electromagnetic induction studies recent developments in electromagnetic induction studies of the lithosphere and the asthenosphere are reviewed attention is given to geoelectrical studies of active tectonic areas in terms of the major zones of crustal extension the basin and range province along western regions of north america and the rio grande rift studies have also been performed of tectonic activity around iceland the salton trough and cerro prieto and the subduction zones of the cascade mountains volcanic belt where magnetotelluric and geomagnetic variation studies have been done geomagnetic variations experiments have been reported in the central appalachians and submarine electromagnetic studies along the juan de fuca ridge controlled source electromagnetic and dc resistivity investigations have been carried out in nevada hawaii and in the adirondacks mountains laboratory examinations on the conductivity of representative materials over a broad range of temperature pressure and chemistry are described

exploration of mid ocean ridges with a frequency domain electromagnetic system summary electromagnetic survey methods using controlled sources have been applied to ocean floor and mid ocean ridge surveys and have shed new light on the distribution of fluids within these structures the present state of knowledge of the electrical nature of oceanic crust is reviewed and some alternative mid ocean ridge electromagnetic surveys are simulated using a numerical modelling algorithm to see what further information they could provide this simulation considers the response of both large scale crustal targets such as magma bodies and zones of hydrothermal circulation and also the effect of near surface heterogeneity that may spatially alias the data the magnitude of this heterogeneity is estimated and it is shown that it could be large enough to prevent useful structural constraints being obtained if the effect spatial aliasing was negligible or could be removed a trans ridge experiment could delinate zones of partial melting and hydrothermal circulation within the oceanic crust

electromagnetic active source sounding near the east pacific rise an active source electrical conductivity sounding was carried out near the east pacific rise at 21 n the average electrical conductivity from 2 km beneath the seafloor into the upper mantle was found to be about 0 004 s m there is no evidence of fully molten magma at a distance of 7 km from the spreading axis the inferred conductivity with depth of the crustal rock is consistent with brace s model of connected seawater filled pores in rocks heated from beneath

conductivity and fluids in the oceanic upper mantle abstract three deep electromagnetic soundings in the north pacific ocean were re examined our inversion of existing data showed the presence of conductors below the three sites in very good agreement with previous results we showed that h 2 o rich fluids as well as partial melting account for the conductivities observed this led us to determine temperatures ranging firstly from 1050 to 1150 c when only h 2 o rich fluids were present and secondly from 1300 to 1350 c for partial melting only we suggest that the upper mantle below the two oldest sites 30 and 72 ma is characterized by about 1 of partial melting in the presence of about 1 of h 2 o rich fluids below the third site that nearest to the ridge zone 1 ma the degree of partial melting might reach 10 the presence of 1 of h 2 o rich fluids would reduce the effect of the asthenospheric outflows on the lithospheric magmatism this agrees with the principle of thermodynamic decoupling of the asthenosphere and lithosphere the weakening effect of fluids particularly water on upper mantle rheology is also discussed we suggest that the asthenosphere and lithosphere might be decoupled mechanically which if true would affect small scale convection below the oceanic lithosphere

the effect of hydrocarbon saturation on resistivity distribution and csem response abstract the controlled source electromagnetic csem method can be used to map resistivity differences in the sub surface with the purpose of identifying hydrocarbon filled reservoirs prior to drilling exploration wells in this study 15 exploration wells in the norwegian sea were analysed to investigate the relationship between reservoir properties hydrocarbon saturation resistivity and subsequently csem response two discovery wells linerle and falk with similar reservoir properties but different reservoir resistivity were further compared and evaluated through forward csem modelling to identify possible causes of the observed contrasting resistivities the results demonstrate that hydrocarbon saturation which can significantly affect the resistivity of hydrocarbon filled reservoirs may change rapidly over short distances controlling factors include 1 oil density and viscosity 2 reservoir quality and 3 the location of the reservoir relative to the transition zone associated with the oil water contact owc high oil density and poor reservoir quality commonly result in a thicker saturation transition zone than low oil densities and high permeability if the reservoir section is located within the transition zone the average resistivity can be significantly reduced low average resistivity in the linerle reservoir is found to be caused by low hydrocarbon saturation as a combined result of high density oil reservoir heterogeneity and that the reservoir section is situated within the oil water transition zone above the owc higher resistivity in the falk reservoir which has similar thickness and burial depth is attributed to better reservoir properties i e higher permeabilities lighter oil and the location of the reservoir section well above the saturation transition zone associated with the owc the ability to understand and predict key fluid and reservoir parameters is critical for appropriate application of the csem technology in hydrocarbon exploration

increasing the sensitivity of controlled source electromagnetics with synthetic aperture controlled source electromagnetics csem has been used as a derisking tool in the hydrocarbon exploration industry we apply the concept of synthetic aperture to the lowfrequency electromagnetic field in csem synthetic aperture sources have been used in radar imaging for many years using the synthetic aperture concept big synthetic sources can be constructed by adding the response to small sources building blocks in different ways and consequently big sources with different radiation patterns can be created we show that the detectability of hydrocarbons is significantly enhanced by applying synthetic aperture to csem data more challenging targets such as deep reservoirs 4km belowsea floor can be detected the synthetic aperture technique also increases the sensitivity of the field to subsurface targets in the towing streamer acquisition we also show that a pseudovertical source orthogonally distributed dipole pairs can be constructed synthetically and that the detection capability of this pseudovertical source is increased by applying field steering the synthetic aperture concept opens a new line of research in csem with the freedom to design suitable synthetic aperture sources for a given purpose

in situ electrical resistivity and bulk porosity of the oceanic crust costa rica rift in situ electrical resistivity was measured in dsdp hole 504b to a depth of 1 013 m into oceanic basement apparent resistivities are about 10 m in about 600 m of pillow lavas sharply increasing to nearly 1 000 m in the underlying dykes bulk porosities calculated from resistivities show a distinct layering apparently corresponding to geophysical layers 2a 2b and 2c

lithostratigraphic hydrothermal and tectonic setting of the boundary volcanogenic massive sulfide deposit newfoundland appalachians canada formation by subseafloor replacement in a cambrian rifted arc the boundary volcanogenic massive sulfide vms deposit 0 5 mt 3 5 cu 4 zn 1 pb 34 0 g t ag in the tally pond group central newfoundland represents one of the best preserved subseafloor replacement style vms deposits in the northern appalachian orogen the deposit is hosted within a late cambrian 510 ma volcanic sequence consisting predominantly of rhyolitic flows and associated volcaniclastic rocks footwall strata are dominated by rhyolitic lapilli tuff tuffs lesser rhyolite flows and in situ rhyolite breccias the hanging wall consists of massive quartz bearing flow banded lobe and breccia facies rhyolite the deposit occurs at and below the contact between these two units and comprises pyrite chalcopyrite and lesser sphalerite and other minerals massive mineralization contains abundant clasts of the surrounding host rocks including chlorite sericite quartz altered rhyolite lapilli and ash hydrothermally altered rocks consist of variably intense chlorite with lesser sericite and quartz chlorite alteration occurs in a discordant geometry likely representing hydrothermal upflow zones and chlorite sericite quartz occur as blankets that are parallel to the volcanic stratigraphy likely representing alteration associated with replacement the hanging wall rhyolite flows also contain moderate to intense pervasive quartz and sericite alteration both the hanging wall and footwall are characterized by strong na2o sr depletions k2o mgo fe2o3 ba enrichments high alteration index values e g ba sr chlorite carbonate pyrite index ccpi alteration index ai and enrichments in base metals and volatile metals e g cu zn pb hg the presence of abundant remnant wall rock and host rock clasts within the ore intricate sulfide replacement of laminated porous tuff and sand dikes replacement fronts in host lithofacies and intense alteration in both the footwall and hanging wall where the hanging wall is preserved are all features consistent with formation of the bulk of the boundary deposit via subseafloor replacement the deposit likely formed as a result of cooling of metal bearing hydrothermal fluids mixing with ambient seawater and pore water entrained seawater within the volcanic rocks at a permeability interface between young unlithified highly permeable footwall volcaniclastic rocks and relatively impermeable hanging wall rhyolitic flows in our model this permeability boundary was an important feature that promoted subseafloor replacement within the deposit immobile element systematics of rhyolitic rocks from the boundary deposit lack major differences in primary petrochemistry between hanging wall and footwall strata all rocks from the deposit area are subalkalic with transitional zr y ratios 2 8 4 5 la sm ratios 1 normalized to upper crust and primitive mantle normalized signatures with slightly light rare earth element lree enriched patterns with flat heavy ree hree and negative nb ti and eu anomalies these geochemical features coupled with existing nd pb isotope data zircon inheritance patterns and geologic information are consistent with rhyolitic rocks at the boundary deposit having formed by re melting of arc basement with continental crust or recycled continental crust present in the source region it is likely that the deposit and its associated rhyolitic host rocks formed within a cambrian continental or pericontinental rifted arc along the margin of ganderia within the iapetus ocean

crustal processes at spreading centers recent discoveries and research on mid ocean ridges mor s in the last four years have greatly increased our knowledge of this fundamental type of plate boundary the last iugg review of mor research was in 1975 aumento et al 1975 and its omission as a topic in 1978 may have reflected a feeling that spreading centers were reasonably well understood since that time however there has been a surfeit of important new observations and exciting research and there is now a turmoil of innovative ideas about the processes which create and leave their imprint on two thirds of the earth s crust new seismic measurements have been made and interpreted as evidence for the existence of shallow axial magma chambers amc beneath intermediate total opening rate 5 9 cm yr and fast 9 18 cm yr spreading centers fig 1 yet there is controversy as to whether they are steady state whether they persist beneath slow ridges as well in the steady state what the dimensions and shape of the chamber are its relation to oceanic crustal stratigraphy and its relation to observations in ophiolites black smoker hydrothermal vents have been discovered in the pacific spewing forth 350 c fluids blackened by polymetallic sulfide precipitates this discovery is embroiled in debate as to how long the vents last whether they occur at slow spreading rates as well whether the enormous heat flux of the vents freeze the magma chamber even at fast spreading rates their influence on geochemical balances in the ocean and the value of the sulfide deposits commercially it has been found that our initial view of mor s as two dimensional linear structures has served us well as an initial working model but must be significantly modified to account for variations in tectonic volcanic and hydrothermal activity along strike as well as for episodicity of these processes deep sea drilling project dsdp samples have raised fundamental questions about formation of the magnetic source layer hydrothermal circulation and fault mechanisms along the mor spreading centers which penetrate existing lithosphere along strike as propagating rifts have modified our views of how plate boundaries reorient themselves and how v shaped magnetic and topographic trends are formed we have also found that transform faults may modify spreading center processes for some distance away from their intersections with mor s and that rigid plate transform faults may fail to develop at fast spreading centers under certain conditions resulting in en echelon overlapping of spreading centers this review will focus on these developments and several others only example references to these topics will be given in the text and many important topics will not be discussed directly e g petrology and geochemistry mantle convection research on lithosphere older than 1 m y however i have tried to compile and have included an extensive bibliography for u s research on a broader range of mor topics for mid 1978 to mid 1982 which is the main purpose of this report pre 1978 references are occasionally included where appropriate abstracts are not included

hydrothermal circulation at the cleft vance overlapping spreading center results of a magnetometric resistivity survey we report on a magnetometric resistivity sounding carried out in the overlapping spreading center between the cleft and vance segments of the juan de fuca ridge the data collected reveal a strong three dimensionality in the crustal electrical resistivity structure on wavelengths of a few kilometers areas of reduced crustal electrical resistivities with values approaching that of seawater are seen beneath the neovolcanic zones of both active spreading centers we interpret these reduced resistivities as evidence of active hydrothermal circulation within the uppermost 1 km of hot young oceanic crust

the ramesses experiment iii controlled source electromagnetic sounding of the reykjanes ridge at 57 45 n summary a controlled source electromagnetic sounding survey centred on an axial volcanic ridge avr segment of the reykjanes ridge at 57 45aen was performed as part of the ramesses experiment low frequency 0 35 11 hz electromagnetic signals were transmitted through the crust to an array of horizontal electric field recorders at the seafloor to ranges of 15 km from the source which was a 100 m long horizontal electric dipole towed at heights of 50 80 m from the seafloor coincident seismic and magnetotelluric studies were conducted during the rest of the ramesses experiment data were interpreted using a combination of 1 d forward modelling and inversion and iterative forward modelling in two dimensions on the axis of the avr the resistivity at the seafloor is 1 v m there is a steep resistivity gradient in the upper few hundred metres of the crust with the resistivity reaching approximately 10 v ma t a depth of 500 m in order to explain the low resistivities the upper layer of the crust must be heavily fractured and saturated with sea water the resistivity increases with distance from the axis as the porosity decreases with increasing crustal age the most intriguing feature in the data is the large diverence in amplitude between fields transmitted along and across the avr axis a significant zone of low resistivity material is required at approximately 2 km depth beneath the ridge crest in order to explain this diverence it is coincident with the low velocity zone required by the seismic data and has a total electrical conductance in excellent agreement with the results of the magnetotelluric study the low resistivity zone can be explained by the presence of a body of partially molten basalt in the crust taken together these results provide the first clear evidence for a crustal magma chamber at a slow spreading mid ocean ridge the data constrain the melt fraction within the body to be at least 20 per cent with a melt volume suycient to feed crustal accretion at this segment of the ridge for of the order of 20 000 years since this body would freeze in the order of 1500 years this finding lends support to the hypothesis that at slow spreading rates crustal accretion is a cyclic process accompanying periodic influxes of melt from the mantle to a crustal melt reservoir

reply sulphur isotope composition of the brunswick no 12 massive sulphide deposit bathurst mining camp new brunswick implications for ambient environment sulphur source and ore genesis the brunswick no 12 massive sulphide deposit occurs within a middle ordovician bimodal volcanic and sedimentary sequence that is thought to have formed in a continental back arc rift covered with a thick succession of carbonaceous hemipelagic and turbiditic sedimentary rocks the deposit consists of three en echelon lenses that are zoned from vent complex to bedded ore and bedded pyrite facies the bedded ore facies has the lowest average 34s values 14 2 but are only slightly less positive than laminated pyrite in footwall sedimentary rocks 34smean 15 1 34s values for the bedded sulphides show an upward increase from 14 2 in bedded ore to 16 5 in bedded pyrite average 34s values for vent complex 15 8 and underlying stringer sulphides 16 1 are consistently more positive than those for bedded ore in carbonaceous shales and siltstone of the patrick brook formation that underlie the deposit 34s values that range between 13 8 and 25 6 and the similarity of these values to those of the bru

electrical resistivity structure of the valu fa ridge lau basin from marine controlled source electromagnetic sounding summary in december 1995 we carried out a comprehensive controlled source electromagnetic survey of the valu fa ridge at 22u25k si n the lau basin the valu fa ridge is ab ack arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity seismic studies have imaged a melt lens at an average depth of 3 2 km below the seafloor surrounded by a zone of lowered seismic velocity interpreted as a region of partial melt in the crust the electromagnetic experiment was part of a multidisciplinary study which included wide angle and reflection seismics bathymetry and potential field measurements electromagnetic signals at frequencies between 0 25 and 40 hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea bottom receivers at ranges of up to 20 km from the source over 80 hr of data consisting of the magnitude of the horizontal electric field at the seafloor were collected these data have extremely low scatter compared to similar data from previous surveys the data were interpreted using a combination of 1 and 2 d forward modelling and inversion the vertical resistivity gradient in the upper crust at the valu fa ridge is abnormally low with resistivities of less than 10 v m observed throughout layer 2 of the crust to a depth of 3 km this is significantly more conductive at depth than the axis of the slow spreading reykjanes ridge at 57u45kn and the fastspreading east pacific rise at 13un where similar data sets have been collected in the past although the structure of layer 2 is well constrained by the electromagnetic data its extremely low resistivity causes rapid attenuation of electromagnetic signals diffusing through it and hence the data are not sensitive to the structure in layer 3 in particular the structure of the melt lens or surrounding low velocity zone the seismic velocity structure of the valu fa ridge determined from the coincident wide angle seismic study is similar to that observed at other mid ocean ridges with a steep seismic velocity gradient through layer 2 although overall velocities are slightly lower the seismic velocity anomaly calculated relative to an average off axis structure is also small this suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity however hot and or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity since the conductive region extends unbroken from 3 km depth to the seafloor it is probable that these fluids circulate to or close to the magma chamber itself

sulphur and lead isotope geochemistry of sulphide minerals from the zn pb cu ag au lemarchant volcanogenic massive sulphide vms deposit newfoundland canada abstract the lemarchant zn pb cu ag au volcanogenic massive sulphide vms deposit in the newfoundland appalachians canada is an important example of a precious metal bearing vms deposit the deposit contains vms and sulphosalt rich mineral assemblages which are paragenetically distinct initial zn pb sulphosalt mineralization was deposited from low temperature 300 c less oxidized hydrothermal fluids with lower sulphur activity in situ microanalyses of pyrite chalcopyrite and galena yield a wide range of 34s values 6 4 to 15 1 sulphides from paragenetically early lower temperature zn pb sulphosalt assemblages have the lowest average 34s values 4 6 whereas the highest 34s values occur in the late stage high temperature cu rich sulphide assemblages variability in 34s occurs in all styles of mineralization and is consistent with s derived from thermochemical reduction of seawater sulphate igneous leaching and potentially disproportionated magmatic so2 although it is difficult to distinguish explicitly between leached igneous and magmatic sources of sulphur mineralogical and geochemical data from lemarchant suggest a possible contribution of magmatic so2 during the deposition of epithermal style mineralization at the earlier stages of deposit formation lead isotope compositions of galena show little variability with style of mineralization suggesting similar pb sources throughout ore deposition most pb at lemarchant is derived from underlying neoproterozoic sandy brook group volcanic rocks with potential input of juvenile pb from the lake ambrose basaltic rocks of the tally pond group and or from more juvenile neoproterozoic basement rocks

splitting marine controlled source electromagnetic responses into sea and subseafloor contributions grounding the air wave abstractwe have found how the effects of the air wave in marine controlled source electromagnetic csem methods gradually vanish in the sea for shallow waters and how at the same time they gradually grow below the seafloor in an effort to comprehend existing detectability definitions the transition from sea to land is smooth because the sea becomes a thin conductive layer when the water depth is smaller than the skin depth in the sea we consider the problem of detecting resistive layers at depth associated with hydrocarbon reservoirs particularly in shallow water explorations and specifically on how the air wave affects detection our analysis is based on an integral representation of the electric field in terms of its sensitivity to changes in the electrical conductivity of a 1d profile two dimensional images of the integrands are obtained by plotting the integrand as a function of depth for different offsets results include the expected growth of the inhibiting effect of the sea as the water d

formation of volcanogenic massive sulfide deposits the kuroko perspective abstract the main objective of this paper is to identify the geochemical hydrological igneous and tectonic processes that led to the variations in the physical size geometry and chemical mineralogy metal ratios and zoning characteristics of volcanogenic massive sulfide deposits with respect to space from a scale of mining district size area to a global scale and time from a all volcanogenic massive sulfide deposits vmsds appear to have formed in extensional tectonic settings such as at mid ocean spreading centers backarc spreading centers and intracontinental rifts and failed rifts all vmsds appear to have formed in submarine depressions by seawater that became ore forming fluids through interactions with the heated upper crustal rocks submarine depressions especially those created by submarine caldera formation and or by large scale tectonic activities e g rifting become most favorable sites for the formation of large vmsds because of hydrological physical and chemical reasons the fundamental processes leading to the formation of vmsds include the following six processes 1 1 intrusion of a heat source typically a 10 3 km size pluton into an oceanic crust or a submarine continental crust causes deep convective circulation of seawater around the pluton the radius of a circulation cell is typically 5 km the temperature of fluids that discharge on the seafloor increases with time from the ambient temperature to a typical maximum of 350 c and then decreases gradually to the ambient temperatures in a time scale of 100 to 10 000 years the majority of sulfide and sulfate mineralization occurs during the waxing stage of hydrothermal activity 2 2 reactions between low temperature t 2 4 as disseminated gypsum and anhydrite in the country rocks 3 3 reactions of the modified seawater with higher temperature rocks at depths during the waxing stage cause the transformation of the seawater to metal and h 2 s rich ore forming fluids the metals and sulfide sulfur are leached from the county rocks the previously formed gypsum and anhydrite are reduced by fe 2 bearing minerals and organic matter providing additional h 2 s the mass of high temperature rocks that provide the metals and reduced sulfur is typically 10 11 tons 40 km 3 in volume the roles of magmatic fluids or gases are minor in most massive sulfide systems except for so 2 to produce acid type alteration in some systems 4 4 reactions between the ore forming fluids and cooler rocks in the discharge zone cause alteration of rocks and precipitation of some ore minerals in the stockwork ores 5 5 mixing of the ore forming fluids with local seawater within unconsolidated sediments and or on the seafloor causes precipitation of primitive ores with the black ore mineralogy sphalerite galena pyrite barite anhydrite 6 6 reactions between the primitive ores with later and hotter hydrothermal fluids cause transformation of primitive ores to matured ores that are enriched in chalcopyrite and pyrite variations in the mineralogical and elemental characteristics the geometry and the size of submarine hydrothermal deposits are controlled by the following four parameters 1 a the chemical and physical characteristics of seawater composition temperature density which depend largely on the geographical settings e g equatorial evaporating basins 2 b the chemical and physical characteristics of the plumbing system lithology fractures 3 c the thermal structure of the plumbing system which is determined largely by the ambient geothermal gradient and the size and temperature of the intrusive and 4 d the physical characteristics of the seafloor depth basin topography for example the submarine hydrothermal deposits developed in basaltic plumbing systems are generally poor in pb and ba compared to those developed in felsic plumbing systems the lower temperature systems are generally poorer in sulfides but richer in iron oxides and sulfates the higher temperature and larger hydrothermal systems tend to produce chalcopyrite and pyrite rich ores contrasts in the metal ratios between the noranda type archean vmsds and the younger vmsds reflect the differences in the geothermal gradient of the plumbing systems the submarine hydrothermal deposits developed in the near equatorial regions tend to form large continuous bedded type ores because of the likeliness of creating large stratified basins the basic processes of submarine hydrothermal mineralization have remained essentially the same throughout the geologic history from at least 3 5 billion year ago to the present

controlled source electromagnetic sounding of the oceanic lithosphere the attenuation of ionospheric signals in the frequency range 0 06 24 hz by sea water effectively precludes using the magnetotel luric method to study the electrical structure of the upper oceanic lithosphere we have carried out a dipole dipole electromagnetic sounding in the north pacific by injecting electromagnetic signals into the ocean and sea bed the crust at the site is 25 myr old and has a thin sediment cover the technique similar to that used in earlier work1 2 involves dragging a horizontal dipole antenna along the sea floor the electric fields that propagated through the resistive basement were detected by seafloor receivers at ranges of 10 65 km as the ambient electric field is very small varying from 10 18 v2 m 2 hz 1 at 0 1 hz to 10 24 v2 m2 hz 1 above 1 hz ref 3 the controlled source signals could be easily monitored our data are consistent with a simple one dimensional earth model consisting of a 3 7 km thick crustal layer of moderate conductivity 0 001 s m 1 underlain by a thicker region of very low conductivity 2 10 5 s m 1 the results suggest an upper mantle water content of at most 0 1 by volume

transient electromagnetic responses in seafloor with triaxial anisotropy summary electrical anisotropy of young oceanic crust at mid ocean ridges is detectable by observation of the rate and geometry of the diffusion of electromagnetic fields the anisotropy in electrical properties arises from the presence of conductive seawater in an interconnected network of mostly ridge parallel cracks in this paper we first justify the choice of a triaxial model to represent young oceanic crust with three distinct electrical conductivities in the vertical strike and spreading directions we then present an algorithm to calculate the transient electromagnetic responses generated by an electric dipole source over such a triaxially anisotropic seafloor we show that if the transient passages are measured with three distinct electric dipole dipole configurations it is possible to discern all three unknown conductivities independently of each other

use of marine controlled source electromagnetic sounding for sub basalt exploration the problem of imaging sedimentary structure obscured by high velocity layers such as carbonate basalt or salt using conventional seismic techniques is well known when this problem is encountered in offshore areas marine electromagnetic data can provide valuable alternative and complementary constraints on the structure we concentrate on the use of marine controlled source electromagnetic csem sounding in the frequency domain the feasibility of mapping sub basalt sedimentary structure using this technique is explored by means of modelling studies which mimic a type of survey which could readily be employed in practice as a starting point the problem is addressed in terms of 1d resistivity structure we show that sub basalt sediments can be detected and their depth of burial quantified to within 200 m in the examples shown the presence of small scale inhomogeneities in the basalt which cause much of the scattering in seismic data is shown to have no appreciable effect on the ability of the csem data to detect the sediments the modelling is then extended to 2 5 dimensions again the presence of sub basalt sediments can be readily detected and their properties and geometry quantified especially if the electromagnetic data are combined with constraints from complementary geophysical techniques

heat flow on the western flank of the east pacific rise at 21 n we report 48 new heat flow measurements on the western flank of the east pacific rise at 21 n the stations were taken in 1 to 30 m of sediment on 0 4 to 1 4 m y old crust the low average measured value of 173 mw 2 including 12 tilted minimum values is about 1 3 the mean heat flow predicted for crust of this age range by cooling plate theory this may be partly due to a bias of measurement locations in sediment ponds nevertheless 4 km running averages of measured heat flows projected normally to the axis display an oscillatory trend suggestive of cellular hydrothermal circulation in the basement we sampled no more than one cycle of the apparent heat flow modulation which is of 15 km wavelength and 200 mw m2 amplitude this variation appears to be uncorrelated with the topographic variation of 10 km wavelength assuming that the porosity sensitive electromagnetic results of young and cox 1981 reflect a permeability distribution that restricts the depth of circulation to 1 4 km average temperatures of circulation may be 50 60 c deeper higher temperature circulation is allowable if a more uniform permeability depth distribution holds

the duck pond and boundary cu zn deposits newfoundland new insights into the ages of host rocks and the timing of vhms mineralization the duck pond cu zn pb ag au deposit in newfoundland is hosted by volcanic rocks of the cambrian tally pond group in the victoria lake supergroup in conjunction with the nearby boundary deposit it contains 4 1 million tonnes of ore at 3 3 cu 5 7 zn 0 9 pb 59 g t ag and 0 9 g t au the deposits are hosted by altered felsic flows tuffs and volcaniclastic sedimentary rocks and the sulphide ores formed in part by pervasive replacement of unconsolidated host rocks u pb geochronological studies confirm a long suspected correlation between the duck pond and boundary deposits which appear to be structurally displaced portions of a much larger mineralizing system developed at 509 3 ma altered aphyric flows in the immediate footwall of the duck pond deposit contained no zircon for dating but footwall stringer style and disseminated mineralization affects rocks as old as 514 3 ma at greater depths below the ore sequence unaltered mafic to felsic volcanic rocks that occur structurally above the o

the role of granites in volcanic hosted massive sulphide ore forming systems an assessment of magmatic hydrothermal contributions assessment of geological geochemical and isotopic data indicates that a significant subgroup of volcanic hosted massive sulphide vhms deposits has a major or dominant magmatic hydrothermal source of ore fluids and metals this group which is typically characterised by high cu and au grades includes deposits such as those in the neoarchean doyon bousquet laronde and cambrian mount lyell districts these deposits are distinguished by aluminous advanced argillic alteration assemblages or metamorphosed equivalents intimately associated with ore zones in many of these deposits 34ssulphide is low with a major population below 3 34ssulphate differs from coexisting seawater and 34ssulphate sulphide 20 30 these characteristics are interpreted as the consequence of disproportionation of magmatic so2 as magmatic hydrothermal fluids ascended and cooled and as a definitive evidence for a significant magmatic hydrothermal contribution other characteristics that we consider diagnostic of significant magmatic hydrothermal input into vhms ore fluids include uniformly high 3 times modern seawater values salinities or very 18o enriched 18o 5 ore fluids we do not consider other criteria e g variable salinity moderately high 18ofluid 2 5 34ssulphide near 0 metal assemblages or a spatial association with porphyry cu or other clearly magmatic hydrothermal deposits that have been used previously to advocate significant magmatic hydrothermal contributions to be diagnostic as they can be produced by non magmatic processes known to occur in vhms mineral systems however in general a small magmatic hydrothermal contribution cannot be excluded in most vhms systems considered conclusive data that imply minimal magmatic hydrothermal contributions are only available in the paleoarchean panorama district where coeval seawater dominated and magmatic hydrothermal systems appear to have been physically separated this district which is characterised by chloritic and sericitic alteration assemblages and lacks aluminous advanced argillic alteration assemblages is typical of many vhms deposits around the world suggesting that for garden variety vhms deposits a significant magmatic hydrothermal contribution is not required other than deposits associated with advanced argillic alteration assemblages the only deposit for which we ascribe a major magmatic hydrothermal contribution is the devonian neves corvo deposit this deposit differs from other deposits in the iberian pyrite belt and around the world in being extremely sn rich with the sn closely associated with cu and in having formed from high 18o rich fluids 18ofluid 8 5 we consider these characteristics particularly the last as diagnostic of a significant magmatic hydrothermal contribution our analysis indicates that two subgroups of vhms deposits have a major magmatic hydrothermal contribution cu au rich deposits with aluminous alteration assemblages and reduced very sn rich deposits in which sn was introduced in a high temperature ore assemblage comparison with normal vhms deposits suggests that these subgroups of vhms deposits may form in specialised tectonic environments the cu au rich deposits appear to form adjacent to magmatic arcs an environment conducive to the generation of hydrous oxidised melts by melting metasomatised mantle in the wedge above the subducting slab this contrasts with the back arc setting of normal vhms deposits in which relatively dry granites in this contribution we use the term granite sensu latto formed by decompression melting drive seawater dominated hydrothermal circulation the tectonic setting of highly sn rich vhms deposits such as neves corvo is less clear however thick continental crust below the ore hosting basin may be critical as it is in other sn deposits

marine controlled source electromagnetic sounding 2 the pegasus experiment the marine controlled source electromagnetic sounding method developed over the past 15 years at scripps institution of oceanography employs a towed seafloor electric dipole transmitter of moment 4 104 am and multiple free vehicle seafloor electric field recorders a survey of 40 ma normal oceanic lithosphere in the northeast pacific using frequencies of 0 25 to 24 hz and synchronous stacking of 0 25 to 12 hour duration detected signals at transmitter receiver ranges between 5 and 95 km one dimensional electrical conductivity structure is recovered from the data using the occam process of nonlinear regularized inversion repeated inversion of a model terminated with an essentially infinite conductor or resistor demonstrates that the maximum depth of inference for this experiment is about 30 km well into the upper mantle with bounds placed on conductivity to depths of 60 km structure shallower than about 1 km is comparable to that obtained by a similar experiment on the east pacific rise and by borehole logging with a sharp increase in resistivity at depths of 600 800 m although strictly our experiment is sensitive only to integrated square root of conductivity or total attenuation in the surface layers the lower crust and upper mantle has a resistivity between 2 and 7 104 m and a transverse resistance of at least 109 m2 suggesting at most 0 3 volume fraction of free water in the lower crust and some form of conductivity enhancement over mineral conductivity in the uppermost mantle although resolution is weak below 30 km our data are compatible with a dry olivine model of mantle conductivity temperature

the electrical conductivity distribution beneath vancouver island a region of active plate subduction magnetotelluric mt and geomagnetic depth sounding gds data were recorded at 25 locations across vancouver island canada over the subducting juan de fuca plate the gds data respond to a number of local conductive features at frequencies above 10 hz while at longer periods the in phase and quadrature induction arrows are colinear and are aligned approximately normal to the continental shelf with a maximum amplitude response near the southwest coast interpretation of the mt data show the presence of a conducting zone at depths greater than 20 km beneath vancouver island and correlating with a strong seismic reflector extensive testing with two dimensional models shows the mt and gds data are consistent with a proposed geoelectrical structure containing a conducting layer extending from the pacific ocean dipping beneath vancouver island and in electrical contact with the mainland conductive region the layer has a horizontal conductance of 200 s that may decrease to 100 s beneath the northeast side of vancouver island related geophysical studies suggest the conductive zone is above the actual thrust surface of the juan de fuca plate the conducting layer is most likely the result of cracks and pores filled with saline fluids which are supplied bymore water subducted with the oceanic crust and by dehydration reactions the presence of fluids has significant implications for thrust earthquakes and for metamorphic reactions that occur in subduction zones the conductance of the upper mantle between 90 and 390 km is best modeled at approximately 1 500 s less

neoproterozoic and cambrian arc magmatism along the eastern margin of the victoria lake supergroup a remnant of ganderian basement in central newfoundland abstract the eastern margin of the composite and structurally complex victoria lake supergroup has previously been mapped where the silurian rogerson lake conglomeratic rocks unconformably overlie cambrian tally pond volcanic rocks however the recognition that the form of this contact changes between an unconformity and a brittle ductile fault in conjunction with the neoproterozoic age of the crippleback intrusive suite dictate that the above interpretation of the nature of this margin requires revision these discrepancies are resolved by the recognition of a neoproterozoic largely bimodal volcanic sequence sandy brook group which is unconformably overlain by rogerson lake rocks to the east with these combined units in fault contact with the tally pond group to the west the sandy brook group and crippleback intrusive suite form a terrane of ca 563 ma rocks with continental arc like geochemical signatures the distinctly negative nd 563 values for the felsic rocks mean 3 of this terrane imply derivation from a crystalline basement older than the largely pan african crust that underlies contemporaneous volcanic rocks in west avalonia but similar to that inferred for the gander zone thus this terrane is likely to be the leading edge of the peri gondwanan microcontinent ganderia the ca 511 ma tally pond group arc magmatism represent some of the oldest products of iapetean subduction in newfoundland although the tally pond group are only observed to be in fault contact with the ca 50 million year older sandy brook group the high proportion of felsic volcanic rocks sandy brook crippleback age inherited zircons and pb isotopic data all support the inference that the tally pond group was formed on sandy brook crippleback continental crust

induction of currents in long submarine cables by natural phenomena the influence of induced currents on long submarine cables has been observed for nearly a century the currents originate from time variations in the magnetic field of external origin magnetospheric atmospheric tides and earth and ocean tides steady earth currents of internal origin have also been proposed as a possible source the induced currents will depend upon the length depth and geomagnetic location of the cables present understanding of the phenomena is reviewed also discussed are potential scientific problems which might be addressed with data from such long cables and possible limitations on the information obtained such problems include studies of 1 the deep conductivity structure of the earth 2 telluric currents flowing in mid oceanic ridges 3 the spatial extent of external source fields and 4 a possible role of electromagnetic eddy currents in fluctuations of the earth s spin rate

origin of the eskay creek precious metal rich volcanogenic massive sulfide deposit fluid inclusion and stable isotope evidence the eskay creek deposit is an unusual polymetallic au ag rich volcanogenic sulfide sulfosalt deposit located in the iskut river area of northwestern british columbia canada economic concentrations of precious and base metals are contained in the 21 zone which is divided into a number of subzones as of december 31 1998 total production and proven probable reserves are 1 9 million tons mt at 60 2 g t au 2 652 g t ag 3 2 percent pb 5 2 percent zn and 0 7 percent cu the 21b zone which contains the bulk of the reserves began production in 1995 the mineralization occurs mainly as well preserved stratiform clastic beds of sulfide sulfosalt debris and also as discordant footwall quartz sulfide veins the hydrothermal system that formed the eskay deposit was low temperature

controlled electromagnetic sources for measuring electrical conductivity beneath the oceans 1 forward problem and model study exact closed form expressions for the electromagnetic induction fields produced by vertical and horizontal current sources in the conducting ocean overlying a one dimensional earth are derived from the maxwell equations numerical methods for the evaluation of the solutions are given including correction for the finite size of real sources simple models of the electrical conductivity structure of the ocean crust and lithosphere are deduced from geologic petrologic and laboratory data and their electromagnetic response is modeled horizontal electric dipole sources produce much larger field amplitudes than their vertical counterparts for a given frequency and range and the horizontal electric field offers superior received signal performance reflections of electromagnetic waves from the sea surface and thermocline must be considered for low enough frequencies or long ranges estimates of the ambient noise level from natural electromagnetic sources in the frequency range 0 01 10 hz are presented the ability of controlled sources to determine features of the conductivity of the ocean crust and upper mantle especially low conductivity zones is demonstrated if the mantle conductivity is low enough horizontal ranges of 50 km and conductivity estimates to over 20 km depth can be achieved

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

three methods for mitigating airwaves in shallow water marine controlled source electromagnetic data in the past several years marine controlled source electromagnetic mcsem techniques have been applied successfully in deep water depth 1 km for oil and gas exploration the application of this technology in shallow water is challenged however because of airwaves that mask the signal from the target reservoir at depth based upon the understanding that an airwave is a lateral wave which can be analytically expressed in a dual half space resistivity model we propose three airwave mitigation approaches to reduce the effects of these airwaves on mcsem data in the em x bucking approach the effect of the airwaves can be bucked out from two measurements by using the analytic expression of the airwave the frequency derivative de dfreq approach takes advantages of the unique characteristics of the airwaves in frequency domain enhancing the reservoir signals while suppressing the airwave the magnetotelluric mt stripping method uses the plane wave feature of the airwaves and subtraction of

electric dipole fields over an anisotropic seafloor theory and application to the structure of 40 ma pacific ocean lithosphere summary seismic anisotropy has been detected in the oceanic crust and upper mantle and likewise it is geologically reasonable to expect that a certain amount of lateral anisotropy exists in sea oor electrical properties anisotropy in earth properties can often lead to surprising eiects on geophysical responses that are not anticipated from simple isotropic theories here we investigate the eiects of lateral anisotropy on the frequency domain controlled source electromagnetic csem response of a uniaxially conducting non magnetic sea oor excited by a horizontal electric dipole whose moment is oriented obliquely with respect to the electrical strike direction a paradox of anisotropy is observed in which the sea oor electric eld strength is enhanced in the most conductive direction of the sea oor this enhancement is opposite to what one would expect based on naive isotropic theory we also show that it is possible in certain circumstances to extract the along strike electrical conductivity from marine controlled source electromagnetic data using only isotropic modelling the extraction of across strike conductivity however requires full anisotropic modelling the physical insight into electromagnetic induction in uniaxial media that is presented here should greatly assist the geological interpretation of marine csem experimental data applying our algorithm to the pegasus data set csem data collected over 40 ma paci c ocean lithosphere produces a model with conductivity in the fossil spreading direction that is seven times greater than the conductivity perpendicular to spreading strain aligned mineralogical fabric as predicted by tectonic modelling would explain our result with enhanced conductivities caused by hydrogen conduction along the olivine a axis or connected accumulations of trace conductors such as graphite or magnetite

using csem to monitor production from a complex 3d gas reservoir a synthetic case study the marine controlled source electromagnetic csem method is most commonly used in targeted exploration or appraisal studies as a tool for detecting and delineating hydrocarbon reservoirs see for example constable and srnka 2007 whereas seismic surveys can detect the structures that may contain hydrocarbons with great accuracy distinguishing hydrocarbon fluids from water within these structures is more problematic originally developed in the late 1970s young and cox 1981 the csem method uses a high powered horizontal electric dipole hed to transmit a low frequency electromagnetic signal through the sea floor to an array of multicomponent electromagnetic receivers bulk electrical resistivity of the sea floor can be determined at scales of a few tens of meters to depths of several kilometers in many cases this resistivity is driven by the fluid content and saturation of subsurface formations

a numerical comparison of time and frequency domain marine electromagnetic methods for hydrocarbon exploration in shallow water in shallow water the frequency domain controlled source electromagnetic method is subject to airwave saturation that strongly limits the sensitivity to resistive hydrocarbon targets at depth it has been suggested that time domain csem may offer an improved sensitivity and resolution of these deep targets in the presence of the airwave in order to examine and test these claims this work presents a side by side investigation of both methods with a main focus on practical considerations and how these effect the resolution of a hydrocarbon reservoir synthetic noisy data for both time domain and frequency domain methods are simulated using a realistic frequency dependent noise model and frequency dependent scaling for representative source waveforms the synthetic data studied here include the frequency domain response from a compact broadband waveform the time domain step response from a low frequency square wave and the time domain impulse response obtained from pseudo random binary sequences these data are used in a systematic resolution study of each method as a function of water depth relative noise and stacking length the results indicate that the broadband frequency domain data have the best resolution for a given stacking time whereas the time domain data require prohibitively longer stacking times to achieve similar resolution

geology mineralogy and lithogeochemistry of metalliferous mudstones associated with the lemarchant volcanogenic massive sulfide deposit tally pond belt central newfoundland the cambrian lemarchant zn pb cu au ag bimodal felsic volcanogenic massive sulfide vms deposit tally pond group central newfoundland canada contains metalliferous mudstones that occur either immediately on top or laterally along strike from massive sulfide mineralization or as interflow mudstones within hanging wall basaltic rocks the mudstones are brown to black graphite rich and locally have intercalations of siliciclastic volcaniclastic and or amorphous chert layers and in some cases fine lamina of organic matter the main sulfide phases are pyrite framboidal and euhedral and pyrrhotite with minor chalcopyrite sphalerite arsenopyrite and galena barite is a common sulfate the metalliferous mudstones occur at various stratigraphic levels and have variable inputs of hydrothermal high fe al and base metal values and detrital components lower fe al and base metal values with positive shale normalized eu anomalies eu eu 1 negative ce anomalies ce ce 1 and an average y ho ratio of 28 3 these signatures suggest precipitation from reduced high temperature hydrothermal vent fluids in an oxygenated water column coupled with the hydrothermal particles having a short residence time within the hydrothermal plume i e a vent proximal setting deposition from an oxygenated water column is also supported by the presence of abundant barite in both the lemarchant massive sulfides and the metalliferous mudstones as well as locally preserved marcasite in the mudstones redox sensitive trace elements as u v mo cr ni and co were scavenged as oxyanions onto fe oxyhydroxides during mudstone formation and these elevated levels of scavenged redox sensitive trace elements cause apparent anoxic signatures immobile element systematics la th zr 10 and th la sc indicate that the detrital components of the lemarchant mudstones were mixtures of both continental and mafic sources most likely deposited within a pericontinental rifted arc environment our research illustrates that the lithogeochemistry of metalliferous mudstones can identify vent proximity and provides insights into the physiochemical conditions of the deposit formation and the ambient environment these results have implications for similar precious metal bearing vms deposits globally

controlled source electromagnetic sounding in shallow water principles and applications the marine controlled source electromagnetic csem method is being applied to the problem of detecting and characterizing hydrocarbons in a variety of settings until recently its use was confined to deepwater water depths greater than approximately 300 m because of the interaction of signals with the atmosphere in shallower water depths the purpose of this study was to investigate using a simple 1d analytical analysis the physics of csem in shallow water this approach demonstrates that it is difficult to simply decouple signals that have interacted with the earth from those that have interacted with the air using either frequency domain or time domain methods stepping away from wavelike approaches which if applied without care can be misleading for the diffusive fields of csem we demonstrate an effective way to mitigate the effect of the air in shallow water surveys by decomposing the em signal into modes and using only the mode least affected by interaction with the atmosphere such decompos

correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential pearce nicholas denton j s perkins w t westgate j a 2007 correlation and characterisation of individual glass shards from tephra deposits using trace element laser ablation icp ms analyses current status and future potential journal of quaternary science 22 7 pp 721 736 rae2008

on the electrical nature of the axial melt zone at 13 n on the east pacific rise the first controlled source electromagnetic experiment directly on a ridge with the potential to identify the presence of an axial melt body beneath a fast spreading center was conducted at 13on on the east pacific rise epr in 1989 transmission for 36 hours was achieved by a deep towed horizontal electric dipole source of moment 6000 am operating at frequencies between 1 4 and 8 hz signals from the source were recorded by seven seafloor electric field receivers positioned both along the ridge crest and 5 km to the east on 100 000 year old crust data above ambient noise levels were obtained at ranges of up to 10 kin the results of modeling observed electric field amplitudes reveal that resistivities in the uppermost crust are very low 1 tim indicating a heavily fractured high porosity surficial layer below this topmost layer the upper 2 km of crust is found to be moderately resistive 100 tim we find no evidence for a large conductive axial melt body with dimensions on the order of kilometers in the middle or upper crust if a partial melt body is present which is continuous along strike and which comprises a connected and therefore conductive melt texture it must be of very limited volumetric extent this picture is consistent with recently proposed models of a thin sill like melt lens with across strike dimensions of no more than 1 km and probably with smaller vertical extent the larger region below the sill characterized by low seismic velocities must contain at best a very small melt fraction distributed in isolated pockets providing further evidence that the epr at 13on is currently in a state of relative magmatic quiescence

the marine controlled source electromagnetic method in shallow water we analyzed marine controlled source electromagnetic csem acquisition in shallow water by analytic and numerical calculations the problem at hand was the fact that the amplitude of the airwave due to a horizontal electric dipole becomes larger as the water depth is reduced the amplitude of the scattered field from a buried thin resistor would be difficult to extract from the increased total field if the scattered field amplitude were independent of water depth however we identified and explained a compensating effect where the amplitude of the scattered field increases with reduced water depth this amplification effect makes marine csem surveys in shallow water feasible we derived an approximate expression for the total field for a simple but realistic conductivity model this expression explains why the amplitudes of the term that describes the background field contribution and the term that gives the scattered field from the thin resistor increase when the water depth is reduced our results and sensitivity analysis indicated that the sensitivity of marine csem data acquired in a water depth of 40 m is comparable to the sensitivity of marine csem data acquired in a water depth of 300 m

variations in style of volcanism and massive sulfide deposition in early to middle ordovician island arc sequences of the newfoundland central mobile belt the newfoundland central mobile belt records the early paleozoic birth development and destruction of the iapetus ocean following opening of the ocean in the cambrian a compressional plate margin developed in the early ordovician and an island arc was formed above an east dipping subduction zone volcanism was active at this plate margin from late tremadocian to early caradocian at which time subduction ceased and the products of the volcanism were blanketed by argillite and flysch the pre caradocian island arc volcanic sequences of the central mobile belt are host to numerous volcanogenic massive sulfide occurrences both the volcanic rocks and the associated massive sulfide occurrences show distinct contrasts across the volcanic belt in the northern part of the belt notre dame bay area submarine dominantly mafic volcanic rocks host pyritic cu zn deposits associated with local felsic domes these sequences pass southward into the victoria river area where volcanic rocks are characterized by laterally extensive felsic pyroclastic sheets which host polymetallic zn pb cu deposits in southern newfoundland the hermitage flexure area partially emergent preponderantly felsic volcanic rocks host pb zn ag massive sulfide mineralization formed during the waning stages of volcanism lead isotope ratios in massive sulfide deposits in the hermitage flexure area are consistently more radiogenic than in deposits hosted by more mafic volcanic sequences to the north this coupled with geologic evidence suggests that these sequences formed under the influence of sialic crustal material it is inferred that the early middle ordovician island arc volcanism in the newfoundland central mobile belt occurred in both oceanic and continental margin settings the compositions and geologic environments of the associated massive sulfide deposits reflect these variations in tectonostratigraphic settings time and space relationships within the volcanic arc during the interval of active volcanism are not entirely clear all early to middle ordovician volcanic rocks in central newfoundland may have formed from contemporaneous volcanic activity if this is the case simultaneous volcanism must have occurred both in oceanic environments and at or near the continental margin alternatively the volcanism may have been slightly diachronous with an early episode of ensimatic mafic volcanism followed by migration of the volcanic centers toward the continental margin the massive sulfide deposits of the pre caradocian island arc exhibit lead isotope ratios which are distinctly more radiogenic than those in the buchans roberts arm and post caradocian cutwell groups this appears to support geologically based inferences that the latter sequences are not part of the main phase of island arc activity in central newfoundland

the determination of resistivity and porosity of the sediment and fractured basalt layers near the juan de fuca ridge summary in the summer of 1984 an electrical survey using magnetometric off shore electrical sounding moses was conducted at two sites in middle valley part of the northern juan de fuca ridge complex moses has been designed to minimize the difficulties inherent in electrical surveys of the crust below the electrically conductive sea layer site 1 at 48 32n 128 42w is in the central part of the turbidite filled basin using a two layer model of conductive sediments overlying a fractured basalt basement the sediment resistivity and thickness were found to be 0 82 0 06 m and 1800 300 m respectively the basement resistivity although not well constrained by the data is consistent with the results obtained at site 2 site 2 located at 48 10n 128 50w has a thinner sediment layer which appears to vary with position the sediment conductivity thickness product is the parameter determined by the data if the sediment resistivity were the same as at site 1 the sediment thickness would be 140 30 m to the se of site 2 and 240 55 m to the nw the fractured basalt basement has a resistivity of 8 5 3 4 m and is at least 1000 m thick using temperature corrected pore fluid resistivity the calculated porosity is found to vary from 62 per cent at the top to 21 per cent at the base of the sediments and is 8 per cent in the basement these values are in good agreement with estimates from seismic velocities for a thick turbidite sequence in a nearby sediment filled basin and determined for layer 2a b basalts in dsdp hole 504b respectively

provenance of exhalites associated with the lemarchant volcanogenic massive sulphide vms deposit central newfoundland canada insights from nd isotopes and lithogeochemistry neodymium isotope data on exhalites and tuffs from the cambrian lemarchant volcanogenic massive sulphide vms deposit provide insights into the tectonic environment of the tally pond group canada new data from exhalites from the lemarchant area show evolved values of end 513 6 0 to 1 8 whereas the associated volcanic rocks have end 513 of 0 4 to 1 4 the lemarchant exhalite end compositions overlap the underlying ganderian neoproterozoic sandy brook group end t 6 5 to 1 9 and crippleback intrusive suite end t 5 9 to 5 2 the evolved nd isotopic signatures suggest that the volcanic rocks of the tally pond group were formed upon ganderian arc basement which itself was possibly built upon or proximal to the gondwanan amazonian margin erosion of older crustal material and tally pond group volcanic rocks together with coeval eruption of the volcanic rocks released nd rich detritus into the water column uptake of eroded detrital and scavenged nd resulted in mixed nd sources juvenile and evolved which are archived in the exhalites the results of this study are not only of significance for occurrences of exhalites within the tally pond group but also have exploration implications for vms districts globally

a magnetotelluric sounding across vancouver island detects the subducting juan de fuca plate magnetotelluric1 mt and geomagnetic2 depth sounding measurements have been made at 18 sites 7 with remote reference3 across vancouver island fig 1 beneath which the juan de fuca plate is underthrusting4 5 vancouver island is part of the old accretionary wrangellia terrane6 and is located in the central portion of a 350 km forearc region that extends from the sediment filled trench7 to the garibaldi volcanic belt8 gravity9 and seismic refraction studies10 suggest that the descending plate has a shallow northeasterly dip of 8 16 with perhaps higher dips beneath northeastern vancouver island a high resolution seismic reflection survey11 along lines 1 and 3 fig 1 located a zone of strong acoustic reflections the e horizon11 near the top of the subducting juan de fuca plate at depths of 23 34 km our model of the broad band mt data defines a sloping highly conducting zone also near the top of the subducting plate at the same depths as the seismic e horizon the high electrical conductivity in this zone the e conductor is the result of saline fluid within the pore spaces of sedimentary and mafic materials of the upper oceanic crust that have been subducted to those depths beneath vancouver island these are the first data which clearly define a dipping conductive layer associated with the boundary region between converging plates they have significant implications for thrust earthquakes and metamorphic reactions that occur in subduction zones

microanalysis of tephra by la icp ms strategies advantages and limitations assessed using the thorsmork ignimbrite southern iceland abstract micron scale analysis of vesicular volcanic glass can be problematic because thin vesicle walls and junctions limit the area available for analysis subsurface vesicles limit the vertical thickness available microcrysts at or below the surface may contaminate glass analyses and some glasses show compositional banding in addition distal tephra are very small 10 100 m and material may be sparse we have analysed the mpi ding reference glasses and natural tephra samples pumice scoria and fiamme from the thorsmork ignimbrite southern iceland using laser ablation inductively coupled plasma mass spectrometry la icp ms three different reduction strategies are used averaging uncertainty weighting and log linear regression we then assess the data quality achieved using the various strategies using our technique we show that the main limiting factor on data quality is precision particularly for natural tephra analyses at 20 000 cps relative standard deviations rsds in the thorsmork tephra are 5 10 approximately twice those achieved in the mpi ding glasses 3 5 at the same conditions rhyolitic pumice and fiamme from the thorsmork ignimbrite are compositionally homogenous the proximal deposit also contains subordinate basalt scoria therefore the deposit is bimodal the thorsmork rhyolite correlates with the north atlantic ash zone 2 naaz2 tephra described in a marine sediment core lacasse and garbe schonberg 2001 jvgr 170 113 147

vertical vertical controlled source electromagnetic instrumentation and acquisition abstract vertical vertical controlled source electromagnetic is an alternative to other techniques for providing three dimensional resistivity images of the subsurface it utilizes a large and powerful vertical dipole transmitter and arrays of e field receivers with vertical and horizontal dipole sensors the necessary instrumentation and acquisition procedures which differ strongly from other controlled source electromagnetic methods are described in the paper

geochemistry of hydrothermal fluids from axial seamount hydrothermal emissions study vent field juan de fuca ridge subseafloor boiling and subsequent fluid rock interaction hydrothermal fluids collected from the ashes vent field in 1986 1987 and 1988 exhibit a very wide range of chemical composition over a small area 60 m in diameter compositions range from a 300 c gas enriched 285 mmol kg co2 low chlorinity 33 of seawater fluid to a 328 c relatively gas depleted 50 mmol kg co2 high chlorinity 116 of seawater fluid the entire range of measured compositions at ashes is best explained by a single hydrothermal fluid undergoing phase separation while rising through the ocean crust followed by partial segregation of the vapor and brine phases other mechanisms proposed to produce chlorinity variations in hydrothermal fluids precipitation dissolution of a chloride bearing mineral or crustal hydration cannot produce the covariation of chlorinity and gas content observed at ashes there is good agreement of the measured fluid compositions with compositions generated by a simple model of phase separation in which gases are partitioned according to henry s law and all salt remains in the liquid phase significant enrichments in silica lithium and boron in the low chlorinity fluids over levels predicted by the model are attributed to fluid rock interaction in the upflow zone depletions in iron and calcium suggest that these elements have been removed by iron sulfide and anhydrite precipitation at some time in the history of the low chlorinity fluids the distribution of low and high chlorinity venting is consistent with mechanisms of phase segregation based on differential buoyancy or relative permeability the relatively shallow depth of the seafloor 1540 m and the observed chemistry of ashes fluids are consistent with phase separation in the sub critical or near critical region

sensitivity study of marine csem data for reservoir production monitoring using numerical and analytical modeling we assess the feasibility of marine controlled source electromagnetic csem data for monitoring the flooding front during water flooding of an oil reservoir we discuss the ability of time lapse csem data to resolve changes in the electric conductivity small enough to be of interest for monitoring purposes measurement and modeling errors are discussed briefly and analytical calculations comparing time lapse signals with a certain type of time lapse modeling errors are performed numerical calculations performed with a volume integral equation method are used to study the effect on the electromagnetic em fields of relevant conductivity changes the numerical investigation includes robustness with respect to survey parameter values discrimination between different front shapes and ability to overcome time lapse error simulated signals are found to be strong enough to overtake typical measurement errors and are fairly robust toward perturbations of survey parameters it is found analytically and numerically that certain modeling errors experience a high degree of time lapse cancellation

an introduction to marine controlled source electromagnetic methods for hydrocarbon exploration early development of marine electromagnetic methods dating back about 80 years was driven largely by defense military applications and use for these purposes continues to this day deepwater frequency domain electric dipole dipole controlled source electromagnetic csem methods arose from academic studies of the oceanic lithosphere in the 1980s and although the hydrocarbon exploration industry was aware of this work the shallow water environments being explored at that time were not ideally suited for its use low oil prices and increasingly successful results from 3d seismic methods further discouraged investment in costly alternative geophysical data streams these circumstances changed in the late 1990s when both statoil and exxonmobil began modeling studies and fieldtrials of csem surveying in deep water around 1000 m or deeper specifically for characterizing the resistivity of previously identified drilling targets trials offshore angola in 2000 2002 by both these companies showed that

reconstructing the quaternary evolution of the world s most active silicic volcanic system insights from an 1 65 ma deep ocean tephra record sourced from taupo volcanic zone new zealand abstract the taupo volcanic zone tvz central north island new zealand is the most frequently active quaternary rhyolitic system in the world silicic tephras recovered from ocean drilling programme site 1123 41 47 16 s 171 29 94 w 3290 m water depth in the southwest pacific ocean provide a well dated record of explosive tvz volcanism since 1 65 ma we present major minor and trace element data for 70 quaternary tephra layers from site 1123 determined by electron probe microanalysis 1314 analyses and laser ablation inductively coupled plasma mass spectrometry 654 analyses trace element data allow for the discrimination of different tephras with similar major element chemistries and the establishment of isochronous tie lines between three sediment cores 1123a 1123b and 1123c recovered from site 1123 these tephra tie lines are used to evaluate the stratigraphy and orbitally tuned stable isotope age model of the site 1123 composite record trace element fingerprinting of tephras identifies 4 5 m and 7 9 m thick sections of repeated sediments in 1123a 49 0 53 5 mbsf metres below seafloor and 1123c 48 1 56 0 mbsf respectively these previously unrecognised repeated sections have resulted in significant errors in the site 1123 composite stratigraphy and age model for the interval 1 15 1 38 ma and can explain the poor correspondence between 18o profiles for site 1123 and site 849 equatorial pacific during this interval the revised composite stratigraphy for site 1123 shows that the 70 tephra layers when correlated between cores correspond to 37 38 individual eruptive events tephras 7 of which can be correlated to onshore tvz deposits the frequency of large volume tvz derived silicic eruptions as recorded by the deposition of tephras at site 1123 has not been uniform through time rather it has been typified by short periods 25 50 ka of intense activity bracketed by longer periods 100 130 ka of quiescence the most active period at least 1 event per 7 ka occurred between 1 53 and 1 66 ma corresponding to the first 130 ka of tvz rhyolitic magmatism since 1 2 ma 80 of tephras preserved at site 1123 and the more proximal site 1124 were erupted and deposited during glacial periods this feature may reflect either enhanced atmospheric transport of volcanic ash to these sites up to 1000 km from source during glacial conditions or more speculatively that these events are triggered by changes in crustal stress accumulation associated with large amplitude sea level changes only 8 of the 37 38 site 1123 tephra units 20 can be found in all three cores and 22 tephra units 60 are only present in one of the three cores whether a tephra is preserved in all three cores does not have any direct relationship to eruptive volume instead it is postulated that tephra preservation at site 1123 is patchy and influenced by the vigorous nature of their deposition to the deep ocean floor as vertical density currents at this site at least 5 cores would need to have been drilled within a proximity of 10 s to 100 s of metres of each other to yield a 99 chance of recovering all the silicic tephras deposited on the ocean surface above it in the past 1 65 ma

decomposition of electromagnetic fields into upgoing and downgoing components this paper gives a unified treatment of electromagnetic em field decomposition into upgoing and downgoing components forconductiveandnonconductivemedia wheretheelectromagneticdataaremeasuredonaplaneinwhichtheelectricpermittivity magneticpermeability andelectricalconductivityareknown constants with respect to space and time above and below the plane of measurement the medium can be arbitrarily inhomogeneousandanisotropic in particular the proposed decomposition theory applies to marine em low frequency data acquired for hydrocarbon mapping where the upgoing components of the recordedfield guided and refracted from the reservoir that are of interest for the interpretation the direct source field the refracted airwave induced by the source the reflected field from the sea surface and most magnetotelluric noise traveling downward just below the seabed are field components that are considered to be noise in electromagneticmeasurements the viability and validity of the decomposition method is demonstrated using modeled and real marine em data also termed seabed logging sbl data the synthetic data are simulated in a model that is fairly representative of the geologic area wheretherealsblwerecollected theresultsfromthesynthetic data study therefore are used to assist in the interpretation of the realdatafromanareawith320 mwaterdepthaboveaknowngas province offshore norway the effect of the airwave is seen clearly in measured data after field decomposition just below the seabed the upgoing component of the recorded electric field has almost linear phase indicating that most of the effect of the airwavecomponenthasbeenremoved

synthetic aperture controlled source electromagnetics controlled source electromagnetics csem has been used as a de risking tool in the hydrocarbon exploration industry although there have been successful applications of csem this technique is still not widely used in the industry because the limited types of hydrocarbon reservoirs csem can detect in this paper we apply the concept of synthetic aperture to csem data synthetic aperture allows us to design sources with specific radiation patterns for different purposes the ability to detect reservoirs is dramatically increased after forming an appropriate synthetic aperture antenna consequently the types of hydrocarbon reservoirs that csem can detect are significantly extended because synthetic apertures are constructed as a data processing step there is no additional cost for the csem acquisition synthetic aperture has potential for simplifying and reducing the cost of csem acquisition we show a data example that illustrates the increased sensitivity obtained by applying synthetic aperture csem source

controlled source electromagnetic three dimensional grid modelling based on a complex resistivity structure of the seafloor effects of acquisition parameters and geometry of multi layered resistors a comprehensive controlled source electromagnetic csem modelling study based on complex resistivity structures in a deep marine geological setting is conducted the study demonstrates the effects of acquisition parameters and multi layered resistors on csem responses three dimensional 3d finite difference time domain fdtd grid modelling is used for csem sensitivity analysis interpolation of normalized csem responses provides attributes representing relative sensitivity of the modelled structures modelling results show that fine grid 1 1 km receiver spacing provides good correlations between csem responses and the modelled structures irrespective of source orientation the resolution of csem attributes decreases for receiver spacing 2 2 km when using only in line data broadside data in the grid geometry increase data density by 100 approximately 200 by filling in in line responses and improve the resolution of csem attributes optimized source orientation i e oblique to the strike of an elongated resistor improves the structural definition of the csem anomalies for coarse grid geometries receiver spacing 3 3 km the study also shows that a multi resistor anomaly is not simply the summation but a cumulative response with mutual interference between constituent resistors the combined response of constituent resistors is approximately 50 higher than the cumulative response of the multi resistor for 0 5 hz at 4000 m offset a gradual inverse variation of offset and frequency allows differentiation of csem anomalies for multi layered resistors similar frequency offset variations for laterally persistent high resistivity facies show visual continuity with varying geometric expressions 3d grid modelling is an effective and adequate tool for csem survey design and sensitivity analysis

upper cambrian to upper ordovician peri gondwanan island arc activity in the victoria lake supergroup central newfoundland tectonic development of the northern ganderian margin the exploits subzone of the newfoundland appalachians comprises remnants of cambro ordovician peri gondwanan arc and back arc complexes that formed within the iapetus ocean the exploits subzone experienced at least two accretionary events as a result of the rapid closure of the main portion of the iapetus tract the penobscot orogeny c 480 ma which juxtaposed the penobscot arc c 513 486 ma with the gander margin and c 450 ma collision of the victoria arc c 473 454 ma with the annieopsquotch accretionary tract that juxtaposed the peri laurentian and peri gondwanan elements along the red indian line the newly recognized pats pond group forms a temporal equivalent to other lower ordovician intra oceanic complexes of the penobscot arc the pats pond group c 487 ma has a geochemical stratigraphy that is consistent with rifting of a volcanic arc an ensialic setting is indicated by low end values end 0 3 to 0 5 near the stratigraphic base and its abundant zircon inheritance c 560 ma and 0 9 1 2 ga the spatial distribution of tremadocian arc back arc complexes indicates that the penobscot arc is best explained in terms of a single east dipping subduction zone this model is favored over west dipping models in that it explains the distribution of the penobscot arc elements continental arc magmatism and the obduction of back arc penobscot ophiolites without requiring subduction of the gander margin or subduction reversal the newly recognized wigwam brook group c 454 ma disconformably overlies the pats pond group and records the youngest known phase of ensialic arc volcanism end 4 1 in the victoria arc which is also related to east dipping subduction thus the penobscot and the overlying victoria arc are reinterpreted in terms of a single relatively long lived east dipping subduction zone beneath the peri gondwanan microcontinent of ganderia the cessation of arc volcanism towards the top of the wigwam brook group and the subsequent syn tectonic sedimentation in the badger group constrain the arrival of the leading edge of ganderia with the ensialic arc complexes to the laurentian margin to c 454 ma

csem technology as a value driver for hydrocarbon exploration abstract the use of the controlled source electromagnetic csem method as a risk reduction tool in marine hydrocarbon exploration is gaining increased acceptance in the oil industry this is related to the ability to map resistivity contrasts in the sub surface and thus aid the detection of hydrocarbons which are typically more resistive than surrounding rocks whereas acoustic seismic energy allows for mapping sub surface structures that may contain hydrocarbons electromagnetic em energy can often say something about the fluids contained within the structures numerous successful csem case stories have been published over the past several years however there are also quite a few stories about failure cases although not well documented in the literature such failure cases may reflect the lack of understanding of the csem technology and how csem data can act as a value driver in order to understand this it is necessary to handle uncertainties and decisions associated with the technology these include geological uncertainties noise models survey designs forward modelling parameters inversion migration parameters and pre processing of real data a proper handling of these uncertainties and decisions will aid in defining and constraining the chance of geologic success and geologic success case net present value for prospects prior to drilling wells as such the csem technology has a significant potential to increase exploration efficiency if applied correctly

erratum to variations of sulphur isotope signatures in sulphides from the metamorphosed ming cu au volcanogenic massive sulphide deposit newfoundland appalachians canada the ming deposit is an early ordovician bimodal mafic cu au volcanogenic massive sulphide vms deposit in the newfoundland appalachians that was metamorphosed to upper greenschist lower amphibolite facies conditions and deformed in the silurian and devonian the ming deposit consists of several spatially proximal ore bodies of which the 1806 zone 1807 zone ming south up plunge and down plunge and the lower footwall zone are the focus of this paper the ore bodies have similar stratigraphic sequences the ore bodies can be divided into 1 a silicified horizon that caps the massive sulphides 2 semi massive to massive sulphides and 3 sulphide mineralization in a rhyodacitic footwall sulphide mineralization in a rhyodacitic footwall includes a sulphide stringers immediately below the semi massive to massive sulphides and b chalcopyrite pyrrhotite pyrite stringers distally from semi massive to massive sulphides in the lower footwall zone pyrite chalcopyrite pyrrhotite arsenopyrite and galena were analysed by in situ secondary ion mass spectrometry sims for sulphur isotope compositions the isotopic signatures of pyrite chalcopyrite pyrrhotite and arsenopyrite fall within a limited range of 2 8 to 12 0 for semi massive to massive sulphides and sulphide mineralization in the footwall the silicified horizon capping the semi massive to massive sulphides has higher 34s 5 8 19 6 especially for pyrrhotite mean 17 2 2 2 n 8 the sulphur isotope composition of galena is more heterogeneous especially within semi massive to massive sulphides and sulphide stringers ranging from 0 8 to 17 3 mean 6 1 4 3 n 35 and 7 6 to 17 1 mean 13 7 5 3 n 3 respectively geothermometric calculations give insufficient formation and metamorphism temperatures for neighbouring mineral pairs because sulphides were not in isotopic equilibrium while deposited in early ordovician or re equilibrated during silurian devonian metamorphism respectively therefore original isotopic compositions of sulphides at the ming deposit have been preserved modelling of the source of sulphur shows that 1 reduced seawater sulphate and 2 sulphur leached from igneous wall rock and or derived from magmatic fluids are the main sources of sulphur in the ming deposit the influence of igneous sulphur igneous wall rock magmatic fluids increases with temperature and is an important sulphur source for the semi massive to massive sulphides and footwall mineralization in addition to a contribution from thermochemical sulphate reduction tsr of seawater it is difficult to distinguish between sulphur leached from igneous rocks and magmatic fluid related sulphur and it is possible that both sources contributed to the ores at the ming deposit in addition to igneous sulphur the heavy isotopes of the silicified horizon are consistent with the sulphur in this horizon being derived only from thermochemical sulphate reduction of early ordovician seawater sulphate

role of metalliferous mudstones and detrital shales in the localization genesis and paleoenvironment of volcanogenic massive sulphide deposits of the tally pond volcanic belt central newfoundland canada the cambrian tally pond volcanic belt in central newfoundland contains numerous volcanogenic massive sulphide vms deposits and prospects associated with exhalative metalliferous mudstones deposits in the belt are bimodal felsic vms deposits that are both base metal bearing e g duck pond boundary and base metal and precious metal bearing lemarchant at the lemarchant deposit metalliferous mudstones are stratigraphically and genetically associated with mineralization in the remainder of the tally pond belt detrital shales occur predominantly in the northeastern part of the belt mostly as unrelated mid ordovician structural blocks in the upper sections of the cambrian volcanic stratigraphy but locally also are intercalated with metalliferous mudstones their relationships to massive sulphides are less obvious with many spatially but not necessarily genetically related to mineralization upper cambrian to lower ordovician black shales from bell island which represent pelagic sedimentation not associated with hydrothermal activity and volcanism are compared with the tally pond belt mudstones and shales exhalative mudstones like those at lemarchant have elevated fe al and base metal values and have shale normalized negative ce and positive eu anomalies indicative of deposition from high temperature 250 c hydrothermal fluids within an oxygenated water column mudstones and shales sampled from other tally pond prospects have more variable signatures ranging from hydrothermal to nonhydrothermal black shales no positive eu anomalies flat rare earth element patterns low fe al and base metal contents to those that have mixed signatures accordingly mudstones from areas with a lemarchant like hydrothermal and vent proximal character are more attractive exploration targets than mudstones and shales with predominantly detrital signatures