the effect of carbonate cementation on permeability heterogeneity in fluvial aquifers an outcrop analog study abstract spatial variations in permeability have been widely recognized as a dominant control on solute transport and petroleum migration outcrop studies are one means of improving our understanding of the nature of permeability heterogeneity and the underlying geologic controls to date these studies have either neglected the importance of cementation or investigated only the modern cemented permeability without explicitly considering how cementation has affected permeability two outcrops of channel sand deposits in the plio pleistocene sierra ladrones formation new mexico are investigated to quantify 1 the effect of cementation on permeability at a given location and 2 the patterns of cementation and their impact on permeability heterogeneity we analyzed approximately 400 spatially located samples 10 cm 3 each from the two outcrops cementation is typically weak to moderate filling less than 60 of the original pore space permeability and cement content are bimodally distributed and negatively correlated permeability reduction is investigated by measuring modern permeability minus cement permeability minus cement porosity and volumetric cement content permeability is reduced locally by up to three orders of magnitude by combining models of permeability for uncemented sediments and cemented sediments we derive a model of cemented permeability k c that depends primarily on three factors uncemented permeability k u original porosity and cement fraction our results suggest that accounting for porosity reduction only may result in an overestimation of k c conversely accounting for porosity reduction increased specific surface area and increased tortuosity appears to result in an underestimation of k c accounting for porosity reduction and increased specific surface area provide the best estimate of k c cementation is highly variable spatially and is poorly correlated with lithofacies variogram analysis of cementation and permeability indicates that the spatial distribution of cementation is the dominant control on the permeability correlation structure the spatially variable cementation also results in an order of magnitude increase in the variance of log permeability applying stochastic models of effective flow and transport properties to the results of this study suggest that cementation has a relatively small effect on effective permeability but can potentially increase dispersivity by two orders of magnitude

crustal extension ag pb zn veins in the xiong ershan district southern north china craton constraints from the shagou deposit the shagou vein type ag pb zn deposit in the xiong ershan district southern margin of the north china craton is hosted within amphibolite facies metamorphic rocks of the late archean to early paleoproterozoic taihua group the ag pb zn veins are localized in ne to nne trending brittle faults and typically display symmetrical zoning consisting of siderite quartz sphalerite galena and quartz calcite from the margin toward the center of each vein ore related hydrothermal alteration is well developed on both sides of the veins dominated by silicification sericitization chloritization and carbonatization sericite separates extracted from a major ag pb zn vein yield a 40 ar 39 ar plateau age of 140 0 1 0 ma 1 and isochron age of 141 1 1 6 ma 1 indicating that mineralization occurred at the beginning of early cretaceous field and textural relationships indicate four hydrothermal stages marked by assemblages of quartz siderite stage i quartz sphalerite ankerite stage ii quartz galena silver minerals ankerite stage iii and quartz calcite stage iv respectively silver minerals are abundant in all veins and are composed of in paragenetic order argentiferous tetrahedrite polybasite jalpaite argentite and native silver these silver minerals commonly occur as replacements of galena chalcopyrite and other sulfides or as fillings of microfractures in sulfides and quartz microthermometric measurements of primary fluid inclusions in quartz carbonates and sphalerite from various hydrothermal stages indicate that ore minerals were deposited at intermediate temperatures 267 157 c from aqueous carbonic to aqueous fluids with moderate salinities 7 2 15 9 wt nacl equiv coexisting galena sphalerite pair yields sulfur isotope equilibrium temperatures of 205 to 267 c consistent with the overall homogenization temperatures of fluid inclusions the microthermometric data also indicate that both fluid mixing and fluid rock interaction were important mechanisms for ore precipitation carbonate minerals siderite ankerite calcite spanning the entire mineralization history have 13 c v pdb values of 5 2 to 1 4 and 18 o v smow of 10 9 to 15 0 corresponding to calculated values for the ore fluids of 6 5 to 1 8 and 1 4 to 5 4 respectively 34 s v cdt values of sulfide minerals pyrite sphalerite galena range from 1 1 to 5 5 consistent with a deep seated sulfur source galena separates have 206 pb 204 pb ratios of 17 472 to 17 813 207 pb 204 pb ratios of 15 411 to 15 498 and 208 pb 204 pb ratios of 38 178 to 38 506 the isotope data together with geological and geochronological evidence favor a primary metamorphic source for sulfur and other components in the ore fluids a synthesis of available data suggests that the shagou deposit is a typical vein type ag pb zn deposit that formed under an extensional geodynamic setting associated with tectonic reactivation of the north china craton during the late mesozoic a time period that is manifested by pervasive magmatism widespread formation of metamorphic core complexes and development of faulted basins throughout the eastern part of the craton metamorphic devolatilization of the meso neoproterozoic marine sedimentary rocks previously subducted beneath the xiong ershan district facilitated by extensive magmatism and elevated heat flow due to lithospheric extension could have provided large amounts of ore fluids responsible for the ag pb zn mineralization the ne to nne trending faults affiliated with the transcrustal machaoying fault may have acted as pathways for the upward migration of deep seated metamorphic fluids mixing of the metamorphically derived fluids with meteoric waters ultimately resulted in deposition of the ag pb zn veins in brittle extensional structures

exhumation driven devolatilization as a fluid source for orogenic gold mineralization at the damang deposit ghana gold mineralization at the damang deposit is unique among currently known orogenic gold deposits in ghana comprising gold hosted within metasediments of the tarkwaian system and contained in a subhorizontal extensional quartz vein array that formed during regional compression the damang region has an extended paragenesis involving numerous structural metamorphic igneous and metasomatic events orogenic gold mineralization occurred late in the geologic paragenesis at damang postdating regional metamorphism and an earlier episode of hydrothermal alteration locally termed pink hematite alteration associated with the intrusion of mafic sills and dikes this earlier pink hematite alteration event involves extensive silicification that changed the rheology of the altered rocks and promoted later fracturing following peak regional metamorphism at around 2005 ma the damang region underwent a short period of rapid exhumation as constrained through numerical thermal modeling of existing pressure temperature time data this exhumation triggered the generation of a subhorizontal fracture array that was fed by fluids released through decompression driven metamorphic devolatilization the interaction between these fluids and the host rock resulted in precipitation of gold in association with sulfide carbonate potassic alteration halos around quartz veins such postpeak metamorphic exhumation driven devolatilization is unlikely to be a singular occurrence and represents a potentially important source of fluid for orogenic gold deposits elsewhere in ghana west africa and globally

a review of te and se systematics in hydrothermal pyrite from precious metal deposits insights into ore forming processes abstract pyrite is one of the most common minerals in many precious and base metal hydrothermal ore deposits and is an important host to a range of trace elements including au and co and the semi metals as se sb te and bi as such in many hydrothermal ore deposits where pyrite is the dominant sulphide phase it can represent a major repository for these elements furthermore the concentrations and ratios of au as and co in pyrite have been used to infer key ore forming processes however the mechanisms controlling the distribution of te and se in pyrite are less well understood here we compare the te and se contents of pyrite from a global dataset of carlin type orogenic au and porphyry epithermal deposits to investigate 1 the potential of pyrite to be a major repository for these elements and 2 whether te and se provide insights into key ore forming processes pyrite from carlin type low sulphidation and alkaline igneous rock hosted epithermal systems is enriched in te and se compared to pyrite from high sulphidation epithermal and porphyry cu deposits orogenic au pyrite is characterised by intermediate te and se contents there is an upper solubility limit for te as a function of as in pyrite similar to that established for au by reich et al 2005 and this can be used to identify te present as telluride inclusions which are common in some epithermal porphyry and orogenic au deposits physicochemical fluid parameters such as ph redox and temperature as well as crystal chemistry control the incorporation and concentration of se and te in pyrite neutral to alkaline fluids have the ability to effectively mobilise and transport te fluid boiling in porphyry epithermal systems as well as wall rock sulphidation and oxidation in carlin type and orogenic au deposits can effectively precipitate te in association with pyrite and au in contrast se concentrations in pyrite apparently vary systematically in response to changes in fluid temperature irrespective of ph and f o 2 hence we propose that the se contents of pyrite may be used as a new geo thermometer for hydrothermal ore deposits furthermore the comparison of bulk ore and pyrite chemistry indicates that pyrite represents the major host for te and se in carlin type and some epithermal systems and thus pyrite can be considered to be of economic interest as a potential source for these elements

influence of host rock texture on the morphology of carbonate concretions in a meteoric diagenetic environment abstract the integrated stratigraphic petrographic geochemical study of quaternary marine terraces that have undergone well constrained sea level and water table oscillation s over geologically short time periods can provide important clues on diagenetic processes at the marine continental interface the punta di i cani marine terrace quaternary saint florent corsica unconformably overlies the well cemented low permeability arenites of the miocene monte s angelo formation and is erosively overlain by unlithified high permeability holocene continental sediments the terrace is composed of a retrogradational fining upward succession from fluvial pebbly sandstone to beach and shoreface deposits the entire thickness of the terrace hosts concretions made of low mg calcite concretion geometry is linked to the sedimentological facies of the host sediment i massive fluvial pebbly sandstone at the base of the terrace contains knobby subhorizontal concretions somewhat resembling turbidite flute casts ii very low angle cross bedded well sorted beach deposits contain pipy subhorizontal concretions iii trough crossbedded upper shoreface deposits in the upper part of the terrace contain a subtle diagenetic lineation on the laminae locally obliterated by a patchy isotropic cementation overall the degree of cementation decreases from the bottom to the top of the terrace cement chemistry and morphology as well as stable carbon and oxygen isotopic signatures indicate that the concretions formed by precipitation of meteoric phreatic water flowing toward the coast elongate concretions are invariably parallel to the topographic gradient consistent with the notion that the growth of concretions was controlled by advection smectitic vadose cement is locally present its crosscutting relationships with calcite cement point to periodic oscillations of the water table the results of this study i show that elongate concretions can be used as interstitial paleoflow indicators and ii suggest that concretion geometry in an advection dominated diagenetic environment can depend on the small scale permeability distribution as suggested by the fact that each type of concretion is invariably associated with a specific sedimentary facies

relationship between fluvial bounding surfaces and the permeability correlation structure results of three meter scale outcrop studies suggest a quantifiable relationship between fluvial bounding surfaces and the permeability correlation structure mapping of bounding surfaces and in situ measurements of permeability were obtained in three types of deposits a low energy channel deposit a paleosol and a high energy channel deposit for the low energy channel deposit only second order surfaces separating lithofacies were mapped the variogram exhibits anisotropic exponential behavior and an analysis of variance anova suggests that the within facies variation dominates over between facies variation the paleosol contained no internal bounding surfaces and exhibited a bellshaped variogram consistent with a smoothly varying style of heterogeneity both firstand second order bounding surfaces were mapped in the high energy channel deposit results of the anova suggest that the between facies variation dominates over the within facies variation a model of spatial correlation using superposition of independent processes acting over different scales provides a good fit to the data these three studies suggest that fluvial bounding surfaces provide a geological basis for conceptualizing and modeling heterogeneity in alluvial deposits

twin lake tracer tests setting methodology and hydraulic conductivity distribution following evaluation of three potential sites a fluvial sand aquifer with high 0 7 1 6 m d 1 linear average groundwater flow velocities was instrumented with a fully penetrating injection well and 82 monitoring installations over a 40 m flow path length two natural gradient tracer migration experiments have employed pulse injections of a nonreactive radioisotope 131i in a study of hydraulic conductivity structure and dispersive characteristics of the flow system the use of a gamma emitting radionuclide in these experiments allowed measurement of tracer distribution by scanning fully penetrating boreholes with a scintillation detector this technique provided many advantages in data collection over conventional tracers including unprecedented resolution of the vertical distribution of tracer measurements without pumping and the attendant concerns for flow system perturbation and minimal manpower for sample analysis tracer breakthroughs at monitors along the axis of flow revealed the presence of heterogeneities in aquifer hydraulic conductivity at two scales over the 40 m path length six regions with hydraulic conductivities substantially different from adjacent strata were identified thicknesses of these regions varied from 0 2 to more than 3 m their lateral extents in the direction of groundwater flow ranged from 15 m to more than 40 m smaller strata with an average thickness of 0 34 m and lengths of less than 10 m were detected within all six of the hydraulic conductivity regions but there was no evidence of increases in local dispersivity with increased transport distance as a result of these variations the six hydraulic conductivity regions do however have measurable effects on whole aquifer dispersion stratification characteristics of the tested aquifer believed to have been deposited in a sandy braided river are consistent with those observed in similar modern environments estimation of the magnitude and spatial structure of hydraulic conductivity variations in aquifers of concern would be greatly aided by studies of the stratification characteristics in well exposed modern or lithified sedimentary analogues

paleoproterozoic gold deposits hosted by albite and carbonate altered tonalite in the chirano district ghana west africa paleoproterozoic birimian supergroup rocks of southwest ghana host numerous mesothermal lode gold deposits including the 38 million ounce moz ashanti deposit at obuasi and other multimillion ounce deposits at bogoso konongo prestea bibiani and in the yamfo sefwi region gold deposits of the chirano district are localized along the chirano lode horizon a 9 km segment of the chirano and related shear zones on the southeast margin of the sefwi bibiani belt economically significant mineralization at chirano is largely hosted by altered tonalite intrusions weaker alteration and subparts per million gold grades extend into adjacent birimian mafic igneous rocks and tarkwaian group sedimentary rocks pink albite alteration albite quartz pyrite hematite rutile leucoxene chalcopyrite gold and later pale carbonate alteration ankerite sericite quartz pyrite hematite gold fineness 910 940 ag au telluride characterize the gold deposits of the chirano district alteration assemblages and the bi te and mo contents of the mineralized rocks at chirano are more typical of lode gold deposits than orthomagmatic gold deposits alteration assemblages at chirano contrast with the graphite carbonate pyrite arsenopyrite pyrrhotite assemblages that dominate the lode gold deposits in southwest ghana which are hosted by graphitic shear zones and birimian sedimentary rocks differences between alteration assemblages at chirano and those in graphitic shear zones may reflect the different wall rock assemblages with which the hydrothermal fluid equilibrated rather than different fluid sources mineralization in the chirano district formed during a relatively late episode of small displacement brittle cataclasis on the chirano lode horizon after reverse ductile shearing associated with regional scale shortening this structural history is similar to that documented at the larger ashanti and bogoso deposits on the opposite side of the kumasi basin timing relationships are compatible with all three deposits forming during a single mineralizing event late in the eburnean 2 orogeny at chirano limited fault vein geometric relationships imply a component of reverse movement during mineralization whereas sinistral strike slip movement accompanied mineralization at ashanti and bogoso no consistent correlation was observed between the asymmetry of bends and splays in the strike of the chirano lode horizon and the location or style of individual gold deposits unlike at ashanti and bogoso the structural settings of deposits in the chirano district indicate that an exploration strategy which assumes mineralization occurred during sinistral offset in the host structures similar to relationships at ashanti and bogoso may result in significant targets being ignored

a fluid inclusion and stable isotope study at the loulo mining district mali west africa implications for multifluid sources in the generation of orogenic gold deposits the world class loulo mining district 15 5 moz resource in the birimian terrane of western mali contains a range of mineralogically diverse styles of orogenic gold mineralization the district is distinguished by as rich orebodies as at yalea as well as tourmaline bearing fe rich orebodies as at gara new fluid inclusion and stable isotope data presented here constrain the nature of the hydrothermal fluids responsible for these different types of mineralization and point towards the role of multifluid sources metamorphic and magmatic in the formation of orogenic gold deposits microthermometric and laser raman studies from yalea main and two other similar orebodies loulo 3 and baboto reveal the dominance of coexisting co 2 n 2 ch 4 type 1 and h 2 o nacl type 2 fluid inclusions these inclusions show evidence of fluid unmixing from reduced quartz faylite magnetite qfm buffered low salinity 10 wt nacl equiv co 2 rich h 2 o nacl n 2 ch 4 primary ore fluids the combination of microthermometric data and geothermometry based on ore and alteration assemblages indicate trapping temperatures and pressures of 270 to 340 c at 1 4 to 1 8 kbar the p t x state of the yalea style mineralizing fluids along with the 18 o fluid compositions of 8 8 to 10 7 is consistent with the derivation of auriferous fluids during greenschist facies regional metamorphism of the host terrane similar fluid compositions were previously reported elsewhere in the birimian crust of west africa and in other orogenic gold districts worldwide the precipitation of gold from the h 2 s rich metamorphic fluid is primarily linked to phase separation of the ore fluid which is controlled by p t fluctuations and or fluid rock interaction with carbonaceous host sedimentary rocks confirmed by low 13 c values of 21 7 to 15 8 fluid inclusion investigations from gara and a similar style orebody yalea north indicate the presence of coexisting co 2 n 2 ch 4 type 1 and mixed salinity 5 21 wt nacl equiv co 2 rich h 2 o nacl inclusions type 3 inclusion assemblages also contain common oxidized hematite magnetite hm buffered high temperature 400 c hypersaline 35 50 wt total dissolved solids metalliferous na fe ca cu ni w pb zn multiphase h 2 o rich co 2 nacl fecl 2 inclusions type 4 this inclusion type has not been previously reported in other birimian terranes the composition of the brines along with carbon isotope data 13 c of 14 4 to 4 5 suggests a magmatic input to the gara style hydrothermal system the coexistence in the fluid inclusion assemblages of the magmatic brines with yalea style co 2 rich metamorphic fluids and the positive correlation between salinity and homogenization temperatures suggest mineralization was locally controlled by fluid mixing the interaction of these two chemically contrasting fluids explains the distinctive petrographic characteristics of the gara style orebodies this includes the growth of widespread multi stage fe 3 rich tourmaline b rich granite source and sodic alteration and ore assemblages consisting of abundant nickeloan pyrite ree phosphates ni co pb zn minor trace sulfides and scheelite gold deposition in the gara style hydrothermal system is related to physical and chemical changes of the two fluids during mixing e g decreases in f o2 and t in the brines and retrograde boiling of the co 2 component in the metamorphic fluids a salting out effect

the saqisan mine a paleokarst uranium deposit south china abstract the saqisan uranium deposit located in guangxi province south china is hosted mainly by a dark gray limestone solution breccia unit and paleocavern fill sedimentary rocks the deposit is the largest known breccia hosted uranium deposit in china and was initially described as of synsedimentary shallow marine origin however recent open pit operations at the saqisan mine have uncovered new exposures that necessitate a thorough change in the genetic interpretation of the deposit the new exposures suggest that the occurrence of saqisan ores in sediment filled solution breccia and paleokarst cavities of limestone is in part similar to that of solution collapse breccia pipe type uranium deposits in arizona united states the interconnected paleokarst passages and caverns were filled with consolidated mineralized clastic sedimentary rocks most of the orebodies were deposited in the interconnected paleokarst passages and caverns two stages of primary uranium mineralization are recognized the early stage i mineralization is the most important for uranium as it accounts for 60 of saqisan s total uranium tonnage stage i mineralization formed low grade ore the late stage ii mineralization which accounts for the remaining 40 of saqisan s total uranium tonnage is hosted by strongly silicified foliated and mylonitized host limestones the uranium in stage i ores predominantly occurs as micro sized pitchblende in the clay rich breccia matrix the uranium in stage ii ores mainly occurs as pitchblende veinlets 0 1 2 mm wide and as disseminated pitchblende grains ranging from less than 0 1 to 1 mm in size sulfur isotope data indicate that biogenic pyrite in the host carbonate may have been the major source of sulfur in the mineralizing fluids oxygen and carbon isotope data show that most of the participated co 2 and hco 3 during stage i mineralization was derived from bicarbonate dissolved out of the marine host carbonate oxygen and hydrogen isotope data combined with determinations of temperature and salinity for the inclusion fluids of stage ii veinlet quartz show that the ore bearing fluids of stage ii mineralization are apparently devoid of co 2 and ch 4 and were originally saline meteoric ground water these basinal brines which were from connate waters trapped in the paleozoic carbonates are believed to have flowed along aquifers of the basinal sequence and up preexisting subvertical faults to produce the observed style of mineralization by chemical interaction with host carbonates it is postulated that the high grade uranium ore of stage ii is the product of multiple superimposed reconcentration of uranium from the early synpaleokarstic ore stage i by low to middle temperature 110 251 c uranium enriched hydrothermal brines driven by a regional heating event such as the late yanshanian tectonism the heat source for the saqisan hydrothermal system could have been related to conversion of mechanical energy into heat during the late yanshanian regional tectonic deformation

source and evolution of ore forming hydrothermal fluids in the northern iberian pyrite belt massive sulphide deposits sw spain evidence from fluid inclusions and stable isotopes a fluid inclusion and stable isotopic study has been undertaken on some massive sulphide deposits aguas tenidas este concepcion san miguel san telmo and cueva de la mora located in the northern iberian pyrite belt the isotopic analyses were mainly performed on quartz chlorite carbonate and whole rock samples from the stockworks and altered footwall zones of the deposits and also on some fluid inclusion waters homogenization temperatures of fluid inclusions in quartz mostly range from 120 to 280 c salinity of most fluid inclusions ranges from 2 to 14 wt nacl equiv a few cases with th 80 110 c and salinity of 16 24 wt nacl equiv have been also recognized in addition fluid inclusions from the soloviejo mn fe jaspers 160 190 c and 6 wt nacl equiv and some late to post hercynian quartz veins 130 270 c and 4 wt nacl equiv were also studied isotopic results indicate that fluids in equilibrium with measured quartz 18ofluid 2 to 4 chlorites 18ofluid 8 14 dfluid 45 to 27 whole rocks 18ofluid 4 7 dfluid 15 to 10 and carbonates 18oankerite 14 5 16 13cfluid 11 to 5 evolved isotopically during the lifetime of the hydrothermal systems following a waxing waning cycle at different temperatures and water rock ratios the results fluid inclusions 18o d and 13c values point to a highly evolved seawater along with a variable but significant contribution of other fluid reservoirs such as magmatic and or deep metamorphic waters as the most probable sources for the ore forming fluids these fluids interacted with the underlying volcanic and sedimentary rocks during convective circulation through the upper crust

the origin and hydrothermal mobilization of carbonaceous matter associated with paleoproterozoic orogenic type gold deposits of west africa abstract the chemical and physical properties of carbonaceous matter cm were studied in paleoproterozoic metasediment hosted orogenic type gold deposits in burkina faso the inata deposit in mali the syama deposit and in ghana the obuashi and bogoso deposits two types of cm occur in all the studied deposits metamorphosed and hydrothermal metamorphosed cm prevails in all the deposits hydrothermal cm occurs in small veinlets in hydrothermally altered rocks and in quartz veins or forms irregular accumulations parallel or sub parallel to c type cleavage within the shear zones the origin of hydrothermal cm which occurs in paragenesis with au bearing arsenopyrite or pyrite seems to have been due to supersaturation of hydrothermal fluids with carbon at the deeper or middle crustal levels the isotopic composition of carbon in bulk cm 33 1 to 26 2 vpdb indicates its biogenic origin the isotopic composition of carbon in hydrothermal carbonates ranges from 14 5 to 4 4 vpdb which suggest mixing of carbon derived from a deep seated source with carbon derived from an organic source the interaction of hydrothermal fluids with metamorphosed cm could be one of the causes of the reduction of hydrothermal fluids and formation of the respective mineralization the optical properties and raman spectra of the metamorphosed cm particles in the individual studied mineral deposits differ considerably the temperatures calculated on the basis of the raman spectra of metamorphosed cm vary between 280 and 440 c depending on the thermometer used and correspond to temperatures of metamorphism of upper sub grenschist and greenschist facies the temperatures calculated for hydrothermal cm at the individual deposits are only slightly lower compared to the metamorphosed cm at the same deposits which indicates approximately the same temperature of the metamorphic and hydrothermal processes

architecture of the sierra ladrones formation central new mexico depositional controls on the permeability correlation structure statistical models of hydrogeological heterogeneity are often used in aquifer and reservoir characterization the number of data required to estimate objectively the spatial correlation structure of permeability however is often prohibitive the objective of this study was to develop a better understanding of how information about depositional processes can be used to characterize hydrogeological heterogeneity an outcrop of the fluvial interfluvial sierra ladrones formation of new mexico was studied for this purpose on the basis of previous studies of paleogeography and our own field observations deposits of the sierra ladrones formation are interpreted as marginal ancestral rio grande flood plain and tributary deposits architectural elements were mapped over a 0 16 km sup 2 peninsular outcrop of pliocene pleistocene deposits of the central albuquerque basin geostatistical analysis of the architectural element map data indicates non orthogonal anisotropy in the horizontal direction the orientations of the strongest n30 degree w and weakest n90 degree e correlation correspond to the orientation of the tributary system and the ancestral rio grande flood plain respectively in the vertical direction the correlation structure exhibits exponential behavior corresponding to the average element thicknesses the results demonstrate that information about depositional environment can be used to help to quantify statistically subsurface heterogeneity 28 refs 9 figs 1 tab

mixed gas hydrate structures at the chapopote knoll southern gulf of mexico abstract in underwater hydrocarbon seepage environments gas hydrates are considered to play a significant role as shallow gas reservoirs and buffers for light hydrocarbon expulsion here we report on mixed hydrate structures from the chapopote knoll in the southern gulf of mexico and discuss several options on how a mixture of structure i si and structure ii sii gas hydrate may occur in nature locally resolving microscopic methods are needed to characterize the coexistence of different hydrate structures at geological hydrate deposits we used raman spectroscopy x ray diffraction and gas chromatography for our investigations gas hydrates were found within the matrix and pores of the asphalts extruded at the seafloor two of the three hydrate pieces investigated comprised only si formed mostly from methane in contrast one piece comprised an intimate mixture of both si and sii with sii representing ca 25 wt and si ca 75 wt of the hydrate present the two structures were closely associated within individual grain agglomerates the crystallites of sii were significantly larger than of si suggesting differences in the nucleation density or different crystallite ages the structural coexistence may be a result of one or more processes i de mixing into two hydrate structures during the growth from the gas phase which provides an additional degree of freedom for lowering the free energy in the system ii fractionated crystallization with a subsequently changing molecular composition iii crystallization from separated gas bubbles with different hydrocarbon compositions and water and iv partial transformation from sii to si after hydrate nucleation ceasing when a thermodynamically stable state was reached the presented work will affect future assessments of natural hydrate deposits at thermogenic hydrocarbon systems as it shows that both hydrate types i and ii can be present at a certain geological site and may provide a lingering strength to the system if conditions fall below the si stability limit this suggests that present worldwide hydrate occurrences are likely to be underestimated

origin of mineralizing fluids of the sediment hosted navachab gold mine namibia constraints from stable o h c s isotopes the navachab gold mine in the damara belt of central namibia is characterized by a polymetallic au bi as cu ag ore assemblage including pyrrhotite chalcopyrite sphalerite arsenopyrite bismuth gold bismuthinite and bismuth tellurides gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near vertical sequence of shelf type metasedimentary rocks including marble calcsilicate rock and biotite schist the sequence has been intruded by abundant syntectonic lamprophyre aplite and pegmatite dikes documenting widespread igneous activity coeval with mineralization the majority of quartz from the veins has 18o values of 14 to 15 per mil v smow the total variations in 18o values of the biotite schist and calcsilicate rock are relatively small 12 14 whereas the marble records steep gradients in 18o values 17 21 the lowest values being recorded at the vein margins despite this there is no correlation between 18o and 13c values and the carbonate content of the rocks indicating that fluid rock interaction alone cannot explain the isotopic gradients in addition the marble records increased 13c values at the contact to the veins possibly related to a change in the physicochemical conditions during fluid rock interaction gold is interpreted to have precipitated in equilibrium with metamorphic fluid 18o 12 14 d 40 to 60 at peak metamorphic conditions of ca 550 c and 2 kbars consistent with isotopic fractionations between coexisting calcite garnet and clinopyroxene in the alteration halos the most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the damaran metapelites that underwent prograde metamorphism at amphibolite to granulite facies grades although there is no isotopic evidence for the contribution of magmatic fluids they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients ca 80 c km 1 in the mine area

large scale natural gradient tracer test in sand and gravel cape cod massachusetts 1 experimental design and observed tracer movement a large scale natural gradient tracer experiment was conducted on cape cod massachusetts to examine the transport and dispersion of solutes in a sand and gravel aquifer the nonreactive tracer bromide and the reactive tracers lithium and molybdate were injected as a pulse in july 1985 and monitored in three dimensions as they moved as far as 280 m down gradient through an array of multilevel samplers the bromide cloud moved horizontally at a rate of 0 42 m per day it also moved downward about 4 m because of density induced sinking early in the test and accretion of areal recharge from precipitation after 200 m of transport the bromide cloud had spread more than 80 m in the direction of flow but was only 14 m wide and 4 6 m thick the lithium and molybdate clouds followed the same path as the bromide cloud but their rates of movement were retarded about 50 relative to bromide movement because of sorption onto the sediments

graphitization of organic matter and fluid deposited graphite in palaeoproterozoic birimian black shales of the kaya goren greenstone belt burkina faso west africa palaeoproterozoic black shales form an essential part of the birimian volcanosedimentary belt in burkina faso west africa the mean rmax values and the atomic h c values of the bulk carbonaceous matter bcm together with rock structures and mineral assemblages indicate that these carbon rich rocks were metamorphosed to sub greenschist and low grade greenschist facies x ray diffraction reveals that the 002 graphite peak width in half maximum fwhm ranges from 0 43 to 0 71 2 in sub greenschist facies and from 0 27 to 0 41 2 in greenschist facies rocks but the d 002 values in both groups of rocks are approximately the same 3 35 a the bcm of individual samples is composed of particles with very variable shape reflectance and raman spectra type i particles that predominate in sub greenschist facies are fine grained irregular or elongate bodies 1 to 3 m in size their maximum reflectance rmax ranges between 2 5 and 8 2 and raman parameters r1 and r2 range from 0 5 to 1 4 and 0 5 to 0 8 respectively type ii particles are lath shaped up to 40 m large bodies commonly arranged parallel to white mica flakes the number of these particles increases from sub greenschist to greenschist facies maximum reflectance varies between 6 and 11 2 and r1 and r2 raman parameters range from 0 05 to 0 7 and from 0 1 to 0 5 respectively type iii particles occur in hydrothermally altered and sheared rocks these are nodular aggregates composed of grains up to 10 m in size this type of particles has very high reflectance rmax 11 15 and its raman spectra indicate a very high degree of structural ordering corresponding to well ordered graphite type i particles represent original organic matter in the metasediments type ii particles are believed to have been formed either in situ by solid state transformation of type i particles or by crystallization from metamorphic fluids gradual graphitization of the type i organic particles and the growth of lath shaped type ii particles from a fluid phase is assumed to have taken place under the peak metamorphic conditions associated with the burial of birimian sediments during thrust tectonism progressive tectonic accretion and crustal thickening during the d1 event of the eburnean orogeny the growth of equant high reflectance postkinematic nodular aggregates of type iii particles is ascribed to the reduction of co2 rich fluids during a hydrothermal event associated with late eburnean d2 exhumation and strike slip movements type i carbonaceous particles were only slightly affected by high temperature low pressure contact metamorphism during intrusion of late eburnean magmatic bodies whereas formation of type ii or iii particles was not recorded in contact metamorphosed rocks at all

stable c o and s isotope record of magmatic hydrothermal interactions between the faleme fe skarn and the loulo au systems in western mali the gara yalea and gounkoto au deposits of the 17 moz loulo mining district largely hosted by the kofi series metasediments are located several kilometers to the east of the 650 mt fe skarn deposits in the adjacent faleme batholith the au deposits are interpreted to have formed through phase separation of an aqueous carbonic fluid which locally mixed with a hypersaline brine of metaevaporite origin recognition of an intrusive relationship between the faleme batholith and kofi series opens the possibility that the fe skarns and au deposits are part of the same mineral system in this paper we combine new 13c 18o and 34s data from the karakaene ndi skarn au occurrences along the western margin of the kofi series and zircons within plutonic rocks of the faleme batholith we combine these with existing data from the loulo au deposits to model the contribution of magmatic volatiles to au mineralization c and o isotope compositions of auriferous carbonate quartz sulfide veins from the loulo au deposits have wide ranges 13c 21 7 to 4 5 and 18o 11 8 to 23 2 whereas values from carbonate veins in kofi series au prospects close to the faleme batholith and the karakaene ndi fe skarn deposit have more restricted ranges 13c 16 8 to 3 7 18o 11 4 to 17 2 and 13c 3 0 1 18o 12 6 1 respectively kofi series dolostones have generally higher isotopic values 13c 3 1 to 1 3 and 18o 19 1 to 23 3 pyrite from kofi series au prospects adjacent to the faleme batholith have a wide range of 34s values 4 6 to 14 2 similar to pyrite from the karakaene ndi skarn 2 8 to 11 9 whereas 34s values of pyrite and arsenopyrite from the loulo deposits are consistently 6 comparison of the c and o isotope data with water rock reaction models indicates the loulo au deposits formed primarily through unmixing of an aqueous carbonic fluid derived from the devolatilization of sedimentary rocks with an organic carbon component isotopic data are permissive of the hypersaline brine that enhanced this phase separation including components derived from both kofi series evaporite horizons interlayered with the dolostones and a magmatic hydrothermal brine this magmatic hydrothermal component is particularly apparent in o c and s isotope data from the gara deposit and au prospects immediately adjacent to the faleme batholith

relationship between oriented calcite concretions and permeability correlation structure in an alluvial aquifer sierra ladrones formation new mexico abstract fluvial deposits of the sierra ladrones formation pliocene pleistocene contain abundant elongate calcite concretions that are interpreted to have precipitated in the saturated zone parallel to the direction of groundwater flow the orientation of these concretions is coincident with the orientation of the permeability correlation structure as determined from architectural element mapping and permeameter measurements in cases where the paleo groundwater flow closely corresponds with the direction of fluvial deposition measurement of concretion orientations in alluvial rocks and sediments may provide a means of rapidly estimating the major axis of the permeability correlation structure over large areas

an oxygen and carbon isotopic study of multiple episodes of fluid flow in the dalradian and highland border complex stonehaven scotland the carbon and oxygen systematics of rocks north of stonehaven scotland were studied to provide new constraints on the nature and timing of metamorphic and post metamorphic fluid infiltration carbon and oxygen isotopic data were collected from carbonated spilites and carbonate bearing veins in the highland boundary fault from dalradian metacarbonate layers and pervasively carbonated schists and from three generations of carbonate bearing veins and a quartz porphyry dyke within the dalradian isotopic evidence for syn metamorphic fluid infiltration in the dalradian is preserved in quartz calcite veins in the upper chlorite zone and in staurolite zone metacarbonates 18 o values for coexisting quartz 18 o quartz 13 6 and calcite 18 o calcite 11 6 in a vein from the upper chlorite zone are consistent with precipitation of both minerals from a single fluid at c 375 c oxygen isotopic data from staurolite zone metacarbonate layers 18 o calcite 10 4 to 12 9 suggest major isotopic exchange with an external fluid equilibrated with a quartzo feldspathic reservoir such as the dalradian metasediments at metamorphic temperatures low negative carbon isotope data from these metacarbonate layers 13 c calcite 12 4 to 17 7 may indicate a component of oxidized organic material in the infiltrating fluid post metamorphic fluids derived from or equilibrated with a high temperature silicate reservoir such as felsic igneous rocks were responsible for precipitation of post metamorphic vein carbonate carbonation of pelitic schists and alteration of metamorphic feldspar porphyroblasts from barrow s famous spotted chloritoid schists in an area geographically associated with several carbonated quartz porphyry dykes some veins in the biotite and garnet zones have been conduits for multiple generations of fluid flow quartz from this generation of veins preserves a metamorphic isotopic signature 18 o quartz 11 2 to 13 2 whereas dolomite and calcite are considered to be post metamorphic based on textural evidence and isotopic data indicating extreme oxygen isotopic disequilibrium between vein quartz and carbonate minerals 18 o dolomite 23 7 to 24 2 18 o calcite 25 3 18 o quartz 11 2 to 13 2 variably depleted carbon isotopes from carbonate minerals in these veins 13 c dolomite 8 1 to 13 4 13 c calcite 12 2 may reflect input of oxidized organic material in the mineralizing fluid isotopic data from carbonate veins and variably carbonated spilites from the highland boundary fault are consistent with hydrothermal alteration of basalts by seawater which had its carbon isotope systematics modified by addition of magmatic co 2 and or oxidation of organic carbon such alteration may have occurred on the seafloor or during emplacement in the highland boundary fault

tectono magmatic controls of post subduction gold mineralisation during late caledonian soft continental collision in the southern uplands down longford terrane britain and ireland a review abstract the southern uplands down longford terrane sudlt within the british caledonides hosts several economically significant gold occurrences including a 45 km long gold trend in central ireland that includes the proven deposit at clontibret however the region is relatively underexplored for gold and the well constrained geology and tectonic history of the terrane provide a firm geological framework for investigating the roles of tectonic magmatic and metamorphic processes in gold mineralisation during the transition from subduction of oceanic lithosphere to soft thin skinned continental collision the sudlt is an ordovician silurian fore arc accretionary complex that evolved into a foreland basin fold and thrust belt during continental collision in earliest devonian times comparable terranes in phanerozoic orogenic belts globally host significant orogenic gold deposits however the sudlt exhibits a very low metamorphic grade and mineralisation exhibits similarities with orogenic intrusion related and post subduction porphyry gold systems gold in the sudlt predominantly occurs as a lattice constituent of arsenopyrite and pyrite within quartz carbonate veins and disseminated within phyllic and propylitic potassic alteration haloes around quartz carbonate veins and some minor intrusions fluid inclusion data indicate that the gold was deposited from a low salinity mesothermal 330 c carbonic fluid of mixed magmatic metamorphic origin consistent with caledonian orogenic conditions gold mineralisation occurred at shallow crustal depths of less than 10 km and exhibits a broad spatial association with major ne sw trending caledonoid shear zones d1 economically significant gold mineralisation in the sudlt is most commonly hosted by transverse nw se and n s trending fractures d3 constrained to between 418 and 410 ma that cut anchizone epizone facies volcaniclastic turbiditic metasedimentary host rocks the mineralised d3 structures cut and are cut by broadly coeval polyphase diorite and granodiorite intrusions gold mineralisation is associated with the early dioritic i type metaluminous oxidised phase of trans suture suite magmatism that straddles the iapetus suture but predates the later emplacement of s type granitic magma the host structures were subsequently reactivated during late palaeozoic mesozoic and cenozoic times and host younger base metal deposits pb zn cu sn sb mineralisation occurred in early devonian times 418 and 410 ma following arrival of the avalonian continental margin at the subduction trench at 420 ma coeval with the onset of regional transtension post subduction lithospheric mantle delamination and k lamprophyric and mafic to intermediate calc alkaline magmatism gold in the sudlt therefore provides a case study for relationships between post subduction porphyry and orogenic gold mineralisation in soft continental collision zones the transient geodynamic setting between subduction and transtension common during soft continental collision provided the critical elements of the mineralising system a metasomatically fertilised mantle source transient geodynamics favourable lithospheric architecture for the rapid transfer of mass and energy from subcrustal to upper crustal levels thin skinned deformation and relatively low exhumation favours the preservation of mineral deposits in the upper crust

hydrothermal alteration and mineralization in the neves corvo volcanic hosted massive sulfide deposit portugal ii oxygen hydrogen and carbon isotopes the oxygen hydrogen and carbon isotope compositions of hydrothermally altered rocks and minerals from massive and stockwork ores of the neves corvo volcanic hosted massive sulfide deposit have been used to infer its thermal history and the origins of the hydrothermal fluids quartz and cassiterite separated from the stringer and massive cassiterite ores of the corvo orebody tin corridor have 18o values of 12 5 to 13 6 and 1 4 to 0 4 per mil respectively quartz cassiterite oxygen isotope fractionation indicates temperatures of 174 to 207 c avg 191 c for precipitation of this mineral pair the calculated oxygen isotope composition of the fluid 18oh2o 0 4 to 0 7 avg 0 1 suggests that cassiterite precipitation resulted from extensive mixing of a tin bearing fluid with seawater at sea floor hydrothermal vents the whole rock oxygen isotope composition of the hydrothermally altered felsic volcanic rocks hosting the sulfide stockwork of the corvo orebody varies from the central stockwork chloritic alteration zone i 18o 8 7 9 6 to the peripheral zones sericitic and paragonitic alteration zones iia 18o 11 6 13 1 and iib 18o 11 0 14 1 the 18o values of quartz also increase from alteration zone i avg 13 9 to alteration zone iia avg 14 4 to alteration zone iib avg 15 0 quartz from the hanging wall jasper unit yielded higher 18o values of 15 5 to 17 9 per mil chlorite from alteration zone i has 18o values of 6 1 to 8 4 per mil avg 7 4 sericite has 18o and d values that vary slightly from 18o 9 3 to 10 1 per mil avg 9 9 and d 59 to 47 per mil avg 53 in alteration zone i to 18o 9 8 to 11 4 per mil avg 10 5 and d 50 per mil in alteration zone iia to 18o 9 4 to 11 7 per mil avg 10 6 and d 61 to 36 per mil avg 45 in alteration zone iib siderite associated with waning hydrothermal activity has low 13c values 11 1 to 5 8 suggesting the incorporation of oxidized organic carbon from the footwall sequence and or the addition of magmatic carbon in alteration zone i quartz chlorite fractionation indicates temperatures of 249 to 402 c avg 335 c and 18oh2o values of 6 1 to 10 2 per mil avg 8 3 quartz sericite pairs gave isotopic equilibrium temperatures of 250 to 289 c avg 269 c and 18oh2o values of 5 8 to 7 0 per mil avg 6 4 in alteration zone iia and 170 to 318 c avg 223 c and 18oh2o 1 4 to 8 3 per mil avg 4 5 in alteration zone iib the oxygen isotope data show that in the core of the feeder system alteration zone i the temperature and the 18oh2o values were very high the d values calculated for the ore fluids d 37 to 11 are also significantly lower than seawater outward from the central stockwork alteration zones iia and iib the ore fluid was depleted in 18o and the temperatures were lower the oxygen hydrogen and carbon isotope signatures of the neves corvo ore fluids are best explained by incorporation of magmatic and or metamorphic fluids into a seawater dominated hydrothermal system magmatic fluids in particular were a likely source for the tin and much of the copper in the neves corvo ores

constraints on the timing of late eburnean metamorphism gold mineralisation and regional exhumation at damang mine ghana the damang gold deposit in southwest ghana is unique among known deposits in ghana comprising gold mineralisation in two distinct styles early gold hosted in a stratigraphically controlled auriferous quartz pebble conglomerate horizon is overprinted by later gold mineralisation contained in a sub horizontal fault fracture quartz vein array a multi system geochronological study is used to constrain the timing of igneous activity regional metamorphism gold mineralisation and the thermal history at damang u pb analysis of zircons from birimian volcaniclastic and intrusive rocks constrain volcanism and associated intrusive activity at 2178 0 9 3 ma and 2164 6 8 0 ma respectively which is consistent with previous studies the age of formation of staurolite grade amphibolite facies peak metamorphic mineral assemblages at 2005 26 ma is provided by u th total pb epma analysis of metamorphic monazite grains in the tarkwa phyllite measured 40ar 39ar biotite ages range between 1980 9 ma and 1898 11 ma argon diffusion modelling with the programme diffarg suggests that this age range could be achieved by a period of rapid cooling at a rate of approximately 17 c ma followed by a prolonged period of much slower cooling at a rate of 0 15 c ma the period of rapid cooling is interpreted to represent localised exhumation of the damang host rocks during the latest stage of the eburnean orogeny at the time of hydrothermal gold mineralisation given these age constraints hydrothermal gold mineralisation is inferred to have occurred between approximately 2030 ma and 1980 ma these ages constrain metamorphism fluid flow and gold mineralisation at damang and are the youngest currently recognised in the birimian of sw ghana

grain size measurements of natural gas hydrates abstract methane hydrates are present in marine seep systems and occur within the gas hydrate stability zone very little is known about their crystallite sizes and size distributions because they are notoriously difficult to measure crystal size distributions are usually considered as one of the key petrophysical parameters because they influence mechanical properties and possible compositional changes which may occur with changing environmental conditions variations in grain size are relevant for gas substitution in natural hydrates by replacing ch 4 with co 2 for the purpose of carbon dioxide sequestration here we show that crystallite sizes of gas hydrates from some locations in the indian ocean gulf of mexico and black sea are in the range of 200 400 m larger values were obtained for deeper buried samples from odp leg 204 the crystallite sizes show generally a log normal distribution and appear to vary sometimes rapidly with location

the geology and mineralogy of the loulo mining district mali west africa evidence for two distinct styles of orogenic gold mineralization loulo is a world class orogenic gold mining district in the birimian terrane of western mali orebodies are located along second or higher order shears associated with the senegal mali shear zone with gold mineralization largely linked to a transtensional event the mine camp is divided into two distinct styles of gold deposit on the basis of differing geologic characteristics one group is typified by the gara deposit whereas the other by the yalea deposit both deposit styles are hosted by similar rock types calcareous graywackes and calciticdolomitic marbles gara style orebodies occur as sulfide disseminations or ankerite rich shear vein stockworks hosted in folded tourmalinized sandstones and breccias mainly within 2 km of the senegal mali shear zone these deposits are characterized by intense multi stage albitization and tourmalinization pre syn and postmineralization gold lodes are fe rich dominated by nickeloan pyrite contain cu ni co minor and trace sulfides e g chalcopyrite gersdorffite pentlandite cobaltite millerite and show consistently high levels of p ree w bearing phases apatite monazite xenotime and scheelite base metal concentrations show a marked increase in marble host rocks with the formation of nickeloan pyrite cobaltite clausthalite ores in contrast yalea style deposits are associated with quartz ankerite vein lodes and disseminated sulfide stringer zones mineralization occurs along highly altered tourmaline absent brittle ductile shears up to 8 km away from the senegal mali shear zone wall rock alteration is characterized by addition of k 2 o cao co 2 h 2 o and sio 2 with mineral assemblages consisting of chlorite sericite carbonate quartz albite ore paragenesis is enriched in as mainly as multistage growth of arsenopyrite and arsenian pyrite base metal sulfides scheelite and ree phosphates are extremely rare the diversity in the ore paragenesis is controlled by a dynamic hydrothermal system that sourced fluids and metals from different reservoirs within the region the as rich yalea style deposits have characteristics typical of birimian gold mineralization in ghana and auriferous fluids are likely derived from the dewatering of sedimentary rocks during regional metamorphism on the other hand the polymetallic fe b rich gara style orebodies show atypical features for birimian gold mineralization instead field relations and the mineralogy and geochemistry of the gara style lodes indicate a strong hydrothermal influence from surrounding intrusive stocks with possible links between gold mineralization and iron oxide skarn development in the region the data collected at loulo highlight the diverse nature of orogenic gold deposits especially in west africa this style of ore deposit can form from a variety of fluid sources both metamorphic and magmatic

a probability model to detect carbonate cementation in sandstones and other enigmatic wireline facies abstract zones of carbonate cementation identified within lower paaratte formation sandstones of the eastern otway basin victoria southeastern australia can be quantitatively detected by a wireline log based probability model though trained on these zones the same model appears to accurately predict carbonate cementation within late cretaceous to eocene reservoir sandstones of the latrobe group supersequence of the gippsland basin 100s of km to the east predicted carbonate cementation matches published evidence a regional petrographic study and provides a plausible interpretation for corresponding sections of formation micro imager data acquired more recently these latrobe group sandstones are thought to have once been pervasively cemented prior to development of the secondary porosity responsible for providing reservoirs to the main petroleum system of the basin however model predictions imply that discrete heavily cemented zones remain which must have acted and must still act as local obstructions to reservoir fluid migration these zones would also likely react with carbonic acid formed at plume fronts generated by dedicated co2 storage operations of the future the cemented zones that have been predicted are sparse spatially sporadic and cannot be resolved by 3d seismic reflection survey data at present day reservoir depths these predictions therefore emphasise the difficulty in mapping the distribution of carbonate cemented zones however they provide calibration data for future mapping systems two different tests of the probability log model were undertaken one benchmarks its predictions against those of a neural network trained using the original model development dataset from the otway basin predictive performance of the neural network model is significantly worse than that of the probability log model when making predictions using wireline data acquired at a well in the gippsland basin the second test demonstrates that the general probability log modelling approach is amenable to classifying other cryptic electro facies for example those representing enigmatic or ambiguous rock types not easily interpreted from common wireline log data using standard theory this test presents a case study by which the probability log modelling approach predicts the presence of both basic and evolved felsic igneous sill intrusions within the stratigraphic succession of the faroe shetland basin of nw europe

field study of dispersion in a heterogeneous aquifer 3 geostatistical analysis of hydraulic conductivity observations of the spatial variability of hydraulic conductivity at a tracer test site located at columbus air force base in mississippi are presented direct measurements of hydraulic conductivity of the heterogeneous alluvial aquifer at the site were made using borehole flowmeter logging slug tests and a laboratory permeameter to test undisturbed soil cores indirect methods estimating hydraulic conductivity were also evaluated including soil grain size analyses surface geophysical surveys and mapping of sediment facies the spatial covariance of the 2187 hydraulic conductivity values obtained with the borehole flowmeter method was examined the log hydraulic conductivity variance ln2k and the horizontal and vertical correlation scales h and v of 4 5 12 8 m and 1 6 m respectively were estimated assuming second order stationarity of the conductivity field the covariance parameters are uncertain with bounding values that are 24 76 above or below the estimate covariance parameters estimated with more limited nonflowmeter data were within the same magnitude as those obtained using the extensive flowmeter data suggesting that data from a variety of methods may be used to provide approximate values of the autocovariance parameters nonstationarity of the ln k field was examined by removing three dimensional polynomial trend surfaces and calculating variograms of the residuals significantly lower estimates for ln2k h and v of 2 7 4 8 m and 0 8 m respectively were obtained from the third order log conductivity residuals after trend removal the bounding parameter values differ 15 44 from the estimated values accounting for unsteady flow and the uncertainty in the covariance parameters of the third order log conductivity residuals the calculated longitudinal and horizontal transverse macrodispersivities ranged from 1 5 m to 1 8 m and 0 3 m to 0 6 m respectively

fluid inclusion and isotopic constraints on the origin of the paleoproterozoic yinachang fe cu ree deposit southwest china the yinachang deposit in the kangdian fe cu metallogenic province southwest china contains approximately 20 mt ore 41 9 to 44 5 wt fe and 15 mt ore 0 85 to 0 97 wt cu with potentially significant rees orebodies are hosted in the late paleoproterozoic dongchuan group and consist mainly of massive and banded replacement ores the paragenetic sequence of this deposit includes pre ore na fe alteration stage i fe ree mineralization dominated by magnetite and siderite with subsidiary apatite and fluorite stage ii and cu ree mineralization with chalcopyrite ankerite biotite and subordinate apatite fluorite allanite and synchysite stage iii this sequence is similar to those of many iron oxide copper gold iocg deposits elsewhere there are two distinct types of fluid inclusions within apatite of both stages ii and iii type 1 fluid inclusions with liquid and vapor phases have relatively low homogenization temperatures 98 345 c and salinities 4 7 16 2 wt nacl equiv whereas type 2 fluid inclusions with one or more solid phases a liquid phase and a vapor phase have higher homogenization temperatures 273 500 c and salinities 36 2 59 8 wt nacl equiv fluids in sulfur isotope equilibrium with chalcopyrite of stages iii have 34 s cdt values ranging from 2 8 to 2 7 consistent with a magmatic origin for the sulfur thus sulfur isotopie compositions imply that magmatic hydrothermal fluids probably represented by the hot and saline type 2 fluid inclusions were involved in the ore forming process although 34 s close to 0 might also mean that sulfur was leached from igneous rocks by nonmagmatic fluids siderite of stage ii and ankerite of stage iii have 13 c pdb values ranging from 12 4 to 7 5 and from 8 5 to 4 4 respectively the large variation of 13 c pdb values reveals a strong interaction between the magmatic hydrothermal fluids and the dolostone in the ore hosting sequence calculated 18 o smow values of the fluids in stages ii and iii are broadly similar and vary from 11 1 to 17 1 avg 13 5 such values are higher than that of magmatic hydrothermal fluids and thus may indicate extensive fluid wall rock interaction and or mixing of magmatic hydrothermal fluids with shallow level nonmagmatic fluids this interpretation is consistent with the radiogenic sr isotope compositions of bulk ores 87 sr 86 sr i 0 710404 0 734034 and fluorite separated from ores 87 sr 86 sr i 0 709851 0 723658 these nonmagmatic fluids are probably representative of the type 1 fluid inclusions bulk ores and mineral separates have nd t values ranging from 8 1 to 0 4 mostly 0 9 to 0 4 similar to the coeval mafic intrusions in the region 2 0 to 2 8 this similarity suggests that rees in the yinachang deposit might have been derived dominantly from mantle derived magmas on the other hand relatively low nd t values 5 5 and 8 1 of some samples indicate insignificant ree contributions from wall rocks the yinachang deposit is temporally associated with regional mantle derived mafic magmatism and may have a genetic relationship with it fluids derived from deep seated magmas may have supplied abundant sulfur and carbon as well as some of the ore metals in this deposit the mafic magmatism could also have provided heat to induce the shallow level nonmagmatic fluids to interact with the sedimentary volcanic sequence thus leaching additional ore metals into the ore forming system fluid mixing can effectively trigger saturation and deposition of ore minerals our work highlights that nonmagmatic fluids are not a prerequisite for cu au ree mineralization but may facilitate metal deposition in magmatic hydrothermal iocg systems

appearance and preservation of natural gas hydrate from hydrate ridge sampled during odp leg 204 drilling abstract the state of preservation of natural gas hydrate samples recovered from 6 sites drilled during odp leg 204 at southern summit of hydrate ridge oregon margin has been investigated by x ray diffraction xrd and cryo scanning electron microscopy cryo sem techniques a detailed characterization of the state of decomposition of gas hydrates is necessary since no pressurized autoclave tools were used for sampling and partial dissociation must have occurred during recovery prior to the quench and storage in liquid nitrogen samples from 16 distinct horizons have been investigated by synchrotron x ray diffraction measurements at hasylab hamburg a full profile fitting analysis rietveld method of synchrotron xrd data provides quantitative phase determinations of the major sample constituents such as gas hydrate structure i si hexagonal ice ih and quartz the ice content ih in each sample is related to frozen water composed of both original existing pore water and the water from decomposed hydrates hydrate contents as measured by diffraction vary between 0 and 68 wt in the samples we measured samples with low hydrate content usually show micro structural features in cryo sem ascribed to extensive decomposition comparing the appearance of hydrates at different scales the grade of preservation seems to be primarily correlated with the contiguous volume of the original existing hydrate the dissociation front appears to be indicated by micrometer sized pores in a dense ice matrix

concretions in glacial sediments at seglvatnet norway abstract concretions in pleistocene glacial sediments at seglvatnet norway include a wide variety of forms the simplest of which apparently reflects relatively uniform growth rates in different directions the preferred orientation of others may be associated with water movement through the sediments concretion shape generally is influenced by the grain size and bedding of the enveloping sediments some concretions may have formed during or soon after deposition of the sediments others are forming at present

mapping of prospectivity and estimation of number of undiscovered prospects for lode gold southwestern ashanti belt ghana in the southwestern part of the ashanti belt the results of fractal and fry analyses of the spatial pattern of 51 known mines prospects of mostly lode gold deposits and the results of analysis of their spatial associations with faults and fault intersections suggest different predominant structural controls on lode gold mineralisation at local and district scales intersections of nne and nw trending faults were likely predominantly involved in local scale structural controls on lode gold mineralisation whilst nne trending faults were likely predominantly involved in district scale structural controls on lode gold mineralisation the results of the spatial analyses facilitate the conceptualisation and selection of spatial evidence layers for lode gold prospectivity mapping in the study area the applications of the derived map of lode gold prospectivity and a map of radial density of spatially coherent lode gold mines prospects results in a one level prediction of 37 undiscovered lode gold prospects the applications of quantified radial density fractal dimensions of the spatial pattern of spatially coherent lode gold mines prospects result in an estimate of 40 undiscovered lode gold prospects the study concludes finally that analysis of the spatial pattern of discovered mineral deposits is the key to a strong link between mineral prospectivity mapping and assessment of undiscovered mineral deposits

new constraints on the genesis of the giant dayingezhuang gold silver deposit in the jiaodong district north china craton abstract the dayingezhuang gold silver deposit in the jiaodong district eastern margin of north china craton is hosted in the ne to nne trending zhaoping fault along the contact between precambrian metamorphic rock jiaodong group and mesozoic intrusion linglong granite although many studies have addressed the genesis of this deposit the mineralization age the sources of ore forming fluids and materials the ore genesis is still controversial particularly on the relationship between au and ag in this paper new dating and in situ isotopic data are presented to provide tight constraints on the timing and ore forming processes of the dayingezhuang deposit mineral assemblages of dayingezhuang deposit can be divided into four stages the quartz pyrite stage i the au bearing quartz siderite pyrite stage ii the ag bearing quartz siderite polymetallic sulfide stage iii and the quartz carbonate stage iv the ag mineralization is texturally later than the gold mineralization and only develops locally in the orebody i sericite from sulfide bearing ores has a 40ar 39ar plateau age of 119 1 1 2 ma and pre ore mafic dikes yield a zircon u pb age of 122 7 2 6 ma the new dating results consistently indicate that the dayingezhuang deposit formed at 120 ma and is synchronous with the regional gold mineralization event in the jiaodong district the c and o isotopic values of siderite and calcite from stage ii to iv vary from 5 4 to 2 2 13cv pdb and 7 8 to 12 1 18ov smow respectively and the calculated values of them for ore forming fluids are 7 1 to 2 9 and 4 1 to 5 7 respectively the 34scdt values of pyrite from stage i to iii range from 4 8 to 9 0 pyrite from stage i to iii has 206pb 204pb values of 17 181 17 339 207pb 204pb of 15 414 15 504 208pb 204pb of 37 729 37 984 these isotopic results combined with geological and geochronological evidence collectively indicate that the ore forming fluids and metals of dayingezhuang were mainly derived from the deep mantle derived magma typically represented by intermediate mafic dikes and mixed with meteoric water subsequently these data also indicate that the stage iii ag mineralization was not a late event independent from the stage i and ii au mineralization the dayingezhuang deposit may not be related to the activation of detachment faults around the linglong massif at 130 4 ma as previously suggested but a product of the destruction of ncc accompanied with the crust mantle interaction and lithospheric thinning at 120 ma

the filon norte orebody tharsis iberian pyrite belt a proximal low temperature shale hosted massive sulphide in a thin skinned tectonic belt the filon norte orebody tharsis iberian pyrite belt is one of the largest pyrite rich massive sulphide deposits of the world the present structure of the mineralization consists of an internally complex low angle north dipping thrust system of variscan age there are three major tectonic units separated by thick fault zones each unit with its own lithologic and hydrothermal features they are internally organized in a hinterland dipping duplex sequence with high angle horses of competent rocks igneous and detritic rocks and massive sulphides bounded by phyllonites the mineralization is within the lower unit and is composed of several stacked sheets of massive sulphides and shales hosting a stockwork zone with no obvious zonation the intermediate unit is made up of pervasively ankeritized shales and basalts spilites here hydrothermal breccias are abundant the upper unit is the less hydrothermally altered one and consists of silicified dacites and a diabase sill the tectonic reconstruction suggests that the sequence is inverted and the altered igneous rocks were originally below the orebody carbon oxygen and sulphur isotopes in the massive sulphides and hydrothermal rocks as well as the mineral assemblage and the paragenetic succession suggest that the sulphide precipitation in the sea floor took place at a low temperature 150 c without indication at least in the exposed section of a high temperature copper rich event sporadic deep subsea floor boiling is probably responsible for the formation of hydrothermal breccias and the wide extension of the stockwork its co au enrichment is interpreted as being related with the superposition of some critical factors such as the relationship with black shales the low temperature of formation and the boiling of hydrothermal fluids the present configuration and thickness of the orebody is due to the tectonic stacking of a thin and extensive blanket 2 4 km2 of massive sulphides with low aspect ratio they were formed by poorly focused venting of hot modified seawater equilibrated with underlying rocks into the seafloor massive sulphide precipitation took place by hydrothermal fluid quenching bacteriogenic activity and particle settling in an unusual restricted euxinic and shallow basin brine pool with a low detritic input but with important hydrothermal activity related to synsedimentary extensional faulting resedimentation of sulphides seems to be of major importance and responsible for the observed well mixed proximal and distal facies the tectonic deformation is largely heterogeneous and has been mostly channelled along the phyllonitic tectonized shales deformation bands thus sedimentary and diagenetic textures are relatively well preserved outside the deformation bands in the massive sulphides superimposed variscan recrystallization is not very important and only some early textures are replaced by metamorphic tectonic ones the stockwork is much more deformed than the massive sulphides the deformation has a critical effect on the present morphology of the orebody and the distribution of the ore minerals this deposit is a typical example of the sheet like shale hosted anoxic low temperature and zn rich massive sulphides developed in a ensialic extensional basin

oriented concretions ionian coast italy evidence of groundwater flow direction abstract highly elongate aligned calcite cemented concretions are present in exposed shallow marine pleistocene sands and gravels along the basilicata coast of the ionian sea southern italy concretions are shaped like rods pencils and cigars 0 5 3 cm in diameter and thin blades 031 cm thick and 1 5 cm wide most have aspect ratios greater than 5 and some reach 30 most concretions are in parallel laminated beds but a few are in cross stratified and ripple cross laminated beds concretions cut across foresets and inclined laminae the standard deviations of long axes at each of six localities with more than eight measurements are less than 7 the standard deviation of the orientation of concretions from 30 localities over a 400 km2 area is less than 17 previous interpretations for elongate carbonate concretions in sandstones include growth in the direction of groundwater flow growth parallel with current oriented grains and or paleocurrents and selective cementation in the troughs of wave ripples the basilicata concretions are oriented perpendicular to the local coastline and parallel with the direction of present day groundwater flow the groundwater flow direction was probably the same during the pleistocene the evidence indicates that the concretions grew parallel with groundwater flow and were not influenced by sedimentary structures or grain fabric the isotopic values 13c and 18o range from 10 to 1 pdb and from 5 to 3 pdb respectively and textures of calcite cement in the concretions indicate that the calcite was precipitated from meteoric groundwater nannobacteria may have mediated precipitation of calcite

paleogeographic significance of oriented calcareous concretions in the triassic katberg formation south africa abstract elongated calcareous concretions in sandstones of the fluvial katberg formation beaufort group south africa display a distinct preferred orientation that coincides with the regional paleoslope as deduced from cross stratification and primary current lineations these concretions formed after deposition of the sandstone but before the strata were affected by tectonic movement presumably by groundwater flowing down the regional paleoslope similar sandstone hosted concretions in other non marine successions could potentially yield valuable information on paleo groundwater flow and by inference depositional paleoslopes

field study of dispersion in a heterogeneous aquifer 1 overview and site description results are presented for a large scale natural gradient tracer experiment conducted in a heterogeneous alluvial aquifer at a site near columbus mississippi the study was initiated with a 48 hour pulse injection of 10 m3 of groundwater containing bromide and three organic tracers pentaflourobenzoic acid o trifluoromethylbenzoic acid and 2 6 diflourobenzoic acid over a 20 month period seven comprehensive samplings of the tracer plume were performed at approximately 1 to 4 month intervals using an extensive three dimensional sampling well network the dominant feature of the tracer plume that evolved during the study was the highly asymmetric concentration distribution in the longitudinal direction this asymmetry was produced by accelerating groundwater flow along the plume travel path that in turn resulted from an approximate 2 order of magnitude increase in the mean hydraulic conductivity between the near field and far field regions of the site the columbus study is distinct from previous natural gradient experiments because of the extreme heterogeneity of the aquifer the large scale spatial variations in groundwater velocity and the extensive set of hydraulic conductivity measurements for the aquifer

cyclonic brine flow pattern recorded by oriented gypsum crystals in the badenian evaporite basin of the northern carpathian foredeep abstract in the middle miocene badenian within sulfate evaporites formed in the northern carpathian foredeep basin of poland ukraine and the czech republic the apices of the primary bottom grown gypsum crystals selenites are similarly aligned over broad areas and have a common azimuth this crystal orientation is interpreted as the product of a consistent direction of a brine inflow during gypsum crystallization and such a unique commonality is used as a tool for paleocurrent analysis in order to recognize both the local and the regional brine paleocurrents in the basin earlier sedimentologic and stratigraphic studies proved that these beds of oriented selenites were deposited at the same or nearly the same time in a brine not more than a few meters deep it is interpreted that the oriented selenites recorded only the mean flow within the basin mainly from the periods of mixing in the dry season lowstands when the whole brine column down to the bottom was ca sulfate oversaturated the measured paleocurrent vectors showed that the brines flowed along the marginal zone of the carpathian foredeep basin predominantly in a counterclockwise direction this longshore flow is interpreted as a typical cyclonic circulation analogous to those present in recent lakes and semi closed basins in the northern hemisphere where counterclockwise mean water drift is directly or indirectly forced by earth rotation the coriolis effect the detected brine flow directions do not reflect downslope gravity flow of dense brines into the central basinal depths or subbasins that were filling with the halite facies

the coupled geochemistry of au and as in pyrite from hydrothermal ore deposits the ubiquity of au bearing arsenian pyrite in hydrothermal ore deposits suggests that the coupled geochemical behaviour of au and as in this sulfide occurs under a wide range of physico chemical conditions despite significant advances in the last 20years fundamental factors controlling au and as ratios in pyrite from ore deposits remain poorly known here we explore these constraints using new and previously published empa la icp ms sims and pixe analyses of as and au in pyrite from carlin type au epithermal au porphyry cu cu au and orogenic au deposits volcanogenic massive sulfide vhms witwatersrand au iron oxide copper gold iocg and coal deposits pyrite included in the data compilation formed under temperatures from 30 to 600 c and in a wide variety of geological environments the pyrite au as data form a wedge shaped zone in compositional space and the fact that most data points plot below the solid solubility limit defined by reich et al 2005 indicate that au1 is the dominant form of au in arsenian pyrite and that au bearing ore fluids that deposit this sulfide are mostly undersaturated with respect to native au the analytical data also show that the solid solubility limit of au in arsenian pyrite defined by an au as ratio of 0 02 is independent of the geochemical environment of pyrite formation and rather depends on the crystal chemical properties of pyrite and post depositional alteration compilation of au as concentrations and formation temperatures for pyrite indicates that au and as solubility in pyrite is retrograde au and as contents decrease as a function of increasing temperature from 200 to 500 c based on these results two major au as trends for au bearing arsenian pyrite from ore deposits are defined one trend is formed by pyrites from carlin type and orogenic au deposits where compositions are largely controlled by fluid rock interactions and or can be highly perturbed by changes in temperature and alteration by hydrothermal fluids the second trend consists of pyrites from porphyry cu and epithermal au deposits which are characterised by compositions that preserve the au as signature of mineralizing magmatic hydrothermal fluids confirming the role of this sulfide in controlling metal ratios in ore systems

air bubbles and clathrate hydrates in the transition zone of the ngrip deep ice core abstract field studies of air bubble and clathrate inclusions in the ngrip ice core reveal two stages of clathrate formation in the transition zone translucent primary clathrates with rough surfaces and irregular shapes formed from individual bubbles split up and form smaller specimens deeper down in the transition zone 1100 m clathrate metamorphosis sets in giving rise to the formation of transparent secondary clathrates with smooth and regular often poly hedral or faceted shapes rod like crystals of great length 2 mm often show signs of separation into smaller units that fragment into smaller specimens these findings imply that the simplified picture of one bubble forming one clathrate has to be modified

the massawa gold deposit eastern senegal west africa an orogenic gold deposit sourced from magmatically derived fluids the massawa gold project is situated on the senegalese side of the highly prospective productive palaeo proterozoic birimian kedougou kenieba inlier which hosts several world class orogenic gold deposits districts in western mali e g loulo and sadiola the massawa ore body has a strike length of at least 4 km and a current resource of 3 61 moz at a grade of 2 8 g t 1 the ore body is structurally controlled and located within a package of low grade regionally metamorphosed volcaniclastic sediments agglomerates tuffs and ash tuffs quartz feldspar and lithic wackes carbonaceous shales hydrothermal breccias and gabbro and porphyry sills these rocks have undergone pervasive silica alteration followed by a sericite ankerite chlorite alteration event related to mineralization two major styles of mineralization are recognized at massawa from field and laboratory studies the first stage of sulphide au mineralization is associated with disseminated arsenopyrite pyrite which follows shear zones in the sedimentary and volcano sedimentary host rocks the second stage consists of quartz stibnite tetrahedrite veining distinguished by coarse visible gold and represents a late stage overprint on the primary mineralization the two stages of gold mineralization are separated by a phase of quartz molybdenite veining a distinctive base metal trace assemblage is linked to stibnite formation including multiple sb phases such as chalcostibite zinkenite roshchinite aurostibite jamesonite and robinsonite secondary ion mass spectroscopy based gold deportment data indicate that up to 90 of stage 1 gold is held as a solid solution within either arsenopyrite or arsenian pyrite stable isotope data yield 34ssulphide values of between 0 and 4 1 and 18oh2o values of 5 5 10 9 for all stages of mineralization suggesting a magmatic fluid influence this is consistent with field data that suggest that mineralization is synchronous with emplacement of a sequence of concordant felsic sheets that mineralization occurred at shallow 6 km depths is suggested both by the presence of stibnite and by fluid inclusion studies low temperature homogenization temperatures between 150 c and 230 c h2o nacl fluids 6 wt nacl equiv and coeval co2 ch4 inclusions observed in both phases of mineralization indicate trapping conditions of 220 315 c at 1 1 65 kbar a combination of phase petrology fluid inclusion and stable isotope data suggests deposition of gold from low salinity magmatic fluids most probably released from felsic rocks similar to those emplaced into the massawa sequence during mineralization

timing and structural controls on gold mineralization at the bogoso gold mine ghana west africa the bogoso gold deposit comprises 20 open pits scattered along 18 km of a major fault zone which separates birimian supergroup turbiditic sedimentary rocks to the northwest from tarkwaian group rocks to the southeast individual pits are centered on 200 to 400 m long ore shoots which are located in three broad structural settings steeply pitching relatively high grade orebodies are commonly localized in left hand dilational jogs in the strike of the controlling faults these jogs often coincide with intersecting minor faults gold is largely confined to the graphitic mylonite which defines the fault and imbricated slices of carbonate altered rock gently dipping mineralized quartz vein stockworks and disseminated auriferous pyrite and arsenopyrite form broader zones between more steeply dipping bounding faults in the vicinity of right hand bends and splays in these faults smaller zones of disseminated sulfide mineralization coincide with areas where bedding and or foliation in the adjacent birimian sedimentary rocks is strongly discordant with the adjacent mineralized fault generally where the faults cut the short limbs of tight upright east trending folds approximately 75 percent of the gold extracted from the bogoso district has been mined from a cluster of pits that coincides with the densest and most highly connected concentration of larger faults in the bogoso concession the majority of ore shoots are located along the largest of these faults the narrower faults generally host smaller and less frequent ore shoots the fault network originated during an early episode of regional scale shortening involving thrusting and isoclinal folding the structural setting and geometry of the ore shoots implies that mineralization occurred during a later minor episode of sinistral strike slip movement on the controlling faults after development of the fault network the undeformed nature of the auriferous sulfides implies that this was the last phase of ductile brittle deformation on the host faults this conclusion is consistent with earlier work indicating that the giant ashanti deposit 150 km along strike from bogoso also developed during a period of small displacement sinistral offset late in the history of the host fault network

intraplate mafic magmatism degasification and deposition of mercury the giant almaden mercury deposit spain revisited abstract the giant almaden mercury deposit spain is hosted by the lower silurian criadero quartzite in turn this ore bearing rock unit is cross cut by the so called frailesca unit a diatreme body of basaltic composition the geochemical characteristics of the silurian to devonian almaden district volcanic units indicate that these rocks originated from an enriched evolving mantle source that ultimately yielded basanites nephelinites to rhyolites through olivine basalts pyroxene basalts trachybasalts trachytes very scarce rhyolites and quartz diabases the silurian intraplate alkaline volcanism developed in submarine conditions which triggered widespread hydrothermal activity resulting in hg ore formation and pervasive alteration to carbonates the 18 o 13 c and 34 s isotopic signatures for carbonates and pyrite suggest different sources for carbon and sulfur including magmatic and organic for the former and magmatic and sea water for the latter the most important and efficient natural source of mercury on earth is by far the volcanic activity which liberates mercury via quiescent degassing and catastrophic plinian events when eruptions can overwhelm the atmospheric budget of hg thus we suggest that co 2 degasification and coeval distillation of mercury from the volcanic rocks fed the huge hydrothermal system that led to massive deposition of mercury at almaden build up of hg 0 gas in magmatic chambers during waning rifting in the late ordovician followed by renewed volcanism in the early silurian would have resulted in massive degasification of the accumulated mercury part of this mercury went into the criadero quartzite leading to formation of the huge almaden deposit and others e g el entredicho along the same stratigraphic level progressive depletion of the deep seated magmatic hg stock would have resulted in a drastic reduction in ore deposit size after the lower silurian when smaller deposits formed e g las cuevas

oriented carbonate concretions in a paleoaquifer insights into geologic controls on fluid flow oriented carbonate concretions appear to provide a powerful new tool in the evaluation of flow through heterogeneous aquifers calcite cement forms elongate bodies of indurated rock that have distinct and mappable spatial orientations we have mapped 200 of these concretion orientations as well as the hydrogeologic characteristics of the host sediments in outcrops of the pliocene pleistocene sierra ladrones formation in central new mexico the average azimuthal concretion orientations is 150 which is consistent with the southeasterly groundwater flow suggested by past and present basin geometry using a first order approximation the dimensionless transverse velocity is estimated from the orientation data the spatial correlation structure of the dimensionless transverse velocity is distinctly different in the two units investigated however both are consistent with existing stochastic models the differences between the units and the similarities with theoretical models suggest that concretion orientations can be used to study how fluids flow through heterogeneous systems over scales ranging from centimeters to tens of kilometers

evolution of hydrothermal fluids in the ashanti gold belt ghana stable isotope geochemistry of carbonates graphite and quartz the oxygen and carbon isotope systematics of ankerite siderite calcite quartz and graphite from the bogosu and prestea mining districts of the ashanti gold belt ghana are used to speculate on the processes leading to hydrothermal alteration and gold deposition the geologic environment of this turbidite and graywacke hosted gold system was a sediment dominated accretionary complex the gold ores are localized within the ashanti structural belt which we suggest became a conduit for deep seated fluids during uplift and dilatancy ankerite and siderite in mineralized sedimentary rocks are broadly similar in oxygen and carbon isotope compositions to corresponding minerals in the metasedimentary country rocks evidence of alteration that preceded gold mineralization is best preserved in spatially associated altered mafic dikes the compositions of fluids responsible for the gold mineralization and associated alteration have been reconstructed using temperatures from carbonate mineral geothermometry alteration of country rocks occurred under rock dominant greenschist facies conditions the isotopic composition of the least altered mafic dikes was influenced by interaction with fluids generated from birimian greenschist facies metasedimentary rocks later extensive carbonate alteration of mafic dikes adjacent to the structural conduit occurred during slow upward migration of fluids lateral diffusion of hydrogen into the structural conduit permitted partial conversion of co 2 to methane resulting in 13 c enrichment of both the residual co 2 and the deposited carbonate minerals rapid even explosive expulsion of deep seated fluids and co 2 h 2 o phase separation occurred episodically throughout the evolution of the hydrothermal system this caused precipitation of sulfides arsenides and gold at relatively high crustal levels and low ambient pressure and temperature during mineralization fluid ascent was too rapid for isotopic and chemical reequilibration between the deep seated ore fluid and adjacent country rocks the ore fluid had chemical and isotopic compositions consistent with deep seated metamorphic fluids that were subsequently modified by preferential partitioning of 18 o and 13 c into a gas phase the composition of the deep seated metamorphic fluid determined using chemical equilibria co 2 h 2 o rich 7 25 mole co 2 is in approximate agreement with that required to produce the isotopic depletion of the residual fluid via phase separation