role of fluid mixing and wallrock sulfidation in gold mineralization at the semna mine area central eastern desert of egypt evidence from hydrothermal alteration fluid inclusions and stable isotope data abstract the semna gold deposit is one of several vein type gold occurrences in the central eastern desert of egypt where gold bearing quartz veins are confined to shear zones close to the boundaries of small granitoid stocks the semna gold deposit is related to a series of sub parallel quartz veins along steeply dipping wnw trending shear zones which cut through tectonized metagabbro and granodiorite rocks the orebodies exhibit a complex structure of massive and brecciated quartz consistent with a change of the paleostress field from tensional to simple shear regimes along the pre existing fault segments textural structural and mineralogical evidence including open space structures quartz stockwork and alteration assemblages constrain on vein development during an active fault system the ore mineral assemblage includes pyrite chalcopyrite subordinate arsenopyrite galena sphalerite and gold hydrothermal chlorite carbonate pyrite chalcopyrite and kaolinite are dominant in the altered metaggabro whereas quartz sericite pyrite kaolinite and alunite characterize the granodiorite rocks in the alteration zones mixtures of alunite vuggy silica and disseminated sulfides occupy the interstitial open spaces common at fracture intersections partial recrystallization has rendered the brecciation and open space textures suggesting that the auriferous quartz veins were formed at moderately shallow depths in the transition zone between mesothermal and epithermal veins petrographic and microthermometric studies aided recognition of co 2 rich h 2 o rich and mixed h 2 o co 2 fluid inclusions in the gold bearing quartz veins the h 2 o co 2 inclusions are dominant over the other two types and are characterized by variable vapor liquid ratios these inclusions are interpreted as products of partial mixing of two immiscible carbonic and aqueous fluids the generally light 34 s of pyrite and chalcopyrite may suggest a magmatic source of sulfur spread in the final homogenization temperatures and bulk inclusion densities are likely due to trapping under pressure fluctuation through repeated fracture opening and sealing conditions of gold deposition are estimated on basis of the fluid inclusions and sulfur isotope data as 226 267 c and 350 1100 bar under conditions transitional between mesothermal and epithermal systems the semna gold deposit can be attributed to interplay of protracted volcanic activity dokhan volcanics fluid mixing wallrock sulfidation and a structural setting favoring gold deposition gold was transported as au bisulfide complexes under weak acid conditions concomitant with quartz sericite pyrite alteration and precipitated through a decrease in gold solubility due to fluid cooling mixing with meteoric waters and variations in ph and f o 2

quartz vein fabrics coupled to elevated fluid pressures in the stawell gold deposit south eastern australia shear and extensional veins formed during the reactivation of the magdala shear system at stawell in western victoria australia contribute to the formation of the auriferous central and basalt contact lodes within this shear system is a range of fault rocks accompanied by steep dipping 65 quartz rich laminated shear veins and relatively flat lying extensional veins both vein sets appear to have been a primary source for the host rock permeability during fluid flow in a regime of significant deviatoric stresses the macro and microstructures suggest that the dilatancy that produced mineralized veins formed under conditions of overpressure generated by fluid infiltration late in a tectonic regime a new microfabric analysis technique is used to investigate the quartz rich veins which allows rapid integration of the microstructure with the crystallographic preferred orientations cpos both the shear and extensional quartz veins have a random cpo with 120 dihedral angles between the quartz quartz grains which is typical of a metamorphic equilibrium microfabric the microstructures indicate that the quartz has undergone extensive grain adjustment in the solid state with grain shape and size affected by interfacial solution pressure solution effects these features are consistent with inferences from experimental rock deformation studies where grain boundary migration is enhanced in a water rich environment the onset of solution transfer processes pressure solution developed as the quartz microfabric stabilized and continued to modify the cpo and microstructure significantly it is concluded that grain growth and pressure solution are coupled diffusive mass transfer processes related to fluctuations in pore fluid pressures in a region undergoing deformation at near lithostatic pressures

synthetic fluid inclusions vii re equilibration of fluid inclusions in quartz during laboratory simulated metamorphic burial and uplift p t conditions inferred from fluid inclusions in metamorphic rocks often disagree with the values predicted from mineral equilibria calculations these observations suggest that inclusions formed during early stages of regional metamorphism continue to re equilibrate during burial and subsequent uplift in response to differential pressure p t conditions accompanying burial and uplift were experimentally simulated by initially forming pure h2o inclusions in quartz at elevated temperatures and pressures and then re equilibrating the inclusions in the presence of a 20 wt nacl solution such that final confining pressures ranged from 5 kbar above to 4 kbar below the initial internal pressure of the inclusions at the temperature of re equilibration in all samples re equilibrated at confining pressures below the internal pressure some inclusions were formed that had compositions of 20 wt nacl and densities in accord with the final p t conditions additionally some inclusions were observed to contain fluids of intermediate salinities between 0 and 20 wt nacl densities of these inclusions were also consistent with formation at the re equilibration p t conditions the remainder of the fluid inclusions observed in these samples contained pure h2o and their homogenization temperatures corresponded to densities intermediate between the initial and final p t conditions in short term experiments 7 days where the initial internal overpressure exceeded 1 kbar no inclusions were found that contained the original density and none were found to have totally re equilibrated instead most h2o inclusions re equilibrated until their internal pressures were between 750 and 1500 bars above the confining pressure regardless of the initial pressure differential in a long term experiment 52 days inclusions re equilibrated at a lower confining pressure than the initial internal pressure displayed homogenization temperatures corresponding to a range in final internal pressures between 0 kbar i e total re equilibration and 1 2 kbar above the confining pressure in experiments where the confining pressure during re equilibration exceeded the initial internal pressure densities of pure h2o inclusions increased to values intermediate between the initial and final p t conditions additionally these inclusions were generally surrounded by a three dimensional halo of smaller inclusions also of intermediate density resulting in a texture similar to that previously ascribed to decrepitation from internal overpressure in extreme cases where confining pressures were 4 5 kbar above the initial pressure the parent inclusion almost completely closed leaving only the three dimensional array of small 5 m inclusions the outline of which may be several times the volume of the original inclusion groups of such inclusions closely resemble textures commonly observed in medium to high grade metamorphic rocks inclusions containing 10 and 42 wt nacl solutions trapped at 600 c and 3 kbar were re equilibrated at 600 c and 1 kbar for 5 days in dry argon to evaluate the importance of h2o diffusion as a mechanism of lowering the inclusion bulk density salinities of re equilibrated inclusions obtained from freezing point depressions and halite dissolution temperatures indicate that original compositions were preserved density changes similar to those previously described were noted in these experiments in inclusions showing no visible microfractures therefore density variations observed in inclusions in this study re equilibrated under rapid deformation conditions are considered to result from a change in the inclusion volume without significant loss of contents by diffusion or leakage

fossil fluid reservoir beneath a duplex fault structure within the central range of taiwan implications for fluid leakage and lubrication during earthquake rupturing process in order to understand the kinematics which likely facilitated the speedy rupturing process of the 1999 mw 7 6 taiwan chichi earthquake we examined exposed rocks in the taiwan slate belt where the pressure and temperature conditions most resembled the hypocentre of the chi chi earthquake i e subgreenschist facies field observations and composition analyses of the silicified vein rich zones beneath the duplex structure suggest that impermeable slate layers may serve as cap rocks for confining deep seated fluids these fluids most likely come from the taiwan metamorphic complex at deeper depths by the dehydration and decarbonation reactions or partial melting in addition the gouge zone of a link fault above the detachment also indicates the presence of overpressured fluids during faulting it is probable that episodic leakage of the confined fluid reservoirs may provide essential fluids for fault lubrication during earthquake ruptures

fluid rock reaction weakening of fault zones the presence of weak phyllosilicates may explain the low shear strengths of fault zones if they define well developed fabrics the growth of phyllosilicates is favored in meteoric water dominated granitic fault systems where mineral aqueous fluid equilibria predict that modal phyllosilicate will increase via feldspar replacement reactions in deeper more alkaline rock dominated regimes the reactions reverse and feldspars tend to replace phyllosilicates in mg rich mafic rocks however phyllosilicates chlorite biotite can replace stronger framework and chain silicates in both shallower approximately 10 km meteoric h sub 2 o dominated and in deeper alkaline rock dominated regimes where these phyllosilicates precipitate in active fault zones they contribute directly to reaction softening because low temperature deformation of phyllosilicates is not governed by frictional processes alone but can occur by pressure independent dislocation glide the strength of phyllosilicate rich fault rocks can be low at all depths low strain rate creep during interseismic periods can align phyllosilicate grains in foliated gouge and phyllonites where preferred orientations are strong and contiguity of phyllosilicates is large strengths of rocks within fault zones may approach minimum strengths defined by single phyllosilicate crystals fault zones containing localized high concentrations of phyllosilicates with strong preferred orientations in well defined folia can exhibit aseismic slip especially where mafic mg rich rocksmore occur along the fault like parts of the san andreas fault 104 refs 6 figs 1 tab less

double vein ore bearing vs ore free structures in the xitian ore field south china implications for fluid evolution and mineral exploration abstract two types of spatially and temporally associated hydrothermal veins ore bearing and ore free occur as double vein structures in the xitian ore field south china here we investigated the genetic relationship material provenance fluid properties and prospecting significance of the co developed veins in the granite related fault controlled vein type xiangdong w goudanlan w chaling pb zn and xinggao fluorite deposits of the xitian ore field focusing on two common minerals quartz and zircon petrographic cl microthermometric and h o isotopic analyses of stage i and stage ii quartz in both the ore bearing and ore free veins of these deposits document two episodes of hydrothermal activity the first episode involved a high temperature up to 370 high salinity up to 28 nacl equiv high 18o 2 1 to 7 2 and high 2h 82 to 54 mean 68 n 13 fluid that corresponds to the main magmatic hydrothermal stage of the deposits and the second episode involved a low temperature down to 90 low salinity down to 1 nacl equiv low 18o 8 8 to 5 3 and low 2h 83 to 60 mean 73 n 6 fluid that represents an accessary mineralization stage variations in fluid inclusion types and compositions indicate that the processes of fluid mixing boiling cooling and degassing played important roles in mineralization and that magmatic fluid became less important than meteoric water along during the formation of ore free veins la icpms u pb dating reveals totally different age populations for these veins zircons from the ore bearing quartz veins yield older and more variable ages pre devonian 400 ma n 68 that are similar to that of the local basement sedimentary rocks whereas zircons from the ore free quartz veins yield younger and less variable ages triassic 250 200 ma n 31 and jurassic 170 140 ma n 13 that are identical to those of the host granite zircons in addition the ore free veins show lower temperatures salinities and total rees as recorded by quartz but greater alteration as reflected by cl and trace element features of hydrothermally altered zircons and wider vein size than the ore bearing veins we propose a mineral water separation model to explain the formation of the double vein structures in this model ore bearing veins and ore free veins formed in succession during faulting events ore bearing veins formed first in the smaller fractures and were the dominant sites of mineralization whereas the ore free veins served as dissipating passageways for residual water and silica in larger fractures we suggest that the mineral water separation model has good application prospects in mineral exploration and that zircons can be applied as an indicative mineral to differentiate the mineralization potential of different types of quartz veins

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

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

brittle failure mode plots for compressional and extensional tectonic regimes abstract equations governing different macroscopic modes of brittle rock failure extensional fracturing extensional shear fracturing compressional shear failure and reshear of existing faults can be represented on plots of differential stress vs effective vertical stress for a set of material properties such plots can be constructed for different tectonic regimes and correlated to depth for particular fluid pressure conditions allowing easy evaluation of the physical controls on brittle rock failure and ready comparison of the fields occupied by the three failure modes in different tectonic settings they emphasize the relative ease in terms of differential stress and fluid pressure levels of deforming a rock mass by brittle fracturing and faulting in extensional regimes compared with compressional aside from their relevance to general structural mechanics these generic failure plots have wide ranging application to understanding the initial development and progressive evolution of fault fracture systems both in sedimentary basins and as hosting structures for hydrothermal mineralization in different tectonic settings

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

gold quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting gold quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime in the val d or district of the abitibi greenstone belt canada quartz tourmaline carbonate veins form a vein field 30 15 km in the hanging wall of a crustal scale fault zone which was the main channelway for upward migration of the deeply generated fluids the veins occur in small high angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks they have formed incrementally during active reverse faulting in response to crustal shortening at depths corresponding to those at the base of the seismogenic zone in actively deforming crust detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures p of the order of 200 mpa and for cyclic stress reversals during vein formation and provide good support for the fault valve model a comparison of vein characteristics with standard earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake 4 m 3 or less the large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system it is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes m 6 nucleating near the base of the seismogenic regime along the nearby crustal scale fault either as aftershocks or as a precursory smarm

synthetic fluid inclusions viii vapor saturated halite solubility in part of the system nacl cacl2 h2o with application to fluid inclusions from oceanic hydrothermal systems abstract halite solubility along part of the vapor saturated liquidus in the system nacl cacl 2 h 2 o has been determined using the synthetic fluid inclusion technique data allow the construction of liquidus isotherms for temperatures up to 500 c and bulk compositions containing 60 wt total salt and as much as 25 wt cacl 2 combined with previous data for the binary system nacl h 2 o and for the ternary system nacl cacl 2 h 2 o in the low salinity low temperature region a preliminary ternary phase diagram can be constructed that remains incomplete only in the cacl 2 rich region results are applied to the interpretation of saline fluid inclusions from quartz veins in oceanic metagabbros and can be applied to many other natural inclusions containing aqueous solutions with nacl and cacl 2 the dominant solutes microthermometric measurements at equilibrium of the melting temperature of ice tm ice and of the dissolution temperature of halite tm halite are sufficient to determine the bulk composition of the nacl cacl 2 h 2 o fluid

load strengthening versus load weakening faulting increases in shear stress along a fault during loading to failure cannot generally occur without changes in the normal stress across the fault n the fault loading parameter n n pf distinguishes situations of load strengthening n 0 where the frictional shear strength of faults increases as tectonic shear stress rises from load weakening environments n t6 0 where it decreases compressional faulting in tectonic regimes with v 3 is always load strengthening unless fluid pressure is rapidly increasing extensional faulting in regimes where v 1 is load weakening unless fluid pressure is dropping rapidly strike slip faulting in terrains where v 2 can be either load weakening or load strengthening the particular case where n 0 so that frictional shear strength stays constant during fault loading is a very special situation corresponding to direct shear load strengthening strike slip faulting appears to correlate with tectonic transpression and load weakening with transtension differing loading characteristics of faults in different tectonic regimes must induce varying patterns of cyclic fluid redistribution accompanying the seismic cycle with implications for earthquake recurrence and precursory groundwater phenomena

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

evolution of metamorphic fluids at the variscan fold and thrust belt in eastern belgium abstract quartz veins in the southern greenschist facies metamorphic border of the stavelot venn massif belgium have been examined a detailed petrographical study of these quartz veins indicates the presence of eleven quartz stages a microthermometric analysis shows the expulsion of three main fluid types during the palaeozoic in the first quartz stage i of caledonian age mixed aqueous gaseous h 2 o co 2 nacl fluid inclusions are present the post caledonian and pre variscan second stage ii is characterised by gaseous co 2 n 2 inclusions occurring along inter crystal grain boundaries and by aqueous h 2 o nacl intra crystal inclusions the quartz stages iii vi which formed during the variscan period contain primary mixed aqueous gaseous h 2 o co 2 nacl inclusions all with an equal bulk chemical composition post variscan quartz stages vii xi could not be investigated microthermometrically the combination of microthermometry chlorite geothermometry and geothermal gradients reveals minimum trapping conditions of 200 mpa and 200 c for stage i quartz and 175 275 mpa and 385 435 c for quartz stages iii vi the trapping conditions of stage ii quartz could not be determined due to the strong recrystallisation of the quartz veins at the lower metamorphic variscan thrust front distinct temperature anomalies are associated with thrust faults as indicated by conodont colour alteration index cai vitrinite reflectance data chlorite geothermometry and fluid inclusion microthermometry the expulsion of metamorphic fluids generated at the southern part of the stavelot venn massif could have caused the distinct temperature anomalies along variscan thrust faults alternatively some of these anomalies could have been induced by deeply circulating meteoric fluids

pyrite textures and trace element compositions from the granodiorite hosted gold deposit at jonnagiri eastern dharwar craton india implications for gold mineralization processes

three stage formation of greenstone hosted orogenic gold deposits in the val d or mining district abitibi canada evidence from pyrite and tourmaline abstract orogenic gold deposits are the most widespread type of gold deposit worldwide defining important exploration targets in precambrian greenstone belts here we refine the model for orogenic gold formation in the world class val d or mining district quebec canada using geochemical isotopic and mineralogical data from quartz tourmaline carbonate qtc veins from several deposits across the district multi element ag as au b ba co mg mo ni and pb pyrite mapping as well as major trace and rare earth element variations in tourmaline define a three stage paragenesis across several deposits the first reduced phase crystallized the quartz carbonate gold association in the veins the second oxidized phase favored tourmaline and barite crystallization barite was not previously reported from this district the third phase involved a reduced fluid and the crystallization of non auriferous as co ni rich cubic pyrite and tourmaline boron isotope ratios of tourmaline vary from 15 6 to 7 7 which is attributed to the mixing of at least two fluids one related to a deep metamorphic source and the other to shallower possibly relict formation water a systematic regional pattern in b isotope variation is found with lighter values in the volcanic rocks and heavier ones in or close to intrusions which suggests different mixing proportions in the vein field from ne to sw

hydrothermal fluid venting along a seismogenic detachment fault in the moresby rift woodlark basin papua new guinea we investigate the conditions of hydrothermal alteration occurring along an active seismogenic normal fault dipping 30 in the moresby rift woodlark basin papua new guinea petrological and fluid inclusion studies were conducted in the samples recovered from the ocean drilling program leg 180 that cored the footwall scarp and hanging wall of the fault chemical analyses of the thermodependent si al substitutions in chlorites show that synrift alteration is coeval with a temperature rise from 200 c to 300 c in the footwall of the detachment whereas no thermal event is recorded in the hanging wall in the same samples fluid inclusions indicate that footwall rocks followed an isothermal exhumation path from 5 7 km to the subsurface taken together the results strongly suggest that the 100 c temperature rise results from the upward migration of h2o nacl fluids expelled by dehydrating metamorphic rocks from the lower crust fluid release began with the onset of detachment faulting and is still actively occurring nowadays the fact that low angle earthquakes also occur now at 5 7 km along the fault suggests that the dehydration of the metamorphic rocks and the subsequent circulation of hot hydrothermal fluids may be responsible for the weakening of the middle crust and the formation of a detachment in the moresby rift

the effect of fluid chemistry on quartz microcrack lifetimes abstract metamorphic porosity consists of a branching hierarchy of primary features grain boundaries secondary features 3 and 4 grain junction tubules tertiary features microcracks and higher order features fractures while some of these features are rarely open to bulk fluid flow e g boundaries higher order features are open to fluid flow for timescales which vary significantly as a function of temperature pressure and fluid and rock chemistry the kinetics of microcrack healing in quartz between 300 600 c at 200 mpa increases as a function of fluid chemistry in the following order co 2 h 2 o 2 o m nacl 2 m cacl 2 6 m nacl changes in fluid chemistry which increase quartz solubility correlate with increased rates of healing these observations suggest that local redistribution of quartz occurs by solution precipitation driven by gradients in curvature along the crack surface and is possibly rate limited by pore fluid diffusion however the effect of crack geometry is larger than the effect of fluid chemistry at elevated temperatures and pressures aqueous fluids with elevated nacl concentrations can penetrate pervasively into polycrystalline quartz along a branching hierarchy consisting of episodically open fractures connected to 3 and 4 grain junction tubule networks while pure water or h 2 o co 2 fluid can only infiltrate pervasively along episodically open fractures 1 at lower temperatures and pressures 3 and 4 grain tubule networks in polycrystalline rocks may be closed to all fluids limiting fluid infiltration to microfacture networks lifetimes of quartz microcracks in all these fluids p fluid p confining is geologically short even at temperatures as low as 200 c

the significance of linear trends and clusters of fault related mesothermal lode gold mineralization gold deposits from the laverton region western australia form both linear trends and cluster like distributions in map view associated with regional scale shear zone systems clusters of deposits are common and they form 5 5 km domains spaced every 15 to 20 km in contrast there is also a linear trend of mineralization that can be traced continuously for 20 km along a shear zone in detail mineralization is found in a range of different rock types and is hosted in fault shear zone networks that are smaller than the adjacent regionalscale shear zones by several orders of magnitude a mixture of different fault types comprise these mineralized fault shear networks often within the same gold field although thrust and reverse faults are a common component of the richer deposits e g sunrise dam wallaby granny smith the seismogenic behavior of fault systems during mineralization can explain these observations illustrated by simple stress transfer models that show the location of aftershock distributions around different fault types strike slip ruptures on regional scale structures can repeatedly trigger restricted clusters of aftershocks these aftershock clusters describe vertical pipe like domains in three dimensions in contrast regional scale thrust or reverse ruptures trigger continuous more diffuse domains of aftershocks along the full strike of the rupture thus ruptures along regional strike slip structures are able to trigger ruptures on preexisting thrust faultsshears at the deposit scale when they do so the thrust aftershocks generate a vertical domain of elevated permeability over the depth of the crust where the individual flow paths are likely to be highly tortuous increasing the potential for fluid rock interaction or fluid mixing the existence of linear trends and cluster like distributions of deposits at laverton may be consistent with two different ages of mineralization in this gold camp the results also show that distributions of deposits provide an indication of the kinematics of the regional fault systems that they relate to and that stress transfer modeling is a simple non computationally demanding technique that is easily adapted to exploration for fault related mineral deposits

fluid participation in deep fault zones evidence from geological geochemical and 18o 16o relations fluid incursion into fault zones and their deeper level counterparts brittle ductile shear zones is examined in a number of different crustal environments at the grenville front translation was accommodated along two mylonite zones and an associated boundary fault the high mz ii and low temperature mz i mylonite zones formed at 580 to 640 c and 430 to 490 c respectively in the presence of fluids of metamorphic orgin indigenous to the immediate rocks a population of posttectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative 18o at 200 to 300 c the water may have been derived from downwards penetration into fault zones of low 18o precipitation on a mountain range induced by continental collision with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults at lagoa real brazil archaean gneisses overlie proterozoic sediments along thrust surfaces and contain brittle ductile shear zones locally occupied by uranium deposits following deformation at 500 to 540 c in the presence of metamorphic fluids and under conditions of low water to rock ratio shear zones underwent local intense oxidation and desilicification all minerals undergo a shift of 10 indicating discharge of meteoric water recharged formation brines in the underlying proterozoic sediments up through the archaean gneisses during overthrusting about 1000 km3 of solutions passed through these structures at yellow knife a series of large scale shear zones developed by brittle ductile mechanisms involving volume dilation with migration of 5 wt volatiles into the shear zone from surrounding metabasalts this early deformation involved no departures in redox state or whole rock 18o from background states of fe2 fe 0 72 and 7 to 7 5 respectively attesting to conditions of low water rock shear zones subsequently acted as high permeability conduits for pulsed discharge of 9 km3 of reduced metamorphic hydrothermal fluids at 360 450 c the west bay fault a late major transcurrent structure contains massive bull quartz that grew at 200 300 c from fluids of 2 to 6 possibly formation brines in general flow regimes in these fault and shear zones follow a sequence from conditions of high temperature and pressure with locally derived fluids at low water to rock ratios during initiation of the structures to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs later in the tectonic evolution and at shallower crustal levels there was incursion of oxidizing fluids from near surface reservoirs into the faults

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

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

pressure temperature and timing of mineralization of the sedimentary rock hosted orogenic gold deposit at klipwal southeastern kaapvaal craton south africa gold mineralization in the klipwal shear zone ksz at the klipwal gold mine is confined to laminated quartz carbonate lodes stringers and associated alteration in sandstone and siltstone of the delfkom formation in the upper mozaan group of the mesoarchaean pongola supergroup the moderately dipping brittle ductile ksz strikes n s with an oblique reverse sinistral sense of shear the deformational events that are recognized include an early compressional phase that produced anastomosing shears defined by shear fabrics with numerous shear parallel laminated quartz carbonate fault fill veins and in places extensional quartz vein stockworks and a late brittle reactivation phase that produced fault breccias displacing earlier extensional veins three closely spaced economic reefs lodes are developed the main r reef constitutes the ksz while the j and h reefs represent footwall splays alteration comprises chlorite muscovite epidote feldspar and carbonates along with pyrite arsenopyrite and chalcopyrite pyrrhotite an inner alteration zone is dominated by laminated quartz carbonate veins with alternating quartz carbonate rich and muscovite chlorite rich laminae whereas the proximal zone is characterized by alteration halos of k feldspar albite epidote chlorite and muscovite along with carbonates and associated quartz veins chlorite thermometry from the inner and proximal zones yielded temperatures of 267 to 312 c arsenopyrite compositions provide temperatures in the same range 255 to 318 c fluid inclusion microthermometry and raman spectrometry of quartz veins in the mineralized reefs reveal the presence of metamorphogenic aqueous gaseous fluid with an average salinity of 6 5 wt nacl equiv fluid compositions and estimated pressure temperature p t range 1 1 to 2 5 kbar at 255 to 318 c are typical of orogenic gold deposits devolatilization during the regional facies metamorphism of the pongola supergroup is considered the likely fluid forming event with fluid flow focused into a compressional jog of the ksz shear induced pressure fluctuations generated a phase separation of the initial aqueous gaseous fluid producing a gaseous and low salinity aqueous fluid this together with fluid rock interaction and a decrease in fo2 lead to sulfide and gold precipitation at klipwal re os data from six sulfide samples constrain the age of sulfide precipitation and by inference gold mineralization to 2563 84 ma with an initial 187os 188os 0 29 0 08 mswd 0 38 this age is distinctly younger than the post pongola granites 2863 2721 ma ruling out the association of granite emplacement with mineralization this would suggest that mineralization is linked to the regional d3 folding event which reactivated the ksz after emplacement of the post pongola granites and that final brittle post mineralization reactivation is related to karoo age faulting low initial os values suggest that ore fluid interacted with mafic rocks leaching non radiogenic os the likely source being the deeper seated nsuze group volcanics and or the greenstone belts that underlie the pongola supergroup

ion probe and fluid inclusion evidence for co seismic fluid infiltration in a crustal detachment we have investigated the geochemical pattern of fluid infiltration in the extensional detachment of tinos island cyclades greece ion microprobe o isotope analyses and fluid inclusion studies have been conducted in strain fringes developing around pyrite blasts in the mylonite of the shear zone micro scale traverses in quartz and calcite fibres show that 18o increases from 17 18 to 20 21 in 1 mm towards the blast drops of 3 in 200 m then rises again in the direction of growth 18o variations are interpreted as transient influxes of exotic fluids into the shear zone between periods of closed system buffering by the host rock fluid inclusions trapped in the fibres show fluctuating salinities 0 4 wt nacl eq and densities that reflect drops of the pore pressure from lithostatic 1 to hydrostatic 0 4 values during fringe growth isotopic and microthermometric data are consistent with models of seismic pumping developed for compressive shear zones we therefore suggest that co seismic pore pressure variations developed suction forces sufficient to drive large scale fluid migration in the tinos detachment as in convergent tectonic settings

hydrothermal alteration and mineralization in the faina greenstone belt evidence from the cascavel and sertao orogenic gold deposits abstract the faina greenstone belt is located in central brazil and contains the cascavel and sertao gold deposits 500 000 oz au total resource and past production the bulk of the gold is hosted in quartzite and biotite schist at cascavel and carbonaceous schist and banded iron formation at sertao the metamorphic grade of the host rocks is greenschist facies cascavel is a vein hosted gold deposit with orebodies mainly controlled by f2 fold hinges with minor mineralization in d3 shear zones hydrothermal alteration in quartzite is characterized by distal quartz k feldspar muscovite intermediate quartz fuchsitic mica biotite pyrite and proximal quartz white mica pyrite chalcopyrite the biotite schist is characterized by distal quartz chlorite calcite and proximal white mica biotite tourmaline siderite pyrite native gold is located within intensely deformed quartz veins and locally as free gold disseminated in proximal alteration zones sertao is a shear zone hosted gold deposit with orebodies controlled by d3 shear zones including gold bearing shear veins and surrounding hydrothermally altered wallrocks hydrothermal alteration is characterized by distal quartz ankerite chlorite pyrite chalcopyrite and proximal quartz white mica siderite pyrite arsenopyrite chalcopyrite gold is in equilibrium with arsenopyrite and within arsenopyrite disseminated in the proximal alteration zone chlorite geothermometry suggests temperatures of 335 407 c and 328 397 c for the gold bearing hydrothermal fluids in the sertao and cascavel deposits respectively arsenopyrite geothermometry on arsenopyrite within gold bearing veins in the sertao deposit yields temperatures between 310 and 480 c structures controlling hydrothermal alteration and gold mineralization in both deposits are developed during a regional d2 event and subsequently deformed during a d3 event hydrothermal alteration minerals are consistently replacing metamorphic minerals the alteration assemblages in both deposits are similar albeit their modal mineralogy is controlled by the host rock composition therefore both the cascavel and sertao deposits are interpreted as orogenic mesozonal gold systems comparable with other orogenic gold systems in brazil and worldwide

deep structure of an archaean greenstone terrane more than half of the continental crust was formed before about 2 500 million years ago1 yet there is still considerable uncertainty regarding the structure and evolution of the ancient archaean cratons2 7 modern analogues proposed for the setting of archaean greenstone belts include aborted intra cratonic rifts continental margins and various oceanic settings2 7 alternatively archaean tectonics may have been quite different from the plate tectonics we see today2 4 6 the abitibi belt in canada is the largest archaean greenstone ternane3 and has been a touchstone for many models because its surface geology is relatively well known8 10 it contains significant gold and base metal deposits10 and it is less deformed and has suffered less erosion than most archaean ter ranes here we present deep seismic reflection data from part of the abitibi belt which reveal extensive subhorizontal and gently dipping reflection zones truncated by steeply inclined discontinuities which project to major fault zones at the surface our preliminary model of the abitibi belt comprises a mostly juvenile magmatic crust similar to that exposed in the nearby kapuskasing uplift11 12 dissected and imbricated by deep penetrating discontinuities associated with thrust and strike slipe faulting in view of the similarity of greenstone belts worldwide2 7 13 our seismic sections provide new constraints on the general processes that have affected archaean metavolcanic terranes

structural evolution of the orogenic gold deposits in central victoria australia the role of regional stress change and the tectonic regime abstract this review synthesizes the main characteristics of the orogenic gold deposits in western and central victoria australia and presents new insights into the relationships between orogen scale deformation and the 12 hard rock gold deposits that have each produced 30 t au we examine five of these large deposits stawell ballarat bendigo fosterville and costerfield from the start of the delamerian orogeny at ca 515 ma to the end of the tabberabberan orogeny at ca 370 ma the eastern continental margin of gondwanan australia experienced both accretionary growth and the development of the lachlan orocline central western victoria lies on the western edge of this oroclinal structure and is partly underlain by a proterozoic continental fragment the selwyn block all of the 30 t orogenic gold deposits in the region lie either on or to the west of the selwyn block from the delamerian orogeny onwards the deformation front progressively moved eastwards and the principal stress direction rotated clockwise from ne sw through e w and ultimately to n s so that the initial faulting and folding created pathways and traps for the subsequent mineralizing fluids through the brittle to brittle ductile crust thus the 455 440 ma benambran orogeny was the major mineralizing event at stawell and resulted in some early mineralization in the bendigo zone the 420 410 ma bindian orogeny gave rise to significant additional mineralization at ballarat bendigo and fosterville while the 385 370 ma tabberabberan orogeny drove mineralization at costerfield and a second bonanza event at fosterville the largest gold deposits formed where the fluid pathways either remained open or reopened in this rotating stress field allowing multiple pulses of gold rich fluids

crack seal patterns records of uncorrelated stress release variations in crustal rocks statistical properties of crack seal veins are investigated with a view to assessing stress release fluctuations in crustal rocks crack seal patterns correspond to sets of successive parallel fractures that are assumed to have propagated by a subcritical crack mechanism in the presence of a reactive fluid they represent a time sequence record of an aseismic and anelastic process of rock deformation the statistical characteristics of several crack seal patterns containing several hundreds of successive cracks have been studied samples were collected in three different areas gold bearing quartz veins from abitibi in canada serpentine veins from the san andreas system in california and calcite veins from the apennine mountains in italy digitized pictures acquired from thin sections allow accurate measurement of crack seal growth increments all the samples show the same statistical behaviour regardless of their geological origin the crack seal statistical properties are described by an exponential distribution with a characteristic length scale and do not show any spatial correlation they differ from other fracture patterns such as earthquake data which exhibit power law correlations gutenberg richter relationship crack seal series represent a natural fossil record of stress release variations less than 50 bars in the crust that show a characteristic length scale associated with the resistance of rock to effective tension and no correlation in time

stable isotope geochemistry of the archean val d or canada orogenic gold vein field a systematic study of the auriferous quartz veins of the val d or vein field abitibi quebec canada demonstrates that the c o s isotope composition of silicate carbonate borate oxide tungstate and sulphide minerals have a range in composition comparable to that previously determined for the whole superior province the oxygen isotope composition of quartz from early quartz carbonate auriferous veins ranges from 9 4 to 14 4 whereas later quartz tourmaline carbonate veins have 18oquartz values ranging from 9 2 to 13 8 quartz carbonate veins have carbonate 18o 6 9 12 5 13c 6 2 1 9 and pyrite 34s 1 2 and 1 9 isotope compositions comparable to those of quartz tourmaline carbonate veins 18o 7 9 11 7 13c 8 0 2 4 34s 0 6 6 0 18oquartz values in quartz tourmaline carbonate veins have a variance comparable to analytical uncertainty at the scale of one locality irrespective of the type of structure the texture of the quartz or its position along strike across strike or down dip a vein in contrast the oxygen isotope composition of quartz in quartz tourmaline carbonate veins displays a regional distribution with higher 18o values in the south central part of the vein field near the cadillac tectonic zone and which 18o values decrease regularly towards the north another zone of high 18o values in the northeast corner of the region and along the trace of the senneville fault is separated by a valley of lower 18o values from the higher values near the cadillac tectonic zone oxygen isotope isopleths cut across lithological contacts and tectonic structures this regional pattern in quartz tourmaline carbonate veins is interpreted to be a product of reaction with country rocks and mixing between 1 a deep seated hydrothermal fluid of metamorphic origin with minimum 18o 8 5 13c 0 6 and 34s 0 4 and 2 a supracrustal fluid most likely archean seawater with a long history of water rock exchange and with maximum 18o 3 9 13 c 5 6 and 34s 5 0

fluid expulsion and dilatancy pumping during thrusting in the pyrenees pb and sr isotope evidence pb and sr isotope data have been collected from fluid inclusions trapped in syntectonic quartz veins associated with the gavarnie thrust in the central pyrenees and from lithologies in the footwall and hanging wall of the thrust these data suggest the occurrence of three distinct phases of fluid movement related to different periods in the structural evolution of the thrust zone during the first phase fluid interacting with the silurian graphitic slates in the immediate hanging wall of the gavarnie thrust moved along a carbonate mylonite zone beneath the thrust enriching the deforming carbonates in 87 sr and 207 pb the second phase of fluid movement occurred during imbricate thrusting and intense veining beneath the gavarnie thrust isotope data from quartz hosted fluid inclusions show a trend toward high 208 pb 204 pb ratios with increasing 207 pb 204 pb and 206 pb 204 pb and decreasing 87 sr 86 sr the inclusions contain fluids equilibrated with several source lithologies deformed cretaceous and silurian rocks within the thrust zone devonian phyllites located above the silurian slates in the thrust hanging wall and triassic redbeds located beneath cretaceous limestones in the thrust footwall the mixture of fluids reflects both the transient chemical interaction of previously isolated rock types in the footwall and hanging wall via fault and fracture networks and dilatancy pumping of fluid into opening veins the last phase of fluid movement resulted in the precipitation of late carbonate vein fills and a return to more local fluid movement as stresses relaxed and fracture networks healed overpressured fluids trapped within and partially expelled from the thrust zone during phase 1 were still present during phases 2 and 3 this suggests that average permeabilities within the mylonites were low and that transient high permeability zones related to fracture networks were of limited extent

fluid characteristics of vein and altered wall rock in archean mesothermal gold deposits certain trends in alteration are commonly observed near archean mesothermal gold deposits this study was designed to monitor the changes in composition of fluid inclusions using solid probe mass spectrometry by 1 establishing and verifying systematic changes in the composition of the fluid during fluid rock interaction 2 characterizing the composition and behavior of fluids in the vein and the wall rock in order to establish a link between them 3 elucidating new insights on the role of fluid immiscibility in gold deposition and 4 scrutinizing the results for potential use in exploration three representative sites of gold mineralization were chosen a large deposit with appreciable amounts of free gold sigma mine in val d9or a second deposit with a smaller amount of free gold norbeau mine in chibougamau and a third area of gold mineralization with altered wall rock and little veining tadd prospect in chibougamau fluid inclusions were carefully selected to sample different alteration zones and the gold mineralization at each site in most of the cases varying degrees of unmixing of co 2 h 2 o were noted mass spectra results show that during alteration all generations of inclusions at all sites contained co 2 and h 2 o for the alteration hosted deposit a decrease in x sub co 2 could be mapped outward from the mineralized zone the most striking result was that fluid inclusions associated with the main mineralizing episode at sigma showed a nearly complete segregation into co 2 and h 2 o end members this can be interpreted as the result of extreme fluid pressure fluctuations this characteristic was absent from the norbeau veins where free gold exists in very limited amounts a test on a high grade ore shoot from the doyon mine resulted in a pattern similar to sigma we suggest extreme pressure fluctuations were limited in vein hosted deposits like norbeau and gold was precipitated essentially from fluid rock mineral interaction unmixing was the most likely cause of free gold deposition mineralizing systems may have undergone major fracturing events at time of free gold deposition this study suggests high co 2 h 2 o ratios are related to gold mineralization in altered rock hosted deposits but that the ratio may be an unreliable guide in vein hosted deposits because of the variable unmixing pattern of co 2 h 2 o fluids decrepitometry may prove a useful and rapid method for identifying co 2 h 2 o unmixing and bonanza gold potential

structural analysis of faults related to a heterogeneous stress history reconstruction of a dismembered gold deposit stawell western lachlan fold belt australia in the internationally significant victorian goldfields a complex system of faults dismembers the 5 million ounce magdala gold deposit these faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio the fault geometry is strongly controlled by the pre existing rheology faults have propagated around the flanks of an antiformal basalt dome along earlier ductile cleavages and the margins of porphyry dykes many of the faults do not have andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions much of the faulting is associated with the emplacement of porphyry dykes additional gold mineralisation related to plutonism and late stage deformation post dating the intrusion of the stawell pluton systematic mapping of extension veins associated with faults striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation this successfully predicted the offset direction of the currently mined magdala ore body beneath the studied system of faults the use of extension veins was a critical aspect of the analysis if striations on the fault surfaces had solely been used the offset direction of the new golden gift orebody would not have been correctly ascertained the palaeostress history was delineated via use of compression and tension dihedra stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped the calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing 1 and 3

structural controls on granitoid hosted gold mineralization and paleostress history of the edikan gold deposits kumasi basin southwestern ghana the 9 moz total aggregate gold endowment at the edikan mine kumasi basin ghana is contained within a cluster of orogenic gold deposits located along the akropong fault zone the granitoid hosted orebodies at edikan e g ag2 ag3 fobinso esuajah essentially an interconnected mesh of gold bearing quartz veins formed during deformation event d3edk which postdates the penetrative regional d2edk deformation the gold bearing quartz veins developed in and adjacent to n s and nw se trending low angle thrust faults that crosscut lithological contacts and earlier formed steeply dipping d2edk faults our paleostress analysis shows that the d3edk deformation during which the mineralized fault system developed was characterized by a wnw ese hybrid compression that evolved to a strike slip regime this progressive deformation is best described with the following stress regimes wnw ese transpression pure compression t1 associated with low angle thrusting subsequent transpression strike slip t2 and later strike slip transtension t3 associated with steeply dipping strike slip faulting the bulk of the granitoid hosted gold mineralization at edikan is associated with two principal sets of gold bearing quartz veins including low angle fault fill veins controlled by thrusts and shallow dipping oblique extension veins that developed during t1 the activation of the reverse and sinistral strike slip faults led to the development of restraining jogs characterized by abundant shallow and steeply dipping gold quartz veins with moderately ne plunging ore shoots the geometry of the mineralized fault fracture meshes is consistent with fault valve behavior in a horizontal compressive stress regime under sustained conditions of supralithostatic fluid pressures at low differential stress

hydrothermal transport and depositional processes in archean lode gold systems a review abstract compelling evidence now exists for the formation of archean lode gold deposits over a substantial range in metamorphic conditions and crustal depth the commonality of ore fluid isotopic and geochemical characteristics the extreme fluid rock ratios within actual ore zones and the observed depth continuity within individual orebodies all suggest that these deposits formed within vertically extensive crustal scale hydrothermal systems calculations based on new thermodynamic data and estimated ore fluid conditions suggest that gold would have been transported as au hs 2 over most of this range gold transport in the ore fluids that formed some diopside and diopside k feldspar class deposits t 550 c would have been as aucl2 whereas the au hs complex may have been significant in some of the lower temperature 270 c less alkaline ore fluids the high solubility of gold in lode gold ore fluids at amphibolite facies conditions 10 s to 1000 s ppb suggests that fluids may have been undersaturated in gold within their source regions promoting the ore fluid s capacity to retain gold in solution over the large transport distances involved ore depositional mechanisms were also likely to vary between crustal levels fluid rock interactions and wallrock sulphidation in particular appear to be the most important type of precipitation mechanism however with decreasing depth of formation the scope of other depositional mechanisms notably phase separation and fluid mixing becomes greater

crustal scale shear zones and their significance to archaean gold mineralization in western australia many large archaean epigenetic gold deposits show a broad spatial relationship to regional lineaments in greenstone belts although in detail they are sited in subsidiary brittle ductile fault structures fluids originating from a deep source follow a complex path and re equilibrate with different lithologies and with metamorphic fluid during migration to higher crustal levels temperature and pressure conditions at or below the amphibolite greenschist metamorphic boundary where most gold deposits are located favour the establishment of brittle ductile and brittle subsidiary structures the preferred structural setting of gold deposits physical gradients between the regional ductile structures and more brittle subsidiary structures ensure transient strongly localized fluid flow into the latter where lower temperatures and suitable host rocks with high fe fe mg ratios favour gold deposition the multi source origin and continuous re equilibration of the fluid with crustal rock which includes granitoid and greenstone belt lithologies of different ages is reflected in the diverse isotopic and geochemical signature of the gold deposits

an integrated approach to the search for gold in a mature data rich brownfields environment a case study from sigma lamaque quebec abstract the greater than 1300 participants in the 2015 integra gold rush challenge the biggest gold exploration crowdsourcing competition to date were given access to six terabytes of historic data collected at the c 12 moz au sigma lamaque gold deposits val d or district quebec and asked to identify targets with the highest potential for the next big gold discovery at the property this paper describes our approach to exploration targeting at sigma lamaque with particular emphasis on i integrating and interrogating the available data ii identifying mappable expressions key ingredients of ore forming processes using spatial data modelling in particular inversion modelling and advanced filtering of the available geophysical data iii developing a conceptual model in the framework of a mineral systems approach and iv translating the conceptual model into an effective tool for targeting gold in the val d or district

structure and origin of the gold mineralization in the nacimiento block the los burros deposits central california abstract the los burros gold deposit is located within the franciscan complex of the nacimiento block california although this deposit was mined during the late 19th and early 20th centuries its origin and its relationship with the regional metamorphism and deformation are unclear in order to better understand the mode of formation of the deposit and its possible link with a post subduction thermal anomaly we performed a detailed geological and structural mapping within the los burros district coupled with a u th he thermochronological study on apatite and zircon fluid inclusion and stable isotope o and c analyses of the ore deposit provide constraints on the temperature and potential source of the fluid s responsible for the los burros gold formation our results demonstrate that the los burros au deposit formed from a metamorphic aqueous co2 ch4 fluid at temperatures of 300 c results from raman spectroscopy of carbonaceous materials confirm the presence of a thermal anomaly in the range 260 320 c near the los burros mining district as previously inferred by vitrinite reflectance techniques our structural and preliminary thermochronological analyses show that the los burros au deposit consists of a series of meter wide quartz calcite sulfide en echelon veins formed during a late transpressive event related to san andreas fault activity and the subduction of a slab window during the past 30 ma our new data suggest that the thermal anomaly recorded in the vicinity of the los burros mining district is not associated with a magmatic derived hydrothermal system such as described within the epithermal mclaughlin deposit in northern california but rather with a late transpressive deformation event that uplifted deeply buried metamorphic rocks possibly connected to a local metamorphic overprint

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

p t x conditions of hydrothermal fluids and precipitation mechanism of stibnite gold mineralization at the wiluna lode gold deposits western australia conventional and infrared microthermometric constraints the wiluna lode gold deposits are located in the archean wiluna greenstone belt in the northern sector of the norseman wiluna belt in the yilgarn craton of western australia they are hosted in subgreenschist facies meta basalts and controlled by the wiluna strike slip fault system and associated shear veins and breccias the 13 individual lode gold deposits have produced around 115 t au from 1901 to 1946 and 1986 to today historically they also produced 38 3 t as and 3 5 t sb gold formed in two stages stage 1 gold pyrite arsenopyrite is finely disseminated in the wallrock and breccia fragments whereas stage 2 gold stibnite is located in massive shear veins and breccia matrix as fracture fill and in banded colloform textured veins stibnite gold orebodies only occur in some of the deposits e g moonlight and northern part of the west lode and also display a restricted vertical extent being preserved only in the uppermost 200 m of stibnite bearing lodes petrographic conventional and infrared microthermometric and laser raman analysis on stibnite bearing quartz veins and breccias reveal that the antimony and gold rich hydrothermal fluid was of mixed h2o nacl co2 ch4 type microthermometric measurements reveal maximum homogenization temperatures of 340 c average 290 25 c and a wide range of salinities between 0 2 and 23 eq wt nacl aqueous carbonic fluid inclusions contain variable xco2 ch4 0 03 to 0 82 with the carbonic phase containing a maximum xch4 of 0 21 combined petrographic and microthermometric evidence suggests that the fluid inclusion properties reflect fluid immiscibility of a low salinity medium xco2 ch4 homogeneous parent fluid at about 290 c and pressures between 700 and 1 700 bar fluid immiscibility was triggered by cyclic pressure release during fault zone movement the decompression adiabatic cooling of the hydrothermal fluids shifted the ore fluid to lower temperatures significantly reduced the degree of stibnite undersaturation and caused stibnite to precipitate the deposition of stibnite reduced the ore fluid h2s concentration thereby destabilized gold bisulfide complexes in solution and caused gold precipitation locally this mechanism explains the intimate spatial association of stibnite and gold in quartz veins and breccias in the stibnite gold orebodies at wiluna

the bronzewing lode gold deposit western australia p t x evidence for fluid immiscibility caused by cyclic decompression in gold bearing quartz veins abstract the bronzewing lode gold deposit is located in the yandal greenstone belt in the yilgarn craton of western australia gold mineralization is hosted in tholeiitic basalt that is metamorphosed to greenschist facies individual ore bodies are controlled by a complex gold bearing quartz vein system that comprises shear and extension veins formed during a progressive d2 deformation event gold is localized in quartz along fractures and deformed grain boundaries and is interpreted to have formed late in the formation of the veins detailed petrography and microthermometry on primary pseudosecondary and secondary fluid inclusions trapped in gold bearing shear and extension veins revealed five types of fluid inclusions 1 co2 ch4 h2o nacl inclusions of variable salinity 0 1 to 17 5 eq wt nacl containing between 10 and 99 mol co2 and molar volumes that range from 22 to 76 cm3 2 h2o nacl inclusions of variable salinity 0 4 to 22 1 eq wt nacl 3 co2 ch4 inclusions with up to 58 mol ch4 and molar volumes between 54 and 73 cm3 4 ch4 h2o inclusions with ch4 ranging from 80 to 90 mol and 5 ch4 inclusions with low molar volumes of 19 to 23 cm3 types 1 2 and 3 constitute a fluid inclusion assemblage that occurs consistently in primary pseudosecondary and secondary trails and clusters within the gold bearing quartz vein system these co existing fluids are interpreted to have formed by fluid immiscibility of a low salinity homogeneous parent fluid at about 300 c and 1500 bars locally type 2 fluid inclusions exhibit total homogenization temperatures that are on average 100 c less than the co genetically trapped type 1 aqueous carbonic inclusions this discrepancy is interpreted to have involved co2 effervescence in response to fluid pressure fluctuations types 4 and 5 fluid inclusions are rare and are only present locally in secondary trails and clusters in extension veins significant pressure fluctuations but relatively constant homogenization temperatures for types 1 2 and 3 fluid inclusions suggest that cyclic decompression of the hydrothermal fluids due to seismic activity along the shear zones that host the gold bearing veins triggered fluid immiscibility the process of fluid immiscibility and subsequent lowering of gold solubility is interpreted to be the most efficient precipitation mechanism for gold in the d2 shear zone hosted vein system at bronzewing

bicarbonate rich fluid inclusions and hydrogen diffusion in quartz from the libcice orogenic gold deposit bohemian massif abstract unusual paleofluid composition is reported for the libcice orogenic type gold deposit located in the central bohemian plutonic complex cbpc czech republic unexpected bicarbonate rich fluids and their complex chemistry variations characterize primary fluid inclusions from the main gold bearing quartz vein a detailed microthermometry laser raman micro spectroscopy lrms and sem cathodoluminescence study was used to understand the complex chemistry and p t x history the results indicate the presence of h 2 o and h 2 o co 2 ch 4 n 2 h 2 s fluids the latter displaying variations of the co 2 ch 4 ratio in the gaseous phase from 6 8 to 0 06 variation of the ch 4 content across single grains and between different levels of the mine was recorded the presence of nahcolite h 2 up to 6 mol and ethane 0 0 2 mol in the fluids were also discovered by lrms potential models for the formation of different types of fluids present in the deposit are discussed including the genesis of hco 3 rich fluids as well as h 2 and c 2 h 6 presence in the primary fluid inclusions the potential influence of organic matter bearing sediments as well as the impact of the intrusion of cbpc re equilibration and or re speciation of fluid inclusions during the post entrapment history is considered to have the main impact on the complex paleofluid chemistry based on the thermodynamic modeling h 2 diffusion into the fluid inclusions was shown to be the main reason for the ch 4 variation on the scale of a single grain as well as across the whole vein although the exact processes of production formation of hco 3 and h 2 at the libcice deposit remain open to discussion reactions in the c o h system are considered to be a possible formation mechanism this work also contributes to our understanding of the importance of post entrapment modifications and reactions in the c o h system on interpretation deciphering the processes in orogenic type deposits

formation of archean batholith hosted gold veins at the lac herbin deposit val d or district canada mineralogical and fluid inclusion constraints the lac herbin deposit consists of a network of mineralized parallel steep reverse faults within the synvolcanic bourlamaque granodiorite batholith at val d or in the archean abitibi greenstone belt there are two related quartz tourmaline carbonate fault fill vein sets in the faults which consist of subvertical fault fill veins associated with subhorizontal veins the paragenetic sequence is characterized by a main vein filling ore stage including quartz tourmaline carbonate and pyrite hosted gold chalcopyrite tellurides pyrrhotite and cubanite inclusions most of the gold is located in fractures in deformed pyrite and quartz in equilibrium with chalcopyrite and carbonates with local pyrrhotite sphalerite galena cobaltite pyrite or tellurides petrography and microthermometry on quartz from the main vein filling ore stage reveal the presence of three unrelated fluid inclusion types 1 gold bearing aqueous carbonic inclusions arranged in three dimensional intragranular clusters in quartz crystals responsible for the main vein filling stage 2 barren high temperature aqueous moderately saline inclusions observed in healed fractures postdating the aqueous carbonic inclusions and considered as a remobilizing agent of earlier precipitated gold in late fractures and 3 barren low temperature aqueous high saline inclusions in healed fractures similar to the crustal brines reported throughout the canadian shield and considered to be unrelated to the gold mineralization at the lac herbin deposit the aqueous carbonic inclusions are interpreted to have formed first and to represent the gold bearing fluid which were generated contemporaneous with regional greenschist facies metamorphism in contrast the high temperature aqueous fluid dissolved gold from the main vein filling ore stage transported and reprecipitated it in late fractures during a subsequent local thermal event

genesis of the auriferous c quartz tourmaline vein of the siscoe mine val d or district abitibi subprovince canada structural mineralogical and fluid inclusion constraints the c quartz tourmaline vein of the siscoe deposit is a classic example of a high grade archean gold lode and averages 45 g t au locally attaining 221 g t au this vein cuts all dikes the main regional foliation s2 and several auriferous quartz carbonate veins and is interpreted to be a shear vein which formed during the development of a late to post d2 reverse fault the vein is composed of alternating tourmaline and quartz rich layers with minor amounts of rutile scheelite apatite chlorite muscovite carbonate mainly calcite pyrite chalcopyrite tetradymite and native gold vein filling occurred during the development of a reverse fault through a process involving repeated episodes of opening mineral deposition and plastic and brittle deformation the vein geometry was controlled by local rheologic heterogeneities or by a west northwest east southeast compressive event the wall rock alteration is characterized by the occurrence of tourmaline pyrite and calcite the timing of gold deposition and the gold fineness in the c quartz tourmaline vein differ from those of the wall rock in the vein native gold 4 7 7 3 wt ag was the latest phase to deposit and commonly occurs in fractures that cut across the quartz and tourmaline layers whereas in the wall rock gold 7 9 13 2 wt ag started to precipitate just before or synchronously with pyrite consequently the textural and mineral chemical evidence suggests that the fluids in the vein and wall rock were not in equilibrium with each other during gold deposition and that the mechanisms of precious metal precipitation were different in the two environments in the wall rock gold started to precipitate synchronously with sulfides due to reaction of hydrothermal fluids with magmatic ilmenite this reaction caused the destabilization of gold bisulfide complexes and prompted gold deposition however sulfides are very rare in the c quartz tourmaline vein and gold deposited after the main episode of vein filling suggesting that its precipitation was not controlled by pyrite deposition the occurrence of aqueous carbonic ac fluid inclusions in healed fractures that radiate from gold grains suggests that these fluids were associated with the ore forming process two types of aqueous carbonic fluid inclusions were identified h2o nacl co2 bearing salinities between 4 and 7 wt nacl equiv and h2o nacl co2 ch4 bearing salinities varying between 7 and 9 wt nacl equiv both types homogenize at t 190 c based on structural textural and fluid inclusion evidence we propose that gold deposition in the c quartz tourmaline vein took place after the main episode of vein filling as a result of continued fracturing and mixing of these two fluids

the evolution of the southern abitibi greenstone belt in light of precise u pb geochronology this paper summarizes u pb zircon ages for the archean southern abitibi greenstone belt and integrates them together with zircon and baddeleyite data for three new samples into a general framework for the evolution of the region new ages are reported for the ghost range gabbroic complex 2713 super 7 sub 5 ma and for a pyroclastic volcanic rock of the gauthier group 2700 or 3 ma a previously determined age of 2701 super 3 sub 2 ma for the felsic volcanic skead group is confirmed by magmatic zircons from a second sample that also contains a xenocrystic component or 2720 ma indicating that the skead volcano was built on top of an older volcanic sequence the compiled ages show that the southern abitibi greenstone belt developed between 2750 and 2670 ma the earliest ages date remnants of volcanic complexes that occur at the periphery of the belt major preorogenic magmatism was confined mainly to the period 2720 to 2700 ma in general each episode was characterized by the essentially coeval formation of komatiitic tholeiitic and calc alkalic igneous rocks taking into consideration the known geologic geochemical and isotopic data this association is consistent with evolution in paired active are and back are systems older crust was absent from the region but it may have interacted with these arcs and rift systems farther to the west the extensional regime was followed by a period of compression that caused folding and thrusting emplacement of calc alkalic plutons and deposition of turbidites between 2700 and 2688 ma the magmatism became predominantly alkalic during the main timiskaming period between 2681 and 2676 ma this event was accompanied by the deposition of alluvial fluvial sequences and was followed by renewed compression that caused polyphase folding and thrusting probably between approximately 2676 and 2670 ma lamprophyre dikes were associated with these final stages of deformation postorogenic activity includes faulting and episodes of hydrothermal activity locally associated with gold mineralization ages in the range 2630 to 2580 ma for hydrothermal minerals e g titanite and rutile camflo suggest a correlation of these events with lower crustal magmatism and metamorphism as recorded for example in the adjacent kapuskasing zone

fluid mixing and recycling during pyrenean thrusting evidence from fluid inclusion halogen ratios abstract syntectonic fluids have been sampled through fluid inclusion microthermometry and crush leach analyses cations and halogens from a 50 km n s transect through the central southern pyrenees the fluid inclusions are contained in syntectonic quartz veins in triassic redbeds cretaceous carbonates and hercynian basement rocks with some calcite and dolomite data from limestones and evaporites in more external parts of the belt the main datasets come from 1 alpine shear zones cutting the neouvielle granodiorite in the hercynian axial zone at the north end of the transect 2 an imbricate zone beneath the alpine gavarnie thrust at the pic de port vieux 3 several localities in the footwall and hangingwall of the gavarnie thrust on the southern margin of the axial zone the inclusion fluids generally decrease in salinity from 27 35 at the northern end of the transect to 7 22 on the southern margin of the axial zone the majority of the inclusions have cl br ratios lower than seawater and are interpreted as relict fluids after seawater evaporation and halite precipitation in the upper trias this interpretation is supported by cl br na systematics which are consistent with a change from halite to halite sylvite precipitation with progressive evaporation fluids in the basement shear zones are interpreted to have essentially the same evaporitic origin as those still contained in sedimentary formations although it is possible that final concentration of brines in the neouvielle massif involved retrograde hydration reactions with removal of water by precipitation of hydrous minerals the fluids are also very similar in salinity and halogen chemistry to those found in veins associated with mesozoic pb zn f deposits which predate the thrusting the lower salinities seen at the southern margin of the axial zone are interpreted to reflect mixing of the brines with a higher level fluid connate or meteoric water circulating within the mesozoic carbonates of the higher thrust sheets at one locality where triassic evaporites are still present high cl br ratios at relatively low salinities are present in inclusions within the underlying triassic redbeds but low cl br ratios at higher salinities are seen lower in the sequence this is consistent with dissolution of halite by a dilute fluid but with limited penetration downwards we suggest that the fluid history of the pyrenees evolved through a series of stages 1 upper triassic evaporite formation with sinking of brines into underlying redbeds and fractured basement rocks 2 circulation of brines with formation of pb zn deposits along faults at some time between the triassic and the upper cretaceous 3 renewed extension with erosion of triassic rocks in many areas and further drawing down of triassic brines into the basement 4 deposition of u cretaceous and palaeocene carbonates containing connate waters of marine origin 5 formation of the pyrenean thrust belt with overpressuring and expulsion of the brines along shear zones and faults 6 creation of topography with a high level circulation system in the mesozoic thrust sheets driven largely by topography at the southern margin of the axial zone there was limited mixing of the deeper overpressured brines with these more dilute hydrostatically pressured fluids an important point is that because of their density hypersaline brines are difficult to expel from the upper crust and may be involved in a succession of alteration and mineralisation events in the same general area over hundreds of millions of years

fluid inclusions tectonic indicators abstract during the first half of the 20th century fluid inclusions have been studied in ore geology in order to determine the chemistry and physical parameters density of the mineralising fluids apart from a few pioneering works in the middle of the century fluid inclusions were not used as structural markers before the late 1980s during the same time experiments as well as observations of natural samples have shown that the volume shape and density of fluid inclusions may change when submitted to pressure temperature conditions different from their trapping isochores in view of these experiments and based on natural examples from the higher himalaya metamorphic rocks it is evident how fluid inclusions can be used as thermobarometric structural and tectonic markers points of interest for further studies on fluid inclusions in nature and experiments are also stressed

geothermal mineralized scales in the pipe system of the geothermal piancastagnaio power plant mt amiata geothermal area a key to understand the stibnite cinnabarite and gold mineralization of tuscany central italy the co2 rich geothermal fluids produced in the piancastagnaio geothermal field mt amiata geothermal area southern tuscany italy show temperatures up to 360 c and pressures of about 200 bar at depths of around 3 500 m giolito ph d thesis universita degli studi di firenze italy pp 1 147 2005 caco3 and or sio2 dominated scales are deposited in the pipes leading to the pressure and atmospheric separators of the geothermal wells high content of metastibnite and or stibnite in both calcite and silica scales and sb contents of up to 50 mg l in the fluids indicate their mineralising potential the red or black colours of the scales depend on the predominance of red metastibnite or black stibnite respectively in our condensation experiments as well as during deposition of the scales metastibnite is the first sb2s3 mineral to form in a second stage metastibnite is transformed to stibnite during depressurization the hg content of geothermal fluids partitions preferentially into the gas phase whereas sb and as remain in the liquid phase this separation explains the often observed areal separation of hg and sb mineralization the multistage deposition of sb in the mining district of tuscany is due to a periodic restoration of the permeability of the ore bearing faults by microseismic events and subsequent host rock brecciation the still ongoing microseismic events are induced by the accumulation of high pressure co2 rich fluids along faults followed by mechanical failure of the faults

evidence of upgrading of gold tenor in an orogenic quartz carbonate vein system by late magmatic hydrothermal fluids at the madrid deposit hope bay greenstone belt nunavut canada abstract evidence of secondary gold enrichment due to the addition of new gold into an earlier orogenic quartz carbonate vein deposit by magmatic hydrothermal fluids is strongly suggested for the madrid deposit hosted in the hope bay greenstone belt in nunavut canada the conclusion is based on an extensive in situ microanalytical protocol sem confocal raman microspectroscopy microthermometry decrepitate mound analysis la icp ms cathodoluminescence sims not previously applied to gold systems this approach was used to characterize the mineralogy and fluid inclusion systematics associated with the upgrading event mineralization comprised of only ag bearing gold electrum 89 8 at au avg n 8 is present throughout all investigated laminated and brecciated orogenic quartz veins however in high grade vein intersections where gold grades are locally elevated up to 122 g t assemblages containing tennantite tetrahedrite chalcopyrite electrum 80 7 at au avg n 15 ag pb au tellurides occur that are texturally late stage relative to electrum only mineralization subdomains of quartz coeval with this later assemblage are optically and texturally distinct from earlier orogenic quartz this late mineral assemblage is absent in all low grade vein intersections 1 g t au avg examined where only electrum is identified quartz hosted fluid inclusions h2o nacl co2 of intermediate salinity 16 7 1 2 wt nacl equiv n 93 were identified only in the high grade vein samples and are present along healed planes associated with tennantite tetrahedrite chalcopyrite electrum ag pb au telluride assemblages in situ sims 18o analyses of quartz combined with temperature constraints from mineral equilibria show that early orogenic vein quartz and late quartz subdomains associated with gold upgrading precipitated from fluids with similar 18oh2o values of 4 5 12 4 n 10 and 5 5 to 11 8 n 13 respectively isotopic data suggests that meteoric water was a negligible component in fluids responsible for gold precipitation microthermometry and raman spectroscopy show that upgrading fluids were distinct in composition compared to the earlier metamorphic fluids h2o nacl co2 ch4 n2 4 6 1 6 wt nacl equiv n 33 and later canadian shield basement brines h2o nacl 22 4 1 2 wt nacl equiv n 12 which have also been identified in the fluid inclusion record at madrid laser ablation icp ms analyses indicate the gold upgrading fluids are enriched in as sb zn pb and lile cs ba rb sr these data are consistent with late fluids derived from an evolved magmatic hydrothermal system trace element mapping of pyrite coupled with principle component analysis of the data confirms a strong correlation between au and ag te sb bi w as in upgraded veins whereas only au and as strongly correlate in low grade veins the study suggests that gold upgrading either involving newly introduced gold or remobilization of existing gold can be linked to the incursion of a late magmatic hydrothermal fluid that post dated formation of the main orogenic type auriferous quartz vein system the geological setting and mineral chemical features suggest an intrusion related i e porphyry or intermediate sulfidation epithermal mineralization style for the later event this work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high grade gold deposits in orogenic settings

influence of structural setting on sulphur isotopes in archean orogenic gold deposits eastern goldfields province yilgarn western australia the published mean 34s values of ore related pyrites from orogenic gold deposits of the eastern goldfields province yilgarn craton lie between 4 and 4 as for orogenic gold deposits worldwide most deposits have positive means and a restricted range of 34s values but some have negative means and wider ranges of 34s values wall rock carbonation and back mixing of similar source fluids with different fluid pathways can explain some of the more negative 34s signatures however structural setting appears to be the most important factor controlling ore fluid oxidation state and hence the distribution of 34s values in gold related pyrites shear hosted deposits appear to have experienced fluid dominated processes such as phase separation whereas stockwork vein hosted or disseminated deposits formed under conditions of greater rock buffering at victory defiance in particular negative 34s values are more common in gently dipping dilational structures compared to more compressional steeply dipping structures it appears most likely that fluid pressure fluctuations during fault valve cycles establish different fluid flow regimes in structures with different orientations rapid fluid pressure fluctuations in dilational structures during seismic activity can cause partitioning of reduced gas phases from the ore fluid during extreme phase separation and hence are an effective method of ore fluid oxidation leading to large local fluctuations in oxidation state it is thus not necessary to invoke mixing with oxidised magmatic fluids to explain 34s signatures indicative of oxidation in any case available robust geochronology in the eastern goldfields province does not support the direct involvement of oxidised magmatic fluids from adjacent granitic intrusions in orogenic gold genesis thus negative mean 34s values and large variations in 34s values of ore related pyrites in world class orogenic gold deposits are interpreted to result from multiple mechanisms of gold precipitation from a single ubiquitous ore fluid in varying structural settings rather than from the involvement of oxidised ore fluids from a different source such signatures are indicative but not diagnostic of anomalously large orogenic gold systems

fluid regimes fault valve behavior and formation of gold quartz veins the morro do ouro mine ribeira belt brazil abstract fluid inclusion studies petrographic analyses and structural analyses for neoproterozoic auriferous veins from the morro do ouro mine ribeira belt southeastern brazil are presented the deposit is classified as an orogenic gold deposit the vein type au mineralization at the mine is restricted to structurally controlled domains in a low grade calymmian ca 1500 1450 ma metasedimentary sequence that also hosts syngenetic au mineralization two main auriferous quartz vein systems are present i nw trending subvertical extensional veins and ii ne trending subvertical veins parallel to the axial surface of tight folds the mineralized veins are adjacent to a major dextral transcurrent fault zone and their structural relationships indicate that this fault is severely misoriented for frictional reactivation the nw trending veins present a fluid inclusion assemblage dominated by co 2 n 2 ch 4 inclusions h 2 o co 2 n 2 ch 4 nacl cacl 2 inclusions and h 2 o nacl cacl 2 inclusions both with highly variable salinities 0 5 to 52 wt nacl equivalent while the ne trending veins are dominated by h 2 o co 2 n 2 ch 4 nacl cacl 2 and h 2 o nacl cacl 2 inclusions also of highly variable salinities 1 to 45 wt nacl equivalent the entrapment of fluids occurred at temperatures between 225 and 240 c in the nw trending veins and at approximately 208 c in the ne trending veins involving processes of fluid immiscibility and mixing between fluids of contrasting compositions the nw trending extensional veins were formed predominantly under pressure fluctuating between near lithostatic and strongly supralithostatic conditions 100 250 mpa during pre seismic failure stages the ne trending veins precipitated predominantly under subhydrostatic to lithostatic pressure conditions 10 150 mpa following the discharge of fluids from different reservoirs along the ruptured zone after earthquake rupture stages the hydrothermal fluids were probably enriched with gold through interactions with the host rocks and its precipitation is attributed to changes in physicochemical properties due to fluid immiscibility and mixing between fluids of highly contrasting salinities as a consequence of cyclic fluctuations in the values of fluid pressure and tectonic stresses accompanying episodes of seismogenic fault activity

p t conditions of mineralization in the jonnagiri granitoid hosted gold deposit eastern dharwar craton southern india constraints from fluid inclusions and chlorite thermometry abstract gold mineralization at jonnagiri dharwar craton southern india is hosted in laminated quartz veins within sheared granodiorite that occur with other rock units typical of archean greenstone granite ensembles the proximal alteration assemblage comprises of muscovite plagioclase and chlorite with minor biotite and carbonate which is distinctive of low to mid greenschist facies the laminated quartz veins that constitute the inner alteration zone contain muscovite chlorite albite and calcite using various calibrations chlorite compositions in the inner and proximal zones yielded comparable temperature ranges of 263 to 323 c and 268 to 324 c respectively gold occurs in the laminated quartz veins both as free milling native metal and enclosed within sulfides fluid inclusion microthermometry and raman spectroscopy in quartz veins within the sheared granodiorite in the proximal zone and laminated auriferous quartz veins in inner zone reveal the existence of a metamorphogenic aqueous gaseous h2o co2 ch4 salt fluid that underwent phase separation and gave rise to gaseous co2 ch4 low saline 5 wt nacl equiv aqueous fluids quartz veins within the mylonitized granodiorites and the laminated veins show broad similarity in fluid compositions and p t regime although the estimated p t range 1 39 to 2 57 kbar at 263 to 323 c compare well with the published p t values of other orogenic gold deposits in general considerable pressure fluctuation characterize gold mineralization at jonnagiri factors such as fluid phase separation and fluid rock interaction along with a decrease in f o2 were collectively responsible for gold precipitation from an initial low saline metamorphogenic fluid comparison of the jonnagiri ore fluid with other lode gold deposits in the dharwar craton and major granitoid hosted gold deposits in australia and canada confirms that fluids of low saline aqueous carbonic composition with metamorphic parentage played the most dominant role in the formation of the archean lode gold systems

textural evidence for recent co seismic circulation of fluids in the nojima fault zone awaji island japan abstract the hirabayashi borehole awaji island japan was drilled by the geological survey of japan gsj 1 year after the hyogo ken nanbu kobe earthquake 1995 mjma 7 2 this has enabled scientists to study the complete sequence of deformation across the active nojima fault from undeformed granodiorite to the fault core in the fault core different types of gouge and fractures have been observed and can be interpreted in terms of a complex history of faulting and fluid circulation above the fault core and within the hanging wall compacted cataclasites and gouge are cut by fractures which show high apparent porosity and are filled by 5 50 m euhedral and zoned siderite and ankerite crystals these carbonate filled fractures have been observed within a 5 5 m wide zone above the fault but are especially abundant in the vicinity 1 m of the fault the log normal crystal size distributions of the siderite and ankerite suggest that they originated by decaying rate nucleation accompanied by surface controlled growth in a fluid saturated with respect to these carbonates these carbonate filled fractures are interpreted as the result of co seismic hydraulic fracturing and upward circulation of fluids in the hanging wall of the fault with the fast nucleation of carbonates attributed to a sudden fluid or co2 partial pressure drop due to fracturing the fractures cut almost all visible structures at a thin section scale although in some places the original idiomorphic shape of carbonates is modified by a pressure solution mechanism or the carbonate filled fractures are cut and brecciated by very thin gouge zones these features are attributed to low and high strain rate mechanisms respectively the composition of the present day groundwater is at near equilibrium or slightly oversaturated with respect to the siderite calcite dolomite and rhodochrosite taken together this suggests that these fractures formed very late in the evolution of the fault zone and may be induced by co seismic hydraulic fracturing and circulation of a fluid with a similar composition to the present day groundwater they are therefore potentially related to recent earthquake activity

single zircon age constraints on the tectonic juxtaposition of the archean abitibi greenstone belt and pontiac subprovince quebec canada abstract zircons from metasediments and granitoids in the high grade lacorne block within the low grade archean abitibi greenstone belt have been dated by the single zircon pb evaporation technique yielding 207pb 206pb minimum ages detrital zircons in the mature clastic metasediments of the lacorne block display a range of ages from 2691 8 ma to 3042 6 ma the younger zircon ages 2691 8 2695 4 2715 16 and 2719 4 ma thus impose an upper limit for deposition and indicate that the high grade lacorne block is not basement to the abitibi supracrustal sequence 2747 2680 ma existence of abundant 69 older detrital zircons 2750 ma suggests in turn that the abitibi supracrustal rocks are not the source of the lacorne sediments two generations of granitoids occur in the lacorne block an early monzodiorite monzonite granodiorite syenite series mmgs 2671 4 to 2675 24 ma and a younger s type garnet muscovite granite series gmg 2630 20 to 2655 8 ma this contrasts with granitoid magmatism in the abitibi greenstone belt which ended at 2675 ma the pontiac subprovince to the south of the abitibi greenstone belt shares all of the above features of the lacorne block including detrital zircon ages as well as the composition and timing of granitoid magmatism this is interpreted to indicate that the lacorne block was originally part of the same tectonic terrane as the pontiac subprovince after development of the mmgs magmatism 21670 2680 ma the pontiac subprovince locally underthrust the abitibi greenstone belt and crustal thickening promoted partial melting of underthrust pontiac metasediments to form the 2644 13 ma s type granites later differential uplift exposed the lacorne block as a high grade tectonic window in the abitibi greenstone belt

ore forming fluids in archean gold bearing quartz veins at the sigma mine abitibi greenstone belt quebec canada archean gold bearing quartz veins at the sigma mine occur in deformed metavolcanics intruded by two generations of preore porphyries the veins were emplaced during a late compressional tectonic event and they consist primarily of quartz tourmaline carbonates pyrite and scheelite vein minerals have been deformed and are also cut by abundant healed fractures outlined by fluid inclusions in the veins gold typically occurs in late fractures and in recrystallized quartz microthermometric sem eda leachate analysis and gas chromatographic techniques were used to study fluid inclusions in quartz most fluid inclusions at sigma occur in late fractures and are secondary the fluid inclusions are commonly found in fractures parallel to or less commonly within gold bearing fractures and are genetically related to the gold three types of fluid inclusions are identified 1 high salinity na ca cl aqueous inclusions 25 34 wt salt with small amounts of co 2 some containing halite calcite and nahcolite as daughter minerals 2 co 2 rich inclusions containing small amounts of h 2 o and ch 4 and 3 low salinity h 2 o co 2 inclusions 10 equiv wt nacl with co 2 contents which are variable but commonly in the range of 15 to 30 mole percent co 2 leachate analyses show that in addition to na and ca the aqueous fluids contain minor amounts of k fe mg and li homogenization temperatures of h 2 o co 2 inclusions with constant phase ratios range from 285 degrees to 395 degrees c those of aqueous inclusions range from 60 degrees to 295 degrees c the aqueous and co 2 rich inclusions are interpreted to have trapped two coexisting fluids resulting from the unmixing of a homogeneous h 2 o co 2 parent fluid itself occasionally trapped in late fractures the ore bearing fluid is interpreted to be a low salinity 10 equiv wt nacl fluid containing 15 to 30 mole percent co 2 at minimum temperatures in the range of 285 degrees to 395 degrees c the relatively low homogenization temperatures of aqueous inclusions are interpreted to be the result of co 2 effervescence from a co 2 saturated aqueous fluid due to pressure fluctuations at the time of entrapment it is also proposed that the onset of gold deposition is related to the onset of fluid unmixing

variscan sb au mineralization in central brittany france a new metallogenic model derived from the le semnon district numerous sb au deposits are known in the european variscan belt and are often associated with the latevariscan hydrothermal events linked to postorogenic extension and strike slip faults through the whole belt thecentral part of the armorican massif central brittany hosts several sb au deposits such as the le semnondeposit however little is known about its formation its structural control or hydrothermal history in order toprovide new constraints on the sb au mineralization framework of the variscan armorican massif structural geochronological and chemical analyses have been performed on the le semnon sb au deposit first wehighlight that sb au mineralization consists of fault fill and flat extensional vein systems resulting from smallscalefault valve mechanism controlled by fluid overpressure at shallow depth indeed this deposit is characterizedby the presence of dolerite dykes which acted as the plumbing system for fluids within impermeableslates the low permeability of the slates permitted the establishment of local supralithostatic fluid pressure in apredominantly hydrostatic regime coupled with new micro textural evidences a reappraisal of the parageneticevolution and new physico chemical data 40ar 39ar and u pb analyses show that the le semnon sb aumineralization was emplaced around 360 ma and therefore indicates an early hydrothermal event in thevariscan history of the armorican massif moreover absolute dating of mafic magmatism coupled with hydrothermalalteration associated with the deposition of sb au mineralization strongly suggest that the emplacementof the sb au mineralization was coeval with the emplacement of the host rock doleritic dyke furthermore the high temperature recorded in fluid inclusions and the paragenetic evolution suggests an elevatedthermal anomaly in central brittany thus we propose that the emplacement of the widespread maficmagmatism at ca 360 ma played a major role in the mobilization of fluid flows and the redistribution of metalsin the armorican crust

fluid infiltration into fault zones chemical isotopic and mechanical effects fluid infiltration into fault zones and their deeper level counterparts brittle ductile shear zones is examined in diverse tectonic environments in the 2 7 ga abitibi greenstone belt major tectonic discontinuities with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism during reverse motion on the structures accommodating shortening of the belt these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids such fluids were characterised by 18o 6 2 d 50 20 13c 4 4 and temperatures of 270 to 450 c probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism hydrothermal fluids were more radiogenic 87sr 86sr 0 7010 to 0 7040 and possessed higher than did contemporaneous mantle komatiites or tholeiites and thus carried a contribution from older sialic basement a provinciality of87sr 86sr and 13c is evident signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres mineralised faults possess enrichments of large ion lithophile lil lil elements including k rb ba cs b and co2 and rare elements such as au ag as sb se te bi and w fluids were characterised by xco2 0 1 neutral to slightly acidic ph low salinity 3 wt k na 0 1 they carried minor ch4 co and n2 and they underwent transient effervescence of co2 during decompression clastic sediments occupy graben developed at fault flexures the40ar 39ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years

geochemical and mineralogical diagnosis on gold ores a case study from the cadillac larder lake fault zone abitibi canada abstract numerous orogenic gold deposits have been discovered within the archean abitibi greenstone belt of canada these include the francoeur wasamac astoria zulapa goldex lamaque lac herbin and beaufor deposits all of which are located within the cadillac larder lake fault zone cllfz these gold deposits are hosted by various lithologies and vary in terms of style of mineralization sulfide abundance and hydrothermal alteration these variations mean that exploration for new deposits has been difficult in this area using common exploration criteria this study presents the results of a principal component analysis pca based approach combining geochemical and mineralogical data to quickly identify different types of gold mineralization within the cllfz including assessing the use of this approach in mineral exploration the first principal components in both classification methods used during this study pce1 and pcm1 representing element geochemical and mineralogical classification respectively can efficiently discriminate between mineralization and barren altered rocks the second principal components pce2 and pcm2 can be used to constrain the host rocks altered vs barren and style of mineralization disseminated vs vein hosted respectively finally pce3 and pcm3 give more detailed information on the nature of the mineralization in an area possible reflecting variations in the composition of ore forming fluids all of the above indicates that the two pce and pcm classification methods are complementary which means that using approaches that combine these methods will provide a comprehensive understanding of the nature of studied samples and or deposits information extracted from the results of the pca undertaken during this study may be beneficial for mineral exploration in unknown districts such as glaciated areas where traditional geological fieldwork is challenging this study also defines element and mineral mineralization indices based on pce1 and pcm1 which enables evaluation of the extent and style of gold mineralization in an area this study indicates that pca based automated mineralogical and geochemical classification methods could be highly useful in exploration for gold deposits

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

interpretation of post entrapment fluid inclusion re equilibration at the three mile hill marvel loch and griffins find high temperature lode gold deposits yilgarn craton western australia abstract fluid inclusions that are interpreted to be related to vein filling of quartz veins at the three mile hill and marvel loch gold deposits in amphibolite facies rocks and the griffins find deposits in granulite facies rocks in the yilgarn craton of western australia are dominantly low salinity h 2 o co 2 ch 4 mixtures or co 2 ch 4 fluids at each deposit inclusions have a wide range of compositions with respect to both the aqueous carbonic ratio and the co 2 ch 4 ratio of the carbonic phase and most fluids could not have been in equilibrium with the mineral assemblage in the vein and adjacent rock inclusion densities suggest a range of p t conditions of entrapment and are in general not consistent with the conditions of vein formation indicated by vein assemblages diffusional addition of h 2 into inclusions diffusional loss of h 2 o and reduction of inclusion volume are possible during cooling and uplift along the inferred p t path the inclusion populations could have been derived from an originally uniform population of low salinity aqueous dominated h 2 o co 2 inclusions by a combination of these processes inclusion modification as the cause of the complex inclusion populations is supported by relations of molar volume to composition and to an extent by variations in fluid salinity if inclusion re equilibration is the cause of inclusion variability it was of variable intensity within a single vein system and within individual clusters of inclusions in some samples and is suggested to have been a function of the local petrological and textural environment

fluid pressure transients on seismogenic normal faults abstract fluid inclusions in hydrothermally altered footwall rocks of the dixie valley fault nevada and the wasatch fault utah indicate that pore fluid pressure fluctuated minimum entrapment pressures for fluid inclusions consisting of h2o co2 nacl ranged from 295 mpa to 60 mpa in the temperature range 350 to 170 c on the wasatch fault and from 158 to 35 mpa in the temperature range 350 to 200 c on the dixie valley fault scatter in the pressure estimates at constant temperature is interpreted as paleo fluid pressure transients at depths of up to 11 km on the wasatch fault and 3 to 5 km on the dixie valley fault observed pressure transients range from 5 mpa within the limits of error in pressure determination to 120 mpa on the wasatch fault and 7 to 126 mpa on the dixie valley fault the pressure transients are greatest on both faults in the temperature range 270 to 310 c the fluids represented by fluid inclusions play a key role in nucleation and propagation of earthquake ruptures high fluid pressures may initiate rupture then dilatancy pore pressure reduction and dilatant hardening may arrest the rupture however decompression of the fluids and phase separation produces a decrease in fluid bulk modulus of 41 to 90 which reduces the dilatant hardening effect and may permit ruptures to propagate

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

linked fluid and tectonic evolution in the high himalaya mountains nepal fluid inclusions were studied in a quartz lens from the structurally highest unit of the himalaya mountains in nepal from a textural geometrical chemical and isotopic point of view six types of fluid inclusions were distinguished one of these types consists of annular inclusions this shape is attributed to a confining pressure increase in a non isotropic stress field two successive stress fields were deduced from the orientation of the inclusion planes relative to the schistosity the bulk composition of the fluid was dominated by co2 84 mol and h2o the composition remained constant during the whole history of the sample indicating that it was buffered by the carbonaceous host rock and or that one single fluid was reworked in situ by decrepitation stable isotope of fluids and minerals indicate 1 that fluids were buffered by surrounding rocks for o and c and 2 that at least two types of water metamorphic and meteoric were involved finally a p t t e path is proposed for the sample taking into account the southward thrusting along the main central thrust the northward tectonic denudation of the himalaya mountains inducing tectonic burying below the annapurna range and lastly rapid uplift

fluid flow within the damage zone of the boccheggiano extensional fault larderello travale geothermal field central italy structures alteration and implications for hydrothermal mineralization in extensional settings the neogene extensional province of southern tuscany in central italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post orogenic silicic magmatism characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit known as filone di boccheggiano hosted within the damage zone of the boccheggiano fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and more generally to provide inferences on fault controlled hydrothermal fluid flow in extensional settings we provide a detailed description of the fault zone architecture and alteration mineralization associated with the boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies this investigation shows that the boccheggiano ore consists of an adularia illite type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the boccheggiano fault the low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross fault flow with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability fluid inclusion characteristics indicate that ore minerals were deposited between 280 and 350 c in the upper levels of the brittle extending crust lithostatic pressure in the order of 0 1 gpa abundant vapour rich inclusions in ore stage quartz are consistent with fluid immiscibility and boiling and quartz ore vein textures suggest that mineralization in the boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions results from this study i predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension and ii indicate the rate of secondary structural permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes these features intimately link ore grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block s of major extensional fault zones

40ar 39ar age constraints on the thermal history of the archean abitibi greenstone belt and the pontiac subprovince implications for terrane collision differential uplift and overprinting of gold deposits 40ar 39ar mineral age spectra of granitic and metamorphic rocks in conjunction with existing conventional zircon geochronology indicate that at least two major late archean thermal events affected tectonic blocks of the abitibi southern volcanic zone svz and the juxtaposed pontiac subprovince the earlier thermal activity 2690 2670 ma was accompanied by the intrusion of voluminous syntectonic plutons and caused low pressure greenschist facies metamorphism in the svz and intermediate pressure metamorphism in the pontiac subprovince the second thermal event 2660 2630 ma was coeval with the emplacement of syncollisional s type garnet muscovite granites in the pontiac subprovince and the higher grade lacorne block of the abitibi svz and reset the k ar systems in preexisting rocks magmatic amphibole from the syntectonic round lake batholith 2695 ma u pb zircon age of the abitibi svz has a slightly disturbed ar release spectrum with an upper plateau age of 2669 6 ma signifying that the low gra

a model for turbulent hydraulic fracture and application to crack propagation at glacier beds glaciological observations of under flooding suggest that fluid induced hydraulic fracture of an ice sheet from its bed sometimes occurs quickly possibly driven by turbulently flowing water in a broad sheet flow taking the approximation of a fully turbulent flow into an elastic ice medium with small fracture toughness we derive an approximate expression for the crack tip speed opening displacement and pressure profile we accomplish this by first showing that a manning strickler channel model for resistance to turbulent flow leads to a mathematical structure somewhat similar to that for resistance to laminar flow of a power law viscous fluid we then adapt the plane strain asymptotic crack solution of desroches et al 1994 and the power law self similar solution of adachi and detournay 2002 for that case to calculate the desired quantities the speed of crack growth is shown to scale as the overpressure in excess of ice overburden to the power 7 6 inversely as ice elastic modulus to the power 2 3 and as the ratio of crack length to wall roughness scale to the power 1 6 we tentatively apply our model by choosing parameter values thought appropriate for a basal crack driven by the rapid drainage of a surface meltwater lake near the margin of the greenland ice sheet making various approximations perhaps relevant to this setting we estimate fluid inflow rate to the basal fracture and vertical and horizontal surface displacements and find order of magnitude agreement with observations by das et al 2008 associated with lake drainage finally we discuss how these preliminary estimates could be improved

stress controls on intrusion related gold lodes wonga gold mine stawell western lachlan fold belt southeastern australia the wonga gold lodes 300 000 oz au in the victorian gold fields of eastern australia formed at a depth of less than 7 km within the seismogenic zone the wonga gold deposit is hosted by mudstone dominant lithofacies that define the core of the regional synform these are underlain by sandstone dominant lithofacies that are interlayered with mafic schist the lodes occur within the contact aureole of a late stage granitic body that intrudes an earlier phase of the fractionated pluton in contrast to the nearby 440 ma magdala orogenic gold lodes 5 moz au this smaller deposit formed at ca 400 ma and has been classified as intrusion related the orogenic and intrusion related gold deposits formed at different stages in the structural and magmatic evolution of the fold belt and at different crustal levels compared to the magdala lodes the wonga lodes have a close temporal and spatial relationship with the intrusive rocks and a distinct structural style and mineralogy including acicular arsenopyrite stibnite gold telluride alloys bismuth silver and late stage molybdenite the wonga lodes formed between two distinct phases of regional faulting and the kinematics of the mineralized structures cannot be correlated with any preserved far field regional paleostresses instead they are inferred to reflect the local state of stress at the point that magmatic related fluid overpressure occurred pervasive fluid overpressure drove deformation across a broad zone producing massive mineralized quartz vein structures that included angular clasts of wall rock a discrete overpressure event is suggested rather than repeated crack seal behavior the lodes represent an early stage in the development of a fault zone characterized by a series of discontinuous discrete shear failures the geometry of the lodes is strongly dependent on structural position and rheology northeast dipping preexisting weaknesses such as dike margins and variably developed crenulation cleavages produced localized shear failure defined by dextral reverse slip structures the hanging wall structures 30 to 60 se dipping extension veins and extensional shear veins the link structures diverge from these creating en echelon lode geometries there is a sudden change over the top of a regional plunge reversal defined by the dominant fold axes from shear failure the hanging wall structures to extensional shear and then extensional failure the links this occurs along strike in the same structural corridor and within the same stratigraphic unit the explicit finite difference code flac was used to model the local state of stress at the point where an influx of magmatic fluids raised the pore fluid pressure the model predicts that a zone of slightly lower differential stress was associated with the plunge reversal and this variation in differential stress is interpreted to have caused the drastic change in lode style the modeling highlights that only subtle differences in the elastic moduli of the rocks are required to produce the observed stress geometry

structural controls on gold mineralization on the margin of the yilgarn craton albany fraser orogen the tropicana deposit western australia the tropicana gold deposit is located adjacent to the margin of the yilgarn craton in the albany fraser orogen western australia the deposit is hosted in granulite facies quartzo feldspathic gneisses of the archean tropicana gneiss ore bodies comprise biotite pyrite alteration concentrated in shear zones that formed during ne sw shortening in the late archean and clearly postdate the formation and deformation of high grade gneiss fabrics d1 and d2 the orientation of the ore bodies is controlled by the shear zones that are in turn localised by the gneissic banding mineralization also involved solution and coeval microfracturing and veining of more competent pegmatitic units the mineralizing event d3 was followed by at least two further deformations which reactivated and overprinted the biotite fabrics with sericite and chlorite created new shear zones and affected gold distribution d5 consisted of dextral shear on e w shear zones which subdivide the deposit into five major structural domains the importance of structurally controlled permeability at tropicana is similar in cratonic lode gold deposits as is the protracted deformation fluid flow history like renco mine in zimbabwe tropicana gold deposit was formed by hydrothermal fluid flow peripheral to the craton economic gold mineralization was clearly post peak metamorphism

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

geodynamic setting of mesothermal gold deposits an association with accretionary tectonic regimes mesothermal gold provinces of phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs a recurring sequence of transpressive deformation uplift late kinematic mineralization and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust mesothermal gold camps in the superior province are spatially associated with large scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes these boundary structures are characterized by the sequence of significant horizontal shortening uplift late kinematic mineralization and shoshonitic lamprophyres and therefore may have the same geodynamic significance as phanerozoic counterparts in this model thermal re equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime over time scales of 10 to 40 m y is a necessary precursor to gold mineralization hydrothermal fluids are released along boundary faults and their splays during uplift the uniform temperature low salinity and mole co2 signify uniform source conditions whereas the variable o c sr and pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction related crustal underplating and deep late metamorphism rather than magmatic or metamorphic events in the supracrustal rocks secular variations in the generation of archean proterozoic and phanerozoic mesothermal au provinces reflect the timing of collisional orogenies within terranes of these eras

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

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

fluid inclusion evidence for p v t x evolution of hydrothermal solutions in late alpine gold quartz veins at brusson val d ayas northwest italian alps

fluid inclusion and stable isotope o h c and s constraints on the genesis of the serrinha gold deposit gurupi belt northern brazil the serrinha gold deposit of the gurupi belt northern brazil belongs to the class of orogenic gold deposits the deposit is hosted in highly strained graphitic schist belonging to a paleoproterozoic 2 160 ma metavolcano sedimentary sequence the ore zones are up to 11 m thick parallel to the regional nw se schistosity and characterized by quartz carbonate sulfide veinlets and minor disseminations textural and structural data indicate that mineralization was syn to late tectonic and post metamorphic fluid inclusion studies identified early co2 ch4 n2 and co2 ch4 n2 h2o nacl inclusions that show highly variable phase ratios co2 homogenization and total homogenization temperatures both to liquid and vapor interpreted as the product of fluid immiscibility under fluctuating pressure conditions more or less asso ciated with postentrapment modifications the ore bearing fluid typically has 18 33mol of co2 up to 4mol of n2 and less than 2mol of ch4 and displays moderate to high densities with salinity around 4 5wt nacl equiv mineralization occurred around 310 to 335 c and 1 3 to 3 0 kbar based on fluid inclusion homogenization temperatures and oxygen isotope thermometry with estimated oxygen fugacity indicating relatively reduced conditions stable isotope data on quartz carbonate and fluid inclusions suggest that veins formed from fluids with 18 oh2o and dh2o 310 335 c values of 6 2 to 8 4

petroleum geochemistry of the froy field and rind discovery norwegian continental shelf implications for reservoir characterization compartmentalization and basin scale hydrocarbon migration patterns in the region abstract petroleum geochemistry provides an excellent tool for understanding reservoir characteristics with respect to oil flow and tracing out elements of basin scale migrational patterns and reservoir compartmentalization organic geochemical characterization has been carried out on 580 core extracts and 9 oils from 11 wells of the froy field and rind discovery in the south viking graben two different drainage areas and source rock systems have been identified for the hydrocarbons in the froy field and the rind discovery on the basis of differences in maturity and gas to oil ratio in addition to facies differences like c 28 sterane content and the ratio of bisnorhopane to bisnorhopane plus norhopane results show that the rind discovery contains oil of higher maturity compared to the oil from froy field the petroleum in the rind discovery has a more terrestrial character and is interpreted to have been derived from source rocks other than the draupne formation in its typical anoxic distal development a likely candidate is the heather formation in addition a small contribution of immature oil has been identified in a separate small sub compartment in the froy field the petroleum in this sub compartment shows a high pristane to phytane ratio and a very prominent odd to even predominance well 25 14 drilled northeast of the froy field penetrates a water bearing hydrostatic structure the detailed geochemical characterization including petroleum inclusion studies have shown that this structure never received oil or gas nor did it have any communication with the froy field hence it is inferred that this structure never communicated with the hydrocarbon producing basin one of the two compartments in the rind discovery is hydrostatic and contains petroleum of a comparatively large maturity span whilst residual core bitumen in a high pressured neighboring compartment has a very small maturity span and contains presently only water as producible phase the ratios of the 20s to 20s 20r isomers of c 29 steranes the ratio of c 20 to c 20 c 28 triaromatic steroids and the c 28 triaromatic steroid to c 28 triaromatic steroid c 29 monoaromatic steroid ratio in the latter compartment have almost uniform values indicating a single oil pulse i e a short filling history whereas in the other compartment these ratios vary over a large range indicating a longer filling history as both compartments contain petroleum with the same source rock facies signatures it is suggested that these at some time were part of the same migration system communication between these compartments is inferred to have been disrupted before seal failure occurred in the structure which was penetrated by well 25 15r2 the analyses carried out indicate a high degree of chemical homogeneity in the petroleum of the froy field reservoir which in comparison with the rind discovery is more similar to extracts from typical anoxic developed distal shales of the upper jurassic source rock in the viking graben i e the kimmeridge equivalent draupne formation however the upper reservoir units in the northern well 25 5 a7 contain extracts with lower maturity and more terrestrial characteristics compared with the extracts in the deeper reservoir sections in addition well 25 5 1 which penetrates the highest point of the froy structure contains petroleum of slightly higher maturity than found elsewhere in the froy field the results further suggest that the faults in the froy field including the main e w trending fault separating the northern and the southern parts of the of the froy field did not adversely affect lateral communication during filling to conclude there is no petroleum geochemical evidence to suggest that intra reservoir faults have caused significant compartmentalization of the froy field except for the n s trending fault which separates a small block to the west from the froy field main reservoir thus with respect to petroleum production it is concluded that primary sedimentological features are more important in terms of exerting a first order control for determining the vertical and lateral flow of hydrocarbons in this reservoir than faults

the structure of shear related vein type gold deposits a review abstract the structure of vein type gold deposits is defined by the shapes and geometrical relationships of mineralized bodies the form of the mineralization making up these bodies and the sequence of vein forming events most mineralized zones occur within or are spatially associated with shear zones especially shear zones in larger systems of intersecting shear zone sets they range in shape from tabular to linear and in form from disseminated to breccia to stockwork or sheeted veinlet zones to single veins there typically is a complex history of mineral deposition which overlaps and is genetically related to the deformation that generated the host structural zone the purpose of this paper is to consider the structural features of vein type deposits in relation to the geometrical properties and evolution of veinhosting fractures particularly shear zones the evidence indicates that veins are localized by tectonically generated dilatancy in an environment of low mean stress caused by high fluid pressure it is proposed that a major cause of tectonic dilation of shear zones is the interference between intersecting shears during bulk inhomogeneous flattening by movement on systems of intersecting shear zone sets