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

the role of granitic intrusions in the evolution of the abitibi belt canada abstract the abitibi greenstone belt canada constitutes a segment of a broad volcanic arc that experienced three distinct periods of volcanic activity peaking at 2730 2725 2718 2714 and 2705 2700 ma during all three volcanic episodes which were generated by subduction related processes mantle plume derived komatiitic volcanic rocks were also added to the composite abitibi arc plume activity resulted in local regional extension and restricted later volcanism within the belt north south shortening produced thrusts and folds followed by se trending dextral strike slip faults between 2700 and 2692 ma the principal belt wide deformation subsequent e w dextral strike slip faulting limited to the major faults continued to 2690 2680 ma destor porcupine fault and even to 2680 2670 ma cadillac larder fault extensional structures resulting from the thickened crust developed around 2660 ma granitic intrusions are associated with all the volcanic episodes and phases of deformation and have been used to constrain the age of many of these events their role in stabilizing and adding to the volcanic crust is perhaps less well appreciated large synvolcanic tonalite dominated intrusions of the tonalite trondhjemite granodiorite ttg suite are associated with the three major periods of volcanic activity of the abitibi greenstone belt and are genetically related to the volcanism these synvolcanic intrusions occur both as high level flat cauldron subsidence complexes and as batholithic masses at the base of the oceanic crust tonalite gneiss dated from 2714 to 2703 ma occurring in structural culminations is considered to be arc underplated material the gneiss crystallized at depth and in contrast to the other tonalites does not appear to have intruded volcanic rocks the gneisses also record the earliest compressional deformation numerous intrusions of island arc affinity both the ttg and monzodiorite mzd suites form stocks and small plutons emplaced during the late phases of major deformation these are more numerous than voluminous and most of the tonalite end members of the suites probably intruded deeper within the sequence retarded by the now thickened crust large tonalite intrusions dominate the uplifted deeper sections adjacent to the abitibi belt in the kapuskasing uplift and grenville parautochthon and presumably make up much of the deep crust of the abitibi belt imaged in lithoprobe seismic profiles small stocks and dyke complexes characterize the late localized strike slip deformational phase these intrusions are porphyritic syenites and related rocks including some alkaline compositions garnetiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2660 ma thus plutonic rocks are a volumetrically major overall component of the abitibi belt and were implicated in every phase of its development from early volcanic arc magmatic activity through the various stages of deformation to final stabilization as continental crust

tectonic controls on the endowment of neoarchean cratons in volcanic hosted massive sulfide deposits evidence from lead and neodymium isotopes review and analysis of initial lead isotope ratios from archean volcanic hosted massive sulfide vhms and lode gold deposits and of neodymium isotopes from igneous rocks from the geologic provinces that host these deposits identifies systematic spatial and temporal patterns both within and between the provinces the abitibi wawa subprovince of the superior province is characterized by highly juvenile lead and neodymium most other archean provinces however are characterized by more evolved isotopes although domains within them can be characterized by juvenile isotope ratios the analysis indicates that the endowment measured as the quantity of metal contained in geologic resources per unit surface area of vhms and komatiite associated nickel sulfide deposits is related to the isotopic character and therefore the tectonic history of provinces that host these deposits provinces with extensive juvenile crust have significantly higher endowment of vhms deposits possibly as a consequence of higher heat flow and extension related faults provinces with evolved crust have higher endowment of komatiite associated nickel sulfide deposits possibly because such crust provided either a source of sulfur or a stable substrate for komatiite emplacement in any case initial radiogenic isotope ratios can be useful in predicting the endowment of archean terranes for vhms and komatiite associated nickel sulfide deposits limited data suggest similar relationships may hold in younger terranes lead isotope data may provide constraints on supercraton reconstructions in the archean as lead isotope systematics appear to be highly provincial similar isotope systematics between the superior province and the eastern goldfields superterrane may indicate a linkage during the neoarchean consistent with the similar geologic and metallogenic histories of these provinces

upper mantle processes beneath the 2 7 ga abitibi belt canada a trace element perspective abstract mantle processes associated with volcanism in the abitibi belt remain poorly understood despite numerous detailed studies of its volcano sedimentary successions this paper employs log plots of nb y and zr y to distinguish the sources of well defined abitibi volcanic suites and to clarify processes in the upper mantle during greenstone belt development mafic rocks of the southern volcanic zone in the abitibi belt define two main trends on log plots of nb y versus zr y trends for the kidd munro assemblage and malartic block correspond to mixtures of strongly deleted plume asthenosphere and adakitic melts the shared trace element systematics of these volcanic successions confirm they are parts of a single authochthonous supracrustal assemblage that extended across the southern abitibi belt the younger kamiskotia assemblage and blake river group correspond to crustal extension episodes within an abitibi island arc trace element plots establish that they were derived from mantle plume related asthenosphere and not the late archean morb source upper mantle associated with island arc assemblages elsewhere in the southern superior province the data suggests widespread lateral flow of mantle plume asthenosphere occurred in the vicinity of the abitibi arc mantle mixing with adakitic melts continued between 2716 and 2698 ma the data are not consistent with wet komatiite models that generate ultramafic volcanic rocks from typical upper mantle following hydration of a mantle wedge chemical mixing trends for mafic magma sources indicate melt hybridization of the abitibi mantle wedge the trends require a slab melt origin for adakitic magmas rather than internal differentiation of underplated mafic crust comparisons with other 2 7 ga greenstone belts suggest that mantle plume interaction with archean subduction zones as proposed for the abitibi belt can be distinguished in the nb zr y systematics of associated mafic volcanic rocks

crustal geometry of the abitibi subprovince in light of three dimensional seismic reflector orientations we provide precise estimates of reflector orientations beneath the archean abitibi subprovince using two distinct approaches based on lithoprobe seismic reflection data in the first we use the dip of reflections observed on intersecting profiles to establish the three dimensional orientation of reflective structures in the second the strikes and dips of reflectors are estimated in the crooked parts of seismic reflection profiles by calculating a measure of coherency along the traveltime trajectories defined by a particular azimuth dip depth and medium velocity mid crustal reflectors define two areas with distinctive geometry reflectors beneath the southern abitibi belt are oppositely dipping and convergent at depth providing a v shape aspect to the greenstone rocks other reflectors beneath the northern abitibi belt are in general subparallel dipping at an average of 30 toward the north these north dipping reflectors are partly disrupted by a low reflectivity zone which is attributed to

pericontinental crustal growth of the southwestern abitibi subprovince canada u pb hf and nd isotope evidence the abitibi subprovince is widely regarded as a classic example of neoarchean continental crust formed in an ensimatic setting however sialic crust as old as 2 93 ga occurs in the adjacent wawa subprovince and the two subprovinces are thought to have formed a contiguous block since the initiation of abitibi magmatic activity at 2 75 ga a detailed geochemical u pb geochronological and nd hf isotope study of plutonic and volcanic rocks from the southwestern abitibi provides evidence for the involvement of older crust during magma genesis near the abitibi wawa boundary kapuskasing structural zone inherited zircons dated at ca 2 85 ga occur both in 2747 ma tonalite and 2700 ma granodiorite of the rice lake batholith part of the larger kenogamissi plutonic complex igneous zircons from this batholith display large intrasample variations in 176 hf 177 hf which we attribute to mixing of magmas derived from mantle juvenile crust and older crustal sources nd isotope data for felsic volcanic and plutonic rocks in this region indicate a dominantly juvenile source however two older felsic volcanic units 2739 and 2729 ma show nd isotope evidence of significant crustal contamination post tectonic plutonic units that were emplaced at 2676 and 2662 ma also contain inherited zircons but none are older than 2 76 ga in contrast tonalitic units dated at 2744 and 2715 ma from the round lake plutonic complex located 150 km east of the kapuskasing structural zone lack zircon inheritance and display less intrasample hf isotope variability in zircons our data suggest that the influence of older sialic crust during abitibi volcanic and plutonic activity is restricted both in space and time similar evidence for an older crustal influence occurs elsewhere in the southwestern abitibi near the kapuskasing structural zone the data collectively suggest that an approximately 75 km wide belt of abitibi crust adjacent to this structure was underlain by older wawa crust from 2 75 ga onward this crustal geometry is consistent with development of the abitibi sub province at the edge of a wawa protocraton perhaps as the result of rifting of this older continental block

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

shallow high resolution seismic imaging at the ansil mining camp in the abitibi greenstone belt volcanic rocks in and around the ansil mining camp host a large number of massive sulfide mineral deposits a high resolution seismic reflection profile was shot across the camp with the objective of mapping the contacts between the different volcanic units at which most of the ore bodies have been found numerous exploration boreholes define the geology to a depth of 1600 m and allow a precise comparison with the recorded reflections geophysical logs obtained in one deep borehole suggest that reflection coefficients between the andesite rhyolite units of the volcanic stratigraphy are around 0 05 but few corresponding reflections can be identified in the seismic data those reflections in the surface seismic profile that can be correlated with the subsurface geology originate from diorite sills at which reflection coefficients are between 0 05 and 0 11 we suggest that reflections are observed from the diorite sills because the sills were intruded as sheets some along fault planes resulting in inte

archean continental assembly in the southeastern superior province of canada between 1988 and 1993 seismic reflection and refraction surveys were acquired across the medium to high grade opatica plutonic gneiss belt the low grade abitibi greenstone belt and the pontiac metasedimentary belt all of which form part of the late archean superior province shallowly north dipping reflections define a structural style consistent with the northward underthrusting and accretion over about 30 ma of various exotic terranes against a backstop provided by the opatica belt this rapid southward growth of the archean protocraton was driven by at least one north dipping subduction zone as revealed by north dipping reflections that extend to 65 km depth in the upper mantle below the opatica belt in contrast to the mainly orthogneissic opatica and pontiac belts the midcrust of the abitibi belt comprises metasedimentary and igneous rocks plus imbricated units of unknown affinity relict midcrustal accretionary complexes of substantial size whieh are indicative of primary suture zones are interpreted near the northern and southern limits of the abitibi belt an interpreted basal decollement and significantly older ages in the north suggest that the upper crustal greenstone rocks are allochthonous evidence of large scale extension appears to be confined to the southern volcanic zone of the abitibi which developed into a half graben as the original suture zone was reactivated in extension unusually high seismic p wave velocities 7 5 8 2 kms 1 are present in the lower 8 km of the abitibi crust and they correlate well with a downward reduction in seismic reflectivity attributable to late modification of the deepest part of the crust crustal xenolith studies suggest that this process may be linked to early proterozoic magmatism

intracrustal extension of an archean orogen revealed using single grain u pb zircon geochronology the metaplutonic wawa gneiss domain in the southern superior province is part of an uplifted block that was exhumed during the proterozoic exposing the lower crustal 30 km levels of the late archean 2720 to 2670 ma abitibi wawa orogen previous structural and geophysical studies of the midcrustal wawa gneiss domain reveal subhorizontal shear zones that formed during orogen parallel extension and correspond to gently dipping seismic reflectors present at depth elsewhere in the orogen in order to bracket the age of this extensional deformation the ages of metre scale igneous bodies that crystallized before and after extension have been determined in four outcrops using single grain u pb zircon dating techniques the results constrain the time of extension to the interval between 2660 and 2645 ma and show that deformation was coeval with pegmatite emplacement partial melting of tonalite gneisses as old as 2925 35 ma and regional circa 2660 ma granulite facies metamorphism of adjacent more deeply eroded crust this extension and associated high grade metamorphism in the wawa gneiss domain occurred 10 to 25 m y after the cessation of regional compression major transcurrent fault movement and sediment deposition in the upper structural levels of the orogen the middle and lower crust was therefore mechanically decoupled from the upper crust during this period of orogen parallel extension this intracrustal style of deformation is interpreted to be the consequence of gravitational forces working on a thermally softened middle crust the detachment of lithosphere from the orogen base is the favored mechanism for generating these forces and for introducing the heat that caused synextension metamorphism cyclic construction and delamination of orogen lithosphere could account for even younger metamorphic strain events evident in nearby exposures of lower crust

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

cathodoluminescence characteristics of archean volcanogenic massive sulfide related quartz noranda ben nevis and matagami districts abitibi subprovince canada 1 the cathodoluminescence cl of quartz from ore stockwork veins and interstitial fillings between lava pillows from the 2 7 ga noranda ben nevis and matagami volcanogenic massive sulfide vms districts abitibi greenstone belt has been investigated using the hot cathode technique hc1 lm system to assess the potential of these various sample types to host primary seafloor vms related fluids trapped as inclusions in minerals with primary depositional morphologies the cl responses indicate that the various quartz types are of hydrothermal origin and are therefore a potential host for primary hydrothermal fluid inclusions most notable is a transient t 120 s blue cl characteristic of hydrothermal quartz observed in most samples cl characteristics are similar over 250 km indicating coherent nonrandom behavior furthermore in ore and stockwork material from the matagami and noranda districts respectively cl reveals primary concentric growth zoned quartz that predates pulses of sulfide deposition clear evidence that the quartz is undeformed and directly related to vms mineralization these growth zones are not apparent in transmitted light in addition ore and stockwork quartz commonly show a very unstable t 30 s yellow cl coincident with microfractures and grain boundaries defining areas affected by secondary hydrothermal activity in ore from matagami local zones of nontransient brown cl may reflect strain zones associated with the deformation and recrystallization of the massive sulfide mound and indicate that such modifications can be recognized and are minor in the investigated cases cl clearly reveals pseudo hexagonal apparently zoned structures in sulfide mineralized breccia pipe quartz from the ben nevis area these structures and their host quartz characterized by a very unstable t 20 s bright yellow cl are interpreted as recrystallized quartz that has undergone rapid growth from a strongly supersaturated solution and noncrystalline precursor the cl also clearly reveals colloform crustiform textures indicative of open space filling these textures are not visible optically

3d reconstruction of volcanic and ore forming environments of a giant vms system a case study from the kidd creek mine canada abstract the archean north rhyolite is a bimodal suite of rocks located directly northwest of but structurally separate from the kidd creek volcanogenic massive sulfide deposit which has produced 140 4 million tonnes of ore grading 2 29 wt cu 6 15 wt zn 0 22 pb and 86 2 g t ag the stratigraphy of the north rhyolite is identical to that of the kidd creek mine consisting from oldest to youngest of ultramafic flows aphyric rhyolite quartz porphyritic rhyolite and basalt flows divisible into a low tio2 basalt unit 1 55 wt with minor interflow graphitic argillites the north rhyolite is interpreted to be the folded continuation of the kidd creek mine stratigraphy similar alteration patterns in the north rhyolite and the kidd creek mine stratigraphy as well as significant copper and zinc values within the thickest portions of rhyolite domes and ridges of the north rhyolite suggests the potential for economically significant mineralization within the north rhyolite stratigraphy correlation between alteration mineralization and the rhyolite domes and ridges demonstrate the volcanic structures through which the rhyolite was extruded also acted as hydrothermal fluid pathways and were controlling structures in ore formation a 3d volcanic reconstruction showing the paleo seafloor at the time of ore formation illustrates that the rhyolite flows of the north rhyolite were emplaced into the same subsidence structure as the strata of the kidd creek mine area this 3d model generated by translating complex geological data acquired through core logging and underground mapping indicates the volcanic setting of and ore forming environment at kidd creek is much larger than previously thought and provides new spatial data that can be used in exploration targeting

seismic imaging of the geologic framework and structures related to volcanogenic massive sulfide deposits in the archean rouyn noranda district quebec canada two seismic reflection profiles acquired by xstrata canada in the noranda mining camp were reprocessed and interpreted with the objective of providing key information on the geologic contacts and structures associated with volcanogenic massive sulfide vms deposits at depth the amulet and ribago seismic profiles run approximately from east to west and cross the volcanic rocks of the noranda formation which host most of the ore deposits in this camp the seismic data interpretation relies strongly on a detailed three dimensional geologic model built from an extensive number of exploration boreholes available in this area and is further supported by physical rock property measurements from in situ borehole logging data some reflections observed on the amulet and ribago seismic profiles correlate with rhyolite andesite or silicified andesite andesite contacts that host the prospective exhalites of the noranda formation in particular the silicified andesite andesite contact hosting the c contact exhalite is imaged clearly down to a vertical depth of 1 100 m along the ribago profile the processing sequence included dip moveout dmo corrections and poststack migration the seismic data reprocessing allowed the identification of two diffractions that correlate with known sulfide bodies intersected in boreholes located close to the ribago profile one of these diffractions at approximately 1 200 m depth coincides with the main massive sulfide intersection of the subeconomical ribago orebody diorites cause several reflections observed within the flavrian pluton some diorite units also have sufficient acoustic impedance contrast to produce strong reflections when juxtaposed against volcanic rocks however such reflections do not predominate in the amulet and ribago profiles possibly because reflective diorite units are not so significant or have limited lateral continuity in this part of the noranda formation the reconciliation of the detailed three dimensional geologic model and two dimensional seismic data was not necessarily a straightforward task a significant complication results from the inherent limitations of two dimensional seismic imaging techniques in a complex three dimensional geologic environment nevertheless results presented in this paper indicate that seismic methods can image prospective contacts and deep seated massive sulfide mineralization and can be a valuable exploration tool in the noranda mining camp

descriptive oceanography during the frontal air sea interaction experiment medium to large scale variability medium and large scale oceanographic variability in the sargasso sea is examined during the frontal air sea interaction experiment fasinex focusing primarily on processes that influence the formation of subtropical fronts from fall to spring the mean meridional gradient of meridional ekman transport in the subtropical convergence zone stcz enhances the meridional sea surface temperature ts gradients between 26 o and 32on in the presence of this enhanced mean gradient baroclinic eddies with zonal wavelengths of 800 km and periods of 200 days exert the dominant influence on the formation of subtropical fronts at medium and large scales these eddies generate westward propagating ts anomaly features with the same dominant wavelengths and periods they are confined between 26 o and 32on and have amplitudes that occasionally exceed ioc ts fronts tend to be found within bands 200 km wide that roughly follow the periphery of these anomaly features deformation in the horizontal eddy current field is primarily responsible for the existence of these frontal bands the migration of the strong front originally bracketed by the fasinex moored array was related to the westward propagation of the larger scale eddy anomaly frontal band pattern the moored array was located within a warm anomaly feature during most of the experiment which produced exceptionally warm conditions in the upper ocean these anomalies are confined between 26 o and 32on not only because the relatively large seasonal mean tsy there allows horizontal eddy currents to force strong anomalies but also because the baroclinic eddies with wavelengths of 800 km and periods of 200 days are confined to the stcz large meridional variability exists in many properties of the eddy field much of which can be traced to the influence of the sargasso sea mean current field on eddy variability

upb zircon ages for the missanabie renabie area and their relation to the rest of the michipicoten greenstone belt superior province ontario canada abstract the missanabie renabie area is the eastern portion of the michipicoten greenstone belt ten new u pb zircon ages confirm and extend the chronostratigraphy established previously for the michipicoten and the adjoining mishibishu and gamitagama greenstone belts in northern ontario the michipicoten belt evolved between 2661 and 2889 ma the igneous activity was episodic the bimodal cyclic volcanism occurred at 2700 2750 2900 ma extensive plutonic activity occurred at 2670 ma and synchronously with the three volcanic cycles the pattern of ages in the michipicoten belt is significantly different from that in the abitibi belt the felsic volcanism in this area is dated by the renabie crystal tuff at 2740 ma and by the cradle lakes porphyry at 2736 ma these ages are typical of cycle 2 volcanism in the michipicoten belt a deformed quartz feldspar porphyry at conboy lake has an age of 2675 ma which correlates with a well documented regional plutonic event it is possible that this rock unit is synvolcanic with a hitherto unrecognized volcanic episode in this belt the conboy lake tonalite a pluton in the wabatongushi lake granitoid complex which lies to the north of the supracrustal rocks has an age of 2688 ma the nudulama tonalite of the missanaibi lake batholith to the east of the supracrustal rocks has an age of 2741 ma the southern flank of the greenstone belt is bordered by the ogasiwi lake granitoid complex the hand lake tonalite in this complex has an age of 2723 ma four plutons internal to the belt are dated as follows ruby lake stock 2661 ma lochalsh bay stock 2670 ma ash lake pluton 2679 ma and rennie lake stock 2668 ma the four internal plutons dated here together with previously dated eight similar post tectonic to syntectonic plutons were emplaced between 2661 and 2679 ma along the northern margin of the greenstone belt most of the deformation in the supracrustal rocks developed between 2661 and 2741 ma but a period of late deformation related to the gold mineralization can be bracketed more precisely between 2661 and 2679 ma

composite synvolcanic intrusions associated with precambrian vms related hydrothermal systems large subvolcanic intrusions are recognized within most precambrian vms camps of these 80 are quartz diorite tonalite trondhjemite composite intrusions the vms camps spatially associated with composite intrusions account for 90 of the aggregate sulfide tonnage of all the precambrian intrusion related vms camps these low alumina low k and high na composite intrusions contain early phases of quartz diorite and tonalite followed by more voluminous trondhjemite they have a high proportion of high silica 74 sio2 trondhjemite which is compositionally similar to the vms hosting rhyolites within the volcanic host rock successions the quartz diorite and possibly tonalite phases follow tholeiitic fractionation trends whereas the trondhjemites fall within the composition field for primitive crustal melts these transitional m i type primitive intrusive suites are associated with extensional regimes within oceanic arc environments subvolcanic composite intrusions related to the archean sturgeon lake and noranda and paleoproterozoic snow lake vms camps range in volume from 300 to 1 000 km3 three have a sill morphology with strike lengths between 15 and 22 km and an average thickness between 1 500 and 2 000 m the fourth has a gross stock like shape the vms deposits are principally restricted to the volcanic strata above the strike length of the intrusions as are areally extensive thin exhalite units the composite intrusions contain numerous internal phases which are commonly clustered within certain parts of the composite intrusion these clusters underlie eruptive centers surrounded by areas of hydrothermal alteration and which contain most of the vms deposits early quartz diorite and tonalite phases appear to have intruded in rapid succession evidence includes gradational contacts magma mixing and disequilibrium textures they appear to have been emplaced as sill dike swarms these early phases are present as pendants and xenoliths within later trondhjemite phases the trondhjemite phases contain numerous internal contacts indicating emplacement as composite sills common structural features of the composite intrusions include early xenolith phases abundant small comagmatic dikes fractures and veins and in places columnar jointing internal phases may differ greatly in texture from fine to coarse grained aphyric and granophyric through seriate to porphyritic mineralogical and isotopic evidence indicates that early phases of each composite intrusion are affected by pervasive to fracture controlled high temperature 350 450 c alteration reflecting seawater rock interaction trondhjemite phases contain hydrothermal magmatic alteration assemblages within miarolitic cavities hydrothermal breccias and veins this hydrothermal magmatic alteration may in part be inherited from previously altered wall rocks two of the four intrusions are host to cu mo rich intrusive breccias and porphyry type mineralization which formed as much as 14 ma after the main subvolcanic magmatic activity the recognition of these precambrian subvolcanic composite intrusions is important for greenfields vms exploration as they define the location of thermal corridors within extensional oceanic arc regimes which have the greatest potential for significant vms mineralization the vms mineralization may occur for 2 000 m above the intrusions in some cases vms mineralization has been truncated or enveloped by late trondhjemite phases of the composite intrusions evidence that much of the trondhjemitic magmatism postdates the principal vms activity is a critical factor when developing heat and fluid flow models for these subseafloor magmatic hydrothermal systems

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

u pb geochronology of the eastern abitibi subprovince part 2 noranda kirkland lake area u pb zircon ages for 15 volcanic and plutonic units in the noranda and kirkland lake areas help constrain the history of volcanism plutonism sedimentation and deformation in the south central part of the abitibi belt volcanism occurred over an interval of at least 50 ma beginning with the deposition of the volcanic and volcaniclastic units within the pacaud structural complex at 2747 ma following a period of apparent quiescence magmatism resumed at 2730 2725 ma with the eruption of volcanic rocks in the normetal and lac abitibi area from 2715 until about 2698 ma volcanism occurred sporadically throughout much of the area culminating in the eruption of the blake river group from 2703 to 2698 ma several large intrusive bodies yield ages that indicate that they are plutonic equivalents of the blake river group plutons that are considered to have been emplaced during the kenoran orogeny give ages that are only slightly younger than the youngest volcanic units of the blake river group emphasizing

petrology and u pb geochronology of footwall porphyritic rhyolites from the wolverine volcanogenic massive sulfide deposit yukon canada implications for the genesis of massive sulfide deposits in continental margin environments porphyritic rhyolite sills form an important component of the footwall of the wolverine volcanogenic massive sulfide vms deposit yukon canada and occur proximal to mineralization in the immediate deposit area wolverine lynx zone and at similar stratigraphic levels along strike fisher puck and sable zones porphyritic rhyolites are of two types an older quartz feldspar porphyritic qfp rhyolite suite and a younger feldspar porphyritic fp suite both the qfp and fp suites of intrusions are semiconcordant suggesting a silllike morphology and are altered and crosscut by veinlet mineralization suggesting that they are pre to synmineralization the margins of qfp suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks whereas the fp suite of intrusions shows well developed chilled margins indicative of emplacement into fully solidified sedimentary rock these features suggest that the qfp suite of intrusions represents an older phase of rhyolitic magmatism whereas the fp suite represents a younger event this is supported by u pb zircon ages which indicate a 352 4 1 5 ma emplacement age for the fp suite and a 347 to 346 ma emplacement for the fp suite two ages at 347 8 1 3 and 346 0 2 2 ma both suites of porphyries have inherited proterozoic zircon and have ratios of la smucn 1 and nb thucn 1 ucn upper continental crust normalized indicating derivation from and or extensive interaction with ancient upper continental crustal materials the fp suite however has elevated high field strength element hfse and rare earth element ree contents high zircon saturation temperatures and higher nb ta ratios and lower ti sc ratios than the qfp suite these features are interpreted to reflect that the fp suite of magmas was hotter 900 c melts with a greater mantle component in their genesis both suites however are interpreted to have formed due to basaltic upwelling crustal melting and crust mantle mixing during ensialic back arc basin activity the presence of mantle heat within the wolverine basin from 352 to 347 to 346 ma a minimum of 5 m y suggests that sustained mantle heat flow was critical to the genesis of the wolverine porphyries it is also suggested that this sustained mantle heat was responsible for the wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin type vms environments e g bathurst iberian pyrite belt the wolverine porphyries are among the most hfse and ree enriched felsic rocks associated with vms mineralization globally the high hfse and ree concentrations in these rocks are interpreted to be due to

fluid temperature and salinity characteristics of the matagami volcanogenic massive sulfide district quebec fluid inclusions hosted within various lithologic units of the 40 million metric ton mt matagami district abitibi greenstone belt preserve samples of archean volcanogenic massive sulfide vms and postvolcanogenic massive sulfide hydrothermal fluid microthermometric measurements on ore hosted primary two phase liquid vapor inclusions from matagami s south limb deposits indicate that the vms hydrothermal fluid was highly saline 16 2 4 7 wt nacl cacl2 equiv 1 n 230 and of moderate temperature trapping temperature tt 208 32 c 1 n 230 a fluid with these characteristics is capable of transporting 5 10 4 m 30 ppm zn as zncl3 and zncl4 2 chloride complexes however the low temperature of this fluid precluded efficient cu transport 3 ppm which may in part explain the relatively cu poor nature of the matagami deposits calculated densities of this ore fluid as high as 1 10 g cm3 are consistent with a bottom hugging brine model however a subset of the data indicate a fluid less dense than ambient seawater suggesting that buoyant hydrothermal plumes were also present a microthermometrically determined high cacl2 content xnacl 0 55 molar na ca 2 3 1 for the vms ore hosted primary fluid is consistent with an archean hydrothermal fluid more ca rich than modern day seawater quartz epidote veins located in the hydrothermal cracking zone of the bell river complex host primary liquid vapor halite inclusions these inclusions are interpreted to be samples of the deep seated equivalent to the vms ore hosted hydrothermal fluid described above microthermometry indicates that these inclusions trapped a high temperature brine tt 373 44 c 1 n 92 38 2 1 9 wt nacl equiv 1 n 92 we interpret this brine to be a phase separated product of modified model seawater 3 2 wt nacl an exsolved magmatic fluid or a combination thereof deep within the hydrothermal system at 650 to 670 c and a near lithostatic pressure of 90 mpa phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone the occurrence of high temperature brine overlain by lower temperature salinity fluid suggests a two cell convection model consistent with metal mass balance calculations for the south limb the high salinity of the ore hosted fluid inclusions indicates two possibilities 1 a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection 2 archean seawater itself was very saline and of variable salinity with the cooling of the bell river complex lower temperature fluids dominantly of seawater origin circulated deep within the hydrothermal system modified by water rock interaction yet not phase separated these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid vapor fluid inclusions homogenization temperature th 242 17 c 9 1 1 6 wt nacl equiv 1 n 14 hosted within the bell river complex quartz epidote vein material post and or waning stage vms hydrothermal activity is evident from the presence of quartz epidote veins crosscutting wabassee group hanging wall rocks microthermometry on quartz hosted primary liquid vapor fluid inclusions suggests that this activity occurred at relatively low temperatures th 76 177 c n 212 over a wide range of salinity 6 0 32 4 wt nacl cacl2 equiv n 212 and with a high apparent cacl2 content xnacl 0 06 these fluid inclusion data illustrate the importance of subsea floor chemical and physical processes directly related to metal transport and deposition in vms systems in particular phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits

timing and characteristics of the archean subaqueous blake river megacaldera complex abitibi greenstone belt canada abstract the archean blake river group of the abitibi greenstone belt represents a megacaldera complex which evolved over 8 11 m y from approximately 2704 to 2696 ma the early misema caldera developed from a series of amalgamated shield volcano complexes identified by the remnants of mafic dyke and sill systems these remnants are found in the jevis clericy montsabrais renault clifford tannahill and colnet regions where summit calderas are delineated by circular ring dyke structures the secondary 330 trending new senator caldera formed within the envelope of the misema caldera and exhibits a box work graben type structure finally the felsic dominated 070 striking noranda caldera well known for its vms endowment represents the final collapse of the megacaldera complex deformation has overprinted the calderas but structural patterns can be used in order to reconstruct the pre existing volcanic architecture structures such as mafic ring dyke complexes and rhyolitic dome flow complexes have nucleated fold geometries and synvolcanic fractures were reactivated within zones of ductile deformation precise u pb geochronological analyses were conducted in selected areas with specific emphasis on the misema and new senator calderas felsic volcanism progressed throughout the evolution of the megacaldera complex and intermediate to felsic units have been dated to limit this evolution the misema caldera formed between 2704 and 2702 ma via the amalgamation of shield volcanoes that were probably active prior to 2704 ma the new senator caldera was generated between 2702 and 2700 ma during paroxysmal felsic volcanism followed by the collapse of the noranda caldera culminating between 2700 2696 ma the misema caldera was generated by a gravitational stress field consistent with the formation of ring and radial dyke architecture whereas the se trending new senator caldera is more compatible with a sw trending principal compression direction related to oblique convergence in the abitibi belt the noranda caldera is interpreted as a ne rift structure that formed in the final stages of megacaldera evolution

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

an archean oceanic felsic dyke swarm in a nascent arc the hunter mine group abitibi greenstone belt canada abstract the 2730 ma old hunter mine group hmg a dominantly felsic subaqueous volcanic sequence was formed during early arc construction in the abitibi greenstone belt quebec canada the western part of the hmg contains a felsic dyke swarm up to 1 5 km wide and traceable up section for 2 5 km five distinct generations were identified 1 aphanitic to feldspar phyric dykes 2 quartz feldspar phyric dykes with nd values suggest that an older sialic substrate was not involved in their genesis we infer that the felsic rocks were generated by melting of mafic oceanic crust the swarm was emplaced during nascent oceanic island arc development and was related to rifting of the arc the conformably overlying morb like basalts and basaltic komatiites of the stoughton roquemaure group used the same conduits and further indicate splitting of the arc hmg and associated parts of the abitibi greenstone belts bear a strong resemblance to modern rifted intraoceanic arcs of the western pacific

petrogenesis of the neoarchean granitoids and crustal oxidation states in the western shandong province north china craton abstract na rich and k rich granitoid rocks contain geological signatures that are crucial for understanding the global evolution and oxidation states of the archean crust here we present an integrated investigation of the lithology geochronology and geochemistry of neoarchean plutonic granitoid gneisses and supracrustal rocks from xintai to feixian within the western shandong province wsp granite greenstone belt of the north china craton ncc six main lithological assemblages are identified within the wsp granite greenstone belt 1 the 2 69 2 68 ga tonalite trondhjemite granodiorite ttg gneisses which were derived from the partial melting of low k mafic rocks at lower crust have low mgo contents 0 68 2 11 wt and high la yb n ratios 16 7 92 2 the 2 53 ga granodioritic gneisses exhibit a low mgo content 0 87 1 91 wt but high k2o content 3 06 4 66 wt their magma originated from the partial melting of high k mafic rocks 3 the 2 54 2 49 ga monzogranitic gneisses show the highest sio2 content 71 67 75 26 wt and their magma was formed from the partial melting of metagreywackes 4 the 2 56 2 52 ga ttg gneisses also have low mgo content 1 42 1 69 wt and relatively low la yb n ratios 17 1 23 their magma was produced by the partial melting of low k mafic rocks at lower crustal levels 5 the 2 53 2 52 ga quartz syenites have the highest alkali content 9 66 11 23 wt but relatively low mgo content 0 71 1 50 wt and their magma was generated by the partial melting of juvenile basaltic rocks associated with recycled sediments 6 an 2 73 ga xenolith of plagioclase amphibolite is characterized by high positive ehf t2 values from 5 2 to 8 8 and fractionated chondrite normalized ree patterns indicating it may have been derived from a depleted mantle source enriched either by the addition of melt or by fluid metasomatism the oxygen fugacity values of the crust derived neoarchean granitoid magmas are calculated based on a zircon trace element oxybarometer the results show that the log f o2 values for the 2 7 ga ttg gneisses that range from fmq 7 4 to fmq 5 5 with an average value of fmq 0 5 whereas the various 2 5 ga granitoid gneisses have log f o2 values ranging from fmq 4 3 to fmq 8 9 with an average value of fmq 3 3 in conjunction with mineralogical results we consider that the 2 7 ga continental crust was in a relatively more reduced state than the 2 5 ga continental crust in the wsp which may be associated with differences in their source regions and compositions

an archean megacaldera complex the blake river group abitibi greenstone belt abstract the archean blake river group of the southern abitibi greenstone belt is defined as a subaqueous megacaldera compelling evidence include 1 a radial and concentric organization of synvolcanic mafic to intermediate dykes 2 an overall dome geometry defined by the volcanic strata 3 a peripheral distribution of subaqueous volcaniclastic units 4 a zonal distribution of carbonate alteration and 5 a distinct annular synvolcanic inner and outer ring fault pattern three caldera forming events have been identified 1 the early misema caldera 2 the new senator caldera and 3 the well known noranda caldera the multi vent brg mafic volcanic complex developed on a monotonous sequence of tholeiitic basalts forming a submarine plain and experienced a first major collapse that created the giant misema caldera 80 km in diameter an endogenic dyke swarm intruded the synvolcanic fractures and an underlying magma chamber developed major volcaniclastic units were generated by local volcanic centers and summit calderas formed along the outer and inner ring faults this fault system was used as a conduit for co2 rich hydrothermal activity renewed volcanic activity was associated with a resurgent central dome inside the misema caldera this second collapse event created the 35 km by 14 km nw trending new senator caldera this caldera was produced by multi step sagging after the underlying magmatic chamber migrated to the se and formed the flavrian powel plutons the final collapse resulted in the formation of the noranda caldera that generated a well developed 070 trending fracture pattern associated with several vms deposits the multiple caldera setting provides an effective model to explain the presence of vms mineralization along the synvolcanic fractures associated with the three collapse episodes however the importance of the new senator caldera fracture pattern is emphasized because of its role in the positioning of the giant horne mine deposit

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

bimodal cyclical archean basalts and rhyolites from the michipicoten wawa greenstone belt ontario geochemical evidence for magma contributions from the asthenospheric mantle and ancient continental lithosphere near the southern margin of the superior province abstract the petrographic and geochemical variations in the middle to late archean volcanic rocks of the michipicoten wawa greenstone belt mgb ontario record bimodal basalt rhyolite magmatic activity at approximately 2 9 2 75 and 2 7 ga along what has previously been interpreted to be a southward dipping convergent plate margin which is underlain by continental crust cycle 1 komatiitic basalts have elevated mg number 0 51 0 83 compared to cycle 2 and 3 tholeiites mg number 0 44 0 56 and dominantly flat ree patterns with negative ta nb anomalies la ta 4 35 cycles 2 and 3 are more evolved than cycle 1 but all three cycles have broadly similar ratios of the incompatible trace elements compared to modern oceanic basalts the basaltic members of cycles 1 3 have a moderate ta nb anomaly la ta 4 46 and low la yb n 1 18 1 56 low la ta is a feature of oceanic lavas whereas high la ta is a feature of magmas which contain contributions from a crustal reservoir the mafic volcanic rocks are believed to be derived from asthenospheric mantle with assimilation of older continental crust the felsic volcanic rocks have elevated th nb 0 7 1 6 and ce yb n 10 2 16 4 and are enriched in light rare earth elements lree and large ion lithophile elements lile relative to rhyolites from other superior province greenstone belts we suggest that the rhyolites record the geochemical traits of their sources and these could be interpreted to be moderate to high degree melts of tonalitic upper crust rather than fractionated magmas or partial melts derived from the underlying mafic volcanics this contrasts with models for many other rhyolites in superior province greenstone belts such as the southern volcanic zone of the abitibi greenstone belt where variations require a more mafic crustal source to generate the fractionated low la sm gd yb and high zr th rhyolites believed to be locally associated with volcanogenic massive sulphide deposits the absence of significant volumes of compositionally intermediate volcanic rocks and the evidence for crustal contributions to mgb cyclical magmatism raises the possibility that these are archean analogues of the phanerozoic flood basalts which straddle the margins of continental shield areas such as the java ontong sequence and are not relics of either archean oceanic crust or subduction related volcanics

magmatic and tectonic emplacement of the pukaskwa batholith superior province ontario canadathis article is one of a series of papers published in this special issue on the theme of geochronology in honour of tom krogh the neoarchean pukaskwa batholith consists of pre syn and post tectonic phases emplaced over an interval of 50 million years pre tectonic phases are broadly synvolcanic and have a high al tonalite trondhjemite granodiorite ttg affinity interpreted to reflect derivation by partial melting of basaltic crust at lower crustal or upper mantle depths minor syn tectonic phases slightly post date volcanism and have geochemical characteristics suggesting some involvement or interaction with an ultramafic mantle source component magmatic emplacement of pre and syn tectonic phases occurred in the midcrust at paleopressures of 550 600 mpa and these components of the batholith are thought to be representative of the midcrust underlying greenstone belts during their development subsequent to emplacement of the syntectonic phases and likely at approximately 2680 ma the pukaskwa batholith was uplifted as a structural dome relative to flanking greenstone belts synchronously with ongoing regional sinistral t

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

nd isotope evolution of archaean plutonic rocks in southeastern superior province the opatica plutonic belt abitibi greenstone belt and pontiac subprovince represent a major proportion of the southeastern superior province which was formed and accreted rapidly between approximately 2 9 and 2 8 ga plutons in these belts are grouped into four types i trondhjemite tonalite granodiorite ttg suite 2 82 2 69 ga ii monzodiorite mzd suite 2 697 2 669 ga iii late alkaline granitoid alk suite 2 68 2 67 ga and iv anatectic granite and monzonite ana suite 2 69 2 64 ga the four suites are represented in all belts and show similar petrography and geochemistry in terms of nd isotope composition the ttg mzd and alk suites are typical of destructive plate margin magmatism and have 1 4 initial end 3 7 values which are very similar to that of the abitibi mantle end 2 5 the lower values for the ana suites end 0 1 to 2 4 result from recycling of crustal components in the opatica belt the ana granitoids fall on the nd isotope evolution curve defined b

u pb geochronology of the opatica tonalite gneiss belt and its relationship to the abitibi greenstone belt superior province quebec u pb ages for zircon monazite and titanite from samples collected from two transects across the opatica tonalite gneiss belt in the superior province of quebec indicate that the belt contains rocks that are significantly older than those in the adjacent low grade northern abitibi belt tonalite and tonalite gneiss which make up most of the belt formed over an interval of 100 ma from pre 2800 to 2702 ma five samples have ages of ca 2807 13 2773 23 and 2702 3 ma zircon growth at 2718 2721 ma in the two oldest samples may record an early period of high grade metamorphism in the belt hornblende diorite monzodiorite and tonalite plutons were intruded at 2693 2696 ma particularly along the southern boundary with the abitibi belt these include the canet pluton at ma the lac ouescapis pluton at 2693 2 ma and the barlow pluton at 2696 3 ma pink granite plutons and dykes are the youngest intrusive rocks in the belt three samples have yielded zircon ages of 2690 2 and 2686 4 ma

geochemical and u pb zircon geochronological constraints on the development of a late archean greenstone belt at birch lake superior province canada abstract this paper presents new high precision u pb zircon ages that place constraints on the timing of magmatic activity volcanic and plutonic deformation and gold mineralization within the late archean birch confederation lakes greenstone belt within the uchi subprovince of the superior province geochemical data including sm nd isotopic analyses place additional constraints on the nature of the magmatic activity three volcanic rocks from the birch lake area have similar ages 2742 2 1 8 1 7 2739 1 2 8 and 2741 7 2 3 2 2 ma and together with previously published data suggest that a major proportion of the volcanic rocks of the greenstone belt were erupted in a narrow time interval from 2742 to 2735 ma older volcanic rocks recognized to the south of the birch lake area are either absent or of limited extent plutonic rocks include the pre to syn tectonic horseshoe island stock 2725 0 2 5 ma the syn tectonic mainprize lake batholith 2724 3 1 8 1 5 ma and the late to post tectonic shabumeni lake stock 2710 8 1 9 1 3 ma and okanse lake pluton 2700 8 1 9 1 4 ma these ages and structural relationships indicate that the development of a d2 regional penetrative fabric at least in part post dates the emplacement of the horseshoe island stock this is consistent with the interpretation that d2 deformation developed synchronously with and perhaps in response to the emplacement of the mainprize lake batholith an earlier d1 penetrative deformational event as well as possible large scale subhorizontal structures d0 developed subsequent to the deposition of the youngest volcanic rocks and prior to d2 plutons are younger than and unrelated to volcanism in contrast to volcanic rocks plutons show no indication of having interacted with older felsic crustal rocks and are interpreted to be derived from partial melting of metasomatized mantle and subducted tholeiitic basalt the okanse lake pluton has affinities with the sanukitoid suite and is closely comparable in age to similar plutons over a wide area it is proposed that a 2700 ma sanukitoid magmatic event may reflect melting of previously metasomatized mantle in response to thermal re equilibration associated with the transition from an earlier subduction dominated regime to a later post collisional stage culminating in the stabilization of the northern superior province

evolution of the archean abitibi greenstone belt and adjacent terranes new insights from geochronology geochemistry structure and facies analysis

constraints on the geodynamic evolution of the southern superior province u pb la icp ms analysis of detrital zircon in successor basins of the archean abitibi and pontiac subprovinces of ontario and quebec canada abstract to investigate the tectonic development of the southern superior province new u pb laser ablation inductively coupled mass spectrometry analysis of detrital zircon was conducted on graywacke samples from successor basins of the abitibi and pontiac subprovinces the three successor basin successions investigated are the 2690 2685 ma porcupine assemblage and the 2679 2669 ma timiskaming assemblage of the abitibi subprovince and the 2682 ma sedimentary rocks of the pontiac subprovince predominately neoarchean zircon grains 80 95 of the total zircon population reflect local sources in the abitibi and pontiac subprovinces the successor basins were deposited within 100 ma of the majority of the detrital zircon grains primary crystallization ages which is consistent with patterns observed at modern convergent margin settings porcupine assemblage samples contain 5 mesoarchean zircon while pontiac subprovince and timiskaming assemblage samples contain 18 and 13 respectively no local mesoarchean sources exist in the abitibi or pontiac subprovinces suggesting input from an adjacent hinterland the higher proportion of mesoarchean zircon in the timiskaming assemblage relative to the porcupine assemblage suggests that detritus from the hinterland was more prevalent during the later stages of collision probably as a result of progressive uplift and denudation of the hinterland the high proportion of mesoarchean zircon in the pontiac subprovince sedimentary rocks may reflect an additional mesoarchean source comparison to previously published geochronology data for the superior province indicates that the hinterland was to the northwest and is in part composed of the winnipeg river marmion and opatica subprovinces patterns of contractional deformation successor basin formation and felsic plutonism progressively young to the south in the southern superior province and are interpreted in terms of accretionary processes detrital transport across subprovince or terrane boundaries progressive hinterland emergence and foreland directed propagation of the deformational front recorded by successor basins is similar to that observed in modern style collisional or accretionary orogens

pb isotopic geochemistry of granitoids and gneisses from the late archean pontiac and abitibi subprovinces of canada abstract initial pb isotopic compositions were determined on k feldspars for thirty granitoid plutons and gneiss complexes in the abitibi and the pontiac subprovinces of canada the gneisses cover a wide spectrum of initial pb isotopic compositions and undoubtedly demonstrate that very radiogenic reservoirs were present in the pontiac subprovince and along its southern boundary with the grenville province in the late archean the gra nitoid rocks also define a large spectrum covering 0 3 unit of the 207 pb 204 pb ratio ranging from compositions typical of mafic mantle melts to those of sedimentary banded iron formations the older intrusives emplaced between 2 72 and 2 68 ga were principally derived from juvenile mantle sources they may have been formed in a volcanic arc environment but they still frequently record the signature of crustal precursors which may have been represented by subducted sediments or by older assimilated crustal components a sharp change in the provenance of the granitoid magmas occurred close to 2 68 ga an age that also corresponds to an episode of alkaline activity and fault related sedimentation for the following 40 ma magmatism chiefly resulted from melting of crustal precursors in a thickened crustal lithosphere the pb isotopic compositions of these crustal melts demonstrate that the basement of the pontiac subprovince contains a major component of rocks older than 3 0 ga in comparison the abitibi subprovince may not contain basement components older than 2 9 ga or else they represent a negligible fraction of the crust this supports the concept that the abitibi and pontiac subprovinces formed as discrete crustal segments tectonically juxtaposed finally the grenville front tectonic zone was metamorphosed and partially melted in the late archean and is probably the deep crustal equivalent of the pontiac subprovince

in situ hydroclastic fragmentation of subaqueous ponded lavas new senator caldera abitibi greenstone belt quebec canada abstract subaqueous ponded lavas have been recognized within the structure of the new senator caldera in the blake river group of the abitibi greenstone belt quebec canada sub vertical to near vertical dips of extrusive volcanic facies within the southern sector of the caldera permit the identification of the internal architecture of an archean subaqueous volcanic complex detailed facies analyses of the dominantly mafic sequences have shown two localities with atypical primary volcanic structures and facies inconsistent with those observed at modern subaqueous mafic seamounts combined with the use of facies specific geochemical analyses a coherent model for the two localities accounting for all observed volcanic structures and features has been developed mapping completed at a scale of 1 100 at localities 1 and 2 reveals a series of sub parallel to parallel hyaloclastite rich horizons separating aphanitic to medium grained ponded units several primary volcanic structures are observed within the hyaloclastite horizons and are identified as pillowed forms cigar forms and v shaped structures ponded units range in thickness from 2 5 to 5 m with the exception of one unit which is approximately 30 m thick in central portions this thick unit is medium grained and has a sub ophitic texture units at locality 1 strike approximately nw while those at locality 2 strike ene several families of mafic dykes cross cut locality 1 and trend e w and nne both volcaniclastic and effusive facies have similar compositions affinities and trace and rare earth element signatures all facies have a tholeiitic affinity range in composition from basalt to andesite and have morb type trace and rare earth element signatures locality 1 appears slightly more evolved than locality 2 with later dykes and a late sill being the most evolved facies volcanic facies at localities 1 and 2 are consistent with subaqueous ponded lavas bounding synvolcanic structures and volcanic facies validate this interpretation and indicate these ponded lavas are hosted within a large synvolcanic depression at the summit of a mafic shield complex primary volcanic structures within hyaloclastite horizons formed via in situ fragmentation when water entered the system as ponded lavas began to cool water entered the system through synvolcanic fractures and encountered semi molten pockets of lava leading to hydroclastic fragmentation of the lavas v shaped and cigar structures formed during more energetic events and changed laterally into pillowed forms as energy dissipated from the system

the relation between iron formation and low temperature hydrothermal alteration in an archean volcanic environment abstract the uppermost section of the hunter mine group hmg 2728 ma a bimodal volcanic complex in the abitibi greenstone belt contains both oxide and carbonate facies banded iron formation bif this paper explores the relationship between volcanic activity and the development of the two types of iron formation the oxide facies represented by chert jasper magnetite iron formation is widespread and recurs at several stratigraphic levels field evidence suggests it deposited directly on the sea floor during periods of volcanic quiescence and is most probably derived from fluids seeping from volcaniclastic sediments and the compaction of felsic shard rich tuffs seeping occurred along synsedimentary and synvolcanic faults and then along bedding planes carbonate siderite facies iron formation in contrast formed locally below a silica cap rock by in situ low temperature hydrothermal replacement of chert tuff beds and even of the oxide formation thus the two types of iron formation although spatially and stratigraphically juxtaposed are not the result of the same process and only the carbonate facies appears to be directly related to a low temperature hydrothermal volcanic process the contrasting genesis of oxide and carbonate iron formations indicates that current simplistic models relating banded iron formation to volcanic massive sulphide deposits need to be re evaluated the notion that oxide iron formation represents a distal manifestation of volcano generated hydrothermal activity may be the exception and not the rule because as indicated by this study bif occurs in the central part of a volcanic edifice rather than on the flanks

low temperature alteration at the world class laronde penna archean au rich volcanogenic massive sulfide deposit abitibi subprovince quebec canada evidence from whole rock oxygen isotopes the oxygen isotope composition of 169 whole rock samples from the world class laronde penna au rich volcanogenic massive sulfide deposit range from 6 5 to 22 0 fractional crystallization from basalt to rhyolite accounts for a variation of approximately 2 in primary whole rock 18o values the remaining variance in whole rock 18o values found at laronde penna must therefore be ascribed to water rock exchange with hydrothermal fluids the metamorphosed hydrothermally altered host rocks have been subdivided according to mineralogical assemblages which display weak covariance with 18o values and whole rock geochemical alteration indices water rock equilibrium exchange models indicate that the high 18o values that characterize the laronde penna deposit are compatible with low temperature 150oc hydrothermal alteration at high water rock oxygen atomic ratios 1 50 zones of lower 18o values stratigraphically beneath the principal ore lens 20 north indicate local zones of higher temperature hydrothermal alteration the oxygen isotope composition of the hydrothermal fluid is estimated to have been 5 as a consequence of mixing between seawater 18o 0 and a component of magmatic water degassed from the volcanic and intrusive rocks associated with the laronde penna deposit

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

fluid chemistry and evolution of hydrothermal fluids in an archaean transcrustal fault zone network the case of the cadillac tectonic zone abitibi greenstone belt canada detailed fluid geochemistry studies on hydrothermal quartz veins from the rouyn noranda and val d or areas along the transcrustal cadillac tectonic zone ctz indicate that unmineralized with respect to gold sections of the ctz contained a distinct co2 dominated h2s poor hydrothermal fluid in contrast both gold mineralized sections of the ctz e g at orenada 2 and associated higher order shear zones have a h2o co2 c h 4 nacl hydrothermal fluid their co2 h2s ratios indicate h2s rich compositions the br cl compositions in fluid inclusions trapped in these veins indicate that hydrothermal fluids have been equilibrated with the crust oxygen isotope ratios from hydrothermal quartz veins in the ctz are consistently 2 more enriched than those of associated higher order shear zones which are inter preted to be a function of greater fluid rock ratios in the ctz and lower fluid rock ratios and more efficient equilibration of the hydrothermal fluid with the wall rock in higher order shear zones an implication from this study is that the lower metal endowment of the transcrustal ctz when compared with the higher metal endowment in higher order shear zones ratio of about 1 1000 may be the result of the lack of significant amounts of h 2o h2 sr ich fluids in most of the ctz in contrast gold mineralization in the higher order shear zones appear to be controlled by the high h 2s activity of the aqueous fluids because gold was likely transported in a bisulfide complex and was deposited during sulfidation reac tions in the wall rock and phase separation in the quartz veins

three dimensional visualization of the archean horne and quemont au bearing volcanogenic massive sulfide hydrothermal systems blake river group quebec the hydrothermal system architecture related to the formation of the contemporaneous au bearing horne and quemont volcanogenic massive sulfide vms deposits was visualized by employing kriging methods to map whole rock oxygen isotope compositions zones of silica addition and loss and water contents in two and three dimension zones of alteration were mapped in three dimensions in the vicinity of the steeply dipping horne deposit to depths of as much as 2 km in all nearly 300 samples were analyzed for oxygen isotopes and supplemented by previously published whole rock analyses contents of sio2 h2o mgo al2o3 and s from chemical analyses of nearly 5 000 samples within the two and three dimensional study regions were used separately and in combination with the oxygen isotope data for modeling and hydrothermal mapping purposes the horne and quemont deposits formed within a similar time frame but in different magmatic hydrothermal systems distinguished by their mapped hydrothermal architecture the quemont deposit appears to be centered on the powell pluton which intruded late into an apparent volcanic filled rift graben structure although structural complexities are apparent we infer mineralizing high temperature upflow in the footwall of the quemont deposit to have emanated from a reaction zone above the powell pluton and its precursors beneath a zone of extensive silicification faulting on the andesite fault and horne creek faults plus erosion has removed evidence of the upflow zone in the hydrothermal system of the horne deposit areas of silicification correspond in general with isotopic evidence of lower temperature alteration such alteration east of the quemont deposit signaled the waning of hydrothermal activity the suggested cooling for the most part promoted the precipitation of silica in the case of the horne deposit mixing of metalliferous hydrothermal fluid with cold seawater in the permeable footwall rocks in an apparently relatively stratigraphically stable and long lived hydrothermal system evidently led to marked footwall silicification the silicified footwall may have contributed to an increased efficiency of sulfide precipitation in the horne deposit continued intrusion and some post vms hydrothermal activity is recorded in the hanging wall section to the horne deposit our data suggest that deposition of the 10 million ounces moz of au within the horne deposit was syngenetic and not the product of subsequent hydrothermal activity

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

preferential dissolution of 234u and radiogenic pb from α recoil damaged lattice sites in zircon implications for thermal histories and pb isotopic fractionation in the near surface environment abstract zircons from two archean samples a 2732 ma trondhjemite and a 2728 ma dacite both from the western superior province were washed in hydrofluoric acid hf at moderate temperatures zircon residues and hf wash solutions were analyzed for u pb isotope systematics and 234u 238u ratios in an attempt to study the effect of hf washing on selective dissolution of radiogenic isotopes in areas damaged by recoil on a conventional concordia diagram the datum from one analyzed wash solution plots below concordia near the array of discordant data from magnetic unabraded zircon fractions which record a lower concordia intersection of about 230 ma data points from hf wash residues of two different zircon fractions from the trondhjemite are also discordant and collinear with near concordant data from abraded unleached fractions which record the true age of crystallization however this array has a negative lower concordia intercept of 749 ma the residue and wash from hf treatment of a single zircon fraction from the dacite are collinear with the crystallization age of the sample as defined by near concordant data from unleached fractions this line has a lower concordia intersection of 1388 ma also negative but distinct from the trondhjemite residue line the discordance of strongly washed residues is proportional to the calculated recoil dosage of the grains leach residues show negative 234u 238u anomalies while the leaches themselves show 234u excess on the order of 10 these results can be explained by the bulk dissolution of altered and metamict crystal domains during the hf wash accompanied by leaching of 234u and radiogenic pb from recoil damaged lattice sites in alteration free zircon residues studies were also carried out on zircons washed in nitric acid these show small ca 0 5 negative 234u anomalies this suggests that small amounts of radiogenic pb can be leached from damaged lattice sites by nitric acid the negative age lower concordia intercepts of pb loss lines defined by hf leach residues can be explained if early radiation damage has been annealed so that only more recently formed radiogenic pb atoms are in damaged crystal lattice sites and are susceptible to leaching since this pb fraction is relatively young the residue pb is biased toward anomalously old values a model is proposed for early burial and heating of the rocks above the annealing temperature for recoil damage followed by uplift and cooling several hundred million years after crystallization this model predicts a cooling time of 2500 20 ma for the trondhjemite and 2295 35 ma for the rhyolite comparison with metamorphic and cooling histories from ar ar studies suggest that the long term annealing temperature for low levels of recoil damage in zircon is in the range 250 300 c approximately the same as for fission tracks these results demonstrate that hf and possibly to a small extent hno3 can preferentially leach radioactive daughter elements from damaged zircon lattice sites in addition to dissolving metamict domains the threshold recoil dosage for dissolution of damaged zircon appears to be slightly in excess of 1 5 1015 decay events mg the isotopic composition of the leached pb is sensitive to the annealing history of the zircon thus in addition to supplying the magmatic crystallization age of the sample the hf leaching of zircon might be useful for constraining its later low temperature thermal history leaching from recoil damaged sites also occurs in the natural environment and this may lead to significant enhancement of 206pb 204pb in groundwater

a special issue on archean magmatism volcanism and ore deposits part 2 volcanogenic massive sulfide deposits preface this is the second of a two part special issue of economic geology devoted to archean ore deposits the first part of the special issue vol 107 no 5 contains six papers that discuss craton to deposit scale controls on komatiite associated ni cu pge deposits barnes and fiorentini 2012 collins et al 2012 fiorentini et al 2012 heggie et al 2012a b taranovic et al 2012 and six papers that discuss the diversity and key aspects of greenstone hosted au deposits cote mantha et al 2012 garde et al 2012 leclerc et al 2012 mercier langevin et al 2012b schneider et al 2012 sherlock et al 2012 a general introduction to the special issue and an overview of some implications for exploration for greenstone hosted au and komatiite associated ni cu pge deposits in ancient belts is given in mercier langevin et al 2012a the ten papers in this second part of the special issue discuss specific aspects of archean volcanogenic massive sulfide vms deposits the archean which represents more than one third of earth s history was a prolific period for the formation of vms deposits franklin et al 2005 most of these deposits formed within a comparatively narrow time interval between 2740 and 2690 ma in the neoarchean approximately 8 of the global vms tonnage production reserves and geologic resources is contained within deposits of archean age fig 1a given the low degree of preservation of archean crust compared to phanerozoic crust e g veizer 1988 the abundance of archean vms deposits is remarkable proterozoic vms deposits account for approximately 20 of the global vms tonnage whereas the phanerozoic deposits contain 72 of the global tonnage the latter is due in large part to giant vms deposits of the iberian pyrite belt in spain and portugal the urals in russia the rudny

2 7ga juvenile crust formation in the north china craton taishan xintai area western shandong province further evidence of an understated event from u pb dating and hf isotopic composition of zircon abstract the evolution of the north china craton ncc is well known for a marked 2 55 2 50 ga tectonothermal event however supracrustal and intrusive rocks of 2 75 2 70 ga are in fact widely distributed in the western shandong province the most important area of archaean basement in the eastern part of the ncc this paper reports shrimp u pb dating and la icpms hf isotopic composition of zircons from 2 75 2 70 ga supracrustal and trondhjemite tonalite granodiorite ttg rocks in that area three fine grained hornblende biotite gneiss samples known locally as leptynite with meta volcanic or volcanosedimentary rock protoliths and five ttg samples have shrimp zircon u pb ages varying from 2 75 to 2 70 ga and 2 74 to 2 71 ga respectively zircons from most of the samples have high positive hf t values 4 7 to 10 0 and t dm hf ages 2 85 2 60 ga similar to their zircon u pb ages this indicates that the rocks represent largely juvenile crustal additions derived from depleted mantle only a short time before however some granitoids show hf t zircon values of 13 6 to 5 1 and t dm hf of 3 51 2 80 ga therefore the strong 2 75 2 70 ga tectonothermal event in the western shandong province involved not only juvenile addition to the continental crust but also intracrustal recycling of older components combined with craton wide data it is shown that the ncc is similar to many other cratons around the world where tectonothermal events of 2 7 ga are well developed however the main difference is that in the ncc superimposed 2 5 ga tectonothermal events were much stronger than in most other cratons

age constraints and characteristics of subaqueous volcanic construction the archean hunter mine group abitibi greenstone belt the archean hunter mine group hmg is a 6 7 km thick south facing volcanic sequence at the southern limit of the northern volcanic zone abitibi greenstone belt the hmg is composed of 1 felsic lava flows and endogenous and exogenous domes with autoclastic breccia deposits 2 pyroclastic fire fountain products and high to low concentration volcaniclastic sediment gravity flows of autoclastic and pyroclastic origin interstratified with iron formation 3 mafic pillowed and brecciated flows 4 a prominent felsic dyke swarm and 5 abundant mafic intrusions the stratigraphy is complex as inferred by detailed volcano sedimentary facies mapping a 5 7 km wide n trending felsic dyke swarm coupled with abundant synvolcanic and synsedimentary faults chaotic breccias and mineralization suggest a central volcanic depression which is consistent with a caldera structure voluminous effusive activity and explosive fire fountaining eruptions contributed to caldera development rifting of this arc edifice is supported by the presence of felsic dykes and mafic tholeiitic sills the lithofacies and overlying komatiites flows are consistent with a deep water marine setting u pb age determinations help constrain the hmg caldera formation to 6 million years the sampled quartz feldspar phyric dykes yielded u pb zircon ages of 2728 3 4 3 4 and 2728 9 0 8 ma and together with a previously published dyke age of 2729 6 1 4 ma from the same dyke swarm a case is made that the swarm evolved over 1 million years 2728 6 2729 6 ma a lobate flow and flow breccia revealed similar ages of 2724 6 4 6 1 7 and 2727 6 4 2 2 0 ma whereas the up to 1 km thick gabbro diorite roquemaure sill precedes both felsic dykes and flows as indicated by the 2731 8 2 2 2 0 ma age the 2706 4 5 7 4 3 ma feldspar quartz phyric dykes occurred late in the history of the hmg and were not associated with subaqueous volcanic construction the 207pb 206pb ages of analyzed zircon fractions have an inherited component within error of the oldest crystallization ages obtained for the hmg complex collectively these age determinations and inheritance as well as stratigraphy support a tripartite division into a 2734 2730 ma lower formational stage a 2732 ma middle formational stage and a 2730 2728 ma upper formational stage the study shows that age constraints without detailed facies mapping may lead to erroneous conclusions on volcanic construction and that felsic volcanic dykes and flows may inherit an earlier component of their own volcanic edifice

glacial rebound of the british isles i preliminary model results summary observations of sea level change from localities around the british coastline indicate that major spatial and temporal variations have occurred over the past 15000 10000yr these observations provide a valuable data set for testing models of glacial rebound and for estimating the earth s response to surface loading as well as for placing broad constraints on models of the ice sheet over the northern british isles in late devensian time simple models have been developed to examine the criteria required for a high precision rebound model suitable for an inversion of the observations of sea level change for earth and ice model parameters for such a model to have a precision of better than 1 m these requirements include 1 introduction of the fennoscandian and more distant ice sheets into the model both as contributions to the sea level rise and to the crustal deformation of the associated changing ice and water loads 2 an expansion of the ice and meltwater loads to very high spherical harmonic degree including terms up to about 240 for the water load term 3 the development of higher iteration solutions of the sea level equation in order to model accurately the meltwater load contribution to sea level change 4 the introduction of loading cycles before the attainment of the last glacial maximum for both the british ice sheet and the other major but more distant ice sheets and 5 the introduction of time dependent coastlines during the period of rapid global sea level rise the sea level predictions are strongly dependent on both earth model and ice model parameters but because observations are available from a wide range of locations within and beyond the former ice sheet margins some separation of the two types of parameters is possible for example the models exclude the possibility that a substantial ice load occurred over the north sea between scotland and norway in late devensian time and that the maximum ice thickness over northern great britain is unlikely to have exceeded about 1500 m during the last glacial maximum the earth model parameters that can be resolved are the lithospheric thickness and the upper mantle viscosity a high resolution model incorporating the above model criteria and used to infer ice and earth model parameters from the observations of sea level change is developed in the accom panying paper

synvolcanic and younger plutonic rocks from the blake river group implications for regional metallogenesis the blake river group of the abitibi greenstone belt is host to the well known volcanogenic massive sulfide vms deposits of the noranda camp quebec canada in the noranda camp massive sulfide deposits are spatially associated with the coeval flavrian powell intrusive complex which is interpreted to have driven hydrothermal circulation that formed the deposits in the camp in the western blake river group in ontario there are also abundant intrusive rocks yet relatively insignificant accumulations of vms deposits these intrusive rocks are younger 2686 9 1 2 and 2688 5 2 3 ma and are associated with porphyry style cu mo au mineralization with re os molybdenite ages of 2682 4 5 ma the intrusions are also smaller in areal extent and do not have a temporally extended polyphase history like the flavrian powell intrusive complex the intrusions of the western blake river group can also be distinguished by lithogeochemical attributes and a distinct petrological history compared to the flavrian powell intrusive complex they have lower total y yb th hfse and ree with higher zr y la yb sm yb al2o3 yb and la smucn ratios and lower zr nb zr th zr la zr tio2 and nb thucn ratios petrologic modeling is consistent with the western blake river group intrusive rocks being generated as relatively low temperature melts 40 km in the crust where garnet is stable in the residue e g garnet amphibolite residue in contrast modeling of the flavrian powell intrusive complex is consistent with melting at shallower levels in the crust e g 800o to 850oc the occurrence of high temperature magmatism at high levels within this crust that is coeval with volcanic activity explains the occurrence of numerous vms deposits with the flavrian powell intrusive complex as this intrusive complex would have generated abundant heat at high levels in the crust to drive hydrothermal circulation in contrast the intrusions of the western blake river group were generated at depth and at lower temperature increasing the probability of heat loss upon emplacement from depth hence decreasing the probability of hydrothermal circulation at high levels in the crust and likely explaining the paucity of vms mineralization associated with these intrusive rocks the petrological attributes of the flavrian powell intrusive complex and western blake river group may be useful in delineating potentially fertile versus less prospective intrusive complexes in greenfields exploration areas notably the western blake river group intrusions have geochemical features similar to some phanerozoic porphyry cu mo au associated intrusive systems consistent with the observed mineralization and highlighting the potential of porphyry cu mo au systems of late archean age elsewhere in the abitibi

age and tectonic setting of the jingangku besshi type volcanogenic massive sulfide deposit from the northern shanxi north china craton abstract the jingangku volcanogenic massive sulfide deposit is one of the largest archean deposits in china it is located in the wutai greenstone belt in the northern part of the central orogenic belt in the north china craton the deformed deposit is hosted in a suite of neoarchean metamorphosed volcano sedimentary rocks mica quartz schist plagioamphibolite plagioclase schist and bif the orebodies are characterized by massive pyrite and contain varied amounts of chalcopyrite pyrrhotite and sphalerite the country rock of the jingangku vms ores shows intense chloritization silicification sericitization and carbonation the vms hosted pyrite and copper deposit is represented by the jingangku deposit that yielded a la icp ms zircon age of 2500 15 ma mswd 0 87 the other two vms hosted pyrite and copper deposits are represented by the houping and haoditang deposits that formed between 2494 ma and 2507 ma the late neoarchean vms deposits can be interpreted to have formed in a forearc setting and subsequently tectonically transported in melange belts during continental collision quite similar to the younger besshi type deposits in japan analyses of fluid inclusions from ore bearing magnetite quartzite showed the composition of metamorphic fluid to be h2o nacl co2 ch4 n2 h2 the analysis also showed that during the peak of metamorphism the fluid was characterized by medium high temperature 322 473 and low salinity 2 2 6 74 sulfide orebodies are closely associated with the magnetite quartzite and therefore it is proposed that both the sulfide and magnetite quartzite were derived from the same material source and have the same formation age and underwent the same history of deformation and metamorphism consequently the jingangku deposit most likely formed from submarine exhalative sedimentation and later superimposed and transformed by metamorphic fluid

the role of fluids during formation and evolution of the southern superior province lithosphere an overview models for fluid flow and hydrothermal alteration for the abitibi greenstone belt are reviewed in the light of lithoprobe results in the region in the abitibi greenstone belt eruption of volcanic sequences over 2750 2700 ma was accompanied by pervasive low temperature hydrothermal alteration at high water rock ratios giving systematic 18o enrichment archean ambient ocean water bottom temperatures were likely ca 30 c and 18o 0 1 chert iron formations precipitated from low temperature hydrothermal discharge base metal massive sulphide deposits formed at or near the seafloor from focussed discharge of high temperature 300 400 c fluids in convective cells sited above subvolcanic intrusions the ore fluids were evolved seawater that had undergone compositional and isotopic evolution by high temperature low water rock exchange with the volcanic pile to nacl 3 7 wt or cacl2 nacl up to 30 wt brines of 18o 0 8 these volcanic associated hydrothermal deposits are associated with greenst

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