geochemical identification of episodes of gold mineralisation in the barberton greenstone belt south africa abstract the morphology and mineral chemistry of gold and associated sulphides at sheba fairview and new consort gold mines in the barberton greenstone belt bgb identify two main types of mineralization the first type occurs associated with sulphides mainly pyrite either as inclusions 10 30 m or as sub microscopic gold the second gold type consists of large gold grains 100 m within the silicates mostly quartz la icp ms studies reveal that some gold and associated sulphide grains contain high values of cl br na and i the elemental relationships reflect the different chemistry and precipitation processes of possible source fluids and identify several episodes of mineralisation in the study area one of them formed due to a boiling process in a supercritical hydrothermal environment this paper reports on the compositional characteristics of these gold grains the significance of the halogen contents and the implications for possible sources of the gold and associated sulphides

combining in situ isotopic trace element and textural analyses of quartz from four magmatic hydrothermal ore deposits this study couples in situ 16o 17o and 18o isotope and in situ trace element analyses to investigate and characterize the geochemical and textural complexity of magmatic hydrothermal quartz crystals euhedral quartz crystals contemporaneous with mineralization were obtained from four magmatic hydrothermal ore deposits el indio au ag cu deposit summitville au ag cu deposit north parkes cu au deposit and kingsgate quartz mo bi w deposit the internal features of the crystals were imaged using cathodoluminescence and qualitative electron microprobe maps quantitative isotopic data were collected in situ using 157 nm laser ablation inductively coupled plasma mass spectrometry for 40 trace elements in quartz and sensitive high resolution ion microprobe for 3 isotopes in quartz imaging revealed fine oscillatory zoning sector zoning complex macromosaic textures and hidden xenocrystic cores in situ oxygen isotope analyses revealed a 18o range of up to 12 4 0 3 in a single crystal the largest isotopic range ever ascribed to oscillatory zonation in quartz some of these crystals contain a heavier 18o signature than expected by existing models while sector zoned crystals exhibited strong trace element variations between faces no evidence for anisotropic isotope fractionation was found we found 1 isotopic heterogeneity in hydrothermal quartz crystals is common and precludes provenance analysis e g d 18o using bulk analytical techniques 2 the trace element signature of quartz is not an effective pathfinder toward noble metal mineralization and 3 in three of the four samples both textural and isotopic data indicate non equilibrium deposition of quartz

paleostress and statistical analysis using quartz veins from mineralized and non mineralized zones application for exploration targeting abstract quartz vein orientations thickness and spacing data are analyzed from 10 transects each of mineralized and non mineralized zones of gadag region which is a gold bearing terrain within the dharwar craton southern india paleostress analysis reveals that the veins emplaced when fluid pressure pf varied from pf 2 intermediate principle stress to pf 2 to pf

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

paragenesis and geochemistry of ore minerals in the epizonal gold deposits of the yangshan gold belt west qinling china six epizonal gold deposits in the 30 km long yangshan gold belt gansu province are estimated to contain more than 300 t of gold at an average grade of 4 76 g t and thus define one of china s largest gold resources detailed paragenetic studies have recognized five stages of sulfide mineral precipitation in the deposits of the belt syngenetic diagenetic pyrite py0 has a framboidal or colloform texture and is disseminated in the metasedimentary host rocks early hydrothermal pyrite py1 in quartz veins is disseminated in metasedimentary rocks and dikes and also occurs as semi massive pyrite aggregates or bedding parallel pyrite bands in phyllite the main ore stage pyrite py2 commonly overgrows py1 and is typically associated with main ore stage arsenopyrite apy2 late ore stage pyrite py3 arsenopyrite apy3 and stibnite occur in quartz calcite veins or are disseminated in country rocks post ore stage pyrite py4 occurs in quartz calcite veins that cut all earlier formed mineralization electron probe microanalyses and laser ablation inductively coupled plasma mass spectrometry analyses reveal that different generations of sulfides have characteristic of major and trace element patterns which can be used as a proxy for the distinct hydrothermal events syngenetic diagenetic pyrite has high concentrations of as au bi co cu mn ni pb sb and zn the py0 also retains a sedimentary co ni ratio which is distinct from hydrothermal ore related pyrite early hydrothermal py1 has high contents of ag as au bi cu fe sb and v and it reflects elevated levels of these elements in the earliest mineralizing metamorphic fluids the main ore stage py2 has a very high content of as median value of 2 96 wt and au median value of 47 5 ppm and slightly elevated cu but relatively low values for other trace elements arsenic in the main ore stage py2 occurs in solid solution late ore stage py3 formed coevally with stibnite contains relatively high as median value of 1 44 wt au fe mn mo sb and zn and low bi co ni and pb the main ore stage apy2 compared to late ore stage arsenopyrite is relatively enriched in as whereas the later apy3 has high concentrations of s fe and sb which is consistent with element patterns in associated main and late ore stage pyrite generations compared with pyrite from other stages the post ore stage py4 has relatively low concentrations of fe and s whereas as remains elevated 2 05 3 20 wt which could be interpreted by the substitution of as for s in the pyrite structure these results suggest that syngenetic diagenetic pyrite is the main metal source for the yangshan gold deposits where such pyrite was metamorphosed at depth below presently exposed levels the ore forming elements were concentrated into the hydrothermal fluids during metamorphic devolatilization and subsequently during extensive fluid rock interaction at shallower levels these elements were precipitated via widespread sulfidation during the main ore stage

petrogenesis of triassic granitoids in the fengxian taibai ore cluster western qinling orogen central china implications for tectonic evolution and polymetallic mineralization abstract the petrogenesis and tectonic setting of the triassic granitoids and their relationship with the coeval au and pb zn polymetallic mineralization in the western qinling orogen wqo in central china remain debated the fengxian taibai ore cluster ftoc in the eastern part of the wqo hosts large scale triassic granitoids and numerous polymetallic deposits providing an excellent opportunity to constrain the controversies mentioned above in this work we choose the huahongshuping granodiorite pluton guji biotite monzogranite pluton taoerling biotite monzogranite dike dongtangzi granite porphyry dike and dagou dioritic dikes in the ftoc for a combined study on the zircon u pb geochronology whole rock major trace elements and sr nd isotopes la icp ms zircon u pb dating yields ages of 225 ma 224 ma and 227 217 ma for the huahongshuping pluton guji pluton and granitoid dikes respectively all these granitoids have metaluminous and high k calc alkaline series affinities and are enriched in large ion lithophile elements lile light rare earth elements lree and depleted in high field strength elements hfse with a marked lree hree fractionation la yb n 10 66 32 42 fractionation of lree la sm n 2 72 6 60 and negative eu anomalies eu 0 45 0 99 they have initial sr isotopic ratios of 0 7062 to 0 7082 and negative end t values of 9 42 to 8 09 the above features indicate that the ftoc granitoids were mainly derived from the partial melting of the neoproterozoic meta volcanic rocks induced by the decrease in pressure and the addition of water dehydrated from biotite with direct contribution of mantle materials combined with previous data on sedimentation deformation and metamorphism and tectonics we suggest that the wqo underwent the early mesozoic tectonic evolution from the subduction at 248 235 ma through syn collision at 235 225 ma to post collision at 225 195 ma integrating with the ore geology geochronology mineralogy and quartz and carbonate h o and in situ s isotope data of sulfides from different mineralization stages at representative au and pb zn deposits in literatures we proposed a conceptive model that the post collision process and granitic magmatic activity played an important role in the polymetallic mineralization within the ftoc

mixing of metamorphic and surficial fluids during the uplift of the hercynian upper crust consequences for gold deposition a detailed geochemical study of fluids from representative quartz sealed faults hosting late hercynian gold concentrations shows that fluids percolating the mineralised faults had two main distinct reservoirs one was a quite shallow and the other rather deep seated both fluids have lost a great part of their original geochemical signature through interactions with host metamorphic formations early fluids present during the primary sealing of the faults by quartz are considered to have effectively equilibrated with the metamorphic pile and then predominantly flowed upwards along the faults they are characterised by ch4 co2 h2o ratios rather typical of fluids equilibrated with graphite and moderate to medium chlorinities with a high br cl ratio the striking feature of the gold bearing quartz is that gold is not synchronous within any quartz deposition and appears located in late microfractures and associated with pb bi sb sulphosalts and sulphides these late stages are characterised by fluids whose salinities decrease to very low values indicating their progressive dilution by waters of more surficial origin in the fault system the long lived activity of the fault favoured the connection between two distinct fluid reservoirs at a critical time during the basement uplift the fluids evolved through two main driving mechanisms which were responsible for the au deposition i decrease in temperature accompanying decompression from supra lithostatic to hydrostatic conditions which yielded in some instances volatile unmixing in the faulted systems ii mixing of the resulting fluids with waters entering the hydrological systems from shallower reservoirs in addition to dilution and fluid mixing which are favourable factors for decreasing the gold solubility the presence of microfractured sulphides could have enhanced gold precipitation through electrochemical processes d 2002 elsevier science b v all rights reserved

modeling singular mineralization processes due to fluid pressure fluctuations abstract mineralization in the earth s crust can be regarded as a singular process resulting in large amounts of mass accumulation and element enrichment over short time or space scales the elemental concentrations modeled by fractals and multifractals show self similarity and scale invariant properties we take the view that fluid pressure variations in response to earthquakes or fault rupture are primarily responsible for changes in solubility and trigger transient physical and chemical variations in ore forming fluids that enhance the mineralization process based on this general concept we investigated mineral precipitation processes driven by rapid fluid pressure reductions by coupling mineralization to a cellular automaton model to reveal the nonlinear mechanism of the orogenic gold mineralization process using simulation in the model fluid pressure can increase to the rock failure condition which was set as lithostatic pressure at a depth of 10 km 270 mpa due to either porosity reduction or dehydration reactions rapid drops in pressure resulting from fault rupture or local hydrofracture may induce repeated gold precipitation the geochemical patterns generated by the model evolve from depletion to enrichment patterns and from spatially random to spatially clustered structures quantified by multifractal models and geostatistics results show how metal elements self organize to form high metal concentration patterns displaying self similarity and scale invariance these transitions are attributed to the growth and coalescence of sub networks with different fluid pressures up to the percolation threshold resulting in a wide range of fluid pressure reductions and gold precipitation in the form of clusters the results suggest that cyclic evolution of fluid pressure and its effects on gold precipitation systems can effectively mimic the repeated mineralization superposition process and generate complex geochemical patterns characterized by a multifractal model the nonlinear behavior exhibits scale invariance and self organized critical threshold where mineral phase separations result from fluid pressure reductions associated with fault failure

the multiple sulfur isotope architecture of the golden mile and mount charlotte deposits western australia the golden mile and mount charlotte deposits in the kalgoorlie terrane western australia display three main mineralization styles fimiston comprised of interconnected shear zones associated with ankerite pyrite hematite magnetite gold telluride alteration oroya made up of breccia bodies with v muscovite ankerite pyrite pyrrhotite gold telluride alteration and mount charlotte which consists of vein arrays with symmetrical ankerite sericite albite pyrite pyrrhotite gold alteration pyrite is in equilibrium with gold in all three mineralization styles and has been selected as a proxy to record the sulfur source of the mineralizing fluids as well as the nature of the hydrothermal processes the 34s 33s and 36s analyses on pyrite grains from the different mineralization styles including oxidized and reduced sulfide oxide assemblages reveal 1 a large variation in 34s from 12 6 to 23 5 and 2 a previously unrecognized occurrence of anomalous 33s and 36s signatures from 1 0 to 1 1 and from 2 3 to 0 9 respectively it is argued that the mineralizing fluids that formed the golden mile and mount charlotte deposits record mixing among three components mantle sulfur oxidized seawater sulfur e g so4 and reduced elemental sulfur e g s8 petrographic evidence in conjunction with 33s and 36s data suggest that mif s was acquired during the deposition of shales and basalts present in the kalgoorlie terrane and later mixed with mantle derived sulfur during the mineralization events the negative 34s values that predominate in fimiston style mineralization are consistent with a prevalence of oxidized fluids during the ore forming process as reflected by the presence of hematite pyrite magnetite gold assemblages conversely the positive 34s values that dominate in the mt charlotte and oroya mineralization styles reflect a reducing environment as reflected by the presence of pyrite pyrrhotite gold assemblages

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

relationships between gold and pyrite at the xincheng gold deposit jiaodong peninsula china implications for gold source and deposition in a brittle epizonal environment the xincheng gold deposit hosted by the early cretaceous 132 to 123 ma guojialing type granitoids in northwest jiaodong peninsula southeast north china craton formed about 2 billion years later than regional metamorphism of the archean jiaodong basement rocks the xincheng deposit comprises mineralized zones with three types of hydrothermal pyrite associated with gold tellurides and a variety of sulfides py1 as disseminated euhedral to subhedral grains in altered granitoids around quartz veins py2 as subhedral grains with brittle cataclastic textures and fractures in quartz pyrite veins and py3 as subhedral partially corroded crystals in sulfide rich veins or veinlets all three generations of pyrite are unzoned and have low trace element contents including very low lattice bound gold contents py1 0 180 ppm py2 0 053 ppm py3 0 060 ppm given that there is 10 to 15 pyrite in the ore zone at xincheng its very low gold content indicates that it contributes 0 2 of gold to the 7 75 g t gold in the orebody instead over 99 of the gold is present as discrete electrum and or gold total range 0 02 59 silver grains which are largely sited in fractures at all scales in pyrite other ore minerals and quartz importantly visible gold in py3 is also sited on solution corroded pyrite grains the pyrite textural and geochemical data indicate that it is impossible to derive the high gold grade orebodies through local remobilization of originally lattice bound gold in pyrite instead the gold is interpreted to have been deposited through sulfidation reactions and phase separation of a h2o co2 ore fluid during progressive brittle cataclastic deformation associated with seismic activity and regional sinistral transtensional shear movement this concomitant fluid infiltration and deformation caused episodic deposition and fracturing and corrosion of earlier formed pyrite and deposited visible gold in dilational cracks the coupled development of the transtensional rather than normal transpressional setting and precipitation of gold within dilational veins and wall rock alteration facilitated the deposition of visible gold and an exceptionally high gold tenor all deposit characteristics indicate that the xincheng gold deposit is a member of the epizonal orogenic deposit class

water vaporization promotes coseismic fluid pressurization and buffers temperature rise we investigated the frictional properties of carbonate rich gouge layers at a slip rate of 1 3 m s under dry and water saturated conditions while monitoring temperature at different locations on one of the gouge host rock interfaces all experiments showed a peak frictional strength of 0 4 0 7 followed by strong slip weakening to steady state values of 0 1 0 3 experiments which used a pore fluid with a constant drainage path to the atmosphere showed the development of a temperature plateau beyond 100 c contemporaneous with the dynamic slip weakening and consistent with thermodynamic considerations of ongoing vaporization of pore water upon pore fluid vaporization the pore pressure increases while the temperature is buffered endothermically such that the pore water moves along the liquid vapor transition curve in a pressure temperature phase diagram pore fluid phase transitions of this kind are expected to occur in natural earthquakes at relatively shallow crustal levels enhancing fluid pressurization while impeding the achievement of high temperatures therefore the operation of vaporization may help explain the low downhole temperature anomalies obtained shortly after large earthquakes

fault valve behaviour in optimally oriented shear zones an example at the revenge gold mine kambalda western australia abstract quartz vein systems developed in and adjacent to shear zones host major gold deposits in the kambalda region of the norseman wiluna greenstone belt at the revenge mine two groups of mineralised reverse shear zones formed as conjugate near optimally oriented sets during ese subhorizontal shortening adjacent to a major transpressional shear system the shear zones developed at temperatures of about 400 c in a transitional brittle ductile regime deformation was associated with high fluid fluxes and involved fault valve behaviour at transiently near lithostatic fluid pressures during progressive evolution of the shear system early brittle and ductile deformation was overprinted by predominantly brittle deformation brittle shear failure was associated with fault dilation and the formation of fault fill veins particularly at fault bends and jogs a transition from predominantly brittle shear failure to combined shear along faults and extension failure adjacent to faults occurred late during shear zone evolution and is interpreted as a response to a progressive decrease in maximum shear stress and a decrease in effective stresses the formation of subhorizontal stylolites locally subvertical extension veins and minor normal faults in association with thrust faulting indicates episodic or transient reorientation of the near field maximum principal stress from a subhorizontal to a near vertical attitude during some fault valve cycles local stress re orientation is interpreted as resulting from near total shear stress release and overshoot during some rupture events previously described fault valve systems have formed predominantly in severely misoriented faults the shear systems at revenge mine indicate that fault valve action and associated fluctuations in shear stress and fluid pressure can influence the mechanical behaviour of optimally oriented faults

ore shoots in folded and fractured rocks insights from 3d modelling of the fosterville gold deposit victoria australia abstract at the fosterville gold deposit high concentrations of visible and refractory gold are hosted in arrays of quartz and quartz carbonate veins they are distributed in elongate zones to form south plunging ore shoots within a folded and fractured turbidite sequence here we distinguish the influence of faults versus folds at fosterville by assessing the deposit scale structural framework and hydrothermal fluid flow using a combination of 3d implicit modelling structural analysis and 3d fault slip dilation tendency analyses gold assay and bedding trend data from a large drillhole dataset were spatially interpolated and used to analyse and measure the orientation of ore bodies and folded host rocks field measurements of fold axis orientations were compared to bedding measurements in oriented drillcore bingham axial analysis of bedding trends and orientation data from a 3d implicit model both field and drill core structural data show that ore shoots at fosterville do not parallel the first order gently 10 south plunging folds instead the orientation of the ore shoots is coincident with an intersection lineation between steeply west dipping bedding parallel faults and moderately dipping linkage faults that obliquely cut across the main structural grain resulting in ore shoot plunges larger than 20 slip and dilation tendencies suggest that linkage faults had a higher likelihood of activity in the inferred far field stress regimes at the time of mineralisation relative to other structures at fosterville these linkage faults are therefore interpreted to have been preferential fluid flux conduits in the system despite appearing to be subordinate structures at the scale of the deposit this new structural model for fosterville highlights that fluid flow and ore shoot formation are strongly controlled by obliquely oriented structures that are unlike the main structural grain of first order folds and faults

the engineer mine british columbia an example of epithermal au ag mineralization with mixed alkaline and subalkaline characteristics abstract the engineer mine epithermal precious metal deposit in british columbia canada is related to eocene sloko skukum group ssg volcanism and according to previous studies contains roscoelite in intimate association with electrum roscoelite a vanadian mica is considered characteristic of low sulfidation epithermal deposits related to alkaline magmatism this contradicts the fact that the ssg volcanics are subalkaline in order to address this ambiguity and to accurately classify the style of epithermal mineralization at the engineer mine we conducted detailed petrographic mineralogical geochronological fluid inclusion and stable isotope studies the principal ore assemblage of the engineer mine epithermal veins precipitated in response to boiling during a hydrothermal event at 49 90 ma 0 25 ma during this event electrum arsenopyrite pyrite chalcopyrite sphalerite lollingite tetrahedrite group phases allargentum acanthite hessite dyscrasite stibarsen galena and an unidentified ag rich phase were deposited in conjunction with amorphous silica platy and rhombic calcite k feldspar and vanadian illite fluid inclusion and stable isotope data suggest that the ore forming fluid was boiling at 220 c during vein mineralization and had an isotopic composition derived from local meteoric water based on these results the engineer mine is classified as an epithermal low sulfidation deposit which shares similarities with alkaline and subalkaline epithermal low sulfidation deposits this is attributed to the fact that the ssg volcanic rocks are borderline subalkaline to alkaline in character and that the sedimentary host rocks are vanadium bearing these sedimentary rocks contributed the bulk of the vanadium to the engineer mine epithermal system the presence of roscoelite at the engineer mine could not be confirmed during this study the mica referred to as roscoelite in previous publications instead is vanadian illite to our knowledge the only alkaline low sulfidation epithermal precious metal deposit that contains v mica which exclusively qualifies as true roscoelite is the porgera deposit papua new guinea

physical chemical properties and metal budget of au transporting hydrothermal fluids in orogenic deposits we use a set of analytical techniques for fluid inclusion analysis to determine the bulk properties of the ore fluids from five orogenic gold deposits the most common ore fluid is a low salinity 0 4 6 5 wt nacl twoto three phase aqueous carbonic fluid its th total range is 200 400 8c and total homogenization occurs by bubble or liquid disappearance a less common aqueous fluid of similar bulk salinity and t h total is also documented microchemical data show that the ore fluid in the five deposits is chemically uniform and made of na subordinate k and b and minor to trace amounts of cu as li sr rb ba cs sb and au range 0 5 5 mg g thermodynamic modelling carried out for one of the studied deposits indicates that the ore fluid was in equilibrium with vein minerals at the time of gold deposition and that its vapour phase was distinctly enriched in au b as and sb the proposed mechanism of gold precipitation is a combination of fluid decompression and boiling combining these results with 3d geometrical reconstructions of one of the studied deposits sigma we estimate that a minimum ore fluid volume of c 0 1 1 0 km is sufficient to generate a world class orogenic deposit orogenic deposits provide about a third of the global gold production goldfarb et al 2001 frimmel 2008 these deposits are formed within active continental margins in orogenic belts where major tectonic boundaries juxtapose metamorphosed volcanic plutonic and sedimentary terranes the ore bodies consist of au mineralized vein networks filled with quartz carbonates and other silicates that in places crosscut the stacked terranes of the continental margins the veins are dilated portions of vertically extensive brittle ductile to ductile fracture networks that form due to the advection of aqueous hydrothermal fluids within the continental crust ridley et al 1996 groves et al 2003 these ore fluids are generated in still undefined source regions deep in the crust and transported to shallower levels 3 15 km depth through the vein networks along which they deposit gold a number of studies kerrich et al 2000 ridley diamond 2000 groves et al 2003 showed that although these networks do not occur systematically in all orogens they have a set of important geological features in common which include 1 formation during accretionary orogenic events 2 occurrence relatively close to regional tectonic lineaments especially along lower order splays of these structures 3 spatial association with granitoid intrusions 4 occurrence within some metamorphic belts with gold precipitation being synto postmetamorphic in age 5 relatively low sulphide abundance within the veins 6 carbonate sericitealbite sulphide hydrothermal alteration of host rocks postdating metamorphism 7 poorly developed mineral and metal zoning within the ore bodies and 8 genesis from low to moderate salinity ore fluids over a large temperature range 250 700 8c but mostly within 300 350 8c these common traits occur systematically in deposits from different terrains having distinct ages suggesting that a common geological process and au transporting fluid are responsible for their formation here we present a database of fluid inclusion properties from five typical orogenic deposits which show most of the geological characteristics listed above the database was constructed with the aim of determining as accurately as possible the properties of the au transporting fluid in this geological environment in particular laser ablation inductively coupled plasma mass spectrometry la icp ms of single fluid inclusions paragenetically coeval with vein gold provides the concentration of a suite of metals including au in this fluid this dataset when combined with 3d geometrical reconstructions of one of these deposits offers new constraints on the volume of the ore from garofalo p s ridley j r eds gold transporting hydrothermal fluids in the earth s crust geological society london special publications 402 http dx doi org 10 1144 sp402 8 the geological society of london 2014 publishing disclaimer www geolsoc org uk pub ethics by guest on april 3 2014 http sp lyellcollection org downloaded from

geochemical evidence for relict degassing pathways preserved in andesite andesitic arc volcanoes degas large quantities of volatiles evidence for vapour transport in the erupted lavas is rarely preserved and poorly understood but is crucial for understanding eruption style we present geochemical evidence for the transport of metal bearing vapour in shear zones preserved in lavas erupted from soufriere hills volcano montserrat textural evidence suggests that shear induced brittle failure occurred in a narrow zone at the conduit wall or in the lava dome elevated metal concentrations cu au ag pb zn within the zones indicate that the fractures acted as a transient pathway for metal bearing magmatic gases during slip frictional heating to temperatures of 1000 c caused partial melting at the slip surface resorption of volatiles and metals into the partial melt preserved the geochemical signature of magmatic vapour in the shear zone cordierite which is highly unusual in volcanic rocks crystallised from the peraluminous partial melt with metal bearing sulphides and oxides the shear zones provide the first geochemical evidence for vapour segregation and transport through viscous andesite magmas and provide an insight into controls on eruption style

vaporization of fault water during seismic slip laboratory and numerical studies as well as field observations indicate that phase transitions of pore water might be an important process in large earthquakes we present a model of the thermo hydro chemo mechanical processes including a two phase mixture model to incorporate the phase transitions of pore water occurring during fast slip i e a natural earthquake in order to investigate the effects of vaporization on the coseismic slip using parameters from typical natural faults our modeling shows that vaporization can indeed occur at the shallow depths of an earthquake irrespective of the wide variability of the parameters involved sliding velocity friction coefficient gouge permeability and porosity and shear induced dilatancy due to the fast kinetics water vaporization can cause a rapid slip weakening even when the hydrological conditions of the fault zone are not favorable for thermal pressurization e g when permeability is high at the same time the latent heat associated with the phase transition causes the temperature rise in the slip zone to be buffered our parametric analyses reveal that the amount of frictional work is the principal factor controlling the onset and activity of vaporization and that it can easily be achieved in earthquakes our study shows that coseismic pore fluid vaporization might have played important roles at shallow depths of large earthquakes by enhancing slip weakening and buffering the temperature rise the combined effects may provide an alternative explanation for the fact that low temperature anomalies were measured in the slip zones at shallow depths of large earthquakes

three dimensional modelling of alteration zones based on geochemical exploration data an interpretable machine learning approach via generalized additive models abstract the inference of deep alterations plays an important role in the analysis of deep geological processes and the targeting of buried mineral resources however the construction of three dimensional 3d models of alteration zones based on geochemical data is challenging due to the diverse geological processes and scarcity of information related to deep alterations in this paper we propose an interpretable machine learning method to infer 3d geological models of alteration zones based on geochemical data from the dayingezhuang area china to expose the information hidden in geochemical data that is indirectly and weakly related to alteration zones a generalized additive model gam is used it learns a nonlinear intelligible model associating geochemical anomalies extracted by pca with alteration features the model learned via gam and is then combined with a bayesian framework to infer the thickness of alterations in a no drilling area thus enabling 3d modelling of deep alteration zones with such an intelligible model geologists are capable of inspecting the reliability of the model and better understanding the association between the distribution of geochemical elements and deep alterations the inference results show that the relationship between geochemical anomalies and alteration depth is significant while the relationship with alteration thickness is complex the results of this study provide the underlying insights needed to guide the analysis of deep geological mineralization processes

silica hydrate preserved with δ18o rich quartz in high temperature hydrothermal quartz in the high sulfidation copper gold deposit at el indio chile quartz microcrystals from the el indio au ag cu deposit chile preserve a rare glimpse into the high temperature evolution of silica here we show for the first time that aggregates of euhedral quartz microcrystals preserve cryptocrystalline cores that contain silica hydrates opal and moganite we propose that these phases are metastable remnants of progressive dehydration from a precursor silica hydrate phase evidence for sequential dehydration to from silica hydrate to quartz silica hydrate opal moganite quartz is provided by shrimp 18o microanalytical data that show oscillatory isotopic zoning from 3 6 to 16 2 18o 0 5 coupled with k and al variations we estimate that the precursor silica hydrate deposited between 480 680 c and contained 32 63 wt h2o silica hydrate is metastable with respect to quartz and forms during rapid deposition of silica at high silica supersaturation a consequence of rapid expansion of magmatic fluid into the fracture array that hosts the el indio copper gold deposit modern understanding of ore forming fluids in hydrothermal ore deposits is largely underpinned by the assumption that quartz and its included fluids faithfully record depositional conditions the discovery of silica hydrate affects the paragenetic and geochemical interpretation of quartz and included fluids quartz matured from silica hydrate would record pseudo primary fluid inclusions such that homogenization temperatures record retrograde rather than depositional conditions while 18o data may bias fluid provenance interpretation within sub volcanic systems

the mesoproterozoic abra polymetallic sedimentary rock hosted mineral deposit edmund basin western australia abstract abra is a blind sedimentary rock hosted polymetallic fe pb zn ba cu au ag bi w deposit discovered in 1981 located within the easterly trending jillawarra rift sub basin of the mesoproterozoic edmund basin capricorn orogen western australia the edmund basin contains a 4 10 km thick succession of siltstone sandstone dolomitic siltstone and stromatolitic dolomite the age of the edmund group is between 1 66 and 1 46 ga the abra polymetallic deposit is hosted in siltstone dolostone sandstone and conglomerate of the irregully and kiangi creek formations but the mineralised zones do not extend above an erosion surface marking the change from fluvial to marine facies in the lower part of the kiangi creek formation the abra deposit is characterised by a funnel shaped brecciated zone interpreted as a feeder pipe overlain by stratiform stratabound mineralisation the stratiform stratabound mineralisation includes a red zone and an underlying black zone the red zone is characterised by banded jaspilite hematite galena pyrite quartz barite and siderite the jaspilite and hematite cause the predominant red colouration the black zone consists of veins and rhythmically banded sulphides laminated and or brecciated hematite magnetite fe rich carbonate and scheelite in both zones laminations and bands of sulphide minerals fe oxides barite and quartz commonly exhibit colloform textures the feeder pipe stringer zone merges with black zone and consists of a stockwork of fe carbonate quartz barite pyrite magnetite and chalcopyrite exhibiting fluidised and or jigsaw textures the abra mineral system is characterised by several overprinting phases of hydrothermal activity from several stages of brecciation and fluidisation barite and sulphide veining to barren low temperature chalcedonic epithermal regime veining hydrothermal alteration minerals include multi stage quartz chlorite prehnite fe rich carbonate and albite albite na metasomatism is an early alteration phase whereas fe rich carbonate is a late phase fluid inclusion studies indicate that the ore fluids had temperatures ranging from 162 to 250 c with salinities ranging from 5 8 to about 20 wt nacl in the course of our studies microthermometric and raman microprobe analyses were performed on fluid inclusions in carbonate quartz and barite grains fluid inclusions in quartz show homogenisation temperatures ranging from 150 to 170 c with calculated salinities of between 3 7 and 13 8 wt nacl the sulphur isotopic system shows 34 s values ranging from 19 4 to 26 6 for sulphides and from 37 4 to 41 9 for barite vogt and stumpfl 1987 austen 2007 sulphur isotope thermometry between sulphides and sulphide barite pairs yields values ranging from 219 to 336 c austen 2007 galena samples were analysed for pb isotope ratios which have been compared with previous pb isotopic data the new pb isotope systematics show model ages of 1650 1628 ma consistent with the formation of the host edmund basin re os dating of euhedral pyrite from the black zone yielded an age of 1255 ma this age corresponds to the 1320 1170 ma mutherbukin tectonic event in the gascoyne complex this event is manifested primarily along a wnw trending structural corridor of amphibolite facies rocks about 250 km to the northwest of the abra area it is possible that the re os age represents a younger re activation event of an earlier sedex style system with a possible age range of 1640 1590 ma a genetic model for abra is proposed based on the above data the model involves two end members ore forming stages the first is the formation of the sedex style mineral systems followed by a second multi phase stage during which there was repeated re working of the mineral system guided by seismic activity along major regional faults

multi stage cu remobilization of the huping metamorphic hydrothermal deposit in the southern north china craton abstract the zhongtiao region is a well endowed cu metallogenic province in the southern north china craton and contains a large number of early precambrian cu deposits of different styles most of which have been subjected to the ca 1 85 ga regional metamorphism however whether there was any role for the proterozoic metamorphism on the extensive cu mineralization is yet to be well understood this paper presents an integrated investigation on the field geology mineral chemistry fluid inclusion and c h o s isotopes of the huping metamorphic hydrothermal deposit in the neoarchean paleoproterozoic zhongtiao metamorphic terranes and reveals multi stage cu remobilization that formed and upgraded the cu deposits in the region we identified three stages of syn to post metamorphic cu remobilization transportation in the huping cu deposit including i early mechanical transport ii intermediate metamorphic hydrothermal remobilization and iii late meteoric fluid incursion in stage i sulfides were remobilized and transported internally under lower greenschist facies metamorphism forming the veinlet disseminated ores and chalcopyrite avg 34s 2 1 similar to the chlorite amphibole schist wall rocks in stage ii the immiscibility of metamorphic fluids avg 18o 4 6 and d 56 6 from the metamorphic terranes triggered the precipitation of higher 34s chalcopyrite avg 8 0 led by the incorporation of extra sulfur during the fluid migration in stage iii incursion of meteoric fluids into the waning metamorphic hydrothermal system formed the barren quartz calcite veins avg 18o 0 09 and 13c 0 88 during these cu remobilization processes the earlier formed sulfides were chemically stable during the mechanical transfer by regional metamorphism and deformation subsequently the immiscibility of metamorphic fluids induced significant cu remobilization and precipitation in the metamorphic hydrothermal system we thus propose a model of multi stage mechanical and chemical cu remobilization for the huping cu deposit which highlights the crucial impacts of metamorphic remobilization processes on the regional cu mineralization in the zhongtiao region

fluid pressure dominated orogenic gold mineralization under low differential stress case of the yaoure gold camp cote d ivoire west africa the 1 5 moz yaoure gold deposit which is located within the birimian greenstone belts of central cote d ivoire west africa is associated with two stages of economic gold mineralization both ore forming stages occurred during protracted e w directed shortening associated with the eburnean orogeny the first mineralization stage is hosted by fault fill veins along conjugate ene striking dextral and wnw striking sinistral strike slip fault arrays this ore stage is associated with a polymetallic assemblage including mo bi pb w te ag and au and cryptic alteration in wall rocks the second and main gold mineralization stage is associated with a low displacement fault fracture vein mesh formed during a reverse faulting episode this second gold phase is associated with a polymetallic assemblage including w and au pb and is characterized by pronounced albite ankerite pyrite hydrothermal alteration in wall rocks shear veins associated with this second mineralization episode formed during episodic fluid pressure variations under low differential stress within pre existing low angle reverse fault planes permeability destruction by albite ankerite pyrite hydrothermal sealing of these low angle reverse faults resulted in fluid pressure build up and ultimately in the development of extension veins under low differential stress in the late stages of the structural evolution of the deposit this study presents the structural evolution of a polyphase orogenic gold deposit and highlights that the interplay between fault zone activity hydrothermal alteration and fluid pressure fluctuations at low differential and deviatoric stresses were key in controlling fluid flow and the formation of ore within optimally oriented faults

incursion of meteoric waters into the ductile regime in an active orogen rapid tectonic uplift on the alpine fault new zealand elevates topography regional geothermal gradients and the depth to the brittle ductile transition and drives fluid flow that influences deformation and mineralisation within the orogen oxygen and hydrogen stable isotopes fluid inclusion and fourier transform infrared ft ir analyses of quartz from veins which formed at a wide range of depths temperatures and deformation regimes identify fluid sources and the depth of penetration of meteoric waters most veins formed under brittle conditions and with isotope signatures 18oh2o 9 0 to 8 7 vsmow and d 73 to 45 vsmow indicative of progressively rock equilibrated meteoric waters two generations of quartz veins that post date mylonitic foliation but endured further ductile deformation and hence formation below the brittle to ductile transition zone 6 8 km depth preserve included hydrothermal fluids with d values between 84 and 52 indicating formation from meteoric waters ft ir analyses of these veins show no evidence of structural hydrogen release precluding this as a source of low d values in contrast the oxygen isotopic signal of these fluids has almost completely equilibrated with host rocks 18oh2o 2 3 to 8 7 these data show that meteoric waters dominate the fluid phase in the rocks and there is no stable isotopic requirement for the presence of metamorphic fluids during the precipitation of ductilely deformed quartz veins this requires the penetration during orogenesis of meteoric waters into and possibly below the brittle to ductile transition zone

flash vaporization during earthquakes evidenced by gold deposits fluids flowing through cavities in earth s crust can deposit gold thermo mechanical modelling of a fluid filled cavity that expands suddenly during an earthquake shows that the drop in pressure would cause the fluid to vaporize and deposit the gold almost instantaneously

new constraints on the genesis and geodynamic setting of the wulong gold deposit liaodong peninsula northeast china evidence from geology geochemistry fluid inclusions and c h o s pb isotopes the wulong lode gold deposit is located in the liaoning province northeast part of north china craton gold ore bodies are mainly hosted in the late jurassic granite and structurally controlled by

initiation and development of the pennine basal thrust swiss alps a structural and geochronological study of an exhumed megathrust abstract the pennine basal thrust pbt is an exhumed megathrust developed during continental collision from late eocene to miocene to trace its evolution five samples with indications for up to three microstructurally diachronous white mica generations were investigated by laser in situ and step heating 40 ar 39 ar dating three deformation related crystallization ages can be distinguished 1 d 1 characterized in the pbt hanging wall by an s 1 foliation defined by white mica chloritoid began at or before 38 0 ma 2 d 2 formed a pervasive s 2 cleavage and synchronous white mica rich veins dated at 27 ma 3 d 3 produced an s 3 crenulation cleavage and chlorite white mica veins dated at 23 ma older ages of 96 ma footwall and 115 ma hanging wall are interpreted as minimum ages for the detrital component finally discrete faulting produced fault gouge with an illite k ar age of 19 ma a simplified back restored reconstruction provides a tectonic context for the dated structures in this framework d 1 occurred during middle to late eocene tectonic accretion after late eocene initiation of continental collision d 2 reflects oligocene top to nw shearing with both in and out of sequence thrusting d 3 then developed from 23 to 19 ma during the progressive deactivation of the pbt

a holistic model for the origin of orogenic gold deposits and its implications for exploration the term orogenic gold deposits has been widely accepted but there has been continuing debate on their genesis early syn sedimentary or syn volcanic models and hydrothermal meteoric fluid models are now invalid magmatic hydrothermal models fail because of the lack of consistent spatially associated granitic intrusions and inconsistent temporal relationships the most plausible models involve metamorphic fluids but the source of these fluids is equivocal intra basin sources within deeper segments of the hosting supracrustal successions the underlying continental crust subducted oceanic lithosphere with its overlying sediment wedge and metasomatized lithosphere are all potential sources several features of precambrian orogenic gold deposits are inconsistent with derivation from a continental metamorphic fluid source these include the presence of hypozonal deposits in amphibolite facies domains their anomalous multiple sulfur isotopic compositions and problems of derivation of gold related elements from devolatilization of dominant basalts in the sequences the phanerozoic deposits are largely described as hosted in greenschist facies domains consistent with supracrustal devolatilization models a notable exception is the jiaodong gold deposits of china where ca 120 ma gold deposits are hosted in precambrian crust that was metamorphosed over 2000 million years prior to gold mineralization other deposits in china are comparable to those in the massif central and elsewhere in france in that they are hosted in amphibolite facies domains or clearly post date regional metamorphic events imposed on hosting supracrustal sequences if all orogenic gold deposits have a common genesis the only realistic source of fluid and gold is from devolatilization of a subducted oceanic slab with its overlying gold bearing sulfide rich sedimentary package or the associated metasomatized mantle wedge with co2 released during decarbonation and s and ore related elements released from transformation of pyrite to pyrrhotite at about 500 c although this model satisfies all geological geochronological isotopic and geochemical constraints and is consistent with limited computer based modeling of fluid release from subduction zones the precise mechanisms of fluid flux are model driven and remain uncertain from an exploration viewpoint the model re emphasizes the ubiquitous occurrence of orogenic gold deposits in subduction related orogenic belts and importance of continental scale lithosphere tapping fault and shear zones to focus large volumes of auriferous fluid it confirms the importance of the consistent spacing between world class deposits broadly equivalent to the depth of the moho as derived from empirical observations

direct observation and measurement of au and ag in epithermal mineralizing fluids abstract colloidal gold particles have been observed in c 300 c low salinity fluids from the arapucandere intermediate sulphidation epithermal base metal au deposit in nw turkey this is the first time colloidal gold has been recorded in an ancient mineralizing fluid growth in veins of large euhedral quartz crystals after the deposition of sulphides occurred in a number of stages from the introduction of fresh pulses of fluid the quartz overgrowths have a fibrous texture which facilitated trapping of large elongate fluid inclusions between the quartz fibres which grew perpendicular to the crystal faces of the pre existing quartz episodic periods of intense trapping of fluid inclusions occurred throughout the growth of quartz trapped within primary fluid inclusions are numerous particles of gold the largest observed is c 1 m but most are smaller bse element mapping shows these to contain au ag cu hg la icp ms profiles of the fluid inclusions confirm au and ag is not present in solution being present as numerous particles we have quantified the concentration of gold in fluid inclusions which are orders of magnitude greater than has been previously measured or thought likely in crustal fluids the average ag concentration is c 32 ppm and au is c 41 ppm but the maximum concentrations of both may reach several 100 s to 1000 ppm calcite forms a coating on the inner surface of the inclusions and barite pyrite galena sphalerite and unidentified minerals are also present with the au ag particles it is clear that the au ag particles could not have precipitated in the fluid inclusions therefore they must have precipitated elsewhere and been carried with the hydrothermal fluid the high concentrations and their colloidal nature have implications for the enrichment of gold in mineral deposits

geology hydrothermal fluids h o s pb isotopes and rb sr geochronology of the daxintun orogenic gold deposit in heilongjiang province ne china abstract the newly discovered daxintun sb au deposit heilongjiang province ne china is located at the north eastern margin of the central asian orogenic belt and it is hosted by neoproterozoic to early cambrian two mica schist and marble the antimony bearing minerals include stibnite famatinite tetrahedrite boulangerite freibergite and minor andorite gold occurs largely as native gold microparticles 0 1 20 m with high fineness 974 983 accompanied by quartz stibnite calcite and pyrite fluid inclusions in ore bearing quartz and calcite phases mainly comprise liquid vapor and minor co2 bearing three phase types and the gas phases contain co2 ch4 and h2s the daxintun fluid inclusions homogenize at 111 8 353 4 c and are of low salinity 0 53 12 51 wt nacl equiv the 18osmow of the daxintun quartz falls into a wide range 4 2 16 9 yet the dsmow falls into a narrow range 108 0 to 87 3 suggesting that the ore forming fluids formed from metamorphic fluids that mixed with meteoric water the stibnite 34s ranges between 0 9 and 5 1 mean 2 8 the stibnite has 206pb 204pb of 18 234 18 306 207pb 204pb of 15 537 15 561 and 208pb 204pb of 38 103 38 227 implying a metamorphic fluid source the conditions of gold and antimony precipitation are comprised of neutral ph and fluid mixing with decreasing fs2 and fo2 zircon u pb dating of the fine grained daxintun granite yields a late jurassic age 154 0 2 0 ma and the daxintun diorite dike yields a weighted mean age of 122 1 1 3 ma stibnite rb sr dating suggests that the sb au mineralization has an age of approximately 152 8 2 4 ma the observed ore deposit geology as well as new data on the ore forming fluids h o s pb isotopes and rb sr geochronology suggest that the daxintun sb au deposit is best described as an orogenic type gold deposit that formed during subduction of the palaeo pacific and the mongolia okhotsk oceans during the late jurassic

scaling the 3 d mohr circle and quantification of paleostress during fluid pressure fluctuation application to understand gold mineralization in quartz veins of gadag southern india abstract in this study orientations of 157 quartz veins occurring in metabasalts of the gadag region dharwar craton southern india are used to plot the 3 d mohr stress circle which provides information about relative stress fluid pressure pf conditions as well as stress state during pf fluctuation to scale the 3 d mohr circle vein orientation data are integrated with a available estimates from fluid inclusions of highest recorded pf 390 mpa and lowest recorded pf 50 mpa and b intrinsic rupture criterion that empirically quantify rock properties based on the scaled 3 d mohr circle the absolute magnitudes of the three principal stresses are quantified for high and low pf of 157 veins investigated here 14 veins are identified as having favourable orientation for dilation at high as well as low pf these 14 veins have a mean strike of 150 which is similar to the orientation of the gold bearing quartz lodes reported in the region the effective normal stress n n pf prevalent during dilation of fracture fabric anisotropy with 150 strike is calculated to be 11 5 mpa at high pf and 1 0 mpa at low pf thus it is interpreted that in the gadag region a change in n of 10 5 mpa prevailed during pf fluctuation and associated separation of gold from the fluid

evolution of gold bearing splays of crustal scale fault zones of the south central archean abitibi subprovince constraints from the kirana deformation zone kirkland lake ontario canada abstract we explored the effects of brittle ductile deformation processes on the localization of strain and hydrothermal fluid flow throughout the formation and evolution of a fault zone this was accomplished through detailed structural analysis of the kirana deformation zone kdz a splay of the archean crustal scale orogenic gold hosting larder lake cadillac deformation zone llcdz in ontario canada investigations included mapping quantification of fault related breccia by particle size distribution analysis microstructural and petrographic characterization and kinematic analysis of brittle ductile structures the kdz records three principal evolutionary stages stage 1 involved seismogenic processes that resulted in the formation of a 50 m wide fault core and damage zone stage 2 is characterized by transpressional ductile shear localized along texturally favorable mechanically weakened zones that formed during stage 1 and or along zones where coeval fluid flow facilitated reaction weakening native gold is observed within the mylonitized fault core that is interpreted to have formed syn to late ductile shear all structures are crosscut by small scale faults and shear veins with disseminated alteration halos defining stage 3 the kdz demonstrates how early fault processes may control the spatial pattern and temporal progression of strain and hydrothermal fluid localization in orogenic gold bearing fault splays

colloidal gold and silica in mesothermal vein systems some of the textural features of mesothermal gold quartz veins may be best explained by the initial precipitation of amorphous silica gel colloid which subsequently crystallizes to quartz this can occur in brittle ductile shear zones where a significant fluid pressure drop occurs during stick slip failure such a process rapidly supersaturates the hydrothermal fluid with respect to amorphous silica which precipitates instead of quartz owing to favorable kinetics depressurization also commonly leads to fluid unmixing and destabilization of soluble gold complexes however the presence of colloidal silica can stabilize gold colloid allowing further transport of particulate gold in suspension in the hydrothermal fluid silica gel would be highly unstable under mesothermal conditions and would undergo rapid syneresis and crystallization to form quartz solid impurities would tend to be expelled toward grain boundaries this model can account for the primary anhedral aggregate textures typical of mesothermal quartz veins the concentration of gold along grain boundaries and the formation of discrete gold nuggets and the rare occurrence of low order silica polymorphs and relict spheroidal structures the transport of gold in colloidal form may be one reason for the frequently consistent bulk grade distribution in gold quartz vein systems over many hundreds of metres in some cases kilometres of depth in addition the formation of charged colloidal particles may help to explain the attraction of gold grains to specific mineral surfaces

dilatancy induced fluid pressure drop during dynamic rupture direct experimental evidence and consequences for earthquake dynamics abstract fluid pressure and flow in the crust are key parameters controlling earthquake physics since earthquake slip is linked to spatio temporal localisation of deformation it is expected that the local fluid pressure around the fault plane could potentially impact the dynamic strength of the slipping fault zone coseismic fluid pressure drops have been inferred from field studies notably in gold deposits which are thought to be formed by this process but reliable quantitative predictions are still lacking here experimental results are presented where local on and off fault fluid pressure variations were measured in situ during dynamic rock fracture and frictional slip under upper crustal stress conditions during the main rupture the on fault fluid pressure dropped rapidly to zero indicating partial vaporisation and or degassing further deformation produced stick slip events systematically associated with near instantaneous drops in fluid pressure providing direct experimental support of the concept of seismic suction pump in situ fluid volume and wave speed measurements together with microstructural investigations show that dilatancy is the process driving fluid pressure drops during rupture and slip extrapolation of the laboratory results indicate that dilatancy induced fluid pressure drops might be a widespread phenomenon in the crust counteracting thermal pressurisation as a weakening mechanisms in freshly fractured rock

fluid p t x characteristics and evidence for boiling in the formation of the phoenix uranium deposit athabasca basin canada implications for unconformity related uranium mineralization mechanisms abstract the phoenix uranium deposit in the southeastern athabasca basin canada is a typical unconformity contact hosted uranium deposit characterized by an association with reactivated basement faults graphite rich rocks in the underlying basement and a pervasive clay alteration halo surrounding the mineralized zones petrographic results suggest that the mineralizing hydrothermal system was characterized by alternating desilicification and silicification events uraninite and clay size minerals tourmaline kaolinite illite and minor chlorite mainly precipitated in the desilicification periods whereas hydrothermal quartz mainly drusy quartz formed during the silicification periods primary fluid inclusions in the hydrothermal quartz are inferred to represent the ore forming fluids even though quartz did not co precipitate with uraninite the coexistence of multiple types of fluid inclusions liquid dominated biphase vapor dominated biphase vapor only and halite bearing triphase within individual fluid inclusion assemblages is interpreted to indicate fluid boiling and heterogeneous trapping bulk fluid inclusion volatile analysis by mass spectrometry indicates h2o as the dominant species with less than 1 mol non aqueous volatiles that show a compositional trend typical of fluid boiling microthermometric and cryogenic raman spectroscopic analyses indicate that the mineralizing fluids are of h2o nacl cacl2 mgcl2 composition with salinities ranging mainly from 23 4 to 31 1 wt the liquid dominated biphase inclusions excluding those interpreted to have resulted from heterogeneous trapping have homogenization temperatures from 90 to 157 c these data are generally consistent with the classical diagenetic hydrothermal model in which basinal brines that extracted uranium from the basin or the basement were channeled along reactivated basement faults and precipitated uraninite near the unconformity through reaction with reducing agents however the relatively low fluid inclusion homogenization temperatures documented in this paper together with fluid boiling inferred from fluid inclusion assemblages suggest that the deposit may have formed in a shallower environment 2 5 km than assumed in the conventional diagenetic hydrothermal model 5 km abrupt fluid pressure drops during episodic faulting may have resulted in fluid boiling flash vaporization changing the fluid ph and promoting uraninite precipitation with the alternating liquid vapor conjugates of the boiling system resulting in alternating precipitation of quartz and uraninite respectively

origin and evolution of ore fluid and gold deposition processes at the giant taishang gold deposit jiaodong peninsula eastern china abstract the early cretaceous taishang gold deposit is the largest deposit in the linglong goldfield jiaodong peninsula with a pre mining endowment of 326 t of gold it is hosted in the 165 150 ma linglong biotite granite and controlled by the nee to ne trending potouqing fault near the northern end of the regional zhaoping fault system hydrothermal alteration is well developed in the footwall of the potouqing fault and is characterized by a narrow zone of sericitization followed by a wider zone of silicification and an even wider zone of potassic alteration the main orebodies no i orebody comprise disseminated and stockwork veinlet systems although about 5 of the resources are in larger quartz veins no ii orebody four stages have been identified for both styles of mineralization on the basis of cross cutting relationships and mineralogical and textural characteristics pyrite quartz sericite stage 1 quartz pyrite stage 2 quartz pyrite base metal sulfide stage 3 and quartz carbonate stage 4 gold was mainly deposited in stages 2 and 3 with minor amounts in stage 1 petrographical microthermometric and laser raman spectroscopic studies on fluid inclusion assemblages in quartz and calcite from the four stages reveal three types of primary fluid inclusions type 1 h 2 o rich aqueous carbonic type 2 co 2 rich aqueous carbonic and rare type 3 carbonic inclusions stage 1 quartz primary inclusions are only type 1 inclusions with an estimated composition of 88 h 2 o 10 co 2 4 5 wt nacl equiv and 0 5 ch 4 with trapping at 336 c and 1 7 kbars the gold rich stages 2 and 3 from both orebodies typically contain primary fluid inclusion assemblages with both type 1 and 2 inclusions which show similar phase transition temperatures and were trapped between 246 and 294 c the stage 4 quartz and calcite contain only primary type 1 inclusions which are estimated to have a composition of 93 h 2 o 6 0 co 2 3 6 wt nacl equiv and trace amount of ch 4 and were trapped at temperatures of 236 c the 34 s values of hydrothermal pyrite from the four stages have a narrow range from 4 5 to 8 0 and are within the ranges for whole rock sulfides from the archean jiaodong group and magmatic pyrite from mesozoic granitoid and intermediate basic dikes the 18 o values of hydrothermal quartz range mainly from 10 9 to 12 5 and remain constant for all four stages calculated fluid 18 o values are 1 3 10 0 the d water values calculated from hydrothermal sericite range from 60 to 45 considering the fluid inclusion compositions 18 o and d compositions of ore forming fluids and regional geological events the most likely ultimate potential fluid and metal reservoirs would be the paleo pacific oceanic slab and its overlying sediments which were thrust below the high grade metamorphic rocks of the jiaodong peninsula the initial ore forming fluids were medium high temperature co 2 rich and low salinity h 2 o co 2 nacl ch 4 homogeneous fluids over the duration of the hydrothermal system the fluid remained fairly consistent in p t x although becoming slightly more water rich during final post ore activity fluid immiscibility occurred during stages 2 and 3 ore deposition at pressures that fluctuated strongly from 1700 to 580 bars during hydrofracturing the hs 2 ion was the most probable gold transporting complex at taishang wall rock sulfidation and episodic pressure drops with associated fluid unmixing and other chemical changes were the two main mechanisms of ore deposition

paleomagnetic evidence for rotation of the iberian peninsula and the external betic cordillera southern spain abstract paleomagnetic investigations of jurassic cretaceous and tertiary sites in the external betic cordillera indicate that while the observed remanence vectors have inclinations which are conformable with the expected inclinations for stable iberia declinations deviate significantly these results indicate that 1 the mesozoic limestones of the penibetic have been rotated clockwise with respect to stable iberia and 2 there is a differential rotation between the cretaceous sites dec 61 11 and the jurassic sites dec 38 8 the differential rotation of 23 between the late jurassic and late cretaceous is consistent with the cretaceous anticlockwise rotation of the iberian peninsula with respect to eurasia it follows that during this time period this part of the betic mobile belt waspart of the continental margin of the iberian peninsula a study which was confined to a single unbroken stratigraphic section west of ronda indicates that most of the anticlockwise rotation 20 probably occurred between the cenomanian and the campanian the anticlockwise rotation of these rocks must have been followed by a clockwise rotation of at least 60 to account for the observed clockwise declination anomalies

orogenic gold formation in an evolving decompressing hydrothermal system genesis of the samut gold deposit eastern desert egypt abstract samut is a vein type gold mineralization in the eastern desert of egypt which shares many geologic features with other orogenic deposits in this region the deposit has a long history of mining activity dating back to ancient times but so far has received little attention from the standpoint of geologic study here we present new field petrographic microthermometric and microanalytical data from the samut gold deposit to evaluate its genesis and fluid evolution vein quartz at samut shows abundant evidence of precipitation coinciding with early phases of ductile deformation that show transitions to increasing brittle fracturing and fragmentation through time in some samples brecciation of the vein quartz is intense and shows evidence of repeated episodes of fragmentation this brittle style of deformation culminates with latest stage brittle opening and loss of confining pressure resulting in open space filling textures backscattered electron images and electron microprobe analysis reveal three varieties of pyrite in the mineralized quartz veins py i py ii and py iii laser ablation icpms analysis of pyrites from mineralized quartz veins reveals that the earlier py i is low in arsenic and gold whereas py ii and py iii are enriched in both elements anhedral py ii is particularly gold rich and gives way to blocky euhedral py iii that is rich in arsenic but not as rich in gold compared to py ii fluid inclusions in the least brittle deformed vein quartz indicate that the veins formed at minimum temperatures of 300 c early in the hydrothermal system the calculated fluid pressures at homogenization of the aqueous carbonic inclusions furnish a wide range throughout the paragenesis perhaps on the order of 1 to 3 kbar the 34s values of pyrites imply sulfur isotopic composition of the mineralizing fluid on the order of 0 6 to 4 9 overlapping with the broad range of 34s values calculated for ore fluids in orogenic gold deposits worldwide together these results support a general model in which gold and sulfide deposition was coinciding with an ongoing syn deformational vein formation and tending towards the late stages of vein growth prior to loss of confinement free milling gold appears to be mostly related to the later stages of brittle fracturing and re sealing of the veins during repeated fluid release as the background fluid pressure evolved from earlier dominantly lithostatic to later fully hydrostatic conditions

fluid inclusion geochemistry and ar ar geochronology of the cenozoic bangbu orogenic gold deposit southern tibet china abstract located along the southern part of the yarlung zangbo suture zone in southern tibet bangbu is one of the largest gold deposits in tibet auriferous sulfide bearing quartz veins are controlled by second or third order brittle fractures associated with the regional qusong cuogu zhemulang brittle ductile shear zone fluid inclusion studies show that the auriferous quartz contains aqueous inclusions two phase and three phase co2 bearing inclusions and pure gaseous hydrocarbon inclusions the co2 bearing inclusions have salinities of 2 2 9 5 nacleq and homogenization temperatures th of 167 336 c the d 18o and 13c compositions of the bangbu ore forming fluids are 105 5 to 44 4 4 7 to 9 0 and 5 1 to 2 2 respectively indicating that the ore forming fluid is mainly of metamorphic origin with also a mantle derived contribution the 3he 4he ratio of the ore forming fluids is 0 174 to 1 010 ra and 40ar 36ar ranges from 311 9 to 1724 9 calculations indicate that the percentage of mantle derived he in fluid inclusions from bangbu is 2 7 16 7 these geochemical features are similar to those of most orogenic gold deposits dating by 40ar 39ar of hydrothermal sericite collected from auriferous quartz veins at bangbu yielded a plateau age of 44 8 1 0 ma with normal and inverse isochronal ages of 43 6 3 2 ma and 44 3 ma respectively this indicates that the gold mineralization was contemporaneous with the main collisional stage between india and eurasia along the yarlung zangbo suture which resulted in the development of near vertical lithospheric shear zones a deep metamorphic fluid was channeled upward along the shear zone mixing with a mantle fluid the mixed fluids migrated into the brittle structures along the shear zone and precipitated gold sulfides and quartz because of declining temperature and pressure or fluid immiscibility the bangbu is a large scale cenozoic syn collisional orogenic gold deposit

isotope geochemistry and genesis of the liyuan gold deposit shanxi north china abstract the liyuan lode gold deposit is located in the shanxi province central north china craton gold orebodies are mainly hosted in archean metamorphic rocks and structurally controlled by the nne trending faults gold occurs in disseminated and auriferous quartz sulfide veins and veinlets within hydrothermally altered rocks mineralization can be divided into three intervals 1 quartz pyrite stage 2 quartz polymetallic sulfides stage and 3 quartz carbonate stage gold formed mainly in the middle stage sericite sample associated with the middle stage pyrite from phyllic alteration zones yields an well defined 40ar 39ar plateau age of 133 3 1 2 ma which is remarkably consistent with zircon u pb age 133 4 1 1 ma of the quartz porphyry dikes in the liyuan mine indicating a close relationship between gold mineralization and granitic magmatism in the area the sulfur isotopic compositions of pyrite py1 2 from the early stage have a narrow range from 0 3 to 4 1 indicating a deep seated magmatic source however the sulfides py3 5 sphalerite galena from the middle stage have lower 34sv cdt values of 7 2 to 3 0 which is thought to be a result of fluid oxidation during gold mineralization the pb isotopic compositions of sulfides from liyuan ores have 206pb 204pb 207pb 204pb and 208pb 204pb ratios of 16 671 16 860 15 236 15 255 and 41 452 44 159 respectively indicting a high thorium lower continental crust source of ore forming materials the 13cvpdb and 18ovsmow values of late stage calcite ranging from 7 1 to 5 8 and 12 2 to 13 7 respectively illustrating a deep seated magmatic source of carbon bearing portion given the absence of granite bodies at the present mining levels we speculate that the ore forming fluids may have been exsolved from a concealed granite pluton at greater depth based on the regional tectono magmatic events ore geology geochronologic and isotopic data obtained in this study together with the previous published fluid inclusion data we suggest that the liyuan deposit is an orogenic gold deposit that is probably related to the early cretaceous granite magmatism in the interior of the north china craton

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

tectonics magmatism and paleo fluid distribution in a strike slip setting insights from the northern termination of the liquine ofqui fault system chile this study addresses the interplay between strain stress fields and paleo fluid migration in the southern andean volcanic zone svz the svz coexists with the margin parallel liquine ofqui fault system lofs and with nw striking andean transverse faults atf to tackle the role of different fault fracture systems on deformation distribution and magma fluid transport we map the nature geometry and kinematics of faults veins and dikes at various scales fault slip data analysis yields stress and strain fields from the full study area data base regional scale and fault zones representative of each fault system local scale regional scale strain analysis shows kinematically heterogeneous faulting local strain analyses indicate homogeneous deformation with ne trending shortening and nw trending extension at nne striking liquine ofqui master fault zones strain axes are clockwise rotated at second order fault zones with ene trending shortening and nnw trending stretching the atf record polyphasic deformation conversely stress field analysis at regional scale indicates a strike slip dominated transpressional regime with n64 e trending 1 and n30 w trending 3 deformation is further partitioned within the arc through nne striking dextral reverse faults ne striking dextral normal faults and nw striking sinistral reverse faults with normal slip activation the regional tectonic regime controls the geometry of ne striking dikes and volcanic centers ne striking faults record local stress axes that are clockwise rotated with respect to the regional stress field nne and ne striking faults are favorably oriented for reactivation under the regional stress field and show poorly developed damage zones conversely nw striking fault systems misoriented under the regional stress field show multiple fault cores wider damage zones and dense vein networks deformation driven by oblique subduction is partially partitioned into strike slip and shortening components the trench parallel component is mostly accommodated by ns striking right lateral faults of the lofs trench perpendicular shortening is accommodated by sinistral reverse atf and dextral reverse nne striking faults we conclude that the svz records a deformation history coeval with magma fluid migration

pressure fluctuations phase separation and gold precipitation during seismic fracture propagation gold mineralization at croagh patrick western ireland occurs in quartz veins associated with three synchronous oblique tensile shear systems the veins evolved by progressive thickening of isolated en echelon lensoid tensile fractures that show a power law relation between their length and thickness a change in power law exponent marks the linking of the en echelon arrays by large oblique tensile veins fluid inclusion data show that gold was deposited due to unmixing of an h 2 o co 2 nacl fluid at 320 240 c and 160 30 mpa primarily due to decreases in fluid pressure evidence for phase separation is only observed in the large linking veins and is abundant close to intersections of the vein arrays these sites are where high gold grades occur the distribution of gold is believed to reflect both spatial and temporal variations in fluid pressure fluctuation during seismic fracture propagation controlled by the vein growth mechanism and zones of maximum dilation

constraints on depositional conditions and ore fluid source for orogenic gold districts in the west qinling orogen china implications from sulfide assemblages and their trace element geochemistry abstract yangshan mian lue ning and manaoke represent three large orogenic gold districts in the triassic west qinling orogen the third largest gold province in china 1100 t gold resource although their component gold deposits have similar geological characteristics representative deposits from these three gold districts have contrasting ore mineral assemblages and gold speciation suggestive of diversity in gold source and or ore forming depositional conditions or processes decrease in p t conditions of ore formation and metamorphic grades of host rocks in deposits from the mian lue ning through yangshan to manaoke districts is matched by variation in their mineral paragenesis from pyrite chalcopyrite sphalerite galena native gold through pyrite arsenopyrite stibnite to scheelite pyrite arsenopyrite stibnite realgar invisible gold in the deposits of the yangshan and manaoke gold districts contrasts with native gold in the mian lue ning gold district concentrations of ag as au bi cu pb sb tl in hydrothermal pyrite show a gradual systematic increase from mian lue ning through yangshan to manaoke pyrite in the huachanggou gold deposit from the mian lue ning district has high co and low as and au concentrations in contrast with pyrite which is zoned with respect to as and au in gold deposits from the other two districts these variations between gold deposits in the different districts could be due to variable local ore fluid sources derived from the different host rock sequences during metamorphic devolatilization as also allowed by mass balance calculations however previously unmetamorphosed voluminous source rocks that have experienced amphibolite facies metamorphism with liberation of gold and other metals during conversion of gold bearing pyrite into pyrrhotite are limited it appears more likely that an external deeply sourced fluid most logically from devolatilization of the pyrite bearing sediment wedge above the triassic subduction zone associated with the formation of the west qinling orogen was responsible for gold mineralization in this model variation between districts was due to fluid interactions with contrasting host rock sequences under contrasting p t conditions

high temperature gas solid reactions in calc silicate cu au skarn formation ertsberg papua province indonesia the 2 7 3 ma ertsberg east skarn system indonesia adjacent to the giant grasberg porphyry copper deposit is part of the world s largest system of cu au skarn deposits published fluid inclusion and stable isotope data show that it formed through the flux of magma derived fluid through contact metamorphosed carbonate rock sequences at temperatures well above 600 c and pressures of less than 50 mpa under these conditions the fluid has very low density and the properties of a gas combining a range of micro analytical techniques high resolution qemscan mineral mapping and computer assisted x ray micro tomography an array of coupled gas solid reactions may be identified that controlled reactive mass transfer through the 1 km3 hydrothermal skarn system vacancy driven mineral chemisorption reactions are identified as a new type of reactive transport process for high temperature skarn alteration these gas solid reactions are maintained by the interaction of unsatisfied bonds on mineral surfaces and dipolar gas phase reactants such as so2 and hcl that are continuously supplied through open fractures and intergranular diffusion principal reactions are a incongruent dissolution of almandine grossular to andradite and anorthite an alteration mineral not previously recognized at ertsberg and b sulfation of anorthite to anhydrite these sulfation reactions also generate reduced sulfur with consequent co deposition of metal sulfides diopside undergoes similar reactions with deposition of fe enriched pyroxene in crypto veins and vein selvedges the loss of calcium from contact metamorphic garnet to form vein anhydrite necessarily results in fe enrichment of wallrock and does not require fe addition from a vein fluid as is commonly assumed

geology and structural evolution of the muruntau gold deposit kyzylkum desert uzbekistan abstract the muruntau gold deposit in the kyzylkum desert of uzbekistan is the largest single deposit 1100 tonnes of gold of the class of low sulfide syndeformation synigenous gold deposits formed in the brittle ductile transition zone of the crust within transpressional shear zones hosted by the cambrian to ordovician besopan suite the ores were deposited in pre existing thrust fault and metamorphism related permeabilities and in synmineralization dilational zones created in a large fault related fold the besopan suite is a 5 000 m thick sequence of turbiditic siltstones shales and sandstones the ore is primarily localized at the base of the besopan 3 unit which is a 2 000 m thick series of carbonaceous shales siltstones sandstones and cherts initial gold deposition took place within the sangruntau tamdytau shear zone which was developed along the stratigraphic contact between the besopan 3 and besopan 4 units during the mineralization process folding of the besopan suite and a left step adjustment in the sangruntau tamdytau shear zone were caused by two concurrent events 1 the activation of the left lateral muruntau daugyztau shear zone that developed at nearly a 90 angle to the preceding shear zone and 2 the intrusion of granitoid plutons these structural events also resulted in the refocusing of hydrothermal fluid flow into new zones of permeability

physico chemical controls on ore deposition in the arapucandere pb zn cu precious metal deposit biga peninsula nw turkey abstract arapucandere is one of a number of similar cu zn pb au ag epithermal deposits in the biga peninsula which are mineralogically and tectonically similar fluid inclusions have very low salinities between 1 7 and 0 wt nacl and a wide range of temperatures from 360 to 160 c there was extensive boiling and flashing of the hydrothermal fluids which initiated mineral deposition the range of temperatures is consistent with emplacement of the veins at 700 m depth with the pressure decreasing from near lithostatic to near hydrostatic and a decrease in temperature to 250 c due to adiabatic expansion of the fluids there is evidence of a limited amount of boiling but the temperature and pressure decrease was close to the liquid vapour curve flashing of the fluids was caused by sudden drops to sub hydrostatic pressures and even lower temperatures mineralization was caused by these pressure related temperature decreases as there is no evidence of cooling and dilution of the ore fluids 34 s values of sulphides indicate a magmatic source but the more negative than usual values also suggest boiling affected isotopes d and 18 o of the fluids indicate a mixing between meteoric waters and magmatic fluids with the large range of d due to boiling la icp ms analyses of fluid inclusions reveal high cu zn pb concentrations in the fluids despite their low salinity transported as chloride complexes exceptional pressure and temperature decrease causing the fluids to flash was likely to have been in response to earthquakes

silica gel in a fault slip surface field evidence for palaeo earthquakes high velocity friction experiments have shown an almost complete loss of strength associated with silica gel formation on slip surfaces the identi cation of frictional silica gel products in palaeo seismic faults is however problematic because there are multiple natural sources of silica gel and recrystallization of gel to quartz complicates preservation the importance of gel formation on natural faults is therefore unknown here we report a structurally distinct and semi continuous 0 5 10 mm thick layer of microcrystalline quartz along a major carbonate hosted fault the olive fault in the naukluft nappe complex namibia the quartz layer is distinguished by ow banding like textures and unusual cathodoluminescence characteristics the layer consists of 2 20 m hexagonal quartz crystals which include distinct crystalline pore bearing micro to nano spheres separated by pore geometries indicative of volumetric contraction and with grain boundaries enriched in aluminium we interpret these features to indicate that the quartz crystals formed from recrystallization and dehydration of a silica gel because it is found in a carbonate hosted fault and crosscuts lithological layering the silica source is not from comminution of local wall rocks rather the gel likely formed from reshear of a quartz coated fault surface or incremental shear slip associated with precipitation of silica driven by co seismic pressure drops this example of fault related silica gel may have formed by a different mechanism than the gels produced in high velocity friction experiments but once formed may have comparable rheological effects

p t x constraints on the koru epithermal base metal au deposit biga peninsula nw turkey abstract the koru deposit is a typical intermediate sulfidation base metal au example of volcanic volcaniclastic hosted mineralization in the biga peninsula and northwestern turkey ore deposition was associated with the collisional and post collisional tectonics related to the closure of the tethys ocean galena baryte and quartz are main minerals accompanied by minor amounts of sphalerite pyrite chalcopyrite covellite and marcasite the homogenization temperature of fluid inclusions indicates two distinct fluid pulses one at a temperature commensurate with epithermal mineralization and boiling near boiling conditions at c 350 c with the second approximately 150 200 c lower salinity in both instances was from 11 0 to 0 2 wt nacl the d and 18o values of water in equilibrium with early quartz and fluid inclusions plot close to the magmatic water box indicating the source of the high temperature fluid was magmatic d and 18o values from early and late baryte trend towards the meteoric water line mwl but this is not due to mixing with meteoric water rather equilibration with alteration assemblages at decreasing temperature la icp ms analyses of fluid inclusions reveal high cu zn pb concentrations in the fluids despite their low salinity transported as chloride complexes the range of temperatures within the early quartz and sphalerite mineralization can be explained by pressure variations during vein and fracture opening

linking high grade gold mineralization to earthquake induced fault valve processes in the porgera gold deposit papua new guinea processes responsible for high grade gold enrichment are of significant economic interest and have long been elusive structural and geophysical models postulate that earthquake induced fault valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems although a comprehensive geochemical record of these processes has been lacking the porgera gold deposit papua new guinea provides an opportunity to examine the complex processes surrounding high grade au mineralization in structurally controlled hydrothermal systems new high resolution trace element and sulfur isotope data obtained in situ by laser ablation inductively coupled plasma mass spectrometry and sensitive high resolution ion microprobe stable isotope analysis of pyrite across a well constrained paragenetic sequence reveal a temporal evolution in fluid chemistry the data uncover a stratigraphy of repeated high au negative 34 s and low au positive 34 s zones within individual pyrite crystals present in the highest grade gold event stage ii results provide the first clear geochemical documentation of rapid complete switching in ore forming processes preserved in individual pyrite crystals throughout an evolving hydrothermal system it is postulated that geochemical variations are attributable to rapid au deposition during pressure release due to fault failure followed by switching to more reduced rock buffered metal poor fluids from adjacent sediments as the rupture seals the cyclicity of this process is reflected in the pyrite zonation seen at the porgera deposit this study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways revealing that deposition of exceptionally high grade ore at the porgera deposit was the result of multiple events in a fluctuating ore forming environment

underground fumaroles excess heat effects in vein formation we examine the heat extracted from a fractured hydrothermal system following the sudden dilation of fractures in the host rock this dilation increases the permeability of the system and veins form along irregular fractures where the opening is localized for simplicity we will consider one such vein in this paper fluid in the system is assumed to be initially close to vapor saturation and in thermal equilibrium with the surrounding host rock the rapid opening of the vein leads to approximately isenthalpic vapor separation and causes a pressure drop to propagate through the system as the two phase fluid in the fracture system is constrained to the vapor pressure curve the depressurization is accompanied by a rapid cooling of the fluid fluid flow is also induced from a hydrothermal reservoir toward the dilated discharge vein the temperature difference between the host rock and the two phase fluid in the fracture system drives heat from the rock into the fluid the excess heat 1 1 derived from the host rock in this manner evaporates additional liquid in the two phase flow which develops substantially larger vapor fractions than those due only to the rapid initial adiabatic depressurization of the fluid such additional vapor separation therefore leads to high solute concentrations in the liquid phase and potentially results in faster deposition of minerals in the fracture system and local veins a model is developed here to describe the minimum excess heat transfer i e the worst case scenario into the two phase saturated liquid vapor mixture as it is drawn horizontally from the hydrothermal reservoir toward the discharge vein excess heat effects are shown to be effective within tens to hundreds of meters of actively forming veins and a principal cause of oscillatory banding and lamination fabrics this transient heat extraction process applies to a wide range of shallow crustal deposit styles associated with brittle and brittle ductile deformation these deposit styles range from epithermal bonanza veins sea floor massive sulfide deposits mesothermal veins in shear zones to the higher temperature veins and stockworks of hypothermal and porphyry mineralization 1 fn 1

contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations the echassieres district massif central france abstract the echassieres district of central france hosts diverse magmatic and magmatic hydrothermal deposits of rare metals mostly related to the well known beauvoir granite tungsten mineralization crops out at three distinct wolframite occurrences the two most important of which are related to two distinct magmatic bodies emplaced ca 335 and 310 ma monnier et al 2019 the mineralization occurred at 335 ma formed during a hydrothermal episode marked by precipitation of topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist fluid inclusions in topaz and quartz display similar features i e all have low salinity contain significant amount of licl display constant liquid vapor ratios and homogenized within a narrow temperature range th 380 c no evidence for fluid pressure variations was observed and temperature decrease is considered to be the main cause for wolframite deposition the younger w mineralization is related to greisenizing fluids that altered the beauvoir granite and generated several quartz topaz and apatite veins all greisen related fluid inclusions display low salinity however th are spread from ca 190 to 400 c and several populations exhibit heterogeneous liquid vapor ratios while others consist of only vapor rich fluid inclusions respectively these populations are interpreted to have been trapped during boiling or flashing vaporization of the fluid in contrast with the other regional veins flashing was particularly intense in the mazet veins which host the bulk of the last wolframite generation consequently it is proposed that flashing is the key factor that triggered w precipitation this work highlights the role of two physical parameters pressure and temperature whose variations played a preponderant role on wolframite mineralization it documents in depth an example of greisen fluid evolution providing critical information on w behavior in orthomagmatic fluids and on greisen related rare metal deposits

geochronological and sulfur isotopic evidence for the genesis of the post magmatic deeply sourced and anomalously gold rich daliuhang orogenic deposit jiaodong china the daliuhang gold deposit 20 t gold is located in the central penglai qixia belt of the giant jiaodong gold province eastern china the ore hosting guojialing granodiorite and pegmatite were formed at 129 0 0 6 ma and 126 2 0 6 ma respectively syn ore monazite with a u pb age of 120 5 1 7 ma represents the timing of gold mineralization given at least 5 m y between magmatism and mineralization a genetic relationship to magmatic hydrothermal activity is negated noble gas isotopes of pyrite have crust mantle mixed 3he 4he 1 13 to 1 50 ra and air like 40ar 36ar 327 574 together with the broadly positive correlation between 3he and 36ar it is inferred that the initial ore forming fluids were deeply sourced from the sedimentary wedge overlying the subducted plate and overlying mantle during early cretaceous paleo pacific plate subduction the pre ore and post ore pyrites have low 34s values 3 7 5 6 and 5 3 6 4 respectively whereas ore related pyrites have higher 34s values especially in as au rich domains 7 8 8 3 these positive 34s values also suggest that the initial ore fluid and some of the sulfur component were derived via subduction related devolatilization the elevated 34s values of the ore related pyrite are partly ascribed to mass fractionation and partly to a contribution from sulfur leached from crustal host rocks this interpretation is also supported by neodymium isotope ratios of monazite end 120 ma 13 7 to 11 6 which correlate well with the ore hosting guojialing granodiorite this study highlights the combined roles of deeply derived fluids and intense interaction with upper crustal rocks in the formation of jiaodong gold deposits

natural and anthropogenic radionuclide distributions in the nansen basin artic ocean scavenging rates and circulation timescales abstract determination of the naturally occurring radionuclides 232 th 230 th 228 th and 210 pb and the anthropogenic radionuclides 241 am 239 240 pu 134 cs and 137 cs in water samples collected across the nansen basin from the barents sea slope to the gakkel ridge provides tracers with which to characterize both scavenging rates and circulation timescales in this portion of the arctic ocean large volume water samples 15001 were filtered in situ to separate particulate 0 5 m and dissolved th isotopes and 241 am thorium 230 displays increases in both particulate and dissolved activities with depth with dissolved 230 th greater and particulate 230 th lower in the deep central nansen basin than at the barents sea slope dissolved 228 th activities also are greater relative to 228 ra in the central basin residence times for th relative to removal from solution onto particles are 1 year in surface water 10 years in deep water adjacent to the barents sea slope and 20 years in the eurasian basin deep water lead 210 in the central basin deep water also has a residence time of 20 years with respect to its removal from the water column this texture of scavenging is reflected in distributions of the particle reactive anthropogenic radionuclide 241 am which shows higher activities relative to pu in the central nansen basin than at the barents sea slope distributions of 137 cs show more rapid mixing at the basin margins barents sea slope in the south gakkel ridge in the north than in the basin interior cesium 137 is mixed throughout the water column adjacent to the barents sea slope and is present in low but detectable activities in the eurasian basin deep water in the central basin at the time of sampling 1987 the surface water at all stations had been labeled with 134 cs released in the 1986 accident at the chernobyl nuclear power station in the 1 year since the introduction of chernobyl 134 cs to the nansen basin it had been mixed to depths of 800 m at the barents sea slope and to 300 m in the central basin prechernobyl inventories of 137 cs as well as 239 240 pu are 10 times those expected from global atmospheric fallout from nuclear weapons testing and are derived principally from releases from the sellafield u k nuclear fuel reprocessing facility on the irish sea based on the sources of 137 cs to the nansen basin mixing time scales are 9 18 years for the upper water column to 1500 m and 40 years for the deep water these mixing time scales combined with more rapid scavenging at the basin margin relative to the central basin produce residence times of particle reactive radionuclides in the nansen basin comparable to other open ocean areas e g north west atlantic despite the presence of permanent ice cover and long periods of low light levels that limit productivity in the arctic

structural control and genesis of gold deposits in the liaodong peninsula northeastern north china craton abstract two distinct styles of gold mineralization occurred in the liaodong peninsula in the northeastern north china craton one formed an altered rock type gold deposit in the west and the other formed a quartz vein type deposit in the east geological investigations and structural analysis show that these deposits may have originated from two distinct tectono magmatic hydrothermal systems the former is an e w trending contractional extensional structural system formed in the late triassic which includes the deposits in baiyun jianshangou and maoling the latter is related to high angle strike slip faults formed in the early cretaceous and includes the wulong gold deposit stress induced tectonic analyses show that the principal stress in the western district was nearly n s oriented contractional stress initially which subsequently changed to extensional stress this might have been the result of the collision and subsequent extension of the siberia craton north china craton and yangzi craton in the late triassic the principal stress in the eastern district was nearly nw se oriented contractional and sinistral shear stress initially which subsequently changed to extensional stress which may be the result of regional extension associated with the westward subduction of the paleo pacific plate in the early cretaceous the structural control of the gold mineralization is manifested at all scales from regional to mines to orebodies that is the grading of the ores was controlled by different generations or orders of deformation gold ores were mainly formed under brittle or brittle shear deformation conditions in particular with extensive ore bearing catalasite flows which were generated by both the brittle or brittle shear failure along major fault systems and related late triassic and early cretaceous magmatic hydrothermal activity this genetic origin for gold mineralization and ore bearing catalasite flows may provide support for the mechanism of flash vaporization in mineralization processes that form mesothermal gold systems

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

fluid evolution of the neoarchean pampalo orogenic gold deposit e finland constraints from la icpms fluid inclusion microanalysis abstract the sources of fluids and precious metals in orogenic gold deposits remain controversial and a key question is whether auriferous magmatic hydrothermal fluids are a decisive ingredient for economic orogenic gold mineralization contributions of magmatic hydrothermal fluids have been proposed based on indirect arguments such as isotope data or emplacement of granites coevally with gold deposits by contrast complete elemental concentration data for orogenic gold fluids which could be used to compare them directly with those of magmatic hydrothermal fluids are not yet available this study reports the major and trace element chemistry of fluid inclusions from the neoarchean pampalo orogenic gold deposit located in the hattu schist belt in eastern finland three successive generations of gold mineralized and barren veins provide the paragenetic and chronological framework for establishing the fluid evolution history of the pampalo deposit based on petrography microthermometry and la icpms fluid inclusion microanalysis five major fluid types were identified the data shows that all fluid types despite characteristic differences carry the signature of orogenic gold fluids i e low to intermediate salinity presence of co 2 ch 4 or n 2 high s contents enrichment in b as sb and w and low pb and zn concentrations gold concentrations vary significantly between the oldest ca 0 6 g g fluid related to the zones with highest gold grades in the deposit and later fluid types

potential earthquake triggering in a complex fault network the northern south island new zealand summary a synthetic seismicity computer model of multiple interacting faults has been used to investigate potential earthquake triggering in the alpine marlborough buller fault network northern south island new zealand of particular interest is whether large characteristic earthquakes on the central 300 km long segment of the alpine fault average recurrence time of 240 yr might trigger events to the north and northeast possible temporal clustering of other large events and stress shadows the synthetic seismicity model generates long homogeneous catalogues that sample a wide range of possible stress interactions and stress states allowing statistical analysis the catalogues used here include 1500 central alpine fault events of m 7 8 and approximately 2 000 000 other events of magnitudes down to 4 8 these other events occur on four additional segments of the alpine fault on 30 segments of the major marlborough and buller faults and on 4650 randomly distributed smaller faults the synthetic seismicity model is of the quasi static type governed by the coulomb failure criterion with extensions to better account for rupture propagation and fault healing true rate and state friction and viscoelastic relaxation are not yet included mechanical properties are adjusted so that most events rupture an entire fault segment i e the faults behave characteristically the regional b value of 1 thus arises mainly from the fault size distribution the driving mechanism and fault properties are iteratively adjusted so that the resulting long term slip rates single event displacements and recurrence times match the observed real world values the major results are as follows i large alpine fault earthquakes m 7 8 on the central segment have recurrence times with a broad distribution from 81 to 601 yr average 234 yr ii the regional moment release rate is constant over periods of 1000 yr but is quite variable on scales of a few hundred years iii large events on the central alpine fault will usually propagate at least part way along strike to the northeast past the junctures with the marlborough faults that splay off to the east iv thirty seven per cent of such large alpine fault events lafes will also instantaneously trigger significant activity m 6 5 on one or another of the other major faults here instantaneous means with a 0 to approximately 3 yr delay v characteristic events initiating on other major faults also have a high chance of triggering activity elsewhere vi following the short period of increased risk of triggered events a stress shadow effect predominates probably reflecting the mutually inhibitory nature of parallel strike slip faults and the need for a constant long term moment release rate

an integrated mineral system model for the gold deposits of the giant jiaodong province eastern china abstract the giant jiaodong gold province with more than 5000 t of proven gold resources is unique in that its mesozoic gold deposits are hosted in a precambrian basement block there has long been debate on the source of gold the mechanism for its extreme enrichment and a holistic genetic model for these spectacular deposits in order to improve understanding of these factors a mineral system model is proposed for the jiaodong gold deposits for the first time it is based on a detailed review of the architecture and composition of the underlying mantle lithosphere the geodynamic setting at the time of gold mineralization the geological and geochemical features of the deposits themselves and mechanisms of their preservation regional geodynamic reconstructions combined with the nature of mesozoic magmatic systems show that late mesozoic breakoff of the subduction slab and rollback of the paleo pacific plate caused asthenosphere upwelling and lithosphere extension and thinning robust geochronological studies demonstrate that the jiaodong gold deposits formed at ca 120 ma during a change of subduction direction of the paleo pacific plate and the synchronous transition of tectonic regime from regional compression to transpression or transtension prior to peak extension caused by thinning of the lithosphere geophysical exploration data and hf nd isotope mapping of the jiaodong peninsula reveal a relatively simple ancient crustal structure cut by near vertical shear or fault zones including the lithosphere scale tan lu and wulian yantai faults that provided the first order architecture for emplacement of mantle magmas and sub crustal fluids in terms of fertility the significant older pre mineralization crustal metamorphism combined with sulfur lead and strontium isotope ratios indicate a dominant sub crustal source for the ore components for the jiaodong gold deposits consistent with evidence for volatile and gold fertilized metasomatized mantle lithosphere beneath the north china block helium ar and c o isotope ratios implicate mantle sources of ore fluids with contamination by crustal components en route to crustal depositional sites in terms of crustal architecture combined geophysical structural mineralogical fluid inclusion and geochemical studies show that the deeply sourced ore fluids and metals advected upwards through lithosphere scale faults to deposit gold via sulfidation and phase separation in crustal scale nne ne trending fault systems at mid to upper crustal levels preservation of disseminated to stockwork style deposits and auriferous quartz pyrite vein deposits was enhanced by deep crustal formation combined with limited uplift and exhumation of the province the jiaodong gold deposits although commonly hosted in granite intrusions clearly post date them on the basis of robust and reliable geochronology the spatial but non temporal relationships of gold deposits with granites their strong structural control and their other characteristics clearly place them in the orogenic gold group it is suggested because of their uniqueness that they be termed jiaodong type orogenic gold deposits

genesis of the selinsing gold deposit peninsular malaysia constraints from mineralogy geochemistry and in situ sulfur isotope compositions of sulfides abstract as a renowned orogenic lode gold deposit the selinsing gold deposit is located in the central belt of peninsular malaysia geologically the gold deposit is characterized by auriferous quartz veins and fine grained gold minerals coexisted with sulfides and the ore body is hosted by low grade metamorphic sedimentary rocks and volcanic rocks furthermore gold mineralization is closely related to tectonic deformation four main stages of vein formation have been identified namely 1 quartz pyrite arsenopyrite vein 2 quartz pyrite gold vein 3 quartz stibnite pyrite vein and 4 quartz chlorite vein the occurrence of gold includes two types i e visible gold and invisible gold the former is featured by coarse grained gold millimeter scale such as auriferous quartz veins and fine grained gold micron scale such as native gold calaverite and electrum the latter is contained in pyrite and arsenopyrite in the form of nano scale native gold particles au0 and lattice gold au1 the geothermometer of arsenopyrite and chlorite as well as temperature sulfur fugacity diagram indicate that the temperature and sulfur fugacity of hydrothermal fluid decrease gradually considering that the sulfur fugacity drops gradually and aqueous inclusions coexist with carbonic inclusions it is inferred that sulfuration reaction in which sulfur is consumed to produce sulfides and fluid immiscibility or phase separation may be responsible for gold precipitation tested by la mc icp ms the in situ sulfur isotope compositions of sulfides formed in different stages are homogenous about 1 to 1 and the calculated h2s dominated hydrothermal fluid is also consistent about 2 to 0 in sulfur isotope compositions which reflect the mantle or ultrabasic rock and basic rock or magmatic rocks may be a potential source of sulfur combined with possible source and tectonic evolution of peninsular malaysia possible metallogenic processes are proposed to explain genesis of the selinsing gold deposit and other gold deposits in the central belt i e ore forming fluid was differentiated from oceanic crust and or fertilized mantle wedge or ore forming fluid may be sourced from concealed magmatic rocks that formed by subduction of the palaeo tethys toward the east malaysia block

the structural history and mineralization controls of the world class geita hill gold deposit geita greenstone belt tanzania the geita hill gold deposit is located in the archean geita greenstone belt and is one of the largest gold deposits in east africa the geita greenstone belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the geita hill gold deposit deformation involved early stages of ductile shearing and folding d1 to d5 during which episodic emplacement of large diorite intrusive complexes sills and dykes occurred these ductile deformation phases were followed by the development of brittle ductile shear zones and faults d6 to d8 the last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes within the greenstone belt and the emplacement of large granitic bodies now forming the margins of the greenstone belt early folded lamprophyre dykes and later lamprophyre dykes crosscutting the folded sequence are common although volumetrically insignificant the gold deposit formed late during the tectonic history of the greenstone belt post dating ductile deformation and synchronous with the development of brittle ductile shear zones that overprinted earlier structural elements the main mineralizing process involved sulfide replacement of magnetite rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions the intersection between the brittle ductile d6 geita hill shear zone and different structural elements of ductile origin e g fold hinges and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization

metallogenesis of the paleoproterozoic piaba orogenic gold deposit sao luis cratonic fragment brazil piaba is an orogenic gold deposit 3 5 moz of the sao luis cratonic fragment in north northeastern brazil the deposit is epizonal mesozonal and associated with the development of a subvertical strike slip fault that cut across rhyacian metavolcano sedimentary rocks and granitoids the metavolcano sedimentary sequence comprises carbon bearing schists andesite dacite ultramafic rocks and felsic tuffs formed at 2240 5 ma to 2227 33 ma that were intruded at 2214 3 ma by fine grained subvolcanic granophyric granodiorite whole rock geochemistry indicates that the hosting metavolcano sedimentary sequence formed in a subduction related arc back arc setting which has previously been interpreted as the early arc stage of an accretionary orogen 2240 2214 ma that was followed by voluminous subduction related calc alkaline arc magmatism 2168 2145 ma and by a collisional phase 2100 15 ma that produced several bodies of peraluminous granites the hydrothermal alteration of the host rocks produced early distal carbonization and hematitization and proximal ore related chlorite sericite carbonate sulfide alteration two styles of mineralization occurring in association are present 1 a network of thin quartz sulfide veinlets and subordinate narrow breccia veins with refractory to free milling gold dissemination in hydrothermally altered host rocks and 2 subordinate low sulfide au quartz veins with free milling gold fluid inclusions and stable isotopes characterize the mineralizing solution as a relatively reduced log fo2 30 to 35 neutral to slightly alkaline ph 5 to 6 2 low salinity average 5 nacl aqueous carbonic xco2 5 to 59 mol xh2o 40 to 93 mol xn2 and xch4 1 mol metamorphic fluid 18oh2o 5 5 to 8 9 dh2o 8 to 69 and suggest that gold mineralization occurred at 250 to 330 c and 1 25 to 2 8 kbar in response to phase separation and fluid rock interactions that produced sulfidation and minor carbonatization sulfur 34s 2 7 to 3 8 is from an undetermined source probably magmatic oxidized or sedimentary lead isotope compositions of gold ore related pyrite and k feldspar from regional rocks suggest that collision related granitoids of ca 2100 15 ma are the most likely sources for pb in the ore although mantle and lower crustal sources are also observed 40ar 39ar in hydrothermal sericite yielded a plateau age of 1980 13 ma which is similar to the pb pb model age of gold of 1959 ma these are considered the minimum age for gold mineralization whereas the maximum age of 2009 11 ma is given by a small gold hosting intrusion in the same region

hydrothermal rare earth element xenotime mineralization at maw zone athabasca basin canada and its relationship to unconformity related uranium deposits a reply

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

opening the magmatic hydrothermal window high precision u pb geochronology of the mesoproterozoic olympic dam cu u au ag deposit south australia

formation of gold deposits a metamorphic devolatilization model a metamorphic devolatilization model can explain the enrichment segregation timing distribution and character of many goldfields such as those found in archean greenstone belts slate belts and other gold only provinces in this genetic model hydrated and carbonated greenschist facies rocks particularly metabasic rocks are devolatilized primarily across the greenschist amphibolite facies boundary in an orogenic setting devolatilization operates on the scale of individual mineral grains extracting not just h2o and co2 but also s and in turn au elevated gold in solution is achieved by complexing with reduced s and by h2co3 weak acid buffering near the optimal fluid ph for gold solubility the buffering is more important than being at the point of maximum gold solubility low salinity ensures low base metal concentrations in the auriferous metamorphic fluid migration of this fluid upwards is via shear zones and or into hydraulic fracture zones in rocks of low tensile strength the geometry of the shear zones dictates the kilometre scale fluid migration paths and the degree of fluid focusing into small enough volumes to form economic accumulations of gold deposition of gold from solution necessitates breakdown of the gold thiosulphide complex and is especially facilitated by fluid reduction in contact with reduced carbon bearing host rocks and or by sulphidation of wallrocks to generate iron bearing sulphide and precipitated gold as such black slate carbon seams banded iron formation tholeiitic basalt magnetite bearing diorite and differentiated tholeiitic dolerite sills are some of the important hosts to major goldfields gold deposition is accompanied by carbonation sulphidation and muscovite biotite alteration where the host rock is of suitable bulk composition the correlation of major gold deposits with rock type even when the gold is primarily in veins argues for rock dominated depositional systems not fluid dominated ones as a consequence a general role in gold deposition for fluid mixing temperature decrease and or fluid pressure decrease and boiling is unlikely although such effects may be involved locally several geological features that are recorded at gold only deposits today reflect subsequent modifications superimposed upon the products of this generic metamorphic devolatilization process overprinting by higher grade metamorphism and deformation and or palaeo weathering may provide many of the most obvious features of goldfields including their mineralogy geochemistry geometry small scale timing features geophysical response and even mesoscopic gold distribution

breccia formation by particle fluidization in fault zones implications for transitory rupture controlled fluid flow regimes in hydrothermal systems breccias in the rusey fault cornwall uk and the roamane fault porgera gold deposit papua new guinea provide insights about the dynamics of fluid flow and flow velocities when fault ruptures breach overpressured reservoirs of hydrothermal fluid these faults contain cockade like breccias in which rock fragments are mantled by spheroidal overgrowths of hydrothermal minerals at rusey the overgrowths are quartz dominant whereas in the roamane fault overgrowths are composed of calcite quartz or gold rich pyrite although none of the rock fragment cores of accretionary spheroids are in contact with their neighbours the spheroidal overgrowths do contact each other and are at least partially cemented together the hydrothermal overgrowths mostly comprise either outwards coarsening crystals that radiate from the surface of the core rock fragment or finer grained inequigranular to mesh like intergrowths concentric textural banding and oscillatory growth zones are present in some hydrothermal mantles the breccias occur as fault parallel layers and lenses each up to several tens of centimeters thick adjacent layers are characterized by texturally distinct ranges of clast sizes and different proportions of clasts to hydrothermal overgrowths in the rusey fault many texturally distinct breccia layers are present within a 3m wide fault core some layers truncate others and many breccia layers exhibit grainsize grading or banding clasts in the breccias include fragments of wall rock veins and various fault damage products including fragments of earlier generations of cemented breccia brecciation was episodic and separated by periods of cementation the distinctive textures of the breccias are interpreted to have formed by fluidization of fault damage products in a high fluid flux regime hydrothermal coatings developed while clasts were in a suspended state during fluid ascent through dilatant fault segments layered breccias record multiple episodes of particle fluidization and indicate that the faults provided conduits for repeated transitory fluid upflow particle size distributions indicate that fluid velocities during fluidization were in the range 0 1 ms 1 to 1 ms 1 the maximum flow rates correspond to fluid fluxes of 10 to 100 l s 1 per meter strike length of fault through dilatant fault apertures up to several tens of centimeters wide such high flow rates characteristically induce intense swarm seismicity rather than mainshock aftershock seismicity and have implications for the dynamics and rates of formation of fault related hydrothermal ore deposits

orogenic gold common or evolving fluid and metal sources through time abstract orogenic gold deposits of all ages from paleoarchean to tertiary show consistency in chemical composition they are the products of aqueous carbonic fluids with typically 5 20 mol co 2 although unmixing during extreme pressure fluctuation can lead to entrapment of much more co 2 rich fluid inclusions in some cases ore fluids are typically characterized by significant concentrations of ch 4 and or n 2 common estimates of 0 01 0 36 mol h 2 s a near neutral ph of 5 5 and salinities of 3 7 wt nacl equiv with na k ca mg this fluid composition consistency favors an ore fluid produced from a single source area and rules out mixing of fluids from multiple sources as significant in orogenic gold formation nevertheless there are broad ranges in more robust fluid inclusion trapping temperatures and pressures between deposits that support a model where this specific fluid may deposit ore over a broad window of upper to middle crustal depths much of the reported isotopic and noble gas data is inconsistent between deposits leading to the common equivocal interpretations from studies that have attempted to define fluid and metal source areas for various orogenic gold provinces fluid stable isotope values are commonly characterized by the following ranges 1 18 o for precambrian ores of 6 to 11 and for phanerozoic ores of 7 to 13 2 d and 34 s values that are extremely variable 3 13 c values that range from 11 to 2 and 4 15 n of 10 to 24 for the neoarchean 6 5 to 12 for the paleoproterozoic and 1 5 to 10 for the phanerozoic secular variations in large scale earth processes appear to best explain some of the broad ranges in the o s and n data fluid rock interaction particularly in ore trap areas may cause important local shifts in the o s and c ratios the extreme variations in d mainly reflect measurements of hydrogen isotopes by bulk extraction of waters from numerous fluid inclusion generations many which are not related to ore formation radiogenic isotopes such as those of pb sr nd sm and os measured on hydrothermal minerals are even more difficult to interpret for defining metal source particularly as the low salinity ore fluids transport limited amounts of these elements and significant amounts of these may be locally added to the minerals during alteration reactions at the sites of gold deposition noble gas and halogen data are equally equivocal fluid exsolution from granitoids emplaced into the upper and middle crust metamorphism of the crust or fluids entering trans crustal fault zones from below the crust all remain as permissive scenarios associated with orogenic gold formation as the abundant geochemical data are equivocal however geological and geochronological data weigh heavily against a magmatic hydrothermal model in the upper to middle crust there is no universal temporal association between orogenic gold and magmatism and where there is an overlap in age there is no specific type of magmatism consistently associated with gold formation nor element zonation around any specific pluton a crustal metamorphic model for fluid and metal sources is very consistent with geological geochronological and geochemical data although metamorphism on a regional scale that releases these components into major fault zones can be associated with many processes along active continental margins these can include crustal thickening and radiogenic heating slab rollback and heating during crustal extension or subduction of a spreading ridge heating the base of an accretionary prism in rare examples where phanerozoic orogenic gold deposits are hosted in precambrian high grade metamorphic terranes fluids and metals must however enter a transcrustal fault system from a sub crustal source this could either be a devolatilized subducted relatively flat perhaps stalled slab and its overlying sediment or the corner of the fertilized mantle wedge that releases a fluid during a thermal event

nanosims element mapping and sulfur isotope analysis of au bearing pyrite from lannigou carlin type au deposit in sw china new insights into the origin and evolution of au bearing fluids abstract sulfur isotope signatures of au bearing pyrite from lannigou au deposit a typical carlin type au deposit in sw china provide valuable information about the origin of the ore forming minerals analysis by nanosims was used to determine s isotope compositions of au bearing pyrite and to map the grain scale distributions of au cu as and s in pyrite from the deposit based on different textural pattern of pyrites revealed by back scattered electron bse images they are divided into three types py 1 diagenetic pyrite without core rim structure py 2 pyrite with an au free core and a rhythmically zoned au bearing rim py 3 au bearing pyrite with rhythmic zoning across the entire grain the element distributions and s isotope compositions of four paragenetic stages are recognized on the basis of textural observation py 1 grains and the au free homogeneous cores of zoned crystals were formed in stage 1 while the au bearing rims of the zoned crystals with rhythmic zonation of as and cu and to a lesser degree au were formed in two superimposed stages stage 2 formed the inner zone that is enriched in as alone and stage 3 formed the outer zone that is enriched in both au and as other sulfides such as realgar cinnabar and stibnite are formed in the last stage the relationship between au and as distributions in pyrite rim is complicated changing from coupled to decoupled at the nanoscale such complexity is interpreted to reflect fluctuation of fluid composition and temperature with time which in turn affect the modes of occurrence of as and au it is inferred that as mainly occurs in the crystal lattice replacing s whereas au is mainly present as nanoparticles that were trapped in pyrite during crystal growth the au bearing rims of the zoned pyrite crystals are characterized by highly variable 34s values from 1 1 to 18 1 which exceed the values of the triassic calcareous host rocks 10 14 in contrast the 34s values of the au free cores of zoned pyrite crystals vary over a narrower interval and are mainly between 6 and 12 close to the values of pyrite crystals in the sedimentary country rocks our new analyses also reveals that the 34s values of the au bearing fluids generally increase during the formation of the deposit the observed s isotope variations are consistent with mixing between a magmatic related fluid with mantle like 34s value 0 and a sedimentary or deep basin brine fluid with elevated 34s value 18 with an increasing contribution from the latter with time the notably varied values of 34s and the disseminations of au and other trace elements such as as and cu in pyrite crystals indicate that the process responsible for au precipitation in this deposit occurred in an open hydrothermal system

age and geologic setting of quartz vein hosted gold mineralization at curraghinalt northern ireland implications for genesis and classification acknowledgments we are grateful to dalradian gold ltd for providing the sections for petrographic analysis geochemical data and general support we would also like to thank the following john still alison sandison and jenny johnston of the school of geosciences university of aberdeen for assistance with the sem studies js and with preparing figures as and jj nerc for ongoing funding of the argon isotope facility at suerc jim imlach and ross dymock at suerc for technical assistance and martin lee at the school of geographical and earth sciences at the university of glasgow for use of the sem cl equipment the paper has benefitted significantly from comments by the official reviewers and unofficial reviews by garth earls jamie wilkinson mark cooper and adrian boyce and detailed conversations with ian alsop structural geology of the sperrins and nyree hill and gawen jenkin gold mineralization in the caledonides the authors are entirely responsible for the conclusions expressed

radiogenic isotopes record a drop in a bucket a fingerprint of multi kilometer scale fluid pathways inferred to drive fault valve behavior abstract minerals precipitated from fluids carry a chemical signature of fluid origin and transport history we use this concept to test the hypothesis that a subset of veins precipitated in the loma blanca fault zone rio grande rift nm usa record fault valve behavior previous studies of this site show syntectonic calcite veins formed in response to post seismic fluid discharge following repeated ground rupturing earthquakes u th ages on veins constrain the timing of fault slip over a period of 400 000 years the radiogenic isotope sr pb and u data we report here demonstrate that a cluster of earthquakes that occurred at 430 ka when fault slip frequency increased by nearly one order of magnitude corresponds with a brief period of particularly large and rapid variations in post seismic fluid chemistry isotopic variations are consistent with fluid input from the socorro magma body a large active mid crustal basalt intrusion 19 km below the surface the short duration of the earthquake cluster