rodinian supercontinent rifting and fragmentation of archaean continents began in the late archaean and continued into the proterozoic passive margin deposits such as those of the hamersley basin accumulated on isolated fragments of archaean crust these numerous fragments were subsequently assembled into three cratons by 1830 ma a west australian craton was established by collision of the archaean pilbara and yilgarn cratons which were joined along the capricorn orogen similarly a south australian craton developed by amalgamation of the proto gawler and proto curnamona cratons along the kimban orogen a north australian craton appears to have formed by accretion of numerous crustal fragments including the kimberley pine creek lucas and altjawarra cratons with sutures marked by the king leopold halls creek tennant creek and proto isan orogens the southern margin of the north australian craton was the site of repeated terrane accretion and orogenic activity between 1880 ma and 1400 ma this included an orogenic event at 1880 1850 ma the strangways 1780 1730 ma argilke 1680 1650 ma and chewings 1620 1580 ma orogenies and the intracratonic anmatjira uplift 1500 1400 ma intracratonic rifting at 1750 to 1710 ma and 1640 to 1600 ma produced the mcarthur basin and related minor basins parts of which were deformed by the isan orogeny at 1600 and 1530 ma rifting along the line of the capricorn orogen led to deposition in the overlying intracratonic bangemall basin between 1630 and 1300 ma along the eastern margin of the south australian craton the 1670 to 1600 ma olarian orogeny marks interaction with now obscured continental crust to the east tectonic activity between 1300 and 1100 ma led to the assembly of proterozoic australia as an early component of the supercontinent of rodinia this first involved the amalgamation of the west australian and north australian cratons followed by collision with the south australian craton the centralian superbasin developed over the junction of the north south and west australian cratons between 830 and 750 ma rifting to the east formed the adelaide geosyncline at 830 ma this was followed by the breakup of rodinia with the rifting apart of laurentia and gondwanaland along the eastern margin of proterozoic australia at 750 ma and the subsequent formation of the palaeo pacific ocean after the breakup of rodinia a series of northeast southwest compressional events followed by periods of relaxation reflect the assembly of a new supercontinent old lines of weakness were reactivated culminating in the intracratonic king leopold paterson petermann ranges and pinjarra orogenies between 620 and 540 ma subsequent reactivation continued into the phanerozoic with the widespread eruption of continental flood basalts and the formation of intracratonic basins 540 530 ma

improved accuracy of u pb zircon dating by selection of more concordant fractions using a high gradient magnetic separation technique abstract a loop of soft iron wire or a paper clip or a ferromagnetic grid mounted between the poles of an electromagnet picks up and allows further magnetic separation of zircons previously found to be non magnetic on a frantz isodynamic separator tests on previously analysed samples indicate that most such zircons that are fairly discordant say 10 can be picked up and isolated from associated grains that are more concordant tests on new samples indicate that even when most grains can be picked up the last few percent of the sample contain less uranium and are more concordant than the bulk sample the degree of discordance is the dominant factor affecting the uncertainty of u pb zircon ages both because of the error amplification in projections and because the assumption of a simple two stage system may not be valid only by eliminating or reducing discordance can errors approaching the uncertainty in a single analysis say 2 m y for 2700 m y rocks be achieved rutile normally concentrated with zircon as non magnetic has been successfully removed from a small amount of low uranium zircon using the high intensity separation technique

the simpson strzelecki and tirari deserts development and sand provenance abstract sands of the simpson strzelecki and tirari deserts central australia can be divided into two main groups on the basis of their physical and chemical characteristics colour grainsize heavy minerals quartz oxygen isotope composition zircon u pb ages the first group encompasses the strzelecki tirari and southeastern simpson deserts while the second occupies the northern and the western simpson desert the boundary between the two groups corresponds approximately to the northern most extent of the kallakoopah lakes several lines of evidence suggest derivation of the sands mainly from local bedrock with very little subsequent aeolian transport ultimate protosources for the sands in order of importance are for the southeastern simpson tirari and strzelecki deserts the tasman orogenic system new england and lachlan fold belts georgetown inlier musgrave and arunta blocks gawler and curnamona cratons and for the north and western simpson desert arunta musgrave and mount isa blocks and tennant creek inlier sediment from the tasman orogenic system includes an additional exotic component from palaeozoic sediments probably derived mainly from antarctica sediment transport from these protosources across the several hundred kilometres to the surficial sedimentary basins was dominantly by fluvial not aeolian means quaternary aeolian transport or reworking has been minimal serving only to form the dunes by vertical corrasion of underlying sedimentary rocks or residual products of local basement weathering the deserts have received some recent localised sediment input from modern fluvial systems

revisiting the paleomagnetism of the 1 476 ga st francois mountains igneous province missouri 1 a paleomagnetic investigation of the st francois mountains igneous province in southeastern missouri provides a key 1476 16 ma paleomagnetic pole for laurentia the pole 13 2 s 219 0 e dp 4 7 dm 8 0 is considered primary on the basis of positive conglomerate inverse baked contact and fold tests an analysis of 1470 1430 ma poles from laurentia highlights key differences between poles obtained from the belt supergroup electra lake gabbro and cratonic north america paleolatitudes based on the lower belt supergroup poles are enigmatic as two previous studies yielded a difference of 10 our new pole combined with an analysis of previous results favors the higher latitude interpretation for the lower belt supergroup paleolatitudes from the younger belt rocks indicate lower latitudes than coeval rocks from elsewhere in laurentia for which there has been no adequate explanation a comparison of the st francois mountain pole with similar age poles from baltica siberia and australia allow first order tests of proposed continental configurations paleomagnetic data from australia are compatible with proposed rodinia reconstructions whereas paleomagnetic data from baltica are not we are unable to rigorously test the alternative suggestion that places siberia against the western margin of laurentia due in part to large errors associated with siberian paleomagnetic data

a fluid inclusion and stable isotope study of the moonta copper gold deposits south australia evidence for fluid immiscibility in a magmatic hydrothermal system abstract the copper gold ores of the moonta mineral field in the eastern part of the archaean mesoproterozoic gawler craton south australia are vein deposits occupying fractures and shear zones within the palaeoproterozoic moonta porphyry the veins show a spatial relationship with mesoproterozoic granites equivalents of the hiltaba suite granites that are widespread in the gawler craton the ores were deposited in four hydrothermal stages separated by episodes of fracturing the first two stages were dominated by fe oxides the third by fe sulfides and the final stage by a cu fe co au zn pb s assemblage three types of fluid inclusions are present in quartz associated with the oxide and sulfide minerals type 1 fluid inclusions are two phase vapour rich with homogenization temperatures up to 473 c and low to moderate salinity 2 to 22 wt nacl equivalent type 2 are two phase liquid rich with lower homogenization temperatures up to 269 c and low to moderate salinities 0 7 to 22 wt nacl equivalent type 3 are liquid rich solid s bearing with homogenization temperatures up to 467 c and high salinities 28 to 55 wt nacl equivalent the coexistence of types 1 and 3 suggests that these fluid inclusions resulted either from trapping of boiling fluids or represent two immiscible fluids most likely derived from a magma the fluid history reflected in fluid inclusion characteristics was complex involving variable amounts of boiling cooling and mixing in the vein system salinity homogenization temperature relationships indicate mixing of the hypersaline fluid and the vapour rich fluid with a surface derived fluid of low temperature and low to moderate salinity d h 2 o values 22 to 31 and 18 o h 2 o values 2 9 to 5 1 calculated from analyses of chlorite and 18 o h 2 o values calculated from analyses of quartz 0 6 to 8 8 on the basis of mean to maximum fluid inclusion homogenization temperatures are intermediate between magmatic water and surface derived fluids i e seawater or meteoric water the narrow range of 18 o values of quartz 10 8 to 12 7 may be explained by mixing of a magmatic fluid and either seawater or a low latitude moderately evolved meteoric water with a coupled decrease in fluid temperature and 18 o content similar ranges for 34 s values of ore sulfides 2 3 to 6 4 and disseminated sulfides in the moonta porphyry and interdigitated doora schist 1 5 to 4 6 suggest assimilation of crustal sulfur by the granitic magma or ore fluids the combined fluid inclusion and stable isotope data support previous proposals based on field and mineralogical evidence for a genetic relationship between the moonta ores and the mesoproterozoic granitoids

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

surface and subsurface facies architecture of a small hydroexplosive rhyolitic centre in the mesoproterozoic gawler range volcanics south australia abstract at menninnie dam south australia a drilling program has revealed a complete section through the subsurface feeder system and erupted products of a small hydroexplosive rhyolitic centre within the mesoproterozoic gawler range volcanics porphyritic rhyolite intruded near vertical faults in the palaeoproterozoic basement and at less than a few hundred metres depth interacted with fault hosted hot groundwater hydrofracturing of the wall rock occurred in advance of and at the margins of the rhyolitic intrusions the rhyolitic intrusions have peperitic margins and grade into discordant lithic rich pb facies the advancing fragmentation front intersected the palaeosurface triggering phreatic eruptions that deposited a poorly sorted lithic rich explosion breccia rhyolite then rose to the surface through the intrusive breccias and shallow seated magma water interaction occurred in the conduit within 800 m from the inferred source the products include muddy sandstone and pumice breccia at the end of the eruption rhyolitic lava was extruded in the form of a small dome the presence of contemporaneous pb zn ag mineralisation in the wall rocks suggests that an active hydrothermal system may have been involved in the formation of the menninnie dam hydroexplosive volcanic centre

lithology and hydrothermal alteration control the distribution of copper grade in the prominent hill iron oxide copper gold deposit gawler craton south australia the prominent hill iron oxide copper gold iocg deposit located in the gawler craton of south australia contains ca 278 million metric tons mt of ore at 0 98 cu 0 75 g t au and 2 5 g t ag in contrast to the predominantly granite hosted olympic dam iocg deposit prominent hill is mainly within unmetamorphosed sedimentary rocks comprising coarse clastic to laminated argillaceous lithologies with some volcaniclastic components and variable carbonate including local massive dolomite essentially unmetamorphosed sedimentary rocks and structurally underlying mafic to intermediate composition lavas inferred to be members of the lower gawler range volcanics host the economically mineralized hematite breccias the volcanic sedimentary package was downfaulted and tilted along a major east west fault north of which similar but regionally low grade metamorphosed rocks were affected by subeconomic skarn mineralization and on a more regional scale of the mount woods domain intruded by granitic and gabbroic bodies hydrothermal alteration and mineralization at prominent hill involved pervasive and texturally destructive replacement of formerly calcareous dolomitic and siliciclastic breccia components hydrothermal alteration minerals comprise hematite magnetite siderite ankerite quartz sericite chlorite kaolinite fluorapatite fluorite barite ree u minerals including monazite uraninite and coffinite together with cu sulfides including chalcopyrite bornite and chalcocite in the highest grade ore brecciation and replacement caused mechanical mixing as well as chemical alteration of primary lithologies such that sedimentary contacts became obscured mass balance calculations identify al ti si and zr as least mobile components during hematite chlorite sericite to weak hematite quartz alteration because zr was not regularly assayed in drill cores we use concentration ratios of ti al and si from the deposit scale assay database to delineate the distribution of lithochemical units prior to hydrothermal alteration and cu mineralization the resulting lithochemical model based on one horizontal and five vertical cross sections is used as a basis for mapping alteration patterns calculated from molar fe si fe si al k na and k al ratios these chemical patterns in conjunction with mineral stoichiometry indicate that the spatial distribution of hematite chlorite variably phengitic sericite and or illite kaolinite quartz bearing alteration is superimposed on the pattern of interpreted lithologic contacts the alteration patterns confirm visual logging results showing that hematite enrichment correlates only partially with the distribution of cu grades of 0 25 wt a subvertical body of complete replacement by hematite and quartz with consistent but subeconomic gold enrichment forms a cu barren core in the central and eastern parts of the deposit zones of increasing k al and k na ratios extend upward and westward from this cu barren core transgressively overprinting lithologic contacts the degree of hematite quartz replacement can be measured by a hematite quartz alteration index here termed the hmsi value fe si fe si al which inversely correlates with the normal probability for cu grade areas of highest cu grade 1 wt spatially correlate with irregular zones having intermediate molar alteration indices 0 34 k al 0 40 20 k na 36 and hmsi 0 98 hematite breccias and cu ore deposition developed after tilting of the host sequence into its present steep orientation as indicated by geopetal structures within the breccia matrix thus the economic mineralization occurred late in the deformational history of the region and after extrusion of the lower gawler range volcanics the formation of the prominent hill orebodies occurred during or after upthrusting of deeper seated rocks containing subeconomic cu in skarns north of the fault faulting as well as ore formation may be related to orogenic processes in the central and northern part of the mount woods domain iron oxide introduction was decoupled from and at least partly preceded hydrothermal deposition of high grade cu geochemical and petrographic data indicate that economic cu mineralization occurred together with mildly acidic hematite chlorite sericite siderite alteration of originally carbonate illite and feldspar bearing sedimentary rocks the presence of copper enrichment with an intermediate degree of cation leaching from the host rocks indicates that ph neutralization of initially highly acidic metal transporting fluids was an essential factor causing cu sulfide deposition distinct ranges in na k al ratios and low hmsi values offer potential as exploration indicators pointing toward higher ore grades these results from prominent hill are consistent with recently published mineralogical studies at the giant olympic dam deposit indicating similar ore depositional controls despite lithologically different host rocks

rejuvenation vs recycling of archean crust in the gawler craton south australia evidence from u pb and hf isotopes in detrital zircon abstract zircon grains from modern and ancient drainages of the gawler craton south australia have been analysed to define the relative contributions of juvenile sources and recycled crust to magmatic activity as a function of time and to constrain the role of mantle inputs during the proterozoic rejuvenation of the archean gawler craton the integration of hf isotope data with u pb age spectra shows that the crustal evolution was dominated by long periods of crustal reworking and that crust generated in the paleoarchean 3 2 3 5 ga was largely reworked during proterozoic time four periods of juvenile mantle input can be recognised at ca 2540 ma 1853 ma 1595 ma and 1169 ma the 1169 ma event is not previously recognised in the gawler craton but the absence of other important age peaks argues against the transport of these zircons from the musgrave block only minor proportions of the magmatic zircons with paleoproterozoic to mesoproterozoic ages have strongly juvenile signatures e hf 4 indicating that crustal reworking was the dominant process with relatively minor mantle input comparison of the crustal evolution with that of nearby australian terranes suggests that although the gawler craton shows some similarities with the mount isa block and the georgetown inlier in neoarchean paleoproterozoic time it evolved independently between ca 1800 and 1550 ma

implications for proterozoic plate margin evolution from geophysical analysis and crustal scale modeling within the western gawler craton australia abstract complexly deformed late archean to mesoproterozoic rocks of the western gawler craton are poorly exposed thus reducing the ability to extract meaningful geological structural and tectonic information this study focuses on shear zone analysis via a constrained integration of qualitative aeromagnetic and bouguer gravity data interpretation with quantitative two dimensional forward modeling this integration provides a means to interpret the crustal architecture as well as constrain a sequence of events which aid the interpretation of a tectonic history for the western gawler craton results indicate a polyphase shearing history dominated by a crustal scale array of predominantly west dipping shear zones the initiation of east west trending sz1 structures was likely coincident with ca 1750 1720 ma crustal extension high heat flow and deposition on thinned areas of archean crust with the development of a dense possibly underplated lower crustal component sz1 r2 dextral transpressional movements reactivated sz1 during the ca 1720 1670 ma kimban orogeny which is interpreted to record collision between the gawler craton and the north australian craton during this event basins marginal to and in the interior of the archean crustal block of the western gawler craton were inverted and the dense lower crust was vertically offset causing some of the long wavelength 30 50 km bouguer gravity anomalies presently observed shear zone activity is constrained until ca 1680 ma during which time subduction rollback established a new arc position at the southern margin of the archean crust with magmatism persisting until ca 1670 ma sz3 structures overprint and offset the interpreted axis of the kimban orogeny within the archean crust these structures record a major phase of crustal scale sinistral strike slip movement associated with north south shortening between ca 1630 and 1540 ma prior to their reactivation at ca 1450 ma this sz3 r4 reactivation is interpreted to cause southwest directed crustal transport which was focused as west side up movements internal to the archean continent whereas dextral strike slip was focused at a major rheological boundary between thick archean crust to the south and thick paleoproterozoic basins to the north

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

metamorphic response in orogens of different obliquity scale and geometry abstract investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets to illustrate the problems in documenting flow vectors we present integrated structural metamorphic datasets from two transpressional systems the kaoko belt in namibia and the kalinjala shear zone in south australia these orogens experienced widely differing metamorphic responses to transpressional deformation integration of kinematic and metamorphic datasets from the kaoko belt indicate shallow up plunging extrusion trajectories in the orogen core and show that the maximum stretching direction pattern matches the inferred flow vectors high grade domains 800 840 c and 7 0 8 0 kb in the orogen core developed low angle upward verging maximum stretching direction trajectories whereas a low grade domain 575 600 c and 5 0 5 5 kb in the orogen core has downward verging lineation trajectories the barometric differential between these high grade and low grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high grade domains that were extruded obliquely for the amount of lateral shear estimated for the orogen core the kalinjala shear zone in south australia contrasts strongly with the kaoko belt in this example the high grade and high strain shear zone core of the orogen experienced high t high p metamorphism with low thermal gradients of 21 26 c km and steep decompressive p t paths the lower grade external domains experienced lower t lower p metamorphism with high thermal gradients of 35 37 c km sub horizontal maximum stretching directions do not match the vertical extrusional flow in the high grade core that is indicated by the metamorphic data this comparison shows that in general and on a gross scale maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis in some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow consequently information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record these two datasets must be used in concert to obtain realistic constraints on first order material flow trajectories at orogenic scales the horizontal component of flow is typically best recorded by structural fabrics maximum stretching direction and sense of shear whereas the vertical component is typically best recorded by metamorphic information such as p t paths temperature over depth ratio g and metamorphic field gradients i e t p and g across the orogen

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

u pb id tims baddeleyite ages and paleomagnetism of 1 79 and 1 59 ga tholeiitic dyke swarms and position of the rio de la plata craton within the columbia supercontinent abstract the tandilia terrane southernmost fringe of the rio de la plata craton is an igneous and metamorphic complex produced by an accretionary orogeny 2 25 2 02 ga calc alkaline acidic dykes with e w strike and a major shear zone with similar orientation are related with the late orogeny stage as supported by field relations in a previous study the acid dykes gave 40 ar 39 ar ages of 2007 24 ma to 2020 24 ma a n and nw trending tholeiitic dyke swarm tandil swarm is also present in the tandilia terrane one sample from the nw trending subset previously gave a u pb id tims baddeleyite age of 1588 11 ma new precise u pb id tims baddeleyite dating of both n and nw trending tandil dykes yielded crystallization ages of 1589 3 ma 1588 3 ma and 1588 3 ma significantly older tholeiitic dykes known as the florida swarm occur in the northern rio de la plata craton for which a u pb id tims baddeleyite age of 1790 5 ma was previously reported consequently intermittent rifting 1 79 1 59 ga took place after tectonic stabilization of the late paleoproterozoic lithosphere proto rio de la plata craton the available geochemical data for the 1 59 ga tandil dykes define low and high tio 2 trends although only the low tio 2 subgroup is firmly dated both the tandil and florida dykes have geochemical and nd sr characteristics consistent with derivation from heterogeneous mantle sources that underwent metasomatic effects the tandil dykes may be linked with the 1 57 0 02 ga capivarita anorthosite which occurs to the east of the northern part of the craton correlatives on other crustal blocks may include those in baltica such as bimodal rock association including the breven hallefors and aland aboland diabase dykes and in the reconstructed gawler craton nw laurentia dolerites bimodal magmatism and iocg deposits contemporary within plate bimodal associations are also present in the sw amazonian craton paleomagnetic data for the 1790 ma florida dykes allows three possible reconstruction scenarios for the position of rio de la plata craton i in the southern hemisphere linked to the pampia terrane and the amazonia rio apa landmass as close as they are in present day ii in the northern hemisphere as a nearest neighbor to the reconstructed nw laurentia and gawler blocks proto australia or iii in the northern hemisphere near the boundary of amazonia proto amazonian craton and baltica positions 2 and 3 are consistent with all three criteria paleomagnetic poles and proximity to matching ca 1790 ma and 1590 ma mafic magmatism our data are consistent with the idea that the columbia supercontinent started major break up at 1 59 ga

palaeomagnetism of the ejecta bearing bunyeroo formation late neoproterozoic adelaide fold belt and the age of the acraman impact abstract a new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late neoproterozoic bunyeroo formation in the adelaide fold belt geosyncline south australia which host an extensive horizon of shock deformed rock fragments and microtektite like material of probable impact origin thermal step demagnetisation of 116 samples of red shale and siltstone from six sections sites revealed a high temperature component with a bedding corrected site mean direction of remanence of d 56 6 i 29 3 95 10 7 that gives a pole at 18 1 s 16 3 e dp 6 5 dm 11 8 the high temperature component provides a positive tectonic fold test 99 level of confidence the bunyeroo high temperature remanence direction is near the remanence direction d 50 i 40 indicated by modelling the subsurface magnetic source of the central high amplitude anomaly at acraman australia s largest confirmed meteorite impact structure 220 350 km west of the adelaide fold belt and also is close to the mean direction d 48 3 i 54 7 95 5 2 determined for surface melt rock from acraman statistical tests show that the virtual geomagnetic poles indicated by the directions for the subsurface central magnetic source and surface melt rock at acraman may be regarded as subsets of the bunyeroo palaeomagnetic pole position indicating that the three pole positions are statistically indistinguishable the results imply that the subsurface magnetic source and surface melt rock acquired their remanence in the ambient geomagnetic field during cooling after the impact when structural disturbance had ceased while the bunyeroo formation was accumulating the agreement among the various remanence directions argues strongly that the ejecta horizon in the bunyeroo formation was derived from acraman the present findings confirm that the acraman impact occurred in the late neoproterozoic about 590 ma which is the age of the bunyeroo formation provided by rb sr whole rock shale dating of equivalent and contiguous strata in the adelaide fold belt region the bunyeroo palaeomagnetic data give a palaeolatitude of 15 indicating a low palaeolatitude for the acraman impact and supporting other findings that the adelaide fold belt occupied low to equatorial palaeolatitudes during the late neoproterozoic