hydrothermal alteration of basalts beneath the bent hill massive sulfide deposit middle valley juan de fuca ridge burial of midocean ridges by clastic sediments particularly at continental margins profoundly affects the geometry and chemistry of hydrothermal circulation and mineralization in the upper crust middle valley the sediment covered northern extension of the bare rock endeavour segment of the juan de fuca ridge is host to the base metal rich cu zn bent hill massive sulfide deposit at a water depth of 2 400 m the 9 mt bent hill deposit is a steep sided body 200 m across and 100 m thick ocean drilling program odp hole 856h penetrates through the massive sulfide and underlying feeder zone extending to a total depth of 500 m below sea floor through the base of the strongly recrystallized quartz chlorite sediment pile and into the uppermost volcanic basement the basaltic rocks beneath the bent hill deposit include narrow sills intruded into indurated sediments a volcanic flow erupted on top of sediments and pillow lavas below the lowermost sediments recovered similar styles of alteration are present in both the sills and flows and alteration is dominated by the effects of large scale hydrothermal upflow rather than hydrothermal activity associated with individual eruptions or intrusions the basalts are slightly to completely altered to greenschist facies secondary minerals principally quartz chlorite and titanite with subsidiary epidote cu fe sulfides and rare actinolite there are steep mineralogical chemical and isotopic alteration gradients from the highly altered basalt sediment interfaces down to the less altered flow interiors suggesting the channeling of hydrothermal fluids along the basalt sediment boundaries alteration is reflected in intense metasomatic changes in the basalts assuming immobile tio 2 the most intensely altered basalts have undergone about 20 percent mass loss during recrystallization to chlorite quartz rocks with depletions in silica alumina and alkali alkali earth and base metals chloritized pillow margins with strongly light rare earth element enriched chondrite normalized patterns la sm n 1 5 cf fresh basalts 0 7 that mimic profiles for midocean ridge hydrothermal fluids require fluid rock exchange with large quantities of hydrothermal fluid w r 27 000 oxygen isotope compositions of chlorite quartz rocks 18 o 1 8 2 4 suggest that alteration occurred between 320 and 370 c strontium isotope compositions of the altered basalts and the chlorite quartz rocks are not homogeneous and range from 87 sr 86 sr ratio 0 7037 to 0 7046 there is a strong mode in 87 sr 86 sr ratio at 0 7038 suggesting that much of the alteration occurred by isotopic exchange with a hydrothermal fluid of that composition this ratio is significantly lower than that measured for 265 c fluids venting from the nearby odp mound 87 sr 86 sr 0 7044 the occurrence of epidote and isocubanite in the chloritized glassy pillow margins suggests that these rocks may retain a record of the high temperature 350 c hydrothermal fluid responsible for the formation of the overlying bent hill massive sulfide deposit the strontium isotope composition of the chloritized glassy pillow margins and hence the mineralizing fluid is slightly more radiogenic 87 sr 86 sr 0 7046 this composition could result from the addition of 15 percent of a pelagic or sedimentary component to the 87 sr 86 sr ratio 0 7038 fluid responsible for most of the sr isotope exchange with the upper basement the sediments beneath the bent hill deposit are also strongly recrystallized to quartz and chlorite although their strontium isotope compositions are much lower than those in pelagic or terrigenous sediments in the region 87 sr 86 sr 0 709 0 720 the range of compositions 87 sr 86 sr 0 7046 0 7060 has little overlap with that of the altered basalts and chlorite quartz rocks from the sills and uppermost basement this lack of overlap suggests that the sediments either retain some of their original sedimentary strontium or that there is a range of fluid compositions in the sediment pile beneath the bent hill deposit sediments from the margins of middle valley far from zones of active black smoker venting odp site 855 have 87 sr 86 sr ratios with a significant hydrothermal component 0 7059 0 7086 these ratios indicate that the subsurface hydrology of middle valley is dominated by evolved fluids rather than seawater and that recharge into this system is not through boundary faults or through the sedimentary blanket as suggested by previous models rather exposed basement rocks that form the flanks of middle valley are the most likely zones of regional seawater recharge to the deep high temperature hydrothermal systems

a miocene coarse volcaniclastic mass flow deposit in the shimane peninsula sw japan product of a deep submarine eruption a subaqueous volcaniclastic mass flow deposit in the miocene josoji formation shimane peninsula is 15 16 m thick and comprises mainly blocks and lapilli of rhyolite and andesite pumices and non to poorly vesiculated rhyolite it can be divided into four layers in ascending order layer 1 is an inversely to normally graded and poorly sorted lithic breccia 0 3 6 m thick layer 2 is an inversely to normally graded tuff breccia to lapilli tuff 6 11 m thick this layer bifurcates laterally into minor depositional units individually composed of a massive lithic rich lower part and a diffusely stratified pumice rich upper part with inverse to normal grading of both lithic and pumice clasts layer 3 is 2 5 3 m thick and consists of interbedded fines depleted pumice rich and pumice poor layers a few centimeters thick layer 4 is a well stratified and well sorted coarse ash bed 1 5 2 m thick the volcaniclastic deposit shows internal features of high density turbidites and contains no evidence for emplacement at a high temperature the mass flow deposit is extremely coarse grained dominated by traction structures and is interpreted as the product of a deep submarine explosive eruption of vesicular magma or explosive collapse of lava

intrinsic elasticity of a textured transversely isotropic muscovite aggregate comparisons to the seismic anisotropy of schists and shales 1 the anisotropic properties of phyllosilicate rocks are investigated through the modeling of the pore free intrinsic elasticity of a textured muscovite aggregate the elastic constants were calculated for textures as defined using orientation distribution functions odfs ranging from perfectly aligned to completely random the odfs were described using a gaussian distribution the width of which is controlled by the standard deviation of the orientations of the crystal c axis with respect to the sample foliation plane under the assumption that the sample has hexagonal transversely isotropic symmetry the values were obtained using the widely used voigt reuss hill averaging techniques as well as the geometric mean average gma the gma is more physically meaningful in that it requires that the elastic constants determined in either the stiffness or the compliance domains be invertible comparisons between these different averages indicate that strictly the voigt and reuss values do not necessarily provide the limiting bounds as is expected for the isotropic case despite this some characteristics of the elastic wave p wave anisotropy are only weakly dependent on the averaging procedure employed the calculated elastic constants are converted to the anisotropic and parameters in order to allow for comparison to existing measurements in the literature however these intrinsic results are not intended to provide a measure of the properties of phyllites per se but to provide a reference against which more sophisticated models that incorporate porosity layering and composition may be constructed

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

textures of the hellyer volcanic hosted massive sulfide deposit tasmania the aging of a sulfide sediment on the sea floor the relatively undeformed polymetallic massive sulfide ore of the hellyer deposit retains many textures developed during the early crystallization history common textures in the least modified ore away from the main feeder zone include pyrite framboids concentric and radial textured spheres dendritic and fibrous forms breccias mineral banding veins and reniform and crustification textures many are typical of early crystallization within a medium at high degrees of supersaturation some result from open space filling and others indicate disturbances of the sulfide mass possibly due to tectonic activity and volume adjustment the banding could be due to current winnowing most of these textures are found in modern midocean ridge and kuroko massive sulfide deposits but at hellyer the framboids are much more common hexagonal outlines in ore sphalerite indicate derivation of much of the sphalerite by inversion of wurtzite the high content of chalcopyrite in the wurtzite derived sphalerite appears in most cases to have developed by exsolution rather than replacement or coprecipitation although there is local evidence of both processes having operated possibly resulting from early asymmetrical crystal growth comparison of hellyer ore and typical kuroko ores reveals other differences than framboid abundance e g metal contents and ratios mineralogy and sulfur isotope compositions the most significant of which are the relatively high salinities of the hellyer ore fluids these together with evidence of a coeval basin imply sulfide deposition in a brine pool as previously proposed the development of a quenched sulfide mud at the bottom of a basin filled with spent ore fluid with little or no involvement of seawater explains the different mineral content e g lack of fe oxides scarcity of barite in the massive sulfide and the pyrite arsenopyrite assemblage the high metal content lack of chimney structures and differences in sulfur isotope compositions the sulfide mud dominated by fe s phases would be an ideal growth medium for framboidal pyrite the weight of experimental evidence concerning the development of framboidal pyrite points to involvement of metastable precursor fe s phases such as mackinawite and greigite in a weakly oxidized environment oxygen may have been added to the reduced acid brine pool as a result of inward diffusion of oxygen from overlying seawater which is shown to have been oxic by the presence of sessile benthic dendroids in the shales overlying the ore deposit however the level of oxidation was insufficient to form marcasite

substorm wave base felsic hydroclastic deposits in the archean lac des vents volcanic complex abitibi belt canada abstract volcaniclastic deposits of the 2 3 km thick archean lac des vents volcanic complex are an integral part of major submarine volcanic construction the volcanic edifice which formed on a subaqueous basalt plain is comparable to modern seamounts resting on the ocean floor the initial 770 m of the mafic felsic edifice subject of this study is composed of massive brecciated and pillowed basalts massive to brecciated felsic lava flows and abundant felsic fragmental rocks of hydroclastic origin four distinct volcaniclastic lithofacies constitute the latter 1 the pumice lapilli tuff lithofacies 2 the lapilli tuff breccia lithofacies characterized by two sublithofacies 3 the turbidite tuff and tuff breccia lithofacies and 4 the volcanic sandstone and breccia lithofacies these four volcaniclastic lithofacies are considered to be the result of explosive and non explosive hydrovolcanic fragmentation processes operating at depths below storm wave base 200 m primary deposition or limited remobilization of unconsolidated hydroclastic debris is shown by the preservation of delicate clasts and volcanic textures and heat retention structures the principal transport agents are high concentration sediment gravity flows occurring under laminar and turbulent flow conditions high and low density turbiditic tuffs and fine grained tuff fallout deposits are related to either the dissipating stages of volcanic eruptions or slumping of syneruptive volcanic debris on the flanks of a subaqueous volcanic edifice ubiquitous interstratification of volcaniclastic turbidites shale and pillowed basalt flows with the felsic lava flows and fragmental debris favours subaqueous deposition these features combined with the absence of wave induced sedimentary structures imply deposition in water depths in excess of 200 m viscous feldspar phyric massive and brecciated felsic flows and associated volcaniclastics cross cut by felsic dykes suggest vent proximity the abundance of breccia size hydroclastic debris is consistent with this interpretation collectively these criteria argue for subaqueous fragmentation and deposition of volcaniclastics of inferred hydroclastic origin close to the central vent area at depths below storm wave base

the late archaean melita complex eastern goldfields western australia shallow submarine bimodal volcanism in a rifted arc environment abstract the melita volcanic complex is a late archaean bimodal rhyolite basalt volcanic succession within the gindalbie terrane in the eastern goldfields province of the yilgarn craton the melita complex has been dated by ion probe at 2683 3 ma 95 and forms part of a distinctive 2681 2692 ma volcanic association that records bimodal basalt rhyolite and calc alkaline intermediate silicic volcanism at several discrete volcanic centres and which locally hosts volcanic massive sulphide mineralisation teutonic bore approximately 3 km of stratigraphic thickness is exposed in the melita area the upper 1 1 5 km of the exposed succession is dominated by subaqueously resedimented volcaniclastic sandstones and breccias rhyolite flows and sills primary subaerial pyroclastic deposits including ignimbrites have not been identified in this study although subaerial explosive activity is indicated by the occurrence of accretionary lapilli and the abundance of vitric material shards and pumice fragments in resedimented deposits the lower part of the succession is dominated by pillowed to massive basalt lavas and in situ and resedimented mafic hyaloclastites mafic extrusive and intrusive rocks are tholeiitic with trace element concentrations similar to modern arc tholeiites felsic volcanic rocks at melita are dacite to high silica rhyolite they are highly enriched in incompatible elements particularly high field strength element enriched compared to other felsic associations in the eastern goldfields province representing evolved partial melts of heterogeneous intermediate arc type crust the volcanic facies and geochemistry of volcanic rocks at melita are consistent with those observed in modern intra arc or arc rift settings and the succession is interpreted to represent the initial stages of back arc rifting within a complex convergent margin

physical volcanology and geochemistry of a late archaean volcanic arc kurnalpi and gindalbie terranes eastern goldfields superterrane western australia abstract late archaean 2715 2704 ma calc alkaline volcanic complexes in the kurnalpi terrane are identical in facies and geochemistry to modern intra arc volcanic complexes preserved volcanic rocks and associated feldspathic sedimentary rocks represent proximal to medial submarine facies of submarine to locally emergent intra oceanic arc related volcanoes the least altered volcanic rocks are dominantly andesite but range from basalt and basaltic andesite to rare rhyolite and are the product of magmas similar in composition to those of modern arcs intermediate magmas were most likely derived from large ion lithophile element lile enriched mafic magmas by fractional crystallisation with or without assimilation processes the kurnalpi terrane thus provides an example of a late archaean volcanic arc in which hydrated mantle wedge melting was the main mechanism for magma genesis ttd suite volcanism derived from slab melting was either absent or played a subordinate role the adjacent gindalbie terrane contains younger 2692 2680 ma bimodal basalt rhyolite volcanic complexes with associated quartz rich sedimentary rocks and calc alkaline intermediate silicic volcanic complexes volcanosedimentary basement to the gindalbie volcanic sequences is deeply eroded equivalents of the calc alkaline volcanic complexes their associated sedimentary rocks and younger basaltic sequences found in the kurnalpi terrane gindalbie basalt rhyolite successions locally contain volcanic hosted massive cu zn mineral deposits rhyolites derived by low pressure fractional crystallisation or melting of andesites are more abundant than in the kurnalpi volcanic complexes facies associations and the local occurrence of distinctive incompatible element enriched mildly peralkaline a type rhyolites are consistent with rifting of a mature arc system the two terranes most likely represent volcanic evolution of the same late archaean volcanic arc but are different tectonic fragments that were welded together by 2660 ma

geochemistry and tectonic setting of two contrasting archaean felsic volcanic associations in the eastern goldfields western australia abstract archaean felsic volcanic rocks from two areas in the eastern yilgarn craton of western australia comprise dacite and rhyolite with subordinate basalt and andesite one suite from one of the areas black flag group is dominated by subaqueous dacite lava lobes and their reworked products the other suite from the other area melita and jeedamya volcanics consists of abundant rhyolitic and rhyodacitic pyroclastic deposits with less abundant dacitic lava flows both suites are intercalated with volumetrically subordinate basic and intermediate volcanic rocks black flag group dacites and rhyolites have steep ree patterns la yb cn about 40 and no eu anomaly high sr and al2o3 low nb and y hence high sr y melita and jeedamya dacites and rhyolites have flatter ree patterns la yb cn between 2 and 9 negative eu anomalies eu eu 0 4 0 6 and higher zr nb y and lower sr hence lower sr y on chemical grounds mafic rocks in both areas are genetically unrelated to felsic volcanic rocks black flag group andesites dacites and rhyolites are probably unrelated by fractional crystallization but resulted from 22 28 partial melting of amphibolite leaving a residual mineralogy of amphibole plagioclase garnet ilmenite clinopyroxene alternatively andesites may have been derived by partial melting of a metasomatised mantle source jeedamya rhyodacite could have been produced by 35 vapour absent melting of tonalite leaving a residual mineralogy of plagioclase orthopyroxene amphibole ilmenite the source would have had a slightly lree enriched pattern with a positive eu anomaly residual garnet in the source of black flag group rocks requires melting at depths greater than 35 km which is below the base of the present tectonically thinned crust the most likely source is subducted oceanic crust with black flag group rocks erupted in a back arc basin melita and jeedamya felsic volcanic rocks resulted from melting of thicker continental crust adjacent to the back arc

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

rutile as a pathfinder for metals exploration abstract the use of detrital rutile as an indicator of mineralisation is contingent on the identification of trace element signatures that uniquely distinguish rutile grains derived from mineralisation from those sourced from barren rocks in order to discriminate rutile sources principal component analysis was applied to a total of 3149 la icp ms analyses of rutile from ore environments au au cu pegmatites carbonatites lamprophyres vhms and barren igneous and metamorphic rocks hierarchical clustering of the results of principal component analysis pca determined sb and cr exhibit the most chemical variation in ore derived rutile rutile sourced from au deposits and enriched in sb can be distinguished from rutile from pegmatites which are enriched in nb ta and sn discriminating rutile from barren rocks can be done primarily using w and fe contents which can also provide insight on source rocks based on metamorphic grade and degree of crustal input clustering all rutile chemistries based on pca of 12 elements identified nine chemically distinct groups from ore mineralisation and barren igneous and metamorphic rocks primarily using variation in w and cr contents two groups are solely derived of rutile from au ores high versus low sb rutile of the remaining seven groups one group is comprised mainly of rutile from pegmatites high nb ta sn four groups are comprised solely of rutile from barren rocks and two groups are a mix of ore and non ore rutile rutile from au ore similar to the giant kalgoorlie and big bell au deposits is distinctive compared to rutile from all other rocks and can be distinguished from other ores and barren environments using multi element clustering of pca analysis thus rutile chemistry of detrital rutile grains is a valuable tool in au exploration

the mount morgan gold copper mine and environment queensland a volcanogenic massive sulfide deposit associated with penecontemporaneous faulting the mount morgan mine was closed in july 1981 having yielded 237 896 kg of gold 7 648 533 oz and 360 616 metric tons of copper from 50 million metric tons of ore an average recovered grade of 4 75 g metric ton au and 0 72 percent cu it has been australia9s largest single lode gold orebody and is the fourth largest gold producing area after the kalgoorlie golden mile and the bendigo and ballarat areas woodall 1979 the mine comprises the main pipe orebody of pyritic massive sulfides and the adjacent sugarloaf orebody of siliceous stringer mineralization when the host stratigraphy is reoriented with respect to regional dip and local faulting the two orebodies are shown to form a pipelike volcanogenic massive sulfide configuration at a high angle to the stratigraphy minor stratiform pyritic mineralization adjacent to the stratigraphic top of the orebody grades downdip away from the orebody to a sequence of interbedded crystal tuffs and cherty rocks with some jasperoid beds the associated alteration halo of the massive sulfide orebody has been intruded by the late devonian mount morgan tonalite the ore and alteration have been subjected to metamorphism and overprinted by later alteration and additional mineralizing phases mostly related to the complex tonalite intrusion the immediate host stratigraphy shows gross thickening and change of character on either side of the orebody suggesting that the orebody formed along a penecontemporaneous fault the fault appears to be part of a major resurgent cauldron structure

cross borehole tomography in anisotropic media anisotropy has significant effect on traveltime cross borehole tomography even relatively weak anisotropy cannot be ignored if accurate velocity estimates are desired since isotropic traveltime tomography treats anisotropy as inhomogeneity traveltime data in our examples were synthetically generated by a ray tracing code for anisotropic media and the computed quasi p wave traveltimes were subsequently inverted using the dual tomography technique carrion 1991 the results of the tomographic inversion show typical artifacts due to the anisotropy and that accurate imaging is impossible without taking the anisotropy into account

architecture of the neoarchaean jaguar vhms deposit western australia implications for prospectivity and the presence of depositional breaks abstract the duration and nature of breaks in archaean volcanosedimentary packages recently re evaluated in the abitibi greenstone belt has been assessed at jaguar an archaean cu zn rich volcanic hosted massive sulphide vhms deposit in the teutonic bore volcanic complex tbvc the eastern goldfields of the yilgarn craton western australia the deposit is hosted by coherent volcanic facies and associated volcaniclastic facies with minor non volcanic facies the hypothesis that vhms ore lies on the intersection of volcanic centres and depositional breaks is found to be valid for the jaguar deposit with the site being an active volcanic complex at footwall time with further dome development close to the final ore location at mineralised package time and then reoccurring volcanic complexes andesitic and basaltic at several points in the hangingwall succession broadly tied into the ore position at jaguar the entire stratigraphic sequence faces west with no major structural complications making it ideal for stratigraphic analysis the footwall evidences prolific volcanism with no intervening sedimentation the mineralised package mp consists of intercalated dacite breccia conglomerate sandstone and mudstone the footwall volcanic rocks and the mp dacite appear to be differentiation products from the same magma chamber a major volcanic geochemical boundary is identified between the footwall mp tholeiitic to transitional affinities and the immediate hangingwall succession calc alkaline the hangingwall package includes four distinct volcanic units intercalated with mass flow deposits and graphitic argillites vhms mineralisation occurred across a distinct change in the geochemistry of erupted volcanic facies at a time of increased volcaniclastic sedimentation and extension this break is comparable to member level hiatuses seen in the abitibi gb i e the most brief of breaks because there is no silicification of sedimentary unit tops by silicic seawater exposure and there is minimal development of black shales a third time sensitive criteria was the development of peperite on the base of hanging wall hw andesite lavas the mp closely resembles breaks within interpreted deeper water domains of the abitibi gb for instance the deloro tisdale assemblage interface south of timmins the recognition of the change in the rate of emplacement of volcanic units and particularly the change in chemistry of volcanic rocks are critical exploration lithologic guides to use in the absence of chemical sediment deposits exhalites although crustal extension and basin subsidence were factors in the shaping of the succession their effects were too subtle to use as an exploration tool a corollary of the recognition of a member level vhms bearing interval in the teutonic bore volcanic complex is that like the kidd munro assemblage stratigraphy abitibi gb where several ore bearing depositional breaks are known below the great kidd creek deposit there is scope for the discovery of further potentially larger deposits on higher formation or group level breaks in the tbvc

nature age and tectonic setting of granitoid hosted orogenic gold deposits of the jiaodong peninsula eastern north china craton china the jiaodong gold province along the southeastern margin of the north china craton has combined production and resources of 900 tonnes of gold more than 95 of the gold in this province is hosted in mesozoic granitoids which intruded late archean metamorphic basement rocks episodically at ca 160 150 and 130 126 ma the deposits of the jiaodong peninsula form one of the largest provinces of granitoid hosted lode gold deposits in the world the jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second or third order faults and as disseminated and stockwork style mineralization hosted in alteration zones along major regional scale faults mineralization took place during brittle reactivation d3 of pre existing d2 ductile shear zones wall rock alteration is dominated by k feldspar and sericite with also sulfide minerals and extensive silicification closest to the orebodies the alteration was accompanied by the introduction of volatiles fe mg k rb ag as au bi and sb enrichment of base metals in the gold lodes is generally low although locally significant gold deposition took place at temperatures of 250 to 350 c for most gold deposits and the ore fluids are co2 bearing and have calculated salinities of 6 13 wt nacl equiv deposits from the zhao ye xixia and muping rushan belts have similar if not identical structural controls alteration patterns ore mineral assemblages and stable isotope signatures lead isotope data indicate that the ore stage leads were buffered by their host rocks the granitoid hosted gold deposits in the zhao ye belt are most radiogenic the greenstone hosted deposits in the xixia belt is the least radiogenic and the granitoid hosted deposits in the muping rushan belt have intermediate compositions gold mineralization in the zhao ye xixia and muping rushan belts in the jiaodong gold province appears to be broadly contemporaneously at ca 130 120 ma as constrained by shrimp u pb zircon dating on host rocks and a post gold dike as well as by ar ar rb sr and k ar dating of hydrothermal alteration minerals and ores this early cretaceous age is about 100 million years younger than the peak of ultra high pressure metamorphism in the qinling dabie sulu orogen between the north china and yangtze cratons and about 500 km south of the gold province however it overlaps with the extension and exhumation stage of the collisional orogeny and the subduction of the pacific plate beneath the eurasian continental crust whereas the majority of the jiaodong gold deposits occur on the margin of the north china craton some also occur in the suture zone between the two cratons they are significantly landward relative to the subduction zone of the pacific plate compared with typical orogenic lode gold deposits globally lode gold deposits of ca 130 120 ma occurred on both sides of the tan lu fault zone available data favor a temporal link of gold mineralization with pacific plate subduction

geochemistry of the host rocks of the volcanogenic massive sulfide deposit at que river tasmania que river is a moderate sized high grade polymetallic volcanogenic massive sulfide deposit located in the cambrian mount read volcanics of western tasmania several subvertical lenses of massive sulfide occur in a sequence dominated by variably altered intermediate to silicic volcaniclastic rocks the sequence was deformed and regionally metamorphosed to prehnite pumpellyite facies during the devonian approximately 240 host rock samples have been analyzed in an attempt to determine their primary geochemical affinities and to evaluate the geochemical effects of alteration on binary variation diagrams elements such as zr nb y ti and al define linear fields that characteristically project toward the origin the linear behavior of these elements on such plots is in marked contrast to that of almost all other elements which show very complex distribution patterns the linear patterns are interpreted to reflect immobility of the components mentioned with movement up and down the linear arrays being controlled by concentration or dilution due to subtraction or addition of other components thus primary ratios between immobile components have been preserved and can therefore be used to infer the primary geochemical affinities of the altered rocks the intersection point of the linear trends defined by the que river rocks with a low pressure fractional crystallization trend defined by cenozoic orogenic lavas probably provides the best estimate of the initial composition of the rocks prior to alteration in a plot of zr tio 2 vs nb y the que river rocks define a restricted area dominantly in the fields defined by modern andesite and dacite rhyodacite basalts and rhyolites are apparently absent in terms of their primary geochemical affinities the host rocks of the mineralization are probably best described as a high k calc alkaline suite similar in many ways to modern continental margin andesitic rocks however the que river rocks are by comparison abnormally depleted in ti and enriched in transition metals such as ni cr and v the geochemistry of alteration is complex and not readily amenable to quantitative evaluation elements such as fe mg and mn have been added to the system whereas ca and na have been removed both si and k have been partly remobilized during alteration among the major and minor elements mn shows the greatest relative enrichment over its inferred primary abundance however the mn anomalies are sporadic and do not define a neat halo although most of the observed geochemical alteration appears to have accompanied mineralization the andesites have been modified from their inferred primary compositions probably during the regional metamorphism hydrothermal alteration is not symmetrical about the ore lenses although the presently observed displacement between the massive sulfide lenses and the alteration halo is probably due largely to the devonian deformation it is possible that there is also a primary component the apparent immobility of al implies that the abundant phyllosilicates whose development is clearly related to mineralization do not represent simple chemical precipitates or sediments rather their development would appear to be controlled by the local availability of immobile al

the geochemistry of late archaean microbial carbonate implications for ocean chemistry and continental erosion history trace element concentrations and combined sr and nd isotope compositions were determined on stromatolitic carbonates microbialites from the 2 52 ga campbellrand carbonate platform south africa shale normalised rare earth element and yttrium patterns of the ancient samples are similar to those of modern seawater in having positive la and y anomalies and in being depleted in light rare earth elements in contrast to modern seawater and microbialite proxies the 2 52 ga samples lack a negative ce anomaly but possess a positive eu anomaly these latter trace element characteristics are interpreted to reflect anoxic deep ocean waters where unlike today hydrothermal fe input was not oxidised and scavenged and rare earth elements were not coprecipitated with fe oxyhydroxides the persistence of a positive eu anomaly in relatively shallow campbellrand platform waters indicates a dramatic reversal from hydrothermally dominated archaean to continental erosion dominated phanerozoic rare earth element flux ratio the dominant hydrothermal input is also expressed in the initial sr and nd isotope ratios there is collinear variation in sr nd systematics which range from primitive values 87sr 86sr of 0 702386 and end of 2 1 to more evolved crustal ratios mixing calculations show that the range in trace element ratios e g y ho and initial isotope ratios is not a result of contamination by trapped sediment but that the chemical and isotopic variation reflects carbonate deposition in an environment where different water masses mixed calculated nd flux ratios yield a hydrothermal input into the 2 52 ga oceans one order of magnitude larger than continental input such a change in flux ratio most likely required substantially reduced continental inputs which could in turn reflect a plate tectonic causation e g reduced topography or expansion of epicontinental seas

trace element geochemistry of ore associated and barren felsic metavolcanic rocks in the superior province canada reply archaean felsic metavolcanic rocks in the superior province of the canadian shield may be divided into three major groups on the basis of trace element abundances and ratios 1 fi felsic metavolcanic rocks are dacites and rhyodacites characterized by steep ree patterns with weakly negative to moderately positive eu anomalies high zr y low abundances of high field strength elements e g hree y zr hf and high abundances of sr examples occur in the bowman subgroup and skead group in the abitibi belt in the kakagi lake lake of the woods shoal lake and sturgeon lake areas of the wabigoon belt and in the confederation lake area of the uchi belt none of these horizons as known hosts base metal sulphide deposits 2 fii felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping ree patterns with variable eu anomalies moderate zr y and intermediate abundances of hfs elements and sr examples occur in the misema subgroup of the abitibi belt in the wabigoon lake a

impact of fabric microcracks and stress field on shale anisotropy summary few data are available on shales in terms of seismic to ultrasonic properties and anisotropy although all are important with regards to imaging problems often encountered in such lithologies additionally mechanisms causing changes in these properties are not well documented due to the fine grain size of such materials and time required for testing under controlled pore pressure conditions the results presented here are derived from a set of experiments run on muderong shale with pore pressure control in order to evaluate the effect of stress magnitude and stress anisotropy on ultrasonic response this shale was noted to have a linear velocitymean effective stress response and extremely high anisotropy both likely the result of the presence of fluid filled cracks in a low permeability porous medium changes in velocity and vp vs ratios are dependent on both stress and smectite content s wave velocity is significantly affected by the presence of smectite in this and other shales and at low stress 20 mpa is less sensitive to stress change than p wave velocity vp vs ratios are noted to increase in this shale up to 20 mpa effective stress then decrease slightly due to stress induced loss of interlayer water in smectite intrinsic anisotropy comes from composition a strong compaction fabric and the presence of microfractures changes to ultrasonic anisotropy are the result of the magnitude of the stresses their orientation with respect to the fractures and the degree of stress anisotropy

lode gold deposits and archean mantle plume island arc interaction abitibi subprovince canada abstract in combination with seismic interpretations and geochronological constraints the association of juvenile arc type low ti tholeiitic basalts with komatiites in the southeastern abitibi subprovince canada supports a history of subduction step back following late archean mantle plume island arc interaction the resulting paired collision zones preserved abundant komatiites and numerous massive sulphide deposits and established the critical metallogenic features to concentrate the majority of canada s precambrian gold resources in a small area of the southern abitibi subprovince

the elastic anisotrophy of shales shales constitute about 75 of the clastic fill sedimentary basins and have a decisive effect on fluid flow and seismic wave propagation because of their low permeability and anisotropic microstructure the elastic stiffnesses of a shale with partially oriented clay particles is expressed in terms of the coefficients w sub lmn in an expansion of the clay particle orientation distribution function in generalized legendre functions application is made to the determination of the anellipticity of shales for transverse isotrophy the anellipticity quantifies the deviation of the p wave slowness curve from an ellipse and is shown to depend on a single coefficient w sub 400 in the expansion of the clay particle orientation distribution function if w sub 400 is small the anellipticity may be neglected as is apparently the case for a near surface late tertiary shale studied by winterstein and paulson strongly aligned clay particles result in a positive value of w sub 400 and a positive anellipticity in agreement with the majority of the field measurements however less well ordered shales could have a significantly positive second moment w sub 200 but only a small positive or even negative value of w sub 400 for such shales the anellipticity would be small or negative despite amore preferred alignment of clay particles in the bedding plane numerical examples of clay particle orientation distribution functions leading to zero or negative anellipticity are given less

geology distribution and classification of gold deposits in the western qinling belt central china gold deposits of the western qinling belt occur within the western part of the qinling dabie sulu orogen which is located between the precambrian north china and yangtze cratons and east of the songpan ganzi basin the early paleozoic to early mesozoic orogen can be divided into northern central and southern zones separated by the shangdan and lixian shanyang thrust fault systems the northern zone consists of an early paleozoic arc accreted to the north china craton by ca 450 ma the central zone which contains numerous orogenic gold deposits is dominated by clastic rocks formed in a late paleozoic basin between the converging cratonic blocks the southern zone is characterized by the easternmost exposure of triassic sedimentary rocks of the songpan ganzi basin these early to late triassic turbidities in part calcareous of the immense songpan ganzi basin also border the western qinling belt to the west carlin like gold deposits are abundant 1 along a westward extension of the southern zone defined by a window of early paleozoic clastic rocks extending into the basin and 2 within the easternmost margin of the basinal rocks to the south of the extension and in adjacent cover rocks of the yangtze craton triassic and early jurassic synkinematic granitoids are widespread across the western qinling belt as well as in the songpan ganzi basin orogenic lode gold deposits along brittle ductile shear zones occur within greenschist facies highly deformed devonian and younger clastic rocks of the central zone mainly coarse grained gold along with pyrite pyrrhotite arsenopyrite and minor base metal sulfides occur in networks of quartz veinlets brecciated wall rock and are disseminated in altered wall rock isotopic dates suggest that the deposits formed during the late triassic to middle jurassic as the leading edge of the yangtze craton was thrust beneath rocks of the western qinling belt many gold bearing placers are distributed along the river systems that flow south from the lode bearing central zone carlin like gold deposits have only been identified during the last decade in the southern zone of the western qinling and in the northeastern corner of the songpan ganzi basin the deposits mainly contain micron diameter gold in arsenical pyrite are characterized by the common occurrence of cinnabar stibnite realgar and orpiment exhibit strong silicification carbonatization pyritization and decalcification dissolution textures and are structurally controlled the lack of reactive host lithologies may have prevented development of large 100 tonnes of gold stratigraphically controlled orebodies which are typical of the carlin deposits in the western usa these deposits are hosted by triassic turbidities and shallow water carbonates and an early paleozoic inlier in the songpan ganzi basin that extends in an east west belt for about 300 km rather than true carlin deposits these carlin like deposits may be some type of shallow crustal i e epithermal hybrid with features intermediate to nevada style carlin deposits and the orogenic gold deposits to the immediate north these carlin like deposits also overlap in age with the early mesozoic orogenic gold deposits and therefore also formed during the final stages of collision between the cratons and intermediate basin closure

bedded jaspers of the ordovician lokken ophiolite norway seafloor deposition and diagenetic maturation of hydrothermal plume derived silica iron gels sedimentary beds of jasper red hematitic chert in the ordovician lokken ophiolite of norway are closely associated with volcanogenic massive sulphide vms deposits the jaspers occur in the immediate hangingwall and laterally peripheral to the large lokken 25 30 mt and small hoydal 0 1 mt vms deposits and are exposed discontinuously for several kilometres along strike massive or laminated types predominate jasper sulphide debris flow deposits are also abundant near vms deposits the jaspers contain hematite rich laminae showing soft sediment deformation structures and microtextural evidence that record the presence of a colloidal precursor and an origin as gels early textures include 1 straight or curved chains of hematitic filaments 3 10 m in diameter and 20 100 m long 2 branching networks of 15 25 m thick tubular structures surrounded by cryptocrystalline hematite and filled with quartz and euhedral hematite 3 small up to 10 m spherules composed of cryptocrystalline hematite and silica and 4 up to 50 m silica spherules with hematitic cores the small filaments seem to have been deposited in varying proportions in the primary laminae possibly together with hematitic and siliceous microspheroids diagenetic changes are represented by polygonal syneresis cracks and the presence of cryptocrystalline originally opaline silica chalcedony quartz carbonate and cryptocrystalline hematite and or goethite forming botryoidal masses and spheroids 10 m to 5 mm in diameter coarser euhedral grains of quartz carbonate and hematite are integral parts of these textures bleached silica rich jaspers preserve only small relics of fine grained hematite rich domains and locally contain sparse pockets composed of coarse euhedral hematite epidote

primary silica granules a new mode of paleoarchean sedimentation in the modern silica cycle dissolved silica is removed from seawater by the synthesis and sedimentation of silica biominerals with additional sinks as authigenic phyllosilicates and silica cements fundamental questions remain however about the nature of the ancient silica cycle prior to the appearance of biologically mediated silica removal in neoproterozoic time the abundance of siliceous sedimentary rocks in archean sequences mainly in the form of chert strongly indicates that abiotic silica precipitation played a significant role during archean time it was previously hypothesized that these cherts formed as primary marine precipitates but substantive evidence supporting a specific mode of sedimentation was not provided we present sedimentologic petrographic and geochemical evidence that some and perhaps many archean cherts were deposited predominately as primary silica grains here termed silica granules that precipitated within marine waters this mode of silica deposition appears to be unique to archean time and provides evidence that primary silica precipitation was an important process in archean oceans understanding this mechanism promises new insights into the archean silica cycle including chert petrogenesis microfossil preservation potential and archean alkalinity budgets and silicate weathering feedback processes

evolution of the archaean granite greenstone terranes of the eastern goldfields western australia shrimp upb zircon constraints abstract thirty five high precision shrimp u pb zircon dates with an average 95 confidence error of 6 ma have been obtained from the granite greenstone terranes in the southern part of the eastern goldfields of western australia these dates establish the timing of major deformation metamorphism and granite intrusion events and test inferred stratigraphic relationships within the greenstones these new data reveal that felsic volcanic rocks were erupted throughout the southern part of the eastern goldfields between c 2713 and c 2672 ma analytically indistinguishable dates of c 2705 ma were obtained for felsic volcanic rocks interbedded with komatiitic lavas at three sites dates on felsic volcanic rocks within tectonostratigraphic packages that include komatiites are also consistent with an inferred eruption age for these komatiites of c 2705 ma komatiitic lavas in the southern part of the eastern goldfields were probably emplaced during a single voluminous eruption that resulted in the flooding of a vast area of the late archaean landscape filling the topographic lows around older felsic volcanic vents the stratigraphic complexity within tectonostratigraphic domains is attributed to the existence of an undulating topography during the eruption of the mafic and ultramafic lavas and to the rapid and localized deposition of felsic volcanic rocks close to a number of isolated volcanic centres prior to and after the komatiite lava eruption event between c 2685 and c 2675 ma early pre d2 intrusive equivalents of the felsic volcanic rocks were emplaced as thick granite sheets into the base of the greenstone sequences regional east west compression between c 2675 and c 2657 ma reactivated early extensional structures and resulted in the stacking of the greenstones this was accompanied by further intrusion of granite within developing antiforms burial of greenstone slices by stacking and the effect of hydrothermal fluids emanating from both the granites and buried greenstones resulted in regional metamorphism to greenschist facies prior to c 2640 ma monzogranite and granodiorite emplaced near the western southern and eastern margins of the greenstones were deformed and metamorphosed to upper amphibolite facies and converted into gneisses during d2 and d3 compression events movement along large scale transcurrent d3 shear zones had ceased by c 2635 ma remnants of a c 2958 to c 2930 ma greenstone sequence preserved near norseman and ravensthorpe in the southeastern yilgarn craton and xenocryst zircons ranging in age from c 2910 2730 ma in the c 2713 2672 ma felsic volcanic rocks indicate that the exposed greenstone sequences were deposited on continental basement the eastern goldfields granite greenstone terranes were formed by the closure of a series of narrow back arc rift basins that developed along a continental margin and above an active subduction zone located to the east of the rift basins

new field and laboratory evidence for the origin of hyaloclastite flows on seamount summits new field observations with the submersible alvin and photographic evidence from a study of the summits of seamounts near the east pacific rise show that hyaloclastite deposits occur commonly hyaloclastite outcrops were found on six volcanoes at depths from 1240 to 2500 m these new observations plus laboratory study of new hyaloclastite specimens extend the results of previous studies most of the hyaloclastite samples are of hydrovolcanic eruptive origin but a few show evidence of a predominantly sedimentary origin primarily from morphology we identify several vent areas from which hyaloclastite presumably erupted the surface appearance of the hyaloclastite deposits varies with distance to these vents leading us to propose a facies model for deep sea hyaloclastites on seamount summits hyaloclastites of hydromagmatic origin exhibit weak normal grading and bedding parallel alignment of platy shards they consist of blocky sliver and fluidal basalt glass shards and lithics in a matrix that contains pelagic sediment the shards themselves are remarkably free of even the tiniest crystals and are usually chemically homogeneous we propose that the shards form mainly by cooling contraction granulation but cannot rule out the possibility of limited steam explosivity hyaloclastites are closely associated with submarine pahoehoe and we propose that a very rapid eruption rate promoting clastic dominated versus flow dominated eruptive behavior is the dominant control on hyaloclastite formation we propose that shard formation occurs during submarine lava fountaining gravitational instability of the resulting slurry of shards sea water and possibly steam causes gravity flow that carries the shards outward from the vent further field and modelling studies are needed to test these ideas and more quantitatively constrain the ascent mechanism eruption dynamics and deposition of deep sea hyaloclastites

gold deposits of the northern margin of the north china craton multiple late paleozoic mesozoic mineralizing events the northern margin of the north china craton is well endowed with lode gold deposits hosting a resource of approximately 900 tonnes t of gold the 1 500 km long region is characterized by east trending blocks of metamorphosed archean and proterozoic strata that were episodically uplifted during variscan indosinian and yanshanian deformational and magmatic events at least 12 gold deposits from the daqinshan yan liao includes the zhangjiakou yanshan and chifeng gold districts and changbaishan gold provinces contain resources of 20 100 t au each most deposits are hosted in uplifted blocks of precambrian metamorphic rocks although felsic paleozoic and mesozoic plutons are typically proximal and host 30 of the deposits the lodes are characterized by sulfide poor quartz veins in brittle structures with low base metal values and high au ag ratios although phyllic alteration is most common intensive alkali feldspar metasomatism characterizes the wulashan dongping and zhongshangou deposits but is apparently coeval with variscan alkalic magmatism only at wulashan stepwise 40ar 39ar geochronology on 16 samples from gangue and alteration phases combined with unpublished shrimp u pb dates on associated granitoids suggest that gold mineralizing events occurred during variscan indosinian and yanshanian orogenies at circa 350 250 200 180 150 and 129 ma however widespread permo triassic 250 ma and early jurassic 180 ma thermal events caused variable resetting of most of the white mica and k feldspar argon spectra as well as previously reported k ar determinations compiled and new stable isotope and fluid inclusion data show that most 18o values for ore stage veins range from 8 to 14 indicating a fluid in equilibrium with the precambrian metamorphic basement rocks d values from fluid inclusions range widely from 64 to 154 which is indicative of a local meteoric component in some veins and highly variable 34s data 7 to 17 even within individual deposits indicate various local country rock sources for sulfur fluid inclusions from all districts show variable homogenization temperatures between 240 and 400 c and are consistently characterized by low salinity h2o co2 ch4 n2 solutions although the data are largely consistent with that from orogenic gold veins intrusion related veins and epithermal veins are also recognized the multiple episodes of mineralization are coincident with episodic tectonic reactivations and associated magmatism along the northern margin of the north china craton electronic supplementary material to this paper can be obtained by using the springer link server located at http dx doi org 10 1007 s00126 001 0239 2

was the deposition of large precambrian iron formations linked to major marine transgressions geochemical evidence suggests the iron and silica rich precipitates that compose large neoarchean and paleoproterozoic iron formations formed along a chemocline between iron rich deep water and iron poor surface water sedimento logical evidence indicates many large iron formations accumulated in marine shelf environments during major transgressions we suggest this occurred because the chemocline could only impinge on continental shelves during times of sea level rise or highstand on the other hand the stratigraphic record indicates that deposition of iron formations ceased during regressions in some basins and transgressions in others to explain this we propose that iron concentrations in the precambrian ocean were greatest at mid water depths in a zone on the order of 100 m thick radiometric age dates from the hamersley basin of western australia suggest major iron formation deposition had a periodicity of ca 20 to 125 m y close to second order cycles in phanerozoic sea level driven by long te

geomechanical and ultrasonic characterization of a norwegian sea shale anisotropy of velocity in shaly overburden is known to cause significant problems for geophysical interpretation including depth conversion and fluid identification in addition mechanical and dynamic elastic shale behavior is not well understood because few tests have been performed on well preserved samples multiple stage triaxial tests were performed upon horizontal core plugs of a shale from the norwegian sea with a view to evaluating rock strength and the evolution of ultrasonic response during rock deformation in addition standard rock physical properties were characterized as well as composition the shale microfabric is seen to be strongly laminated with alternating thick clay rich laminae and thin silt rich laminae occasional microfractures are also noted parallel to these laminations the shale has low friction coefficient and cohesive strength and shows anisotropy of these parameters when the maximum principal stress is oriented parallel to and at 45 to the microfabric the orientation of the maximum principal stress parallel to the intrinsic fabric and microcracks was seen to significantly impact on velocity normal to the fabric as stress parallel to the fabric increased s wave anisotropy was significantly affected by the increasing stress anisotropy stress orientation with respect to fabric orientation was therefore found to be an important control on the degree of anisotropy of dynamic elastic properties in this shale

australian volcanic hosted massive sulfide deposits features styles and genetic models the cambro ordovician period has yielded the major development of volcanic hosted massive sulfide vhms deposits in australia resulting in about 12 million metric tons of contained metal copper lead and zinc concentrated in the mount read volcanics tasmania and the mount windsor volcanics queensland the archean 4 million metric tons of metal and the silurian 3 5 million metric tons of metal constitute the next important episodes while the devonian and permian have produced isolated deposits the deposits range from cu type to zn cu type to zn pb cu type the zn cu type deposits are restricted to the archean and silurian whereas the cu type and zn pb cu type occur sporadically throughout the time span from early archean to permian in general terms the major vhms bearing districts are calc alkaline in character and display a thick basal pile of rhyolitic volcanics 1 3 km thick including lavas epiclastics and subvolcanic intrusions which is overlain by a polymodal sequence containing various proportions of rhyolite dacite andesite basalt and sediments the major deposits are commonly located at the top contact of the rhyolite pile or within the lower part of the overlying polymodal sequence there is a wide range in variability of styles of australian vhms deposits including mounds pipes sheets layered deposits stacked deposits stockwork and disseminated deposits distal reworked deposits and cyclic layered deposits although these various styles show a range of features there is a consistent theme across the spectrum of metal zonation alteration mineralogy alteration chemistry sulfur isotopes macrotextures microtextures and host volcanic relationships which strongly suggests that they all belong to the one genetic group of ore deposit the classic mound style deposits such as hellyer have a series of key features subsets of which are represented in the other deposit styles the mound deposits are considered to form from deposition of metal sulfides on the sea floor immediately around the hydrothermal vent growth of the mound occurs by upward replacement of sulfide assemblages stable at higher fluid temperatures leading to zone refining and lead zinc silver gold enrichment in the outer and upper parts of the mound departure from the classic mound style of deposit is related principally to three key factors the chemistry of the ore fluid salinity temperature f02 and ah2s the nature permeability and chemistry of the volcanic pile and the sea hoor environment sea hoor topography and seawater depth these factors control the aspect ratio of the deposit the extent of stringer zone development the degree of subsea floor replacement mineralization the nature and spatial development of footwall alteration and the development and style of related distal mineralization sulfur isotope studies of australian vhms deposits indicate that reduced seawater sulfate is a major source of sulfur in the deposit whereas lead and strontium isotope results are compatible with the metals being derived by seawater convection and leaching from the volcanic pile and basement rocks this conclusion is supported by the application of various leaching models which demonstrate the availability of an adequate source of both base and precious metals in the footwall volcanic sequences the input of metals directly from volcanic magma chambers is not precluded by the available data and one likely scenario involves the contribution of gold and copper from a magmatic vapor plume rising to mix with convective seawater fluids which contribute lead zinc silver and gold leached from the footwall volcanics and basement rocks the relative importance of the magmatic input compared with the seawater convective input may help to explain the spectrum of deposit styles arid their spatial relationship to volcanic centers or adjacent sedimentary basins

the silica cycle in the precambrian abstract whereas the modem silica cycle is heavily dependent on the biology of silica secreting organisms such as the diatoms the precambrian silica cycle was dominated by inorganic reactions among dissolved silica clay and zeolite minerals and organic matter there is no evidence for deposition of a layered amorphous silica but abundant evidence for diagenetic silicification in the neoproterozoic the time period chosen for this analysis the influx of silica to the neoproterozoic ocean would have been governed as today by the balance among tectonism weathering and hydrothermal input the evidence of neoproterozoic rocks favors tectonic and weathering regimes not very dissimilar to those of the early phanerozoic and hydrothermal inputs significantly altered at certain periods one flux would have been very different the present diffusional influx of silica from interstitial waters into the oceans would have been altered to an efflux from the oceans to interstitial waters reactions of dissolved silica with inorganic phases would have controlled silica concentrations at a level of about 60 ppm this figure is based on experimental determinations of solubility and silica sorption on clay minerals and zeolites as silica in interstitial waters was lowered by opal ct precipitation a diffusional efflux from the ocean into the sediment was set up silicification would have started in early diagenesis as opal ct precipitated from interstitial waters diffusion of silica from overlying seawater declined as the sediment was buried a few hundred meters and the diffusion path lengthened during this still relatively shallow burial silicification continued but this time powered by hydrologie transport of dissolved silica from coastal plain sediments during transgressions and regressions thus the major removal of silica from the neoproterozoic ocean took place by diagenetic reactions

geochemical characteristics of the triassic tethys turbidites in northwestern sichuan china implications for provenance and interpretation of the tectonic setting the triassic tethys graywacke slate turbidites in the northwestern sichuan china have been analysed for major and trace elements and petrographically studied compared to phanerozoic turbidites the turbidites in this study are quartz intermediate average 55 in composition and are characterized geochemically by their moderate fe2o 3 mgo 9 28 2 84 tio2 0 72 0 17 contents and al2o3 sio2 0 25 0 10 ratios moderate abundances of ferromagnesian trace elements co 15 8 ppm cr 113 72 ppm ni 14 9 ppm sc 12 4 ppm etc and moderate contents of incompatible elements such as th 11 3 ppm u 3 0 7 ppm zr 170 64 ppm hf 4 1 ppm and total rees 159 33 ppm in general the slates show systematically higher fe2o 3 mgo sc co and eueu but lower sollasc and thsc than the associated graywackes suggesting that various provenance components may separate into different grain size fractions during sedimentary sorting processes that is the more mafic materials tended to incorporate into the sedimentary record for the clay size fraction framework modes and geochemical data indicate that the turbidites were mainly derived from a recycled orogenic provenance characterized chiefly by sedimentary metasedimentary rocks and granite gneisses similar to the upper continental crust but with a variable admixture of continental island arc volcanic components flysch deposition took place in a back arc basin situated between an active continental margin the kunlun qinling fold belt and a continental island arc the yidun island arc weathering conditions in the source area significantly influenced the composition and distribution of elements in the sediments with the elapse of time during sedimentation the degree of chemical weathering in the provenance became intense while the tectonic activity decreased gradually

2 7 ga plume associated vhms mineralization in the eastern goldfields superterrane yilgarn craton insights from the low temperature and shallow water ag zn au nimbus deposit economic volcanic hosted massive sulfide vhms deposits of the archaean yilgarn craton of western australia are restricted to zones of juvenile crust as revealed through regional nd pb and hf isotopic variations and the geochemistry of felsic volcanic rocks interpreted as archaean paleo rift zones one of these runs n s through the eastern goldfields superterrane broadly coincident with the kurnalpi terrane and is associated with the high grade ca 2690 ma teutonic bore jaguar and bentley deposits plus sub economic vhms mineralization further south to date only small historic cu occurrences e g anaconda and barren pyritic lenses have been recognized in the older 2 7 ga plume dominated lower stratigraphy of the eastern goldfields superterrane the nimbus ag zn au deposit 12 1 mt at 52 g t ag 0 9 zn and 0 2 g t au is located approximately 10 km east of kalgoorlie near the margin of the kurnalpi terrane its origin has been contentious for a number of years with previous models favouring seafloor sub seafloor vhms mineralization or a high sulfidation fault hosted system we present a detailed account of the deposit its host stratigraphy and associated hydrothermal alteration plus two new shrimp u pb zircon ages pb isotope galena and o isotope zircon constraints compared to other vhms occurrences in the yilgarn craton the nimbus deposit is unusual in terms of its tectono stratigraphic position the geochemistry of its host sequence i e fi affinity felsic volcanic rocks ocean plateau like low th basalts mineralogy e g abundance of ag sb pb as bearing sulfosalts high hg low cu and quartz carbonate sericite dominated alteration assemblages classification of nimbus as a shallow water and low temperature vhms deposit with epithermal characteristics i e a hybrid bimodal felsic deposit is consistent with its position near the margin of the kurnalpi paleo rift zone and radiogenic 238u 204pb values the recognition that the nimbus deposit is associated with c 2705 ma plume magmatism opens new areas for vhms exploration in the eastern goldfields superterrane over a strike length exceeding 500 km

woodlawn zn pb cu sulfide deposit new south wales australia an interpretation of ore formation from field observations and metal zoning the woodlawn zinc lead copper silver deposit new south wales australia contains two styles of mineralization polymetallic massive sulfides and footwall chalcopyrite mineralization stockwork copper ore the deposit was discovered following roadside reconnaissance soil geochemical sampling in the late 1960s at the start of production in 1978 in situ ore reserves amenable to open cut mining totaled 6 3 million metric tons of complex ore with an average grade of 1 7 percent cu 5 5 percent pb 14 4 percent zn and 89 ppm ag and 3 7 million metric tons of stockwork copper ore averaging 1 9 percent cu the deposit occurs in middle to upper silurian pyroclastics volcanogenic sediments and carbonaceous shales intruded by diabase sills on a regional scale the silurian rocks occupy synclinorial zones over a basement of upper ordovician metasediments with early devonian granitoids in adjacent anticlinorial zones in the main lens of massive sulfides pyrite is the major mineral with variable sphalerite galena and chalcopyrite and minor arsenopyrite tetrahedrite tennantite pyrrhotite marcasite and electrum silicate gangue minerals include talc quartz chlorite and phlogopite the stockwork ore is a network of fracture filling assemblages of pyrite and chalcopyrite with subordinate to minor sphalerite galena pyrrhotite quartz calcite chlorite and barite stockwork ore is the center of an aureole of hydrothermal alteration around the deposit reconstruction of the structural depositional setting indicates that sulfide deposition occurred on an unconformity within a submerged graben metalliferous fluid beneath the sea floor migrated from the south bounding 760 fault updip in a northerly direction through permeable volcanic horizons the north bounding 790 fault was apparently active in a growth sense during the later stages of sulfide deposition the ore horizon hanging wall sequence and 790 fault were displaced in a right lateral sense by the 750 fault during regional deformation a preliminary interpretation of ore formation is developed from field observations and blast hole base metal zoning relationships above the sea floor base metal zonation patterns outline mounds of chalcopyrite and pyrite capped by massive sphalerite galena ore beneath the mounds copper concentration patterns 0 5 wt in the footwall are interpreted as a network of fossil feeder channels the confluence regime of the convection cell streamlines the depositional history of the main woodlawn lens and stockwork copper zone is divided into three stages i precipitation and deposition ofpyrite chalcopyrite from early hot silica rich metalliferous solutions mixing with seawater to form sulfide mounds adjacent to vents current dispersion of colloidal galena and sphalerite quench precipitated higher in the plume ii deposition of sphalerite and galena in a basinal low north of the mounds from cooler fractionated negatively buoyant ore solutions and iii onlapping sedimentation of sulfides over mounds choking off venting ore solutions followed by subsurface hydrothermal alteration and diffusion of solutions through unconsolidated sulfides to form zn pb rich chimney structures above and to the north of mounds woodlawn s detailed metal zoning architecture provides an interesting comparison between paleozoic volcanogenic massive sulfide mineralization and modern sea floor hydrothermal activity despite the different tectonic settings and metal contents similarities can be drawn between off axial sulfide structures on the east pacific rise and the woodlawn cu rich mounds

geologic setting of the teutonic bore massive sulfide deposit archean yilgarn block western australia the teutonic bore massive sulfide deposit occurs in a sequence of archean mafic and felsic volcanic rocks in the northeastern yilgarn block western australia felsic volcanic rocks form part of a regionally continuous linear belt consisting of locally emergent volcanic centers separated by interbedded aquagene tuffs and water lain epiclastic sedimentary rocks mafic and felsic rocks in the area have been extensively intruded by gabbro and subsequently by high level granitoids believed to be cogenetic with the rhyolitic rocks regionally the deposit lies in a linear zone of complex structure and lithology characterized by repeated periods of extensional tectonism rhyolite and cogenetic granitoids are related to earlier phases of extension whereas subsequent activity led to the formation of a suite of saturated to undersaturated syenites and fault bounded units of polymictic conglomerate derived largely from the rhyolite and high level granitoids the teutonic bore deposit consists of one relatively small steeply dipping lens of predominantly banded massive sulfides mainly pyrite sphalerite chalcopyrite and galena underlain by concordant and discordant stringer mineralization the mineralization occurs within a sequence of pillowed basalt and tuffaceous sediment 0 to 150 m stratigraphically above rhyolitic volcanic and volcaniclastic rocks primary textures within the massive sulfides and underlying chert indicate depositional instability active growth faulting controlled the deposition of felsic epiclastic sedimentary rocks subsequent basaltic rocks and the development of the third order basin which provided the site for deposition of massive sulfides the faults probably provided the conduits for hydrothermal solutions these fluids spread laterally and vertically prior to exhaling at the rock seawater interface during a volcanic hiatus features within the local environment are compatible with regional extension regional extension resulted in bimodal magmatism and the associated rifting formed basins suitable for the deposition of extensive epiclastic sedimentary rocks the bimodal volcano plutonic host at teutonic is similar to many examples from the archean of canada but the style of felsic volcanism the carbonate rich alteration and the lead and silver enriched ore are most similar to deposits in the sturgeon lake area

archean synvolcanic intrusions and volcanogenic massive sulfide at the genex mine kamiskotia area timmins ontario u pb geochronology and detailed field mapping has resulted in a new subdivision of the kamiskotia volcanic complex into lower and upper kamiskotia strata with a 3 5 m y volcanic hiatus between the two the hiatus is interpreted as an unconformity in common with newly recognized unconformities throughout the greenstone belt the genex and other volcanogenic massive sulfide vms deposits in the area occur within the upper kamiskotia strata volcanic facies mapping and reconstruction indicate that the genex vms deposit formed within a volcanic graben where synvolcanic mafic and intermediate sills and dikes with peperitic and locally pillowed contacts define a proximal volcanic vent area sulfide mineralization at genex occurs in three zones the first hosted in pillow breccia and hyaloclastite the second occurring at the contact between felsic tuff and an intermediate intrusion and the third hosted within the intermediate intrusion the mineralization represents subsea floor replacement sulfides localized within zones of higher primary permeability although the synvolcanic mafic intrusions are not directly related to the genex vms mineralization they are indicative of a high heat flow thermal regime the synvolcanic faults that controlled the emplacement of synvolcanic dikes also provided conduits for the hydrothermal fluids responsible for the mineralization and alteration at genex this high level mafic dike and sill complex was previously correlated with the upper zone of the kamiskotia gabbroic complex but is now interpreted to be younger and correlative with the upper kamiskotia volcanic complex new geochronological data also indicate that the kamiskotia gabbroic complex is older than the vms hosting upper kamiskotia strata and was emplaced into the lower kamiskotia volcanic complex this negates the possibility of a genetic relationship between the genex and other kamiskotia vms deposits within the kamiskotia gabbroic complex the small size of the synvolcanic intrusions indicates that they were also not the heat source for the genex hydrothermal alteration system however their spatial coincidence suggests that this part of the kamiskotia volcanic complex was the focus of long lived intrusive activity and high heat flow that defined a thermal corridor encompassing the vms deposits in the kamiskotia volcanic complex

tectonic evolution of the east qinling mountains china in the palaeozoic a review and new tectonic model east asia is a composite assemblage built up from various plates and the qinling mountains are prominent among the orogenic belts dividing these plates because of its complex and prolonged history interpretation of the geology of the qinling mountains has led to a variety of tectonic models we review and summarize recently published data and previous work and develop a new tectonic model for the eastern part of this mountain range four tectonic zones are recognized from north to south the lesser qinling the north qinling the central qinling and the south qinling zones the lesser and north qinling zones are interpreted as a late proterozoic to early palaeozoic passive continental margin bordering the north china block the central qinling separated from the lesser north qinling zone by a suture zone mainly consists of volcanic arcs and a high grade metamorphic complex the south qinling zone forms part of the northern margin of the yangtze block and has a long latest proterozoic to middle triassic depositional history as revealed by its sedimentary cover available data do not support the hypothesized existence of a major suture divide between the central and the south qinling zones these two zones seem to have collectively formed an early palaeozoic destructive continental margin on the northern edge of the yangtze block with the central qinling as an island arc system and the south qinling as a wide highly attentuated marginal basin late ordovician to early silurian collision between the north china and the yangtze block built up the palaeozoic orogenic belt in the northern part of the qinling and gave rise to the embryonic east asian continent yet the south qinling was transformed to a remnant basin and remained undeformed until the late triassic when the palaeo tethys collision to the south of the yangtze block drove the yangtze block to move northeastwards deformation in a transpressive regime during this late event largely reconstructed the qinling belt and hence further complicated its tectonics

autochthonous development of superior province greenstone belts abstract models for superior province greenstone belt development have changed over the last 30 years from ensimatic generally autochthonous models to suspect allochthonous terrane models based on the plate tectonic paradigm models intermediate between these end members such as autochthonous terrane models have not been prominent within the superior province there is a secular variation from early ca 3 ga platformal assemblages with overlying rift related komatiite tholeiite units representing breakup of ca 3 ga continental fragments to late ca 2 7 ga pull apart basins autochthonous terranes should demonstrate some sort of logical order such as that described above whereas allochthonous terranes will be largely oceanic in character and should be essentially random in their occurrence rather than exhibiting secular order in the face of this situation this paper reviews evidence from throughout the superior province that suggests many but not all greenstone assemblages developed autochthonously or parautochthonously the earliest greenstone assemblages in the superior province are generally quartzite and carbonate bearing platform sequences in the north caribou terrane and the marmion terrane deposited unconformably upon granitoid or greenstone basement the evidence for autochthonous parautochthonous development includes the presence of unconformities between keewatin type greenstone units such as 1 basal unconformities between ca 3 ga granitoid or volcanic basement and slightly younger platformal assemblages 2 between ca 2 9 ga platformal assemblages and younger platformal and andean arc assemblages stratigraphic patterns such as adjacent greenstone belt assemblages representing sharply differing geodynamic settings with some indication of palinspastic order or progression in the northern central and southern superior province are considered suggestive of orderly autochthonous progression from platforms through rifting of continental fragments and late assembly during the kenoran orogeny petrogenetic data indicating involvement of older granitoid crust in genesis of greenstone assemblages of a variety of ages throughout the superior province suggests the presence of older basement implying autochthonous or parautochthonous development of many assemblages younger greenstone assemblages in the abitibi wawa subprovince and the north caribou terrane contain xenocrystic zircons similar in age to stratigraphically underlying greenstone assemblages this pattern has been interpreted to represent autochthonous greenstone development the current above enumerated evidence for autochthonous development of greenstone assemblages is generally indirect few actual unconformities between keewatin type greenstone assemblages have been recognized in the superior province to aid in future recognition of unconformities in keewatin type greenstone assemblages examples of unconformities of varying degrees of subtlety in archean sequences are reviewed archean unconformities ranging from sharp angular discordances in the northern superior province to subtle stratigraphic patterns to weathering related geochemical mobilization along inter formational contacts are described these styles of unconformity in archean sequences suggest previously unrecognized major disconformities and unconformities may well exist between keewatin type assemblages in the superior province generally and the abitibi wawa subprovince specifically the most important disconformity suggested in the abitibi wawa subprovince is between the ca 2726 ma deloro and the ca 2708 ma tisdale assemblages the presence of unconformities and disconformities coupled with the indirect lines of evidence cited above suggest an autochthonous origin for many superior province greenstone belts this autochthonous origin involves either autochthonous or parautochthonous evolution of assemblages at least in part in place that is development of ca 3 ga platform sequences in selected locales followed by in place evolution of a succession of plume oceanic and arc assemblages culminating in late pull apart basin sequences implications of the autochthonous model for greenstone belts of the superior province particularly the abitibi wawa subprovince include the future ability to recognize unconformity bounded sequences potentially improving regional scale correlation

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

subsea floor replacement in volcanic hosted massive sulfide deposits abstract recent research on volcanic hosted massive sulfide vms deposits indicates that syngenetic subsea floor replacement ores form an important component of many deposits in the context of vms deposits subsea floor replacement can be defined as the syn volcanic formation of sulfide minerals within pre existing volcanic or sedimentary deposits by infiltration and precipitation in open spaces fractures inter and intra granular porosity as well as replacement of solid materials there are five criteria for distinguishing subsea floor replacement in massive sulfide deposits 1 mineralized intervals are enclosed within rapidly emplaced volcanic or sedimentary facies lavas intrusions subaqueous mass flow deposits pyroclastic fallout 2 relics of the host facies occur within the mineral deposit 3 replacement fronts occur between the mineral deposit and the host lithofacies 4 the mineral deposit is discordant to bedding and 5 strong hydrothermal alteration continues into the hanging wall without an abrupt break in intensity criteria 1 3 are diagnostic of replacement whereas criteria 4 and 5 may suggest replacement but are not alone diagnostic because clastic sulfide ores contain accessory rock fragments collected by the parent sediment gravity flow s during transport criteria 2 can only be applied to massive semi massive disseminated or vein style deposits and not clastic ores the spectrum of vms deposit types includes deposits that have accumulated largely subsea floor and others in which sedimentation and volcanism were synchronous with hydrothermal activity and precipitation of sulfides occurred at and below the sea floor over the life of the hydrothermal system deposits that formed largely subsea floor are mainly hosted by syn eruptive or post eruptive volcaniclastic facies gravity flow deposits water settled fall autoclastic breccia however some subsea floor replacement vms deposits are hosted by lavas and syn volcanic intrusions sills domes cryptodomes burial of sea floor massive sulfide by lavas or sediment gravity flow deposits can interrupt sea floor mineralization and promote subsea floor replacement and zone refining the distance below the sea floor at which infiltration and replacement took place is rarely well constrained with published estimates ranging from less than 1 to more than 500 m but mainly in the range 10 200 m the upper few tens to hundreds of metres in the volcano sedimentary pile are the favoured position for replacement as clastic facies are wet porous and poorly consolidated in this zone and at greater depths become progressively more compacted dewatered altered and less amenable to large scale infiltration and replacement by hydrothermal fluids furthermore sustained mixing between the upwelling hydrothermal fluid and cold seawater is regarded as a major cause of sulfide precipitation in vms systems and this mixing process generally becomes less effective with increasing depth in the volcanic pile the relative importance of subsea floor replacement in vms systems is related principally to four factors the permeability and porosity patterns of host lithofacies sedimentation rate the relative ease of replacement of host lithofacies especially glassy materials and early formed alteration minerals during hydrothermal attack and physiochemical characteristics of the hydrothermal fluid

u pb zircon ages of granitoid rocks in the north dabie complex eastern dabie shan china the north dabie complex ndc comprises a high temperature metamorphic suite dominated by amphibolite facies granitoid orthogneiss intruded by voluminous granites u pb zircon dating on these rocks yields the following new results 1 two of the granitoid orthogneiss samples have early cretaceous protolith emplacement ages of 133 7 2 3 ma and 134 0 2 8 ma 2 an undeformed granodiorite intrusive into granitoid gneiss yields an emplacement age at 125 6 0 3 ma these early cretaceous ages entirely post date late triassic early jurassic continental subduction and exhumation evident in the neighboring ultrahigh and high pressure metamorphic terrains to the south in the dabie shan hence they do not support previous tectonic models that infer paleozoic or triassic protolith emplacement ages for these granitoid orthogneisses 3 a mylonitic granite from the northern boundary of the ndc yields a concordant age of 756 6 0 8 ma which is interpreted as magma emplacement age the concordance of this late

seismic anisotropy of shales the seismic properties and preferred clay mineral orientation of a suite of shales are investigated using laboratory velocity measurements as a function of confining pressure x ray diffraction techniques and electron microprobe backscatter bse imaging the velocity measurements indicate that these shales are transversely isotropic with the main symmetry axis perpendicular to bedding anisotropy at elevated pressures caused mainly by preferred orientation of clays illite parallel to bedding ranges from 20 vp and 19 vs for a sample of new albany shale to 30 vp and 35 vs for a sample of chattanooga shale the degree of clay mineral alignment in the shales is constrained by orientation indices produced using simple x ray diffraction techniques a strong positive correlation is found between the degree of preferred orientation as expressed in the orientation indices and seismic anisotropy bse images of the shale fabrics confirm in a qualitative manner the results of the x ray study to investigate wave propagation in the shales elastic constants of each sample are calculated and used to produce phase and group wave velocity surfaces which describe variation in velocity as a function of angle to the bedding normal the calculated velocity surfaces constrained by independent velocity measurements display a lack of shear wave splitting at near normal incidence in even the most anisotropic shales for the highly anisotropic chattanooga shales group velocity surfaces differ significantly from corresponding phase velocity surfaces

emplacement origins of coarsely crystalline mafic rocks hosted in greenstone belts examples from the 2 7 ga yilgarn craton western australia coarsely crystalline mafic rocks commonly referred to as dolerite and diabase make up a substantial proportion of archean greenstone belts and are hosts to major orogenic gold deposits yet their internal architecture and contact relationships remain poorly documented and their emplacement origins are assumed rather than supported with evidence we present results from drill core logging of several late archean coarsely crystalline packages from the eastern goldfields superterrane of the yilgarn craton western australia including the golden mile dolerite these large mafic bodies comprise mostly medium to coarsely crystalline mafic rocks but also include a variety of other products of differentiation the bodies range from 75 to 460 m thick and extend for tens of kilometers are primarily hosted in mudstones and basalt and appear to be concordant to stratigraphy as such this study excludes mafic dykes and extremely thick and internally complex bodies such as the bushveld complex from base to top the internal stratigraphy generally consists of a lower unit of aphyric basalt dolerite gabbro and peridotite pyroxenite then a voluminous middle dolerite gabbro and an upper quartz dolerite gabbro and a topmost aphyric basalt granophyric veins and domains are common in the upper third of the mafic body contacts are gradational between all lithofacies with the exception of granophric veins which have sharp contacts within this overall stratigraphic framework there is variability in the relative thickness of lithofacies number of lithofacies as well as repetition of some lithofacies several examples also include basalt in the interior in well exposed examples the key evidence for distinguishing intrusions from extrusions is provided by the top contact however the top contact from these examples is ambiguous to overcome this gap we review the character of well studied thick ponded lavas from the literature most aspects of the internal character are not unique to intrusions although the maximum groundmass grain size excluding granophyric veins of the archean examples 3 mm is significantly coarser than that for any known ponded lavas 1 mm the ability to distinguish intrusions from extrusions has important implications for reconstructing archean greenstone stratigraphy and mineral exploration

the fluid evolution of the nimbus ag zn au deposit an interplay between mantle plume and microbial activity abstract the nimbus ag zn au volcanic hosted massive sulfide vhms deposit represents an exceptional orebody in the yilgarn craton of western australia it is located in a zone of juvenile crust running n s through the eastern goldfields superterrane however unlike most other vhms occurrences associated with the 2690 2680 ma rift sequence teutonic bore jaguar bentley erayinia it is hosted by the ca 2705 ma plume related stratigraphy which is more typically associated with komatiite hosted nickel sulfide mineralization the nimbus deposit displays hybrid vhms epithermal characteristics resulting from low temperature and shallow water conditions which developed a quartz carbonate sericite dominated alteration assemblage in the dacite host rocks sulfide mineralization comprises pyrite sphalerite galena arsenopyrite ag sb pb bi sulfosalts and rare chalcopyrite in this study we applied a combination of in situ analytical techniques to monitor the evolving sulfur isotope signature of the mineralizing fluids and propose that the nimbus vhms deposit developed as a result of a bimodal fluid history in the first stage the hydrothermal system powered by plume related magmatism deposited a series of barren pyrite lenses with colloform textures their variable but consistently negative mass independent sulfur isotope fractionation signature 33s 0 81 reflects the interaction between mantle derived magmatic fluids and archean seawater the ubiquitous presence of carbon rich porous textures and the remnants of carbonaceous nests in colloform pyrite also indicate a significant contribution of microbial sulfate reducers during the incipient stages of sulfide precipitation the gradual formation of colloform lenses eventually sealed the hydrothermal system establishing a physical barrier that limited the interaction with seawater this process promoted the onset of higher temperature and pressure conditions required to form the high grade zn ag mineralization sourcing sulfur and fluids largely from a magmatic reservoir 33s 0 09 whereas the identification of a predominantly magmatic sulfur source is supported by sulfur isotope signatures the recognition of a magmatic origin of the fluid may only be inferred indirectly it is consistent with the observed quartz sericite alteration style and the proposed mechanism of sulfide deposition via decompression which does not require any interaction with seawater the trace element distribution of the ore related sulfides also supports a closed hydrothermal system developed through multiple fluid pulses indeed this ore forming scenario is commonly favored by high confining pressures which at nimbus were established following the deposition of the colloform pyrite lenses that progressively sealed the hydrothermal system creating the favorable conditions for mineralization

tectonics and distribution of gold deposits in china an overview gold exploration in china has expanded rapidly during the last two decades since a modern approach to economic development has become a national priority china currently produces 180 tonnes t of gold annually which is still significantly less than south africa usa and australia however china is now recognized as possessing significant gold resources in a wide range of mineral deposit types present estimates of gold resources in china exceed 4 500 t which comprise 60 in gold only deposits more than 25 in base metal rich skarn porphyry and vein deposits and more than 10 in placer accumulations the major gold provinces in china formed during the main episodes of phanerozoic tectonism such tectonism involved interaction of china s three major precambrian cratons north china tarim and yangtze or south china when combined with cathaysia block with the angara or siberian kazakhstan kyrgyzstan and indian cratons resulting collisions included deformation of accreted oceanic sequences between the cratonic blocks the most important ore forming orogenies were 1 the late paleozoic variscan 405 270 ma which led to amalgamation of the angara north china and yangtze cratons 2 the indosinian 270 208 ma which led to the collision of north china and south china cratons 3 the yanshanian 208 90 ma which was largely influenced by the subduction of the izanagi pacific plates beneath eastern china and 4 the himalayan 90 ma indentation of the indian continent into eurasia no important precambrian gold systems are recognized in china mainly because of reworking of exposed precambrian rocks by these younger orogenies but there are a few caledonian 600 405 ma gold bearing systems in northern xinjiang most of china s orogenic epithermal and carlin like gold deposits are in the reworked margins of major cratonic blocks and in metasedimentary rock dominated fold belts adjacent to these margins accordingly the major gold provinces are present along the northern southeastern and southern margins of the north china craton along the southwestern and northwestern margins of the yangtze craton in the tianshan and altayshan orogenic belts in northern xinjiang and throughout the southeastern china fold belt gold placer deposits derived from these primary deposits are concentrated in the northernmost part of northeastern china and along the northwestern margin of the yangtze craton the major provinces with significant gold in porphyry related copper systems and base metal skarns are present in the yangtze river area along the northeastern and southeastern margin of the yangtze craton in the fold belt in southwestern china and scattered through northern china three quarters of the chinese gold only deposits occur within the north china craton margins half are located in the uplifted precambrian metamorphic rocks and most of the remainder are hosted in the phanerozoic granitoids that intruded the reworked precambrian terranes the abundance of granite hosted gold contrasts the north china craton with other precambrian cratons such as those in western australia central canada and zimbabwe where gold is mainly hosted in the archean greenstone belts this difference may be explained by the multiple episodes of phanerozoic tectonism along the north china craton margins resulting from the collision of the angara north china and south china cratons and from subduction of the izanagi pacific oceanic plates underneath the eastern china continent

deep submarine pyroclastic eruptions theory and predicted landforms and deposits submarine pyroclastic eruptions at depths greater than a few hundred meters are generally considered to be rare or absent because the pressure of the overlying water column is sufficient to suppress juvenile gas exsolution so that magmatic disruption and pyroclastic activity do not occur consideration of detailed models of the ascent and eruption of magma in a range of sea floor environments shows however that significant pyroclastic activity can occur even at depths in excess of 3000 m in order to document and illustrate the full range of submarine eruption styles we model several possible scenarios for the ascent and eruption of magma feeding submarine eruptions 1 no gas exsolution 2 gas exsolution but no magma disruption 3 gas exsolution magma disruption and hawaiian style fountaining 4 volatile content builds up in the magma reservoir leading to hawaiian eruptions resulting from foam collapse 5 magma volatile content insufficient to cause fragmentation normally but low rise speed results in strombolian activity and 6 volatile content builds up in the top of a dike leading to vulcanian eruptions we also examine the role of bulk interaction steam explosivity and contact surface steam explosivity as processes contributing to volcaniclastic formation in these environments we concur with most earlier workers that for magma compositions typical of spreading centers and their vicinities the most likely circumstance is the quiet effusion of magma with minor gas exsolution and the production of somewhat vesicular pillow lavas or sheet flows depending on effusion rate the amounts by which magma would overshoot the vent in these types of eruptions would be insufficient to cause any magma disruption the most likely mechanism of production of pyroclastic deposits in this environment is strombolian activity due to the localized concentration of volatiles in magma that has a low rise rate magmatic gas collects by bubble coalescence and ascends in large isolated bubbles which disrupt the magma surface in the vent producing localized blocks bombs and pyroclastic deposits another possible mode of occurrence of pyroclastic deposits results from vulcanian eruptions these deposits being characterized by the dominance of angular blocks of country rocks deposited in the vicinity of a crater should be easily distinguishable from strombolian and hawaiian eruptions however we stress that a special case of the hawaiian eruption style is likely to occur in the submarine environment if magmatic gas buildup occurs in a magma reservoir by the upward drift of gas bubbles in this case a layer of foam will build up at the top of the reservoir in a sufficient concentration to exceed the volatile content necessary for disruption and hawaiian style activity the deposits and landforms are predicted to be somewhat different from those of a typical primary magmatic volatile induced hawaiian eruption specifically typical pyroclast sizes might be smaller fountain heights may exceed those expected for the purely magmatic hawaiian case cooling of descending pyroclasts would be more efficient leading to different types of proximal deposits and runout distances for density flows would be greater potentially leading to submarine pyroclastic deposits surrounding vents out to distances of tens of meters to a kilometer in addition flows emerging after the evacuation of the foam layer would tend to be very depleted in volatiles and thus extremely poor in vesicles relative to typical flows associated with hawaiian style eruptions in the primary magmatic gas case we examine several cases of reported submarine volcaniclastic deposits found at depths as great as 3000 m and conclude that submarine hawaiian and strombolian eruptions are much more common than previously suspected at mid ocean ridges furthermore the latter stages of development of volcanic edifices seamounts formed in submarine environments are excellent candidates for a wide range of submarine pyroclastic activity due not just to the effects of decreasing water depth but also to 1 the presence of a summit magma reservoir which favors the buildup of magmatic foams enhancing hawaiian style activity and episodic dike emplacement which favors strombolian style eruptions and 2 the common occurrence of alkalic basalts the co2 contents of which favor submarine explosive eruptions at depths greater than tholeiitic basalts these models and predictions can be tested with future sampling and analysis programs and we provide a checklist of key observations to help distinguish among the eruption styles

late archaean synorogenic basins of the eastern goldfields superterrane yilgarn craton western australia abstract siliciclastic supersequences of the kalgoorlie and gindalbie terranes post date 2660 ma deformation uplift and erosion associated with the amalgamation of arc related terranes but were tectonically buried to greenschist or amphibolite facies during the orogenic shortening that accreted the eastern goldfields superterrane to the yilgarn craton the supersequences are synclinal remnants of late stage basins that were linear zones of subsidence adjacent to north trending strike slip faults fluvial facies merougil navajo jones creek scotty creek and yandal basins are dominated by sheet sandstones that represent the deposits of medial braid plains but proximal channel and bar complexes of cobble and boulder conglomerates are also preserved submarine facies kurrawang basin range from boulder to pebble conglomerates but are dominantly bouma turbidites with rare black shales and record the filling of a proximal canyon that fed a distal submarine fan axial depositional systems are preferentially preserved in all basins but transverse systems are preserved at the base of some fluvial deposits the gross vertical succession in all basins is from proximal up to distal the basins developed during two tectonic stages separated by an intervening period of uplift tilting and erosion with the southern kalgoorlie terrane recording change from a stage i fluvial to a stage ii deep marine basin stage ii fluvial deposits are preserved in the northern kalgoorlie terrane and in the gindalbie terrane sparse palaeocurrents establish sediment dispersal was to the south or southeast implying uplift to the north or northwest controlled palaeoslopes stacking of lithofacies is aggradational and is consistent with limited lateral migration of facies tracts in fault confined basins similar lithofacies and aggradational styles of facies stacking characterise syncollisional sequences of the cenozoic coastal range basin of taiwan and the present day syncollisional basins of southeastern taiwan are good analogues of late stage basins of the kalgoorlie and gindalbie terranes

subaqueous pyroclastic flows and ignimbrites an assessment an assessment of the literature on subaqueous pyroclastic flows and their deposits shows that the term pyroclastic flow is frequently used loosely to describe primary hot gas rich pyroclastic flows mass flows which resulted from the transformation of gassupported flows into water supported ones and secondary mass flows carrying redeposited pyroclastic debris based on subaerial pyroclastic flows the term pyroclastic flow should be restricted to demonstrably hot gas rich mass flows of pyroclastic debris using this definition very few examples of subaqueous pyroclastic deposits with evidence for hot emplacement and of having been wholly submerged have been described in the majority of these cases the evidence for a hot state of emplacement and for the subaqueous nature of the host depositional environment is inadequate the only unequivocal cases of hot pyroclastic flow deposits with adequate supporting evidence are the ordovician nearshore shallow marine ignimbrites of ireland and wales and miocene ignimbrites of southwest japan resulting from the passage of subaerially erupted pyroclastic flows into shallow water other possible examples are near vent dense clast deposits in the donzurobo formation of japan possible submarine intra caldera ponded ignimbrite successions in california and wales and near vent pumiceous deposits of ramsay island wales all other purported cases are either clearly the result of water supported mass flow transportation and deposition debris avalanches debris flows turbidity currents or lack adequate supporting evidence regarding the heat state or the palaeoenvironment only the shallow marine ignimbrites of ireland and wales show adequate evidence of welding but even these could have been nearly wholly exposed above sea level when welding occurred we conclude that when pyroclastic flows enter water they are generally disrupted explosively and or ingest water and transform into water supported mass flows and we suggest the various scenarios in which this occurs there is no evidence to suggest that welding in wholly subaqueous environments is common

fluidal clast breccia generated by submarine fire fountaining trooper creek formation queensland australia abstract a distinctive monomictic breccia composed of fluidal and blocky basaltic andesite clasts occurs in a cambro ordovician submarine volcanic succession in northern queensland australia associated with this fluvial clast buccia facies are coherent facies and coarse and fine breccia facies of the same composition the fluidal clast breccia facies is internally massive and 250 m thick varying only in the ratio of fluidal clasts to blocky clasts fluidal clasts range in size from 2 cm to 170 cm and have moderately to highly vesicular cores and thick up to 1 cm non vesicular formerly glassy rims blocky clasts are highly vesicular to non vesicular subaqueous fire fountain breccias are distinguished from subaerial fire fountain breccias by thick glassy margins on fluidal clasts the lack of welding and agglutination and the distinctive association of highly vesicular fluidal clasts with non vesicular angular blocky clasts recognition of submarine fire fountain breccias in volcanic successions constrains the eruption style proximity tens of metres to source and environment of deposition

age and tectonic setting of late archean greenstone gneiss terrain in henan province china as revealed by single grain zircon dating the authors report precise u pb zircon ages for single grains of a metarhyodacite from the late archean dengfeng greenstone belt in henan province china near the southern margin of the north china craton most grains belong to an igneous population whose u pb isotopic systematics define a straight line intersecting concordia at 2512 12 ma and this is interpreted as the time of crystallization of the original greenstone volcanics several grains are distinctly older between 2576 9 and 2945 44 ma and the authors interpret the older grains as xenocrysts of pre greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava the xenocrysts suggest evolution of the dengfeng greenstone belt in a continental environment that may be represented by the taihua high grade gneisses bordering the dengfeng greenstones and for which the authors obtained ages of 2806 7 and 2841 6 ma the data add evidence to the now widely held concept that most archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination in such cases conventional zircon dating may not always record the precise age of rock formation

resolution analysis for stereotomography in media with elliptic and anelliptic anisotropy stereotomography is a tomographic method that uses slowness vector components to improve traveltime inversion we extend it to general anisotropic media and implement it for media with elliptic and anelliptic anisotropy such media depend on only three elastic parameters which makes stereotomography less sensitive to ambiguity resulting from limited coverage of surface seismic experiments than it would be in transversely isotropic or orthorhombic media the corresponding approximations of the slowness surface restrict the validity of the present approach to qp events and mild anisotropy our numerical analysis demonstrates that stereotomography can be used to estimate macrovelocity models in the presence of anisotropy for elliptic anisotropy the parameter that is most difficult to estimate is the orientation of the symmetry axis for anelliptic anisotropy estimation of stereotomography parameters is possible in principle however it will be mostly ill conditioned in practice even for low noise levels better results are achieved if transmission events from multiple offset vertical seismic profiling experiments are included in the inversion

the filon norte orebody tharsis iberian pyrite belt a proximal low temperature shale hosted massive sulphide in a thin skinned tectonic belt the filon norte orebody tharsis iberian pyrite belt is one of the largest pyrite rich massive sulphide deposits of the world the present structure of the mineralization consists of an internally complex low angle north dipping thrust system of variscan age there are three major tectonic units separated by thick fault zones each unit with its own lithologic and hydrothermal features they are internally organized in a hinterland dipping duplex sequence with high angle horses of competent rocks igneous and detritic rocks and massive sulphides bounded by phyllonites the mineralization is within the lower unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation the intermediate unit is made up of pervasively ankeritized shales and basalts spilites here hydrothermal breccias are abundant the upper unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill the tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody carbon oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature 150 c without indication at least in the exposed section of a high temperature copper rich event sporadic deep subsea floor boiling is probably responsible for the formation of hydrothermal breccias and the wide extension of the stockwork its co au enrichment is interpreted as being related with the superposition of some critical factors such as the relationship with black shales the low temperature of formation and the boiling of hydrothermal fluids the present configuration and thickness of the orebody is due to the tectonic stacking of a thin and extensive blanket 2 4 km2 of massive sulphides with low aspect ratio they were formed by poorly focused venting of hot modified seawater equilibrated with underlying rocks into the seafloor massive sulphide precipitation took place by hydrothermal fluid quenching bacteriogenic activity and particle settling in an unusual restricted euxinic and shallow basin brine pool with a low detritic input but with important hydrothermal activity related to synsedimentary extensional faulting resedimentation of sulphides seems to be of major importance and responsible for the observed well mixed proximal and distal facies the tectonic deformation is largely heterogeneous and has been mostly channelled along the phyllonitic tectonized shales deformation bands thus sedimentary and diagenetic textures are relatively well preserved outside the deformation bands in the massive sulphides superimposed variscan recrystallization is not very important and only some early textures are replaced by metamorphic tectonic ones the stockwork is much more deformed than the massive sulphides the deformation has a critical effect on the present morphology of the orebody and the distribution of the ore minerals this deposit is a typical example of the sheet like shale hosted anoxic low temperature and zn rich massive sulphides developed in a ensialic extensional basin

submarine volcanism eruption styles products and relevance to understanding the host rock successions to volcanic hosted massive sulfide deposits the increasing hydrostatic pressure of the water column with increasing water depth in subaqueous environments limits the ability of superheated volatiles to expand instantaneously against the ambient pressure explosive submarine eruptions are only likely in water depths less than 1 km and generally less than 500 m thus refuting models of highly explosive very deep water calderas popularly proposed as the host volcanic centers for the kuroko volcanic hosted massive sulfide vhms deposits of japan large volumes of blocky breccias can be produced by quench fragmentation during submarine lava eruptions and such deposits may have been mistaken for pyroclastic deposits in many cases in the past pyroclastic debris may occur in the host rock successions of some vhms deposits and may even be very voluminous however facies characteristics and theoretical constraints indicate that these are usually the products of mass flow deposition from shallow water or basin margin volcanic centers the maximum water depths for submarine explosive eruptions coincide approximately with the minimum pressures and water depths required to prevent boiling of mineralizing hydrothermal fluids in the stockwork before the fluids reach the sea floor the key elements in evaluating the prospectivity of ancient volcanic successions for vhms deposits appear to be deep water sediments and lavas or shallow intrusions in an extensional basin setting pyroclastic debris in many cases at least appears to be an accidental externally introduced component there is little evidence that explosive submarine calderas are essential as host volcanic centers to vhms deposits

the scuddles cu zn prospect an archean volcanogenic massive sulfide deposit golden grove district western australia the scuddles volcanogenic massive sulfide prospect together with the nearby gossan hill golden grove prospect constitute the largest known potentially economic cu zn sulfide deposits in the archean yilgarn block of western australia enclosed in a submarine sequence of felsic to intermediate volcanics epiclastics and exhalative chemical sediments the scuddles deposit lies in a regionally extensive mineralized horizon this horizon is up to 80 m thick and is dominated by sulfidic and cherty exhalites and epiclastics a sequence of felsic to intermediate lavas pyroclastics and epiclastics lies above the mineralized horizon with felsic pyroclastics and epiclastics beneath pyroclastic rocks may have been resedimented by mass flow processes the entire sequence has sustained the effects of hydrothermal alteration and regional metamorphism and dips steeply to the west lying on the east limb of a major synclinal structure due to alteration major element values of the rocks are not an accurate guide to their primary composition however abundances of ti zr y nb and v are commonly unaffected and confirm an andesitic to rhyolitic spectrum zoning of ore metals in the scuddles mineralized horizon is typical of that in many precambrian felsic volcanic associated exhalative sulfide deposits the top is defined by a laminated magnetite bearing iron formation enriched in fe mn and base metals which conformably overlies several lenses of massive fe zn sulfides in places containing significant pb ag au and cu grading downward into pyritic sulfides in turn the massive sulfides overlie a zone of epigenetic stockwork stringer and disseminated cu fe sulfides in the middle and lower portions of the mineralized horizon massive to disseminated exhalative sulfides are in places accompanied by quartz chlorite talc and other silicates carbonates and magnetite also interpreted to be at least partly exhalative in origin the lower cu fe sulfide zone is considered to have formed immediately below the sea floor perhaps occupying an extensive feeder system for hot springs the upper zn fe sulfide zone may have formed by precipitation out of hydrothermal plumes as they vented into and mixed with seawater ore mineral deposition was attended by the development of a volumetrically large 10 km long several hundred meters thick strata bound zone of hydrothermal alteration well represented in the footwall and mineralized horizon upper hanging wall rocks are little altered but elsewhere the sequence displays moderate to intense feldspar destructive development of quartz chlorite sericite and locally abundant carbonate talc and sulfides these mineralogical changes are accompanied by pervasive depletion of na ca and sr and locally k the mineralized horizon and much of the footwall is enriched in fe mg mn base and precious metals as s co 2 and h 2 o interpreted to be the result of addition from exhalative sources the strata bound alteration zone represents a volume of hydrothermal flux in which major sites of sulfide deposition at scuddles and gossan hill were focused zones of fe mg base metal enrichment below and adjacent to positions of submarine metal bearing exhalations precipitation of exhalites occurred under relatively quiescent conditions in the mineralized horizon however the stratigraphic sequence indicates that at other times active mass flow deposition was taking place

global comparisons of volcanic associated massive sulphide districts although volcanic associated massive sulphide vms deposits have been studied extensively the geodynamic processes that control their genesis location and timing remain poorly understood compar

did the character of subduction change at the end of the archean constraints from convergent margin granitoids large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite trondhjemite granodiorite ttg in the archean shifting to a combination of ttg calc alkaline granitoid and graywacke control thereafter geochemical differences between ttg and high silica adakites do not require production of most ttg magmas in descending slabs changes in the ratio of ttg to calc alkaline granitoids after 2 5 ga indicate that archean subduction zones must have differed from younger subduction zones in two very important ways 1 a deep mafic crust served as a ttg magma source either as thickened crust or in descending slabs and 2 they did not give rise to significant volumes of calc alkaline magma thickened mafic crust in the late archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during late archean mantle thermal events

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

basaltic sill sediment complexes in young spreading centers genesis and significance in contrast to mature mid oceanic ridges where magmatic activity is little affected by the slow accumulation of sediments in young spreading centers such as that of the guaymas basin in the gulf of california the basaltic magma of great magmatic pulses forms dikes and sills within the uppermost few hundred metres of soft sediments in general younger dikes and sills are injected next to or on top of the contact zone of the older ones in this manner a distinctive sill sediment complex is built up the sediments of which are rather compacted and partly metamorphosed despite the low burial depth the thickness of this transitional zone between the sheeted dike complex seismic layer 2 and younger sediment seismic layer 1 is controlled chiefly by the rates of sedimentation and spreading if the half spreading rate is approximately one order of magnitude greater than the sedimentation rate the sill sediment complex can reach a thickness of only a few hundred metres and the depth of the spreading trough remains approximately constant sedimentation rates approaching or surpassing the spreading rate cause filling up of the basins which probably hampers the injection of magma into sediments sill sediment complexes similar to those in the gulf of california are also expected to occur at the passive margins of older oceanic basins as well as orogenic belts

the 4d evolution of the teutonic bore camp vhms deposits yilgarn craton western australia abstract the teutonic bore camp comprised of the teutonic bore jaguar and bentley deposits is one of the most significant volcanic hosted massive sulphide vhms camps in western australia despite being extensively studied only recently there have been advances in the understanding of the mechanism that drove the formation of mineralisation it has been recognized by recent studies that the volcanic hosted deposits from the teutonic bore camp represent replacement type vhms systems with significant input of fluids and metals from a magmatic source this paper tests the existing hypothesis that the nearby penzance granite acted as the metals source and or thermal engine driving the development of these ore deposits new age constraints on the formation of the host volcanic sequence at the bentley deposit and the crystallization of the penzance granite allows for the construction of a 4d evolutionary model for the ore system a new u pb shrimp monazite age of 2681 9 4 5 ma indicates that the penzance granite post dates the host stratigraphy at bentley ca 2693 ma and is probably coeval with mineralisation all zircons penzance bentley units i and iii have very similar hf i with most values between 1 and 6 slightly higher than the hf i of zircons from other granites and volcanics within the kurnalpi terrain and indicative of juvenile sources the mean th u ratios are 0 7 and 0 6 for the penzance and bentley zircons respectively all zircons have similar ce nd cn ratios the chemical similarities between the zircons from the granite and the volcanic rocks at bentley support a shared magmatic source between the penzance and the teutonic bore camp sequence the penzance granite is the likely source of heat and potentially metals which drove the vhms mineralisation at the teutonic bore camp

approximate dispersion relations for qp qsv waves in transversely isotropic media to decouple qp and qsv sheets of the slowness surface of a transversely isotropic ti medium a sequence of rational approximations to the solution of the dispersion relation of a ti medium is introduced originally conceived to allow isotropic p wave processing schemes to be generalized to encompass the case of qp waves in transverse isotropy the sequence of approximations was found to be applicable to qsv wave processing as well although a higher order of approximation is necessary for qsv waves than for qp waves to yield the same accuracy the zeroth order approximation about which all other approximations are taken is that of elliptical ti which contains the correct values of slowness and its derivative along and perpendicular to the medium s axis of symmetry successive orders of approximation yield the correct values of successive orders of derivatives in these directions thereby forcing the approximation into increasingly better fit at the intervening oblique angles practically the first o

setting of zn cu au ag massive sulfide deposits in the evolution and facies architecture of a 1 9 ga marine volcanic arc skellefte district sweden the skellefte mining district occurs in an early proterozoic mainly 1 90 1 87 ga svecofennian magmatic province of low to medium metamorphic grade in the baltic shield in northern sweden the district contains over 85 pyritic zn cu au ag massive sulfide deposits and a few vein au deposits and subeconomic porphyry cu au mo deposits the massive sulfide deposits mainly occur within and especially along the top of a regional felsic dominant volcanic unit attributed to a stage of intense extensional continental margin arc volcanism from facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow water areas surrounded by deeper seas all the major massive sulfide ores occur in below wave base facies associations however some ores occur close to stratigraphic intervals of above wave base facies associations and the summits of some volcanoes that host massive sulfides emerged above sea level intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism uplift resulting in subregional disconformities and then differential uplift and subsidence resulting in a complex horst and graben paleogeography uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels a few massive sulfide ores formed within the basal strata of this second stage the horst and graben system was filled by prograding fluvial deltaic sediments and mainly mafic lavas and during this stage the skellefte district was a transitional area between renewed arc volcanism of more continental character to the north and subsidence and basinal mudstone turbidite sedimentation to the south this whole volcanotectonic cycle occurred within 10 to 15 m y we define 26 main volcanic sedimentary and intrusive facies in the skellefte district the most abundant facies are 1 normal graded pumiceous breccias which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris 2 porphyritic intrusions and 3 mudstone and sandstone turbidites facies associations define seven main volcano types which range from basaltic shields to andesite cones and rhyolite calderas despite this diversity of volcano types most massive sulfide ores are associated with one volcano type subaqueous rhyolite cryptodome tuff volcanoes these rhyolite volcanoes are 2 to 10 km in diameter 250 to 1 200 m thick at the center and are characterized by a small to moderate volume rhyolitic pyroclastic unit intruded by rhyolite cryptodomes sills and dikes massive sulfide ores occur near the top of the proximal near vent facies association the remarkable coincidence in space and time between the ores and this volcano type indicates an intimate genetic relationship between the ores and the magmatic evolution of the volcanoes many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits we suggest that massive sulfides in the skellefte district span a range in ore deposit style from deep water sea floor ores to subsea floor replacements to shallow water and possibly subaerial synvolcanic replacements facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes

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

the genesis of the archaean welcome well volcanic complex western australia the welcome well volcanic complex east of leonora western australia is interpreted to be the eroded remnant of an archaean stratovolcano andesitic flows and intercalated mudflow deposits comprising the volcanic centre give way to coarse poorly sorted lithic wackes that were deposited in alluvial fans skirting the lower slopes or base of the subaerial volcanic edifice these deposits are succeeded both laterally and vertically by fine grained subaqueous turbiditic sediments that are intercalated with pillowed tholeiitic basalts

evolution of the southern abitibi greenstone belt based on u pb geochronology autochthonous volcanic construction followed by plutonism regional deformation and sedimentation abstract new mapping and u pb zircon geochronological results in the southern abitibi greenstone belt sagb support an autochthonous regional stratigraphy comprised of nine supracrustal assemblages rather than the collage of allochthonous terranes proposed in recent publications based on lithological and geochronological criteria these supracrustal assemblages also encompass the swayze shining tree and montcalm greenstone belts indicating that despite separation by large granitic intrusions they are in fact part of one extensive greenstone belt in conjunction with the mapping the geochronological results support coherent upward facing stratigraphic sections with about 20 of the samples containing inherited zircons with ages similar to those found in underlying assemblages the seven oldest assemblages represent semi continuous volcanism from 2750 to 2697 ma the volcanism is compositionally diverse ranging from komatiite and tholeiitic basalt to calc alkaline mafic to felsic lavas intimate intermingling of the different magma clans occurs throughout much of the stratigraphic section regional fault control on the distribution of the volcanic assemblages provides evidence for early dip slip movement associated with volcanic extension dating back to at least 2725 ma the two youngest assemblages are dominantly sedimentary and were unconformably deposited on the volcanic assemblages in close proximity to regional faults the earlier sedimentary assemblage consists of turbidites and minor iron formation deposited from 2696 to 2692 ma and the youngest one of subaerial conglomerates fluvial sandstones and alkalic to shoshonitic volcanic rocks ranging in age from 2687 to 2675 ma the sedimentary assemblages are broadly contemporaneous with the emplacement of syntectonic granitic plutons regional folding and reactivated movement on the regional faults probably related to accretion of the abitibi to the superior province craton autochthonous repetition of different geodynamic environments over the 50 million years of volcanic activity with five dominantly tholeiitic komatiitic assemblages and two dominantly calc alkaline assemblages as well as the extensive intermingling of these different magma clans in a number of the assemblages appears to be unique to the archean it suggests complex large scale and long lived interaction between mantle plumes and subduction zone magmas the nd isotope data indicate a derivation of the diverse volcanic magmas from homogeneously depleted sources with end 2 5 0 5 demonstrating a lack of contamination by ancient enriched components

geochemistry of volcanic rocks associated with cu zn and ni cu deposits in the abitibi subprovince volcanogenic massive sulfide vhms deposits in the abitibi subprovince are preferentially associated with volcanic successions containing 150 m thicknesses of felsic volcanic rocks approximately 50 by area of volcanic terranes and are found within volcanic sequences of at least three distinct affinities group i which is host to greater than half of the volcanogenic massive sulfide deposits by tonnage and which comprises only approximately 10 percent by area of volcanic terranes is composed of bimodal tholeiitic basalt basaltic andesite and high silica rhyolite the basaltic andesites and high silica rhyolites are characterized by high high field strength element and heavy rare earth element ree contents low light to heavy ree ratios most with la n yb n 0 8 3 and strong negative eu anomalies the kamiskotia matagami and chibougamau lower cycle volcanogenic massive sulfide areas all of which are also underlain by large synvolcanic gabbroic complexes are associated with group i volcanic sequences the kidd creek potter normetal and horne deposits are also included in this category group ii which is host to one third of the volcanogenic massive sulfide deposits by tonnage and which is also approximately 10 percent by area of volcanic terranes is composed of bimodal transitional tholeiitic to calc alkalic andesitc and rhyolite characterized by intermediate high field strength element contents and slightly higher ree ratios la n yb n 1 4 noranda camp excluding the horne deposit and val d or camp group iii is host to only one known deposit the selbaie mine which is unusual in that much of its mineralization cuts stratigraphy the selbaie mine sequence contains calc alkalic andesite rhyolite with relatively low high field strength element and ree contents and higher ree ratios la n yb n 3 9 the vast majority of volcanogenic massive sulfide bearing mafic and felsic volcanic rocks in the abitibi subprovince have la n yb n 5 barren volcanic sequences are group iv calc alkalic basaltic andesite to rhyodacite with low high field strength element and relatively high ree ratios la n yb n 8 20 represented by the upper skead group the quebec hunter mine group and the upper cycle chibougamau rocks and group v mafic to felsic alkalic volcanic rocks with high ree ratios la n yb n 12 62 represented by the timiskaming opemisca and ridout series rocks group i is most similar to thickened oceanic rift suites e g galapagos spreading center iceland east rift group ii is similar to suites in rifled island arcs e g hokuroku district japan and group iii is comparable to continental arc suites e g southern volcanic zone central chile groups iv and v are comparable to arc related suites derived from metasomatized mantle with variable amounts of crustal contamination e g setouchi area japan roman province italy known ni cu deposits in volcanic sequences are hosted exclusively in komatiitic flows and hypabyssal sills represented by chill compositions with high mgo contents 20 35 wt anhydrous very low incompatible element contents and depleted signatures la n sm n 0 5 0 8 zr y 2 5 alexo marbridge and shaw dome deposits in comparison to most barren komatiites in the abitibi subprovince their depleted trace element signatures indicate minimal crustal assimilation which would appear contrary to the widely held view that crustal contamination e g addition of silica sulfur oxygen of a mantle derived melt is necessary for sulfur saturation and magmatic sulfide segregation this problem may be reconciled by considering the dynamics of komatiite flows lesher and arndt 1990 where contaminated present address 23 euclid avenue ottawa ontario canada kis 2w2 komatiite liquid segregated and deposited sulfide and was subsequently swept downstream and replenished by uncontaminated magma over the deposit as the flow ebbed and cooled when considered together with geologic features such as evidence for thermal erosion in the footwall langmuir deposits and the presence of sulfidic footwall rocks shaw dome deposits the distinctive depleted geochemical signature of mineralized komatiites is useful in exploration for ni cu deposits

geochemistry of the la erma and qiongmo au se deposits in the western qinling mountains china abstract the microdisseminated gold deposits at la erma and qiongmo are typically hosted in cambrian chert formation in western qinling mountains china both in the host rocks and the gold ores selenium is present in high abundance as selenide minerals or isomorphous admixtures in sulfides with independent selenium orebodies can be delineated in some localities intergrowth of selenides and native gold is commonly observed selenium is also noticed in considerably high content in fluid inclusions in quartz and barite which are closely related to gold mineralization studies show that gold and selenium may have been transported together in ore solutions as au s se complex in view of the wide difference in electrode potential between selenium and sulfur the two elements would be separated under oxidizing conditions selenium would mostly occur as isomorphic mixtures in sulfides during the early stage of mineralization when the environment is more reducing but would precipitate as selenides during the late oxidizing stage simultaneous enrichment of gold and selenium would be favored by boiling of the ore solution or mixing of ore solution with oxygen rich water at shallow depths

products of neptunian eruptions a common pyroclastic facies in subaqueous volcanic successions comprises massive to graded very thick several to tens of meters laterally extensive several kilometers beds of nonwelded pumice lapilli with volumes ranging to tens of cubic kilometers this facies may be overlain by laminated ash or bimodal ash and giant 1 m pumice clasts and underlain by coarse lithic breccia the association is inferred to be the typical product of sustained magmatic volatile driven explosive eruptions from vents at water depths of 1300 200 m we propose the term neptunian for such eruptions and their products the eruption column rapidly mixes with the surrounding water cools increases in density and collapses while remaining under water lithic clasts that are too heavy to be entrained in the column are deposited close to the source forming a neptunian lithic breccia pumice lapilli are rapidly waterlogged and form the dominant component in the collapsing column and in eruption fed water supported density currents neptunian density currents hot buoyant giant pumice clasts continue to rise and may reach the water surface before being waterlogged and settling along with temporarily suspended ash forming neptunian suspension deposits eruption magnitude fragmentation mechanisms and juvenile pyroclast characteristics especially vesicularity are very similar in neptunian and plinian style eruptions but column behavior differs primarily because of the contrasting physical properties of the ambient fluid water versus air

anisotropic velocity analysis1 results from walkaway vsp and shale laboratory experiments show that shale anisotropy can be significantly anelliptic heterogeneity and anellipticity both lead to non hyperbolic moveout curves and the resulting ambiguity in velocity analysis is investigated for the case of a factorizable anisotropic medium with a linear dependence of velocity on depth more information can be obtained if there are several reflectors the method of dellinger et al for anisotropic velocity analysis in layered transversely isotropic media is examined and is shown to be restricted to media having relatively small anellipticity a new scheme based on an expansion of the inverse squared group velocity in spherical harmonics is presented this scheme can be used for larger anellipticity and is applicable for horizontal layers having monoclinic symmetry with the symmetry plane parallel to the layers the method is applied to invert the results of anisotropic ray tracing on a model sand shale sequence for transversely isotropic media with small anisotropy the scheme reduces to the method of byun et al and byun and corrigan the expansion in spherical harmonics allows the p phase slowness surface of each layer to be determined in analytic form from the layer parameters obtained by inversion without the need to assume that the anisotropy is weak

geologic setting of volcanic associated massive sulfide deposits in the kamiskotia area abitibi subprovince canada the upper archean volcanic succession in the kamiskotia area abitibi greenstone belt timmins region hosts a series of past producing copper zinc volcanic associated massive sulfide vms deposits all of these occur within a restricted east facing stratigraphic interval in the upper part of the kamiskotia volcanic complex new u pb ages for this interval ranging from 2701 1 1 4 to 2698 6 1 3 ma and an age of 2703 1 1 2 ma from the lower part of the kamiskotia volcanic complex indicate that the complex is likely part of the blake river assemblage 2701 2697 ma rather than the older tisdale assemblage 2710 2703 ma the kamiskotia volcanic complex consists largely of felsic and mafic lava flows and vms mineralization appears to have generally developed at or near the sea floor close to inferred synvolcanic faults new u pb ages of 2714 6 1 2 and 2712 3 2 8 ma from the northeast facing volcanic succession in the northern part of the study area loveland macdiarmid and thorburn townships indicate that it forms part of the kidd munro assemblage 2719 2710 ma a west northwest trending faulted contact is inferred between this older succession and the kamiskotia volcanic complex rocks to the south the kidd munro assemblage rocks are coeval with the kidd volcanic complex which hosts the giant kidd creek vms deposit 30 km to the east of the study area the lower part of the succession in south central loveland township consists of high silica fiiib rhyolites these rocks are geochemically similar to ore associated fiiib rocks from kidd creek and seem likely to represent the most prospective part of this succession future exploration in the kamiskotia volcanic complex is probably best focused on the along strike extension of the vms hosting interval and in particular on areas close to the intersections of synvolcanic faults mafic and felsic volcaniclastic strata which can be replaced by vms mineralization and felsic coherent facies flows and or domes appear to be important potential targets