the three dimensional morphology of arrays of echelon and sigmoidal mineral filled fractures data from north cornwall arrays of mineral filled echelon sigmoidal fractures in cobbles of sandstone from the crackington formation of north cornwall were examined by cutting serial sections in three dimensions each array consists of a set of blade like mineral filled fractures these blades project as a fringe from the single but composite fracture that forms through their linkage linkage takes place as the strips of rock bridges between blades fail through cross fracture cross fractures give rise to the steps that mark the margins of the composite fractures so formed bridges show little penetrative deformation arrays are not zones of high ductile strain displacement is a consequence of dilatation echelon fractures were formed and opened by intruding hydrothermal fluid

felsic magmatic phases and the role of late stage aplitic dykes in the formation of the world class cantung tungsten skarn deposit northwest territories canada abstract a field and petro chemical classification of felsic magmatic phases fmps at the world class cantung w skarn deposit was undertaken to document the evolution of magmatism and the relationships between different fmps metasomatism and mineralization early fmps include moderately differentiated zr hf 18 26 ti zr 14 15 biotite monzogranitic plutons and early biotite rich granitic dykes and compositionally similar quartz feldspar porphyry dykes late highly fractionated zr hf 8 17 ti zr 3 13 fmps sourced from a deeper monzogranitic intrusion include 1 leucocratic biotite or tourmaline bearing dykes derived from localized entrapments of residual magma and 2 sub vertical ne trending aplitic dykes derived from a larger segregation of residual fluid and incompatible element enriched magma the aplitic dykes have textures morphologies spatial associations and a pervasive calcic metasomatic mineral assemblage ca plagioclase quartz or clinozoisite indicative of syn mineralization emplacement very late stage overpressuring and initiation of sub vertical fractures into the overlying plutonic carapace and country rocks by supercritical magmatic fluid led to an interaction with calcareous country rocks that resulted in an increased aca2 in the fluid and the concurrent precipitation of w skarn residual magma also ascended with and quenched in equilibrium with the magmatic fluid to from the aplitic dykes then was metasomatized by the fluid as it interacted with calcareous country rocks overall highly fractionated and moderately to very highly undercooled fmps at cantung provide evidence for a large and evolving felsic magmatic system at depth that segregated and maintained a stable fluid and incompatible element enriched residual magma until the latest stages of crystallization the detailed study of fmps associated with magmatic hydrothermal mineral deposits allow us to refine our understanding of these mineralizing systems and better define metallogenic and exploration models for intrusion related mineralization

role of permeability and storage in the initiation and propagation of natural hydraulic fractures 1 joint sets within sedimentary basins are commonly interpreted to have formed by tensile failure in conditions where pore fluid pressure was elevated such tensile fractures are inferred to be a part of the process that relieves high fluid pressure by locally increasing rock permeability in spite of the importance of this feedback mechanism the detailed mechanics of hydraulic fracture genesis remain poorly understood we describe the results of both experimental and numerical studies of hydraulic fracture genesis on the basis of an experimental protocol that combines rock extension with elevated pore fluid pressure such that the hydraulic fracture criterion is met in the sample interior this is achieved by simultaneously dropping both minimum stress and external pore fluid pressure inducing a large fluid pressure drop between the sample interior and its ends poroelastic modeling suggests that the pore fluid pressure is highest close to but not at the sample ends and is locally maintained at levels that meet the hydraulic fracture criterion for up to 50 s application of this experimental protocol to an impure sample of sandstone resulted in the generation of several hydraulic fractures subparallel to the maximum principal stress fracturing did not occur in a drained test on the same sample demonstrating that the elevated pore fluid pressure was critical to fracture formation to better understand the experimental results we explore the role of rock permeability and storage on fracture processes using a numerical model that directly couples a lattice boltzmann model for fluid mechanics with a discrete element model for solid mechanics like the experiment the numerical simulations produced opening mode fractures when operated to replicate the conditions of the experiment fractures preferentially occur in portions of the model inferred to be mechanically weak local fluid pressure gradients strongly influence the state of stress in the solids and thereby fracture growth increasing the model permeability increases fracture propagation rate decreases sample deformation and increases fracture spacing sample deformation increases and fracture spacing decreases with increasing overpressure it appears that bulk forcing of the solid via fluid seepage forces is important in fracture genesis explaining the key roles of permeability and diffusivity in the hydrofracture process

relay zone geometry and displacement transfer between normal faults recorded in coal mine plans abstract overlap lengths separations and throw gradients were measured on 132 relay zones recorded on coal mine plans throws on the relay bounding fault traces are usually 2 m and individual structures are recorded on only one seam throw gradients associated with relay zones are not always higher than on single faults but asymmetry of throw profiles is diagnostic of relay zones bed geometries around larger faults in opencast mines are used to assess the displacement accommodated by shear in the vertical plane normal to the faults and displacement transfer accommodated by shear in the fault parallel plane three dimensional structure is defined for two relay zones each recorded on five seam plans these relay zones are effectively holes through the fault surfaces and overlap occurs between salients or lobes of the parent fault surfaces lobes initially terminated at tip lines but as the faults grew gradually rejoined the main fault surfaces along branch lines this type of relay zone originates by bifurcation of a single fault surface at a locally retarded tip line and is an almost inevitable result of a tip line irregularity

vein morphology host rock deformation and the origin of the fabrics of echelon mineral veins abstract a system of sigmoidal echelon veins from a sample of sandstone from the upper carboniferous culm sequence of southwest england is described veins are separated from one another by strips of sandstone and divided internally by thin seams with crack seal fabrics the latter extend as thin veins into the sandstone host rock without change of fabric seams appear to be merely parts of crack seal veins formed in a first phase of deposition in only minutely opened fractures this phase ended as rates of fracture opening greatly increased to allow for this widespread opening host rock between dilatating fractures sandstone strips and seams had to be deformed this deformation was limited however to rotation bending and fracture shear displacement was a function of dilatation not zone parallel ductile shear strain the textures of the quartz and carbonate aggregates filling the sigmoidal veins show that second phase crystallization took place into cavities opening more rapidly than growth was able to fill them growth for the greater part took place from fibres in seams and not off vein walls of the sandstone host rock coarsest aggregates fill the arcs of folds in seams where rates of vein opening might be expected to have been highest and the scope for competitive cavity growth greatest

exploiting forward scattering for detecting submerged proud half buried unexploded ordnance laboratory underwater bistatic scattering measurements are reported for free proud and half buried unexploded ordnances for 0 and 90 source angles forward echoes are larger than backscattered returns and half burial significantly decreases the latter but not the former results agree with analytic predictions borrowed from radar the forward echo and source signal are separated by measurements made with and without the target a method not possible in a target search for this a method is described that uses knowledge of the source location and the hyperbolic character in time cross range of the signals received at points along a line

formation and interpretation of dilatant echelon cracks the relative displacements of the walls of many veins joints and dikes demonstrate that these structures are dilatant cracks we infer that dilatant cracks propagate in a principal stress plane normal to the maximum tensile or least compressive stress arrays of echelon crack segments appear to emerge from the peripheries of some dilatant cracks breakdown of a parent crack into an echelon array may be initiated by a spatial or temporal rotation of the remote principal stresses about an axis parallel to the crack propagation direction near the parent crack tip a rotation of the local principal stresses is induced in the same sense but not necessarily through the same angle incipient echelon cracks form at the parent crack tip normal to the local maximum tensile stress further longitudinal growth along surfaces that twist about axes parallel to the propagation direction realigns each echelon crack into a remote principal stress plane the walls of these twisted cracks may be idealized as helicoidal surfaces an array of helicoidal cracks sweeps out less surface area than one parent crack twisting through the same angle thus many echelon cracks grow from a single parent because the work done in creating the array as measured by its surface area decreases as the number of cracks increases in cross sections perpendicular to the propagation direction echelon cracks grow laterally each crack overlapping its neighbors until the mechanical interaction of adjacent cracks limits this growth dilation of each crack pinches the tips of adjacent cracks into an asymmetrical form and introduces local stresses that can cause lateral growth along a curving sigmoidal path sigmoidal echelon cracks may link at tip to plane intersections leaving a step in the through going crack wall the geometry of dilatant echelon cracks may be used to infer spatial or temporal changes in the orientation of principal stresses in the earth

displacement geometry in the volume containing a single normal fault fault displacements measured in coal mines and from seismic data are used to develop a model describing the near field displacements associated with an ideal single normal fault displacement on a fault surface ranges from a maximum at the center of the fault to zero at the edge or tip line the tip line is elliptical with the shorter axis of the ellipse parallel to the displacement direction contours of equal displacement form concentric ellipses centered on the point of maximum displacement displacement gradients vary with fault size and with mechanical properties of host rock fault radius to maximum displacement ratios range from 5 to 500 plotting of displacement contour diagrams and knowledge of displacement gradients are useful in interpreting seismic reflectio data both for quality control of interpretations and for quantitative extrapolation of limited data displacements associated with faulting decrease systematically with increasing distance along the normal to the fault surface this decrease is seen as reverse drag in both hanging wall and footwall hanging wall rollover and tilting of the reflectors cannot be used to distinguish listric from planar normal faults even where fault block rotation can be demonstrated neither listric fault geometry nor a flat detachment surface is geometrically necessary because faulting is accommodated by ductile deformation rigid fault bounded blocks cannot exist except in some special circumstances related to a free surface the displacements within the rock volume affected by a single fault are not simply related to regional extension apparent horizontal extension by faulting varies from one layer to another and a significant proportion of the extension in a basin may be due to ductile deformation

fracture propagation associated with dike emplacement at the skaergaard intrusion east greenland abstract extensive magmatic activity including several episodes of dike emplacement was associated with early tertiary continental rifting in eastern greenland exposures of the skaergaard intrusion host a large number of these dikes as well as a variety of hydrothermal veins one vein set has a characteristic outcrop expression and is filled with silica rich chlorites relative timing constraints as well as the distribution and geometric relations of these veins relative to the mafic dikes indicate that at least some of these fractures formed during the emplacement of the dikes that cut the intrusion calculations based on elastic theory permit modeling of dike geometries observed in the field and prediction of fracture geometries that could be associated with the inflation of these dikes extensional fractures may form in the area ahead of the dike termination for a variety of loading conditions the predicted width to length ratio for the dike as well as the size of the predicted fracture zone increases with increasing magma pressure within the dike and or increasing differential between the remote principal stresses for dike geometries similar to those observed in the field the predicted fracture zone is roughly elliptical in cross section and is centered in front of the dike termination the size of the zone perpendicular to the dike trend varies from less than one meter to several meters field observations indicate that dike parallel fractures are locally offset normal to the dike margin by distances of a few centimeters to a few meters our analysis implies that at least some of these fractures initiated and propagated ahead of the dikes as they were emplaced the cooling of these dikes initiated localized hydrothermal systems that formed the vein minerals currently filling the fractures

genesis of the xiaolonghe quartz vein type sn deposit sw china insights from cathodoluminescence textures and trace elements of quartz fluid inclusions and oxygen isotopes abstract quartz textures revealed by scanning electron microscope cathodoluminescence sem cl images suggest that quartz in the xiaolonghe sn deposit formed in at least three fluid episodes quartz q1 in the pre ore silicification zone has homogeneous texture high cl intensity and high ti content 19 53 ppm fluid inclusions hosted in q1 are vapor rich v1 type and homogenized as supercritical fluid at 570 c quartz q2 in the syn ore topaz cassiterite quartz veins has lower cl intensity and is characterized by euhedral growth zones with lower ti content 1 3 12 ppm fluid inclusions hosted in q2 topaz and cassiterite are liquid rich fluid inclusions of moderate salinity 15 wt nacl equiv l1 type which homogenized at 423 c the post ore quartz q3 closely associated with sericitization of topaz generally crosscuts q2 and has dark cl intensity and much lower ti concentration the upper limits of t p conditions of pre ore and syn ore stage are constrained by the intersection of nacl h2o fluid inclusion isochores and ti in quartz isopleths and the lower limits are based on microthermometry results of fluid inclusions combined with the 18ofluid value 8 3 of q1 it indicates that the pre ore supercritical fluid was directly exsolved from a granitic magma at t 570 600 c and p 0 70 0 77 kbars corresponding to a maximum depth of 2 8 km under lithostatic pressure the syn ore subcritical fluid 8 0 18ofluid 6 3 mixed with some meteoric water was under 423 450 c 0 32 0 45 kbars the transition from the pre ore supercritical fluid to the syn ore subcritical fluid indicates boiling condition probably at 512 c 0 62 kbars which was likely caused by hydraulic fracturing under high pressure both the fluid boiling and fluid mixing contribute to the cassiterite precipitation in xiaolonghe sn deposit in the post ore stage the coexisting s type and v2 type fluid inclusions in q3 indicates a second fluid boiling at 269 c 0 05 kbars which was probably caused by decompression due to rapid denudation of the overlain strata the low salinity 5 5 wt nacl equiv l2 type fluid inclusions hosted in fluorite homogenized at 199 c and

a fractal relationship between vein thickness and gold grade in drill core from la codosera spain analysis of quartz vein thicknesses and gold grades in drill core from western spain indicates that both approximate a power law distribution suggesting an underlying fractal relationship log log plots of cumulative frequency n against vein thickness t show systematic changes in relation to gold grade the exponent d in a power law of the form n c t super d is lower for higher gold grades indicating that a greater proportion of the dilation is taking place on the thicker veins which are interpreted as providing interconnected conduits for gold bearing fluids thus monitoring the vein thickness distribution may provide a means of discriminating mineralized and barren structures in vein hosted mineral deposits

the initiation and growth of en echelon veins abstract numerical models are used to understand the evolution of mode i opening fractures from spatially random distributions in a brittle elastic material en echelon arrays commonly develop because mechanical fracture interaction promotes growth for this geometry this provides a new mechanism for en echelon vein formation in rock which is distinctly different than previously proposed mechanisms it is suggested that some macroscopic en echelon vein arrays may have served as zones of weakness that localized later shear zone development in a manner analogous to that observed by experimentalists examining micro cracking and subsequent shear rupture of rocks loaded under compression sigmoidally shaped veins and vein fillings are explicitly modeled showing that they can form in response to the mechanical interaction of neighboring fractures which redirects the propagation path numerical comparison of sigmoidal veins formed by brittle fracture and by ductile shear zones demonstrates some of the pitfalls of failing to correctly identify the mechanism of formation

complex vein systems as a data source in tectonics an example from the ugab valley nw namibia abstract neoproterozoic metaturbidites in the lower ugab domain namibia contain a complex network of four sets of quartz calcite veins overprinted by km scale folds associated with four regional foliations the veins formed by fluid overpressure predating the main deformation deformation structures developed at the junction of two mobile belts during the assembly of gondwana the ns kaoko belt and the ew trending damara belt km scale ns trending folds were initiated during ew constriction in the kaoko belt while their further development and all subsequent events are related to constriction in the ew damara belt with coeval sinistral strike slip in the kaoko belt deformation of the veins and development of four orthogonal foliations are due to gradual changes in the bulk tectonic framework rather than separate orogenic events the veins are deformed in a complex manner allowing a full 3d reconstruction of regional sequence of events the local complex tectonics could be reconstructed because of the perfect local exposure and the multitude of veins it illustrates the potential complexity of tectonic events and structural evolution in apparently simple slate belts

fluid rock interaction is decisive for the formation of tungsten deposits tungsten mineralization is typically associated with reduced granitic magmas of crustal origin while this type of magmatism is widespread economic tungsten deposits are highly localized with 90 produced from only three countries worldwide therefore the occurrence of reduced magmatism while necessary for tungsten enrichment seems to be insufficient to form such rare deposits here we explore the mechanisms that lead to wolframite precipitation and evaluate whether they may exert a decisive control on tungsten global distribution tungsten differs from other rare metals enriched in magmatic hydrothermal ore deposits because it is transported as an anionic species precipitation of the main tungstate minerals scheelite cawo 4 and wolframite fe mn wo 4 thus depends on the availability of calcium iron or manganese we demonstrate quantitatively that magmatic fluids at panasqueira portugal provide tungsten in solution whereas the host rock contributes the iron required to precipitate wolframite the combination of special source conditions with specific reactive host rocks explains why major wolframite deposits are rare and confined to a few ore provinces globally

persistence of quartz disequilibrium in groundwater flows this paper shows how constraints can be placed on the prediction of quartz equilibration distance in subsurface water flows this is achieved here by coupling kinetic equations for the simple sio2 quartz h2o water system to various groundwater flow parameters compaction flow aquifer convection flow and hydrothermal system this approach determines 1 the order of magnitude of the downstream distance necessary for equilibrium to be attained when a flowing solution is initially out of equilibrium and 2 the minimum flow velocity required to sustain a sufficient quartz undersaturation or oversaturation over a given distance geological conditions under which specific water rock interactions take place can be constrained with those calculations water flow in most subsurface environments as idealized here tends to equilibrate with the quartz rich hosting medium on a very short distance 1 m major exceptions are 1 the topmost part of a compacting pile 2 shallow and fast flowing aquifers and 3 most hydrothermal systems

bistatic above critical angle scattering measurements of fully buried unexploded ordnance uxo and clutter laboratory grade bistatic scattering measurements are conducted in order to examine the acoustic response of realistic fully buried unexploded ordnance uxo from above critical angle insonification between 2 and 40 khz a 127 mm diameter rocket uxo a 155 mm diameter artillery shell a natural rock of approximately the same size and a cinder block are fully buried in water saturated medium grained sand mean grain diameter 240 m at depths of 10 cm below the water sediment interface a two dimensional array of bistatic scattering measurements is generated synthetically by scanning a single hydrophone in steps of 3 cm over a 1 m 1 m patch directly above the targets at a height of 20 cm above the water sediment interface three dimensional volumetric acoustic images generated from the return waveforms reveal scattering components attributed to geometric and elastic scattering as well as multiple scattering interactions of returns between the sediment water interface and the buried objects the far f

fractal vein distributions within a fault fracture mesh in an exhumed accretionary melange chrystalls beach complex new zealand a well developed fault fracture mesh is observed in the chrystalls beach complex an accretionary melange within the otago schist on the south island of new zealand in this study an analysis of vein thicknesses and clustering of veins is presented both shear and extension veins have a power law thickness distribution measures of vein spacing best fit a power law distribution but a small data set limits this interpretation to a small fractal range vein clustering varies from random to moderately clustered between outcrops and is the greatest where a large proportion of relatively competent blocks occurs within the melange fractures are distributed within the melange matrix and this localized deformation requires heterogeneity in rheology and or fluid pressure distribution whereas pervasive distributed deformation occurs in relatively homogeneous rock the overall trend of this deformation being mainly accommodated by thin veins required that new fractures formed preferentially over refracturing existing veins which highlights the distributed nature of deformation within a fault fracture mesh the predominance of new fractures may result from vein material being stronger than the cleaved wall rock such that wall rock failure occurred instead of reopening of pre existing shear and extension veins

transpressional folding and associated cross fold jointing controlling the geometry of post orogenic vein type w sn mineralization examples from minas da panasqueira portugal the world class w sn panasqueira deposit consists of an extensive subhorizontal vein swarm peripheral to a late orogenic greisen cupola the vein swarm consists of hundreds of co planar quartz veins that are overlapping and connected laterally over large distances various segmentation structures a local zigzag geometry and the occurrence of straight propagation paths indicate that they exploited a regional joint system a detailed orientation analysis of the systematic joints reveals a geometrical relationship with the subvertical f2 fold generation reflecting late variscan transpression the joints are consistently orthogonal to the steeply plunging s0 s2 intersection lineation both on the regional and the outcrop scale and are thus defined as cross fold or ac joints the joint system developed during the waning stages of the variscan orogeny when already uplifted to an upper crustal level veining reactivated these cross fold joints under the conditions of hydraulic overpressures and low differential stress the consistent subperpendicular orientation of the veins relative to the non cylindrical f2 hinge lines also when having an inclined attitude demonstrates that veining did not occur during far field horizontal compression vein orientation is determined by local stress states variable on a meter scale but with the minimum principal stress consistently subparallel to fold hinge lines the conspicuous subhorizontal attitude of the panasqueira vein swarm is thus dictated by the geometry of late orogenic folds which developed synchronous with oroclinal buckling of the ibero armorican arc

power law vein thickness distributions and positive feedback in vein growth the thicknesses of veins of a single generation from the kodiak accretionary complex show a power law distribution with an exponent of d 1 33 this value d 1 indicates that a significant portion of the vein related extension is accommodated by the thinnest veins thick vein segments are typically confined to southeast dipping brittle ductile shear zones textural observations of veins within these zones indicate a progressive evolution from crack seal to euhedral growth of quartz with an increase over time in the duration that fractures remained open model simulations of vein thickness distributions show that a constant growth model results in a negative exponential distribution and that it is necessary to assume size proportional growth to produce a power law distribution textures and power law thickness distributions for the kodiak veins are thus consistent with an increasing time averaged growth rate as vein thickness increased comprising a positive feedback the positive feedback resulted in the progressive evolution from fractures to brittle ductile shear zones and ultimately to faults

tectonics magmatism hydrothermalism and sets of flat joints locally filled by snw aplite pegmatite and quartz veins southeastern border of the serra de estrela granitic massif beira baixa portugal abstract sets of flat joints controlling barren or sn w mineralized veins with aplite pegmatite and or quartz fillings form an interesting but difficult and controversial problem in fact at least three more or less independent parameters influence the formation of these vein type deposits 1 the creation of flat sets 2 the origin of opening s and 3 their filling s relatively few studies have been made of these domains and even if their authors did determine similar successions of events i e creation opening and filling of the joints they disagreed and even came to drastically contrary conclusions concerning the mode of the origin and the opening of flat joints these different hypotheses are summarized and compared with our own results the latter were obtained from observations of flat joint sets and of mineralized or unmineralized quartz and or aplite pegmatite veins occupying parts of these sets on the southern flank of the serra de estrela and by comparison with already known flat sets locally with veins of the iberian variscan belt the following conclusions are drawn 1 1 sets of flat joints are very common structures in the iberian variscan chain 2 2 the sets are locally opened and filled by veins either with or without mineralization which is present only in close proximity of granitic intrusions especially near late well differentiated leucogranitic cupolas elsewhere the joints remain closed and barren 3 3 the genetic history of the flat joints is clearly independent of their opening s and filling s these joints originated long before the opening and infilling the later having been two sub synchronous or penecontemporaneous mechanisms namely the consequence of shrinkage of the granites and hydraulic over pressuring of different fluids clearly several genetic stages were involved 4 4 the origin of flat joint systems can be related to shearing movements associated with weak to very weak tectonics which developed when the main structuration of the belt by the first syn metamorphic phase of folding was already achieved and when the belt was partially uplifted and eroded 5 5 the formation of the flat joints terminated just before the intrusion of the latest granites stephanian leucogranites and thus before the injection of the residual magmatic liquids aplites pegmatites or of hydrothermal sn w mineralizing fluids in fact some late biotitic leucogranites filled some scarcely opened joints

can magnetotelluric data resolve lower crustal conductors abstract it is commonly accepted that the lower crust is electrically conductive the most commonly used measurement technique is magnetotellurics mt using natural plane waves but also a few controlled source experiments have been carried out to study this problem previous work using mt and controlled source data has shown that in areas devoid of a sedimentary layer a hypothetical homogeneous lower crust can be resolved accurately adding a conductive surface layer degrades the resolving power of mt and controlled source data we have studied this problem quantitatively by varying the conductances product of the conductivity and thickness of the sedimentary and lower crustal layers both mt data and the data calculated with a controlled source grounded dipole technique are used for investigation we show that for sedimentary conductances of 200 2000 s mt data combined with controlled source data may identify a conducting lower crust provided its resistivity is less than a few hundred m but its thickness and resistivity cannot be resolved

energy release in subvolcanic environments implications for breccia formation mechanical pdelta v energy is released from hydrous magmas in subvolcanic environments by the two generally sequential processes of second boiling and subsequent decompression the second boiling process h 2 o saturated melt crystals h 2 o vapor releases sufficient pdelta v energy in the model system previously established by burnham 1979 and burnham and ohmoto 1980 to cause fracture failure of even the strongest wall rocks tensile strength approximately 3 77 x 10 10 ergs kg super 1 to depths of 4 or 5 km following fracture failure moreover expansion of already exsolved h 2 o and exsolution of additional h 2 o upon decompression into the model fracture system releases additional pdelta v energy 9 7 x 10 10 ergs kg super 1 of magma sufficient to lift an equivalent mass of rock nearly 1 km in the absence of frictional resistance this latter energy although a conservative value is model dependent to the extent that it is affected by changing any one of the following six independent model parameters 1 bulk composition of the magma hornblende biotite granodiorite in the model 2 depth of emplacement of the magma body approximately 2 3 km p t 0 6 kb 3 mass fraction of h 2 o in the initial melt f o w 0 027 2 7 wt h 2 o 4 mass fraction of melt in the magma at the time of emplacement f o m 1 0 100 liquid 5 tensile strength of the wall rocks 3 77 x 10 10 ergs kg super 1 and 6 depth to the top of the fracture once formed approximately 2 3 km p f 0 3 kb although all of these parameters except the first are of major importance in the formation of hydrothermal breccias generally the mass fraction of h 2 o in the initial melt f o w is regarded as the single most important factor in the formation of breccia bodies in typical porphyry type systems f o w not only determines primarily whether or not the second boiling reaction releases enough pdelta v energy to produce fractures it also determines the magnitude of the decompressional energy released into the fractures if formed magmas in which f o w 0 02 to 0 04 at liquidus temperatures generally release sufficient pdelta v energy not only to produce extensive porphyry type fracture systems but also to produce the breccia bodies these systems commonly contain magmas in which f o w 0 02 on the other hand generally do not release sufficient energy for extensive breccia formation except in structurally weak wall rocks at depths shallower than approximately 2 km magmas in which f o w raquo 0 04 at near liquidus temperatures are incapable of emplacement in the subvolcanic environment

effect of deep reaching tectonics on conductivity structure in the pannonian basin can magnetotelluric data constrain tectonic interpretations abstract magnetotelluric mt deep soundings carried out along the pannonian geotraverse detected anomalous conductivity structures below the narrow szolnok flysch belt with a characteristic width of about 20 km both in the lower crust and the upper mantle here seismic investigations determined a strike slip zone the main fault zone of which penetrates below the mohorovicic discontinuity to a depth of 35 40 km although the similarity in these two independent tectonic indications is high the change in the conductivity structure is questioned and is studied from the point of view of mt field distortions it is concluded that the resolving power of mt methods is strongly reduced by the geologic noise and so the tectonic interpretation of deeper horizons becomes uncertain

thermochronology of economic mineral deposits dating the stages of mineralization at panasqueira portugal by high precision 40 39 ar age spectrum techniques on muscovite 40 ar 39 ar age spectrum dates for 13 muscovites have been used to reconstruct the thermal history thermochronology of the panasqueira portugal tin tungsten deposit a deposit spatially associated with a belt of hercynian plutons muscovite samples with an age difference as small as 2 2 m y 0 7 of the age are statistically distinct statistics are even better for comparison of multiple samples from separate events that is a difference of 0 9 m y 0 3 can be resolved in this approximately 300 m y old deposit the major tin and tungsten ore forming stages which are the oxide silicate stage the main sulfide stage and greisenization occurred between 296 3 or 0 8 1 sigma and 291 6 or 0 8 m y 1 sigma the first substage of the oxide silicate stage was a short lived thermal pulse at 296 3 or 0 6 m y the fluids responsible may have emanated from the known granite cupola the main sulfide stage was active at 294 5 or 0 9 m y as a slightly longer lived pulse with oldest evidence for this stage 295 8 or 0 6 m y coming from areas farthest away from the known cupola and youngest evidence 293 5 or 0 8 m y closest to the cupola a second substage of the oxide silicate stage occurred as a short lived thermal pulse at 292 9 or 0 7 m y synchronous with greisenization of the cupola and alteration of the silica cap at 292 1 or 0 4 m y the duration of activity of the oxide silicate stage the main sulfide stage greisenization and alteration of the silica cap based on the ages of all 13 muscovites was greater than 4 2 or 0 5 m y 1 sigma minor argon loss from all dated muscovites occurred during later reheating probably during the longer lived pyrrhotite alteration stage a single center the known cupola had a prolonged role and was the source for main sulfide stage oxide silicate stage ii greisenization and alteration of the silica cap and possibly oxide silicate stage i and the pyrrhotite alteration stage however a separate source for these latter two stages cannot be ruled out this study is an example of a new and powerful application of 40 ar 39 ar age spectrum dating of muscovite because of the high precision demonstrated in this study it is now possible to establish time constraints necessary for solving some of the long standing problems in economic geology beyond this the unique geologic situation of panasqueira has allowed us to quantify the thermal characteristics of muscovite published fluid inclusion data have been used to estimate a muscovite argon closure temperature of approximately 325 degrees c during rapid cooling or short reheating and a temperature of approximately 270 degrees c during slow cooling or extended reheating argon loss patterns displayed by all dated muscovites resulted from reheating after original closure the mechanism for this argon loss appears to have been argon transport by volume diffusion thus 40 ar 39 ar age spectrum dating of muscovite can be used to evaluate thermal conditions controlling argon diffusion as well as age duration and number of episodes of mineralization

extensional failure and hydraulic valving at minas da panasqueira portugal evidence from vein spatial distributions displacements and geometries abstract at panasqueira portugal exceptional exposure and demonstrable vein connectivity allow robust characterisation of brittle elastic failure mechanisms in intrusive related environments extensional failure was driven by cycles of fluid injection hydraulic valving and vein growth under conditions with v 1 and differential stress

how long can a hydrothermal system be sustained by a single intrusive event published numerical calculations show that the convective systems which produce near surface geothermal systems or ore deposits are likely to cool their intrusion heat source in at most a few tens of thousands of years even if the intrusion is large generally hydrothermal circulation near surface geothermal activity and magmatic intrusion should be closely tied in time and space long lived approximately 1 m y hydrothermal activity normally suggests multiple pulses of intrusion with associated pulses of hydrothermal circulation however if the permeability of the intruded environment is just above that which allows convection and the volume of the intrusion is very large calculations presented here show that a single episode of intrusion can sustain hydrothermal circulation and near surface geothermal activity for approximately 800 000 yr for a deep sill heat source these unusually long lived hydrothermal systems can vent through a few very widely separated discharge sites and have the potential to produce isolated massive sulfide deposits of unusually large size

subcritical scattering from buried elastic shells buried objects have been largely undetectable by traditional high frequency sonars due to their insignificant bottom penetration further even a high grazing angle sonar approach is vastly limited by the coverage rate dictated by the finite water depth making the detection and classification of buried objects using low frequency subcritical sonar an interesting alternative on the other hand such a concept would require classification clues different from the traditional high resolution imaging and shadows to maintain low false alarm rates a potential alternative even for buried targets is classification based on the acoustic signatures of man made elastic targets however the elastic responses of buried and proud targets are significantly different the objective of this work is to identify analyze and explain some of the effects of the sediment and the proximity of the seabed interface on the scattering of sound from completely and partially buried elastic shells the analysis was performed using focused array processing of data from the goats98 experiment carried out jointly by mit and saclantcen and a new hybrid modeling capability combining a virtual source or wave field superposition approach with an exact spectral integral representation of the green s functions for a stratified ocean waveguide incorporating all multiple scattering between the object and the seabed among the principal results is the demonstration of the significant role of structural circumferential waves in converting incident evanescent waves into backscattered body waves emanating to the receivers at supercritical grazing angles in effect making the target appear closer to the sonar than predicted by traditional ray theory

full paleostress tensor reconstruction using quartz veins of panasqueira mine central portugal part i paleopressure determination abstract paleostress tensor restoration methods are traditionally limited to reconstructing geometrical parameters and are unable to resolve stress magnitudes based on previous studies we further developed a methodology to restore full paleostress tensors we concentrated on inversion of mode i fractures and acquired data in panasqueira mine portugal where optimal exposures of mineralized quartz veins can be found to carry out full paleostress restoration we needed to determine 1 pore paleo pressure and 2 vein attitudes the present contribution focuses specifically on the determination of pore pressure to these aims we conducted an extensive fluid inclusion study to derive fluid isochores from the quartz of the studied veins to constrain p t conditions we combined these isochores with crystallisation temperatures derived from geochemical analyses of coeval arsenopyrite we also applied the sphalerite geobarometer and considered two other independent pressure indicators our results point to pore pressures of 300 mpa and formation depths of 10 km such formation depths are in good agreement with the regional geological evolution the obtained pore pressure will be merged with vein inversion results in order to achieve full paleostress tensor restoration in a forthcoming companion paper

chemical and boron isotopic composition of hydrothermal tourmaline from the panasqueira w sn cu deposit portugal abstract tourmaline is a locally abundant hydrothermal mineral in the wallrocks surrounding the w sn cu mineralized veins in the panasqueira deposit portugal and a minor phase within the veins themselves tourmaline chemical and boron isotope compositions have been determined from three settings 1 pervasive fine grained tourmalinization zones in wallrocks within 10 cm of the veins 2 coarser tourmaline in wallrock hosted fault zones and 3 needle shaped tourmaline from late stage vugs in the quartz veins all tourmalines from panasqueira have ferromagnesian compositions with significant octahedral aluminum contents and variable x site vacancies tourmaline compositions show significant chemical variations at the grain and sample scale but are homogeneous on the deposit scale tourmaline is typically optically and chemically zoned showing significant increases in fe and f and decrease in mg ca and al from core to rim moreover as a general trend al increases and mg decreases with proximity to the vein contact the observed chemical variation seems to have been controlled mainly by the vectors femg 1 x alna 1 r2 1 x al oh na 1 r2 1f 1 r2 oh f al 1o 1 and x al2o na 1 r2 2 oh f 1 the total range in tourmaline 11b values from the panasqueira w sn cu deposit is from 3 7 to 12 7 including tourmaline from the three settings the calculated b isotope composition of the hydrothermal fluid based on the average tourmaline composition of 9 and an estimated alteration temperature in the wallrocks based on ti in quartz thermometry of 500 c is 7 1 this isotopic composition is permissive of a boron source from local metasediments or an s type granite but the high boron concentration in the wallrocks with 50 vol tourmaline along with the typical element association of panasqueira w sn f nb ta rb makes a granitic source more likely late stage tourmaline needles in vugs within the mineralized veins show the same patterns and ranges in both chemical and boron isotope composition suggesting that exsolved magmatic fluids were injected in at least two pulses the variations in chemical and boron isotopic compositions of tourmaline combined with estimates from ti in quartz thermometry suggest that hydrothermal ore formation is associated with fluid cooling and geochemical fluid rock interactions

antitaxial crack seal vein microstructures and their relationship to displacement paths abstract the microstructures developed in an example of a syntectonic crack seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack seal vein growth this observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation it is demonstrated that crack seal processes can lead to the development of both irregular and laminated vein microstructures as well as fibrous microstructures the types of microstructures which develop in crack seal veins and the extent to which they reflect the opening paths of veins depend on a range of factors the most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures and the location of sites of material accretion during successive crack seal increments

a deep crustal electromagnetic sounding in the georgia piedmont electromagnetic soundings in the georgia piedmont using a horizontal loop source indicate an upper layer with conductivity variations from less than 2 10 5 to 2 5 10 4 s m extending to about 15 km underlain by a zone with conductivity of more than 10 3 s m at one site the conductance of any intermediate layer is limited to 2 s to a depth of 15 km the conductivities of the upper layer agree with those expected from the surface geology and the increase in conductivity occurs at a depth which correlates generally with reflections detected by the consortium for continental reflection profiling s seismic profile of the area a possible source of the high conductivity is water saturated sediments trapped when the region was overthrust in the appalachian orogeny

fault fluid evolution in the xitian w sn ore field south china constraints from scheelite texture and composition abstract understanding fault fluid evolutionary processes is crucial for understanding the genesis of vein type deposits scheelite and quartz are common hydrothermal minerals in w sn ore field faults in the xitan ore field nanling range south china scheelite textural and compositional features were studied and two types of scheelite were identified type i represented by scheelite in the xihu fault is intergrown with quartz has oscillatory zonation variable zonation band width and is homogeneous light grey color in cathodoluminescence images and type ii represented by scheelite in the goudalan fault is intergrown with quartz that shows multi stage growth has oscillatory zonation and each zonation band is the same width scheelite grains of both types have homogeneous major element distribution e g w and ca however trace element contents e g mg nb are slightly variable and rare earth element ree and y contents gradually decrease by an order of magnitude from the core to the rim in both types of scheelite from core to rim the chondrite normalized ree patterns of type i scheelite change from flat curves with no or slightly positive eu anomalies to intense concave shapes with intense positive eu anomalies the middle rees gradually become depleted indicating the mree fractionation associated with early scheelite precipitation in a relatively closed fault fluid system in contrast the chondrite normalized ree patterns of type ii scheelite grains are similar all showing weak mree depletion from the core to rim implying recurrent fluid pulses in an open fault fluid system the ree patterns in the cores of the type i scheelite are similar to those of the scheelite from jurassic granites related to the w sn mineralization suggesting that the initial ore forming fluids were exsolved from the jurassic magma the ree patterns in the rims of type ii scheelite are consistent with those of scheelite from the goudalan w deposit suggesting that the fault fluid system experienced a similar evolution to the ore forming fluid system combining the results of this work with previous quartz h o isotopic and fluid inclusion studies it can be concluded that fluid mixing occurred in both the fault fluid system and the ore forming fluid system triggering large scale w precipitation

electromagnetic studies in the fennoscandian shield electrical conductivity of precambrian crust abstract electromagnetic em investigations of the 1980s in the fennoscandian baltic shield produced an unique and unified em data set studies include regional investigations by the magnetovariational mv method with large lateral sampling distance investigations of anomalous conductivity structures by magnetotelluric mt soundings and other em and electrical methods audio mt soundings d c dipole dipole and vlf resistivity profilings with shorter sampling distance and studies of the near surface conductivity by airborne em surveys the variety of methods provide an ability to map efficiently crustal conductivity structures from a regional scale of hundreds of kilometres down to local details of some metres in the anomalous structures the precambrian of the fennoscandian shield is characterized by roughly nw se directed elongated belts of conductors which separate more resistive crustal blocks the latter serve as transparent windows through which to probe deep electrical structure and belts of conductors as tectonic markers of ancient orogenic zones including 1 the kittila vetrenny poyas conductor 2 the lapland granulite belt and inari pechenga imandra varzuga conductors 3 the archaean proterozoic boundary conductor and 4 the southern finland conductor the conductive belts orogenic conductors indicate places where crustal masses collided and were finally sealed together enhanced conductivity in the orogenic conductors is caused primarily by an electronic conducting mechanism in graphite and sulphide bearing metasedimentary rocks estimations of the lower crustal conductivity indicate a laterally heterogeneous lower crust in the fennoscandian shield archaean lower crust seems to be in general more resistive than the early proterozoic lower crust of the karelian and svecofennian domains the lower crust in the southwestern part of the svecofennian domain and in the sveconorwegian domain seems to be more resistive than in the central part of the svecofennian domain

the character and distribution of mineralized fractures in the kodiak formation alaska implications for fluid flow in an underthrust sequence the kodiak formation an accreted slate belt in southwest alaska experienced two major episodes of fracturing and mineralization in the late cretaceous early tertiary the earlier of these episodes is related to underthrusting of sediments below a major decollement d1 whereas a later episode is marked by development of fold and thrust structures and a regionally pervasive slaty cleavage within the accretionary wedge d2 d1 veins are restricted to sand layers and are concentrated at the highest structural level of the kodiak formation the most pervasive d1 fractures occur within a melange zone where the kodiak formation is thrust beneath the older uyak complex microstructures in the melange suggest that these veins formed during the later stages of a deformation that involved layer parallel shear in conjunction with progressive lithification d2 veins cut sand and shale layers and are concentrated at the lowest structural levels of the kodiak formation microstructures within these veins indicate that d2 veins are hydrofractures that were either 1 intermittentiy sealed along the length of the crack e g in the smallest veins 0 5 cm width fracturing coincided with development of a slaty cleavage in a regime of progressive simple shear from highest to lowest structural levels there is a gradual increase in measured finite strain the abrupt appearance of pervasive d2 veins and a greater noncoaxiality of strain related to southeast directed thrusting these observations suggest that d2 veins formed above a decollement that was active at the base of a late cretaceous early tertiary accreted package in total the d1 and d2 veins suggest substantial fluid flow within an ancient subduction complex with development of hydrofractures first in the footwall and then in the hanging wall of a basal decollement that stepped from shallow to deeper structural levels

inferring paleostresses from natural fracture patterns a new method we introduce a method to infer the remote differential stress magnitude from the curvature of overlapping echelon fracture traces the curving paths of overlapping echelon cracks imply the predominance of local crack induced stresses over remote stresses during propagation nearly straight crack paths imply the controlling influence of a remote compressive crack parallel differential stress this method is used to interpret complex joint patterns mapped in sedimentary rock it is also applied to the problem of fracture pattern generation using computer models

lithogeochemical studies of aureoles around the panasqueira tin tungsten deposit portugal multi element lithogeochemical surveys of aureoles surrounding tin tungsten bearing quartz vein systems have been utilized infrequently in exploring for these deposits at panasqueira beira beixa portugal a wrich quartz vein mineralization occurs in the beira schists surrounding a greisenized cupola and offers a suitable test case for such lithogeochemical studies therefore shale samples were collected at a density of about 6 samples per km2 pronounced anomalies of tin and tungsten have been detected in these samples together with those of other elements coeval with the mineralizing processes

are large differential stresses required for straight fracture propagation paths abstract fracture growth geometries in linear elastic materials are sensitive to the ratio between the remote differential stress or t stress and the fracture driving stress two mechanically interacting fractures follow straighter paths for greater values of this ratio we demonstrate that the degree of curvature that develops between two mechanically interacting fractures is also controlled by such factors as sub critical fracture growth and fracture surface roughness sub critical fracture growth limits the development of fracture curvature by decreasing the fracture driving stress needed for propagation fracture surface roughness can affect the development of fracture curvature by limiting the magnitude of the shear displacement discontinuity that can be accommodated along the fracture thus propagation paths for two mechanically interacting fractures can be quite straight even in the presence of small to moderate differential stresses consequently the absence of fracture curvature does not necessarily imply a high differential stress state

evolution and paragenetic context of low δd hydrothermal fluids from the panasqueira w sn deposit portugal new evidence from microthermometric stable isotope noble gas and halogen analyses of primary fluid inclusions abstract the evolution paragenetic context and origin of remarkably low d hydrothermal fluids from the hercynian panasqueira w sn cu ag deposit have been investigated through a combined microthermometric stable isotope halogen and noble gas fluid inclusion study large variations in d between 60 and 134 have been observed in primary fluid inclusions from growth zones in a quartz crystal pa66 paragenetically constrained to the main sulfide stage mss of ore formation the same fluids exhibited relatively constant fluid inclusion homogenisation temperatures 254 to 260 c salinities 7 4 to 8 7 wt nacl equivalent and calculated fluid 18o 3 8 to 4 4 all the fluids exhibited 40ar excesses mean molar br cl and i cl ratios varied from 2 3 10 3 4 2 10 3 and 8 10 4 10 10 4 respectively with the more br rich fluids being associated with the more deuterium depleted fluids the low palaeolatitude of panasqueira throughout the main stages of ore formation and the overwhelming predominance of meteoric fluids in the main late and post ore mineralising fluids with d of 40 to 65 lends little support for any models involving low d palaeometeoric water the limited range of 40ar cl ratios 1 1 10 5 1 6 10 5 also precludes boiling during the mss or mixing of meteoric water with highly fractionated deuterium depleted magmatic waters as viable mechanisms for producing the low d fluids the high br cl and i cl of the mss fluids indicate significant fluid interactions with organic rich sediments or metasediments in particular regionally abundant carboniferous coals or coaly sediments that are inferred to be depleted in deuterium by around 85 compared to palaeometeoric water the large range of d observed in the mss fluids may be explained by isotopic exchange of palaeometeoric water with these coals at varying water rock wt wt ratios between 0 02 and 0 002 with fluid oxygen isotopic compositions controlled by exchange with the beira schists such a model is consistent with the requirements that any satisfactory model must take into account viz i the relative constancy of temperature salinity 40ar and fluid 18o during excursions to low d ii the high i cl and br cl ratios observed and iii the coincidence of these low d fluids with the onset of precipitation of polymetallic sulfides and phosphates under reducing conditions the model indicates that meta sedimentary sources of ore forming components became increasingly important during the evolution of the panasqueira sn w deposit and accounts in part for the observed progression from oxide dominated to base metal sulfide dominated parageneses

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

bistatic scattering from submerged unexploded ordnance lying on a sediment the broadband bistatic target strengths tss of two submerged unexploded ordnance uxo targets have been measured in the nrl sediment pool facility the targets a 5 in rocket and a 155 mm projectile were among the targets whose monostatic tss were measured and reported previously by the authors bistatic ts measurements were made for 0 target front and 90 target side incident source directions and include both backscattered and forward scattered echo angles over a complete 360 with the targets placed proud of the sediment surface for the two source angles used each target exhibits two strong highlights a backscattered specular like echo and a forward scattered response the ts levels of the former are shown to agree reasonably well with predictions based on scattering from rigid disks and cylinders while the levels of the latter with predictions from radar cross section models based on simple geometric optics appropriately modified the bistatic ts levels observed for the proud case provide comparable or higher levels of broadband ts relative to free field monostatic measurements it is concluded that access to bistatic echo information in operations aimed at detecting submerged uxo targets could provide an important capability

broadband acoustic scattering measurements of underwater unexploded ordnance uxo in order to evaluate the potential for detection and identification of underwater unexploded ordnance uxo by exploiting their structural acoustic response we carried out broadband monostatic scattering measurements over a full 360 degrees on uxo s two mortar rounds an artillery shell and a rocket warhead and false targets a cinder block and a large rock the measurement band 1 140 khz includes a low frequency structural acoustics region in which the wavelengths are comparable to or larger than the target characteristic dimensions in general there are aspects that provide relatively high target strength levels approximately 10 to 15 db and from our experience the targets should be detectable in this structural acoustics band in most acoustic environments the rigid body scattering was also calculated for one uxo in order to highlight the measured scattering features involving elastic responses the broadband scattering data should be able to support feature based separation of uxo versus false targets and identification of various classes of uxo as well

electric resistivity structure of the siljan impact region based on 65 magnetotelluric mt measuring stations a cross section of the electric conductivity structure in the siljan impact area located in the central part of sweden is presented static sh

characterization and timing of the different types of fluids present in the barren and ore veins of the w sn deposit of panasqueira central portugal the panasqueira w sn deposit is the largest quartz vein type deposit of the iberian peninsula and the most important wolframite deposit in western europe the ore veins are almost exclusively sub horizontal besides ore bearing sub horizontal veins the panasqueira mine also contains barren quartz veins there are essentially two generations of barren quartz quartz contemporaneous with the earliest regional metamorphism qi and recrystallized quartz contemporaneous with the thermal metamorphism related to the granite intrusion qii fluid inclusion studies microthermometry and raman were undertaken in order to distinguish fluids contemporaneous with the barren quartz from those contemporaneous with the ore bearing quartz qiii fluid inclusion data indicate that the barren and ore bearing quartz fluids are dominantly aqueous 93 to 98 mol h2o with a nearly constant bulk salinity 8 to 12 wt eq nacl with the quantity of volatile component determined by raman spectrometry higher in qiii but never greater than 5 mol however the co2 ch4 n2 ratio is different for each type of quartz volatiles are dominated by ch4 10 to 96 mol zch4 and or n2 3 to 87 mol zn2 in the barren quartz and by co2 60 to 73 mol zco2 in ore bearing quartz the bulk chemical composition of the fluids in qiii is comparable to that found commonly in hydrothermal fluids associated with wolframite mineralization where na k ca and hco3 cl so4 a dispersion in th 226 to 350 c found in qiii together with a variation in the degree of filling 0 5 to 0 7 and with the consequent variation of fluid densities 0 70 to 0 79 may result from changes in the fluid pressure regime below lithostatic pressure suggesting vein filling related to tectonic events

industrial interference magnetotellurics an example from the tangshan area china in order to attack the problem of industrial interference in magnetotellurics we apply the general theory of weidelt and christensen describing the electromagnetic fields of a grounded electric dipole over a horizontally stratified earth the impedance tensor for dipole fields is proven to be independent of the dipole orientation within the near field the elements of the impedance tensor are real quantities and are independent of period because of this the logarithm of the apparent resistivity versus the logarithm of period rises with a slope of 45 degrees and the phase is zero we study the inversion of the controlled source impedance tensor to reveal earth structure at a given distance from the source using a nonlinear least squares technique we present an interpretation of magnetotelluric data in the tangshan area of china as a practical application of the technique sedimentary thicknesses of about 600 m are estimated and agree well with conventional schlumberger soundings the upper resistive crust thickness about 13 km overlies a more conductive lower crust whose resistivity is below a few hundred omega m

crustal conductors along the sveka profile in the fennoscandian baltic shield finland summary electrical properties of the crust in the central fennoscandian baltic shield are examined along the 435 km magnetotelluric profile sveka a 2 d geoelectric model shows large variations in the crustal resistivity with the model resistivities ranging from 0 1 to 35 000 m the main geoelectrical features of the crust and upper mantle are 1 a resistive archaean lower crust conductance 30 s 2 a conductive palaeoproterozoic lower crust beneath the central finland granitoid complex 170 900s free h2o rich fluids trapped into the lower crust seem less plausible causes for enhanced conductivity as long retention times are required and no possible sources seem to be available to produce fluids into the dry lower crust most likely causes for permanent enhancement of lower crustal conductivity include the conductive fill of ductile shear zones conductive graphite bearing lamellae conduction in mafic rocks perhaps via conduction in magnetite or conductive solid remnants of an ancient shear zone between two crustal blocks thin graphitic grain boundary films precipitated from co2 rich fluids also seem less plausible 3 a very resistive 10 000 35 000 m upper crust of the palaeoproterozoic central finland granitoid complex and the archaean domain 4 narrow and elongated conductors 0 1 100 m beneath the palaeoproterozoic kainuu ksb and tampere tsb schist belts are caused by graphite bearing metasedimentary rocks buried deep into the crust in the collisions of crustal terranes during the formation of the palaeoproterozoic svecofennian crust the 500 km long tsb conductor represents an internal svecofennian terrane boundary whereas the ksb conductor is a part of a discontinuous archaean proterozoic boundary conductor 5 resistive upper mantle there are no indications of an upper mantle conducting layer within the uppermost 300 km in the central part of the fennoscandian shield

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

regional fractures i a mechanism for the formation of regional fractures at depth in flat lying reservoirs 1 regional fractures may form slowly at depth in extension and parallel to regional horizontal tectonic compression such fracture sets can be common in reservoirs despite the absence of flexure of the strata and are commonly important contributors to reservoir permeability such fractures make good targets for deviated wells high formation pore pressure is crucial to creating low effective confining stresses and brittle rock properties in the reservoirs conditions conducive to fracturing despite deep burial but pore pressure does not exceed the least principal stress and does not cause tensile fracturing the mechanism of natural hydraulic fracturing is incompatible with the characteristics of most regional fracture sets rather many regional fractures form duri g far field compression initiate at locally induced tensile stresses caused by flaws in the rock and propagate in the plane of the maximum and intermediate compressive stresses the differential stress required for initiation and propagation of regional fractures is well below that necessary for shear failure in the presence of high pore pressure fractures can be held open at depth by a tectonic basinwide dilatancy of the strata and the open void space commonly becomes mineralized

ore related breccias in volcanoplutonic arcs an overview of breccias related to a variety of base metal precious metal and lithophile element deposits in volcanoplutonic arcs permits definition of six possible mechanisms for subsurface brecciation 1 release of magmatic hydrothermal fluids from high level hydrous magma chambers during second boiling and subsequent decompression generates a spectrum of breccia types in which fragments may suffer collapse and or ascent single or multiple intrusion related breccia pipes and pre and intermineral breccias in porphyry copper deposits provide widespread examples 2 magmatic heating and expansion of meteoric pore fluids may lead to brecciation commonly of late or postmineral age and including pebble dikes in porphyry type and related deposits magmatic heating of rocks saturated with seawater may cause submarine hydrothermal eruptions late in the emplacement histories of kuroko type massive sulfide deposits many of the resultant breccias underwent limited sedimentary transport overpressuring of heated fluids beneath semipermeable partly self sealed cap rocks may lead to brecciation and subaerial hydrothermal eruptions in shallow epithermal precious metal settings magmatic heating or tectonic disturbance may have triggered brecciation 3 interaction of cool ground waters with subsurface magma can generate phreatomagmatic explosions postmineral phreatomagmatic diatremes associated with porphyry systems and premineral diatremes with epithermal precious or base metal deposits were generated in this manner these attained the palcosurface to produce pyroclastic base surge and fall deposits that accumulated as tuff rings around maar craters 4 magmatic hydrothermal brecciation may lead to disruption of rocks through to the paleosurface decompression and fragmentation and eruption of the top part of an underlying magma chamber pre and postmineral magmatic diatremes of this sort are inferred to accompany a few porphyry type and other base and precious metal systems they were manifested at the palcosurface by accumulations of pyroclastic fall and flow deposits 5 breccias may result from mechanical disruption of wall rocks during subsurface movement of magma any intrusion related deposit may include such intrusion breccias 6 tectonic breccias resulting from fault displacement may accompany any type of ore deposit a continuum exists between many of these breccia types and it is difficult to identify unique criteria for their unambiguous distinction

lithospheric and upper mantle structures results of electromagnetic soundings in europe abstract information on electrical conductivity of the subsurface at a crustal scale is inferred mainly from magnetotelluric mt and magnetometer data the latter include both magnetometer array studies mv and geomagnetic depth soundings gds locally controlled source data e g data from d c and very low frequency vlf resistivity surveys frequency soundings and airborne electromagnetic mappings provide information on near surface structures e g on exposures of deeper conductors the variety of electromagnetic methods provides an efficient zooming ability for structures from a regional scale of hundreds of kilometres to small local details of some metres thus both the mapping of large scale crustal conductivity structures and a detail study of the anomalous structures are possible the electrical conductivity of the crust and upper mantle is described within the plate tectonic framework examples of structures which become electrically conductive at different phases of the process are shown using the results of electromagnetic soundings in various tectonic environments in europe it is shown how the crustal conductors can be interpreted as ancient and modern tectonic markers these include structures from the fennoscandian shield precambrian extensional basins and collisional zones the carpathians scotland and ireland suture zones and the pannonian basin extensional basin and thrust zones as well as conductors detected along the european geotraverse e g the terrain boundaries and the decollement of the variscan thrusting an example which considers the lower crustal conductivity from the fennoscandian shield shows rather large lateral variations in conductance of the lower crust varying from 1 s to several hundreds of siemens estimates of the depth of the asthenospheric conducting layer within the european territory also vary considerably shallowest depths are detected beneath extensional regions the pannonian basin and the rhinegraben 40 80 km whereas depths of 200 km or over or the absence of a conducting layer are detected beneath the alps and old precambrian cratons among the key areas where electromagnetic investigations should play an essential role are the deep structure of the east european platform boundaries of archaean and proterozoic terrains the structure of the teisseyre tornquist zone the nature of the present day continental lower crust and the characteristics of the electrical asthenosphere beneath europe

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

mechanisms of dilatant en echelon crack formation in bedded chalk abstract this study focuses on various styles of dilatant out of plane en echelon cracks which are associated with bed restricted burial joints that cut slightly deformed sediments in the beer sheva syncline there is a positive linear fit of en echelon crack length to layer thickness r2 0 928 in fringes which were formed by discontinuous breakdowns in joints lithological properties influence the breakdown process the breakdown of joints into en echelon cracks in chalk beds is influenced by both local conditions and remote stresses there are two distinct cases of opposite senses of segment rotation in a single fringe first in a curved fringe and second in a straight fringe the former can best be assigned to local deviations of the minimum principal direction from perpendicularity to the parent joint whereas the latter appears to reflect changes in remote stresses two of the three sets of en echelon segmentation which were distinguished in the beer sheva syncline are correlated with two distinct tectonic associations the k iii k i ratio increases from the layer centre towards the layer boundaries and this is the driving motivation for both the gradual increase of barb overlap from the centre towards the layer boundaries and the en echelon segmentation along the layer boundaries

structural and geochemical controls on the development of turbidite hosted gold quartz vein deposits wattle gully mine central victoria australia gold quartz vein deposits in the wattle gully mine arefault related structures developed in a low grade metamorphosed ordovician quartz rich flysch sequence the major mineralization is associated with a high angle reverse fault wattle gully fault that has a maximum displacement of 40 m over 10 6 metric tons t of vein quartz averaging 10 5 g t au occur within a dilatant jog in this structure fold geometry has controlled fault geometry and resulted in the localization of the jog segment where the wattle gully fault traverses an east dipping fold limb to link two more steeply dipping bedding parallel fault sediments on adjacent west dipping fold limbs gold mineralization is also associated with several minor east clipping reverse faults and a bedding parallel fault whose development is related to flexural slip during fold tightening mineralization occurs within fault fill veins and extension veins adjacent to faults the geometry and internal structures of veins indicate that vein growth has been controlled by repeated fluctuations in fluid pressure and shear stress associated with fault valve behavior vein mineralogy is dominated by quartz with minor carbonates chlorite white mica albite and apatite arsenopyrite and pyrite are the major sulfide phases but sphalerite galena chalcopyrite and pyrrhotite are also present the highest au grades tend to be localised in vein systems that are in proximity to carbonaceous slates wall rock alteration is minor and restricted mainly to narrow zones adjacent to major vein systems sulfur isotope fractionation between galena and sphalerite indicates mineralization occurred at temperatures around 300 degrees c the delta 34 s compositions of pyrite and arsenopyrite are nearly uniform throughout the mine 1 5 2 8 but are generally not in equilibrium with the delta 34 s values of associated base metal sulfides 0 7 5 3 the delta 18 o values for vein quartz vary between 15 6 and 18 0 per mil and correspond to delta 18 o values