gold speciation and transport in geological fluids insights from experiments and physical chemical modelling this contribution provides an overview of available experimental thermodynamic and molecular data on au aqueous speciation solubility and partitioning in major types of geological fluids in the earth s crust from low temperature aqueous solution to supercritical hydrothermal magmatic fluids vapours and silicate melts critical revisions of these data allow generation of a set of thermodynamic properties of the auoh aucl 2 auhs and au hs 2 complexes dominant in aqueous hydrothermal solutions however other complexes involving different sulphur forms chloride and alkali metals may operate in high temperature sulphur rich fluids vapours and melts the large affinity of au for reduced sulphur is responsible for au enrichment in s rich vapours and sulphide melts which are important gold sources for hydrothermal deposits thermo dynamic speciation and partitioning data and their comparison with au and s contents in natural fluid inclusions from magmatic hydrothermal gold deposits provide new constraints on the major physical chemical parameters temperature pressure salinity acidity redox and ubiquitous fluid components sulphur carbon dioxide arsenic affecting au concentration transport precipitation and fractionation from other metals in the crust the availability and speciation of sulphur and their changes with the fluid and melt evolution are the key factors controlling gold behaviour in most geological situations

low temperature thermochronology of the northern cordillera real ecuador tectonic insights from zircon and apatite fission track analysis the post early cretaceous tectonic development of the northern andean segment north of 5 s is loosely constrained by a series of partially and fully reset k ar ages new apatite and zircon fission track data from three traverses across the cordillera real of ecuador reveal distinct periods of accelerated crustal cooling during the cenozoic fission track ages range from 65 to 14 ma zircon and 44 to 8 ma apatite with apatite mean track lengths ranging from 14 6 to 12 1 m these results record rapid cooling rates during 43 30 ma 23 15 ma and 10 0 ma occurring at temperatures below 275 c the events reflect up to 9 km of crustal exhumation in various regions of the cordillera in response to relative changes in the kinematics of the farallon nazca and south american plates the exhumation history of the cordillera real was not continuous but rather evolved during separate phases these phases persisted for 10 myr and are partly coincident with tectonic phases of the central andes although extend for longer time periods possibly as a result of isostatically driven crustal exhumation large spatial variations in the zircon and apatite fission track data within the cordillera real indicate that local fault bounded massifs have experienced different thermal histories since 43 ma major variations are not restricted to the regional scale and exist within individual terranes faulted slivers within terranes demonstrate highly variable exhumation histories suggesting that deformation was not dominated by terrane bounding faults additionally n s variations in exhumation depths suggest that ne sw trending faults have been active since 10 9ma

statistical microthermometry of synthetic fluid inclusions in quartz during decompression reequilibration fluid inclusion microthermometric data are often reported as homogenization temperature frequency histograms interpretation of such histograms for a single fluid inclusion assemblage fia of non reequilibrated fluid inclusions is usually straightforward and provides an accurate determination of the original density th of that fia however interpretation of such histograms for reequilibrated inclusions is more problematic decompression experiments using synthetic inclusions in natural quartz and conducted at 2 5 kbar and 600 700 c with a maximum internal overpressure of 2 kbar indicate that histogram shape reflects the sample s p t history our results further indicate that the mean mode range standard deviation extreme values etc all have a significance with respect to the p t history of the sample thus a mound shaped unimodal histogram with low range is indicative of a nearly isochoric cooling p t path a unimodal histogram that is slightly skewed to the right and with a low standard deviation but high range results from inclusion deformation in the plastic regime high temperature low strain rates fluid inclusions deformed plastically show no correlation between size and density histogram outliers should not be ignored and may be used to determine an isochore that passes close to the conditions of entrapment minimum th or close to the final reequilibration conditions maximum th the histogram mean th value corresponds to an isochore that represents the internal overpressure about 1 kbar that can be maintained over geologic time by a majority of reequilibrated fluid inclusions a multimodal histogram with high range and high standard deviation indicates inclusion brittle deformation low temperature high strain environments fluid inclusions deformed in a brittle manner show strong positive correlation between size and density histograms produced in the laboratory show many similarities to histograms for natural samples offering the hope that laboratory results may be used to interpret p t histories of natural samples

relationship between diagenesis and reservoir quality in sandstones of the garn formation haltenbanken mid norwegian continental shelf a quantitative model is proposed for the dependence of porosity and permeability on diagenesis in subarkosic arenites of the middle jurassic garn formation porosity variation is controlled by the combined effects of compaction and quartz cementation in general more porosity has been lost by compaction than by quartz cementation but compactional porosity loss appears to have occurred relatively early in the burial history before burial below 2 km below the sea floor thus progressive reduction of the total porosity with increasing burial below 2 km results mainly from quartz cementation permeability is a function of the abundance of intergranular macroporosity which is interpreted to be a measure of effective porosity at depths less than 3 5 km below the sea fl or intergranular macroporosity generally comprises more than 50 percent of the total porosity but at greater depth as total porosity decreases below about 16 percent the ratio of effective to ineffective porosity drops below one this depth corresponds to the onset of extensive illitization and grain dissolution which results in major reorganization of the pore system generation of secondary porosity by feldspar dissolution appears not to have been beneficial to reservoir quality in the garn formation but it is regarded as an intrinsic part of the overall process of porosity destruction with increasing diagenesis

constraints on the magnitude and patterns of ocean cooling at the last glacial maximum observation based reconstructions of sea surface temperature from relatively stable periods in the past such as the last glacial maximum represent an important means of constraining climate sensitivity and evaluating model simulations1 the first quantitative global reconstruction of sea surface temperatures during the last glacial maximum was developed by the climate long range investigation mapping and prediction climap project in the 1970s and 1980s refs 2 3 since that time several shortcomings of that earlier effort have become apparent4 here we present an updated synthesis of sea surface temperatures during the last glacial maximum rigorously defined as the period between 23 and 19 thousand years before present from the multiproxy approach for the reconstruction of the glacial ocean surface margo project5 we integrate microfossil and geochemical reconstructions of surface temperatures and include assessments of the reliability of individual records our reconstruction reveals the presence of large longitudinal gradients in sea surface temperature in all of the ocean basins in contrast to the simulations of the last glacial maximum climate available at present6 7

synthetic fluid inclusions in natural quartz iii determination of phase equilibrium properties in the system h2o nacl to 1000 c and 1500 bars phase equilibria in the system h2o nacl have been determined to 1000 c and 1500 bars using synthetic fluid inclusions formed by healing fractures in inclusion free brazilian quartz in the presence of the two coexisting immiscible h2o nacl fluids at various temperatures and pressures petrographic and microthermometric analyses indicate that the inclusions trapped one or the other of the two fluids present or mixtures of the two salinities of the two coexisting phases were obtained from heating and freezing studies on those inclusions which trapped only a single homogeneous fluid phase results of the present study are consistent with previously published data on the h2o nacl system at lower temperatures and pressures and indicate that the two phase field extends well into the p t range of most shallow magmatic hydrothermal activity as a consequence chloride brines exsolved from many epizonal plutons during the process of second boiling should immediately separate into a high salinity liquid phase and a lower salinity vapor phase and produce coexisting halite bearing and vapor rich fluid inclusions this observation is consistent with results of numerous fluid inclusion studies of ore deposits associated with shallow intrusions particularly the porphyry copper deposits in which halite bearing and coexisting vapor rich inclusions are commonly associated with the earliest stages of magmatic hydrothermal activity

composition and evolution of fluids forming the baiyinnuo er zn pb skarn deposit northeastern china insights from laser ablation icp ms study of fluid inclusions the baiyinnuo er skarn deposit is one of the largest zn pb deposits in northeastern china with 32 74 million metric tons mt resources averaging 5 44 zn 2 02 pb and 31 36 g t ag the deposit formed in three stages the preore stage prograde skarn minerals with minor magnetite the synore stage sulfides and retrograde skarn minerals including calcite and minor quartz and the postore stage late veins composed of calcite quartz fluorite and chlorite cutting the above mineral assemblages in this study we analyzed the composition of single fluid inclusions using laser ablation inductively coupled plasma mass spectrometry la icp ms to 1 determine the composition of the fluids and the evolution through the stages 2 infer the fluid and metal sources and 3 explore the metal deposition mechanisms the preore fluids trapped in pyroxene have higher homogenization temperatures 432 504 c higher salinity 36 5 46 1 wt nacl equiv and higher concentrations of zn 0 9 wt pb 1 4 wt and other elements e g na k li as rb sr cs ba cl and br than synore mineralizing fluids 370 c 10 wt nacl equiv 450 ppm zn and 290 ppm pb the postore fluids show lower temperatures 250 c and a rather dilute composition 4 wt nacl equiv 33 ppm zn and 24 ppm pb geochemically the fluids of all paragenetic stages in baiyinnuo er have magmatic signatures based on the element mass ratios including elevated k na zn na and rb na ratios lower ca k ratios and combined cl br na k ratios which are distinctively different from basinal brines inclusion fluids in preore stage show little variation in composition between 510 and 430 c indicative of a closed cooling system in contrast the major components of the syn and postore fluids including cl na and k decrease and correlate with a drop of homogenization temperatures from 370 to 200 c indicating a dilution by mixing with groundwater the baiyinnuo er mineralizing fluids trapped in sphalerite have higher ca k mass ratios avg 0 78 than other proximal magmatic hydrothermal systems 0 06 0 52 but lower than that of the distal el mochito skarn avg 6 4 probably reflecting a gradually weakened magmatic signal away from the causative intrusions the metal contents in preore fluids are significantly higher than those in synore fluids but no mineralization occurred this confirms that the early fluids were although enriched in metals not responsible for ore precipitation most likely due to their high temperature and high salinities one important factor controlling zn pb mineralization was mixing with groundwater which resulted in temperature decrease and dilution that significantly reduced the metal solubility thereby promoting metal deposition another main driving force was the interaction with carbonate wall rock that buffered the acidity generated during the breakdown of zn and pb cl complexes and the precipitation of sulfides phase separation occurred in both the preore and the early part of the synore stages but no evidence indicated that it caused metal deposition the prograde minerals and retrograde minerals including ore minerals coexisting in the same samples could have been caused by two or more successive pulses of hydrothermal fluids released from residual melts of a progressively downward crystallizing magma each fluid produced a series of proximal high temperature prograde to distal low temperature assemblages with the lower temperature assemblages of later fluids overprinting the higher temperature assemblages at most locations

an autochthonous geological model for the eastern andes of ecuador abstract we describe a traverse across the cordillera real and sub andean zone of ecuador poorly known areas with very little detailed mapping and very little age control the spine of the cordillera comprises deeply eroded triassic and jurassic plutons the roots of a major arc emplaced into probable palaeozoic pelites and metamorphosed volcanic rocks the w flank comprises a jurassic submarine basaltic andesitic volcanic sequence which grades up into mixed jurassic cretaceous volcanic and sedimentary rocks of the inter andean valley the sub andean zone on the e flank of the cordillera comprises a newly recognized cretaceous basin of cleaved mudrocks quartz arenites and limestones east of the syndepositional cosanga fault the cretaceous basin thins into a condensed sequence that is indistinguishable from the rocks of the adjacent hydrocarbon bearing oriente basin the principal penetrative deformation of the cordillera real was probably latest cretaceous palaeocene it telescoped the magmatic belts but shortening was largely partitioned into the pelites between plutons the plutons suffered inhomogenous deformation some portions completely escaped tectonism the pelites conserve two foliations the earliest comprises slaty cleavage formed under low or sub greenschist conditions the later is a strong schistosity defined by new mica growth it largely transposed and obliterated the first both foliations may have developed during a single progressive deformation we find inappropriate recent terrane models for the cordillera real and sub andean zone of ecuador instead we find remarkable similarities from one side of the cordillera to the other including a common structural history in place of sutures we find mostly intrusive contacts between major plutons and pelites triassic to cretaceous events occurred on the autochthonous western edge of the archaean guyana shield the latest cretaceous paleocene deformation is interpreted as the progressive collision of an oceanic terrane s with the south american continent young fault movements have subsequently juxtaposed different structural levels through the cordillera real orogen

alteration and mineralization at the zhibula cu skarn deposit gangdese belt tibet abstract the zhibula cu skarn deposit contains 0 32 mt cu metal with an average grade of 1 64 and is located in the gangdese porphyry copper belt in southern tibet the deposit is a typical metasomatic skarn that is related to the interaction of magmatic hydrothermal fluids and calcareous host rock stratiform skarn orebodies occur at the contact between tuff and marble in the lower jurassic yeba formation alteration zones generally grade from a fresh tuff to a garnet bearing tuff a garnet pyroxene skarn and finally to a wollastonite marble minor endoskarn alteration zonations are also observed in the causative intrusion which grade from a fresh granodiorite to a weakly chlorite altered granodiorite a green diopside bearing granodiorite and to a dark red brown garnet bearing granodiorite prograde minerals which were identified by electron probe microanalysis include andradite grossularite of various colors e g red green and yellow and green diopside retrograde metamorphic minerals overprint the prograde skarn and are mainly composed of epidote quartz and chlorite the ore minerals consist of chalcopyrite and bornite followed by magnetite molybdenite pyrite pyrrhotite galena and sphalerite three types of fluid inclusions are recognized in the zhibula deposit including liquid rich two phase inclusions type l vapor rich two phase inclusions type v and daughter mineral bearing three phase inclusions type s as the skarn formation evolved from prograde stage i to early retrograde stage ii and later retrograde stage iii the ore forming fluids correspondingly evolved from high temperature 405 667 c high salinity up to 44 0 wt nacl equiv and high pressure 500 600 bar to low moderate temperature 194 420 c moderate high salinity 10 1 18 3 and 30 0 44 2 wt nacl equiv and low moderate pressure 250 350 bar isotopic data of 34 s 0 1 to 6 8 estimated 34 s fluids 0 7 d h2o 91 to 159 and 18 o h2o 1 5 to 9 2 suggest that the ore forming fluid and material came from magmatic hydrothermal fluids that were associated with miocene zhibula intrusions fluid immiscibility likely occurred at the stage i and stage ii during the formation of the skarn and mineralization fluid boiling occurred during the stage iii which is the most important cu deposition mechanism for the zhibula deposit

ore genesis and hydrothermal evolution of the kendenggao er copper molybdenum deposit western tianshan evidence from isotopes s pb h o and fluid inclusions abstract the boluokenu range is one of the most important metallogenic belts in northwest china and it contains numerous fe cu mo au etc deposits the kendenggao er cu mo deposit is located in the eastern segment of the boluokenu metallogenic belt within the western tianshan orogeny skarn and cu mo orebodies occur as lamellae or lenses along the contacts between different units of the upper carboniferous dongtujinhe formation and the aimujin granitic intrusions based on field investigations and petrographic observations three main metallogenic stages of skarn formation and ore deposition have been distinguished 1 the pre ore stage is characterized by the formation of garnet wollastonite and diopside and high temperature hypersaline hydrothermal fluids with oxygen isotopic compositions that are similar to those of magmatic fluids these fluids were most likely generated by the separation of brine from the silicate melt due to the immiscibility of primary magmatic fluids that occurred at a depth of approximately 1 km with inferred lithostatic pressure conditions of 300 bars 2 the main ore stage coincides with the replacement of garnet and diopside by vesuvianite epidote sulfides and quartz the hydrothermal fluids of this stage are represented by v type vapor rich fluid inclusions that coexist with l type liquid rich inclusions the oxygen isotope 18oh2o values of the main ore stages are in a narrow range of 7 5 9 9 which indicates that the mineralization related fluid is also of magmatic origin based on the fluid inclusion and stable isotopic data it can be concluded that the ore forming fluid evolved by the boiling of the magmatic fluids that likely occurred at a depth of 1 km and hydrostatic pressure of 100 bars and that this was the predominant mechanism for the large scale precipitation in the kendenggao er cu mo deposit 3 the post ore stage is characterized by the development of calcite quartz and chlorite the fluids during this stage are represented by the l type inclusions which mainly show low salinity and low homogenization temperatures these fluids yield the lowest 18oh2o values 2 8 to 3 7 the above data indicate that the post ore stage fluids originated from the mixing of residual main ore stage fluids with various proportions of external water low salinity low temperature no boiling occurred at this stage at the low temperatures the 34s values of the sulfides range from 4 3 to 1 9 and the 206pb 204pb 207pb 204pb and 208pb 204pb values of the sulfides range from 18 220 to 18 554 15 608 to 15 812 and 38 080 to 39 055 respectively the s and pb isotopic compositions indicate that the ore materials were mainly derived from magmas taken together these geologic geochemical and isotopic data confirm that kendenggao er is a typical skarn deposit

potassium leached zones at the contacts of the garn formation mid norwegian continental shelf evidence for the role of shale in sandstone diagenesis abstract

origin and evolution of hydrothermal fluids in the taochong iron deposit middle lower yangtze valley eastern china evidence from microthermometric and stable isotope analyses of fluid inclusions abstract the middle lower yangtze river valley is one of the most important metallogenic belts in china hosting numerous cu fe au mo deposits the taochong deposit is located in the northern part of the fanchang iron ore district of the middle lower yangtze river metallogenic belt the fe orebody is hosted by middle carboniferous to lower permian limestones skarns and fe orebodies occur as tabular bodies along interlayer gliding faults at some distance from the inferred granitic intrusions field evidence and petrographic observations indicate that the three stages of hydrothermal activity the skarn iron oxide main mineralization stage and carbonate stages all contributed to the formation of the taochong iron deposit the skarn stage is characterized by the formation of garnet and pyroxene with high temperature hypersaline hydrothermal fluids with isotopic compositions similar to those of typical magmatic fluids these fluids were probably generated by the separation of brine from a silicate melt instead of the product of aqueous fluid immiscibility the iron oxide stage coincides with the replacement of garnet and pyroxene by actinolite chlorite quartz calcite and hematite the hydrothermal fluids at this stage are represented by saline fluid inclusions that coexist with vapor rich inclusions with anomalously low d values 66 to 94 the decrease in ore fluid 18 o water with time and decreasing depth is consistent with the decreases in fluid salinity and temperature the fluid d values also show a decreasing trend with decreasing depth both fluid inclusion and stable isotopic data suggest that the ore fluid during the main period of mineralization was evolved by the boiling of various mixtures of magmatic brine and meteoric water this process was probably induced by a drop in pressure from lithostatic to hydrostatic the carbonate stage is represented by calcite veins that cut across the skarn and orebody locally producing a dense stockwork this observation indicates the veins formed during the waning stages of hydrothermal activity the fluids from this stage are mainly represented by a variety of low salinity fluid inclusions as well as fewer high salinity inclusions these particular fluids have the lowest 18 o water values 2 2 to 0 4 and a wide of range of d values 40 to 81 which indicate that they were originated from a mixture of residual fluids from the oxide stage various amounts of meteoric water and possibly condensed vapor low temperature boiling probably occurred during this stage we also discuss the reasons behind the anomalously low d values in fluid inclusion water extracted by thermal decrepitation from quartz at high temperatures and suggest that calcite data provide a possible benchmark for adjusting low d values found in quartz intergrown with calcite

the geochemistry of zoning in skarn minerals at the king island dolphin mine trends of chemical zoning in crystals of garnet pyroxene scheelite and amphibole in 34 samples collected from the pluton9s contact updip to 500 m from the contact in the main ore horizon at the king islnd scheelite dolphin mine show systematic variations garnet cores up to 400 m from the contact are the most andradite rich reaching values of 98 mole percent nearest the contact andradite values decrease systematically both for a particular part of the garnet crystals away from the contact and across individual grains to the outer edges the mole percent andradite in core and outer core zones decreases to the 500 m mark while in the midsection and edge zones the values decrease to 400 m and then increase a similar relationship occurs in pyroxenes which nearest the contact contain hedenbergite values of 98 mole percent scheelite occurring as inclusions in garnet reach values of scheelite 65 powellites 34 for core zones nearest the pluton while scheelite interstitial to garnet have maximum values of scheelite 72 powellite 27 powellite contents decrease both toward the edges of individual grains and along the traverse to the 400 m mark where a reversal of trend occurs continuing to 500 m from the contact amphibole compositions show a linear decrease of the ratio fe super 2 fe super 2 mg mn from 98 nearest the contact to 88 at 390 and then decreasing abruptly to 65 at 500 m fluid inclusion filling temperatures vary systematically in time and distance from the pluton with a maximum near 800 degrees c for garnet midsections nearest the pluton to temperatures as low as 180 degrees c for interstitial calcite and quartz the salinities decrease from near 65 weight percent total dissolved salts in the early formed minerals i e garnet nearest the pluton to less than 3 weight percent for later formed minerals farthest from the contact i e calcite these decreases occur out from the contact and progressively toward the end of the paragenetic sequence scheelite initially crystallized at higher temperatures but interstitial scheelite precipitation occurred from a concentrated na k ca cl solution approximately 10 molal total dissolved salts at temperatures of approximately 500 degrees c at the pluton9s contact to 300 degrees c at a distance of 500 m from the contact chemical variations of fluid compositions for all but garnet core zones beyond 400 m from the contact occurred due to mixing of solutions derived from the northern boundary fault the interface between solutions which have permeated through and reacted with the developing skarn unit and those derived from the fault marks the maximum outward edge from the pluton of boiling of the solutions the highest concentrations of w ore reversals or inflection points in mineral composition profiles and changes in the elemental ratios of constituents in the brine solutions hot saline solutions initially cooled to approximately 380 degrees c on entering the colder marble horizon flow along this horizon caused the loss of heat dilution and chemical change of the solution this latter in response to both reactions with the marble and hornfelsic units as well as the precipitation of the skarn permeability was produced by the dissolution of caco 3 to produce cacl sub 2 aq and faulting with time thermal equilibrium was reached between the evolving fluid and the contact rocks until boiling of the solution occurred with the loss of a co sub 2 rich vapor during this event the northern boundary fault was tectonically active so that primary fluid now entered the forming skarn both from the fault and the contact of the pluton mo rich scheelite began to precipitate at the time of boiling subsequent to garnet pyroxene crystallization after the boiling event and contemporaneous with the crystallization of amphibole some quartz mo rich scheelite not armored in garnet was dissolved redistributed and redeposited as mo poor scheelite and molybdenite this type of scheelite forms most of the ore being mined

mobility and h2o loss from fluid inclusions in natural quartz crystals petrographic observations on quartz crystals from the mole granite australia and other localities shed new light on the mechanisms of post entrapment modification of fluid inclusions these modifications include migration away from pseudosecondary trails changes in fluid salinity and density shape distortion and the formation of sweat haloes around strongly deformed inclusions increases in fluid salinity which usually are associated with inclusion migration indicate water losses of up to 50 however la icp ms analysis of unmobilized and mobilized inclusions of the same trail reveals basically unchanged ratios of major and trace element cations with the exception of li which seems to be incorporated into the crystal lattice during migration despite the fact that all these modifications are closely related to deformation processes they occur not only in mechanically deformed quartz but also in free standing crystals in the latter samples stress has been generated internally as a result of brazil twinned growth and compositional zonation these observations and their interpretation leads to a list of practical criteria that should help in differentiating between reliable and suspect fluid inclusions in other samples

hydrothermal evolution and ore genesis of the laozuoshan au skarn deposit northeast china constrains from mineralogy fluid inclusion and o c s pb isotope geochemistry abstract the laozuoshan mine located in the central jiamusi block is the largest gold skarn deposit in northeast china it comprises three ore zones east middle west with complex stratigraphic and structural controls the skarn type orebodies mainly occur in the east and middle ore zones along the contact zones between hercynian granite and precambrian metasedimentary rocks the earliest stage of alteration is characterized by potassium hornfels subsequently prograde skarn formation occurred characterized by garnet in the grossular andradite series and pyroxene in the hedenbergite diopside series this was followed by a retrograde skarn overprint of hydrous silicates and ore minerals three types of garnet with distinct textures and elemental compositions are recognized which indicate fluctuating physicochemical conditions in the hydrothermal fluids the fluids associated to the prograde skarn are characterized by high temperature and high salinity which underwent fluid boiling that led to precipitation of minor pyrite and arsenopyrite the main gold and sulfide mineralization is related to the retrograde skarn overprint where cooling and strong fluid rock interactions occurred gold precipitation is estimated to have occurred at temperatures of 380 470 c based on the arsenopyrite geothermometer and fluid inclusion microthermometry the fluid inclusion and c o isotope data indicate that the ore forming fluids were dominated by magmatic water during the prograde skarn stage and the main ore forming stages mixing between meteoric and magmatic fluids occurred during the latest stages of the retrograde skarn formation both sulfur and lead isotope compositions of sulfides support a magmatic origin for the ore forming materials on the basis of the available geological and geochemical evidence we propose that the laozuoshan deposit is a reduced gold skarn deposit genetically related to the granitic pluton

synthetic fluid inclusions in natural quartz i compositional types synthesized and applications to experimental geochemistry synthetic fluid inclusions having a wide range of geologically applicable compositions in the c o h s na k ca cl cu fe system have been formed by healing fractures in inclusion free natural quartz and by precipitating new quartz overgrowths on an original seed crystal inclusion types synthesized include 1 1 liquid rich pure h2o inclusions of primary and secondary origin 2 2 pure h2o inclusions with the critical density 3 3 liquid rich inclusions containing undersaturated aqueous solutions of nacl or kcl or cacl2 or mixtures of the three salts 4 4 h2o nacl inclusions containing halite daughter minerals 5 5 h2o nacl kcl inclusions containing halite and sylvite daughter minerals 6 6 h2o co2 inclusions of various compositions containing liquid h2o and either co2 vapor or co2 liquid or both at 25 c 7 7 h2o co2 nacl inclusions containing an aqueous phase liquid and vapor co2 and halite at 25 c 8 8 c o h inclusions containing liquid h2o a co2 ch4 liquid phase and graphite daughter minerals at 25 c 9 9 h2o nacl inclusions that contain a chalcopyrite daughter mineral and 10 10 inclusions representing trapping of the coexisting immiscible fluids in the h2o nacl h2o co2 nacl and na c o h systems the inclusions exhibit uniform phase ratios at room temperature and the temperatures of various phase changes within individual inclusions agree with those predicted from experimental and theoretical data indicating that the inclusion fluid has the same composition and density as the parent solution these miniature autoclaves thus may be used to study various problems related to fluid inclusion research to calibrate analytical equipment used to study natural inclusions and to study phase equilibria solubility and pvt relations of a variety of chemical systems

fluid inclusion and stable isotope constraints c o h on the dagbasi fe cu zn skarn mineralization trabzon ne turkey abstract the dagbasi fe cu zn skarn mineralization developed along the contact between the block and lens shaped limestones of the lower cretaceous berdiga formation and the upper cretaceous dagbasi granitoid the exoskarn type mineralization is characterized by prograde stage garnet and pyroxene while the retrograde stage is characterized by epidote tremolite actinolite and chlorite quartz and calcites were observed in both stages of the skarn development the ore minerals mainly consist of magnetite and hematite with a lesser amount of pyrrhotite pyrite chalcopyrite sphalerite and minor galena the homogenization temperatures th and salinity values of the prograde stage halite bearing fluid inclusions are in the range of 412 514 c and 48 8 61 8 wt nacl equ respectively the second stage liquid and vapor rich fluid inclusion assemblage reveals that boiling at temperatures of 353 458 c took place after the formation of halite bearing fluid inclusions final stage liquid rich fluid inclusions were characterized by low th 160 and 327 c and salinity values 0 5 and 6 2 wt nacl equ the decreasing salinity trend of the fluid inclusions versus th indicated that meteoric water was involved in the hydrothermal solutions eutectic temperatures te of the prograde stage fluid inclusions were found to be cacl2 dominated while retrograde stage inclusions contained different salt combinations rather than a specific salt type the minimum trapping pressures of the early stage brine fluid inclusions were calculated to be between 710 and 884 bar while later stage inclusions had much lower trapping pressures between 195 and 445 bar the 18o isotopes of prograde stage quartz garnet and pyroxenes are close to the composition of the hydrothermal solutions of magmatic sources moreover retrograde stage quartz epidote tremolite actinolite and calcite minerals and their equilibrated solutions were found to be highly depleted by 18o isotopes therefore the fluid inclusion and stable isotope constraints suggest that the hydrothermal solutions of magmatic origin were responsible for the prograde skarn stage while a mixture of magmatic and meteoric solutions were responsible for the ore formation in a shallow skarn environment

geology mineralization and geochronological constraints of the mirador cu au porphyry district southeast ecuador the mirador porphyry cu au district is located in the southernmost sector of the northern andean cordillera in zamora chinchipe province southeastern ecuador the district contains two significant porphyry cu au ag mo deposits mirador and mirador norte and an interconnected series of narrow mineralized structures known collectively as chancho the principal mineralization in the porphyries is disseminated to blebby chalcopyrite developed primarily in potassic alteration with overlying chalcocite supergene enrichment zones prior to radiometric dating presented in this study these deposits were considered late jurassic based on close similarity and therefore assumed age equivalence with the well dated panantza san carlos and sutzu porphyry copper deposits located 40 km to the north new u pb zircon ages confirm a middle jurassic age for granodiorite of the zamora batholith at this location 163 8 1 9 ma and late jurassic ages for subvolcanic intrusions 156 2 1 0 and 153 1 1 3 ma re os molybdenite ages average 156 1 0 ma and indicate coeval mineralization between mirador and mirador norte mineralization and related sub volcanic igneous activity are closely associated at mirador and represent the same temporal event recorded at panantza san carlos and sutzu as well as coeval porphyry skarn and epithermal au mineralization extending at least 80 km south defining a north south cu au metallogenic belt spanning over 120 km

sedimentation and tectonics of the sylhet trough bangladesh the sylhet trough a sub basin of the bengal basin in northeastern bangladesh contains a thick fill 12 to 16 km of late mesozoic and cenozoic strata that record its tectonic evolution stratigraphic sedimentologic and petrographic data collected from outcrops cores well logs and seismic lines are here used to reconstruct the history of this trough the sylhet trough occupied a slope basinal setting on a passive continental margin from late mesozoic through eocene time subsidence may have increased slightly in oligocene time when the trough was located in the distal part of a foreland basin paired to the indo burman ranges oligocene fluvial deltaic strata barail formation were derived from incipient uplifts in the eastern himalayas subsidence increased markedly in the miocene epoch in response to western encroachment of the indo burman ranges miocene to earliest pliocene sediments of the surma group were deposited in a large mud rich delta system that may have drained a significant proportion of the eastern himalayas subsidence rates in the sylhet trough increased dramatically 3 8 times from miocene to pliocene pleistocene time when the fluvial tipam sandstone and dupi tila formation were deposited this dramatic subsidence change is attributed to south directed overthrusting of the shillong plateau on the dauki fault for the following reasons 1 pliocene and pleistocene strata thin markedly away from the shillong plateau consistent with a crustal load emplaced on the northern basin margin 2 the shillong plateau is draped by mesozoic to miocene rocks but pliocene and younger strata are not represented suggesting that the massif was an uplifted block at this time 3 south directed overthrusting of the shillong plateau is consistent with gravity data and with recent seismotectonic observations sandstone in the tioam has a marked increase in sedimentary lithic fragments compared to older rocks reflecting uplift and erosion of the sedimentary cover of the shillong plateau if the dauki fault has a dip similar to that of other himalayan overthrusts then a few tens of kilometers of horizontal tectonic transport would be required to carry the shillong plateau to its present elevation uplift of the shillong plateau probably generated a major 300 km westward shift in the course of the brahmaputra river

fluid inclusions in skarns carbonate replacement deposits fluid inclusions in ore skarn minerals reflect the physiochemical nature of the solutions present during the skarn forming process because of the dense nature of skarn minerals and the dynamic processes operative during skarn genesis sufficient primary fluid inclusions are usually present ore skarn solutions as opposed to metamorphic skarn or ore vein solutions have much higher cacl2 contents and usually very high formation temperatures 500 c and salinities 40 wt t d s temperatures and salinities generally decrease away from the solution source both in time and space the gradients found at greater distances from the source in distal far from contact skarns tend to be less e g 210 350 c for a particular skarn stage to that in proximal near contact skarns e g 400 650 c this information is useful for delineating the parts of such a hydrothermal system temperatures also tend to decrease with time which is reflected by the superimposition of various overprinting retrograde mineral stages in a few areas e g naica mexico intermittent boiling of ore solutions occurs periodically elevating both temperatures and salinities but commonly boiling only occurs early in skarn genesis just after an early commonly lower temperature phase most fluid inclusions represent a mix of exhaust or reacted solutions with minor unreacted or new pre ore solution components limited data on the distribution of elements present in fluid inclusions that do not normally take part in skarn genesis na k and cl indicate that their proportions reflect the nature of the associated pluton high kc1 contents are found in skarns adjacent to high k granitoids whereas high nacl contents are found in skarns adjacent to calcic granitoids in many examples daughter minerals present in minor proportions in opened fluid inclusion cavities reflect the metal characteristics of the ore solution small rare earth metal tungsten zinc and copper daughter minerals have been identified the temperature and or salinity data for skarns of different metal or geological type is not particularly useful to delineate whether a skarn locality is part of a more complex as yet unexplored system solutions in pb zn skarns tend to be lower temperature 150 400 c and more dilute 30 wt t d s than in other skarn types but exceptions occur

magmatic dominated fluid evolution in the jurassic nambija gold skarn deposits southeastern ecuador the jurassic approximately 145 ma nambija oxidized gold skarns are hosted by the triassic volcanosedimentary piuntza unit in the sub andean zone of southeastern ecuador the skarns consist dominantly of granditic garnet ad20 98 with subordinate pyroxene di46 92hd17 42jo0 19 and epidote and are spatially associated with porphyritic quartz diorite to granodiorite intrusions endoskarn is developed at the intrusion margins and grades inwards into a potassic alteration zone exoskarn has an outer k and na enriched zone in the volcanosedimentary unit gold mineralization is associated with the weakly developed retrograde alteration of the exoskarn and occurs mainly in sulfide poor vugs and milky quartz veins and veinlets in association with hematite fluid inclusion data for the main part of the prograde stage indicate the coexistence of high temperature 500 c to 600 c high salinity up to 65 wt eq nacl and moderate to low salinity aqueous carbonic fluids interpreted to have been trapped at pressures around 100 120 mpa corresponding to about 4 km depth lower temperature 510 300 c and moderate to low salinity 23 2 wt eq nacl aqueous fluids are recorded in garnet and epidote of the end of the prograde stage the microthermometric data th from 513 c to 318 c and salinity from 1 0 to 23 wt eq nacl and 18o values between 6 2 and 11 5 for gold bearing milky quartz from the retrograde stage suggest that the ore forming fluid was dominantly magmatic pressures during the early retrograde stage were in the range of 50 100 mpa in line with the evidence for co2 effervescence and probable local boiling the dominance of magmatic low saline to moderately saline oxidizing fluids during the retrograde stage is consistent with the depth of the skarn system which could have delayed the ingression of external fluids until relatively low temperatures were reached the resulting low water to rock ratios explain the weak retrograde alteration and the compositional variability of chlorite essentially controlled by host rock compositions gold was precipitated at this stage as a result of cooling and ph increase related to co2 effervescence which both result in destabilization of gold bearing chloride complexes significant ingression of external fluids took place after gold deposition only as recorded by 18o values of 0 4 to 6 2 for fluids depositing quartz below 350 c in sulfide rich barren veins low temperature 300 c meteoric fluids 18owater between 10 0 and 2 0 are responsible for the precipitation of late comb quartz and calcite in cavities and veins and indicate mixing with cooler fluids of higher salinities about 100 c and 25 wt eq nacl the latter are similar to low temperature fluids 202 74 5 c with 18o values of 0 5 to 3 1 and salinities in the range of 21 1 to 17 3 wt eq cacl2 trapped in calcite of late veins and interpreted as basinal brines nambija represents a deep equivalent of the oxidized gold skarn class the presence of co2 in the fluids being partly a consequence of the relatively deep setting at about 4 km depth as in other au bearing skarn deposits not only the prograde stage but also the gold precipitating retrograde stage is dominated by fluids of magmatic origin

fluid infiltration effects on stable isotope systematics of the susurluk skarn deposit nw turkey abstract a calcic skarn deposit occurs along the contact zone between oligo miocene cataldag granitoid and mesozoic limestones in susurluk northwestern turkey the skarn zone with little or no retrograde stage is represented by fluid inclusions with high homogenization temperatures up to 600 c and a wide range of salinity 12 to 70 wt nacl pluton derived fluids facilitated occurrence of continuous prograde reactions in the country rocks particularly in the proximal zone and oxygen isotopic depletion in calc silicate and calcite minerals 18o of anhydrous minerals within proximal and distal zones indicate that skarn forming fluids had a magmatic origin the 18o values are 5 93 9 08 mean 6 8 for garnet 4 08 9 94 mean 6 4 for pyroxene 4 89 7 92 mean 6 4 for wollastonite and 6 65 8 28 mean 7 5 for vesuvianite temperatures estimated by isotopic compositions of mineral pairs are significantly lower than those measured from the fluid inclusions indicating that isotopic equilibrium is not preserved between the skarn minerals 18o and 13c values are systematically depleted from marbles to skarn carbonates calc silicate forming reactions and permeability increase triggered by volatilization and consequent strong infiltration of h2o rich siliceous fluids into the system promoted fluid rock interaction causing isotopic resetting and isotopic depletion of silicates e g pyroxene and wollastonite and skarn calcites

u pb re os and 40ar 39ar geochronology of the nambija au skarn and pangui porphyry cu deposits ecuador implications for the jurassic metallogenic belt of the northern andes new u pb re os and 40ar 39ar dates are presented for magmatic and hydrothermal mineral phases in skarn and porphyry related ores from the nambija and pangui districts of the subandean zone southeastern ecuador nambija has been one of the main gold producing centers of ecuador since the 1980s due to exceptionally high grade ores average 15 g t but frequently up to 300 g t au pangui is a recently discovered porphyry cu mo district the geology of the subandean zone in southeastern ecuador is dominated by the i type subduction related jurassic zamora batholith which intrudes triassic volcanosedimentary rocks the zamora batholith is in turn cut by porphyritic stocks which are commonly associated with skarn formation and or porphyry style mineralization high precision u pb and re os ages for porphyritic stocks u pb zircon associated prograde skarn u pb hydrothermal titanite and retrograde stage skarn re os molybdenite from veins postdating gold deposition of the nambija district are all indistinguishable from each other within error 145 ma and indicate a late jurassic age for the gold mineralization previously gold mineralization at nambija was considered to be early tertiary based on k ar ages obtained on various hydrothermal minerals the new jurassic age for the nambija district is slightly younger than the 40ar 39ar and re os ages for magmatic hydrothermal minerals from the pangui district which range between 157 and 152 ma mineralization at nambija and pangui is associated with porphyritic stocks that represent the last known episodes of a long lived jurassic arc magmatism 190 to 145 ma a jurassic age for mineralization at nambija and pangui suggests that the northern andean jurassic metallogenic belt which starts in colombia at 3 n extends down to 5 s in ecuador it also adds a new mineralization style au skarn to the metal endowment of this belt

ore genesis and hydrothermal evolution of the baiyinnuo er zinc lead skarn deposit northeast china evidence from isotopes s pb and fluid inclusions the baiyinnuo er zinc lead deposit 32 74 mt at 5 44 zn 2 02 pb and 31 36 g t ag located in the south segment of the great xing an range is the largest zn pb deposit in northern china skarn and orebodies mainly occur between the different units of the huanggangliang formation or within the contact zone between the intrusions and permian marble several phases of igneous rocks exposed within the mining areas and among them the yanshanian plutonic rocks which intruded into limestone of the early permian huanggangliang formation are interpreted to be the source of ore since their pb isotope compositions 206pb 204pb 18 25 18 35 207pb 204pb 15 50 15 56 and 208pb 204pb 38 14 38 32 are highly consistent with the sulfides including sphalerite galena and chalcopyrite 206pb 204pb 18 23 18 37 207pb 204pb 15 47 15 62 and 208pb 204pb 37 93 38 44 sulfur isotope values of the sulfides give a narrow 34s interval of 6 1 to 4 6 mean 5 4 n 15 suggesting the ore forming fluid is of magmatic origin three main paragenetic stages of skarn formation and ore deposition have been recognized based on petrographic observation which are the preore stage garnet clinpyroxene wollastonite magnetite sulfides the synore stage sulfides epidote quartz calcite garnet and the postore stage calcite chlorite quartz fluorite several fluid evolution episodes can be inferred from microthermometric results at the baiyinnuo er zn pb deposit 1 immiscibility preore stage coexistence of halite bearing brine inclusions s1 type 44 wt nacl equiv and vapor rich fluid inclusions v type sharing the same homogenization temperatures 470 c confirms that fluid unmixing occurred under lithostatic pressures of 400 bars 1 5 km and the brine is considered to account for most prograde skarn minerals e g clinopyroxene 2 overpressure trapping preore stage brine inclusions homogenized by halite dissolution s2 type postdated the immiscible assemblages this type of inclusions is characterized by high but variable minimum trapping pressures 150 3 000 bars relative to s1 type inclusions and can be explained as a result of entrapment under overpressuring condition 3 boiling the presence of both vapor and liquid inclusions i e v and l type in the same assemblages within synore stage quartz calcite and sphalerite indicates the occurrence of fluid boiling 350 c at hydrostatic pressures of 150 bars and depth of 1 5 km which resulted in large scale mineralization in the baiyinnuo er zn pb deposit 4 mixing with external fluids fluid inclusions scattered within postore stage calcite or secondary trails in synore stage minerals show low homogenization temperatures 260 c and both decreasing for l type and increasing for cacl2 bearing inclusions i e lc type trends for salinities as homogenization temperatures decrease implying addition of both meteoric water low temperature low salinity and basinal brines low temperature ca rich respectively systematic fluid inclusion studies also indicate that the mineralization related fluid is of magmatic origin prograde minerals formed during the preore stage fluid immiscibility while sulfides deposition occurred during the synore stage fluid boiling mixing with external fluids began as the hydrothermal system cooled to 300 c when the main metal precipitation process had ended

raman microspectrometry of fluid inclusions abstract for many kinds of fluid inclusions the coupling of microthermometry and raman microspectrometry is still the only viable option to obtain compositions of single fluid inclusions a review is given on the basis of 16 years of experience and helped with about 120 references of the instrumentation analytical conditions and methodology of the application of raman microspectrometry to gaseous aqueous and hydrocarbon inclusions and their daughter minerals

cretaceous and tertiary terrane accretion in the cordillera occidental of the andes of ecuador new field geochronological geochemical and biostratigraphical data indicate that the central and northern parts of the cordillera occidental of the andes of ecuador comprise two terranes the older pallatanga terrane consists of an early to late cretaceous oceanic plateau suite late cretaceous marine turbidites derived from an unknown basaltic to andesitic volcanic source and a tectonic melange of probable late cretaceous age the younger macuchi terrane consists of a volcanosedimentary island arc sequence derived from a basaltic to andesitic source a previously unidentified regionally important dextral shear zone named the chimbo toachi shear zone separates the two terranes regional evidence suggests that the pallatanga terrane was accreted to the continental margin the already accreted cordillera real in campanian times producing a tectonic melange in the suture zone the macuchi terrane was accreted to the pallatanga terrane along the chimbo toachi shear zone during the late eocene probably in a dextral shear regime the correlation of cretaceous rocks and accretionary events in the cordillera occidental of ecuador and colombia remains problematical but the late eocene event is recognised along the northern andean margin

diagenesis and reservoir quality of bhuban sandstones neogene titas gas field bengal basin bangladesh abstract this study deals with the diagenesis and reservoir quality of sandstones of the bhuban formation located at the titas gas field of bengal basin petrographic study including xrd cl sem and bse image analysis and quantitative determination of reservoir properties were carried out for this study the sandstones are fine to medium grained moderately well to well sorted subfeldspathic arenites with subordinate feldspathic and lithic arenites the diagenetic processes include clay infiltration compaction and cementation quartz overgrowth chlorite kaolinite calcite and minor amount of pyrite dolomite and k feldspar overgrowth quartz is the dominant pore occluding cement and generally occurred as small euhedral crystals locally as large pyramidal crystals in the primary pores pressure solution derived from grain contact is the main contributor of quartz overgrowths chlorite occurs as pore lining and pore filling cement in some cases chlorite helps to retain porosity by preventing quartz overgrowth in some restricted depth interval pore occlusion by calcite cement is very much intense kaolinite locally developed as vermiform and accelerated the minor porosity loss due to pore occlusion kaolinite chlorite enhances ineffective microporosity kaolinite is a by product of feldspar leaching in the presence of acidic fluid produced during the maturation of organic matter in the adjacent miocene or deeper oligocene source rocks the relation between diagenesis and reservoir quality is as follows the initial porosity was decreased by compaction and cementation and then increased by leaching of the metastable grains and dissolution of cement good quality reservoir rocks were deposited in fluvial environment and hence quality of reservoir rocks is also environment selective porosity and permeability data exhibit good inverse correlation with cement however some data points indicate multiple controls on permeability reservoir quality is thus controlled by pore occluding cement textural parameters grain size pore size and sorting and depositional environment the reservoir finally resumed partly its pre cementation quality after development of secondary porosity

formation of anhydrous and hydrous skarn in cu au ore deposits by magmatic fluids most skarn ore deposits are characterized by two distinctly different alteration styles an early prograde stage with anhydrous minerals such as garnet and pyroxene forms from relatively high temperature hypersaline liquid a later retrograde stage with hydrous minerals such as epidote amphibole and chlorite plus sulfide ore minerals forms from lower temperature lower salinity fluids these two alteration stages commonly have been thought to reflect a dominance of magmatic and meteoric water respectively with relevance to the source of ore metals we report data from two different skarn systems one being part of the world s largest cu au resource stable isotope compositions of anhydrous and hydrous alteration minerals from both deposits indicate a magmatic source for both the prograde and retrograde stages 18o averages 5 0 per mil for garnet range 3 4 7 2 6 5 per mil for pyroxene 4 3 8 2 and 7 1 per mil for amphibole 4 3 8 7 the d values of late amphibole are more complex with magmatic values 77 to 78 for one deposit and both magmatic and lighter values for another deposit that could be explained either by magmatic degassing or by limited mixing with meteoric water we conclude that the differences in fluid composition prograde versus retrograde stages resulted from a magmatic fluid that intersected its solvus during the early stage creating vapor and hypersaline liquid whereas in the later stage this magmatic fluid did not intersect its solvus because it followed a different cooling path this late low salinity liquid only boiled once its vapor pressure curve was reached causing sulfide ore to precipitate during the retrograde stage

mineral zoning and gold occurrence in the fortuna skarn mine nambija district ecuador the fortuna oxidized gold skarn deposit is located in the northern part of the nambija gold district southern ecuador it has been subdivided into four mineralized sites covering a distance of 1 km which are named from north to south cuerpo 3 mine 1 mine 2 and southern sector massive skarn bodies occur in k na metasomatized volcanic and volcaniclastic rocks of the triassic piuntza unit they appear to result from selective replacement of volcaniclastic rocks very minor presence of bioclast relicts suggests the presence of subordinate limestone endoskarn type alteration with development of na rich plagioclase k feldspar epidote actinolite anhedral pyroxene and titanite affects a quartz diorite porphyritic intrusion which crops out below the skarn bodies in mine 2 and the southern sector endoskarn alteration in the intrusion grades into a k feldspar biotite magnetite assemblage k alteration suggesting that skarn formation is directly related to the quartz diorite porphyritic intrusion the latter being probably emplaced between 141 and 146 ma the massive skarn bodies were subdivided into a dominant brown garnet skarn a distal green pyroxene epidote skarn and two quartz rich varieties a blue green garnet skarn and light green pyroxene garnet skarn which occur as patches and small bodies within the former skarn types the proximal massive brown garnet skarn zone is centered on two 060 trending faults in mine 2 where the highest gold grades 5 10 g t were observed it grades into a distal green pyroxene epidote skarn zone to the north cuerpo 3 granditic garnet shows iron enrichment from the proximal to the distal zone diopsidic pyroxene exhibits iron and manganese enrichment from proximal to distal zones the retrograde stage is weakly developed and consists mainly of mineral phases filling centimeter wide veins vugs and interstices between garnet and pyroxene grains the main filling mineral is quartz followed by k feldspar epidote calcite and chlorite with minor sericite apatite titanite hematite pyrite chalcopyrite and gold metal and sulfur contents are low at fortuna and the highest gold grades coincide with high hematite abundance which suggests that retrograde stage and gold deposition took place under oxidizing conditions fluid inclusions from pyroxene indicate precipitation from high temperature high to moderate salinity fluids 400 to 460 c and 54 to 13 wt eq nacl which result probably from boiling of a moderately saline 8 wt eq nacl magmatic fluid later cooler 180 to 475 c and moderate to low saline fluids 1 to 20 wt eq nacl were trapped in garnet epidote and quartz and are interpreted to be responsible for gold deposition chlorite analysis indicates temperature of formation between 300 and 340 c in accordance with fluid inclusion data it appears thus that gold was transported as chloride complexes under oxidizing conditions and was deposited at temperatures around 300 c when transport of chloride complexes as gold carriers is not efficient

sedimentology of the bengal shelf bangladesh comparison of late miocene sediments sitakund anticline with the modern tidally dominated shelf abstract characterisation of latest miocene facies of the bay of bengal reveals a range of tidal and fluvially dominated sediments that represent sedimentation on a broad shallow shelf that was strongly influenced by tidal and seasonal cyclicity these tidally influenced sediments are excellently exposed within growth anticlines of the westward migrating chittagong tripura fold belt ctfb and permit detailed study of the late miocene bengal shelf for comparison with the modern subaqueous deltaic sediments at the outcrop scale facies associations record a subtidal shelf migrating intertidal subtidal bedforms tidal and incised channels the subtidal shelf comprises extensive sand sheets interbedded with thick clay horizons while the migrating intertidal subtidal bedforms exhibit a decrease in bedform size from large dunes to small ripple scale cross stratification that reflect a decrease in current velocity and sand silt grade sediment supply as the bedforms migrated rootlet horizons within these intertidal subtidal bedforms may represent a lower delta plain environment the tidal channel facies consists of metre scale sandstone beds that are rich in locally derived clay clasts with up to 0 5 m of incision into a rhizolith horizon below incised channel facies records a coarse pebbly sandstone with clasts derived from a reworked clastic source with metre scale scour at the base of this facies association into laminated and rhizolith clays a 534 m one and two dimensional section within the sitakund anticline north of chittagong is characterised by 20 100 m thick tidally dominated progradational parasequences mudstones with isolated sand lenses and abundant trace fossils of the cruziana ichnofacies coarsen upwards to fine medium grained sandstones deposited in shallower high energy environments these sands were deposited under both uni directional and bi directional currents resulting in dune scale cross stratification and are overlain by 2 5 cm thick clay drapes considered to be of a thickness greater than that typical of deposition from a single slackwater episode the main controls on sedimentation are the seasonally influenced river and sediment discharge onto the shelf from the himalayan source region combined with the influence of daily spring neap tides channel and bedform migration and minimal wave action over the shallow shelf the modern bay of bengal shelf provides an analogue for interpreting processes recorded within the miocene succession allowing construction of a depositional model for the late miocene shelf of the brahmaputra ganges delta system with sedimentation dominated by tidal and seasonal cyclicity

the compositional limits of fluid immiscibility in the system h2onaclco2 as determined with the use of synthetic fluid inclusions in conjunction with mass spectrometry abstract the compositional limits of fluid immiscibility in the system nacl h2o co2 were investigated from 500 to 700 c at pressures of 1 2 and 3 kbar synthetic fluid inclusions formed in quartz prisms were equilibrated with high temperature high pressure fluids in hydrothermal pressure vessels the inclusions were analyzed optically by mass spectrometry and by microthermometry the fields of immiscibility were defined as a function of temperature and pressure mass spectrometry was used to validate the existence of the immiscible fluids and to determine the co 2 h 2 o ratios of the two coexisting phases microthermometry i e the measurement of the melting temperature of solid sodium chloride was used in conjunction with the results of the mass spectrometry measurements to define the tie lines within the two phase fields isochores and the location of a portion of the isopleth were determined for composition nacl8 9 h2o76 1 co215 0

emplacement depth and carbon dioxide rich fluid inclusions in intrusion related gold deposits phanerozoic intrusion related gold deposits have many consistent characteristics such as a spatial temporal and geochemical association with moderately reduced predominantly ilmenite series i type intrusions the deposits exhibit a range of characteristics that vary over a wide range of emplacement depths 7 km deposits in shallow crustal settings 5 km have characteristics of mesothermal environments and are hosted by plutons containing sheeted veins greissen and disseminated gold fluid characteristics also vary with depth deposits in shallow environments contain high temperature 350 c immiscible brine 30 wt nacl equiv and low salinity 5 wt nacl equiv vapor that commonly contains carbon dioxide deposits in deeper environments contain abundant low salinity carbon dioxide rich aqueous fluids 10 wt nacl equiv that in some deposits are postdated by moderate to high salinity brines 10 40 wt nacl equiv these contrasting fluid types are interpreted to be magmatic in origin and are the result of the complex interplay between exsolution of different volatiles carbon dioxide water and chlorine from felsic magmas emplaced at different crustal levels experimental studies have shown that carbon dioxide will exsolve from felsic magmas at much higher pressures than water and chlorine due to its lower solubility in the melt thus deposits formed from felsic magmas in deeper environments will contain abundant carbon dioxide rich fluids subsequent fluids released from the magma are likely to be water rich and more saline felsic magmas in shallow environments will contain lower concentrations of carbon dioxide that will partition into the vapor phase during phase separation between brine and vapor

ore genesis of the duotoushan fe cu deposit eastern tianshan nw china constraints from ore geology mineral geochemistry fluid inclusion and stable isotopes abstract the duotoushan fe cu deposit is located in the western segment of the aqishan yamansu metallogenic belt in eastern tianshan xinjiang china magnetite orebodies are mainly developed in the upper carboniferous volcano sedimentary rocks and are mostly stratabound pyrite re os dating has yielded an isochron age of 312 24 ma mswd 1 5 five hydrothermal and one supergene alteration mineralization stages have been recognized from the spatial distribution of alteration zoning mineral assemblages and textural relationships the earliest alteration of albite ab84 98or2 16 amphibole was related to the nearby albitization granite porphyry prograde alteration was developed on the footwall of the intermediate mafic volcanic rocks and formed a garnet ad60 95gr5 40 clinopyroxene hd0di93jo7 to hd10di83jo7 assemblage alteration associated with magnetite can be further divided into three sub stages consisting of magnetite amphibole titanite stage iii a magnetite amphibole epidote stage iii b and epidote magnetite quartz amphibole titanite pyrite stage iii c mineral assemblages the secondary amphibole is dominantly actinolite pyrite with minor chlorite chalcopyrite veins cut the magnetite ore followed by later quartz calcite hematite veinlets based on fluid inclusions fis and stable isotopes studies the pre ore stage stage ii was formed at high temperature 500 c and originated from magmatic hydrothermal fluids 18 o h 2 o 7 1 d h 2 o 85 two garnet samples have similar 18 o h 2 o values 5 6 and 6 4 but considerably lower d h 2 o values 134 and 139 indicating the overprinting by late meteoric water iron ore and hydrous minerals assemblages and the associated fluids had overprinted the early anhydrous minerals early magnetite mineralization stage stage iii a shows clear magmatic water input as constrained from the 18 o h 2 o 6 9 to 7 8 and d h 2 o 85 to 72 values later magnetite stage was magmatic hydrothermal fluid dominated but with mixing of heavy d h 2 o as indicated by stable isotope compositions 18 o h 2 o 6 1 to 7 8 d h 2 o 47 to 32 sulfur isotope values of the stage iv pyrite give a 34s interval of 8 8 to 12 1 suggesting significant addition of external basin fluid seawater or evaporite sulfur source to magmatic fluid the low temperatures 83 213 c average 135 c generally medium high but variable salinities 9 9 27 2 wt nacl equiv average 21 3 wt and ca mg rich components of stage v fluids show clear features of involvement of basinal brines seawater and minor meteoric water in the late hydrothermal stages we infer that the temperature drop of the hydrothermal fluids may have been the main cause of massive magnetite precipitation at the duotoushan deposit whereas increasing ph values decreasing temperature and fluid mixing maybe responsible for cu precipitation ore deposit geology mineral geochemistry and fluid evolution confirm that the duotoushan fe cu deposit is different from typical skarns and may represent a distinct group of fe cu deposits formed during the aqishan yamansu back intra arc basin inversion which show many similarities with the central andean iocg deposits

ore genesis and hydrothermal evolution of the huanggang skarn iron tin polymetallic deposit southern great xing an range evidence from fluid inclusions and isotope analyses abstract the southern great xing an range is one of the most important metallogenic belts in northern china and contains numerous pb zn ag cu sn fe mo deposits the huanggang iron tin polymetallic skarn deposit is located in the sn polymetallic metallogenic sub belt skarns and iron orebodies occur as lenses along the contact between granite plutons and the lower permian huanggangliang formation marble or dashizhai formation andesite field evidence and petrographic observations indicate that the three stages of hydrothermal activity i e skarn oxide and sulfide stages all contributed to the formation of the huanggang deposit the skarn stage is characterized by the formation of garnet and pyroxene and high temperature hypersaline hydrothermal fluids with isotopic compositions that are similar to those of typical magmatic fluids these fluids most likely were generated by the separation of brine from a silicate melt instead of being a product of aqueous fluid immiscibility the iron oxide stage coincides with the replacement of garnet and pyroxene by amphibole chlorite quartz and magnetite the hydrothermal fluids of this stage are represented by l type fluid inclusions that coexist with v type inclusions with anomalously low d values approximately 100 to 116 the decrease in ore fluid 18oh2o values with time coincides with marked decreases in the fluid salinity and temperature based on the fluid inclusion and stable isotopic data the ore fluid evolved by boiling of the magmatic brine the sulfide stage is characterized by the development of sphalerite chalcopyrite fluorite and calcite veins and these veins cut across the skarns and orebodies the fluids during this stage are represented by inclusions with a variable but continuous sequence of salinities mainly low salinity inclusions these fluids yield the lowest 18oh2o values and moderate d values 1 6 to 2 8 and 101 to 104 respectively the data indicate that the sulfide stage fluids originated from the mixing of residual oxide stage fluids with various amounts of meteoric water boiling occurred during this stage at low temperatures the sulfur isotope 34s values of the sulfides are in a narrow range of 6 70 to 4 50 mean 1 01 and the oxygen isotope 18o values of the magnetite are in a narrow range of 0 1 to 3 4 both of these sets of values suggest that the ore forming fluid is of magmatic origin the lead isotope compositions of the ore 206pb 204pb 18 252 18 345 207pb 204pb 15 511 15 607 and 208pb 204pb 38 071 38 388 are consistent with those of k feldspar granites 206pb 204pb 18 183 18 495 207pb 204pb 15 448 15 602 208pb 204pb 37 877 38 325 but significantly differ from those of permian marble 206pb 204pb 18 367 18 449 207pb 204pb 15 676 15 695 208pb 204pb 38 469 38 465 which also suggests that the ore forming fluid is of magmatic origin

textural evolution of synthetic fluid inclusions in quartz during reequilibration with applications to tectonic reconstruction experimental reequilibration of synthetic 10 wt nacl h2o inclusions in natural quartz reveals that reequilibration textures show distinct differences depending upon the p t path followed by the inclusion after formation these differences combined with other geological information may be used to determine whether the sample rock followed a dominantly isothermal or isobaricp t path following entrapment the intensity and style of inclusion reequilibration features is related to the direction and magnitude of the departure of thep t path from the original isochore for the inclusion thus fluid inclusion reequilibration textures not only permit inclusionists to determine whether the rocks followed an isothermal or isobaric retrogradep t path but also the magnitude of departure of this path from one that is isochoric

stable isotope and fluid inclusion constraints on the source and evolution of ore fluids in the hongniu hongshan cu skarn deposit yunnan province china the hongniu hongshan cu skarn deposit 77 8 mt at 1 8 cu is located in the central part of the zhongdian porphyry and skarn cu belt in southwestern china skarn and orebodies occur mainly between the different units of the upper triassic qugasi formation or within altered limestone adjacent to late cretaceous intrusions 78 76 ma three main paragenetic stages of skarn formation and ore deposition have been recognized on the basis of petrographic observations 1 pre ore stage hornfels with diopside di87 72hd12 7 small scale endoskarn with reddish grossular adr22 57gr78 43 diopside di83 92hd7 15 vesuvianite and abundant exoskarn with red brown andradite adr75 98gr2 22 sahlite di28 41hd58 71 and wollastonite 2 syn ore stage retrograde minerals sulfides pyrite chalcopyrite pyrrhotite molybdenite galena and sphalerite quartz and calcite and 3 post ore stage calcite veins sulfur isotope values of sulfides are relatively high with an average 34s 4 9 n 40 suggesting that the ore forming fluid was magmatic and that the sulfides precipitated from a relatively reducing ore fluid the coexistence of silicate melt and primary fluid inclusions in quartz phenocrysts of the mineralization related quartz monzonite porphyry indicates the simultaneous entrapment of fluid and melt and records the process of the aqueous fluid exsolving from the crystallizing melt the initial single phase fluid has a salinity of 8 8 to 12 7 wt nacl equiv and homogenization temperatures of 566 to 650 c corresponding to pressures of 680 to 940 bar and lithostatic depth of 2 5 to 3 5 km the primary fluid inclusions in the pre ore stage garnet and pyroxene composed of coeval vapor rich v type and halite bearing s i and s ii types containing sylvite inclusions 32 79 total wt salts share similar homogenization temperatures 450 550 c indicative of the occurrence of fluid unmixing under lithostatic pressures of 550 to 780 bar 2 0 km depth primary fluid inclusions trapped in syn ore quartz calcite and epidote show the common development of s type inclusions 37 3 wt nacl equiv with coexisting v type liquid rich l type and co2 bearing c i type inclusions all of which have homogenization temperatures of 300 to 400 c and trapping pressures of 100 to 400 bar 1 5 km depth brine inclusions homogenized by halite dissolution after vapor disappearance in both the pre and syn ore stages are interpreted to have been trapped under overpressured conditions 1 520 bar oxygen isotope analyses were conducted on garnet wollastonite epidote quartz and calcite the pre ore stage garnet and wollastonite have 18ofluid values of 5 6 to 8 1 whereas the syn ore stage epidote quartz and calcite have more variable 18ofluid values in the range of 3 9 to 17 5 the 18ofluid values of the post ore stage vein calcite 15 2 21 3 are much higher than both the pre and syn ore stages the vapor phase of inclusions contains h2s ch4 and c2h6 in the syn ore stages all these observations reveal that 1 the formation of the cu skarn deposit was dominated by a magmatic hydrothermal system 2 multiple fluid pulses contributed to the formation of the pre and syn ore stage skarn minerals and sulfides and 3 the increase in ph due to the neutralization of the acidic fluid could be the main factor controlling the large scale ore deposition in hongniu hongshan

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

diagenesis and reservoir quality of the sherwood sandstone triassic corrib field slyne basin west of ireland abstract the triassic sherwood sandstone in the corrib field slyne basin west of ireland on the european continental margin is a dry gas reservoir with a mercia mudstone top seal petrographic analysis combined with x ray diffraction stable isotope fluid inclusion and core analysis have been used to assess timing of growth and origin of mineral cements whether sandstone diagenesis involved mass flux the controls on reservoir quality and how reservoir quality is likely to vary in more deeply buried sections depositional and early diagenetic characteristics of the sherwood in corrib are typical of a semi arid fluvial environment containing groundwater of meteoric origin early diagenesis included the development of copious dolomite cement in the form of dolocrete as well as the formation of abundant clay while less volumetrically important grain coating haematite and k feldspar cement also grew burial diagenesis witnessed the initial growth of minor chlorite and albite quartz overgrowths and ankerite followed and are the most important burial cements growing over a temperature interval between 100 and 1165 c albite commenced growth at about 90 c quartz cement at 100 c and ankerite at 110 c these cements reached the zenith of their development at 105 110 c for albite 125 130 c for quartz and 135 145 c for ankerite siderite and anhydrite are relatively minor late stage cements the formation water has been consistently highly saline during burial and in terms of mineral cement derived oxygen stable isotope values is likely to be a diagenetically evolved version of the initial depositional water suggesting long term stagnation of the pore fluids the diagenetic reactions that formed burial cements all seem to be essentially isochemical including the ankerite that has carbon isotope characteristics of the cannibalisation of dolocrete reservoir quality is mainly controlled by early diagenetic dolomite and clay although grain size is also important because only sandstones with 200 m grain size have 50 md permeability both shallower and deeper sections than corrib may have similar porosity and permeability since temperature dependent diagenetic controls on reservoir quality e g quartz and illite cement are peripheral

trench slope controlled deep sea clastics in the exposed lower surma group in the southeastern fold belt of the bengal basin bangladesh abstract deltaic to shallow marine neritic depositional settings have until now been the accepted interpretation for the neogene surma group of rocks exposed throughout the southeastern fold belt of the bengal basin the present study revises the earlier views and proposes some new insights on the basis of detailed field studies carried out in the sitapahar anticline rangamati area and in the mirinja anticline lama area it is here proposed that the surma group succession represents an overall basinward progradation from deep marine to coastal marine depositional settings sedimentological evidence strongly suggests that the lower part of the surma group represents a slope apron the growth of which is thought to have been governed by a westward migrating accretionary prism complex within the active margin setting of the indo burmese plate convergence thin packages of distinct turbidity current generated deposits together with some slump and debris flow deposits contained within thicker intervals of mudstone characterize the lowermost exposed unit of the surma group in the sitapahar anticline the overlying slope deposits are essentially mudstone dominated comparable deep sea clastics with thicker intervals of sandstone turbidites contained within a submarine canyon are present in the mirinja anticline this new model suggests that it may be timely to re evaluate the existing stratigraphic and tectonic framework of the eastern bengal basin