fissure fills along faults variscan examples from gower south wales the extent to which persistent rather than transient fissures wide planar voids can exist along upper crustal faults is important in assessing fault permeability to mineral and hydrocarbon bearing fluids variscan late carboniferous faults cutting dinantian lower carboniferous limestones on the gower peninsula south wales host clear evidence for fissures up to several metres wide evidence includes dendritic hematite growth and elongate calcite growth into open voids spar ball and cockade breccia formation laminated sediment infill and void collapse breccias detailed mapping reveals cross cutting geometries and brecciation of earlier fissure fills showing that fissures were formed during rather than after active faulting fissures therefore probably formed by geometric mismatch between displaced fault walls rather than by solution widening along inactive faults

intermediate sulfidation epithermal gold base metal deposits in tertiary subaerial volcanic rocks sahinli tespih dere lapseki western turkey abstract gold in the sahinli and tespih dere intermediate sulfidation gold base metal deposits in western turkey occurs in relatively deep epithermal quartz veins along with base metal minerals which have epithermal textures including plumose quartz vug infills comb and cockade textures and matrix supported milled breccias the total sulfide content of the veins in the area is variable ranging from fluid inclusions in main stage quartz at sahinli are only liquid rich with homogenization temperatures ranging from 220 to 322 c and the majority of th values between 250 and 300 c salinity ranges from 4 3 to 6 9 wt nacl equiv first ice melting temperatures tmf between 24 5 and 19 0 c indicate that the fluids were dominated by nacl h2o during mineralization the relatively higher average th at the tespih dere deposit 295 c is attributed to a relatively deeper level of exposure calculated 18o values indicate that ore forming hydrothermal fluids in the study area had 18oh2o ranging from 1 1 to 9 7 average 3 8 strongly 18o enriched compared with present day hydrothermal meteoric water in the area 18o 8 5 d values of fluid inclusions in quartz range from 58 to 93 and d values of clay minerals and alunite from 40 to 119 d values from intermediate argillic alteration average 68 in the study area are very similar to d values of the present day local geothermal system average d 54 whereas d values from advanced argillic alteration average d 33 are very different from the present day local geothermal system the 34s values in samples from the sahinli and tespih dere deposits average 2 9 for pyrite 3 3 for chalcopyrite 5 4 for sphalerite and 7 6 for galena these data are consistent with derivation of the sulfur from either igneous rocks or possibly from local wallrock

strontium isotopic evidence for the origin of siderite ankerite and magnesite mineralizations in the eastern alps metasomatic magnesite siderite and ankerite mineralizations within paleozoic sediments of the northern greywacke zone and its metamorphic counterpart in the crystalline basement upper austroalpine unit were investigated with respect to their strontium isotopic composition the results support a sedimentary s l model for the magnesites within visean platform carbonates of the veitsch nappe and an epigenetic hydrothermal model for the siderites and ankerites in devonian platform carbonates of the noric nappe and from huttenberg the fe transporting fluids can be characterized either as magmatogene or more probably as metamorphogene by increasing the stage of recrystallization and or remobilization due to at least two later metamorphic overprints sr in the various ore minerals beame enriched in radiogenic 87sr

structural controls on the base metal vein deposits of the northampton complex western australia abstract the northampton complex is a proterozoic inlier near the west coast of western australia 450 km north of perth the copper lead and zinc vein type deposits occupy dilational sites along brittle ductile shear zones there are four types of structural settings of the ore deposits 1 lens structures 2 shear breccia link shear structures extensional strike slip duplexes 3 intersecting brittle ductile shears and 4 curved brittle ductile shears a model involving n s dextral wrenching along the darling mobile belt best explains the various features of the deposits in this model proterozoic granulite facies paragneisses of the northampton complex were fractured with the development of predominantly extensional fractures and normal faults and to a lesser extent riedel both r and r restraint and principal displacement shears since previous isotopic dating has not resolved the age of mineralisation crosscutting relationships between mineralisation the older dolerite dykes 650 800 ma and younger 330 trending sinistral shears 500 650 ma are used to constrain the timing to between 650 ma and 800 ma ne sw shortening across the paterson and king leopold mobile belts between 650 ma and 800 ma may also have been associated with the dextral movements along the darling mobile belt

binge purge oscillations of the laurentide ice sheet as a cause of the north atlantic s heinrich events ice rafted debris in sediment cores from the north atlantic suggests that the laurentide ice sheet lis periodically disgorged icebergs in brief but violent episodes which occurred approximately every 7 000 years here i propose that heinrich events i e what these episodes are called were caused by free oscillations in the flow of the laurentide ice sheet which arose because the floor of hudson bay and hudson strait is covered with soft unconsolidated sediment that forms a slippery lubricant when thawed the proposed heinrich event cycle has two phases the growth phase occurs when the sediment is frozen and the lis is stranded immobile on a rigid bed the volume of the lis slowly grows during this phase at a rate dictated by snow accumulation the purge phase occurs when the basal sediment thaws and a basally lubricated discharge pathway i e an ice stream such as those which occur in west antarctica today developes through hudson strait the volume of the lis rapidly equilibrates to the reduced basal friction during this phase by dumping icebergs into the labrador sea the periodicity t k sl2g 2 7000 years of the proposed heinrich event cycle is a function of the thermal conductivity and diffusivity of ice k and respectively the atmospheric sea level temperature sl in degrees celsius and the excess geothermal heat flux defined by g g k where is the atmospheric lapse rate and g is the geothermal heat flux agreement between the predicted t and the apparent periodicity implied by the marine record is the main virtue of the free oscillation mechanism i propose an alternative mechanism in which heinrich events are forced by periodic variations in external climate is implausible because periodic atmospheric temperature perturbations are strongly attenuated with depth in an ice sheet

a contribution to the ore genesis of the magnesite deposit of eugui navarra spain in the area of the magnesite deposits of eugui navarra spain studies on illite crystallinity the degree of graphitization of carbonaceous material measurements of vitrinite reflectivity and fluid inclusions have been carried out on dolomites magnesites schists and carbonaceous matter these rocks have suffered metamorphism of very low to low grade the magnesite appears generally concordant with the namurian dolomitic rocks showing a typically banded structure the genetic model proposed involves an early mg concentration during sedimentation syndiagenetic dolomitization lateral circulation of saline solutions and formation of diagenetically crystallized rhythmites dcr and final compaction the formation of magnesite took place under the conditions of low pressure and temperatures close to 150 c very similar to all strata bound ore deposits deformation and regional metamorphism only caused minor removal recrystallization and transformation of the clay minerals and carbonaceous matter

transient permeability and reseal hardening in fault zones evidence from dilation breccia textures t is well established that earthquake faulting can create permeability along a fault zone in high competence rocks by mismatch of the fault walls and by secondary fracture in a surrounding damage zone and that this permeability is created repeatedly during successive earthquake cycles less well proven is that such permeability is transient being episodically reduced by precipitation of cements in the fracture porosity the textures of carbonate dilation breccias formed at around 1 7 km depth on the dent fault zone nw england lend support to this economically important concept of transient fracture permeability the key observation is that many breccias reflect only a single episode of brecciation and reseal a generally applicable explanation of such single phase breccias is that they were resealed in the interval between major earthquakes that this reseal made the breccia stronger that the intact rock and that subsequent brecciation in the same rock volume was inhibited this reseal hardening model implies that transient permeability in fault zones may last no longer than the recurrence times of large earthquakes and that the permeability conduits will change position in the damage zone with time unless focused at a major fault jog or termination

how ice sheets flow melt waters at the bases of glaciers and ice sheets play an important role in their flow behavior in her perspective hulbe 1 highlights the report by fahnestock et al 2 who have identified high basal melting rates below a part of the greenland ice sheet as the cause for fast ice sheet flow they attribute the observation to higher than expected geothermal heat flow but melt waters are not uniformly distributed underneath the ice many regions are frozen to the bedrock determining the conditions under which basal melting occurs is important for understanding ice sheet histories and predicting their future behaviors 1 http www sciencemag org cgi content full 294 5550 2300 2 http www sciencemag org cgi content short 294 5550 2338

a new genetic model for the north african metasomatic siderite deposits based on published accounts of the diagenetic evolution of evaporite diapirs and geological preliminary geochemical and fluid inclusion data a new genetic model is presented for the north african metasomatic siderite deposits

genesis of cockade breccias in the tectonic evolution of the cirotan epithermal gold system west java the pliocene cirotan gold deposit provides an exceptional example of mineralized breccias and in particular cockade breccias analysis of these breccias approached through both field observation and image analysis on photographs made it possible to determine their method of graded formation the mineralized structure as much a 25 m thick in places is a right lateral strike slip fault that evolved to a normal fault at the end of the system s development it consists of juxtaposed breccia bodies separated by major slip or septa which formed at a major inflection of the host fault the breccia bodies all show a similar internal organization from footwall to hanging wall one passes from a siliceous breccia with angular clasts to a thick infill of cockade breccia showing a reverse grading with small centimetre size cockades aspect ratios of 1 10 passing to larger up to 1 m more equant cockades aspect ratios 1 3 in contact with the hanging wall the different stages of cockade formation demonst

tributaries of west antarctic ice streams revealed by radarsat interferometry interferometric radarsat data are used to map ice motion in the source areas of four west antarctic ice streams the data reveal that tributaries coincident with subglacial valleys provide a spatially extensive transition between slow inland flow and rapid ice stream flow and that adjacent ice streams draw from shared source regions two tributaries flow into the stagnant ice stream c creating an extensive region that is thickening at an average rate of 0 49 meters per year this is one of the largest rates of thickening ever reported in antarctica

magnesite deposits and occurrences in the west carpathians of czecho slovakia abstract magnesite occurrences in the west carpathians a part of the alpine orogenic system have been known since the second half of the 19th century they are hosted by cambrian silurian devonian carboniferous permian lower and upper triassic sedimentary units economically most important are crystalline magnesite veitsch type and locally associated talc in the devonian sequence of the veporides and in the caboniferous of the gemerides yearly production 2 3 million tonnes traditionally the slovak magnesite deposits had been interpreted as hydrothermal metasomatic related to magmatically derived fluids more recently a sedimentary diagenetic origin of the magnesite with mg derived in full or in part from the contemporaneous submarine basaltic volcanism is considered more probable after deposition and diagenesis the magnesites and their host rocks were metamorphosed in greenschist carboniferous of the gemerides to amphibolite devonian of the veporides facies

source levels of shallow explosive charges experimental measurements are reported in this article for the source levels of 0 82 kg signals underwater sound sus charges detonated at nominal depths of 18 3 91 and 183 m the data were recorded in the farfield using a high resolution digital data acquisition system that provided good reproduction of both the high and low frequency components of the signal from the explosion the results are presented in 1 3 octave bands from 5 630 hz and represent the mean values of samples of at least 30 charges for each depth group the standard deviations were within 1 db for most frequency bands analysis of the distribution of explosion depths indicated that the average detonation depths were consistently deeper than the nominal depths this result was attributed to the effect of the relatively cold water temperatures at the experimental site in addition to the measured source levels estimates for 0 82 kg sus charges at 244 m were scaled from the experimental data using a method suggested by hughes r hu

metallogenetic evolution of the east alpine paleozoic basement paleo metallogenetic sketch maps of the east alpine area are presented for the ordovician silurian the devonian lower carboniferous and the upper carboniferous permian mineralization and tectonic evolution are then discussed in the frame of a plate tectonic model compatible with recent results in western europe

flow rates in the axial hot springs of the east pacific rise 21 n implications for the heat budget and the formation of massive sulfide deposits abstract the first in situ measurements of flow rates in submarine hot springs temperatures between 275 and 350 c were made in the hot springs of the east pacific rise at 21 n during november 1981 the flow rates ranged between 0 7 and 2 4 m s we estimate that the total rates at which heat and mass are transported from the three hot spring areas obs national geographic and southwest vents are ca 2 2 108 w and 150 kg of fluid s respectively the very high rate of heat loss precludes the existence of hot springs as steady state features of the east pacific rise at 21 n we estimate that hot springs are active for a maximum of ca 40 000 years chemical and or mechanical clogging of the vents can drastically reduce the lifetime of the hot springs using a simple model of plume development 1 we calculated that less than 3 of the sulfide particles that are entrained in the hydrothermal plumes at 21 n settle from the plumes before dispersal by a lateral submarine current at a height of ca 250 m above the seafloor 2 the time that is required to form the sulfide mounds that surround the hot springs by the accumulation of sulfide particles that settle from the plumes is ca 70 85 years the current rate at which sulfide particles settle from the hydrothermal plumes and accumulate near the hot springs appears to be too small to lead to the formation of large massive sulfide deposits such as the mavrouni deposit on cyprus 3 within the time limits that are suggested by our thermal energy balance calculations

shear wave velocity structure of shallow water sediments in the east china sea the results of an experiment designed to measure the shear wave velocity structure of the shallow water sediments at three locations in the east china sea in water depths ranging from 40 m to 150 m are reported both vertically polarized and horizontally polarized shear wave velocities were measured by placing both source and receiver on the seafloor the recorded data consist of interface waves of love wave and stoneley scholte wave type and both horizontally polarized sh and vertically polarized sv body waves with the aid of synthetic seismogram analysis the travel times and amplitudes of these phases were used to constrain the velocity structure of the seabed to depths of 50 m these results show that velocities range from less than 50 m s at the seafloor to greater than 400 m s at subseafloor depths of only 50 m vertical velocity gradients of up to 200 and 15 s 1 are required in the upper 1 and 10 m of the seabed respectively comparison of profiles carried out at the same approximate location shows that the velocity structure of the upper 10 m varies by up to 80 over lateral distances of less than a few kilometers in the data bandwidth of 5 30 hz shear q values range from 10 20 attenuation factor of 0 07 0 14 m 1 at 20 hz at the seafloor to 40 attenuation factor of 0 004 m 1 at 20 hz at depth the shear velocities immediately below the seafloor differ by less than a factor of 2 from the values predicted by applying the biot theory to the geotechnical parameters measured on cores taken at the experiment locations in two of three cases the differences between the predicted and measured values are less than 8

the importance of pressurized subglacial water in separation and sliding at the glacier bed l effet de l eau sous glaciaire sous pression sur le processus de glissement est examine a partir d un parametre appele indice de decollement du lit qui exprime les contraintes perpendiculaires au lit et la pression d eau les donnees proviennent de trois glaciers de formes differentes une loi puissance du type de weertman avec un exposant voisin de 3 modifiee pour tenir compte de l effet de la pression des eaux sous glaciaires apparait comme etant la meilleure

gangue mineral 87 sr 86 sr ratios and the origin of mississippi valley type mineralization the 87 sr 86 sr ratio measurements in mississippi valley type gangue minerals and host marine carbonates ranging in age from upper cambrian to mississippian show that in northwestern ohio where the mineralization is sparse the minerals precipitated from fluids in local isotopic equilibrium with the host carbonates in which the minerals were emplaced in contrast most gangue minerals from districts of major mineralization in illinois kentucky virginia and tennessee contain strontium with higher 87 sr 86 sr ratios than the carbonate hosts and appear to have crystallized from fluids containing strontium from silicate minerals in the sedimentary succession no evidence has been found for a magmatie component in the ore fluid strontium data from elmwood tennessee show that the early ore fluids like those of northwestern ohio were in isotopic equilibrium with the host carbonates but that they were mixed with and displaced by introduced fluids containing more radiogenic strontium these findings are consistent with the two fluid mixing model for the mississippi valley mineralization where a fluid indigenous to the host carbonates and containing reduced sulfur was mixed with introduced fluids analogous to metal bearing and low sulfide oil field brines with an extensive history of chemical exchange with clays and other silicate minerals in the sedimentary succession

greenland ice sheet surface properties and ice dynamics from ers 1 sar imagery c band synthetic aperture radar sar imagery from the european space agency s ers 1 satellite reveals the basic zonation of the surface of the greenland ice sheet the zones have backscatter signatures related to the structure of the snowpack which varies with the balance of accumulation and melt at various elevations the boundaries of zones can be accurately located with the use of this high resolution imagery the images also reveal a large flow feature in northeast greenland that is similar to ice streams in antarctica and may play a major role in the discharge of ice from the ice sheet

a fracture loop thermal balance model of black smoker circulation abstract this time dependent model examines the thermal balance between hot rocks and hydrothermal circulation during the formation of volcanogenic ore deposits in order to establish whether the major heat supply to black smoker systems comes from the rock or from an underlying magma chamber flow is modelled as an open thermosyphon in a single fault which is parallel and very close to the mid ocean ridge axis this configuration is based on seafloor and ophiolite evidence of black smoker systems and their structural location the fault is represented in the model by a series of pipes carrying turbulent flow which is driven by the buoyancy difference between upflow and downflow hydraulic resistance to flow is predominantly in the discharge zone the fault is allowed to propagate downwards the horizontal conductive heat flux from the rock is computed analytically with the water flow treated digitally so that the heat exchange at the rock water interface at every position in the loop can be calculated for each time step the model thus provides the exit water temperature and mass flow rate throughout the lifespan of a hot spring enabling the resulting ore mass to be estimated results show that this system cannot obtain sufficient heat from a thin layer of rock such as the 1 2 km thick layer of lavas and dykes overlying an axial magma chamber to sustain long lived ore forming black smokers if the circulation penetrates through a greater volume of hot rock to the base of a recently solidified magma chamber ore masses of a few million tonnes can be formed if conditions are particularly favourable we conclude that systems producing larger ore masses must tap a magmatic heat supply

intense fracturing and fracture sealing induced by mineral growth in porous rocks article i nfo mineral precipitation in the pores of a rock may exert a force which is called crystallization pressure this process has been studied experimentally and results bring a new look on the way fractures may develop and seal in natural systems cylindrical core samples of porous limestone and sandstone were left for several weeks in contact with an aqueous solution saturated with sodium chloride at 30 or 45 c under axial normal stress in the range 0 02 0 26 mpa the fluid was allowed to rise in the core samples by capillary forces up to a controlled height where evaporation and precipitation occurred the uniaxial deformation of the samples was measured using high resolution displacement sensors the samples were characterized using computed x ray tomography allowing therefore imaging in 3d the intensity and localization of the damage two kinds of damage could be observed firstly small rock fragments were peeled from the sample surface secondly

dispersion of interface waves in sediments with power law shear speed profiles i exact and approximate analytical results in the upper tens of meters of ocean bottom unconsolidated marine sediments consisting of clay silt or fine sand with high porosity are almost incompressible in the sense that the shear wave velocity is much smaller than the compressional wave velocity the shear velocity has very large gradients close to the ocean floor leading to strong coupling of compressional and shear waves in such soft sediments the weak compressibility opens an avenue for developing a theory of elastic wave propagation in continuously stratified soft sediments that fully accounts for the coupling elastic waves in soft sediments consist of fast waves propagating with velocities close to the compressional velocity and slow waves propagating with velocities on the order of the shear velocity for the slow waves the theory predicts the existence of surface waves at the ocean sediment boundary in the important special case of the power law depth dependence of shear rigidity phase and group velocities of the interface waves are shown to scale as a certain power of frequency an explicit exact solution was obtained for the surface waves in sediments characterized by constant density and a linear increase of shear rigidity with depth that is for the case of shear speed proportional to the square root of the depth below the sediment water interface asymptotic and perturbation techniques were used to extend the result to more general environments theoretical dispersion relations agreed well with numerical simulations and available experimental data and as demonstrated in a companion paper d m f chapman and o a godin j acoust soc am 110 1908 2001 led to a simple and robust inversion of interface wave travel times for shear velocity profiles in the sediment

carbonate hosted zinc lead deposits in the lower cambrian of hunan south china a radiogenic pb sr isotope study zinc lead deposits hosted by lower cambrian carbonate rocks near huayuan in the hunan guizhou orogenic belt southern china have been studied by means of pb and sr isotopes the deposits are characterized by two mineralization stages 1 a main economic stage comprising cockade style sphalerite galena pyrite marcasite fluorite barite and calcite and 2 a subsequent noneconomic stage with nodular rosette sphalerite saddle dolomite and bitumen both ore stages are postdated by a generation of late calcite main stage sulfides 206pb 204pb 18 162 18 225 display a steep linear trend on conventional pb pb diagrams which indicates binary mixing of lead components bearing the imprint of ancient 207pb dominated crustal rocks and reservoirs with elevated th u ratios possibly located in the lower crust nodular rosette sphalerites fall outside this line but reflect a second mixing trend between mineralizing fluids characterized by 206pb 204pb 18 3 and lead isotopically similar to specific main stage cockade sphalerites suggesting either remobilization of pb from main stage ores or contemporaneous mixing of three distinct lead components in the course of an evolving hydrothermal event the 87sr 86sr ratios of diagenetic and hydrothermal carbonate cements and barites range from 0 70905 to 0 71006 and increase systematically with the paragenetic sequence indicating progressive mixing of marine sr derived from the lower cambrian carbonate host rocks with radiogenic sr supplied by external mineralizing fluids in order to identify possible local metal sources u pb and rb sr analyses were performed on six whole rock samples from the lower cambrian sequence and their time integrated pb and sr isotope evolution was compared to the isotopic signature of the mineralization whereas shales and black shales of the cambrian balang and niutitang formations underlying the ore bearing qingxudong carbonate unit appear to be possible external sources for the radiogenic sr found in hydrothermal carbonates the u pb data do not permit these shales to be considered as primary source rocks for the ore lead both the pb and the sr isotopes indicate mineralizing fluids that had interacted with various external metal sources some of them most probably being located in sedimentary or crystalline basement rocks this refutes prevailing genetic models favoring the hydrocarbon and metal rich lower cambrian niutitang black shales as the most likely source reservoir for the zn and pb in the deposits of the huayuan district the pb and sr isotope mixing trends observed for the ore and gangue preciptates reflect small scale heterogeneous fluid systems that were not capable of producing large deposits this may indicate that the huayuan district is not a favorable target for future exploration of economic ore deposits it also limits the prospects of successful radiometric dating of the mineralization since conventional geochronologic models require initial isotopic homogeneity

waveform analysis of scholte modes in ocean sediment layers summary in an effort to determine the characteristics of seismic noise on the ocean bottom and its relationship to the structure of the sea floor we have adapted the method of nonlinear waveform fitting to accommodate multidimensional models shear velocity j and shear damping qs and have applied it to invert several records of interface waves scholte 1958 from the thumper experiment off southern california waveform fitting is a very powerful tool to determine the s velocity in the top few metres of the sediment starting from 30 m s l at the top clay layer the s velocity increases with a gradient of 2 8 m sk1 m over the first 150 m of sediment a theoretical estimation of the source strength gives coherent estimates of qs as a function of depth for distances between 400 and 1070m from the source the qs models are characterized by very low values 10 20 in the top three metres but by values in excess of 100 below that level the results confirm the identification of the noise as harmonics of interface waves in the area of this experiment the largest noise amplitudes belong to the fundamental mode and penetrate to a depth of about 20 m into the sediment the overtone energy can be appreciable too and is noticeable to about 80m depth the qs structure confirms the strong influence that the sea floor structure has on the noise spectrum the high attenuation at frequencies above 3 4 hz suppresses noise propagation and produces low noise at higher frequencies similarly high attenuation in the asthenosphere suppresses noise propagation below 0 1 hz

basal mechanics of ice stream b west antarctica 2 undrained plastic bed model based on the results of our studies of the physical conditions beneath ice stream b we formulate a new analytical ice stream model the undrained plastic bed model henceforth the upb model mathematically the upb model is represented by a non linear system of four coupled equations which express the relationships among ice sliding velocity till strength water storage in till and basal melt rate we examine this system of equations for conditions of ice stream stability over short timescales that permit holding ice stream geometry constant less than hundreds of years temporal variability is introduced into the upb model only by the direct dependence of till void ratio changes e t on the basal melting rate m r since till strength b e and ice stream velocity u b b change as long as till void ratio varies the first condition for ice stream stability is that of constant till water storage 0 the second condition for ice stream stability arises from the feedback between ice stream velocity till strength and the basal melting rate which depends on shear heating m r u b b this is the weak till condition which requires that in a steady state till strength is a small fraction of the gravitational driving stress b n 1 1 d the salient feature of the upb model is its ability to produce two thermo mechanically controlled equilibrium states one with a strong bed and slow ice velocities ice sheet mode and one with a weak bed and fast ice velocities ice stream mode this bimodality of basal conditions is consistent with the available observations of subglacial conditions beneath slow and fast moving ice in west antarctica basal conditions that do not correspond to these two steady states may occur transiently during switches between the two stable modes the upb model demonstrates that ice streams may be prone to thermally triggered instabilities during which small perturbations in the basal thermal energy balance grow leading to generation or elimination of the basal conditions which cause ice streaming

a multidisciplinary study of invisible gold in arsenopyrite from four mesothermal gold deposits in siberia russian federation characterization of gold in four siberian mesothermal gold deposits was done through a multidisciplinary investigation including ore microscopy inaa sims epma and 197 au moessbauer spectroscopy the deposits are olympiada and veduga in the enisei mountain range and nezhdaninskoye and sentachan in the verkhoyansk kolyma area in which vein and disseminated orebodies are hosted by sedimentary sequences metamorphosed to amphibolite olympiada and greenschist facies ore minerals include arsenopyrite and pyrrhotite or pyrite as major constituents with minor scheelite and sphalerite chalcopyrite and native gold as minor constituents with or aurostibite galena gudmundite tetrahedrite stibnite and berthierite some of the gold mineralization is metallurgically refractory in all four deposits largely due to the presence of invisible gold in arsenopyrite and to a lesser extent in pyrite several generations of arsenopyrite are recognized in the auriferous zones of the veduga nezhdaninskoye and sentachan deposits textural observations indicate late deposition of native gold with respect to arsenopyrite no evidence was found of arsenopyrite recrystallization and the crystallization of native gold is considered to result from an evolution of fluids rather than redeposition of gold wide variations in the gold contents of arsenopyrite occur within deposits within samples and within individual grains inna analyses of individual arsenopyrite grains 30 70 mu m in size show that the gold concentration ranges from 197 au moessbauer spectra of arsenopyrite concentrates from the veduga nezhdaninskoye and sentachan deposits exhibit mainly a component with isomer shifts between 3 3 and 3 6 mm s which is interpreted as strong evidence of the presence of chemically bound gold in the arsenopyrite crystal structure the spectra of samples from the veduga deposit also contain minor amounts of native gold for which negative isomer shifts near 1 2 mm s are expected metallic gold dominates in the 197 au moessbauer spectra of samples from the olympiada deposit but varying amounts of chemically bound gold are also visible in the spectra some attributed to aurostibite it is concluded that the ores from these deposits are refractory partly because the gold occurs in arsenopyrite as chemically bound gold as well as submicroscopic metallic particles probably as small as 2 nm this study also shows that au bearing arsenopyrite is usually depleted in as with respect to its stoichiometric composition but correlation of au enrichment to fe deficiency was not found it is inferred from 197 au moessbauer spectroscopy that substitution of fe super 3 for au super 3 is most likely the temporal change in different modes of gold occurrence i e as native gold or invisibly in sulfides in the mineralized zones and veins is interpreted to result from an evolution of fluids rather than to be a recrystallization phenomenon dissemination of au bearing sulfides within the altered halos infers that a pervasive fluid rock interaction was an effective mechanism for deposition of minerals and gold absorption of gold complexes onto sulfide surfaces is also suggested to have played an important role in the deposition of native gold during the latest episodes in the formation of the deposits

physical modelling of a surface wave survey over a laterally varying granular medium with property contrasts and velocity gradients laboratory experiments using laser based ultrasonic techniques can be used to simulate seismic surveys on highly controlled small scale physical models of the subsurface most of the time such models consist in assemblies of homogeneous and consolidated materials to enable the physical modelling of unconsolidated heterogeneous and porous media the use of granular materials is suggested here we describe a simple technique to build a two layer physical model characterized by lateral variations strong property contrasts and velocity gradients we use this model to address the efficiency of an innovative surface wave processing technique developed to retrieve 2 d structures from a limited number of receivers a step by step inversion procedure of the extracted dispersion curves yields accurate results so that the 2 d structure of the physical model is satisfactorily reconstructed the velocity gradients within each layer are accurately retrieved as well confirming current theoretical and experimental studies regarding guided surface acoustic modes in unconsolidated granular media

quantifying fault breccia geometry dent fault nw england a geometric classification of fault breccia borrowed from the cave collapse literature has been suggested as an alternative to available genetic classifications here image analysis is used to explore geometric discrimination between the visually assigned classes of crackle breccia mosaic breccia and chaotic breccia using samples from the well understood dent fault northwest england clast sphericity and surface roughness show some correlation with the breccia classes but particle size distributions and their fractal dimension show none a more useful parameter is the percentage of sample area occupied by clasts crackle breccia has 75 clasts mosaic breccia 60 75 clasts and chaotic breccia has 60 clasts the eye is also good at judging the tessellation goodness of fit of clasts and a semi quantitative approach to assessing this parameter is explored the average degree of rotational misfit of the clasts is strongly related to breccia class crackle breccia involves less than 10 average rotation mosaic breccia 10 20 and chaotic breccia more than 20 rotation comparison charts are provided for semi quantitative classification of fault breccias

high geothermal heat flow basal melt and the origin of rapid ice flow in central greenland age depth relations from internal layering reveal a large region of rapid basal melting in greenland melt is localized at the onset of rapid ice flow in the large ice stream that drains north off the summit dome and other areas in the northeast quadrant of the ice sheet locally high melt rates indicate geothermal fluxes 15 to 30 times continental background the southern limit of melt coincides with magnetic anomalies and topography that suggest a volcanic origin

isotopic fronts in hydrothermally mineralized carbonate rocks advective dispersive fluid flow through permeable and porous rock causes systematic alteration of the infiltrated rock o and sr isotopes can be used as tracers to monitor exchange processes between mineralizing hydrothermal solutions and carbonate host rocks fluid infiltration into rock of initially uniform isotopic composition leads to characteristic changes in 18o and 87sr in the rock that depend on infiltration distance fluid velocity diffusivity and reaction kinetics for fast fluid solid equilibrium during advective flow the shape of such alteration profiles can be described by the dimensionless peclet number p e which expresses the fluid flux infiltration distance and diffusivity in the case of mineralization from low to moderate temperature solutions isotope exchange is not instantaneous but kinetically controlled and a similar parameter the damkohler number n d expressing fluid flux isotope exchange rate and infiltration distance becomes more useful n d and p e values have been estimated for various examples of carbonatehosted hydrothermal mineralization in the eastern alps austria and the benue trough nigeria by modelling o and sr isotope data from ore or gangue minerals and host rock the values obtained enable a rough differentiation between pervasive and channelled fluid flow as the prevailing transport mechanism to be made results indicate that both highly channelled fluid flow through fractures without significant interaction with the wall rock and pervasive fluid flow with infiltration distances into the percolated host rock of the order of 100 m occur near mineral deposits in the latter case the extent of o and sr isotopic alteration of the host rock can be used as an effective exploration tool

cockade breccia product of mineralisation along dilational faults cockade breccias are fault fills in which individual clasts are completely surrounded by concentric layers of cement they occur particularly in low temperature near surface hydrothermal veins at least six mechanisms have been proposed for the formation of cockade breccia like textures but only two repeated rotation accretion and partial metasomatic replacement of clast minerals have been supported by detailed evidence a typical example of cockade breccia from the gower peninsula south wales shows clear evidence for the rotation accretion mechanism in particular overgrown breakage points in cement layers where cockades were previously touching each other and rotated geopetal infills of haematitic sediment based on the available evidence it is proposed that cockade textures result from low rates of cement growth compared to high rates of dilational fault slip seven criteria are given for the correct identification of cockade breccias

crystallization pressure versus crack seal as the mechanism for banded veins banded fibrous veins are often pointed to as evidence for episodic crack opening driven by oscillations in fluid pressure or bulk strain advances in understanding the geochemistry of precipitation data on veins and experiments suggest that pressure due to growing crystals may be an alternate explanation for many observations on these types of veins we propose that some veins originate at sites of precipitation and then propagate due to the pressure exerted by the crystal growth as materials precipitate the vein walls are pushed apart the resulting veins have shapes typical for mode i cracks except that mechanically crystallization pressure replaces the role of internal fluid pressure in their propagation a nonzero remote differential stress serves to align veins mechanical and geochemical considerations suggest that this process will be most important in fine grained rocks such as greenschist grade pelites where diffusion from sites of dissolution rather than advection is the dominant mass transport process veins may owe their orientation to tectonism but their initiation and growth are due to processes that supersaturate the pore fluid veins formed by this mechanism involve cracking to the extent that precipitation forces an original flaw to extend during precipitation cyclic quartz mica bands may indicate geochemical self organization at the vein wall driven by pressure solution enhanced supersaturation in the pore fluid and nonlinear precipitation kinetics at the vein wall

chaotic breccia zones on the pembroke peninsula south wales evidence for collapse into voids along dilational faults chaotic breccias and megabreccias locally called gash breccias hosted within the pembroke limestone group visean mississippian lower carboniferous of southwest wales are re mapped along with spatially related crackle and mosaic breccias of thirteen studied megabreccia bodies seven lie along steep nnw or nne striking strike slip faults originating during north south variscan late carboniferous shortening though reactivated during later extension four bodies are conformable with e w striking steeply dipping bedding and two have irregular or indeterminate margins the bedding parallel zones are interpreted as the dilational tips of listric normal faults and the cross strike faults as transtensional transfer zones sub horizontal clast fabrics suggest brecciation by gravitational collapse into opening fissures rather than by cataclasis along the faults most fissures have geometrically matched margins produced by this dilational faulting and only locally have the indented margins indicating solutional processes the most likely age for the main fissure extension and fill is late triassic based on analogous dated fills at the eastern end of the bristol channel basin the pembroke megabreccias blur the distinction between fault rocks formed by deformation and those formed by redeposition along fault zones

shallow water sediment properties derived from high frequency shear and interface waves low frequency sound propagation in shallow water environments is not restricted to the water column but also involves the subbottom thus as well as being important for geophysical description of the seabed subbottom velocity attenuation structure is essential input for predictive propagation models to estimate this structure bottom mounted sources and receivers were used to make measurements of shear and compressional wave propagation in shallow water sediments of the continental shelf usually where boreholes and high resolution reflection profiles give substantial supporting geologic information about the subsurface this colocation provides an opportunity to compare seismically determined estimates of physical properties of the seabed with the ground truth properties measurements were made in 1986 with source detector offsets up to 200 m producing shear wave velocity versus depth profiles of the upper 30 50 m of the seabed and p wave profiles to lesser depths measurements in 1988 were made with smaller source devices designed to emphasize higher frequencies and recorded by an array of 30 sensors spaced at 1 m intervals to improve spatial sampling and resolution of shallow structure these investigations with shear waves have shown that significant lateral and vertical variations in the physical properties of the shallow seabed are common and are principally created by erosional and depositional processes associated with glacial cycles and sea level oscillations during the quaternary when the seabed structure is relatively uniform over the length of the profiles the shear wave fields are well ordered and the matching of the data with full waveform synthetics has been successful producing velocity attenuation models consistent with the subsurface lithology indicated by coring results both body waves and interface waves have been modeled for velocity attenuation as a function of depth with the aid of synthetic seismograms and other analytical techniques some results give strong evidence of anisotropy and lateral heterogeneity in shear velocity of the upper 5 10 m of sediments and of extremely high velocity gradients in the topmost 1 2 m possibly exceeding 30 s 1

measurements of past ice sheet elevations in interior west antarctica a lateral moraine band on mount waesche a volcanic nunatak in marie byrd land provides estimates of past ice sheet surface elevations in west antarctica helium 3 and chlorine 36 surface exposure ages indicate that the proximal part of the moraine up to 45 meters above the present ice surface was deposited about 10 000 years ago substantially later than the maximum ice extent in the ross embayment the upper distal part of the moraine may record multiple earlier ice sheet high stands a nonequilibrium ice sheet model predicts a delay of several thousand years in maximum ice levels at mount waesche relative to the maximum ice extent in the ross sea the glacial geologic evidence coupled with the ice sheet model indicates that the contribution of the ross sea sector of the west antarctic ice sheet to holocene sea level rise was only about 3 meters these results eliminate west antarctic ice as the principle source of the large meltwater pulse during the early holocene

on the arrangement of solid inclusions in fibrous veins and the role of the crack seal mechanism the arrangement of solid inclusions in veins was examined in three different rock types and tectonic settings common to all samples is a new type of sinusoidal inclusions associated with inclusion bands oriented parallel to the vein wall and inclusion trails at high angle to the vein wall crosscutting relationships of inclusion bands and the sinusoidal inclusions imply that inclusion bands do not reflect individual crack seal increments in our veins inclusion bands are not a sufficient criterion to infer the crack seal mechanism further evidence for solid inclusions not formed by the classic crack seal mechanism is given by inclusion bands in gypsum crystals grown in soft clay during incorporation solid inclusions can become folded or boudinaged depending on their orientation with respect to the opening direction the complex displacement field within the vein is recorded by curved crystal fibres which track the opening direction based on the arrangement of solid inclusions within the vein we suggest that our veins grew continuously during syntectonic vein formation solid inclusions formed by a fluctuation of adhesion at the vein wall interface rather than by crack sealing and vein growth was at least partly driven by the force of crystallisation q 2005 elsevier ltd all rights reserved

origin of siderite mineralization in western guizhou sw china constrains from rees c o sr and s isotopes abstract numerous siderite deposits in western guizhou of sw china are mainly hosted in the middle devonian dolostones the larger caiyuanzi cyz and xiongxiongjia xxj deposits have an ore reserve in excess of 300 mt with an average ore grade of 35 total iron there are banded and massive siderite ores and sulfide siderite ores the major ore mineral is magnesian siderite and gangue minerals are ankerite dolomite kaolinite quartz and carbon there are also metallic minerals including pyrite chalcopyrite and minor galena tetrahedrite and enargite rare earth elements rees and carbon oxygen and strontium isotope data indicate a marine origin of the host dolostones and a hydrothermal metasomatic origin of the siderite mineralization the variations of rees and carbon and oxygen isotopic compositions of siderite from the cyz and xxj deposits reflect the variable mineralization processes and different fluid rock ratios the lree depletion of siderite from the cyz deposit could be due to crystallographic control whereas the lree depletion along with mree enrichment and positive eu anomalies of siderite from the xxj deposit was possibly controlled by both crystallography and fluid composition siderite and calcite have 13 c pdb and 18 o smow values ranging from 8 to 4 01 and from 13 42 to 20 07 respectively the variations in c and o isotopic compositions of siderite and calcite may reflect metasomatic replacement of marine carbonates by hydrothermal fluids at different fluid rock ratios dolostones have 87 sr 86 sr initial values of 0 70844 0 70920 whereas siderite and calcite have values ranging from 0 71396 to 0 71708 negative correlations of initial sr isotope ratios versus carbon and oxygen isotopes of the host dolostones siderite and calcite indicated that carbonate minerals were metasomatic products of marine carbonates by hydrothermal fluids leaching from the proterozoic basement rocks iron was possibly derived from the basement leaching rather than from the continental weathering the 34 s cdt values 19 9 to 13 6 of sulfides associated with siderite indicated that the sulfur was probably derived from the marine sulfates through either bacterial sulfate reduction or organic mediated thermochemical reduction

texture mineralogy and geochemistry of hydrothermally altered submarine volcanics recovered southeast of cheshire seamount western woodlark basin the western woodlark basin lies within a gold rich metallogenic province this area is characterized by detachment faults that record ongoing extension and act as major pathways for the circulation of hydrothermal fluids dredging from the flanks of a submarine ridge southeast of cheshire seamount western woodlark basin retrieved hydrothermally altered monomictic to polymictic crackle mosaic and chaotic breccias with at least 30 clasts n2 mmin diameter the precursor rocks are andesitic to rhyolitic in composition but have been intensely hydrothermally altered with about 90 of the volcanic glass replaced by secondaryminerals the breccias show five generations of quartz growth with the first being related to magmatic processes and the remaining four to alteration stages including silicification chloritization illitization sericitization albitization and sulfidation needle like crystals ofmordenite zeolite withmultiple growth centers growon the fourth generation of quartz notable textural variants in the breccias are vesicles perlitic cracks and zoned alteration halos that mantle the rims of clasts electron microprobe analyses on chlorite from breccia samples have identified clinochlore as the main chlorite type and indicate a formation temperature in the range of 210 304 c this and the elevated au as ag hg zn pb sb contents of a mineralized sample indicate hydrothermal alteration temperatures n200 c suggesting that these breccias may represent the upflow zone of a hydrothermal system and highlight the potential for seafloor massive sulfides in the area the breccias show elevated contents of immobile trace elements and lree as well as a depletion in ta and nb suggesting that the precursor rocks were formed in a rift related suprasubduction environment

strontium isotopes in magnesites from permian and triassic strata eastern alps abstract the sr isotopic composition of sediment hosted magnesites in permian and scythian series of upper austroalpine units eastern alps has been determined the results suggest diagenetic metasomatic magnesite formation by mg rich pore solutions the depositional environment of the magnesite bearing rocks is reflected by different isotopic compositions with initial 87 sr 86 sr ratios close to contemporaneous sea water in marine sediments 0 7071 0 7083 and higher ratios being typical for lacustrine and coastal environments influenced from the hinterland 0 7133 0 7139 coarser grained recrystallized magnesite and magnesite veins show a distinct increase in their sr isotopic ratios 0 7202 0 7220 which can be attributed to metamorphic fluids of eoalpine age the findings of these magnesite occurrences which have been affected only by very low grade metamorphism are compared with similar results from spar magnesites associated with metamorphosed paleozoic sedimentary rocks a similar genetic model is proposed for the first magnesite mineralization in those paleozoic strata but with variably intense later recrystallisation during metamorphism

fluid evolution in the baia mare epithermal gold polymetallic district inner carpathians romania the baia mare district romania represents a complex segment of the neogene quaternary subduction related volcanic chain of the carpathians in this province the calc alkaline volcanism and associated mineralization are closely related to a major strike slip fault and an underlying pluton the ore deposits of the district have many features typical of low sulfidation or adularia sericite epithermal deposits the combination of fluid inclusion microthermometry raman spectroscopy bulk crush leach analysis laser ablation optical emission spectrometry la oes of individual fluid inclusions and stable isotope data has been used to characterize the ore forming solutions involved with gold polymetallic mineralization in three of the largest deposits ore fluids at baia sprie cavnic and sasar deposits had temperatures of 320 to 150 c and salinities in the range 0 21 wt nacl equiv during phreatomagmatic activity fracturing and periods of vein dilation the pressure fluctuations may have caused boiling the hydrothermal fluids were na k li ca chloride solutions with lesser amounts of so4 and f the na k ratios and the stable isotope compositions are consistent with equilibration of the fluids with country rocks at progressively lower temperatures the halogen ratios br cl 103 in the range 0 13 1 21 together with sulfur isotopic systematics 34s 0 3 5 2 per mil are consistent with a dominantly magmatic origin of the solutes and probably of the ore fluids themselves

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model ore geology reviews 24 267 298 a reply

organic and inorganic geochemistry of ljubija siderite deposits nw bosnia and herzegovina the ljubija siderite deposits hosted by a carboniferous sedimentary complex within the inner dinarides occur as stratabound replacement type ore bodies in limestone blocks and as siderite sulfides veins in shale three principal types of ore textures have been recognized including massive dark siderite and ankerite siderite with zebra texture and siderite veins the ore and host rocks have been investigated by a combination of inorganic major trace and rare earth element concen trations organic characterization of hydrocarbons includ ing biomarkers and stable isotope geochemical methods isotope ratios of carbonates sulfides sulfates kerogen and individual hydrocarbons new results indicate a marine origin of the host carbonates and a hydrothermal metasomatic origin of the fe mineralization the differ ences in ore textures e g massive siderite zebra siderite are attributed to physicochemical variations e g changes in acidity temperature and or salinity of the mineralizing fluids and to the succession and intensity of replacement of host limestone vein siderite was formed by precipitation from hydrothermal fluids in the late stage of mineralization the equilibrium fractionation of stable isotopes reveals higher formation temperatures for zebra siderites around 245 c then for siderite vein around 185 c sulfur isotope ratios suggest permian seawater or permian evaporites as the main sulfur source fluid inclusion composition confirms a contribution of the permian seawater to the mineralizing fluids and accord with a permian mineralization age organic geochemistry data reflect mixing of hydrocarbons at the ore site and support the hydrothermal metasomatic origin of the ljubija iron deposits

continental basinal origin of ore fluids from southwestern massif central fluorite veins albigeois france evidence from fluid inclusion and stable isotope analyses the most important fluorspar mining district in france is located in the palaeozoic basement of the albigeois in southwestern french massif central the massive fluorite is hosted within large e w striking fractures crosscutting cambro ordovician clastics associated with large zones of hypersilicified tectonic breccia which form the wall of the mined deposits fluid inclusion data for pre fluorite and fluorite stage fluids have salinities between 20 26 wt nacl equiv with homogenisation temperatures between 85 170 c furthermore low first ice melting temperatures around 50 c indicates the presence of significant cacl2 and possibly mgcl2 together with nacl calculated fluid 18o for pre fluorite quartz ranges from 9 1 to 5 2 with d between 55 to 64 placing the data directly on the present day meteoric water line fluorite stage fluids have 18o between 0 1 to 3 2 and d ranging from 53 to 75 indicating an interacted meteoric fluid origin combining the fluid inclusion and stable isotope data illustrates that the main fluorite depositing fluid has characteristics typical of a basinal brine the authors have no evidence that a magmatic system was involved in the deposit genesis the proposed model highlights that mineralisation was related to major mesozoic extensional events coinciding with the gradual opening of the atlantic and tethys oceans in order to account for the chemistry of the fluids and the siting of the deposits the authors postulate a genetic relationship with local continental evaporite bearing basins coincident with and controlled by the e w fractures

modeling explosion generated scholte waves in sandy sediments with power law dependent shear wave speed experimental measurements of scholte waves from underwater explosions collected off the coast of virginia beach va in shallow water are presented it is shown here that the dispersion of these explosion generated scholte waves traveling in the sandy seabed can be modeled using a power law dependent shear wave speed profile and an empirical source model that determines the pressure time series at 1 m from the source as a function of tnt equivalent charge weight

the melting of floating ice raises the ocean level summary it is shown that the melting of ice floating on the ocean will introduce a volume of water about 2 6 per cent greater than that of the originally displaced sea water the melting of floating ice in a global warming will cause the ocean to rise if all the extant sea ice and floating shelf ice melted the global sea level would rise about 4 cm the sliding of grounded ice into the sea however produces a mean water level rise in two parts some of the rise is delayed the first part while the ice floats is equal to the volume of displaced sea water the second part equal to 2 6 per cent of the first is contributed as it melts these effects result from the difference in volume of equal weights of fresh and salt water this component of sea rise is apparently unrecognized in the literature to date although it can be interpreted as a form of halosteric sea level change by regarding the displaced salt water and the meltwater even before melting as a unit although salinity changes are known to affect sea level all existing analyses omit our calculated volume change we present a protocol that can be used to calculate global sea level rise on the basis of the addition of meltwater from grounded and floating ice of course thermosteric volume change must be added

siderite mineralization of the gemericum superunit western carpathians slovakia review and a revised genetic model ore geology reviews 24 267 298 a discussion radvanec et al 2004 provided a revised version of their metamorphic hydrothermal model for siderite and associated mineral deposits of the western carpathians compared to the previous version grecula 1995 grecula et al 1996 the upgraded model emphasizes the dominant role of retrograde variscan metamorphic processes and admits remobilization of the variscan siderite and other ore components during the alpine orogenic cycle similar to the previous versions the variscan ore forming processes genetically related to metamorphism are believed to dominate not only in the siderite vein and metasomatic deposits but also in the superimposed vein sulphides p 292 the revised model is unsustainable in light of recent knowledge the major pitfalls are summarized and commented as follows

peloids in stratabound zn pb deposits and their genetic importance peloids from sediment hosted zn pb deposits in belgium ireland and poland are composed of zn calcite siderite smithsonite silica pyrite melnikovite sphalerite galena thiosulphates and zn as bearing limonite the size of peloids is between 7 and 60 m the core and the shell of peloids are composed of different minerals and the shell is usually coarsely crystalline peloids are present in collapse breccias of karst cavities stromatactis cavities debris flow breccias and fractures cross cutting carbonates hosting the mineralization in places peloids form a major microtexture of the sulfide mineralization peloids are cemented and replaced by sulfides organic acids extracted from sulfide peloids are composed of amido acids considered to be characteristic of bacterial origin the size and specific texture of peloids are independent of mineralogy location and age of the mineralization therefore the peloids disussed are considered as forms induced by bacterial activity rather than forms precipitated inorganically

palaeoseismic events recorded in archaean gold quartz vein networks val d or abitibi quebec canada archaean gold quartz vein deposits are commonly hosted in high angle reverse shear zones and are interpreted to have formed in a regime of horizontal compression and high fluid pressure environment this paper presents the results of a combined structural and fluid inclusion study on three gold quartz vein deposits of the val d or area abitibi quebec consisting of subhorizontal extensional veins and e w steeply dipping shear veins crack seal structures tourmaline fibres stretched quartz crystals and open space filling textures indicate that the subhorizontal veins formed by hydraulic fracturing under supralithostatic fluid pressure co2 rich and h2o nacl fluid inclusions interpreted as two coexisting immiscible fluids occur typically in microcracks of different orientations interpreted to have formed in the 1 2 plane horizontal co2 rich fluid inclusion planes are contemporaneous with the opening of these veins 3 vertical vertical h2o nacl fluid inclusion planes as well as some microstructures such as deformed minerals indicate that the same extensional veins have experienced episodic vertical shortening 3 horizontal alternating with the opening events deformation and slip opening also occurred in shear veins in which preferred orientation of fluid inclusion planes is not clear except that the h2o nacl fluid inclusion planes tend to be oriented at high angles to the slip direction the successive opening and collapse events in subhorizontal extensional veins are correlated with deformation and slip opening events in shear veins respectively and are attributed to cyclic fluid pressure fluctuations in the system they are thus consistent with the fault valve model sudden drop in fluid pressure from supralithostatic to lower values induces fluid unmixing and occurs immediately post failure following seismic rupturing along the shear zone sealing of the shear veins allows the fluid pressure to build up again and the subhorizontal extensional veins to form the intermittent change in orientation of 3 in horizontal extensional veins is ascribed to a total shear stress release along the neighbouring shear veins after rupture the complex internal structures of the gold quartz veins provide an excellent record of archaean seismic activity along high angle reverse faults

geometry and formation of gypsum veins in mudstones at watchet somerset sw england gypsum veins and faults were studied in red mudstones of the upper triassic mercia mudstone group in the bristol channel basin exposed in e w trending cliffs at watchet on the somerset coast sw england in nodular gypsum horizons individual gypsum nodules are connected by subhorizontal gypsum veins in evaporite free mudstone layers however dense anastomosing networks of gypsum veins occur in a 300 m long profile dissected by 28 mostly normal faults with small displacements 24 faults have veins following them indicating palaeofluid transport along the fault planes ninety seven cross cutting relationships and mostly perpendicular vein fibres indicate that the veins are primarily extension fractures the thickest veins in the vein network are subhorizontal 160 measurements indicating a vertical orientation of the minimum principal compressive stress horizontal basin compression such a stress state may have existed during basin inversion associated with alpine compression late cretaceous to early tertiary i propose that the gypsum veins are the result of hydrofracturing in the gypsum nodules then presumably consisting of anhydrite overpressure was generated related to the hydration of anhydrite to gypsum stress concentration around the nodules led to rupturing and injection of thin subhorizontal hydrofractures some of the calcium sulphate saturated fluids were then transported upwards along the faults and gained access to evaporite free mudstone layers where dense anastomosing vein networks developed most veins were arrested during their propagation by layers with contrasting mechanical properties stress barriers some veins however propagated through the barriers along faults to shallower levels the dense networks of mineral veins observed in watchet indicate that hydrofractures can generate a very high temporary permeability in fluid reservoirs

siderite deposits in northern italy early permian to early triassic hydrothermalism in the southern alps abstract we present a minero petrographic geochemical and geochronological study of siderite orebodies from different localities of the southern alps northern italy siderite occurs as veins cutting the variscan basement and the overlying lower permian volcano sedimentary cover collio fm and as both veins and conformable stratabound orebodies in the upper permian verrucano lombardo and bellerophon fms and lower triassic servino and werfen fms sedimentary sequences of the lombardian and the venetian alps all types of deposits show similar major and rare earth ree element patterns suggesting a common iron mineralizing event the compositions of coexisting siderite fe rich dolomite and calcite suggest formation from hydrothermal fluids at relatively high temperature conditions 250 c geochemical modelling supported by ree analyses and by literature and new 1 3 c and 18 o isotopic data suggests that fluids responsible for the formation of siderite in the variscan basement and in the overlying lower permian cover were derived from dominant fresh water which leached fe and c from volcanic rocks mainly rhyolites rhyodacites and organic carbon bearing continental sediments on the basis of u th pb microchemical dating of uraninite associated with siderite in the val vedello and novazza deposits lombardian alps the onset of hydrothermalism is constrained to 275 13 ma early mid permian i e it was virtually contemporaneous to the plutonism and the volcanic sedimentary cycle reported in the same area orobic basin the youngest iron mineralizing event is represented by siderite veins and conformable orebodies hosted in lower triassic shallow marine carbonatic successions in this case the siderite forming fluids contained a seawater component interacted with the underlying permian successions and eventually replaced the marine carbonates at temperatures of 250 c the absence of siderite in younger rocks suggests an early triassic upper limit for the iron pulse in the southern alps which would thus predate the middle triassic magmatism based on the overlap between hydrothermalism extensional tectonics and in part magmatism the genesis of siderite in the southern alps may be related to plutonic activity and or magmatic underplating occurring since the permian in a geodynamic scenario preluding the opening of the neo tethys

multi stage fluid incursion in the palaeozoic basement hosted saint salvy ore deposit nw montagne noire southern france abstract the saint salvy vein hosted zn ge deposit occurs in an e w fault system which flanks the southern margin of the late variscan sidobre granite and cross cuts cambrian black shales of the palaeozoic basement comprehensive mineralogical and geochemical studies of vein samples have revealed four mineralizing events m1 m4 related to late and post variscan tectonic events a further late stage event may be related to weathering m1 skarn deposits and m2 patchily mineralized quartz veinlets are associated with granite emplacement quartz contains low salinity h2o co2 nacl dominated fluids 6wt nacl equiv of relatively high temperature 300 580 c trapped under moderate to high pressure estimated m1 fluid d and calculated fluid 18o plot within the metamorphic water field there appears to be no involvement of magmatic fluids by contrast m3 barren quartz and m4 zinciferous economic mineralization stages have h2o co2 nacl cacl2 fluid inclusions with high salinities 23 25 wt nacl equiv and low temperatures 80 140 c which were trapped under low pressure conditions the high salinity and nacl cacl2 content of both m3 and m4 indicates that their parent fluids leached evaporitic salts m3 fluids are meteoric water dominated falling close to the meteoric water line d and 18o averaging 64 and 8 respectively m4 fluids have highly distinctive d averaging 101 and calculated fluid 18o varying from 1 2to 7 1 the unusually low d composition of m4 suggests the involvement of organic fluids in which h is derived directly or indirectly from organic matter the relatively high 18o of m4 fluids indicates that considerable isotopic exchange with sedimentary material took place displacing the 18o from the meteoric water line the data imply interaction of meteoric waters with evaporite and hydrocarbon bearing sedimentary sequences most probably the adjacent aquitain basin the main economic mineralization m4 stage took place during a tensional event probably coincident with the lias dogger transition calculated 34sh2s of m4 sulphide 5 4to 8 2 is almost identical to 34s of local cambrian sulphides 4 7to 9 4 suggesting a genetic link abundant siderite associated with m4 sphalerite has 13c ranging from 2 6to 4 4 indicating that carbon was sourced from sedimentary carbonate mobilized by or equilibrated with the hydrothermal fluid late stage sulphides exhibit extraordinary and highly distinctive 34s sphalerite has extremely low 34s 42 5to 50 5 whereas pyrite has an extraordinary large range from 33 2 to 74 3 closed system sulphate reduction is held to be responsible for the extremely high 34s whereas more open system reduction produces the very low values the coincidence of isotopically low 13c 7 6to 11 9 for co genetic calcite suggests the involvement of organic matter in the reduction process

red bed and basement sourced fluids recorded in hydrothermal mn fe as veins sailauf germany a la icpms fluid inclusion study the hydrothermal mn fe as vein mineralization at sailauf germany hosts a complex sequence of oxide and carbonate minerals that record a protracted fluid history the mineralization is related to a major unconformity that separates permian zechstein sedimentary rocks from underlying variscan crystalline basement the hydrothermal veins contain two principal mineralization stages which are mn oxides associated with calcite and hematite associated with mn calcite the fluid evolution of the hydrothermal system has been reconstructed from fluid inclusion petrography microthermometry and la icpms microanalysis coupled with stable isotope geochemistry of carbonate and oxide minerals the fluid inclusions are high salinity sodic calcic brines and the bulk fluid properties show no major differences between the mn oxide and the hematite stage la icpms analysis of major and trace elements demonstrates that the mineralization formed from chemically distinct fluid pulses characterized by variations in their k li b pb and zn concentrations the fluid that precipitated the mn oxide stage has anomalous pb zn and li b ratios which closely resemble fluids found in fracture fillings in red beds of the permian rotliegend basin by contrast the fluids associated with the hematite stage have pb zn and li b ratios typical of crustal fluids that were derived from interaction with crystalline basement both fluids possess characteristic element ratios including cl br but variable absolute concentrations of most metals this suggests that both fluids were modified by mixing with a common metal depleted brine that had a similar cl br ratio most likely a formation water from the overlying zechstein sedimentary rocks the sailauf mineralization provides insight into the protracted post variscan fluid evolution at the basement cover interface the compositionally anomalous fluid that precipitated the mn oxides is comparable to brines derived from interaction with red beds and likely represents the ore fluid of kupferschiefer type sediment hosted cu deposits conversely the fluid that deposited the hematite mineralization resembles fluids that typically form basement and sediment hosted pb zn deposits

long range correlations of microseism band pressure fluctuations in the ocean we investigate the spatial coherence of underwater ambient noise using a yearlong time series measured off ascension island qualitative agreement with observed cross correlations is achieved using a simple range dependent model constrained by earlier active tomographic studies in the area in particular the model correctly predicts the existence of two weakly dispersive normal modes in the microseism frequency range with the group speed of one of the normal modes being smaller than the sound speed in water the agreement justifies our interpretation of the peaks of the measured cross correlation function of ambient noise as modal arrivals with dispersion that is sensitive to crustal velocity structure our observations are consistent with scholte to moho head wave coupled propagation with double mode conversion occurring due to the bathymetric variations between receivers we thus demonstrate the feasibility of interrogating crustal properties using noise interferometry of moored hydrophone data at ranges in excess of 120 km

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets

the archean lode gold deposits at wiluna western australia high level brittle style mineralization in a strike slip regime the wiluna lode hosted gold deposits comprise a group of individual archean lode hosted gold deposits located in the northern part of the norseman wiluna belt western australia collectively they are the fifth largest gold producer in the yilgarn block the deposits are hosted in a series of very low grade metamorphosed prehnite pumpellyite facies mafic and ultramafic lava flows with abundant primary igneous textures preserved lode hosted gold mineralization is controlled by the wiluna strike slip fault system which consists of north and northeast trending and west and east dipping sets of principal displacement faults and associated subsidiary structures the movement on all faults was dextral with minor oblique reverse or oblique normal motion and maximum apparent displacement of 1 350 m the lode deposits are centered on structural inhomogeneities in the principal displacement faults e g divergent bends and dilational jogs including horsetail splays and or subsidiary structures e g overstepping and intersecting faults the plunge of the ore shoots is generally moderately to steeply to the south and is controlled by the intersections between 1 lithologic layering and fault planes 2 two or more fault planes 3 a fault plane and an earlier formed barren or weakly mineralized quartz reef that is semiconcordant to the regional stratigraphy and 4 a fault plane and a felsic intrusion the shape and orientation of ore shoots becomes more complex downdip where they are controlled by additional features such as drag folds and link structures fault hosted ore breccias including implosion and net veined breccias and brecciated veins are significant loci of gold mineralization in all lode deposits they contain mainly quartz chalcedony carbonates fuchsite and arsenopyrite pyrite and or stibnite mineralization occurs also in shear veins and extensional veins and finely disseminated along foliation planes of the mineral fabric formed oblique to the principal displacement faults textures of implosion breccias at wiluna indicate a history of incremental extensional opening across dilational sites and multiple episodes of brecciation and hydrothermal recementation quartz textures such as vugs cavities comb textures and cockade textured quartz are ubiquitous indicating the preservation of open space due to low effective confining pressure geologic relations indicate that the mineralized strike slip fault system was generated late in the tectonic history of the greenstone belt and that high grade ore zones formed late in the strike slip deformation event the very low grade metamorphic facies and the brittle style of deformation in the host rocks combined with the style of mineralization indicate that the fault and mineralizing system formed in a shallow near surface crustal environment textural evidence combined with the tectonic setting of the lode deposits favors the suction pump mechanism as a fluid infiltration mechanism for the lode hosted gold deposits within the wiluna strike slip fault system