the teutonic bore deposit western australia a lead isotope study of an ore and its gossan the teutonic bore deposit occurs in an archaean greenstone belt within the eastern goldfields province of the yilgarn block in western australia the ore is hosted by basaltic rocks and consists of a conformable massive sulfide lens underlain by a thick zone of pyritic stringer ore the zone of oxidation reaches a depth of 90 100 m with the development of secondary copper sulfides the lead isotopic compositions of six samples of massive sulfide three mineral separates from the ore and eight gossan samples collected from the open cut were determined by standard mass spectrometric techniques four of the massive sulfide samples all three mineral separates and seven of the eight gossan samples have lead isotopic compositions identical to each other within experimental error these results confirm the findings of earlier studies that the lead isotopic signature of a massive sulfide ore is transferred to its gossan and provide additional data suggesting the usefulness of lead isotopic determinations in ore prospect evaluation the teutonic bore leads plot below the average global lead evolution curves for the uranogenic isotopes 206pb and 207pb suggesting that the lead in the ore contains a significant mantle component this feature of the isotopic data is consistent with the idea of a mantle plume origin of the eastern goldfields greenstone belts

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

a test of recent inferences of net polar ice mass balance based on long wavelength gravity abstracta comprehensive analysis of satellite datasets has estimated that the ice sheets of greenland west antarctica the antarctic peninsula and east antarctica experienced a net mass loss of 100 92 gt yr 1 over the period 1992 2000 and 298 58 gt yr 1 over the period 2000 11 representing an increase of 198 109 gt yr 1 between the two epochs the authors demonstrate that the time rate of change of the degree four zonal harmonic of earth s gravitational potential provides an independent check on these mass balances that is less sensitive to uncertainties that have contaminated previous analyses of the degree 2 zonal harmonic e g due to ongoing glacial isostatic adjustment gia solid earth body tides and core mantle coupling for the period 2000 11 the signal implied by the ice sheet mass flux cited above is 3 8 0 6 10 11 yr 1 whereas the change in the harmonic across the two epochs is 2 3 1 1 10 11 yr 1 in comparison using satellite laser ranging slr data the authors

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

incompatible element enrichment in archean basalts a consequence of contamination by older sialic crust rather than mantle heterogeneity high abundances of incompatible elements in mgo rich archean basalts from western australia which were thought to have been derived from an enriched mantle source are probably the result of contamination of primitive magmas by small amounts of older sialic crust because these lavas represent some of the more extreme cases of enrichment in incompatible elements observed in archean basalts it may be possible to explain much of the trace element variation previously ascribed to a heterogenous archean mantle by variable crustal contamination of mantle derived magmas recent calculations of thermal erosion rates predict that high temperature high mgo magmas abundant in the archean are most susceptible to contamination therefore a cautious reappraisal of isotopic and trace element data from archean greenstone belts is required before the real extent of archean mantle heterogeneity can be established

emplacement and metamorphism of archaean mafic volcanics at kambalda western australia geochemical and isotopic constraints abstract extensive exploration and mining at kambalda western australia has revealed a conformable mafic ultramafic succession consisting of a sulphide ore bearing ultramafic layer located between metabasalt units geochemical and rb sr u pb isotopic analyses have been carried out on two 70 140 m sections of drill core from the metabasalts and u pb isotopic analyses have been made on sulphide ore samples greenschist metamorphism of the metabasalts is dated at 2610 30 m y and was approximately isochemical except for addition of h 2 o and co 2 and a differential mobilization of k and rb within the units the addition of h 2 o and co 2 is believed to have taken place shortly after extrusion of the metabasalts in a submarine environment resulting in an early greenschist metamorphism the later 2600 m y event is correlated with a high thermal regime also detected at kalgoorlie 50 km to the north a pb pb isochron age of 2720 105 m y may represent the metamorphic event or the time of emplacement of the metabasalts new sm nd data c hauvel et al c laque l ong et al suggest the mafic volcanics are 3200 m y old if true large scale homogenization of pb isotopes is required alternatively the mafic sequence may be only slightly older than 2800 m y and the sm nd data does not have time significance

the importance of breaching as a mechanism of subaqueous slope failure in fine sand large bank failures comprising up to 10 6 m 3 of sediment are common features along steep channel banks in estuaries and large rivers that consist of clean fine sands and are mostly assumed to be generated by sudden liquefaction of large masses of very loosely packed sand another less commonly recognized type of failure is manifested by the gradual retrogression of a very steep wall steeper than the angle of repose instead of the voluminous surging plastic sediment water flow or hyperconcentrated density flow sensu mulder alexander 2001 generated by liquefaction this type of failure known as breaching by dredging companies and hydraulic engineers produces a sustained quasi steady turbidity current to date sedimentologists have not recognized the process of breaching as such in this paper it is suggested that breaching may be the origin of many thick massive sand layers known from ancient deposits from various environments notably in some turbidite successions possible differences in the sedimentary structure of the deposits produced by breach failures vs liquefaction slope failures liquefaction flow slides can be deduced from a knowledge of the sediment transport processes initiated by the failure a field study is presented on some poorly structured beds in the eocene vlierzele sands in belgium which are supposed to have originated from liquefaction failures but are reinterpreted to be the products of breaching it is postulated that the local steep slope disturbance required to initiate an active breach can be produced by a small liquefaction slope failure liquefaction flow slide failure or local erosion by river or tidal channel flow at the initial stage of the failure event

can lamprophyres resolve the genetic controversy over mesothermal gold deposits associations between calc alkaline lamprophyres and mesothermal gold deposits archean to tertiary in which the lamprophyres are coeval as well as cospatial with mineralization are increasingly recognized worldwide our suggested hypothesis regards lamprophyres as transporting agents for au from au rich sources in the deep mantle which then undergo extensive crustal interactions generating felsic magmas or releasing their au into metamorphic hydrothermal systems such a model not only may reconcile the conflict between the extant magmatic and metamorphic models for mesothermal deposits but also may resolve the inconclusive relations between gold deposits and felsic porphyry granitoid intrusions because lamprophyres may act as parents to both the gold lamprophyre association implies that exceptionally deep seated magmatism repeatedly accompanied gold mineralization in postcollisional orogenic island arc oblique subduction or graben environments this has important implications for the late stage evolution of mineralized archean greenstone belts in particular and for gold deposit genesis in general

crustal scale shear zones and their significance to archaean gold mineralization in western australia many large archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts although in detail they are sited in subsidiary brittle ductile fault structures fluids originating from a deep source follow a complex path and re equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels temperature and pressure conditions at or below the amphibolite greenschist metamorphic boundary where most gold deposits are located favour the establishment of brittle ductile and brittle subsidiary structures the preferred structural setting of gold deposits physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient strongly localized fluid flow into the latter where lower temperatures and suitable host rocks with high fe fe mg ratios favour gold deposition the multi source origin and continuous re equilibration of the fluid with crustal rock which includes granitoid and greenstone belt lithologies of different ages is reflected in the diverse isotopic and geochemical signature of the gold deposits

a two stage model for the formation of the granite greenstone terrains of the kalgoorlie norseman area western australia abstract accurate dating of granites and greenstones from the kalgoorlie norseman area shows that there is considerable overlap in time between the formation of granites and greenstones however within any given area the oldest granites are always younger than the oldest greenstones it is suggested that the long lived 100 ma thermal anomaly in the uppermost asthenosphere that gave rise to the greenstones is also responsible for regional metamorphism and widespread crustal melting leading to the production of granites the time lag between greenstone and granite formation reflects the time taken to conduct heat from the mantle to the lower crust anatexis of the lower crust produces a layer of light granitic magma that may temporarily stop the ascent of basaltic magma where this happens the basaltic magma ponds at the base of the crust supplying additional heat and accelerating the melting process eventually the granitic melt layer becomes unstable and rises within the crust areas intruded by granite become topographic highs with topographic lows forming above areas where light melt has been withdrawn from the lower crust when the topographic highs protrude above sea level they are eroded to shed sediments into the topographic lows which contain the basaltic greenstones thus the change from a basalt dominated lower greenstone succession to a sediment and felsic volcanic dominated upper whitestone sequence seen in many granite greenstone terrains results from the intrusion of granitic magma into the upper crust the crust of the kalgoorlie norseman area formed in two distinct stages separated by a time break of up to 700 ma the first stage involved the formation of primitive crust probably above subduction zones this primitive crust was characterized by low rb sr and high sm nd ratios the second stage resulted from a major change in mantle convection that produced thermal upwellings in the mantle below the continents this led to the eruption of basaltic magmas and to metamorphism and anatexis of the lower crust to produce granitic magmas melting reset the rb sr sm nd and u pb systems giving the impression that the second event which occurred at 2 7 ga was a major crust forming event it was in fact a crustal reworking process although some basalt was added to the crust at this time although the two stage history of the kalgoorlie norseman area is clearly seen in the lead isotopes and in zircons it is not readily discernible from the rb sr and sm nd systematics due to mantle like ratios of those elements in the primitive crust

relationships between calc alkaline acidic and basic mantle derived magmas in late archaean composite dykes kambalda goldfield western australia abstract composite dykes from the north dome area kambalda comprise basic spessartite hornblende plagioclase lamprophyre 56 sio 2 with mg numbers cr and ni contents typical of primitive mantle derived rocks and more acidic plagioclase phyric quartz andesite 66 sio 2 together with felsic net veins and hornblendic patches suggesting local differentiation the spessartites can be regarded chemically as volatile lile and incompatible element enriched versions of siliceous high magnesian basalts from the local greenstone sequence a pb pb isochron on one dyke yields an age of 2636 27 ma making this the oldest composite dyke so far dated field and geochemical evidence including identical pb initial ratios for the constituent rock types virtually indistinguishable ree spectra and tight linear trends on plots such as y versus ti and ce cr versus ti cr suggests that the two magmas lamprophyric and acidic were genetically related and were emplaced contemporaneously into the same fracture quantitative least squares modelling of whole rock and mineral compositions suggests that the acidic components resulted mainly from fractionation of hornblende from parental spessartite magmas together with a possible contribution from mixing of the same spessartite magma with crustally derived acidic e g trondjhemitic magma the latter is represented by separate dykes and stocks in the kambalda area close similarities between the kambalda composite dykes and archaean tertiary composite dykes in other parts of the world imply a persistent role for mantle derived magmas such as lamprophyres in the genesis of the dominant intermediate acidic e g granitoid magmas in many calc alkaline provinces

the sm nd age of kambalda volcanics is 500 ma too old although sm nd isotopic analyses of volcanics from kambalda western australia form a 3 2 ga linear array pb pb analyses of the same suite give an isochron age of 2 73 0 03 ga and a 2 7 ga model age for the associated sulphides we suggest that the 3 2 ga age is incorrect and that the sm nd array results from mixing between depleted mantle and either older continental crust or enriched mantle

lead isotope constraints on the age and source of gold mineralization in the archean yilgarn block western australia model mineralization ages based on whole earth lead isotope growth curves are used to date sulfides mainly galena with minor fe sulfides and pb tellurides related on one hand to stratiform base metal and ni cu mineralization and on the other to predominantly lode type gold mineralization in greenstone belts of the archean yilgarn block western australia galenas related to undoubted syngenetic volcanogenic base metal mineralization and fe sulfides to komatiite associated ni cu deposits yield pb model ages that are broadly consistent with sm nd pb pb and u pb zircon greenstone belt geochronology such deposits from the murchison province give model ages of ca 3 0 b y or greater whereas those of the eastern goldfields province give model ages of ca 2 85 to 2 80 b y for the northeastern part of the province and ca 2 75 to 2 70 b y for the highly mineralized norseman wiluna belt the majority of galenas related to gold mineralization record model ages of ca 2 85 to 2 75 b y for the murchison province and ca 2 75 to 2 70 b y for the southern cross province with most deposits from the norseman wiluna belt having model ages around or below ca 2 7 b y a few examples of gold mineralization all but one economically insignificant have galenas with more complex multistage lead isotope histories with mineralization ages lying in the proterozoic lead model ages of galenas from gold deposits in the murchison province are consistently younger then those related to syngenetic mineralization those from gold deposits in the southern cross province broadly coincide with the best estimates of the age of peak metamorphism and those from the norseman wiluna belt are indistinguishable or slightly younger than the ages of syngenetic base metal and ni cu mineralization in the same belt the lead isotope compositions of most gold related galenas yield 238 u 204 pb mu values that are consistent with a previously proposed epigenetic model for economically important gold mineralization in this model lead and by inference gold was derived from basalts and komatiites by major hydrothermal systems which developed due to devolatilization of the greenstone pile during regional metamorphism from between 250 m y to less than 50 m y after eruption the small deposits that give younger mineralization ages may be related to or affected by minor thermal events which occurred later in the history of the yilgarn block and which were also responsible for resetting rock and mineral rb sr systems the contrast in isotopic histories between galenas and possibly fe sulfides from major and minor gold deposits may have application as an exploration tool significantly higher 238 u 204 pb values for sulfides from the norseman gold deposits provide support for the suggestion that crust older than 3 0 b y existed prior to formation of the greenstone belts of the yilgarn block

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

criteria for the recognition of metamorphosed or altered lamprophyres a case study from the archaean of kambalda western australia abstract the ability to distinguish ancient lamprophyres from other mafic and ultramafic intrusive rock types has become increasingly important lamprophyres have been used as marker horizons in the mapping of several precambrian terranes they have been recognized for decades as possible indicators of gold mineralization and their presence has implications for the tectonic interpretation of ancient greenstone belts whilst fresh undeformed lamprophyres are readily recognized metamorphosed altered and or deformed examples are more problematical the intrusive ages of kambalda lamprophyres are constrained by available field and geochronological evidence including direct dating of one kersantite to the period 2684 2660 ma these rocks therefore belong to the world s oldest group of alkaline rocks and lamprophyres so far recognized which also includes contemporaneous examples from canada zimbabwe and probably tanzania despite their extreme age and low to medium grade regional metamorphic history these rocks still preserve characteristic lamprophyric textures chemistry and mineralogy only where metamorphism has been overprinted by gold related alteration and deformation do the lamprophyres cease to be recognizable petrographically however their unique chemistry persists and despite losses of the most mobile elements from presumed magmatic compositions altered and deformed specimens still retain far higher contents of sr k ba th nb p zr and ce than basalts of the local greenstone succession even lamprophyres which have been converted to biotite plagioclase carbonate pyrite schists retain this distinctive chemical signature during both alteration and metamorphism mobile elements k rb sr ba display far larger changes than archetypal immobile elements ce nb zr p ti clearly sanctioning traditional preference for the latter in assessing altered rocks however the surprising loss of nb during biotitization of hornblende indicates that negative nb anomalies conventionally regarded as indicators of subduction related magmatism can also be induced purely by alteration processes

age and origin of granitic rocks in the kalgoorlie norseman region of western australia implications for the origin of archaean crust three granites from the norseman area at stennet rocks buldania rocks and east of lake johnston have u pb zircon ages of 2691 8 95 c l 2689 22 and 2661 8 ma respectively one stennet rocks contains 3100 3200 ma cores providing support for oversby s 1975 argument that all three plutons were derived by anatexis of much older basement common leads within the zircons show evidence of having been derived from within the rock and represent mixtures of initial rock leads and radiogenic leads the unusually 208pb rich compositions of the common leads suggest the breakdown of a th rich phase perhaps uranothorite as a major contributor to the radiogenic component available zircon geochemical and isotopic data are used to develop a two stage model for archaean crustal development in this region with formation of primitive crust of broadly andesitic composition from depleted mantle or basalt beginning at 3400 ma followed by extensive thermal reworking and possibly continued growth over the period 3000 3300 ma in the second stage at 2700 ma anatexis resulted in large scale intra crustal differentiation giving a thick upper granitic layer and a lower crust residual from anatexis it is this second major thermal event that is recorded by most geochronometers implicit in the andesitic crustal growth model is that because the sm nd and rb sr ratios of newly formed archaean crust are close to mantle values this process is transparent to the sm nd and rb sr isotope systems until recycling of incompatible element rich sediment becomes important following the late archaean production of a wide spread differentiated upper crust

the genesis and tectonic control on archaean gold deposits of the western australian shield a metamorphic replacement model abstract gold deposits occur in greenstone belts world wide and contribute to anomalously high gold production from archaean terranes as in other cratons archaean gold mineralization of western australia represents a complex array of deposit styles despite this most deposits are clearly epigenetic and large deposits have a number of features in common including their strong structural controls distinctive wallrock alteration fe sulphide k mica albite ca mg fe carbonates consistent metal associations au ag as sb w b low base metals commonly fe rich host rocks great depth extension and lack of appreciable vertical zonation these shared characteristics combined with their ubiquitous occurrence indicate that archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic porphyry intrusions although these gold systems involved low salinity lowdensity reduced near neutral h 2 o co 2 fluids carrying gold as reduced sulphur complexes the origin of the fluids is equivocal most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic granitoid or felsic porphyry orggins but the lack of consistent spatial relationships between specific volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids the metamorphic replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile with high geothermal gradients inhibiting significant melting co 2 possibly formed by the decarbonation of early alteration related to mantle degassing along crustal scale synbasinal fault zones auriferous fluids were channelled along greenstone scale faults in part developed during reactivation of crustal scale faults in a strike slip regime gold deposition occurred largely under greenschist facies conditions about 300 400 c 1 2 kb in response to decreasing gold solubility with declining temperature however a major control on gold deposition was fluid wallrock interaction many large deposits formed by sulphidation of fe rich host rocks with synchronous deposition of fe sulphides and gold however the variable nature of gold depositing reactions including lowering of fo 2 and ph allowed a multitude of small and some large deposits to form wherever that fluid circulation occurred in consequence several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks there are few constraints on the source of components au s k co 2 added to gold deposits but even giant deposits such as the golden mile kalgoorlie could have formed from a realistic greenstone source volume ca 8 8 5 km convective circulation of fluids could have contributed to the generation of high fluid rock ratios on the regional scale the markedly heterogeneous distribution of large gold deposits gold productivity and host rocks to deposits can be accommodated by the metamorphic replacement model the most favourable conditions for development of auriferous hydrothermal systems operated in younger ca 2 7 0 1 ga rift phase greenstones where greatest extension and crustal thinning produced high geothermal gradients crustal scale synbasinal faults and rapid extrusion and burial of volcanics including abundant komatiites iron rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits the least favourable conditions existed in older ca 3 5 3 4 ga platformphase greenstones where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites commonly in very shallow water environments this allowed intense synvolcanic alteration of both gold source rocks and potential host rocks the generally smaller gold deposits formed mainly in ultramafic or greywacke hosts younger ca 3 0 ga platform phase greenstones appear intermediate in nature but unlike other greenstones have significant epigenetic gold deposits in originally oxide facies bif which were deposited on relatively deep water platforms similar controls appear to exist on a world scale with gold mineralization peaking at ca 2 7 0 1 ga in response to development of major rift zones in thickened relatively mature continental crust interestingly the giant witwatersrand goldfield formed at about the same time

source of pb in orogenic lode gold mineralisation pb isotope constraints from deep crustal rocks from the southwestern archaean yilgarn craton australia southern cross was one of the earliest gold mining centres in western australia over 142 tonnes of gold have been produced from the district and on a gold per hectare basis the southern cross greenstone belt in the southwestern yilgarn craton is the most productive of western australia s archaean greenstone belts the sw yilgarn craton is characterised by high grade amphibolite to granulite facies metamorphism extensive granitoid magmatism and older greenstone volcanism ages compared to the well known greenschist facies metamorphism and younger 2 7 ga eruption ages which dominate in the eastern goldfields province the pb isotope compositions of deep seated granitoids in the sw archaean yilgarn craton which were emplaced coeval with a craton wide major orogenic lode gold mineralization event at about 2 64 2 63 ga have been determined for 96 whole rock and 24 k feldspar samples the pb isotope data of the granitoids are consistent with a crustal origin for their genesis probably by reworking partial melting of older continental crust the pb isotope composition of greenstones which are the main host rocks for gold mineralisation and pyrites from the komatiite hosted syngenetic ni deposits in the amphibolite facies forrestania greenstone belt have also been determined with initial pb isotope ratios higher than that for the eastern goldfields province the pb isotopic character of the orogenic lode gold deposits in the region is intermediate between coeval granitoid and greenstone pb indicating that the ore fluids contained metals from both reservoirs the pb in the ore fluid of the most deeply formed deposit griffin s find overlaps the isotopic composition of coeval granitoids indicating the deep seated granitoid magmatism was the primary source for pb in the ore fluids

the porphyry gold association in the norseman wiluna belt of western australia implications for models of archaean gold metallogeny abstract porphyritic minor intrusions felsic intermediate porphyries and lamprophyres occur in the vicinity of many gold deposits in the archaean norseman wiluna belt of western australia but are not restricted to this environment unlike the abitibi belt of canada there is no correlation between the presence of such intrusions and the economic status of a deposit the timing of intrusive activity with respect to spatially associated mineralization is variable with mainly instances of pre sometimes 10 ma earlier and some examples of post ore emplacement being recorded in many cases the porphyry gold association reflects the importance of structural control to the localization of mineralization intrusions may exploit the same zones of crustal weakness as the ore fluids although such a relationship need not imply contemporaneity or may act as structural traps for mineralization owing to rheological contrast between intrusion and country rock the composition and petrogenesis of the minor intrusions are both variable and there is no evidence of a specific suite of intrusions preferentially associated with mineralization spatial temporal and petrogenetic considerations preclude direct evolution of the gold bearing fluids from the minor intrusions and militate against magmatic theories of ore genesis rather both the igneous activity and the mineralization are viewed as expressions of a deep seated tectono thermal event that affected the yilgarn block in the late archaean

archean gold relation to granulite formation and redox zoning in the crust the archean was the principal period for gold mineralization these rocks compose 12 of the exposed crust but have produced more than half of the world9s gold most major lode deposits formed at 2 7 0 2 ga this corresponds to the principal time of crustal thickening and stabilization an essential part of which was formation of granulite the anhydrous refractory base for the crust upward streaming of mantle co 2 along shear zones in the lower crust is one means for dehydrating amphibolite to form granulite and is accompanied by removal of large ion lithophile lil elements to higher crustal levels archean lode gold deposits were formed along major shear zones at intermediate crustal depths they are enclosed by zones of co 2 metasomatism many of them kilometres in extent 13 c data indicate that the co 2 is juvenile some major archean deposits have been shown to have formed from relatively oxidized hydrothermal fluids co 2 streaming with lil element depletion could only have occurred under relatively oxidized conditions that permitted a free co 2 vapor to exist in the lower crust such conditions would also have favored dissolution of gold and associated sulfide and their transfer to the mid crust in a co 2 h 2 o fluid the h 2 o coming from dehydration of amphibolite

the relationship between archaean gold mineralization and spatially associated minor intrusions at the kambalda and norseman gold camps western australia lead isotope evidence many archaean mesothermal gold deposits are spatially associated with felsic to lamprophyric minor intrusions and it has been suggested that magmatic processes related to such intrusions may be important in the genesis of these deposits a comparison of the pb isotopic signature of gold related galenas from kambalda and norseman with that of spatially associated minor intrusions at the time of mineralization indicates that the ore fluid pb cannot have been derived solely from the intrusions or their source regions for both study areas the galena pb isotopic compositions are bracketed by those of local volcanic mafic and intrusive largely felsic rock types this is consistent with the ore fluid having derived metallic components from the crust or crustally derived granitic rocks and the mantle or mantle derived rocks of the greenstone succession via metamorphic dewatering or mantle crustal degassing interaction of granite derived magmatic fluids with greenstone lithologies could plausibly produce a similar array of pb isotopic signatures the norseman data as a whole are more radiogenic than the kambalda data for broadly synchronous mineralization reflecting the greater abundance of older granitic rocks with respect to mafic ultramafic rocks in the norseman district the provinciality exhibited by the pb isotopic composition of the ore fluid indicates that the gold mineralizing process formed galena whose pb isotopic composition was very sensitive to local variations in crustal pb isotopic composition either within the source region of the fluid or along fluid conduits

ground melting and ocellar komatiites a lead isotopic study at kambalda western australia stratigraphically and geographically restricted ocellar komatiite flows at kambalda western australia appear to represent the products of ground melting of sulphidic sediments by komatiites in lava channels that localized the fe ni cu sulphide ores an immiscible sulphide liquid formed and gravitationally separated from the melted sediment xenomelt the resultant buoyant silicate liquid being partly or wholly assimilated by the turbulently convecting komatiite magma rarely the xenomelt gravitationally migrated to the top of flows and overflowed into the less turbulent lava levees where it collected to form a separate layer overlying a komatiitic layer within a single flow there was selective preservation of the hybrid felsic layer as an upper ocellar unit within an ocellar komatiite flow in lava levees flanking lava channels the ocellar unit is enriched in elements previously concentrated in the sediments and shows u th pb isotopic systematics akin to the underlying sediments moreover the partitioning relationships of u and pb between the immiscible xenomelt and sulphide liquid enhances the range of u pb ratios for components of the ocellar unit thus allowing sufficient spread of modern uranogenic pb isotopic ratios to form isochrons albeit imprecise ones the range and similarity of model th u data from these flows 2 8 3 9 and adjacent sulphidic sediments 2 3 4 4 mostly 2 8 3 9 contrasts with the generally invariable th u within kambalda ultrabasic basic flows 3 6 3 9 and further supports the ground melting hypothesis

archean gold mineralization and oxidized hydrothermal fluids sulfate minerals hematite and isotopically fractionated sulfides are relatively rare in rocks of archean age several archean au deposits however contain one or more of these features indicative of oxidized hydrothermal fluids they include hemlo mcintyre hollinger macassa lake shore and ross which are all in ontario canada and kalgoorlie in australia examples of lesser deposits are in hislop township and in the matachewan area ontario canada with the exception of hemlo these are quartz carbonate vein deposits spatially and temporally associated with felsic plutonism because metamorphic fluids and archean surface waters were mostly reducing the most feasible general source for the fluids was felsic magmas parent magmas were relatively oxidized hydrous and co 2 rich the oxidized character may have favored the incorporation of au into the partial melt and its partitioning into magmatic aqueous fluids high contents of co 2 are conducive to the separation of an aqueous fluid from magma at greater depth than otherwise fluid inclusion data support the formation of major archean au vein deposits at deeper crustal levels than more recent magmatic hydrothermal mineralization such as porphyry cu deposits co 2 rich fluids can unmix into two phases at relatively high temperatures and pressures a saline aqueous liquid and a low salinity h 2 o co 2 liquid this can cause partitioning of cl complexed base metals from other metals including au sb and as the extensive carbonatization around most deposits has consistent delta 13 c values in the range of juvenile magmatic co 2

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