zircon xenocrysts from the kambalda volcanics age constraints and direct evidence for older continental crust below the kambalda norseman greenstones abstract the hangingwall basalt at kambalda western australia contains zircons that have been shown by ion microprobe analyses to have very high u and th contents and a wide variety of crystallization ages nearly all of these zircons certainly are xenocrysts a few might relate to intrusive veinlets the age of the youngest xenocrysts 2693 50ma 2 shows that the eruptive age of the basalt cannot exceed 2743 ma this confirms that the apparent sm nd isochron giving 3200 ma 1 2 for kambalda mafic and ultramafic rocks is a mixing line 2 between unrelated components enriched and depleted in light rare earth elements mixing probably occurred at depth by erosion of 3200 3500 ma old felsic crust from the walls of the hwb conduits the zircon xenocryst ages are the first direct evidence for the presence of very old felsic crust in the eastern yilgarn block the latter implies that the kalgoorlie norseman greenstone sequences were formed in a continental rather than an oceanic environment

geochemistry and nd isotope systematics of archaean granites of the eastern goldfields yilgarn craton australia implications for crustal growth processes abstract the archaean granites and granitic gneisses of the eastern goldfields in the yilgarn craton western australia can be divided into two major groups high ca and low ca and three minor groups high hfse high field strength elements mafic and syenitic the high ca group 68 77 sio 2 with high al 2 o 3 na 2 o and sr and low y shares many features with typical archaean tonalite trondhjemite suites but has higher k 2 o rb and th contents the low ca group 70 76 sio 2 differs from the high ca group in having lower al 2 o 3 cao and na 2 o but higher k 2 o rb th zr y la and ce contents granites of the high hfse mafic and syenitic groups form a minor component 10 20 of the eastern goldfields granites the siliceous 74 77 sio 2 high hfse granites are restricted to a narrow nnw trending zone and are characterised by high tio 2 total feo mgo y zr and ce but only moderate rb th and pb contents the a type syenites 50 68 sio 2 are distinguished by their high total alkalies and mainly occur along tectonic lineaments the mafic group 55 70 sio 2 is lithologically diverse and exhibits a wide range of k 2 o rb th la and ce contents granite emplacement occurred between 2 69 and 2 60 ga contemporaneous with or post dating greenstone formation nd values for the high ca mafic and syenitic groups are similar mostly 0 0 to 2 5 whereas those for the low ca group show a pronounced polarity from 4 5 in the west to 2 0 in the east nd depleted mantle model ages for the low ca granites range from 3 2 ga in the west to 2 75 ga in the east model ages for the high ca group 2 75 2 9 ga overlap with ages for inherited zircons from these granites the high ca granites appear to have been derived at high pressures by partial melting of a mafic to intermediate source subducted oceanic crust crustal underplate or thickened crust or remelting of an older tonalitic source derived by such a process if the high ca granites were largely crust derived then their extensive source rocks must have formed less than 250 ma prior to granite formation the low ca granites appear to be the products of crustal reworking probably from a tonalitic to granodioritic protolith that was progressively younger towards the east perhaps in the order of several hundred million years younger or involved an additional younger component the localisation of the crustally derived high hfse granites suggests either geochemical zonation of the crust or specific tectonic processes such as rifting the mafic granites and possibly the syenites imply a significant mantle contribution and new crustal growth although the extent is equivocal the crust of the eastern goldfields appears to have been generated by both lateral and vertical accretion over a significant period from or before 3 0 ga until the time of granite emplacement

the nature and distribution of upper cenozoic ignimbrite centres in the central andes abstract a wide variety of upper cenozoic ignimbrite centres have been identified and studied on landsat imagery of the central andes some have been investigated in the field lower miocene ignimbrite centres are distributed along an arc which in north chile diverges markedly from the line of the present trench and parallels an arc of lower miocene plutons in the eastern cordillera of bolivia much of the ignimbrite erupted since the late miocene has been erupted around the faulted margins of the altiplano basin of western bolivia particularly where the western and eastern cordilleras converge typical examples of ignimbrite centres are described some centres take the form of ignimbrite shield volcanoes other ignimbrite sheets are more widely dispersed the degree of dispersal probably relates to the height of the eruption column partial or total caldera formation sometimes took place and a few of these calderas have resurgent centres late stage lava extrusions are common and some of these are located on caldera ring fractures

neoarchean orogenic magmatic and hydrothermal events in the kalgoorlie kambalda area western australia constraints on gold mineralization in the boulder lefroy golden mile fault system the boulder lefroy golden mile blf gmf fault system is the most intensely mineralized structure 2150 t au to 2015 in the archean yilgarn craton western australia the fault system links the kalgoorlie and kambalda mining districts in the eastern goldfields province a continental margin orogen subdivided into the western kalgoorlie ensialic rift and the eastern kurnalpi volcanic arc after rifting the 2 73 2 66 ga greenstone greywacke succession in the kalgoorlie kambalda area underwent five phases of orogenic deformation predominantly during ene wsw shortening d1 upright folding at ca 2680 ma d2 sinistral strike slip faulting at 2678 2663 ma d3 folding of late conglomerate turbidite successions at 2665 2655 ma d4 dextral strike slip faulting at 2655 2640 ma and d5 east northeast striking normal faulting regional prehnite pumpellyite to greenschist facies burial metamorphism took place during d1 and d3 crustal thickening and amphibolite facies aureoles formed around granite batholiths during and after d3 at 400 100 mpa pressure the d2 blf offsets d1 folds by 12 km sw side south and contains a porphyry dyke 2676 7 ma boudinaged by transtensional oblique slip along a line pitching 21 southeast the blf is linked by transverse d2 thrusts to other sinistral faults recording strike slip until 2663 7 ma late d2 strike slip movement alternated with early d3 shortening d3 thrusts accommodated strain in fault blocks of rigid mafic ultramafic volcanic rocks consolidated during d1 while the sedimentary rocks in d3 synclines were foliated at high strain biotite sericite alteration and gold pyrite mineralization in the blf gmf system took place at 11 4 km burial depth in faults active during d2 and d3 the golden mile 1708 t au and other deposits are associated with stocks and dykes of high mg monzodiorite tonalite porphyry part of a late orogenic 2665 2645 ma mantle derived suite of adakitic affinity hornblende and apatite compositions indicate that these intrusions are characterized by high water contents 5 6 wt h2o in melt by high oxidation states nno 1 0 to 2 4 log units and by igneous anhydrite some stocks contain pervasive anhydrite pyrite mineralization of low gold grade 0 4 g t biotite sericite pyrite ore bodies such as those at kanowna belle 140 t au also replace faulted metamorphic rocks above batholith domes cored by plutons of the monzodiorite suite the d4 strike slip faults are barren at kambalda but control gold quartz vein ore at kalgoorlie 2651 9 ma and au ag breccia ore at black flag 2648 6 ma

oxidized and reduced mineral assemblages in greenstone belt rocks of the st ives gold camp western australia vectors to high grade ore bodies in archaean gold deposits hydrothermal sulfide oxide gold mineral assemblages in gold deposits in the archaean st ives gold camp in western australia indicate extremely variable redox conditions during hydrothermal alteration and gold mineralization in space and time reduced alteration assemblages pyrrhotite pyrite occur in deposits in the southwest of the camp e g argo junction deposits and moderately to strongly oxidized assemblages magnetite pyrite hematite pyrite occur in deposits in the central corridor in the northeast e g north orchin revenge deposits reduced mineral assemblages flank the central corridor of oxidized deposits and locally cut across it along e w trending faults oxidized mineral assemblages in the central corridor are focused on gravity lows which are interpreted to reflect abundant felsic porphyritic intrusions at about 1 000 m below present surface hydrothermal magnetite predates and is synchronous with early phases of gold associated albite carbonate pyrite biotite chlorite hydrothermal alteration later stage gold associated pyrite is in equilibrium with hematite the spatial distribution and temporal sequence of iron sulfides and oxides with gold indicate the presence of at least two spatially restricted but broadly synchronous hydrothermal fluids with contrasting redox states sulfur isotope constraints support the argument that the different mineral assemblages reflect differences in redox conditions the 34s values for pyrite for the st ives gold camp range between 8 4 and 5 1 with the negative values occurring in oxidized magnetite rich domains and slightly negative or positive values occurring in reduced pyrrhotitic domains preliminary spatial and paragenetic analysis of the distribution of iron sulfides and oxides in the st ives camp suggests that gold grades are highest where the redox state of the hydrothermal alteration assemblages switches from relatively reduced pyrrhotite pyrite to relatively oxidized magnetite pyrite and hematite pyrite both in space and time gold deposition is inferred to have occurred where fluids of contrasting redox state mixed

catastrophic debris avalanche deposit of socompa volcano northern chile between 10 000 and 500 yr ago the socompa volcano in northern chile experienced a catastrophic collapse of a 70 deg sector of the original cone causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano the deposits cover some 490 sq km and have a minimum volume of 15 cu km parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high the primary avalanche traveled northwestward over sloping ground before coming to rest transiently forming a prominent marginal ridge and then slid away northeastward to form a secondary flow overriding much of the primary avalanche deposit abundant prismatic jointed dacite blocks within the debris avalanche deposit and a thin fine grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes

late archaean deep marine volcaniclastic sedimentation in an arc related basin the kalgoorlie sequence of the eastern goldfields superterrane yilgarn craton western australia abstract the kalgoorlie sequence 2658 ma is a 3000 m thick succession of volcaniclastic sedimentary rocks and subordinate rhyolite dacite andesite and basalt spatially and temporally associated with subvolcanic sills and dykes of rhyolite and dacite gabbro sills and i type granitoids although basalts are tholeiitic the felsic rocks have ttd affinity the sequence comprises mostly deep marine channel levee turbidite complexes and hemipelagites resedimented syneruptive volcaniclastic deposits are associated with coherent lavas but almost all of the sedimentary fill was recycled from prior cycle volcanic and volcaniclastic sources provenance was mainly dacitic rhyolitic with minor andesitic basaltic and polymictic compositions volcano bound and fault bound architectures and two styles of basin filling are identified progradational or aggradational transverse fans and slope aprons and retrogradational longitudinal fans of a linear trough retrogradation in longitudinal systems is consistent with basin lengthening and deepening six depositional sequences are grouped in four unconformity bounded tectonic stages that record uplift subsidence cycles coeval mafic and felsic magmatism indicate that the tectonic stages represent extensional tectonic settings whereas volcanogenic provenance spatially linked volcanism and plutonic magmatism and deep marine environments support an intra arc site cyclic uplift subsidence and basin lengthening are more consistent with strike slip controls on basin formation than with orthogonal extension the tectonic setting is interpreted to have been a pull apart basin associated with strike slip faulting and amalgamating arc related terranes similar to the modern philippines archipelago the strong similarity between archaean volcaniclastic and modern arc related volcaniclastic environments implies comparable tectonic settings and therefore comparable controlling tectonic factors

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

the largest au deposits in the st ives goldfield yilgarn craton western australia may be located in a major neoarchean volcano sedimentary depo centre the largest neoarchean gold deposits in the world class st ives goldfield western australia occur in an area known as the argo junction region e g junction argo and athena why this region is so well endowed with large deposits compared with other parts of the st ives goldfield is currently unclear because gold deposits at st ives are hosted by a variety of lithologic units and were formed during at least three different deformational events this paper presents an investigation into the stratigraphic architecture and evolution of the argo junction region to assess its implications for gold metallogenesis the results show that the region s stratigraphy may be subdivided into five regionally correlatable packages mafic lavas of the paringa basalt contemporaneously resedimented feldspar rich pyroclastic debris of the early black flag group coarse polymictic volcanic debris of the late black flag group thick piles of mafic lavas and sub volcanic sills of the athena basalt and condenser dolerite and the voluminous quartz rich sedimentary successions of the early merougil group in the argo junction region these units have an interpreted maximum thickness of at least 7 130 m and thus represent an unusually thick accumulation of the neoarchean volcano sedimentary successions it is postulated that major basin forming structures that were active during deposition and emplacement of the voluminous successions later acted as important conduits during mineralisation therefore a correlation exists between the location of the largest gold deposits in the st ives goldfield and the thickest parts of the stratigraphy recognition of this association has important implications for camp scale exploration

4d architecture and tectonic evolution of the laverton region eastern yilgarn craton western australia abstract the laverton region located in the eastern yilgarn craton eyc western australia is second only to the kalgoorlie region for gold endowment the integration of high density potential field data regional and camp scale seismic reflection data regional and mine scale structural analysis and geochronologically constrained stratigraphy provided new insights into the 4d architecture and tectonic evolution of laverton region at a regional scale the map patterns of laverton are dominated by the mt margaret dome in the northwest and the kirgella dome in the southeast these domes are flanked to the east and west by north northwest striking shear zones with the central zone between the two domes being dominated by north to north northeast striking sigmoidal shear zones these distinctly different strikes to the shear zones developed early in the tectonic evolution and resulted in a favourable architecture for late stage orogenic gold mineralisation a series of regional seismic reflection lines have imaged the sub surface architecture of the laverton region with a major crustal penetrating shear extending to the moho mapped as the laverton tectonic zone ltz the ltz consists of a basal fault the north northwest striking celia fault and a series of steeper faults for example the north to north northeast striking childe harold fault and far eastern fault which sole at depth into the celia fault beneath the laverton region lower angle faults imaged in the seismic reflection data develop parallel to granite gneiss domes that postdate the north northwest striking and north to northeast striking faults the construction and integration of the new 3d map with the structural stratigraphic and geochronological analysis has defined six main deformation events and their effects on the architectural evolution of the region this analysis suggests that early formed faults are genetically associated with late stage orogenic gold mineralisation regional and mine scale structural analysis at the wallaby and sunrise dam deposits indicate that gold deposition occurred synchronously during the d4b event in both deposits both mines are separated by the ltz which records sinistral and later d5 dextral movement the kinematic model which best describes the observed architecture is a restraining step over within a sinistral strike slip system a structural model consistent with analogue modelling of strike slip systems is proposed where two early extensional basement faults were reactivated during d4b northwest southeast contraction as low angle thrusts in a pop up zone this architecture has been mapped in 3d in the region between wallaby and sunrise dam where both deposits are located on the low angle thrusts on opposing sides of a sigmoidal structure developed during sinistral strike slip movement this strike slip step over architecture provided significant opportunity for dilation and fluid flow late dextral movement of the basement faults during northeast southwest contraction d5 reactivated the low angle thrusts at sunrise dam producing extensional movement and further dilation within the overall restraining step over this event is not significant at wallaby mine a detailed 3d stress analysis at sunrise dam revealed that the most favourable fault segments to reactivate during the d4b event were orientated at 30 towards 310 a comparison of the spatial distribution of gold defined from extensive production and resource estimation drilling with the favourably orientated segments of faults was confirmed the analysis also highlighted down dip extensions beyond the coverage of the drilling of favourably oriented segments which may provide additional targets during future near mine exploration the same approach was applied to the regional 3d map and new targets generated beneath the major salt lake between wallaby and sunrise dam

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

an early thrust duplex in the kalgoorlie kambalda greenstone belt eastern goldfields province western australia abstract extensive regional mapping of the stratigraphic sequence throughout the kalgoorlie kambalda greenstone belt in western australia has outlined the geometry of early subhorizontal thrust faults these thrusts stacked and repeated the stratigraphy to form a major thrust duplex south of kambalda the roof thrust to the duplex becomes a major thrust flat to the north at kalgoorlie later transpressional deformation f2 upright folds d3 sinistral shear zones and folding has rotated and displaced the d1 thrusts both sub horizontal deformation and subsequent transpression indicate that compressional tectonics rather than vertical tectonics involving solid state granitoid diapirs were crucial in the development of this granite greenstone terrain

kilometre scale structural setting of ore shoots in the frasers gold deposit macraes mine new zealand abstract the hyde macraes shear zone in southern new zealand contains the circa 10 million ounce macraes gold deposit one of the larger phanerozoic orogenic gold deposits discovered to date globally approximately 50 of this 10 million ounce resource is hosted by 5 major ore shoots up to 400 m wide and 1500 m long in the frasers area at the southern end of the mine higher grade 1 5 g t au ore shoots are located along and immediately below the hangingwall shear the principal strand of the hyde macraes shear zone at the frasers deposit they typically trend parallel to the intersection of the shear and foliation in the underlying schist commonly where the foliation dips more steeply that the overlying hangingwall shear especially thick zones of higher grade mineralised rock are located between the hangingwall shear and underlying second order splay shears whose position correlates with minor right hand bends in the strike of the overlying hangingwall shear lower grade

new insights into archean granite greenstone architecture through joint gravity and magnetic inversion archean tectonic evolution and metallogenic endowment in cratonic areas are intrinsically linked to lithological diversity and complex architectures unfortunately reliable models of the deep structure of such areas are scarce due to poor surface exposure and limited availability of drill holes and high resolution geophysical information by combining multiple geophysical data sets the strategy of cross gradient joint geophysical inversion permits improved subsurface imaging of geological heterogeneity and structure in this work we showcase the application of cross gradient joint inversion of gravity and magnetic data to characterize the heterogeneity of the leonora district in the eastern yilgarn craton western australia our results harmonize both geophysical data types into integrated images of the top 15 km of the crust and successfully predict observations made by a seismic experiment and surface geological mapping

precise u pb zircon dating of a post d2 meta dolerite constraints for rapid tectonic development of the southern adelaide fold belt during the cambrian numerous morb like mafic dykes plugs and sills intruded the kanmantoo group and older meta sediments of the southern adelaide fold belt from pre d1 to post d2 during the delamerian orogeny a post d2 meta dolerite yields a precise u pb zircon age of 510 2 ma this age combined with a published shrimp zircon age of 526 4 ma reveals a rapid tectonic evolution of the fold belt during the cambrian events including the thick kanmantoo group9s deposition major mafic magmatism low p high t regional metamorphism and multiple deformations all took place from about 526 to 510 ma this rapid evolution supports the suggestion that the heat source for the metamorphism originated from continental extension associated with mantle upwelling and mafic magmatism

tectono stratigraphy of late archaean greenstone terranes in the southern eastern goldfields western australia abstract late archaean greenstone terranes in the southern part of the eastern goldfields are regionally extensive faultbounded entities defined on the basis of small but distinct differences in stratigraphy and structure the greenstones comprise volcano sedimentary successions that were all deposited at the same time c 2720 2675 ma on sialic crust hence the terranes are interpreted to represent contemporaneous probably adjacent basins these basins were affected by the same history of granite plutonism and regional deformation c 2675 2620 ma early subhorizontal deformation is poorly understood but includes at least locally significant thrust stacking d1 with a south to north movement direction this was followed by regional east west extension affecting all supracrustal successions above a province wide detachment now lying at depths between 4 and 7 km this detachment may have evolved from the greenstone basement contact but only after extensive reworking clastic basins developed along or above major extensional faults but no volcanism is associated with this extension subsequent east west d2 shortening resulted in regional scale thrusting folding and associated late stage transcurrent fault movements the d2 event is contemporaneous with a major thermal event represented by voluminous granite emplacement c 2660 ma and associated peak regional metamorphism in the greenstone succession predominantly monzo granite derived from older sialic crust was emplaced throughout the period of basin formation and deformation early synvolcanic granite c 2719 2710 ma is a minor component late synvolcanic granite c 2685 2675 ma is widespread and syn d2 granite c 2660 ma is abundant there are a few small late tectonic intrusions c 2640 ma to 2600 ma the lateral lithostratigraphic persistence of the contemporaneous basins the syn d2 low pressure regional metamorphism and the province wide monzo granite magmatism suggest that the greenstone basins possibly developed in a tectonic setting similar to that of failed rifts above aborted spreading centres the basins closed in response to far field compressional stresses

age constraints on recycled crustal and supracrustal sources of archaean metasedimentary sequences eastern goldfields province western australia evidence from shrimp zircon dating abstract shrimp u pb zircon dating of sedimentary volcanic and subvolcanic intrusive rocks from unconformity bounded sequences in the eastern goldfields province defines maximum depositional ages for those sequences the kambalda sequence which records a backarc basin was deposited prior to 2700 ma and from at least 2715 ma the spargoville sequence which records an arc adjacent volcano bound basin was deposited between 2700 and 2683 ma the kalgoorlie sequence which records an intra arc rift basin comprises two tectonic stages with depositional ages of 2681 2670 and 2661 2655 ma deposition of the spargoville and kalgoorlie sequences overlapped with the intrusion of granitoids and subvolcanic porphyries thereby indicating that those granitoids were emplaced during crustal extension younger submarine fan kurrawang sequence and braided fluvial merougil and jones creek sequences deposits of a remnant ocean basin were deposited at when coupled to palaeocurrent data provenance dating of detrital zircon suites fingerprints southeast oriented amalgamation of felsic detritus from multiple sources within longitudinal depositional systems and longitudinal reworking of felsic detritus from single sourced transverse systems most felsic detritus was recycled from coeval volcanoplutonic arcs and the orogen to the remnant ocean basin sequences with zircon ages ranging from 2730 to 2660 ma older felsic sources with age ranges of 3570 3130 3030 2900 2870 2770 and 2760 ma could have been structural enclaves within the volcanoplutonic arcs or orogen but were more likely within a distant cratonic basement when coupled with stratigraphic data it is concluded that detrital zircon age distributions preserve evidence for the sedimentary recycling of coeval volcanoplutonic arc or orogenic sources uplifted basin floor and dissected arc successions and cratonic basement virtually all source terrains have apparently vanished presumably through the combination of tectonic and sedimentary recycling and tectonic severance increased use of detrital zircon suites from archaean sedimentary sequences is advocated in order to quantify the balance between sedimentary recycling of felsic rocks from contemporaneous volcanoplutonic sources which themselves document crustal growth in arc environments and from relict volcanoplutonic and crustal basement sources which document the degree of crustal recycling

kinematic history of the boulder lefroy shear zone system and controls on associated gold mineralization yilgarn craton western australia the boulder lefroy shear zone developed in the ca 2 7 to 2 65 ga archean greenstone sequence of the kalgoorlie terrane in the yilgarn craton is remarkable for the number and importance of gold deposits it hosts these include from south to north the following gold districts with tons of contained au st ives 253 t hampton boulder jubilee 123 t golden mile 1 821 t plus mount charlotte 219 t and paddington broad arrow 112 t detailed structural studies reveal that the shear zone system developed in two main stages the first related to a phase of north northwest south southeast trending folding and thrusting regional d 2 and the second related to sinistral shearing regional d 3 that reactivated and linked the preexisting north north west south southeast trending network of thrust planes sinistral movement resulted in only limited displacement less than 12 km and evolved from a ductile shearing in broad zone toward semibrittle shearing along narrow planes regional d 4 the structures suggest a temporal evolution from crustal thickening during d 2 through lateral escape during d 3 to the development of brittle structures during d 4 the fimiston and oroya lodes at the golden mile in kalgoorlie are also exceptional in their gold endowment and associated alteration and were most likely developed during d 2 all other deposits along the shear zone system were formed within 25 m y of the fimiston and oroya lodes late in the tectonic history of the terrane either during d 3 or d 4 intense mineralization within and around the shear zone resulted from multiscale focusing of mineralizing fluids at a regional scale fluids were focused into the shear zone system and its large anticlines which expose deeper mafic and ultramafic rocks at a local scale fluids were further focused into structural pathways resulting from strain induced permeability in rocks with contrasting rheology

late archaean felsic alkaline igneous rocks in the eastern goldfields yilgarn craton western australia a result of lower crustal delamination numerous felsic alkaline intrusions were emplaced in the granite greenstone terrain of the eastern goldfields yilgarn craton between c 2650 and 2630 ma intrusion coincided with the waning stages of voluminous regional granitic magmatism the alkaline rocks are localized along and adjacent to major crustal structures that separate lithostratigraphic greenstone terranes and are divided into four geographically and compositionally distinct suites viz the mount monger emu claypan and ninnis suites the rocks have features in common with a type granites including high total alkalis an anhydrous primary mineralogy an apparent anorogenic setting and enrichment in some hfse although they also have low concentrations of nb and y they show a range of features that together point to a crustal source e g the alkaline rocks are not associated with voluminous mafic magmatism they have high lile concentrations they carry zircon xenocrysts of similar age to whole rock sm nd model ages they show evidence for regional source heterogeneity suites have constant and distinct la y and nb y ratios and the rocks show many compositional similarities to the slightly older voluminous crust derived granites extreme enrichments in sr and ba in the mount monger suite however suggests that their source may have been metasomatized before or during melting intrusion of the alkaline rocks heralds a poorly recognized third tectonothermal event in the evolution of the eastern part of the yilgarn craton and was broadly synchronous with the intrusion of voluminous felsic fractionated granites crustal extension and widespread au mineralization the late archaean geology of the eastern goldfields shows many features in common with phanerozoic collisional belts thought to be profoundly influenced by processes of lithospheric delamination or convective thinning this newly recognized third tectonothermal event in the eastern goldfields may relate to the delamination or convective thinning of a dense garnet rich lower crust that resulted from extraction of earlier voluminous granite magma and it is possible that such processes have had a greater influence on late archaean crustal evolution than previously recognized

quantified spatial relationships between gold mineralisation and key ore genesis controlling factors and predictive mineralisation mapping st ives goldfield western australia abstract determining the spatial relationships between various geological features and mineralisation is not only important for understanding the ore genesis of mineral deposits but can also help to guide mineral exploration by providing predictive mineral maps in this gis based study we quantify the spatial relationships between gold mineralisation and plausible controlling factors in the central part of the st ives area western australia we weigh plausible control factors including rock types lithological boundaries and faults using gold occurrences in drill cores and then apply the weights to the geological data to generate a predictive map for the entire st ives area the three major findings of this study are 1 all major gold deposits are controlled by faults and small scale fault systems have a stronger correlation with gold mineralisation than large faults 2 felsic to intermediate intrusive rocks show strong correlations with gold mineralisation whereas metamorphic mafic rocks greenstones possibly acted as part of the broad regional environment for the mineral province rather than as a factor controlling the exact locations of the deposits and 3 our predictive mapping indicates that the southeast part of the st ives goldfield has a high potential for discovering new gold mineralisation

the invincible deposit early gold mineralisation truncated by unaltered c 2665 ma conglomerate st ives eastern goldfields western australia abstract two distinct styles of mineralisation are recognised at the invincible deposit with bedding parallel shear related gold mineralisation type 1 and shallow dipping mineralised extension veins type 2 the different styles of mineralisation reflect formation during two discrete deformation events type 1 mineralisation is hosted in laminated mudstone of the upper black flag group and mineralised quartz veins and breccia zones are distributed throughout the mudstone rather than being associated with a single shear mineralised zones are associated with intense sodic alteration the early mineralised structures are strongly modified by east west horizontal d2 3 compression in contrast type 2 extension veins are undeformed and are best developed in the massive sandstone conglomerate footwall but also overprint type 1 veins in the mudstone unit throughout the mine the early type 1 mineralisation is truncated by unaltered conglomerate of the c 2665 ma merougil formation a 10 20 difference in strike dip is commonly present and indicates that the main mineralising event at invincible occurred prior to erosion and deposition of the merougil formation the strong modification of the mineralisation by horizontal east west d2 3 compression is also consistent with formation early in the deformation history during the c 2672 2660 ma d1 event gold deposition in the egt was episodic occurring over a prolonged period during extension and compression bedding vein relationships indicate southwest side down kinematics during the early mineralising event in weakly mineralised zones sub vertical extension veins cut across beds that dip 70 to the southwest in strongly mineralised zones bedding is sub vertical and large quartz breccia veins develop along the bedding above moderately west dipping shears that extend across the steep bedding the location of the steep strongly mineralised zones where bedding is sub vertical reflects enhanced dilation in these zones during southwest side down movement southwest side down movement could represent extensional slip on the laminated mudstone beds if the bedding was tilted into a developing basin early merougil basin the location of the invincible system at the eastern edge of the merougil basin likely reflects an association with early basin controlling faults

linking mainland australia and tasmania using ambient seismic noise tomography implications for the tectonic evolution of the east gondwana margin for nearly half a century a number of conflicting tectonic models have been postulated to explain the enigmatic geological relationship between tasmania and victoria with a view to unifying our understanding of the evolution of the eastern margin of gondwana in australia in this study ambient noise data from an array of 24 broadband seismometers is used to produce a high resolution 3 d crustal shear wave velocity model of bass strait the key to understanding the missing link we apply a novel transdimensional and hierarchical bayesian inversion approach to construct group velocity maps in the period range of 2 30 s and subsequently invert group velocity dispersion for 3 d shear wave velocity structure this allows us to image for the first time the entire crust beneath bass strait in high detail and elucidate the geometry and position of key crustal features with corroboration from complementary datasets the three sedimentary basins related to the failed rifting event associated with the australia antarctica breakup in particular bass basin clearly emerge from the tomographic solution model a key feature of the 3 d shear wavespeed model is a distinct mid lower crustal nw se high velocity zone which extends from northwestern tasmania to south central victoria confirming a proterozoic geological connection we also image three north south high velocity belts that appear to span bass strait with some interruption from velocity variations possibly related to more recent tectonic events these belts are consistent with recent gravity and magnetic maps and may indicate the presence of an exotic precambrian terrane the selwyn block the model also images the crustal velocity structure of the southern stawell and bendigo zones and their internal large scale multi layer characters a legacy of their early paleozoic intra oceanic origins another high velocity anomaly imaged in the mid lower crust is an east west lineament beneath the northern part of bass strait which may be an intrusive feature associated with the failed rift

lithologic mapping using random forests applied to geophysical and remote sensing data a demonstration study from the eastern goldfields of australia the eastern goldfields of western australia is one of the world s premier gold producing regions however large areas of prospective bedrock are under cover and lack detailed lithologic mapping away from the near mine environment exploration for new gold prospects requires mapping geology using the limited data available with robust estimates of uncertainty we used the machine learning algorithm random forests rf to classify the lithology of an underexplored area adjacent to the historically significant junction gold mine using geophysical and remotesensing data with no geochemical sampling available at this reconnaissance stage using a sparse training sample 1 6 of the total ground area we produce a refined lithologic map the classification is stable despite including parts of the study area with later intrusions and variable cover depth and it preserves the stratigraphic units defined in the training data we assess the uncertainty associated with this new rf classification using information entropy identifying those areas of the refined map that are most likely to be incorrectly classified we find that information entropy correlates well with inaccuracy providing a mechanism for explorers to direct future expenditure toward areas most likely to be incorrectly mapped or geologically complex we conclude that the method can be an effective additional tool available to geoscientists in a greenfield orogenic gold setting when confronted with limited data we determine that the method could be used either to substantially improve an existing map or produce a new map taking sparse observations as a starting point it can be implemented in similar situations with limited outcrop information and no geochemical data as an objective data driven alternative to conventional interpretation with the additional value of quantifying uncertainty

scale integrated architecture of a world class gold mineral system the archaean eastern yilgarn craton western australia abstract world class mineral systems such as those found in the archaean eastern yilgarn craton are the product of enormous energy and mass flux systems driven by lithospheric scale processes these processes can create big footprints or signatures in the lithosphere which can be observed at a range of scales and via a variety of methods including geophysics isotopes tectonostratigraphy and geochemistry we use these datasets to describe both the architecture structure of world class gold systems of the yilgarn craton and the signatures of their formation by applying an understanding of the most critical elements of the process and their signatures new areas especially undercover may be targeted more predictably than before knowledge of the major architectural elements of the yilgarn craton has increased greatly over the last decade through the collection of a wide range of geophysical geochemical and isotopic data these data range in scale from 1 lithospheric scale which provide information on the entire craton and their underlying volumes down to depths in excess of 350 km through 2 regional scale which provide information at the province scale and to depths of 30 40 km to 3 mine and camp scale which provide information on the top few kilometres of the crust the architecture structure in the upper mantle and lower crust can be inferred from broadband seismic tomography and analysis of long wavelength gravity data geophysical data such as magnetotellurics and deep crustal reflection seismic also provide evidence for the signatures of the flow of fluids through this architecture the pathways and they illustrate that the scale of the system is many orders of magnitude larger than the immediate deposit itself of particular importance is the role of deep crustal penetrating shear zones or faults which link the mantle with domes in the upper crust together these data provide first order insights into the physical and chemical properties of the yilgarn craton these insights provide geodynamic clues and constraints as to why the yilgarn is so well endowed in metals such as gold groves et al 1989 the intersection of favourable architectural features provides connections between the mantle and the upper crust in the kalgoorlie region possibly explaining why this area is so well endowed in comparison to others

the world class wallaby gold deposit laverton western australia an orogenic style overprint on a magmatic hydrothermal magnetite calcite alteration pipe gold mineralisation at the wallaby gold deposit is hosted by a 1 200 m thick mafic conglomerate the conglomerate is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic monzonite monzonite syenite syenite porphyry to late stage carbonatite in the mine area a pipe shaped zone of actinolite magnetite epidote calcite amec alteration overprints the conglomerate gold mineralisation associated with dolomite albite quartz pyrite alteration is hosted in a series of sub horizontal structurally controlled zones that are largely confined within the magnetite rich pipe the deposit has a current ore reserve of 2 0 moz au and a total resource of 7 1 moz au

archaean basic volcanism in the eastern goldfields province yilgarn block western australia abstract petrographic petrological and geochemical studies have demonstrated the presence of three distinctive basic volcanic suites in the eastern goldfields province yilgarn block western australia these are termed the high magnesian series basalts hmsb the low magnesian series basalts lmsb and the siliceous high magnesian series basalts shmsb the hmsb and shmsb constitute differentiation series which contain both high mgo 9 5 14 wt and low mgo the hmsb and lmsb were derived from source mantle regions which were variably depleted in the incompatible elements archaean komatiites were derived from similarly depleted source regions and it is argued that the main petrogenetic difference between these three volcanic suites was the degree of partial melting from which they were derived the depleted nature of the source regions may have been induced by earlier small degree source mantle regions of the shmsb were variably enriched in the incompatible elements and water and parental magmas of the shmsb were derived from moderately hydrous conditions of partial melting the relative proportions of each basalt suite varies considerably between the layered successions examined for example the basic volcanics overlying the komatiites at kambalda are shmsb while the footwall volcanics consist predominantly of hmsb with subordinate lmsb however the norseman succession where no ultramafic volcanics are known to occur is comprised mainly of lmsb with a smaller proportion of hmsb

biotite and apatite as tools for tracking pathways of oxidized fluids in the archean east repulse gold deposit australia fluid pathways in hydrothermal ore deposits can potentially be traced through alteration zoning patterns and the chemistry of mineral phases i e major trace elements and isotopic composition which provide insight into the physico chemical gradients within and adjacent to pathways world class neoarchean gold deposits of the eastern yilgarn can be zoned with respect to alteration assemblages but in many cases fluid pathways are poorly understood which makes it difficult to identify the nature and source of fluids this study conducted detailed core logging sem mineral mapping of alteration assemblages and microprobe analysis of biotite and apatite in order to identify chemical gradients across the east repulse gold deposit of the st ives gold camp eastern yilgarn western australia results show that the east repulse deposit is vertically zoned with relatively oxidized sulfate rich alteration in the footwall of the deposit anhydrite celestine and or barite sulfide rich alteration in the core of the deposit pyrite magnetite with minor barite and celestine and relatively reduced sulfide only assemblages pyrite pyrrhotite with minor pentlandite millerite and cobaltite in the hanging wall of the deposit gold was identified as inclusions in pyrite and associated biotite implying a link between biotite pyrite alteration and gold mineralization the abundance of f in biotite and apatite progressively decreases from the footwall of the deposit into the mineralized core this trend of decreasing f in biotite and apatite with decreasing depth is consistent with an hypothesis that oxidized f and k bearing fluids were focused along subvertical pathways beneath the deposit within or subjacent to granitoid dikes the biotite apatite geothermometer shows that these fluids were relatively hot 480 60 c at the time of alteration these relatively high temperature oxidized f and k bearing fluids may have been derived from a proximal magmatic source

geochronological constraints on pre syn and postmineralization events at the world class cleo gold deposit eastern goldfields province western australia recent geological and geochronological studies have led to a reevaluation of whether or not the majority of the lode gold deposits of the yilgarn craton formed nearly synchronously during an archean craton wide hydrothermal event as previously proposed the majority of reliable data indicate that gold mineralization took place at ca 2640 to 2625 ma however recent work in the far north of the eastern goldfields province provides structural evidence for instances of earlier gold mineralization and geochronological data for interpreted postore rocks that are considerably older than 2640 ma the documentation of xenotime and monazite in association with ore minerals at the cleo deposit provides a valuable means of determining the absolute age of gold mineralization at the cleo deposit high grade western lodes veins crosscut the sunrise shear zone at a high angle with only centimeter scale offset given that the stratigraphic sequence indicates at least several hundred meters offset across the shear zone the minimal offset of the western lodes veins indicates that gold mineralization was late in the history of movement in the shear zone the absolute ages of pre syn and postmineralization elements in the mine stratigraphic succession have been determined using a variety of geochronological techniques fuchsite micas formed in the sunrise shear zone yield an 40ar 39ar isochron age of 2667 19 ma rhyodacite porphyry dikes postdate the formation of the shear zone but predate the main phase of gold mineralization dike intrusion is constrained to 2674 3 ma by shrimp ii u pb analysis of zircons from these dikes rhenium os analysis of molybdenite from quartz chalcopyrite molybdenite veins that crosscut the porphyry dikes gives an age of formation at 2663 11 ma consistent with their relationship to the porphyry dikes both the dikes and these veins are crosscut by gold bearing western lodes veins mutually crosscutting relationships with ore minerals indicate that microscopic xenotime and monazite grains were deposited during ore formation in the western lodes ore zones shrimp ii u pb analyses of xenotime and monazite grains indicate that western lodes mineralization took place at 2654 8 ma at least 9 m y after the intrusion of the porphyry dikes 95 confidence evidence for substantial temporal separation between intrusive activity and gold mineralization is recorded by other researchers for other deposits associated with granitic intrusions in the leonora laverton area phlogopitic micas from a lamprophyre that crosscuts the whole succession including the sunrise shear zone yield an ar ar plateau age of 2080 4 ma this study constrains the timing of multiple events at cleo including the main phase of gold mineralization during a relatively short span of time however in the context of the larger debate about the existence of a dominant 2640 to 2625 ma period of gold mineralization and the extent of a proposed widespread pre 2660 ma mineralization event in the yilgarn craton the data from cleo are equivocal the 2654 8 ma age for mineralization being significantly older than 2640 ma but not significantly pre 2660 ma even if the maximum possible age is accepted

predictive mineral discovery in the eastern yilgarn craton western australia an example of district scale targeting of an orogenic gold mineral system abstract the eastern yilgarn craton eyc is one of the world s premier gold provinces subject to over a century of mineral exploration prolonged interest in the terrane has led to the assembly of multiple world class data sets suitable for testing district scale targeting methodologies district scale targeting is concerned with identifying a mineral camp 60 km 60 km in size within a prospective region or province 1000 km 1000 km in size exploration at the district scale necessitates the development of predictive exploration models which can be applied to large regions recent advances in the study of the geodynamic evolution and 3d architecture of the eyc together with an understanding of their interrelationship with the orogenic gold mineral system has resulted in identification of critical mineralisation processes responsible for the region s rich gold endowment here we describe and map these critical processes using them as a basis for district scale targeting we relate gold mineralisation to three temporally constrained geodynamic periods integrated with regional hydrothermal alteration unlike many targeting methodologies this methodology does not incorporate the location of known gold deposits in the analysis yet it predicted 75 of known gold mineralisation in 5 of the area the methodology allows critical mineralisation processes to be identified and mapped through time and space these critical processes are mostly generic and can be applied to other granite greenstone orogenic gold regions such as the abitibi in canada an important outcome of this work for the eyc is the identification of a number of new target areas not known currently for significant gold mineralisation in what is otherwise thought to represent a mature terrane for gold exploration

multi stage enrichment processes for large gold bearing ore deposits abstract a review of previous studies of the world s large hydrothermal gold deposits indicates that the largest deposits tend to show complicated parageneses where multiple gold enrichment events and processes have been involved in the deposit generation these observations suggest that multistage processes may even be a requirement for the formation of large deposits in some deposits e g witwatersrand boddington cadia sukhoi log or carlin the different enrichment processes occur millions of years apart in others such as many large porphyry deposits the different stages are much closer in time in many deposits particularly sedimentary hosted deposits early diffuse enrichment occurs within a particular province that is then upgraded by more focused processes e g sukhoi log kalgoorlie the presence of this early diffuse enrichment could explain the tendency for gold deposits to cluster into camps this model has important implications as the presence or absence of multiple gold events could be used to discriminate at the exploration and feasibility stages between small deposits with single stage ore genesis and more complicated deposits with multistage enrichment and the potential for larger gold endowment

the tectonic setting and evolution of the 2 7 ga kalgoorlie kurnalpi rift a world class archean gold province the yilgarn craton results from three major mantle input events at ca 3 0 2 9 2 8 and 2 7 ga that have interacted with 3 0 ga continental crust zircon geochronology and sm nd isotopic data subdivide the craton into an older yilgarn proto craton and the younger more primitive eastern goldfields superterrane egst formation of the kalgoorlie kurnalpi rift kkr within the egst was associated with the 2 7 ga event which exploited weakened crust at the eastern margin of the yilgarn proto craton where thick sequences of komatiite and basalt were erupted between ca 2710 and 2690 ma in the kalgoorlie terrane calc alkaline volcanism in the kurnalpi terrane began at ca 2730 ma and continued to ca 2690 ma overlapping rifting and plume related volcanism in the kalgoorlie terrane deposition of siliciclastic sedimentary rocks within basins at ca 2660 resulted from an intra orogenic extensional event and coincided with the transition from high ca to low ca granite magmatism and peak emplacement of intrusions with a metasomatised mantle source component most aspects of the kkr are satisfied by broadly coincident plume related magmatism in the kalgoorlie terrane and westward subduction to the east of the burtville terrane geochemical characteristics of 2730 2700 ma calc alkaline volcanism and 2685 2630 ma low sio2 and alkali rich intrusions support models for a continental margin subduction zone setting world class gold deposits formed in reactivated margins of the kkr which became flux zones for mantle derived magmas hydrothermal fluids and heat during 2675 2620 ma orogenesis the orogenic gold mineralisation can be subdivided into proximal intrusion related and distal source deposits

architecture and geodynamic evolution of the st ives goldfield eastern yilgarn craton western australia abstract a new structural evolution consisting of both extensional and contractional events has been defined for the st ives goldfield in the south central kalgoorlie terrane of the eastern yilgarn craton in western australia these events shaped the development of the fault architecture which controlled the location of the regional anticlines the magmatic centres and the deposition of the archaean greenstone successions the fundamental grain of the st ives goldfield is north northwest trending this trend is marked by faults which developed during d1 extension which was oriented east northeast west southwest across these faults we map major stratigraphic changes in the thickness and composition of units especially of the previously undivided black flag group volcaniclastic rocks the centre of the st ives goldfield is dominated by the kambalda anticline this north northwest trending regional fold was likely established early during the d1 extensional history and was fully established during subsequent east northeast oriented d2 contraction the regional anticline is an important architectural element because 1 magmatism and gold mineralising fluids were focussed into this domed region and 2 deformation was partitioned across the limbs and crest of this structure the d3 event involved regional uplift and extension resulting in the formation of late basins merougil conglomerate locally and the emplacement of granitoids sourced from a metasomatised mantle wedge mafic type porphyries the most significant gold event in terms of endowment occurred during d4b sinistral strike slip shearing and associated thrusting e g tramways and republican thrusts these thrusts were previously interpreted as the first contractional structures to deform the area d1 but are here reinterpreted as relatively late d4b in this d4b event the north northwest trending faults underwent sinistral strike slip shearing and were linked across the kambalda anticline by accommodation structures represented by generally east to east northeast trending thrusts reactivation of d1 transfer structures may have influenced the location of these later accommodation structures late stage mineralisation during d5 was the result of dextral strike slip brittle shearing

element mobility and spatial zonation associated with the archean hamlet orogenic au deposit western australia implications for fluid pathways in shear zones mass balance equations can be used to understand metasomatism here we use 2726 multi element chemical analyses to quantify the effect of metasomatism associated with au mineralisation at the hamlet gold mine hamlet yilgarn craton western australia principal components analysis is first used to identify covarying elements and six groups of elements are identified 1 relatively immobile elements with zr and hf providing an appropriate reference frame for mass balance calculations 2 elements associated with mafic rocks such as ca co fe ge mg mn and zn 3 elements related to potassic alteration ba cs k rb and tl 4 elements related to sodic alteration be na and sr 5 chalcophile elements including bi weakly associated with au and 6 au te and s representing the main au mineralising event a statistically rigorous approach is then used to quantify element addition and depletion and provides 95 confidence limits for changes in mass to achieve this the basaltic host rocks are divided into four subunits based on cr content sample subsets are then created to select altered rocks and least altered protolith rocks using discrimination diagrams pairings of least altered altered samples from each basalt subunit are used to construct matrices via bootstrapping mass balance results from these matrices indicate similar element mobility associated with au mineralisation across the four basalt subunits with up to 24 mass loss key potassic and sodic group elements are enriched in the au mineralised zones at the expense of elements with a mafic association spatial patterns of mass change are linear and repetitive within the steeply dipping hamlet shear zone interpreted to represent a mesh like geometry of metasomatism within the plane of the shear zone localised enrichment of k na and bi are related to au distribution while ca depletion zones reflect the movement of reactive fluid along the shear plane

contemporaneous ultramafic and felsic intrusive and extrusive magmatism in the archaean boorara domain eastern goldfields superterrane western australia and its implications abstract two komatiite emplacement styles are recognized in the boorara domain of the kalgoorlie terrane eastern goldfields superterrane yilgarn craton western australia 1 large volume dominantly cumulate textured komatiite bodies interpreted as high level intrusions based on the presence of symmetrical spinifex cumulate textural zonation together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite 2 extrusive komatiite lavas exhibit asymmetrical textural zonation with the top chilled flow margin preserving a fine grained fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine now serpentine orthocumulate texture above a thin basal chilled margin the intrusive komatiite unit is observed to be intercalated with quartz and feldspar phyric dacite the two lithologies exhibit primary irregular contacts against each other komatiite apophyses have been injected into the dacite and magma mingling has occurred these features are indicative of liquid liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism the dacite was emplaced first and the komatiite intruded the dacite before it solidified the reconstructed stratigraphy of the boorara domain comprises a basal unit of quartzofeldspathic turbidite beds conformably overlain by high mg tholeiite basalt the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions these greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence the tectonic scale processes operating in and around the boorara domain depocentre include derivation of the komatiite unit from a likely mantle plume source the contemporaneous dacite magmatism is interpreted to be the result of melting of pre existing archaean crust during plume ascension although the exact composition of the primitive crust is yet to be determined

the 16 september 1986 eruption of lascar volcano north chile satellite investigations landsat thematic mapper tm images acquired in 1984 and 1985 revealed a pronounced thermal anomaly on lascar volcano north chile subsequent images showed that the anomaly was persistent but variable and that after a significant eruption on 16 september 1986 it was weaker and divided into several sources tm studies and other observations of lascar indicate that the persistent thermal anomaly may be due to high temperature fumaroles within the summit crater goes weather satellite images and field investigations confirm that the 16 september event was a short lived vulcanian type eruption which produced an ash column that reached 15 km altitude the ash cloud can be tracked on geos images to about 400 km downwind and covered an area 112 000 km2 transport occurred in the upper troposphere at speeds up to 180 km h ash fall from the plume was well sorted and moderately fine grained md 200 m formation and fallout of ash as aggregates was unimportant in the deposition of the ash layer although small the lascar 16 september eruption is significant because few historic eruptions have been recorded in the central andes little would have been known of the eruption in the absence of remote sensed data

geodynamics of the eastern yilgarn craton abstract over the last decade there have been significant advances in our understanding of the stratigraphy magmatism deformation metamorphism and timing of mineralisation in the eastern yilgarn craton eyc of western australia the integration of these disciplines has enabled a holistic review of the tectonic history of the eyc which favours a paraautochthonous tectonic model a significant advance has been the recognition of a 2810 ma rifting event off the eastern margin of the youanmi terrane which set up the north northwest trending architecture of the eyc rifting was followed by a 100 myr long phase of tectonic activity initiated by the establishment of a convergent margin at 2715 ma characterised by a west dipping subduction zone to the east of the eyc subduction was associated with the deposition of 2715 2670 ma volcanic stratigraphy and the emplacement of voluminous tonalite trondhjemite granodiorite magmatism which resulted in magmatic thickening of the crust volcanism was terminated by a 5 myr pulse of east northeast contraction followed by mid orogenic extension possibly linked to lithospheric and lower crustal delamination the lack of ultra high pressure metamorphism and the presence of high geothermal gradients preclude this contractional event from recording continent continent collision mid orogenic extension at 2665 ma resulted in the introduction of metasomatised mantle melts mafic granites and syenites deposition of siliciclastic basins which record metamorphic anticlockwise ptt paths and the start of significant economic gold mineralisation in the eyc high stretching factors and or delamination associated with this extension event resulted in significant heat input into the base of the crust which eventually led to the emplacement of low ca crustal melt granites and cratonisation of the eyc major gold mineralisation postdates mid orogenic extension and was associated with renewed compression transpression 2650 ma and the development of steep predominantly north northwest striking sinistral and later north to north northeast striking dextral strike slip faults during low ca granite emplacement

discrimination of iron alteration minerals in visible and near infrared reflectance data field and laboratory studies were conducted to relate the mineralogy of rock and soil samples containing hematite fe2o3 goethite feooh and jarosite kfe3 so4 2 oh 6 to their reflectance properties in visible and near infrared wavelengths field reflectance measurements of regolith containing one or more of these minerals were made in the goldfield nevada mining district with a four channel radiometer the mineralogy and reflectance properties of regolith samples collected from these field measurement sites were then determined by laboratory studies mossbauer spectroscopy was used to identify the iron minerals present and their relative proportions the reflectance spectra of samples containing hematite and goethite were characterized by a reflectance minimum near 900 nm the location of this minimum could be accounted for by the relative proportions of hematite and goethite present in the samples under certain conditions it may be feasible to estimate the relative proportions of hematite and goethite on the earth s surface by determining the location of this near infrared reflectance minimum in high spectral resolution remote sensing data in addition the field measurements suggest that the ratio of band passes at 852 and 982 nm may be used to distinguish qualitatively among hematite goethite and jarosite thus maps of the distribution of these minerals may be derived from scanner images acquired in those band passes

a paleozoic suture in eastern gondwanaland a new tectonic interpretation of southeastern australia tasmania and north victoria land antarctica is given in which a major n s trending lineament representing a suture zone is recognized separating essentially shallow water shelf and continental deposits of latest cambrian or early ordovician age from deeper water ordovician flysch deposits lateral offset of this lineament which is interpreted as a suture along which thrust telescoping and major strike slip faulting occurred could adequately explain the present distribution of major rock types in the three areas and also removes a number of problems previously encountered in correlation

granite cored domes and gold mineralisation architectural and geodynamic controls around the archaean scotia kanowna dome kalgoorlie terrane western australia abstract granite cored domes are associated with many of the larger gold deposits of the archaean eastern yilgarn craton of western australia the scotia kanowna dome is eroded to sufficiently deep levels to provide insights into the role granite cored domes play in controlling fluid flow and gold deposition at the centre of the scotia kanowna dome is a granite batholith which is surrounded by outward dipping greenstone belts and associated shear zones this upper crustal dome sits above mid crustal domes providing a series of stacked geometries favourable to focussed fluid flow a number of small to medium sized gold deposits occur on the limbs and the centre of the dome and the world class kanowna belle gold mine occurs on the nose of the dome at least three separate gold mineralising events are defined each of regional significance which can be correlated with other well known gold deposits of the eastern yilgarn craton the palaeostress across this region was dominated by both maximum contractional and extensional vectors oriented perpendicular to the north northwest trending grain the resultant strain is seen in the architecture of the scotia kanowna dome with its margins and associated shear zones controlling the distribution of stratigraphy which is relatively linear and parallel to the margin throughout most of the tectonic history the flanking shear zones were high strain domains that recorded multiple stages of contraction with thrust and mostly sinistral strike slip shear kinematics in contrast the dome nose region was a relatively low strain domain as it was located in the strain shadow of the regional contractional events for example the early extensional record has been preserved at the nose with late stage basins formed in the hangingwall to the extensional faults adding to the stratigraphic complexity of the local region these late basins also may have contributed to the fluid budget in contrast along the flanks of the domes there is no record of these late basins or the extensional faults that controlled their deposition the architecture of the dome played an important role in controlling mineralisation during a significant deviation in the maximum contractional stress vector to an east southeast west southwest orientation at this time basins in the nose region underwent inversion with thrusting to the north and northwest shortening was accommodated along the dome flanks by sinistral strike slip shearing which was relatively free to move due to the mostly linear stratigraphy combined with limited anisotropy and perturbations along the flank however the stratigraphic and geometrical complexity around the dome nose resulted in localised maximum dilation and gold deposition at this time the dome nose area was also a favoured locus for gold enriched dominantly mafic type magmas which are seen as the extensive porphyry swarms in the region these felsic intermediate rocks also may have contributed to the complex fluid budget and are considered as indicators of particular fertility for gold they may have been the primary source for much of the metal endowment the architecture of granite cored domes has played a critical role in focussing magma as well as mineralising fluids into regions of maximum dilation they did this by virtue of their inherent competence and by influencing the position and geometry of shear zones fluid pathways and hence distribution of stratigraphy depositional sites across the region the scotia kanowna dome is a good example of this effect and inferences can be made in favour of similar controls by concealed domes such as beneath kalgoorlie

the age of the kambalda greenstones resolved by ion microprobe implications for archaean dating methods abstract a chert horizon within the archaean basalt pile at kambalda western australia contains zircons from air fall tuffs erupted as the sediment was laid down ion microprobe analyses of these magmatic zircons constrain the long sought age of the kambalda lavas to 2692 4ma 2 and show that regional metamorphism and deformation of the greenstone pile occurred within 30 ma of deposition rare zircon xenocrysts are evidence of pre greenstone crust as old as 3450 ma exposed remnants of which are yet to be identified subsets of both the magmatic and xenocryst zircons show evidence of hitherto unsuspected low temperature u and or pb mobility in the proterozoic long after stabilisation of the craton the ion microprobe data unravel complexities of isotopic inheritance and alteration illustrating the hazards of using bulk or whole rock samples in archaean isotopic investigations identification of interflow cherts as a precisely dateable rock type opens the way to direct ion probe dating of mafic ultramafic piles and close numerical age control of the complex but ill exposed stratigraphic and structural relations in this richly mineralised greenstone belt

pyrite compositions from vhms and orogenic au deposits in the yilgarn craton western australia implications for gold and copper exploration abstract the archaean yilgarn craton western australia is a world class metallogenic province hosting considerable resources of au ag ni cu zn and fe here we present trace element compositions of pyrite from 30 orogenic au and 5 volcanic hosted massive sulphide vhms deposits across the yilgarn pyrites from vhms deposits tend to have higher sn se cu pb bi and lower ni relative to orogenic deposits vhms deposit pyrites commonly have co ni as 100au te au se te orogenic gold deposits could be subdivided based on association of au with as or te pyrites from au as ores generally have pb bi 5 se te 5 pb sb 100 and the majority of au is refractory in pyrite structure at the same time au te association pyrites are characterised by lower values of pb bi se te and tl te higher values of ag au pb sb and au generally resides in inclusions of different compositions our data can be used at the exploration stage to distinguish between vhms vs orogenic au signatures for all studied deposits inclusion populations are summarised with implications for au and ag deportment orogenic au deposits from the yilgarn mostly have multistage formation histories reflected in the presence of multiple generations of pyrites however only some deposits record multiple high au mineralisation events

sm nd isotopic evidence for the provenance of sediments from the adelaide fold belt and southeastern australia with implications for episodic crustal addition abstract in south australia late proterozoic and cambrian sediments were deposited in basins formed within archaean to early mid proterozoic basement these sequences were subsequently deformed and uplifted by the cambro ordovician delamerian orogeny to form the adelaide fold belt in using the adelaide fold belt to address models for lithospheric evolution it is necessary to understand the mechanisms of basin formation and whether the sequences merely reflect reworking of existing cratonic material or if there were new crustal additions this study presents geochemical and isotopic data on the sediments and basement rocks to complement existing data on the delamerian igneous rocks as a means of investigating these questions the data reveal that the archaean mid proterozoic basement rocks of the gawler craton have nd depleted mantle model ages which average about 2 6 ga and nd values at 800 ma in the range 9 to 24 or at 500 ma from 11 to 28 the late proterozoic adelaidean sediments however have an average model age of 1 9 ga and initial nd values in the range 5 to 11 the sequence can be divided into lowermost rift phase sediments derived from erosion of local gawler craton material and a subsequent widely distributed thermal sag phase sedimentation whose isotopic signature is not compatible with the basement this implies that a more primitive or juvenile source was accessed via broadening of the provenance region the cambrian sediments have slightly older model ages 2 1 ga and lower nd 9 to 13 and can be interpreted as mixtures of basement and reworked adelaidean detritus the early palaeozoic sedimentary sequences of the lachlan fold belt to the east have isotopic compositions very like the south australian late proterozoic and cambrian sequences they have an average model age of 1 85 ma but much higher k 2 o na 2 o this indicates that the widespread flysch of eastern australia is the result of recycling of the late proterozoic and cambrian sediments by erosion of the adelaide fold belt with some modification by the mixture of the cambro ordovician mafic and felsic magmatic rocks in this orogen our data suggest regional intracontinental crustal evolution combined with recycling of older crustal material with new additions from the mantle during orogenesis and to a lesser extent episodes of extension and sedimentation

volcano monitoring using short wavelength infrared data from satellites data from remote sensing satellites operating in the short wavelength infrared can be used to measure temperatures of about 150 c and above the gain settings of the landsat thematic mapper tm sensors are such that to record pixel integrated temperatures from 150 c to over 1000 c requires the use of several spectral bands if the radiant source occupies less than a whole pixel as is the case for fumaroles or a crusted lava surface with incandescent cracks then the pixel integrated temperature is less than the actual surface temperature of the hot areas and appears to be different at each wavelength by determining pixel integrated temperature in two spectral bands we can estimate both the temperature and size of hot areas which occupy less than a complete pixel in some cases our observations provide the only available data on the state of activity of a particular volcano in others they support and refine field observations our landsat tm data reveal otherwise unobserved precursor and subsequent activity related to the september 1986 eruption of lascar volcano chile and supplement other data on the march 1986 eruption of augustine volcano alaska they also indicate the continued presence of lava lakes at erta ale ethiopia in the absence of any ground based observations and document minor eruptive activity at mount erebus antarctica landsat multispectral scanner data have been used to measure the variation in temperature along a lava flow on sierra negra galapagos islands which occurred in 1979

structural event framework for the eastern yilgarn craton western australia and its implications for orogenic gold abstract the nnw trending tectonic grain of the eastern yilgarn craton eyc was established as a result of predominantly ene wsw directed extension d1 and d3 and e ene w wsw d2 d4 to ne sw directed d5 contraction the result has been a succession of nnw striking temporally discrete fabric elements which can be difficult to interpret reliably at any single location despite this many past workers interpreted the nnw striking fabric as the result of only one regional contractional event and used it as a marker for correlating structural events across the region in order to unravel the complexity this paper presents a new sixfold d1 d6 deformation nomenclature based on 10 000 new mesoscale structural observations including their kinematic analysis and cross cutting relationships these mesoscale data were referenced with regional 3d map patterns stratigraphic magmatic metallogenic considerations and deep seismic reflection images this integrated geodynamic architectural approach is applicable to solving structural event histories in other polydeformed terrains gold mineralisation occurred during the first five events but was particularly concentrated from d3 onwards the d3 event marked the most profound change in the tectonic evolution of the eyc with changes in greenstones granites and tectonic mode lithospheric extension and core complexes with the first significant gold deposited within extensional shear zones that dissect the crust later contraction d4 was imposed at a high angle to the previously established anisotropic architecture the outcome was the creation of a new dynamic permeability framework which resulted in gold mineralisation during nnw striking sinistral strike slip faulting and associated thrusting a further stress switch d5 further modified the architecture resulting in n to nne striking dextral strike slip faulting and the final period of gold mineralisation before late stage extension d6

oceanic plateaus the fragmentation of continents and mountain building many anomalous rises in today s oceans may be submerged continental fragments detached from previous continents ancient island arcs or basaltic piles formed by hot spots and spreading centers these rises are embedded in their respective moving oceanic plates and are fated to be consumed at active margins where such rises are being consumed at present e g the nazca ridge they cause cessation of volcanism disruption of the downgoing slab and possible shifts in plate boundary configuration many past rises including numerous continental fragments have been recognized within mountain belts as allochthonous terranes they constitute a large portion of the orogenic belts in the north pacific from mexico through western north america alaska east siberia japan and in new zealand the orogenic deformation in these belts is possibly the result of the accretion of the allochthonous terranes many terranes have been accreted with substantial deformation also in the alpine chain well before major continent continent collisions it is suggested therefore that the accretion of fragments may be the common process of the deformation phase of mountain building subduction of normal oceanic crust may be insufficient for deformation whereas full continent continent collision may not be necessary the general validity of this conclusion depends critically on whether allochthonous terranes caused orogenic deformation in the andes or not most of the accreted fragments with continental affinities in the mesozoic cenozoic orogenic belts of the world can be traced back to the breakup of gondwana beginning with a pacifica domain in the permian through a larger india domain in the early mesozoic and continuing through the separation of the somalia plate in the near future the reasons for this 250 million year breakup process are not known but some kind of thermal process possible of mantle wide scale is implied

seismic structure of the southeast australian lithosphere from surface and body wave tomography abstract teleseismic arrival time residuals from the wombat transportable seismic array experiment are inverted to construct a high resolution 3 d p wave velocity model of the upper mantle beneath southeast australia in order to address one of the principal limitations of teleseismic tomography that long wavelength structure is filtered out when data from multiple arrays operating at different times are used an initial model with lower spatial resolution derived from surface tomography is constructed to preserve the broad scale features that would otherwise be lost although the absolute velocities of the final model are not strongly constrained due to the assumption of radial vp vs ratios and differences in regularisation the relative variations appear robust across all scales these reveal a wealth of features that can be related to the geology and tectonic history of the region the most significant being 1 an easterly dipping velocity transition zone which involves a higher velocity delamerian orogen extending beneath the western subprovince of a lower velocity lachlan orogen 2 a distinct region of low velocity in the upper mantle north of melbourne which can be associated with recent quaternary hot spot volcanism 3 a gradual east southeast decrease in velocity towards the coast which is consistent with lithospheric stretching and thinning near a passive margin and 4 a zone of high velocity north of adelaide that may correspond to the presence of the palaeoproterozoic curnamona province at depth these results have important implications for the palaeozoic evolution of the east margin of gondwana the subsequent break up of australia and antarctica and opening of the tasman sea

formation and reactivation of the cambrian kanmantoo trough se australia implications for early palaeozoic tectonics at eastern gondwana s plate margin the kanmantoo group is a thick and largely metamorphosed sedimentary succession that filled an isolated arcuate cambrian basin kanmantoo trough which formed within continental gondwana and now lies on the southern margin of the present australian continent kanmantoo group sediments unconformably overlie neoproterozoic and older cambrian rocks we consider that the geometry of the southern part of this trough was influenced by strike slip movement along an intra continental tear fault to the north the basin changes to a style dominated by orthogonal extension and eventually tapers and dies out normal to the tear fault balanced sections show that the kinematic style and strain distribution developed during early palaeozoic inversion was controlled by the specific architecture of the kanmantoo trough early palaeozoic tear faulting could have linked contrasting subduction polarities along the then contiguous palaeo pacific margin of gondwana the kanmantoo trough is considered to have formed at a passive margin related to east directed subduction in what is now the australian continent in contrast west directed subduction formed an active margin at contiguous parts of current antarctica kanmantoo group sediments were derived from the south by erosion of a grenvillean province mixed with sediments eroded from the emergent active margin of gondwana the inception localization and sedimentation in the kanmantoo trough reflects a complex interaction of tectonic processes along the encroaching ross delamerian orogen

tectonic setting of cambrian rifting volcanism and ophiolite formation in western tasmania abstract in western tasmania small precambrian regions are surrounded by a ramifying system of troughs filled with cambrian sedimentary and volcanic rocks and ophiolite complexes the volcanic associations include a rift related olivine tholeiite association dacite rhyolite and andesite associations and a low ti high mg andesite tholeiite ophiolite association and may have formed during a long lived period of crustal thinning punctuated by episodes of crustal rupturing magmatism and small scale rifting such extensional tectonism could occur in an active continental margin associated with strike slip faulting of regional scale and the volcanic associations may together constitute an igneous assemblage characteristic of magmatism in a transcurrent tectonic regime within an active continental margin undergoing break up the western tasmanian cambrian palaeogeography and volcanism preserve a transitional stage between the late proterozoic kanmantoo regime of sedimentary basins with little volcanism developed at the rifting margin of the proterozoic craton and the tectonic regime of the palaeozoic lachlan fold belt where the cambrian volcanic rocks are dominated by island arc associations and the rift related olivine tholeiite association is absent eastern australian lithosphere may have grown by the insertion of newly formed igneous complexes within the stretched and rifted continental margin as well as by the accretion of terrenes and the addition of packets of subduction complexes which developed off shore

the structure of the upper mantle beneath the delamerian and lachlan orogens from simultaneous inversion of multiple teleseismic datasets abstract distant earthquake data recorded by seven sub arrays of the ongoing wombat rolling seismic array deployment in southeast australia are combined for the first time to constrain 3 d variations in upper mantle p wavespeed via teleseismic tomography the seven arrays comprise a total of 276 short period recorders spaced at intervals of approximately 50 km thus allowing unprecedented resolution of the upper mantle over a large region in the mantle lithosphere immediately below the crust 50 km depth dominant variations in velocity tend to strike east west and share little resemblance to palaeozoic boundaries in the shallow crust inferred from surface geology and potential field data a broad region of elevated wavespeed beneath northern victoria may represent the signature of underplated igneous rocks associated with detachment faulting during the break up of australia and antarctica a distinct low velocity anomaly in southern victoria appears to correlate well with the quaternary newer volcanic provinces towards the base of the mantle lithosphere the dominant structural trend becomes north south and five distinct velocity zones become apparent of particular note is a transition from higher wavespeed in the west to lower wavespeed in the east beneath the stawell zone implying that the proterozoic lithosphere of the delamerian orogen protrudes eastward beneath the western subprovince of the lachlan orogen this transition zone extends northwards from southern victoria into central new south wales the northward limit of the arrays and is one of the dominant features of the model further east there is a transition from lower to higher wavespeeds in the vicinity of the boundary between the western and central subprovinces of the lachlan orogen which has several plausible explanations including the existence of a proterozoic continental fragment beneath the wagga omeo zone the presence of elevated wavespeeds beneath the melbourne zone in victoria although not well constrained due to limited data coverage provides some support to the selwyn block model which proposes a northward extension beneath bass strait of the proterozoic core of tasmania

using the landsat thematic mapper to detect and monitor active volcanoes an example from lascar volcano northern chile the landsat thematic mapper tm offers a means of detecting and monitoring thermal features of active volcanoes using the tm we have discovered a prominent thermal anomaly on lascar volcano northern chile data from two short wavelength infrared channels of the tm show that material within a 300 m diameter pit crater was at a temperature of at least 380 c on two dates in 1985 the thermal anomaly closely resembles in size and radiant temperature the anomaly over the active lava lake at erta ale in ethiopia an eruption took place at lascar on september 16 1986 tm data acquired on october 27 1986 revealed significant changes within the crater area we find that lascar is in a much more active state than any other volcano in the central andes and for this reason it merits further careful monitoring our studies show that the tm is capable of confidently identifying thermal anomalies less than 100 m in size at temperatures of above 150 c and thus it offers a valuable means of monitoring the conditions of active or potentially active volcanoes particularly those in remote regions

the role of early formed structures on the development of the world class st ives goldfield yilgarn wa a revised structural interpretation for the victory to kambalda area of the world class st ives goldfield in the archean yilgarn craton has mapped out the distribution of wnw trending faults within this area of the field these previously cryptic wnw trending structures had been identified in gravity data and also by isopach thickness variations the wnw trending faults acted as transfers syn gold mineralization although only discrete segments of these faults were active during the main stage of gold mineralization where mineralized the faults transferred strain from a complex combination of block on block movement associated with thrusting and strike slip movement on nw and n trending faults along some segments n trending mineralized faults terminate against the wnw trending faults many of the wnw trending faults correlate with major strike changes on regional and camp scale faults and they are domain boundaries for the critical n trending fault segments that host high grade gold within contractional jogs the wnw trending faults also show evidence for an older deformation history prior to main stage gold which may extend back to early basin development associated with ultramafic and mafic volcanism they are inferred to have been a series of early wnw trending normal faults and breached relay ramps associated with oblique rifting along an older nnw trending basement boundary

influence of basin architecture on the style of inversion and fold thrust belt tectonics the southern adelaide fold thrust belt south australia abstract the southern adelaide fold thrust belt involves neoproterozoic adelaidean strata deposited during protracted rift and sag phases and an overlying cambrian kanmantoo sequence represented by a thick wedge of turbidites rapidly deposited in an eastwards deepening basin deeply incised into the underlying rocks the belt was deformed during the delamerian orogeny developed at the southeastern margin of the australian craton during early palaeozoic nw directed displacement during this shortening event the margin of the kanmantoo basin was reactivated and formed a significant strain guide that controlled key features of the fold thrust belt strain integrated and restored sections reveal contrasting geometric and kinematic styles along the strike of the belt a transpressional zone is developed at the steep basin margin in the south oriented oblique to the structural grain the central parts of the belt are dominated by strong buttressing growth fault reactivation and basement involved footwall shortcut thrusting in the northern part of the belt shortening is accommodated by homogeneous folding above a master de collement these variations in regional structural style and internal strain partitioning reflect significant variations in the original kanmantoo basin margin geometry rheological parameters control the partitioning of strain associated with pervasive fabrics for example cleavage and transposed bedding folding is accompanied by negative line length changes shortening in the northern part of the belt in its thrusted central part pelites commonly show positive line length changes psammites were least affected by line length changes and thus provide key beds for section balancing pervasive strain makes a full restoration of cross sections impossible however incorporating regional strain data assists establishment of balanced and restorable sections which provide a powerful tool in understanding both pre deformational basin geometry and its shortening

tectonic affinity of the mathinna group in the lachlan fold belt abstract pre carboniferous rocks of tasmania comprise two disparate terranes which were brought together during the middle palaeozoic the eastern tasmania terrane consists of middle palaeozoic granitoids and the mathinna group a turbidite succession of ordovician to early devonian age the older part of the adjacent western tasmania terrane the only part of the southern tasmanides with exposed precambrian basement is related to the kanmantoo fold belt and older tectonic provinces located northwest of tasmania the younger part of the western tasmania terrane comprises upper cambrian to lower devonian terrestrial to shallow marine siliciclastics and carbonates a marked contrast to the coeval deep water deposits of the mathinna group the contact between the two terranes is concealed by the tamar valley in northern tasmania the main folds in the eastern tasmania terrane verge to the northeast whereas structures in the adjacent western tasmania terrane indicate tectonic transport to the southwest with the break in structural facing coinciding with the tamar valley reinvestigation of the structure of the westernmost eastern tasmania terrane where there is a large recumbent f1 fold 10 km extent in profile view raises the possibility that a second generation of structures previously regarded as of only local importance could have caused rotation of the upright to steeply west dipping axial surfaces of f1 folds to recumbent attitudes during southwesterly directed thrusting in this interpretation recumbent or near recumbent zones could reflect ramps in a sole thrust and the mid devonian imbricate thrust zone near beaconsfield on the west bank of the tamar river would be either the frontal thrust system or a splay off the sole thrust which continues westward beneath the exposed precambrian badger head block the implications of this new interpretation are that docking of the eastern and western tasmania terranes must have occurred after the ordovician but before the middle devonian during the consolidation phase there were two deformations of the mathinna group the first which occurred in the early devonian prior to emplacement of the st marys porphyrite 388 1 ma produced upright to locally overturned folds verging to the northeast the second which took place during the early middle devonian before deposition of givetian cave deposits resulted in the mathinna group being thrust southwestward this sense of thrusting is opposite to the direction of thrusting at the same time in the melbourne zone of victoria where the fold and thrust belt verges eastward away from the craton a concealed devonian transfer fault zone in bass strait could account for this change of polarity

timing of orogenic gold mineralisation in northeastern tasmania implications for the tectonic and metallogenetic evolution of palaeozoic se australia new 40ar 39ar data from sedimentary rock hosted orogenic gold deposits in northeastern tasmania constrain most ore formation to between 395 ma and 385 ma these 385 395 ma ages for the formation of orogenic gold agree well with an inferred early to middle devonian timing for peak deformation and folding across much of northeastern tasmania data from micas within alteration halos in some deposits give dates of 420 430 ma these dates confirm the occurrence of an earlier silurian phase of deformation and suggest that at least some of the mineralisation was possibly generated during this event gold mineralisation hosted by middle devonian post tectonic granites may be genetically related to magmatism following orogeny but these deposits formed virtually synchronously with peak deformation related systems early to middle devonian deformation in northeastern tasmania also reactivated older structures in western tasmania and the formation of quartz vein hosted gold mineralisation there based on geological structural tectonic and metallogenetic similarities northeastern tasmania is interpreted as a lateral equivalent of the turbidite dominated fold thrust belt of the western lachlan orogen however unlike victoria where the sedimentary rock sequence developed on oceanic crust northeastern tasmania was probably underlain by thinned proterozoic crust either as part of a promontory along the gondwana margin or as a microcontinental fragment this may have protected the palaeozoic succession from large scale pre devonian orogeny with collision not beginning until the middle devonian these variations in the structural and tectonic evolution and the timing of deformation and ore formation can explain the difference in contained gold and the distribution and number of major orogenic gold deposits within the palaeozoic of northeastern tasmania

ordovician contourites in the lachlan fold belt eastern australia abstract the ordovician sequence in the northeastern lachlan fold belt consists of three lithologic units representing an overall fining upward succession from turbidites to black shale the lowest unit dominated by turbidite beds represents a small segment of an extensive ordovician submarine fan sequence that covered much of southeastern australia the overlying unit consists predominantly of ripple bedded sandstone with beds 3 5 cm thick these beds are texturally mature containing heavy minerals mica and silt concentrated on the cross laminae the ripple bedded unit has a mean palaeocurrent trend to 321 which is at 60 to sole marks and almost normal to the trend of ripple cross bedding in the underlying and associated turbidite beds this unit is here interpreted as a sandy contourite sequence that can be attributed to contour currents moving northwards along the eastern margin of gondwana from areas of subpolar downwelling off southern africa and america the two major fining upward successions in the ordovician sequence in southeastern australia may reflect eustatic sea level rises during the middle and late ordovician and thus provide evidence of interglacial periods in the darrawilian and late gisbornian to bolindian the contourites would have accumulated in the latter stages of the glacial periods after sea level had risen sufficiently to cut off major sediment supply to the submarine fan but before the temperature had risen to the extent of preventing deep thermohaline circulation

alteration patterns linked to high grade gold mineralization at the wattle dam deposit western australia abstract sharp physio chemical gradients occur in archean gold systems yet they are often poorly defined these physio chemical gradients can provide windows into fluids that deposit high grade gold mineralization recognizing different alteration styles and fluid types in mineral systems helps define processes that led to the deposition of high grade gold here we report on the architecture and chemical gradients across the komatiite hosted wattle dam gold deposit in the eastern yilgarn craton western australia which is one of australia s highest grade gold deposits the deposit notably contains millimeter to centimeter thick gold only veins with an absence of quartz whole rock geochemistry shows that high grade gold veins are hosted in komatiite flow margins facies with olivine orthocumulate komatiites in the footwall interbedded graphitic shale close to the lode and komatiite basalt in the hangingwall the komatiite flow margin facies that hosts high grade gold veins have been pervasively altered by stage 1 biotite actinolite stage 2 gold veins cross cut the biotite actinolite altered komatiite and these veins contain thin

on kalgoorlie australia timmins porcupine canada and factors in intense gold mineralisation abstract the golden mile gold deposits of kalgoorlie eastern goldfields province yilgarn craton australia 46 moz au and the gold deposits in the timmins porcupine camp southern abitibi province superior craton canada 63 7 moz au are earth s two biggest archaean lode gold camps while similarities at the camp scale are dominant they differ in that the golden mile is a compact camp of one major deposit with two nearby subordinate deposits separated by less than 10 km along strike whereas timmins porcupine consists of a relatively large camp with 30 important mines distributed over 20 km strike length seismic data show a detachment fault at the base of the supracrustal rocks at kalgoorlie at between 5 and 9 km but none is identified in the timmins porcupine camp map scale patterns ages and lithologies are superficially similar the gabbros in kalgoorlie and conglomerates in timmins porcupine are distinctive rock types but served similar competence contrast functions late orogenic sediments are an important host at timmins porcupine for competence contrast reasons but are apparently absent in the golden mile area gold deposit formation in both camps is of the same age 2 67 ga with a long duration of mineralisation porphyries albitites and lamprophyres are characteristic features but played no major genetic role both camps have early elements of mineralisation that have an epithermal character in cu au w v 3 and f o 2 and both camps lie in belts locally characterised by the abundance of telluride minerals however none of these characteristics seem to be necessary or sufficient for intense gold mineralisation syn mineralisation transpressive deformation in both camps involved the imbrication of thrust slices followed by strike slip movement along a major crust scale fault with a dilatational curve in its trend in the centre of both camps the various deposits in each of the two camps were formed from immediately before to just after strike slip deformation and opening of late orogenic sedimentary basins both areas are characterised by an early episode of intense thrust faulting extensive intersecting fracture arrays likely developed as thrusting gave way to strike slip faulting these arrays may have enabled access to fertile source rock and also served as fluid conduits throughout deformation moreover both terranes are characterised by a relatively short pre mineralisation crustal history thin ned subcontinental lithospheric mantle and input of juvenile mantle derived material the pre mineralisation crustal history and evolution of the host terranes thus may have been the most important single factor in the formation of giant orogenic gold deposits at kalgoorlie and timmins porcupine

character distribution and tectonic significance of accretionary terranes in the central alaska range the central part of the alaska range near mount mckinley is composed of nine separate tectonostratigraphic terranes that were accreted in southern alaska during late mesozoic time these terranes now form long linear fault bounded belts that are subparallel to the denali fault on the north but oblique to the fault on the south the postaccretion right lateral offset along the denali fault system is about 200 km from north to south the major terranes are 1 yukon tanana terrane metasedimentary and metavolcanic rocks mostly undated but including rocks of known late paleozoic age polymetamorphosed with terminal events in late mesozoic 2 pingston terrane isoclinally folded upper triassic deep water silty limestone quartzite and carbonaceous slate folded with upper paleozoic phyllite chert tuff and minor limestone 3 mckinley terrane upper paleozoic flysch chert and minor limestone intruded by large gabbro sills and dikes and overlain by thick piles of triassic porphyritic pillow lava the top of the section is thick sequence of upper mesozoic conglomerate flysch chert and phyllite 4 dillinger terrane very thick sequence of strongly folded lower paleozoic micaceous sandstone turbidites graptolitic shale and deep water limestone locally overlain unconformably by jurassic fossiliferous sandstone or triassic pillow basalt 5 windy terrane heterogeneous assemblage of serpentinite basalt tuff and chert ophiolite with paleozoic and mesozoic flysch and blocks of mid paleozoic fossiliferous limestone 6 mystic terrane predominantly upper paleozoic flysch and conglomerate but also includes lower paleozoic graptolitic shale pillow basalt and shallow water limestone and upper paleozoic fossiliferous limestone sandstone chert and undated pillow basalt 7 chulitna terrane upper devonian ophiolite overlain by upper paleozoic chert volcanic conglomerate limestone and flysch capped by lower triassic limestone and upper triassic red beds basalt and interbedded basalt and limestone later mesozoic rocks are sandstone chert and argillite 8 westfork terrane jurassic chert sandstone conglomerate and triassic and jurassic crystal tuff 9 broad pass terrane upper paleozoic chert tuff and argillite with blocks of devonian and older limestone locally associated with serpentinite these diverse terranes of mixed oceanic and continental affinities are now juxtaposed to form a complex sequence of folded and faulted rootless nappes major suture zones between them are occupied by intensely deformed upper mesozoic flysch all of the terranes differ markedly from wrangellia the southermost coherent accreted terrane in alaska paleomagnetic measurements from triassic rocks prove that wrangellia originated near the equator probably in the southern heimsphere paleolatitudes for terranes in the central alaska range are not yet determined but other lines of evidence biogeographic and lithologic suggest large scale northern transport for some terranes

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia part iii signatures of tectonic escape in an arc continent collision zone abstract siliciclastic supersequences in the eastern goldfields superterrane are remnants of basins that were linear zones of subsidence between intra terrane faults fluvial deposits record proximal to medial braid plains deep marine canyon and fan deposits were linked across intra terrane compartments only by mud rich deposits axial depositional systems dominate with only minor evidence for slope aprons although transverse and axial systems are preserved in some fluvial deposits basin development occurred during two stages separated by an inversion event with the kalgoorlie terrane recording change from a fluvial to a deep marine basin and the kurnalpi terrane change from distal to proximal deep marine facies tracts sparse palaeocurrents establish that uplift to the north controlled sediment dispersal lithofacies stacking is aggradational whereas abrupt upwards change in facies tracts is back stepping within depositional sequences and between depositional sequence sets but forward stepping between depositional sequences the stratigraphic architecture is atypical of synorogenic basins but consistent with a strike slip regime characterised by pulsed uplift subsidence steady state basin lengthening and subsidence punctuating catastrophic subsidence and lengthening and tectonic anchoring the basins are interpreted to have been similar to graben and half graben of the western anatolian extensional province of turkey with linear zones of subsidence developing adjacent to strike slip oblique slip or normal dip slip faults controls on stratigraphic patterns are considered to have been regional rather than at the scale of each basin the envisaged tectonic setting is similar to projected collision between the philippine archipelago west of the philippine fault and continental crust of the south china sea but the orogenic architecture is similar to that of accreted superterranes in the canadian cordillera interpreted north or northwest convergence between the burtville terrane to the east and the ancestral yilgarn continent to the northwest produced a south propagating orogen with the kalgoorlie gindalbie and kurnalpi terranes trapped between in a tectonic escape corridor tectonic escape to the south between bounding strike slip faults produced back stepping of facies tracts that simulated basin lengthening whereas forward stepping of facies tracts was the response to south propagating uplift

rb sr dating of differentiated cleavage from the upper adelaidean metasediments at hallett cove southern adelaide fold belt abstract upper adelaidean psammites from the foreland part of the late proterozoic adelaide fold belt at hallett cove have suffered one major deformation which produced upright folds that plunge gently south axial to these folds is a well developed differentiation cleavage inferred to have resulted from constant volume plane strain deformation by solution transfer and now defined by alternating p phyllosilicate rich and q quartz rich domains geochemical analyses show that cleavage development resulted in a 2 4 fold fractionation of rb sr between the p and q domains thus the age of deformation is datable and isotopic analyses show the p and q domains form a 531 32 ma isochron this age is statistically older than the pervasive deformation and metamorphism in the more interior parts of the fold belt inferred from intrusive ages to have occurred at 485 6 ma and thus suggests that deformation either propagated across the fold belt from external to interior zones or that it occurred episodically the age data allow but do not demand that cleavage formation predated the deposition of the overlying cambrian kanmantoo group sediments

application of the landsat thematic mapper to the identification of potentially active volcanoes in the central andes abstract landsat thematic mapper tm images have been used to make the first comprehensive study of the central andean volcanic province 14 s 28 s one of the world s largest zones of active volcanism more than 60 major volcanoes have been identified as potentially active whereas only 16 had been previously cataloged most of these are large composite cones up to 6000 m in height some of which threaten nearby settlements apart from its consistent coverage the chief merit of the landsat tm in making this study has been spatial resolution provided by the 30 m pixel size this has been particularly important for the recognition of subtle glacial morphological features such as valley and terminal moraines which can be used to determine relative ages of the volcanoes on the tm images we have classified a volcano as potentially active if it has i a well preserved summit crater ii pristine lava flow texture and morphology iii flank lava flows with low albedo and iv evidence of post glacial

the geology of the kambalda nickel field western australia the kambalda nickel field contains the major concentration of volcanic peridotitc associated nickel ores in the archean of australia it occupies an area of 400 km 2 underlain by two sequences of ultramafic mafic and felsic volcanics and sedimentary rocks which can be broadly correlated with previously erected regional stratigraphic sections most ores occur in the lower kambalda sequence at the base of the kambalda ultramafic rocks the ultramafic rocks of the upper bluebush sequence contain only minor mineralization four phases of deformation occurred over an interval of more than 300 m y resulting in a dominant north northwest structural trend faulting was active during original volcanism and continued intermittently until after mafic felsic dike intrusion the ultramafic rocks have been serpentinized and all rocks have exerienced low strain upper greenschist to lower amphibolite facies metamorphism and variable carbonation and potash metasomatism the kambalda ultramafic rocks are of variable thickness and in places exceed 1 000 m the lower member of the ultramafic rocks occupying approximately one third of the sequence comprises thicker high magnesium flows i e generally 36 volatile free mgo commonly separated by sulfidic quartz albite sediments thinner lower magnesium flow units characterize much of the upper member the ores occur in 24 shoot complexes each containing multiple ore surfaces caused by structural dislocation of a few grouped ribbonlike original orebodies eighty percent is contact ore occurring at the base of the lowermost ultramafic flow and generally occupying elongate troughs in the footwall basalt ultramafic contact these troughs are thought to be original volcanic depressions which focused later deformation the remainder is hanging wall ore usually occurring directly above contact ore at the base of the second or third ultramafic flow hanging wall orebodies occasionally grade into stratigraphically equivalent interflow sediments the ores contain varying proportions of massive matrix and disseminated sulfides the hypogene mineral assemblage is pyrrhotite pentlandite pyrite chalcopyrite but millerite is locally significant the massive ores in particular are commonly severely deformed and some have been remobilized tens of meters from their original contact position metamorphism has caused recrystallization of the ores and some redistribution of elements there is commonly marked variation of mineralogy and s ni ratios between contact and overlying hanging wall ores and within and between original ore surfaces within shoots but the relative contributions of primary composition and metamorphism to this variation are uncertain in ore environments the footwall basalt ultramafic contact is more deformed than in non ore environments the basal ultramafic flows are thicker and richer in magnesium the trend toward lower magnesium flows upward through the ultramafic formation is less well developed and there is better textural development and compositional differentiation within flows non ore environments usually contain interflow sediments whereas there is generally a sediment free prism of ultramafic rocks above ore environments most ores are interpreted to have originated by either simultaneous or sequential extrusion of sulfide melts and ultramafic lavas rich in olivine phenocrysts along active linear fissures in a basaltic ocean floor volcanic exhalations probably contributed to minor ore in sulfidic sediments

field examples of strike slip fault terminations in mongolia and their tectonic significance deformation at the ends of large intracontinental strike slip faults that do not simply link other major structures often involves rotations about a vertical axis we use earthquake slip vectors surface rupture in earthquakes and geomorphology to examine the ends of three major strike slip faults in mongolia in these places a simple pattern is seen consisting of a thrust fault on one side with a displacement that decreases away from the strike slip fault consistent with local rotational deformation strike slip faults that terminate in this way allow the style of faulting to change spatially within a deforming area for example from dominantly strike slip to dominantly dip slip while still accommodating the overall deformation required by larger scale regional motions such a change in fault style should also be accompanied by a change in the rotation rate about a vertical axis which may be detected paleomagnetically the kind of strike slip fault termination described here may have consequences for how large strike slip fxaults evolve and grow and for the variation in displacement along their length