evolution of the south central segment of the archean abitibi belt quebec part iii plutonic and metamorphic evolution and geotectonic model textural criteria permit distinction between the pre kenoran and kenoran phases of plutonism and metamorphism the pre kenoran plutons and pre kenoran metamorphic phases are directly related to the volcanic evolution synvolcanic tonalite trondhjemite plutons and swarms of mafic and felsic dykes core central volcanic complexes the volcanic rocks underwent three types of pre kenoran metamorphism namely a pervasive alteration a thermal contact metamorphism that affected narrow aureoles around synvolcanic plutons and a high intensity hydrothermal alteration that affected cross cutting pipes in central volcanic complexes synkinematic kenoran metamorphism resulted in the growth of minerals chlorite actinolite etc parallel to schistosities synkinematic metamorphic grade ranges form the pumpellyite prehnite facies to the amphibolite facies late to post kinematic metamorphic phases resulted in the growth of minerals across schistosities syn to post kinematic plutons are not voluminous in the part o

structural analysis and metamorphism of the barlow fault zone chibougamau area neoarchean abitibi subprovince implications for gold mineralization faults and deformation zones are important features of archean terranes because of their significance for structural evolution and the formation of large gold districts in the chibougamau area no

thermobarometry in a subgreenschist to greenschist transition in metabasites of the abitibi greenstone belt superior province canada mineral equilibria in the system cao mgo al2o3 sio2 h2o provide a basis for mapping of four reaction isograds and one bathograd in the low pressure transition from subgreenschist to greenschist facies most of the matachewan area of the abitibi greenstone belt is in the lower pressure bathozone as indicated by the widespread occurrences of the subassemblage prh chl the higher pressure bathozone is indicated by two occurrences of pmp act ep qtz but in these samples the bathograd is displaced to anomalously low pressure by the high fe content of the coexisting minerals this illustrates the need to analyse coexisting minerals calculate activities of end member species and compute p t curves for individual samples before interpreting the isograd bathograd pattern petrographic and microprobe analysis indicates that great care must be taken in the selection of equilibrium assemblages pyroxene phenocrysts in one sample are replaced by the assemblage pmp act ep chl qtz whereas prh act ep chl qtz occurs in the groundmass compositional variation may be more cryptic as in a sample of metabasaltic hyaloclastite that contains two spatially distinct univariant assemblages prh pmp ep chl qtz and prh act ep chl qtz within the devitrified matrix whereas chlorite compositions are similar in both assemblages prehnite and epidote in the latter assemblage are significantly richer in fe and poorer in al accordingly the rock is interpreted to contain two distinct univariant assemblages rather than one invariant assemblage prh pmp act ep chl qtz the displaced univariant curves for this sample intersect at 2 2 kbar and 250 c taking account of all thermobarometric implications the low grade limit of the greenschist facies is at 250 270 c and 2 2 5 kbar corresponding to depths of 7 8 km comparison of apparent p t conditions on both sides of the larder lake cadillac break a regional co2 metasomatized fault zone that is spatially associated with many archaean gold deposits provides an upper limit of not more than c 1 km for post metamorphic south side up dip slip displacement

numerical simulation of shoaling broad crested internal solitary waves abstractthe interaction between fully nonlinear miyata choi camassa mcc internal solitary waves and topographic slopes are modeled by direct numerical simulation the immersed boundary method is

a detachment fold model for fault zones in the late archean abitibi greenstone belt abstract the abitibi belt is one of the largest and most extensively studied late archean greenstone belts the structural geology of the abitibi belt consists of one generation of upright to slightly overturned doubly plunging first order folds with half wavelengths of 20 60 km and e w striking steeply dipping fault zones that are parallel to the fold limbs two of the main fault zones are continuous for hundreds of kilometers previous tectonic models for the abitibi belt interpret the fault zones to have formed as extensional growth faults bounding a volcanic sedimentary basin which were reactivated as thrusts during subsequent crustal shortening other models propose that the fault zones represent tectonic sutures implying that the abitibi belt is a collage of exotic terranes however distinct geological terranes have not been geologically demonstrated we propose a new detachment fold model for the deformational history of the southern abitibi belt in ontario that explains the formation of the fault zones during the single well documented folding event that deformed the entire region the internal structure of the fault zones documented here with emphasis on the porcupine destor fault zone consists of isoclinally folded strongly schistose highly metamorphosed rock cross cut by numerous fault segments we interpret that the upper crust greenstones was folded above a proposed detachment in the lower part of the volcanic stratigraphy the fault zones would be in essence highly evolved detachment anticlines ultramafic metavolcanic rock that crops out within the fault zones would represent material from the detachment horizon that was emplaced in the cores of the detachment anticlines the numerous segments that make up the mapped fault zones would be linked faults that formed within the isoclinal detachment anticlines to accommodate folding of the rheologically complex greenstones the detachment fold model is compared to the results of analogue experiments designed to investigate crustal scale folding using viscous and frictional materials detachment folds are produced in the brittle upper crustal analogue on the limbs of folds formed in the ductile middle and lower crust analogues the experimentally produced structures scale to the structures in the study area and indicate the detachment fold model for the southern abitibi is mechanically viable

the timing of prograde metamorphism in the pontiac subprovince superior craton implications for archean geodynamics and gold mineralization abstract the pontiac subprovince is located in the superior province south of the abitibi subprovince the metasedimentary rocks of the pontiac group are characterized by a barrovian metamorphic gradient increasing from north to south from biotite through garnet to staurolite zone conditions the pontiac subprovince has been interpreted as an accretionary wedge or alternatively as an exotic terrane that was tectonically docked to the abitibi subprovince during archean subduction however few studies have attempted to characterize its pressure temperature time path to test this hypothesis and little is known about the timing relationship between regional metamorphism and the gold mineralization an e w lepidoblastic ductile micaceous foliation defines the regional fabric related to the second episode of deformation garnet and staurolite porphyroblasts are interpreted to be late to post kinematic to the regional fabric locally cordierite or andalusite porphyroblasts fully pseudomorphed by a polymineralic assemblage of muscovite feldspar and quartz are wrapped by the main foliation indicating that a potential low pressure high temperature metamorphism m1 preceded the main episode of deformation and barrovian metamorphism m2 pressure and temperature p t forward thermodynamic modelling on a water saturated pseudosection yielded peak conditions of 550 600 c and 5 6 kbar during a prograde clockwise p t path for m2 lu hf dating of garnet from three different locations within the staurolite zone yielded a weighted average age of 2657 5 4 4 ma 95 confidence level that is inferred to be representative of these conditions the age of garnet growth calls into question the previous tectonic interpretation of the abitibi and pontiac subprovinces and is coherent with the new rafted ribbon continent model of bedard 2018 because the canadian malartic gold mineralization is older or synchronous with the garnet growth episode in the staurolite zone south of the ore body this garnet age also provides an estimate of the minimum age of the mineralization event previously dated at 2664 11 ma with re os on molybdenite

provenance of igneous clasts in conglomerates of the archaean timiskaming group kirkland lake area abitibi greenstone belt canada four types of igneous clasts from timiskaming conglomerates of the kirkland lake area are identified calc alkaline porphyry trachyte k2o n2o 7 25 wt trondhjemite and tholeiitic basalt clasts derived from quartz carbonate veins and carbonatized porphyries and komatiites indicate co2 rich hydrothermal activity along the larder lake cadillac fault before sedimentation calc alkaline porphyry the predominant clast is similar in appearance to feldspar porphyry intrusions widely exposed in the area but the two are not related the porphyry clasts are cogenetic with diorite granodiorite intrusions and volcanic rocks of the greenstone belt andesite clasts are only observed near the base of the assemblage and the ratio of basalt to andesite increases up stratigraphic sections the change suggests progressive erosion of arc related volcanic rocks during sedimentation composition texture and mineralogy of the trondhjemite clasts are consistent with their derivation from the marginal phases of

u pb ages and tectonic significance of late archean alkalic magmatism and nonmarine sedimentation timiskaming group southern abitibi belt ontario the paper presents u pb ages for zircons of the calc alkalic to alkalic igneous suite and associated alluvial fluvial sedimentary rocks of the timiskaming group in the late archean abitibi greensto

plutonism in the southern abitibi subprovince a tectonic and petrogenetic framework studies of plutonic rocks in archean terranes such as the southern abitibi subprovince have been an important component in the development of an accretionary tectonic model for the superior province in the southern abitibi subprovince pretectonic approximately 2 75 2 70 ga low k tholeiitic calc alkaline dioritic to tonalitic plutons e g bourlamaque batholith flavrian pluton are associated with volcanism and are interpreted to be derived from both partial melting of hydrated mantle during oceanic arc formation and fractionation of tholeiitic to calc alkaline basalt andesite pretectonic synvolcanic tholeiitic intrusions e g dundonald sill kamiskotia gabbroic complex 2 72 2 70 ga show in situ fractionation trends and locally have mixed magma textures with contemporaneous calc alkaline granitoids they are believed to be derived from a midocean ridge basalt morb like depleted mantle with little evidence for contamination by an older crustal component pre to syntectonic approximately 2 72 2 69 ga porphyry and tonalite plutons e g timmins porphyries round lake batholith have geochemical characteristics typical of archean tonalite trondhjemite suites considered to be derived from melting of basaltic rocks at high metamorphic grades late to post tectonic 2 69 ga diorite monzonite and syenite plutons e g watabeag batholith lamaque stocks otto stock have large ion lithophile element enriched calc alkaline to sho shonitic compositions these intrusions and associated timiskaming type volcanic rocks are attributed to melting of large ion lithophile element enriched mantle in the late stages of subduction late to post tectonic biotite muscovite granite intrusions contain a significant contribution of evolved crust e g s type characteristics of the preissac lacorne batholith in the southern abitibi subprovince the tectonic setting of mantle and crust derived intrusions is broadly comparable to magmatic processes in phanerozoic arcs and accreted terranes the plutonic rocks record a transition from pre to synaccretionary subduction related plutonic suites to late to postaccretionary suites derived from enriched mantle and crustal sources this framework provides a basis for understanding mineral deposits associated with intrusive rocks of the southern abitibi subprovince which range from ni cu sulfides in layered synvolcanic tholeiitic intrusions e g kamiskotia gabbroic complex ontario to volcanogenic sulfides in early tholeiitic and calc alkaline sequences e g flavrian pluton noranda district quebec to the spatial association of gold with late large ion lithophile element enriched intrusions e g lamaque plugs val d or district quebec

the lithoprobe abitibi grenville transect two billion years of crust formation and recycling in the precambrian shield of canada we summarize the results of lithoprobe studies in the neoarchean southeastern superior province and the mesoproterozoic grenville province in the southeastern precambrian shield of canada through two composite cross sections based on seismic reflection data which define dramatically different styles of crust formation and tectonic accretion in the neoarchean and mesoproterozoic in the neoarchean the structures at the surface are steep with discontinuous and flatter structures at depth much of the crust appears to be juvenile and the predominant process of crustal growth is inferred to have been subduction accretion of primitive crust in a prograding arc system in the mesoproterozoic surface structures are shallow and the seismic character of the crust is continuous over the entire cross section archean parautochthonous rocks and reworked archean crust comprise a very significant proportion of the preserved crust in the mesoproterozoic and provided the backstop to the grenvillian orogeny result

seafloor hydrothermal fluids ben nevis area abitibi greenstone belt implications for archean 2 7ga seawater properties abstract a central part of the 2 7 ga abitibi greenstone belt between the destor porcupine and cadillac larder lake structures has been subjected to sub greenschist facies metamorphism t the majority of observed fluid inclusions are two phase and liquid dominated with a vapor bubble volume of 10 microthermometry on 58 samples 1100 fluid inclusions and eutectic temperatures between 45 and 30 c indicate the presence of at least one significant divalent cation in the fluids hence salinity is calculated as wt cacl2 equiv the peak of the salinity distribution of inclusions from vesicles and interstitial and drainage cavity quartz falls between 4 and 10 wt cacl2 equiv a small subset of inclusions yielded salinities between 18 and 22 wt cacl2 equiv most homogenization temperatures fall in an interval between 110 and 160 c a geothermal gradient estimated from the data of 10 15 c km is reasonable and consistent with the interpretation of the area as a regional seafloor recharge zone primary porosity in the modern ocean floor does not show the rapid precipitation of quartz that is indicated for the archean however the global occurrence of silica rich banded iron formations and the lack of silica consuming marine organisms in the precambrian indicates that marine geochemistry and sio2 precipitation mechanisms may have been different during the precambrian the total salinity of a fluid is dependent primarily on its total chlorinity hence water rock interactions with ion exchange cannot be the only reason for the higher salinities encountered in this study indications for boiling or high temperature phase separation are not evident suggesting that these options are not a reason for the observed high salinities our data are in agreement with six other data sets from the 3 75 ga isua greenstone belt w greenland the 3 24 ga pilbara region w australia the 3 2 ga barberton greenstone belt south africa 2 the 2 7 ga matagami area abitibi greenstone belt canada and the proterozoic 2 2 ga ongeluk formation south africa the consistency of fluid inclusion data suggests that archean and early proterozoic seawater may have been more saline and potentially stratified compared to modern seawater

review of archean supracrustal assemblages of the southern abitibi greenstone belt in ontario canada products of microplate interaction within a large scale plate tectonic setting abstract previously formulated plate convergent models account for many aspects of the large scale regularity and parallelism between different subprovinces of the archean superior province such models do not however appear to adequately explain the within subprovince evolution of 2 75 2 68 ga supracrustal assemblages of the late archean southern abitibi greenstone belt principal supracrustal assemblage classes include l assemblages dominated by tholeiitic metabasalt and metakomatiite with varying amounts of tholeiitic intermediate to felsic metavolcanic rocks these assemblages probably formed in extensional settings the ratio of basalt to komatiite and the ratio of basalt komatiite to intermediate felsic metavolcanic rocks may be a function of mantle temperature and crustal thickness at the site of extension respectively 2 assemblages dominated by intermediate to felsic fragmental and or effusive rocks are generally of calcalkalic affinity and likely formed in island arc settings 3 assemblages containing a substantial component of iron formation may represent older supracrustal assemblages that form structural and or stratigraphic basement to some of the younger supracrustal assemblages 4 assemblages consisting almost wholly of turbidite facies metasedimentary rocks appear to postdate volcanic activity related to the previous assemblages deposition of these sediments broadly coincides with the onset of regional deformation these assemblages may represent distal deposits related to an emerging collisional orogen 5 assemblages consisting of alluvial fluvial metasedimentary rocks with or without spatially associated alkalic metavolcanic rocks are associated with crustal scale shear zones and are the youngest assemblages within the greenstone belt alluvial fluvial assemblages likely represent the final emergence of the consolidating orogen geochronological data and assemblage distribution suggest that the supracrustal assemblages of the southern abitibi greenstone belt may represent the amalgamation of products of diverse and intricate microplate tectonic settings repeated in time and space prior to the impingement of the large scale plate tectonic regime that dictated the final large scale configuration of the archean superior province

geochemical characteristics of granitoid rocks from within the archean michipicoten greenstone belt wawa subprovince superior province canada implications for source regions and tectonic evolution three distinct tonalite trondhjemite granodiorite ttg magma series occur syngenetically with bimodal mafic felsic cyclical volcanics in the middle to late archean michipicoten greenstone belt mgb canada between 2 72 and 2 60 ga two additional series are recognised as pre tectonic late tectonic and post tectonic syenite quartz monzonite granitoid stocks smg series and an alkali feldspar syenite alkali feldspar quartz syenite stock ss series the synvolcanic ttg series such as the hawk lake granitic complex and the jubilee lake stock are characterised by moderate to low k2o 2 5 wt rb 60 ppm and sr concentrations 400 ppm moderate tio2 levels 0 1 1 0 wt lasm 2 12 and gdyb 0 8 2 9 moderate to large negative eu anomalies and small negative ta nb troughs on ocean ridge granite org normalised plots the ttg series are compositionally similar to synvolcanic mgb felsic volcanics of cycles 1 to 3 the high lasm lata and low zrth are features of sialic crust a contribution of ancient sialic crust in the genesis of the felsic volcanics and ttg series granitoids of all three felsic volcanic cycles and synvolcanic ttg stocks is required in the mgb the parental magma like that of the ttg series of the abitibi greenstone belt agb may originate in oceanic mantle the smg series have elevated k2o 2 0 6 3 wt rb 40 280 ppm ba 10 2800 ppm and sr 250 1350 ppm steep ree patterns reflected in elevated lasm 3 12 and gdyb 1 8 5 0 without eu anomalies a moderate to large negative ta nb trough on ocean ridge granite normalised plots and low tio2 0 5 wt and cao na2o k2o the smg series is compositionally different to the mgb felsic volcanic rocks and to the ttg series granitoids the source is tentatively placed in the mantle wedge overlying a southward dipping subducted plate of oceanic crust and the elevated lile signature is attributed to metasomatism of this source by fluids derived from the subducted slab of oceanic crust the smg series granitoids like those of the abitibi are different when compared to the sanukitoids in that they are compositionally more evolved however they may be related to these rocks by low pressure fractionation and may be derivatives of sanukitoid like magmas the alkali feldspar syenite alkali feldspar quartz syenite represented by the herman lake stock of the ss series has a primitive composition 60 wt sio2 with low ree abundances and relatively flat lree and hree but steep mree profiles these rocks are strongly lile enriched but do not develop pronounced ta nb troughs on org normalised profiles these rocks may be derived by low degrees of partial melting of a depleted mantle wedge previously metasomatised by alkali rich fluids released from a subducting slab the elevated tiy and zry of these rocks appear to suggest that small amounts of partial melting were responsible for the genesis of these rocks the granitoid rocks record an overall progression from contributions made from the continental crust in a continental margin environment to magmas dominated by contributions from subducted crust or the overlying variably metasomatised mantle wedge during the terminal phase of arc continent collision

morphological evolution of beach cusps and associated swash circulation patterns abstract a numerical model capable of simulating the motion of water particles on beach cusp morphology under the influence of an initial velocity and gravity is presented the model indicates that the typical swash pattern on beach cusps is three dimensional with wave uprush diverging at the cusp horns resulting in concentrated backwash streams in the embayment the degree of horn divergence is an increasing function of the parameter e s 2 where e quantifies the prominence of the beach cusps s is the horizontal swash excursion length and is the cusp spacing the numerical experiments are supplemented with detailed field measurements of beach cusp morphological change the field data include three types of morphological response each characterised by a particular pattern of swash circulation that can be delineated using e s 2 when e s 2 beach cusp morphology is large and or subdued in relation to the swash length swash circulation is essentially two dimensional oscillatory flow and results in steepening of the beachface and infilling of the cusp embayments for e s 2 0 015 to 0 15 wave uprush is deflected from the cusp horns and flows into the embayments where it exits in a concentrated backwash stream horn divergent flow the ensuing swash backwash inequality reinforces cusp development and maintains existing cusp morphology when e s 2 0 15 beach cusps are small and or pronounced in relation to the length of the swash overtopping and ponding of the cusp horn takes place and swash circulation is from the embayment to the horn horn convergent flow as a result cusp horns are eroded and accretion occurs in the embayments

diapirism during regional compression the structural pattern in the chibougamau region of the archean abitibi belt quebec the major fold pattern of the chibougamau region in the archean abitibi belt quebec is the result of two fold phases the principal f1 synclines first formed as subsiding fault troughs and were shortened in ns direction and molded around volcanic centers that had been consolidated by large synvolcanic plutons they have a general easterly direction but curve locally to the ne or se n trending f1 folds are locally present in the anticlinoria and are thought to have been produced by the upwelling of basement gneiss and or synvolcanic plutons the f2 folds have rather regular easterly trends their axial plane schistosity s2 is the predominant planar fabric element basin margin faults were transformed into east trending thrust faults during the f1 and f2 folding

an extensional fault model for the early development of greenstone belts with reference to a portion of the abitibi belt ontario canada abstract detailed geological mapping of a small part of the archaean abitibi greenstone belt northwestern ontario suggests that the mafic and ultra mafic lavas were extruded in a submarine environment in which the volcanism was intimately associated with deformation of the growing volcanic pile the deformation led to the subsidence and progressive tilting of the lavas and to the development of discontinuities a review of selected areas of quaternary volcanism suggests that subsidence of the lava pile during volcanism is not unusual and that activity is frequently also associated with normal faulting in both volcanic and non volcanic extensional regimes deformation is dominated by the development of listric faults we argue that greenstone belts such as the abitibi were formed in extensional regimes and we show that a simple geometric model involving fissure volcanism and synvolcanic rotation of the lavas on listric faults accounts for the major primary features of greenstone belts and for some associated geophysical properties

subaqueous calderas in the archean abitibi greenstone belt an overview and new ideas abstract the 300 700 km abitibi greenstone belt in canada contains numerous world class archean volcanogenic massive sulfide vms deposits yet documentation of archean subaqueous calderas hosting such deposits is lacking the modern sunrise deposit in the myojin knoll caldera of the izu bonin arc shows that submarine calderas are first order sites for vms the hunter mine and normetal calderas as well as the blake river megacaldera complex are reviewed with respect to geometry regional geology physical volcanology dyke emplacement and hydrothermal carbonate alteration these subaqueous calderas are placed into the geodynamic context of the abitibi greenstone belt the abitibi belt displays a complex history of oceanic arc formation evolution collision and fragmentation with numerous types of volcanic edifices and sedimentary basins controlled by oblique subduction and mantle plumes analogue models yielding new insights into the sequence of caldera forming events and their synvolcanic fault patterns are applied to explain archean examples underpressure experiments develop a consistent sequence of fault patterns that compare favourably with natural caldera events during subsidence two sets of faults propagate from the magma chamber margin 1 early reverse outward dipping faults and 2 normal inward dipping faults the largest massive sulfide deposits form preferentially at the caldera margin based on experimental studies and detailed facies mapping the hunter mine caldera is of the segmented piecemeal variety and the normetal caldera is akin to a piston structure the blake river megacaldera contains 1 the e w trending misema 2 the nw trending new senator and 3 the ne trending noranda calderas with the latter two being nested graben type calderas consistent with oblique archean subduction the poorly documented hydrothermal carbonate alteration is discussed and a new exploration model for calderas is presented the alteration study was evaluated at three levels microprobe analysis geochemical staining of drill core and whole rock analyses there were two principal stages of alteration related to vms formation an early silicification phase developed either at depth and or at the seawater interface whereby silicification was bedding parallel in fine grained volcaniclastic deposits near the water surface a pervasive and extensive semi conformable carbonate alteration zone with a discordant focused root zone along faults evolved subsequently three distinct carbonate pairings were observed 1 proximal siderite sideroplesite fe ankerite next to the vms deposit 2 an intermediate ankerite fe dolomite zone and 3 a distal calcite dolomite zone the carbonate phase is associated with the mineralizing event transitions between carbonate species are subtle and changes are indicated by mineral assemblage overlap the silica carbonate hydrothermal alteration pattern is a replacement product and is far more widespread than has been documented this alteration can be easily confused with primary carbonates and chemical precipitation deposits e g banded iron formation if detailed volcano sedimentary facies mapping with alteration has not been conducted and if outcrop is insufficient

two stage evolution in an archean tonalite suite the taschereau stock abitibi quebec canada the taschereau stock occurs north of timmins and val d or quebec in the abitibi greenstone belt of the superior province this late archean composite pluton is composed mainly of diorite tonalite trondhjemite cut by granitic rocks gold molybdenum occurrences are associated with a zone of albite rich rocks surrounding the granitic rocks diabase dykes and shear zones postdate all rock units field and geochemical evidence suggests that the taschereau stock was emplaced diachronously trace element geochemical modelling shows that trace element abundances rare earth elements ti zr of taschereau granitic rocks are consistent with partial melting of preexisting taschereau tonalitic rocks and implies that these two rock types are not end members of a single magma that evolved through fractional crystallization

volcanic stratigraphy and structure in the hunter creek fault area rouyn noranda quebec the hunter creek fault hcf has been considered in the past as a major synvolcanic fault marked by a change in volcanic facies and separating two geologically distinct blocks within the noranda volcanic complex the north mine and the central mine sequences detailed mapping in the hcf area in conjunction with petrographic and geochemical studies has revealed that most of the volcanic units defined south of the fault zone also occur on its northern side field and drill hole correlations indicate that the rocks are cut by a n240 n250 reverse slip fault dipping approximately 70 toward the northwest major and trace element data suggest that least altered volcanic rocks are of tholeiitic affinity and that there is no calc alkaline unit in the hcf area

conditions and timing of metamorphism in the southern abitibi greenstone belt quebec regional metamorphism ranging in grade from the subgreenschist facies to the greenschist amphibolite facies transition affects all archean supracrustal rocks 2677 ma in the rouyn noranda area contact metamorphic minerals associated with the posttectonic preissac lacorne batholith 2643 ma show no evidence of a regional retrograde event accordingly the age of regional metamorphism can be bracketed between 2677 and 2643 ma three reaction isograds were mapped in subgreenschist facies metabasites dividing the low grade rocks into three metamorphic zones the pumpellyite actinolite zone the prehnite pumpellyite zone and the prehnite epidote zone in addition the pumpellyite actinolite epidote quartz bathograd corresponding to a pressure of approximately 200 mpa occurs on both sides of the porcupine destor fault low pressure regional metamorphism is also indicated both by the occurrence of an actinolite oligoclase zone and the persistence of pre regional metamorphic andalusite the coincidence

partial least squares discriminant analysis of trace element compositions of magnetite from various vms deposit subtypes application to mineral exploration abstract the petrography and mineral chemistry of magnetite from fifteen volcanogenic massive sulfide vms deposits in canada and the lasail vms deposit in oman as well as from two vms associated banded iron formations bif austin brook new brunswick canada and izok lake nunavut canada were investigated using optical microscopy electron probe micro analyzer epma and laser ablation inductively coupled plasma mass spectrometry la icp ms the method of robust estimation for compositional data robcompositions was applied to investigate geochemical censored data among thirty seven elements analyzed by epma and or la icp ms in magnetite from the studied deposits bedrock lithologies only the results for si ca zr al mg ti zn co and ni contain pls da shows that variable compositions of magnetite slightly discriminate different studied deposits bedrock lithologies the geochemical observations suggest that the variation in magnetite chemistry from different vms settings might be sourced from differences in 1 the composition and temperature of parental magmas or hydrothermal fluids 2 the composition of host bedrocks 3 the composition of co forming minerals and 4 oxygen fugacity pls da distinguishes magnetite compositions from the studied vms deposits and bifs from that of the other ore deposit types including ni cu porphyry cu mo au iron oxide copper gold iron oxide apatite and the bayan obo ree fe nb deposit magnetite from the vms settings on average contains lower concentrations of si zr al mg ti zn co and ni relative to that from the other mineral deposit types pls da of magnetite data from vms deposits and bifs of the bathurst mining camp as well as pls da of magnetite compositions from various mineral deposit types yield discrimination models for application to mineral exploration for vms deposits using indicator minerals in quaternary lithified sedimentary rocks

moho reflectivity patterns a comparison of canadian lithoprobe transects abstract the complex nature of the continental moho may be strongly influenced by the tectonic and magmatic history of the lithosphere observations of moho variability from near vertical incidence seismic reflection surveys suggest that it may be possible to correlate moho reflectivity and lithospheric evolution as an initial step in attempting to determine such correlations we qualitatively compare seismic reflection data from canadian lithoprobe transects that sample regions of exceptionally diverse age and tectonic history the seven selected datasets were collected using consistent acquisition parameters facilitating effective and meaningful comparisons among transects the moho reflections exhibit considerable variability however in virtually all recent transects a distinct moho is observed even in regions where lower crust is considered to have stabilized in the early proterozoic or archean this suggests not only that relationships between moho reflectivity and tectonics can be extended into the archean but that tectonic age is poorly correlated with moho reflectivity the qualitative comparisons do not clearly link a specific tectonic process with a distinctive reflection pattern however well defined moho reflections are associated with lower crustal deformational processes including compression transpression and extension this indicates that ductile shear and the rheologies of the lower crust moho transition and upper mantle play important roles in influencing reflective character indistinct moho reflectivity may be associated with regions that have experienced little ductile deformation or with processes that have disrupted reflective laminations however the majority of incoherent moho reflections appear related to signal degradation associated with data acquisition the study demonstrates that the qualitative characteristics of moho reflectivity are difficult to correlate with tectonic history and thus use as an interpretive tool isolating quantitative reflection characteristics of the moho from distortions imposed by the overlying crust is a challenging problem but appears necessary to further understanding of the crust mantle transition

petrology of the archean pontiac and kewagama sediments and implications for the stratigraphy of the southern abitibi belt we present an interpretation of the petrology of two archean sedimentary groups in the southeastern portion of the abitibi greenstone belt 1 the pontiac sediments which represent an assemblage of quartz rich sandstones and 2 the kewagama sediments which are quartz poor and rich in volcanic fragments the pontiac sediments display a north south decrease in bed thickness sand shale ratio and grain size suggesting derivation from a northern source relative to their present position petrographic and rare earth element ree analysis indicates that three provenances contributed to the pontiac 1 a mature craton 2 light heavy ree fractionated igneous rocks probably of tonalite trondhjemite composition and 3 ultramafic mafic volcanics in contrast to the pontiac sediments the kewagama sediments were derived from acid to mafic volcanic rocks ratio of 3 1 with minor contributions from plutonic rocks and from graded sedimentary rocks possibly the pontiac the ree data indicate a source frac

oblique archean subduction accretion and exhumation of an oceanic arc during dextral transpression southern volcanic zone abitibi subprovince canada abstract the archean abitibi subprovince divided into northern nvz and southern svz volcanic zones is characterized by the crustal scale destor porcupine manneville fault zone dpmfz which links the svz and nvz and the cadillac larder lake fault zone cllfz which separates the abitibi and pontiac subprovinces the dpmfz and cllfz represent major deformation zones that record over 60 million years of archean deformation the dextral transpressional phase in the nvz which represents incipient svz deformation occurred between 2700 and 2692 ma and is characterized by southeast se trending dextral faults e g macamic fault synorogenic flysch sedimentation is focussed at the interface between the svz nvz and pontiac abitibi subprovinces the se trending parfouru fault linking the two major trench fault systems is interpreted as a reactivated leaky transform fault and is associated with synorogenic flysch sedimentation the early monzodioritic suite of the preissac lacorne batholith was related to dpmfz thrusting ca 2692 2690 ma the change from early thrusting to transcurrent motion is recorded in the duparquet strike slip basin which formed between 2690 and 2680 ma along the dpmfz and the granada pull apart basin which evolved between 2680 and 2670 ma along the cllfz the timing of these basins indicates a diachronous evolution renewed thrusting affected the granada basin but not the northern duparquet strike slip basin so that a southward migrating deformation front is inferred late exhumation resulted in extension along both fault zones and was responsible for the juxtaposition of medium and low grade metamorphic rocks the monzogranitic suite of the lamotte pluton which occurred between 2660 and 2642 ma is linked to exhumation final dextral transpression post dating exhumation produced shearing and folding the svz of the abitibi subprovince shows the salient attributes of modern oblique orogenic collisions with alternating phases of thrusting and strike slip movement along crustal scale faults during dextral transpression

u pb geochronology of archean metasedimentary rocks in the pontiac and abitibi subprovinces quebec constraints on timing provenance and regional tectonics abstract u pb isotopic ages of single detrital zircons have been obtained from metasandstones in the northern part of the pontiac subprovince and from the cadillac kewagama and lac caste groups within the southern abitibi subprovince youngest detrital zircon ages from these sequences are 2685 2686 2687 and 2693 ma respectively the lac fourniere pluton which intrudes pontiac group metasedimentary rocks is dated at 2682 1 ma a similar age was previously measured from a pluton that cuts the kewagama group these data bracket deposition of greywacke in the pontiac and abitibi subprovinces to 2685 3 ma in contrast the youngest zircon found in a volcaniclastic unit interbedded with timiskaming group metawacke is 2673 2 ma while the age of a pluton that intrudes the timiskaming is 2672 1 ma thus the timiskaming group is distinctly younger than the other metasedimentary rocks the normalized probability distributions of detrital zircon ages from the pontiac and the abitibi metasedimentary rocks as well as of dated igneous rocks from the southern abitibi subprovince all show that their major zircon populations formed in the age range 2685 2710 ma the distributions for the abitibi metaturbidites and dated igneous rocks are very similar suggesting that metasedimentary rocks in the abitibi belt were mostly derived from the surrounding granite greenstone terrain the largest peak of the pontiac distribution is a few million years younger than that from the abitibi metasedimentary rocks but there is a correlation between major peaks and both groups show similar age ranges for older pre abitibi components 2750 2850 and 3020 3030 ma deposition of the ca 2685 ma sediments was virtually syn deformational based on previous field geochemical and isotopic studies as well as the detrital zircon age data the pontiac group may represent a foreland basin that may have developed from an accretionary prism the source of much of the sediment may have been an uplifted extinct continental arc to the north the arc was mostly eroded into the sediment basin and perhaps tectonically overridden by a southward advancing nappe complex comprised of its back arc basin now partly preserved in the southern volcanic zone of the abitibi subprovince metaturbidites exposed between metavolcanic assemblages within the greenstone belt may represent overturned unconformities at the structural base of the greenstones these were brought up by high angle faults which also localized deposition of non marine timiskaming type sediments it is tentatively suggested that pontiac type metasedimentary rocks extend most of the way beneath the abitibi subprovince the oldest part of the basin emerging as the quetico group in the western superior province

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

effects of greenschist facies metamorphism and related deformation on the mobrun massive sulfide deposit quebec canada the mobrun zn cu ag au deposit in the noranda mining camp is hosted by archean mafic and felsic submarine volcanic rocks the deposit comprises three massive sulfide complexes the main and satellite lenses near surface and the 1100 orebody at depth the rocks have been subjected to lower greenschist facies metamorphism and related deformation which resulted in changes in ore textures development of shear zones and veins systems remobilization of gold and formation of a new mineral electrum within the orebodies both mechanical and chemical processes operated to produce secondary textures and structures resulting from brittle deformation ductile deformation and annealing the specific deformation mechanisms include brittle failure and cataclastic flow dislocation glide dislocation creep and solution precipitation creep the main and satellite lenses are characterized by excellent preservation of primary sulfides deposited from and reworked by synvolcanic hydrothermal fluids these orebodies were affected to a limited degree by mechanical processes of deformation in contrast the 1100 orebody is characterized by a higher degree of development of textures and structures related to metamorphism and deformation especially those formed by chemical processes the differences may be due to the greater depth of the 1100 orebody relative to the other lenses as regional metamorphic isograds are subhorizontal and more extensive interaction between metamorphic fluids and the 1100 lens

genesis of the auriferous c quartz tourmaline vein of the siscoe mine val d or district abitibi subprovince canada structural mineralogical and fluid inclusion constraints the c quartz tourmaline vein of the siscoe deposit is a classic example of a high grade archean gold lode and averages 45 g t au locally attaining 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear vein which formed during the development of a late to post d2 reverse fault the vein is composed of alternating tourmaline and quartz rich layers with minor amounts of rutile scheelite apatite chlorite muscovite carbonate mainly calcite pyrite chalcopyrite tetradymite and native gold vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening mineral deposition and plastic and brittle deformation the vein geometry was controlled by local rheologic heterogeneities or by a west northwest east southeast compressive event the wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite the timing of gold deposition and the gold fineness in the c quartz tourmaline vein differ from those of the wall rock in the vein native gold 4 7 7 3 wt ag was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers whereas in the wall rock gold 7 9 13 2 wt ag started to precipitate just before or synchronously with pyrite consequently the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments in the wall rock gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite this reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition however sulfides are very rare in the c quartz tourmaline vein and gold deposited after the main episode of vein filling suggesting that its precipitation was not controlled by pyrite deposition the occurrence of aqueous carbonic ac fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore forming process two types of aqueous carbonic fluid inclusions were identified h2o nacl co2 bearing salinities between 4 and 7 wt nacl equiv and h2o nacl co2 ch4 bearing salinities varying between 7 and 9 wt nacl equiv both types homogenize at t 190 c based on structural textural and fluid inclusion evidence we propose that gold deposition in the c quartz tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids

geodynamic evolution of the southern abitibi and pontiac terranes evidence from geochemistry of granitoid magma series 2700 2630 ma four distinct granitic series developed during the evolution of the archean abitibi southern volcanic zone svz 1 a minor synvolcanic tonalite trondhjemite granodiorite ttg series was emplaced at 2700 ma which has extremely low k2o 1 5 wt rb 50 ppm and sr 200 ppm higher nb y and sc flat rare earth element ree patterns and negative eu anomalies 2 a voluminous tonalite granodiorite granite quartz monzonite tggm series developed syntectonically at 2695 2685 ma and displays low k2o 1 3 wt rb 10 50 ppm ba 1000 ppm and u enhanced sr depletion of ta nb and ti and strongly fractionated ree patterns la ybn 49 21 3 a late tectonic quartz syenite quartz monzonite granite smg series was emplaced from 2685 to 2675 ma and is grossly similar to the tggm but has lower cao k2o na2o and greater concentrations of rb ba th and u 4 a late tectonic to posttectonic alkali feldspar syenite alkali feldspar quartz syenite ss series was emplaced

geochemistry and origin of archean felsic metavolcanic rocks central noranda area quebec canada felsic magma petrogenesis was studied by analyzing 24 stratigraphically controlled archean andesite to rhyolite lava flows of both tholeiitic and calc alkalic affinity from the upper noranda subgroup quebec using instrumental neutron activation and x ray fluorescence techniques the lavas have moderate values of la yb n 0 9 3 8 and low values of 100 th zr 1 according to calculations following batch partial melting and rayleigh fractional crystallization models both the calc alkalic and tholeiitic felsic volcanic rocks are probably products of shallow level fractional crystallization of mafic parental magmas formed respectively by lower 7 for calc alkalic and higher 14 for tholeiitic degrees of partial melting of a primitive mantle source contribution to the magma genesis from plausible crustal materials was negligible a back arc type diapirism is geochemically suggested for the tectonic model of origin of noranda felsic magmas in conformity with geological observations felsic volca

pb isotope compositions of pyrite from the c quartz tourmaline vein of the siscoe gold deposit val d or quebec constraints on the origin and age of the gold mineralization the siscoe mine was one of the richest in the val d or mining camp producing 27 5 tonnes t of gold and 9 5 t of silver from 1929 to 1949 the c quartz tourmaline vein at the siscoe mine which is the youngest auriferous vein on the property is an excellent example of a high grade archean lode gold deposit it contains an average of 45 g t au and locally up to 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear structure that formed during the development of a late to post d2 reverse fault the vein comprises mainly alternating tourmaline and quartz rich layers and gold occurs commonly in late fractures cutting the vein the visible wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite in this study we report the pb isotope compositions of hydrothermal pyrite separates taken from the c quartz tourmaline vein and its wall rock pyrite is abundant in the wall rock where it occurs as grains with inclusions of chalcopyrite gold tetradymite pyrrhotite rutile silicates and carbonates in the vein it forms euhedra generally devoid of inclusions locally the euhedra are fractured and the fractures are filled with native gold calcite and tetradymite textural relationships suggest that pyrite precipitated synchronously with and or after gold in the wall rock but predated gold in the vein pyrite separates from the vein and its wall rock yielded similar 206pb 204pb 207pb 204pb and 208pb 204pb ratios 13 63 13 83 14 58 14 62 and 33 42 33 76 respectively which are more radiogenic than those reported for volcanic supracrustal and syntectonic plutonic rocks of the southern abitibi subprovince but are very similar to those of the late to posttectonic s type granites in the pontiac subprovince and lacorne block this suggests that the pb originated from a reservoir similar to that of the magmas which produced these granites or alternatively that it was leached from the latter the pb ratios yield a model age of 2 57 0 07 ga which is interpreted to date the cooling of the hydrothermal system that introduced and or remobilized gold into the c quartz tourmaline vein these results when considered in the context of the evolution of the southern abitibi indicate that widespread hydrothermal circulation occurred between 2 64 and 2 55 ga after the collision of the pontiac and abitibi subprovinces and was caused by dewatering and s type magmatism in the thickened unstable continental crust this long lived hydrothermal system promoted remobilization of gold from earlier veins and possibly introduction of gold from deeper zones enhancing gold resources in the siscoe deposit

metamorphism denudation and sea level in the archean and cooling of the earth abstract over long enough time scales an antagonistic balance acting between tectonics uplift and rifting and the sedimentary cycle erosion and deposition probably keeps continental surfaces close to sea level we argue that this balance constitutes the primary mechanism for keeping long term continental freeboard constant by regulating the continental thickness an important implication is that the long term uplift or down warp history of ancient continental segments provides direct information on secular changes in the isostasy of the adjacent ocean basins through time the amount of uplift can be estimated from the regional metamorphic grade the burial pressures at present exposure of ten undisturbed archean granite greenstone segments are estimated to be 1 5 0 5 kb from this we infer a maximum long term uplift and erosion of 5 2 km for stable continent since 3 0 ga implying a mean continental thickness of 46 km at the time of crustal stabilization for a constant mean ocean depth isostasy requires a palaeo oceanic crust 14 2 km thick in the middle archean alternatively if little continent existed then the oceans would have been shallower and the palaeo oceanic crust 21 2 km thick assuming plate spreading did occur in the archean potential temperatures in the shallow mantle at 3 ga were 90 30 c or 170 20 c for no continent hotter than those prevailing today these temperatures translate into time averaged cooling rates for the shallow mantle of 57 c ga 1 and are comparable to estimates of the present day cooling rate for the whole earth the low burial pressures are a strong argument against the existence of catastrophic mantle flush events in the geological past which have been predicted by some mantle convection models taken together these constraints suggest a plume related geological setting i e oceanic plateau like flood basalt for the initial stage of greenstone belt evolution

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

synthesis of the structural evolution and associated gold mineralization of the cadillac fault abitibi canada abstract the cadillac fault zone cfz is a 250 km long east trending crustal discontinuity associated with numerous world class gold deposits in the archean abitibi subprovince in quebec the fault follows the boundary between the abitibi and the pontiac subprovinces and can be divided into four segments 1 rouyn 2 joannes 3 malartic and 4 val d or each displays different attitudes and structural styles the malartic segment strikes se whereas the other segments are e trending abrupt changes in strike or smaller scale faults represent limits between the segments structural styles include four generations of foliation with numerous shear sense indicators and folding specifically related to each foliation structural features can be correlated from one segment to another but display clear variations in attitude or expression a stretching lineation signature is definitely associated with each segment in the malartic segment the predominant e trending schistosity displays an angular relationship with the contact between groups and bedding suggesting that the se trending architecture of this segment was already emplaced prior to ductile deformation the evidence suggests a model where the cfz originates from several independent structures inherited from volcanism prior to timiskaming deformation and abitibi pontiac accretion the proto cfz was then linked up during the timiskaming deposition forming high strain zones inherited structures form the frame on which each deformation event was overprinted with different responses depending on segmentation the structural styles of each segment can be integrated in a structural evolution model for the cfz the n s shortening led to the development of a dominantly e striking penetrative foliation and folding during the subsequent extensional stage the cfz was reactivated with normal kinematics during the pontiac uplift a third deformation event is represented by the local development of nw trending cleavage axial planar to the s shaped minor folds finally a late nw se shortening event produced a dextral strike slip movement with ne striking vertical foliation and development of z shaped folds on bedding surfaces and anterior fabrics linkage of the cfz had a major impact on hydrothermal activity in the southern abitibi as it channeled fluids along large scale pathways distribution of gold bearing intrusions can also be related to segmentation and inherited structures of the cfz this association is expressed by gold mineralization styles present in several longitudinal fields reflecting fault segmentation

sm nd geochemistry and u pb geochronology of the preissac and lamotte leucogranites abitibi subprovince the lacorne block in the southern volcanic zone of the abitibi subprovince is composed of interleaved metavolcanic and metasedimentary rocks that are intruded by syn to posttectonic diorites granodiorites and granites these rocks form the lacorne lamotte and preissac plutons which can be divided into an early suite of dioritic granodioritic rocks and a later suite of s type leucocratic granites with an estimated age of 2640 ma this study presents sm nd data and u pb monazite and titanite ages for the late leucocratic granites of the preissac and lamotte plutons a biotite muscovite monzogranitic phase of the lamotte pluton is dated at 2647 2 ma but similar phases of the preissac pluton are dated at 2681 2660 ma these ages extend the period of leucogranitic plutonism for this area to 40 ma and suggest that the age of collision of the abitibi and the pontiac subprovinces occurred before 2685 ma the end values for the leucogranites range from 1 to 3 and suggest an origin largely through melti

regional scale hydrothermal alteration in the central blake river group western abitibi subprovince canada implications for vms prospectivity the late archean blake river group is a thick succession of predominantly mafic volcanic rocks within the southern zone of the abitibi greenstone belt it contains a number of silicic volcanic centers of different size including the large noranda volcanic complex which is host to 17 past producing volcanogenic massive sulfide deposits the noranda complex consists of a 7 to 9 km thick succession of bimodal mafic and felsic volcanic rocks erupted during five major cycles of volcanism massive sulfide formation coincided with a period of intense magmatic activity cycle iii and the formation of the noranda cauldron hydrothermal alteration in these rocks is interpreted to reflect large scale hydrothermal fluid flow associated with rapid crustal extension and rifting of the volcanic complex the alteration includes abundant albite chlorite epidote and quartz silicification which exhibit broad stratigraphic and structural control and correlate with previously mapped whole rock oxygen isotope zonation the mine sequence volcanic rocks are characterized by abundant iron rich chlorite fe fe mg 0 5 hydrothermal amphibole ferroactinolite and coarse grained epidote of clinozoisite composition 10 wt fe2o3 lacking the clinozoisite solid solution alteration in the mine sequence volcanic rocks persists along strike well beyond the limits of the main ore deposits as far as several tens of kilometers and can be readily distinguished from greenschist facies metamorphic assemblages at a regional scale the lack of similar alteration in the pre cauldron sequences is consistent with limited 18o depletion and suggests that the early history of the volcanic complex did not support large scale high temperature fluid flow in these rocks comparisons with a much smaller barren volcanic complex in nearby ben nevis township reveal important differences in the alteration mineralogy between volcanoes of different size with implications for area selection during regional scale mineral exploration the ben nevis complex consists of a 3 to 4 km thick succession of mafic intermediate and felsic volcanic rocks centered on a small subvolcanic intrusion alteration of the volcanic rocks comprises mainly low temperature assemblages of prehnite pumpellyite magnesium rich chlorite fe fe mg 10 wt fe2o3 and calcite actinolite magnetite alteration occurs proximal to the intrusive core of the complex but the limited extent of this alteration indicates only local high temperature fluid circulation adjacent to the intrusion a distal zone of carbonate alteration is located 4 6 km from the center of the volcano although iron bearing carbonates are present locally within this zone the absence of siderite argues against a high temperature origin for this alteration these observations do not offer positive encouragement for the existence of a fossil geothermal system of sufficient size or intensity to have produced a large massive sulfide deposit

depositional gaps in abitibi greenstone belt stratigraphy a key to exploration for syngenetic mineralization models of greenstone belt development are crucial for exploration allochthonous models predict belts to be a collage of unrelated fragments whereas autochthonous models allow for prediction of syngenetic mineral deposits within specific stratigraphic intervals superior province greenstone belts consist of mainly volcanic units unconformably overlain by largely sedimentary timiskaming style assemblages and field and geochronological data indicate that the abitibi greenstone belt developed autochthonously we describe major revisions to stratigraphy of the abitibi greenstone belt and the implications of an autochthonous development of the volcanic stratigraphy for exploration for syngenetic mineralization the abitibi greenstone belt is subdivided into seven discrete volcanic stratigraphic episodes on the basis of groupings of numerous u pb zircon ages of pre 2750 2750 to 2735 2734 to 2724 2723 to 2720 2719 to 2711 2710 to 2704 and 2704 to 2695 ma we present revised lithotectonic and or stratigraphic nomenclature using these time intervals including 1 isotopic inheritance in younger episodes which indicates that the older episodes 2750 2735 and 2734 2724 ma had greater extent than is presently seen 2 dikes feeding younger volcanic episodes 2706 ma cutting older volcanic units 2734 2724 ma and 3 2710 to 2704 ma mafic to ultramafic sills intruding the 2719 to 2711 ma episode changes to the nomenclature include the identification of pre 2750 ma volcanic episode supracrustal fragments in the northern and southern abitibi greenstone belt and subdivision of the 2719 to 2711 ma 2710 to 2704 ma and 2704 to 2695 ma episodes into lower and upper parts we present the results of this lithostratigraphic subdivision as the first geochronologically constrained stratigraphic and or lithotectonic map of the abitibi greenstone belt many of the volcanic episodes are intercalated with and capped by a relatively thin sedimentary interface zone dominated by chemical sedimentary rocks stratigraphic and geochronological analysis of these zones indicates discontinuous deposition with localized gaps of 2 to 27 m y between volcanic episodes the zones consist of up to 200 m of iron formation chert breccia heterolithic debris flows of volcanic provenance sandstone and or argillite and conglomerate modeling of the time required for deposition of the volcanic units based on rates of magma production in modern arc and plume environments is on the order of 103 to 104 years whereas the time interval between basalt rhyolite cycles is 106 years the sedimentary interface zones are therefore interpreted as condensed sections zones with very low rates of sedimentation in a basinal setting or zones with negligible rates of sedimentation marked by silicification of existing rock types the sedimentary interface zones are therefore considered submarine correlative conformities disconformities or unconformities separating the equivalent of group level stratigraphic and lithotectonic units the unconformity bounded stratigraphic model provides a new regional to deposit scale interpretive model for use in exploration for syngenetic mineralization

metamorphic evolution of the late archaean cadillac tectonic zone mcwatters abitibi belt quebec archaean greenstone belts are often cut by major shear zones for example the cadillac tectonic zone ctz of the southern abitibi region in north western quebec at mcwatters the ctz contains slices of metavolcanic units bounded by corridors of highly strained and altered rocks mineral assemblages of the metabasites record the metamorphic evolution of the ctz the mcwatters metabasalts and metagabbros have similar chemistry but different mineral assemblages consisting of variable amounts of actinolite hornblende chlorite albite epidote quartz carbonates titanite biotite rutile magnetite ilmenite and sulphides the different mineral assemblages which coexist in a single tectonic slice can be divided into three types characterized by a presence of hornblende and actinolite b presence of actinolite and epidote and c absence of amphibole and epidote partial replacements indicate that these mineral assemblages are not in equilibrium the hornblende of the least altered and deformed samples of the type a assemblage is a relict of a prograde metamorphic event contemporaneous with the development of the main schistosity the prograde conditions are estimated at p 5 kbar t 475 c with low pf the more altered and deformed samples of the type c assemblage record a later retrograde metamorphic event conditions of the later event are estimated at p 4 kbar t 400 c with higher pf widespread calcite precipitation occurred during a later episode the diversity of the mineral assemblages results from permeability variations along the high strain zones of the ctz

archaean basin development and crustal extension in the northeastern yilgarn block western australia abstract supracrustal sequences in the northeastern yilgarn block western australia form two contrasting lithotectonic associations most sequences were deposited in subaqueous to subaerial environments in a basin 120 km in width the preserved succession reaches a maximum of e 10 km in thickness in the central part of the basin although there is no record of the basement on which it was deposited in the later stages of its development the basin was intruded by calc alkaline granitoids that are in part comagmatic with calc alkaline volcanic rocks in the upper part of the stratigraphic succession brittle fracture normal strike faults and conjugate cross faults open upright folds and peak low strain metamorphism were coincident with granitoid emplacement the eastern and western margins of the basin were greatly modified during a period of increased crustal extension marked initially by the generation of a mildly peralkaline volcanic plutonic suite the volcanic members of which were deposited in linear faultbounded depressions a major growth fault indicated by thin persistent units of oligomictic and polymictic conglomerate and localized intense metasomatism was probably contemporaneous with felsic volcanic activity subsequent sediments derived largely from granitic source areas were deposited in narrow grabens the youngest felsic plutonic rocks in the area include small bodies of alkali feldspar syenite many of which are found near the margins of the late sediment filled grabens tectonism at the eastern basin margin extended into the proterozoic in contrast to sequences at the centre of the basin those at its margin have undergone isoclinal folding ductile deformation and high strain metamorphism the geological framework of the northeastern yilgarn block emphasizes the importance of crustal extension in the development of younger archaean supracrustal sequences and provides valuable support for extension dependent greenstone belt evolutionary models e g groves and batt

lithofacies associations and structural evolution of the archean rio das velhas greenstone belt quadrilatero ferrifero brazil a review of the setting of gold deposits abstract the rio das velhas greenstone belt is located in the quadrilatero ferrifero region in the southern extremity of the sao francisco craton central southern part of the state of minas gerais se brazil the metavolcano sedimentary rocks of the rio das velhas supergroup in this region are subdivided into the nova lima and maquine groups the former occurs at the base of the sequence and contains the major au deposits of the region new geochronological data along with a review of geochemical data for volcanic and sedimentary rocks suggest at least two generations of greenstone belts dated at 2900 and 2780 ma seven lithofacies associations are identified from bottom to top encompassing 1 mafic ultramafic volcanic 2 volcano chemical sedimentary 3 clastic chemical sedimentary 4 volcaniclastic association with four lithofacies monomictic and polymictic breccias conglomerate graywacke graywacke sandstone graywacke argillite 5 resedimented association including three sequences of graywacke argillite in the north and eastern at greenschist facies and in the south at amphibolite metamorphic facies 6 coastal association with four lithofacies sandstone with medium to large scale cross bedding sandstone with ripple marks sandstone with herringbone cross bedding sandstone siltstone 7 non marine association with the lithofacies conglomerate sandstone coarse grained sandstone fine to medium grained sandstone four generations of structures are recognized the first and second are archean and compressional driven from nne to ssw the third is extensional and attributed to the paleoproterozoic transamazonian orogenic cycle and the fourth is compressional driven from e to w is related to the neoproterozoic brasiliano orogenic cycle gold deposits in the rio das velhas greenstone belt are structurally controlled and occur associated with hydrothermal alterations along archean thrust shear zones of the second generation of structures sedimentation occurred during four episodes cycle 1 is interpreted to have occurred between 2800 and 2780 ma based on the ages of the mafic and felsic volcanism and comprises predominantly chemical sedimentary rocks intercalated with mafic ultramafic volcanic flows it includes the volcano chemical sedimentary lithofacies association and part of the mafic ultramafic volcanic association the cycle is related to the initial extensional stage of the greenstone belt formation with the deposition of sediments contemporaneous with volcanic flows that formed the submarine mafic plains cycle 2 encompasses the clastic chemical sedimentary association and distal turbidites of the resedimented association in the eastern sector of the quadrilatero ferrifero it was deposited in the initial stages of the felsic volcanism cycle 2 includes the coastal and resedimented associations in the southern sector in advanced stages of subduction in this southern sedimentary cycle it is also possible to recognize a stable shelf environment following the felsic volcanism cycle 3 comprises sedimentary rocks of the volcaniclastic and resedimented lithofacies associations largely in the northern sector of the area the characteristics of both associations indicate a submarine fan environment transitional to non marine successions related to felsic volcanic edifices and related to the formation of island arcs cycle 4 is made up of clastic sedimentary rocks belonging to the non marine lithofacies association they are interpreted as braided plain and alluvial fan deposits in a retroarc foreland basin with the supply of debris from the previous cycles

accretion of japanese island arcs and implications for the origin of archean greenstone belts the present day region of arc arc collision between the izu bonin arc and mainland japan honshu arc in the western pacific called the izu collision zone or icz provides a useful kinematic model for the development of many archean greenstone belts the compositional assemblage thermal structure and structural style of the crust in the icz are very similar to those inferred for many archean greenstone belts in both cases thick young and flaky arc crust with a high heat flux is in collision with another crustal block the icz has resulted in the imbrication of very thick slices of crust and associated sediments juxtaposed against older accretionary prisms many archean greenstone belts show similar geological features and we suggest that the incremental imbrication of the crust was a common tectonic style in some archean convergent margins

inverse age stratification in the archaean crust of the superior province evidence for infra and subcrustal accretion from high resolution u pb zircon and monazite ages abstract new u pb mineral ages from the agawa migmatite terrain in the southern superior province canada together with published ages from neighbouring high and low grade terrains reveal a pattern of progressive younging of plutonic and metamorphic processes with depth and provide insights into the mechanisms controlling late archaean cratonisation the following ages are reported 1 about 2670 ma for a first zircon generation about 2660 ma for a second zircon generation and 2649 2 ma for monazite all from a leucocratic layer in migmatites 2 2666 2 ma for a first zircon generation 2663 1 5 ma for a second zircon generation and 2642 3 ma for titanite all from an amphibolite 3 2654 4 ma for monazite from a granodiorite and 4 2637 2 ma for monazite from a granite together with petrographic observations and chemical and morphological features of the zircons the ages indicate the occurrence of a protracted period of magmatism and metamorphism commencing with migmatisation at 2670 2665 ma followed by injection of calc alkalic melts and metasomatism at 2663 2660 ma and concluded by the intrusion of granites and pegmatites and extensive hydrothermal activity between 2660 and 2635 ma the timing and the style of plutonism and metamorphism in the agawa migmatite terrain contrast with the situation observed in the adjacent but tectonically higher level batchawana greenstone belt where major metamorphism occurred about 2675 ma ago and plutonism was concluded at 2668 ma ago on the other hand it resembles the evolution of the high grade portion of the kapuskasing zone where the orogenic activity continued until after 2630 2620 ma ago these age relationships are interpreted as reflecting downward vertical accretion of the archaean crust the lower crust experienced magmatism and metamorphism long after conclusion of these processes at shallower crustal levels the late tectonic regime produced granulites at depth transferring granitic material rich in h 2 o and heat producing elements to intermediate crustal levels these processes were probably accompanied by crustal thickening through the addition of mantle derived material

archaean metasedimentary rocks from the northwestern pontiac subprovince of the canadian shield chemical characterization weathering and modelling of the source areas abstract the northwestern pontiac subprovince is dominated by a monotonous greywacke sequence metamorphosed and deformed during the late archaean kenoran orogeny limited chemical weathering of the source rocks is suggested by 1 low values of the chemical index of alteration 2 no preferential leaching of ca sr and ba 3 well defined correlations among mgo k2o and al2o3 and 4 small variations of cr ni the source was dominantly felsic as indicated by high th sc average 0 39 la co 1 06 zr cr 0 7 zr y 11 2 and sio2 mgo 19 4 the composition of these metasedimentary rocks can be modelled by mixing of three end member components 1 tonalites trondhjemites 65 in order to explain the sio2 al2o3 th u zr sr and ba abundances na k ratios strong depletion in hree and hfse relative to lree and lile low th y and k2o and fractionated la yb 19 4 2 komatiites 5 in order to explain the high cr 220 ppm and ni 66 ppm contents and 3 tholeiitic basalts 30 accounting for the observed tio2 0 62 wt v 144 ppm and sc 17 ppm concentrations the source region may be considered as a deeply eroded greenstone belt intruded by abundant tonalite trondhjemite plutons possibly now located to the south and east in the grenville province or possibly to the north underlying the dominantly volcanic abitibi subprovince

origin and incremental evolution of brittle ductile shear zones in granitic rocks natural examples from the southern abitibi belt canada abstract metre scale shear zones developed in the mooshla granitic pluton exhibit a plethora of internal mesoand microstructures revealing their mode of nucleation growth and termination the subvertical e w brittle ductile shear zones are developed in the most isotropic part of the intrusion and consist of phyllonites characterized by mylonitic fabrics and well developed down dip mineral lineations they are exposed on a single large and flat outcrop perpendicular to the mineral lineation these brittle ductile shear zones are in close spatial association with a fracture system along which they have nucleated and propagated joints fractures and brittle faults contain abundant evidence of ductilely deformed and recrystallized minerals suggesting that they underwent ductile shearing after their formation the deformation was accompanied by the influx of a metasomatic fluid which altered original fe mg bearing minerals and plagioclase into quartz epidote chlorite sericite and carbonate that precipitated in dilatant fractures and narrow breccia zones we recognize two different types of lateral terminations i shear zones that terminate into joints and fractures that are parallel or oblique to the shear direction and ii curved segments of shear zones that connected with neoformed splay fractures at high angle to the shear direction in the latter case sharp bending and severe perturbation of the shear zone orientation occur at the shear zone termination where ductile shearing is transfered into an adjacent shear zone via dilatational fractures this demonstrates that shear zones tended to grow towards each other and coalesce to form a well developed anastomosing network we propose that narrow dilatational fractures and joints acted as paleo weakness planes along which fluid rock interaction and resulting reaction softening occurred the deformation scenario involves episodic fluid drawn into dilatant sites responsible for cyclic fluid pressure fluctuation in the system dilatancy related fracturing and reaction softening lead to deformation under brittle to plastic conditions and to shear strain localization