polymetallic epithermal fissure vein mineralization topia durango mexico part ii silver mineral chemistry and high resolution patterns of chemical zoning in veins

scaling invariance of crack surfaces the morphology of fractured rock surfaces is studied in terms of their scaling invariance fresh brittle fractures of granite and gneiss were sampled with a mechanical laboratory profilometer and 1 1 dimensional parallel profiles were added to build actual maps of the surfaces a first step in the scaling invariance description is a self affine analysis using three independent methods the root mean square and the maximum minimum difference of the height are shown to follow a power law with the sample length the return probability and the fourier spectrum are also computed all these approaches converge to a unique self affine exponent 0 80 analysis over a broad statistical set provides a reproducibility error of 0 05 no significant differences between the isotropic granite and the markedly anisotropic gneiss appear for the scaling exponents an analysis of the profilometer shows that the two main drawbacks of the setup are not significant in these analyses the systematic errors of the scaling analysis are estimated for the different methods isotropy of the scaling invariance within the mean fracture plane is shown either with the result obtained from different fracture orientations or with the two dimensional fourier spectrum of the surface topography itself the analysis is brought further into the multifractal framework the structure functions are shown to have power law behavior and their scaling exponent varies nonlinearly with the moment order finally the corresponding conserved process belongs to a universal multifractal class with 1 5 for the levy index and c1 0 3 for the fractal codimension of the mean singularities the three indices and c1 completely characterize the scale invariance the multifractal behavior is significant for physical properties which depend on high order moments like contact according to this study and that of other groups the self affine exponent is constant over a large range of scales and for different fracture modes and various materials this opens the possibility that there exists a form of universality in the cracking process it appears that only the prefactor of the roughness is dependent on material and mode

mechano chemical interactions in sedimentary rocks in the context of co2 storage weak acid weak effects abstract due to the corrosive nature of dissolved co2 the potential short or long term alteration of rock properties represents a major issue in several sites where natural co2 circulation is observed as well as in reservoirs targeted for storage of anthropogenic co2 to date this has been primarily studied from a transport chemical perspective with laboratory evidence of microstructural modifications together with the consequences for flow properties compared to the transport chemical aspects the mechanical chemical aspects have been less investigated though it is to be expected that mechanical properties e g elastic properties failure parameters and time dependent mechanical behaviour could potentially be affected in a similar manner to hydraulic parameters yet since co2 is a weak acid the ph drop is expected to be moderate with a likely lower limit close to 4 0 the buffering of ph by calcite minerals present in most reservoirs targeted for storage may further limit the ph drop as well as confining it to a localized rock volume around the injection well this leads to the question of the magnitude and time spatial scales of chemically mediated mechanical processes during co2 sequestration the authors propose to address this issue by reviewing recent laboratory based studies restricted to sedimentary rocks namely reservoir rocks carbonate or sandstone intact or fractured caprocks and fault rocks key findings include the following 1 the short term impact on the elastic and inelastic behaviour of intact caprocks remains limited 2 shear strength weakening is likely to be respectively low and low to moderate for shale clay rich and anhydrite rich faults but without modifying slip stability in either case 3 the largest impact is located within carbonate reservoirs but with a broad range of reported responses depending on hydrodynamic conditions closed or open and on dissolution regime uniform or channelling and 4 creep experiments confirm that co2 induced dissolution may enhance long term compaction of carbonate reservoirs but the magnitude of acceleration varying from non significant to 50 times depends to a large extent on site specific conditions grain size ph temperature effective stress state etc which renders any direct extrapolation from laboratory to reservoir scale difficult finally some directions for future research studies are discussed

a new method for determining the hydraulic aperture of rough rock fractures using the support vector regression abstract understanding fluid flow in rock fractures is relevant to many engineering applications including underground nuclear waste disposition oil and gas exploitation dam and slope design as well as contaminant transport rough rock fracture is usually idealized as two parallel plates separated by the hydraulic aperture the determination of which is key to solving seepage problems in fractured rock using the cubic law the mechanical aperture and fracture morphology are the main aspects affecting the hydraulic aperture here an innovative hydraulic aperture prediction equation for two dimensional fractures is established based on the mechanical aperture and fracture morphology six parameters were firstly selected to predict the hydraulic aperture including three mechanical aperture parameters and three fracture morphology parameters a training database was then established from numerical experiments of 300 fracture models which were built based on barton s 10 standard profiles and their combinations the numerical experiments were conducted with fluent software based on the database a hydraulic aperture prediction model was established using the support vector machine theory and the particle swarm optimization algorithm which was transformed into an explicit equation finally 20 natural fractures were used to validate the performance of this equation with the comparisons of four existing empirical equations the results show that this method worked well in predicting the hydraulic aperture this method provides an alternative and effective way to directly determine the hydraulic aperture using the geometry information of rough rock fractures

structural controls on the auriferous shear zones of the saint yrieix district massif central france evidence from the le bourneix and laurieras gold deposits at least 2 000 occurrences of gold mineralization including four economic deposits occur in the saint yrieix district in the massif central france two of these the le bourneix and laurieras deposits are currently being mined the mineralization consists of quartz lenses with sulfides and native gold the lenses are several meters thick and 300 m in length and occur within a shear zone several kilometers long in para and orthogneisses of the hercynian basement metallogenic and structural investigations in the saint yrieix district and in le bourneix mine have made it possible to relate the various episodes of mineralization to the sequence of tectonic events the lenses of vein quartz were emplaced during phases of extension their distribution being controlled by anastomosing mylonitic bands formed during a preceding tectonic phase the mylonitization was accompanied by sericitization and the introduction of disseminated pyrite arsenopyrite rutile and organic matter along the planes of mylonitic foliation gold fine grained galena and sulfosalts were introduced later impregnating the microsaccharoidal quartz resulting from cataclasis of initially barren vein quartz subsequent partial remobilization of the gold bearing lenses during phases of extension caused coarsening of the gold with the appearance of argentiferous nuggets the emplacement of the quartz lenses in brittle ductile shear zones and the diachronism between the introduction of the vein quartz and that of the gold are characteristic of this mineralization and make it possible to propose guides for exploration

deposit scale structural controls on orogenic gold deposits an integrated physical process based hypothesis and practical targeting implications the application of deposit scale structural targeting to orogenic gold deposits is primarily concerned with locating volumes of higher grade mineralisation ore shoots within a broader host structural system although empirical correlations between structural heterogeneities and ore shoots have long been noted and certain key physical processes elucidated no integrated system based synthesis has been previously published however linking the existing understanding with important recent advances in understanding of the dynamic physical processes of emplacement of hydrothermal ore systems now provides the opportunity to develop such a synthesis the most important of these new concepts includes the idea that deposits form in vertically extensive conduit systems linking source to sink and the related concept of the phenomena of injection driven swarm behaviour integration of all of these elements leads to the proposal of a coherent physical process based model for the structural emplacement of orogenic gold systems and the local formation of gold enriched volumes this model considers that an orogenic gold deposit must be viewed from the perspective of three different geometric elements each with a particular meaning in terms of physical process and which typically relate to different scales these elements include a the integrated swarm volume isv which is the rock volume which bounds the limits of the mineralisation hosting structural system and is effectively the geological entity that defines a deposit b fluid pathways which are 3d pathways through the isv where dynamic ore fluid emplacement has been repeatedly strongly focused and c ore shoots which are localised volumes of high grade gold deposition that are interpreted to represent volumes of anomalous dilation or second order valve sites within host fluid pathways consideration of this new framework suggests significant potential to improve the effectiveness of structural targeting at the deposit scale in particular ore shoots are only expected within fluid pathways and equivalent dilatant zones outside such pathways are predicted to be barren a key theme is that the rheological architecture of the host rock mass is the most important predictive input to targeting both at the scale of emplacement of the ore hosting conduit system and the scale of localised ore shoots

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part i geologic relations and formation of the vein system the sigma deposit consists of an archean gold bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes the veins cut all rock types and are composed chiefly of quartz and tourmaline with minor amounts of carbonates pyrite chlorite scheelite and free gold they were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes the veins were not significantly affected by any subsequent metamorphism or tectonism two principal types of veins occur in the mine subvertical veins and subhorizontal veins they are contemporaneous and cogenetic subvertical veins occur within steeply dipping east west trending ductile shear zones along which reverse subvertical displacements have taken place the veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears subhorizontal veins occupy extensional fractures which developed between ductile shear zones preferentially in the more competent host rocks these veins were formed by one or several episodes of open space filling geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones a structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation the z axis trending north south and horizontal the y axis plunging 10 degrees west and the x axis plunging 80 degrees east the deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north south trending compression postdating regional deformation

kilometre scale structural setting of ore shoots in the frasers gold deposit macraes mine new zealand abstract the hyde macraes shear zone in southern new zealand contains the circa 10 million ounce macraes gold deposit one of the larger phanerozoic orogenic gold deposits discovered to date globally approximately 50 of this 10 million ounce resource is hosted by 5 major ore shoots up to 400 m wide and 1500 m long in the frasers area at the southern end of the mine higher grade 1 5 g t au ore shoots are located along and immediately below the hangingwall shear the principal strand of the hyde macraes shear zone at the frasers deposit they typically trend parallel to the intersection of the shear and foliation in the underlying schist commonly where the foliation dips more steeply that the overlying hangingwall shear especially thick zones of higher grade mineralised rock are located between the hangingwall shear and underlying second order splay shears whose position correlates with minor right hand bends in the strike of the overlying hangingwall shear lower grade

impact of channel like erosion patterns on the frequency magnitude distribution of earthquakes reactive flow at depth either related to underground activities like enhancement of hydrocarbon recovery and co2 storage or to natural flow like in hydrothermal zones can alter fractures topography which might in turn change their seismic responses depending on the flow and reaction rates instability of the dissolution front can lead to a wormhole like pronounced erosion pattern in a fractal structure of rupture process we question how the perturbation related to well spaced long channels alters rupture propagation initiated on a weak plane and eventually the statistical feature of rupture appearance in frequency magnitude distribution fmd contrary to intuition a spatially uniform dissolution is not the most remarkable case since it affects all the events proportionally to their sizes leading to a downward translation of fmd the slope of fmd b value remains unchanged the parameter space study shows that the increase of b value of 0 08 is statistically significant for optimum characteristics of the erosion pattern with spacing to length ratio of the order of 1 40 large magnitude events are more significantly affected leading to an imbalanced distribution in the magnitude bins of the fmd the larger the spacing the lower the channel s influence besides a spatial analysis shows that the local seismicity anomaly concentrates in a limited zone around the channels this opens perspective for detecting these eroded regions through high resolution imaging surveys

predictive distribution of fault fill and extensional veins example from the sigma gold mine abitibi subprovince canada this paper presents the results of a geometric and structural study aimed at developing a predictive model to account for the distribution of gold bearing veins at the sigma gold deposit abitibi subprovince canada results indicate that third order shear zones define a well organized system comprising two sets of parallel striking moderately to steeply south dipping curviplanar structures intersecting at constant 15 to 25 angles fault fill veins are better developed within the curved segments of the shear zones that represent large zones of enhanced permeability most extensional veins were formed later as tension gashes mainly distributed along conjugate corridors of low reverse faults typical of andersonian type faulting

bridge structures as sense of displacement criteria in brittle fault zones abstract fault zones often occur as arrays of en echelon fractures there are two main types of en echelon patterns involving right or left stepping fractures in such arrays first generation fractures define compressive or tensile bridges the structural features of such natural bridges at any scale can be used as criteria to determine the sense of displacement small compressive bridges provide criteria based on the increase in linear density of solution seams related to the degree of asymmetry of the fault trace and to the sequence of formation of the different types of en echelon fractures the criteria deduced from large compressive bridges involve upward displacement of the material by folds and hill or mountain ranges asymmetry of the fault profile and relative chronology of the segments tensile bridges yield criteria based upon vein fillings asymmetry and relative chronolog in large scale structures some pull apart basins are shown to be bounded by normal faults parallel to but outside the strike slip segments from a comparison of analytical models with well known large scale fault zones a new criterion can be proposed in fault zones formed through coalescence of en echelon fractures the bridging segments are wide if facing leeward of the bulk movement sense and narrow if forward facing

ore controls in the charters towers goldfield ne australia constraints from geological geophysical and numerical analyses the approach taken in this paper namely synthesising a wealth of previous information with new data and a genetic model in combination with integrated numerical analyses led to new insights into the geological controls on the localisation of auriferous veins and residual prospectivity of the charters towers goldfield ne australia the method also has implications for the assessment of other mature goldfields worldwide despite a number of different ore controls having operated within the charters towers goldfield the controlling factors can be linked to a single genetic model for orogenic granitoid hosted lode gold mineralisation in a brittle deformation regime d4 of ne sw to nne ssw shortening under conditions of supralithostatic fluid pressure and low stress difference spatial autocorrelation results suggest district scale alignment of the auriferous veins parallel to and overlapping with the ese wnw to e w striking charters towers ravenswood lineament a major crustal boundary in the basement to the ravenswood batholith at the camp scale auriferous veins have abundance and proximity relationships with nw se nnw sse ne sw and ene wsw oriented lineaments suggesting that structures that controlled gold deposition in one camp did not necessarily control mineralisation in other camps fractal dimensions obtained with the box counting method range from 1 02 to 1 10 whereas veins in the charters towers city camp are characterised by a significantly higher fractal dimension of 1 28 this discrepancy may be taken to imply that most or all outcropping and near surface deposits within the charters towers city camp have been found and that new discoveries are more likely to occur at greater levels of depth or outside the boundaries of this camp the new understanding has implications for the assessment of the residual prospectivity of the charters towers goldfield where large areas of prospective rock types and structures e g approximately 40 of the charters towers ravenswood lineament are hidden under cover this parameter space was inaccessible to the historic prospectors and has received relatively little attention from recent explorers the following steps are suggested for the development of a targeting strategy for lode gold exploration in areas of the goldfield under cover 1 identify from geological and geophysical data the ene wsw 15 and nnw sse 15 striking structures and geological boundaries within a 20 km wide corridor parallel to and centred upon the charters towers ravenswood lineament the potential control on gold deposit distribution at the regional to district scale 2 interpret from geological and geophysical data the distribution of pre middle devonian granitoids within these areas that are the preferred host rocks of the payable gold deposits 3 deduce from geophysical data the ene wsw 15 and nnw sse 15 striking structures that cut or bound the intrusions identified in step 2 4 locate segments along the structures identified in step 3 that deviate most from the geometry of a straight line e g potential bends or splays and or intersect other structures or geological contacts or both and 5 define and rank potential targets within the prospective areas identified in step 4 and systematically test the best ones

melange and sediment hosted gold bearing quartz veins hodgkinson gold field queensland australia gold bearing quartz veins in the hodgkinson gold field are hosted in subgreenschist facies siluro devonian sandstone conglomerate carbonaceous shale clast in matrix rock chert and spilite mineralization occurs in brittle to brittle ductile shear zones which crosscut two previous deformations and melange vein systems are isolated in distinct structural and lithologic domains by postmineralization faulting scheelite quartz veins may locally overprint the gold quartz mineralization but stibnite gold quartz mineralization is restricted to separate domains the strongest mineralized areas define elongate elliptical ore shoots which plunge steeply down or across the dip of the host shear zones melange hosted lodes are parallel in strike but have opposite dips to the melange cleavage sandstone hosted shear zones lie within reactivated f 2 axial planes or are refracted through bedding planes splaying fissure intersections changes in attitude and lithologic contacts are the main ore shoot controls the gold quartz vein textural types are ribbon buck breccia assimilation textured and comb quartz multiple injection of quartz and multiple shear zone movement have produced complex zones of quartz separated by seams of gouge arsenopyrite galena sphalerite pyrite pyrrhotite and gold 600 840 fine constitute 5 percent of the ore shoots and occur as millimeter scale clusters in quartz fluid inclusions in quartz are liquid and liquid vapor types and give salinity ranges of 3 to 11 equiv wt percent nacl with a slight salinity decrease through time co 2 and other gases are not abundant homogenization temperatures of 170 degrees to 305 degrees c have been pressure corrected to 270 degrees to 355 degrees c based on estimated formation pressure of about 1 kbar measured and calculated stable isotope compositions of the fluid depositing gold quartz are delta 18 o 10 or 2 per mil delta d 100 or 20 per mil although meteoric water contributions cannot be discounted geologic characteristics isotopic data quartz types sulfide mineralogy and ore controls favor gold deposition from an upwardly migrating homogeneous metamorphic or distal magmatic fluid depletion of d is attributed to exchange reactions involving organic matter in the metasediments and reduced gas species rather than introduction of meteoric water gold mineralization accompanied brittle tectonism which was contemporaneous with regional plutonism and took place late in the evolution of the melange

fluid flow in synthetic rough fractures and application to the hachimantai geothermal hot dry rock test site fracture profiles from the hachimantai geothermal hot dry rock hdr test site in northern japan have been measured and analyzed to characterise their geometrical properties these properties have been used to create a population of numerical synthetic fractures that were tuned to imitate all the geometric and statistical properties of the natural fracture described in a companion paper such fractures have been used as input boundary conditions in three types of modeling 1 simple elastic normal closure relating the aperture of the fracture to applied normal load 2 hagen poiseuille calculations of fluid transmissivity in the fracture as a function of normal load fracture fluid pressure and temperature 3 two dimensional flow modeling within the rough walled fracture using reynolds equation the modeled closure of these fractures provided a realistic relationship between normal and fracture fluid pressure and aperture when this pressure aperture relationship was combined with a hagen poiseuille approach to calculating fluid transmissivity in the fracture we obtained results which when compared with data from field transmissivity tests at the hachimantai site showed that the functional dependence of fluid transmissivity with fracture fluid pressure was well modeled but overestimated by a factor of about 2 reynolds equation flow modeling was carried out in the synthetic fracture to ascertain the extent to which the hagen poiseuille law was overestimating the transmissivity due to the rough surfaces affecting the fluid flow in the fracture when the calculations were corrected for this effect the overestimation in the fluid transmissivity was reduced considerably

structural controls on the distribution of gold bearing quartz veins in the arltunga gold field northern territory australia the arltunga gold field in central australia is a carboniferous epigenetic vein type gold deposit which is hosted by quartzite quartzofeldspathic gneiss calc silicate and mica schist it is situated within a large ductile shear zone that forms the floor thrust for a series of nappes the arltunga nappe complex gold and other minerals were deposited within quartz veins by fluids that were channeled along the shear zone during the waning stages of greenschist facies metamorphism a geometric and genetic relationship existed between the evolving structure of the cooling shear zone and progressively emplaced quartz veins with associated gold and sulfide mineralization southward thrusting was inhibited late in the history of the shear zone and subsequent strain was partitioned along north verging kink zones the kinked anisotropy occurred at high angles to the kink boundary within the shortening field of the regionally distributed strain pattern this resulted in the development of simple kink geometries associated with minor buckling which rotated the laminated rock along the kink zones into the extensional field the accompanying dilatancy caused the deposition of extensional gold barren quartz veins along the kink zones that can be divided into two types en echelon arrays of dilational veins that formed as tension gashes in relatively incompetent mica schists and tensional veins parallel to the kink zone boundaries with progressive deformation and waning metamorphic temperatures strain was further localized within the kink zones as an anastomosing system of narrow steep south dipping reverse faults these faults reflect the transition from ductile to brittle structures and mainly occur in the more competent rock types such as the quartzite the mica poor quartzofeldspathic gneiss and the extensional quartz veins where the fracture system transects these lithologies extensive brecciation caused dilatant zones that host deposits of gold together with quartz pyrite chalcopyrite calcite and minor silver many of the earlier extensional veins that contained vugs filled in with comb quartz became mineralized the preferred occurrence of gold in the monomineralic quartzites and the conspicuous lack of wall rock alteration in all other lithologies suggests only limited control of the wall rock on gold deposition general retrograde metamorphic conditions in particular the fall in ambient temperatures probably forced the gold out of solution in the same temperature range in which brittle structures were initiated as a result of the irregular distribution of the anastomosing fault pattern and competent units within the preexisting ductile shear gold mineralization is highly irregular and discontinuous making exploration of the arltunga gold field difficult

x ray tomography characterization of fracture surfaces during dissolution 1 the changes of fracture surfaces geometry and extend are studied using x ray tomography during aperture increase due to co2 rich fluid percolation dissolution experiments were conducted on two micritic rock samples one pure calcite end member and one with typical composition for marine carbonates 85 calcite high resolution digital images of the fracture geometry allow quantifying the surface properties changes over four spatial scales with a resolution of 4 91 m fracture surfaces are self affine with an initial dimension of 2 5 dissolution of the pure calcite sample is clearly a process of homogeneous chemical erosion of the surface elevation fractal dimension and specific surface remains constant 1 5 times the planar surface conversely for the 85 calcite sample initial topographic surfaces of the fracture walls evolve rapidly toward non topographic interfaces displaying overhangs due the preferential dissolution of the carbonate grains in this case the conventional definition of the effective aperture must be revisited such structures can only be assessed from 3d observations as dissolution progresses the specific surface increases strongly more than 5 times the planar surface and probably faster than the reactive surface

carbon dioxide injection and resultant alteration of the weber sandstone rangely field colorado 1 geologic interpretations made during the current enhanced oil recovery project at rangely field rio blanco county colorado have shown the relationships between alteration of reservoir mineralogy and texture changes in produced water composition and increased problems with oil production the bottom hole ph of weber brine has decreased from approximately 7 5 to 4 5 since the initiation of co 2 injection into the pennsylvania permian weber sandstone in the central portion of the field much of the weber is partially cemented with carbonate minerals including ferroan calcite and ferroan dolomite with authigenic clay minerals predominately illite and mixed layer smectite illite also present these minerals are affected by the lowered ph of the co 2 flood changes in the chemistry of produced water are associated with alteration of reservoir mineralogy the co 2 flood has caused substantial increases in the concentrations of iron calcium and magnesium in the produced brine from the dissolution of carbonate cements and perhaps authigenic clays and detrital feldspars produced water from some wells has experienced large changes in barium and sulfate concentrations an increase in bromide concentration indicates that previously segregated portions of the reservoir which contained very saline original formation brine have been accessed by the carbonated water hypotheses based on the water chemistry changes were confirmed in core flood experiments these experiments also indicated that there was no net change in permeability following co 2 injection

cretaceous eocene sedimentation tectonism and biofacies in the bengal basin india abstract the bengal basin located in the eastern part of the indian subcontinent is one of the thickest sedimentary basins in the world the basin started downwarping in the early cretaceous and the sedimentation has since been continuous from an initial transgressive marine phase lasting almost up to end of the eocene the basin has evolved through a subsequent regressive marine and a later continental deltaic phase in the absence of detailed information available from the deeper basinal parts an analysis of the sedimentary and tectonic history and biofacies of the regions forming the western and northwestern shelves of the bengal basin pertaining to the initial phase of its evolution is attempted this is achieved through the synthesis of all the geological information available on the basis of recent exploration and research conducted on the sequences developed in west bengal meghalaya mikir hills and assam india the subsidence of the bengal basin was the result of differential adjustments of the crystalline basement along certain well established trends and was initiated soon after the gondwana tectonism sedimentation and volcanicity the earlier sediments were not typically marine but were more paralic to estuarine in nature the entire bengal basin except the area now comprising assam came under the influence of the first regional marine transgression during the late campanian maestrichtian the coarse clastic sediments were deposited under shallow coastal marine conditions a more widespread marine transgression took place during the early middle eocene which covered almost the entire shelf area including assam during this transgression the sediments belonging to the orthoquartzite carbonate association were formed in the shelf the sea started receding in the late eocene during which the entire bengal shelf came under the influence of heavy unloading of fine clastics the top of this fine clastic shaly sequence is an important sedimentary datum in the bengal shelf which marks the end of the first phase in the evolutionary history of the basin

measurement and analysis of dissolution patterns in rock fractures 1 nuclear magnetic resonance imaging nmri is applied to noninvasively measure flow and dissolution patterns in natural rough walled water saturated halite fractures three dimensional images of water density and flow velocity acquired with nmri allow quantification of the developing fracture morphology and flow patterns the flow patterns are correlated strongly to the local apertures and the large scale wall roughness the correlations of the dissolution patterns to the fracture morphology flow patterns and mineralogical composition of the rock matrix are a function of the overall dimensionless damkohler number at high damkohler numbers the dissolution patterns are dominated by the flow structure in addition at high damkohler numbers buoyancy stratified flow becomes important in such cases the dissolution patterns also depend on the orientation and elevation of the fracture walls resulting in preferential upward dissolution at low damkohler numbers the dissolution patterns depend mainly on the mineralogical composition of the rock matrix these findings suggest that coupled flow and dissolution processes are much more complex and unpredictable than commonly assumed even under simplified conditions

evolution of planar fractures in limestone the role of flow rate mineral heterogeneity and local transport processes abstract the processes that affect the evolution of fracture geometry and reactive transport in limestone fractures were evaluated using an integrated experimental and modeling approach flow through experiments were performed to investigate the alteration of three artificially made planar limestone fractures which were exposed to hcl acidified water flow ph 3 8 0 1 at three different rates reactive infiltration instability developed in the fractures because of small fracture aperture variations and naturally occurring heterogeneity in the rock i e the presence of clay spots of higher porosity the dissolution regime changed from conical to dominant and ramified wormhole with increasing flow rate i e peclet number maps of fracture aperture obtained from x ray micro tomography imaging together with measurements of permeability and analyses of fluid chemistry provided experimental constraints for modeling numerical simulations were performed using a 2 5d continuum reactive transport model to examine the development of flow patterns and the reactivity and permeability evolution over time and to interrogate the impacts of fluid reactivity mineral heterogeneity and diffusive boundary layer the modeling results showed that fracture alteration is very sensitive to initial fluid reactivity and mineralogical heterogeneity in addition to flow rate the changes of local transport processes during fracture opening also affected the reactivity of the fractured samples

evaluation of the impact of co2 co contaminant gas aqueous fluid and reservoir rock interactions on the geologic sequestration of co2 lowering the costs of front end processes e g separation in the geologic sequestration of co2 can dramatically lower the overall costs one possible approach is to sequester less pure co2 waste streams that are less expensive or require less energy to separate from flue gas or a coal gasification process etc the objective of this research is to evaluate the impacts of co2 itself as well as an impure co2 waste stream on geologic sequestration using reaction progress models reactive transport simulators and analogous reactive transport experiments run in a plug flow reactor specifically we are investigating the potential for co injecting the sox nox and h2s present in coal fired waste streams along with the co2 we present here the results of reactive transport simulations made to investigate the long term impact of dissolved co2 h2s and so2 on carbon sequestration in the frio fm tx the results suggest that addition of relatively large amounts of h2s to a co2 injection should not adversely impact injectivity or sequestration compared to the injection of co2 alone the co injection of so2 however could produce significantly different results if conditions are such that so2 can oxidize to sulfate in the long term the simulations suggest that carbonate mineral formation can sequester a significant fraction of the injected carbon through the formation of calcite magnesite and dawsonite

the detrital record of orogenesis a review of approaches and techniques used in the himalayan sedimentary basins the sediment archive of material eroded from an active tectonic region and stored in adjacent basins can provide a valuable record of hinterland tectonism especially when information in the source region itself is obscured by later metamorphism or removed by tectonism or erosion using the sediment record to document tectonism is a well established approach but more recently there has been a burgeoning of the number of isotopic techniques which can be applied to detrital material in particular single grain analyses thus the scope for application of detrital studies to a number of different tectonic problems has widened considerably in this review the example of sediments eroded from the himalayan orogen and preserved in the suture zone basin foreland basin remnant ocean basins and deep sea fans is used to illustrate the approach techniques as diverse as petrography heavy mineral xrf and sr nd studies single grain dating by ar ar u pb and fission track methodologies and single grain sm nd and pb isotopic analyses are described the paper documents how the sediment record can be used to determine the thermal and tectonic evolution of the orogen constrain mechanisms of continental deformation exhumation rates and palaeodrainage

gold quartz veins and auriferous granite at the oriental mine alleghany district california in the vicinity of serpentinite wall rocks the three fissure veins at the oriental mine alleghany district california contain small high grade ore shoots carrying hundreds of troy ounces of gold per ton in addition minable zones of albitized wall rock granite ore contain an average of 0 2 ounces of gold per ton in the veins most native gold is associated with arsenopyrite but in the granite it occurs as microscopic inclusions within the pyrite various geothermometers and paragenetic relations suggest that vein gold deposition occurred during the end stages of mineralization at temperatures between approximately 200 degrees and 300 degrees c at pressures between 670 and 2 500 bars and at depths exceeding 2 5 km during the early stages of mineralization when most hydrothermal alteration and gold deposition in granite probably occurred temperatures were above 340 degrees c and possibly as high as 491 degrees c muscovite paragonite calcite dolomite dawsonite and albite fluid inclusion daughter minerals have been identified in situ by refractive index measurements and by x ray powder diffraction the near neutral to alkaline character of the mineralizing fluids implied by the presence of 0 34 m dissolved dawsonite and the low equivalent nacl contents of fluid inclusions suggest that gold may have been complexed with sulfur or arsenic rather than occurring as chloride complexes wall rock alteration effects include envelopes of intense carbonatization and desilication surrounding the principal gold quartz veins but also occurring along splits and fractures emanating from the main vein structures for example in granite the main alteration effects dissolution of original quartz and formation of albite occur along nearly flat lying fractures chiefly in the footwall of the oriental vein chemical analyses suggest that desilication of granite and removal of silica during ankeritization of mafic minerals in the other rock types liberated sufficient amounts of silica to form the vein quartz at the oriental mine beyond the fringes of the main alteration envelopes is an outer essentially pervasive alteration zone characterized by sericitization and chloritization which grades into greenschist facies country rocks and may predate mineralization gold in high grade ore shoots may have been precipitated by reactions involving reducing fluids derived from the wall rock serpentinite near which all high grade vein ores occur limited fluid inclusion data for ore shoot quartz however seem to be consistent with the alternative possibility of temperature controls for ore deposition

a test of the effectiveness of pore scale fluid flow simulations and constitutive equations for modelling the effects of mineral dissolution on rock permeability abstract macro scale transport properties of rocks such as permeability are bulk parameters combining the contributions of various properties only properly defined at the pore scale since pore scale processes are known to modify the rock properties it is legitimate to ask if constitutive equations based on macro scale properties e g porosity permeability formation factor etc can properly describe their effect in a previous experimental study lamy chappuis et al 2014 we found that the effect of mineral dissolution on permeability could be much higher than predicted by such semi empirical constitutive equations here we directly solve pore scale fluid flow in high resolution 2 5 m 3d models of a rock s geometry before and after mineral dissolution in order to evaluate permeability and how it is changed this methodology is limited by the resolution of the micro ct images used to define the rock geometry which leads to significant overestimates of absolute permeability but it does produce a much closer match to the change in permeability due to mineral dissolution than the constitutive equations this is possible because the dissolution features which enhance permeability are large enough to be adequately resolved and produce a significant change in permeability

influence of aperture variability on dissolutional growth of fissures in karst formations the influence of aperture variability on dissolutional growth of fissures is investigated on the basis of two dimensional numerical simulations the logarithm of the fissure aperture before dissolution begins is modeled as a gaussian stationary isotropic random field the initial phase of dissolutional growth is studied up to the time when turbulent flow first occurs at a point within the fissure the breakthrough time the breakthrough time in variable aperture fissures is smaller than that in uniform fissures and decreases as the coefficient of variation of the aperture field increases in comparing uniform and variable aperture fissures in limestone the breakthrough time with 0 1 is about a factor of 2 smaller than that in a uniform fissure the breakthrough time is reduced by about an order of magnitude with 2 0 the mechanism leading to reduced breakthrough times is the focusing of flow into preferential flow channels which are enlarged at a faster rate than the surrounding regions of slower flow dissolution channels are narrower and more tortuous as increases investigations of the influence of reaction rate reveal that the influence of aperture variability is more pronounced in rapidly dissolving rock in uniform fissures in rapidly dissolving minerals breakthrough times are very long since water becomes saturated with respect to the mineral within a short distance of the entrance to the flow path however in variable aperture fissures breakthrough occurs rapidly because of selective growth along preferential flow channels which progressively capture larger fractions of the total flow these results partly explain why conduits develop rapidly in gypsum although previous one dimensional studies suggest that conduit growth will not occur

post eocene biofacies palaeoenvironments and palaeogeography of the bengal basin india abstract the bengal basin is known for the extensive development of a thick cretaceous tertiary sedimentary sequence in the eastern part of india the basin has undergone several phases of transgressions regressions periodic uplifts and localised negative movements throughout the period of its evolution the sedimentation up to the end of the eocene was largely under the influence of a major transgressive phase which was later replaced by a regressive phase on account of the uplift and resulting marine retreat in this paper an analysis of the microfaunal distribution biofacies and sedimentary and tectonic history is made for reconstruction of the palaeoenvironmental conditions and palaeogeography of the bengal basin relating to the post eocene regressive phase of evolution during the oligocene early miocene the open marine conditions existing since the eocene were progressively modified on account of the regression into more brackish lagoonal and estuarine types a typical prograding delta started to spread over a large part of the marginal area of the basin the rocks of the barail group formed under the transitional environments of the delta are found to occur in almost all the major geo provinces of the basin the sedimentary environments and characteristics of the rocks were ultimately controlled by the configurations of the respective geo provinces the negative movements in the miocene along certain marginal areas were associated with the marine transgressive phase giving rise to the surma group this transgressive phase was however short lived and its impact was not equally felt in all areas the northeastern region was still uplifted which favoured an almost continuous existence of the deltaic phase by the end of miocene a greater part of the basin underwent a strong tectonic upheaval which was possibly the precursor of the main himalayan orogeny the intermontane depressions formed in the uplifted areas were the depositional sites for the tipam group and younger beds in plio pleistocene times the rapidly eroded debris from the highlands spread over a wide area towards the south under the influence of the river system of the ganga and brahmaputra which has transformed the original basinal areas into a large modern delta complex

applications of x ray computed tomography in the geosciences x ray computed tomography ct is a technique that allows non destructive imaging and quantification of internal features of objects it was originally developed as a medical imaging technique but it is now also becoming widely used for the study of materials in engineering and the geosciences x ray ct reveals differences in density and atomic composition and can therefore be used for the study of porosity the relative distribution of contrasting solid phases and the penetration of injected solutions as a non destructive technique it is ideally suited for monitoring of processes such as the movement of solutions and the behaviour of materials under compression because large numbers of parallel two dimensional cross sections can be obtained three dimensional representations of selected features can be created in this book various applications of x ray ct in the geosciences are illustrated by papers covering a wide range of disciplines including petrology soil science petroleum geology geomechanics and sedimentology

measurement of aperture distribution capillary pressure relative permeability and in situ saturation in a rock fracture using computed tomography scanning we develop an experimental technique that uses computed tomography ct scanning to provide high resolution measurements of aperture distribution and in situ saturation along with capillary pressure and relative permeability for the same rough walled fracture we apply this technique to an induced fracture in a cylindrical basalt core undergoing water drainage we find that the sum of the water and gas relative permeabilities is much 1 at intermediate saturations and the water relative permeability shows a sharp change over a narrow range of average water saturation in situ saturation maps show channeling of gas and significant retention of the wetting phase water the capillary pressure initially increased and then decreased with decreasing water saturation although this type of capillary behavior is atypical for unsaturated flows we find that the sizes of the gas filled apertures are consistent with the measured capillary pressure

fault zone reactivation kinematics and mechanisms the kinematics and mechanisms of fault zone reactivation are reviewed reactivation is dependent upon fault zone orientation and the existence of weak mylonites along these zones the recognition of reactivation within mylonite zones and the softening processes that first concentrate deformation into these and secondly provide a weak medium for reactivation are discussed attention is given to the darling mobile zone western australia and the redbank zone see obee white this symposium as examples of reactivated zones

the role of fluids in syntectonic mass transport and the localization of metamorphic vein type ore deposists abstract in low to medium grade metamorphic environments the mobilization of species is closely associated with the operation of solution precipitation creep processes during foliation development such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields mass transfer occurs via solute diffusion in an essentially static fluid network or by transport of dissolved species in a migrating fluid precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces grain scale microcracks and veins sink sites may be close to or distantly removed from source sites the transport of material during deformation involving solution precipitation creep is largely controlled by the mobility of metamorphic fluids in regions of high fluid mobility significant mass transfer may occur over large distances fluid flow mass transfer paths and the development of sink sites in such cases are constrained by the structurally controlled development of high fluid pressure domains and the consequent mechanical enhancement of permeability by hydraulic fracture processes and the development of grain scale dilatancy these processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions a proportion of syntectonically mobilized material is precipitated to form vein type deposits in hydraulic fracture arrays which develop in high fluid pressure environments the microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization the formation of syntectonic gold quartz vein deposits in central victoria australia illustrates the importance of structural controls on both the large scale migration of metamorphic fluids and the development of sites suitable for the precipitation of metamorphically mobilized species

the influence of fracture geometry variation on non darcy flow in fractures under confining stresses abstract to investigate the influence of geometric characteristics of deformable rough fractures under confining stresses on the behaviors of non darcy flow four fractured sandstone specimens were used for hydraulic tests in the experiments according to the experimental results of the relationships between the hydraulic gradient and the flow rate it is demonstrated that the forchheimer s equation can offer a good description of the non darcy flow in rough fractures in addition the coefficients a and b in forchheimer s equation are sensitive to the fracture geometric characteristics and their values also increase as the confining stress rises mainly owing to the reduction of the hydraulic aperture and the heterogeneous distribution of the interconnected void areas with the confining stress rising then the surface and interior geometric properties of rough fractures were quantitatively characterized with the peak asperity height and the box counting fractal dimension d of the heterogeneous distribution of the interconnected void areas respectively furthermore an empirical relationship between the fractal dimension d and the fracture apertures was constructed according to the experimental results lastly a quantitative model was proposed to represent the relationship between the fracture geometric characteristics and the non darcy coefficient this model was further used to link the non linear coefficient b of forchheimer s equation and the critical reynold number r e c with the fracture geometric characteristics the proposed models were validated by the experimental data and would be helpful to characterize the non darcy flow behavior in rough fractures under various confining stresses

two phase flow properties of a sandstone rock for the co2 water system core flooding experiments and focus on impacts of mineralogical changes the two phase flow characterization co2 water of a triassic sandstone core from the paris basin france is reported in this paper absolute properties porosity and water permeability capillary pressure relative permeability with hysteresis between drainage and imbibition and residual trapping capacities have been assessed at 9 mpa pore pressure and 28 c co2 in liquid state using a single core flooding apparatus associated with magnetic resonance imaging different methodologies have been followed to obtain a data set of flow properties to be upscaled and used in large scale co2 geological storage evolution modeling tools the measurements are consistent with the properties of well sorted water wet porous systems as the mineralogical investigations showed a nonnegligible proportion of carbonates in the core the experimental protocol was designed to observe potential impacts on flow properties of mineralogical changes the magnetic resonance scanning and mineralogical observations indicate mineral dissolution during the experimental campaign and the core flooding results show an increase in porosity and water absolute permeability the changes in two phase flow properties appear coherent with the pore structure modifications induced by the carbonates dissolution but the changes in relative permeability could also be explained by a potential increase of the water wet character of the core further investigations on the impacts of mineral changes are required with other reactive formation rocks especially carbonate rich ones because the implications can be significant both for the validity of laboratory measurements and for the outcomes of in situ operations modeling

fault fracture density and mineralization a contouring method for targeting in gold exploration abstract a widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore forming process in archaean greenstone hosted deposits high density fracturing was accompanied by enhanced fluid flow during fault fracture network development producing regional scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas fracture density is both increased and decreased during faulting and fault healing and fracture density accumulates over time in zones of high palaeo fluid flow localised zones where the density of fracturing is increased by deformation become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation the ora banda mining centre in western australia contains significant gold deposits that appear to demonstrate a close association between high density fracturing and gold precipitation fracture density in the ora banda mines was enhanced by fault fault intersections fault contact intersections and changes in fault geometry the mine scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales hence these relationships may be used in targeting for gold exploration contouring the density of fracturing in a region provides a semi quantitative way to rank areas for exploration and uses data from mapping drilling and high quality geophysical data as a basis for analyses fracture density contouring is complementary to other prospectivity analysis methods

3d analysis of geometry and flow changes in a limestone fracture during dissolution summary the effects of reactive transport on fracture geometry and fluid flow were investigated through an integrated experimental and modelling approach a fractured limestone sample 90 calcite was injected with an acidic co2 rich solution over a period of 55 h to induce carbonate dissolution the changes in fracture geometry and related parameters are reported for six data sets obtained from synchrotron x ray micro tomography experiments a series of algorithms was used to extract the aperture and fracture walls from 3d images and allowed quantification of the geometry changes with an optical resolution of 4 91 m in addition measurement of fluid chemistry hydraulic tests and computation of navier stokes flow constrained the characterisation of the dissolution process the effects of reactive transport on fracture geometry and fluid flow were then discussed the presence of silicates in the rocks led to heterogeneous dissolution at the micro scale despite dissolution appearing to be quite homogeneous at sample scale no formation of preferential flow pathways was noticed although heterogeneous dissolution at the micro scale led to fracture walls and aperture decorrelation and to modification of the flow velocity profiles in the fracture

regional dome evolution and its control on ore grade distribution insights from 3d implicit modelling of the navachab gold deposit namibia abstract we introduce a novel approach to analyse and assess the structural framework of ore deposits that fully integrates 3d implicit modelling in data rich environments with field observations we apply this approach to the early palaeozoic navachab gold deposit which is located in the damara orogenic belt namibia compared to traditional modelling methods 3d implicit modelling reduces user based modelling bias by generating open or closed surfaces from geochemical lithological or structural data without manual digitisation and linkage of sections or level plans instead a mathematically defined spatial interpolation is used to generate 3d models that show trends and patterns that are embedded in large drillhole datasets in our 3d implicit model of the navachab gold deposit distinctive high grade mineralisation trends were identified and directly related to structures observed in the field the 3d implicit model and field data suggest that auriferous semi massive sulphide ore shoots formed near the inflection line of the steep limb of a regional scale dome where shear strain reached peak values during fold amplification this setting generated efficient conduits and traps for hydrothermal fluids and associated mineralisation that led to the formation of the main ore shoots in the deposit both bedding parallel and highly discordant sets of auriferous quartz sulphide veins are interpreted to have formed during the later lock up stage of the regional scale dome additionally pegmatite dykes crosscut and remobilise gold mineralisation at the deposit scale and appear to be related to a younger joint set we propose that kilometre scale active folding is an important deformation mechanism that influences the spatial distribution and orientation of mineralisation in ore deposits by forming structures traps and pathways for fluids at different preferred sites and orientations we also propose that areas that experience high shear strain located along the inflection lines of folds can act as preferred sites for syn deformational hydrothermal mineralisation and should be targeted for regional scale exploration in fold and thrust belts our research also suggests that examination of existing drillhole datasets using 3d implicit modelling is a powerful tool for spatial analysis of mineralisation patterns when combined with fieldwork this approach has the potential to improve structural understanding of a variety of ore deposits

nomenclature concepts and classification of oreshoots in vein deposits abstract oreshoots are discrete hypogene masses usually hosted within a planar channel surface lode or conduit which may be either a shear zone fissure fault zone or lithologic bed or unit such as a contact oreshoots are characterized by breadth strike 1000 m and dip and plunge 100 500 m lengths and have higher metal contents than the adjacent parts of the host conduit the mass of most oreshoots ranges between 1 10 6 and 2 10 4 tonnes there is a tendency for oreshoots to be thicker and richer in the center rather than to have uniform grade distributions the thickness of the oreshoots may be between 0 25 and 1 75 m in shear zone hosted deposits to up to 60 m in replacement deposits several conduits may connect to form vein systems vein systems have common fluid sources which result in general homogeneity of alteration mineralization types and oreshoot control and therefore commonly share the same plumbing system the internal constituents usually reflect unique episodes relating to ore formation the main intern constituents in oreshoots are mineralization gangue and alteration these constituents usually mix with each other in complex patterns the relationships between which may be used to interpret the processes of oreshoot formation the term ground preparation represents the effect of various events in the geologic history of an ore district or oreshoot area that have assisted in enhancing the rocks so that oreshoots can preferentially form in certain areas or geometries several types of ground preparation can be recognized 1 sequential deformation that produces a grain in the rock 2 severe faulting and jointing which augments permeability and areas where ore minerals can precipitate and 3 interplay between ore fluid and deformation to produce an oreshoot controls of oreshoot location and shape are usually due to dilatant zones caused by changes in attitude splays lithologic contacts and intersections in addition conceptual parameters such as district fabric magic distances and stacking are also used to describe the geometry of oreshoots controls in vein systems and the location and geometry of oreshoots within vein systems can be predicted by a number of qualitative concepts such as internal and external plunges district plunge district stacking conduit classification gradients and warps these concepts have a practical and empirical application in most districts where they are useful in the exploration for ore but are of such broad and general application that they can rarely be explained definitively

displacement gradients on fault surfaces abstract the maximum displacements and the dimensions of single faults are systematically related and the variation in displacement from a maximum at the centre of a fault to zero at an elliptical tip line loop is described by a simple theoretically derived expression these relationships are used to examine the theoretical ranges and distributions of rates of change of displacement on fault surfaces the dimensions and maximum displacement of a fault can be estimated from a limited number of displacement gradient measurements estimates are further constrained by a knowledge of the effective shear modulus of the rocks containing a fault an understanding of displacement gradients can be applied to problems commonly encountered in mining operations and in the interpretation and use of seismic reflection data in addition to field problems a comparison of theoretical displacement gradients with measured displacement gradients from british coalfield faults shows that i fault surface ellipticities are between 1 5 and 2 5 and ii gross displacement gradients expressed as the ratio of maximum displacement long axis radius of the fault ellipse range from 0 02 to 0 002 the measured displacement gradients indicate that the shear moduli of the faulted rocks range from 3 to 11 gpa which is within the known range for coal measure rocks displacement gradients along an array of fault segments are similar to those of single faults for analysis of displacement gradients it is necessary to sum the discontinuous and continuous components of displacement in extreme cases all the displacement may be accommodated by continuous deformation

detrital zircon u pb age and hf isotopic composition from foreland sediments of the assam basin ne india constraints on sediment provenance and tectonics of the eastern himalaya abstract synorogenic palaeogene neogene sediments of the assam foreland basin were derived by erosion of adjacent crustal and orogenic sources following the greater india eurasia collision since 55 ma to constrain source sediment influx and its relation to himalayan tectonics from pre to post collision time detrital zircon u pb geochronology and their hf isotopic compositions were carried out the varying detrital zircon spectral patterns analyzed from the paleogene jaintia barail and neogene surma and tipam groups with sediment petrography track source sediment derived from cratonic india gangdese and eastern transhimalayan batholiths and the eastern himalaya these sources are tested against cenozoic paleopositions proposed for the northeastward motion of the indian plate precollisional cratonic detritus to middle to late eocene sylhet formation shifted to tethyan himalaya and arc sources of the gangdese and eastern transhimalayan batholiths to late eocene kopili and barail formations consistent with the proposed paleoposition proximal to the indus yarlung suture this sylhet kopili formation transition within the jaintia group reflects one of the earliest himalayan hinterland exhumation stages during the late eocene major shift in provenance to higher himalayan crystalline and arc detritus is recorded from the surma group constraining mid miocene himalayan tectonic exhumation from the eastern himalaya late miocene tipam group preserves sediment of higher himalayan crystalline detritus ophiolite and likely lesser himalayan rocks

sedimentation and basin fill history of the neogene clastic succession exposed in the southeastern fold belt of the bengal basin bangladesh a high resolution sequence stratigraphic approach abstract the tertiary basin fill history of the bengal basin suffers from oversimplification the interpretation of the sedimentary history of the basin should be consistent with the evolution of its three geo tectonic provinces namely western northeastern and eastern each province has its own basin generation and sediment fill history related mainly to the indo burmese and subordinately to the indo tibetan plate convergence this paper is mainly concerned with facies and facies sequence analysis of the neogene clastic succession within the subduction related active margin setting oblique convergence in the southeastern fold belt of the bengal basin detailed fieldwork was carried out in the sitapahar anticline of the rangamati area and the mirinja anticline of the lama area the study shows that the exposed neogene succession represents an overall basinward progradation from deep marine through shallow marine to continental fluvial environments based on regionally correlatable erosion surfaces the entire succession 3000 m thick has been grouped into three composite sequences c b and a from oldest to youngest composite sequence c begins with deep water base of slope clastics overlain by thick slope mud that passes upward into shallow marine and nearshore clastics composite sequence b characteristically depicts tide dominated open marine to coastal depositional systems with evidence of cyclic marine regression and transgression repetitive occurrence of incised channel tidal inlet tidal ridge shoal tidal flat and other tidal deposits is separated by shelfal mudstone most of the sandbodies contain a full spectrum of tide generated structures e g herringbone cross bedding bundle structure mud couplet bipolar cross lamination with reactivation surfaces tidal bedding storm activities appear to have played a subordinate role in the mid and inner shelf region rizocorallium rosselia planolites and zoophycos are the dominant ichnofacies within the shelfal mudstone this paralic sedimentation of neogene succession in the study area can serve as a good point of reference for tide dominated regressive shelf depositional systems the top of the composite sequence b is marked by a pronounced erosion surface indicating the final phase of marine regression followed by the gradual establishment of continental fluvial depositional systems represented by composite sequence a in this composite sequence stacked channel bars of low sinuosity braided rivers gradually pass upsequence into high sinuosity meandering river deposits a sequence stratigraphic approach has been adopted to interpret the basin fill history with respect to relative sea level changes and to subdivide the rock record into several sequences and units systems tracts and parasequences based on identified bounding discontinuities such as transgressive erosion surface tes regressive erosion surface res marine flooding surface mfs and incised valley floor ivf this approach provides new insight for both exploration and exploitation strategy for hydrocarbon plays that may prove vital to the oil companies engaged in exploration activities in the bengal basin it is strongly recommended here that the traditional lithostratigraphic classification of this part of the basin which is based on the assam stratigraphy be abandoned or at least revised a tentative allostratigraphic scheme is presented and it is suggested that to formalize the scheme further study both surface and subsurface is needed

investigation of porosity and permeability effects from microstructure changes during limestone dissolution we studied experimentally the dissolution of a porous limestone core during co2 enriched water injection we measured the changes in porosity and permeability arising from modifications of the pore network geometry and the fluid rock interface a methodology based on periodic x ray microtomography imaging was implemented to record the evolution of the time and scale dependent microstructures with a spatial resolution of 4 91 m two processes were successively involved in the rapid permeability increase of the sample as documented from microscale to core scale measurements

geology and depositional history of cenozoic sediments of the bengal basin of bangladesh abstract the bengal basin lies on the eastern side of the indian subcontinent and occupies the northern part of the bengal geosyncline sedimentation started with the breakup of gondwanaland cretaceous fluvial and deltaic sedimentation was slowly taken over by open marine and deep sea fan sedimentation by the mid eocene the basin was witnessing the maximum marine transgression and most of the stable shelf was in carbonate regime as part of the extended tethyan carbonate shelf the deeper part of the basin was dominated by deep sea fan sedimentation fed by clastic turbidites from the northeast with limited carbonate turbidites from the shelf the collision of the indian plate with the tibetan plate and with the burmese plate in the miocene resulted in a rapid switch in sedimentation pattern in the bengal basin the basin structure and sedimentation were both strongly influenced by the collision pattern of the plates and by the uplift of the himalayas with the miocene collision of the indian plate there was a change from flysch to molasse sedimentation the present day basin configuration with the ganges bramaputra delta on the north and the bengal deep sea fan on the south was established in the pliocene the stratigraphic and tectonic framework of the bengal basin suggest that the deposition of hydrocarbon source rocks was associated with the flysch sedimentation whereas formation of traps and deposition of most of the reservoir rocks in the basin was associated with mid pliocene tectonic movements and with the molasse sedimentation the pre mid miocene sediments now deeply buried in the deeper part of the bengal basin and in the eastern folded belt are the major source of gas of the eastern folded belt of the basin

the paleogene record of himalayan erosion bengal basin bangladesh a knowledge of himalayan erosion history is critical to understanding crustal deformation processes and the proposed link between the orogen s erosion and changes in both global climate and ocean geochemistry the most commonly quoted age of india asia collision is 50 ma yet the record of paleogene himalayan erosion is scant either absent or of low age resolution we apply biostratigraphic petrographic geochemical isotopic and seismic techniques to paleogene rocks of the bengal basin bangladesh of previously disputed age and provenance our data show that the first major input of sands into the basin in the 1 km thick deltaic barail formation occurred at 38 ma our biostratigraphic and isotopic mineral ages date the barail formation as spanning late eocene to early miocene and the provenance data are consistent with its derivation from the himalaya but inconsistent with indian cratonic or burman margin sources detrital mineral lag times show that exhumation of the orogen was rapid by 38 ma the identification of sediments shed from the rapidly exhuming southern flanks of the eastern central himalaya at 38 ma provides a well dated accessible sediment record 17 myr older than the previously described 21 ma sediments in the foreland basin in nepal discovery of himalayan detritus in the bengal basin from 38 ma 1 resolves the puzzling discrepancy between the lack of erosional evidence for paleogene crustal thickening that is recorded in the hinterland 2 invalidates those previously proposed evidences of diachronous collision which were based on the tenet that himalayan derived sediments were deposited earlier in the west than the east 3 enables models of himalayan exhumation e g by mid crustal channel flow to be revised to reflect vigorous erosion and rapid exhumation by 38 ma and 4 provides evidence that rapid erosion in the himalaya was coincident with the marked rise in marine 87sr 86sr values since 40 ma whether 38 ma represents the actual initial onset of vigorous erosion from the southern flanks of the east central himalaya or whether older material was deposited elsewhere remains an open question

nondestructive high resolution visualization and measurement of anisotropic effective porosity in complex lithologies using high resolution x ray computed tomography abstract new methods are presented that allow high resolution x ray computed tomography hrxct to be utilized for imaging and measuring the effective porosity field of mineralogically complex specimens at resolutions in the 5 100 m range these methods extend previous established approaches by allowing sample displacement between successive imaging runs and eliminating the need for a highly attenuating tracer a highly detailed map of partial porosity can be generated by scanning a sample in a dry state and infiltrated by distilled water and performing a three dimensional alignment of the two data sets this map can then be used to obtain a number of quantitative analyses such as the frequency distribution of partial porosity and directional analysis to infer flow anisotropy it can also be used for petrographic study of the relationship of porosity to evolving mineralogy in a dynamic system these methods are demonstrated on a series of altered volcanic rocks from a deep sea hydrothermal field

investigation of flow in water saturated rock fractures using nuclear magnetic resonance imaging nmri the application of nuclear magnetic resonance imaging nmri to the direct three dimensional measurement of flow in rough walled water saturated rock fractures is presented for the first time the study demonstrates the abilities of nmri to noninvasively measure rock water interfaces and water flow velocities in these fractures and investigates the effects of wall morphology on flow patterns inside a typical rock fracture two and three dimensional flow encoded spin echo pulse sequences were applied the stability and reproducibility of the water flow patterns were confirmed by analyzing two dimensional velocity images a variety of geometrical and hydraulic features were determined from three dimensional velocity images including the rock water interfaces the fracture aperture distribution and the critical aperture path velocity profiles and volumetric flow rates flow and stagnant regions and the critical velocity path in particular the effects of a sharp step discontinuity of the fracture walls and the applicability of the cubic law were examined as a result of the complex three dimensional geometry velocity profiles are generally parabolic but often highly asymmetric with respect to the fracture walls these asymmetric velocity profiles are clustered together with significant correlations they are not just local random phenomena however theoretical considerations indicate that the effects of the measured asymmetry on volumetric flow rates and hydraulic conductivities are insignificant in that the overall flow inside rough fractures still obeys the cubic law the features discussed in this study emphasize the strong heterogeneity and the highly three dimensional nature of the flow patterns in natural rock fractures and consequently the need for three dimensional flow analysis

a study of dissolution and water bearing characteristics of the restricted platform dolomite facies in the karst areas of guizhou china the restricted platform facies dolomites are widely distributed in the northern and northeastern guizhou china and they are the main lithology of karst groundwater and a primary target for locating drinking water resources in the province of guizhou the geological history and deposition environment of the restricted platform dolomite facies vary significantly across the province making it more difficult to evaluate the dissolution and water bearing capacity of karstic dolomites this paper explores a quantitative approach to assessing the solubility of four typical geological strata using analytical hierarchy process ahp modeling properties including cao mgo clast content porosity permeability and fracture density are investigated and used as the major criteria in the ahp analysis to render the rankings for solubility among the studied formations hydrogeological characteristics are also discussed overall the presented results demonstrate the feasibility of ahp as a modeling tool to provide a quantitative approach when a wide range of properties need to be considered the explored approach can have strong implications for the exploitation and utilization of karst groundwater in guizhou china

a correlation between the evolution of mineralizing fluids and the geomechanical development of a shear zone as illustrated by the henderson 2 mine quebec two ore zones one enclosed within a shear zone zone b and the other in a subsidiary structure zone d at onset of the shear zone subsidiary fracture patterns are developed in second order faults in the following sequence at peak strength riedel shears are formed which propagate out into the walls of the shear zones producing the zone d structure after peak strength restraint p shears are developed in the thrust attitude within the shear zone principal displacement shears d developed toward residual strength in the direction of movement the continuation of the shear displacement gives rise to schistosities within the main shear zone while the main shear zone and the p and d shears within the main shear zone were in continual movement the subsidiary zone d structure once formed retained a simple fracture pattern and moved little in comparison to the main shear zone mineralizing fluids introduced into this shear environment show a similar pattern of evolution with a wider range of homogenization and halite disappearance temperatures in the more active zone b than within zone d systematic variations of ca na and ca mg indicate migration modified journal abstract

impact of coating development on the hydraulic and transport properties in argillaceous limestone fracture 1 results are reported for an acidic water flow through experiment conducted in a fractured argillaceous limestone sample 73 carbonates the change in fracture geometry and related parameters is reported for six data sets obtained from synchrotron x ray microtomography experiments high resolution three dimensional images of the sample allowed quantification of the changes in fracture morphology at a spatial resolution of 6 m mineral mass loss and permeability changes in the sample were also determined several physico chemical phenomena were identified during the experiment initial smooth fracture surfaces evolved rapidly toward rough surfaces with uneven clay coverage due to the preferential dissolution of carbonate minerals compared to clay minerals whose dissolution rate is about 106 slower a microporous clay coating progressively developed at the fluid rock interface during heterogeneous dissolution of the fracture while the global dissolution rate of the fracture walls exponentially decreased the increase in surface roughness and the presumed reorganization of clays caused a progressive reduction in permeability during the last flow through stage a large decrease in sample permeability was attributed to the large removal of clay particles this process was responsible for a dramatic collapse of the fracture walls near the sample inlet and led to the development of preferential flow pathways the development of the clay coating also acted as a barrier to flow and mass transfer between calcite grains and bulk solution and affected transport processes within the fracture

geological geophysical and inherited tectonic imprints on the climate and contrasting coastal geomorphology of the indian peninsula abstract the indian subcontinent comprises accreted cratonic fragments that underwent subsequent rifting subduction collision processes along major mobile belts it is also a storehouse of a prolonged history of crustal evolution involving repeated episodes of magmatism volcanism sedimentation and metamorphism and the formation of intracratonic and foreland basins the geologic geomorphic evolution of the peninsula is recorded in relic faults suture zones that confine paleozoic mesozoic and cenozoic sedimentary basins and landforms in addition escarpments plateaus waterfalls deltas planation surfaces and strandlines are either aligned parallel and or formed over them asymmetric relief across the western eastern parts of the peninsula orographic effect on the precipitation pattern and climatic zones provide important constrains on the geologic and geomorphic evolution of the region though extensive continental and coastal deposition occurred during permo triassic and early cretaceous only the downwarped grabens of gondwana basins preserved the sedimentary records occurrences of extensive cenozoic deposits offshore and neogene holocene deltaic deposits along the entire east coast as opposed to the absence of comparable deltaic sequences in the west coast are linked to intense and recurrent episodes of denudation indicating first order control of basement structures over landscape evolution inheritance and continuum of basement structures over climate and landscape evolution of peninsular india can also be witnessed by the unique and diverse drainage patterns of major rivers while most of the major rivers follow a general easterly direction the landscape morphology of their catchments indicates youthful character and tectonically active nature the east flowing rivers show an asymmetry between the areal extents of drainage basin size and delta whereas the west flowing rivers are short and straight and their catchments are miniscule in comparison with the rivers flowing toward east the western ghats escarpment forms an orographic barrier and acts as the main divide between these two drainage systems there is a disparity between the quanta of sediment discharge as well in addition the eastern and western parts of india across the western ghats escarpment show significant differences in terms of climatic zones humid to per humid on the west while it is arid to subhumid on the east the west coast is characterized by 3000 mm year annual rainfall that often reaches 5000 mm year dominance of sw monsoon winds occurrence of narrow here we propose a conceptual tectono geomorphic model demonstrating the inheritance and continuum of gondwanan structural trends on the transient landscape of peninsular india

strain refraction in layered systems abstract strain refraction across competence contrasts is presented as a simple model consisting of two components a homogeneous strain and a heterogeneous simple shear for newtonian materials the ratio of the layer parallel simple shear component in adjacent layers is the inverse of their viscosity ratio strong changes in ellipsoid size shape and orientation are predicted across viscosity contrasts the geological implications of strain refraction theory are considered within the context of the cleavage strain debate the particular relationships of relative competence and strain revealed by the refraction model may contribute to the problem of why cleavages of different morphologies in rocks of different lithologies and kinematic histories should appear to be subparallel to the xy planes of measured strain ellipsoids competent rocks should develop dominantly layer orthogonal strain and incompetent layers shear dominated deformation a variety of structural features ranging from cleavage refraction changing lineation orientations folds transected by cleavage changes from coaxial to non coaxial deformation and ramp flat fault geometry may be the result of stress and strain refraction in rocks

pore scale simulation of carbonate dissolution in micro ct images we present a particle based method to simulate carbonate dissolution at the pore scale directly on the voxels of three dimensional micro ct images the flow field is computed on the images by solving the incompressible navier stokes equations rock fluid interaction is modeled using a three step approach solute advection diffusion and reaction advection is simulated with a semianalytical pore scale streamline tracing algorithm diffusion by random walk is superimposed while the reaction rate is defined by the flux of particles through the pore solid interface we derive a relationship between the local particle flux and the independently measured batch calcite dissolution rate we validate our method against a dynamic imaging experiment where a ketton oolite is imaged during co2 saturated brine injection at reservoir conditions the image calculated increases in porosity and permeability are predicted accurately and the spatial distribution of the dissolution front is correctly replicated the experiments and simulations are performed at a high flow rate in the uniform dissolution regime pe 1 and peda 1 thus extending the reaction throughout the sample transport is advection dominated and dissolution is limited to regions with significant inflow of solute we show that the sample averaged reaction rate is 1 order of magnitude lower than that measured in batch reactors this decrease is the result of restrictions imposed on the flux of solute to the solid surface by the heterogeneous flow field at the millimeter scale

influence of the flow rate on dissolution and precipitation features during percolation of co2 rich sulfate solutions through fractured limestone samples abstract calcite dissolution and gypsum precipitation are expected to occur when injecting co2 in a limestone reservoir with sulfate rich resident brine if the reservoir is fractured these reactions will take place mainly in the fractures which serve as preferential paths for fluid flow as a consequence the geometry of the fractures will vary leading to changes in their hydraulic and transport properties in this study a set of percolation experiments which consisted of injecting co2 rich solutions through fractured limestone cores was performed under p 150 bar and t 60 c flow rates ranging from 0 2 to 60 ml h and sulfate rich and sulfate free solutions were used variation in fracture volume induced by calcite dissolution and gypsum precipitation was measured by x ray computed microtomography xcmt and aqueous chemistry an increase in flow rate led to an increase in volume of dissolved limestone per unit of time which indicated that the calcite dissolution rate in the fracture was transport controlled moreover the dissolution pattern varied from face dissolution to wormhole formation and uniform dissolution by increasing the flow rate i e pe from 1 to 346 fracture permeability always increased and depended on the type of dissolution pattern

analytical dual energy microtomography a new method for obtaining three dimensional mineral phase images and its application to hayabusa samples abstract we developed a novel technique called analytical dual energy microtomography that uses the linear attenuation coefficients lacs of minerals at two different x ray energies to nondestructively obtain three dimensional 3d images of mineral distribution in materials such as rock specimens the two energies are above and below the absorption edge energy of an abundant element which we call the index element the chemical compositions of minerals forming solid solution series can also be measured the optimal size of a sample is of the order of the inverse of the lac values at the x ray energies used we used synchrotron based microtomography with an effective spatial resolution of 200 nm to apply this method to small particles 30 180 m collected from the surface of asteroid 25143 itokawa by the hayabusa mission of the japan aerospace exploration agency jaxa a 3d distribution of the minerals was successively obtained by imaging the samples at x ray energies of 7 and 8 kev using fe as the index element the k absorption edge of fe is 7 11 kev the optimal sample size in this case is of the order of 50 m the chemical compositions of the minerals including the fe mg ratios of ferromagnesian minerals and the na ca ratios of plagioclase were measured this new method is potentially applicable to other small samples such as cosmic dust lunar regolith cometary dust recovered by the stardust mission of the national aeronautics and space administration nasa and samples from extraterrestrial bodies those from future sample return missions such as the jaxa hayabusa2 mission and the nasa osiris rex mission although limitations exist for unequilibrated samples further this technique is generally suited for studying materials in multicomponent systems with multiple phases across several research fields

structural controls in the distribution of gold at how mine bulawayo zimbabwe the how mine is situated in the southeastern quadrant of the bulawayo greenstone belt within sediments and volcaniclastics of the late archean dacitic greenstone formation of the upper bulawayan group sited on the northern limb of a northeast verging syncline the mine comprises several ore zones which occur as an en echelon array of parallel steeply plunging linear shoots these shoots are confined within an extensional right stepping northerly trending strike slip duplex faulting has followed preferred lithologic contacts mineralization is dominated by a pyrite gold association and occurs as disseminations within altered mylonites felsites and tuffs as fabric replacement within tuffs and associated with veins in siltstones and iron formations accessory magnetite in tuffs and magnetite layers in iron formation are replaced by pyrite and ore textures suggest an epigenetic origin to the mineralization alteration is widespread and dominated by carbonation sulfidization and propylitic alteration permeability within the duplex was generated by slip and dilation of the synclinal axialplanar cleavage which lies at a high angle to the strike of the duplex early ductile deformation is overprinted by later brittle faulting which cuts early mineralization the linear ore zones parallel the intersection lineation between the preexisting cleavage and the duplex fault shear zones in the north area of the mine ore zones are developed within the fault bounded tuff unit e g the north 180 ore zone but in the south the ore zones parallel the hanging wall fault shear zone and transgress major lithologic contacts

reconstruction of the fossil hydrothermal system at lake city caldera colorado u s a constraints for caldera hosted geothermal systems abstract reconstruction of the physiochemical characteristics of fossil hydrothermal systems can help guide exploration for modern geothermal or mineral resources in similar settings the 22 9 ma lake city caldera in colorado u s a is well exposed and contains an exhumed fossil shallow hydrothermal system in this study alteration mineralogy vein textures and fluid inclusions are used to characterise the temperature and composition spatiotemporal variability and structural controls of the hydrothermal system at paleo depth equivalent between 1 and 2 km the hydrothermal system was dominantly moderate temperature up to 290 c low salinity 20 wt nacleq fluid inclusions within the caldera centre we interpret that the resurgent syenite intrusions provided little magmatic fluid input and had cooled significantly by the time the hydrothermal system had established in contrast in the eastern portion of the caldera late distinct magma batches provided high temperature up to 540 c and hypersaline up to 65 nacl eq magmatic fluid input above intrusions our conceptual hydrothermal model emphasizes the importance of discontinuity intersections in facilitating permeability in caldera settings we also recognize the contrasting hydrothermal manifestations of a waning degassed magma batch left over from a caldera forming eruption compared to fresh volatile rich magma

4d multi scale imaging of reactive flow in carbonates assessing the impact of heterogeneity on dissolution regimes using streamlines at multiple length scales abstract we have experimentally investigated the impact of heterogeneity on the dissolution of two limestones characterised by distinct degrees of flow heterogeneity at both the pore and core scales the two rocks were reacted with reservoir condition co2 saturated brine at both scales and scanned dynamically during dissolution first 1 cm long 4 mm diameter cores were scanned during reactive flow with a 4 m voxel size between 10 and 71 times using 4d x ray micro tomography ct over the course of 90 min second 3 8 cm diameter 8 cm long cores were reacted at the same conditions inside a reservoir condition flow apparatus and imaged using a medical grade x ray computed tomography scanner xct each sample was imaged 13 times over the course of 90 min at a 250 250 500 m resolution these larger cores were then scanned inside a ct at a 27 m voxel size to assess the alteration pore space heterogeneity after reaction both rock types exhibited channel widening at the mm scale and progressive high porosity pathway dissolution at the cm scale in the more heterogeneous rock dissolution was more focussed and progressed along the direction of flow additionally the dissolution pathways contained a distinct microstructure captured with the ct that was not visible at the resolution of the xct where the reactive fluid had not completely dissolved the internal pore structure this microstructure was further analyzed by performing a direct simulation of the flow field and streamline tracing on the image voxels we found that at the larger scales the interplay between flow and reaction significantly affects flow in the unreacted regions of the core when flow is focussed in large reacted channels this focusing is carried through to the unreacted parts of the rock where flow continues to be confined to preferential pathways after passing the reaction front this focussing effect is greater with increasing pore space heterogeneity indicating that the representative elementary volume rev for dissolution is far greater than the dissolution front itself this study of scale dependence using in situ 4d tomography provides insight into the mechanisms that control local reaction rates at the mm and cm scales furthermore this work suggests that under these conditions at larger scales it is likely to be structural heterogeneity that dominates the pattern of dissolution and therefore the evolution of high permeability pathways

an experimental study of co2 brine rock interaction at in situ pressure temperature reservoir conditions abstract a detailed investigation of co 2 brine rock interactions through a core flooding laboratory experiment was carried out under simulated reservoir conditions 100 c and 24 mpa changes in the ionic chemistry of the outlet solution combined with core scanning electron microscopy sem and bulk rock x ray diffraction xrd analysis of the core pre and post experiment reveal new insights into co 2 brine rock interactions minerals such as potassium k feldspar albite calcite and ankerite are variably dissolved after the experiments calcite is the mineral most affected by dissolution followed by ankerite whereas dissolution of feldspar minerals is minimal small amounts of kaolinite and solid phases were generated as a result of the experiment the solid phases are mainly comprised of c o na cl al and si and are presumed to be the transitional products in the formation of carbonate minerals the very low fluid penetration rate in the experiment resulted in significantly reduced contact area between the acidic co 2 fluid and minerals in the cores resulting in low rates of feldspar dissolution core permeability decreased substantially throughout the experiment although core porosity remained unchanged the permeability reduction is the result of precipitation of new mineral phases e g kaolinite and solid phases and potentially also the presence of clay particles released by the dissolution of carbonate cement which have then been transported in the fluid flow path and accumulated at pore throats the results provide new insights into co 2 trapping mechanisms in depleted oil and gas reservoirs and into the potential formation damage that may result from massive injections of co 2 into reservoirs during enhanced oil recovery programs

development of a fracture network in crystalline rocks during weathering study of bishop creek chronosequence using x ray computed tomography and 14c pmma impregnation method the progressive development of porosity during subsoil weathering of granodiorite clasts was studied at the bishop creek moraine chronosequence in east central california fractures and other pores were examined using two complementary imaging techniques x ray computed tomography xrct and a 14c pmma 14c polymethylmethacrylate method the well known xrct method allows the investigation of three dimensional 3 d pore space 14c pmma is a less known method based on the complete impregnation of pore space with 14c doped pmma and subsequent autoradiograph of a rock section these imaging methods allow us to decipher the evolution of pore space in the granodiorite during the 120 k y weathering period the 14c pmma imaging technique was found to be more suitable for following the evolution of the whole sequence from intact bedrock to saprock in terms of crack density porosity and aperture working with hand specimens this method was adapted to detect both the low aperture fractures microcracks and macrocracks only a slight and progressive increase in total fracture density was observed during the whole weathering period however this trend does not hold if macrocracks and microcracks are separated microcrack density slightly decreases whereas macrocrack density increases due to a progressive expansion of microcracks the total porosity of the rock increases during weathering and is correlated to the progressive aperture increase of all types of cracks this evolution is accompanied by a change of crack morphology and connectivity and an overall increase in intragranular porosity of biotite and plagioclase aggregates

an overview of the sedimentary geology of the bengal basin in relation to the regional tectonic framework and basin fill history abstract the bengal basin in the northeastern part of indian subcontinent between the indian shield and indo burman ranges comprises three geo tectonic provinces 1 the stable shelf 2 the central deep basin extending from the sylhet trough in the northeast towards the hatia trough in the south and 3 the chittagong tripura fold belt due to location of the basin at the juncture of three interacting plates viz the indian burma and tibetan eurasian plates the basin fill history of these geo tectonic provinces varied considerably precambrian metasediments and permian carboniferous rocks have been encountered only in drill holes in the stable shelf province after precambrian peneplanation of the indian shield sedimentation in the bengal basin started in isolated graben controlled basins on the basement with the breakup of gondwanaland in the jurassic and cretaceous and northward movement of the indian plate the basin started downwarping in the early cretaceous and sedimentation started on the stable shelf and deep basin and since then sedimentation has been continuous for most of the basin subsidence of the basin can be attributed to differential adjustments of the crust collision with the various elements of south asia and uplift of the eastern himalayas and the indo burman ranges movements along several well established faults were initiated following the breakup of gondwanaland and during downwarping in the cretaceous by eocene because of a major marine transgression the stable shelf came under a carbonate regime whereas the deep basinal area was dominated by deep water sedimentation a major switch in sedimentation pattern over the bengal basin occurred during the middle eocene to early miocene as a result of collision of india with the burma and tibetan blocks the influx of clastic sediment into the basin from the himalayas to the north and the indo burman ranges to the east rapidly increased at this time and this was followed by an increase in the rate of subsidence of the basin at this stage deep marine sedimentation dominated in the deep basinal part while deep to shallow marine conditions prevailed in the eastern part of the basin by middle miocene with continuing collision events between the plates and uplift in the himalayas and indo burman ranges a huge influx of clastic sediments came into the basin from the northeast and east throughout the miocene the depositional settings continued to vary from deep marine in the basin to shallow and coastal marine in the marginal parts of the basin from pliocene onwards large amounts of sediment were filling the bengal basin from the west and northwest and major delta building processes continued to develop the present day delta morphology since the cretaceous architecture of the bengal basin has been changing due to the collision pattern and movements of the major plates in the region however three notable changes in basin configuration can be recognized that occurred during early eocene middle miocene and plio pleistocene times when both the paleogeographic settings and source areas changed the present basin configuration with the ganges brahmaputra delta system on the north and the bengal deep sea fan on the south was established during the later part of pliocene and pleistocene and delta progradation since then has been strongly affected by orogeny in the eastern himalayas pleistocene glacial activities in the north accompanied sea level changes in the bay of bengal

structural control of greenstone hosted gold mineralization by transcurrent shearing a new interpretation of the kalgoorlie mining district western australia abstract the kalgoorlie mining district encloses the golden mile and mt charlotte mining areas and has produced 1200 metric tons of gold it is classed as one of the largest archaean gold deposits in the world the district is located in the norseman wiluna greenstone belt of the yilgarn craton western australia this greenstone belt is characterized by nnw trending transcurrent shear zones some of which are traceable over more than 500 km strike length mineralization in the district is controlled by second third and fourth order structures located between principal shear zones of regional extent an early deformation phase of regional folding d1 associated with prograde regional metamorphism of low grade preceded two phases of wrenching the geometry of mineralized shear zones in both mining areas as well as field and microstructural studies allow us to distinguish an early phase of transpressional sinistral shearing d2 from a later phase of transcurrent dextral shearing d3 the earlier sinistral shear system in the golden mile area was in part reactivated but its gross original geometry was preserved these observations are consistent with clay model experiments carried out by the authors hydrothermal carbonate sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event mineralization within individual shear zones forms tabular orebodies whereas pipe shaped ore shoots occur at the intersection of shear zones the ore shoots contain high grade telluride mineralization locally relative timing relationships show that mineralization postdates the peak of regional metamorphism hydrothermal activity in the district was preceded by the intrusion of calc alkaline porphyry dykes of granodioritic to tonalitic composition and accompanied by the intrusion of lamprophyre dykes

a paleo brahmaputra subsurface lithofacies analysis of miocene deltaic sediments in the himalayan bengal system bangladesh abstract the bengal foreland basin contains a succession of up to 16 km of dominantly deltaic deposits eroded from the eastern himalayas and the indo burman ranges and carried by major river systems similar to the present day ganges and brahmaputra analysis of electric logs and core descriptions acquired during oil and gas exploration in bangladesh allows construction of lithofacies maps which constrain depositional patterns of miocene strata compilations of sand thickness and sand shale ratio of the miocene surma group show that lower to middle miocene strata of the bhuban formation accumulated in a large elongate trough sand thickness and percentage both decrease markedly away from this depocenter which describes a large scale bend running initially westward from rashidpur northeast bengal basin and curving southward toward the bengal fan middle to upper miocene strata of the boka bil formation show a similar geographic trend in deposition of coarsest and thickest sediment but the major depocenter had shifted northward relative to that of the bhuban formation by some 30 km passing near beani bazar these trends suggest that deltaic deposits of the surma group filled the sylhet trough of the northeast bengal basin from the east published seismic data from western bangladesh show that additional large channels also contributed materials to the bengal basin from the northwest during the late miocene but these channels resulted in very little accumulation in the northwestern part of the basin probably due to restricted subsidence of underlying continental crust this study suggests that there was a major drainage system similar to the modern brahmaputra river during miocene time which carried orogenic sediments eroded from the uplifted terranes of the eastern himalayas and indo burman ranges to the eastern bengal delta

archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec part ii vein paragenesis and hydrothermal alteration the sigma deposit consists of an archean gold bearing vein system in deformed volcanic rocks intruded by two generations of porphyries the veins cut all rock types and wall rock alteration is superposed on greenschist facies metamorphic assemblages there are two related vein sets subvertical veins in ductile shears and subhorizontal veins in extensional fractures between shear zones this paper reports on the paragenesis of extensional veins and related hydrothermal alteration tension veins are composed chiefly of quartz and tourmaline and subordinate amounts of carbonates chlorite pyrite pyrrhotite scheelite and free gold the veins formed by one or more episodes of open space filling the paragenetic sequence consists essentially of successive deposition of carbonate scheelite tourmaline pyrite pyrrhotite carbonate chlorite and quartz most of the gold and tellurides was introduced into fractures and along grain boundaries of recrystallized vein minerals during a postfilling episode of deformation veins are surrounded by zoned alteration envelopes consisting of an outer zone of cryptic alteration characterized by a chlorite carbonate white mica assemblage and an inner zone of visible alteration characterized by a carbonate white mica outer subzone and a carbonate albite inner subzone alteration zones represent reaction fronts advancing into the wall rocks and resulted from progressive interaction of wall rocks with a single hydrothermal fluid the chlorite carbonate white mica zone is characterized by replacement of epidote by calcite and white mica accompanied by fixation of co 2 and k and by release of small amounts of si in carbonate white mica and carbonate albite subzones of visible alteration chlorite biotite and white mica were replaced by calcite albite and quartz variable amounts of gold tourmaline pyrite pyrrhotite and ilmenite were also introduced visible alteration zones are characterized by additions of ca co 2 na b s p and au and by important losses of mg fe al and h 2 o mass balance calculations show that ca na si co 2 s and b in vein minerals were contributed from the hydrothermal fluid whereas fe mg and al were derived from the walls the development of alteration envelopes began with the infiltration of hydrothermal fluids into open extensional fractures and it ceased before complete filling of the veins when continuous mineral coatings formed along vein walls deposition of carbonate accompanied development of cryptic alteration including initial co 2 additions to the wall rocks subsequent deposition of tourmaline pyrite and pyrrhotite carbonate chlorite accompanied the progressive development of visible alteration which encroached on cryptically altered rocks visible alteration is characterized by additions of ca and co 2 to the wall rocks and by release of mg fe and al later bulk quartz deposition which filled the vein openings generally took place after complete development of the alteration envelopes except for this late quartz deposition of vein minerals is attributed to chemical changes in the fluid resulting from progressive wall rock alteration bulk quartz deposition is attributed to progressive cooling of the hydrothermal fluid

structural controls on gold bearing quartz mineralization in a duplex thrust system hyde macraes shear zone otago schist new zealand the hyde macraes shear zone in east otago new zealand is a low angle north to northeast dipping thrust system in which gold bearing quartz veins have been deposited mineralization accompanied deformation in the shear zone which is preferentially developed in graphitic pelitic schist the shear zone is a duplex system dominated by steep hinterland dipping thrust faults between well defined shallow dipping upper and lower bounding thrusts imbrication and stacking have occurred in the shear zone especially where intrashear schist is psammitic and relatively brittle this stacking contributes to lateral thickness changes from 2 up to 125 m mineralized veins formed mainly in dilational jogs or localized extensional zones on the relatively shallow dipping parts of the intrashear thrust faults late stage shallow south dipping extensional veins crosscut the duplex structures but are related to the same overall deformation from analysis of the shear zone structure it is possible to make predictions about the geometry of mineralized schist within unexposed parts of the shear zone this is a potentially useful tool for further prospecting and mining in hydrothermally mineralized duplex systems

characterization of organic matter and depositional environment of tertiary mudstones from the sylhet basin bangladesh abstract the sylhet basin of bangladesh is a sub basin of the bengal basin it contains a very thick up to 22 km tertiary stratigraphic succession consisting mainly of sandstones and mudstones the sylhet succession is divided into the jaintia paleocene late eocene barail late eocene early miocene surma middle late miocene tipam late miocene pliocene and dupitila groups pliocene pleistocene in ascending order the origin of the organic matter om and paleoenvironment of deposition have been evaluated on the basis of c n s elemental analysis rock eval pyrolysis and gas chromatography mass spectrometry gc ms analysis of 60 mudstone samples collected from drill core and surface outcrops total organic carbon toc content ranges from 0 11 to 1 56 sulfur content is low in most samples toc content in the sylhet succession varies systematically with sedimentation rate with low toc caused by clastic dilution produced by high sedimentation rates arising from rapid uplift and erosion of the himalaya the om in the succession is characterized by systematic variations in pristane phytane pr ph oleanane c30 hopane n c29 n c19 alkane tm ts 17 h 22 29 30 trisnorhopane 18 h 22 29 30 trisnorhopane and sterane c29 c27 c28 c29 ratios during the middle eocene to pleistocene based on biomarker proxies the depositional environment of the sylhet succession can be divided into three phases in the first middle eocene to early miocene deposition occurred completely in seawater dominated oxic conditions with abundant input of terrestrial higher plants including angiosperms the second phase middle to late miocene consisted of mainly freshwater anoxic conditions along with a small seawater influence according to eustasic sea level change with diluted om derived from phytoplankton and a lesser influence from terrestrial higher plants oxygen poor freshwater conditions prevailed in the third phase post late miocene planktonic om was relatively abundant in this stage while a high angiosperm influx prevailed at times tmax values of ca 450 c vitrinite reflectance ro of ca 0 66 and methylphenanthrene index mpi 3 of ca 1 indicate the om to be mature the lower part middle eocene to early miocene of the succession with moderate toc content and predominantly terrestrial om could have generated some condensates and oils in and around the study area

archean lode gold deposits fluid flow and chemical evolution in vertically extensive hydrothermal systems abstract the range of conditions of formation of lode gold deposits from the sub greenschist to the lower granulite facies in archean greenstone belts and the generally steeply plunging vertically continuous pipe like or tabular geometries of individual deposits indicate long distance hydrothermal fluid advection along well defined channelways in the upper and middle crust from presently available gold solubility data destabilisation of gold bisulphide complexes through h 2 s loss from the fluid to the wallrock was the dominant gold precipitation mechanism within these hydrothermal systems as a whole this inference is supported by the s au ratios of ores sulphur and au precipitation in the hydrothermal system is estimated to be relatively inefficient with only 10 50 of s or au contained in the fluid precipitated over any kilometre length of fluid channelway the relative inefficiency of gold precipitation allowed mineralisation over a significant depth range in a crustal profile

high resolution non destructive measurement and characterization of fracture apertures abstract we measure at high resolution the fracture aperture of several consolidated materials and statistically characterize the aperture distribution for future studies on single and multiphase flow and transport through fractured porous media the images of the real fractures in granite and sandstone rocks are obtained with a computer aided tomography cat x ray scanner the minimum pixel size is 0 27 0 27 mm but fractures as small as 35 m can be accurately measured using a calibration standard for the variation in ct number due to the missing rock mass in a given pixel the distribution of fracture apertures is best described by a lognormal function this was also corroborated by comparing the theoretical to actual ratio of geometric mean to arithmetic mean apertures the mechanical roughness of the fractures ranged from 0 17 to 1 5 the variability in fracture aperture displayed only short range structure with correlation lengths of 0 8 to 7 mm at maximum which is at most one tenth of the minimum dimension width of the fracture plane fracture aperture distribution is rather heterogeneous potential flow channels can be deduced from the fracture aperture distribution

rapid porosity and permeability changes of calcareous sandstone due to co2 enriched brine injection reservoir injectivity and storage capacity are the main constraints for geologic co2 sequestration subject to safety and economic considerations brine acidification following co2 dissolution leads to fluid rock interactions that alter porosity and permeability thereby affecting reservoir storage capacity and injectivity thus we determined how efficiently co2 enriched brines could dissolve calcite in sandstone cores and how this affects the petrophysical properties during computerized tomography monitored flow through reactor experiments calcite dissolved at a rate largely determined by the rate of acid supply even at high flow velocities which would be typical near an injection well the porosity increase was accompanied by a significant increase in rock permeability larger than that predicted using classical porosity permeability models this chemically driven petrophysical change might be optimized using injection parameters to maximize injectivity and storage

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

initial conditions or emergence what determines dissolution patterns in rough fractures dissolution of fractured rocks is often accompanied by the formation of highly localized flow paths while the fluid flow follows existing fractures in the rock these fissures do not in general open uniformly simulations and laboratory experiments have shown that distinct channels or wormholes develop within the fracture from which a single highly localized flow path eventually emerges the aim of the present work is to investigate how these emerging flow paths are influenced by the initial aperture field we have simulated the dissolution of a single fracture starting from a spatially correlated aperture distribution our results indicate a surprising insensitivity of the evolving dissolution patterns and flow rates to the amplitude and correlation length characterizing the imposed aperture field we connect the similarity in outcomes to the self organization of the flow into a small number of wormholes with the spacing determined by the length of the longest wormholes we have also investigated the effect of a localized region of increased aperture on the developing dissolution patterns a competition was observed between the tendency of the high permeability region to develop the dominant wormhole and the tendency of wormholes to spontaneously nucleate throughout the rest of the fracture we consider the consequences of these results for the modeling of dissolution in fractured and porous rocks