mineralogical and stable isotope studies of gold arsenic mineralisation in the sams creek peralkaline porphyritic granite south island new zealand at sams creek a gold bearing peralkaline granite porphyry dyke which has a 7 km strike length and is up to 60 m in thickness intrudes camptonite lamprophyre dykes and lower greenschist facies metapelites and quartzites of the late ordovician wangapeka formation the lamprophyre dykes occur as thin 3 m slivers along the contacts of the granite dyke 18omagma values 5 to 8 vsmow of the a type granite suggest derivation from a primitive source with an insignificant mature crustal contribution hydrothermal gold sulphide mineralisation is confined to the granite and adjacent lamprophyre metapelite country rocks have only weak hydrothermal alteration three stages of hydrothermal alteration have been identified in the granite stage i alteration high fo2 consisting of magnetite siderite biotite stage ii consisting of thin quartz pyrite veinlets and stage iii low fo2 consisting of sulphides quartz and siderite veins and pervasive silicification the lamprophyre is altered to an ankerite chlorite sericite assemblage stage iii sulphide veins are composed of arsenopyrite pyrite galena sphalerite gold chalcopyrite pyrrhotite rutile graphite three phases of deformation have affected the area and the mineralised veins and the granite and lamprophyre dykes have been deformed by two phases of folding the youngest of which is early cretaceous locally preserved early formed fluid inclusions are either carbonic showing two or three phases at room temperature liquid co2 ch4 liquid h2o co2 vapour or two phase liquid rich aqueous inclusions some of which contain clathrates salinities of the aqueous inclusions are in the range of 1 4 to 7 6 wt nacl equiv final homogenisation temperatures th of the carbonic inclusions indicate minimum trapping temperatures of 320 to 355 c which are not too different from vein formation temperatures of 340 380 c estimated from quartz albite stable isotope thermometry 18o values of stage ii and iii vein quartz range from 12 and 17 and have a bimodal distribution 14 5 and 16 with stage ii vein quartz accounting for the lower values siderite in stage iii veins have 18o 12 to 16 and 13c values 5 relative to vpdb unlike those from wangapeka formation metasediments 13cbulk carbon values of 24 to 19 and underlying arthur marble marine carbonates 18o 25 and 13c 0 calculated 18owater 8 to 11 at 340 c and delta 13 text c rm co 2 5 values from vein quartz and siderite are consistent with a magmatic hydrothermal source but a metamorphic hydrothermal origin cannot be excluded 34s values of sulphides range from 5 to 10 relative to cdt and also have a bimodal distribution modes at 6 and 9 correlated with stage ii and stage iii mineralisation respectively the 34s values of pyrite from the arthur marble marine carbonates range from 3 to 13 and wangapeka formation range from 4 to 9 5 indicate that they are potential sources of sulphur for sulphides in the sams creek veins another possible source of the sulphur is the lithospheric mantle which has positive values up to 14 ages of the granite lamprophyre alteration mineralisation and deformation in the region are not well constrained which makes it difficult to identify sources of mineralisation with respect to timing our mineralogical and stable isotope data does not exclude a metamorphic source but we consider that the source of the mineralisation can best be explained by a magmatic hydrothermal source assuming that the hydrothermal fluids were sourced from crystallisation of the sams creek granite or an underlying magma chamber then the sams creek gold deposit appears to be a hybrid between those described as reduced granite au bi deposits and alkaline intrusive hosted au mo cu deposits

archean lamprophyre dikes of the superior province canada distribution petrology and geochemical characteristics shoshonitic lamprophyre dikes of late archean age are a volumetrically minor yet widespread component of magmatism in the superior province and preferentially occur along major structures that delineate subprovince boundaries lamprophyre swarms are most prominently developed in fault graben where they are associated with alkali gabbro and trachytes and molasse sediments mineralogically the dikes are characterized by zoned phlogopite or hornblende phenocrysts restriction of feldspar to the groundmass globular segregations of k feldspar and calcite panidiomorphic texture and the sporadic presence of crustal xenoliths compositionally fresh dikes have contents of sio2 47 51 wt tio2 0 55 0 60 and p2o5 0 36 1 58 and k2o na2o ratios 1 5 2 3 and mg numbers 78 80 comparable to phanerozoic shoshonitic lamprophyres unaltered dikes are characterized by extreme enrichments of the lithophile elements k rb ba cs th u and light rare earth elements lree and enhanced abundances of cr 650 920 ppm co 37 42 ni 264 355 and sc 22 28 mid ocean ridge basalt normalized plots display 1 consistent enrichments of k rb and ba relative to sr and lree 2 variable enrichments of rb and ba relative to k and 3 troughs at ta nb and ti compared with fresh rocks altered samples have systematically lower k rb ba and sr contents but similar ree distributions and profiles from th yb and sc ni on normalized diagrams suites of post archean ultramafic lamprophyres in the superior province are enriched in large ion lithophile elements lile lree and compatible elements relative to their archean counterparts and lack troughs at ta nb and ti emplacement of the lamprophyres was diachronous from north 2705 ma to south 2674 ma probably being related to localized extension during the successive accretion of individual subprovinces archean lamprophyre magmas were derived from a depleted mantle source variably enriched in lile and lree by sediments and altered oceanic crust with small variable degrees of crustal contamination whereas the post archean lamprophyres lacked a plate margin setting

archean turbidite hosted orogenic gold mineralization in the gadag greenstone belt western dharwar craton peninsular india abstract turbidite hosted orogenic gold mineralization in the archean gadag greenstone belt of the western dharwar craton forms a major auriferous zone central auriferous zone extending over a strike length of about 12 km in the gadag duplex the turbidite sequence comprises thick inter bedded medium to coarse grained lithic graywacke and thin laminated layers of fine grained carbonaceous phyllite gold bearing quartz veins impregnate preferentially along the en echelon shear planes fractures and schistosity planes auriferous quartz veins are enveloped by the altered wall rocks mineralogy of the auriferous zone is dominated by gangue minerals like quartz ankerite chlorite sericite and carbonaceous matter with subordinate plagioclase monazite and xenotime are the important accessory minerals arsenopyrite and pyrite are the major sulfide minerals but pyrrhotite chalcopyrite sphalerite galena and scheelite are also present gold in native state occurs within quartz silicates and arsenopyrite notable distinctions in mineral assemblage texture and in chemical compositions of altered wall rocks compared to the precursor host rock in the study area implies that the metasomatism and wall rock alterations are the results of pervasive infiltration and intense interaction between hydrothermal fluids and the surrounding host rocks over a prolonged period sulfides carbonates carbonaceous matter k2o mgo cao cr ni cu pb zn as and higher values of gold 0 98 4 72 ppm are added into the altered wall rocks immediately enveloping the auriferous quartz vein bodies the chondrite normalized ree pattern of altered wall rocks exhibits enriched lree lan ybn av 9 54 with prominent negative eu anomaly the observed variation in geochemical characteristics and mineral assemblages in the alteration zones indicates differential response of the host rock and intensity of alteration depending on the composition of host rocks and hydrothermal fluids the auriferous hydrothermal fluids were of low salinity 2 0 to 6 6 wt nacl dominated by co2 h2o about 30 mol co2 with moderate densities 0 7 to 1 04 g cm3 and gold deposition occurred over a wide temperature range between 175 c and 325 c gold deposition was influenced by fluid mixing phase separation and redox reactions mixing between co2 h2o fluids and more reduced fluids which evolved during fluid reaction with adjacent carbonaceous wall rocks was the key factor causing gold deposition the formation of the gadag duplex deformation folds and reverse strike slip faults discontinuities was caused by the compression associated with subduction related tectonic processes during the initial period of intrusive magmatism 2 555 6 ma regional metamorphism occurred in the entire greenstone belt while during later period hydrothermal fluids responsible for gold mineralization probably were derived from metamorphic processes as well as from intrusive granites such fluids channeled through the thrust in host turbidite sequence carrying dissolved gold associated metals and sulfur ultimately were precipitated in a reducing environment in the splays to the thrust in the gadag duplex at about 2 522 6 ma resulting in retrograde alteration assemblages

discovery of concealed ore bodies at the dongping gold deposit northern china revealed by the study of ore controlling structures abstract the dongping gold deposit is the largest gold deposit in the hebei province china with proven gold reserves of more than 100 tons after the main gold ore bodies were mined out the dongping gold deposit faced a shortage in gold reserves with an uncertain future as a result we conducted systematic studies on the ore controlling structures at the dongping gold deposit with the aim of finding additional concealed ore bodies should these be present gold mineralization at dongping is controlled by two stages of maximum principal compressional stress 1 nw trending stress that produced the ne to nw trending structures and sub vertical stress that produced the circular structure respectively the sub vertical compressional stress may have resulted from the emplacement of a concealed intrusive body which also led to horizontal extensional stress generating structures surrounding the intrusion development of such structures potentially increased the permeability of the local crust facilitating the mobilisation of gold bearing hydrothermal fluids that subsequently precipitated along the structures forming the gold ore bodies we propose that the deep part of the dongping gold deposit at the lithological contact with the concealed intrusive bodies is prospective for the exploration of additional reserves which is supported by our subsequent drilling and the discovery of concealed gold ore bodies beneath the deposit

structural setting and geochemistry of archaean i type granites in the bardoc coolgardie area of the norseman wiluna belt western australia abstract regional structures in the bardoc coolgardie area of the archaean norseman wiluna greenstone belt in western australia developed during early large scale thrusting followed by a transpressional regime characterized by enewsw shortening and large scale sinistral strike slip faults structural relationships between granites and greenstone successions indicate emplacement of granitic rocks throughout this deformation history emplacement mechanisms include solid or liquid state diapirism forceful dilation of country rocks and passive stoping i type granites are common and include early and late tectonic intrusions the widespread occurrence of i type granodiorite and monzogranite probably derived by partial melting of lower continental crust or subducted oceanic crust suggests a continental margin setting for the belt alkalic quartz diorite occurs as rounded enclaves in granodiorite and is enriched in ba rb sr ce la p and f which suggests a mantle derived volatile component these quartz diorite melts may have formed by partial melting of metasomatized mantle or alternatively mantle degassing may have lowered solidus temperatures in the source region and thus induced melting emplacement of hot volatilerich quartz diorite melts into the lower crust may have lowered solidus temperatures in the source rocks for granodiorite and monzogranite recent models for archaean gold mineralization suggest that auriferous fluids contained co 2 derived from lower crustal or mantle sources in the bardoc coolgardie area co 2 may have been flushed out during partial melting of the lower continental crust however the origin of the gold in the mineralizing fluids requires some other explanation because gold mineralization is generally a late stage event and took place over a considerably shorter time interval than the formation and emplacement of i type granites

geochronology of the porgera gold deposit papua new guinea resolving the effects of excess argon on k ar and age estimates for magmatism and mineralization abstract mesothermal epithermal gold mineralization at porgera in the highlands of papua new guinea png occurs in structurally controlled veins and disseminations which overprint and cross cut a suite of shallow level comagmatic mafic alkaline stocks and dykes the porgera intrusive complex pic and their sedimentary host rocks conventional k ar apparent ages of twelve hornblende separates from eight different intrusions scatter between 7 and 14 ma but four biotite separates are concordant at 6 02 0 29 ma 2 40 ar 39 ar step heating experiments on six of the hornblende separates reveal saddleshaped age spectra which indicate the presence of excess 40ar one of these samples yields a well defined plateau with an apparent age of 5 96 0 25 ma 2 this age is identical to that of the biotites and suggests an age of 6 0 0 3 ma 2 for emplacement of the pic older apparent ages obtained from conventional k ar and 40 ar 39 ar analyses are explained by excess 40ar contamination conventional k ar analyses of six separates of hydrothermal illite and roscoelite associated with gold mineralization yield apparent ages of between 5 1 and 6 1 ma and indicate that ore deposition occurred within 1 ma of magmatism at porgera evidence for the evolution of a magmatic volatile phase and the presence of excess 40ar both in the intrusives and in hydrothermal fluids associated with the orebody suggest that magmatic fluids may have had some involvement in metallogenesis but the exact nature of this involvement is not yet clear late miocene magmatism and mineralization at porgera are thought to have occurred shortly prior to or during the initiation of continent arc collision and to pre date associated pliocene uplift and foreland deformation in the highlands

chemical and boron isotope composition of tourmaline from the kiaka orogenic gold deposit burkina faso west african craton as a proxy for ore forming processes the kiaka orogenic gold deposit burkina faso located in the paleoproterozoic domain of the west african craton is characterized by a two stage gold mineralization hosted in volcano sedimentary metamorphic rocks that was formed during the eoeburnean 2 20 2 13 ga and eburnean 2 13 2 05 ga orogenic cycles these two stages include an early disseminated low grade gold mineralization and a late vein hosted high grade gold mineralization paragenetic studies indicate that the first gold stage was coeval with the deposition of hydrothermal tourmaline the aim of this paper is twofold i to determine the processes responsible for deposition of the economic disseminated gold mineralization and ii to identify the source of the mineralizing fluids for this purpose we performed an in situ study on tourmaline by combining electron probe microanalysis and secondary ion mass spectrometry measurements of boron isotopes hydrothermal tourmaline hosted in metamafic volcanic rocks and metagreywackes has a dravite composition but shows different 11b values falling within the two intervals of 25 1 to 22 0 and 19 6 to 15 1 respectively our results suggest that tourmaline formed from a distal high temperature ca 400 c reduced and low salinity hydrothermal fluid that interacted with the local host rocks based on the modeling of tourmaline fluid boron isotope fractionation we propose a metamorphic fluid origin derived from devolatilization of deeply buried muscovite schists during the regional prograde to peak metamorphism prior ca 2 13 ga this metamorphic fluid rock interaction model may possibly extend to other orogenic gold deposits in the west african craton

paleoproterozoic potassic ultrapotassic magmatism morro do afonso syenite pluton bahia brazil abstract the morro do afonso syenitic pluton masp is a small bimodal suite of lamprophyric and syenitic rocks located in the serrinha nucleus sao francisco craton northeastern brazil the unit consists predominantly of alkali feldspar syenite with numerous mafic xenoliths and dykes that are predominantly lamprophyric the syenite shows geochemical and isotopic features typical of the roman potassic magmas i e low tio2 up to1 2 low k2o al2o3 0 33 0 49 strong enrichment in lile and high lile hfse ratios fractionation mainly of clinopyroxene and amphibole explains the transition from lamprophyre to syenite magma mixing is required to explain the most evolved compositions masp syenites have paleoproterozoic crystallization ages 2 11 ga the nd isotope composition of mafic and felsic phases are similar e n d mafic 2 36 to 2 61 and e n d felsic 2 47 and they have archaean model ages tdm 2 56 2 58 ga that match the age of an inherited zircon grain ca 2 6 ga masp is similar to many other post orogenic syenitic plutons distributed over a wide area of sao francisco craton sfc which are closely comparable in age and also typically associated with ultrapotassic lamprophyre the present results suggest that a major proportion of the potassic ultrapotassic syenitic rocks of the sfc were emplaced in a narrow time interval from 2111 to 2080 ma limiting the time of metamorphism peak of the transamazonian orogeny to at least 2 1 ga these plutons show some evidence of having interactions with older felsic crustal rocks and are interpreted to be derived from partial melting of lithospheric mantle that was enriched by subduction

the nature and dimensions of regional and local gold related hydrothermal alteration in tholeiitic metabasalts in the norseman goldfields the missing link in a crustal continuum of gold deposits the archaean lode gold deposits at norseman western australia consist of auriferous quartz veins in dextral reverse ductile brittle shear zones within tholeiitic metabasalts of upper greenschist to amphibolite facies metamorphic grade three types of deposits northern central southern are delineated on the basis of their spatial distribution veining style alteration mineraloty and metamorphic grade of host rocks northern deposits hosted in upper greenschist to lower amphibolite facies rocks comprise massive to laminated quartz veins with selvedges of quartz chlorite calcite biotite plagioclase assemblages central deposits hosted in lower amphibolite facies rocks consist of laminated to massive quartz veins with selvedges of quartz actinolite biotite plagioclasecalcite assemblages southern deposits hosted in middleamphibolite facies metabasalts consist of banded quartz diopside calcite microcline zoisite veins all deposits exhibit variable ductile deformation of veins and contiguous alteration haloes consistent with a syn deformational genesis at high temperatures from northern to southern deposits the alteration assemblages are indicative of higher temperatures of formation and there are progressively greater degrees of dynamically recovered textures in alteration and gangue minerals these observations imply that a thermal variation of gold related hydrothermal alteration exists within the norseman terrane over a distance of 40 km with tnorthern tcentral tsouthern this thermal zonation is corroborated by t xco 2 phase relations between vein selvedge assemblages which signify formation temperatures of approximately 420 475 c 470 495 c and 500 c for northern central and southern deposits respectively the sum of structural petrographic and mineral chemistry data indicates that the alteration assemblages formed in high temperature open hydrothermal systems and have not been subsequently metamorphosed the thermal differences between the deposit groups may reflect 1 a temperature gradient at relatively constant p corresponding to the proximity of the deposits to regional granitoid complexes or 2 formation of the deposits at progressively deeper crustal levels from north to south in either case the deposits represent a continuum of gold deposition from upper greenschist to amphibolite facies now exposed in an oblique section through the archaean crust at norseman

geological geochronological and h o isotopic constraints on the genesis of the tongjing cu au deposit in the ningwu basin east china abstract the tongjing cu au deposit is a medium sized deposit within the ningwu volcanic basin east china and is hosted by cretaceous volcanic rocks of the dawangshan and niangniangshan formations the veined and lenticular cu au orebodies are spatially and temporally related to the volcanic and subvolcanic rocks of the niangniangshan formation in the ore district the wall rock alteration is dominated by silicification siderite alteration carbonation sericitization chloritization and kaolinization on the basis of field evidence and petrographic observations two stages of mineralization are recognized 1 a siderite quartz sulfide stage stage 1 associated with the formation of chalcopyrite and pyrite in a quartz and siderite gangue and 2 a quartz bornite stage stage 2 cutting the stage 1 phases stage 1 is the main mineralization stage quartz that formed in stage 1 has 18 o h2o values of 4 3 to 3 5 with d values of fluid inclusion waters of 97 1 to 49 9 indicating that the ore forming fluids were derived from early magmatic fluids and may have experienced oxygen isotopic exchange with meteoric water during stage 1 mineralization la mc icp ms zircon u pb dating of the mineralization related nosean bearing phonolite and nosean bearing phonolitic brecciated tuff at tongjing yields ages of 129 8 0 5 ma and 128 9 1 1 ma respectively these results are interpreted as the crystallization age of the volcanic rocks of the niangniangshan formation a hydrothermal sericite sample associated with cu au mineralization at tongjing yields a plateau 40 ar 39 ar age of 131 3 1 3 ma these results confirm a genetic link between the volcanism and associated cu au mineralization the tongjing cu au deposit in the ningwu basin is genetically and possibly tectonically similar to alkaline intrusion related gold deposits elsewhere in the world

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

tectonic and economic implications of trace element 40ar 39ar and sm nd data from mafic dykes associated with orogenic gold mineralisation in central victoria australia abstract mafic to intermediate dykes are spatially and temporally closely associated with major post tectonic granitic complexes in the western lachlan orogen of se australia these dykes which range petrographically from basaltic to andesitic are concentrated within several north to northwest trending zones and were emplaced during two broad intervals of extensive magmatic activity during the silurian devonian period geochemical and sm nd isotopic characteristics of these mafic intrusives are consistent with their formation in a complex subduction related tectonic setting interaction between mantle wedge material sinking oceanic crust and input from the overlying continental crust resulted in the petrological and geochemical variations displayed by these and more felsic dykes throughout the study region field evidence and 40 ar 39 ar data show that in the eastern part of the stawell zone and in the northwest portion of the bendigo zone mafic dyke were intruded between 410 and 400 ma late silurian early devonian further emplacement in the bendigo zone and the eastern part of the melbourne zone took place at between 375 and 365 ma middle to late devonian episodic mantle derived magmatism was possibly related to step wise rollback slab detachment or changes in the angle and rate of westward subduction in response to periodically occurring accretionary pulses a close spatial and temporal relationship also exists between the dykes and orogenic gold mineralisation in the central victorian gold province mafic to intermediate dykes both crosscut and are host to mineralisation in a number of goldfields although there is little evidence for a direct genetic association the two processes are linked by the common utilisation of translithospheric structures which facilitated the rapid ascent into shallow crustal levels of both mantle derived magma and crustal scale ore forming fluid systems previous studies have suggested that transfer of heat into the crust via ascending mafic mantle magmas can provide a thermal engine which triggers and sustains extensive crustal melting thus explaining the commonly observed close association of mafic to intermediate and felsic intrusive suites this study supports the viability of this mechanism and in addition indicates that a causal link exists between the formation of mantle magmas in collisional zones and the generation of orogenic gold deposits

auriferous arsenopyrite pyrite and stibnite mineralization from the siflitz guginock area austria indications for hydrothermal activity during tertiary oblique terrane accretion in the eastern alps

archean polyphase deformation in the lake johnston greenstone belt area implications for the understanding of ore systems of the yilgarn craton abstract a structural and geochronology study on the 2900 ma lake johnston greenstone belt ljgb has defined multiple deformation events quantitative u pb age constraints have been obtained on the ductile deformation and peak metamorphic assemblages the d 1lj event is associated with the development of fold nappes during interpreted nne ssw shortening u pb shrimp dating on monazite from a dyke folded by the d 1lj event gives an age of 2634 1 ma providing an upper age limit on d 1lj this age could have been reset by the d 2lj metamorphic event a major metamorphic thermal event post dates the d 1lj event and is inferred to be the result of elevated temperatures coeval with the emplacement of granitoid intrusions the d 2lj event is associated with peak metamorphic temperatures and reflects nne ssw to ne sw shortening associated with dextral shearing a dyke that intrudes and crosscuts d 2lj fabrics yield a u pb shrimp age on monazite at 2629 1 ma and provides a lower age limit on the d 2lj event d 3lj is defined by crenulation cleavages developed during e w shortening the d 4lj episode is defined by brittle deformation and a series of n to ne trending steep dipping dextral faults formed during craton scale shortening under a ne sw directed far field stress the brittle d 5lj event is characterized by e w shortening with dip slip reverse n s faults the new d 2lj deformation age is younger than previously constrained ages for major deformation in the 2900 ma greenstone belts deformation within the ljgb was broadly coeval with the waning stage of formation of gold deposits in adjacent greenstone belts most peak magmatic and metamorphic events preserved in the greenstone belts have been constrained to ca 2640 2650 ma the presence of such a high temperature deformation event recorded at ca 2630 ma in the ljgb belt has implications for the thermal history fluid formation and geodynamic context of the development of these gold systems the 2630 ma age correlates with late stage and post gold emplacement of pegmatite dykes in surrounding terranes the ljgb is interpreted to have been a mobile belt associated with high grade metamorphism that was inboard west of and active during the formation of major world class gold systems in the eastern goldfields terranes the d 1lj to d 2lj changes in the kinematics of the system i e d 1lj to d 2lj may correlate with a stress switch linked to the influxes of gold bearing fluids in adjacent greenstone belts

les gisements d or des tonalites archeennes abitibi quebec de nombreux gisements ont ete mis en exploitation depuis une dizaine d annees dans des plutons granitiques de la ceinture de roches vertes archeennes de l abitibi la plupart d entre eux sont encaisses par des facies sodiques dans la zone sud est de cette ceinture il existe deux types de concentrations des zones filoniennes et des disseminations a caractere porphyrique

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

petrogenesis of cretaceous adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china implications for geodynamics and cu au mineralization both adakitic and shoshonitic igneous rocks in the luzong area anhui province eastern china are associated with cretaceous cu au mineralization the shaxi quartz diorite porphyrites exhibit adakite like geochemical features such as light rare earth element lree enrichment heavy ree hree depletion high al2o3 mgo sr sr y and la yb values and low y and yb contents they have low end t values 3 46 to 6 28 and high 87sr 86sr i ratios 0 7051 0 7057 sensitive high resolution ion microprobe shrimp zircon analyses indicate a crystallization age of 136 3 ma for the adakitic rocks most volcanic rocks and the majority of monzonites and syenites in the luzong area are k rich or shoshonitic and were also produced during the cretaceous 140 125 ma they are enriched in lree and large ion lithophile elements and depleted in ti and nb and ba and exhibit relatively lower end t values ranging from 4 65 to 7 03 and relatively higher 87sr 86sr i ratios varying between 0 7057 and 0 7062 the shoshonitic and adakitic rocks in the luzong area have similar pb isotopic compositions 206pb 204pb 17 90 18 83 207pb 204pb 15 45 15 62 and 208pb 204pb 38 07 38 80 geological data from the luzong area suggest that the cretaceous igneous rocks are distributed along ne fault zones e g tanlu and yangtze river fault zones in eastern china and were likely formed in an extensional setting within the yangtze block the shaxi adakitic rocks were probably derived by the partial melting of delaminated lower crust at pressures equivalent to crustal thickness of 50 km i e 1 5 gpa possibly leaving rutile bearing eclogitic residue the shoshonitic magmas in contrast originated mainly from an enriched mantle metasomatized by subducted oceanic sediments they underwent early high pressure 1 5 gpa fractional crystallization at the boundary between thickened 50 km lower crust and lithospheric mantle and late low pressure 1 5 gpa fractional crystallization in the shallow 50 km crust the adakitic and shoshonitic rocks appear to be linked to an intra continental extensional setting where partial melting of enriched mantle and delaminated lower crust was probably controlled by lithospheric thinning and upwelling of hot asthenosphere along ne fault zones e g tanlu and yangtze river fault zones in eastern china both the shoshonitic and adakitic magmas were fertile with respect to cu au mineralization

gold mineralisation and alteration of penakacherla schist belt india constraints on archaean subduction and fluid processes abstract in the dharwar craton southern india gold deposits are found mostly along the six arcuate shear zones passing through late archaean greenstone belts 2 7 ga one such shear zone complex extends for about 400 km within and along the ramagiri hungund schist belt the penakacherla sector of this shear zone is excellently exposed enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration metamorphism and deformation under ne sw compression associated with archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists continued compression generated a 50 100 m wide shear zone complex consisting of mafic phyllonites advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified k metasomatic assemblage mainly consisting of chlorite amphibole k mica plagioclase ankerite quartz fe oxides pyrite chalcopyrite and arsenopyrite networks of quartz and carbonate veinlets a few millimeters to a few centimeters thick formed along the foliation planes giving rise to microscopic alteration envelope in which individual veinlet systems are merged into one another to form a composite alteration system gold is found within these quartz veinlets mafic phyllonites and at their mutual contacts hydrothermal fluids have modified the primary major minor trace and lree compositions of host rocks such that their mutual behaviour became non systematic some hfse and hree also show minor mobility but the overall ree pattern generally resembles that of the precursor mafic volcanic rocks mass and volume loss gain by si and ca has made significant impact on al ti and zr abundances which are generally immobile during hydrothermal alteration however element pairs such as zr hf v sc and nb ta maintain primary inter element ratios although their absolute abundances are drastically diluted similarly ree in highly silicified and carbonatised samples are reduced but patterns remain similar to those of relatively least altered mafic phyllonites with la n yb n between 1 and 3 in some samples lree enrichment is observed elevating in la n yb n from 3 to 11 pathfinder elements and base metals such as as cd cu pb zn and sb have been added along with the au and ag 13 c of carbon varies from 16 to 21 suggesting a biogenic origin whereas coexisting pyrite 34 s ranges from 1 to 3 pointing towards the involvement of magmatic or average crustal sulphur overall concentrations of k rb sr ba nb ta ti cs cr co v y and sc and many of the ratios such as k rb la sc la yb indicate that metamorphism devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity

lithostratigraphy and composition of 2 1 ga greenstone belts of the west african craton and their bearing on crustal evolution and the archean proterozoic boundary we have determined the lithostratigraphy of three early proterozoic 2 1 ga birimian greenstone belts in the west african craton and measured the concentrations of major and trace elements in samples of volcanic rocks from those belts the nangodi belt largely consists of tholeiitic basalt flows with mg numbers of 37 to 69 and flat to slightly lree depleted chondrite normalized ree patterns overlain by sodic na 2 o k 2 o 3 7 29 calc alkaline dacitic tuffs and lavas that are hree depleted la yb 4 5 8 9 times chondrites yb 3 9 8 7 times chondrites the volcanic sequence is capped by calc alkaline andesitic crystal tuffs with high mg numbers 66 75 and a high la yb ratio 10 5 times chondrites similar to archean and modern sanukitoids the dixcove belt largely consists of tholeiitic hyaloclastites and pillow lavas mg numbers 52 61 that grade upward into high mg mg numbers 59 calc alkaline andesitic tuffs and flows the winneba belt largely consists of massive tholeiitic h

geochemical oxygen and neodymium isotope compositions of metasediments from the abitibi greenstone belt and pontiac subprovince canada evidence for ancient crust and archean terrane juxtaposition abstract the abitibi greenstone belt agb and pontiac subprovince ps in the southwestern superior province are adjacent greenstone plutonic and metasedimentary dominated terranes respectively separated by a major fault zone metasediments from these two contrasting terranes are compared in terms of major and trace element and o and nd isotope compositions and detrital zircon ages the following two compositional populations of metasediments are present in the low grade abitibi southern volcanic zone 1 1 a mafic element enriched population mep characterized by flat depleted ree patterns enhanced mg cr co ni and sc low incompatible element contents and minor or absent normalized negative troughs at nb ta and ti and 2 2 a low mafic element population lmep featuring lree enriched patterns enhanced rb cs ba th and u contents and pronounced normalized negative troughs at nb ta and ti these geochemical features are interpreted to indicate that the mep sediments were derived from an ultramafic and mafic dominated oceanic provenance whereas the lmep sediments represent mixtures of mafic and felsic arc source rocks the ps metasediments are essentially indistinguishable from abitibi lmep on the basis of majorelement and transition metal abundances suggesting comparable types of source rocks and degrees of maturity but are distinct in terms of some trace elements and o isotope compositions the pontiac metasediments are depleted in 18o and enriched in cs ba pb th u nb ta hf zr and total ree and also have higher ratios of rb k cs rb ba rb ta nb th la and ba la relative to the abitibi lmep two subtypes of ree patterns have been identified in ps metasediments both have moderately fractionated distributions with la yb n 3 9 17 and 7 8 13 6 respectively but the first has minor eu anomalies eu eu 0 89 1 16 whereas the second features pronounced negative anomalies eu eu 0 52 0 81 coupled with high concentrations of incompatible elements such as k rb th u nb and ta the first subtype is interpreted to be derived from provenances of mixed mafic and felsic volcanic rocks whereas the eu depleted type has geochemical features that are typical of post archean sediments or archean k rich granites and volcanic equivalents abitibi metasediments have nd 2 7 ga values of 0 71 to 1 83 corresponding to model tchur 2 82 2 55 ga with the majority around 2 65 ga close to the depositional age of 2 7 ga in contrast ps metasediments are characterized by nd 2 7 ga 0 95 to 3 51 corresponding to t dhur 2 7 to 3 1 ga consistent with a detrital zircon age spectrum of 2 69 ga to 3 1 ga pontiac subprovince and abitibi metasediments have contrasting provenances suggesting their separate initial geodynamic settings and later tectonic juxtaposition major and trace elements o and nd isotopic compositions and detrital zircon ages collectively suggest that some of the ps metasediments were derived mainly from juvenile rocks whereas others require various degrees of contribution from older 3 0 3 5 ga evolved crust mixed with juvenile 2 7 ga rocks older components were derived either directly from erosion of an ancient continental arc or from contemporaneous volcanic rocks which may contain recycled older components if the ps represents in part an accretionary prism to the abitibi southern volcanic zone then oblique subduction may have been involved which allowed the pontiac subprovince to juxtapose or sample exotic source rocks

hydrothermal gold mineralization at the hira buddini gold mine india constraints on fluid evolution and fluid sources from boron isotopic compositions of tourmaline we determined the boron isotope and chemical compositions of tourmaline from the hira buddini gold deposit within the archean hutti maski greenstone belt in southern india to investigate the evolution of the hydrothermal system and to constrain its fluid sources tourmaline is a minor but widespread constituent in the inner and distal alteration zones of metabasaltic and metadacite host rocks associated with the hydrothermal gold mineralization the hira buddini tourmaline belongs to the dravite schorl series with variations in al fe fe mg ca ti and cr contents that can be related to their host lithology the total range of 11b values determined is extreme from 13 3 to 9 0 but 95 of the values are between 4 and 9 the boron isotope compositions of metabasalt hosted tourmaline show a bimodal distribution with peak 11b values at about 2 and 6 the wide range and bimodal distribution of boron isotope ratios in tourmaline require an origin from at least two isotopically distinct fluid sources which entered the hydrothermal system separately and were subsequently mixed the estimated 11b values of the hydrothermal fluids based on the peak tourmaline compositions and a mineralization temperature of 550 c are around 1 and 10 the isotopically lighter of the two fluids is consistent with boron released by metamorphic devolatilization reactions from the greenstone lithologies whereas the 11b rich fluid is attributed to degassing of i type granitic magmas that intruded the greenstone sequence providing heat and fluids to the hydrothermal system

the mount bruce megasequence set and eastern yilgarn craton examples of late archaean to early proterozoic divergent and convergent craton margins and controls on mineralization abstract the late archaean to early proterozoic mount bruce megasequence set formerly the mount bruce supergroup overlies and is restricted to the pilbara craton and comprises two megasequences which comprise a total of six supersequences or supersequence packages that formed in different but related tectonic settings the chichester range megasequence which is the older of the two megasequences comprises two supersequences overlain by a supersequence package the lower two supersequences are dominated by tholeiitic flood basalt and terrigenous clastic sedimentary rock with only minor placer gold and uranium mineralization these formed during the breakup of a late archaean craton between about 2 77 and 2 70 ga the overlying supersequence package is dominated by mudrock carbonate sedimentary rock iron formation and was deposited in a divergent marginal setting bordering an open ocean between about 2 69 and 2 60 ga the hamersley range megasequence comprises clastic sedimentary rock iron formation carbonate sedimentary rock and felsic volcanic rock was deposited between 2 47 ga and 2 9 ga volcano sedimentary assemblages gold mineralization occurred during the late stages of this orogeny which involved transfer of strain from the eastern yilgarn limited crustal thickening metamorphism and granitoid emplacement accretion into orogenic belts provides the best chance of preserving mineralization formed in deep marine environments such as komatiite associated ni and volcanogenic massive sulphide mineralization and explains the common close spatial association of these deposits with gold mineralization in greenstone belts

the alkaline porphyry associated yao an gold deposit yunnan china rare earth element and stable isotope evidence for magmatic hydrothermal ore formation the yao an vein type gold deposit is located in the ailaoshan jinshajiang alkaline intrusive belt yunnan province china and is associated both in time and space with 33 5 1 0 ma old alkaline intrusions the gold mineralization is associated with potassic wall rock alteration the ree distribution patterns of secondary k feldspar are generally similar to those of the igneous perthite but with about seven times higher total ree abundances the alteration is ascribed to a high ree magmatic fluid derived from the yao an alkaline intrusive suite the hydrothermal yao an gold deposit formed during two gold bearing stages i e a sulfide pyrite stage and a sulfide oxide stage pyrite specularite the ree abundance of early stage i pyrite is relatively high with strong enrichment in lree la yb n of 40 290 generally positive eu anomalies eu eu 0 86 1 55 and ree patterns very similar to those of secondary feldspar in contrast the later stage ii pyrite has much lower ree concentrations and lower la yb n of 5 5 11 8 eu eu of 0 49 0 76 and flat chondrite normalized spidergram patterns the stage i pyrite has 34s in the range of 2 2 to 3 2 and overlaps with regionally distributed pyrite in least altered syenite porphyry in contrast stage ii pyrite has much higher 34s values between 7 8 and 16 5 carbon isotope data for four samples from stage ii revealed 13c pdb values between 6 and 8 these stable isotope and ree data suggest that magmatic fluids of the alkaline intrusions caused both potassic alteration and stage i sulfide mineralization the system evolved from stage i to stage ii mineralization by influx of meteoric fluids with relatively heavier sulfur although calcite carbon isotope data suggest that the co2 remained dominantly of magmatic origin

plume and arc magmatism in the abitibi subprovince implications for the origin of archean continental lithospheric mantle abstract based on eclogitic mantle xenoliths that appear to be the residue of slab melts it is commonly argued that the formation of distinctive lithospheric keels beneath archean cratons occurred by the tectonic accretion of oceanic crust these models are complicated by evident buoyancy of the keels the varied compositions of the mantle xenolith suite as a whole and a compositional constraint that a large portion of the keels originated in mantle plume melting events resolution of these issues is complicated by the long development history of most cratons 100 s of m y and the multiple cycles of magmatism they represent in contrast the single 50 m y period of arc volcanism in the abitibi subprovince and the immediate onset of orogeny and craton stabilisation avoids many of these uncertainties chemical and isotopic data demonstrate that syn volcanic to syn tectonic adakitic abitibi granitoids were derived via slab melting deep seismic reflection profiles image singular slab like features along the moho these are interpreted as the remnants of oceanic slabs emplaced and torn during orogeny nonetheless their presence directly contradicts subcretion models of keel formation because they occupy regions that should correspond to the top of an earlier formed subcreted keel post adakite shoshonitic granitoids and lamprophyres also attest to the presence of a metasomatised mantle wedge throughout the entire period of abitibi arc magmatism diamond bearing lamprophyres in the abitibi and adjacent wawa subprovince are compositionally equivalent to paleoproterozoic diamondiferous rocks and characteristic of adakite generating shallow subduction rather than evidence of pre existing sub cratonic keels abitibi mantle plume and subduction related volcanism also demonstrates the continuous presence of shallow asthenospheric mantle beneath the belt over a 50 m y period komatiites and then boninites erupted along the central part of the belt halfway through its evolution and rifted arc style volcanism dominated the belt within a few m y of orogeny none of these events are consistent with the development of a progressively thickened continental lithospheric keel the geodynamic evolution of the abitibi belt demonstrates a fact recognised in some xenolith studies the characteristic association of deep keels and archean cratons was not developed gradually during greenstone belt volcanism rather it was initiated by the coupling of shallow imbricated plume and arc related crust with the buoyant residue of mantle plume melting that rose from depths of 80 km to 400 km subduction was active along the southern and eastern margin of the superior province craton before and after ascent of the abitibi plume renewal of subduction in the abitibi as marked by the occurrence of the 2716 ma boninitic volcanic suite delayed the ascent of the deep keel and its coupling to the composite crust final ascent of the plume residue occurred following orogeny and slab loss and may have included entrainment of fragmented slab residue into the rising diapir the coupling of shallow crust and deep keel occurred soon after and the keel s presence beneath the subprovince is recognised by phanerozoic diamond deposits xenolith chemistry and geophysical data

archean tectonic and metallogenic evolution of the superior province of the canadian shield abstract the superior province of the canadian shield has mainly experienced an archean tectonic history comprising several cycles of volcanism plutonism and tectonism although the rocks of the superior province span more than 500 million years of archean time most mineral deposits formed in an interval of 100 million years or less corresponding to a period of accretion of oceanic and pre existing continental fragments in the late archean at 2 7 ga in accord with recent tectonic syntheses which argue for the accretionary origin of the superior province the metallogenic history of the superior province can also be interpreted in the same context provided secular anomalies in the early history of the earth are acknowledged for relatively unique deposit types such as komatiitic nickel deposits and voluminous iron formations volcanogenic massive sulphide iode gold and granitoid associated deposits as well as ni cu pge cr and ti deposits associated with mafic ultramafic intrusions are readily interpreted in terms of the construction and accretion of late archean island arcs

characterization of fluids associated with gold mineralization and with regional high temperature retrogression of granulites in the limpopo belt south africa abstract the hout river shear zone forms the terrane boundary between the granite greenstone terrane of the kaapvaal craton and the granulite terrane of the limpopo belt it dips steeply northward and flattens to the south and was responsible for allowing hot granulites to spread up and over the cooler greenstone terrane this shear zone acted as a conduit for deep seated infiltrating fluids into still hot granulites in the hangingwall producing a regional retrograde orthoamphibole isograd and an associated zone of rehydrated granulites thermodynamic constraints on the composition of fluids involved in retrograde anthophyllite formation in pelitic granulites on the isograd supported by the results of a fluid inclusion study and by direct textural evidence in which olivine in closely associated ultramafic granulites has been partially replaced by hypersthene and magnesite suggest that the hydrating fluid contained at least 70 mol co2 and that this fluid infiltrated at p tconditions of about 620 c and 6 kbar pressure the s13c values of co2 extracted from magnesite vary between 5 5 and 6 0 and suggests a deep seated possibly mantle source for the co2 rich retrograde fluid fluid inclusion studies from two lode gold deposits osprey and louis moore in the same high grade area and three deposits from the immediately adjacent sutherland greenstone belt fumani klein letaba and birthday at the northern edge of the kaapvaal craton show that co2 rich fluids with similar characteristics were also associated with epigenetic gold mineralization all of the ore bodies are located within ew trending northward dipping ductile shear zones with an oblique to reverse sense of movement these relationships suggest that there is a direct link between the source of fluids involved in gold mineralization the fluids that formed the zone of rehydration in the southern marginal zone of the limpopo belt and southward verging faulting during the exhumation of the granulite terrane at about 2670 ma

mantle derivation of archean amphibole bearing granitoid and associated mafic rocks evidence from the southern superior province canada amphibole bearing late archean 2 73 2 68 ga granitoids of the southern superior province are examined to constrain processes of crustal development the investigated plutons which range from tonalite and diorite to monzodiorite monzonite and syenite share textural mineralogical and geochemical attributes suggesting a common origin as juvenile magmas despite variation in modal mineralogy the plutons are geochemically characterized by normative quartz high al2o3 15 wt na rich fractionation trends mol na2o k2o 2 low to moderate rb generally 150 ppm are comagmatic with the evolved granitoids and indicate that the suites are mantle derived isotopic studies of archean monzodioritic rocks have shown lree enrichment and initial 143nd 144nd ratios indicating derivation from mantle sources enriched in large ion lithophile elements lile shortly before melting mineral assemblages record lower ph2o with increased alkali contents of the suites this evidence in conjunction with experimental studies suggests that increased alkali contents may reflect decreased ph2o during mantle melting these features indicate that 2 73 ga tonalitic rocks are derived from more hydrous mantle sources than 2 68 ga syenitic rocks and that the spectrum of late archean juvenile granitoid rocks is broader than previously recognized comparison with phanerozoic and recent plutonic suites suggests that these archean suites are subduction related

archean geodynamics and the abitibi pontiac collision implications for advection of fluids at transpressive collisional boundaries and the origin of giant quartz vein systems abstract giant quartz vein systems of all ages from the archean to cenozoic are characterised by similarities of geodynamic setting local structure paragenesis metal budget p t t paths and chemical fluid dynamic and isotopic properties that collectively imply a singular hydrothermal process such quartz vein provinces form in regional brittle ductile shear zones that define terrane boundaries including closure of back arc basins and arc continent or continent continent collisions examples include the svz of the archean abitibi belt the norseman wiluna belt yilgarn block kolar schist belt india foothills metamorphic belt california and coast ranges megalineament in the cordillera and the cenozoic monte rosa district insubric line evidence for an archean collisional environment comes from the abitibi and pontiac subprovince tectonic boundary which hosts giant quartz vein systems in this region the amphibolite facies lacorne tectonic block in the archean abitibi greenstone belt is anomalous with respect to the prevalent low grade supracrustal sequences in neighbouring blocks the lacorne block has mature clastic sediments with a zircon provenance age spectrum from 3040 2691 ma and two granitic magma series a late syntectonic monzodiorite monzonite granodiorite syenite series formed over 2685 2670 ma similar to phanerozoic volcanic arc granites and post tectonic garnet muscovite granites emplaced at 2650 2630 ma which compositionally resemble phanerozic collisional s type granites the pontiac subprovince to the south of the abitibi greenstone belt shares all the above features with the lacorne block and collided with and was locally thrust under the abitibi belt with differential uplift of the lacorne block following collision to generate a tectonic window following collision of allochthonous terranes the conjuction of large volumes of subcreted oceanic crust and sediments between accreted terranes displaced isotherms rise and metamorphic dehydration are all necessary conditions for forming giant quartz vein systems metamorphic fluids are expelled along the terrane boundary structures at deep levels and focussed into second and higher order splays at mid crustal levels where quartz and au precipitation occurs hydrothermal fluids that formed the giant quartz veins are remarkably uniform in h o c and sr isotopic compositions albeit with small provincial variations the veins and gold precipitated at 270 360 c from fluids with low salinity and moderate co2 at 2 3 kbar in the brittle ductile transition

timing of gold mineralization at red lake northwestern ontario canada new constraints from u pb geochronology at the goldcorp high grade zone red lake mine and the madsen mine the goldcorp high grade zone at the red lake mine has reserves proven and probable established at 1 775 million tons mt at an average grade of 80 6 g t au new u pb geochronologic data combined with detailed mapping and crosscutting relationships provide timing constraints and new insights into the formation of the exceptionally rich goldcorp high grade zone the results show that the main stage of the high grade mineralization in the high grade zone formed before 2712 ma and that a second stage of gold mineralization much smaller in terms of total gold content but spectacular in terms of grade formed after 2702 ma this second stage is attributed to gold remobilization caused by the enhanced thermal gradient and deformation associated with the emplacement of the cat island pluton 2699 ma approximately 7 km east of the deposit or by the ca 2 63 to 2 66 ga postorogenic regional thermal event indicated by hornblende muscovite and biotite ar ar cooling ages from the uchi subprovince it is proposed that the main stage of high grade mineralization formed between ca 2723 to 2712 ma possibly synchronous with emplacement of the dome and mckenzie island stocks the abino granodiorite and hammell lake batholith as well as with penetrative d2 main stage regional deformation lamprophyre dikes spatially associated with the deposit postdate main stage mineralization by at least 10 m y a postore granodiorite dike from the madsen mine the second largest deposit in the district was dated at 2698 1 ma this age is similar to a postalteration and late deformation diorite dike at the creek zone showing near the former starratt olsen mine which was dated at 2696 2 ma the crosscutting relationships confirm that the main stage of mineralization at the madsen mine is pre 2698 ma a folded and metamorphosed polymictic conglomerate located stratigraphically above the campbell red lake deposit and deposited after ca 2747 but before 2712 ma is correlated with the newly recognized huston assemblage crosscutting and overprinting relationships between the conglomerate and the alteration indicate protracted multistage aluminous and iron carbonate quartz veining and or alteration event s before and after deposition of the conglomerate the age of the conglomerate confirms that the campbell red lake deposit is proximal to a folded regional unconformity consistent with the empirical relationship between large gold deposits and unconformities elsewhere in archean greenstone belts this paleosurface represents a key first order exploration target and it is proposed that areas of high potential for gold exploration in red lake occur in the balmer assemblage rocks within 500 m to 1 km of the unconformity

investigating sulfur pathways through the lithosphere by tracing mass independent fractionation of sulfur to the lady bountiful orogenic gold deposit yilgarn craton abstract as sulfur is the primary complexing ligand for gold transport in aqueous fluids recent work has demonstrated that the archean systems can preserve mass independent fractionation of sulfur mif s to yield near zero values 33s1 0 0 5 in such cases interpretation of the geological significance of such isotopic signatures may be compromised as it is known that mass dependent fractionation of sulfur mdf s can yield small 33s values herein as 33sb often owing to geobiological reactions involving sulfur in this study we reassess the range of possible values of 33sb from the traditionally accepted value of 0 0 2 to demonstrate that this range is not constant but rather directly linked to the degree of 34s fractionation we present a multiple sulfur isotope dataset from the neoarchean lady bountiful orogenic gold deposit from the yilgarn craton western australia this deposit contains pyrite with an isotopic composition of 34s 2 1 0 8 and 33s 0 14 0 06 2sd on the mean the analysis of these results as well as of a data compilation from other 2 65 ga orogenic gold deposits of the yilgarn craton indicates that 33s1 values are too large to be the result of purely mdf s processes in addition when compared to a compilation of 3400 33s0 measurements of archean sedimentary rocks which shows highly variable positive and negative signals the remarkably constant positive mif s1 signature of gold deposits from the yilgarn craton with 33s1 values between 0 and 0 6 is evidence that the source of the auriferous fluids equilibrated at depth by mixing with a non mif s0 bearing sulfur reservoir by enhancing the application of mif s1 33s1 as a powerful and indelible tracer to understand the source and emplacement pathways of gold bearing fluids through the lithosphere the outcomes from this study demonstrate the recorded 33s1 signature in the auriferous fluids reflects the presence of sulfur that was at least partially sourced from an archean sediment reservoir

the jurassic danba hypozonal orogenic gold deposit western china indirect derivation from fertile mantle lithosphere metasomatized during neoproterozoic subduction the genesis of hypozonal orogenic gold deposits is highly controversial with both crustal metamorphic fluids and variously derived sub crustal fluids suggested as fluid and metal sources the lower jurassic danba gold deposit a rare phanerozoic hypozonal orogenic gold deposit on the western margin of the yangtze craton china provides an opportunity to resolve these controversies danba is characterized by biotite amphibole alteration and pyrrhotite dominant ore assemblages formed at 500 600 c the 34s values of ore related pyrrhotite range from 3 1 to 9 9 in sharp contrast to those of syn sedimentary pyrrhotite from devonian host rocks which are between 6 8 and 9 5 the pb isotope compositions of ore related pyrrhotite with 206pb 204pb 17 85 18 25 and 207pb 204pb 15 48 15 67 are less radiogenic and more variable than those of unaltered host rocks these differences combined with the hypozonal feature of the deposit indicate that ore metals and sulfur were derived from a sub crustal rather than a local crustal source the pge patterns of ore related pyrrhotites are similar to those of sulfides in baltic mantle lithosphere that was metasomatized by aqueous fluids in terms of marked enrichment of pd and ru but significantly differ from those of magmatic intrusions and related magmatic hydrothermal deposits this suggests that the metals that formed the danba deposit were transported via aqueous fluid from metasomatized mantle lithosphere rather than from a magmatic source the 18o of mantle fluid calculated from hydrothermal quartz and biotite ranges from 10 to 12 the anomalously high s and o isotope ratios and variable pb isotope ratios together with pge data support a model in which the inferred mantle lithosphere source was significantly hydrated metasomatized and fertilized by fluid derived from subducted oceanic sediments with elevated 34s and 18o values geological evidence suggests that this subduction event was neoproterozoic in age this study defines a model in which hydrated and metasomatized mantle lithosphere was formed during early subduction but it was over 500 million years later during later lower jurassic asthenosphere upwelling that metal and sulfur rich fluid was released into the crust to form the danba gold deposit

metallogeny of the syenite related dongping gold deposit in the northern part of the north china craton a review and synthesis abstract the dongping gold deposit is located at the northern margin of the north china craton in hebei province china it is the largest alkaline pluton related au deposit in china the ore deposit is hosted in the 400 386 ma la icpms and shrimp zircon u pb method shuiquangou syenite intrusion which cuts archean aged metamorphic rocks the ore bodies consist of a set of en echelon lenses and veins controlled by shear zones the ores can be divided into auriferous quartz vein stockwork and disseminated ores two stages of gold mineralization can be distinguished the first stage mineralization consists of high grade 10 g t grayish quartz veins enveloped by stockwork and disseminated ores whereas the second stage mineralization occurs as low grade studies of fluid inclusions in quartz indicate that the ore forming fluids have low salinity 0 7 8 1 nacl equivalent in the h2o co2 nacl system and have homogenization temperatures varying from 310 to 350 c the ore forming fluids have 18o values varying from 1 7 to 6 9 with an average of 2 9 and d values from 108 to 66 5 with an average of 85 indicating a magmatic origin with some involvement of meteoric fluid the ore forming fluids have initial 87sr 86sr385ma ratios of 0 705 0 706 and 3he 4he ratios of 0 3 to 5 2 ra ra being the atmospheric ratio of 1 0 indicative of a mantle contribution of the fluids the ores have bulk 34s values varying from 1 to 2 similar to those of the host syenites 34s 1 8 3 and the archean metamorphic rocks 34s 0 04 to 4 4 lead isotope analyses of sulfides stock and disseminated ores quartz veins fresh host syenites and the archean metamorphic rocks have a linear correlation of 208pb 206pb and 207pb 206pb indicating a genetic link between the gold mineralization and the syenites hydrothermal zircons from the high grade ores of the first stage can be distinguished from magmatic zircons based on internal textures on cathodoluminescence images and rare earth element patterns these zircons have been dated at 389 1 0 to 385 5 7 ma whereas those from the second stage low grade auriferous quartz vein were dated at 140 ma la icpms and sim u pb methods we propose a model for the formation of the dongping gold deposit involving a late magmatic hydrothermal process related to a devonian mantle sourced syenites with a late jurassic early cretaceous hydrothermal overprint

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

secular variation in greenstone sequence development emphasizing superior province canada abstract evidence from the superior province of the canadian shield suggests the existence of several distinct lithostratigraphic associations in greenstone belts 1 quartz arenite and carbonate bearing sequences 2 mafic ultramafic volcanic sequences 3 mafic to felsic volcanic cycles and 4 timiskaming type sequences of fluviatile sediments and calc alkaline to alkaline volcanics recently recognized quartz arenite bearing sequences in the sachigo and wabigoon subprovinces are from base to top quartz arenite stromatolite bearing carbonate oxide facies iron formation and komatiite or tholeiite primary structures in the quartz arenites indicate shallow water deposition and represent mature sedimentation granitoid provenance of these sediments is postulated based upon petrography of coarse arenites and clasts of associated conglomerates the few dated superior province quartz arenite bearing sequences are 2 85 ga the shallow water origin of the primary structures and biota and the areal extent of the quartz arenite and carbonate sequences invite comparison with platform sediments at passive margins platform sequences unconformably overlie 3 ga granitoid rocks and older mafic volcanics where original lower contacts are preserved in two localities mafic volcanic sequences contain sheet flows iron formations and deep water argillites based on physical volcanology and sedimentology these sequences are compared to modern deep water mafic plain and hawaiian volcanism they generally underlie mafic to felsic volcanic cycles and overlie platform sequences mafic sequences are 2740 2700 ma in age mafic to felsic volcanic cycles consist of deep water tholeiitic flows overlain by shallow water to subaerial intermediate to felsic pyroclastics and flows the facies style of volcanism petrography and geochemistry of the mafic to felsic cycles most closely resemble the characteristics of crust in continental magmatic arc systems this type of lithostratigraphic association varies in age from 2775 to 2700 ma in the superior province with minor exceptions timiskaming type sequences consist of fluviatile conglomerate immature arenites and wackes with subaqueous to subaerial volcanics of variable chemistry longitudinal and lateral variation within fault bounded basins is extreme and the sequences have generally been affected by only late stages of deformation the combination of rock types facies distribution the fault bounded nature of the repositories and the locus of sedimentation along zones of transcurrent displacement suggest timiskaming sequences are best compared with modern pull apart basins superior province timiskaming sequences are within the superior province platform sequences are the earliest postorogenic lithostratigraphic unit in greenstone belts followed by mafic plain sequences arc volcanics and pull apart basins we suggest that platform sequences based upon limited evidence are preceded by an orogeny and that pull apart basins represent a subsequent orogeny the lithostratigraphic units above have distinct metallogenic associations 1 platform sequences based upon compilation of several shields may contain detrital au and u and oxide facies iron formations associated felsic volcanics contain no recognized volcanogenic cu zn deposits in the superior province the following metallogenic associations are noted 2 mafic plain sequences contain komatiite flow related ni deposits cu ni and platinum group deposits are associated with flows and coeval intrusions 3 arc volcanic sequences contain volcanogenic cu zn sulphide deposits and scarce deposits of porphyry type cu mo and 4 timiskaming sequences and related plutonism are related to lode au

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

metamorphic fluids in the deep crust most of the inferences about fluid action in the deep crust are drawn from field petrographic and geochemical studies in the granulite facies terrains deductions on the nature of fluids are made from major and minor element distributions and isotopic patterns in metamorphic rocks calculations of fluid mineral equilibria based on observed assemblages fluid inclusions and field evidence of fluid pathways discussion of these features has focused attention on several major problems concerning the timing flow mechanisms and origin of the fluids a key issue concerns episodicity versus secular activity of fluids there are strong arguments that most fluid action in the deep crust is linked to thermal deformational events including magmatism and regional meta morphism discrete radiometric ages of high grade terrains often define relatively short periods of crystallization or recrystallization the late archean granulite facies metamorphism of southern india is an example where u pb systems of zircons buhl et al 1983 whole rock u pb data peucat et al 1 987 and sm nd systems of whole rocks and minerals bernard griffiths et al 1 987 all show a major high temperature recrys tallization event inferred by several workers to have involved pervasive fluid action centered closely around 2 5 ga another geochemical indi cation of episodic fluids comes from stable isotope studies of progressive metamorphic sequences as in the damara orogen namibia where whole rock b 180 increases slightly at isograds representing arrested dehydration events but is monotonous between isograds hoernes hoffer 1 985

transition from steady to unsteady plinian eruption column the vei 5 4 6 ka fogo a plinian eruption sao miguel azores abstract the 4 6 ka fogo a trachytic plinian eruption on sao miguel island azores portugal is composed by four main phases 1 initial phreatomagmatic activity which emplaced thin surge deposits with accretionary lapilli 2 fully buoyant steady plinian activity which generated a prominent trachytic basal fall out deposit 3 partial collapsing activity which led to the emplacement of two trachytic trachydacitic intra plinian ignimbrites pink and black ignimbrites and associated surge deposits inter stratified within two fall out deposits 4 vent widening and fully collapsing activity which caused the emplacement of the climactic brown trachydacitic ignimbrite the deposits of these four main phases have been organised into three members fga1 fga2 and fga3 sub divided into 15 stratigraphic or emplacement units the fluctuations from a steady to unsteady plinian eruption column parallel the change in composition of the magma from trachytic to trachydacitic although minimal the change in composition resulted in a marked change in colour of the juvenile clasts from white at the base first phase to pink and light dark grey banded and black second and third phases and to dark brown fourth phase density analysis of juvenile clasts revealed that the change in composition did not significantly affect the density of the juvenile pumice pyroclasts the vesicularity of which increases from 70 to 85 up through the sequence the pink and the black intra plinian ignimbrites were emplaced only within the narrow paleo valleys of the southern flank with maximum thicknesses of 9 m and 5 m respectively transitioning onto adjacent topographic highs where thin veneer surge layers were deposited the fall out deposits inter bedded between the ignimbrites lack basal reverse grading indicating that the eruption column continued to be sustained during the partial collapses generating the intra plinian ignimbrites the climactic dark brown ignimbrite was emplaced radially with a maximum thickness of 40 m the absence of oxidation surfaces or evidence of reworking within the fogo a deposits indicates that there were no time breaks during the different phases of the eruption the on land bulk volume of the three ignimbrites has been estimated around 3 2 km3 while the volume of the intra plinian fall out deposits is 0 003 km3 these bulk volume estimations summed with the basal fall out volume calculated by walker and croasdale 1970 makes a total bulk volume for the fogo a deposits of 4 4 km3 indicating a volcanic explosively index of 5 higher than previously estimated this result is fundamental for the forecasting a possible scenario for a future eruption

timing of magmatism and deformation in the kamiskotia kidd creek area western abitibi subprovince canada abstract the kamiskotia kidd creek area comprises well preserved granitoid greenstone belt lithologies of the western most abitibi subprovince similar in composition and structural style to other late archean granitoid greenstone terranes e g kalgoorlie norseman western australia belingwe zimbabwe slave province canada it contains the kidd creek volcanogenic massive sulfide deposit and lesser deposits of the kamiskotia district and lode gold deposits associated with the destor porcupine fault zone near the prolific porcupine gold district the magmatic and tectonic history for the area is provided through detailed structural mapping and high precision u pb geochronology field observations support the following sequence of magmatic and structural events in the kamiskotia area 1 formation of older mafic and intermediate volcanic rocks followed by the emplacement of the kamiskotia gabbroic complex kgc and cogenetic massive sulfide bearing bimodal volcanic rocks kv also accompanied by coeval intrusion of granitoid a into the crystallizing kgc 2 regional bulk rotation about a sub horizontal axis of this stratigraphy to form a ne facing monocline 3 emplacement of three large granitoid intrusions imparting their respective contact strain aureoles on the stratigraphy 4 regional sub horizontal bulk north south shortening that formed a non penetrative east west flattening fabric with an element of 5 late dextral transpression that formed high strain zones related to western destor porcupine fault zone dpfz within less competent volcanic units and 6 emplacement of alkalic dikes associated with the late stages of transpressive deformation u pb ages help substantiate this chronology and aid in the correlation of kamiskotia lithologies across the southern abitibi subprovince zircon ages for the kgc and a kamiskotia rhyolite are 2707 2 ma and 2705 2 ma respectively these ages are coeval with the reid rhyolite with a zircon age of 2706 2 ma but are significantly younger than the kidd creek rhyolite and the prosser rhyolite with zircon ages of 2717 2 ma data refined by the addition of an abraded concordant analysis and 2716 6 3 ma respectively two of the granitoid intrusions have zircon ages of 2696 1 5 and 2694 4 ma the older granitoid intrusion has a titanite population with a 207 pb 206 pb minimum age of 2692 5 ma which may represent a protracted cooling history or a metamorphic event after the setting of the zircon u pb system two zircon fractions from the younger granitoid intrusion exhibit inheritance with 207 pb 206 pb minimum ages up to 2926 ma indicating the presence of significantly older crust in this region timing of north south compression and late transpression may be constrained by a u pb garnet titanite age on a late to post tectonic alkalic dike at 2687 3 ma

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

petrology of the rainy lake area minnesota usa implications for petrotectonic setting of the archean southern wabigoon subprovince of the canadian shield the rainy lake area in northern minnesota and southwestern ontario is a late archean 2 7 ga granite greenstone belt within the wabigoon subprovince of the canadian shield in minnesota the rocks include mafic and felsic volcanic rocks volcaniclastic chemical sedimentary rocks and graywacke that are intrucded by coeval gabbro tonalite and granodiorite new data presented here focus on the geochemistry and petrology of the minnesota part of the rainy lake area igneous rocks in the area are bimodal the mafic rocks are made up of three distinct suites 1 low tio2 tholeiite and gabbro that have slightly evolved mg numbers 63 49 and relatively flat rare earth element ree patterns that range from 20 8 x chondrites ce ybn 0 8 1 5 2 high tio2 tholeiite with evolved mg numbers 46 29 and high total ree abundances that range from 70 40 x chondrites ce ybn 1 8 3 3 and 3 calc alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive mg numbers 79 63 enriched light rare earth elements lree and depleted heavy rare earth elements hree these three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma they resulted from melting of heterogeneous archean mantle the felsic rocks are made up of two distinct suites 1 low al2o3 tholeiitic rhyolite and 2 high al2o3 calc alkaline dacite and rhyolite and consanguineous tonalite the tholeiitic felsic rocks are high in y zr nb and total ree that are unfractionated and have pronounced negative eu anomalies the calcalkaline felsic rocks are depleted in y zr and nb and the ree that are highly fractionated with high lree and depleted hree and display moderate negative eu anomalies both suites of felsic rocks were generated by partial melting of crustal material the most reasonable modern analog for the paleotectonic setting is an immature island arc the bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre and syntectonic granitoid rocks however the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments the low tio2 tholeiite is similar to both n type mid ocean ridge basalt morb and low k tholeiite from immature marginal basins the calc alkaline basaltic andesite is like that of low k calc alkaline mafic volcanic rocks from oceanic volcanic arcs however the high tio2 tholeiite is most similar to modern e type morb which occurs in oceanic rifts the conundrum may be explained by 1 rifting of a pre existing immature arc system to produce the bimodal volcanic rocks and high tio2 tholeiite 2 variable enrichment of a previously depleted archean mantle to produce both the low and high tio2 tholeiite and the calc alkaline basaltic andesite and or 3 enrichment of the parental rocks of the high tio2 tholeiite by crustal contamination

diverse middle neoarchean granitoids and the delamination of thickened crust in the western shandong terrane north china craton abstract middle neoarchean granitoids contain crucial clues for understanding crust mantle interactions and further investigating the mechanism of the compositional transformation of the crust in the neoarchean in the western shandong terrane wst diverse neoarchean magmatism is recognized including 2 62 ga tonalitic gneisses 2 59 ga granodioritic gneisses and 2 61 2 58 ga monzogranitic gneisses which generally intruded into 2 70 ga tonalite trondhjemite granodiorite ttg gneisses the 2 70 ga ttg gneisses show high sio2 contents 70 17 72 89 wt but low mgo contents 0 58 0 69 wt and high la yb n ratios 30 48 and originated from partial melting of low k mafic rocks at lower crustal levels the 2 62 ga tonalitic gneisses exhibit low k2o na2o ratios 0 10 0 39 but high la yb n 7 60 12 7 and sr y 18 8 48 ratios and mg 47 57 with high ehf t2 values ranging from 2 7 to 7 4 these rocks were derived from dehydration melting of subducted oceanic slabs the 2 61 2 58 ga monzogranitic gneisses show high sio2 70 35 75 12 wt and k2o 4 35 5 81 wt contents but low mgo 0 20 0 77 wt contents with positive ehf t2 values 2 0 6 6 suggesting they were derived from partial melting of juvenile greywackes the 2 59 ga granodioritic gneisses exhibit high sio2 66 34 73 04 wt k2o 2 23 4 32 wt and mgo 1 39 3 51 wt contents with dispersed ehf t2 values 0 2 5 3 indicating a mixing process between crust and mantle sourced magmas the 2 59 ga granodioritic gneisses show consistent major and trace element compositions i e high mg lack of eu anomalies and uniform chondrite normalized ree patterns and accordingly their magmatic precursors were derived from partial melting of delaminated lower crustal materials the lithological assemblages in the wst became increasingly diverse between 2 71 2 58 ga changing from ttg gneisses to arc related rock series and then to delamination related granodioritic gneisses crust derived monzogranitic and trondhjemitic gneisses and mantle derived tholeiitic basalts and diorites by integrating these data with a reduction in the continental crustal thickness from 2 70 ga to 2 55 ga a subduction delamination model is proposed in which a long term lateral accretion beginning at 2 7 ga and persisting through 2 68 2 62 ga led to the delamination of the lower crust at 2 61 2 58 ga which may have further triggered the transition of the continental crust composition from basaltic to andesitic

factors in the formation of the giant kalgoorlie gold deposit abstract kalgoorlie golden mile plus mt charlotte accounts for more than half the gold won from the 2000 deposits that comprise the widespread gold mineralization of the archaean yilgarn block of western australia it is also one of the largest gold deposits globally 1800 t au production and reserves an analysis of the various stages of formation of the kalgoorlie deposits from source through solution of gold in hydrothermal fluids and their infiltration along channelways through to gold precipitation at depositional sites reveals several features that make kalgoorlie favourably disposed as a site for a major gold deposit however none alone account for its anomalously large size it is concluded that the giant kalgoorlie gold deposit owes its size and unique position in western australian gold production history to favourable host rock chemical composition favourable host rock thickness and mechanical properties and to the favourable geometry of its host units and hosting greenstone belt at favourable pt conditions within the greenschist facies collectively these factors together with proximity to the regional scale boulder lefroy fault allowed highly focussed fluid flow on both a deposit and district scale in this regard it is significant that the structural setting of gold mineralization and the greenstone architecture at the largest canadian archaean deposit timmins are remarkably similar to kalgoorlie orebody size distribution histograms for both the yilgarn block and the abitibi belt canada are highly skewed with extremely few giant deposits but numerous deposits of 10 tonnes gold existing this topology can be accounted for in terms of system efficiency the probability of all six factors in the ore forming system being optimized high heat flow large fluid reservoir proximity to major structure efficient fluid focussing and large and compositionally favourable host lithology is extremely small in contrast the probability of two processes coupling to give low efficiency is very large resulting in numerous small deposits kalgoorlie appears to have formed by the same basic processes that formed the plethora of smaller yilgarn gold deposits however it represents a mineralized domain in which there was a conjunction of a number of favourable parameters both on a regional and deposit scale that produced high system efficiency and allowed the effective fluid focussing and gold deposition that formed this giant deposit

noble metal abundances of late archean 2 7 ga accretion related shoshonitic lamprophyres superior province canada abstract shoshonitic magmatism occurred in transpressive geodynamic settings within the superior province during the diachronous accretion of allochthonous subprovinces over 2720 to 2680 ma which culminated in stabilization of this archean craton the dikes are interpreted to have formed from low degrees of partial melting of depleted mantle metasomatically enriched in lile and lree during a prior subduction accretion event selenium contents of the dikes range from 50 130 ppb and s from 150 1000 ppm with s se ratios of 1150 18 000 signifying open system behaviour of s during autometasomatic and late hydrothermal alteration based on the se contents and assuming magmatic s se ratios of 5 000 primary s contents of the alkaline magmas were undersaturated at 650 ppm mgo and ni are correlated and ir during evolution of the shoshonitic magmas olivine pyroxene controlled ir concentrations and probably rh platinum and palladium correlate with ti implying that these pge were controlled by timagnetite fractionation

shoshonitic lamprophyre dykes and their relation to mesothermal ausb veins at hillgrove new south wales australia abstract the hillgrove mineral field in the southern part of the new england orogen of northeastern new south wales australia contains numerous mesothermal au sb vein systems calc alkaline shoshonitic lamprophyre cal dykes are also associated with mineralisation with dilational lode structures acting as conduits for dyke intrusion which has occurred before and after major quartz stibnite veining dykes include minette and vogesite compositions and were emplaced in the late permian 247 255 ma at the same time as regionally extensive i type magmatism in the new england orogen least altered dykes are enriched in mg k ba rb sr zr th cr and ni relative to i type intrusives although chemical affinities are evident between lamprophyres and the more mafic members of the high k moonbi plutonic suite hillgrove lamprophyres are commonly enriched in sb as hg au w and bi with respect to average cal compositions evidence indicates this is most likely due to contamination of magma during intrusion through mineralised structures rather than a primary magmatic feature partially resorbed xenocrystic stibnite occurs in dykes which have intruded lode structures probably facilitated by the low melting point of stibnite 550 c and its incorporation into the magma carbon and oxygen isotopic data from carbonates in least altered post lode lamprophyres are indistinguishable from carbonate in altered dykes and veins implying that hydrothermal interaction continued after dyke intrusion although it is unlikely that lamprophyre dykes have been a direct source for mineralisation at hillgrove the close temporal and spatial relation of dykes mesothermal au sb veins and i type intrusions are interpreted to be manifestations of the post collisional setting and influx of mantle derived heat and partial melts into the new england orogen during the late permian

the scale of ore depositional systems an important restraint on epigenetic vs remobilized syngenetic origins for archaean mesothermal gold deposits there is general consensus that the majority of gold deposits in the important group hosted by volcanic or intrusive host rocks within archaean granitoid greenstone terrains have an epigenetic origin however there is controversy surrounding those stratabound deposits hosted by fe rich sedimentary rocks such as banded iron formation bif for which epigenetic syngenetic and remobilized syngenetic lateral secretion origins have all been proposed in western australia such controversial gold deposits in fe rich sedimentary units have similar ore mineralogies ore element ratios and in places alteration assemblages and on the craton scale show very similar structural controls to the other deposits equivalent relationships are recorded for similar deposits in other cratons all western australian deposits occur along kilometre scale shear or fault zones linked to trans craton obliqueto strike slip shear zones that were a focus for i carbonation with a mantle like isotopic signature and ii emplacement of high level a and i type granitoids felsic porphyries and or calc alkaline lamprophyres the scale of the mineralizing systems and the broad contemporaneity of mineralization as indicated by radiogenic isotope studies is much larger than that envisaged in conceptual secretion models which involve volumetrically small locally enriched source rocks instead archaean gold mineralization in western australia at least is probably the result of high fluid flux during deformation hightemperature metamorphism and magmatism which may be related to tectonism at convergent plate margins as in modern examples there is strong evidence for similar tectonic and mineralizing processes from the canadian shield

u pb garnet and titanite age for the bristol township lamprophyre suite western abitibi subprovince canada a u pb age of 2687 3 ma has been determined for a garnetite dike using melanite garnet and titanite mineral separates this is the first primary igneous u pb age reported using garnet the garne

the crustal continuum model for late archaean lode gold deposits of the yilgarn block western australia most archaean gold ores belong to a coherent genetic group of structurally controlled lode deposits that are characteristically enriched in au with variable enrichments in ag as w sb bi te b and pb but rarely cu or zn and are surrounded by wallrock alteration haloes enriched in k lile and co2 with variable na and or ca addition evidence from the yilgarn block of western australia combined with similar evidence from canada and elsewhere indicates that such deposits represent a crustal continuum that formed under a variety of crustal regimes over at least a 15 km crustal profile at pt conditions ranging from 180 c at 1 kb to 700 c at 5 kb individual deposits separated by tens to hundreds of kilometres collectively show transitional variations in structural style of mineralisation vein textures and mineralogy of wallrock alteration that relate to the pt conditions of their formation at varying crustal depths specific transitions within the total spectrum may be shown also by deposits within gold camps although nowhere is the entire continuum of deposits recorded from a single gold camp or even greenstone belt