u pb evidence for abitibi gold mineralization postdating greenstone magmatism and metamorphism two widely held theories for the origin of archaean lode gold deposits the magmatic1 and metamorphic replacement2 models predict that gold emplacement should have occurred near the end of late syn kinematic greenstone magmatism here we report that at the camflo mine in the abitibi subprovince of canada u pb ages for rutile and titanite in auriferous alteration 2 625 7 myr are much younger than the age of the host quartz syenite 2 680 4 myr and ages for regional magmatism 2 670 myr 3 and metamorphism 2 680 myr 4 mineral associations and pb pb systematics indicate that the hydrothermal minerals record crystallization ages that are probably coeval with the gold mineralization these ages and others from the abitibi subprovince4 6 rule out a direct connection between gold and magmatism or metamorphism associated with regional folding of the greenstone belt they are broadly contemporaneous with metamorphism at deep crustal levels7 and may record late devolatilization of the lower crust

a brittle failure mode plot defining conditions for high flux flow stress and fluid pressure conditions for the initial formation within intact rock of faults shear fractures extension fractures and extensional shears and for the reshear of existing faults may all be represented on a generic failure plot of differential stress 1 3 versus least effective compressive stress 3 3 pf scaled to nominal tensile strength t half the cohesive strength plots of this kind may be used to define maximum sustainable overpressure in different tectonic environments and the structural conditions under which the flow of large fluid volumes may occur through fault fracture meshes containing gaping extension and extensional shear fractures they may also be adapted to different tectonic regimes and correlated to depth for particular fluid pressure conditions high flux flow through distributed fault fracture meshes requires the tensile overpressure condition 3 3 which can only be achieved in the absence of throughgoing cohesionless faults that are well oriented for frictional reactivation in the prevailing stress field high flux flow through distributed fault fracture meshes intrinsically a transient pulsing phenomenon may therefore occur as follows 1 in effectively intact low pemeability crust devoid of throughgoing favorably oriented faults 2 where existing faults have become severely misoriented in the prevailing stress field and 3 where existing faults have regained cohesive strength through hydrothermal cementation

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

cone penetration test calibration for erksak beaufort sea sand interpretation of the cone penetration test in sands is generally based on empirical calibrations from tests in large diameter calibration chambers although interpretation of these calibration data for clean sands in terms of the state parameter is expected to be broadly applicable to other sands material specific correlations are desirable for many projects this paper describes a series of calibration chamber tests carried out on a sand dredged from the beaufort sea for construction of artificial islands this erksak sand is a uniformly graded subrounded medium grained sand with a fines content of 3 6 the testing chamber described is 1 4 m in diameter and allows independent control of vertical horizontal and back pressures on the sand sample samples of the sand were prepared by moist compaction to preserve the fines content and then back pressure saturated the chamber test data are presented and confirm that the erksak sand fits the general trends observed for other sands very well a method i

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

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

geochemistry and diagenetic history of the ordovician lower head formation sandstones western newfoundland canada the lower head formation in the parson s pond area western newfoundland canada comprises siltstones with very fine grained to fine grained sandstones petrography confirms that these sandstones are matrix rich essentially wackes with detrital minerals including quartz feldspar biotite and numerous accessory minerals observed petrographic features suggest that the lower head sediments are the distal product of erosion with sediment sourced from the dashwoods microcontinent and lushs bight oceanic tract and thoroughly mixed in an earlier basin prior to final deposition in a trench slope basin the lower head formation sandstones have low porosity with early diagenetic cements c1 and later calcite in crosscutting calcite veinlets c2 petrographic isotopic and fluid inclusion analyses indicate that c1 cements formed during the early stages of diagenesis both 13c and 18o isotopes for the c1 calcite cements are isotopically heavier than the c2 calcite veins fluid inclusion homogenization te

significance of alteration assemblages for the origin and evolution of the proterozoic nabarlek unconformity related uranium deposit northern territory australia the proterozoic nabarlek unconformity related uranium deposit in the alligator rivers uranium field is hosted by paleoproterozoic amphibolite grade metamorphosed semipelitic sedimentary rocks and amphibolite schist high grade ore is confined to the nabarlek fault a reverse fault shear zone that crosscuts a series of interbedded muscovite quartz biotite schists and amphibolite petrographic studies on polished thin sections combined with electron microprobe analyses x ray diffraction fluid inclusion data o h and u pb isotope values as well as 40 ar 39 ar dating have identified up to eight significant fluid events beginning with the precipitation of early quartz veins during uplift of the myra falls metamorphics at 1830 ma and ending with limited uraninite recrystallization during reactivation of the nabarlek fault between 1380 and 750 ma quartz veins that likely formed toward the end of the top end orogeny represent the earliest recorded fluid event fluid inclusion data and 18 o and d values indicate that these veins formed from basement derived fluids that may have been heated by a cooling nabarlek granite during circulation through reverse faults shear zones the next fluid event represented by fine grained sericite and chlorite occurred when fluids passed into these faults and altered the metamorphic minerals following the exhumation of the basement and deposition of the kombolgie subgroup at ca 1760 ma the intrusion of the oenpelli dolerite at ca 1720 ma resulted in the local remobilization of silica and the precipitation of quartz associated with minor pyrite and dolomite around the reverse faults uranium mineralization is associated with an inner and outer alteration halo that extends as far as 1 km from the nabarlek fault alteration in the outer halo began as early as 1700 ma and is dominated by chlorite and sericite which formed when a 200 c fluid flowed into the nabarlek fault from the overlying kombolgie subgroup u pb and 207 pb 206 pb dating reveals that massive uraninite precipitated at ca 1640 ma and formed together with illite and hematite at ca 200 c chlorite was not coeval with uraninite precipitation stable isotope values indicate that the pre and synore alteration assemblage formed from basinal brines with 18 o fluid and d fluid values of 2 2 and 25 10 per mil respectively reactivation of the nabarlek fault at ca 1360 1100 and 900 ma is indicated by u pb and 207 pb 206 pb dating of uraninite these ages correlate with the intrusion of the maningkorrirr phonolitic dikes and the derim derim dolerite at ca 1316 40 and 1324 4 ma respectively the amalgamation of australia and laurentia during the grenville orogen at ca 1140 ma and the breakup of rodinia between 1000 and 750 ma fluid incursions associated with these events precipitated much of the chlorite that has previously been related to uraninite precipitation drusy quartz veins that host high salinity fluid inclusions and sulfides particularly galena also formed after the initial uraninite forming event finally erosion of the kombolgie subgroup and subsequent weathering of the deposit resulted in the recent formation of kaolinite and numerous secondary uranium minerals these data constrain individual events more precisely than previous studies and thus advance the current genetic model to a level that takes into account the multiple stages of fluid overprinting that occurred over a period of at least 800 m y

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

internal structure of the cadillac tectonic zone southeast of val d or abitibi greenstone belt quebec the kirkland lake larder lake cadillac break is a major fault zone of the abitibi greenstone belt well known for its spatially associated gold camps a detailed structural analysis of this fault zone in the val d or area has shown the presence of a 200 750 m wide zone of high strain defined here as the cadillac tectonic zone ctz which includes the narrower schist zone generally regarded as the cadillac break the boundaries of the ctz which coincide with major lithologic contacts strike approximately east west and dip 80 to the north the internal structure of the ctz is best analysed in terms of two increments of the same progressive deformation the d1 strain increment is characterized by an east west striking subvertical s1 foliation at low angle in strike to the bound aries of the ctz and containing subvertical elongation lineation primary lithological contacts have been transposed into s1 and folded by f1a folds f1b intrafolial folds of s1 ranging from noncylindrical subhorizontal f

systematic stress variations in the southern san joaquin valley and along the white wolf fault implications for the rupture mechanics of the 1952 ms 7 8 kern county earthquake and contemporary seismicity the southern san joaquin valley ssjv stress state is characterized by systematic variations in the regional principal stress directions and relative magnitudes as well as local and possibly temporal variations that appear to be the result of the 1952 ms 7 8 kern county earthquake the regional maximum horizontal principal stress shmax orientation in the san joaquin valley systematically rotates from ne sw compression along the western margins of the valley to n s compression in the ssjv this n s shmax stress direction in the ssjv is consistent with active development of a e w trending structural fabric of fold axes and shallow thrust and the reverse dip slip motion observed during the kern county earthquake on the south southeast dipping white wolf fault contemporary seismicity 30 40 years after the mainshock clusters in two separate areas along the white wolf fault the southwest region where the 1952 earthquake nucleated and the northeast region near where the earthquake rupture terminated earthquakes in the southwest and northeast regions show a diversity of focal mechanisms that include strike slip reverse oblique and to a lesser extent normal slip inversion of earthquake focal mechanisms for in situ stress in the southwest region indicates a strike slip reverse stress regime with s1 oriented approximately perpendicular to the ruptured fault plane implying low frictional strength in the nucleation zone of the 1952 earthquake inversion of focal plane mechanisms in the northeast region indicates a strike slip stress regime with s2 nearly perpendicular to secondary 1952 rupture planes also implying low frictional strength these results indicate near complete stress drops for fault planes associated with the 1952 earthquake and some of the contemporary earthquakes implying fault surfaces which are frictionally weak i e slip planes subparallel to principal stress planes based on the observed stress state in the southern san joaquin valley much of the seismicity may be the result of elevated fluid pressures within these active fault zones

fluid inclusion constraints on the kinematics of footwall uplift beneath the brenner line normal fault eastern alps dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear each class of model predicts a different incremental strain history that should be recorded in the footwall in the tauern window eastern alps postmylonitic structures in the footwall of the brenner line normal shear zone predominantly consist of closely spaced steep west down and east down microfaults formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear in contrast formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes the field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed however both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion lined microfaults with the macroscopic west down and east down faults obtaining density data for the inclusions and correlating these data with previously obtained geochronologic data the results indicate that the west down structures formed at depths of 10 20 km and temperatures 450 c in the mid to late oligocene and that the east down structures formed at 2 to 10 km depth and temperatures of 300 50 c in the mid miocene these data support the hypothesis that a rolling hinge was present in the footwall of the brenner line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear the depths at which west down and east down faulting occurred coupled with the angle of dip of the brenner line yield a minimum lateral displacement on the fault of 15 26 km approximately coeval ductile shearing and brittle faulting at depths of 15 20 km and temperatures in excess of 400 c may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation

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

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

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

the tectonic evolution of the abitibi greenstone belt of canada based on structural geochemical sedimentological and geochronological studies we have formulated a model for the evolution of the late archaean abitibi greenstone belt of the superior province of canada the southern volcanic zone svz of the belt is dominated by komatiitic to tholeiitic volcanic plateaux and large bimodal mafic felsic volcanic centres these volcanic rocks were erupted between approximately 2710 ma and 2700 ma in a series of rift basins formed as a result of wrench fault tectonics the svz superimposes an older volcanic terrane which is characterized in the northern volcanic zone nvz of the abitibi belt and is approximately 2720 ma or older the nvz comprises basaltic to andesitic and dacitic subaqueous massive volcanics which are cored by comagmatic sill complexes and layered mafic anorthositic plutonic complexes these volcanics are overlain by felsic pyroclastic rocks that were comagmatic with the emplacement of tonalitic plutons at 2717 2 ma the tectonic model envisages the svz to have formed in a series of rift basins which dissected an earlier formed volcanic arc the nvz analogous rift environments have been postulated for the hokuroko basin of japan the taupo volcanic zone of new zealand and the sumatra and nicaragua arcs the difference between rift related submergent volcanism in the svz and emergent volcanism in the nvz resulted in the contrasting metallogenic styles the former being characterized by syngenetic massive sulphide deposits whilst the latter was dominated by epigenetic porphyry type cu au deposits

puzzles and the maximum effective moment mem criterion in structural geology abstract the essential difference in the formation between conjugate brittle shear fractures and ductile shear zones is that the intersection angle of conjugate faults in the contractional quadrants in the former is acute usually 60 and obtuse usually 110 in the latter the mohr coulomb failure criterion is an experimentally validated empirical relationship which structural geologists use to interpret the stress directions based on the orientation of the brittle shear fractures however a simple application of this criterion assuming that the principal stresses are vertical or horizontal throughout the crust fails to explain crustal scale low angle normal faults high angle reverse faults and certain types of conjugate strike slip faults that have intersection angles in the compressional quadrants greater than 90 although the maximum effective moment mem criterion has been used to analyze structures in several natural cases and obtained some new evidence in nature and experiments it is not yet commonly used to interpret non mohr coulomb features widely distributed in the crust it is important to review the mem criterion and its implications for explaining the formation mechanism of naturally occurring faults and shear zones behavior of a rock depends on its mechanical properties the latter in turn depends on pressure and temperature conditions related to crustal depth and strain rates conjugate fractures with obtuse angles ca 110 in the contractional direction at different scales and at different crustal levels are consistent with the prediction of the mem criterion and suggest deformation at low strain rates thus strain rate clearly plays an important role in the style of crustal deformation and the many observed departures from the mohr coulomb failure criterion

stress state evolution of the brittle upper crust during compressional tectonic inversion as defined by successive quartz vein types high ardenne slate belt germany abstract in the frontal part of the rhenohercynian fold and thrust belt high ardenne slate belt germany two successive types of quartz veins oriented normal and parallel to bedding respectively are interpreted to reflect the early variscan compressional tectonic inversion of the ardenne eifel sedimentary basin fracturing and sealing occurred in lower devonian siliciclastic multilayers under very low grade metamorphic conditions in a brittle upper crust a geometrical and microthermometric analysis of these veins has helped to constrain the kinematic and pressure temperature conditions of both vein types allowing the reconstruction of the stress state evolution in a basin during tectonic inversion it is demonstrated that bedding normal extension veins which developed under low differential stresses and repeatedly opened and sealed crack seal under near lithostatic fluid pressures reflect the latest stage of an extensional stress regime bedding parallel veins which developed at differential stresses that were still low enough to allow the formation of extension veins cross cut the bedding normal veins and preceded the regional fold and cleavage development these veins show a pronounced bedding parallel fabric reflecting bedding normal uplift and bedding parallel shearing under lithostatic to supra lithostatic fluid pressures during the early stages of a compressional stress regime this kinematic history corroborates that fluid overpressures are easy to maintain during compressional tectonic inversion at the onset of orogeny

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

extensional failure and hydraulic valving at minas da panasqueira portugal evidence from vein spatial distributions displacements and geometries abstract at panasqueira portugal exceptional exposure and demonstrable vein connectivity allow robust characterisation of brittle elastic failure mechanisms in intrusive related environments extensional failure was driven by cycles of fluid injection hydraulic valving and vein growth under conditions with v 1 and differential stress

antitaxial crack seal vein microstructures and their relationship to displacement paths abstract the microstructures developed in an example of a syntectonic crack seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack seal vein growth this observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation it is demonstrated that crack seal processes can lead to the development of both irregular and laminated vein microstructures as well as fibrous microstructures the types of microstructures which develop in crack seal veins and the extent to which they reflect the opening paths of veins depend on a range of factors the most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures and the location of sites of material accretion during successive crack seal increments

evolution of the south central segment of the archean abitibi belt quebec part ii tectonic evolution and geomechanical model in this paper we describe the relations between the paleogeographic and tectonic evolution of the southwestern part of the archean abitibi and bellecombe belts volcanism in the abitibi belt created a very thick anisotropic plate composed of competent volcanic rocks and broken by the duparquet destor break the depocenters of the upper division of diverse volcanic rocks subsided about 10 km relative to their surroundings and some central volcanic complexes within this division were consolidated by synvolcanic plutons and their thermal metamorphic aureole the cadillac break a normal fault separated the abitibi and bellecombe belts the latter consisted of comparatively incompetent sedimentary rocks on top of a basement composed of ultramafic mafic flows north south compression of the volcanic terrain during the kenoran orogeny produced a set of flexure folds f1 that curve around the consolidated cores of central volcanic complexes generally in an easterly direction synclinoria nucleated at the dee

microcracks in rocks a review abstract in the past decade the number of studies about microcracks in rocks has rapidly increased this review of recent work concentrates on microcracks in rock as separate entities emphasizing microcrack morphogenesis kinematics dynamics population statistics and observational techniques cracks are produced when the local stress exceeds the local strength the local stress may be augmented by twin lamellae interactions kink bands and deformation lamellae stress concentrations at grain boundary contacts and around intracrystalline cavities local strength may be reduced along cleavage planes along grain boundaries and along any internal surface as a result of corrosion by chemically active fluids dislocations appear not to be a significant factor for crack nucleation below about 500 c in silicates spatial and temporal changes in temperature can also induce microcracking as a result of differential thermal expansion between grains with different thermoelastic moduli and thermal conductivities the amount of quartz in the rock has a significant effect on thermally induced microcracks because of its large and variable thermal expansivity the application of hydrostatic pressure between 100 and 200 mpa effectively closes most cracks but the closure may not be uniform if crack wall asperities exist hydrostatic pressure appears to stabilize cracks and make crack growth more difficult the number and average size of mechanically induced microcracks is greater in rock deformed at higher pressures the application of a deviatoric stress field on the boundaries of a rock mass results on a microscopic scale in a very complex stress system which greatly affects nucleation and propagation paths the relative amount of intragranular and intergranular cracking appears to depend upon mineralogy rock type and stress state the vast majority of stress induced microcracks in rocks appear to be extensional statistically they are predominantly oriented within 30 of the macroscopic maximum stress direction crack densities increase as macroscopic deviatoric stress increases above a threshold level crack size distributions may be either lognormal or exponential fracture in rock under compressive boundary loads is a result of the coalescence of many microcracks not the growth of a single crack some crack configurations are more favorable for coalescence than others as deviatoric stress increases and rock failure is approached the microcrack population changes spatially from random to locally intense zones of cracking away from the fault the crack density dies off rapidly to the background level a few grains away under lesser deviatoric stresses slow subcritical microcrack growth can occur as a result of stress aided corrosion at the crack tip the rate governing mechanism may be either the chemical reaction rate or the rate at which water can get to the crack tip important details still remain to be worked out

microfracturing and regional stress field a study of the preferred orientations of fluid inclusion planes in a granite from the massif central france abstract a study of the deformation of a granitic massif indicates a relationship between palaeostress fields and the geometry of microfractures as defined by fluid inclusion trails that is healed microfractures a statistical study of the distribution of the trails leads to the following conclusions trails exhibit several distinct preferred orientations on the scale of a grain which may be observed in many samples the orientations of the trails are similar to those of micro and mesoscale fractures in the granite and the dominant trail direction is parallel to the main direction of regional shortening thus fluid inclusion trails are mode i cracks which can be used as excellent markers of palaeostress fields

water sill hypothesis for the origin of certain veins in the meguma group nova scotia canada certain bedding parallel quartz veins some gold bearing in the cambrian ordovician meguma group nova scotia are believed to indicate the former presence of water sills the veins contain primary subhedral quartz crystals which tend to be oriented normal to the vein walls and display brazil twins twinned crystals commonly show hexagonal sections with dark solid inclusion rich centers and white fluid inclusion rich rims viewed in a plane perpendicular to bedding the dark centers of crystals exhibit millimetre thick quartz plates bounded by bedding parallel micrometre thick inclusion bands of white micas the inclusion bands are aligned parallel to the vein walls and terminate within tapered crystals that have an outer zone of milky quartz the quartz veins are on limbs and hinges of folds particularly anticlines from these observations we conclude that 1 the solid inclusion banded quartz resulted from episodic hydraulic extension fracturing and incremental growth of dispersed quartz bridges across water sills 2 the milky quartz filled the vein by growing in optical continuity with the solid inclusion banded quartz 3 localization of certain bedding parallel quartz veins is due to ponding of water beneath aquacludes in very broad domes formed by differential compaction in the meguma group 4 during tectonic shortening these intensely veined rocks localized some of the major anticlines

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

paleostress history of the basin and range province in western utah and eastern nevada from healed microfracture orientations in granites healed microfractures in quartz grains in 14 to 169 ma granites from the basin and range province in western utah and eastern nevada record a history of paleostress the microfracture data are in good agreement with the orientations of paleostresses determined for this same region by using orientations of dikes emplaced into only slightly older igneous host rocks samples generally have two or three steeply dipping microfracture sets one set is related to the tectonic stress field at the time of magma emplacement it strikes northeast southwest in 31 169 ma samples and east northeast west southwest in 25 30 ma samples rotates to east west by about 22 ma and strikes northwest southeast in the youngest samples about 14 ma plutons younger than about 14 ma are not exposed at the surface in western utah and eastern nevada other crack sets in the samples could have formed during cooling of the pluton during a post cooling regional thermal or tectonic event or during some local stress event a strong north south trend in these other crack sets may suggest a formation during basin and range extension

multistage au as sb mineralization and crustal scale fluid evolution in the kwekwe district midlands greenstone belt zimbabwe a combined geochemical mineralogical stable isotope and fluid inclusion study the kwekwe district comprises the central part of the archean midlands greenstone belt of zimbabwe and represents one of few archean gold provinces in the world hosting gold deposits associated with massive antimony mineralization the deposits typically occur along late archean transcrustal deformation zones mineralization is characterized by an early au as stage 1 followed by a pronounced au sb stage 2 and a low grade sb bearing stage 3 four gold deposits within the kwekwe district have been studied the primrose and globe and phoenix gold deposits display typical features of archean orogenic lode gold systems such as fluid inclusions with low salinity mixed aqueous carbonic fluids formation temperatures between 300 and 400 c and a common stable isotope composition of fluid and mineral precipitates deposits of this type formed in the brittle ductile crustal transition zone at 1 5 to 3 0 kbars in contrast gold mineralization at jojo and especially the indarama gold deposits probably formed at lower temperatures 300 c and from dominantly aqueous early moderate to late high salinity fluids at indarama microthermometric studies indicate stibnite gold mineralization at temperatures between 120 and 230 c at about 0 8 to 1 8 kbars low formation temperatures and extremely low 34s values of pyrite 52 and stibnite 36 of the latest mineralization stage at indarama reveal reactivation of the hydrothermal system within a near subsurface organic rich epithermal or hot spring type environment during the late archean or even early proterozoic such low 34s values have not been reported previously from gold deposits in archean greenstone belts we propose that hydrothermal activity and gold mineralization has not only taken place at different crustal levels but also at different times

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

mixing of h2o co2 in fluid inclusions geobarometry and archean gold deposits abstract a modified redlich kwong mrk equation of state for h 2 o and co 2 was used to calculate bulk molar volumes mole fractions and co 2 densities for mixed volatile fluid inclusions the results allow very useful p t diagrams to be constructed for fixed co 2 density and contoured with volume percent co 2 observed in the laboratory these diagrams permit rapid qualitative or quantitative determination of the p t path for a bulk inclusion isochore above the solvus in the system h 2 o co 2 nacl the calculations suggest peak metamorphic pressures of 3 5 kb for some archean greenstone gold deposits insights gained from this work may require reinterpretation of pressure estimates in published fluid inclusion studies

the complexity of 3d stress state changes during compressional tectonic inversion at the onset of orogeny compressional tectonic inversions are classically represented in 2d brittle failure mode bfm plots that illustrate the change in differential stress s1 2 s3 versus the pore fluid pressure during orogenic shortening in these bfm plots the tectonic switch between extension and compression occurs at a differential stress state of zero however mostly anisotropic conditions are present in the earth s crust making isotropic stress conditions highly questionable in this study theoretical 3d stress state reconstructions are proposed to illustrate the complexity of triaxial stress transitions during compressional inversion of andersonian stress regimes these reconstructions are based on successive late burial and early tectonic quartz veins which reflect early variscan tectonic inversion in the rhenohercynian foreland fold and thrust belt high ardenne slate belt belgium germany this theoretical exercise predicts that no matter the geometry of the basin or the orientation of shortening a transitional wrench tectonic regime should always occur between extension and compression to date this intermediate regime has never been observed in structures in a shortened basin affected by tectonic inversion our study implies that stress transitions are therefore more complex than classically represented in 2d ideally a transitional wrench regime should be implemented in bfm plots at the switch between the extensional and compressional regimes extension mode i fractures are classically interpreted to form at low differential stresses less than four times the tensile strength t of rock e g secor 1965 they open perpendicular to the minimum principal stress s3 and propagate in the plane of the maximum s1 and intermediate s2 principal stresses the alignment of fractures has therefore often been used in orogenic belts to determine the orientation of the palaeostress field at the time of fracturing cox et al 2001 gillespie et al 2001 laubach et al 2004 if veins which initiated as extension fractures are uniform in trend over a large area then they can also be used as such a palaeostress indicator e g boullier robert 1992 cosgrove 2001 gillespie et al 2001 mazzarini et al 2010 van noten sintubin 2010 different cross cutting or successive vein generations are even more interesting because they have the potential 1 to serve as a tool to decipher changes in the overall stress regime during multiple phases of fracturing and 2 to define the p2t conditions at which these stress changes occurred on the one hand different crosscutting fracture and or vein generations have been used in many studies to deduce local stress field rotation in a consistent remote stress regime jackson 1991 crespi chan 1996 stowell et al 1999 laubach diaz tushman 2009 wiltschko et al 2009 on the other hand successive cross cutting veins i e vein sets that are oriented differently with respect to bedding have often been used to determine a transition in regional stress regime from extension to compression or reversely manning bird 1991 boullier robert 1992 teixell et al 2000 hilgers et al 2006b van noten et al 2008 2011 laubach diaz tushman 2009 alternatively cross cutting vein sets are observed in close relationship to faults in which they represent fluctuating fluid pressures during fault valve activity and are related to short lived local stress transitions in a consistent remote stress regime boullier robert 1992 sibson 1995 nielsen et al 1998 muchez et al 2000 collettini et al 2006 local mutually cross cutting vein sets may also develop in the presence of mechanical anisotropy around fault planes in a consistent remote stress field healy 2009 fagereng et al 2010 in all these studies transitions between regional stress regimes are defined as tectonic inversions a positive compressional tectonic inversion corresponds to a transition from an extensional to a compressional stress regime while a negative extensional tectonic inversion reflects the transition from compression to extension from healy d butler r w h shipton z k sibson r h eds 2012 faulting fracturing and igneous intrusion in the earth s crust geological society london special publications 367 51 69 http dx doi org 10 1144 sp367 5 the geological society of london 2012 publishing disclaimer www geolsoc org uk pub ethics by guest on august 9 2012 http sp lyellcollection org downloaded from

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

a note on fault reactivation reactivation of existing faults whose normal lies in the 1 3 plane of a stress field with effective principal compressive stresses 1 2 3 is considered for the simplest frictional failure criterion n n p where and n are respectively the shear and normal stresses to the existing fault p is the fluid pressure and is the static friction for a plane oriented at to 1 the stress ratio for reactivation is 1 3 1 cot 1 tan this ratio has a minimum positive value at the optimum angle for reactivation given by 12tan 1 1 but reaches infinity when 2 beyond which 3 0 is a necessary condition for reactivation an important consequence is that for typical rock friction coefficients it is unlikely that normal faults will be reactivated as high angle reverse faults or thrusts as low angle normal faults unless the effective least principal stress is tensile

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

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

paleostress directions from the preferred orientation of closed microfractures fluid inclusion planes in sandstone east texas basin u s a abstract closed extensional microfractures are useful indicators of paleostress directions in shallowly buried quartz cemented quartz arenites of the lower cretaceous travis peak formation a tabular sandstone and shale unit approximately 670 m thick that was deposited in the gradually subsiding northern gulf of mexico basin the closed microfractures are syndiagenetic fluid inclusion rich planes that have an ene strike that is subparallel to contemporary quartz filled extension veins and regional normal faults indicating that closed microfractures contain the maximum paleostress direction these microstructures are potentially useful for mapping paleostress trajectories in passive margin and platform settings particularly where samples are obtained from core previous experimental studies of microfracture closure suggest that microfractures evolved into fluid inclusion planes by dendritic quartz precipitation in east texas microfracture closure may have occurred at temperatures as low as 85 c and depths as shallow as 1500 m

high pressure and temperatures during the early stages of tungsten deposition at panasqueira revealed by fluid inclusions in topaz the variscan vein type panasqueira w sn cu deposit one of the main tungsten deposits in western europe has a long and complicated geological history the first vein infillings which consist of the quartz wolframite association as well as the first generation of topaz underwent significant deformation as a consequence most fluid inclusions of the earliest hydrothermal event are deformed and destroyed two preserved fluid inclusion assemblages are however found in the topaz overgrowth band and are dense aqueous carbonic inclusions as well as dense co2 dominated fluid inclusions the p t conditions of fluid trapping are constrained by using the intersection between isochores as well as graphite water equilibrium data and yield the following trapping conditions 500 20 c and 250 20 mpa these p t conditions are incompatible with fluid unmixing fluid chemistry results from water graphite equilibrium probably in metapelites at two distinct temperatures around 450 500 c for the predominant aqueous carbonic fluid and higher temperatures of maximal 550 c for the co2 rich fluid enriched in n2 these p t estimates are consistent with deep crustal levels around 8 10 km depth and a high geothermal gradient around c 60 c km 1 the ascending non magmatic fluids enriched in volatiles are essential in the ore genesis the high thermal gradients may be related either to new magma pulse after the formation of the panasqueira granite intrusion or to anomalous heat flux produced by the hot fluids ascending from migmatitic levels present at greater depth this hypothesis necessitates to consider the role of a crustal weakness which is attested both by the successive intrusions of several granitic magmas at the same place and the presence of inherited quartz filled structures so called seixo bravo found only in the panasqueira area