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

on the resource evaluation of marine gas hydrate deposits using sea floor transient electric dipole dipole methods methane hydrates are solid nonstoichiometric mixtures of water and the gas methane they occur worldwide in sediment beneath the sea floor and estimates of the total mass available there exceed 1016kg since each volume of hydrate can yield up to 164 volumes of gas offshore methane hydrate is recognized as a very important natural energy resource the depth extent and stability of the hydrate zone is governed by the phase diagram for mixtures of methane and hydrate and determined by ambient pressures and temperatures in sea depths greater than about 300 m the pressure is high enough and the temperature low enough for hydrate to occur at the seafloor the fraction of hydrate in the sediment usually increases with increasing depth the base of the hydrate zone is a phase boundary between solid hydrate and free gas and water its depth is determined principally by the value of the geothermal gradient it stands out on seismic sections as a bright reflection the diffuse upper boundary is not as well mar

the abundance of seafloor massive sulfide deposits the possibility of mining seafl oor massive sulfide deposits has stirred debate about the sustainable use of this new resource and whether commercial development is worth the risk among the outstanding questions is how many deposits might be accessible to deep sea mining more than 300 sites of high temperature hydrothermal venting have been identifi ed since the discovery of black smokers but signifi cant massive sulfide accumulation has been found at only 165 of these sites estimates of the total number of vent fields and associated mineral deposits based on plume studies and deposit occurrence models range from 500 to 5000 we have used new deposit occurrence data from 10 000 km of ridge arc and backarc spreading centers to estimate the amount of massive sulfide in the easily accessible neovolcanic zones of the global oceans the total accumulation in these areas is estimated to be on the order of 6 108 tonnes containing 3 107 tonnes of copper and zinc this is similar to the total discovered copper and zinc in cenozoic massive sulfi de deposits mined on land but is insuffi cient to satisfy a growing global demand for these metals

geological fate of seafloor massive sulphides at the tag hydrothermal field mid atlantic ridge deep sea mineral deposits potentially represent vast metal resources that could make a major contribution to future global raw material supply increasing demand for these metals many of which are

active and relict sea floor hydrothermal mineralization at the tag hydrothermal field mid atlantic ridge the tag hydrothermal field is a site of major active and inactive volcanic hosted hydrothermal mineralization in the rift valley of the slow spreading mid atlantic ridge at 26 degree n the axial high is the principal locus of present magmatic intrusions the tag field contains three main areas of present and past hydrothermal activity 1 an actively venting high temperature sulfide mound 2 two former high temperature vent areas 3 a zone of low temperature venting and precipitation of fe and mn oxide deposits the volcanic centers occur at the intersections between ridge axis parallel normal faults and projected axis transverse transfer faults the intersections of these active fault systems may act as conduits both for magmatic intrusions from sources beneath the axial high that build the volcanic centers and for hydrothermal upwelling that taps the heat sources radiometric dating of sulfide samples and manganese crusts in the hydrothermal zones and dating of sediments intercalated with pillow lava flows in the volcanic center adjacent to the active sulfide mound indicate multiple episodes of hydrothermal activity throughout the field driven by heat supplied by episodic intrusions over a period of at least 140 times 10 sup 3 yr the sulfide deposits are built by juxtaposition and superposition during relatively long residence times near episodic axial heat sources counterbalanced by mass wasting in the tectonically active rift valley of the slow spreading oceanic ridge hydrothermal reworking of a relict hydrothermal zone by high temperature hydrothermal episodes has recrystallized sulfides and concentrated the first visible primary gold reported in a deposit at an oceanic ridge

high resolution magnetics reveal the deep structure of a volcanic arc related basalt hosted hydrothermal site palinuro tyrrhenian sea high resolution magnetic surveys have been acquired over the partially sedimented palinuro massive sulfide deposits in the aeolian volcanic arc tyrrhenian sea surveys flown close to the seafloor using an autonomous underwater vehicle auv show that the volcanic arc related basalt hosted hydrothermal site is associated with zones of lower magnetization this observation reflects the alteration of basalt affected by hydrothermal circulation and or the progressive accumulation of a nonmagnetic deposit made of hydrothermal and volcaniclastic material and or a thermal demagnetization of titanomagnetite due to the upwelling of hot fluids to discriminate among these inferences estimate the shape of the nonmagnetic deposit and the characteristics of the underlying altered area the stockwork we use high resolution vector magnetic data acquired by the auv abyss geomar above a crater shaped depression hosting a weakly active hydrothermal site our study unveils a relatively small nonmagnetic deposit accumulated at the bottom of the depression and locked between the surrounding volcanic cones thermal demagnetization is unlikely but the stockwork extends beyond the limits of the nonmagnetic deposit forming lobe shaped zones believed to be a consequence of older volcanic episodes having contributed in generating the cones

marine dipole dipole controlled source electromagnetic and coincident loop transient electromagnetic experiments to detect seafloor massive sulphides effects of three dimensional bathymetry seafloor massive sulphides smss are regarded as a potential future resource to satisfy the growing global demand of metals including copper zinc and gold aside from mining and retrieving profitable amounts of massive sulphides from the seafloor the present challenge is to detect and delineate significant sms accumulations which are generally located near mid ocean ridges and along submarine volcanic arc and backarc spreading centres currently several geophysical technologies are being developed to detect and quantify sms occurrences that often exhibit measurable contrasts in their physical parameters compared to the surrounding host rock here we use a short fixed offset controlled source electromagnetic csem system and a coincident loop transient electromagnetic tem system which in theory allow the detection of sms in the shallow seafloor due to a significant electrical conductivity contrast to their surroundings in 2016 csem and tem experiments were carried out at several locations near the trans atlantic geotraverse hydrothermal field to investigate shallow occurrences of massive sulphides below the seafloor measurements were conducted in an area that contains distinct sms sites located several kilometres off axis from the mid atlantic ridge some of which are still connected to hydrothermal activity and others where hydrothermal activity has ceased based on the quality of the acquired data both experiments were operationally successful however the data analysis indicates bias caused by three dimensional 3d effects of the rough bathymetry in the study area and thus data interpretation remains challenging therefore we study the influence of 3d bathymetry for marine csem and tem experiments focusing on shallow 3d conductors located beneath mound like structures we analyse synthetic inversion models for attributes associated with 3d distortions of csem and tem data that are not sufficiently accounted for in conventional 1d tem and 2d csem interpretation schemes before an adequate quantification of sms in the region is feasible these 3d effects need to be studied to avoid over underestimation of sms using the acquired em data the sensitivity of csem and tem to bathymetry is investigated by means of 3d forward modelling followed by 1d tem and 2d csem inversion of the synthetic data using realistic error conditions subsequently inversion models of the synthetic 3d data are analysed and compared to models derived from the measured data to illustrate that 3d distortions are evident in the recorded data sets

reduced crustal magnetization beneath the active sulfide mound tag hydrothermal field mid atlantic ridge at 26 n abstract a detailed near bottom magnetic field survey was carried out by the submersiblealvin over the actively venting mound located within the tag trans atlantic geotraverse hydrothermal field on the mid atlantic ridge at 26 08 n 44 49 w three dimensional analysis of these data clearly shows a distinct zone of reduced magnetization directly beneath the active mound this magnetization low is consistent with the highly altered upflow zone of a hydrothermal vent system that feeds the actively venting mound structure in contrast the sea surface magnetic anomaly is associated with a broad 2 8 km magnetization low elongated along the axis that includes both the active and inactive mounds the short wavelength 250 m near bottom magnetic anomaly over the active mound is far too small to produce the long wavelength 8 km sea surface magnetic anomaly at tag however and even a collection of mounds with similar magnetic structure cannot produce the magnetic moment needed to generate the sea surface anomaly other hypotheses including reversely magnetized crust and structurally thinned crust could account for the sea surface anomaly but are considered unlikely the existence of vigorous hydrothermal activity at tag and the lack of microseismic activity in the tag area suggests that thermal demagnetization is the prime contributor to the sea surface anomaly the thermal halo associated with a largely solid but still hot intrusion would provide sufficient demagnetization on a kilometer scale to produce the long wavelength sea surface anomaly pervasive alteration at depth would also be an important factor in the destruction of crustal magnetization and is the only way that such a long wavelength magnetic signal could be preserved in the crust the overall model of crustal magnetization at a hydrothermal field with discrete zones of demagnetization in the upper crust and a broader zone of demagnetization at depth is consistent with studies of hydrothermal systems in ophiolite suites these studies show narrow alteration pipes in the upper crust feeding the exhalative seafloor deposits and pervasive alteration at depth which commonly have associated late stage intrusive bodies 1 while detailed magnetic surveys may provide some clues to the location of oceanic hydrothermal upflow zones only drilling will ultimately test these hypotheses

heat flow and mineralogy of tag relict high temperature hydrothermal zones mid atlantic ridge 26 n 45 w submersible investigations employing heat flow measurements 12 stations sampling and imagery of the two relict high temperature hydrothermal zones of the tag field the alvin and mir sulfide zones elucidate relations between heat sources and mineralization including an active sulfide mound that has been the focus of prior studies values of heat flow in the mir zone and at the margin of the active mound are inversely proportional to distance from adjacent volcanic centers this observation supports the hypothesis that intrusions at volcanic centers adjacent to the high temperature hydrothermal zones supply the heat to drive hydrothermal activity the chronology of hydrothermal deposits in the different zones indicates that the intrusions are episodic with field wide high temperature hydrothermal events recurring at intervals of tens of thousands of years while activity at individual zones may recur at intervals of hundreds to thousands of years a sequence of hydrothermal deposits ranges to at least 140 000 years ago from massive sulfides forming at the active mound to recrystallization of sulfides in the active and relict zones to pyritization of an inactive mound in the alvin zone low temperature mineral phases precipitate before during and after the sulfides

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

reduced crustal magnetization beneath relict hydrothermal mounds tag hydrothermal field mid atlantic ridge 26 n submersible magnetic profiles over the mir and alvin relict hydrothermal zones of the tag hydrothermal field on the mid atlantic ridge at 26 n show magnetic anomalies that are consistent with the presence of reduced crustal magnetization these magnetic anomalies are similar to the anomaly found associated with the tag active sulfide mound and suggests that hydrothermal alteration rather than thermal demagnetization is responsible for these reduced magnetization zones magnetic field mapping thus can be useful in determining the subsurface extent of upflow zones and alteration pipes of midocean ridge hydrothermal systems

hydrothermal sulfide accumulation along the endeavour segment juan de fuca ridge highlights high resolution auv bathymetry is used to identify hydrothermal sulfides on the seafloor 1 2 mt of massive sulfide is identified along 24 km of mid ocean ridge length massive sulfide accumulated at a rate of 400 t yr efficiency of sulfide deposition is 5 of total mobilized metals and reduced sulfur current global seafloor sulfide estimates likely underestimate amount present by about a factor of four abstract hydrothermal sulfide deposits that form on the seafloor are often located by the detection of hydrothermal plumes in the water column followed by exploration with deep towed cameras side scan sonar imaging and finally by visual surveys using remotely operated vehicle or occupied submersible hydrothermal plume detection however is ineffective for finding hydrothermally inactive sulfide deposits which may represent a significant amount of the total sulfide accumulation on the seafloor even in hydrothermally active settings here we present results from recent high resolution autonomous underwater vehicle based mapping of the hydrothermally active endeavour segment of the juan de fuca ridge in the northeast pacific ocean analysis of the ridge bathymetry resulted in the location of 581 individual sulfide deposits along 24 km of ridge length hydrothermal deposits were distinguished from volcanic and tectonic features based on the characteristics of their surface morphology such as shape and slope angles volume calculations for each deposit results in a total volume of 372 500 m3 of hydrothermal sulfide sulfate silica material for an equivalent mass of 1 2 mt of hydrothermal material on the seafloor within the ridge s axial valley assuming a density of 3 1 g cm3 much of this total volume is from previously undocumented inactive deposits outside the main active vent fields based on minimum ages of sulfide deposition the deposits accumulated at a maximum rate of 400 t yr with a depositional efficiency proportion of hydrothermal material that accumulates on the seafloor to the total amount hydrothermally mobilized and transported to the seafloor of 5 the calculated sulfide tonnage represents a four fold increase over previous sulfide estimates for the endeavour segment that were based largely on accumulations from within the active fields these results suggest that recent global seafloor sulfide resource estimates which were based mostly on the sizes and distribution of hydrothermally active deposits may be similarly underestimating the amount of sulfide along the global submarine neovolcanic zones

the endemic trends of liassic ammonite faunas of portugal as the result of the opening up of a narrow epicontinental basin abstract during several stages the sinemurian and pliensbachian ammonite faunas of the lusitanian basin coastal portugal have a pronounced endemic trend these trends concern several lineages without narrow phyletic relationship belonging to asteroceratinae ptycharietites and pompeckioceras phricodoceratinae pseudophricodoceras and epideroceras polymorphitidae dayiceras and eoderoceratidae metaderoceras frequently the species most derived from these endemic lineages display a pronounced morphological convergence these species posses an involute shell with rather slender ribbing this converging trend is probably the result of a morphofunctional factor the lusitanian endemic trends can only be understood within their biogeographical framework the sequence of endemic events occurs during a critical phase in the geological history of the lusitanian basin the settlement of a definite marine environment besides this early extension of open marine conditions affects a narrow basin chopped up by several transform faults it is within this particular paleogeographical framework that the main causes of these endemic events must be looked for

seafloor massive sulphide exploration using deep towed controlled source electromagnetics navigational uncertainties deep towed geophysical surveys require precise knowledge of navigational parameters such as instrument position and orientation because navigational uncertainties reflect in the data and therefore in the inferred geophysical properties of the sub seafloor we address this issue for the case of electrical conductivity inferred from controlled source electromagnetic data we show that the data error is laterally variable due to irregular motion during deep towing but also due to lateral variations in conductivity including those resulting from topography to address this variability and quantify the data error prior to inversion we propose a two dimensional perturbation study our workflow enables stable and geologically reliable results for multi component and multi frequency inversions an error estimation workflow is presented which comprises the assessment of navigational uncertainties perturbation of navigational parameters and forward modelling of electric field amplitudes for a homogeneous and then a heterogeneous sub seafloor conductivity model some navigational uncertainties are estimated from variations of direct measurements other navigational parameters required for inversion are derived from the measured quantities and their error is calculated by means of error propagation some navigational parameters show direct correlation with the measured electric fields for example the antenna dip correlates with the vertical electric field and the depth correlates with the horizontal electric field for the perturbation study each standard deviation is added to the navigational parameters forward models are run for each perturbation amplitude deviations are summed in quadrature with the stacking error for a total laterally varying data error the error estimation is repeated for a heterogeneous sub seafloor model due to the large conductivity range several orders of magnitude which affects the forward model the approach enables us to utilize data from several components multiple electric fields frequencies and receivers in the inversion to constrain the final model and reduce ambiguity the final model is geologically reasonable in this case enabling the identification of conductive metal sulphide deposits on the seafloor

resources of the sea floor in his perspective rona reviews recent advances in knowledge of sea floor resources knowledge is expanding from those minerals eroded from land metals gemstones sand and gravel to those minerals concentrated by plate tectonic processes in ocean basins exploration of the oceans is at an early stage and development of marine mining is subject to economic and environmental considerations

near seafloor magnetics reveal tectonic rotation and deep structure at the tag trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n we take advantage of geological constraints from ocean drilling program drill holes and high resolution bathymetry to revisit the near seafloor magnetic anomaly at the trans atlantic geotraverse hydrothermal site mid atlantic ridge 26 n the dipolar anomaly associated with the site is better reduced to the pole if the magnetization vector is tilted by 34 an observation suggesting that the strongly faulted basalt surrounding the site has been rotated by 53 along an axis parallel to the mid atlantic ridge as a probable consequence of the detachment tectonics inferred in this area the faults and the deeper detachment focus and guide the hot ascending hydrothermal fluid magnetic modeling shows that the nonmagnetic stockwork zone is a significant contributor to the observed negative anomaly the rest being accounted for by a deeper source probably related to thermal demagnetization of an ascending hydrothermal pipe beneath the active part of the site

the trans atlantic geotraverse hydrothermal field a hydrothermal system on an active detachment fault abstract over the last ten years geophysical studies have revealed that the trans atlantic geotraverse tag hydrothermal field 26 08 n on the mid atlantic ridge is located on the hanging wall of an active detachment fault this is particularly important in light of the recognition that detachment faulting accounts for crustal accretion extension along a significant portion of the mid atlantic ridge and that the majority of confirmed vent sites on this slow spreading ridge are hosted on detachment faults the tag hydrothermal field is one of the largest sites of high temperature hydrothermal activity and mineralization found to date on the seafloor and is comprised of active and relict deposits in different stages of evolution the episodic nature of hydrothermal activity over the last 140 ka provides strong evidence that the complex shape and geological structure of the active detachment fault system exerts first order but poorly understood influences on the hydrothermal circulation patterns fluid chemistry and mineral deposition while hydrothermal circulation extracts heat from a deep source region the location of the source region at tag is unknown hydrothermal upflow is likely focused along the relatively permeable detachment fault interface at depth and then the high temperature fluids leave the low angle portion of the detachment fault and rise vertically through the highly fissured hanging wall to the seafloor the presence of abundant anhydrite in the cone on the summit of the tag active mound and in veins in the crust beneath provides evidence for a fluid circulation system that entrains significant amounts of seawater into the shallow parts of the mound and stockwork given the importance of detachment faulting for crustal extension at slow spreading ridges the fundamental question that still needs to be addressed is how do detachment fault systems and the structure at depth associated with these systems e g presence of plutons and or high permeability zones influence the pattern of hydrothermal circulation mineral deposition and fluid chemistry both in space and time within slowly accreted ocean crust

vulcan a deep towed csem receiver we have developed a three axis electric field receiver designed to be towed behind a marine electromagnetic transmitter for the purpose of mapping the electrical resistivity in the upper 1000 m of seafloor geology by careful adjustment of buoyancy and the use of real time monitoring of depth and altitude we are able to deep tow multiple receivers on arrays up to 1200 m long within 50 m of the seafloor thereby obtaining good coupling to geology the rigid body of the receiver is designed to reduce noise associated with lateral motion of flexible antennas during towing and allows the measurement of the vertical electric field component which modeling shows to be particularly sensitive to near seafloor resistivity variations the positions and orientations of the receivers are continuously measured and realistic estimates of positioning errors can be used to build an error model for the data during a test in the san diego trough offshore california inversions of the data were able to fit amplitude and phase of horizontal electric fields at three frequencies on three receivers to about 1 in amplitude and 1 in phase and vertical fields to about 5 in amplitude and 5 in phase the geological target of the tests was a known cold seep and methane vent in 1000 m water depth which inversions show to be associated with a 1 km wide resistor at a depth between 50 and 150 m below seafloor given the high resistivity 30 m and position within the gas hydrate stability field we interpret this to be massive methane hydrate

marine mineral exploration with controlled source electromagnetics at the tag hydrothermal field 26 n mid atlantic ridge seafloor massive sulphide sms deposits are of increasing economic interest in order to satisfy the relentless growth in worldwide metal demand the trans atlantic geotraverse tag hydrothermal field at 26 n on the mid atlantic ridge hosts several such deposits this study presents new controlled source electromagnetic csem bathymetric and magnetic results from the tag field potential sms targets were selected based on their surface expressions in high resolution bathymetric data high resolution reduced to the pole magnetic data show negative anomalies beneath and surrounding the sms deposits revealing large areas of hydrothermal alteration csem data sensitive to the electrical conductivity of sms mineralization further reveal a maximum thickness of up to 80 m and conductivities of up to 5 s m sms samples have conductivities of up to a few thousand s m suggesting that remotely inferred conductivities represent an average of metal sulphide ores combined with silicified and altered host basalt that likely dominates at greater depths

discrimination of hydrothermal mound structures using transient electromagnetic methods we present numerical modeling results to support the use of seafloor transient electromagnetic em techniques in characterizing the buried component of active hydrothermal mounds we construct 2 d electrical models of a volcanic hosted sulphide mound and compute the em fields created by a transient electric dipole source deployed on the seafloor the time it takes for fields to diffuse between the source and receivers is diagnostic of the intervening electrical conductivity in the case considered the highly conductive outcropping sulphide mound is essentially invisible to the em probe exposing the moderately conducting alteration and upflow zone below transient em responses are measurably different for the model featuring concentrated upflow compared to the model containing a spatially broad fluid supply beneath the mound

a time domain electromagnetic survey of the tag hydrothermal mound a conductivity survey of the tag mound was made using innovative transient electric dipole dipole instrumentation the receiver was deployed on the sulfide mound 30 m south of the central black smoker while the submersible alvin carried the transmitter on a survey path around the receiver at a radius of approximately 70 m conductivity measurements of the seafloor between the transmitter and receiver were made at 5 minute intervals the survey lasted 4 hours measurements made at 12 sites are initially interpreted in terms of an apparent conductivity of a uniform seafloor a more complex model in which the seafloor has two layers is then used apparent conductivities range from 1 4 to 15 9 s m generally higher than that of seawater the results suggest possible focusing of hydrothermal convection in the northern quadrant of the mound adjacent zones of anhydrite accumulation and fluid convection in the north eastern quadrant heterogeneity and layering in the kremlin zone and unexpected spatial variability in the western quadrant the data from several sites show evidence of distortion attributed to 3 d anomalies

hydrothermal activity on a 105 year scale at a slow spreading ridge tag hydrothermal field mid atlantic ridge 26 n new radiometric dates are presented for the hydrothermal deposits in the alvin and mir relict hydrothermal zones located on the lower east wall of the rift valley within the tag hydrothermal field and placed in context of the evolution of the tag field as a whole over the past 105 years the field includes an active high temperature sulfide mound situated near the base of the east wall and a low temperature zone at middepth on the east wall all within a 5 by 5 kilometer area the oldest hydrothermal deposits sampled in the mir relict zone the low temperature zone and an inactive sulfide mound in the alvin relict zone are low temperature manganese crusts dated at 140 000 125 000 and 74 000 years respectively high temperature activity began about 100 000 years ago in the mir zone a 50 000 year high temperature event took place at the active sulfide mound in the mir zone and at the inactive sulfide mound in the alvin zone this event is followed by sporadic activity at the mir zone and the active sulfide mound until 10 000 and 5000 years ago respectively a reactivation of the system affected the mir zone from about 2000 to 600 years ago the presently active sulfide mound was reactivated only 60 years ago after a quiescence of about 4000 years the radiochronologic record reveals that the hydrothermal activity has been episodic with a major high temperature event throughout the tag field at 50 000 years and other more localized events

the pliensbachian ammonite dayiceras dayiceroides and early jurassic paleogeography uptonia dayiceroides mouterde is placed in the genus dayiceras and its age established as latest jamesoni zone to possibly earliest domerian the species is abundant and associated with faunas of tethyan aspect along the northeastern pacific margin first occurrences in oregon and nevada and new occurrences in british columbia are reported localities at apparently high paleolatitudes are attributed to post early pliensbachian transcurrent fault displacements genetic continuity with a disjunct population in portugal is postulated via a central atlantic seaway here named the hispanic corridor connecting the eastern pacific and western tethys oceans the existence of this corridor during the pliensbachian is supported by several lines of independent paleobiogeographic evidence

p wave velocity measurements for preliminary assessments of the mineralization in seafloor massive sulfide mini cores during drilling operations highlight it is important to develop systems able to detect and classify mineralized zones from waste materials while drilling deep water seismic p wave velocities vp were measured on 40 sms and unmineralized mini cores the porosity was back calculated from vp the results were compared with electrical resistivity measurements using archie s law it is possible to observe that metallic conduction exists abstract deep sea mining exploration for seafloor massive sulfide sms deposits is currently increasing at present most exploration activities are surficial and use indirect methods to identify potential sites and perform first assessments for a proper resource estimate however drilling is inevitable by using seabed drill rigs exploration costs can be reduced considerably sms deposits are normally found at depths between 1000 and 4000 m and in order for deep sea mining to be implemented reliable technologies are needed additionally the development of geophysical systems that can detect and classify mineralized zones from waste materials while drilling could decrease costs and speed up offshore operations by limiting the amount of drilling of unmineralized materials this paper shows how the physical properties of sms can be used to discriminate between host rocks and mineralization seismic p wave velocities vp were measured on 40 sms and unmineralized mini cores by back calculating the porosity from vp comparing the results with electrical resistivity measurements and using archie s law it is possible to observe that metallic conduction exists for deep sea mineral exploration the combination of seismic tests electrical resistivity and magnetic susceptibility could support the preliminary discrimination of mineralized samples in the cores while drilling at the seafloor

tension vein arrays in progressive strain complex but predictable architecture and major hosts of ore deposits abstract most en echelon vein arrays are extensional and can be termed tension vein arrays tvas tvas in major fault and shear zones fszs are subject to progressive deformation this deforms the initial tva whose geometry is well documented in the literature via discrete stages first into progressively more complex architecture involving folding of the tension veins then into progressively more simple architecture ultimately forming pipes during this progression the tva axis new definition rotates within the shear plane from normal to parallel to the displacement vector the deforming tva axis has simpler geometry than the complex tension veins and it can be employed to precisely track the deformation state of the fsz its displacement vector and shear sense through all the strain stages five structural metasomatic stages are defined by discrete steps in the strain evolution tvas are difficult to recognise and are under recognised in ore systems for a number of inherent geological reasons orebodies founded on dilation form parallel to the tva axis which is also parallel to dilational jogs in the parent fsz orebodies formed early in the fsz history are normal to the displacement vector and in progressive shear rotate with the tva axis toward the displacement vector orebodies formed late in the fsz history overprint apparently complex to chaotic vein stockwork which nevertheless has analysable geometry tva hosted orebodies are not necessarily parallel to the displacement vector of the host fsz but occupy elongate orientations over a 90 range within the fsz large orebodies are favourably developed in tvas in unfoliated fszs type 1 shear zones which may form fluid superhighways type 1 shear zones form in predictable circumstances involving particularly host rocktype and crustal position strike slip fszs possess a downdip tva axis and are especially able to tap deep crustal fluid tva hosted orebodies form a major deposit style exploration requires careful analysis of the prospect geometry at an early stage and recognition of the multi faceted tva architecture the relationship between the parent shear zone or fault including fault veins and the component tension veins is complex and search strategies depend critically on the strain stage the type of host fsz and the type of exposure deciphering the architecture of tvas involves a combination of vector orientational parameters and scalar angular relationship parameters this permits ready analysis in oriented rock outcrop and oriented drillcore and in unoriented rock unoriented drillcore mine dumps and float